Gas-phase reactions of lanthanum dication with small alkanes and the photodissociation of LaC2H4n+ and LaC3H6n+ (n = 1 and 2)

Abstract

The gas-phase reactivity of La2+ with Q-Q linear alkanes is investigated and compared to the analogous reactions of La+. La2+ is unreactive with methane and ethane but reacts with propane and butane to give dehydrogenation, alkane loss, and for butane charge-splitting reaction products. Interestingly, charge transfer is not observed in agreement with a simple curve-crossing model. Four ligated species, namely, LaC2H4+, LaC2H42+, LaC3H6+, and LaC3H62+, are photodissociated and the photodissociation thresholds are used directly, or together with information from ligand displacement reactions, to assign thermochemical values for these processes and metal ligand bond energies. The bonding properties of LaC2H2+ and LaC2H22+ are also investigated. A surprising outcome of this study is that the monocation is in some cases more strongly bound to these ligands than the dication. Theoretical calculations by Bauschlicher and co-workers are in good agreement with these results.

Gas-Phase Reactions of Doubly Charged Lanthanide Cations with Alkanes and Alkenes. Trends in Metal(2+) Reactivity

“…The non-observation of 2+ products for Lu 2+ , which has a d 1 state at 0.7 eV, can be explained by its rather high electron affinity as compared with the other Ln 2+ ions (see Table 4), which renders the electron-transfer channel for all the hydrocarbons except methane being highly exothermic (see Table 3). The main overall differences observed between [8] and for the group 3 ion Y 2+ [11], which has a d 1 ground state and an EA close to that of Gd 2+ . As indicated above, there is a close match between our results to those obtained by Freiser and co-workers for the reactions of La 2+ [8].…”

“…The main overall differences observed between [8] and for the group 3 ion Y 2+ [11], which has a d 1 ground state and an EA close to that of Gd 2+ . As indicated above, there is a close match between our results to those obtained by Freiser and co-workers for the reactions of La 2+ [8]. Ce 2+ and Tb 2+ , although less reactive, also show similarities to the La 2+ ion, and Gd 2+ in particular is similar to Y 2+ [11].…”

“…Ce 2+ and Tb 2+ , although less reactive, also show similarities to the La 2+ ion, and Gd 2+ in particular is similar to Y 2+ [11]. All these ions form products resulting only from H 2 , CH 4 or C 2 H 6 eliminations, which Freiser and co-workers took to be indicative of a mechanism for a radical-like d 1 M 2+ ion [8,11]. These authors proposed two possible mechanisms for this type of bond activation involving the d 1 configuration of the doubly-charged metal ion [8,11] may arise from the presence of relatively weak allylic C-H and C-CH 3 bonds in 1-butene, and C-H bonds in propene.…”

“…All these ions form products resulting only from H 2 , CH 4 or C 2 H 6 eliminations, which Freiser and co-workers took to be indicative of a mechanism for a radical-like d 1 M 2+ ion [8,11]. These authors proposed two possible mechanisms for this type of bond activation involving the d 1 configuration of the doubly-charged metal ion [8,11] may arise from the presence of relatively weak allylic C-H and C-CH 3 bonds in 1-butene, and C-H bonds in propene. These reaction channels would appear to be consistent with the postulated abstraction mechanism [8,11].…”

“…These authors proposed two possible mechanisms for this type of bond activation involving the d 1 configuration of the doubly-charged metal ion [8,11] may arise from the presence of relatively weak allylic C-H and C-CH 3 bonds in 1-butene, and C-H bonds in propene. These reaction channels would appear to be consistent with the postulated abstraction mechanism [8,11]. From the results in Tables 1 and 2, it is apparent that for bond activation to occur the Ln 2+ ions must have an accessible d 1 configuration, with an upper energy threshold lying between ca.…”