Tuning the strain and polymerizability of organometallic rings: The synthesis, structure, and ring-opening polymerization behavior of [2]ferrocenophanes with C-Si, C-P, and C-S bridges

A series of novel [2]ferrocenophanes with unsymmetrical C-E bridges has bee n prepared in which the covalent radius of the second-row element, E, and h ence the ring strain present is varied. Species [Fe(eta -C5Me4)(eta -C5H4)C H2ERx] (7, ERx = SiMe2; 8a, ERx = PPh; 8b, ERx = PMes: 9; ERx= S) were synt hesized via reaction of the PMDETA (N,N,K,N',N " -pentamethyldiethylenetria mine) adduct of [(eta -C5H4Li)Fe(eta -C5Me4)CH2Li] with Cl2ERx (E = Si or P ) or S(SO2Ph)(2). Studies of 7-9 by single-crystal X-ray diffraction confir med the presence of ring-tilted structures: for 7, alpha (angle between the planes of the Cp rings) = 11.8(1)degrees; for 8a. alpha (average) = 14.9(3 )degrees; for 8b, alpha (average) = 18.2(2)degrees; and for 9, alpha = 18.5 (1)degrees. The least tilted compound, 7, was found to be resistant to ther mal, anionic, and transition metal catalyzed ROP. In contrast, the signific antly more tilted compounds 8a, 8b, and 9 were all found to polymerize ther mally with small negative values of DeltaH(ROP) Of ca. 10-20 kJ . mol(-1) d etermined by DSC. Whereas thermal ROP of 8a yielded the soluble high molecu lar weight polycarbophosphaferrocene [(r-C5Me4)Fe(eta -C5H4)CH2PPh](n) (11) , species 9 formed the insoluble polycarbothiaferrocene [(eta -C5Me4)Fe(eta -C5H4)CH2S](n) (14). Attempted anionic ROP of 8a and 9 with (BuLi)-Bu-n wa s unsuccessful and treatment of 8a with CF3SO3Me resulted in the formation of the novel phosphonium salt [(eta -C5Me4)Fe(eta -C5H4)CH2PMePh][CF3SO3] ( 13). which was found to be resistant to thermal ROP as a result of its less strained structure (for 13, alpha = 11.4(7)degrees). Treatment of 9 with C F3SO3Me or BF3. Et2O resulted in the first example of cationic ROP for a tr ansition metal-containing heterocycle to yield polycarbothiaferrocene 14. I n the presence of excess 2,6-di-tert-butylpyridine as a selective proton tr ap, ROP of 9 was only observed with CF3SO3Me, and not BF3. Et2O, which indi cated that Me+ and H+ are the probable cationic initiators, respectively. T hermal copolymerization of 9 with trimethylene sulfide resulted in the isol ation of the soluble, high molecular weight, random copolymer [(eta -C5Me4) Fe(eta -C5H4)CH2S](n)(CH2)(3)S](m), 15.