Effect of the β-diketonate ligand on the spin states of [Ni(β-dkt)2(NH2-quin)] complexes

Three new complexes [Ni(β-dkt)2(NH2-quin)] {β-dkt = 2,2,6,6-tetramethylheptane-3,3-dionate (tmhd) 1, hexafluoroacetylacetonate (hfac) 2, 1,3-diphenylpropanedionate (dbm) 3} have been prepared by reacting [Ni(β-dkt)2(H2O)2] with 8-aminoquinoline (NH2-quin). [Ni(tmhd)2(NH2-quin)] is found to be solvatochromic exhibiting a square planar geometry in CH2Cl2, acetone, and acetonitrile with the tmhd ligand acting unusually as a counteranion while in THF and DMSO the complex is octahedral. In contrast, 2 and 3 are octahedral in all solvents. Single crystal X-ray diffraction studies reveal octahedral nickel centres with a cis arrangement of the β-diketonates. The molecular packing consists of hydrogen bonded dimers which in the case of 2 and 3 are connected to one another via π···π and C–H⋯π interactions, respectively. Cyclic voltammetry shows 1 and 3 oxidise irreversibly at 0.46 and 1.17 V, respectively.

Graphical abstractA series of novel [Ni(β-dkt)2(NH2-quin)] complexes is reported. The complexes are octahedral in the solid state but in the case of the electron rich and steric bulky tmhd ligand are square planar in non- and weakly coordinating solvents. The change in coordination geometry is found to radically alter the redox behaviour of [Ni(tmhd)2(NH2-quin)].Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Nickel aminoquinoline β-diketonates as molecular switches. ► Octahedral in the solid state, square planar in solution. ► Geometry dependent on β-diketonate and then the solvent. ► Redox activity of the square planar and octahedral complexes radically different.