Arran A. D. Tulloch

N-Functionalised heterocyclic carbene complexes of silver

N-Functionalised imidazol-2-ylidene carbene complexes of AgI have been prepared by interaction of the corresponding imidazolium salt with Ag2O in dichloromethane or 1,2-dichloroethane in the presence of molecular sieves or with Ag2CO3 in 1,2-dichloroethane. In an analogous way disilver(I) complexes of o-phenylenedimethylenebis(imidazol-2-ylidene) have been obtained. Their structures, as determined by X-ray crystallography, indicate that the carbene ligand acts as monodentate from the carbene end or as a bridge between two different metal atoms. In one case the silver is coordinated by two carbene ligands. In the majority of the structurally characterised complexes the metal centres adopt linear geometries with M–C(carbene) bond lengths in the range of 207–210 pm characteristic of a single bond.

Chiral 2,6-lutidinyl-biscarbene complexes of palladium

Chiral complexes of palladium, 1, with the new tridentate ‘pincer’ ligand 2,6-lutidinyl-biscarbene (C⁁N⁁C), have been prepared; in the solid state they exhibit helical C2 symmetrical structures which are persistent in solution at least up to 80 °C; the chiral nature of 1 has been established by NMR methods using Pirkle’s acid as a chiral discriminating agent; racemic mixtures of 1 are highly active catalysts in Heck coupling reactions.

Pyridine functionalised N-heterocyclic carbene complexes of palladium

The pyridine functionalised N-heterocyclic carbene complexes (C–N)PdMeBr, C–N = 3-R-1-(2-picolyl)imidazolin-2-ylidene, (R = But 1a, R = mes 1b, mes = mesityl) are described; they are excellent catalysts for the Heck arylation; 1b is monomeric with a chelating C–N ligand; the product obtained by interaction of 1a with AgO2CCF3 shows a [Pd(C–N)]4 framework with bridging C–N ligands.

Chelating and ‘pincer’ dicarbene complexes of palladium; synthesis and structural studies

A series of ‘pincer’ 2,6-pyridyl dicarbene complexes of the type [(C–N–C)PdX]+, 4a, 4b, 2,6-lutidinyl dicarbene complexes of the type [(C⁁N⁁C)PdX]+, 4c, 4d, 1,3-xylyl dicarbene complexes of the type [(C⁁C⁁C)PdX], 5a,5b,5c and 1,2-xylyl dicarbene complexes [(CC)PdX2], 8, where carbene is 3-arylimidazol-2-ylidene, aryl = mesityl, 2,6-Pri2C6H3, X = Br or Cl, have been prepared by (i) transmetallation from the corresponding silver complexes, i.e. [(C–N–C)Ag2Cl2], [(C⁁N⁁C)Ag2Cl2] and [(CC)Ag2Cl2] to palladium, (ii) substitution of cod in (cod)PdX2 by the ‘pincer’ (C–N–C) (iii) palladation of bis-imidazolium salts [C(H)⁁C(H)⁁C(H)] in the presence of base. The crystal structures show that the ligands act as chelates in 4–5, while in 8 as both chelate and bridging.

Functionalised and chelate heterocyclic carbene complexes of palladium; synthesis and structural studies

A series of picolyl-functionalised, [(C^N)PdXY], 1a, 2a, 3a and 4, pyridyl-functionalised, [(C–N)PdXY], 5a, 6, 7 and 8, methoxymethyl-functionalised, [(C^O)Pd(μ-X)X]2, 10, and trans-[(C^O)2PdX2], 11, and diethylcarbamoylmethyl-functionalised, trans-[(C^amide)2PdX2], 12, N-heterocyclic carbene (NHC) complexes of palladium(II), (C^N) = (3-R1)[1-(α-picolyl)imidazol-2-ylidene, (C–N) = (3-R1)[1-(2-pyridyl)imidazol-2-ylidene, R1 = But, mesityl, 2,6-Pri2C6H3, (C^O) = (3-R1)(1-methoxymethyl)imidazol-2-ylidene, R1 = 2,6-Pri2C6H3, (C^amide) = (3-R1)(1-diethylcarbamoylmethyl)imidazol-2-ylidene, R1 = 2,6-Pri2C6H3, Y = Me, X = Br or Cl, have been synthesised by interaction of (1,5-cod)PdXY or (1,5-cod)PdX2 with the corresponding silver NHC complexes, or (in certain cases) with the free functionalised NHC ligands. Derivatives of 1a, 2a, 3a and 5a were prepared by substitution of the halide X by weakly coordinating anions. The majority of the complexes described here were characterised by spectroscopic and diffraction methods and show that the (C–N) and (C^N) ligands act as chelates, while the methoxymethyl- and diethylcarbamoylmethyl-functions in 10, 11 and 12, respectively, are dangling. Derivatives of 5a in which the pyridine ring is substituted with electron withdrawing or bulky substituents show only minor variation of the pyridine–palladium distances.