Structural and spectroscopic characterization of iridium trihydride complexes: Evidence for proton-proton exchange coupling

Article abstract of DOI:10.1021/ja00159a001

Protonation of the neutral iridium dihydrides (η-C₅H₅)Ir(L)H₂ with HBF₄.Et₂O affords cationic trihydride complexes of the form [(η-C₅H₅)Ir(L)H₃]BF₄ (L = various phosphine and phosphite ligands). The ¹H NMR spectra of the cations in the hydrige region at low temperature display AB₂X spin systems (X = ³¹P), which are simplified to AB₂ spin systems in the ¹H{³¹P} spectra. The values for J(A-B) derived by computer fitting of the observed spectra are extremely large, ranging from 61 to 1565 Hz. In general, J(A-B) is inversely proportional to the basicity of the ligand L. The J(A-B) values are strongly temperature dependent, increasing rapidly over the temperature range 150-200 K. Coupling constants have not been evaluated at temperatures higher than 200 K due to the onset of A/B site exchange, which causes line broadening and eventual coalescence of the two resonances. In derivatives partially labeled with deuterium in the hydride ligands, no H-D coupling was resolved. In a tritiated derivative, ³H NMR was used to detect J(H-T), which was substantially reduced from the expected value. A neutron diffraction study of [(η-C₅H₅)Ir(PMe₃)H₃]BF₄ (21) has been carried out. Compound 21 crystallizes in the space group P2₁2₁2₁ (Z = 4) with a = 8.840 (2) A, b = 9.971 (1) A, c = 14.278 (2) A, and V = 1258.5 (5) A³ (130 K). The structure of 21 in the solid state is best described as a normal pseudo seven-coordinate trihydride, with the cyclopentadienyl ligand capping a square pyramid comprising the three hydride ligands and the P ligand. The shortest H-H distance between the hydride ligands is 1.674 (14) A. These results are interpreted in terms of the operation of quantum mechanical exchange coupling between the hydrogen nuclei in these complexes.