Sequential reduction of high hydride count octahedral rhodium clusters Rh-6(PR3)(6)H-12BAr4F(2): Redox-switchable hydrogen storage

摘要

Cyclic voltammetry on the octahedral rhodium clusters with 12 bridging hydride ligands, Rh-6(PR3)(6)H-12BAr4F(2) (R = Cy Cy-H12(2+), R = Pr-i Pr-i-H12(2+); BAr4F(-) = B{C6H3(CF3)(2)}(4)(-)) reveals four potentially accessible redox states: Rh-6(PR3)(6)H-12(0/1+/2+/3+). Chemical oxidation did not produce stable species, but reduction of Cy-H12(2+) using Cr(eta(6)-C6H6)(2) resulted in the isolation of Cy-H12(+). X-ray crystallography and electrospray mass spectrometry (ESI-MS) show this to be a monocation, while EPR and NMR measurements confirm that it is a monoradical, S = (1)/(2), species. Consideration of the electron population of the frontier molecular orbitals is fully consistent with this assignment. A further reduction is mediated by Co(eta(5)-C5H5)(2). In this case the cleanest reduction was observed with the tri-isopropyl phosphine cluster, to afford neutral Pr-i-H12. X-ray crystallography confirms this to be neutral, while NMR and magnetic measurements (SQUID) indicate an S =1 paramagnetic ground state. The clusters Cy-H12(+) and Pr-i-H12 both take up H-2 to afford Cy-H14(+) and Pr-i-H14, respectively, which have been characterized by ESI-MS, NMR spectroscopy, and UV-vis spectroscopy. Inspection of the frontier molecular orbitals of S = 1 Pr-i-H12 suggest that addition of H-2 should form a diamagnetic species, and this is the case. The possibility of "spin blocking" in this H-2 uptake is also discussed. Electrochemical investigations on the previously reported Cy-H16(2+) J. Am. Chem. Soc. 2006, 128, 6247 show an irreversible loss of H-2 on reduction, presumably from an unstable Cy-H16(+) species. This then forms Cy-H12(2+) on oxidation which can be recharged with H-2 to form Cy-H16(2+). We show that this loss of H-2 is kinetically fast (on the millisecond time scale). Loss of H-2 upon reduction has also been followed using chemical reductants and ESI-MS. This facile, reusable gain and loss of 2 equiv of H-2 using a simple one-electron redox switch represents a new method of hydrogen storage. Although the overall storage capacity is very low (0.1) the attractive conditions of room temperature and pressure, actuation by the addition of a single electron, and rapid desorption kinetics make this process of interest for future H-2 storage applications.