Phosphinimide ligands, also known as phosphorane iminato ligands, are any of a class of organic compounds of the general formula NPR3−. The R groups represent organic substituents or, in rare cases, halides or NR2 groups. NPR3− is isoelectronic with phosphine oxides (OPR3) and siloxides ([OSiR<sub>3</sub>]−), but far more basic. By varying the R groups on P, a variety of ligands with different electronic and steric properties can be produced, and due to the high oxidation state of phosphorus, these ligands have good thermal stability.[1] Many transition metal phosphinimide complexes have been well-developed as have main group phosphinimide complexes.[2]
In main group phosphinimide complexes, only terminal and μ2-N-bridging bonding modes are observed. The terminally bound bent ligands are primarily commonly have M-N-P bond angles ranging from 120-150°. Both the M-N and N-P bond lengths are appropriate for double bonds. This bonding can best be described by a covalent single bond with an overlaying share of polar bonding. The μ2-N-bridging mode arises when the free electron pair at nitrogen gives rise to dimerization. These dimeric complexes yield different M-N bond lengths depending on the ligands present in the rest of the ligand sphere of M. When the complex contains two or four identical ligands, nearly equal M-N distances are observed, whereas, when different or odd-numbered identical ligands are in the complex, the M-N distances are all of significantly different length.
Phosphonimines with the formula R3P=NSiMe3 are particularly useful. They are prepared by the Staudinger reaction of tertiary phosphines with trimethylsilyl azide:
R3P + N3SiMe3 → R3P=NSiMe3 + N2R3P=NSiMe3 undergoes alcoholysis to give the parent imine:
R3P=NSiMe3 + MeOH → R3P=NH + MeOSiMe3Ammonia can be used in place of alcohol.[3]
Lithium phosphinimides are produced by deprotonation of the parent imine:
R3P=NH + RLi → R3P=NLi + RHThe lithio derivatives, which exist as tetrameric clusters in the solid state, are useful reagents.[4] [5]