Reactive MALDI mass spectrometry of saturated hydrocarbons: A theoretical study

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(1,3-butadiene)]+, has a higher than expected cyclopentadienyl hapticity. In this structure a distance of nearly 0.5nm between the metal core and the butadiene ligand leads to very little electron sharing. Thus the detected ions are better described as non-covalent ion–molecule complexes. In turn, the lack of orbital overlap leads to a low enthalpy giving the cis-butadiene complex a −13.0kJ/mol binding energy and the trans-butadiene binding a −3.8kJ/mol binding energy. These low binding energies lead to low levels of charged alkanes in the reactive ion formation process in agreement with experimental observations. [Copyright &y& Elsevier]