Synthesis and Crystal Structures of Lithium Salts of New Iminophosphide/Phosphinoamide Anions

Aminodiphenylphosphines are deprotonated by alkyllithium reagents to give the corresponding iminophosphide/phosphinoamide ions:  lithium (neopentyl)(diphenylphosphino)amide (3), lithium (isopropyl)(diphenylphosphino)amide (4), and lithium (2,4,6-tri-tert-butylphenyl)(diphenylphosphino)amide (5). Derivative 3 crystallizes in the monoclinic space group P2₁/c with Z = 4, a = 17.717(2) Å, b = 11.167(2) Å, c = 22.151(1) Å, β = 104.35(1)°, R1 = 0.064, and wR2 = 0.184 at −73 °C. Derivative 4 crystallizes in the orthorhombic space group Pna2₁ with Z = 4, a = 20.557(4) Å, b = 10.475(2) Å, c = 17.982(4) Å, R1 = 0.044, and wR2 = 0.089 at −70 °C. Derivative 5 crystallizes in the monoclinic space group C2/c with Z = 8, a = 47.256(6) Å, b = 9.126(2) Å, c = 18.246(2) Å, β = 104.67(1)°, R1 = 0.064, and wR2 = 0.165 at −73 °C. The solid-state structures of compounds 3 and 4 consist of approximately centrosymmetric dimers containing a Li₂N₂ rhombus with each lithium ion bound by both anions through their nitrogen atoms and by an ether solvate to give approximate trigonal planar coordination geometry. In some cases, weak second-sphere dipole interactions are suggested by relatively short P···Li distances. The anions of 3 and 4 adopt cis conformations. In contrast, 5 is a monomer in the solid state. The anion of 5 adopts a trans conformation and the cation, which is bound by two ether solvates, exhibits approximately trigonal planar geometry.