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ABSTRACT: In this article, we report the structural, spectroscopic, and electrochemical properties of the cyanide-bridged complex salts trans-[(NC)RuII(L)4(μ-CN)RuII(py)4Cl]PF6 and trans-[RuII(L)4{(μ-CN)RuII(py)4Cl}2](PF6)2 (L = pyridine or 4-methoxypyridine). The mixed-valence forms of these compounds show a variety of metal-to-metal charge-transfer bands, including one arising from charge transfer between the remote ruthenium units. The latter is more intense when L = 4-methoxypyridine and points to the role of the bridging ruthenium unit in promoting mixing between the dπ orbitals of the terminal fragments.
Inorganic Chemistry 03/2013; · 4.60 Impact Factor
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ABSTRACT: Although it has not proved possible to crystallize the newly prepared cyclam-methylimidazole ligand 1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane (L(Im1)), the trans and cis isomers of an Ni(II) complex, namely trans-aqua{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate) monohydrate, [Ni(C(15)H(30)N(6))(H(2)O)](ClO(4))(2)·H(2)O, (1), and cis-aqua{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate), [Ni(C(15)H(30)N(6))(H(2)O)](ClO(4))(2), (2), have been prepared and structurally characterized. At different stages of the crystallization and thermal treatment from which (1) and (2) were obtained, a further two compounds were isolated in crystalline form and their structures also analysed, namely trans-{1-[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}(perchlorato)nickel(II) perchlorate, [Ni(ClO(4))(C(15)H(30)N(6))]ClO(4), (3), and cis-{1,8-bis[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane}nickel(II) bis(perchlorate) 0.24-hydrate, [Ni(C(20)H(36)N(6))](ClO(4))(2)·0.24H(2)O, (4); the 1,8-bis[(1-methyl-1H-imidazol-2-yl)methyl]-1,4,8,11-tetraazacyclotetradecane ligand is a minor side product, probably formed in trace amounts in the synthesis of L(Im1). The configurations of the cyclam macrocycles in the complexes have been analysed and the structures are compared with analogues from the literature.
Acta crystallographica. Section C, Crystal structure communications 05/2012; 68(Pt 5):m121-6. · 0.78 Impact Factor
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ABSTRACT: The title complexes, [RuCl(C(9)H(21)N(3))(C(12)H(12)N(2)O(2))]ClO(4)·2C(2)H(3)N, (I), and [Ru(C(9)H(21)N(3))(C(12)H(12)N(2)O(2))(H(2)O)](ClO(4))(2)·2H(2)O, (II), display similar structures with the Ru atom in a distorted octahedral environment. In the crystal packing of the chloride complex, (I), the Ru complex molecules are held together in pairs through C-H···Cl interactions of the 4,4'-dimethoxy-2,2'-bipyridine and chloride ligands. In the case of the aqua complex, (II), hydrogen bonding affords a tetrameric hydrogen-bonded network. These two structures are the first examples of complexes with the {Ru(1,4,7-trimethyl-1,4,7-triazacyclononane)} motif and an electron-rich substituted 2,2'-bipyridine ligand.
Acta crystallographica. Section C, Crystal structure communications 05/2012; 68(Pt 5):m127-30. · 0.78 Impact Factor
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ABSTRACT: trans-[(NC)Ru(py)(4)(mu-CN)Ru(py)(4)(NO)](3+) (py = pyridine) is a stable species in aqueous solution. It displays an intense absorption in the visible region of the spectrum (lambda(max) = 518 nm; epsilon(max) = 6100 M(-1) cm(-1)), which turns this compound into a promising agent for the photodelivery of NO. The quantum yield for the photodelivery process resulting from irradiation with 455 nm visible light was found experimentally to be (0.06 +/- 0.01) x 10(-3) mol einstein(-1), almost 3 orders of magnitude smaller than that in the closely related cis-[RuL(NH(3))(4)(mu-pz)Ru(bpy)(2)(NO)](5+) species (L = NH(3) or pyridine, pz = pyrazine, bpy = 2,2'-bipyridine; phi(NO) = 0.02-0.04 mol einstein(-1) depending on L) and also much smaller than the one in the mononuclear compound trans-[ClRu(py)(4)(NO)](2+) (phi(NO) = (1.63 +/- 0.04) x 10(-3) mol einstein(-1)). DFT computations provide an electronic structure picture of the photoactive excited states that helps to understand this apparently abnormal behavior.
