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Temperature-dependent measurements of potential, E degrees', and electron-transfer rate constant, k(s,h), are reported for electrochemical reduction (in 0.3 M TBAPF(6)/CH(3)CN) of a series of oxomolybdenum(V) complexes, [(Tp)MoO(X,Y)], where Tp = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X,Y is a series of bidentate 1,2-disubstituted aliphatic or aromatic ligands in which oxygen donors are replaced sequentially by sulfur. E degrees' values shift in the positive direction, and k(s,h) values increase as O is replaced by S and as the framework of the ligand is changed from aliphatic to aromatic. The electrochemical enthalpy of activation, measured under conditions of zero driving force as DeltaH= -R partial differential[ln(k(s,h))]/ partial differential(1/T) and corrected for an outer-shell component by the mean spherical approximation, is approximately 10 kJ mol(-1) larger for complexes with O versus S donors and with an aliphatic versus aromatic ligand framework. Thus, the rate of Mo(V/IV) electron transfer is modulated primarily by differences in inner-shell reorganization. Following a recent description of electronic structure contributions to electron-transfer reactivity (Kennepohl, P.; Solomon, E. I. Inorg. Chem. 2003, 42, 679 ff), it is concluded that more effective charge distribution over the entire molecular structure, as mediated by electronic relaxation in S versus O and aromatic versus aliphatic systems, is responsible for the influence of ligand structure on the kinetics and thermodynamics of Mo-centered electron transfer. There is no evidence, based on experimentally measured pre-exponential factors, that sulfur donors or an aromatic ligand framework are more effective than their structural counterparts in facilitating electronic coupling between the electrode and the Mo d(xy) redox orbital.

Complexes of molybdenum(VI) with the catecholamide siderophore ligands aminochelin, azotochelin and protochelin undergo electrochemical reduction by sequential Mo(VI)→Mo(V) and Mo(V)→Mo(III) electron transfers in pH 5.40–6.74 pyridine (py) buffer in a manner closely analogous to the cis -dioxo Mo(VI)–catechol complex, MoO 2 (cat) 2 2− . The pH dependence of the electrode half-reaction potentials for Mo–azotochelin and Mo–aminochelin reveals that between one and two protons are transferred in conjunction with Mo(VI) to Mo(V) reduction and between two and three protons in conjunction with Mo(V) to Mo(III) reduction. It appears that the acidity of the protons on the coordinated H 2 O in the Mo V O(OH 2 )(L) 2 − product is enhanced by the amide substituents on the siderophore ligands. Electrode potentials shift in the positive direction with increasing py concentration, consistent with coordination of one py to Mo(V) and two py to Mo(III). The Mo(VI)/Mo(V) and Mo(V)/Mo(III) potentials of Mo–siderophore complexes fall in the ranges of −0.45–−0.55 and −0.75–−0.8 V versus Ag/AgCl (−0.2–−0.3 and −0.5–−0.55 V vs. NHE), respectively, in pH 6.74 py buffer.

The kinetics and thermodynamics of concerted two-electron transfer and metal−metal bond cleavage in the binuclear phosphido-bridged complexes [M2(μ-PPh2)2(CO)8]0/2- [M = Mo (10/2-), W(20/2-)] have been determined by variable scan-rate cyclic voltammetry in 0.3 M TBAPF6/acetone. The reductions of 1° and 2° are accompanied by an increase of 1.08 A in M−M distance and expansion and contraction by 29° of the M−P−M and P−M−P angles, respectively, within an intact M2(μ-PPh2)2 unit. The one-electron electrode potentials of these systems are highly inverted: ΔE°‘ = − = +0.17 V for 10/2- and +0.18 V for 20/2-, and the rate constant of the second heterogeneous electron-transfer reaction is smaller than the first: ksh,2/ksh,1 = 0.1 for Mo and 0.018 for W. Results are consistent with progressive cleavage of the metal−metal bond in two one-electron steps, of which the second is rate-limiting, because it is accompanied by a larger part of the structural change. EHMO calculations reveal that the redox-active orbital i...

