The degree to which 6,6-dimethylfulvene, 6,6-diethylfulvene, and 6-cyclopentylidenefulvene form unbridged calcocenes in reductive coupling reactions with activated calcium were compared. The identification of alkenyl substituents along with alkyl substituents among the unbridged calcocenes confirms that hydrogen atom or proton transfer between dialkylfulvenes and their radical anions or dianions is responsible for the formation of unbridged calcocenes. The tendency to form unbridged calcocenes instead of ansa-calcocenes is higher for 6,6-diethylfulvene and 6-cyclopentylidenefulvene than for 6,6-dimethylfulvene. 1,1′-Dicyclopentenylcalcocene, 1, which is formed in the reaction between cyclopentylidenefulvene and calcium, was purified and crystallographically characterized. Photolysis of 1 leads to [2+2] cyclization of the cyclopentenyl substituents to form 2, an ansa-calcocene with a tricyclo-[3.3.0.0]decane-1,10-diyl bridge.

The reductive coupling of 6-mesitylfulvene, 6-(3,5-di-tert-butylphenyl)fulvene, 6-(1-naphthyl)fulvene, and 6-tert-butylfulvene with HgCl2-activated calcium was examined to determine if increasing the steric bulk of the 6-substituent on the fulvene would enhance selectivity for the rac ansa-calcocene product over the meso isomer. Upon formation, 6-mesitylfulvene readily dimerizes by a Diels−Alder cyclization to form (E,E)-5,10-bis(mesitylmethylene)tricyclo[5.2.1.0]deca-3,8-diene (1), the X-ray crystal structure of which was determined. This fulvene does not react with activated calcium. The other fulvenes are successfully coupled by calcium; however, they afford poor to no selectivity for the rac-calcocene isomers in comparison with less sterically hindered 6-phenylfulvene. A 1:1 ratio of the rac and meso isomers of {1,1‘-(1,2-t-Bu-C2H2)(η5-C5H5)}2Ca (rac-5 and meso-5) was obtained from the coupling of 6-tert-butylfulvene with calcium. The two isomers were separated by selective crystallization, and the X-ray crystal structures of the DME adducts of each isomer were determined. The crystal of rac-5 contained two unique molecules of the complex. One molecule contains a single dicoordinated DME on the calcium. The other molecule contains two DME molecules:  one dicoordinated and the other monocoordinated to the calcium. A variable-temperature 1H NMR study of meso-5 was performed to characterize two concurrent dynamic processes in the molecule. One process involves a rearrangement of the chelating ligand framework between λ and δ conformations. The other involves restricted rotations of the tert-butyl groups in the ethylene bridge.

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