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Dramatic Remote Substitutent Effects on the Electronic Spin State of Bis(scorpionate) Iron(II) Complexes
Angewandte Chemie International Edition
Abstract
Einem molekularen Schraubstock ähnelt der violette Bis(tert-butylscorpionat)-Eisen(II)-Komplex, der bei Raumtemperatur trotz des sterischen Einflusses von sechs Methylgruppen am Moleküläquator vollständig im Low-Spin-Zustand vorliegt. Die sperrigen tert-Butylsubstituenten an den Molekülenden wirken wie Feststellschrauben, die die bevorzugte Bildung des Low-Spin-Komplexes erzwingen.
... This is apparent in the [Fe(RB{mpz} 3 ) 2 ] scorpionates ( Figure 2), which are high-spin for R = H, pz, nBu, iBu, C 4 H 4 I-4 or C 6 H 4 CCH 4 . However, when R = tBu, the compound is low-spin at 300 K but undergoes gradual SCO upon heating in the solid state [55]. This reflects steric repulsion between the tertbutyl substituents and the H5 atoms on the pyrazolyl rings, which promotes the low-spin state by disfavoring the expanded metal coordination sphere adopted by the high-spin form ( Figure 2). ...
... This is apparent in the [Fe(RB{mpz}3)2] scorpionates ( Figure 2), which are high-spin for R = H, pz, nBu, iBu, C4H4I-4 or C6H4CCH4. However, when R = tBu, the compound is low-spin at 300 K but undergoes gradual SCO upon heating in the solid state [55]. This reflects steric repulsion between the tertbutyl substituents and the H5 atoms on the pyrazolyl rings, which promotes the low-spin state by disfavoring the expanded metal coordination sphere adopted by the high-spin form ( Figure 2). ...
... Structures and crystallographic space-filling views of two complexes exhibiting low-spin states stabilized by the highlighted steric repulsions at the periphery of the molecules[55,57]. The orientation of the space-filling views is similar to the structure diagrams. ...
The relationship between chemical structure and spin state in a transition metal complex has an important bearing on mechanistic bioinorganic chemistry, catalysis by base metals, and the design of spin crossover materials. The latter provide an ideal testbed for this question, since small changes in spin state energetics can be easily detected from shifts in the spin crossover equilibrium temperature. Published structure-function relationships relating ligand design and spin state from the spin crossover literature give varied results. A sterically crowded ligand sphere favors the expanded metal–ligand bonds associated with the high-spin state. However, steric clashes at the molecular periphery can stabilize either the high-spin or the low-spin state in a predictable way, depending on their effect on ligand conformation. In the absence of steric influences, the picture is less clear since electron-withdrawing ligand substituents are reported to favor the low-spin or the high-spin state in different series of compounds. A recent study has shed light on this conundrum, showing that the electronic influence of a substituent on a coordinated metal ion depends on its position on the ligand framework. Finally, hydrogen bonding to complexes containing peripheral N–H groups consistently stabilizes the low-spin state, where this has been quantified.
... The archetypal [FeTp 2 ] complex, discovered in 1967 30,31 and extensively studied since then, [32][33][34][35][36] exemplifies the robust spin crossover properties and remarkable stability under light, electromagnetic fields, air and water which are characteristic of this family of compounds. Multiple reports have shown the impact of substitution at the pyrazole rings or at the boron on the spin crossover properties, 27,28,[37][38][39][40][41][42][43][44][45][46] achieving transition temperatures T 1/2 ranging from 85 K for [{(3-Me-pz) 3 BH} 2 Fe] 27 to 365 K for the unsubstituted complex. 35 Most of those studies, however, have been limited to complexes with six identical pyrazoles, and have concentrated on the steric effects of substitution in position 3 (closer to the metal). ...
Di(trispyrazolylborato)iron(II) ([Tp2Fe]) complexes represent one of the most robust class of spin crossover complexes. Their stability renders them particularly suitable for integration in nanoscale devices, e.g. as sensors or information storage units. While prior studies of the functionalization of those derivatives have been focused on the electronic and steric effect of alkyl and -CF3 groups in position 3, a pyrazole exchange reaction between nitropyrazole and either trispyrazolylborate or its iron complex allows the regioselective installation of nitro substituents in positions 3, 4 and 5 of [Tp2Fe] complexes. The degree of substitution can be varied from 1 to 4 functionalized pyrazoles per complex. The amine functionalized analogues are accessed by reduction of the nitro analogues under hydrogen transfer conditions. With the exception of di- and tetra-3-NO2 substituted complexes, all derivatives display spin crossover properties in the solid state, with transition temperatures ranging from 180 to 380 K and showing different degree of abruptness, but no hysteresis. The Slichter-Drickamer model was used to extract empirical thermodynamic transition parameters, allowing a systematic investigation of the influence of stoichiometry, position, and electronic nature of the substitution on the magnetic properties of the complexes. Steric effects dominate for substitution in position 3, but electronic effects are significant for the other positions.
... We see in the bottom figures of Fig. 7 that, except at the border of the lattice, both distances, d x , and d y are invariant by translation, and their values perfectly match the analytical prediction of R HL relax given in Eq. (6), confirming if any the antiferro-elastic ordered character of this phase. On the other hand, it is worth noticing that for frustration parameter values ξ < 1.0, leading to a two-step transition, we could find a value of ∆ and g for which T − ↓ ∼ 0 K, which then consequently leads to an incomplete spin transition, which are often observed experimentally [5,33,39], by changing the size of the ligand or the anion [40][41][42][43][44][45][46][47][48][49][50][51]. If we increase further the frustration ratio, ξ, the HS residual fraction appearing at lowtemperature increases (black curve in Fig. 8) and reaches the value n HS 0.77 for ξ = 1.5. ...
Two- and multi-steps spin transitions are frequently observed in switchable cooperative molecular solids. They present the advantage to open the way for three or several-bits electronics. Despite of extensive experimental studies, their theoretical description was to date only phenomenological, based on Ising models including competing ferro- and antiferro-magnetic interactions, even though it is recognized, that the elastic interactions are at the hearth of the spin transition phenomenon, due to the volume change between the low- and high-temperature phases. To remedy this shortcoming, we designed the first consistent elastic model, taking into account both volume change upon spin transition and elastic frustration. This ingredient revealed to be powerful, since it enabled to obtain all observed experimental configurations in the consistent way. Thus, according to the strength of the elastic frustration, the system may undergo: first-order transition with hysteresis, gradual, hysteretic two-step or multi-step transitions, and incomplete transitions. Furthermore, the analysis of the spatial organization of the HS and LS species in the plateau regions revealed the emergence of complex antiferroelastic patterns going from simple antiferromagnetic-like order to long-range spatial modulations of the high-spin fraction. These results enable to identify the elastic frustration as the fundamental mechanism at the origin of the very recent experimental observations showing the existence of organized spatial modulations of the high-spin fraction inside the plateau of two-step spin transitions.
Di(trispyrazolylborato)iron(II) ([Tp2Fe]) complexes represent one of the most robust classes of spin-crossover complexes. Their stability renders them particularly suitable for integration in nanoscale devices, e.g. as sensors or information storage units. While prior studies of the functionalization of those derivatives have been focused on the electronic and steric effects of alkyl and -CF3 groups in position 3, a pyrazole exchange reaction between nitropyrazole and either trispyrazolylborate or its iron complex allows the regioselective installation of nitro substituents in positions 3, 4 and 5 of the [Tp2Fe] complexes. The degree of substitution can be varied from 1 to 4 functionalized pyrazoles per complex. The amine-functionalized analogues are accessed by reduction of the nitro analogues under hydrogen transfer conditions. With the exception of di- and tetra-3-NO2 substituted complexes, all derivatives display spin crossover properties in the solid state, with transition temperatures ranging from 180 to 380 K and showing different degrees of abruptness but no hysteresis. The Slichter-Drickamer model was used to extract the empirical thermodynamic transition parameters, allowing a systematic investigation of the influence of stoichiometry, position, and electronic nature of the substitution on the magnetic properties of the complexes. The steric effects dominate for substitution in position 3 but the electronic effects are significant for the other positions.
We review here new advances in the synthesis and investigation of iron(II) coordination compounds with tris(pyrazol-1-yl)methane and its derivatives as ligands. The complexes demonstrate thermally induced spin crossover accompanied by thermochromism. Factors that influence the nature and temperature of the spin crossover are discussed.
The asymmetric unit of the title compound, [Fe(C 13 H 18 BN 6 ) 2 ], contains two half independent complex molecules. In each complex, the Fe II atom is located on an inversion center and is surrounded by two scorpionate ligand butyltris(1 H -pyrazol-1-yl)borate molecules that coordinate to the iron(II) ion through the N atoms of the pyrazole groups. The two independent complex molecules differ essentially in the conformation of the butyl substituents. In the crystal, the complex molecules are linked by a series of C—H...π interactions, which generate a supramolecular three-dimensional structure. At 120 K, the average Fe—N bond distance is 1.969 Å, indicating the low-spin state of the iron(II) atom, which does not change upon heating, as demonstrated by high-temperature magnetic susceptibility measurements.
