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The Effect of Coordination on the Reactivity of Aromatic Ligands. XII. Reactions of Coordinated Aniline] Naresh K. Chawla, Don G. Lambert, and Mark M. Jones
Contributionf r o m the Department of Chemistry, Vanderbilt University, Nashville, Tennessee 37203. Received August 15, 1966 Abstract. The rate of bromination of coordinated aniline in [C0(en)~(NH~C~€I~)(H20)13f has been determined and found to be less than that for free aniline by a factor of about 1O1O. The cooi:dinated aniline, in turn, reacts more was prepared rapidly than the anilinium ion by a large factor (about lo6). The ion [CO(~~)~(NH~C~H~)(H~O)]~+ ( C Irate ) ] ~ +of. this aquation reaction over the temperature range 25 by the aquation of [ C O ( ~ ~ ) ~ ( N H ~ C ~ H ~ )The to 40" can be described by the first-order rate constant kl = 1.1 X 1015 exp(26,800/RT). The ion [Co(en)2(NH2C6H5)(HzO)]3+also reacts with water a t a very slow rate t o split off aniline and form [Co(en),(Hz0),]3+. This reaction has a first-order rate constant kl = 3.2 X 1015exp(30,100/RT) over the same temperature range. The rate constant for the bromination of [ C O ( ~ ~ > ~ ( ~ H ~ C ~ H ~over ) ( Hthe ~O temperature )]~+ range 10-25" is given by kz = 4.6 X lo6exp(10,200/RT). Studies o n the rate of iodination of [Pd(NHzC6H5)4]2fwere unsuccessful because of the rapid replacement of coordinated aniline by iodide. the complex were aged in a constant-temperature bath, and at the lthough studies in this series have examined a appropriate times aliquots were taken and put into a solution 0.25 variety of aromatic ligands, few of these lend M in potassium bromide and 0.05 M in hydrochloric acid. Brothemselves to such direct demonstrations of effects as mine was generated using a coulometer, and the concentration of the simplest aromatic ligand which coordinates without free bromine in solution was monitored with a platinum-calomel electrode system and a p ~ l a r o g r a p h . ~The current was measured loss of a proton: aniline. It is with this ligand that the at a constant voltage on the diffusion current plateau and was most unambiguous data can be obtained, but it is also linearly related to the bromine concentration in all calibration unfortunately true that the water-soluble complexes of runs. Aniline itself was found to undergo nearly instantaneous aniline are neither numerous nor well characterized. tribromination, anti the presence of free aniline prevented free A further complication arises from the fact that aniline bromine from accuinulating in the system. After the free aniline had been converteli completely to the tribromide, the bromine is not a notably good donor species, and in many circoncentration increased at a rate corresponding to the current cumstances seems unable to compete for coordination setting of the coulcmeter. From the time required to reach this positions with the anions present in the reaction mixture point, the amount of aniline which had been hydrolyzed from (e.g., with Pt(I1) or Pd(I1) complexes in bromide or [ C O ( ~ ~ ) ~ ( N H ~ C ~ H3+~ )could ( H ~ Obe) ] determined. Since the rate of release of aniline from this complex was extremely siow in comiodide solutions). The present studies were carried parison to the rate cif bromination of free aniline, no correction had out largely on trans-[Co(en)~(NH~C~H~)(Cl)]Cl~H~0, to be applied for the amount of hydrolysis which occurred during because it proved to furnish one of the most direct routes the course of the analysis. for obtaining coordinated aniline in an aqueous enRate of Chloride Loss of rrans-[C~(en)~(NH~C~H~)(Cl)]~+. The vironment. A further advantage, which this compound z+ was rate of chloride h s s from truns-[Co(en)*(NH~CeH~)(Cl)] determined conduct.ometrically over the concentration range 0.88 possessed over others which were considered, was that X 10-3 to 4.4 X 10-3 M , and the data were reduced by standard its only reaction with bromine was aromatic halogenatechniques.6 tion. Rate of Brominaiion of Coordinated Aniline. The bromination
A
