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]-

The addition of [Re2H(CO)9]- to the electronically unsaturated complex [Re2(μ-H)2(CO)8] rapidly and selectively gives the anion [Re4H(W-H)2(CO)I?]' (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