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 [Mo2] is an abbreviation for the quadruply bonded
Mo24+ 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.