III —C Bond Rupture

The relative stability of [L5Cr—CH2R] depends on both L and R. For L = H20, the R = aryl systems are much more stable than those for R = alkyl and if R contains an —OH group on the -carbon atom, the complexes decompose via a ) -elimination process (2) to give alkenes. The rate expression for this heterolysis reaction is Eq. (3) and values for k, and 2 e given in Table 6.3. 

When four of the water molecules bound to Cr(III) are replaced by a macrocyclic ligand (2,3,3,3,-tet = N4), the heterolytic acid-catalyzed path similar to Eq. (5) is entirely suppressed and decomposition proceeds via homolysis, as in Eq. (4). The rate of this reaction (Table 6.4) can be followed by monitoring the decrease in concentration of an added scavanger (e.g., [CoBr(NH3)5] or HTMPO = 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-l-ol) which reacts with both of the products in Eq. (4). At high pH ( 7) the coordinated water molecule deprotonates and the hydroxo 

6 Inert-Metal Complexes Chromium Table 6.4. Continued) 

Complications occur with R = 4-BrCgH4CHf (and R = jec-alkyl) and an additional oxidatively induced chain-reaction mechanism is proposed. This step can be almost eliminated by working with excess 

Homolysis (8) of the Cr(IV) macrocyclic complexes is much slower than for [(H20)5Cr(IV)R] , although precise rate constants have not yet been obtained.  

Complexes containing Cr(III)—C bonds have two modes of decomposition heterolysis [equation (2)] and homolysis [equation (3)]. The homolysis reaction 

The decomposition of pentaaqua(organo)chromium(III) complexes in aqueous solution has been observed to occur by homolysis of the Cr—C bond [Eq. (1)] or by heterolysis (acidolysis) [Eq. (2)] with assistance from some proton source. 

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Propyl Radicals n-CsHi —> CHt + CJli. The models for n-propyl decomposition are given in Appendix III. eA was set at 3.1 as opposed to 1.6 kcal. for isopropyl formation. Values of kt are presented in Figure 12. Table XX summarizes some calculated values for various activation systems. H atom rupture is seen to be orders of magnitude slower than C—C bond rupture. Again, comparison may best be made with the results from chemical activation studies at 25°C. The observed values were ka0 = 8 X I07 and /bO0O = 18 X I07 sec.-1. The data fluctuated... 

Iron(III) oxalate decomposed between 410 and 450 K to give CO and iron(II) oxalate which retained about 10% of the iron(lll) salt [57]. Melting was not detected. The sigmoid a - time curves were identified as being due to a nucleation and growth process. The first half of the reaction was well represented by the Avrami - Erofeev equation (n = 2) and the latter half by the contracting volume equation. Values of were relatively low, 107 to 120 kJ mol , and rate control was ascribed to either electron transfer or C - C bond rupture. 

These occur by intramolecular twist (b) or bond-rupture (c) mechanisms, (See 7.6.1) consult Ref. 75 for full details. Subsequent to this early work, the rearrangements of Co(ida)2, Co(ida)(dien) and Co(III) complexes of N-methyl derivatives of ida and dien have been quantitatively examined, by batch analyses. - For Co(ida)(dien)+, the kinetics of the sequence ... 

A rate and equilibrium study of the cis/trans isomerization of tris(l,l,l-trifluoro-2,4-pentanedionato)chromium(III) in the vapour phase has been reported.761 The equilibrium constant 3.56 was independent of temperature in the range 118-144.8 °C, the same value being measured by both static and kinetic methods. The activation energies for isomerization were both less than 115 kJmol-1 the Cr—O bond energy is 210 kJ mol-1. On these grounds, a bond-rupture mechanism was rejected. A similar mechanism may operate for the isomerization in solution. 

First reported in 1986 (181), the complex [Os(NH3)5(acetone)]2+ and related aldehyde and ketone complexes (177) were the first examples of linkage isomerizations on Os(III/II). In acetone solution, a detailed electrochemical and chemical investigation revealed that the substi-tutionally inert complex, [Os(NH3)5(Tj2-acetone)]2+, is in facile equilibrium with the rj1 form, the former being favored by 21 kJ mol-1. Upon oxidation, the Os—C bond is ruptured, but the Os—O bond remains intact, even in good donor solvents such as dma. Reduction of the i71-acetone-Os(III) species occurs at a potential of 750 mV negative of that of the 172 form in acetone. Subsequent tj1 - 172 isomerization of the ketone occurs with a specific rate of 6 x 103 sec-1 at 20 2°C. 

The ability of 5-oxoniachrysenes 67 to form stable adducts with ammonia, methyl amine, and hydrazine is not usual for 2-benzopyrylium (cf. Section III,C,2), but is often encountered for 1-benzopyrylium (chro-mylium) salts (51 Mil). At the same time, no rupture of the C6—Cring bond was observed for 2-benzo- or 1-benzopyrylium salts under the conditions used. A remote similarity to C6—Crjng bond rupture may be seen in reactions of 2-benzopyrylium salts with sodium azide (Section III,C,2) or with hydrogen peroxide (Section III,C,4,b,ii). 

