Nitro-substituted benzoates of dimolybdenum: The Mo-2(4+) delta to ligand charge transfer band

The preparation and characterization of a series of Mo-2(O2CAr')(4) compounds, where Ar' is a nitrosubstituted phenyl group, are described. The introduction of the NO2 group in the 4- (para) position causes a significant red-shift of a metal-to-ligand charge transfer (MLCT) band such that these compounds are intensely purple in solution: lambda(max) ca. 540 nm with epsilon -15 000 M(-1) cm(-1). The cinnamate derivative (O2CCH=CHPh-4-NO2) shows a further red-shift of this MLCT band while the 3-NO2 (meta) derivatives are only slightly red-shifted compared to the parent benzoate Mo-2(O(2)CPh)(4). The cyclic voltammograms for Mo-2(O(2)CR)(4), where R = Me, (t)Bu, n-octyl, Ph, C6H4-4-NO2, and C6H4-3-NO2 reveal how substituents on R influence the ease of oxidation from the delta orbital. The structure of Mo-2(O2CC6H4-3-NO2)(4) was determined by a single crystal X-ray diffraction study as its pyridine adduct with additional pyridine in the lattice. Crystal data for Mo-2(O2CC6H4-3-NO2)(4)(py)(2) . 2py at -174 degrees C: a = 10.982(2) Angstrom, b = 20.457(4) Angstrom, c = 10.615(2) Angstrom, alpha = 93.82(1)degrees, beta = 90.49(1)degrees, gamma = 98.31(1)degrees, Z = 2, d(calc) = 1.65 g cm(-3) and space group P1. The nature of the MLCT band is discussed in the light of Fenske-Hall MO calculations on Mo-2(O2C6H5)(4), Mo-2(O2C6H4-4-NO2)(4), Mo2(O2CC6H4-3-NO2)(4), aad Mo-2(O2C6H3-2Cl-4-NO2)(4).