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

Complexes [MoCp#(PMe3)2H3] (Cp#=1,2,4-C5H2tBu3, 2 a; C5HiPr4, 2 b) have been synthesized from the corresponding compounds [MoCp#Cl4] (1 a, 1 b) and fully characterized, including by X-ray crystallography and by a neutron diffraction study for 2 a. Protonation of 2 a led to complex [Mo(1,2,4-C5H2tBu3)(PMe3)2H4]+ (3 a) in THF and to [Mo(1,2,4-C5H2tBu3)(PMe3)2(MeCN)H2]+ (4 a) in MeCN. Complex 4 b analogously derives from protonation of 2 b in MeCN, whereas the tetrahydride complex 3 b is unstable. One-electron oxidation of 2 a and 2 b by [FeCp2]PF6 produces the EPR-active 17-electron complexes 2 a+ and 2 b+. 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 (HH=1.36(6) Å). Controlled thermal decomposition of 2 a+ yielded the product of H2 elimination, the 15-electron monohydride complex [Mo(1,2,4-C5H2tBu3)(PMe3)2H]PF6 (5 a), 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 THF. An electrochemical study by cyclic voltammetry provides further evidence for a rapid H2 elimination process from the 17-electron complexes. In contrast to the previously investigated [MoCp*(dppe)H3]+ system (dppe=1,2-bis(diphenylphosphino)ethane; Cp*=pentamethylcyclopentadienyl), the decomposition of 2 a+ by H2 substitution with a solvent molecule appears to follow a dissociative pathway in MeCN.