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]^-

Abstract

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 (E a = 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 C 2 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.

[Re5(μ-H)4(CO)20]− and [Re5(μ-H)5(CO)20], Two Isolobal Analogues of Cyclopentane

“…In addition, spacers (for example, polyphenyls, [6] alkenes, [7] or alkynes [5,8] ) can also be selected to serve as electron-conducting active components to promote long-range electronic coupling between terminal subunits or (for example, saturated hydrocarbons [9] ) to serve as passive connecting components. Systems with unsaturated sp carbon chains (C n ) between two subunits constitute one of the most fundamental classes of one-dimensional molecular wires, however, intense studies in numerous laboratories have focused on the systems with alkynes or acetylenyl bridges.…”

“…tetranuclear clusters, [6,7] by allowing hydrido ± carbonyl rhenates to react with [Re 2 (CO) 9 L], where L is a very labile ligand such as THF or H 2 O (the otherwise labile MeCN ligand is not labile enough to be substituted by neutral [8] or anionic [6] hydride complexes). Recently the di-substituted complex 1,2-eq,eq-[Re 2 (CO) 8 8 ] with diazomethane.…”

Anionic Oligomerization as a Route to Chain Clusters

“…More recently we proved that also carbonylates containing terminal hydrido ligands can add to 1, [22,18] through M-H-Re interactions (Scheme 8). [34] This last achievement has been decisive in discovering the anionic oligomerization of 1.…”

“…However, in the present cases, due to the electron difference between Re and Co, no CoÀCo bond is present in the Co-Cu-Co chains, while the Re atoms are connected through 3c-2e Re-H-Re bonds. This leads to distances between the Re(CO) 4 units that are much shorter than those between the corresponding Co(CO) 4 moieties, with possible 1,3-and 1,4-Re´´´Re interactions [18] that could destabilize the Re chains.…”

“…[18,19] On the other hand, ring opening by the addition of electron donors [20] is penalized by the fact that the number of ring clusters of nuclearity higher than 3 is very limited.…”