Preparation of Stable and Metastable Coordination Compounds:  Insight into the Structural, Thermodynamic, and Kinetic Aspects of the Formation of Coordination Polymers

The reaction of ZnI₂ and pyrimidine in acetonitrile results in the formation of the 1:2 compound ZnI₂(pyrimidine)₂ (1), which consists of discrete tetrahedral building blocks. Slow heating of 1 at 1 °C/min leads to its transformation into the ligand-deficient intermediate 1:1 compound ZnI₂(pyrimidine) (3), which upon further heating decomposes into the most ligand-deficient 2:1 compound (ZnI₂)₂(pyrimidine) (4). In contrast, the 2:3 compound (ZnI₂)₂(pyrimidine)₃ (2) is formed as an intermediate by decomposing 1 using a faster heating rate of 8 °C/min. Compound 2 consists of oligomeric units in which each ZnI₂ unit is coordinated by two iodine atoms and one bridging and one terminal pyrimidine ligand. The crystal structure of compound 3 is built up of ZnI₂ units, which are connected by the ligands into chains. For the thermal transformation of 1 into 3 via 2 as the intermediate, a smooth reaction pathway is found in the crystal structure, for which only small translational and rotational changes are needed. The metastable solvated compound (ZnI₂)(pyrimidine)(acetonitrile)_0.25 (5) consisting of (ZnI₂)₄(pyrimidine)₄ rings is obtained by quenching the reaction of ZnI₂ and pyrimidine in acetonitrile using an antisolvent. On heating, 5 decomposes into a new polymorphic 1:1 compound 6, which consists of (ZnI₂)(pyrimidine) chains. On further heating, 6 transforms into a third polymorphic 1:1 compound 7, which consists of (ZnI₂)₃(pyrimidine)₃ rings, and finally into the 1:1 compound 3. Solvent-mediated conversion experiments reveal that compounds 1−4 are thermodynamically stable, whereas compounds 5−7 are metastable. Time-dependent crystallization experiments unambiguously show that compound 7 is formed by kinetic control and transforms within minutes into compound 6, which finally transforms into 3. Compound 3 represents the thermodynamically most stable 1:1 modification, whereas compounds 6 and 7 are metastable. The different compounds obtained by thermal decomposition and by crystallization from solution represent a snapshot of the species in solution and thus provide insight into the formation of coordination compounds.