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

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-C^eth)][Br] (^MeC(H)-η¹-C^eth = ethylene[(N-methyl)imidazolium][(N-methyl)imidazole-2-ylidene] and ^tBuC(H)-η¹-C^eth = 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-C^eth)][Br] (R = Me, ^tBu) or [RhBr(P^∩P)(^MeC(H)-η¹-C^eth)][Br] (P^∩P = Ph₂PCH₂PPh₂, Ph₂PCH₂CH₂PPh₂, Et₂PCH₂PEt₂), respectively. The resulting diphosphine complexes readily decompose in solution. A series of palladium complexes [PdI_3-n(PR₃)_n(L)][I]_n (n = 1,2) and [PdI(P^∩P)(L)][I]₂ (L = ^tBuC(H)-η¹-C^meth, ^tBuC(H)-η¹-C^eth; ^tBuC(H)-η¹-C^meth = 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-C^meth)] and [PdI₃(^tBuC(H)-η¹-C^eth)] 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-^tBuCC^meth)RhI(COD)] (^tBu CC^meth = 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-C^meth)][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 Å. Reaction of this Rh/Pd complex with CO yields the corresponding dicarbonyl product [PdI₂(PEt₃)(μ-^tBuCC^meth)RhI(CO)₂] via replacement of the COD ligand. The related dicarbene-bridged Ir/Rh complex [IrBr(COD)(μ-^tBuCC^meth)RhBr(COD)] can be generated by reaction of [IrBr(COD)(^tBuC(H)-η¹-C^meth )][Br] with [Rh(μ-OAc)(COD)]₂, while the Pd/Ir complexes [PdI₂(PR₃)(μ-^tBuCC^meth)IrI(COD)] (PR₃ = PPh₃, PMe₂Ph) can be generated by reaction of the monometallic [PdI₂(PR₃)(^tBuC(H)-η¹-C^meth)][I] species with K[N(SiMe₃)₂] in the presence of [Ir(μ-Cl)(COD)]₂. The carbonyl analogues, [PdI₂(PR₃)(μ-^tBuCC^meth)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.