The Ammonium Salt Route to Oligomeric Lanthanum Salt Complexes: Synthesis and X-Ray Crystal Structure of La(NCS)3· 4 HMPA
✍ Scribed by Dr. Donald Barr; Alan T. Brooker; Michael J. Doyle; Dr. Simon R. Drake; Dr. Paul R. Raithby; Dr. Ronald Snaith; Dr. Dominic S. Wright
- Publisher
- John Wiley and Sons
- Year
- 1990
- Tongue
- English
- Weight
- 413 KB
- Volume
- 29
- Category
- Article
- ISSN
- 0044-8249
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✦ Synopsis
Table 1. Kinetic data of four alternative modes of C-C bond cleavage in 1 ( T = 3 0 0 K ) [ a ] . Mode of t , 2 [ b ] AH* AS* ACToo AG,,,[c] cleavage b i n 1 [kcal [cal K -' [kcal [kcal rnol-'] mol-'] mol-'1 mol-'1 Homolytic 1 . 7 ~ 10' 45.6 38 34.2 34 Heterolytic 8 . 2 ~ lo3 22.5 -13 26.4 37 Aniomesolytic 2.0 x 10' 22.2 2 21.6 20 Catiomesolytic 2 . 0 ~ lo-" 7.0 -10 10.0 8
[a] The estimated errors ofenthalpies and entropies of activation for homolysis and heterolysis are f 2 kcal mol-' and i 5 cal K-mol-'. The corresponding estimates for mesolyses are +1 kcalmol-' and f 3 c a l K -' m o l -' [b] Estimated. The actual values measured in the experimentally accessible range are reported in the text. [c] Estimated from redox data ( k 2 kcalmol-I).
molecules between the aromatic rings. The transition state very likely resembles such an ion pair. Therefore, the observed highly negative AS* of heterolysis might be due in part to the compactness of the transition structure. It has to be stressed that 1 is biased by substitution to favor a heterolytic process. A rate increase such as that observed in this case (in comparison to homolysis) is not expected to be a universal phenomenon.
In contrast, the thermodynamically driven acceleration of mesolytic processes is general since radicals undergo oxidation or reduction easier than the corresponding neutral prec u r s o r ~. [ ~~~ As our data indicate, all the scission reactions have activation energies that are only slightly higher than the free-energy changes accompanying the reactions. This is not surprising for the highly endergonic homolysis, but quite remarkable for mesolytic cleavages. In the latter cases, almost all of the thermodynamic advantage gained by electron removal or addition is exhibited in the transition state, even for the very weakly endergonic reaction of 1'@. This observation is of particular significance for designing low-activation-energy processes for fragmentation of C-C bonds. Not only can the cleavage be carried out in a selective manner['51 under mild conditions, but the energy supplied to the system in order to generate the reactive radical ions is likely to result in the largest possible lowering of the activation energy.