[Show abstract][Hide abstract] ABSTRACT: A series of iron dicarbonyl complexes with bipyridine-based PNN pincer ligands were synthesized and characterized by multinuclear NMR spectroscopy (1H, 13C, 15N, 31P), IR spectroscopy, cyclic voltammetry, 57Fe Mössbauer spectroscopy, XPS spectroscopy, and single-crystal X-ray diffraction. The complexes with the general formula [(R-PNN)Fe(CO)2] (5: R-PNN=tBu-PNN=6-[(di-tert-butylphosphino)methyl]-2,2′-bipyridine, 6: R-PNN=iPr-PNN=6-[(diisopropylphosphino)methyl]-2,2′-bipyridine, and 7: R-PNN=Ph-PNN=6-[(diphenylphosphino)methyl]-2,2′-bipyridine) feature differently P-substituted PNN pincer ligands. Complexes 5 and 6 were obtained by reduction of the corresponding dihalide complexes [(R-PNN)Fe(X)2] (1: R=tBu, X=Cl; 2: R=tBu, X=Br; 3: R=iPr, X=Cl; 4: R=iPr, X=Br) in the presence of CO. The analogous Ph-substituted complex 7 was synthesized by a reaction of the free ligand with iron pentacarbonyl. The low-spin complexes 5–7 (S=0) are diamagnetic and have distorted trigonal bipyramidal structures in solution, whereas in the solid state the geometries around the iron are best described as distorted square pyramidal. Compared to other structurally characterized complexes with these PNN ligands, shortened interpyridine CC bonds of about 1.43 Å were measured. A comparison with known examples, theoretically described as metal complexes bearing bipyridine π-radical anions (bpy.−), suggests that the complexes can be described as FeI complexes with one electron antiferromagnetically coupled to the ligand-based radical anions. However, computational studies, at the NEVPT2/CASSCF level of theory, reveal that the shortening of the CC bond is a result of extensive π-backbonding of the iron center into the antibonding orbital of the bpy unit. Hence, the description of the complexes as Fe0 complexes with neutral bipyridine units is the favorable one.
Chemistry - A European Journal 01/2014; · 5.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The electronic structures of a number of iron, cobalt, vanadium, and titanium complexes with the 2,2′-bipyridine (bpy) ligand were considered using the multireference CASSCF and NEVPT2 methods. Many of these systems have been studied in the past using B3LYP and were then found to contain the bpy ligand as a radical anion. For many of the cases, this is contradicted by our multireference calculations. While there are instances where the ligand is indeed a radical anion, in many cases it remains neutral and is involved in backbonding from the metal center. For those cases where CASSCF is too costly, a number of DFT functionals, including the newer double-hybrid functionals, were evaluated against the CASSCF data. It was found that nonhybrid functionals, especially those containing the kinetic energy density τ, were the best at predicting the electronic nature of the complexes. The τ-HCTH and HCTH functionals were the top performers, correctly predicting eleven out of eleven test cases and with the lowest mean unsigned errors (MUE, 7.6 and 7.8 kcal·mol–1, respectively); the M06-L, N12, BLYP, PBE, and TPSS functionals also did well, while B3LYP had significant problems.
Journal of Chemical Theory and Computation 12/2013; 10(1):220–235. · 5.39 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: A combinatorial fluorescent molecular sensor operates as a highly efficient molecular security system. The ability of a pattern-generating molecule to process diverse sets of chemical inputs, discriminate among their concentrations, and form multivalent and kinetically stable complexes is demonstrated as a powerful tool for processing a wide range of chemical "passwords" of different lengths. This system thus indicates the potential for obtaining unbreakable combination locks at the molecular scale.
Journal of the American Chemical Society 10/2013; · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Halogen bonding between complementary organic monolayers was directly observed in an organic environment using force spectroscopy. This non-covalent interaction is significantly affected by the nature of the organic media. We also demonstrated the effect of lateral packing interactions on the optical properties of the monolayers.
