Coordination chemistry of cyclotriphosphazenes

Abstract

The coordination chemistry of cyclophosphazenes is of great interest from the point of view of developing synthetic strategies for anchoring transition metal moieties to P–N polymers that may function as supported catalysts. The reactions of (amino)spirocyclic and pyrazolyl cyclotriphosphazenes with Group 6 metal carbonyls and palladium chloride have been investigated, and the structures of the complexes elucidated by IR and NMR spectroscopy and X-ray crystallography.

(Amino)spirocyclic phosphazenes, N?P?(NMe?)?[NH(CH?)?NH] (n = 2 or 3), form four-membered chelate complexes with metal carbonyls and palladium chloride, in which an exocyclic nitrogen atom and an adjacent phosphazene ring nitrogen atom are bonded to the metal. The PdCl? complex of (1,2-diaminoethane)spirocyclic phosphazene is highly unstable and undergoes a facile hydrolytic cleavage of the five-membered diazaphospholidine ring to yield monometallic and bimetallic complexes, [PdCl?·{HN?P?(NMe?)?(O)-(NHCH?CH?NH?)}]? and [PdCl?·{N?P?(NMe?)?(NHCH?CH?NH?)}]? (0), both of which exhibit novel structural features. A mechanism is proposed for the formation of these complexes from NMR monitoring of the hydrolysis reaction.

Gem-bis(pyrazolyl) cyclotriphosphazenes (dmp = 3,5-dimethyl-1-pyrazolyl), N?P?H?(dmp)? and N?P?(MeNCH?CH?O)?(dmp)?, react with metal carbonyls to give metal tricarbonyl complexes in which the phosphazenes act as novel tridentate NNN-donor ligands via the two pyridinic pyrazolyl nitrogen atoms and an adjacent phosphazene ring nitrogen atom. A nongeminally substituted cis-bis(dmp) cyclotriphosphazene, N?P?(OPh)?(dmp)?, and its palladium chloride complex have also been synthesized, and their structures established by X-ray crystallography. In this complex, the ligand binds to the metal through the two pyridinic pyrazolyl nitrogen atoms; in addition, there is a weak interaction between the metal and a phosphazene ring nitrogen atom.

The investigations demonstrate that even the “rigid” six-membered cyclotriphosphazenes, if suitably substituted, can form transition metal complexes through their ring nitrogen atoms, exocyclic donor sites, or both, and thus offer a vast scope for designing novel ligands.