Influence of triphosphine ligand coordination geometry in Mn(I) hydride complexes [(P∩P∩P)(CO)2MnH] on their kinetic hydricity

Octahedral Mn(I) complexes bearing tridentate donor ligands [(L∩L’∩L’’)(CO)2MnX] 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 [(L∩L’∩L’’)(CO)2MnH] directly involved in the catalytic processes, namely the nature of the donor extremity located in the trans-position of the hydride (CO and L for mer- and fac-configuration, respectively) may influence their hydride transfer ability. Herein, low-temperature IR and NMR spectroscopy studies of two model Mn(I) complexes mer-[(L1)(CO)2MnH] and fac-[(L2)(CO)2MnH] bearing similar triphosphine ligands (L1 = PhP(CH2CH2PPh2)2; L2 = MeC(CH2PPh2)3) in the presence of B(C6F5)3 as 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 fac-[(L1)(CO)2MnH]∙∙∙B(C6F5)3 with facial geometry arising from the mer-to-fac isomerization of the initial mer-[(L1)(CO)2MnH]. The higher reactivity of the fac-hydride derivatives was found to be consistent with catalytic performance of the corresponding Mn(I) bromide complexes in the benchmark ester hydrosilylation.