Chemical Communications 03/2013; · 6.38 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Singly-charged complexes of (8S,9R)-tert-butylcarbamoylquinine (tBuCQN), N-3,5-dinitrobenzoyl-(S,R)-leucine (DNB-S/R-leucine), and alkali metal counter ions (Li(+), Na(+), K(+)) were investigated by density-functional theory. It is shown that the cations prefer formation of an ionic pair with the carboxylate group of DNB-Leu over the formation of a cation-π interaction. The [tBuCQN·DNB-S/R-Leu·Na](+) complex is bound by a Coulombic attraction, a hydrogen bond, a π-π interaction and van der Waals forces. The tBuCQN chiral selector preferentially complexes with the DNB-S-Leu enantiomer, because the favourable stereochemistry allows the stabilization of the complex by at least one binding mode more compared to the complex containing the DNB-R-Leu molecule. Weakening of the binding modes is observed using the lithium counter ion compared to the sodium one. The weakening is more pronounced in [tBuCQN·DNB-R-Leu·Li](+) than in [tBuCQN·DNB-S-Leu·Li](+). The exact opposite effect is observed using the potassium counter ion. Hence, the lithium counter ion enhances the enantioselectivity of tBuCQN while the potassium counter ion reduces the enantioselectivity of tBuCQN.
Physical Chemistry Chemical Physics 03/2013; · 4.20 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The intramolecular gas-phase reactivity of four oxoiron(IV) complexes supported by tetradentate N(4) ligands (L) has been studied by means of tandem mass spectrometry measurements in which the gas-phase ions [Fe(IV)(O)(L)(OTf)](+) (OTf = trifluoromethanesulfonate) and [Fe(IV) (O)(L)](2+) were isolated and then allowed to fragment by collision-induced decay (CID). CID fragmentation of cations derived from oxoiron(IV) complexes of 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane (tmc) and N,N'-bis(2-pyridylmethyl)-1,5-diazacyclooctane (L(8)Py(2)) afforded the same predominant products irrespective of whether they were hexacoordinate or pentacoordinate. These products resulted from the loss of water by dehydrogenation of ethylene or propylene linkers on the tetradentate ligand. In contrast, CID fragmentation of ions derived from oxoiron(IV) complexes of linear tetradentate ligands N,N'-bis(2-pyridylmethyl)-1,2-diaminoethane (bpmen) and N,N'-bis(2-pyridylmethyl)-1,3-diaminopropane (bpmpn) showed predominant oxidative N-dealkylation for the hexacoordinate [Fe(IV)(O)(L)(OTf)](+) cations and predominant dehydrogenation of the diaminoethane/propane backbone for the pentacoordinate [Fe(IV)(O)(L)](2+) cations. DFT calculations on [Fe(IV)(O)(bpmen)] ions showed that the experimentally observed preference for oxidative N-dealkylation versus dehydrogenation of the diaminoethane linker for the hexa- and pentacoordinate ions, respectively, is dictated by the proximity of the target C-H bond to the oxoiron(IV) moiety and the reactive spin state. Therefore, there must be a difference in ligand topology between the two ions. More importantly, despite the constraints on the geometries of the TS that prohibit the usual upright σ trajectory and prevent optimal σ(CH)-σ*(z2) overlap, all the reactions still proceed preferentially on the quintet (S = 2) state surface, which increases the number of exchange interactions in the d block of iron and leads thereby to exchange enhanced reactivity (EER). As such, EER is responsible for the dominance of the S = 2 reactions for both hexa- and pentacoordinate complexes.
Chemistry - A European Journal 07/2012; 18(37):11747-60. · 5.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Fundamental insights into the factors that control the properties and structure of gold nanoparticle (AuNP) based assemblies enable the design and construction of new materials. The dimensions (shape and size) and the optical properties of AuNP assemblies are affected by the electronic properties of the organic cross-linker and the nature of the AuNPs.
