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Dihydrogen Complexes of Rhodium:  [RhH2(H2)x(PR3)2]+ (R = Cy, iPr; x = 1, 2)

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journal contribution
posted on 02.05.2005, 00:00 by Michael J. Ingleson, Simon K. Brayshaw, Mary F. Mahon, Giuseppe D. Ruggiero, Andrew S. Weller
Addition of H2 (4 atm at 298 K) to [Rh(nbd)(PR3)2][BArF4] [R = Cy, iPr] affords Rh(III) dihydride/dihydrogen complexes. For R = Cy, complex 1a results, which has been shown by low-temperature NMR experiments to be the bis-dihydrogen/bis-hydride complex [Rh(H)22-H2)2(PCy3)2][BArF4]. An X-ray diffraction study on 1a confirmed the {Rh(PCy3)2} core structure, but due to a poor data set, the hydrogen ligands were not located. DFT calculations at the B3LYP/DZVP level support the formulation as a Rh(III) dihydride/dihydrogen complex with cis hydride ligands. For R = iPr, the equivalent species, [Rh(H)22-H2)2(PiPr3)2][BArF4] 2a, is formed, along with another complex that was spectroscopically identified as the mono-dihydrogen, bis-hydride solvent complex [Rh(H)22-H2)(CD2Cl2)(PiPr3)2][BArF4] 2b. The analogous complex with PCy3 ligands, [Rh(H)22-H2)(CD2Cl2)(PCy3)2][BArF4] 1b, can be observed by reducing the H2 pressure to 2 atm (at 298 K). Under vacuum, the dihydrogen ligands are lost in these complexes to form the spectroscopically characterized species, tentatively identified as the bis hydrides [Rh(H)2(L)2(PR3)2][BArF4] (1c R = Cy; 2c R = iPr; L = CD2Cl2 or agostic interaction). Exposure of 1c or 2c to a H2 atmosphere regenerates the dihydrogen/bis-hydride complexes, while adding acetonitrile affords the bis-hydride MeCN adduct complexes [Rh(H)2(NCMe)2(PR3)2][BArF4]. The dihydrogen complexes lose [HPR3][BArF4] at or just above ambient temperature, suggested to be by heterolytic splitting of coordinated H2, to ultimately afford the dicationic cluster compounds of the type [Rh6(PR3)6(μ-H)12][BArF4]2 in moderate yield.