New Chain Clusters of Rhenium connected by Re-H-Re interactions: A low-temperature NMR investigation

Low temperature NMR spectroscopy has been used to characterize the mixtures formed in the oligomerization reactions of the unsaturated complex [Re-2(mu-H)(2)(CO)(8)] (1), promoted by hydrido-carbonyl rhenates. Three families of chain clusters, constituted by Re(CO)(4) units connected through Re(mu-H)Re interactions, have been obtained. The first one, of general formula [(CO)(5)Re-{HRe(CO)(4)}(2n+1)], was formed using [HRe2(CO)(9)] as promoter. The nature of the products was confirmed by C-13 NMR of (CO)-C-13 enriched samples. The formation of Re-6 and Re-8 chain clusters was recognized. The other two families have general formula [H-{HRe(CO)(4)}(2n)] and [H-{HRe(CO)(4)}(2n+1)] and were obtained using as initiators [HB(Bu-s)(3)] or [H2Re(CO)(4)], respectively. Mixtures of oligomers with mean chain lengths higher than 10 have been observed. The addition of a strong acid caused H-2 evolution, leading back to the "monomer" 1. For all the three families, each oligomerization step was reversible, with the longer oligomers favoured at the lowest temperatures, where, however, the reactions were very slow, usually preventing the attainment of the equilibrium. Variable temperature NMR spectra revealed a dynamic process involving the terminal H2Re(CO)(4) moiety(ies), that simultaneously exchanges terminal/bridging hydrides and the carbonyls trans to them (Delta G(#) 41-44 kJ mol (1)). At room temperature, the more hydrogen-rich chain clusters also underwent dehydrogenation/cyclization reactions