Inorganic Chemistry 08/2010; 49(15):6925-30. · 4.60 Impact Factor
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ABSTRACT: We report on the synthesis and properties of a family of linear cyanide bridged mixed-valence heptanuclear complexes with the formula: trans-[L(4)Ru(II){(mu-NC)Fe(III)(NC)(4)(mu-CN)Ru(II)L'(4)(mu-NC)Fe(III)(CN)(5)}(2)](6-) (with L and L' a para substituted pyridine). We also report on the properties of a related pentanuclear complex. These oligomers were purified by size exclusion chromatography, characterized by electrospray ionization (ESI) mass spectrometry and elemental analysis, and their linear shape was confirmed by scanning tunneling microscopy (STM). These complexes present a rich electrochemistry associated with the seven redox active centers. The redox potential split of identical fragments indicates that there is considerable communication along the cyanide bridged backbone of the compounds, even for centers more than 3 nm apart. This small attenuation of the interaction at long distances make these cyanide bridged compounds good candidates for molecular wires. Interestingly, the extent of the communication depends on the relative energy of the fragments, as evaluated by their redox potentials, providing a guide for improvement of this interesting property.
Inorganic Chemistry 12/2009; 48(23):11226-35. · 4.60 Impact Factor
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ABSTRACT: Not so elusive: [Fe(II)(CN)(5)(HNO)](3-) has been characterized spectroscopically after the two-electron reduction of nitroprusside (see scheme). The complex is stable at pH 6, slowly decomposing to [Fe(CN)(6)](4-) and N(2)O. It is deprotonated at increasing pH value with oxidation of bound NO(-) to [Fe(II)(CN)(5)(NO)](3-). [Fe(II)(CN)(5)(HNO)](3-) is the first non-heme iron-nitroxyl complex prepared in aqueous solution that is reversibly redox-active under biologically relevant conditions.
Angewandte Chemie International Edition 06/2009; 48(23):4213-6. · 13.45 Impact Factor
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ABSTRACT: The new linear homotrinuclear compound trans-[ClRu(II)(py)(4)(NC)Ru(II)(py)(4)(CN)Ru(II)(py)(4)(NO)](PF(6))(4) was prepared by reaction between the nitro complex trans-[(NC)Ru(II)(py)(4)(CN)Ru(II)(py)(4)(NO(2))](+) and the solvento complex obtained by reaction between [ClRu(II)(py)(4)(NO)](3+) and N(3)(-) in acetone. The trans-[ClRu(II)(py)(4) (NC)Ru(II)(py)(4)(CN)Ru(II)(py)(4)(NO)](4+) ion (I) has been characterized by (1)H NMR and IR spectroscopy (nu(NO) = 1919 cm(-1)). This species displays intense electronic absorptions in the visible region which can be assigned to donor-acceptor charge-transfer transitions (DACT) involving {RuNO}(6)-centered acceptor orbitals and donor orbitals located on the two different neighboring metal centers at ca. 6.7 and 12.6 A distance from the metal in the {RuNO}(6) fragment. Addition of OH(-) to I generated the nitro complex with a second-order rate constant of (12.5 +/- 0.2) x 10(3) M(-1) s(-1) (25 degrees C). Cyclic voltammetry experiments complemented by spectroelectrochemistry in the UV-vis-NIR region reveal that I can be reversibly reduced at 0.49 or 0.20 V vs AgCl/Ag for acetonitrile and water, respectively, and oxidized at 0.71 or 0.57 V vs AgCl/Ag. The spectroscopic and spectroelectrochemical information (UV-vis-NIR, X-band EPR) supplemented with electronic structure computation (DFT) reveals that the one-electron reduction is centered on the nitrosyl moiety to yield a {RuNO}(7) species, while oxidation occurs on the chlororuthenium side of the molecule. Both processes yield significant changes of the electronic spectra which are discussed in parallel with the electronic structure picture as obtained by DFT.
