FLUXIONALITY AND ISOMERISM OF THE BIS(DIHYDROGEN) COMPLEX RUH2(H-2)(2)(PCY3)(2) - INS, NMR, AND THEORETICAL-STUDIES

To study the fluxionality of the bis(dihydrogen) complex RuH2(H-2)(2)( PCy3)(2) (1), NMR spectra were recorded in Freons (mixture of CDCl3, C DFCl2, and CDF2Cl). 1 was found to remain fluxional at all temperature s, but the presence of CDCl3 necessary for its solubilization induces its transformation into, first, RuHCl(H-2)(2)(PCy3)(2) (3) and the new ruthenium(IV) dihydride RuH2Cl2(PCy3)(2) (4). 4 is produced selective ly in pure CDCl3 but reacts further to give a mixture of chloro comple xes. 4 was isolated from the reaction of 1 with aqueous HCI in Et2O an d shows a fluxional process attributed to the interconversion between two symmetrical isomers. The activation parameters of this process wer e obtained by H-1 NMR line shape analysis, as well as those correspond ing to the exchange between 3 and free dihydrogen. The fluxionality of the dihydrogen-hydride system is also evident at a much faster time s cale than that of NMR studies in the inelastic neutron scattering obse rvations of the rotation of the dihydrogen ligands. The geometries and relative energies of several isomers of complexes 1, 3, and 4 were st udied using density functional theory (DFT) and MP2 methods, together with a few coupled-cluster (CCSD-(T)) calculations. In contrast to wha t might have been expected, the two hydrides and the two H-2 units of 1 lie in the same plane, due to the attractive ''cis effect'' created by the hydrides. The two H-2 ligands adopt cis positions in the lowest -energy isomer. Rotation of the two dihydrogen ligands has been analyz ed using DFT calculations. A slight preference for a C-2 conrotatory p athway has been found with a calculated barrier in good agreement with the experimental INS value. Two low-energy isomers of 4 have been cha racterized computationally, both of which have C-2 upsilon, symmetry, consistent with the solution NMR spectra.