1117
with lifetimes long compared to the time required for values observed for reaction 8 suggests strongly that a solvent reorganization, probably are formed in reaction common intermediate is precursor to stable Co"' 8. There is no evident reason why a [ C O ( N H ~ ) ~ ( O ~ ) ] ~ + products in both the photochemical systems and in complex should not have a similar lifetime. reaction 8. We feel the most logical candidate for this In assessing the differences in behavior of the photointermediate is the coordinatively unsaturated Co"' C1- and 2 C1-, we note that chemical systems 1 ammine [ C O ( N H ~ ) ~ ] obtained ~+, in the photochemical Sykes has presented convincing kinetic evidence for systems from [ C O ( N H ~ ) ~ ( O ~and ) ] ~in + reaction 8 from either [Co(NH3)5(N2)I3+ or [CO(NH~)~(NNNNO)]~+.formation of ion pairs between 1 and Cl-.45 For this system, Sykes found the formation constant for the ion In neither the photochemical systems nor reaction 8 pair to be approximately unity when the total ionic is it likely that the original reactants (i.e., 2 or [Costrength was 2.0. All of the anion-incorporation (NH3)5(N3)]2+)are immediate precursors to [Coexperiments described here were carried out with solu(NH3)5]3+. The five-coordinate intermediate can be tions having total ionic strengths considerably lower expected to react with its solvation sphere at a rate at than 2.0. Under our photolysis conditions, therefore, least competitive with reorganization of that solvation extensive ion pairing between 1 and C1- probably sphere. Consequently, [Co(NH3)J3+ produced dioccurs, and results obtained with 1 are expected to rectly from 2 or its excited states (i.e., before the reduced contain contributions from photolysis of the ion pair Co center departed) would react with a solvation sphere as well as photolysis of free 1. Our stoichiometric appropriate to a species having a charge of 4+, while data allow the further conclusion that the [ C O ( N H ~ ) ~produced ]~+ directly from [ C O ( N H ~ ) ~ ( N ~ ) ] incorporation ~+ role of C1- in the photochemistry of the ion pair cannot after reaction with NO+ would react with a solvation be restricted simply to promotion of formation of sphere appropriate to a species having a charge of 2+ or 3+. Since anion incorporation is well known to [ C O ( N H ~ ) ~ O ~because ] ~ + , if this were the case the f C1- would be identical with depend on the charge of the incorporating species,44 values obtained for 1 those obtained for 2 C1-. it is highly unlikely that kx/kw values obtained for photochemical systems and for reaction 8 could be in Acknowledgment. This work was supported by the agreement as close as that observed unless the prePetroleum Research Fund, administered by the Amercursor to [ C O ( N H ~ ) ~ in ] ~ +both cases had the same ican Chemical Society. We are grateful to Professor charge. It has already been pointed that species Henry Taube for his interest in this work and for many such as [ C O ( N H ~ ) ~ ( N ~or ) ] ~[CO(NH~)~(NNNNO)]~+, + helpful discussions and to Messrs. E. Berthiaume and F. Billings for help with some of the experimental work.
+
+
(44) For instance, see G. E. Dolbear and H. Taube, Znorg. Chem., 6, 60 (1967).
+
+
(45) A. G. Sykes, Trans. Faraday Soc., 59, 1334, 1343 (1963).
Oxidations of Aqueous Chromium (11). 11. Chromium-C arbon Bond Willfred Schmidt, James H. Swinehart, and Henry Taube* Contribution f r o m the Departments of Chemistry, Stanford University, Stanford, California 94305, and the University of California at Davis, Davis, California 95616. Received April 11, 1970 Abstract: An investigation of the formation and kinetic stability of complexes containing a chromium-carbon bond in an aqueous medium is described. Pentaaquochromium(II1)ions carbon bonded to small acyclic anions such as are derived from isopropyl alcohol and diethyl ether are formed by the reduction of organic radicals with Cr(I1). The organic radicals are produced by hydrogen abstraction with OH radicals. Aquation rates of the Cr(II1) complexes when the organic ligands contain hydroxyl groups (alcohols) are described by the differential rate law -d In [(HzO)sCrL2+]/dt= kl k2[H+]. The rate laws for complexes having organic ligands which do not have a hydroxyl group (e.g., methyl or the radical derived from diethyl ether) do not show the term independent of hydrogen ion. A mechanism is proposed in which the activated complex contains a solvent molecule in a position so as to take advantage of hydrogen bonding with the organic ligand.
+
As
part of a continuing study of oxidations of aqueous Cr(II),l the formation and kinetic stability of complexes formed when radicals of small acyclic molecules oxidize Cr(I1) have been studied. A recent review article2 has covered the use of Cr(I1) * Address correspondence
to this author at Stanford University. (1) W. G. Schmidt. J. H. Swinehart, and H. Taube, Znorg. Chem., 7, 1984 (1968).
as a reducing agent in a variety of organic reactions. Anet3 first noted and isolated a stable chromium intermediate containing a chromium-carbon bond. This was the benzylchromium ion obtained in the reduction (2) J. R. Hanson and E . Premuzic, Angew. Cbem., In?. Ed. Engl., 7, 247 (1968). (3) F. A. L. Anet and E . LeBlanc, J. Amer. Cbem. Soc., 79, 2649 ( 1957).
Schmidt, Swinehart, Taube 1 Oxidations of Aqueous Chromium(II)
1118
of a benzyl halide by Cr(I1). Additional ways of forming the benzylchromium ion and some related aralkylchromium ions were studied by Kochi and Davis.4 The kinetics of the protonolysis of benzylchromium ion to toluene was investigated under a variety of conditions and the mechanism in an acidic buffer solution has been formulated as follows5
+ e CyHeCH2CrX+ CsH6CH3+ CrX2+
CyH6CH2Cr2+ XCyH6CH2CrX+
+ H+
-3
(1) (2)
where X- is the anionic buffer component. Recently the formation and aquation of a similar ion, pentaaquopyridiomethylchromium(II1) has been inve~tigated.~,' Reduction of simple alkyl halides with hexaaquochromium(I1) has not been successful. However, it has been found that they can be reduced by ethanolamine and ethylenediamine complexes of Cr(II).8a In a recent papersb the reduction of alkyl halides by Cr"(en) is described in some detail, and some properties of alkylethylenediamine-Cr(II1) complexes are recorded. Chromium complexes containing carbon-bonded simple alkyl moieties have been formed (uide infra), but not isolated, by the reduction of hydroperoxides.$ In this reaction an alkoxy radical is considered to be the initial product in the reduction of the hydroperoxide with Cr(I1). The formation of alkoxy radicals has been postulated in the reduction of alkyl hydroperoxides by Fe(1I)'O and Cu(1). These alkoxy radicals can then be further reduced to the corresponding alcoholg or they can undergo a fl scission fragmentation in which an alkyl radical is produced. The tert-butoxy radical, from the reduction of tertbutyl hydroperoxide, fragments into acetone and a methyl radicalI2 0
(CHJ3CO.
11
+ . C H I + CH3-C-CH3
(3)
An alkylchromium ion is formed when the alkyl radical is captured by a Cr(I1) ion. In the present work a series of chromium complexes containing carbonbonded ligands have been formed by using OH radicals t o abstract hydrogen atoms from organic molecules and then capturing the resulting organic radicals with aquochromium(I1). The complexes formed, (H2O)jCrL2+= CrL2+,are in general more labile than Cr(I1I) complexes containing inorganic ligands. However, some are stable enough for isolation by ion exchange and it was possible to study the kinetics of aquation for the entire series. (4) J. I
