Gas-Phase Reactions of Homo- and ... - Institut für Chemie

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1414 Helvetica Chimica Acta ± Vol. 88 (2005) C H bond no longer seems accessible. Instead, with butanenitrile, only the formation of the formal association complex [Fe 2C 4H 7N] ‡ , without indication for any bond activation reactivity, is observed. This behavior is consistent with the scenario proposed in Scheme 3 in that the alkyl chain of butanenitrile is too short to assist the formation of cyclic species. Scheme 3. Model for the 1,1-Elimination from Pentanenitrile by Fe ‡ 2 Given that an exocyclic MCTS is energetically more favorable than an endocyclic variant, the 1,2-elimination of H 2 from pentanenitrile is suggested to proceed via an initial activation of a C H bond in the g position (Fig. 2; 20a). The alternative d-C H bond activiation would lead to 17 (Scheme 3), from which an endocyclic MCTS had to be formed to bring about loss of H 2 via the 1,2-elimination mode. Although not rigorously proven, the initial activation at the g position appears more plausible. However, while for the reaction of Fe ‡ 2 with pentanenitrile the intermediate 20a (Rˆ CH 3) is capable to undergo an additional exocyclic C H bond activation step, from 20b (Rˆ H) elimination of H 2 would imply the formation of an endocyclic MCTS. This latter option does not seem to exist, and H 2 is generated neither from n-C 3H 7CN/Fe ‡ 2 nor from n-C 3H 7CN/FeCo ‡ under thermal conditions. Fig. 2. Proposed structure for the intermediate resulting from the insertion of Fe ‡ 2 in the g- C H bond of pentanenitrile (20a) and butanenitrile (20b) As to the structural and electronic details of all of the RCN/MM' ‡ species and their initial C H bond activation intermediates discussed in this study, for the time being no reliable data are available, and appropriate computational investigations are indicated. This holds true in particular for the complexes with heteronuclear clusters, e.g., FeCo ‡ , as for these systems quite a few interesting reaction scenarios with regard to the nature of the complexation as well as mechanistic details of activation steps are conceivable. Reactions of Ni ‡ 2 and CoNi ‡ Including Some Specific Aspects of C C Bond Activation. In terms of product formation, Ni ‡ 2 shows the richest pattern of all clusters studied, and this is also enlarged in comparison to atomic Ni ‡ . For Ni ‡ 2 , double dehydrogenation of pentanenitrile is observed with high efficiency, whereas simple dehydrogenation prevails for Ni ‡ [5b,c]. This result is in accord with the multiple
Helvetica Chimica Acta ± Vol. 88 (2005) 1415 dehydrogenation of butane by Ni ‡ 2 (up to three times), meanwhile atomic Ni ‡ dehydrogenates this substrate only once [18]. In addition to dehydrogenation of pentanenitrile by Ni ‡ 2 , ionic products with mass differences of Dm ˆ 29 and 30 are observed, which correspond to the combined losses of H 2 ‡ HCN and H 2 ‡ C 2H 4, respectively. Except for the combined H 2/HCN elimination, all other reaction channels of Ni ‡ 2 are also observed for CoNi ‡ . While the reaction efficiency of n-C 4H 9CN/CoNi ‡ is the highest among all systems studied, the major part of reactivity is due to single dehydrogenation in the products formed by the CoNi ‡ cluster, whereas for Ni ‡ 2 mainly double dehydrogenation is observed (Table 2). The consecutive losses of HCN or C 2H 4 after dehydrogenation are proposed to occur as a result of the processes exemplified for Ni ‡ 2 in Scheme 4: after dehydrogenation and subsequent insertion in the C CN bond (21 ! 22) either a b-H (22 ! 23)ora b-alkyl shift (22 ! 24) take place, both involving an allylic position; because the product isotope distributions for 11c/Ni ‡ 2 and 11d/Ni ‡ 2 are the same, the b-H migration (22 ! 23) quite probably is reversible. Scheme 4. Proposed Mechanism for the Eliminations of C 2H 4 and HCN, Respectively, from Pentanenitrile/Ni ‡ 2 Subsequent to Dehydrogenation The resulting intermediates 23 and 24 then lead to the reductive elimination of HCN and the evaporation of C 2H 4, respectively. In agreement with these suggestions, the product ion [Ni 2C 3H 3N] ‡ , for example, undergoes the secondary Reaction 36 in the presence of pentanenitrile. Quite possibly, the resulting ionic product [Ni 2C 7H 11N] ‡ corresponds to a complex that contains an ethene and a pentenenitrile ligand, which are formed in course of HCN loss and bound to the Ni ‡ 2 core. Future studies are indicated to shed further light on questions like structural details or reactivity patterns of these novel metal cluster cations. [Ni 2C 3H 3N] ‡ ‡ n-C 4H 9CN ! [Ni 2C 7H 11N] ‡ ‡ HCN (36) Insertion of a transition-metal cation in a C CN bond has previously been discussed in a different context [25]. In these studies, the detailed electronic structure influence of the transition-metal cations Fe ‡ through Cu ‡ on their reactivities with 2methylbutanenitrile were probed and three major processes were identified: i) remote functionalization, ii) initial insertion into the C CN bond ( allylic mechanism ) in
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Page 7 and 8: kinetic modeling with the ratios H
Page 9: Model 4: D 2 ˆ f 1;1 Helvetica Chi

Gas-Phase Reactions of Homo- and ... - Institut für Chemie

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