The complex [Fe(HL*)2](OTf)2, 1, where HL* = bis(3,5-dimethylpyrazol-1-yl)(3-1 H-pyrazole)methane, was prepared in order to compare its magnetic properties with those of the analogous parent complex, [Fe(HL)2](OTf)2, that lacks methyl groups on pyrazolyl rings and that undergoes spin crossover (SCO) from the low spin (LS) to the high spin (HS) form above room temperature. It was anticipated that this new semibulky derivative should favor the HS state and undergo SCO at a lower temperature range. During this study, six crystalline forms of 1 were prepared by controlling the crystallization conditions. Thus, when reagents are combined in CH3CN, an equilibrium mixture of cis and trans isomers is established that favors the latter below 311 K. The trans isomer can be isolated exclusively as a mixture of solvates, LS trans-1·2CH3CN and HS trans-1·4CH3CN, by cooling CH3CN solutions to -20 °C with the former being favored at high concentrations and short crystallization times. Subsequently, vapor diffusion of Et2O into CH3CN solutions of pure trans-1·2CH3CN gives solvate-free HS trans-1. Subjecting trans-1·2CH3CN to vacuum at room temperature gives microcrystalline trans-1·CH3CN, identified by elemental analysis and its distinct powder X-ray diffraction pattern. If an isomeric mixture of 1 is subject to room-temperature vapor diffusion, then a crystalline mixture of HS isomers cis-1 and trans-1 is obtained. Finally, slowly cooling hot acetonitrile solutions of isomeric mixtures of 1 to room temperature gives large prisms of HS co-1, a species with both cis and trans isomers in the unit cell. The complexes trans-1, trans-1·CH3CN, cis-1, and co-1 undergo SCO below 250 K while trans-1· xCH3CN ( x = 2, 4) solvates do not undergo SCO before desolvation.
Polynorbornenes prepared by vinylic addition polymerization and bearing pendant alkenyl groups serve as skeletons to support trispyrazolylborate ligands (Tpx) built at those alkenyl sites. Reaction with CuI in acetonitrile leads to VA‐PNB‐TpxCu(NCMe) with a 0.8‐1.4 mmol incorporation of Cu per gram of polymer. The presence of tetracoordinated copper(I) ions has been assessed by FTIR studies with the corresponding VA‐PNB‐TpxCu(CO) adducts, that match those for discrete TpxCu(CO). The new materials have been employed as heterogeneous catalysts in several carbene and nitrene transfer reactions, showing a behavior similar to the homogeneous counterparts but also being recycled several times maintaining a high degree of activity and selectivity. This is the first example of supported Tpx ligands onto polymeric supports with catalytic applications.
Herein, we report the synthesis of two novel ionic Ti complexes possessing three [N,O]-type bidentate ligands from the reaction of Fe metallascorpionate ligands possessing extended alcohol groups and TiCl4. The reaction of substituted hydroxyphenyl tetrazole and Fe(ClO4)3 in a molar ratio of 3:1 afforded iron scorpionate metalloligands possessing extended arms, which were characterized by IR spectroscopy and ESI-TOF-MS spectrometry. Their molecular structures were also confirmed as neutral Fe-centered scorpionate complexes by X-ray crystallography, in which the extended alcohol groups adopted a tripodal geometry. Moreover, two different crystals of iron scorpionate metalloligand grown from CH2Cl2 and CH3OH were studied, revealing that, in the latter crystal, the tripod arms are folded and aligned toward the C3-rotational axis of the molecule, whereas the tripod arms are unfolded and spread outward from the rotational axis in the former crystal. These metalloligands are solvatochromatic; a bathochromic shift was observed as the solvent polarity increased. From the reaction, the aforesaid Fe complexes were further reacted with TiCl4 in a molar ratio of 1:1 to produce ionic [TiL3](+)[FeCl4](-) (L = substituted hydroxyphenyl tetrazole) complexes from the transmetalation of Ti and Fe. The complexes were characterized by various analytical methods including UV/vis and IR spectroscopies, electrospray time-of-flight mass spectrometry (ESI-TOF-MS), and X-ray crystallography.
The reaction of three equivalents of 5-(2,4-dihydroxyphenyl)-1H-tetrazole (H3dhptz) with iron(III) perchlorate in the presence of potassium hydroxide afforded the metallascorpionate framework. By simple recrystallization, an interesting coordination polymer, [K4.5Fe(dhptz)1.5(Hdhptz)1.5DMF7(H2O)3.5]n, was obtained. It possesses two potassium ions with either κ³-O or κ³-N binding modes, resulting in an array of K–Fe–K metal ions and a short Fe–K distance (3.427 Å). One scorpionate ligand possesses 4.5 potassium ions at six different positions in the solid state by means of the bridging oxygen atoms of the DMF solvent. Further coordination modes between the potassium ions and water/DMF provide a one-dimensional (1-D) metallascorpionate array and allow the formation of a 3-D hexagonal coordination network.
Identifying and quantifying the individual factors affecting the temperature and properties of the spin crossover in transition metal complexes is a challenging task, because many variables are involved. While the most decisive factor is the crystal field imparted by ligands around the active metal center, some less common actors are intramolecular steric repulsions or non-covalent interactions. A series of three Fe(II) complexes of 1,3bpp derivatives (2-(pyrazol-1-yl)-6-(1H-pyrazol-3-yl)pyridine) have been prepared and characterized crystallographically to probe these effects; [Fe(1,3bpp)2](ClO4)2 (1), [Fe(met1,3bpp)2](ClO4)2 (2) and [Fe(dimet1,3bpp)2](ClO4)2 (3). The ligands exibit none, one or two methyl substituents on the pyrazol-1-yl heterocycle. These groups exert a dramatic effect on the SCO temperature in the solid state, and, most significantly, in solution (with TSCO (3) > TSCO (1) > TSCO (2)). Extensive DFT calculations have unveiled the origin of these effects which lie in the intramolecular non-covalent or steric interactions rather than resulting from crystal field effects.
The employment of halogen-substituted pyrazole ligands in nickel(II) clusters afforded three anionic Ni(II) cubes, (HNEt3)2[Ni8(Xpz)12(OH)6] (X = Cl, 1·2CH3CN; X = Br, 2·2CH3CN; X = I, 3·12CH3CN; pz = pyrazolate). Clusters 1–3 are isolated as dianionic compounds which have similar cube geometry with each vertex occupied by a Ni(II) atom and eight μ4-OH− capped on eight square faces. The Xpz ligands adopt a bidentate mode to bind paired Ni(II) atoms on the edge of the cube. Interestingly, the three Ni8 cubes have similar symmetry, size and shape but pack together to form different lattice symmetry, which should be dictated by the halogen-related supramolecular interactions (C–X⋯H, C–X⋯π and X⋯X halogen bonding) in different crystals. The electrochemistry of 1–3 showed a Ni(II)/Ni(I) redox couple with E1/2 at ca. 900 mV and the oxidization potential order is 1 > 3 > 2 depending on the halogen substituents. 1–3 exhibit excellent electrocatalytic performances toward the oxidation of nitrite. The different packing of Ni8 cubes in 1–3 also has an important influence on their magnetic behaviors. Complex 2 showed antiferromagnetic couplings between the Ni(II) ions within the cubes, whereas 1 and 3 exclusively exhibited mixed ferromagnetic and antiferromagnetic properties, leading to frustration and typical spin glass behavior.
The importance of scorpionate ligands in modern coordination chemistry continues to increase, because of their outstanding versatility, tunability and user-friendliness. Herein, we provide a short overview of recent developments in the classes of scorpionate ligands, derived from pyrazoles, triazoles, imidazoles, oxazolines, thioimidazoles and other similar systems, followed by an in-depth discussion of a new type of robust and tunable scorpionate ligand, tris(2-pyridyl)borates. The structure and synthesis of tris(2-pyridyl)borate (Tpyb) ligands are discussed, and key features of coordination of Tpyb ligands with metal ions, as well as supramolecular crystal packing, transmetallation and applications in metallo-supramolecular polymer chemistry are addressed.
Phenyltris(2-pyridyl)borates (Tpyb) are a promising class of tripodal "scorpionate"-type ligands with potential utility in the development of transition-metal complexes with interesting optical, electronic, or magnetic properties and as building blocks to metallosupramolecular polymers. We report here a new class of "third-generation"-type Tpyb ligands that contain different functional groups attached to the boron-bound aryl moiety. The synthesis, characterization, and metal-ion complexation behavior of ligands with iodo and trimethylsilyl groups are discussed. The electrochemical and absorption characteristics of the corresponding low-spin iron(II) and ruthenium(II) complexes are compared. We demonstrate the further elaboration of iodo derivatives with alkynes via Sonogashira-Hagihara coupling, a process that proceeds with high yield for the iron(II) and ruthenium(II) complexes but not for the free ligand. Borylation of the silyl-substituted ruthenium(II) complex with BBr3 was also investigated. In addition to the expected borylation product Ru(Tpyb-Bpin)2, the replacement of one (major product) or two phenyl groups is observed, suggesting that electrophilic borylation occurs at both the C(Ph)-Si and the C(Ph)-B aromatic carbon atoms. The successful attachment of a range of different functional groups at the periphery of the Tpyb metal complexes is expected to provide opportunities to access new polymeric materials via C-C coupling or click-type reactions.
