Synthesis, structure, and electrochemical properties of sterically protected molybdenum trihydride redox pairs: A paramagnetic stretched dihydrogen complex?

Complexes [MoCp'(PMe3)(2)H-3] (Cp'=1,2,4-C(5)H(2)tB(u)3, 2a; C(5)HiPr(4), 2b) have been synthesized from the corresponding compounds [(MoCpCl4)-Cl-#] (1a, 1b) and fully characterized, including by X-ray crystallography and by a neutron diffraction study for 2a. Protonation of 2a led to complex [Mo(1,2,4-C(5)H(2)tBu(3))(PMe3)(2-)(H-4](+) (3a) in THF and to [Mo(1,2,4-C(5)H(2)tBu(3))(PMe3)(2)(MeCN)H-2](+) (4a) in MeCN. Complex 4b analogously derives from protonation of 2b in MeCN, whereas the tetrahydride complex 3b is unstable. One-electron oxidation of 2a and 2b by [FeCp2]PF6 produces the EPR-active 17-electron complexes 2a(+) and 2b(+). The former is thermally more stable than the latter and could be crystallographically characterized as the PF6- salt by X-ray diffraction, providing evidence for the presence of a stretched dihydrogen ligand (H center dot center dot center dot H = 1.36(6)A). Controlled thermal decomposition of 2a(+) yielded the product of H-2 elimination, the 15-electron monohydride complex [Mo(1,2,4-C5H2,tBu(3))-(PMe3)(2)H]PF6 (5a), which was characterized by X-ray crystallography and by EPR spectroscopy at liquid He temperature. The compound establishes an equilibrium with the solvent adduct in THE An electrochemical study by cyclic voltammetry provides further evidence for a rapid H, elimination process from the 17-electron complexes. In contrast to the previously investigated [MoCp*(dppe)H-3](+) system (dppe = 1,2-bis(diphenylphosphino)-ethane; Cp*=pentamethylcyclopentadienyl), the decomposition of 2a(+) by H-2 substitution with a solvent molecule appears to follow a dissociative pathway in MeCN.