Steric Crowding Effect of 1,3-Diketonates on the Structures and the Solution Behaviors of Nickel(II) Mixed-Ligand Complexes

Seven nickel(II) mixed-ligand complexes were synthesized: Ni(dike)(diam)X, where dike is a 1,3-diketonate such as 3,5-heptanedionate (dedk) or 2,6-dimethyl-3,5-heptanedionate (dmhd), diam is an N-alkylated ethylenediamine such as N,N,N′,N′-tetramethylethylenediamine (tmen) or 1,2-dipiperidinoethane (dipe), and X is BPh4− or NO3−. The crystal structures were analyzed: each nitrate complex has a six-coordinated octahedral structure with a bidentate-coordinated nitrate, and each tetraphenylborate complex has a four-coordinated square-planar structure. The ligand field strength of [Ni(dike)(diam)]BPh4 in the solid state increases in the order: [Ni(dmhd)(dipe)]BPh4 < [Ni(dedk)(dipe)]BPh4 < [Ni(dmhd)(tmen)]BPh4 = [Ni(dedk)(tmen)]BPh4. In solution, the change of the color depending on the donor/acceptor ability of the solvent, i.e., solvatochromism was observed. When dimethyl sulfoxide was added to a solution of [Ni(dike)(diam)]BPh4 in inert 1,2-dichloroethane, the adduct formation constants are in the order: [Ni(dmhd)(dipe)]BPh4 < [Ni(dedk)(dipe)]BPh4