Kinetics of phosphine substitution in CpRu(PPh3)2X (X = Cl, Br, I, N3, and NCO)

Abstract

The kinetics of phosphine substitution in CpRu(PPh₃)₂X (X = Br, 1b, X = I, 1c, X = N₃, 1d, and X = NCO, 1e) have been measured under pseudo-first order conditions in THF solution and compared with data for CpRu(PPh₃)₂X (1a). The relative rate of substitution is found to be 1a > 1d > 1b > 1e > 1c. Substitution rates decrease in the presence of added PPh₃ and are independent of added X consistent with a dissociative process. Activation parameters for 1a–1c (ΔH^† = 113–135 kJ mol⁻¹, ΔS^† = 21–102 J mol⁻¹ K⁻¹) and DFT calculations support a dissociative or dissociative interchange pathway even though negative activation entropies (ΔS^† = −48 ± 16 to −105 ± 5 J mol⁻¹ K⁻¹) are observed for 1d–e. Differences in Ru–ligand bond angles in 1d–e point to different π-acceptor properties of the pseudohalide ligands, contributing to the faster rate of substitution for the azide complexes, 1d relative to the cyanate derivative 1e. Substitution is not observed when X = F, 1f, X = H, 1g, X = SnF₃, 1h, or X = SnCl₃, 1i. Compounds 1b–1e also react with chloroform to yield 1a. The rates of halide exchange are comparable to phosphine substitution for 1c and 1d. The latter reaction is inhibited by excess triphenylphosphine and is unaffected by both radical inhibitors and radical traps suggesting that a radical mechanism is unlikely.