Fully Localized Mixed-Valence Oxidation Products of Molecules Containing Two Linked Dimolybdenum Units: An Effective Structural Criterion

Two previously reported compounds [Mo₂](CH₃O) ₂M(CH₃O)₂[Mo₂] (Cotton, F. A.; Liu, C. Y.; Murillo, C. A.; Wang, X. Inorg. Chem. 2003, 42, 4619), in which [Mo ₂] is an abbreviation for the quadruply bonded Mo₂ ⁴⁺ unit embraced by three (p-anisyl)NC(H)N(p-anisyl) anions and M = Zn (1) or Co (2), have been chemically oxidized. One-electron oxidation products [Mo₂](CH₃O)₂M(CH₃o) ₂[Mo₂](PF₆) (3, M = Zn; 4, M = Co) and the two-electron oxidation product [Mo₂](CH₃O) ₂Zn(CH₃O)(OH)[Mo₂](PF₆)₂ (5) have been isolated and structurally characterized. As expected, oxidations occur at the dimolybdenum units, The mono-charged cations in 3 and 4 have asymmetric molecular structures with two distinct [Mo₂] units. In each case, one of the [Mo₂] units has a lengthened Mo-Mo bond distance of 2.151[1] Å, as expected for one-electron oxidation, whereas the other remains unchanged at 2.115[1] Å. These correspond to bond orders of 3,5 (σ²π⁴δ¹) and 4.0 (σ²π⁴δ²) respectively. The crystallographic results thus show unambiguously that in the crystalline state, the mixed-valence compounds (3 and 4) are electronically localized and the unpaired electron is trapped on one [Mo₂] unit, These results are supported by the EPR spectra, The doubly oxidized compound 5 has two equivalent [Mo₂] units, both with a Mo-Mo bond distance of 2.149(1) Å. EPR and magnetic susceptibility measurements for 5 indicate that there is no significant ferromagnetic or antiferromagnetic spin coupling and the species is valence-trapped.