Evidence for a Dinuclear Mechanism in Alkyne Hydrogenations Catalyzed by Pyrazolate-Bridged Diiridium Complexes

Abstract

The products obtained from the sequential reaction of [Ir2(mu-H)(mu-Pz)2H3(NCCH3)(PiPr3)2] (1) with diphenylacetylene and their subsequent reactions with hydrogen have been investigated in order to deduce the mechanisms operating in the hydrogenation reactions catalyzed by 1. The reaction of 1 with an excess of diphenylacetylene gives cis-stilbene and [Ir2(mu-H)(mu-Pz)2-[eta1-C6H4-2-[eta1-(Z)-C=CHPh]]((Z)-C(Ph) =CHPh](NCCH3)(PiPr3)2] (2), the structure of which has been determined by X-ray diffraction. The formation of 2 involves the intermediate species [Ir2(mu-H)(mu-Pz)2H2((Z)-C(Ph)=CHPh](NCCH3)-(PiPr3)2](3),[Ir2(mu-H)(mu-Pz)2H[(Z)-C(Ph)=CHPh]2(NCCH3)(PiPr3)2] (4), and [Ir2(mu-H)(mu-Pz)2H[eta1-C6H4-2-[eta1-(Z)-C=CHPh](NCCH3)(PiPr3)2] (5), which have been isolated and characterized. These three complexes react with hydrogen to give cis-stilbene and 1 and are possible intermediates of the diphenylacetylene hydrogenation under catalytic conditions. Nevertheless, the rate of formation of 5 is very slow compared with the rate of catalytic hydrogenation, which excludes its participation during catalysis. Compound 2 also reacts with hydrogen in benzene, but in this case the hydrogenation gives 1,2-diphenylethane as the sole organic product. The course of this reaction in acetone has been investigated, and deuteration experiments were carried out. The formation of [Ir2(mu-H)(mu-Pz)2H[eta1-C6H4-2-[eta1-(Z)-C=CHPh]](OC(CD3)2)(PiPr3)2] (6) and [Ir2(mu-H)(mu-Pz)2H[eta1-C6H4-2-[eta1-(Z)-C-CHPh]](NCCH3)(PiPr3)2] (7) was observed under these conditions. The experimental evidence obtained supports two alternative mechanisms for the alkyne hydrogenation catalyzed by 1, one of them being dinuclear and the other mononuclear. The experimental data suggest that the former is favored.

Half-sandwich binuclear carbaborane compounds: Closo-carbaboranes as good σ-donar ligands

“…The reaction of the dimeric metal complexes and 1 equiv of the corresponding lithium compound Li 2 Cab C,C (Scheme 1) results in the formation of the half-sandwich C,C-chelated metal complexes [Cp * Rh(l-Cl)] 2 -(Cab C,C ) (2), [Cp 0 Rh(l-Cl)] 2 (Cab C,C ) (3), [Cp * Ir(l-Cl)] 2 (Cab C,C ) (4) and [(p-cymene)Ru(l-Cl)] 2 (Cab C,C ) (5) as shown in Scheme 1. The complexes 2-5 have been isolated as related stable at room temperature, orange, transparent crystals.…”

“…Crystallographic data and processing parameters are given in Table 1. The ORTEP diagrams of [2][3][4][5] show that the four complexes have similar structures. The dinuclear complexes bridged by one carborane chelate ligand and two chloride atoms.…”

“…In fact, binuclear metal complexes are known to be active catalysts for a variety of transformations, in particular hydroformylation catalyzed by Rh 2 [1] and Ru 2 [2] complexes; alkene [3] and alkyne [4] hydrogenation catalyzed by Ir 2 complexes. One of the main reasons is the interest in the cooperative influence [5] of neighboring metal centers on catalytic reactions.…”

Highly Selective Hydrogenation of Carbon–Carbon Multiple Bonds Catalyzed by the Cation [(C6Me6)2Ru2(PPh2)H2]+: Molecular Structure of [(C6Me6)2Ru2(PPh2)(CHCHPh)H]+, a Possible Intermediate in the Case of Phenylacetylene Hydrogenation

“…[21] The difference, however, in their proposal is the coordination of the alkyne ligand to one metal center, [21] whereas in 2 the alkyne has inserted into a bridging hydride and coordinates to both metal centers. Duckett, Dyson, and co-workers have shown that with parahydrogen, for catalytic hydrogenation reactions with ruthenium clusters, polar solvents favor the catalytic reaction taking place at the intact ruthenium backbone, whereas unpolar solvents favor fragmentation to give mononuclear intermediates.…”

Homo- and Heterobimetallic Precursor Catalysts for the Heck Reaction, and a Proposal for a General Catalytic Cooperativity Index

“…The majority of these systems employ homobimetallic catalyst precursors. Representative examples include bimetallic catalysts for phosphoester hydrolysis (Co 2 , [39] Cu 2 , [40,41] Zn 2 [42] ), nitrile hydration (Ni 2 , [43,44] Pd 2 [45] ), hydroformylation (Rh 2 , [25,46±48] Ru 2 [49] ), alkene [50] and alkyne [51] hydrogenation (Ir 2 ), and asymmetric epoxide ring-opening (Cr 2 ). [52] In addition, cooperative effects between two different metal centres have been invoked but not proven to be operative in the catalytic hydrogenation of cyclohexene (RuIr [53] ) and in hydroformylations (RhPd [49] ).…”