The reaction of PhBCl2 with 1H-1,2,4-λ(3)-diazaphosphole in the presence of NEt3 gives a new scorpionate ligand, phenyl-tris(1,2,4-diazaphospholyl)borate (PhTdap). The coordination behaviour of this ligand toward transition and non-transition metals has been comprehensively studied. In the thallium(i) complex, Tl(PhTdap), κ(2)-N,N bonding supported with intramolecular η(3)-phenyl coordination has been observed in the solid state. Tl(PhTdap) also shows unusual intermolecular π-interactions between five-membered diazaphosphole rings and the thallium atom giving infinite molecular chains in the crystal. In the square planar complex [Pd(C,N-C6H4CH2NMe2)(PhTdap)], κ(2)-bonded scorpionate has been detected in both solution and in the solid state. For other studied compounds with the central metal ion Ti(iv), Mo(ii), Mn(i), Fe(ii), Ru(ii), Co(ii), Co(iii), Ni(ii) and Cd(ii), the κ(3)-N,N,N coordination pattern was observed. Electronic properties of PhTdap and its ligand-field strength were elucidated from UV-Vis spectra of transition-metal species. The CH/P replacement on going from tris(pyrazolyl)borate to the ligand PhTdap causes a slight increase in electronic density rendered to the central metal atom. The following order of ligand-field strength has been established: HB(3,5-Me2pz)3 < PhB(pz)3 < HB(1,2,4-triazolyl) < HB(pz)3 < PhB(1,2,4-triazolyl) < PhTdap. The crystal structures of ten metal complexes bearing the new ligand are reported. The possibility of PhTdap coordination through the phosphorus atom is also briefly discussed.
The functionalization of organic polymers with polydentate ligands offers opportunities in areas ranging from supported catalysts to materials with desirable magnetic, redox-active, stimuli-responsive, and self-healing properties. Herein, we present the synthesis and self-assembly of tris(2-pyridyl)borate (Tpyb)-functionalized homo and block copolymers, prepared via ring-opening metathesis polymerization (ROMP) of (bicyclo[2.2.1]hept-5-en-2-yl)-4-phenyl) (pyridin-1-ium-2-yl)di(pyridin-2-yl)borate (M1) and dimethyl-7-oxabicyclo[2.2.1]hept-5-ene-exo,exo-2,3-dicarboxylate (M2) using Grubbs third-generation catalyst. Controlled polymerization was confirmed by gel permeation chromatography (GPC; also referred to as size exclusion chromatography, SEC) and multinuclear NMR spectroscopy. The solution self-assembly in block-selective solvents (MeOH, THF) was investigated by dynamic light scattering (DLS), scanning electron microscopy (SEM), scanning tunneling electron microscopy (STEM), and transmission electron microscopy (TEM), and the microphase separation in a thin film was imaged by atomic force microscopy (AFM). Hydrolysis of the ester-substituted oxanorbornene block with NaOH led to a new copolymer with carboxylate functionalities that can be dispersed in water. The latter displays multiresponsive properties as each of the individual blocks can be reversibly switched between hydrophobic and hydrophilic states by simple adjustment of pH. Cross-linking of the block copolymer aggregates via metal ion complexation was accomplished, and the feasibility of metal ion exchange was demonstrated by energy-dispersive X-ray spectroscopy (EDX).
This chapter on novel mononuclear spin-crossover (SCO, or spin-transition, ST) complexes gives an overview of some new developments since 2003. The overview is limited to mononuclear systems, focusing on ST complexes with new or unusual coordination number (CN) and coordination spheres. The section on the novel coordination numbers (CN), coordination geometries and metal centres in this chapter starts with detailed reflections on iron(II) SCO complex with an N3O2C2 coordination sphere, where the iron centre has a CN of 7.2, 29. Examples of rare manganese(III)34 and cobalt(II)36 SCO complexes with N4O2 coordination spheres are also given. The chapter also discusses iron complexes with novel ligand donor atoms and new ligand systems. In this discussion, donor atom set is used as a sorting key, starting with a few selected examples with an N6 coordination sphere, followed by a larger section on new systems with an N4O2 coordination sphere and some new approaches.
Characterizing structural distortions in the metastable spin states of d4-d7 transition metal ion complexes is crucial to understand the nature of their bistability and eventually control their switching dynamics. In particular, the impact of the Jahn-Teller effect needs to be assessed for any electronic configuration that could be effectively degenerate, as in e.g. the high-spin (HS) manifold of highly symmetric homoleptic FeII complexes. However, capturing its manifestations remains challenging since crystallization generally alters the molecular conformations and their interconversion. With the rapid progress of ultrafast X-ray absorption spectroscopy, it is now possible to collect data with unprecedented signal-to-noise ratio, opening up for detailed structural characterization of transient species in the homogeneous solution phase. By combining the analysis of picosecond X-ray absorption spectra with DFT simulations, the structure of the photoinduced HS state is elucidated for solvated [Fe(terpy)2]2+ (terpy = 2,2′:6′,2″-terpyridine). This species can be viewed as the average 5B structure in D2 symmetry that originates from a dynamic Jahn-Teller effect in the HS manifold. These results evidence the active role played by this particular instance of vibronic coupling in the formation of the HS state for this benchmark molecule. Ultimately, correlating the interplay between intramolecular and intermolecular degrees of freedom to conformational strain and distortions in real time should contribute to the development of advanced functionalities in transition metal ion complexes.
We report formation of a new metallascorpionate ligand, [FeL3](3-) (IPtz), containing a Fe core and three 5-(2-hydroxyphenyl)-1H-tetrazole (LH2) ligands. It features two different binding sites, oxygen and nitrogen triangles, which consist of three oxygen or nitrogen donors from tetrazole. The binding affinities of the complex for three alkali metal ions were studied using UV spectrophotometry titrations. All three alkali metal ions show high affinities and binding constants (>3 × 10(6) M(-1)), based on the 1:1 binding isotherms to IPtz. The coordination modes of the alkali metals and IPtz in the solid were studied using X-ray crystallography; two different electron-donor sites show different coordination numbers for Li(+), Na(+), and K(+) ions. The oxygen triangles have the κ(2) coordination mode with Li(+) and κ(3) coordination mode with Na(+) and K(+) ions, whereas the nitrogen triangles show κ(3) coordination with K(+) only. The different binding affinities of IPtz in the solid were manipulated using multiple metal precursors. A Fe-K-Zn trimetallic complex was constructed by assembly of an IPtz ligand, K, and Zn precursors and characterized using X-ray crystallography. Oxygen donors are coordinated with the K ion via the κ(3) coordination mode, and nitrogen donors are coordinated with Zn metal by κ(3) coordination. The solid-state structure was confirmed to be a honeycomb coordination polymer with a one-dimensional infinite metallic array, i.e., -(K-K-Fe-Zn-Fe-K)n-.
The reaction of 4-(dibromoboryl)styrene with 2-pyridylmagnesium chloride resulted in the formation of 4-styryl-tris(2-pyridyl)borate free acid (StTypb), a new polymerizable nonpyrazolyl “scorpionate” ligand. StTypb did not undergo self-initiated polymerization under ambient conditions and proved to slowly polymerize through standard radical polymerization at 90 °C. Nitroxide-mediated polymerization (NMP) of StTypb at 135 °C proceeded with good control, resulting in a polymer of Mn = 27400 and PDI = 1.21. The TEMPO-terminated homopolymer successfully initiated the polymerization of styrene, generating an amphiphilic block copolymer with DPn of 1200 and 78 for the PS and the StTypb block, respectively. A similar block copolymer with DPn of 29 and 20 for the PS and the StTypb block respectively was obtained in a reverse polymerization procedure from a PS macroinitiator. The self-assembly of these block copolymers was examined in selective solvents and preliminary metal complexation studies were performed.
Sandwich-like metal complexes (Tpyb)2M (M = Mg, Fe, Mn) that are based on the novel t-butylphenyltris(2-pyridyl)borate ligand were prepared and fully characterized by multinuclear NMR spectroscopy, high-resolution matrix-assisted laser desorption/ionization (MALDI) mass spectrometry, and single crystal X-ray crystallography. The unique steric and electronic nature of the Tpyb ligand led to structural parameters and properties that are quite different to those of previously reported tris(pyrazolyl)borate and tris(2-pyridyl)aluminate complexes. Most importantly, depending on the crystallization procedure, supramolecular structures could be generated with relatively smaller (ca. 4-5 Å) or larger (ca. 8 Å) diameter pores propagating throughout the crystal lattice. Although the supramolecular structures are held together only by weak intermolecular C-H···π interactions, the solvent in the larger channels could be completely removed without any loss of crystallinity or degradation of the framework. Surface area and gas uptake measurements on the Mg complex further confirmed the permanent porosity, while the calculated non-localized density functional theory (NLDFT) pore diameter of 8.6 Å proved to be in excellent agreement with that obtained from single crystal X-ray crystallography. Our new materials are remarkably thermally stable as degradation did not occur up to about 400 °C based on thermogravimetric analysis (TGA), and a sample of the Mg complex showed no loss of crystallinity even after heating to 140 °C under high vacuum for 72 h according to single crystal X-ray diffraction data.
