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

Abstract

Azolium precursors to expanded-ring NHCs incorporating a chiral bicyclic skeleton derived from (1 R )-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 (1 R,5 S )-2,4-dibenzyl-1,8,8-trimethyl-4-aza-2-azoniabicyclo[3.2.1]oct-2-ene hexafluorophosphate, 1a ·HPF 6, 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(η 3 -C 8 H 13 )( 1b )]PF 6, 6b, was obtained via oxidative addition of the azolium salt 1b ·HPF 6 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 η 3 -C 8 H 13 ligand. When 6b was dissolved in CHCl 3 or CH 2 Cl 2 in air, the allyl ligand was lost and a chloride was abstracted from the solvent to give yellow square-planar [Ni( 1b )Cl]PF 6, 7b, the structure of which was confirmed by single-crystal X-ray techniques.