Charge and Spin-State Characterization of Cobalt Bis(o-dioxolene) Valence Tautomers Using Co Kß X-ray Emission and L-Edge X-ray Absorption Spectroscopies.

The valence tautomeric states of Co(phen)(3,5-DBQ)2 and Co(tmeda)(3,5-DBQ)2, where 3,5-DBQ is either the semiquinone (SQ-) or catecholate (Cat2-) form of 3,5-di-tert-butyl-1,2-benzoquinone, have been examined by a series of cobalt-specific X-ray spectroscopies. In this work, we have utilized the sensitivity of 1s3p X-ray emission spectroscopy (Kß XES) to the oxidation and spin states of 3d transition-metal ions to determine the cobalt-specific electronic structure of valence tautomers. A comparison of their Kß XES spectra with the spectra of cobalt coordination complexes with known oxidation and spin states demonstrates that the low-temperature valence tautomer can be described as a low-spin CoIII configuration and the high-temperature valence tautomer as a high-spin CoII configuration. This conclusion is further supported by Co L-edge X-ray absorption spectroscopy (L-edge XAS) of the high-temperature valence tautomers and ligand-field atomic-multiplet calculations of the Kß XES and L-edge XAS spectra. The nature and strength of the magnetic exchange interaction between the cobalt center and SQ- in cobalt valence tautomers is discussed in view of the effective spin at the Co site from Kß XES and the molecular spin moment from magnetic susceptibility measurements.

Ligand redox activity and mixed valency in first-row transition-metal complexes containing tetrachlorocatecholate and radical tetrachlorosemiquinonate ligands.

Ligand noninnocence occurs for complexes composed of redox-active ligands and metals, with frontier orbitals of similar energy. Usually methods of analysis can be used to define the charge distribution, and cases where the metal oxidation state and ligand charge are unclear are unusual. Ligands derived from o-benzoquinones can bond with metals as radical semiquinonates (SQ(•-)) or as catecholates (Cat(2-)). Spectroscopic, magnetic, and structural properties can be used to assess the metal and ligand charges. With the redox activity at both the metal and ligands, reversible multicomponent redox series can be observed using electrochemical methods. Steps in the series may occur at either the ligand or metal, and ligand substituent effects can be used to tune the range of ligand-based redox steps. Complexes that appear as intermediates in a ligand-based redox series may contain both SQ and Cat ligands "bridged" by the metal as mixed-valence complexes. Properties reflect the strength of metal-mediated interligand electronic coupling in the same way that ligand-bridged bimetallics conform to the Robin and Day classification scheme. In this review, we will focus specifically on complexes of first-row transition-metal ions coordinated with three ligands derived from tetrachloro-1,2-benzoquinone (Cl(4)BQ). The redox activity of this ligand overlaps with the potentials of common metal oxidation states, providing examples of metal- and ligand-based redox activity, in some cases, within a single redox series. The strength of the interligand electronic coupling is important in defining the separation between ligand-based couples of a redox series. The complex of ferric iron will be described as an example where coupling is weak, and the steps associated with the Fe(III)(Cl(4)SQ)(3)/[Fe(III)(Cl(4)Cat)(3)](3-) redox series are observed over a narrow range in electrochemical potential.

Synthesis and characterization of V(V)(3,6-DBSQ)(3,6-DBCat)2, a d(0) metal complex with dioxygenase catalytic activity.

Transition-metal complexes containing redox-active quinoid ligands are of interest because of their catalytic capabilities in multielectron, substrate-activation reactions such as dioxygenase catalysis using O(2). The new catecholate complex V(V)(3,6-DBSQ)(3,6-DBCat)(2) (where 3,6-DBSQ = 3,6-di-tert-butylsemiquinone and 3,6-DBCat = 3,6-di-tert-butylcatecholate) was synthesized by combining VO(acac)(2) with 1 equiv of 3,6-DBBQ (where 3,6-DBBQ = 3,6-di-tert-butylbenzoquinone) and 2 equiv of H(2)(3,6-DBCat) in dry methanol under an inert atmosphere. The resultant complex was characterized by single-crystal X-ray diffraction, elemental analysis, near-IR, UV/vis, and electron paramagnetic resonance (EPR) spectroscopy. The crystallography as well as the near-IR and EPR studies suggest that the radical spin is localized on the 3,6-DBSQ ligand at room temperature, making V(V)(3,6-DBSQ)(3,6-DBCat)(2) a type 1 mixed-valence complex. Initial dioxygenase catalysis studies reveal that V(V)(3,6-DBSQ)(3,6-DBCat)(2) is a good dioxygenase precatalyst for the substrate H(2)(3,6-DBCat) with O(2) in ca. 600 total turnovers to >93% intra- and extradiol products with only 1-2% of the undesired benzoquinone autoxidation product.