Preparation, spectroscopic characterization and electrochemical behavior are reported for the six-coordinate monooxo Mo(VI) complexes, MoOL(cat), where L2−=2,2′-thiobis(4-methyl-6-tert-butylphenolate) or 2,2′-thiobis(4,6-di-tert-butylphenolate) and cat2−=catecholate (Cat2−), 4-methylcatecholate (Mecat2−), 4-nitrocatecholate (NO2cat2−), or 3,5-di-tert-butylcatecholate (DTBcat2−). The compounds are prepared by reaction of catechol with Mo2O3L2, the μ-oxo Mo(V) dimer produced by reaction of MoO2(acac)2 with H2L. MoOL(cat) complexes exhibit a MoO stretching vibration (νMoO) at 949–961 cm−1 and a reversible Mo(VI/V) reduction at −0.3 to −0.6 V versus Fc+/0. These features are characteristic of a mononuclear MoO4+ center. The kinetics of the metal-centered electrode reaction is sensitive to the catechol substituent. Under identical experimental conditions, the apparent electron-transfer rate indicated by the voltammetric peak potential separation (ΔEp) increases in the following sequence: DTBcat2−

Preparation and electrochemical and spectroscopic properties are reported for two six-coordinate monooxo Mo(VI) complexes, MoO(Mecat)(PhTSCsal) and MoO(DTBcat)(PhTSCsal), where Mecat 2− =4-methylcatecholate, DTBcat 2− =3,5-di- tert -butylcatecholate and PhTSCsal 2− =the dianion of salicylaldehyde 4-phenylthiosemicarbazone. Each complex exhibits reversible, one-electron reduction of the MoO 4+ center at approximately −0.6 V versus Fc +/0 and irreversible, one-electron oxidations of the coordinated catecholate at approximately 0.8 and 1.1 V versus Fc +/0 in 1,2-dichloroethane. Two infrared absorptions are observed in the MoO stretching region for both complexes. These are thought to arise from geometric isomers produced by meridional coordination of tridentate PhTSCsal 2− and bidentate coordination of an unsymmetrical catechol to MoO 4+ .

The preparation, characterization and electrochemical properties are reported for monooxo Mo(VI) complexes, MoO(cat)(ssp), containing bidentate catecholate (cat 2 = 3,5-di-tert-butylcatecholate, naphthalene-1,2-diolate, phenanthrene-9,10-diolate) and tridentate NOS-donor Schiff base (ssp 2 = N -salicylidene-2-aminobenzenethiolate) ligands. The intensely colored compounds are formed by oxo abstraction from MoO 2 (ssp) with EtPh 2 P in THF followed by oxidative addition of the appropriate quinone. Oxo abstraction leads to a mixture of MoO(ssp) and Mo 2 O 3 (ssp) 2 rather than either of these single products under the experimental conditions. The six-coordinate MoO 4+ species exhibit reversible Mo(VI/V) electrochemistry at a potential ∼0.1 V more positive than that for analogous complexes with the NOO′-donor Schiff base ligand N -salicylidene-2-aminophenolate (sap 2− ).

Electron-transfer energetics of bridged dinuclear compounds of the form [(CO)(4)M(mu-L)](2)(0/1-/2-) (M = Mo, W; L = PPh(2)(-), SPh(-)) were explored using density functional theory coupled to a continuum solvation model. The experimentally observed redox potential inversion, a situation where the second of two electron transfers is more thermodynamically favorable than the first, was reproduced within this model. This nonclassical energy ordering is a prerequisite for the apparent transfer of two electrons at one potential, as observed in many biologically and technologically important systems. We pinpoint the origin of this phenomenon to be an unusually unfavorable electrostatic repulsion for the first electron transfer due to the redox noninnocent behavior of the bridging ligands. The extent of redox noninnocence is explained in terms of an orbital energy resonance between the metal-carbonyl and bridging ligand fragments, leading to a general mechanism by which potential inversion could be controlled in diamond-core dinuclear systems.

Summary Electrochemical reduction of the binuclear dioxo-bridged Mo(V)-cysteine complex, Mo 2 O 4 (cys) 2 2− , has been studied at mercury electrodes in pH 7.5–10 borate, phosphate, and NH 4 + /NH 3 buffers. The complex is reduced in a single diffusion-controlled 4-electron step to a binuclear Mo(III) product at ∼−1.2 V vs . SCE. The product is unstable and dissociates by cleavage of the oxygen bridge bonds. The rate of the decomposition reaction was measured by double potential step chronocoulometry and found to depend on the nature of the buffering medium, but not on pH or buffer concentration. A mechanism consistent with these experimental facts is proposed which involves coordination of the buffer species to the Mo(III) product during electrochemical reduction followed by intra-molecular dissociation of the complex.

