Experimental and Theoretical Investigations of Lithium and Magnesium Derivatives of Bis(<i>tert</i>-butylamido)cyclodiphosph(III/V)- and (V/V)azane Mono- and Ditellurides

Deprotonation of bis(tert-butylamido)cyclophosph(III/III)azane with organolithium or organomagnesium reagents followed by oxidation with elemental tellurium is a viable approach to the preparation of metal cyclodiphosphazane mono- and ditellurides. The reaction of the cyclodiphosph(III)azane [tBu(H)NP(μ-NtBu)2PN(H)tBu] (1) with elemental tellurium in boiling toluene affords the monotelluride [tBu(H)N(Te)P(μ-NtBu)2PN(H)tBu] (9). A similar reaction involving the magnesium salt Mg[tBuNP(μ-NtBu)2PNtBu](THF)2 (2) also yields a monotelluride Mg[tBuN(Te)P(μ-NtBu)2PNtBu](THF)2 (10). By contrast, reaction of the lithium salt Li2[tBuNP(μ-NtBu)2PNtBu](THF)2 (3) with tellurium results in double oxidation and the formation of the ditellurides Li2[tBuN(Te)P(μ-NtBu)2P(Te)NtBu](THF)4 (11) and Li2[tBuN(Te)P(μ-NtBu)2P(Te)NtBu](tmeda)2 (12). Compounds 9−12 have been characterized by multinuclear (1H, 7Li, 13C, 31P, and 125Te) NMR, while 9, 10, and 12 have also been characterized by X-ray crystallography. The structure of 9 reveals a typical cis/endo,exo arrangement, with no intermolecular contacts to tellurium. The seco-heterocubic structure, observed in 2, is retained in 10, with the ligand chelating magnesium in an N,N‘,N‘ ‘-manner. Unique coordination behavior is exhibited by the ditelluride 12, in which the dianionic ligand is attached to the two lithium centers in both Te,Te‘ and Te,N bonding modes. Multinuclear NMR data are consistent with retention of the solid-state structures of 9−12 in solution at low temperatures. The reactivity of cyclodiphosph(III/III)azanes toward chalcogens is rationalized by using theoretical calculations (semiempirical PM3 level of theory), which show an inverse correlation between the charge at the phosphorus center and the ease of oxidation.