Observance of a large conformational change associated with the rotation of the naphthyl groups during the photodimerization of criss-cross aligned CC bonds within a 2D coordination polymer

Abstract

Five coordination polymers [Zn(oba)(4-spy)]2·(4-spy)0.25·(H2O) (1), [Zn2(oba)2(HT-ppcb)]2·(4-spy)0.25 (1a), [Zn(oba)(4-(2-npy))]2·(4-(2-npy))0.25·(H2O)1.25 (2), [Zn(oba)(4-npy)]2·(4-npy)0.25 (3) and [Zn2(oba)2(HT-pncb)]· (4-npy)0.25 (3a) (H2oba = 4,4′-oxydibenzoic acid; 4-spy = (E)-4-styrylpyridine; HT-ppcb = 1,3-bis(4-pyridyl)-2,4-bis(phenyl)cyclobutane; 4-(2-npy) = (E)-4-(2-(naphthalen-2-yl)vinyl)pyridine; 4-npy = (E)-4-(2-(naphthalen-1-yl)vinyl)pyridine; HT-pncb = rctt-1,3-bis(4-pyridyl)-2,4-bis(1-naphthyl)cyclobutane) were structurally determined. In each case, pairs of Zn(II) centres are linked by oba2− anions to form a 2D network. Olefinic pyridyl ligands bound to Zn(II) extend above and below the mean plane of the network and are located within the windows of adjacent parallel networks. Each window contains a pair of vinyl-pyridyl ligands that are tethered to networks that lie above and below and arranged in a head-to-tail manner. 1–3 show different alignments of the CC bond pair achieved by introducing olefinic ligands with different sizes of their terminal groups. In 1, the CC bonds are parallel and close enough for a [2 + 2] cycloaddition reaction to occur upon exposure to UV light. The CC bonds in 2 are too far apart to combine and no reaction occurs upon UV irradiation. In 3, the CC bonds are less than 4.0 A apart and has a criss-cross arrangement. Exposure of 3 to UV light leads to a [2 + 2] cycloaddition reaction. Analysis of the “before” and “after” structures shows a large conformational change associated with the rotation of the olefinic bonds and/or the naphthyl groups when the photodimerization of criss-cross CC bonds is undergone.