Tetranuclear iron(III) complexes of an octadentate pyridine-carboxylate ligand and their catalytic activity in alkane oxidation by hydrogen peroxide.

Reaction of the octadentate ligand 2,6-bis{3-[N,N-di(2-pyridylmethyl)amino]propoxy}benzoic acid (LH) with Fe(ClO4)3 leads to the formation of the tetranuclear complexes [Fe4(mu-O)2(LH)2(ClCH2-CO2)4](ClO4)4 (1), [{Fe2(mu-O)L(R-CO2)}2](ClO4)4 (2 R = C6H5-, 3 R = CH3-, 4, R = ClCH2-). The crystal structures of complexes 1 and 2 reveal that they consist of two Fe(III)2(mu-O)(mu-RCO2)2 cores that are linked via the two LH/L ligands to give a "dimer of dimers" structure. Complex assumes a helical shape, with protonated carboxylic acid moieties of the two ligands forming a hydrogen-bonded pair at the center of the cation. In complexes 2, 3 and 4, central carboxylates of the two ligands bridge the iron ions in each of the two Fe2O units, with an interdimer iron-iron separation of approximately 10 A and an intradimer separation of approximately 3.1 A. The second carboxylate bridge within the Fe2O units is defined by exogenous benzoate (2), acetate (3) or chloroacetate (4) ligands. The aqua complex [{Fe2(mu-O)L(H2O)2}2](ClO4)6 (5) is proposed to have a similar structure, but with the exogenous bridging carboxylates replaced by two terminal water ligands. These complexes exhibit electronic and Mössbauer spectral features that are similar to those of (mu-oxo)diiron(III) proteins as well as other related (mu-oxo)bis(mu-carboxylato)diiron(III) complexes. This similarity shows that these properties are not significantly affected by the nature of the bridging exogenous carboxylate, and that the octadentate framework ligand is essential in stabilizing the "dimer of dimers" structure. This structural feature remains in highly diluted solution (10(-5) M) as evidenced by electrospray ionization mass-spectroscopy (ES MS). Cyclic voltammetric studies of complexes 2 and 5 showed two irreversible two-electron reductions, indicating that the two Fe2O units of the tetranuclear complexes behave as distinct redox entities. Complexes 2, 3 and, especially, the aqua complex 5 are active alkane oxidation catalysts. Catalytic reactions carried out with alkane substrate molecules and hydrogen peroxide predominantly gave alcohols. High stereospecificity in the oxidation of cis-1,2-dimethylcyclohexane supports the metal-based molecular mechanism of O-insertion into C-H bonds postulated for non-heme iron enzymes such as methane monooxygenase.

Binuclear complexes of Co(III) containing extended conjugated bis(catecholate) ligands.

Binuclear complexes of cobalt(III) have been prepared with 3,3',4,4'-tetrahydroxy-benzaldazine (H4thB), 3,3',4,4'-tetrahydroxy-5,5'-dimethoxybenzaldazine (H4thM), and 3,3',4,4'-tetrahydroxydimethylbenzaldazine (H4thA) as bis(catecholate) ligands that link metal ions separated by 16 A through a conjugated bridge. In one case, [Co2(bpy)4(thM)]2+, stereodynamic properties observed in solution by 1H NMR are associated with valence tautomerism, with formation of a labile hs-Co(II) species by electron transfer from the catecholate regions of the bridge. Electrochemical oxidation of the complexes occurs at the bridges as two closely spaced one-electron couples. Chemical oxidation of [Co2(bpy) 4(thB)]2+ with Ag+ is observed to occur as a two-electron process forming [Co2(bpy) 4(thB(SQ,SQ))]4+. Attempted crystallization in the presence of air was observed to result in formation of the [Co(bpy)2(BACat)]+ (H2BACat, 3,4-dihydroxybenzaldehyde) cation by aerobic oxidation. Structural characterization is provided for the H4thM ligand and [Co(bpy)2(BACat)](BF4).

Synthesis of a quinone-functionalized macrocyclic ligand and the intense fluorescence of its zinc complex.

The redox-active quinone-functionalized macrocyclic ligand 1,4,14,17-tetrahydroxyhemiporphyrazine, H2hp(OH)4, has been synthesized and its zinc complex, [Zn(hp(OH)4)(py)], found to exhibit intense fluorescence.

Coordination complexes of molybdenum with 3,6-di-tert-butylcatechol. Addition products of DMSO, pyridine N-oxide, and triphenylarsine oxide to the putative [MoVIO(3,6-DBCat)2] monomer and self-assembly of the chiral [(MoVIO(3,6-DBCat)2)4] square.

