Divergent Solution and Solid-State Structures of Mono- and Dinuclear Nickel(II) Pyridone Complexes

The chelating 2-pyridone ligand (E)-6-(1-((2-(methylthio)­phenyl)­imino)­ethyl)­pyridin-2­(1H)-one (1) has been used to synthesize five nickel­(II) complexes that have been characterized by single-crystal X-ray diffraction, UV/vis and IR spectroscopies, and benchtop magnetometry. Reaction of NiX₂ (X = BF₄, Cl) with 1 yielded the C₂-symmetric halide-bridged Ni dimers [(μ-F)­Ni₂(κN,N,O-1^–)₂(κN,N-1)₂]­(BF₄) (2) and [(μ-Cl)­Ni₂(κN,N,O-1^–)₂(κN,N-1^–)­(κN,N-1)] (3), where 2 is formed via a rare example of fluoride abstraction from BF₄^– by a transition metal. Remarkably, the reaction of Ni­(OAc)₂ with 1 followed by different methods of crystallization yielded three different products: the dimer [(μ-OH₂)­Ni₂(κN,N,O-1^–)­(κN,N-1^–)­(κN,N-1)­(OAc)₂] (4), as well as the monomers [Ni­(κN,N-1^–)₂(MeOH)₂] (5) and [Ni­(κN,N-1^–)­(κN,N-1)­(OAc)­(OH₂)] (6). This observation is emblematic of the soft energy landscape of coordination motifs and nuclearity that pyridone ligands provide with late 3d transition metals. To better understand the solution versus solid-state speciation, solid-state UV/vis reflectance and solution-state UV/vis absorbance spectra were obtained. Surprisingly, the solution-state UV/vis spectra of 4 and 6 each provided nearly identical absorption spectra (λ_max ≈ 360 nm), which matched neither of the solid-state reflectance spectra of 4 or 6 (λ ≈ 590, 790, 970 nm; ³A_2g → ³T_1g, ³A_2g^→1E_2g, and ³A_2g → ³T_2g, respectively). Rather, the solution spectra are consistent only with the spectroscopic features (MLCT) of a conventional square-planar Ni­(II) species.