Inorganic Chemistry 01/2009; 48(2):565-73. · 4.60 Impact Factor
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ABSTRACT: The new linear homotrinuclear compound trans-[ClRuII(py)4(NC)RuII(py)4(CN)RuII(py)4(NO)](PF6)4 was prepared by reaction between the nitro complex trans-[(NC)RuII(py)4(CN)RuII(py)4(NO2)]+ and the solvento complex obtained by reaction between [ClRuII(py)4(NO)]3+ and N3− in acetone. The trans-[ClRuII(py)4 (NC)RuII(py)4(CN)RuII(py)4(NO)]4+ ion (I) has been characterized by 1H NMR and IR spectroscopy (νNO = 1919 cm−1). This species displays intense electronic absorptions in the visible region which can be assigned to donor−acceptor charge-transfer transitions (DACT) involving {RuNO}6-centered acceptor orbitals and donor orbitals located on the two different neighboring metal centers at ca. 6.7 and 12.6 Å distance from the metal in the {RuNO}6 fragment. Addition of OH− to I generated the nitro complex with a second-order rate constant of (12.5 ± 0.2) × 103 M−1 s−1 (25 °C). Cyclic voltammetry experiments complemented by spectroelectrochemistry in the UV−vis−NIR region reveal that I can be reversibly reduced at 0.49 or 0.20 V vs AgCl/Ag for acetonitrile and water, respectively, and oxidized at 0.71 or 0.57 V vs AgCl/Ag. The spectroscopic and spectroelectrochemical information (UV−vis−NIR, X-band EPR) supplemented with electronic structure computation (DFT) reveals that the one-electron reduction is centered on the nitrosyl moiety to yield a {RuNO}7 species, while oxidation occurs on the chlororuthenium side of the molecule. Both processes yield significant changes of the electronic spectra which are discussed in parallel with the electronic structure picture as obtained by DFT.
12/2008;
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ABSTRACT: The complex [Ru(bpy)(AN)4]2+ (bpy = 2,2'-bipyridyl, AN = acetonitrile) has a Ru(II) --> pi(*)(bpy) MLCT band at 388 nm. Upon irradiation on this absorption band, the compound undergoes total regioselective photocleavage yielding complexes fac-[Ru(bpy)(AN)(3)(H(2)O)](2+) and trans-[Ru(bpy)(AN)(2)(H(2)O)(2)](2+) in two consecutive steps with quantum yields of 0.43 and 0.09, respectively. This behavior is a consequence of the stronger sigma-donor ability of the bpy nitrogens that determines the orbital ordering and therefore the nature of the lowest lying 3d-d state responsible for the photochemistry. The two-step photoreaction, which can be followed by UV-vis and NMR spectra, provides a quantitative path to the preparation of trans-polypyridine species with potentially interesting photochemical properties.
