Cyclic and short-chain linear phosphazenes with hindered aryloxy side groups

Abstract

A series of short-chain and cyclic phosphazenes bearing o-dichloro- and o-dimethyl-phenoxy groups have been synthesised, and their structures studied by X-ray crystallography. These compounds are small-molecule models for the corresponding linear high polymers. Structural parameters: N₃P₃Cl₅(OC₆H₃Cl₂-o)1, a= 8.081(4), b= 8.074(2), c= 25.868(3)Å, β= 91.11(2)°, space group P2₁/c, Z= 4; gem-N₃P₃Cl₄(OC₆H₃Cl₂-o)₂2, a= 18.856(5), b= 8.002(2), c= 16.124(2)Å, β= 114.89(2)°, space group P2₁/c, Z= 4; N₃P₃Cl₂(OC₆H₃Cl₂-o)₄4, a= 7.975(3), b= 18.373(4), c= 22.225(3)Å, β= 90.35(2)°, space group P2₁/n, Z= 4; [NP(OC₆H₃Cl₂-o)₂]₃5, a= 12.021(4), b= 19.938(5), c= 18.379(11)Å, β= 90.84(5)°, space group P2₁/n, Z= 4; gem-N₄P₄Cl₄(OC₆H₃Cl₂-o)₄6, a= 10.806(2), b= 10.861(2), c= 17.075(7)Å, α= 89.33(4), β= 80.79(3), γ= 68.44(2)°, space group P1, Z= 2; gem-N₃P₃Cl₄(OC₆H₃Me₂-o)₂7, a= 9.470(3), b= 18.481(12), c= 13.087(8)Å, β= 94.56(4)°, space group P2₁/n, Z= 4; [NP(OC₆H₃Me₂-o)₂]₃8, a= 20.560(5), b= 20.580(9), c= 22.085(4)Å, α=β=γ= 90.0°, space group P4/ncc, Z= 8; OPCl₂NP(OC₆H₃Cl₂-o)₃9, a= 14.319(4), b= 16.837(4), c= 20.660(8)Å, space group Pbca, Z= 8; OPCl(OC₆H₃Me₂-o)NPCl(OC₆H₃Me₂-o)NP(OC₆H₃Me₂-o)₃10, a= 15.212(3), b= 18.965(1), c= 14.559(2)Å, β= 102.96(2)°, space group P2₁/c, Z= 4. Despite the bulkiness of the o-dichloro- and o-dimethyl-phenoxy groups, their introduction into the phosphazene structure followed a geminal substitution pathway during the early stages of the reaction. In the reactions with the linear short-chain oligomers these nucleophiles preferentially attacked at the terminal PCl₃ rather than at the PCl units in the middle of the chain or those associated with the phosphoryl group. The structure of compound 10 revealed a distorted trans-trans conformation for the PN skeleton, in contrast to the cis-trans arrangement generally believed to exist for linear phosphazene high polymers.