We report the syntheses, characterisations, and spin-crossover behaviours of four mononuclear trans-[Fe(L1-L4)2(NCS)2] complexes (L = 4-(p-R1-phenyl)-3,5-bis(2-pyridyl)-4H-1,2,4-triazole and 4-(p-methylphenyl)-3-(p-R2-phenyl)-5-(2-pyridyl)-4H-1,2,4-triazole, () R1 = Cl (L1); () R1 = Me (L2); () R2 = MeO (L3); () R2 = F (L4)). The X-ray single crystal structural analysis reveals that and crystallize in the triclinic space group P1[combining macron], whereas ·0.5H2O and crystallize in the monoclinic space group P21/c, and all complexes possess a similar distorted [FeN6] octahedron with two trans-positions NCS(-) anions. Changing of the distal substituent on the phenyl ring of the triaryltriazoles causes differences in the crystal packing of the complexes that are key to their magnetic properties. Magnetic measurements indicate that shows an abrupt transition with T1/2 = 235 K, displays a 3 K hysteresis loop with T1/2(↓) = 217 K and T1/2(↑) = 220 K, exhibits a gradual spin transition with T1/2 = 156 K and undergoes an incomplete (70%) spin transition with T1/2 = 97 K. The substituted group effects and intermolecular interactions appear to be critical to spin transition modes and temperature in this family.
Tris(2-pyridyl)borates are introduced as a new robust and tunable "scorpionate"-type ligand family. A facile synthesis of this hitherto unknown ligand and its complexation to Fe(II) are described; the optical and electrochemical properties of the resulting iron complex are compared to complexes derived from tris(pyrazolyl)borate, tris(2-pyridyl)aluminate, and corresponding charge-neutral ligands.
Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).
We have identified a pair of structurally similar iron complexes in the oxidation state II that exist
in a low-spin and a high-spin electronic spin state in aqueous media, respectively. The low-spin,
diamagnetic complex (LS, 1) is mute in MRI while the high-spin, paramagnetic complex (HS, 2)
generates considerable contrast in MRI. These results demonstrate that iron(II) complexes,
hitherto neglected for contrast enhancement in MRI, have potential for the design of an MRI
probe that suffers passage from one state to the other under the influence of a targeted
biochemical activity and thus operates in an off–on mode. At 300 MHz (proton resonance
frequency at 7 T field strength) and in phosphate buffer, we found a longitudinal relaxivity (r1) of
1.29 mM�1 s�1 for 2 that, in light of the difference in unpaired electrons of the central metal
atoms (4 for FeII; 7 for GdIII), comes remarkably close to that of gadolinium(III)–DOTA
(2.44 mM�1 s�1), a commercialized MRI contrast agent. Since gadolinium complexes are always
paramagnetic and can therefore not be muted in MRI, the here presented Fe(II)-based system
offers an alternative strategy to develop responsive MRI probes.
In der Übergangsmetallchemie gibt es eine Klasse von Komplexverbindungen, bei denen eine Temperaturerniedrigung einen Wechsel im Spinzustand des Zentralatoms vom High-Spin- in den Low-Spin-Zustand bewirkt. Dabei ändern sich die magnetischen und optischen Eigenschaften, über die der thermische Spinübergang (auch Spincrossover genannt) sehr gut verfolgt werden kann. Dieses Phänomen tritt sowohl in flüssiger Phase als auch im Festkörper auf. Eine herausragende Stellung nehmen Eisen(II) — Spincrossover — Verbindungen ein, in denen der Spinübergang im Festkörper auf sehr unterschiedliche Weise — graduell, abrupt, mit Hysterese oder stufenweise — verlaufen kann und mit Mößbauer- und optischer Spektroskopie, mit magnetischen Suszeptibilitäts- und Wärmekapazitätsmessungen sowie durch Kristallstrukturanalyse intensiv untersucht worden ist. Die kooperative Wechselwirkung zwischen den einzelnen Komplexmolekülen kann befriedigend durch elastische Eigenschaften und durch die Änderung von Gestalt und Volumen der Komplexmoleküle beim Spinübergang erklärt werden. Bei Untersuchungen an Eisen(II)-Spincrossover-Verbindungen konnte man beobachten, daß sich der Low-Spin-Zustand mit grünem Licht in den High-Spin-Zustand umschalten läßt, der bei tiefen Temperaturen eine nahezu unendlich lange Lebensdauer haben kann (LIESST = Light-Induced Excited Spin State Trapping). Mit rotem Licht läßt sich der metastabile High-Spin- wieder in den Low-Spin-Zustand zurückschalten. Der Mechanismus des LIESST-Effekts ist aufgeklärt, die Zerfallskinetik im Detail untersucht und im Rahmen der Theorie strahlungsloser Übergänge verstanden. Anwendungen des LIESST-Effekts in der optischen Informationstechnik sind denkbar.
Transition metal chemistry contains a class of complex compounds for which the spin state of the central atom changes from high spin to low spin when the temperature is lowered. This is accompanied by changes of the magnetic and optical properties that make the thermally induced spin transition (also called spin crossover) easy to follow. The phenomenon is found in the solid state as well as in solution. Amongst this class, iron(II) spin crossover compounds are distinguished for their great variety of spin transition behavior; it can be anything from gradual to abrupt, stepwise, or with hysteresis effects. Many examples have been thoroughly studied by Mössbauer and optical spectroscopy, measurements of the magnetic susceptibilities and the heat capacities, as well as crystal structure analysis. Cooperative interactions between the complex molecules can be satisfactorily explained from changes in the elastic properties during the spin transition, that is, from changes in molecular structure and volume. Our investigations of iron(II) spin crossover compounds have shown that green light will switch the low spin state to the high spin state, which then can have a virtually unlimited lifetime at low temperatures (this phenomenom is termed light-induced excited spin state trapping—acronym: LIESST). Red light will switch the metastable high spin state back to the low spin state. We have elucidated the mechanism of the LIESST effect and studied the deactivation kinetics in detail. It is now well understood within the theoretical context of radiationless transitions. Applications of the LIESST effect in optical information technology can be envisaged.
Octahedral ferrous chelates based on the hydrotris (1‐pyrazolyl) borate ligand can be prepared which are fully high spin, or are fully low spin, or have an intermediate magnetic moment which is strongly temperature dependent. Mössbauer and magnetic susceptibility data characterizing this behavior in the solid state are presented and analyzed. The orbital splitting of the high spin ground state due to a trigonal component in the octahedral crystal field is ∼1000 cm−1 leaving an orbital singlet lowest. The thermal relaxation time between the singlet and quintet manifolds is anomalously long, leading to the observation of separate Mössbauer transitions for the two electronic states.
A series of iron(II) or iron(III) complexes of type FeL2 or [FeL2]+ X−(X = BF4 or BPh4) with L = hydridotris(1H-pyrazol-1-yl)borate(L0), tetrakis(1H-pyrazol-1-yl)borate(L0′), hydridotris(-methyl-1H-pyrazol1-yl)borate(L1), hydridotris(3,5-dimethyl-1H-pyrazol-1-yl)borate(L2), hydridotris(4-chlor-3,5-dimethyl-1H-pyrazol-1-yl)borate(L2Cl), hydridotris(3,4,5-trimethyl-1H-pyrazol-1-yl)borate(L3), hydridotris(1,2,4-1H-triazol-1-yl)borate(Ltz) and hydridotris(3a,7a-benzo-1,2,3-1H-triazol-1-yl)borate(Lbtz) has been prepared. The series has been characterized mainly by th 57Fe Mössbauer effect. The X-ray crystal structures of iron(II)bis[hydridotris(3-methyl-1H-pyrazol-1-yl)borate] (3) and iron(III)bis[hydridotris(1H-pyrazol-1-yl)borate] tetrafluroborate (9) have been resolved. Discrete units with octahedral FeIIN6 or FeIII sites are present in each. The Fen bond length ranges from 2.197(4) to 2.215(4) Å in 3 and from 1.948(6) to 1.964(6) Å in 9. The bond angle ranges from 86.4(2)° to 93.7(2)° in 3 and from 88.3(4)° to 90.9(5)° in 9.
The separation of first-series transition metal and Cd2+ ions has been studied by solvent extraction using methyl substituted poly(pyrazolyl)borates. These ligands did not extract all of the studied metal ions at a lower pH compared to the parent ligands, and the selectivity was high for Cu(II) and low for Fe(II), although the extracted species were A 2M complexes as well (A-: poly(pyrazolyl)borate, M 2+: metal ion) for all the four ligands. An examination of the structures of the nine complexes determined the present and related ones, which were previously determined elsewhere, revealed that 3-methyl groups of the pyrazolyl rings are arrayed around a metal ion, and that the distances between the methyl groups are smaller than the sum of van der Waals radii of the methyl groups. This suggests that the interligand contact of 3-methyl groups decreases the stability of the complexes. Since the geometry in the copper complexes with these ligands is square bipyramid, the increase in steric energy caused by the interligand contact seems to be smaller than that for those complexes having similar ionic radii. Moreover, the spin state of the iron complexes with these ligands is high-spin due to interligand contact. Thus, while the interligand contact causes a general decrease in the stability of all the complexes, the effect is smallest for Cu(II) and largest for Fe(II), resulting in a unique selectivity.
The synthesis, structural, magnetic, and Mössbauer spectral properties of Fe[(C6H5)B(3-Mepz)3]2 (2, pz=pyrazolyl ring) are reported. The single crystal X-ray structural results indicate that at both 294 and 90K Fe[(C6H5)B(3-Mepz)3]2 (2) has a distorted octahedral iron(II) coordination environment with Fe–N bond distances that average ca. 2.18Å, distances that clearly indicate the high-spin nature of the complex at these temperatures. Both the magnetic and Mössbauer spectral results indicate that Fe[(C6H5)B(3-Mepz)3]2 (2) remains high-spin down to 4K. This result is surprising because the closely related Fe[(p-R-C6H4)B(3-Mepz)3]2 [R=I, CCH, CCSiMe3, or CCC6H5] complexes do exhibit electronic spin-state changes on cooling. The absence of a spin-state crossover in Fe[(C6H5)B(3-Mepz)3]2 (2) is the result of its molecular structure in which the three pyrazolyl rings of each tridentate ligand are highly twisted away from an ideal C3v coordination symmetry, a twisting distortion that favors the high-spin form of the complex with longer Fe–N bond distances.