Inorganic Chemistry 02/2008; 47(3):951-6. · 4.60 Impact Factor
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Journal of the American Chemical Society 02/2007; 129(2):278-9. · 9.91 Impact Factor
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ABSTRACT: The NIR donor-acceptor charge transfer (DACT) bands of the series of trinuclear complexes trans-[(NC)5Fe(II/III)(mu-CN)RuIIL4(mu-NC)FeIII(CN)5](5/4-) (L= pyridine, 4-tert-butylpyridine, and 4-methoxypyridine) are analyzed in terms of a simplified molecular orbital picture that reflects the interaction between the donor and acceptor fragments. The degree of electronic coupling between the fragments is estimated by a full fit of the DACT band profiles according to a three-state model inspired in the Mulliken-Hush formalism. The information is complemented with determinations performed on the asymmetric heterotrinuclear species trans-[(NC)5CoIII(mu-CN)RuII(py)4(mu-NC)FeIII(CN)5]4-, whose preparation is reported here for the first time. The analysis of the NIR spectra of the symmetric trans-[(NC)5FeIII(mu-CN)RuIIL4(mu-NC)FeIII(CN)5]4- species reveals a low degree of mixing between the terminal acceptor fragments and the bridging moiety containing RuII, with H12 values between 1.0 x 10(3) and 1.5 x 10(3) cm-1. The reorganization energy contributions seem to be the same for the three species, even when the spectra were recorded in different media. This observation also applies for the CoIII-substituted compound. The computed potential energy surfaces (PES) of the ground state for these complexes show only one stationary point, suggesting that the FeII-RuIII-FeIII (or FeII-RuIII-CoIII) electronic isomers are not thermally accessible. One-electron reduction leads to asymmetric trans-[(NC)5FeII(mu-CN)RuIIL4(mu-NC)FeIII(CN)5]5- compounds with potentially two DACT bands involving the RuII and the FeII donor fragments. These species reveal a similar degree of electronic mixing but the PES shows three minima. We explore the role of the bridging fragment in the long-range thermally induced electron transfer between the distant iron centers. The results suggest that superexchange and hopping might become competitive paths, depending on the substituents in the bridging fragment.
Inorganic Chemistry 01/2007; 45(26):10595-604. · 4.60 Impact Factor
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ABSTRACT: The new compound [Ru(bpy)(tpm)NO](ClO4)3 [tpm = tris(1-pyrazolyl)methane; bpy = 2,2'-bipyridine] has been prepared in a stepwise procedure that involves the conversion of [Ru(bpy)(tpm)Cl]+ into the aqua and nitro intermediates, followed by acidification. The diamagnetic complex crystallizes to exhibit distorted octahedral geometry around the metal, with the Ru-N(O) bond length 1.774(12) A and the RuNO angle 179.1(12) degrees , typical for a {RuNO}6 description. The [Ru(bpy)(tpm)NO]3+ ion (I) has been characterized by 1H NMR and IR spectroscopies (nu(NO) = 1959 cm(-1)) and through density functional theory calculations. Intense electronic transitions in the 300-350-nm region are assigned through time-dependent (TD)DFT as intraligand pi --> pi for bpy and tpm. The dpi --> pi(bpy) metal-to-ligand charge-transfer transitions appear at higher energies. Aqueous cyclic voltammetric studies show a reversible wave at 0.31 V (vs Ag/AgCl, 3 M Cl-), which shifts to 0.60 V in MeCN, along with the onset of a wave of an irreversible process at -0.2 V. The waves are assigned to the one- and two-electron reductions centered at the NO ligand, leading to species with {RuNO}(7) and {RuNO}(8) configurations, respectively. Controlled potential reduction of I in MeCN led to the [Ru(bpy)(tpm)NO]2+ ion (II), revealing a significant downward shift of nu(NO) to 1660 cm(-1) as well as changes in the electronic absorption bands. II was also characterized by electron paramagnetic resonance, showing an anisotropic signal at 110 K that arises from an S = 1/2 electronic ground state; the g-matrix components and hyperfine coupling tensor resemble the behavior of related {RuNO}7 complexes. Both I and II were characterized through their main reactivity modes, electrophilic and nucleophilic, respectively. The addition of OH- into I generated the nitro complex, with k(OH) = 3.05 x 10(6) M(-1) s(-1) (25 degrees C). This value is among the highest obtained for related nitrosyl complexes and correlates with ENO+/NO, the one-electron redox potential. Complex II is a robust species toward NO release, although a conversion to I was observed in the presence of O2. This reaction afforded a second-order rate law with k = 3.5 M(-1) s(-1) (25 degrees C). The stabilization of the NO radical complex is attributed to the high positive charge of the precursor and to the geometrical and electronic structure as determined by the neutral tpm ligand.
