The Influence of the Second and Outer Coordination
Spheres on Rh(diphosphine)<sub>2</sub> CO<sub>2</sub> Hydrogenation
Catalysts
- Publication date
- Publisher
Abstract
A series
of [Rh(PCH<sub>2</sub>X<sup>R</sup>CH<sub>2</sub>P)<sub>2</sub>]<sup>+</sup> complexes was prepared to investigate second
and outer coordination sphere effects on CO<sub>2</sub> hydrogenation
catalysis, where X is CH<sub>2</sub> (dppp) or X–R is N–CH<sub>3</sub>, N–CH<sub>2</sub>COOH (glycine), N–CH<sub>2</sub>COOCH<sub>3</sub> (Gly-OMe), or N–CH<sub>2</sub>C(O)N–CH(CH<sub>3</sub>)COOCH<sub>3</sub> (GlyAla-OMe). All of these complexes were
active for CO<sub>2</sub> reduction to formate, with the N–CH<sub>3</sub> derivative offering an 8-fold enhancement over the dppp derivative,
which is consistent with increased electron density around the metal.
Despite the increase in rate with the addition of the pendant nitrogen,
the addition of electron withdrawing amino acids and dipeptides to
the amine resulted in complexes with reductions in rate of 1 to 2
orders of magnitude, most consistent with a change in p<i>K</i><sub>a</sub> of the pendant amine, resulting in lower activity. Collectively,
the data suggest multiple contributions of the pendant amine in this
catalytic system