INVESTIGATION OF AN ALKOXIDE BETA-HYDROGEN ELIMINATION EQUILIBRIUM AND ISOLATION OF RHENIUM(III) ALKOXO HYDRIDE CLUSTERS

An isopropoxide beta-hydrogen elimination reaction involving a rhenium alkoxo cluster was investigated in detail, and several rhenium alkoxo-hydride clusters were synthesized by using alkoxide beta-hydrogen elimination. Re3(mu-O-i-Pr)3(O-i-Pr)6 was synthesized by reacting dry Re3(mu-Cl)3Cl6(thf)3 with NaO-i-Pr, and chemical and spectroscopic studies, including a direct observation by NMR, indicate that the cluster is in equilibrium with acetone and Re3(mu-O-i-Pr)3(H)(O-i-Pr)5 via reversible isopropoxide beta-hydrogen elimination. Solid-state thermal decomposition of Re3(mu-O-i-Pr)3(O-i-Pr)6 gave Re3(mu-O-i-Pr)3(H)(O-i-Pr)5, and its pyridine adduct, Re3(mu-O-i-Pr)3(H)(O-i-Pr)5(py), was isolated by trapping the hydride cluster from the equilibrium reaction. [Re3Cl3(H)(O-i-Pr)5]2 was prepared from Re3(mu-Cl)3(O-t-Bu)6 and i-PrOH in a process proposed to involve a combination of tert-butoxide/2-propanol exchange and isopropoxide beta-hydrogen elimination, and Re3(mu-OCHEt2)3(H)(OCHEt2)5 was synthesized by reacting Re3(mu-Cl)3Cl6(thf)3 with NaOC(H)Et2. It is proposed that a necessary condition for alkoxide beta-hydrogen elimination in this system is the presence of destabilizing steric interactions in the parent alkoxide clusters. Crystal data for Re3(mu-O-i-Pr)3(H)(O-i-Pr)5 at -75(1)degrees-C: C24H57O8Re3, triclinic, a = 9.865(2) angstrom, b = 11.495(2) angstrom, c = 16.537(4) angstrom, alpha = 88.41(2)degrees, beta = 84.99(2)degrees, gamma = 67.33(2)degrees, P1BAR), Z = 2. Crystal data for Re3(mu-O-i-Pr)3(H)(O-i-Pr)5(py).1/2py at-80(1)degrees-C: C31.5H64.5N1.5O8Re3, monoclinic, a = 24.940(6) angstrom, b = 15.201(5) angstrom, c = 23.664(2) angstrom, beta = 111.84(1)degrees, P2(1)/c, Z = 8.