| Abstract: |
Recently it has been shown that the cobaltocenium cation, prepared by the laser ablation of a CoCp(CO)2/fullerene matrix, may react with alkanes and polyethylenes in the gas phase via a dehydrogenation reaction to produce [Co(Cp)2(alkadiene)]+ ions without chain scission (W.E. Wallace, Chem. Commun. 2007, 4525-4527). To better understand these experimental observations density functional calculations were used to obtain the gas-phase binding energies and molecular structures of cobaltocenium, Co(Cp)2+, with 1,3-butadiene, 2,4 hexadiene and 2,3-hexadiene. Calculations were conducted for both cis and trans molecular configurations, in both singlet and triplet electronic states, and with a variety of cyclopentadienyl hapticities. For 1,3 butadiene the 18 electron rule would predict a [Co(η3 Cp)2(η4 1,3 butadiene)]+, however, the lowest energy structure, [Co(η5 Cp)2(η4 1,3 butadiene)]+, has a higher than expected cyclopentadienyl hapticity. In this structure a distance of nearly 0.5 nm between the metal core and the butadiene ligand leads to very little electron sharing. In turn, the lack of orbital overlap leads to a low enthalpy giving the cis butadiene complex a -3.1 kcal/mol binding energy and the trans-butadiene binding a -0.9 kcal/mol binding energy. These low binding energies lead to low levels of charged alkanes in the reactive MALDI process compromising the sensitivity of the method in agreement with experimental observations. |
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