Influence of triphosphine ligand coordination geometry in Mn(I) hydride complexes [(PPP)(CO)MnH] on their kinetic hydricity.

Octahedral Mn(I) complexes bearing tridentate donor ligands [(LL'L'')(CO)MnX] have recently emerged as major players in catalytic (de)hydrogenation processes. While most of these systems are still based on structurally rigid pincer scaffolds imposing a meridional coordination mode, for some more flexible tridentate ligands a facial arrangement of donor moieties becomes possible. Accordingly, the geometry of the corresponding Mn(I) hydrides [(LL'L'')(CO)MnH] directly involved in the catalytic processes, namely the nature of the donor extremity located in the -position of the hydride (CO and L for - and -configurations, respectively) may influence their hydride transfer ability. Herein, low-temperature IR and NMR spectroscopy studies of two model Mn(I) complexes, -[(L1)(CO)MnH] and -[(L2)(CO)MnH], bearing similar triphosphine ligands (L1 = PhP(CHCHPPh); L2 = MeC(CHPPh)) in the presence of B(CF) as the H abstractor revealed for the first time a higher kinetic hydricity of the tripodal system. Even for the pincer complex, hydride transfer proceeds from the non-covalent adduct -[(L1)(CO)MnH]⋯B(CF) with the facial geometry arising from the -to- isomerization of the initial -[(L1)(CO)MnH]. The higher reactivity of the -hydride derivatives was found to be consistent with the catalytic performance of the corresponding Mn(I) bromide complexes in the benchmark ester hydrosilylation.