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(h 5 -C 5 H 4 ) 2 (CHPh) 2 ] (meso/rac-7) and meso-[Fe(h 5 -C 5 H 4 ) 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 17) undergo thermal ringopening polymerization (ROP) under similar conditions (300 C, 1 h). Thus, thermolysis of 15 yielded polyferrocenylethylene {Fe[h 5 -C 5 H 4 ] 2 [CH(Ph)CH 2 ]} n (16a) with a broad molecular weight distribution (M w ¼ 13,760, PDI ¼ 3.27). Analysis of 16a by MALDI-TOF mass spectrometry suggested that the material was macrocyclic. Thermal treatment of linear polyferrocenylethylenes {Fe[h 5 -C 5 H 4 ] 2 [CH(Ph) CH 2 ]} 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(h 5 -C 5 H 4 ) 2 (CD 2 ) 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 MgCp 2 (Cp ¼ h 5 -C 5 H 5 ) 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.

Recent developments with strained metallocenophanes

“…The thermal reactivity pathways proved consistent for other dicarba[2] ferrocenophanes with similar aryl and alkyl substituents on the dicarba-bridge. [17].…”

“…NaCp) which do not attack at Si [15]. Furthermore, as discussed in the previous section, cleavage of the bridge EeE bond has been demonstrated for a variety of [2]metallocenophanes [1f, 16,17,22], Until recently, the reactivity modes of [n]metallocenophanes were confined to these aforementioned bond-cleavage processes, with the concomitant release of molecular ring-strain. However, in a recent report a new mode of [n]metallocenophane reactivity based on metalemetal bond formation was elucidated.…”

“…A further investigation into the mechanism of thermal ROP was subsequently conducted in which copolymerization experiments were performed using a series of dicarba[2]ferrocenophanes known to undergo thermal ROP [17]. Thus, mixtures of 8 and 10, and also of 10 and the isotopically labelled analogue d 4 -8 [Fe(h 5 -C 5 H 4 )(CD 2 ) 2 ], were exposed to standard thermolysis conditions, affording random, cyclic copolymeric products of moderate molecular weight and with a broad molecular weight distribution, which is typical of PFE homopolymers isolated via this route (M w ¼ 4800e96,000 Da, PDI ¼ 1.1e3.6) [15,19].…”

Strained Ferrocenophanes

“…40 A rationale for the differences in thermal reactivity based on the structural changes that result from differing bridge substitution has been proposed. 40 Species 37−40, with an equal number of non-hydrogen substituents on each carbon of the bridge, possess longer C−C bridge bonds relative to species 34 and 41, which have one or zero non-hydrogen substituent, respectively. Thus, for species 35 and 37−39, weakening of the C−C bridge bond, in combination with the ability of bridge substituents to stabilize the radical intermediates formed from where one or zero non-hydrogen substituent is present, the C−C bond is relatively short, hypothetical radical intermediates formed from homolytic C−C cleavage are not stabilized, and thermal ROP occurs via a heterolytic Fe−Cp bond cleavage mechanism to afford polyferrocenylethylene products.…”

Sandwich Complex‐Containing Macromolecules: Property Tunability Through Versatile Synthesis

“…The versatility of ROP is also evident from the numerous spacers that can be incorporated in the polymeric main chain through this technique. These spacers that include single atom spacers such as Si, P, B, Sn, S, Ge, Ga, Al, Se; two‐atom spacers such as C–C, C–P, C–Si, C–S, C–Ge, C–Sn; as well as a four‐atom butadienylene spacer, affect the properties of macromolecules. For instance, reaction of dilithioferrocene ( 90 ) with the gallium dichloride ( 91 ) yielded the strained gallium‐bridged ferrocenophane ( 92 ), which spontaneously underwent ROP to afford the poly(ferrocenylgallane) ( 93 ) (Scheme ) .…”