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

W. Edward Lindsell, Neil M. Walker, A. S F Boyd

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Variable-temperature (v.t.) 1H and 13C n.m.r. studies on tetranuclear clusters [RhRu3(µ-H)2(µ-CO)(CO)9(?-C 5H5)] (1), [RhRu3(µ-H)2(?-CO)(CO)9(?-C 5Me5)] (2), and [RhRu3(µ-H)4(CO)9(?-C5Me 5)] (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 ?G‡195 = 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 ?G‡223 = 47.5 ± 1.5 kJ mol-1, and µ-H mobility, with ?G‡164 = 29.4 ± 0.2 kJ mol-1. Reactions of (1) with phosphines produce clusters [RhRu3(µ-H)2(µ-CO)(CO)9-n (?-C5H5)(PPh3)n] [n = 1 (4) or 2 (5)] and [RhRu3(µ-H)2(µ-CO)(CO)7(?-C 5H5)(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 ?G‡203 = 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.

Original languageEnglish
Pages (from-to)675-683
Number of pages9
JournalJournal of the Chemical Society, Dalton Transactions
Issue number3
Publication statusPublished - 1988


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