Factors Dictating the Nuclearity/Aggregation and Acetate Coordination Modes of Lutidine-Coordinated Zinc(II) Acetate Complexes

The reactions of Zn(OAc)2·2H2O with various positional isomers of lutidine were explored with a view to understand the factors responsible for the nuclearity/aggregation and acetate coordination modes of the products. The reactions of Zn(OAc)2·2H2O with 3,5-lutidine, 2,3-lutidine, 2,4-lutidine, and 3,4-lutidine in a 1:1 ratio in methanol at ambient temperature afforded three discrete trinuclear complexes [Zn3(OAc)2(μ2-η2:η1-OAc)2(μ2-η1:η1-OAc)2(H2O)2(3,5-lutidine)2] (1), [Zn3(μ2-η1:η1-OAc)4(μ2-η2:η0-OAc)2L2] [L = 2,3-lutidine (2) and 2,4-lutidine (3)], and a one-dimensional coordination polymer [Zn(OAc)(μ2-η1:η1-OAc)(3,4-lutidine)] (4) in 93, 79, 81, and 94% yields, respectively. Complexes 1−4 were characterized by microanalytical, IR, solution (1H and 13C), and solid-state cross-polarization magic angle spinning 13C NMR spectroscopic techniques and single-crystal X-ray diffraction data. Complex 1 is unique in that it contains three types of acetate coordination modes, namely, monodentate, bridging bidentate, and asymmetric chelating bridging. Variable-temperature 1H NMR data indicated that complex 1 partially dissociates in solution, and the remaining undissociated 1 undergoes a rapid “carboxylate shift” even at 218 K. The plausible mechanism of formation of complexes 1−4 was explained with the aid of a point zero charge (pzc) model, according to which the nuclearity/aggregation observed in complexes 1−4 depends upon the number and nature of equilibrating species formed upon dissolution of the reactants in methanol, and these in turn depend upon the subtle basic/steric properties of lutidines. Further, noncovalent interactions play a crucial role in determining the nuclearity/aggregation and acetate coordination modes of the products.