Photochemical reactions of (η5-cyclopentadienyl)bis(t-butylacrylate) rhodium with silanes: Dynamics of isomer interconversion via Rh(η2-silane) species

Abstract

The compound CpRh(C₂H₃CO₂^tBu)₂1 has been synthesised as a mixture of two pairs of interconverting isomers which differ in the relative orientations of the alkene substituents. The four isomers have been fully characterised by NMR spectroscopy. When complex 1 is photolysed in the presence of a silane, HSiR₂R′ [R₂R′ = Et₃, Me₃, HEt₂, (OMe)₃ and Me₂Cl] the corresponding Si–H oxidative addition products CpRh(SiR₂R′)(H)(C₂H₃CO₂^tBu) and CpRh(H)₂(SiR₂R′)₂ are formed. The Rh(III) complexes CpRh(SiR₂R′)(H)(C₂H₃CO₂^tBu) exist in two isomeric forms of comparable energy which interconvert in an intramolecular process that does not involve a reversible [1,3] hydride or [1,3] silyl migration. The hydride ¹H NMR resonances for these species consequently broaden before coalescing into a single peak. For R₂R′ = Et₃, the activation parameters for interchange from the major to minor isomer were ΔH^‡ = 60.2 ± 2 kJ mol⁻¹ and ΔS^‡ = 8 ± 9 J mol⁻¹ K⁻¹, while for R₂R′ = Me₃ and Et₂H, ΔH^‡ = 61.5 ± 1 kJ mol⁻¹, ΔS^‡ = 6 ± 5 J mol⁻¹ K⁻¹, and ΔH^‡ = 61.8 ± 3 kJ mol⁻¹, ΔS^‡ = 12 ± 9 J mol⁻¹ K⁻¹ respectively for conversion from the major isomer to the minor. For these complexes an η²-Rh–H–Si transition state or intermediate is consistent with the evidence. When R₂R′ = (OMe)₃ and Me₂Cl the change in appearance of the hydride resonances is more complex, with the activation parameters for interchange from the major to minor isomer for the former species being ΔH^‡ = 78.3 ± 2 kJ mol⁻¹ and ΔS^‡ = 30 ± 7 J mol⁻¹ K⁻¹ while for Me₂Cl the barrier proved too high to measure before decomposition occurred. The complex spectral changes could be simulated when a discrete η²-Rh–H–Si intermediate was involved in the isomer interconversion process and hence silane rotation in all these systems is proposed to involve two isomers of CpRh(η²-HSiR₂R′)(C₂H₃CO₂^tBu).