A Catalytic Cycle Related to Molybdenum Enzymes Containing [Mo^VIO₂]²⁺ Oxidized Active Sites

Abstract

Interconversion of mononuclear cis-dioxo-Mo(VI) and oxo-Mo(V,IV) complexes of the hydrotris(3,5dimethylpyrazol-1-yl)borate ligand (L) by one-electron and two-electron reactions is described. In the coordinating solvent pyridine (py), LMo VI O 2 (SPh) is reduced by cobaltocene in one-electron steps to stable LMo IV O(SPh)-(py). The compound LMo IV O(SPh)(py)‚0.6MeOH crystallizes in orthorhombic space group Pbca, with a ) 13.790(2) Å, b ) 15.266(2) Å, c ) 27.807(5), V ) 5853(3) Å 3 , and Z ) 8. The complex exhibits a distorted octahedral structure with a facially tridentate ligand L and mutually cis oxo [ModO ) 1.667( 5) Å], pyridine [Mo-N ) 2.184( 5) Å], and thiolate [Mo-S ) 2.390(3) Å] ligands. This and other LMo IV O(SR)(py) (R ) Ph, CH 2 Ph, CHMe 2 ) complexes are also obtained from LMo VI O 2 (SR) via two-electron oxygen atom transfer reactions involving tertiary phosphines in pyridine. In dry solvents, the oxo-Mo(IV) complexes are oxidized by ferrocenium ion to the EPR-active cations [LMo V O(SR)(py)] + which are hydrolyzed rapidly in wet solvents to LMo V O(OH)-(SR). More generally, the complexes LMo VI O 2 X (X ) Cl, Br, NCS, OPh, SPh, SCH 2 Ph, SCHMe 2 ) react with PPh 3 at room temperature to yield OPPh 3 and unstable, coordinatively-unsaturated intermediates LMo IV OX. The latter are oxidized back to LMo VI O 2 X by Me 2 SO or can be trapped in a number of ways, depending on available ligands. For example, the complexes LMo IV OX(solvent) are detected in coordinating solvents, LMo V OClX in chlorinated solvents, LMo V O(OMe)X in MeOH, and [LMo V O] 2 (µ-O) in dry toluene. However, in wet weaklycoordinating solvents, LMo V O(OH)X complexes are produced cleanly and can be oxidized quantitatively to LMo VI O 2 X. Consequently, LMo VI O 2 X complexes are catalysts for the oxidation of PPh 3 by O 2 in the presence of H 2 O. Oxygen isotope tracing shows that H 2 O rather than O 2 is the source of the oxygen atom which is transferred to PPh 3 . This is the first model system which displays the full cycle proposed for oxidizing molybdoenzymes featuring [Mo VI O 2 ] 2+ resting states.

Oxygen Atom Transfer in Models for Molybdenum Enzymes: Isolation and Structural, Spectroscopic, and Computational Studies of Intermediates in Oxygen Atom Transfer from Molybdenum(VI) to Phosphorus(III)

“…[15] These complexes are components of a broad-based model for the combined (catalytic) OAT and coupled electron-proton transfer (CEPT) reactions envisaged for enzymes. [16] Phosphine oxidation in this system has been probed by a number of computational studies. [17] In the first step of the reaction between sterically unencumbered [MoO 2 (SH) 2 (NH 3 ) 2 ] and PMe 3 , nucleophilic attack of PMe 3 on a p* Mo=O orbital perpendicular to the MoO 2 unit and at an Mo À O···P angle of approximately 1308 takes place.…”

Molybdenum and Tungsten Oxidoreductase Models

“…This synthetic system was the first that ran through the full catalytic cycle, featuring the [Mo VI O 2 ] 2+ resting state and closely modelling sulfite oxidase catalysis. [25] The more elusive Mo IV /Mo VI O couple, again with the Tp* ligand, modelling the reduced and oxidised form of the DMSOR family active sites (excluding AO), was first employed as catalyst in both OAT and PCET reactions by Kirk, Basu and their co-workers. [26] 2 18 O was used in the final step proved water to be the source of oxygen in the catalytic process.…”

Novel O,O′‐Donor Oxo‐MoIV Hydrotris(3‐isopropylpyrazolyl)borate Complexes Formed by Chelation of Potentially Hydrogen‐Bonding Phenolate Ligands on Reduction of Dioxo‐MoVI Complexes

“…[5][6][7] The formation of Mo V species and the incorporation of water oxygen into substrates logically involves intermediate cis-oxo(aqua)-Mo IV or cis-oxo(hydroxo)-Mo(IV/V) complexes. [5][6][7] At the Mo V level, cis-dioxoMo V complexes have been isolated and thoroughly characterized but oxo(hydroxo)-Mo V species have only been observed in solution or in combination with their conjugate base.…”

“…[5][6][7] The formation of Mo V species and the incorporation of water oxygen into substrates logically involves intermediate cis-oxo(aqua)-Mo IV or cis-oxo(hydroxo)-Mo(IV/V) complexes. [5][6][7] At the Mo V level, cis-dioxoMo V complexes have been isolated and thoroughly characterized but oxo(hydroxo)-Mo V species have only been observed in solution or in combination with their conjugate base. [8] To date, cis-oxo(aqua)-Mo IV complexes have not been isolated from this system although a related cisoxo(aqua)-W IV complex, viz., [Tp*WO(OH 2 )(MeCCMe)]-(O 3 SCF 3 ) {Tp* = hydrotris(3,5-dimethylpyrazol-1-yl)-borate}, has been reported by Crane et al [9] Other known oxo(aqua)-Mo IV complexes adopt a trans geometry, e.g., [MoO(OH 2 )(CN) 4 ] 2-and [MoO(OH 2 )(dppe) 2 ] 2+ {dppe = 1,2-bis(diphenylphosphanyl)ethane}.…”