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 center dot center dot center dot 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 C-13 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, P-ccn phase). Ambiguous peak splittings and the presence of unexpected resonances in the respective C-13 CPMAS NMR spectra have been successfully explained by the joint approach of X-ray analysis and density functional theory (DFT) chemical shift computations.