Cp* non-innocence and the implications of (η-Cp*H)Rh intermediates in the hydrogenation of CO, NAD, amino-borane, and the Cp* framework - a computational study

In hydrogenation mediated by half-sandwich complexes of Rh, Cp*Rh(iii)-H intermediates are critical hydride-delivery agents. For bipyridine-supported complexes, a unique transformation named ‘Cp* non-innocence’ leads to the appearance of (Cp*H)Rh(i) intermediates, which are purported to exhibit enhanced hydride-delivery capabilities. In this work, DFT calculations performed to compare the role of these complexes in hydrogenation reveal that (Cp*H)Rh(i) intermediates do not compete with the conventional pathway (involving Cp*Rh(iii)-H); instead they can lead to sequential hydrogenation of the Cp* framework, and potentially, catalyst degradation. Thus, caution is warranted when invoking the truly homogeneous nature of hydrogenation catalysis mediated by this popular class of complexes.