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

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.