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.
A pulsed gradient spin echo experiment on [Rh2(1,3-bis-(diphenylphosphino)-propane)2(μ2-Cl)2] complex has been conducted in order to shed light on the supposed monomerisation process of [Rh2(diphosphine)2(μ2-Cl)2] complexes in solution. Such a process should generate a 14-electron [Rh(diphosphine)Cl] complex, which has only been postulated to date. Metathesis experiments on [Rh2(1,3-bis-(diphenylphosphino)-propane)2(μ2-Cl)2] and [Rh2(bis[2-(diphenylphosphino)phenyl]ether)2(μ2-Cl)2] complexes, analysed by 31P NMR, reveal that monomerisation of [Rh2(diphosphine)2(μ2-Cl)2] complexes is not restricted to the case of 1,3-bis-(diphenylphosphino)propane.
Rhodium(iii) thiophosphinito pincer hydrido complexes were synthesised by C-H activation under exceptionally mild conditions at room temperature without additional base or irradiation and fully characterised by multinuclear NMR spectroscopy and X-ray crystallography. C-H activation under these mild conditions contrasts with the reactivity of related systems with POCOP ligands.
The complexes {bis[(2‐diphenylphosphanyl)phenyl] ether‐κ2P,P′}(η4‐norbornadiene)rhodium(I) tetrafluoridoborate, [Rh(C7H8)(C36H28OP2)]BF4, and {bis[(2‐diphenylphosphanyl)phenyl] ether‐κ2P,P′}[η4‐(Z,Z)‐cycloocta‐1,5‐diene]rhodium(I) tetrafluoridoborate dichloromethane monosolvate, [Rh(C8H12)(C36H28OP2)]BF4·CH2Cl2, are applied as precatalysts in redox‐neutral atomic‐economic propargylic CH activation [Lumbroso et al. (2013). Angew. Chem. Int. Ed.52, 1890–1932]. In addition, the catalytically inactive pentacoordinated 18‐electron complex {bis[(2‐diphenylphosphanyl)phenyl] ether‐κ2P,P′}chlorido(η4‐norbornadiene)rhodium(I), [RhCl(C7H8)(C36H28OP2)], was synthesized, which can form in the presence of chloride in the reaction system.
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