Angewandte Chemie International Edition 06/2012; 51(29):7142-5. · 11.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The bimolecular reactivity of xenon with C(7)H(n)(2+) dications (n=6-8), generated by double ionization of toluene using both electrons and synchrotron radiation, is studied by means of a triple-quadrupole mass spectrometer. Under these experimental conditions, the formation of the organoxenon dications C(7)H(6)Xe(2+) and C(7)H(7)Xe(2+) is observed to occur by termolecular collisional stabilization. Detailed experimental and theoretical studies show that the formation of C(7)H(6)Xe(2+)+H(2) from doubly ionized toluene (C(7)H(8)(2+)) and xenon occurs as a slightly endothermic, direct substitution of dihydrogen by the rare gas with an expansion to a seven-membered ring structure as the crucial step. For the most stable isomer of C(7)H(6)Xe(2+), an adduct between the cycloheptatrienyldiene dication and xenon, the computed binding energy of 1.36 eV reaches the strength of (weak) covalent bonds. Accordingly, electrophiles derived from carbenes might be particularly promising candidates in the search for new rare-gas compounds.
Chemistry - A European Journal 03/2011; 17(14):4012-20. · 5.93 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Mono- and binuclear complexes of N,N-dimethylformamide (DMF) with chlorides of the divalent, late 3d metals M = Co, Ni, Cu, and Zn are investigated by means of electrospray ionization (ESI). Specifically, ESI leads to monocations of the type [(DMF)(n)MCl](+) and [(DMF)(n)M(2)Cl(3)](+), of which the species with n = 2 and 3 were selected for in-depth studies. The latter include collision-induced dissociation experiments, gas-phase infrared spectroscopy, and calculations using density functional theory. The mononuclear complexes [(DMF)(n)MCl](+) almost exclusively lose neutral DMF upon collisional activation with the notable exception of the copper complex, for which also a reduction from Cu(II) to Cu(I) concomitant with the release of atomic chlorine is observed. For the dinuclear clusters, there exists a competition between loss of a DMF ligand and cluster degradation via loss of neutral MCl(2) with decreasing cluster stability from cobalt to zinc. For the specific case of [(DMF)(n)ZnCl](+) and [(DMF)(n)Zn(2)Cl(3)](+), ion-mobility mass spectrometry indicates the existence of two isomeric cluster ions in the case of [(DMF)(2)Zn(2)Cl(3)](+) which corroborates parallel theoretical predictions.
[Show abstract][Hide abstract] ABSTRACT: The mixed density functional theory (DFT) and valence bond study described herein focuses on the activation of 17 benzene derivatives by the active species of Cytochrome P450, so-called Compound I (Cpd I), as well as by the methoxy radical, as a potentially simple model of Cpd I (Jones, J. P.; Mysinger, M.; Korzekwa, K. R. Drug Metab. Dispos. 2002, 30, 7−12). Valence bond modeling is employed to rationalize the P450 mechanism and its spin-state selectivity from first principles of electronic structure and to predict activation energies independently, using easily accessible properties of the reactants: the singlet−triplet excitation energies, the ionization potentials of the aromatics, and the electron affinity of Cpd I and/or of the methoxy radical. It is shown that the valence bond model rationalizes all the mechanistic aspects and predicts activation barriers (for 35 reactions) with reasonable accuracy compared to the DFT barriers with an average deviation of ±1.0 kcal·mol−1 (for DFT barriers, see: Bathelt, C. M.; Ridder, L.; Mulholland, A. J.; Harvey, J. N. Org. Biomol. Chem. 2004, 2, 2998−3005). The valence bond modeling also reveals the mechanistic similarities between the P450 Cpd I and methoxy reactions and enables one to make predictions of barriers for reactions from other studies.
Journal of Chemical Theory and Computation 01/2011; 7(2):327. · 5.31 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Under gas phase conditions, the [Cu(PhO)(PhOH)](+) complex is composed of copper(I), a phenoxy radical bound via the oxygen atom, and a phenol bound via the aromatic ring. Effects of additional ligand coordination on the molecular and electronic structure of the complex [Cu(PhO)(PhOH)](+) are investigated by mass spectrometric and quantum chemical means for [Cu(PhO)L](+) (L = H(2)O, CH(3)OH, tetrahydrofuran, NH(3), pyridine, imidazole, 1,2-dimethoxyethylene, N,N,N',N'-tetramethylethylenediamine, pyrrole, and thiophene) and [Cu(PhO)(PhOH)L(n)](+) (L = H(2)O, NH(3), and 4-methylimidazole) models. The nature and number of additional ligands critically influences the spin distribution in the complex, which is sensitively reflected by the phenoxy CO stretching mode.
