Joel I. Dulebohn

The preparation of dimolybdenum(V,V) complexes from molybdenum quadruply bonded metal-metal dimers

Abstract Oxidation of quadruply bonded metal-metal dimers in the presence of good π-accepting ligands results in the formation of MoVMoV compounds of the type [MO2(μ-X)2(Y)(Y′)]2+ (X = O or S; Y,Y′ = O,O; S,S; O,S). Reaction of MO2(O2CCH3)4 with oxygen in the presence of Na2mnt (mnt = 1,2-dicyanoethylene-2,2-dithiolate) gives [MO2(μ-S)2(O)(S)(mnt)2]2− (1). The compound crystallizes in the monoclinic space group P21/c, with cell dimensions a = 19.547(4), b = 15.210(4), c = 18.754(6) A, β = 101.69(2)°, V= 5460(2) A3, and Z = 4. Similarly, oxidation of o-dichlorobenzene solutions of Mo2Cl4(CH3CN)4 and 4,4′-dimethyl-2,2′-dipyridyl (dmpby) or, more directly, the reaction of Mo2Cl4(dmbpy)2 with oxygen leads to the formation of a red solid, which was characterized by X-ray crystallography to be Mo2(μ-O)2(O)2(Cl)2(dmbpy)2 (2). Red diamond crystals, prepared by slow evaporation of CH3CN solutions of 2, are trigonal and in the space group P3121 with cell dimensions a = 16.135(4), b = 16.135(4), c = 10.709(3) A, V = 2414.4(13) A3 and Z = 3. In both structures, the geometry about each of the molybdenum atoms is a distorted square pyramid with terminal oxygen or sulphur atoms at the apices and in a syn conformation. The molybdenum-molybdenum bond distances of 2.858(1) A and 2.562(2) A in structures of 1 and 2, respectively, are typical of other MoVMoV dimers and indicative of a single MoMo bond.

Photoredox Chemistry of Two-Electron Mixed Valence Systems

The chemistry of two-electron mixed valence dimers Mn-1—Mn+1 provides a unique foundation for the design of multielectron photochemical activation schemes. We have recently prepared a homologous Rh2 fluorophosphine series that is based on a Rh0RhII two-electron mixed valence complex, which features a trigonal bipyramidal coordination geometry about a Rh0 center, an octahedral coordination about a RhII center and a Rh···Rh separation characteristic of a single metal-metal bond. This novel compound exhibits long-lived luminescence, and steady state and time-resolved spectroscopic studies reveal a metal localized excited state of dσ* character. The reactivity of the Rh0RhII compound provides a benchmark for two-electron mixed valence bimetallic cores in that two-electron oxidations are promoted at the Rh0 center and two-electron reductions are effected at the RhII center. Within this framework, the strategy for the development of parallel reactivity of electronically excited binuclear complexes can be elaborated. Transient absorption spectroscopy of the quadruply bonded M2X4(PBu3)4 complexes (M = Mo(II), W(II); X = Cl) provides strong evidence for the generation of a two-electron mixed valence MIMIII excited state complex upon δ2 → δδ* excitation. The smooth reaction of electronically excited W2Cl4(dppm)4 with methyl iodide to a W2Cl4(dppm)4(CH3)(I) photoproduct with direct addition of the organic substrate to the metal-metal core is consistent with the reactivity pattern established by the mixed valence Rh0RhII dimer.