American Chemical Society
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Hydrido−Carbonyl Chain Clusters. Synthesis, Solid State Structure, and Solution Behavior of the Tetranuclear Open Cluster Anions [Re4H(μ-H)2(CO)17]- and [Re4(μ-H)(CO)18]-

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journal contribution
posted on 1997-09-16, 00:00 authored by Mirka Bergamo, Tiziana Beringhelli, Giuseppe D'Alfonso, Pierluigi Mercandelli, Massimo Moret, Angelo Sironi
The addition of [Re2H(CO)9]- to the electronically unsaturated complex [Re2(μ-H)2(CO)8] rapidly and selectively gives the anion [Re4H(μ-H)2(CO)17]- (2), containing an open chain tetranuclear metal skeleton, as revealed by a single-crystal X-ray analysis of its [NEt4]+ salt. In the solid state the three metal−metal interactions display a staggered−eclipsed−staggered conformation, while in solution 1H and 13C NMR spectra have shown conformational freedom around the three Re−Re interactions and a dynamic process exchanging the two hydrides bound to the terminal H2Re(CO)4 moiety, as well as the carbonyls trans to them (Ea = 48(1) kJ/mol). A windshield-wiper motion of the H2Re(CO)4 fragment around the two trans diaxial carbonyls, analogous to that previously observed in the related anions [Re3H(μ-H)(CO)13]- and [Re2H2(μ-H)(CO)8]-, is likely responsible for this exchange. The tetrametallic skeleton of the anion 2 in solution easily undergoes fragmentation to trinuclear species. Under CO atmosphere the clean formation of [ReH(CO)5] and [Re3H(μ-H)(CO)13]- has been recognized. The anion 2 is formed (even if in lower yields) also by reaction of [Re2H2(μ-H)(CO)8]- with “Re2(CO)9(THF)”, obtained by treatment of [Re2(CO)10] with Me3NO in THF. A 13C NMR investigation has clarified that such “Re2(CO)9(THF)” reagent is indeed a mixture of three eq-[Re2(CO)9L] species, containing THF, H2O, and, in a minor amount, NMe3, as labile L ligands. The reaction of the same eq-[Re2(CO)9L] species with [Re2H(CO)9]- affords in good yields the tetranuclear cluster anion [Re4(μ-H)(CO)18}]- (3). The single-crystal X-ray analysis of [NEt4]3 has revealed also in this case a Re4 chain, with an all-staggered conformation, of idealized C2 symmetry. The low-temperature 13C NMR spectrum of the carbonyls has shown a higher symmetry in solution, suggesting conformational freedom around all of the Re−Re interactions.