Summary Electrochemical reduction of the binuclear, dioxo-bridged Mo(V) complex Mo 2 O 4 (EDTA) 2− has been studied at mercury electrodes in pH 7–10 borate, phosphate and NH 4 + /NH 3 buffers. The effect of pH, buffer composition and supporting electrolyte concentration on the reduction mechanism has been investigated. The complex is reduced by a concerted four-electron, four-proton transfer to a binuclear Mo(III) product, Mo 2 O 2 (H 2 O) 2 (EDTA) 2− . Protonated buffer species H 3 BO 3 . H 2 PO 4 − and NH 4 + are directly involved in the electrode reaction mechanism, which is postulated to be a pre-equilibrium transfer of two electrons followed by rate-limiting reaction between the proton donor and initial product and subsequent reduction to Mo(III). Interaction of unprotonated buffer species with the Mo(III) product facilitates the removal of protons upon reoxidation to Mo 2 O 4 (EDTA) 2− .

A number of molybdenum complexes involving the formal oxidation states Mo(IV) and Mo(V) have been studied by a self-consistent-field molecular orbital technique. All the complexes were of dodecahedral geometry and had eight sulfurs chelated to the central metal atom. In all, a series of five tetrakis complexes was studied, including the ligands dithiocarbamate (dtc), thioxanthate (txn), 1,1-dicyano-2,2-ethylenedithiolate (i-mnt), 1-cyano-1-carbethoxy-2,2-ethylenedithiolate (ced), and 1,1-dicarbethoxy-2,2-ethylenedithiolate (ded). The 4d orbitals were included on molybdenum, and the empty 3d levels on all sulfur atoms. The results show that the highest occupied molecular orbital in each case has over 90% metal d/sub xy/ character. Further, the energy of this orbital is linearly related to the reversible half-wave potentials for Mo(IV) ..-->.. Mo(V) and Mo(V) ..-->.. Mo(VI) oxidations of the complexes. A further irreversible oxidation observed experimentally also is closely related to the calculated energy levels. Relationships between the calculated results and Mo 3d/sub 5///sub 2/ X-ray photoelectron binding energies, EPR parameters, and charge-transfer absorption energies are discussed. Electrochemical and spectroscopic properties of these MoS/sub 8/ complexes can be understood in terms of a manifold of orbital energies that retain approximately constant spacings between one another and that move up or down in absolute energy in responsemore » to the charge donated or withdrawn by the ligands.« less

Summary Among the chelates formed by ethylenediaminetetraacetic acid (EDTA), those with Mo(VI) and Mo(V) are unusual because the metal-to-ligand ratio is two-to-one. Coulometry, chronopotentiometry and cyclic voltammetry have been used to investigate the electrochemistry of these compounds with the intention of establishing a correlation between the electrochemical results and the unique structural features of the chelates. In the presence of EDTA, the overall reduction of Mo(VI) involves three electrons. The initial electrode reaction, however, is the reduction of Mo(VI) to Mo(IV), which is followed by a disproportionation reaction to give Mo(V) and Mo(III). The two-to-one Mo(V)-EDTA chelate (a diamagnetic, dioxo-bridged species) is reduced through a series of parallel, but entirely independent steps. A third series of compounds arises from the Mo(III) species produced coulometrically from Mo(VI)-EDTA. The uniqueness of the various molybdenum-EDTA species can be seen from cyclic voltammetric experiments and the differences in the rate of Mo(IV) disproportionation. The rate of disproportionation of Mo(IV) is considerably slower for those compounds having oxygen bridges between molybdenum atoms.

Preparation des 2 complexes qui subissent une oxydation chimique reversible a 2 electrons. On observe des reactivites differentes suivant le solvant

The occurrence of molybdenum in at least six metalloenzymes having redox function (nitrogenase, nitrate reductase, xanthine oxidase, aldehyde oxidase, sulfite oxidase, and formate dehydrogenase)1 has created a need for greater understanding of the oxidation-reduction chemistry of this element. This realization prompted us to begin an investigation of electrochemical behavior of suitable inorganic i.e., model) molybdenum complexes with the expectation that the results would be useful in interpreting the redox behavior of this element in its various enzymes. A number of important questions can be addressed using contemporary electro chemical techniques such as cyclic voltammetry, potential step electrolysis, and controlled potential coulometry. These questions include: (1) whether electron transfer into the molybdenum center occurs by multiple steps involving a single electron or by a single multi-electron step; (2) the redox potentials of the electron transfer steps; (3) whether electron transfer leads to species with unusual stability or reactivity; and (4) the extent to which processes in (1)–(3) are influenced by molecular structure and solution environment. Trends have emerged with important implications for the redox properties of molybdoenzymes.