Inverse Isotope Effect for Aldehyde Hydrosilylation Catalyzed by a Hydrido–Silyl–Iridium(III) Complex

Complex {Ir­[SiMe­(o-C6H4SMe)2]­(H)­(PPh3)­(THF)}­[BArF 4] (1) has been shown to be an effective catalyst in the hydrosilylation of aldehydes. Kinetic analysis of the catalysis indicates that the aldehyde (benzaldehyde) undergoes transformation to the silylether (2a) by a pseudo-first order process (298 K, k = (4.6 ± 0.12) × 10–2 s–1 M–1). The activation barriers have also been calculated: ΔH ‡= 8.4 ± 0.9 kJmol–1, ΔS ‡= −160.2 ± 3.6 Jmol–1K–1, and ΔG ‡ 298 K= 56.2 ± 1.2 kJmol–1. Calculation of isotope effects using Et3Si-D instead of Et3SiH and Me2PhSi-D instead of Me2PhSi-H was performed and revealed inverse isotope effects (KIEEt3Si–H(D)= 0.43 ± 0.03; KIEMe3PhSi‑H(D)= 0.51 ± 0.05). A simplified electrophilic mechanism has been proposed. The inverse isotope effects found suggest that the rate-determining step involves the reduction of the carbonyl group.