STRUCTURAL AND SPECTROSCOPIC CHARACTERIZATION OF IRIDIUM TRIHYDRIDE COMPLEXES - EVIDENCE FOR PROTON-PROTON EXCHANGE COUPLING

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 hydride 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 JA-B derived by computer fitting of the observed spectra are extremely large, ranging from 61 to 1565 Hz. In general, 7A-B is inversely proportional to the basicity of the ligand L. The JA-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 JH-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) Å, c = 14.278 (2) Å, and V = 1258.5 (5) Å3 (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) Å. These results are interpreted in terms of the operation of quantum mechanical exchange coupling between the hydrogen nuclei in these complexes. © 1990, American Chemical Society. All rights reserved.