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.