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Lithiation of (tBuNH)3PNSiMe3 and Formation of Tetraimidophosphate Complexes Containing M3O3 Rings (M = Li, K):  X-ray Structure of the Stable Radical {(Me3SiN)P(μ3-NtBu)33-Li(THF)]3(OtBu)}

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posted on 31.05.2004, 00:00 by Andrea Armstrong, Tristram Chivers, Masood Parvez, Gabriele Schatte, René T. Boeré
The reaction of (tBuNH)3PNSiMe3 (1) with 1 equiv of nBuLi results in the formation of Li[P(NHtBu)2(NtBu)(NSiMe3)] (2); treatment of 2 with a second equivalent of nBuLi produces the dilithium salt Li2[P(NHtBu)(NtBu)2(NSiMe3)] (3). Similarly, the reaction of 1 and nBuLi in a 1:3 stoichiometry produces the trilithiated species Li3[P(NtBu)3(NSiMe3)] (4). These three complexes represent imido analogues of dihydrogen phosphate [H2PO4]-, hydrogen phosphate [HPO4]2-, and orthophosphate [PO4]3-, respectively. Reaction of 4 with alkali metal alkoxides MOR (M = Li, R = SiMe3; M = K, R = tBu) generates the imido-alkoxy complexes {Li3[P(NtBu)3(NSiMe3)](MOR)3} (8, M = Li; 9, M = K). These compounds were characterized by multinuclear (1H, 7Li, 13C, and 31P) NMR spectroscopy and, in the cases of 2, 8, and 3THF, by X-ray crystallography. In the solid state, 2 exists as a dimer with Li−N contacts serving to link the two Li[P(NHtBu)2(NtBu)(NSiMe3)] units. The monomeric compounds 8 and 3THF consist of a rare M3O3 ring coordinated to the (LiN)3 unit of 4. The unexpected formation of the stable radical {(Me3SiN)P(μ3-NtBu)33-Li(THF)]3(OtBu)} (10) is also reported. X-ray crystallography indicated that 10 has a distorted cubic structure consisting of the radical dianion [P(NtBu)3(NSiMe3)]•2-, two lithium cations, and a molecule of LiOtBu in the solid state. In dilute THF solution, the cube is disrupted to give the radical monoanion [(Me3SiN)(tBuN)P(μ-NtBu)2Li(THF)2]•-, which was identified by EPR spectroscopy.