Solid-state photochemical [2+2] cycloaddition in a hydrogen-bonded metal complex containing several parallel and crisscross C=C bonds

One-dimensional hydrogen-bonded complex [Zn(bpe)₂(H ₂O)₄]-(NO₃)₂·8/3H ₂O·2/3bpe (1, bpe = 4,4′-bipyridylethylene) containing coordination complex cations [Zn(bpe)₂-(H₂O) ₄]²⁺ with parallel and crisscross double bonds undergoes photochemical [2+2] cycloaddition in the solid state and produces tetrakis(4-pyridyl)cyclobutane (tpcb) in up to 100% yield with rctt-tpcb (2 a) as major and rtct-tpcb (2 b) as minor product. The bpe ligands with crisscross conformation of C=C bonds appear to undergo pedal-like motion prior to photodimerization. Grinding single crystals to powder ac-celerates the pedal motion of crisscrossed olefins in the bpe ligands to parallel alignment and provides the rctt-cyclobutane stereoisomer 2a quantitatively. In addition, 100% photodimerization of ground 1 indicates that the free bpe ligands, which are remote from each other in a pool of water, and NO₃^- ions moved toward each other to give a mixture of rctt- and rtct-tpcb isomers.