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 0.5 nm 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 -13.0 kJ/mol binding energy and the trans-butadiene binding a -3.8 kJ/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.