Lithium and Aluminium Complexes Supported by Chelating Phosphaguanidinates

Diisopropylcarbodiimide, iPrNCNiPr, inserts into the lithium¿phosphorus bond of in situ prepared Ph2PLi(THF)n to afford the lithium salt, [Li(Ph2PC{NiPr}2)(THF)n]x(2a); alternatively, this compound can be made by deprotonation of the neutral phosphaguanidine, Ph2PC{NiPr}{NHiPr}(1a) with nBuLi. Displacement of the THF solvate in 2a is readily achieved with TMEDA to afford Li(Ph2PC{NiPr}2)(TMEDA)(3a). X-Ray crystallographic analyses show that 2a exists as a dimer in the solid state with a folded ladder structure and an N,N chelating phosphaguanidinate, while 3a is monomeric with N,P-coordination of the ligand to lithium. Compound 2a reacts via a transmetallation pathway with AlMe2Cl to afford the dimethylaluminium complex, Al(Ph2PC{NiPr}2)Me2(4a), which can also be prepared by protonation of a methyl group of AlMe3 using 1a. The formation of a series of dialkylaluminium compounds has been investigated employing this latter pathway using both 1a and the N,N-dicyclohexyl analogue, Ph2PC{NCy}{NHCy}(1b), affording Al(Ph2PC{NR}2)Et2(5a, b), Al(Ph2PC{NR}2)iBu2(6a, b) and the diphenylaluminium compound Al(Ph2PC{NiPr}2)Ph2(7a). The oily nature of most of the dialkyl compounds and high sensitivity to oxygen and moisture lead to difficulty in manipulation and characterization; however, NMR spectroscopy indicated highly pure products (>95%) upon removal of the solvent. The molecular structures of the crystalline examples 4a and 7a are reported, showing monomeric aluminium species with symmetrically chelating phosphaguanidinate ligands. The series of aluminium compounds AlLCl2{L =[EC{NiPr}2]¿: A, E = Me; B, E = Me2N; C, E =(Me3Si)2N and D, E = Ph2P} were investigated using density functional theory. In the more simple cases A and B, the delocalized electron density of the metallacycle was represented by a combination of the HOMO and an orbital of lower energy (A, HOMO-5; B, HOMO-6). The HOMO-1 in B was -bonded across the Me2N¿C bond suggesting delocalization of electron density into the metallacycle. In the more complex systems C and D, delocalization within the metallacycle was less extensive due to the (Me3Si)2N- and Ph2P-moieties. A number of occupied orbitals in D, however, display phosphorus lone-pair characteristics, indicating that these species have the potential to behave as Lewis bases in the formation of poly(metallic) systems.