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

Abstract

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 °C dicarba[2]ferrocenophanes meso/rac-[Fe(η⁵-C₅H₄)₂(CHPh)₂] (mesomeso/racrac-7) and meso-[Fe(η⁵-C₅H₄)₂(CHCy)₂] (mesomeso-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[η⁵-C₅H₄]₂[CH(Ph)CH₂]} (15) and [Fe(η⁵-C₅H₄)₂(CH₂)₂] (17) undergo thermal ring-opening polymerization (ROP) under similar conditions (300 °C, 1 h). Thus, thermolysis of 15 yielded polyferrocenylethylene {Fe[η⁵-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[η⁵-C₅H₄]₂[CH(Ph)CH₂]}_n with narrow molecular weight distributions (prepared by photocontrolled ROP) at 300 °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(η⁵-C₅H₄)₂(CD₂)₂] (dd₄4-17) were conducted in order to probe the bond cleavage mechanism. Comparative NMR spectroscopic analysis of the resulting copolymers 18 and dd₄4-18, 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 = η⁵-C₅H₅) at 300 °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.