Synthesis and characterization of hydrotris(pyrazolyl)borate dihydrogen/hydride complexes of rhodium and iridium

Article abstract of DOI:10.1021/ja971627t

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 A? in the iridium complexes and 0.73-0.92 A? 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 hydrode positions with deuterium or tritium results in large temperature-dependent isotope shifts and resolvable J(H-D) or J(H-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 ¹J(H-D) coupling constant (ca. 25 Hz) in the bound dihydrogen ligand.