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

Abstract

The addition of 2 equiv of n BuLi to a solution of t BuN(H)P(μ-N t Bu) 2 PN(H) t Bu ( 1 ) in toluene produces the adduct [LiP(N t Bu) 2 ] 4 · 0.25P 4 (N t Bu) 6 ( 2 · 0.25 3 ), but the reverse addition yields only 2 . The structure of 2 · 0.25 3 has been determined by X-ray crystallography: monoclinic, P 2 1 / a, a = 17.649(5) Å, b = 18.272(4) Å, c = 20.295(5) Å, β = 111.92(2)°, V = 6072(3) Å 3, and Z = 4. The tetrameric cluster 2 consists of two P 2 N 2 rings bridged by a tub-shaped, eight-membered Li 4 N 4 ring. The mean P−N distance in the P 2 N 2 rings (1.78 Å) is significantly longer than the corresponding value of 1.65 Å for the P−N bonds linking the P 2 N 2 rings to the Li 4 N 4 unit. The P 4 (N t Bu) 6 cage 3 in the adduct is formed as the isomer in which two P 2 N 2 rings are bridged by two N t Bu groups. The same tetracyclic isomer of 3 is prepared in 71% yield by the reaction of t BuN(Li)P(μ-N t Bu) 2 PN(Li) t Bu with ClP(μ-N t Bu) 2 PCl in toluene. Variable temperature 31 P{ 1 H} NMR studies of the 16-atom cluster [LiP(N t Bu) 2 ] 4 ( 2 ) at different concentrations in C 7 D 8 indicate that facile dissociation into two molecules of the dimer [LiP(N t Bu) 2 ] 2 occurs above ca. 295 K.