Solid-state Photochemical [2+2] Cycloaddition Reaction of Hydrogen-Bonded Zn(II) Metal Complex Containing Several Parallel C=C Bonds

Abstract

A 2D hydrogen-bonded dinuclear Zn(II) complex, [{Zn(H2O)3(bpe)2}2(bpe)](NO3)4⋅3bpe ⋅14H2O, 1 (bpe = 4,4′-bipyridylethylene) containing coordination complex cations, [{Zn(H2O)3(bpe)2}2(μ-bpe)] 4+ and free bpe and lattice water molecules shows face-to-face, π⋯π stacking of two of the four free bpe molecules with coordinated bpe ligands. Out of eight bpe molecules, six are aligned in parallel fashion with short C ⋯C distances of 3.663–3.814 Å and they undergo photochemical [2 + 2] cycloaddition reaction. The photoreaction conducted on ground sample of 1 in the solid-state affords rctt-tetrakis(4-pyridyl)cyclobutane (rctt-tpcb) product in 75% yield. The molecular movement of free bpe molecules was tested by conducting the photoreaction in ground sample and heated sample of single crystals. The photoreactivity study of 1 indicates that the free bpe molecules are locked between the cationic [{Zn(H2O)3(bpe)2}2(bpe)] 4+ layers. Single crystals of 2D hydrogen-bonded dinuclear Zn(II) complex of 4,4′-bipyridylethylene (bpe) containing coordination complex cation and free non-coordinated bpe molecules undergo photochemical [2+2] cycloaddition reaction via molecular movement which is accelerated by mechanical grinding.