Molybdenum complexes of 3,6-di-tert-butylcatechol have been prepared from the reaction between [Mo(CO)(6)] and 3,6-di-tert-butyl-1,2-benzoquinone. A putative "[MoO(3,6-DBCat)(2)]" monomer is assumed to form initially by reaction with trace quantities of oxygen. Condensation of the reaction mixture leads to the formation of oligomeric products, including the [(MoO(3,6-DBCat)(2))(4)] chiral square isolated by chromatographic separation. Molybdenum centers at the corner of the square are bridged by oxo ligands centered along edges. Four-fold and inversion crystallographic symmetry gives tetramers as either LambdaLambdaLambdaLambda or DeltaDeltaDeltaDelta isomers, and the crystal structure consists of parallel columns of squares with the same chirality. Addition of O-Subst (O-Subst = dmso, pyridine N-oxide, triphenylarsine oxide) ligands to [MoO(3,6-DBCat)(2)] occurs selectively to give cis-[MoO(O-Subst)(3,6-DBCat)(2)] products. All three addition complexes are fluxional in solution. The temperature-dependent stereodymanic behavior of [MoO(dmso)(3,6-DBCat)(2)] has been shown to occur via a trigonal prismatic intermediate (Bailar twist) that conserves the cis disposition of oxo and dmso ligands. Electrochemical and chemical reduction reactions have been investigated for [MoO(dmso)(3,6-DBCat)(2)] with interest in displacement of SMe(2) with formation of cis-[MoO(2)(3,6-DBCat)(2)](2-). Cyclic voltammetry shows an irreversible two-electron reduction for the complex at -0.852 V (vs Fc/Fc(+)). Chemical reduction using CoCp(2) was observed to give a product with an electronic spectrum that is generally associated with cis-[MoO(2)(Cat)(2)](2-) complexes. Structural characterization revealed that the product was [CoCp(2)][MoO(3,6-DBCat)(2)], possibly formed as the product of dmso displacement upon one-electron reduction of [MoO(dmso)(3,6-DBCat)(2)].

Structural systematics for o-C(6)H(4)XY ligands with X,Y= O, NH, and S donor atoms. o-iminoquinone and o-iminothioquinone complexes of ruthenium and osmium.

The structural features of quinone ligands are diagnostic of charge. The o-benzoquinone, radical semiquinonate, and catecholate electronic forms have C-O bond lengths and a pattern of ring C-C bond lengths that point to a specific mode of coordination. This correlation between ligand charge and structure has been extended to iminoquinone and iminothioquinone ligands, giving a charge-localized view of electronic structure for complexes of redox-active metal ions. The radical semiquinonate form of these ligands has been found to be a surprisingly common mode of coordination; however, the paramagnetic character of the radical ligand is often obscured in complexes containing paramagnetic metal ions. In this report, diamagnetic iminosemiquinonate (isq) and iminothiosemiquinonate (itsq) complexes of ls-d(5) Ru(III) with related complexes of osmium are reported. With osmium, the Os(IV)-amidophenolate (ap) redox isomer is formed. Electrochemical and spectral properties are described for Ru(PPh(3))(2)(isq)Cl(2), Ru(PPh(3))(2)(itsq)Cl(2), Os(PPh(3))(2)(ap)Br(2), Os(PPh(3))(2)(atp)Br(2), and Os(PPh(3))(2)(ap)H(2). Crystallographic characterization of Ru(PPh(3))(2)(isq)Cl(2), Ru(PPh(3))(2)(itsq)Cl(2), and Os(PPh(3))(2)(ap)H(2) was used to assign charge distributions.

Methoxide Coordination at the Pocket of [CuII TpCum, Me] and a Simple Model for the Cu Center of Galactose Oxidase.

An unusually negative oxidation potential is found for the tyrosine residue in the center of fungal galactose oxidase. The complex [Cu(TpCum,Me ){O(MeS)C6 H4 }] (see picture on the right; TpCum,Me =hydrotris(pyrazolyl)borate) offers insight into the mode of (cysteinyl-tyrosine) coordination to the copper center, and the reason for the low oxidation potential.

Pocket Semiquinonate Complexes of Cobalt(II), Copper(II), and Zinc(II) Prepared with the Hydrotris(cumenylmethylpyrazolyl)borate Ligand.