Some 3dn (4 ≤ n ≤ 7) transition metal compounds exhibit a cooperative transition between a low-spin (LS) and a high-spin (HS) state.
This transition is abrupt and occurs with a thermal hysteresis, which confers a memory effect on the system. The intersite
interactions and thus the cooperativity are magnified in polymeric compounds such as [Fe(Rtrz)3]A2·nH2O in which the Fe2+ ions are triply bridged by 4-R-substituted-1,2,4-triazole molecules. Moreover, in these compounds, the spin transition is
accompanied by a well-pronounced change of color between violet in the LS state and white in the HS state. The transition
temperatures of these materials can be fine tuned, using an approach based on the concept of a molecular alloy. In particular,
it is possible to design a compound for which room temperature falls in the middle of the thermal hysteresis loop. These materials
have many potential applications, for example, as temperature sensors, as active elements of various types of displays, and
in information storage and retrieval.
Despite the widespread use of tris(pyrazolyl)borate ligands, their isoelectronic, neutral analogs, tris(pyrazolyl)methane ligands, have not been extensively studied. By use of appropriate starting materials, such as [Cu(NCMe)4]PF6 or [Cd2(thf)5](BF4)4, stable cationic complexes of the ligands HC(3,5-Me2pz)3, HC(3-Phpz)3 and HC(3-Bupz)3 can be prepared with the metals copper(I), silver(I), cadmium(II), lead(II) and thallium(I). In many cases isoelectronic groups of complexes, such as [HB(3,5-Me2pz)3]2Cd. {[HC(3,5-Me2pz)3]Cd[HB(3,5-Me2pz)3]}, and {[HC(3,5-Me2pz)3]2Cd}, have been prepared and shown to have very similar structures. The Cd NMR chemical shifts of the three complexes are also very similar. The isoelectronic complexes {[HC(pz)3]2Pb} and [HB(pz)3]2Pb have similar distorted six-coordinate structures. The isoelectronic pair {[HC(3,5-Me2pz)3]2Pb} and [HB(3,5-Me2pz)3]2Pb have very similar octahedral structures in which the lone pair on the lead(ll) is stereochemically inactive. Thus, for most cases the tris(pyrazolyl)methane and tris(pyrazolyl)borate ligands bond to these metals in a similar fashion.
The Fe 2+ complexes based on the hydrotris(1-pyrazolyl)borate ligand provide an example of a "spin equilibrium" between high- and low-spin forms. Fully high-spin, fully low-spin, and complexes of intermediate spin can be produced by appropriate substitution. Optical spectra, susceptibility data, and magnetic resonance experiments leading to the characterization of these equilibria are presented.
The structures of bis[hydrotris(1-pyrazolyl)borato]iron(II) (1), C18H2N12B2Fe, and bis[hydrotris(3,5-dimethylpyrazolyl)borato]iron(II) (2), C30H44N12B2Fe, have been elucidated by single-crystal X-ray diffraction techniques. The iron atom in 1 is in the low-spin state at room temperature, whereas in 2 the iron atom is in the high-spin state at room temperature. Both molecules possess virtual D3d symmetry in the solid state. The average metal-ligand bond distances, 〈Fe-N〉, are 1.973 (7) Å for 1 and 2.172 (22) Å for 2. The 〈Fe-N〉 value for the high-spin complex is thus 0.199 Å longer than the similar value for the low-spin complex. This is one of the largest values observed for the bond length expansion between the low-spin and high-spin states. The averages of the independent N-Fe-N bond angles are 88.3 (2)° for 1 and 86.6 (5)° for 2. Crystal data for 1 are as follows: space group P21/c; Z = 4; a = 12.258 (3), b = 11.606 (2), c = 16.518 (3) Å; β = 107.56(2)°; V = 2240 Å3; R = 5.2% for 1890 reflections. Crystal data for 2 are as follows: space group P1; Z = 1; a = 8.824 (3), b = 10.216 (4), c = 10.787 (4) Å; α = 116.36 (3), β = 85.24 (3), γ = 100.09 (3)°; V = 858 Å3; R = 3.9% for 1819 reflections.
The temperature dependence of the magnetic susceptibility (4.2-300 K) is reported for a series of crystalline salts of first-row transition-metal ions encapsulated by hexaamine ligands of the sar type, namely, [M(sar)]n+, where sar = 3,6,10,13,16,19-hexaazabicyclo[6.6.6]icosane, or [M((X,Y)sar)]n+, where X = Y= 1,8-NH2 1,8-NH3+ or X = 1-CH3, Y = 8-H. The cage complexes of FeIII, CoIII, and NiIII all exhibit low-spin ground states [of 2T2g and 1A1g (Oh) and 2A1g (D4h) origin] whereas that of MnIII is high-spin [5A1g or 5B1g (D4h)]. The complexes of MnII, CoII, and NiII are high-spin [of 6A1g, 4T1g, and 3A2g (Oh) origin], but surprisingly, those of FeII exhibit either a low-spin (1A1g) or a high-spin (5T2g) ground state depending on the nature of the apical substituent and the lattice. Clearly, the magnitude of the ligand field parameters (10Dq and B)† generated by these saturated macrobicycles for the 3d6 FeII is that required for the high-spin/low-spin crossover for six saturated amine ligands. The ground states for VIV (2T2g), VIII (3T1g), CrIII (4A2g), CuII (2B1g in D4h), and ZnII (1A1g) appear to be unambiguous. The magnitude of the zero-field-splitting parameter has been estimated from the low-temperature χ(T) data for the CrIII and high-spin MnIII, MnII, FeII, CoII, and NiII cage complexes. The low-spin (2Eg origin) state for CoII is stabilized by the (aza)capten ligand because of the larger nephelauxetic effect of sulfur combined with appreciable Jahn-Teller splitting of the 2Eg ground state. The magnetic properties in general reflect the reduction in symmetry observed crystallographically for the individual metal ions.
A study of the infrared and Mössbauer spectra and magnetic properties of Fe[HB(pz)3]2 indicate that this nominally low-spin iron(II) compound undergoes a spin-state crossover to the high-spin state accompanied by a crystallographic phase change at about 400 K. The crystallographic phase transition shatters the crystals and leads to a large hysteresis in the magnetic moment upon cooling from 460 to about 250 K. The analyses of the Mössbauer spectra, obtained as a function of temperature during both heating and cooling, indicate the presence of spin-state relaxation on the Mössbauer time scale of 10-8 s. The activation energy for this relaxation process is 7300 cm-1 for freshly sublimed Fe[HB(pz)3]2 and 1760 cm-1 after the crystals have undergone the phase transition. Both the magnetic moments and Mössbauer spectra indicate that between 295 and 430 K the ground state of Fe[HB(pz)3]2 has a temperature-dependent population of both the high-spin and low-spin electronic configurations. The optical absorption spectrum provides further support for the spin-state crossover and DSC and volume expansion studies indicate the presence of the phase transition at the spin-crossover. Analysis of the far-infrared spectrum utilizing 54Fe/57Fe substitution at both room and high temperatures allows unambiguous assignment of the Fe-N stretching bands both in high-spin and low-spin forms. The low-spin Fe-N stretching bands appear at 459, 434.5, and 399.5 cm-1, whereas upon heating the high-spin Fe-N stretching bands appear at 257.5 and 223 cm-1. Measurement of the area of these bands as the temperature is increased reflects the change in the population of the low-spin and high-spin states.
The spin state of Fe(II) poly(pyrazolyl)borate complexes is highly dependent upon substituents on the ligand molecule. While [B(pz)(4)]Fe-2 (1, pz = 1-pyrazolyl) and [PhB(pz)(3)]Fe-2 (2) are in a low-spin state in CHCl3 at ambient temperature, [HB(pz)(3)]Fe-2 (3) is in a spin-crossover state and [HB(3,5-Me(2)pz)(3)]Fe-2 (4) is in a high-spin state. Here, we present the first rational explanation of spin-crossover caused by substituents. X-ray structures of low-spin 1 (the triclinic space group P $($) over bar$$ 1 with a = 11.943(3) Angstrom, b = 12.310(3) Angstrom, c = 9.628(2) Angstrom, alpha = 96.12(2)degrees, beta 101.22(1)degrees, gamma = 100.02(2)degrees, V = 1352.7(5) Angstrom(3), and Z = 2) and 2 (the orthorhombic space group Pca2(1) with a = 18.046(2) Angstrom, b = 8.894(3) Angstrom, c 18.309(4) Angstrom, V = 2938(1) Angstrom(3), and Z = 4) were determined and compared with the reported structures of low-spin 3 and high-spin 4. All the complexes had a trigonally distorted geometry, and the Ligands were tridentate. H-1-NMR suggested that the solution structures of the complexes were similar to the X-ray structures. The key to the issue was the size of the Fe(II) ion. The fourth substituents on the boron atom in 1 and 2 forced a narrow arrangement on the tripod of the coordinated pyrazolyl groups and favored low-spin complex formation with a small Fe(II) ion. For 4, the methyl group at the 3-position of the pyrazolyl ring brought about severe interligand contact around the metal ion and prohibited low-spin complex formation. These contacts were ascertained by means of molecular mechanics calculations. Consequently, poly(pyrazolyl)borates can control the electron; configuration of Fe(II) ion through intra- and interligand contact.
