The reactions of para-halo diaryl diselenides with halogens. A structural investigation of the CT compound (p-FC6H4)2Se2I2, and the first reported "RSeI3" compound, (p-ClC6H4)SeI·I2, which contains a covalent Se-I bondThis paper is dedicated to the memory of the late Dr Steve Godfrey (1966-2011).CCDC 897285-897287. For crystallographic data in CIF or other electronic format see DOI: 10.1039/c2dt31921g

The reactions of the diaryl-diselenides ( p -FC 6 H 4 ) 2 Se 2 and ( p -ClC 6 H 4 ) 2 Se 2 with diiodine have been investigated. Species of stoichiometry "RSeI" are formed when the ratio employed is 1 : 1. The solid-state structure of "( p -FC 6 H 4 )SeI" has been determined, and shown to be a charge-transfer (CT) adduct, ( p -FC 6 H 4 ) 2 Se 2 I 2 , where the Se-Se bond is retained and the diiodine molecule interacts with only one of the selenium atoms. The Se-I bond in ( p -FC 6 H 4 ) 2 Se 2 I 2 is 2.9835(12) Å, which is typical for a (10-I-2) Se-I-I CT system. When diiodine is reacted in a 3 : 1 ratio with ( p -XC 6 H 4 ) 2 Se 2 (X = F, Cl) species of stoichiometry "RSeI 3 " are formed. The structure of "( p -ClC 6 H 4 )SeI 3 " reveals that this is not a selenium( iv ) compound, but is better represented as a selenium( ii ) CT adduct, ( p -ClC 6 H 4 )SeI·I 2 . The Se-I bond to the diiodine molecule is typical in magnitude for a CT adduct, Se-I: 2.8672(5) Å, whereas the other Se-I bond is much shorter, Se-I: 2.5590(6) Å, and is a genuine example of a rarely observed covalent Se-I bond, which appears to be stabilised by a weak Se I interaction from a neighbouring iodine atom. The reaction of ( p -ClC 6 H 4 )SeI with Ph 3 P results in the formation of a CT adduct, Ph 3 PSe( p -ClC 6 H 4 )I, which has a T-shaped geometry at selenium (10-Se-3). By contrast, the reaction of ( p -FC 6 H 4 )SeI with Ph 3 P does not form an adduct, but results in the formation of Ph 3 PI 2 and ( p -FC 6 H 4 ) 2 Se 2 . The first reported "RSeI 3 " compound is a Se( ii ) species, RSeI·I 2 , with both CT and covalent Se-I bonds.