Reactivity of Lanthanide, Group 2, and Group 3 Metal and Metal Oxide Cations with Pentamethylcyclopentadiene:  Gas-Phase Synthesis of Cyclopentadienyl Cations

Abstract

The gas-phase reactivity of lanthanide (Ln+ = La+−Lu+), group 2 (Ca+, Sr+, and Ba+), and group 3 (Sc+ and Y+) cations, and of their corresponding monoxide ions MO+, with pentamethylcyclopentadiene (C5Me5H) was studied by Fourier transform ion cyclotron resonance mass spectrometry. The reactivity of Eu+, Tm+, Yb+, and the alkaline earth metal ions was similar to that observed previously for Sm+, namely formation of the fulvenide ion M(C5Me4CH2)+ as the main primary product and the metallocene ion M(C5Me5)2 + as the main secondary product. With Sc+, Y+, and the remaining lanthanide series ions, several other species were observed in the primary reactions, corresponding to single and multiple losses of neutral molecules such as H2 and CH4. These differences in reactivity appear to correlate with the accessibility of reactive excited state electron configurations of the metal ions. In the case of the metal oxide cations MO+, the reactivity with pentamethylcyclopentadiene appears to be determined by the strength of the M+−O bonds. The ions with the strongest bonds, LaO+, CeO+, PrO+, and NdO+, formed M(C5Me5)(OH)+ as the sole primary product, which reacted further, eliminating water, to give the metallocene ion M(C5Me5)2 +. ScO+, YO+, and the lanthanide series ions SmO+, GdO+−TmO+, and LuO+ yielded MO(C5Me4CH2)+ and M(C5Me4CH2)+ as the primary products in addition to M(C5Me5)(OH)+. These metal oxides gave M(C5Me4CH2)2 + and M(C5Me5)2 + as secondary products. The ions with the weakest M+−O bonds, EuO+, YbO+, CaO+, SrO+, and BaO+, formed MOH+ as a primary product and M(C5Me5)+ as a secondary product.

Metal ion - molecule reactions of Bk+: comparison with Pu+ and Tb+

“…Previous results for lanthanides [23] and actinides [24] are consistent with the distinctive reactivity pattern observed in the present study with HCp * . As noted above, additional products to those tabulated were produced exclusively for Tb + with HCp * , and the reactivity of Tb + was clearly greater than those of Pu + and Bk + .…”

“…Whereas Tb + was somewhat more efficient than the other two metal ions at both processes, the dehydrogenation and demethanation efficiencies were essentially the same for Bk + and Pu + . This result contrasts with the consistently greater relative reactivity of Pu + with other alkenes and is attributed to the distinctive activation mechanism proposed for metal ion -HCp * reactions [23] which was evidently displayed in an FTICR-MS study of lanthanides [23] and a LAPRD study of actinides [34]. For example, whereas Am + is relatively inert towards most alkenes, in accord with its large promotion energy, ∆ P E[Am + ] = 250 kJ mol −1 , Am + is essentially as effective at dehydrogenating HCp * as is Np + [34].…”

“…Specifically, Tb + induced loss Brought to you by | New York University Bobst Library Technical Services Authenticated Download Date | 5/28/15 2:51 PM of 2H 2 , 3H 2 , (CH 4 + H 2 ) and (CH 4 + 2H 2 ) from HCp * . This greater diversity of products from the {Tb + + HCp * } reaction was anticipated based on previous results, where a high reactivity of Tb + was exhibited compared with other lanthanide ions, Ln + [23]. For some reactions, peaks which were evidently due to Tb + reaction products could not be confidently assigned; examples are noted with asterisks in Fig.…”

“…For example, whereas Am + is relatively inert towards most alkenes, in accord with its large promotion energy, ∆ P E[Am + ] = 250 kJ mol −1 , Am + is essentially as effective at dehydrogenating HCp * as is Np + [34]. A mechanism proposed by Marcalo et al [23] accounts for the lanthanide and actinide results. Elimination of H 2 and CH 4 evidently does not require insertion of a "divalent" metal ion, M + * , into a C−H or C−C bond but rather can proceed by coordination of the metal ion to the π-electron system followed by cleavage of the relatively weak C−H and C−CH 3 bonds at the saturated ring carbon (C 5 ).…”

“…Facile β-H abstraction results in H 2 -loss to produce complexes between the metal ion and dehydrogenated hydrocarbon, M + {L−H 2 }.Similarly, activation of carbon-carbon bonds via C−M + −C intermediates results in hydrocarbon cracking. Previous results have revealed that the lanthanide 4 f[19,20,23] and transneptunium actinide 5 f electrons[9][10][11][12][13] do not appreciably participate in C−H or C−C activation, and that two non-f valence electrons are needed at the metal center to enable insertion and formation of two covalent bonds. The electronic excitation processes and associated energies, ∆ P E[M + ],…”