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 [Re 2 H(CO) 9 ] - to the electronically unsaturated complex [Re 2 (μ-H) 2 (CO) 8 ] rapidly and selectively gives the anion [Re 4 H(μ-H) 2 (CO) 17 ] - ( 2 ), containing an open chain tetranuclear metal skeleton, as revealed by a single-crystal X-ray analysis of its [NEt 4 ] + salt. In the solid state the three metal−metal interactions display a staggered−eclipsed−staggered conformation, while in solution 1 H and 13 C NMR spectra have shown conformational freedom around the three Re−Re interactions and a dynamic process exchanging the two hydrides bound to the terminal H 2 Re(CO) 4 moiety, as well as the carbonyls trans to them ( E a = 48(1) kJ/mol). A windshield-wiper motion of the H 2 Re(CO) 4 fragment around the two trans diaxial carbonyls, analogous to that previously observed in the related anions [Re 3 H(μ-H)(CO) 13 ] - and [Re 2 H 2 (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 [Re 3 H(μ-H)(CO) 13 ] - has been recognized. The anion 2 is formed (even if in lower yields) also by reaction of [Re 2 H 2 (μ-H)(CO) 8 ] - with “Re 2 (CO) 9 (THF)”, obtained by treatment of [Re 2 (CO) 10 ] with Me 3 NO in THF. A 13 C NMR investigation has clarified that such “Re 2 (CO) 9 (THF)” reagent is indeed a mixture of three eq -[Re 2 (CO) 9 L] species, containing THF, H 2 O, and, in a minor amount, NMe 3, as labile L ligands. The reaction of the same eq -[Re 2 (CO) 9 L] species with [Re 2 H(CO) 9 ] - affords in good yields the tetranuclear cluster anion [Re 4 (μ-H)(CO) 18 }] - ( 3 ). The single-crystal X-ray analysis of [NEt 4 ] 3 has revealed also in this case a Re 4 chain, with an all-staggered conformation, of idealized C 2 symmetry. The low-temperature 13 C NMR spectrum of the carbonyls has shown a higher symmetry in solution, suggesting conformational freedom around all of the Re−Re interactions.