Molybdenum Hydride and Dihydride Complexes Bearing Diphosphine Ligands with a Pendant Amine: Formation of Complexes with Bound Amines

CpMo(CO)(PNP)H complexes (PNP = (R2PCH2)(2)NMe, R = Et or Ph) were synthesized by displacement of two CO ligands of CpMo(CO)(3)H by the PNP ligand; these complexes were characterized by IR and variable temperature H-1 and P-31 NIVIR spectroscopy. CpMo(CO)(PNP)H complexes are formed as mixture of cis- and trans-isomers. The structures of both cis-CpMo (CO)((PNPEt)-N-Et-P-Me)H and trans-CpMo(CO)((PNPPh)-N-Ph-P-Me) H were determined by single crystal X-ray diffraction. Electrochemical oxidation of CpMo(CO)((PNPEt)-N-Et-P-Me)H and CpMo(CO)-((PNPPh)-N-Ph-P-Me)H in CH3CN are both irreversible at slow scan rates and quasireversible at higher scan rates, with E-1/2 = -0.36 V (vs Cp2Fe+/0) for CpMo(CO)((PNPEt)-N-Et-P-Me)H and E-1/2 = -0.18 V for CpMo (CO) ((PNPPh)-N-Ph-P-Me)H. Hydride abstraction from CpMo-(CO)(PNP)H with [Ph3C](+)[A](-) (A = B(C6F5)(4) or BAr4F; [Ar-F = 3,5-bis(trifluoromethyl)phenyl]) afforded tuck-in [CpMo(CO)(kappa(3)-(PNPEt)-N-Et-P-Me)](+) complexes that feature the amine bound to the metal. Displacement of the kappa(3) Mo-N bond by CD3CN gives [CpMo(CO)(PNP)(CD3CN)](+). The kinetics of this reaction were studied by 31P{H-1} NMR spectroscopy for [CpMo(CO)(kappa(3)-(PNPEt)-N-Et-P-Me)](+), providing the activation parameters Delta H-double dagger = 21.6 +/- 2.8 kcal/mol, Delta S-double dagger = -0.3 +/- 9.8 cal/(mol K), E-a = 22.1 +/- 2.8 kcal/mol. Protonation of CpMo(CO)((PNPEt)-N-Et-P-Me)H affords the Mo dihydride complex [CpMo(CO)(kappa(2)-(PNPEt)-N-Et-P-Me)(H)(2)](+), which loses H-2 to generate [CpMo (CO)(kappa(3)-(PNPEt)-N-Et-P-Me)](+) at room temperature. Our results show that the pendant amine has a strong driving force to form stable tuck-in [CpMo(CO)(kappa(3)-PNP)](+) complexes, and also promotes hydrogen elimination from [CpMo(CO)(PNP)(H)(2)](+) complexes by formation of a Mo-N dative bond. CpMo(CO)(dppp)H (dppp = 1,3-bis(diphenylphosphino)propane) was studied as a Mo diphosphine analogue without a pendant amine, and the product of protonation of this complex gives [CpMo(CO)(dppp)(H)(2)](+).