Metal Exchange Reactions under the Influence of a Cyclophosphazene Template: Iron, Cobalt, and Rhodium Metallophosphazenes

The use of a cyclophosphazene ring as a template for the construction of transition-metal dimers or clusters is described. The thermal reactions of a spiro diiron octacarbonyl bonded phosphazene (2) with Co₂(CO)₈, CoCp(CO)₂, or RhCp(CO)₂(Cp = η ⁵-C₅H₅) lead to the formation of spiro-metallo or metallo cluster phosphazenes in which phosphorus-iron bonds have been replaced by phosphorus-cobalt or phosphorus-rhodium bonds. Reaction of 2 with Co₂(CO)₈yields the mixed-metal cluster 4 in which two cobalt atoms are covalently bound to the bridging phosphazenyl phosphorus atom and the iron atom is coordinatively linked to the ring nitrogen atom. Reaction of 2 with CoCp(CO)₂results in the formation of a phosphazene-bound Fe-Co dimer (5) and a phosphazene-bound Co-Co dimer (6). Similarly, 2 reacts with RhCp(CO)₂to yield a phosphazene-bound Rh-Rh dimer (7). The reaction of 5 with RhCp(CO)₂also leads to the formation of 7, with no evidence for the formation of any Fe-Rh or Co-Rh dimeric species. The new metallophosphazene compounds were characterized by ³¹P NMR, Mossbauer, infrared, and mass spectral techniques and by X-ray crystal structure analyses. These small molecule metallophosphazenes are model systems for the corresponding linear high polymers.