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

Cationic mononuclear Ni(II) complexes of the chelating N-heterocyclic carbe nes (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-e thylene-3, 3'-di-tert-butyldiimidazole-2,2'-diylidene (2)) have been prepar ed and structurally characterized. Reaction between NiCl2(PMe3)(2) and 1 eq uiv 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)de grees, respectively, are comparable to Ni(II) complexes of two-carbon-atom bridging chelating bis-phosphines. Reaction between NiCl2(PMe3)2 or NiBr2(D ME) 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 revea ls a square-planar geometry at the nickel atom, and the dication adopts a t rans double-boat like conformation. An analogous reaction between NiCl2(PMe 3)(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 influen ce of steric factors which determine the relative thermodynamic stability t oward dicarbene substitution of complexes So and 4a is also discussed.