Sulfur and Selenium Activation by Frustrated NHC/B(C6F5)3 Lewis Pairs; Conformational Flexibility of Products

Frustrated Lewis pairs consisting of N-heterocyclic carbenes (NHC) and the borane B(C6F5)(3) react with elemental sulfur or selenium to give products of the type NHC-E-B(C6F5)(3), where E is S or Se. Three such products, two with sulfur and one with selenium, were characterized by X-ray diffraction and shown to exhibit considerable conformational flexibility, as revealed by differing torsion angles in the atom sequence N-C-E-B-C-ipso-C-ortho. In the sulfur derivatives, the S-B bonds are all long (ca. 2.05 angstrom), and the C-S bonds (ca. 1.73 angstrom) are clearly lengthened compared to imidazole-2-thiones. The Se-B distance of 2.2111 angstrom is the first selenone-borane bond length to be determined by X-ray analysis.