The reaction of (dibromoboryl)ferrocene (1a) with pyrazole/NEt3 gave the ferrocenyltri-1-pyrazolylborate ligand 2. Its thallium(I) derivative 2-Tl provides the first example of a polymeric structure with bridging tri-1-pyrazolylborate units in the solid state. The trinuclear iron complex 2-Fe, which is related to 1,1′-terferrocene, was obtained by reaction of 2 with FeCl2. The bis(polydentate) ligand 1,1′-ferrocenediyl-bis(tri-1-pyrazolylborate) (3-Li) was prepared from 1,1′-bis-[bis(dimethylamino)boryl]ferrocene (1c) and a mixture of lithium pyrazolide/pyrazole in refluxing toluene/THF. 3-Li reacts with TlNO3 to give the thallium(I) complex 3-Tl.
This review describes the recent advances made in the syntheses, structures (including the use of synchrotron sources and variable-temperature crystal structures and PXRD determinations of unit cell axes), magnetism, and photomagnetism of dinuclear and 1-D chain spin-crossover compounds of (primarily) iron(II), iron(III) and cobalt(II). It focuses on the work of the author's group and also includes a discussion of related work by other groups. In particular, the early and ongoing work by Kahn, Real, Gütlich and co-workers on bipyrimidine-bridged FeII dinuclear materials has been extended by us and others to include many other new bridging and capping ligands. This had led to a fuller understanding of the spin transitions that involve HS-HS, HS-LS and LS-LS pair states. The same is being achieved for new 1-D chain materials, and notable advances include structural observation of ordered and disordered metal centres and corresponding spin states. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
Three tricyanometalate precursors, (Bu4N)[(PhTp)Fe(CN)3]·H2O (1), (Bu4N)[(MeTp)Fe(CN)3] (2), and (Bu4N)[(iBuTp)Fe(CN)3] (3) [Bu4N+ = tetrabutylammonium cation; PhTp = tris(pyrazolyl)phenylborate; MeTp = methyltris(pyrazolyl)borate; iBuTp = 2-methylpropyltris(pyrazolyl)borate], were successfully synthesized. By using 1–3 as building blocks, four rectangular clusters, [(PhTp)Fe(CN)3Cu(bpy)(H2O)(ClO4)]2·2H2O (4; bpy = 2,2′-bipyridine), [(PhTp)Fe(CN)3Ni(tren)]2(ClO4)2 [5; tren = tris(2-amino)ethylamine], [(MeTp)Fe(CN)3Ni(tren)]2(ClO4)2·2H2O (6), and [(iBuTp)Fe(CN)3Ni(tren)]2(ClO4)2·2H2O·2CH3OH (7), were prepared in parallel and structurally characterized. All clusters show similar square structures, where FeIII and MII (M = CuII or NiII) ions are alternatively located on the rectangle corners. The cyclic voltammograms of FeIII2NiII2 clusters 5–7 reveal two quasireversible iron-centered reduction processes and two quasireversible nickel-centered oxidation processes. Magnetic studies show intramolecular ferromagnetic coupling and appreciable magnetic anisotropy in clusters 4–7. Complexes 5–7 show obvious frequency dependence in the alternating current magnetic susceptibility data, which indicates single-molecule magnet behavior with preexponential factors of τ0 = 4.5 × 10–8 s (5), 6.1 × 10–8 s (6), and 5.0 × 10–8 s (7) and the effective spin-reversal barriers of Ueff = 17.5 K (5), 20.6 K (6), and 20.8 K (7).(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)
The spin crossover phenomenon in molecular inorganic compounds is one of the most spectacular examples of bistability phenomena leading to a switching between the high-spin and the low-spin states of the molecule by several means such as temperature, pressure, light and magnetic field (multi-property molecular switching). In the present micro-review we report our most important findings, both experimental results and theoretical approaches, and discuss possible technological applications for each switching method as well as the recent synthesis of new spin crossover molecular materials.
The known ligands HC(pz)3, HC(3,5-Me2pz)3, HC(3-Phpz)3, and HC(3-tBupz)3 and the new ligand HC(3-iPrpz)3 (pz=pyrazolyl ring) are prepared in CHCl3–H2O using the appropriate pyrazole, an excess of Na2CO3, and tetra-n-butylammonium bromide as the phase transfer catalyst. Using these conditions, good yields of the ligands are consistently obtained. The new ligand PhC(pz)2py (py=pyridyl ring) is prepared in the CoCl2 catalyzed condensation reaction of (pz)2SO and Ph(py)CO. The reaction of HC(pz)3, KOtBu and para-formaldehyde followed by quenching with water yields HOCH2C(pz)3. All of these ligands, except HC(3-tBupz)3, react with [Mn(CO)5]SO3CF3, prepared in situ from Mn(CO)5Br and Ag(SO3CF3), to yield the respective [(ligand)Mn(CO)3]SO3CF3 complex. The carbonyl stretching frequencies and 13C-NMR trends of these complexes indicate that the donor abilities of all of the ligands are fairly similar. The solid state structure of {[HC(3-iPrpz)3]Mn(CO)3}+ shows the HC(3-iPrpz)3 ligand is tridentate with the iso-propyl groups rotated away from the Mn(CO)3 core of the cation relieving any possible steric congestion.
With the description of more and more complex one-and two-dimensional NMR experiments comes the need to develop methods to make a comprehensive interpretation of the various different experiments that can be carried out on the same sample or series of related samples. We present some examples of modelling one-and two-dimensional solid-state NMR spectra of I = 1 2 spin and quadrupolar nuclei, using laboratory-developed software that is made available to the NMR community. Copyright 2001 John Wiley & Sons, Ltd.
A combined top-down/bottom-up approach for nanoscale and microscale assembly of the 3D spin-crossover coordination polymer was demonstrated. Scanning electron microscopy (SEM) images showed clear patterns of various size that exhibited square shapes with sharp borders for feature sizes between 2 μm and 200 nm. Atomic force microscopy (AFM) studies showed that the height of the patterns decreased with a decrease in the lateral size. The main objective was to achieve a high spatial resolution while retaining spin-crossover properties. These properties are tunable as the process allows replacement of the different bricks of the network. Differences in the film thickness as a function of the lateral pattern size have been observed in the sub-micrometer range, which can be explained by differences in wetting as well as in the reaction kinetics within the small reaction volumes.
We present the theoretical and technical foundations of the Amsterdam Density Functional (ADF) program with a survey of the characteristics of the code (numerical integration, density fitting for the Coulomb potential, and STO basis functions). Recent developments enhance the efficiency of ADF (e.g., parallelization, near order-N scaling, QM/MM) and its functionality (e.g., NMR chemical shifts, COSMO solvent effects, ZORA relativistic method, excitation energies, frequency-dependent (hyper)polarizabilities, atomic VDD charges). In the Applications section we discuss the physical model of the electronic structure and the chemical bond, i.e., the Kohn–Sham molecular orbital (MO) theory, and illustrate the power of the Kohn–Sham MO model in conjunction with the ADF-typical fragment approach to quantitatively understand and predict chemical phenomena. We review the “Activation-strain TS interaction” (ATS) model of chemical reactivity as a conceptual framework for understanding how activation barriers of various types of (competing) reaction mechanisms arise and how they may be controlled, for example, in organic chemistry or homogeneous catalysis. Finally, we include a brief discussion of exemplary applications in the field of biochemistry (structure and bonding of DNA) and of time-dependent density functional theory (TDDFT) to indicate how this development further reinforces the ADF tools for the analysis of chemical phenomena. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 931–967, 2001
Photochemisches Schalten der Dielektrizitätskonstante trifft man erstmalig in dem Spin-Crossover-Komplex [Fe(L)(CN)2]⋅H2O an (siehe Bild; L=2,13-Dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca-1(18),2,12,14,16-pentaen). Die elektrische Detektion des photoinduzierten Wechsels des Spinzustands lässt derartige Komplexe für optische Informationsspeicher interessant erscheinen.
The electronic spin-state crossover observed upon cooling and at high-pressure in the iron(II) and cobalt(II) complexes formed with the HB(pz)3
-and HC(pz)3 ligands and their various methyl derivatives span a variety of different behaviors. Specifically [Fe(HB(pz)3)2], which is low-spin at 295K, undergoes a spin state crossover to the high spin state both upon heating to ca. 420K and at high pressure. [Fe(HB(3,5-(CH3)2pz)3)2], which is high-spin at 295K, undergoes a spin state crossover to the low spin state both upon cooling below ca. 195K and at high pressure. In contrast, [Fe(HB(3,4,5-(CH3)3pz)3)2] remains high-spin between 1.9 and 295K but is gradually converted to the low-spin state with increasing pressure. Similarly, [Fe(HC(pz)3)2](BF4)2, which is low-spin at 295K, undergoes a spin-state crossover to the high spin state upon heating. In a parallel fashion, [Fe(HC(3,5-(CH3)2pz)3)2]I2, which is high-spin at 295K, is completely converted to the low-spin state upon cooling. In contrast, [Fe(HC(3,5-(CH3)2pz)3)2](BF4)2, which is high-spin at 295K, exhibits a phase transition upon cooling below 206K in which only one-half of the iron(II) is converted to the low-spin state; the remaining one-half of the iron(II) remains high-spin even upon cooling to 4.2K. This chapter presents a detailed discussion of these spin-state changes and those observed in the related cobalt(II) complexes.
