Protonation and hydrogenation experiments with iridium(0) and iridium(-1) tropp complexes: Formation of hydrides

The reactions of three different tetracoordinated Ir complexes, [Ir(tropp(p h))(2)](n) (n = +1.0, -1), which differ in the formal oxidation state of th e metal from +1 to -1, with proton sources and dihydrogen were investigated (tropp = 5-(diphenylphosphanyl)dibenzo[a,d]cycloheptene). It was found tha t the cationic 16-electron complex [Ir(tropp(ph))(2)](+) (2) cannot be prot onated but reacts with NaBH4 to the very stable 18-electron Ir-I hydride (I rH(tropp(ph))(2)] (5), which is further protonated with medium strong acids to give the 18-electron Ir-III dihydride [IrH,(tropp(ph))(2)](+) (6; pK(5) in CH2Cl2/THF/H2O 1:1:2 ea. 2.2). Both, the neutral 17-electron Ill comple x [Ir(tropp(ph))(2)] (3) and the anionic 18-electron complex [Ir(tropp(ph)) (2)](-) (4) react rapidly with H2O to give the monohydride S. In reactions of 3 with H2O, the terminal Ir-I hydroxide [Ir(OH)(tropp(ph))(2)] (8) is fo rmed in equal amounts. All these complexes, apart from 5, which is inert, d o react rapidly with dihydrogen. The complex 2 gives the dihydride 6 in an oxidative addition reaction, while 3, 4, and 8 give the monohydride 5. Inte restingly, a salt-type hydride (i.e., LiH) is formed as further product in the unexpected reaction with [Li(thf)(x)](+)-Ir(tropp(ph))(2)](-) (4). Beca use 3 undergoes disproportionation into 2 and 4 according to 2 3 reversible arrow 2 + 4 (K-disp = 2.7 . 10(-5)), it is likely that actually the diamag netic species and not the odd-electron complex 3 is involved in the reactio ns studied here, and possible mechanisms for these are discussed.