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

Abstract

The reactions of Zn(OAc)(2).2H(2)O 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).2H(2)O 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 [Zn(3)(OAc)(2)(mu(2)-eta(2):eta(1)-OAc)(2)(mu(2)-eta(1):eta(1)-OAc)(2)(H(2)O)(2)(3,5-lutidine)(2)] (1), [Zn(3)(mu(2)-eta(1):eta(1)-OAc)(4)(mu(2)-eta(2):eta(0)-OAc)(2)L(2)] [L = 2,3-lutidine (2) and 2,4-lutidine (3)], and a one-dimensional coordination polymer [Zn(OAc)(mu(2)-eta(1):eta(1)-OAc)(3,4-lutidine)] (4) in 93, 79, 81, and 94% yields, respectively. Complexes 1-4 were characterized by microanalytical, IR, solution ((1)H and (13)C), and solid-state cross-polarization magic angle spinning (13)C 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 (1)H 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.

3,5-Lutidine coordinated zinc(II) aryl carboxylate complexes: Precursors for zinc(II) oxide

“…The d C = 176.6 ppm value is assigned to the carbonyl carbon of the monodentate acetate [40,64] and the d C = 181.1 ppm value is assigned to the carbonyl carbon of the monodentate acetate 13 C NMR signals whereas complex 5 revealed only one for the carbonyl carbon as the formerly mentioned complexes possess two crystallographically distinct carbonyl carbon whereas the latter possess only one.…”

“…Eq. (1) was applied by us [40] and others [58] to better interpret Dm values of several Lewis base coordinated zinc(II) carboxylate complexes bearing distinct carboxylate coordination modes. According to Eq.…”

“…(1), the variation of 0.01 Å in dr or 1°in hoco gives rise to a change of 16-18 cm À1 in the value of Dm. Sometimes, a significant deviation was observed between the Dm calcd and Dm exp values due to the involvement of carbonyl oxygen in H-bonding [62] and also due to the cage deformation [40,58].…”

“…This artifact could be due to sample dissociation or due to a rapid 'carboxylate shift' process that occurs on a time scale faster than NMR time scale [40,63]. The solid-state CP-MAS 13 C NMR was proven as a useful technique to distinguish various acetate coordination modes of zinc(II) acetate complexes [40,64,65]. Sometimes, even crystallographically different carbonyl carbons of the carboxylate moieties were distinguished with the aid of solid-state CP-MAS 13 C NMR data [65].…”

“…Recently, we have isolated a series Lewis base coordinated zinc(II) acetate and zinc(II) pivalate complexes with di and trinuclear core as well as a one dimensional coordination polymer from the condensation reaction involving Zn(j 2 O,O 0 -OAc) 2 Á2H 2 O (or) Zn(O 2 C t Bu) 2 and Lewis bases in methanol at ambient condition [40,41]. From this study, it was concluded that subtle basic/steric properties of Lewis bases and noncovalent interactions in the crystal lattice influence the nuclearity and carboxylate coordination modes of the products.…”