1H- and 2H-T1 relaxation behavior of the rhodium dihydrogen complex [(triphos)Rh(eta 2-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-H2 complex [(triphos)Rh(eta 2-H2)H2]+ (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-d8)H2]+ (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-H2 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 H2 ligand with a T1min of 38.9 ms in CD2Cl2 (220 K, 400 MHz). An NMR analysis of the bis-deuterated isotopomer [(triphos)RhH2D2]+ (1-d2) did not provide a J(HD) value. At 190 K, the perdeuterated isotopomers [(triphos)RhD3] (2-d3) and 1-d4 show T1min values of 16.5 and 32.6 ms (76.753 MHz), respectively, for the rapidly exchanging deuterides. An analogous 2-fold elongation of T1min is also observed on going from [(triphos)IrD3] to [(triphos)Ir(eta 2-D2)D2]+. A rationale for the elongation of T1min in nonclassical polyhydrides is proposed on the basis of both the results obtained and recent literature reports.