Inorganic Chemistry 10/2006; 45(21):8608-17. · 4.60 Impact Factor
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ABSTRACT: The [Os(III)(CN)6]3- anion is prepared by chemical oxidation in aqueous solution and isolated as yellow prisms of [Ph4P]3[Os(III)(CN)6].6H2O (1). This species crystallizes in the triclinic space group P with cell parameters a = 13.7609(11) A, b = 16.2275(13) A, c = 17.0895(14) A, alpha = 91.4040(10) degrees , beta = 109.3600(10) degrees , gamma = 102.3970(10) degrees , V = 3497.4(5) A(3), and Z = 2. The slightly distorted octahedral moiety displays Os-C and C-N bond lengths that average 2.058 and 1.146 A, respectively. Spin-orbit-coupling splitting of the ground-state term dominates the NIR region of the electronic spectrum and the magnetic behavior of 1. The experimental information points to higher spin delocalization over the coordinated cyanides than in [Fe(III)(CN)6]3-.
Inorganic Chemistry 04/2006; 45(6):2361-3. · 4.60 Impact Factor
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ABSTRACT: The reaction of [Cr(CN)6]3- with a mixture of trans-[Cr(cyclam)(OH)2]Cl, [Cr(cyclam)(OH)Cl]Cl and [Cr(cyclam)Cl2]Cl affords the cyanide bridged dimer, trans-[HO-Cr(cyclam)-NC-Cr(CN)5]-. The tetraphenylphosphonium salt of the anion crystallizes in space group P2(1)/n and shows a bent arrangement of the Cr1-CN-Cr2 unit with the Cr1-CN bond angle at 166.9 degrees and CN-Cr2 at 160.32 degrees . The Cr2-O bond, trans to the hexacyanide fragment, is very short at 1.902 A. Two dimers are held together by two hydrogen bonds connecting the Cr2-OH group of each dimer with one of the NH groups of the cyclam ligand of an adjacent molecule, leading to an almost linear configuration. These dimers of dimers get packed parallel to each other, generating layers separated by the tetraphenylphosphonium cations. Four of the cyanide groups of the anion are engaged in H-bonds with the four water molecules present in the structure or with a NH group of the macrocycle of an adjacent molecule. From magnetic susceptibility measurements, the dimer was found to exhibit antiferromagnetic interaction between the Cr(III) centers with J=-16 cm(-1)(H=-2JS(A)S(B)). Structural and magnetic parameters have been calculated by density functional theoretical methods at the B3LYP level. The exchange coupling constant, J, calculated for the dimer at the X-ray geometry is -23.2 cm(-1) which is in excellent agreement with the experimental value.
Dalton Transactions 03/2006; · 3.84 Impact Factor
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ABSTRACT: We report the synthesis, characterization, and spectroscopic properties of a family of trinuclear cyano-bridged mixed-valent compounds, trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-), trans-[Ru(II)L(4)[CNFe(III)(CN)(5)](2)](4-), and cis-[Ru(II)(bpy)(2)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine, 4-tert-butylpyridine, and 4-methoxypyridine). Tetraphenylphosphonium salts of complexes trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-) (L = pyridine and 4-tert-butylpyridine) crystallize in the space groups C2 and P2(1)/c, respectively, and show a linear arrangement of the metal units and an almost completely eclipsed configuration of the equatorial ligands. An intense band (epsilon approximately 2000-9000 M(-1) cm(-1)) is observed for all of the compounds in the NIR region of the spectrum, not present in the separated building blocks, and strongly solvent dependent. We assign it as a metal-to-metal charge transfer (MMCT) from the Ru(II) to the terminal Fe(III) moieties in the context of a simplified three-center model. The electrochemistry measurements reveal a splitting of the redox waves for the reduction of the iron centers for some of the complexes with a trans configuration between the metal units, ranging from 100 to 260 mV, depending on the substituting pyridine ligand and the solvent, suggesting long-range metal-metal interactions. These interactions are rationalized in terms of the energy matching between the pi-symmetry orbitals of the metals and the cyanide bridge. The one- and two-electron reduced species derived from compounds trans-[Ru(II)L(4)[NCFe(III)(CN)(5)](2)](4-,5-,6-) were characterized in methanolic solution. The mixed-valent Fe(II)-Ru(II)-Fe(III) system exhibits an intense red shifted band in the NIR region of the spectrum, arising from the superposition of MMCT bands from the central Ru(II) to the terminal Fe(III) fragments and from the 1 nm distant Fe(II) to Fe(III) centers.
