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Reactivity and structure of gas phase olefin-transition metal ions by Fourier transform ion cyclotron resonance mass spectrometry Kan, Ziti


The gas phase ion-molecule chemistry of the following organometallic complexes has been examined using Fourier transform ion cyclotron resonance (Fr-ICR) mass spectrometric techniques. [Formula] Positive and negative fragment ions generated by electron impact ionization from each of the complexes were allowed to react with the neutral parent molecule or other selected neutral molecules. Monometallic ions typically reacted with the parent molecule to form multimetallic products such as(c-C₇H₈)Cr₂(CO)⁺,(c-C₇H₈)₂Cr₂(CO)₃⁺,(c-C₇H₈)₂Cr₃(CO)₂⁺and (c C₇H₈)3Cr₄(CO)₃⁺.The ion-molecule reaction pathways were determined by Fr-ICR multiple resonance ion ejection techniques. Abundances of ionic reactants and products were temporally monitored to obtain kinetic data. A general trend that ion reactivity towards the parent molecule increased with increasing electron deficiency on the metal atom was observed. Some ions exhibited unexpected reactivity patterns, despite their large formal electron deficiency, suggesting the presence of unusual bonding configurations in the ions. For complexes 1 to 3, positive fragment ions formed by losing all three CO groups showed anomalously low reactivities. These low reactivities were rationalized by a gas phase ion rearrangement in which a ß-hydride intramolecularly transferred from the olefin ligand to the metal center to yield a metal hydride ion with a less-electron-deficient central metal atom. The metal hydride ions were detected by a specific ion-molecule addition-dehydrogenation reaction with deuterated methanol that was discovered in this research. The addition-dehydrogenation reaction appeared to be a definitive test for metal-hydrogen bonding in gas phase organometallic ions as ions lacking metal-hydrogen bonds underwent either substitution reactions or adduct-formation reactions with methanol. The major driving forces for the 3- hydride transfer appeared to be the high degree of electron unsaturation at the metal center and the formation of aromatic or other highly delocalized organic ligands. The low observed reactivities of some ions from complex 4 were explained by a coordination expansion (ŋ⁴4 to ŋ ⁶) process that also lowered the electron deficiency on the Fe atom. In some of the substituted ferrocene systems, a ring expansion process was inferred. For the negative ion chemistry of complex 3, an a-proton transfer from the cyclohexadienone ligand to the negatively charged metal center (via ketone/enolate tautomerization) was suggested to explain the observed negative ion reactivities. The ion processes observed in this research are of importance for a better understanding of the mass spectra of organometallic complexes and inferentially for a better understanding of the mechanism of C-H and C-C bond activation by gas phase transition metal ions.

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