An iron-cyclopentadienyl bond cleavage mechanism for the thermal ring-opening polymerization of dicarba[2]ferrocenophanes

In order to gain insight into the mechanism for the thermal ring-opening polymerization of strained dicarba[2]ferrocenophanes, the thermal reactivity of selected examples of these species with different substitution patterns has been explored. When heated at 300 degrees C dicarba[2]ferrocenophanes meso/rac-[Fe(eta(5)-C5H4)(2)(CHPh)(2)] (meso/rac-7) and meso-[Fe(eta(5)-C5H4)(2)(CHCy)(2)] (meso-13) were found to isomerize or to undergo disproportionation, respectively. These processes are apparently general for dicarba[2] ferrocenophanes with one or more non-hydrogen substituents at each carbon atom in the dicarba bridge and both appear to involve homolytic cleavage of the C-C bond in the bridge as a key step. In striking contrast, derivatives containing either one or no non-hydrogen substituents on the bridge such as {Fe[eta(5)-C5H4](2)[CH(Ph)CH2]} (15) and [Fe(eta(5)-C5H4)(2)(CH2)(2)] (17) undergo thermal ring-opening polymerization (ROP) under similar conditions (300 degrees C, 1 h). Thus, thermolysis of 15 yielded polyferrocenylethylene {Fe[eta(5)-C5H4](2)[CH(Ph)CH2]}(n) (16a) with a broad molecular weight distribution (M-w = 13,760, PDI = 3.27). Analysis of 16a by MALDI-TOF mass spectrometry suggested that the material was macrocyclic. Thermal treatment of linear polyferrocenylethylenes {Fe[eta(5)-C5H4](2)[CH(Ph) CH2]}(n) with narrow molecular weight distributions (prepared by photocontrolled ROP) at 300 degrees C confirmed that the macrocycles detected form directly, and not as a result of depolymerization. Copolymerizations of 15 with 17 and of 15 with the deuterated species [Fe(eta(5)-C5H4)(2)(CD2)(2)] (d(4)-17) were conducted in order to probe the bond cleavage mechanism. Comparative NMR spectroscopic analysis of the resulting copolymers 18 and d(4)-18, respectively, and of homopolymer 16a, indicated that thermal ROP does not occur via a homolytic C-C bridge cleavage mechanism. A series of thermolysis experiments were conducted with MgCp2 (Cp = eta(5)-C5H5) at 300 degrees C, which resulted in the isolation of ring-opened species formed from 15 and 17, and indicated that the Fe-Cp bonds can be cleaved under the thermal ROP conditions employed. The studies indicated that a chain growth process that involves heterolytic Fe-Cp bond cleavage in the monomers is the most probable mechanism for the thermal ROP of dicarba[2]ferrocenophanes.