3,5-Di-tert-butyl-1,2-semiquinonate (3,5-DBSQ) complexes of Co(II), Cu(II), and Zn(II) have been prepared that contain the hydrotris(cumenylmethyl-pyrazolyl)borate (Tp(Cum,Me)) coligand. Tp(Cum,Me)Zn(3,5-DBSQ) and Tp(Cum,Me)Cu(3,5-DBSQ) were prepared by treating the parent hydroxide, Tp(Cum,Me)M(OH), M = Cu and Zn, with 3,5-di-tert-butylcatechol. Tp(Cum,Me)Co(3,5-DBSQ) was prepared by a reaction between (Tp(Cum,Me))(2)Co and 3,5-DBCat. The identity of (Tp(Cum,Me))(2)Co in this reaction was confirmed by a structure determination [(Tp(Cum,Me))(2)Co: orthorhombic, Pbcn, a = 17.7189(4) Å, b = 17.4806(3) Å, c = 25.7123(6) Å, V = 7964.1(3) Å(3), Z = 4, R(F) = 0.054]. Intersecting cumenyl substituents of the pyrazolylborate ligand encapsulate the Co(II) ion. Structural characterization on all three members of the Tp(Cum,Me)M(3,5-DBSQ) series has been carried out. The complexes of Co(II) and Zn(II) are isomorphous and isostructural [Tp(Cum,Me)Co(3,5-DBSQ): triclinic, P&onemacr;, a = 14.4631(2) Å, b = 18.5438(3) Å, c = 21.6142(2) Å, alpha = 79.8430(10) degrees, beta = 90.0900(10) degrees, gamma = 84.9900(10) degrees, V = 5683.45(13) Å(3), Z = 4, R(F) = 0.072; Tp(Cum,Me)Zn(3,5-DBSQ), triclinic, P&onemacr;, a = 14.261(3) Å, b = 18.760(7) Å, c = 21.710(4) Å, alpha = 80.049(12) degrees, beta = 89.853(8) degrees, gamma = 85.542(12) degrees, V = 5703(3) Å(3), Z = 4, R(F) = 0.064]. Tp(Cum,Me)Cu(3,5-DBSQ) [monoclinic, P2(1)/c, a = 19.3081(3) Å, b = 13.0291(2) Å, c = 21.4783(4) Å, beta = 102.8420(10) degrees, V = 5268.1(2) Å(3), Z = 4, R(F) = 0.071] has a distorted square pyramidal structure, the complexes of Zn and Co have structures that are closer to a trigonal bipyramid. Parent catecholate complexes of all three metals are unusually stable in air but undergo slow oxidation in solution to give the semiquinonate products characterized structurally. Copper(II) and SQ spins of Tp(Cum,Me)Cu(3,5-DBSQ) are located in orthogonal orbitals, and the complex has a S = 1 spin state. The charge distribution in Tp(Cum,Me)Co(3,5-DBSQ) is Co(II)-SQ, rather than the more common Co(III)-Cat, due to surprisingly weak donation by the Tp(Cum,Me) nitrogens.

Iron-Semiquinone, Semiquinone-Semiquinone, and Iron-Iron Magnetic Exchange in Monomeric and Dimeric Ferric Complexes Containing the 3,6-Di-tert-butyl-1,2-semiquinonate Ligand.

Fe(3,6-DBSQ)(3) has been prepared by reacting 3,6-di-tert-butyl-1,2-benzoquinone with Fe(CO)(5). The complex has been characterized structurally [orthorhombic, Pbca, a = 18.277(5) Å, b = 11.634(3) Å, c = 39.903(10) Å, V = 8485(4) Å(3), Z = 8, R = 0.063], electrochemically, and magnetically. Ligand-based redox couples are observed in electrochemical experiments that consist of reversible or quasireversible Cat/SQ steps at negative potentials and irreversible SQ/BQ oxidations at positive potentials. Magnetic measurements show temperature dependence that arises from antiferromagnetic exchange. Data have been fit to an expression that includes the effects of both Fe-SQ and SQ-SQ exchange with the result that J(SQ-SQ) is larger in magnitude than J(Fe-SQ). In methanol, the complex undergoes solvolysis to form [Fe(3,6-DBSQ)(2)(&mgr;-OMe)](2). Structural characterization [triclinic, P&onemacr;, a = 11.441(2) Å, b = 11.514(2) Å, c = 14.552(2) Å, alpha = 67.86(1) degrees, beta = 70.51(1) degrees, gamma = 72.79(1) degrees, V = 1641.8(5) Å(3), Z = 1, R = 0.048] has shown that the molecule is a centrosymmetric dimer with no close intermolecular contacts. The temperature dependence of magnetic measurements has been fit to a model consisting of two interacting S = (3)/(2) centers that arise from strong Fe-SQ exchange with J(Fe-Fe) = -22.4 cm(-1).