SYNTHESIS OF TRIFERROCENYLBENZENES BY TANTALUM(V)-CATALYZED CYCLOTRIMERIZATION OF ETHYNYLFERROCENE - THE CRYSTAL-STRUCTURE OF 1,3,5-TRIFERROCENYLBENZENE

Exclusive, high-yield formation of the cyclotrimers of ethynylferrocen e (1) was observed when TaCl5 in benzene was employed as catalyst. Ana lysis of the resulting isomer mixture by means of HPLC and H-1 NMR rev ealed the presence of 1,3,5-triferrocenylbenzene (sym-2) and 1,2,4-tri ferrocenylbenzene (asym-2) in the ratio of 4:6. A small amount of pure sym-2 isomer was isolated, and its solid-state structure was establis hed by single-crystal X-ray diffraction. The molecules of sym-2 posses s a distorted up-up-down arrangement of the ferrocenyl groups attached to the benzene ring.