Linear free energy relationships in dinuclear compound .2. Inductive redox tuning via remote substituents in quadruply bonded dimolybdenum compounds

Syntheses and characterizations reported for dimolybdenum(II) compounds supported by the diarylformamidinate (ArNC(H)NAr-) ligand, where Ar is XC(6)H(4)(-), with X as p-OMe (1), H (2), m-OMe (3), p-Cl (4), m-Cl (5), m-CF3 (6), p-COMe (7), p-CF3 (8), or Ar is 3,4-Cl2C6H3- (9) or 3,5-Cl2C6H3- (10). The (quasi)reversible oxidation potentials measured for the Mo-2(5+)/Mo-2(4+) couple were found to correlate with the Hammett constant (sigma(X)) of the aryl substituents according to the following equation: Delta E(1/2) = E(1/2)(X) - E(1/2)(H) = 87(8 sigma(X)) mV. Molecular structure determinations of compounds 1,2,5, and 10 revealed an invariant core geometry around the Mo-2 center, with statistically identical Mo-Mo quadruple bond lengths of 2.0964(5), 2.0949[8], 2.0958(6), and 2.0965(5) Angstrom, respectively. Magnetic anisotropies for compounds 1-10 estimated on the basis of H-1 NMR data were similar and unrelated to sigma(X). Similarity in UV-vis spectra was also found within the series, which, in conjunction with the features of both molecular structures and H-1 NMR spectra, was interpreted as the existence of a constant upper valence structure across the series. Results of Fenske-Hall calculations performed for several model compounds paralleled the experimental observations.