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

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 (H 2 [ t Bu-OPO]) with two equivalents of n -BuLi, NaH, or KH in ethereal solutions produces the corresponding alkali metal complexes {[ t Bu-OPO]M 2 (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 [ t Bu-OPO]Li 2 (DME) units linked by two dative O−Li bonds; the two lithium atoms in each monomeric [ t Bu-OPO]Li 2 (DME) are bridged by both phenolate oxygen donors with only one lithium being coordinated to the phosphorus donor. As a result, the Li 4 O 4 moiety in {[ t Bu-OPO]Li 2 } 2 constitutes three successive Li 2 O 2 tetragons fused in an open cubane structure. The coordination of the phosphorus donor to lithium in {[ t Bu-OPO]Li 2 (DME)} 2 is also confirmed by variable-temperature 31 P{ 1 H} and 7 Li{ 1 H} NMR studies. Interestingly, the sodium complex is composed of two [ t Bu-OPO]Na 2 (DME) 2 units linked by a bridging DME; each [ t Bu-OPO]Na 2 (DME) 2 subunit is structurally similar to its lithium analogue though adopting one more coordinated DME. The potassium complex also contains two [ t Bu-OPO]K 2 (THF) 2 moieties linked with two dative O−K bridges; the structure of the monomeric [ t Bu-OPO]K 2 (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 [ t Bu-OPO]K 2 (THF) 2 is π-bound to one of the aryl rings incorporated in the [ t Bu-OPO] 2− backbone.