Protonation and Hydrogenation Experiments with Iridium(0) and Iridium(−1) tropp Complexes: Formation of Hydrides

Abstract

Dedicated to the memory of Professor Luigi M. VenanziThe reactions of three different tetracoordinated Ir complexes, [Ir(tropp ph ) 2 ] n (n 1, 0, À 1), which differ in the formal oxidation state of the metal from 1 to À 1, with proton sources and dihydrogen were investigated (tropp 5-(diphenylphosphanyl)dibenzo[a,d]cycloheptene). It was found that the cationic 16-electron complex [Ir(tropp ph ) 2 ] (2) cannot be protonated but reacts with NaBH 4 to the very stable 18-electron Ir I hydride [IrH(tropp ph ) 2 ] (5), which is further protonated with medium strong acids to give the 18-electron Ir III dihydride [IrH 2 (tropp ph ) 2 ] (6; pK s in CH 2 Cl 2 /THF/H 2 O 1:1:2 ca. 2.2). Both, the neutral 17-electron Ir 0 complex [Ir(tropp ph ) 2 ] (3) and the anionic 18-electron complex [Ir(tropp ph ) 2 ] À (4) react rapidly with H 2 O to give the monohydride 5. In reactions of 3 with H 2 O, the terminal Ir I hydroxide [Ir(OH)(tropp ph ) 2 ] (8) is formed in equal amounts. All these complexes, apart from 5, which is inert, do 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. Interestingly, a salt-type hydride (i.e., LiH) is formed as further product in the unexpected reaction with [Li(thf) x ] -[Ir(tropp ph ) 2 ] À (4). Because 3 undergoes disproportionation into 2 and 4 according to 2 3>2 4 (K disp 2.71 0 À5 ), it is likely that actually the diamagnetic species and not the odd-electron complex 3 is involved in the reactions studied here, and possible mechanisms for these are discussed.Introduction. ± Rh and Ir complexes in their formal oxidation states of 0 and À 1 have a promising potential in bond-activation chemistry (for an early report on Reppetype hydroformylations with Rh 0 carbonyls, see [1]; for CÀH activation, see [2]; for electrochemical CO 2 reduction with [RhCl(dppe) 2 ], see [3]; for electrochemical CO 2 reduction with [MCl(CO)(PPh 3 ) 2 ] (M Rh,Ir), see [4]; for chemical reduction with [BrMg] [Rh(Ph 2 PÀ(Ch 2 ) n ÀPPh 2 ] À (n 2, 3), see [5]; for hydroformylation, see [6]), and they were discussed in the context of the photocatalytic H 2 O splitting in order to produce H 2 as a carrier of chemical energy (for photochemical H 2 O reduction, see [7]). However, relatively little is known about mononuclear complexes containing these metal centres in such formal oxidation states. Assuming that electron-withdrawing ligands should stabilize formally low metal-oxidation states, early work focused on the synthesis of the carbonyl species [M(CO) 4 ] and [M(CO) 4 ] À (M Rh, Ir). However, the paramagnetic d 9 -valence-electron-configured [M(CO) 4 ] complexes were only observed in inert-gas matrices at very low temperatures [8 ± 10]. On the other side, the colorless anionic d 10 -valence-electron-configured [M(CO) 4 ] À ions are quite stable and were

Domino Rhodium/Palladium‐Catalyzed Dehydrogenation Reactions of Alcohols to Acids by Hydrogen Transfer to Inactivated Alkenes

“…Some time ago we reported iridium(I) complexes containing (5 H ‐dibenzo[ a , d ]cyclohepten‐5‐yl)phosphanes (tropp Ph ) as rigid, concave‐shaped, mixed phosphane‐olefin ligands. These complexes show rather high activity in the hydrogenation of imines [turnover frequency (TOF) of >6000 h −1 ] 17. On the other hand, no or very‐low activity was found in transfer hydrogenations.…”

“…The catalysts from Table 1 (entries 3–11 and 13) were used as obtained from the commercial supplier. [Ir(tropp Ph ) 2 ]PF 6 was synthesized according to the reported method 17 . 1 H NMR spectra were recorded at 400, 300, or 250 MHz and chemical shifts were referenced to the residual solvent peak.…”