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)2NMe, R = Et or Ph) were synthesized
by displacement of two CO ligands of CpMo­(CO)3H by the
PNP ligand; these complexes were characterized by IR and variable
temperature 1H and 31P NMR spectroscopy. CpMo­(CO)­(PNP)­H
complexes are formed as mixture of cis- and trans-isomers. The structures of both cis-CpMo­(CO)­(PEtNMePEt)H and trans-CpMo­(CO)­(PPhNMePPh)H were determined by single crystal X-ray diffraction. Electrochemical
oxidation of CpMo­(CO)­(PEtNMePEt)­H
and CpMo­(CO)­(PPhNMePPh)H in CH3CN are both irreversible at slow scan rates and quasireversible
at higher scan rates, with E1/2 = −0.36
V (vs Cp2Fe+/0) for CpMo­(CO)­(PEtNMePEt)H and E1/2 = −0.18
V for CpMo­(CO)­(PPhNMePPh)­H. Hydride
abstraction from CpMo­(CO)­(PNP)H with [Ph3C]+[A]− (A = B­(C6F5)4 or BArF4; [ArF = 3,5-bis­(trifluoromethyl)­phenyl])
afforded “tuck-in” [CpMo­(CO)­(κ3-PNP)]+ complexes that feature the amine bound to the metal. Displacement
of the κ3 Mo–N bond by CD3CN gives
[CpMo­(CO)­(PNP)­(CD3CN)]+. The kinetics
of this reaction were studied by 31P­{1H} NMR
spectroscopy for [CpMo­(CO)­(κ3-PEtNMePEt)]+, providing the activation parameters
ΔH⧧ = 21.6 ± 2.8 kcal/mol,
ΔS⧧ = −0.3 ±
9.8 cal/(mol K), Ea = 22.1 ± 2.8
kcal/mol. Protonation of CpMo­(CO)­(PEtNMePEt)H affords the Mo dihydride complex [CpMo­(CO)­(κ2-PEtNMePEt)­(H)2]+, which loses H2 to generate [CpMo­(CO)­(κ3-PEtNMePEt)]+ at
room temperature. Our results show that the pendant amine has a strong
driving force to form stable “tuck-in” [CpMo­(CO)­(κ3-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]+.