A gas phase study of the structure, reactivity, and thermochemistry of transition organometallic systems by Fourier transform ion cyclotron resonance mass spectrometry

The gas-phase chemistry of three transition metal ion systems have been studied by Fourier transform ion cyclotron resonance mass spectrometry. The structure, reactivity, and thermochemistry of these organometallic systems are essential in verifying theoretical calculations and in the understanding of catalytic processes. The C$\sb6$H$\sb6$ ligand resulting from the reactions of transition metal ions with linear hydrocarbons was postulated to be benzene by the analogous dehydrocyclization process known to occur in solution. An unknown FeC$\sb6$H$\sb6\sp+$ isomer isolated from 1,4-hexadiene was investigated in order to provide evidence for or against gas-phase dehydrocyclization. Three FeC$\sb6$H$\sb6\sp+$ isomers, Fe(benzene)$\sp+$, Fe(fulvene)$\sp+$, and Fe(1,5-hexadiyne)$\sp+$, were the bases for comparison using results from collision-induced dissociation, photodissociation, and ion-molecule reactions with 1,3-butadiene, cyclopentene, and O$\sb2$. Photodissociation and ion-molecule reactions can be used because of the differences in products and/or product ratios. The wavelength dependence in the photodissociation can be used as well. Early transition metals form strong bonds with oxygen. Nb$\sp+$ is then expected to be highly reactive with alcohols; it reacts successively with methanol at least six times. Many interesting products are formed where several isomers are possible, making the determination of probable structures and mechanisms difficult. The reaction products indicate the possibility of cleavage of the different types of bonds in the alcohol molecule during oxidative addition. Nb$\sp+$ is far more reactive than two other second-row metal ions, Mo$\sp+$ and Rh$\sp+$, and also more reactive than its first-row counterpart V$\sp+$. The chemistry of second-row transition metal ions (Y$\sp+$, Nb$\sp+$, Rh$\sp+$, and Ag$\sp+$) with cycloalkenes has opened several interesting questions as to what factors may be responsible for the dehyodrogenation and bond energy trends. The reactivity trend suggests a decreasing ability to dehydrogenate in going from early to late transition metals with the exception of Y$\sp+$. With Y$\sp+$, Nb$\sp+$, and Rh$\sp+$, the highest bond dissociation energy is with the cyclopentadienyl ring, followed by cycloheptatriene, cyclopentadiene, and finally, benzene. The answer seems to lie in the differences in promotion energies, electronic structure, orbital energies, and molecular orbital interactions. However, definite and detailed explanations await further theoretical and experimental work on a variety of metal-ring ligand systems