Nickel(II) Complexes with Tritopic N^imine C^NHC N^amine Pincer Ligands

Tritopic NC^NHC N' ligands containing a central N-heterocyclic carbene (NHC) donor flanked by two chemically-different nitrogen donors (N^imine and N^amine ) were evaluated as potential tridentate pincer-type ligands in Ni^II complexes. The imidazolium chloride [(ImH){C(Me)=NDipp}(C₂ NMe₂ )]Cl (1) and the ammonium-imidazolium dichlorides [(ImH){C(Me)=NDipp}(C₂ NHMe₂ )]Cl₂ (4) and [(ImH){C(Me)=NDipp}(C₃ NHMe₂ )]Cl₂ (5), differing by the length (C₂ or C₃ ) of the alkyl spacer were used as precursors. Oxidative addition of 1 to [Ni(cod)₂ ] afforded the dinuclear bis-NHC Ni^II complex [Ni₂ Cl₂ {μ-C(Me)=NDipp-κ² N^imine ,C}{μ-Im(C₂ NMe₂ )-κ³ N^amine ,C^NHC ,N^imid }{Im[C(Me)=NDipp](C₂ NMe₂ )-κ¹ C^NHC }] (2) resulting from partial ligand rearrangement. A dinuclear byproduct 3 was also isolated. The salt [NiCl{Im[C(Me)=NDipp](C₂ NMe₂ )-κ³ N^imine ,C^NHC ,N^amine }]₂ [NiCl₄ ] (6) containing two N^imine C^NHC N^amine pincer Ni^II complex cations was obtained from 4. In contrast, the reaction of closely related 5 yielded [NiCl{Im[C(Me)=NDipp](C₃ NMe₂ )-κ² N^imine ,C^NHC }{μ-Im[C(Me)=NDipp](C₃ NMe₂ NiCl₃ }-κ² N^amine ,C^NHC }] (7 a), a dinickel(II) complex that contains a N^imine ,C^NHC -chelating and a bridging κ² N^amine ,C^NHC ligand. The molecular structures of 2, 3, 6, 7 a⋅THF, 7 b, an isomer of 7 a, and 8⋅CH₂ Cl₂ have been determined by single-crystal X-ray diffraction. Our results emphasize the often-underestimated importance of seemingly minor changes in the length of the spacers connecting a NHC donor group to its N-bound lateral functionalities.