Strain-Induced Cleavage of Carbon−Carbon Bonds: Bridge Rupture Reactions of Group 8 Dicarba[2]metallocenophanes

Thermal treatment of dicarba[2]ferrocenophanes [Fe(η5-C5H4)2(CMe2)2] (1), rac-[Fe(η5-C5H4)2(CHiPr)2] (rac-5), and meso-[Fe(η5-C5H4)2(CHtBu)2] (meso-7) at 240−300 °C in the melt led to cleavage of the carbon−carbon bond in the bridge. Compounds 1 and rac-5 underwent intramolecular abstraction of H• and yielded ring-opened, vinyl-substituted 1,1ˈ-metallocenes, while meso-7 thermally converted to the more thermodynamically stable rac isomer. The corresponding dicarba[2]ruthenocenophanes [Ru(η5-C5H4)2(CMe2)2] (10), rac-[Ru(η5-C5H4)2(CHiPr)2] (rac-12), and meso-[Ru(η5-C5H4)2(CHtBu)2] (meso-15) underwent analogous thermal carbon−carbon bond cleavage but more readily, consistent with a higher degree of ring strain. In the case of 7 and 15, the stability of the rac isomers relative to the respective meso isomers was confirmed by DFT studies, despite the former species exhibiting slightly higher tilt angles (α/deg) between the two cyclopentadienyl (Cp) rings. Theoretical investigations were used to explore the mechanism of carbon−carbon bond cleavage in dicarba[2]metallocenophanes, confirming the validity of the proposed homolytic bond cleavage mechanism. In addition, the potential role of bis-fulvene metal(0) and ‘tuck-in’ complexes in the bond-cleavage mechanism was assessed. This study also provides insight into the mechanism of the thermal ring-opening polymerization of −CH2CH2− bridged dicarba[2]metallocenophanes and, for the first time, supports a homolytic carbon−carbon bond cleavage pathway.