Strain-induced cleavage of carbon-carbon bonds: bridge rupture reactions of group 8 dicarba[2]metallocenophanes.
Thermal treatment of dicarba[2]ferrocenophanes [Fe(eta(5)-C(5)H(4))(2)(CMe(2))(2)] (1), rac-[Fe(eta(5)-C(5)H(4))(2)(CHiPr)(2)] (rac-5), and meso-[Fe(eta(5)-C(5)H(4))(2)(CHtBu)(2)] (meso-7) at 240-300 degrees 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 1,1'-metallocenes, while meso-7 thermally converted to the more thermodynamically stable rac isomer. The corresponding dicarba[2]ruthenocenophanes [Ru(eta(5)-C(5)H(4))(2)(CMe(2))(2)] (10), rac-[Ru(eta(5)-C(5)H(4))(2)(CHiPr)(2)] (rac-12), and meso-[Ru(eta(5)-C(5)H(4))(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 (alpha/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 -CH(2)CH(2)- bridged dicarba[2]metallocenophanes and, for the first time, supports a homolytic carbon-carbon bond cleavage pathway.
Herbert
David E
DE
School of Chemistry, University of Bristol, Cantock's Close, Bristol, BS8 1TS, United Kingdom.
Gilroy
Joe B
JB
Staubitz
Anne
A
Haddow
Mairi F
MF
Harvey
Jeremy N
JN
Manners
Ian
I
D002244:Carbon / Q000737:chemistry*; D004563:Electrochemistry; D004583:Electrons; D007536:Isomerism; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D011108:Polymers / Q000737:chemistry; D011789:Quantum Theory; D013056:Spectrophotometry, Ultraviolet; D013696:Temperature
D009942:Organometallic Compounds; D011108:Polymers; D002244:Carbon