Intramolecular Se center dot center dot center dot N nonbonding interactions in low-valent organoselenium derivatives: A detailed study by H-1 and Se-77 NMR spectroscopy and X-ray crystallography

A series of novel low-valent organoselenium compounds stabilized by Se...N intramolecular interactions has been synthesized. The existence of Se...N n onbonding interactions was determined by H-1 and Se-77 NMR spectroscopy and X-ray crystallography. These interactions result in an apparent downfield shift of the Se-77 NMR signals; the shift values roughly correspond with th e strength of the Se...N interaction. Single-crystal X-ray studies show tha t the strength of the Se...N interactions depends on the relative electrone gativity of the group attached to the selenium and the nature of the hetero atom. These interactions lengthen the Se-X (X = Se, Cl, Br, I, CH2-) bond t rans to the N...Se bond and increase the possibility of nucleophilic attack on the selenium. In addition to the Se...N intramolecular interaction, Se. ..X (X = Cl, Br, I) intermolecular interactions were observed in [2-(4,4-di methyl-2-oxazolinyl)phenyl]-selenenyl chloride (4), bromide (5), and iodide (6), indicative of the hypervalent (pseudo-high-valent) nature of the sele nium. The strength of the Se...N intramolecular interaction decreases in th e order RSeCl, RSeBr, RSeI, whereas the Se...X intermolecular interaction i ncreases in the same order. The aryl benzylic compounds [2-(4,4-dimethyl-2- oxazolinyl)phenyl]benzyl selenide (7), alpha,alpha'-di[2-(4,4-dimethyl-2-ox azoliny1)-phenylselenenyl]-m-xylene (8), alpha,alpha'-di[2-(4,4-dimethyl-2- oxazolinyl)phenyl-selenenyl]-o-xylene (9), and 1,3,5-tri[2-(4,3-dimethyl-2- oxazolinyl)phenylselenenyl]-2,4,6-trimethylbenzene (10) are unusually stabl e and also show weak Se...N interactions in the solid state. The strong Se. ..N interactions in all the compounds suggest that the sp(2) nitrogen may c oordinate with the selenium more strongly than the sp(3) nitrogen and the r esonance contributions from the five-membered heterocyclic rings further in crease the extent of Se...N interactions between selenium and tertiary nitr ogen. A correlation between the Se...N intramolecular distance and the Se-7 7 chemical shift was attempted.