Substitution reactivity of the hydrido sulfido bridged dirhenium complex [Re2(μ-H)(μ-Snaph)(CO)8] (naph = 2-naphthyl)

Abstract

The substitution reactivity of the hydrido sulfido bridged dirhenium complex [Re₂(μ-H)(μ-Snaph)(CO)₈] 1 (naph = 2-naphthyl) has been investigated. The former complex can be easily substituted with one equivalent of tmno (trimethylamine-N-oxide) followed by addition of a ligand L giving complexes of the general formula [Re₂(μ-H)(μ-Snaph)(CO)₇L] [L = NCMe (2a), NC(Bu^t) (2b), pyridine (2c), PH₃ (2d), PMe₃ (2e), H₂P(p-C₆H₄OMe) (2f), PPh₃ (2g), P(OMe)₃ (2h)]. L is always perpendicularly co-ordinated to one of the Re atoms with respect to the Re–(μ-SR)–Re plane. In case of ligands with small cone angles two isomers are found. They differ in the relative orientation of L with respect to the substituent R attached to the sulfido bridge (syn and anti isomer). Temperature dependent 2D-EXSY NMR experiments with 2a dissolved in CDCl₃ proved that both isomers are in a dynamic equilibrium, which is based on the pyramidal inversion of the bridging sulfur. The thermodynamic data of this process at 298.15 K were determined to be: ΔG^‡ = 68.6 ± 1.2 kJ mol⁻¹, ΔH^‡ = 49.0 ± 4.2 kJ mol⁻¹ and ΔS^‡ = −66 ± 21 J K⁻¹ mol⁻¹. Owing to intramolecular sterical hindrance ligands with large cone angles give only the anti isomer. Disubstituted complexes [Re₂(μ-H)(μ-Snaph)(CO)₆L₂] [L = NCMe (3a), pyridine (3b), PPh₃ (3c)] and [Re₂(μ-H)(μ-Snaph)(CO)₆(μ-L–L)] [L–L = dppm (4a), dppe (4b)] were prepared in a similar way. The reaction of [Re₂(μ-H)(μ-SH)(CO)₈] 5 with one equivalent of tmno and PPh₃ gave [Re₂(μ-H)(μ-SH)(CO)₆(PPh₃)₂] 6 in 38% yield. Compound 6 is rigid on the timescale of ³¹P NMR spectroscopy. The molecular structures of 2a, 2c, 2g, 2h, 3a, 4b and 6 were confirmed by X-ray structure analyses.