Stereochemical non-rigidity in rhodium–triruthenium clusters studied by nuclear magnetic resonance spectroscopy

Abstract

Variable-temperature (v.t.)¹H and ¹³C n.m.r. studies on tetranuclear clusters [RhRu₃(µ-H)₂(µ-CO)(CO)₉(η-C₅H₅)](1), [RhRu₃(µ-H)₂(η-CO)(CO)₉(η-C₅Me₅)](2), and [RhRu₃(µ-H)₄(CO)₉(µ-C₅Me₅)](3) establish various types of stereochemical non-rigidity in solution. Cluster (1) exhibits at least three distinct CO exchange processes: the lowest-energy intramolecular interchange, with ΔG₁₉₅^‡= 35.2 ± 0.3 kJ mol^–1, monitored by v.t. ¹H and ¹³C n.m.r., including ¹³C-{¹H} two-dimensional exchange correlated spectra at 163 K (NOESY sequence), involves a pairwise exchange of eight CO groups resulting from a ‘rocking motion’ of CO ligands around a single RhRu₂ triangular face. Three isomers of cluster (2) are in dynamic equilibrium in solution: one isomer is relatively rigid stereochemically whereas the other two are fluxional and undergo rapid interconversion at all but the lowest temperatures [e.g. 146 K ¹H (200 MHz)]. Two distinct exchange processes are observed for cluster (3): localised site exchange of CO ligands at Ru(CO)₃ centres, with ΔG₂₂₃^‡= 47.5 ± 1.5 kJ mol^–1, and µ-H mobility, with ΔG₁₆₄^‡= 29.4 ± 0.2 kJ mol^–1. Reactions of (1) with phosphines produce clusters [RhRu₃(µ-H)₂(µ-CO)(CO)_9–n(η-C₅H₅)(PPh₃)_n][n= 1 (4) or 2 (5)] and [RhRu₃(µ-H)₂(µ-CO)(CO)₇(η-C₅H₅)(dppe)](6)(dppe = Ph₂PCH₂CH₂PPh₂), accompanied by cleavage products including [Ru₃(CO)₈(dppe)₂]. Varaible-temperature ¹H, ¹³C, and ³¹P n.m.r. studies indicate that (4) exists in solution in two isomeric forms undergoing slow interconversion at low temperatures: the major isomer is more fluoxional with a lowest-energy exchange related to that in (1) with ΔG₂₀₃^‡= 38.0 ± 0.3 kJ mol^–1. Clusters (5) and (6) show n.m.r. spectra of stereochemically rigid species in solution at ambient temperature and structures of these complexes are discussed.