A CASE OF C-H ACTIVATION (ORTHO-METALATION) WHICH IS REVERSIBLE AT 25-DEGREES-C

The reaction of LiC2Ph with Ir(H)(2)ClL2 (L = p(t)Bu(2)Ph) gives Ir(H) 2(C2Ph)L-2, which readily loses H-2 to form the ortho-metalated specie s IrH(eta 2-(C6H4PBu2)-Bu-t)(C2Ph)L. This molecule is unique in showin g (sp(2))C/H reductive elimination/oxidative addition, which is therma lly reversible at 25 degrees C. Line shape analysis of the P-31{H-1} N MR spectra yield Delta H double dagger = 12.3 (+/-0.4) kcal mol(-1) an d Delta S double dagger = -2.0 (+/-1.1) cal deg(-1) mol(-1) for this p rocess. This implicates the 14-electron species Ir(C2Ph)L-2 as highly reactive, yet more easily accessible, than IrCIL2 (which ortho metalla tes essentially irreversibly). The strained four-membered ring in IrH( eta(2)-C6H4P-Bu-t(2))(C2Ph)L reacts readily with PhC2H to give IrH(C2P h)(2)L-2, another unsaturated molecule essentially devoid of pi-stabil ization. The structure of this molecule, a square pyramid with apical hydride (proton NMR chemical shift -44 ppm), minimizes the Lewis acidi ty of such a species. Crystal data (-181 degrees C): a = 9.616(1) Angs trom, b = 23.032(2) Angstrom, c = 8.886(1) Angstrom, alpha = 95.18(1)d egrees, beta = 99.03(1), gamma = 98.16(1), with Z = 2 in space group P (1) over bar.