Experimental and computational studies of the stability and reactivity of a half-sandwich 16-electron spin triplet MoII complex containing a terminal hydroxide ligand

Abstract

Compound CpMo(OH)(PMe₃)₂, 1, a stable monomeric 16-electron organometallic complex with a spin triplet ground state and a terminal hydroxide ligand, is obtained from the reaction of CpMoCl(PMe₃)₃ with KOH. Density functional (B3LYP) geometry optimizations for a CpMo(OH)(PH₃)₂ model system afford a spin triplet ground state with a bent endo hydroxide ligand. An excited singlet state with a bent exo OH ligand is found 2.27 kcal mol^-1 higher in energy. Analogous calculations on other related CpMoX(PH₃)₂ systems provide a spin triplet ground state for X=Cl and H, whereas the compound with X=PH₂ has a singlet ground state. Addition of PH₃ leads to a calculated bond energy of 20.9, 14.3, 7.2 and 6.8 kcal mol^-1 relative to the 16-electron singlet state for H, Cl, PH₂ and OH, respectively. These values are consistent with qualitative expectations based on the strength of the Mo–X π interaction and steric strain in the CpMo(PH₃)₃ and CpMo(PH₃)₂ fragments. Crude estimates of the thermodynamic strength of the Mo–X π interactions have been derived. The calculations also show that an α-H elimination from the hydroxide ligand to afford CpMoH(O)(PH₃) plus PH₃ should be thermodynamically favored but suggest that it is kinetically difficult. Compound 1 slowly decomposes in C₆D₆ in the presence of PMe₃ by a process that is initiated by the activation of a solvent C–D bond. This process produces a mixture of CpMoH(PMe₃)₃, 4, CpMo(η²-CH₂PMe₂)(PMe₃)₂, 5, C₆D₅OH and PMe₃-dⁿ (n=1–9) molecules. The relative isotopic distribution of the free PMe₃-dⁿ molecules can be satisfactorily simulated. The analogous decomposition of CpMo(OD)(PMe₃)₂ (1-OD) revealed a rapid exchange of the deuteroxide D atom and a PMe₃ ligand H atom. The decomposition of 1 in the presence of PMe₃ in C₆D₆ is catalyzed by water or methanol. The molecules are believed to engage in hydrogen bonding interactions with the OH lone pairs, thereby modifying the electronic structure of compound 1 and enhancing the reactivity toward the oxidative addition of the solvent C–D bonds.