Solid-state NMR and X-ray analysis of structural transformations in O-H...N heterosynthons formed by hydrogen-bond-mediated molecular recognition.

The template-mediated, hydrogen-bond-driven co-crystallization of trans-1,2-bis(4-pyridyl)ethylene (bpe) and resorcinol together with 1,2-bis(4-pyridyl)ethane (bpet) yielded two new polymorphs of 2(bpe):2(res) and 2(bpet):2(res) molecular adducts, thereby exploiting the molecular specificity of resorcinol-pyridine O-H...N recognition in the presence of multiple dipyridines. Comprehensive understanding of the subsequent [2 + 2] photodimerization of the known polymorph of 2(bpe):2(res) complex was obtained by applying single-crystal X-ray analysis and (13)C CPMAS solid-state NMR at different levels of conversion, ranging from monomer to the dimer. In addition, removal of the resorcinol template from the 2(bpe):2(res) complex yields a distorted tetrakis(4-pyridyl)cyclobutane, revealing a rather different molecular geometry (orthorhombic, Pccn phase). Ambiguous peak splittings and the presence of unexpected resonances in the respective (13)C CPMAS NMR spectra have been successfully explained by the joint approach of X-ray analysis and density functional theory (DFT) chemical shift computations.