Synthesis and Molecular Structure of Two Cyclotriphosphazenes with Phosphorus-Iron Bonds

Hexafluorocyclotriphosphazene (I), (NPF₂)₃, reacts with sodium dicarbonylcyclopentadienylferrate to form photolytically unstable 1,1-bis(dicarbonylcyclopentadienyliron)-3,3,5,5-tetrafluorocyclotriphosphazene (II). This compound decarbonylates to 1,1-[P-carbonyl-bis(carbonylcyclopentadienyliron)(Fe-Fe)]-3,3,5,5-tetrafluorocyclotriphosphazene (III) on exposure to light. Compounds II and III are the first phosphazenes known that contain direct phosphorus-metal side group bonds. In addition, III is the first phosphazene with a three-membered spirocyclic ring at phosphorus. Compounds II and III are forerunners of a potentially important class of new compounds based on other transition-metal systems. Spectroscopic evidence and X-ray crystallographic analyses were used to confirm the two structures. Compound II crystallizes in a triclinic unit cell of P1 symmetry with lattice constants of a = 9.035 (3) Å, b = 14.287 (15) Å, c = 8.613 (4) Å, a = 104.04 (4)Å, β = 110.44 (1)°, and y = 96.56 (2)°; ρ_calcd = 1.9 g/cm₁ for Z = 2. Least-squares refinement gave R₁ = 0.056 and R₂ = 0.066 for 3906 observed reflections with I > 3σ(I). The distance between the iron atoms in II is 3.922 (1) Å, and the Fe-P-Fe angle is 119.12 (3)°. Compound III crystallizes in a triclinic unit cell of PI symmetry with lattice constants of a = 9.116 (14) Å, b = 14.219 (9) Å, c = 7.735 (17) Å, a = 90.48 (3)°, β = 113.93 (4)6 7, and y = 92.83 (2)6; ρ_calcd = 1.9 g/cm³ for Z = 2. Least-squares refinement gave R₁ = 0.0498 and R₂ = 0.0524 for 2634 observed reflections with I ≥ 3σ(I). The Fe-Fe bond distance in III is 2.593 (1) A and the Fe-P-Fe bond angle is 72.77 (3)°. The N-P-N bond angles at the phosphorus that is bonded to the iron atoms are 109.18 (11) and 110.09 (16)° for compounds II and III, respectively. In both compounds, an alternation of longer and shorter bonds exists for the P-N bonds located at increasing distance from the iron atoms.