Abstract
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 13C 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 13C CPMAS NMR spectra have been successfully explained by the joint approach of X-ray analysis and density functional theory (DFT) chemical shift computations.
Crystal structures of tetrakis(4-pyridyl)cyclobutane, polymorph 2 of 2(bpe):2(res), and polymorph 2 of 2(bpet):2(res) are provided as cif files. Furthermore, solution 13C NMR spectra of resorcinol, trans-1,2-bis(4-pyridyl)ethylene (bpe), 2(bpe):2(res) monomer (ring form, P1̅ phase), recrystallized 2(bpe):2(res) dimer (P21/n phase), tetrakis(4-pyridyl)cyclobutane, the solid-state 13C CPMAS NMR spectrum of as-dimerized 2(bpe):2(res) dimer (P1̅ phase) at 110 °C, as well as the solution 1H NMR spectrum of both 2(bpe):2(res) monomer (chain form) and 2(bpet):2(res) monomer (chain form) are given. In addition, all xyz coordinates obtained from the respective DFT structure optimizations are available. This material is available free of charge via the Internet at http://pubs.acs.org.
