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Type 1 copper (T1Cu) proteins play important roles in electron transfer in biology, largely due to the unique structure of the T1Cu center, which is reflected by its spectroscopic properties. Previous reports have suggested a correlation between a high ratio of electronic absorbance at ∼450 nm to that at ∼600 nm (

A series of mixed-valence nickel-iron dithiolates is described. Oxidation of (diphosphine)Ni(dithiolate)Fe(CO)(3) complexes 1, 2, and 3 with ferrocenium salts affords the corresponding tricarbonyl cations [(dppe)Ni(pdt)Fe(CO)(3)](+) ([1](+)), [(dppe)Ni(edt)Fe(CO)(3)](+) ([2](+)) and [(dcpe)Ni(pdt)Fe(CO)(3)](+) ([3](+)), respectively, where dppe = Ph(2)PCH(2)CH(2)PPh(2), dcpe = Cy(2)PCH(2)CH(2)PCy(2), (Cy = cyclohexyl), pdtH(2) = HSCH(2)CH(2)CH(2)SH, and edtH(2) = HSCH(2)CH(2)SH. The cation [2](+) proved unstable, but the propanedithiolates are robust. IR and EPR spectroscopic measurements indicate that these species exist as C(s)-symmetric species. Crystallographic characterization of [3]BF(4) shows that Ni is square planar. Interaction of [1]BF(4) with P-donor ligands (L) afforded a series of substituted derivatives of type [(dppe)Ni(pdt)Fe(CO)(2)L]BF(4) for L = P(OPh)(3) ([4a]BF(4)), P(p-C(6)H(4)Cl)(3) ([4b]BF(4)), PPh(2)(2-py) ([4c]BF(4)), PPh(2)(OEt) ([4d]BF(4)), PPh(3) ([4e]BF(4)), PPh(2)(o-C(6)H(4)OMe) ([4f]BF(4)), PPh(2)(o-C(6)H(4)OCH(2)OMe) ([4g]BF(4)), P(p-tol)(3) ([4h]BF(4)), P(p-C(6)H(4)OMe)(3) ([4i]BF(4)), and PMePh(2) ([4j]BF(4)). EPR analysis indicates that ethanedithiolate [2](+) exists as a single species at 110 K, whereas the propanedithiolate cations exist as a mixture of two conformers, which are proposed to be related through a flip of the chelate ring. Mössbauer spectra of 1 and oxidized S = 1/2 [4e]BF(4) are both consistent with a low-spin Fe(I) state. The hyperfine coupling tensor of [4e]BF(4) has a small isotropic component and significant anisotropy. DFT calculations using the BP86, B3LYP, and PBE0 exchange-correlation functionals agree with the structural and spectroscopic data, suggesting that the SOMOs in complexes of the present type are localized in an Fe(I)-centered d(z(2)) orbital. The DFT calculations allow an assignment of oxidation states of the metals and rationalization of the conformers detected by EPR spectroscopy. Treatment of [1](+) with CN(-) and compact basic phosphines results in complex reactions. With dppe, [1](+) undergoes quasi-disproportionation to give 1 and the diamagnetic complex [(dppe)Ni(pdt)Fe(CO)(2)(dppe)](2+) ([5](2+)), which features square-planar Ni linked to an octahedral Fe center.

We report the synthesis, crystal structure, physical properties and hydrolysis kinetics of [(O)(tmpa)VIV(μ-O)VV(tmpa)(O)]3+ (tmpa = tris(2-pyridylmethyl)amine), the first cation in its class for which both solid state and solution-phase characteristics are fully consistent with expectations for a Robin−Day type III mixed-valence complex. [V2(tmpa)2(O)3](ClO4)3·CH3CN crystallizes in the triclinic, P1 (No. 2) space group with a = 10.719(2) A, b = 11.609(1) A, c = 19.516(2) A, α = 87.876(9)°, β = 79.295(9)°, γ = 68.743(9)°, and V = 2222.6(6) A3; Z = 2. The two vanadium centers are crystallographically equivalent in a linear, oxo-bridged cation with V−Ob and VO bond lengths of 1.800(1) and 1.597(4) A, respectively. A 15-hyperfine line EPR spectrum (g = 1.9704, A = 46.4 × 10-4 cm-1) was observed for [V2(tmpa)2(O)3]3+, consistent with coupling of a single electron to two equivalent 51V atoms. The mixed-valence complex exhibits an intervalence transfer (IT) band at 1.05 × 104 cm-1 (e = 2.1 × 103 M-1 cm-1, CH3CN...

The one-electron oxidations of a series of diiron(I) dithiolato carbonyls were examined to evaluate the factors that affect the oxidation state assignments, structures, and reactivity of these low-molecular weight models for the H ox state of the [FeFe]-hydrogenases. The propanedithiolates Fe 2(S 2C 3H 6)(CO) 3(L)(dppv) (L = CO, PMe 3, P i-Pr 3) oxidize at potentials approximately 180 mV milder than the related ethanedithiolates ( Angew. Chem., Int. Ed. 2007, 46, 6152). The steric clash between the central methylene of the propanedithiolate and the phosphine favors the rotated structure, which forms upon oxidation. Electron Paramagnetic Resonance (EPR) spectra for the mixed-valence cations indicate that the unpaired electron is localized on the Fe(CO)(dppv) center in both [Fe 2(S 2C 3H 6)(CO) 4(dppv)]BF 4 and [Fe 2(S 2C 3H 6)(CO) 3(PMe 3)(dppv)]BF 4, as seen previously for the ethanedithiolate [Fe 2(S 2C 2H 4)(CO) 3(PMe 3)(dppv)]BF 4. For [Fe 2(S 2C n H 2 n )(CO) 3(P i-Pr 3)(dppv)]BF 4; however, the spin is localized on the Fe(CO) 2(P i-Pr 3) center, although the Fe(CO)(dppv) site is rotated in the crystalline state. IR and EPR spectra, as well as redox potentials and density-functional theory (DFT) calculations, suggest that the Fe(CO) 2(P i-Pr 3) site is rotated in solution, driven by steric factors. Analysis of the DFT-computed partial atomic charges for the mixed-valence species shows that the Fe atom featuring a vacant apical coordination position is an electrophilic Fe(I) center. One-electron oxidation of [Fe 2(S 2C 2H 4)(CN)(CO) 3(dppv)] (-) resulted in 2e oxidation of 0.5 equiv to give the mu-cyano derivative [Fe (I) 2(S 2C 2H 4)(CO) 3(dppv)](mu-CN)[Fe (II) 2(S 2C 2H 4)(mu-CO)(CO) 2(CN)(dppv)], which was characterized spectroscopically.

