Carbene-anchored/pendent-imidazolium species as precursors to di-N-heterocyclic carbene-bridged mixed-metal complexes?

Dalton Transactions Pub Date: 2009-08-04 DOI: 10.1039/B906884H

Abstract

Reaction of a series of linked diimidazolium dibromide salts with one-half equivalent of [Rh(μ-OAc)(COD)]2 under reflux conditions generates a series of carbene-anchored/pendent-imidazolium complexes, [RhBr(COD)(RC(H)-η1-Ceth)][Br] (MeC(H)-η1-Ceth = ethylene[(N-methyl)imidazolium][(N-methyl)imidazole-2-ylidene] and tBuC(H)-η1-Ceth = ethylene[(N-tert-butyl)imidazolium][(N-tert-butyl)imidazole-2-ylidene]) via deprotonation of one end of the diimidazolium salt and coordination of the resulting carbene to Rh. Reaction of these complexes with carbon monoxide or the appropriate diphosphine yields either [RhBr(CO)2(RC(H)-η1-Ceth)][Br] (R = Me, tBu) or [RhBr(P∩P)(MeC(H)-η1-Ceth)][Br] (P∩P = Ph2PCH2PPh2, Ph2PCH2CH2PPh2, Et2PCH2PEt2), respectively. The resulting diphosphine complexes readily decompose in solution. A series of palladium complexes [PdI3-n(PR3)n(L)][I]n (n = 1,2) and [PdI(P∩P)(L)][I]2 (L = tBuC(H)-η1-Cmeth, tBuC(H)-η1-Ceth; tBuC(H)-η1-Cmeth = methylene[(N-tert-butyl)imidazolium][(N-tert-butyl)imidazole-2-ylidene]), containing the linked NHC-imidazolium moiety, have also been prepared by reacting the triiodo complexes, [PdI3(tBuC(H)-η1-Cmeth)] and [PdI3(tBuC(H)-η1-Ceth)] with several mono- and diphosphines. Attempts to generate mixed Rh/Pd complexes using Pd(OAc)2 to deprotonate the pendent arm of several of the above carbene-anchored/pendent-imidazolium complexes of Rh have proven unsuccessful. However, a targeted di-NHC-bridged heterobimetallic complex [PdI2(PEt3)(μ-tBuCCmeth)RhI(COD)] (tBu CCmeth = 1,1′-methylene-3,3′-di-tert-butyldiimidazol-2,2′-diylidene) can be generated by deprotonation of the imidazolium group in [PdI2(PEt3)(tBuC(H)-η1-Cmeth)][I] using half an equivalent of [Rh(μ-OAc)(COD)]2. The X-ray structure determination of this Pd/Rh complex confirms the dicarbene-bridged formulation and shows a metal-metal separation of approximately 6.2 ?. Reaction of this Rh/Pd complex with CO yields the corresponding dicarbonyl product [PdI2(PEt3)(μ-tBuCCmeth)RhI(CO)2] via replacement of the COD ligand. The related dicarbene-bridged Ir/Rh complex [IrBr(COD)(μ-tBuCCmeth)RhBr(COD)] can be generated by reaction of [IrBr(COD)(tBuC(H)-η1-Cmeth )][Br] with [Rh(μ-OAc)(COD)]2, while the Pd/Ir complexes [PdI2(PR3)(μ-tBuCCmeth)IrI(COD)] (PR3 = PPh3, PMe2Ph) can be generated by reaction of the monometallic [PdI2(PR3)(tBuC(H)-η1-Cmeth)][I] species with K[N(SiMe3)2] in the presence of [Ir(μ-Cl)(COD)]2. The carbonyl analogues, [PdI2(PR3)(μ-tBuCCmeth)IrI(CO)2], can be generated via a gentle purge of CO gas. These di-NHC-bridged heterobimetallic species represent some of the first examples of this class and are the first involving palladium.

Graphical abstract: Carbene-anchored/pendent-imidazolium species as precursors to di-N-heterocyclic carbene-bridged mixed-metal complexes
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