Comparative Studies of Thermally Induced Homolytic Carbon-Carbon Bond Cleavage Reactions of Strained Dicarba[2] ferrocenophanes and Their Ring-Opened Oligomers and Polymers

Andrew D. Russell, Joe B. Gilroy, Ian Manners*

*Corresponding author for this work

Research output: Contribution to journalArticle (Academic Journal)peer-review

5 Citations (Scopus)

Abstract

Reactivity studies of dicarba[2]ferrocenophanes and also their corresponding ring-opened oligomers and polymers have been conducted in order to provide mechanistic insight into the processes that occur under the conditions of their thermal ring-opening polymerisation (ROP) (300 degrees C). Thermolysis of dicarba[2]ferrocenophane rac-[Fe((5)-C5H4)(2)(CHPh)(2)] (rac-14; 300 degrees C, 1h) does not lead to thermal ROP. To investigate this system further, rac-14 was heated in the presence of an excess of cyclopentadienyl anion, to mimic the postulated propagating sites for thermally polymerisable analogues. This afforded acyclic [((5)-C5H5)Fe((5)-C5H4)-CH2Ph] (17) through cleavage of both a FeCp bond and also the CC bond derived from the dicarba bridge. Evidence supporting a potential homolytic CC bond cleavage pathway that occurs in the absence of ring-strain was provided through thermolysis of an acyclic analogue of rac-14, namely [((5)-C5H5)Fe((5)-C5H4)(CHPh)(2)-C5H5] (15; 300 degrees C, 1h), which also afforded ferrocene derivative 17. This reactivity pathway appears general for post-ROP species bearing phenyl substituents on adjacent carbons, and consequently was also observed during the thermolysis of linear polyferrocenylethylene [Fe((5)-C5H4)(2)(CHPh)(2)](n) (16; 300 degrees C, 1h), which was prepared by photocontrolled ROP of rac-14 at 5 degrees C. This afforded ferrocene derivative [Fe((5)-C5H4CH2Ph)(2)] (23) through selective cleavage of the H(Ph)CC(Ph)H bonds in the dicarba linkers. These processes appear to be facilitated by the presence of bulky, radical-stabilising phenyl substituents on each carbon of the linker, as demonstrated through the contrasting thermal properties of unsubstituted linear trimer [((5)-C5H5)Fe((5)-C5H4)(CH2)(2)((5)-C5H4)Fe((5)-C5H4)(CH2)(2)((5)-C5H4)Fe((5)-C5H5)] (29) with a H2CCH2 spacer, which proved significantly more stable under analogous conditions. Evidence for the radical intermediates formed through CC bond cleavage was detected through high-resolution mass spectrometric analysis of co-thermolysis reactions involving rac-14 and 15 (300 degrees C, 1h), which indicated the presence of higher molecular weight species, postulated to be formed through cross-coupling of these intermediates.

Original languageEnglish
Pages (from-to)4077-4085
Number of pages9
JournalChemistry - A European Journal
Volume20
Issue number14
DOIs
Publication statusPublished - 1 Apr 2014

Keywords

  • carboncarbon bond cleavage
  • dicarba[2]ferrocenophanes
  • radicals
  • ring-opening polymerization
  • HIGH-MOLECULAR-WEIGHT
  • OPENING POLYMERIZATION
  • ORGANOMETALLIC RINGS
  • BRIDGING POSITIONS
  • BLOCK-COPOLYMERS
  • COMPLEXES
  • SILICON
  • REACTIVITY
  • PYROLYSIS
  • EFFICIENT

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