The Influence of the Second and Outer Coordination Spheres on Rh(diphosphine)₂ CO₂ Hydrogenation Catalysts

Abstract

A series of [Rh­(PCH2XRCH2P)2]+ complexes was prepared to investigate second and outer coordination sphere effects on CO2 hydrogenation catalysis, where X is CH2 (dppp) or X–R is N–CH3, N–CH2COOH (glycine), N–CH2COOCH3 (Gly-OMe), or N–CH2C­(O)­N–CH­(CH3)­COOCH3 (GlyAla-OMe). All of these complexes were active for CO2 reduction to formate, with the N–CH3 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 pK a of the pendant amine, resulting in lower activity. Collectively, the data suggest multiple contributions of the pendant amine in this catalytic system.

Polymer coordination promotes selective CO2reduction by cobalt phthalocyanine

“…In molecular systems, the importance of the hydrogen bonding in the secondary coordination sphere to stabilize reactive intermediates has been demonstrated by several groups 79–85. In particular, second sphere proton relays incorporated into a ligand scaffold can lead to high electrocatalytic activity in reactions that require the delivery of multiple protons 86–89. The P4VP membrane imbues many of these secondary coordination sphere effects to CoPc without the need for synthetically challenging ligand modifications.…”

Transfer Hydrogenation from 2-propanol to Acetophenone Catalyzed by [RuCl2(η6-arene)P] (P = monophosphine) and [Rh(PP)2]X (PP = diphosphine, X = Cl−, BF4−) Complexes

“…Very recently Mannu et al [71] observed that changing the anion Cl − with BF 4 − , the outcome of the reduction of acetophenone in the presence of 2-propanol was different ( complexes is the role of the anion. In the solid state such complexes are usually described as monomeric and cationic, and single-crystal x-ray crystallography studies confirm a distorted squareplanar tetra-coordinated structure, with no contact between cation and anion [75][76][77]. In contrast, this behavior is not so clear in solution, at least for some of these complexes.…”

Incorporation of Pendant Bases into Rh(diphosphine)2 Complexes: Synthesis, Thermodynamic Studies, And Catalytic CO2 Hydrogenation Activity of [Rh(P2N2)2]+ Complexes

“…12, 13 To gain a better understanding of how the series of pendant amine containing disphosphine ligands affect catalytic activity, we measured the hydricities of several of the complexes, estimated their pK a 's, measured their relative affinities for H 2 addition, and measured their rates for CO 2 hydrogenation.…”

“…12 Recently, pendant bases have been incorporated into this type of catalyst through PNP diphosphine ligands, which contain amines in the second coordination sphere and mono-and dipeptides in the outer coordination sphere, and it was found that the addition of an amine in the second coordination sphere increased catalytic rates for CO 2 hydrogenation by increasing the electron density of the metal center, but electron-withdrawing substituents in the outer coordination sphere decreased activity. 13 However, no true Rh analogue to the well-studied Ni(P 2 N 2 ) 2 system has been presented in the literature.…”