The phosphole tetramer 1, obtained by pyrolysis of 1-phenyl-3,4-dimethylphosphole, incorporates two 2,2'-biphospholyl units linked by two P-P bonds. It is possible to cleave one or both of these bonds by naphthalene sodium in THF to yield two dianions, 2 and 4. The reaction of 2 or 4 with dibromomethane gives either a 1,3,6,8-tetraphosphecane 5 or a 1,2,5,7-tetraphosphonane 7 in 45 and 80% yield, respectively. The reaction of 2 with tetrachloroethylene takes an unexpected course with the formation of a 1,2,5,8-tetraphosphacyclodec-6-yne 6, whose structure was determined by X-ray analysis. The reaction of 4 with 1,4-dibromobutane affords, in 80% yield, a 1,2,5,10-tetraphosphacyclododecane 8, whose P-P bond can be selectively cleaved by naphthalene-sodium in THF at low temperature to give further dianion 10. In turn, 10 reacts with dibromomethane or 1,4-dibromobutane to yield the expected 13- and 16-membered macrocycles 11 and 12. Since the phosphorus atoms of these macrocycles are all included in phosphole rings, they readily invert close to room temperature. The macrocycles can therefore adapt their conformations to the stereochemical requirements of the complexed metals. A preliminary study of the coordination chemistry of 5 and 7 has been carried out. Both macrocycles can chelate either one or two Mo(CO)(4) units via their diagonal phosphorus atoms. The X-ray crystal structure analyses of 5.Mo(CO)(4) (15) and 7.[Mo(CO)(4)](2) (17) have been performed. The structure of the cage complex 17 shows a Mo...Mo distance of 5.883 Angstrom. Thus, it seems possible to use this kind of cage for the bimetallic activation of small molecules.