Inorganic Chemistry 11/2004; 43(21):6762-73. · 4.60 Impact Factor
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ABSTRACT: The new complex trans-[NCRu(py)(4)(CN)Ru(py)(4)NO](PF(6))(3) (I) was synthesized. In acetonitrile solution, I shows an intense visible band (555 nm, epsilon = 5800 M(-1) cm(-1)) and other absorptions below 350 nm, associated with d(pi) --> pi(py) and pi(py) --> pi(py) transitions. The visible band is presently assigned as a donor-acceptor charge transfer (DACT) transition from the remote Ru(II) to the delocalized [Ru(II)-NO(+)] moiety. Photoinduced release of NO is observed upon irradiation at the DACT band. Application of the Hush model reveals strong electronic coupling, with H(DA) = approximately 2000 cm(-1). The difference between the optical absorption energy and redox potentials for the donor and acceptor sites (Ru(III,II), 1.40 V, and NO(+)/NO, 0.50 V, vs Ag/AgCl, 3 M KCl, respectively) (hnu - DeltaE(red)) is 1.33 eV, a large value which probably relates to the significant changes in distances and angles for the Ru-N-O moiety upon reduction. UV-vis absorptions, IR frequencies, and redox potentials are solvent-dependent. Controlled potential reduction (of NO(+)) and oxidation (of Ru(II) associated with the dicyano-chromophore) of I afford stable species, [NCRu(II)(py)(4)(CN)Ru(py)(4)NO](2+) (I(red)) and [NCRu(III)(py)(4)(CN)Ru(py)(4)NO](4+) (I(ox)), respectively, which are characterized by UV-vis and IR spectroscopies. I(red) shows an EPR spectrum characteristic of [Ru(II)-NO(*)] complexes. Compound I is electrophilically reactive in aqueous solution above pH 5: values of the equilibrium constant for the reaction [NCRu(py)(4)(CN)Ru(py)(4)NO](3+)+ 2 OH(-) <--> [NCRu(py)(4)(CN)Ru(py)(4)NO(2)](+) + H(2)O, K = 3.2 +/- 1.4 x 10(15) M(-2), and of the rate constant for the nucleophilic addition of OH(-), k = 9.2 +/- 0.2 x 10(3) M(-1) s(-1)(25 degrees C, I = 1 M), are obtained, with DeltaH = 90.7 +/- 3.8 kJ mol(-1) and DeltaS = 135 +/- 13 J K(-1) mol(-1). The oxidized complex, I(ox), shows an enhanced electrophilic reactivity toward OH(-). This addition reaction is followed by irreversible processes, which most probably lead to disproportionation of bound nitrite and other products.
