Comparative studies of thermally induced homolytic carbon-carbon bond cleavage reactions of strained dicarba[2]ferrocenophanes and their ring-opened oligomers and polymers

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°C). Thermolysis of dicarba[2]ferrocenophane rac-[Fe(η5-C5H 4)2(CHPh)2] (rac-14; 300°C, 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 [(η5-C5H5)Fe(η5-C 5H4)-CH2Ph] (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 [(η5-C5H5) Fe(η5-C5H4)(CHPh)2-C 5H5] (15; 300°C, 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(η5-C5H4)2(CHPh) 2]n (16; 300°C, 1 h), which was prepared by photocontrolled ROP of rac-14 at 5°C. This afforded ferrocene derivative [Fe(η5-C5H4CH2Ph)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 [(η5-C5H5)Fe(η5- C5H4)(CH2)2(η5-C 5H4)Fe(η5-C5H 4)(CH2)2(η5-C5H 4)Fe(η5-C5H5)] (29) with a -H2C-CH2- 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°C, 1 h), which indicated the presence of higher molecular weight species, postulated to be formed through cross-coupling of these intermediates. Getting ROPed in: Studies of the mechanistic processes that occur under the conditions of thermal ring-opening polymerization (ROP) of dicarba[2] ferrocenophanes have revealed a homolytic C-C bond cleavage pathway that also proceeds in the absence of ring-strain (see scheme). © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.