Synthesis and reactivity of copper(I) complexes containing a bis(imidazolin-2-imine) pincer ligand

The new pincer ligand 2,6- bis[( 1,3- di- tert- butylimidazolin- 2- imino) methyl] pyridine ( TLtBu) has been prepared in high yield from 2,6- bis( hydroxymethyl) pyridine ( 1) and 1,3- di- tert- butylimidazolin- 2- imine ( 3). Reaction of TLtBu with [ Cu( MeCN)(4)] PF6 affords the highly reactive copper( I) complex [( TLtBu) Cu] PF6, [ 5] PF6, which forms the stable copper( I) isocyanide complexes [ 6a] PF6 ( v(CN) = 2179 cm(-1)) and [ 6b] PF6 ( v(CN) = 2140 cm(-1)) upon addition of tert- butyl or 2,6- dimethylphenyl isocyanide, respectively. For the cations 6a and 6b, DFT calculations reveal ground- state electronic structures of the type [( TLtBu- kappa N-1: kappa N-2) Cu( CNR)] with tricoordinate geometries around the copper atoms. Exposure of [ 5] PF6 to the air readily leads to trapping of atmospheric CO2 to form the square- planar complex [( TLtBu) Cu( HCO3- kappa O)] PF6, [ 7] PF6, with the bicarbonate ligand adopting a rarely observed monodentate coordination mode. In chlorinated solvents such as dichloromethane or chloroform, [ 5] PF6 rapidly abstracts chloride by reductive dechlorination of the solvent to yield [( TLtBu) CuCl] PF6, [ 8] PF6 quantitatively. Reaction of TLtBu with copper( I) bromide or chloride affords complexes 9a and 9b, respectively, for which X- ray diffraction analysis, low-temperature NMR experiments and DFT calculations reveal the presence of a kappa(2)- coordinated ligand of the type [( TLtBu - kappa N-1: kappa N-2) CuX]. In solution, complex 9b undergoes slow disproportionation forming the mixed- valence copper( II)/ copper( I) system [( TLtBu) CuCl][ CuCl2], [ 8] CuCl2 with a linear dichlorocuprate( I) counterion.