Electron paramagnetic resonance (EPR) spectra are presented for iron complexes of catecholate, tironate, and a 3,4-dihydroxyphenylalanine (DOPA)-containing peptide of sequence Ac-Ala-DOPA-Thr-Pro-CONH2 ("AdopaTP"). This peptide was prepared to model potential metal-protein cross-links in the adhesive used by marine mussels, Mytilus edulis, for affixing themselves to surfaces. Spectra are shown for iron bound to each ligand in mono, bis, and tris coordination environments. For example, the catecholate complexes {Fe(cat)}, {Fe(cat)2}, and [Fe(cat)3]3- are provided. Detailed simulations are presented to describe the origin of spectra for the iron-catecholate and iron-peptide species, which show that the spectral features can be accounted for only with the inclusion of D- and E-strain. The spectroscopy of each compound is shown under both anaerobic and aerobic conditions. When exposed to air, the high-spin Fe3+ signal of [Fe(AdopaTP)3]3- decreases and an organic radical is formed. No other sample exhibited an appreciable radical signal. These data are discussed in light of the biomaterial synthesis carried out by marine mussels.

We report the synthesis, crystal structure, physical properties and hydrolysis kinetics of [(O)(tmpa)V(IV)(mgr;-O)V(V)(tmpa)(O)](3+) (tmpa = tris(2-pyridylmethyl)amine), the first cation in its class for which both solid state and solution-phase characteristics are fully consistent with expectations for a Robin-Day type III mixed-valence complex. [V(2)(tmpa)(2)(O)(3)](ClO(4))(3).CH(3)CN crystallizes in the triclinic, Ponemacr; (No. 2) space group with a = 10.719(2) Å, b = 11.609(1) Å, c = 19.516(2) Å, alpha = 87.876(9) degrees, beta = 79.295(9) degrees, gamma = 68.743(9) degrees, and V = 2222.6(6) Å(3); Z = 2. The two vanadium centers are crystallographically equivalent in a linear, oxo-bridged cation with V-O(b) and V=O bond lengths of 1.800(1) and 1.597(4) Å, respectively. A 15-hyperfine line EPR spectrum (g = 1.9704, A = 46.4 x 10(-4) cm(-1)) was observed for [V(2)(tmpa)(2)(O)(3)](3+), consistent with coupling of a single electron to two equivalent (51)V atoms. The mixed-valence complex exhibits an intervalence transfer (IT) band at 1.05 x 10(4) cm(-1) (epsilon = 2.1 x 10(3) M(-1) cm(-1), CH(3)CN) whose energy is insensitive to variations in the solvent; d-d transitions are observed at 1.25 x 10(4) cm(-1) ((2)E-- (2)B(2)) and 1.44 x 10(4) cm(-1) ((2)B(1)-- (2)B(2)). [V(2)(tmpa)(2)(O)(3)](2+), [V(2)(tmpa)(2)(O)(3)](3+), and [V(2)(tmpa)(2)(O)(3)](4+) comprise a redox family related by vanadium (IV,V/IV,IV) and (V,V/IV,V) half-wave reduction potentials of 0.25 and 1.62 V vs NHE (25 degrees C, CH(3)CN, I = 0.1 M), respectively. The hydrolysis reaction: [V(2)(tmpa)(2)(O)(3)](3+) + H(2)O --[V(IV)(tmpa)(O)(OH)](+) + [V(V)(tmpa)(O)(2)](+) + H(+) proceeds with an acid-independent rate constant of 2.37 x 10(-3) s(-1) (25 degrees C; DeltaH() = 71 kJ/mol, DeltaS() = -59 J/mol K). [V(4)O(5)(tmpa)(4){Fe(CN)(6)}](ClO(4))(4).H(2)O was isolated as an unexpected product from the reaction of [V(2)(tmpa)(2)(O)(3)](ClO(4))(2).2H(2)O with equimolar K(3)[Fe(CN)(6)] in aqueous solution. It is proposed that pairs of V(IV)=O and V(V)=O units aggregate with [Fe(CN)(6)](4-) into a charge transfer complex. Hexacyanoferrate(II)-to-vanadium IT charge transfer bands are observed at 1.98 x 10(4) cm(-1) (epsilon = 5.3 x 10(3) M(-1) cm(-1)) and 2.72 x 10(4) cm(-1) (epsilon = 9.3 x 10(3) M(-1) cm(-1)); a 29-line EPR spectrum (g = 1.9737, A = 22.0 x 10(-4) cm(-1)) demonstrates equal delocalization of unpaired electron density over all four V atoms (CH(3)CN solution).