Phosphorus-based ambidentate chelating ligands: pyridyl-N- and imido-N-metal coordination in the Py₂P(NSiMe₃)₂ anion

Abstract

Monoanionic heteroallylic ligand systems [R-N-E-N-R]^- (E = Si(R₂), S(R₂) or S(R), C(R), and P(R₂)) are versatile chelating substituents both in main group and transition metal chemistry as they provide sufficient steric demand and solubility to the products. Their application is only limited by the rigid bite of the ligands as the N···N distance cannot be tuned to the various radii of different metals. In this paper we present the new concept of opening the ligand periphery to additional coordination. The NP(R₂)N^- chelate in classical aminoiminophosphoranates is extended by additional coordination sites in the organic substituents (e.g., 2-pyridyl (Py) instead of phenyl (Ph)). Py₂P{N(H)SiMe₃}(NSiMe₃) (1) is the starting material for a new class of complexes as deprotonated 1 contains along with the NPN- chelate the pyridyl ring nitrogen atoms to generate a side-selective Janus face ligand. In [(THF)Sr{Py₂P(NSiMe₃)₂}₂] (2) and [(4,4'-bipy)Ba{Py₂P(NSiMe₃)₂}₂]_n (3) both pyridyl rings are involved in metal coordination but only one imido nitrogen atom. Hence, the classical NP(Ph₂)N- chelating ligand is converted into a NP(Py₂)N- tripodal ligand. In the coordination to zinc in the complex [Zn{Py₂P(NSiMe₃)₂}₂] (4) one pyridyl ring and one imido nitrogen atom is employed in metal coordination. Pyridyl substitution of the P(V) center gives not only access to new coordination modes but also changes the reactivity of aminoiminophophoranes considerably. [Li(Py₂PNSiMe₃)]₂ (5) is a lithiated phosphanylamine derived from the reduction of 1 with lithium di(trimethylsilyl)amide. Reaction of 1 with lithium organics yields [(THF)₂Li(Py₂P)] (6). Pyridyl substitution facilitates single or even double P=N bond cleavage, unprecedented in alkyl- or aryl-substituted aminoiminophosphoranes. This reduction of P(V) species to P(III) compounds supplies easy access to phosphanylamines and secondary phosphanes.