Density functional calculations using different functionals and basis sets have been carried out to calculate the electronic energy difference between the high- and low-spin isomers of spin crossover complexes with a transition metal center. The reparameterized B3LYP* method is confirmed to be most suitable for the calculation of electronic energy differences between isomers with different spin states. If only changes of the electronic energy difference upon modifications of the complex are considered all employed density functional methods show a similar performance. Basis sets with effective core potentials seem to perform as well as large all electron basis sets. Calculations using the polarizable continuum model have been performed to investigate the effect of solvents. In addition the effect of hydrogen bonding between the spin crossover complex and a water molecule is studied.
The phenomenon of the thermal spin transition, as observed for octahedral transition metal complexes having a d 4 to d 7 electronic configuration, can be fully rationalised on the basis of ligand field theory. In order to arrive at a self-consistent description of the vibronic structure of spin crossover compounds, it is essential to take into account the fact that the population of anti-bonding orbitals in the high-spin state results in a substantially larger metal-ligand bond length than for the low-spin state. Whereas the electron-electron repulsion is not affected to any great extent by such a bond length difference, the ligand field strength for iron(II) spin crossover compounds can be estimated to be almost twice as large in the low-spin state as compared to the one for the high-spin state. In fact, the dependence of the ligand field strength on the metal-ligand distance may be considered the quantum mechanical driving force for the spin crossover phenomenon.
The behaviour of spin crossover compounds is among the most striking and fascinating shown by relatively simple molecular species. This review aims to draw attention to the various ways in which spin crossover phenomena are manifested in iron(II) complexes, to offer some rationalisation for these, and to highlight their possible applications. Typical examples have been selected along with more recent ones in order to give an overall view of the scope and development of the area. The article is structured to provide the basic material for those who wish to enter the field of spin crossover.
In the present article we discuss the cooperative nature of the spin crossover phenomenon in iron(II) complexes, providing a perspective of the state of the art in this area. The first aspect we discuss is the role of the intermolecular interactions, more precisely the π-interactions, in mononuclear complexes. We show that by playing with the nature of the ligands, aliphatic, aromatic, or extended aromatic, it is possible to create stronger cohesive forces and receive a more cooperative response from the compound. In the next step the singular family of bipyrimidine-bridged iron(II) dinuclear compounds is presented as the simplest example of polynuclear spin crossover complexes exhibiting a rich variety of magnetic behaviours, which stem from the synergistic effect between intramolecular and cooperative intermolecular interactions. Finally, a number of polymeric 1-3D architectures of varying dimensionality and topology are discussed in terms of the search for strong cooperative spin crossover systems.
In this paper, we review recent work reported in the field of molecular spin crossover phenomena in dinuclear compounds. Following a comprehensive overview on the synthesis and properties of new iron(II) dinuclear compounds presenting the spin crossover phenomenon, we focus this review on recent efforts made in studying and understanding the photo-physical properties of the {[Fe(L)(NCX)2]2bpym} (L = bt or bpym, X = S or Se) family of compounds. Finally, literature on the different theoretical approaches treating the static and dynamic properties of dinuclear complexes presenting two-step thermal spin transition is briefly summarized.
One of the most important trends in the spin crossover (SCO) field is focused on the synthesis of new molecule-based functional materials in which the SCO properties may be combined with other physical or chemical properties in a synergic fashion. The current stage of investigations regarding interplay and synergic effects between SCO, magnetic coupling, liquid crystalline properties, host–guest interactions, non-linear optical properties, electrical conductivity, and ligand isomerization is highlighted and discussed.
(Figure Presented) Photoswitching of the dielectric constant has been observed for the first time in the spin-crossover complex [Fe(L)(CN) 2]·H2O (L = 2,13-dimethyl-6,9-dioxa-3,12,18- triazabicyclo[12.3.1]octadeca-1(18),2,12, 14,16-pentaene, see picture). The electrical detection of a photoinduced change in spin state could allow the use of such complexes in optical information-storage devices.
Current gradient-corrected density-functional approximations for the exchange energies of atomic and molecular systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy density. Here we report a gradient-corrected exchange-energy functional with the proper asymptotic limit. Our functional, containing only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of atomic systems with remarkable accuracy, surpassing the performance of previous functionals containing two parameters or more.
Langreth and Mehl (LM) and co-workers have developed a useful spin-density functional for the correlation energy of an electronic system. Here the LM functional is improved in two ways: (1) The natural separation between exchange and correlation is made, so that the density-gradient expansion of each is recovered in the slowly varying limit. (2) Uniform-gas and inhomogeneity effects beyond the randomphase approximation are built in. Numerical results for atoms, positive ions, and surfaces are close to the exact correlation energies, with major improvements over the original LM approximation for the ions and surfaces.
Some preliminary results from a model of temperature-sensitive contrast agents are reported. This paramagnetic system switches from a spin S = 0 diamagnetic state to a S = 2 paramagnetic one at a temperature that can be tuned according to chemical composition. The magnetic susceptibility jump and the subsequent R2* effect as a function of temperature have been followed by means of spectroscopy, relaxometry, and imaging, demonstrating sharp and reversible transitions. Potential applications of this kind of system could be found in therapy by hyperthermia or in material science.
Database analysis and molecular mechanics were used to determine the conformational flexibility of tridentate scorpionate ligands. The tris(pyrazolyl)methane and tris(pyrazolyl)borate ligands act like molecular vises, opening their tripodal structure for larger metals and closing around smaller metal ions. Tris(3-tert-butylpyrazolyl)methane has significant preference for larger metal ions than its unsubstituted parent compound. Tris(pyrazolyl)methanes and tris(pyrazolyl)borates have similar conformational flexibilities. Placing sterically hindered groups on the central carbon or boron has only a minor effect on the geometry of the tris(pyrazolyl)methanes and tris(pyrazolyl)borates. However, it does influence the flexibility of the ligands, particularly when they have to open far from their ideal geometry, which commonly occurs.
Sonogashira coupling reactions of terminal alkynes with Fe[(p-IC6H4)B(3-Mepz)3]2 (pz = pyrazolyl ring) yield Fe[(p-PhC2C6H4)B(3-Mepz)3]2 (2), Fe[(p-Me3SiC2C6H4)B(3-Rpz)3]2 (R = H, 3a, R = Me, 3b), and Fe[(p-HC2C6H4)B(3-Mepz)3]2 (R = H, 4a, R = Me, 4b), a series of new complexes containing "third generation" poly(pyrazolyl)borate ligands. Complex 2 undergoes a fairly gradual iron(II) electronic spin-state crossover with a 30 K hysteresis, whereas complex 3b is an unusual example of a complex with equivalent iron(II) sites in the high-spin form that shows an abrupt 50% spin crossover. For complex 4b, 50% of the iron(II) sites undergo a gradual spin-state transition between 185 and 350 K with an activation energy of 1590 +/- 30 cm(-1) and a T(1/2) = 280 K and, for the remaining iron(II) sites, an abrupt cooperative spin-state crossover between 106 and 114 K. The crystal structures of 4b obtained for each of the three distinct electronic spin states reveal two crystallographically different iron(II) sites, and analysis of the molecular/supramolecular structures indicates that the difference in the degree of pyrazolyl ring tilting in the ligands between the two sites, rather than the strength of the intermolecular forces, play a prominent role in determining the temperature of the spin-state crossover.
The new ligands Na[(p-IC6H4)B(3-Rpz)3] (R = H, Me) have been prepared by converting I2C6H4 to IC6H4SiMe3 with Li(t)Bu and SiMe3Cl, and then to IC6H4BBr2 with BBr3 and subsequent reaction with 3 equiv of (un)substituted pyrazole and 1 equiv of NaO(t)Bu. These new ligands react with FeBr2 to give either purple, low-spin Fe[(p-IC6H4)B(pz)3]2 or colorless, high-spin Fe[(p-IC6H4)B(3-Mepz)3]2. Depending upon the crystallization conditions, Fe[(p-IC6H4)B(3-Mepz)3]2 can exist both as two polymorphs and as a methylene chloride solvate. An examination of these polymorphs by variable-temperature X-ray crystallography, magnetic susceptibility, and Mossbauer spectroscopy has revealed different electronic spin-state crossover properties for each polymorph and yields insight into the influence of crystal packing, independent of other electronic perturbations, on the spin-state crossover. The first polymorph of Fe[(p-IC6H4)B(3-Mepz)3]2 has a highly organized three-dimensional supramolecular structure and does not undergo a spin-state crossover upon cooling to 4 K. The second polymorph of Fe[(p-IC6H4)B(3-Mepz)3]2 has a stacked two-dimensional supramolecular structure, a structure that is clearly less well organized than that of the first polymorph, and undergoes an abrupt iron(II) spin-state crossover from high spin to low spin upon cooling below ca. 130 K. The crystal structure of the methylene chloride solvate of Fe[(p-IC6H4)B(3-Mepz)3]2 has a similar stacked two-dimensional supramolecular structure, but the crystals readily lose the solvate. The resulting desolvate undergoes a gradual spin-state crossover to the low-spin state upon cooling below ca. 235 K. It is clear from a comparison of the structures that the long-range solid-state organization of the molecules, which is controlled by noncovalent supramolecular interactions, has a strong impact upon the spin-state crossover, with the more highly organized structures having lower spin-crossover temperatures and more abrupt spin-crossover behavior.
