Disappearing and Concomitant Polymorphism of Nickel(II) Complexes with 6-Hydroxypicolinic Acid. Structural and Density Functional Theory Studies

The recrystallization of the complex [Ni(6-OHpic)2(H2O)2] (1) from a mixture of pyridine and acetone led to the formation of three different polymorphs of [Ni(6-OHpic)2(py)2]. The stable monoclinic polymorph 3 was obtained by evaporation of the solution, while the slow diffusion of acetone into the pyridine solution of 1 yielded the orthorhombic polymorph 2. Further attempts to prepare 2 again were unsuccessful (disappearing polymorph). The new pseudopolymorph [Ni(6-OHpic)2(py)2]·py (4) was obtained under the same conditions as 3, but again not reproducibly (concomitant and a disappearing polymorph). trans-[Ni(6-OHpic)2(py)2] formed one intramolecular O−H···O hydrogen bond and one intermolecular one to afford dimers (3) and infinite zigzag chains (2). When two intramolecular O−H···O hydrogen bonds were present, the dimers were held by π−π stacking (4). Density functional theory calculations showed trans-[Ni(6-OHpic)2(py)2] to be the most stable isomer. The hydrogen bonded dimers are the most stable motif, followed by π−π stacked dimers (9.6 kcal mol−1) and zigzag chains (24.7 kcal mol−1). Thermogravimetric studies showed the initial loss of two coordinated water molecules in 1 and pyridine in 3 in similar temperature ranges.