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(mu-OAc)(COD)](2) under reflux conditions generates a series of carbene-anchored/pendent-imidazolium complexes, [RhBr(COD)((R)C(H)-eta(1)-C(eth))][Br] ((Me)C(H)-eta(1)-C(eth) = ethylene[(N-methyl)imidazolium][(N-methyl)imidazole-2-ylidene] and (tBu)C(H)-eta(1)-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)((R)C(H)-eta(1)-C(eth))][Br] (R = Me, (t)Bu) or [RhBr(P( intersection)P)((Me)C(H)-eta(1)-C(eth))][Br] (P( intersection)P = Ph(2)PCH(2)PPh(2), Ph(2)PCH(2)CH(2)PPh(2), Et(2)PCH(2)PEt(2)), respectively. The resulting diphosphine complexes readily decompose in solution. A series of palladium complexes [PdI(3-n)(PR(3))(n)(L)][I](n) (n = 1,2) and [PdI(P( intersection)P)(L)][I](2) (L = (tBu)C(H)-eta(1)-C(meth), (tBu)C(H)-eta(1)-C(eth); (tBu)C(H)-eta(1)-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, [PdI(3)((tBu)C(H)-eta(1)-C(meth))] and [PdI(3)((tBu)C(H)-eta(1)-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 [PdI(2)(PEt(3))(mu-(tBu)CC(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 [PdI(2)(PEt(3))((tBu)C(H)-eta(1)-C(meth))][I] using half an equivalent of [Rh(mu-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 A. Reaction of this Rh/Pd complex with CO yields the corresponding dicarbonyl product [PdI(2)(PEt(3))(mu-(tBu)CC(meth))RhI(CO)(2)] via replacement of the COD ligand. The related dicarbene-bridged Ir/Rh complex [IrBr(COD)(mu-(tBu)CC(meth))RhBr(COD)] can be generated by reaction of [IrBr(COD)((tBu)C(H)-eta(1)-C(meth) )][Br] with [Rh(mu-OAc)(COD)](2), while the Pd/Ir complexes [PdI(2)(PR(3))(mu-(tBu)CC(meth))IrI(COD)] (PR(3) = PPh(3), PMe(2)Ph) can be generated by reaction of the monometallic [PdI(2)(PR(3))((tBu)C(H)-eta(1)-C(meth))][I] species with K[N(SiMe(3))(2)] in the presence of [Ir(mu-Cl)(COD)](2). The carbonyl analogues, [PdI(2)(PR(3))(mu-(tBu)CC(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.

Oxidative Addition of 2‐Halogenoazoles—Direct Synthesis of Palladium(II) Complexes Bearing Protic NH,NH‐Functionalized NHC Ligands

“…The metric parameters found in trans- [4]BF 4 (Figure 2) fall in the typical range observed for square-planar Pd II trans-diphosphine complexes bearing NHC donors. [13] The C1-Pd-I (178.67(17)8) and P1-Pd-P2 (178.18 (6)8) angles deviate only slightly from linearity indicating the presence of an almost perfectly square-planar complex cation. The bond angle N1-C1-N2 measures 103.8 (5)8 which is smaller than the N-C-N angle in trans- [2]BF 4 but a typical value for complexes bearing imidazoline-derived protic NHC ligands.…”

“…This value is in good agreement with previously recorded resonances for the N-H protons of protic benzimidzolin-2ylidene ligands. [5a,b,d, 6a] The 13 C{ 1 H} NMR spectrum shows a triplet for the C NHC atom at d = 165.9 ppm (t, 2 J C,P = 9.1 Hz in CD 2 Cl 2 /[D 6 ]DMSO) indicating coupling to the two chemically equivalent phosphorus atoms which also implies a trans arrangement of the phosphine donors. Consequently, only one singlet at d = 21.2 ppm was detected in the 31 P{ 1 H} NMR spectrum.…”

“…[4a,b] Complex trans- [4]BF 4 is quite stable at ambient temperature in moderately polar solvents like dichloromethane and THF but decomposes in dimethyl sulfoxide solution after a couple of weeks. Its 1 H NMR spectrum reveals the characteristic resonance for the N-H protons at d = 11.90 ppm and its 13 C{ 1 H} NMR spectrum shows the resonance for the C2 carbene carbon atom at d = 156.5 ppm (t, 2 J C,P = 9.1 Hz in CD 2 Cl 2 /[D 6 ]DMSO) as a triplet featuring (2), Pd-C1 1.963 (16), N1-C1 1.360(18), N2-C1 1.306(18); Cl-Pd-P 90.19 (10), Cl-Pd-C1 176.1(5), P-Pd-P* 178.95 (19), P-Pd-C1 89.77 (10), N1-C1-N2 106.7 (13).…”

“…[1] However, the reaction sequence selected here illustrates the facile alkylation of the coordinated NH,NH-NHC ligands and their synthetic utility. 13 C{ 1 H} NMR spectra (see also the Supporting Information) of complexes trans- [5]BF 4 and trans- [6]BF 4 exhibit a triplet for the carbene carbon resonances at d = 171.9 ppm (t, 2 J C,P = 9.1 Hz in CD 2 Cl 2 ) and slightly downfield-shifted at d = 174.2 ppm (t, 2 J C,P = 9.0 Hz in CD 2 Cl 2 ), respectively. These values are only slightly downfield from the C NHC resonance recorded for complex trans- [2]BF 4 at d = 165.9 ppm (t, 2 J C,P = 9.1 Hz) illustrating that the electronic situation in palladium(II) complexes bearing NH,NH-, NH,NMe-, and NMe,NMe-NHC ligands is rather similar.…”

Heterometallic complexes, tandem catalysis and catalytic cooperativity

“…The sequential deprotonation/coordination of a methyleneor ethylene-bridged di-azolium salt was used by Cowie and coworkers to prepare a series of heterodimetallic complexes with rhodium-palladium, 14a iridium-palladium 21 and rhodiumiridium 14b metal centers. In these studies, the authors were able to prepare complexes from the related di-imidazolium, ditriazolium and mixed imidazolium-triazolium salts to afford the desired heterodimetallic complexes.…”