[Show abstract][Hide abstract] ABSTRACT: The reaction mechanism of copper(II)-mediated naphthol coupling in the presence of TMEDA (N,N,N',N'-tetramethylethylenediamine) is studied using infrared multiphoton dissociation (IRMPD) spectroscopy and DFT calculations. It is shown that the coupling reaction proceeds in ad hoc formed binuclear clusters [(1-H)(2)Cu(2)Cl(TMEDA)(2)](+), where (1-H) is a deprotonated naphthol molecule (methyl ester of 3-hydroxy-2-naphthoic acid). The IRMPD spectra of the isolated cluster in the gas phase reveal that it contains two uncoupled naphtholate subunits and only the irradiation promotes the coupling reaction, which is thus observed as a genuine gas-phase reaction. The driving force for the C-C coupling is a keto-enol tautomerization of the initial coupling product, and the formation of the corresponding binol in the cluster is exothermic by 0.61 eV. In contrast, analogous C-O and O-O couplings are endothermic reactions.
Journal of the American Chemical Society 12/2009; 132(1):281-8. · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: With the use of the model complexes [(PQ)FeCl(CH(3)O)](+), [(phen)FeCl(CH(3)O)](+), and [(PQ)(phen)FeCl(CH(3)O)](+), where PQ is 9,10-phenanthraquinone and phen is 1,10-phenanthroline, the reactivity of phenanthraquinone in complexes with iron(III) is investigated. It is shown that 9,10-phenanthraquinone takes part in redox processes occurring at iron and thereby allows the oxidation of methanolate to formaldehyde. The oxidation is driven by the reduction of iron(III) to iron(II) and 9,10-phenanthraquinone to the semihydroquinone radical or semiquinolate, if the hydrogen atom is transferred from methanolate to chlorine rather than PQ. 1,10-Phenanthroline, on the other hand, acts as an innocent ligand, and the [(phen)FeCl(CH(3)O)](+) complex shows a typical two-state reactivity. The reactivity of [(PQ)(phen)FeCl(CH(3)O)](+) reveals that the hexacoordination of iron energetically facilitates the oxidation of methanolate, and therefore it is proposed that, in the presence of suitable reductants, the mixture of iron(III) and 9,10-phenanthraquinone can lead to the generation of the semihydroquinone radicals, species responsible for the toxicity of PQ. The fragmentation of [(PQ)(phen)FeCl(CH(3)O)](+) also demonstrates a strong binding of phen toward iron(III), which is a reason for using phen as an iron chelator in biochemistry. The structures and reactivities of the complexes are investigated by means of mass spectrometry, infrared multiphoton dissociation spectra, and density functional theory calculations.
[Show abstract][Hide abstract] ABSTRACT: The accuracy of the diatomics-in-molecules (DIM) model for the krypton ionic trimer is examined in a series of ab initio calculations.
In the C2v
symmetry, the ground states of irreducible representations B2 and A1 were calculated using partially spin restricted open-shell coupled cluster method with perturbative triple connections (RHF-RCCSD-T),
the relativistic effective core potential (RECP) and an extended basis set of atomic orbitals. Internally contracted multireference
configuration interaction method (icMRCI) with the extended and restricted basis set was used to generate the potential energy
surfaces (PESs) of the nine electronic states of Kr3+ corresponding to Kr(1S)+Kr(1S)+Kr+(2P) dissociation limit in a wide interval of nuclear geometries. The overall agreement of the accurate ab initio PESs and the
diatomics-in-molecules PESs confirms the quality of the DIM Hamiltonian for the Kr3+ clusters and justifies its use in dynamical and spectroscopic studies of the Kr
+ clusters. Inclusion of the spin–orbit coupling into the ab initio PESs through a semi-empirical scheme is proposed.
