Lithium and aluminium complexes supported by chelating phosphaguanidinates

Abstract

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 π-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.