{Li[P(NtBu)2]}4·0.25P4(NtBu)6:  Solvent and Substituent Effects on the Structures and Reactivity of 1,3-Diaza-2-phosphaallyllithium Complexes

The addition of 2 equiv of nBuLi to a solution of tBuN(H)P(μ-NtBu)2PN(H)tBu (1) in toluene produces the adduct [LiP(NtBu)2]4 ·0.25P4(NtBu)6 (2 ·0.253), but the reverse addition yields only 2. The structure of 2 ·0.253 has been determined by X-ray crystallography: monoclinic, P21/a, a = 17.649(5) Å, b = 18.272(4) Å, c = 20.295(5) Å, β = 111.92(2)°, V = 6072(3) Å3, and Z = 4. The tetrameric cluster 2 consists of two P2N2 rings bridged by a tub-shaped, eight-membered Li4N4 ring. The mean P−N distance in the P2N2 rings (1.78 Å) is significantly longer than the corresponding value of 1.65 Å for the P−N bonds linking the P2N2 rings to the Li4N4 unit. The P4(NtBu)6 cage 3 in the adduct is formed as the isomer in which two P2N2 rings are bridged by two NtBu groups. The same tetracyclic isomer of 3 is prepared in 71% yield by the reaction of tBuN(Li)P(μ-NtBu)2PN(Li)tBu with ClP(μ-NtBu)2PCl in toluene. Variable temperature 31P{1H} NMR studies of the 16-atom cluster [LiP(NtBu)2]4 (2) at different concentrations in C7D8 indicate that facile dissociation into two molecules of the dimer [LiP(NtBu)2]2 occurs above ca. 295 K.