Alkali Metal Complexes of a tert-Butylphosphine-Bridged Biphenolate Ligand

Abstract

The coordination chemistry of group 1 metals with a potentially tridentate, dianionic biphenolate phosphine ligand that carries a phosphorus-bound tert-butyl group is described. Deprotonation of bis(3,5-di-tert-butyl-2-hydroxyphenyl)-tert-butylphosphine (H2[tBu-OPO]) with two equivalents of n-BuLi, NaH, or KH in ethereal solutions produces the corresponding alkali metal complexes {[tBu-OPO]M2(solv)x}2 (M = Li, Na, K; solv = DME, THF). An X-ray diffraction study of the lithium derivative reveals a dimeric structure that contains two [tBu-OPO]Li2(DME) units linked by two dative O−Li bonds; the two lithium atoms in each monomeric [tBu-OPO]Li2(DME) are bridged by both phenolate oxygen donors with only one lithium being coordinated to the phosphorus donor. As a result, the Li4O4 moiety in {[tBu-OPO]Li2}2 constitutes three successive Li2O2 tetragons fused in an open cubane structure. The coordination of the phosphorus donor to lithium in {[tBu-OPO]Li2(DME)}2 is also confirmed by variable-temperature 31P{1H} and 7Li{1H} NMR studies. Interestingly, the sodium complex is composed of two [tBu-OPO]Na2(DME)2 units linked by a bridging DME; each [tBu-OPO]Na2(DME)2 subunit is structurally similar to its lithium analogue though adopting one more coordinated DME. The potassium complex also contains two [tBu-OPO]K2(THF)2 moieties linked with two dative O−K bridges; the structure of the monomeric [tBu-OPO]K2(THF)2 unit, however, is markedly distinct from those of its lighter congeners. In addition to the anticipated fac-coordination from the heteroatoms in the biphenolate phosphine ligand, one of the potassium atoms in monomeric [tBu-OPO]K2(THF)2 is π-bound to one of the aryl rings incorporated in the [tBu-OPO]2− backbone.