The bis-imidazolium salt, 1,1-bis(N-methylimidazolium) acetate bromide, is a convenient precursor for the synthesis of zwitterionic iridium(I) [Ir(cod){(MeIm) 2 CHCOO}] and cationic iridium(III) [IrH(cod){(MeIm) 2 CHCOO}] + compounds (MeIm = 3-methylimidazol-2-yliden-1-yl) having a carboxylate-functionalized bis(NHC) ligand. The [Ir(cod){(MeIm) 2 CHCOO}] compound catalyzes the hydrogenation of CO 2 to formate in water using NEt 3 as base reaching turnover numbers of approximately 1500. Reactivity studies have shown that activation of the catalyst precursor involves the reaction with H 2 in a multistep process that result, under catalytic conditions, in the formation of a dihydrido iridium(III) octahedral [IrH 2 (H 2 O){(MeIm) 2 CHCOO}] species stabilized by the  3-C,C',O coordination of the ligand. DFT studies on the mechanism were carried out to elucidate two possible roles of the base. In the first one, NEt 3 neutralizes only the produced formic acid whereas in the second it assists the proton transfer in heterolytic cleavage of the H 2 molecule. Although this base-involved mechanism is more favourable exhibiting a lower energy span for the overall reaction, the energy barrier obtained from kinetic experiments suggests that both mechanisms could be operative under the experimental reaction conditions. RESULTS AND DISCUSSION Synthesis and reactivity of iridium complexes bearing a carboxylate-functionalized bis(NHC) ligand. The carboxylate-functionalized bis-(imidazolium) salt precursor was prepared according to the general synthetic method entailing the alkylation of N-alkylimidazole with a suitable functionalized alky bromide. 26 Thus, reaction of ethyl dibromoacetate with an excess of Nmethylimidazole in THF at 343 K for 72 h gave a brown slurry from which the salt 1,1'-bis(N-methylimidazolium) acetate bromide, [(MeImH) 2 CHCOO] Br (1), was obtained as a white hygroscopic solid in 79% yield after recrystallization from methanol/acetone (Scheme 1). Scheme 1. Synthesis of 1,1-bis(N-methylimidazolium) acetate bromide. ASSOCIATED CONTENT Supporting Information. NMR spectra for the new compounds and reactivity studies. Detailed information on the determination of activation parameters for the hydrogenation of CO 2. Hydrogen bonds in the molecular structure of 2. Electronic energy, enthalpy, free energy and optimized coordinates for catalytic intermediates and transition states. The Supporting Information is available free of charge on the ACS Publications website.

The catalytic activity in the hydrosilylation of terminal alkynes by the unsaturated hydrido iridium­(III) compound [IrH­(κ3-hqca)­(coe)] (1), which contains the rigid asymmetrical dianionic ONO pincer ligand 8-oxidoquinoline-2-carboxylate, has been studied. A range of aliphatic and aromatic 1-alkynes has been efficiently reduced using various hydrosilanes. Hydrosilylation of the linear 1-alkynes hex-1-yne and oct-1-yne gives a good selectivity toward the β-(Z)-vinylsilane product, while for the bulkier t-Bu-CCH a reverse selectivity toward the β-(E)-vinylsilane and significant amounts of alkene, from a competitive dehydrogenative silylation, has been observed. Compound 1, unreactive toward silanes, reacts with a range of terminal alkynes RCCH, affording the unsaturated η1-alkenyl complexes [Ir­(κ3-hqca)­(E-CHCHR)­(coe)] in good yield. These species are able to coordinate monodentate neutral ligands such as PPh3 and pyridine, or CO in a reversible way, to yield octahedral derivatives. Further mechanistic aspects of the hydrosilylation process have been studied by DFT calculations. The catalytic cycle passes through Ir­(III) species with an iridacyclopropene (η2-vinylsilane) complex as the key intermediate. It has been found that this species may lead both to the dehydrogenative silylation products, via a β-elimination process, and to a hydrosilylation cycle. The β-elimination path has a higher activation energy than hydrosilylation. On the other hand, the selectivity to the vinylsilane hydrosilylation products can be accounted for by the different activation energies involved in the attack of a silane molecule at two different faces of the iridacyclopropene ring to give η1-vinylsilane complexes with either an E or Z configuration. Finally, proton transfer from a η2-silane to a η1-vinylsilane ligand results in the formation of the corresponding β-(Z)- and β-(E)-vinylsilane isomers, respectively.