Synthesis and Characterization of Hydrotris(pyrazolyl)borate Dihydrogen/Hydride Complexes of Rhodium and Iridium

Abstract

Protonation of TpM(PR3)H2 (M = Rh, Ir) complexes with HBF4·Et2O or [H(Et2O)2][B(Ar)4] (Ar = 3,5-(CF3)2C6H3) affords cationic complexes which exhibit a single hydride resonance at all accessible temperatures in the 1H NMR spectrum. Formulation as fluxional dihydrogen/hydride complexes is indicated by short T1(min) values of ca. 22 ms (Ir) and 7 ms (Rh). The relaxation times are consistent with H−H bond lengths of 0.88−1.11 Å in the iridium complexes and 0.73−0.92 Å in the rhodium complexes depending on the relative rate of the dihydrogen rotational motion. In the case of the iridium complexes, partial substitution of the hydride positions with deuterium or tritium results in large temperature-dependent isotope shifts and resolvable JH-D or JH-T coupling constants. Analysis of the chemical shift and coupling constant data as a function of temperature is consistent with a preference for the heavy hydrogen isotope to occupy the hydride rather than the dihydrogen site. This analysis also provides the limiting chemical shifts of the dihydrogen and hydride ligands as well as the 1JH-D coupling constant (ca. 25 Hz) in the bound dihydrogen ligand.