Monoanionic Molybdenum and Tungsten Tris(dithiolene) Complexes: A Multifrequency EPR Study

Numerous Mo and W tris(dithiolene) complexes in varying redox states have been prepared and representative examples characterized crystallographically: [M(S2C2R2)3]z [M = Mo, R = Ph, z = 0 (1) or 1– (2); M = W, R = Ph, z = 0 (4) or 1– (5); R = CN, z = 2–, M = Mo (3) or W (6)]. Changes in dithiolene C–S and C–C bond lengths for 1 versus 2 and 4 versus 5 are indicative of ligand reduction. Trigonal twist angles (Θ) and dithiolene fold angles (α) increase and decrease, respectively, for 2 versus 1, 5 versus 4. Cyclic voltammetry reveals generally two reversible couples corresponding to 0/1– and 1–/2– reductions. The electronic structures of monoanionic molybdenum tris(dithiolene) complexes have been analyzed by multifrequency (S-, X-, Q-band) EPR spectroscopy. Spin-Hamiltonian parameters afforded by spectral simulation for each complex demonstrate the existence of two distinctive electronic structure types. The first is [MoIV(AL35-•)]1- (AL = olefinic dithiolene, type A), which has the unpaired electron restricted to the tris(dithiolene) unit and is characterized by isotropic g-values and small molybdenum superhyperfine coupling. The second is formulated as [MoV(BL36-)]1- (BL = aromatic dithiolene, type B) with spectra distinguished by a prominent g-anisotropy and hyperfine coupling consistent with the (dz2)1 paramagnet. The electronic structure disparity is also manifested in their electronic absorption spectra. The compound [W(bdt)3]1- exhibits spin-Hamiltonian parameters similar to those of [Mo(bdt)3]1- and thus is formulated as [WV(BL36-)]1-. The EPR spectra of [W(AL3)]1- display spin-Hamiltonian parameters that suggest their electronic structure is best represented by two resonance forms {[WIV(AL35-•)]1- ↔ [WV(AL36-)]1-}. The contrast with the corresponding [MoIV(AL35-•)]1- complexes highlights tungsten’s preference for higher oxidation states.