Synthesis and Structures of Aluminum and Magnesium Complexes of Tetraimidophosphates and Trisamidothiophosphates: EPR and DFT Investigations of the Persistent Neutral Radicals {Me2Al[(μ-NR)(μ-NtBu)P(μ-NtBu)2]Li(THF)2}• (R = SiMe3, tBu)

Abstract

Reactions of (RNH)3PNSiMe3 (3a, R = tBu; 3b, R = Cy) with trimethylaluminum result in the formation of {Me2Al(μ-NtBu)(μ-NSiMe3)P(NHtBu)2]} (4) and the dimeric trisimidometaphosphate {Me2Al[(μ-NCy)(μ-NSiMe3)P(μ-NCy)2P(μ-NCy)(μ-NSiMe3)]AlMe2} (5a), respectively. The reaction of SP(NHtBu)3 (2a) with 1 or 2 equiv of AlMe3 yields {Me2Al[(μ-S)(μ-NtBu)P(NHtBu)2]} (7) and {Me2Al[(μ-S)(μ-NtBu)P(μ-NHtBu)(μ-NtBu)]AlMe2} (8), respectively. Metalation of 4 with nBuLi produces the heterobimetallic species {Me2Al[(μ-NtBu)(μ-NSiMe3)P(μ-NHtBu)(μ-NtBu)]Li(THF)2} (9a) and {[Me2Al][Li]2[P(NtBu)3(NSiMe3)]} (10) sequentially; in THF solutions, solvation of 10 yields an ion pair containing a spirocyclic tetraimidophosphate monoanion. Similarly, the reaction of (tBuNH)3PNtBu with AlMe3 followed by 2 equiv of nBuLi generates {Me2Al[(μ-NtBu)2P(μ2-NtBu)2(μ2-THF)[Li(THF)]2} (11a). Stoichiometric oxidations of 10 and 11a with iodine yield the neutral spirocyclic radicals {Me2Al[(μ-NR)(μ-NtBu)P(μ-NtBu)2]Li(THF)2}• (13a, R = SiMe3; 14a, R = tBu), which have been characterized by electron paramagnetic resonance spectroscopy. Density functional theory calculations confirm the retention of the spirocyclic structure and indicate that the spin density in these radicals is concentrated on the nitrogen atoms of the PN2Li ring. When 3a or 3b is treated with 0.5 equiv of dibutylmagnesium, the complexes {Mg[(μ-NtBu)(μ-NHtBu)P(NHtBu)(NSiMe3)]2} (15) and {Mg[(μ-NCy)(μ-NSiMe3)P(NHCy)2]2} (16) are obtained, respectively. The addition of 0.5 equiv of MgBu2 to 2a results in the formation of {Mg[(μ-S)(μ-NtBu)P(NHtBu)2]2} (17), which produces the hexameric species {[MgOH][(μ-S)(μ-NtBu)P(NHtBu)2]}6 (18) upon hydrolysis. Compounds 4, 5a, 7−11a, and 15−17 have been characterized by multinuclear (1H, 13C, and 31P) NMR spectroscopy and, in the case of 5a, 9a·2THF, 11a, and 18, by X-ray crystallography.