LOW-VALENT RHENIUM-OXO COMPLEXES .10. LOW-VALENT BINUCLEAR RHENIUM OXO ACETYLENE COMPLEXES - COMPARISON OF SYMMETRICAL DIMERS CONTAINING AN UNSUPPORTED RE-RE BOND WITH ASYMMETRIC DIMERS WITH BRIDGING OXO ACETYLENE LIGANDS

Reduction of the rhenium(III)-oxo-bis(acetylene) complexes Re(O)I(RC = CR)2 (1) yields mixtures of two isomeric rhenium dimers Re2(O)2(RC = CR)4 (R = Me, Et, Ph, and (t)Bu/H). A wide variety of reducing agents can be used, including (t)BuZnCl, NaC10H8, and Cp2Co. One of the isomers has C2 symmetry (the ''symmetric'' dimers 2) and consists of two Re(O)(RC = CR)2 fragments connected by an unsupported Re-Re bond (2.686 (1) angstrom in the structure of 2a, R = Me). The structural and spectroscopic properties of the Re(O)(RC = CR)2 groups in 2 are very similar to those of Re(O)X(RC = CR)2 compounds, for instance, with rhenium-terminal oxo distances of 1.693 (11) angstrom (av). The other isomer (3) is asymmetric, with one bridging and one terminal oxo group, as revealed by the structure of (EtC = CEt)(O)Re(mu-O)(mu-EtC = CEt)Re(EtC = CEt)2 (3b). The unusual bridging acetylene ligand in 3b is twisted roughly 34-degrees with respect to the Re-Re bond, with Re-C distances that vary from double bond to longer than single bond distances (1.89 (2) to 2.52 (3) angstrom). Compounds 3 are best described as Re(III)/Re(I) mixed-valence species, the Re(III) side resembling Re(O)X(RC = CR)2 compounds and the Re(I) side close to ReX(RC = CR)3, with X = mu-O and one acetylene shared between the two. On heating, compounds 3 cleanly convert to 2, indicating that the symmetric isomers are thermodynamically more stable. This is surprising in light of the normal preference for bridging oxo ligands in low-valent compounds. The Re-Re bond in 2 is cleaved by I2 to regenerate 1. Compounds 2 are rigid on the NMR time scale (like 1), but 3 are fluxional. The fluxional process involves only the two acetylene ligands on the Re(I) side, and is concluded to be a ''turnstile'' rotation that exchanges the end of one acetylene for the opposite end of another. The symmetric dimers are thought to be formed by combination of two [Re(O)(RC = CR)2]. radicals while the asymmetric dimers can result from reaction of Re(I)- and Re(III)-oxo-bis(acetylene) complexes.