Solid-state studies of the Horner-Wittig reagent [{{Ph2P(O)}2CHLi • (THF)}2 • THF] and its mixed-anion intermediate [{{ Ph2P(O)}2CHLi}4 • {Ph2P(O)2Li}2•8C6H6] formed by reaction with dioxygen

Reaction of Ph2P(O)(2)CH2 with one equivalent of RLi (R = Me, Bu) leads to monodeprotonation of the methylene bridge and formation of a Horner-Wittig type reagent. The crystal structure of the resulting lithium complex [{{Ph2P(O)}(2)CHLi . (TH-F)}(2) . THF], 1, reveals a dimeric aggregate, where each lithium is tetracoordinate through bonding to two bridging and one terminally chelating phosphoryl unit, and to a THF molecule. Upon exposing complex 1 to molecular oxygen, an unexpected mixed-anionic species was formed, [{{Ph2P(O)}(2)CHLi}(4).{Ph-2 P(O)(2)Li}(2).8C(6)H(6)] 2. Complex 2 was identified by X-ray crystallography and found to exist in two polymorphic forms that differ only slightly in their metrical parameters. The structure of 2 is an unusual hexalithium cage aggregate containing four Horner-Wittig anions and two diphenyl phosphinate anions. In both complexes 1 and 2 the beta-diphosphine oxide anions were found to contain localized P=0 double bonds and the negative charge was partially delocalized over the P-C-P fragments. Neither complex displays direct metal-C (anion) contacts but the relative planarity of the individual carbanions show significant variation. (C) 2004 Elsevier Ltd. All rights reserved.