Synthesis and Structures of NCN Pincer-Type Ruthenium and Iridium Complexes Bearing Protic Pyrazole Arms

The reaction of the 1,3-bis­(pyrazol-3-yl)­benzenes 1 with [RuCl­(OAc)­(PPh3)3] resulted in selective C–H cleavage at the 2-position of 1 to give the protic NCN pincer-type ruthenium­(II) complexes [RuCl­(R-NCN-LH2)­(PPh3)2] (2; R-NCN-LH2 = 4-R-2,6-bis­(5-tert-butyl-1H-pyrazol-3-yl)­phenyl). Similar cyclometalation with iridium trichloride followed by addition of triphenylphosphine led to the formation of the iridium­(III) analogue [IrCl­(But-NCN-LH2)­(PPh3)]Cl (5). Treatment of the ruthenium complexes 2 with carbon monoxide afforded the carbonyl complexes [Ru­(CO)­(R-NCN-LH2)­(PPh3)2]Cl (4). On the other hand, the pyridine analogue of 1, 2,6-bis­(5-tert-butyl-1H-pyrazol-3-yl)­pyridine (NNN-LH2), reacted with iridium trichloride to yield the protic NNN pincer-type complex [IrCl3(NNN-LH3)] (7). The stronger σ donation of the NCN pincer-type ligand in comparison with the analogous NNN ligand was suggested by the CO stretching frequencies of the ruthenium carbonyl complexes 4 as well as the M–Cl distances. The catalytic activity of the ruthenium complexes 2a,b toward transfer hydrogenation of a ketone was also evaluated.