Preparation and donor properties of the cyclic methylphosphazenes

Abstract

The cyclic methylphosphazenes (NPMe₂)_3–5 form simple salts such as N₃P₃Me₆,HCl. N₄P₄Me₈,2HClO₄, and N₅P₅Me₁₀H₂CuCl₄,H₂O, and complexes such as N₄P₄Me₈,2HgCl₂, N₄P₄Me₈,4AgNO₃ and N₄P₄Me₈,HCl,CuCl₂. All three phosphazenes form quaternary salts N_nP_nMe_2n,Rl (n= 3–5; R = Me or Et) with alkyl iodides. The iodide can be exchanged for other anions such as Cl^– Or Hgl₃^–; the tetramer forms a dipositive cation in N₄P₄Me_9–HZn(NCS)₄. The phosphorus d-orbitals are less electronegative than they are in the halogenophosphazenes, the ring bonds are consequently weakened, and π-charge is concentrated on nitrogen. The molecular structures of N₄P₄Me₈H⁺, N₄P₄Me₉⁺, and N₅P₅Me₁₀H₂²⁺ show that localisation of two electrons by an acceptor induces substantial bond length inequalities, with patterns expected for the perturbation of an otherwise delocalised π-system. The methylphosphazenes begin to absorb strongly near 190 nm, the band being attributable to a ring transition. Its large energy shows that the electronegativities of the phosphorus and nitrogen orbitals are appreciably different, and, as a consequence, methylphosphazenes make no significant use of antibonding orbitals in complexes with transition metals.