Extent of M2 δ to Ligand π-Conjugation in Neutral and Mixed Valence States of Bis(4-isonicotinate)-bis(2,4,6-triisopropylbenzoate) Dimetal Complexes (MM), Where M = Mo or W, and Their Adducts with Tris(pentafluorophenyl)boron

The reaction between W2(TiPB)4, where TiPB = 2,4,6-triisopropylbenzoate, and 2 equiv of 4-isonicotinic acid (nicH) yields the compound W2(TiPB)2(nic)2, 2, and TiPBH. Compound 2 is related to the previously reported molybdenum analog, Mo2(TiPB)2(nic)2, 1. Compounds 1 and 2 react with 2 equiv of B(C6F5)3 in THF to form the adducts M2(TiPB)2(nic-B(C6F5)3)2, 1B (M = Mo) and 2B (M = W), which have been crystallographically characterized as solvates M2(TiPB)2(nic-B(C6F5)3)2·2THF n-hexane. Compounds 1 and 2 are intensely colored due to M2 δ to π* MLCT transitions, and upon complexation with B(C5F5)3 to give 1B and 2B, these bands shift to lower energy and gain in intensity. Each compound shows two one-electron ligand-based reductions with a ΔE1/2 = 120 (1), 300 (1B), 440 (2), and 650 mV (2B). The larger ΔE1/2 values for the tungsten compounds reflect the greater orbital mixing of the metal 5d-based M2 δ and the nic π* LUMO. Reduction of solutions of 1B and 2B with (C5Me5)2Co leads to the anions 1B− and 2B−, which have been characterized spectroscopically by electron paramagnetic resonance (EPR) and UV−vis−NIR absorption. The EPR spectra of 1B− and 2B− are consistent with ligand-based (i.e., organic) radicals. The electronic spectra contain low-energy narrow charge resonance (IVCT) bands at 3800 (1B−) and 4500 cm−1 (2B−), consistent with fully delocalized mixed valence radical anions. The results are compared with electronic structure calculations and with the spectral features of the metal-centered delocalized mixed valence radical cations [(ButCO2)3M2]2-μ2-(O2C−CO2)+, to which they are remarkably similar, as well as with other organic-based mixed valence systems.