Quantitative UV/Vis Spectroscopic Investigations of the In Situ Synthesis of Neutral μ₂‐Chloro‐Bridged Dinuclear (Diphosphine)rhodium Complexes

Abstract

The stepwise in situ formation of neutral μ 2 -chloro-bridged dinuclear (diphosphine)rhodium complexes of the type [{Rh(PP)(μ 2 -Cl)} 2 ] (PP = chelating diphosphine) following the reaction of [{Rh(COD)(μ 2 -Cl)} 2 ] (COD = cyclooctadiene) with the ligands BINAP, SegPhos, DM-SegPhos and DiFluorPhos was investigated quantitatively by stopped-flow UV/Vis spectroscopy at 25.0°C. The second-order rate constants of each step of the consecutive reactions were determined. The investigations were hindered by an unexpected equilibrium [a]

Oxidative Addition of Aryl Halides to Cationic Bis(phosphane)rhodium Complexes: Application in C–C Bond Formation

“…In the case of chlorobenzene, immediately after mixing the reactants, one doublet centred at δ = 73.0 ppm ( J P,Rh = 205.9 Hz), corresponding to the (η 6 ‐arene)rhodium complex [Rh(DIPAMP)(η 6 ‐chlorobenzene)]BF 4 ( 2a ), was detected, beside the signals of the starting complex 1 (Figure ). Such large coupling constants are found for arene complexes as well as for other (phosphane)Rh I complexes with O, or halido ligands, but unambiguous proof is provided by the 31 P‐ 103 Rh{ 1 H} NMR spectrum (Figure S1), which shows a typical 103 Rh chemical shift of δ = –871 ppm, diagnostic of (η 6 ‐arene)rhodium complexes …”

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

“…Although the course of a catalytic reaction is often solvent-specific, rarely has this correlation been attributed to the formation of different catalytic species [51][52][53]. For example, when [Rh(COD)(µ 2 -Cl)] 2 reacts with SEGPhos in a 1:2 ratio at room temperature in THF the desired species, [Rh(SEGPhos)(µ 2 -Cl)] 2 is produced quantitatively, Figure 3a [38,54]. In DCM an additional monomeric cationic species is formed, in which the rhodium center is coordinated by two SEGPhos molecules and the chloride as possible counterion, Figure 3b [38].…”

“…Less is known about the rate of formation of such species and the kinetics of the underlying two-stage ligand exchange process (Scheme 4). Quantitative studies of classical ligand exchange reactions can be accomplished using UV-Vis spectroscopy (diode array in combination with a stopped-flow unit) because during the reaction a color change takes place [54,59].…”

“…The solid state structure of the intermediate [(COD)Rh(µ2-Cl)2Rh(diphosphine)] has been determined for various disphosphines [38,60,61]. Only recently has it been shown that indeed an equilibrium exists between the starting and the product complexes on the one hand and the intermediate on the other (Scheme 5) which of course complicates kinetic investigations [54]. The rate constants of the reaction steps described in Scheme 4 and 5 for the ligands BINAP, SEGPhos, DM-SEGPhos, and Difluorphos are collected in Table 1.…”

“…Table 1. Rate constants of the ligand exchange reactions described in Scheme 4 and 5 for BINAP, SEGPhos, DM-SEGPhos, and Difluorphos [54]. The rate constants of the reaction steps described in Schemes 4 and 5 for the ligands BINAP, SEGPhos, DM-SEGPhos, and Difluorphos are collected in Table 1.…”