Cationic nickel(II) complexes of chelating N-heterocyclic carbenes

Cationic mononuclear Ni(II) complexes of the chelating N-heterocyclic carbenes (CCmeth)-C-tBu and (CCeth)-C-tBu (where (CCmeth)-C-tBu = 1,1'-methylene-3,3'-di-tert-butyldiimidazole-2,2'-diylidene (1) and (CCeth)-C-tBu = 1,2-ethylene-3, 3'-di-tert-butyldiimidazole-2,2'-diylidene (2)) have been prepared and structurally characterized. Reaction between NiCl2(PMe3)(2) and 1 equiv of 1 or 2 gives; the monocationic salts [((CCmeth)-C-tBu)NiCl(PMe3)]Cl (3a) and [((CCeth)-C-tBu)NiCl(PMe3)]Cl (4a), which undergo salt metathesis with TlBPh4 to give [((CCmeth)-C-tBu)NiCl(PMe3)][BPh4] (3b) and [((CCeth)-C-tBu)NiCl(PMe3)][BPh4] (4b). A comparison of the X-ray structures of 3a and 4b shows that the bite angles at the nickel atoms, 84.92(18) and 88.4(4)degrees, respectively, are comparable to Ni(II) complexes of two-carbon-atom bridging chelating bis-phosphines. Reaction between NiCl2(PMe3)2 or NiBr2(DME) and 2 equiv of 1 gives the Ni(II) dicationic salts [Ni((CCmeth)-C-tBu)(2)](-) [X](2) (X = Cl- (5a), X = Br- (5b)). The X-ray structure of 5b reveals a square-planar geometry at the nickel atom, and the dication adopts a trans double-boat like conformation. An analogous reaction between NiCl2(PMe3)(2) and 2 equiv of 2 gives 4a and does not yield the dication [Ni((CCeth)-C-tBu)(2)][Cl](2). Reaction between 4a and 2 equiv of 1 gives 5a, whereas no reaction under analogous conditions occurs between 3a and 2. The influence of steric factors which determine the relative thermodynamic stability toward dicarbene substitution of complexes So and 4a is also discussed.