Sequential Reduction of High Hydride Count Octahedral Rhodium Clusters [Rh6(PR3)6H12][BArF4]2: Redox-Switchable Hydrogen St
详细信息 查看全文
- 作者：Simon K. Brayshaw ; Andrew Harrison ; J. Scott McIndoe ; Frank Marken ; Paul R. Raithby ; John E. Warren ; Andrew S. Weller
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：February 14, 2007
- 年：2007
- 卷：129
- 期：6
- 页码：1793 - 1804
- 全文大小：598K
- 年卷期：v.129,no.6(February 14, 2007)
- ISSN：1520-5126
文摘
Cyclic voltammetry on the octahedral rhodium clusters with 12 bridging hydride ligands, [Rh6(PR3)6H12][BArF4]2 (R = Cy Cy-[H12]2+, R = iPr iPr-[H12]2+; [BArF4]- = [B{C6H3(CF3)2}4]-) reveals fourpotentially accessible redox states: [Rh6(PR3)6H12]0/1+/2+/3+. Chemical oxidation did not produce stablespecies, but reduction of Cy-[H12]2+ using Cr(
6-C6H6)2 resulted in the isolation of Cy-[H12]+. X-raycrystallography and electrospray mass spectrometry (ESI-MS) show this to be a monocation, while EPRand NMR measurements confirm that it is a monoradical, S = 1/2, species. Consideration of the electronpopulation of the frontier molecular orbitals is fully consistent with this assignment. A further reduction ismediated by Co(5-C5H5)2. In this case the cleanest reduction was observed with the tri-isopropyl phosphinecluster, to afford neutral iPr-[H12]. X-ray crystallography confirms this to be neutral, while NMR and magneticmeasurements (SQUID) indicate an S =1 paramagnetic ground state. The clusters Cy-[H12]+ and iPr-[H12] both take up H2 to afford Cy-[H14]+ and iPr-[H14], respectively, which have been characterized byESI-MS, NMR spectroscopy, and UV-vis spectroscopy. Inspection of the frontier molecular orbitals ofS = 1 iPr-[H12] suggest that addition of H2 should form a diamagnetic species, and this is the case. Thepossibility of "spin blocking" in this H2 uptake is also discussed. Electrochemical investigations on thepreviously reported Cy-[H16]2+ [J. Am. Chem. Soc. 2006, 128, 6247] show an irreversible loss of H2 onreduction, presumably from an unstable Cy-[H16]+ species. This then forms Cy-[H12]2+ on oxidation whichcan be recharged with H2 to form Cy-[H16]2+. We show that this loss of H2 is kinetically fast (on themillisecond time scale). Loss of H2 upon reduction has also been followed using chemical reductants andESI-MS. This facile, reusable gain and loss of 2 equiv of H2 using a simple one-electron redox switchrepresents 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 H2 storage applications.
6-C6H6)2 resulted in the isolation of Cy-[H12]+. X-raycrystallography and electrospray mass spectrometry (ESI-MS) show this to be a monocation, while EPRand NMR measurements confirm that it is a monoradical, S = 1/2, species. Consideration of the electronpopulation of the frontier molecular orbitals is fully consistent with this assignment. A further reduction ismediated by Co(5-C5H5)2. In this case the cleanest reduction was observed with the tri-isopropyl phosphinecluster, to afford neutral iPr-[H12]. X-ray crystallography confirms this to be neutral, while NMR and magneticmeasurements (SQUID) indicate an S =1 paramagnetic ground state. The clusters Cy-[H12]+ and iPr-[H12] both take up H2 to afford Cy-[H14]+ and iPr-[H14], respectively, which have been characterized byESI-MS, NMR spectroscopy, and UV-vis spectroscopy. Inspection of the frontier molecular orbitals ofS = 1 iPr-[H12] suggest that addition of H2 should form a diamagnetic species, and this is the case. Thepossibility of "spin blocking" in this H2 uptake is also discussed. Electrochemical investigations on thepreviously reported Cy-[H16]2+ [J. Am. Chem. Soc. 2006, 128, 6247] show an irreversible loss of H2 onreduction, presumably from an unstable Cy-[H16]+ species. This then forms Cy-[H12]2+ on oxidation whichcan be recharged with H2 to form Cy-[H16]2+. We show that this loss of H2 is kinetically fast (on themillisecond time scale). Loss of H2 upon reduction has also been followed using chemical reductants andESI-MS. This facile, reusable gain and loss of 2 equiv of H2 using a simple one-electron redox switchrepresents 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 H2 storage applications.