Linear Free Energy Relationships in Dinuclear Compounds. 2. Inductive Redox Tuning via Remote Substituents in Quadruply Bonded Dimolybdenum Compounds

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Abstract
Syntheses and characterizations are reported for dimolybdenum(II) compounds supported by the diarylformamidinate (ArNC(H)NAr-) ligand, where Ar is XC6H4-, 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 Mo25+/Mo24+ couple were found to correlate with the Hammett constant (σX) of the aryl substituents according to the following equation: ΔE1/2 = E1/2(X) − E1/2(H) = 87(8σX) mV. Molecular structure determinations of compounds 1, 2, 5, and 10 revealed an invariant core geometry around the Mo2 center, with statistically identical Mo−Mo quadruple bond lengths of 2.0964(5), 2.0949[8], 2.0958(6), and 2.0965(5) Å, respectively. Magnetic anisotropies for compounds 1−10 estimated on the basis of 1H NMR data were similar and unrelated to σX. Similarity in UV−vis spectra was also found within the series, which, in conjunction with the features of both molecular structures and 1H 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.

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