[Show abstract][Hide abstract] ABSTRACT: Dinuclear gold(III) clusters with a rhombic Au(2)O(2) core and 2,2'-bipyridyl ligands substituted in the 6-position (bipy(R)) are examined by tandem mass spectrometry. Electrospray ionization of the hexafluorophosphate salts affords the complexes [(bipy(R))Au(mu-O)(2)Au(bipy(R))](2+) as free dications in the gas phase. The fragmentation behavior of the mass-selected dications is probed by means of collision-induced dissociation experiments which reveal an exceptionally pronounced effect of substitution. Thus, for the parent compound with R = H, i.e., [(bipy)Au(mu-O)(2)Au(bipy)](2+), fragmentation at the dicationic stage prevails to result in a loss of neutral H(2)O concomitant with an assumed rollover cyclometalation of the bipyridine ligands. In marked contrast, all complexes with alkyl substituents in the 6-position of the ligands (bipy(R) with R = CH(3), CH(CH(3))(2), CH(2)C(CH(3))(3), and 2,6-C(6)H(3)(CH(3))(2)) as well as the corresponding complex with 6,6'-dimethyl-2,2'-dipyridyl as a ligand exclusively undergo Coulomb explosion to produce two monocationic fragments. It is proposed that the additional steric strain introduced to the central Au(2)O(2) core by the substituents on the bipyridine ligand, in conjunction with the presence of oxidizable C-H bonds in the substituents, crucially affects the subtle balance between dication dissociation under maintenance of the 2-fold charge and Coulomb explosion into two singly charged fragments.
Journal of the American Chemical Society 09/2009; 131(36):13009-19. · 11.44 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The first and second ionization energies of trimethyl substituted analogs of benzene and pyridine are determined by means of mass spectrometry in conjunction with synchrotron radiation. The first ionization energy of 1,3,5-trimethylbenzene amounts to (8.38 ± 0.05) eV and the second ionization energy to (22.8 ± 0.1) eV. The first ionization energy of 2,4,6-trimethylpyridine is determined as (8.65 ± 0.05) eV and the second ionization energy as (23.0 ± 0.1) eV. The ionization energies are compared with those of unsubstituted benzene and pyridine and the effects of the methyl groups are evaluated by means of isodesmic reactions. As expected, it is found that the electron-donating effect of the methyl groups stabilizes neutral pyridine and doubly charged pyridine more than neutral benzene and doubly charged benzene, respectively. Surprisingly, the opposite effect is found for the radical cations, which is ascribed to the unfavorable degenerate electronic structure of benzene radical-cation, which disappears upon the methyl substitution.
Collection of Czechoslovak Chemical Communications 01/2009; · 1.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: This study aims towards a rationalization and generalization of the fragmentation patterns of medium-sized hydrocarbon dications by tandem mass spectrometry. Phenanthrene (C14H10) serves as a precursor for the generation of CmHn2+ dications with m = 9–14 and n = 1–10. Four characteristic channels occur in the metastable-ion spectra: loss of a hydrogen atom, elimination of molecular hydrogen, loss of acetylene, and charge-separation fragmentation (Coulomb explosion). The first two channels dominate the spectra and their relative intensities show some regularities for ions with an even number of carbon atoms. Further, parent ions with an odd number of hydrogen atoms exhibit loss of atomic hydrogen, whereas loss of molecular hydrogen is preferred for dications with an even number of hydrogen atoms. The ions with odd number of carbon atoms do not show any evident regularity for the H and H2 losses. The channels involving CC bond cleavages occur for larger dications with m > 9, but only for some combinations of m and n without an apparent regularity.
International Journal of Mass Spectrometry 11/2008; 277:107-112. · 2.23 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An unprecedented red shift of more than 200 cm(-1) in the vibrational frequency of the C-O bond in the [Cu(PhO)Ln]+ complex (PhO = phenoxy), dependent on the number n of additional ligands L, is reported. Upon change of n from 1 to 2, the spin density is shifted from the aromatic ring to the oxygen and copper atoms, which is reflected in the bond order and thus vibrational frequency of the C-O bond.
Journal of the American Chemical Society 07/2008; 130(23):7186-7. · 11.44 Impact Factor