Activation, Deactivation and Reversibility Phenomena in Homogeneous Catalysis: A Showcase Based on the Chemistry of Rhodium/Phosphine Catalysts

Abstract

In the present work, the rich chemistry of rhodium/phosphine complexes, which are applied as homogeneous catalysts to promote a wide range of chemical transformations, has been used to showcase how the in situ generation of precatalysts, the conversion of precatalysts into the actually active species, as well as the reaction of the catalyst itself with other components in the reaction medium (substrates, solvents, additives) can lead to a number of deactivation phenomena and thus impact the efficiency of a catalytic process. Such phenomena may go unnoticed or may be overlooked, thus preventing the full understanding of the catalytic process which is a prerequisite for its optimization. Based on recent findings both from others and the authors’ laboratory concerning the chemistry of rhodium/diphosphine complexes, some guidelines are provided for the optimal generation of the catalytic active species from a suitable rhodium precursor and the diphosphine of interest; for the choice of the best solvent to prevent aggregation of coordinatively unsaturated metal fragments and sequestration of the active metal through too strong metal–solvent interactions; for preventing catalyst poisoning due to irreversible reaction with the product of the catalytic process or impurities present in the substrate.

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

“…+ (solv. = solvent), already detected for several solvents and diphosphines[79]. Such species can exist in equilibrium with many other intermediates,…”

“…+ (solv. = solvent), already detected for several solvents and diphosphines[79]. Such species can exist in equilibrium with many other intermediates, including dinuclear [Rh2(diphosphine)2(2-Cl)2][79], trinuclear [Rh3(DPPP)3(μ3-OMe)2] +[80], and mononuclear [Rh(PP)X][81,82].…”

Synthesis, Structure, Dynamics, and Enantioface-Selective η3-Benzyl Coordination in the Chiral Rhodium Complexes Rh(diphos*)(η3-CH2Ph)

“…The Cl-bridged dimers [Rh­(diphos*)­(Cl)] 2 ( 16 – 20 ) were prepared from diphos* and [Rh­(COD)­(Cl)] 2 (Scheme ; see Scheme above for the structures of the diphos* ligands). The Me-DuPhos and BINAP derivatives 16 and 20 were known, , Ph-BPE complex 19 was reported in a patent, and i -Pr-DuPhos and Me-FerroLANE complexes 17 and 18 are new. Complexes 17 – 19 were structurally characterized by X-ray crystallography (Figures –).…”

Catalyst Deactivation, Poisoning and Regeneration

“…Some regeneration strategies [11,16], together with solutions to prevent or limit deactivation phenomena [7,9,11,16], are also discussed. Eventually, one review paper [20] analyzes the rich chemistry of rhodium/phosphine complexes, which are applied as homogeneous catalysts to promote a wide range of chemical transformations, showing how the in situ generation of the active species, as well as the reaction of the catalyst itself with other components in the reaction medium, can lead to a number of deactivation phenomena.More in detail, the effect of the gas flow rate on the formation of hotspots during the Catalytic Partial Oxidation of logistic fuels on Rh-based monoliths for the on-board production of syngas is investigated in [6]. Solutions to prevent the irreversible thermal deactivation of Ni/ZrO 2 catalysts during the Dry Reforming of Methane are proposed in [7] by inhibiting the transition of ZrO 2 into its monoclinic phase via modification with La.…”

“…Some regeneration strategies [11,16], together with solutions to prevent or limit deactivation phenomena [7,9,11,16], are also discussed. Eventually, one review paper [20] analyzes the rich chemistry of rhodium/phosphine complexes, which are applied as homogeneous catalysts to promote a wide range of chemical transformations, showing how the in situ generation of the active species, as well as the reaction of the catalyst itself with other components in the reaction medium, can lead to a number of deactivation phenomena.…”