In der Schleife: Ein reversibler Spinübergang ließ sich Raman-spektroskopisch bei Anwendung eines einzelnen Laserpulses (8 ns) in der Hystereseschleife des Spin-Crossover-Komplexes [Fe(pyrazin){Pt(CN)4}] beobachten. Dieses bidirektionale, photoinduzierte Spinumschalten passiert bei Raumtemperatur über einen weiten Bistabilitätsbereich und umfasst Magnetisierungs- wie Farbänderungen (siehe Bild).
This article reviews the most relevant chemical and structural aspects that influence the spin-crossover phenomenon (SCO). Special attention is focussed on the recent development of SCO coordination polymers. The different approaches currently being explored in order to achieve multifunctionality in SCO materials are discussed.
The lithium (1) and thallium (2) salts of five new tert-butyl-tris(3-hydrocarbylpyrazol-1-yl)borate ligands [t-BuTp(R)]- (R = H, a; Me, b; i-Pr, c; t-Bu, d; Ph, e) have been synthesized and characterized. Because of steric congestion at B, the reaction between t-BuBH3Li x 0.5 Et2O and excess 2,5-dimethylpyrazole Hpz(Me2) afforded the bis-pz(Me2) derivative, Tl[t-BuBH(3,5-Me2pz)2] (3) after metathesis with TlNO3. The compounds were characterized by elemental analysis and NMR spectroscopy. The Li salts 1a and 1c exhibit fluxional behavior on the NMR time scale in solution at room temperature. The solid-state 7Li and 11B NMR spectra of 1c suggest that this salt exists as a mixture of axial and equatorial isomers. The partial hydrolysis of 1d afforded the dimeric Li complex {Li[t-BuB(pz(t-Bu))2(mu-OH)]}2 (4). The crystal structure of 4 shows two Li cations encapsulated by the heteroscorpionate [t-BuB(OH)(3-t-Bupz)2]- ligands. A salt elimination reaction between FeCl2(THF)1.5 and 2 equiv of Li[t-BuTp(R)] (R = H, Me) followed by an in situ one-electron oxidation produced good yields of the homoleptic, paramagnetic low-spin iron(III) complexes [Fe(t-BuTp)2]PF6 (5) and [Fe(t-BuTp(Me))2]PF6 (6) that were characterized by elemental analyses, magnetic susceptibility measurements in solution and the solid phase, 1H NMR, high-resolution mass spectrometry, Mössbauer spectroscopy, and single-crystal X-ray diffraction. The crystals are composed of discrete molecular units with the central Fe(III) ion in an almost perfectly octahedral coordination to six nitrogen atoms. Compound 5 has the shortest Fe-N bond lengths ever reported for [Fe(RTp(R)')2]+-type compounds.
Eines nach dem anderen: Dünne Filme der [Fe(pyrazin){M(CN)4}]-Koordinationspolymere (M=Ni, Pd oder Pt) wurden sequenziell durch koordiniertes Binden an Goldoberflächen erhalten (siehe Schema). Diese Multischichten zeigen bei Raumtemperatur Spin-Crossover-Phänomene mit Hysterese.
Iron(II) poly(pyrazolyl)borate complexes have been investigated to determine the impact of substituent effects, intramolecular ligand distortions, and intermolecular supramolecular structures on the spin-state crossover (SCO) behavior. The molecular structure of Fe[HB(3,4,5-Me3pz)3]2 (pz = pyrazolyl ring), a complex known to remain high spin when the temperature is lowered, reveals that this complex has an intramolecular ring-twist distortion that is not observed in analogous complexes that do exhibit a SCO at low temperatures, thus indicating that this distortion greatly influences the properties of these complexes. The structure of Fe[B(3-(cy)Prpz)4]2.(CH3OH) ((cy)Pr = cyclopropyl ring) at 294 K has two independent molecules in the unit cell, both of which are high spin; only one of these high-spin iron(II) sites, the site with the lesser ring-twist distortion, is observed to be low-spin iron(II) in the 90 K structure. A careful evaluation of the supramolecular structures of these complexes and several similar complexes reported previously revealed no strong correlation between the supramolecular packing forces and their SCO behavior. Magnetic and Mössbauer spectral measurements on Fe[B(3-(cy)Prpz)4]2 and Fe[HB(3-(cy)Prpz)3]2 indicate that both complexes exhibit a partial SCO from fully high-spin iron(II) at higher temperatures, respectively, to a 50:50 high-spin/low-spin mixture of iron(II) below 100 K. These results may be understood, in the former case, by the differences in ring-twisting and, in the latter case, by a phase transition; in all complexes in which a phase transition is observed, this change dominates the SCO behavior. A comparison of the Mössbauer spectral properties of these two complexes and of Fe[HB(3-Mepz)3]2 with that of other complexes reveals correlations between the Mössbauer-effect isomer shift and the average Fe-N bond distance and between the quadrupole splitting and the average FeN-NB intraligand dihedral torsion angles and the distortion of the average N-Fe-N intraligand bond angles.
The compound [Fe(tvp)2(NCS)2] · CH3OH, where tvp is 1,2-di-(4-pyridyl)-ethylene, has been synthesized and characterized by x-ray single-crystal diffraction.
It consists of two perpendicular, two-dimensional networks organized in parallel stacks of sheets made up of edge-shared [Fe(II)]4 rhombuses. The fully interlocked networks define large square channels in the [001] direction. Variable-temperature magnetic
susceptibility measurements and Mössbauer studies reveal that this compound shows low-spin to high-spin crossover behavior
in the temperature range from 100 to 250 kelvin. The combined structural and magnetic characterization of this kind of compound
is fundamental for the interpretation of the mechanism leading to the spin crossover, which is important in the development
of electronic devices such as molecular switches.
This tutorial review describes how complexes of iron(II) (and, rarely, other metal ions) can be switched between their high- and low-spin states by different physical stimuli. At low temperatures, it is possible to trap a compound in a metastable excited spin-state which, in favourable cases, may be stable to thermal relaxation below temperatures as high as 130 K. The selective switching and trapping of individual spin centres in polynuclear compounds, and the interplay between spin centres as they relax back to their ground states, are also discussed. Similar phenomena, in which spin transitions are coupled to charge transfer phenomena, can also occur in inorganic and metal-organic cyanometallate compounds and in cobalt-semiquinonate complexes.
Density functional theory has been used to study the electronic spin-state properties of low-spin Fe[HB(pz)3]2, high-spin Fe[HB(3-Mepz)3]2, high-spin Fe[HB(3,5-Me 2pz)3]2, and high-spin Fe[HB(3,4,5-Me 3pz)3]2 complexes that exhibit very different iron(II) electronic spin-sate crossover behaviors with changing temperature and pressure. Excellent agreement is obtained between the experimentally observed Mössbauer-effect quadrupole splittings and isomer shifts of these complexes and those calculated with the B3LYP functional and various different basis sets for both the high-spin and low-spin states of iron(II). The calculations for Fe[HB(pz)3]2 that use the LANL2DZ, 6-31++G(d,p), and 6-311++G(d,p) basis sets for iron all lead to very similar electric field gradients and thus quadrupole splittings. The initial calculations, which were based upon the known X-ray structures, were followed by structural optimization, an optimization that led to small increases in the Fe-N bond distances. Optimization led to at most trivial changes in the intraligand bond distances and angles. The importance of the 3-methyl-H...H-3-methyl nonbonded intramolecular interligand interactions in controlling the minimum Fe-N bond distances and determining the iron(II) spin state both in Fe[HB(3-Mepz)3]2 and in the related methyl-substituted complexes has been identified.
For comprehensive reviews of tris(pyrazolyl)borate ligands and their transition-metal complexes see: a) S. Trofimenko, Scorpi-onates: The Coordination Chemistry of Polypyrazolylborate Ligands, Imperial College Press Santini in Comprehensive Coordination Chemistry II
- C Pettinari
For comprehensive reviews of tris(pyrazolyl)borate ligands and their transition-metal complexes see: a) S. Trofimenko, Scorpi-onates: The Coordination Chemistry of Polypyrazolylborate Ligands, Imperial College Press, London, 1999; b) C. Pettinari, C. Santini in Comprehensive Coordination Chemistry II, Vol. 1 (Eds.: J. A. McCleverty, T. J. Meyer), Elsevier Pergamon, Oxford, 2004, pp. 159; c) “Scorpionate and Related Ligands”: Polyhedron Symposia-In-Print Number 26 (Ed.: G. F. Parkin), Polyhedron 2004, 23, 195; d) S. Trofimenko, Chem. Rev. 1993, 93, 943.
II) with C 3v symmetry, the ring-twisting Fe-N-N-B and ring-tilting Fe-N-N-C torsion angles approach their ideal values of 08 and 1808, respectively
- Ls For
For LS iron(II) with C 3v symmetry, the ring-twisting Fe-N-N-B
and ring-tilting Fe-N-N-C torsion angles approach their ideal
values of 08 and 1808, respectively (see Tables S2 and S3 in the
the Supporting Information).