A unified mechanistic view obtained from the temperature and pressure dependence of the spontaneous, acid-, and base-assisted cyclometallation reactions of dirhodium(II) complexes

Abstract

The reaction of compounds [Rh₂(O₂CMe)(µ-O₂CMe)₂{µ-(XC₆H₃)P(C₆H₄X-p)₂}{P(C₆H₄X′-p)₃}](X = H, X′= H or Me; X = Me, X′= H or Me), having a metallated and an equatorial phosphine in a head-to-tail cisoid configuration, to produce the corresponding doubly metallated compounds [Rh₂(µ-O₂CMe)₂{µ-(XC₆H₃)P(C₆H₄X-p)₂}₂], has been studied in detail. The thermal, acid-, and base-assisted conditions include toluene solution, neat acetic acid, and different bases [PPh₃, P(C₆H₄Me-p)₃ or pyridine] in toluene solution and as a function of temperature, pressure and base concentration (when applicable). The doubly metallated compounds always show retention of the head-to-tail cisoid configuration according to ³¹P NMR spectroscopy. The reported activation parameters allow a clear differentiation between the spontaneous, acid-assisted and base-assisted processes, some of which had been previously studied (ΔH^‡= 80 ± 5, 68 ± 1, 38 ± 3 kJ mol^–1; ΔS^‡=–105 ± 15, –85 ± 3, –150 ± 30 J K^–1 mol^–1; ΔV^‡=–22.7 ± 0.2, –15.3 ± 0.2, –35.0 ± 0.5 cm³ mol^–1 respectively, for the compound with X, X′= H). For the reactions under baseassisted conditions, when long reaction times and/or high added Lewis-base concentrations were applied, the non-metallated phosphine was displaced from equatorial to axial co-ordination. Under these conditions a mixture of the bis adducts of the monometallated [Rh₂(µ-O₂CMe)₃{µ-(XC₆H₃)P(C₆H₄X-p)₂}], and the doubly-metallated compounds was detected by ³¹P NMR spectroscopy. The activation parameters obtained for this slower process agree with those expected for substitution reactions and are interpreted in terms of the steric demands of the mono and bis adducts formed immediately on mixing the reaction solutions.