Katsumoto Yamanouchi

Monomeric molybdenum(V) complexes. 4. The structure of tetraphenylarsonium oxodichloro(N-2-oxophenylsalicylideniminato)molybdate(V), [Ph4As] [MoOCl2(SalphO)]

The structure of [Ph 4 As] [MoOCl 2 (SalphO)], where SalphO is N-2-oxophenylsalicylideniminate dianion, has been determined by X-ray crystallography. The complex crystallizes in the monoclinic space group P 2 1 /n with a = 11.829(2), b = 16.149(3), c = 17.410(3) A, β = 97.485(15)° and Z = 4. The calculated and observed densities and 1.566 and 1.573(10) g cm −3 , respectively. Block-diagonal least-squares refinement of the structure using 4722 independent reflections with I ⩾ 3σ(I) converged at R = 0.0345 and R w = 0.0484. The crystal contains [Ph 4 As] + cations and [MoOCl 2 (SalphO)] − anions. The Mo atom in the anion is in a distorted octahedral coordination environment. A planar terdentate Schiff base ligand occupies meridional positions with the N atom trans to the terminal oxo group (O t ). Two Cl atoms are cis to the O t atom. The Mo atom is displaced by 0.33 A from the equatorial plane toward the O t atom. The MoO t distance is 1.673(3) A. The MoN bond trans to the O t atom is 2.298(4) A. The two MoCl bond lengths are 2.371(1) and 2.408(1) A. The difference of 0.037 A is significant (30 σ). Preparations of the title complex and the related complexes are also described.

Preparation and structure of a compound containing a triply bridged binuclear Mo(V) cation and the [MoOCl4(H2O)]− anion

Abstract The reaction of di-μ-oxo-bis[oxo-N,N-diethyldithiocarbamatomolybdenum(V)], Mo2O4(S2CNEt2)2, with thiophenol produces a red-orange diamagnetic compound which contains thiophenol and diethyldithiocarbamate in a 1:1 ratio. The attempted recrystallisation of this material from CHCl3/CCl2 in the presence of excess thiophenol resulted in an unexpected chemical reaction and the formation of green crystals shown to be [Mo2O2 —(SPh) 2Cl(S2CNEt2)2]+ [MoOCl2(H2O)]-CHCl3 by a single-crystal X-ray structure determination and by supporting physical data. The μ-chloro-di-μ-thiophenolatobis [oxo-N,N-diethyldithiocarbamatomolybdenum(V)] cation contains two [MoO]3+ units which are triply bridged by the S atoms from the two SPh ligands and by the Cl atom. The bridging Cl atom is approximately trans to the terminal O atoms of each Mo. A diethyldithiocarbamato ligand completes the distorted six-coordinate geometry about each Mo. The effective symmetry of the cation is nearly C2v and the Mo…Mo distance is 2.822(2) A. The monomeric oxo-tetrachloroaquomolybdate(V) anion, [MoOCl4(H2O)]-, has approximate C4v symmetry with Mo−O t = 1.641(7) A , Mo−Cl = 2.344(2) − 2.396(4) A and Mo−OH 2 = 2.342(8) A . The compound has a magnetic moment consistent with the presence of one unpaired electron per formula unit.

Monomeric Mo(V) and Mo(VI) complexes with sterically constrained metal centers

Abstract Bray has proposed a chemical mechanism for the reduction of xanthine oxidase by xanthine, which involves a monomeric molybdenum active center having fac stereochemistry [1]. This proposal has simulated our interest in preparation and characterization of monomeric Mo(V) and Mo(VI) complexes constrained fac configuration by polydentate ligands such as hydrotris(3,5-dimethylpyrazolyl) borate hereafter designated as HB(Me 2 pz) − 3 . The HB(Me 2 pz) − 3 ligand has been extensively used to stabilize a variety of low valent molybdenum compounds [2]. Moreover, the same ligand has been found to stabilize the Mo(V) center in MoOCl 2 {HB(Me 2 pz) 3 }( I ) [3]. The relative stability of these compounds is attributed partly to the steric bulk of 3-methyl group on the ligand. Mo(V) complexes of the type MoOXY{HB(Me 2 pz) 3 } (where X = Y = NCS; X = Cl, Y = OR or SPh; X = Y = SPh) have been prepared by the substitution reactions of I and spectroscopically characterized. Esr spectra of Mo(V) centers are sensitive to X and Y. Substitutions by thiolate ligands give smaller A o (Mo) and larger g o values. These substitutions also shift the MoO stretching vibration significantly to lower wave numbers. A preliminary kinetic study has revealed that the rates of ligand substitution are very slow in these complexes, compared to those of known MoOCl 3 L 2 complexes (where L is a monodentate ligand) [4]. Mo(VI) complexes of the type MoO 2 X{HB(Me 2 pz)3} (X = Cl, Br, NCS) have been synthesized for the first time by the reaction of MoO 2 X 2 (X = Cl, Br) or MoO 2 (NCS) 2− 4 with the ligand, and characterized by spectroscopic methods including 95 Mo NMR. Electrochemical studies and structural studies on these Mo(V) and Mo(VI) complexes will also be described.

Structural Studies of Molybdenum Complexes

Molybdenum-containing enzymes are vitally important in nitrogen uptake, nitrogen metabolism and several biochemical reactions.1,2 The detailed coordination environment about the molybdenum atoms of such enzymes is still unknown because no molybdoenzyme has had its structure determined by x-ray crystallography. However, the molybdenum centers of several enzymes have been probed directly by electron paramagnetic resonance (EPR) spectroscopy1 and by x-ray absorption spectroscopy (EXAFS).3–5 Comparison of the spectral data from enzymes with spectral data from coordination compounds suggests that sulfur atoms are coordinated to molybdenum in the enzymes. EXAFS data from xanthine oxidase4 and sulfite oxidase5 in their oxidized forms also indicate the presence of terminal oxo groups attached to the molybdenum. In order to effectively interpret the spectral results from enzymes, it is important to have available a series of compounds whose stoichiometry and stereochemistry have been definitively established by x-ray structure determination. Of particular interest are compounds of sulfur donor ligands and oxomolybdenum(V) species.