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

摘要

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 ℃). Thermolysis of dicarba[2]ferrocenophane rac-[Fe(η~5- C_5H_4)_2(CHPh)_2] (rac-14; 300 ℃, 1 h) 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 [(η~5-C_5H_5)Fe(η~5- C_5H_4)-CH_2Ph] (17) through cleavage of both a Fe-Cp bond and also the C-C bond derived from the dicarba bridge. Evidence supporting a potential homolytic C-C bond cleavage pathway that occurs in the absence of ring-strain was provided through thermolysis of an acyclic analogue of rac-14, namely [(η~5-C_5H_5)Fe(η~5-C_5H_4)(CHPh)_2-C_5H_5] (15; 300 ℃, 1 h), 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(η~5-C_5H_4)_2(CHPh)_2]_n (16; 300 ℃, 1 h), which was prepared by photocontrolled ROP of rac-14 at 5℃. This afforded ferrocene derivative [Fe(η~5-C_5H_4CH_2Ph)_2] (23) through selective cleavage of the -H(Ph)C-C(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 [(η~5-C_5H_5)Fe(η~5-C_5H_4)(CH_2)_2(η~5-C_5H_4)Fe(η~5- C_5H_4)(CH_2)_2(η~5-C_5H_4)Fe(η~5-C_5H_5)] (29) with a -H_2C-CH_2- spacer, which proved significantly more stable under analogous conditions. Evidence for the radical intermediates formed through C-C bond cleavage was detected through high-resolution mass spectrometric analysis of co-thermolysis reactions involving rac-14 and 15 (300 ℃, 1 h), which indicated the presence of higher molecular weight species, postulated to be formed through cross-coupling of these intermediates.