Influence of triphosphine ligand coordination geometry in Mn(<scp>i</scp>) 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 <i>trans</i>-position of the hydride (CO and L for <i>mer</i>- and <i>fac</i>-configurations, respectively) may influence their hydride transfer ability. Herein, low-temperature IR and NMR spectroscopy studies of two model Mn(I) complexes, <i>mer</i>-[(L1)(CO)₂MnH] and <i>fac</i>-[(L2)(CO)₂MnH], bearing similar triphosphine ligands (L1 = PhP(CH₂CH₂PPh₂)₂; L2 = MeC(CH₂PPh₂)₃) in the presence of B(C₆F₅)₃ 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 <i>fac</i>-[(L1)(CO)₂MnH]⋯B(C₆F₅)₃ with the facial geometry arising from the <i>mer</i>-to-<i>fac</i> isomerization of the initial <i>mer</i>-[(L1)(CO)₂MnH]. The higher reactivity of the <i>fac</i>-hydride derivatives was found to be consistent with the catalytic performance of the corresponding Mn(I) bromide complexes in the benchmark ester hydrosilylation.