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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

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journal contribution
posted on 20.02.2001 by Rui Resendes, James M. Nelson, Armin Fischer, Frieder Jäkle, Alexandra Bartole, Alan J. Lough, Ian Manners
A series of novel [2]ferrocenophanes with unsymmetrical C−E bridges has been prepared in which the covalent radius of the second-row element, E, and hence the ring strain present is varied. Species [Fe(η-C5Me4)(η-C5H4)CH2ERx] (7, ERx = SiMe2; 8a, ERx = PPh; 8b, ERx = PMes; 9, ERx = S) were synthesized via reaction of the PMDETA (N,N,N‘,N‘ ‘,N‘ ‘-pentamethyldiethylenetriamine) adduct of [(η-C5H4Li)Fe(η-C5Me4)CH2Li] with Cl2ERx (E = Si or P) or S(SO2Ph)2. Studies of 79 by single-crystal X-ray diffraction confirmed the presence of ring-tilted structures:  for 7, α (angle between the planes of the Cp rings) = 11.8(1)°; for 8a, αaverage = 14.9(3)°; for 8b, αaverage = 18.2(2)°; and for 9, α = 18.5(1)°. The least tilted compound, 7, was found to be resistant to thermal, anionic, and transition metal catalyzed ROP. In contrast, the significantly more tilted compounds 8a, 8b, and 9 were all found to polymerize thermally with small negative values of ΔHROP of ca. 10−20 kJ·mol-1 determined by DSC. Whereas thermal ROP of 8a yielded the soluble high molecular weight polycarbophosphaferrocene [(η-C5Me4)Fe(η-C5H4)CH2PPh]n (11), species 9 formed the insoluble polycarbothiaferrocene [(η-C5Me4)Fe(η-C5H4)CH2S]n (14). Attempted anionic ROP of 8a and 9 with nBuLi was unsuccessful and treatment of 8a with CF3SO3Me resulted in the formation of the novel phosphonium salt [(η-C5Me4)Fe(η-C5H4)CH2PMePh][CF3SO3] (13), which was found to be resistant to thermal ROP as a result of its less strained structure (for 13, α = 11.4(7)°). Treatment of 9 with CF3SO3Me or BF3·Et2O resulted in the first example of cationic ROP for a transition metal-containing heterocycle to yield polycarbothiaferrocene 14. In the presence of excess 2,6-di-tert-butylpyridine as a selective proton trap, ROP of 9 was only observed with CF3SO3Me, and not BF3·Et2O, which indicated that Me+ and H+ are the probable cationic initiators, respectively. Thermal copolymerization of 9 with trimethylene sulfide resulted in the isolation of the soluble, high molecular weight, random copolymer [(η-C5Me4)Fe(η-C5H4)CH2S]n[(CH2)3S]m, 15.