Synthesis and Structures of (p-Halogenophenoxy)phosphazenes: Comparison of the Structures of Cyclic and Linear Short-Chain Species

The crystal structures of OP(OR)₂NP(OR)₃, where OR = O-C₆H₄F-p, O-C₆H₄Cl-p, and O-C₆H₄Br-p, and [NP- (OR)₂]₃, where OR = O-C₆H₄F-p, O-C₆H₄Cl-p, O-C₆H₄Br-p and O-C₆H₄I-p, were determined by X-ray crystallography. These small molecule phosphazenes are structural models for the corresponding high polymers, and their structures provide insight into the conformation and nature of bonding in the polymeric species. Crystals of OP(O-C₆H₄F-p)₂NP(O-C₆H₄F-p)₃ are triclinic with cell dimensions of a = 8.394(2) Å, b = 10.690(3) Å, c = 16.760(4) Å, a = 86.99(2)°, β = 84.82(2)°, γ = 69.66(2)°, space group P[formula omitted], Z = 2. Crystals of OP(0-C₆H₄-Cl-p)₂NP(O-C₆H₄Cl-p)₃ are triclinic with cell dimension of a = 10.513(6) Å, b = 12.574(3) Å, c = 13.359(5) Å, α = 91.83(3)°, β = 106.76(4)°, γ = 104.68(3)°, space group. and Z = 2. Crystals of OP(O-C₆H₄Br-p)₂NP-(O-C₆H₄Br-p) are triclinic with cell dimension of a = 9.839(2) Å, b = 12.373(4) Å, c = 14.286(3) Å, α = 103.74(2)°, β = 91.76(2)°, γ = 100.89(2)°, space group P[formula omitted], and Z = 2. Crystals of the cyclic species are monoclinic. The unit cell dimensions are as follows: [NP(O-C₆H₄F-p)₂] ₃, a = 20.860(5) Å, b = 8.085(12) Å, c = 21.813(5) Å, β = 104.62(3)°, space group P2₁/n, Z = 4; [NP(O-C₆H₄Cl-p)₂]₃, a = 17.746(9) Å, b = 7.623(7) Å, c = 27.856(10) Å, β= 91.76(12)°, space group P2₁/c,Z = 4; [NP(O-C₆H₄Br-p)₂]₃, a = 13.680(5) Å, b = 31.316(7) Å, c = 9.289(3).β = 90.70(3)°, space group P2₁/n, Z = 4; [NP(O-C₆H₄I-p)₂]₃, a = 13.700(6) Å, b = 9.381(11) Å, c = 32.675(16) Å, β = 95.72(4)°, space group P2₁/c, Z = 4. The results show that the P-N bond lengths in both the short-chain and cyclic species are very similar, with an average value of 1.55 Å. Evidence for side group stacking can be detected in the crystal lattice of both the cyclic and linear oligomers, and these arrangements are analogous to those expected in polymer microcrystallites.