Hexakispyrazolylcyclotriphosphazene as a versatile ligand to new metallophosphazenes

Abstract

Phosphazenes, cyclic or polymeric forms, are unique carriers for transition metals owing to their ability to serve as versatile multifunctional ligands and templates. The type of side group attached to the phosphazene chain and transition metals have a powerful effect on the physical and chemical characteristics. As object of our program to better understand the structure-property relationships in poly(organometallic phosphazenes), we have synthesized a series of small molecule cyclic phosphazenes as structural models for the corresponding high polymers. The crystal structure of 1 shows that the six pyrazole groups are randomly oriented. The phosphazene ring is almost planar, with a maximum deviation of 0.034$\mbox{\AA}$ from the best plane. Reaction of 3 with three fold excess amount of $MX_n$ in $CH_2Cl_2$ gave unprecedented mononuclear metallophosphazenes [$LMX_n$] ($MX_n$ = $HgCl_2$ 6, $Mo(CO)_3$ 8), dinuclear metallophosphazenes [$X_nM(\mu-L)MX_n$] ($MX_n\.=\.ZnCl_2$ 4, CuI 5, $CuCl_2$ 9, $CoCl_2$ 10, $MnCl_2$ 11) or patially hydrolyzed metallophosphazene 7 ($MX_n$ = $FeCl_3$). In 4, L coordinates two $ZnCl_2$ units via geminal pyrazolyl $\eta^2-N_2$ core and via an $\eta^3-N_3$ bite, giving distorted tetrahedral(Td) and trigonalbipyramidal(TBP) geometries around zinc atoms, respectively. In 5, the ability of L to provide double tridentate $N_3$ cores involving phosphazene ring nitrogens affords two severely distorted tetrahedral Cu centers disposed on opposite sides of a rather planar phosphazene ring plane with a separation of 6.790(2)$\mbox{\AA}$. In solution at ambient temperature, the compounds 4 and 5 lose their solid state symmetry of $C_1$ and $C_2$ respectively and undergo unique fluxional motions. The compound 4 shows unusual solvent dependency of fluxionality and in acetone, $A_2B$ phosphorus coupling pattern is seen at 188K while in dichloromethane, a very complicated spectral change results. At low temperature limit, the solution symmetry of 5...