Structural Comparison of Lithium Iodide Complexes of Symmetrical and Unsymmetrical [CH₂(PPh₂NSiMe₃)(PPh₂NR)](R = SiMe₃, H) Ligands

Abstract

Compounds [(LiI)1] and [(LiI)2]₂ crystallize in the centrosymmetric space group P2₁/n. They are made up of neutral ligands [H₂C(PPh₂NSiMe₃)₂] (1) and [H₂C(PPh₂NSiMe₃)(PPh₂NH)] (2) and a LiI molecule. In both cases, N,N chelation with lithium is observed. Ligand 2 contains two different nitrogen centres viz., P=N(SiMe₃) and P=N(H), which are coordinated unsymmetrically to lithium (Li–N = 2.055(8) and 2.072(8) Å) to form [{LiI}{CH₂(PPh₂NSiMe₃)×(PPh₂NH)}] as monomer units that are linked via intermolecular coordination between NH and Li (2.097(8) Å) to form a central four-membered ring, Li₂N₂ with four-coordinate lithium atoms. In contrast, [(LiI)1] is monomeric with a three-coordinate lithium centre. This disparity is reflected in the Li–I bond distances (2.699(11) Å for [(LiI)1] and 2.824(7) Å) for [(LiI)2]₂). The dimer [(LiI)2]₂ displays intramolecular C_sp3H–π and intermolecular C_sp2H–π interactions (between phosphorus-substituted phenyl groups).