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Metal Complexes of Chiral NHCs Containing a Fused Six- and Seven-Membered Central Ring

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posted on 28.06.2010, 00:00 by Paul D. Newman, Kingsley J. Cavell, Benson M. Kariuki
Azolium precursors to expanded-ring NHCs incorporating a chiral bicyclic skeleton derived from (1R)-camphor with N,N′-dibenzyl or N,N′-dipyridyl substituents have been prepared and used for the synthesis of a number of metal complexes of the resultant NHCs. The bicyclic framework contains two fused rings, one of which is seven-membered while the other is six-membered. Deprotonation of (1R,5S)-2,4-dibenzyl-1,8,8-trimethyl-4-aza-2-azoniabicyclo[3.2.1]oct-2-ene hexafluorophosphate, 1a·HPF6, produces a solution of “free carbene”, 1a, that can be used for the preparation of [Ag(1a)2]OTf (2a), [Rh(1,5-COD)(1a)Cl] (3a), [Rh(CO)2(1a)Cl] (4a), and [Ir(1,5-COD)(1a)Cl] (5a) complexes. The fused expanded ring NHC ligands display properties and reactivity patterns more akin to those of five-membered NHCs rather than monocyclic six- or seven-membered NHCs. Consistent with this premise, the red Ni(II) pincer complex [Ni(η3-C8H13)(1b)]PF6, 6b, was obtained via oxidative addition of the azolium salt 1b·HPF6 to Ni(1,5-COD)2 in THF. This is the first unambiguous example of a saturated and/or expanded ring NHC oxidatively adding to a low-valent metal center. A single-crystal X-ray structure determination of 6b showed the NHC to be coordinated in a pincer fashion through the carbene and two pyridine nitrogens at a square-pyramidal nickel(II) center. One of the original 1,5-COD ligands has undergone a 1,5 to 1,3 isomerization before migratory insertion of one of the alkene groups into the Ni−H function, generated on oxidative addition of the azolium salt, to give the allylic η3-C8H13 ligand. When 6b was dissolved in CHCl3 or CH2Cl2 in air, the allyl ligand was lost and a chloride was abstracted from the solvent to give yellow square-planar [Ni(1b)Cl]PF6, 7b, the structure of which was confirmed by single-crystal X-ray techniques.