1H- and 2H-T1 relaxation behavior of the rhodium dihydrogen complex [(triphos)Rh(η2-H2)H2]+

Protonation of the classical trihydride [(triphos)RhH3] (2) at 210 K in either THF or CH2Cl2 by either HBF4. OMe2 or CF3SO2OH gives the nonclassical eta(2)-H-2 complex [(triphos)Rh(eta(2)-H-2)H-2](+) (1) [triphos = MeC(CH2PPh2)(3)]. Complex 1 is thermally unstable and highly fluxional in solution. In THF above 230 K, 1 transforms into the solvento dihydride complex [(triphos)Rh(eta(1)-THF-d(8))H-2](+) (5) that, at room temperature, quickly converts to the stable dimer trans-[{(triphos)RhH}(2)(mu-H)(2)](2+) (trans-6). In CH2Cl2, 1 is stable up to 240 K. Above this temperature, the eta(2)-H-2 complex begins to convert into a mixture of trans- and cis-6, which, in turn, transform into the bridging chloride dimers trans- and cis-[{(triphos)RhH}(2)(mu-Cl)(2)](2+) at room temperature. Complex 1 contains a fast-spinning H-2 ligand with a T-1min of 38.9 ms in CD2Cl2 (220 K, 400 MHz). An NMR analysis of the bis-deuterated isotopomer [(triphos)RhH2D2](+) (1-d(2)) did not provide a J(HD) value. At 190 K, the perdeuterated isotopomers [(triphos)RhD3] (2-d(3)) and 1-d(4) show T-1min, values of 16.5 and 32.6 ms (76.753 MHZ), respectively, for the rapidly exchanging deuterides. An analogous 2-fold elongation of T-1min, is also observed on going from [(triphos)IrD3] to [(triphos)Ir(eta(2)-D-2)D-2](+). A rationale for the elongation of T-1min, in nonclassical polyhydrides is proposed on the basis of both the results obtained and recent literature reports.