AH attempts to convert dimer 263 into a dimeric 2-benzopyryIium salt, on treatment with triphenylmethyl or acetyl perchlorate, lead only to the rupture of the newly formed C—C bond and to the regeneration of the initial monomeric salt 261, unlike the behavior of dimers of monocyclic pyrylium cations [73DOK(212)370]. Dimerization may be considered a typical reaction for benzo[c]pyrylium-4-oxides of type 19, which react in dimerizations as 1,3-dipoles by analogy with their behavior in cycloadditions (Section III,E,2). 

Tanaka296 found the relative rates of oxidation of cycloalkanes by Co(III) acetate in acetic acid at 90°C to decrease in the order Cs >C6 > C7-Ci2. He concluded that the rate-controlling step did not involve C—H bond rupture but, instead, formation of a complex between the alkane and Co(III). The relative reactivities were attributed to steric hindrance in the formation of the complex, the structural features of which were not elaborated further. 

If, conversely, compound VII were to be treated as a cyclic amino-ortho ester, a nonconcerted, thermally induced C-N bond rupture that would lead to another zwitterionic structure (IX) and from there to III by way of recombination-fragmentation would also be more feasible, because ortho ester derivatives are prone to such thermal fragmentation (see Scheme 21.3). ... 

In primary processes I and IV the rupture of a C-C bond occurs giving the appropriate free radicals. There is, of course, no need to discuss the mechanism of these steps. However, the mechanisms of reactions II and III are, by no means, well established, though certain possibilities have already been suggested a concerted intramolecular mechanism was preferred by some of the authors, and the intermediate formation of a biradical was proposed by others. 

Elements of Group III attached to the B, has been described,299 and a pyrolysis carried out at 500 °C. The products were investigated mass-spectrometrically, showing that H-containing methanes predominate over those containing D. This indicates that the rate of homolytic rupture of N—C bonds is greater than that for B—C bonds. 

Boldyrev et al. [46] identified the C - C link as the least stable bond, which may be broken as the initial step during oxalate pyrolysis ( 204 - 2CO ). These radicals (COj") may react in three ways (i) revert to oxalate, (ii) form (OCOCOj) COj + CO, or (iii) reduce the cation (-+ M + 2CO2). The common initial step explains the similar values of for decompositions of several oxalates (often about 170 kJ mol ). This is lower than the energy of rupture of the C - C bond... 

The initial or rate limiting step for anion breakdown in metal oxalate decompositions has been identified as either the rupture of the C - C bond [4], or electron transfer at a M - O bond [5], This may be an oversimplification, because different controls may operate for different constituent cations. The decomposition of nickel oxalate is probably promoted by the metallic product [68] (the activity of which may be decreased by deposited carbon, compare with nickel malonate mentioned above [65]). No catalytically-active metal product is formed on breakdown of oxalates of the more electropositive elements. Instead, they yield oxide or carbonate and reactions may include secondary processes [27]. There is, however, evidence that the decompositions of transition metal oxalates may be accompanied by electron transfers. The decomposition of copper(II) oxalate [69] (Cu - Cu - Cu°) was not catalytically promoted by the metal and the acceleratory behaviour was ascribed to progressive melting. Similarly, iron(III) oxalate decomposition [61,70] was accompanied by cation reduction (Fe " - Fe ). In contrast, evidence was obtained that the reaction of MnC204 was accompanied by the intervention of Mn believed to be active in anion breakdown [71]. These observations confirm the participation of electron transfer steps in breakdown of the oxalate ion, but other controls influence the overall behaviour. Dollimore has discussed [72] the literature concerned with oxalate pyrolyses, including possible bond rupture steps involved in the decomposition mechanisms... 

On platinum, the a, -dicarbene mechanism which accounts for the hydrogenolysis of cycloalkanes (Scheme 34) is no longer predominant in the hydrocracking of acyclic alkanes. It has already been emphasized that the internal fission of isopentane and n-pentane is related to the metallocyclobutane bond shift mechanism of isomerization (see Section III, Scheme 29), and that in more complex molecules, the favored rupture of the C-C bonds in a p position to a tertiary carbon atom is best explained by the rupture of an a,a,y-triadsorbed species (see Section III, Scheme 30). The latter scheme can account for the mechanism of hydrocracking of methylpentanes on platinum. Finally, the easy rupture of quaternary-quaternary C-C bonds in... 

Fig. 15 Rupture of a strained Si-O bond in a siloxane hexamer by attack from a water molecule in the surroundings." (i) shows the approach of the water molecule, (ii) shows the point at which the oxygen atom in the water molecule is attracted to the silicon atom inducing rupture, and (iii) shows the rupture products where a proton has been transferred to the ruptured oxygen of the siloxane and the hydroxyl group goes to the silicon. Reprinted with permission from E. M. Lupton, F. Achenbach, J. Weis, C. Brauchle and I. Frank, J. Phys. Chem. B, 2006, 110, 14557-14563. Copyright 2006 The American Chemical Society.

There has been much treatment of the kinetics and mechanism of decarboxylation of carbonatocobalt(III) complexes. Thus the results for decarboxylation of the [Co(tren)(C03)] cation mentioned above conflict to some extent with earlier results and interpretations in this area. Matters are more straightforward for decarboxylation of monodentate carbonate complexes, for instance, of trans-[Co(tn)2(C03)C ]. The reactivity pattern here, similar to that for other monodentate carbonate complexes, with rates on the stopped-flow time scale, corresponds to carbon-oxygen rather than to cobalt-oxygen bond rupture. Three papers deal with decarboxylate of complexes [Co(LLLL)(C03)], each... 

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