Homolysis and acidolysis reactions of (.alpha.-hydroxyalkyl)- and (.alpha.-alkoxyalkyl)chromium(III) complexes: kinetics, steric effects, and bond energies

Abstract

The rates of homolysis of 12 organochromium complexes were determined by the addition of oxidizing scavengers to prevent recombination of Cr2+ and the carbon-centered radicals. The rate constants for complexes derived from aliphatic alcohols span the range 3.7 X 10"5 s_1 for CrCH2OH2+ to ~3 X 102 s_1 for CrC(CH3)(i-C4H9)OH2+ at 25 °C. There is a good correlation between the free energies of activation for homolysis of the organochromium complexes and corresponding substituted symmetrical ethanes. On the basis of the activation parameters and the literature data, estimates of the Cr-C bond dissociation enthalpies were made. Rate constants for the acidolysis reaction, previously known for some of these complexes, were evaluated for the others. Means were developed, by control of reaction conditions, to permit independent measurements of homolysis and acidolysis. The organochromium complex derived from isopropyl ether, CrC(CH3)2OCH(CH3)22+, undergoes a rapid (0.05 < k < 2 s_1) conversion to CrC(CH3)2OH2+ in dilute perchloric acid.Among the most fundamental reactions that a metal-alkyl complex can undergo are the unimolecular (or solvent-assisted) processes leading to the cleavage of a metal-carbon bond. A complicating but intriguing aspect of this chemistry is found in a series of organochromium complexes in which the Cr-C bond may cleave by both heterolytic and homolytic reactions that can be made to occur concurrently under suitable conditions. In the general case that situation can be represented by the competitive dissociation of a M-R bond to M+ + :R" (actually MOH + RH) vs. • + -R. 0\ CrCtCHOlC.HOOH2" CrCICH,i),OH2" range of 1 O' [ox |/M kHbl^' range of 103[ox |/'M *HC/V Co(NH2)J 2* 4-L00.128 ± 0.003 Co(NH2),C12" 2.0-1 1 0.90 ± 0.02 4-10 0.127 ± 0.004 Co(NH,)5 Br2" 1.0-12 0.94 ± 0.03 , , 10-60 0.90 ± 0.07 130-750 -0.13e VO3" 5-99 0.128 ± 0.004 Cu2" 20-100 0.96 ± 0.05d 4-100 0.138 ± 0.01 ld I'e3* 24-69 0.92 ± 0.10d 17-74 0.120 ± 0.016d av 0.92 ± 0.03 av 0.127 ± 0.003°At 25 °C. 1.00 M ionic strength, 1.0 M 2-butanol or 2-propanol. 6 kK = Atobsd -kA, with kA = 0.055 s'1 at [ " | = 0.100 M. as in these experiments. c&jj =&obsd -fcA. with fcA us in Table IV over the range of [ " ] studied for homolysis. 0.10-0.96 M. d By least-squares extrapolation to zero concentration of oxidizing agent.e At high [ , , |. small amounts of a direct reaction are seen, the highest value being kn = 0.15 sM at 0.75 , ,: the value given is extrapolated to [H20, ] = 0. observations in nearly every system. Because they are central to

Kinetics and mechanisms of the reactions of aliphatic free radicals with organocobaloximes and fluoropentaamminecobalt(III) ion, and of iron(III) with methylrhodoxime

“…The products (15) of reaction were those expected from oxidation of both 2+ Cr and 2-hydroxy-2-propy1 radical by the specific oxidant. These results were interpreted (15) as a rate limiting homolysis of the organochromium(III) ion, followed by rapid oxidation of Cr^^(aq) and…”

“…Hydroxy-2-pro&yl radicalThe rate constant for reduction of Cc^NH^j^F^* by "CXCH^jgOH radical Ox was determined via competition with Cc^en)^ whose rate constant for reduction by "[(CH^jgOH radical is known(10). Homolysis of the CrCfCHgjgOH^* complex(15) in the presence of mixtures of Co(NH2)gF^* and Co(en)j^^, and the fate of the Cr^^(aq) and the organic radical produced are represented in Equations 100-103.…”

Kinetic and mechanistic studies of the reactions of selected aliphatic radicals with metal complexes and the decomposition of organoperoxy cobalt complexes

“…The product was confirmed to be cyclopentyl hydroperoxide, because its NMR spectrum was in good agreement with that of the authentic sample. Their data are listed in Entries (10) and (11). The reaction product of 2-propylperoxy(pyridine)cobaloxime with CF^COOH shown in Entry (1) was reasonably recognized as 2-propyl hydroperoxide.…”

“…The (H20)^CrC(CH2)2OH complex is me tastab le and is subject to decomposition by two parallel routes simultaneously. One of them, called acidolysis, is a reaction consisting of heterolytic cleavage of the chromium-carbon bond (10). (I-IO) 2+ Another route leading to the decomposition of (H20)^CrC(CH2)20H is a process with the homolytic dissociation of the chromium-carbon bond as the rate-limiting step.…”

“…•OH + RH > .R + H^O (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18) Cr^"^ -f -R > Cr-R^"*" (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19) For example, CrC(CH^)20H^^ and CrCH()OC2^^^ may be acquired by con ducting this reaction in O.OIM perchloric acid with the presence of 2-propanol at IM or diethyl ether at the saturated concentration (~0.5M), respectively.…”