of coordinated aniline was examined with both [ C O ( ~ ~ ) ~ ( N H ~ C ~ H ~ ) ( Hsubstrates. Z O ) ] ~ + The rate of (Cl)l2+and [ C O ( ~ ~ ) ~ ( N H ~ C ~ H ~ )as decrease of bromine concentration in solutions containing these Preparation of t r ~ n s - [ C o ( e n ) ~ ( ~ ~ C ~ HsHzO. ~ ~ ) ( c This ~)]C l~ comcomplex ions was determined using the potentiometric method of pound was prepared by the method of Meisenheimer and Kiderlen.2 Bell and Robinson; and a Beckman Research Model pH meter. Anal. Calcd for [ C O ( ~ ~ ) Z ( N H Z C ~ H ~.H20: ) C I ] C ~Co, Z 14.87; The same procedure was used to establish the rate of vaporization C, 30.27; H, 6.31; N, 17.66; NH2C6Hs, 23.46. Found: Co, of bromine in blank runs and to search for evidence for the reaction 14.77; C,30.25; H,6.16; N, 17.71; NHzCoH5,23.23. of bromine with [C~i(en)~(Cl)~]+ and its hydrolysis products. These Preparation of [ P ~ ( N H z C ~ H ~ ) ~ ] B ~ [Pd(NHzC6Hs)2Br21a ~.~HZ~. solutions were all 0.05 Min HC1 and 0.25 Min KBr. (10.42 g, 0.023 mole) was mixed with aniline (75 ml) in a 200-ml For bromination of [ C O ( ~ ~ ) ~ ( N H * C ~ H ~a ) solution ( C ~ ) ] ~ con+ flask fitted with an air condenser. This mixture was kept on a taining potassium bromide and hydrochloric acid was allowed to steam bath for 28 hr after which the resultant slurry was filtered. reach the desired temperature in a jacketed vessel through which The residue was washed with several portions of water, and these water from a constant-temperature bath was pumped. A weighed washings were combined with the filtrate and agitated. The water sample of the chloro complex was added and the timer started. layer was separated and evaporated to obtain a residue which was Millivolt readings were then taken at various times using the then recrystallized from 95 ethanol. The yield of the light buff platinum-glass eleztrode system. Even though some of the solid complex was 4.2 g (26z). Anal. Calcd for [Pd(NH& chloro complex is hydrolyzed to the aquo complex during the HS)4]Br2.3H20:C,41.61; H,4.91. Found: C,41.73; H,4.71. time required for bromination, linear plots were obtained because Rate of Aniline Loss from [CO(~~):(NH~C&I,)(H~O)]~+. While the two species are brominated at the same rate (cide infra). free aniline reacts at an extremely rapid rate with bromine in For bromination of [Co(en),(NH2C6HS)(H~0)] a weighed samaqueous solution, the reaction of the coordinated aniline is exple of [Co(en)z(NHzC6HSXCI)]Cl~~H~0 was dissolved in 0.25 M traordinarily slow. As a result, it is possible to use an amperometric kM-xH2Ar > k + S H a + . One implication of blank solution containing trans-[Co(en)~(H~O>]~+. In these results, which is of considerable importance, is such a system the rate of loss of bromine is identical that the net charge on the complex is so diffusely diswith that found when the complex was absent. The tributed that the effective charge adjacent to or on the rate constants for the bromination of the coordinated nitrogen is less than 1, even though it is adjacent to a aniline have an estimated uncertainty of 10% or less. metal ion with an ostensible charge of +3. The atThe rate of bromination of the coordinated aniline is tainment of the transition state for aromatic brominaconsiderably more rapid than the loss of aniline from tion is thus somewhat more difficult to achieve in the the complex. The relative reaction rates found thus complex than in the free ligand. The basis for this completely exclude any alternative reaction scheme may be seen in the comparison of the data obtained on involving only bromination of released aniline. From (9) B. R. Suthers, P. H. Riggins, and D. E. Pearson, J . Org. Chem., these data, the activation energy is found to be 10.2 21, 447 (1962). kcal/mole and frequency factor is 4.6 X lo6 sec-I. (10) N. K. Chawla and M. M. Jones, Inorg. Chem., 3, 1549 (1964).
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Chawla, Lambert, Jones / Reactions of Coordinated Aniline
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methylbenzenes with that presented here on aniline and its derivatives. In each case, this basis is the manner in which the substituents affect the ability of the system to accommodate the positive charge of the transition state tiia structures of the type +R=C
