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

Article abstract of DOI:10.1021/ic990667q

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 η²-H₂ complex [(triphos)Rh(η²-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(η¹-THF-d₈)H₂]⁺ (5) that, at room temperature, quickly converts to the stable dimer trans-[{(triphos)RhH}₂(μ-H)₂]²⁺ (trans-6). In CH₂Cl₂, 1 is stable up to 240 K. Above this temperature, the η²-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(μ-Cl)₂]²⁺ at room temperature. Complex 1 contains a fast-spinning H₂ ligand with a T₁(min) 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 T₁(min) values of 16.5 and 32.6 ms (76.753 MHz), respectively, for the rapidly exchanging deuterides. An analogous 2-fold elongation of T₁(min) is also observed on going from [(triphos)IrD₃] to [(triphos)Ir(η²-D₂)D₂]⁺. A rationale for the elongation of T₁(min) in nonclassical polyhydrides is proposed on the basis of both the results obtained and recent literature reports.