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Syntheses and solid-state structures of trimeric dibenzylamidolithium and its diethyl ether and hexamethylphosphoramide dimeric complexes: an explanation of these structures and evidence for Li CH interactions in both solid and solution phases

ARMSTRONG, D R and Mulvey, Robert and WALKER, G T and BARR, D and SNAITH, R and CLEGG, W and REED, D (1988) Syntheses and solid-state structures of trimeric dibenzylamidolithium and its diethyl ether and hexamethylphosphoramide dimeric complexes: an explanation of these structures and evidence for Li CH interactions in both solid and solution phases. Journal of the Chemical Society, Dalton Transactions (3). pp. 617-628. ISSN 0300-9246

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Abstract

Dibenzylamidolithium, [(PhCH2)2NLi]n′(1), and two of its comlexes, [(PhCH2)2NLi·OEt2]n′(2), and [(PhCH2)2NLi·hmpa]n′(3)(hmpa = hexamethylphosphoramide), have been synthesised and characterised, and shown to meet earlier proposed criteria for useful proton abstraction reagents. The X-ray crystal structures of (1), (2), and (3) have been determined. Their solid-state structures contain central (NLi)n rings [n= 3 for (1), n= 2 for (2) and (3)], and the diminution in ring size from six- to four-membered on complexation of (1) has been rationalised via the results from ab initio calculations on model systems. Recent ring-stacking and ring-laddering principles have also been used to show why such rings cannot associate further, so leaving their lithium atoms with abnormally low co-ordination number [formally only 2 in (1), 3 in (2) and (3)]. Evidence that the co-ordinatively unsaturated Li atoms, in (1) in particular, are thereby prompted to engage in compensating interactions with CH units on the (PhCH2)2N ligands is drawn from both solid-state and solution studies. For the former, the implications of relatively short Li HC distances in the structure of (1) have been probed by molecular orbital bond index (MOBI) calculations which show that Li HC interactions constitute ca. 40% of lithium's valency, and that C–H bonds involved are weakened. In the solution studies, cryoscopic, 7Li n.m.r. spectroscopic, and u.v.–visible spectroscopic measurements have shown that the pink-red solution colours of (1) and (2) are caused by a species common to both, monomeric (PhCH2)2NLi; m.o. calculations on this one-co-ordinate Li species imply that it contains enhanced Li benzyl interactions which shift the charge transfer transition h.o.m.o. (benzyl)→l.u.m.o. (Li)(highest occupied and lowest unoccupied molecular orbitals, respectively) into the visible region.