Inorganic Chemistry 04/2002; 41(7):1930-9. · 4.60 Impact Factor
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ABSTRACT: The new complex, K(3)[Os(CN)(5)NH(3)].2H(2)O, a convenient precursor for the pentacyano-L-osmate(II) series, was prepared and characterized by chemical analysis, cyclic voltammetry, and IR and UV-vis spectroscopies. By controlled aquation in weakly acidic medium, the [Os(CN)(5)H(2)O](3-) ion was generated. Weak absorptions in the UV region for L = H(2)O, NH(3), and CN(-) were found at 287, 272, and 240 nm, respectively, and were assigned to d-d transitions, in terms of a model for tetragonally distorted ions also valid for the members of the iron and ruthenium series. The kinetics of the formation and dissociation reactions of the [Os(CN)(5)L](n-) ions, L = pyridine (py), pyrazine (pz), N-methylpyrazinium (mpz(+)), etc., were studied. At 25.0 degrees C, the formation rate constants for the neutral ligands pz and isonicotinamide were ca. 0.13 M(-1) s(-1) and slightly increased for L = mpz(+) and decreased for isonicotinate. The enthalpies of activation were ca. 22.0 kcal mol(-1), independently of the entering L, and the activation entropies were all positive, ca. 11-13 cal K(-1) mol(-1). The dissociation reactions showed a saturation rate behavior of k(obs) (s(-1)) as a function of the concentration of the scavenger ligand. The specific dissociation rate constant at 25.0 degrees C was 1.06 x 10(-7) s(-1) for L = NH(3) and around 10(-9) s(-1) for py, pz, and mpz(+) (extrapolated to 25.0 degrees C from values measured in the range 60-95 degrees C). These small values are associated with high activation enthalpies (range 30-35 kcal mol(-1)) and positive activation entropies (range 10-20 cal K(-1) mol(-1)). The evidence for both the formation and dissociation processes shows that dissociative mechanisms are operative, as for the iron and ruthenium analogues.
Inorganic Chemistry 02/2002; 41(1):114-20. · 4.60 Impact Factor
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08/1999;
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ABSTRACT: Quantum chemical calculations using density functional theory have been carried out to investigate the basicity of pentaammine(pyrazine)ruthenium(II) and pentacyano(pyrazine)ruthenate(II) ions. Gas phase equilibrium geometries were fully optimized at the local density approximation (LDA) level. Single-point calculations were performed at the optimized gas phase geometries using a generalized gradient approximation (GGA) functional. The basicity of the uncoordinated nitrogen on pyrazine was investigated in gas phase and in aqueous solution, modeling solvent effects using a self-consistent reaction field (SCRF) Onsager model, a discrete electrostatic representation of the water molecules in the first solvation shell, and also a combined SCRF−discrete model within the DFT−GGA methodology. A reasonable level of agreement between theory and experiment is obtained only when using the SCRF−discrete model, suggesting that both specific interactions in the first solvation shell and long-range dielectric effects affect the reactivity of the complexes.
03/1998;
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ABSTRACT: The series of complexes [OsII(CN)5L]n-, with L = pyridine or pyrazine derivatives, were prepared in aqueous solution and, in some cases, as sodium or potassium salts. The main feature in the UV−visible spectra is the appearance of an intense, asymmetric MLCT band, split under spin−orbit coupling. The energies and intensities of the MLCT bands decrease and increase, respectively, with the electron-acceptor ability of L, and strong solvatochromic energy shifts are observed in different organic media. The Os(II) complexes can be oxidized chemically or electrochemically to the Os(III) species; the latter show typical LMCT bands in the visible region, independent of L. The redox potentials for the OsIII,II couples (range 0.6−1.0 V (NHE)), shift positively when L becomes more electron-withdrawing or less basic. Reduction potentials for the bound and free Mepz+ ligand showed similar values, ca. −0.53 V (NHE), for the three [M(CN)5L]n- complexes, suggesting similar back-bonding abilities of Fe, Ru, and Os toward a given L ligand; this is confirmed by the linear plots with unit slope obtained for the energy of the MLCT bands of the [M(CN)5L]n- complexes (M = Fe, Ru) against the values for the [Os(CN)5L]n- complexes. The IR spectra show intense and weak bands at ca. 2050 and 2100 cm-1, associated with equatorial and axial cyanide stretchings, respectively. The dissociation rate constant for pyrazine release from the [Os(CN)5pz]3- ion shows a saturation kinetic behavior, typical of dissociative mechanisms found for the iron and ruthenium analog complexes; the specific dissociation rate constant, k-pz = 2.0 × 10-8 s-1 (25 °C, I = 0.5 M), is about 3 and 4 orders of magnitude slower than the values found for [Ru(CN)5pz]3- and [Fe(CN)5pz]3-, respectively; this is ascribed mainly to the strong σ interaction in the Os−L bond.
10/1996;
