Chromium exchange between chromium(i1) and benzylchromium(iii) ions M. PARRIS

Transcription

1 Chromium exchange between chromium(i1) and benzylchromium(iii) ions I M. PARRIS Chetnistry Department, Carleton Uniaersity, Ottawa, Ont., Canada KIS 5B6 AND A. W. ASHBROOK Eldorado Nuclear Ltd., Albert St., Ottawa, Ont., Canada KIP5G8 Received August 8,1978 M. PARRIS and A. W. ASHBROOK. Can. J. Chem. 57,1233 (1979). The rate of first-order hydrogen-ion independent chromium isotope exchange between benzylchromium(ii1) and chromium(i1) ions is 1.2 x 10-2 M-' s-' at 0 C in a 70% methanol - water solvent, AS* = -28.5eu, AH* = 10.3 kcal M-'. For the corresponding exchange between benzylchromium(iii) and p-chlorobenzylchromium(iii) ions, these figures are 1.2 x 10-3 M-1 s-l, eu, and 10.9 kcal M-l. M. PARRIS et A. W. ASHBROOK. Can. J. Chem. 57, 1233 (1979). La vitesse du premier ordre de l'echange isotopique du chrome, independante des ions hydrogene, du benzylchrome(ii1) et du chrome(i1) est Bgale a 1.2 x M-' s-i a 0 C dans un solvant a 70% mkthanol/eau; le AS* = 28.5 ue et le AH* = 10.3 kcal M-'. Pour I'Bchange correspondant entre les ions benzylchrome(ii1) et p-chlorobenzylchrorne(iii), ces valeurs sont respectivement 1.2 x M-' s-', ue et 10.9 kcal M-'. [Traduit par le journal] 1 It has been suggested (I) that the mixed coupling upon reduction of mixed organic halides by chro- [3] *Benzyl Cr2+ + benzyl *Cr2+ 7 A *Benzyl *Cr2+ + benzyl Cr2+ j mous ion results from an inner sphere exchange, / i [I] Benzyl Cr2+ + *benzyl halide transition state, 1 + Benzyl halide + *benzyl Cr2+ occurring simultaneously with a coupling sequence, 1 [2] Benzyl Cr2+ + *benzyl halide! + Benzyl *benzyl + halo Cr2+ 1 The extent to which these two reactions occur together leads to the postulate that similar intermediates are involved in both reactions (2), in particular a four-centred transition state for the coupling reaction, C6HsqHz.-.-x c ~ H ~ ~ H ~ ~ ~ kinetically indistinguishable from that for the exchange, Evidence for the former transition state was apparently obtained when an optically-active threo-2,3- diphenylbutane resulted from the reduction of the a-phenethylchloride (2). It was of interest to determine the extent of exchange between two different benzylchromium(1ii) species: the more symmetric analogue of reactions [I] and [2], in order to assess the more symmetric four-centred Experimental Section Materials Chromium(II1) perchlorate stock solution was prepared from potassium dichromate, perchloric acid solutions by reduction with either ethanol or formic acid, and used to prepare chromium(i1) solutions by reduction with zinc amalgam. The chromous solutions were always prepared immediately prior to their use, and always contained at least half as many gram-ions of Zn(I1) as Cr(I1). ='Cr tracer was obtained from Atomic Energy of Canada Ltd. as a solution of sodium chromate. The tracer was added to an aliquot of the potassium dichromate and fumed with perchloric acid to remove any chloride. 1,l-Dimethyl-2-phenethylethanol (Eastman) was distilled, bp 62-64OC (1.5 rnrn), benzyl bromide (Eastrnan), 1,l-dimethyl-2-(p-chlorophenyl)ethanol, p-bromobenzyl bromide, p-methylbenzyl bromide, and p-methoxybenzyl bromide (Aldrich) were used without further purification. Ion-exchange resin, Dowex 50-X2 and 50-X8, mesh (Baker) was pretreated as follows: after removal of the fine material the remaining resin was warmed with a dilute alkaline solution of hydrogen peroxide, washed with dilute hydrochloric acid and then with water, and stored under 1 m perchloric acid. I, I-Dimethyl-2-(p-chlorophenyl)ethyl Hydroperoxide This was by an adaptation of the method for 1,ldimethyl-2-phenylethyl hydroperoxide (1). The preparation is /79/ $01.OO/O el979 National Research Council of Canada/Conseil national de recherches du Canada

2 1234 CAN. J. CHEM. VOL.. 57, 1979 hazardous. Recrystallization from petroleum ether gave white needles, mp 51-52"C, yield 28%. Analysis by ceric titration showed greater than 99.5% hydroperoxide. Apparatus Ion-exchange columns were jacketted at O C. A well-type NaI crystal (1 x 1.5 in.) was used as a scintillation detector. Provision was made for continuous monitoring of the column eluate, fraction collection, and counting of individual samples. A Beckman model B spectrophotometer was used for chromium analyses on the column eluate. Analysis Chromium(I1) concentrations were determined by ferric oxidation and ceric back-titration. Free acid was determined, after oxidation of any chromium(i1) present, by passing an aliquot through an ion exchange column in the hydrogen ion form. Total chromium was determined spectrophotometrically at x1, as chromate (3) after alkaline oxidation with hydrogen peroxide (4). The main chromium species (see Fig. 1 A) separated from an exchange between benzylchromium(ii1) and chromium(i1) ions, quenched by ferric ammonium sulphate, was found to be a sulphatochromium(ii1) ion plus a small (<5%) amount of a disulphatochromium(iii) ion, in addition to the hexaaquochromium(111) ion initially present. The sulphato complexes were identified by spectra (6) and by CrS04 analyses on the separated fractions. No ligated ethanol was found even when the benzylchromium(ii1) ion had been prepared originally in 90 vo1.z ethanol-water at O C. Visually, oxidation of chromium(i1) ion by ferric ammonium sulphate did not appear to be slower than by ferric perchlorate for which k = 2 x lo3 M-Is-' at 25"C, and the lack of ligated ethanol must reflect discrimination by chromium(i1) ion against ethanol. Ethanol, bound and unbound, associated with chromic species was determined by the method of Kemp and King (5). Determination of Rate Constants I. Oxidation of Cr(II), Spectrophotometric Method Solvent was measured into the capped, thermostatted cell and the appropriate reagent introduced by means of a hypodermic syringe. 2. Isotope Exchange Reactions The general procedure used was as follows. Exchange between Benzylchromium(III) Ion and Chromium(II) Ion 1,l-Dimethyl-2-phenethyl hydroperoxide or benzyl halide and solvent were nitrogen-flushed in a capped vessel. A stoichiometric quantity of chromium(i1) perchlorate solution was added to produce the benzylchromium(ii1) ion. A known amount of labelled chromium(i1) perchlorate solution was added and after a time the exchange was quenched by addition of a slight excess (2%) of oxygen-free ferric solution before injection of an aliquot into the ion-exchange column. The eluate activity was continuously monitored and Fig. 1 shows typical records obtained. In experiments in which ferric ammonium sulphate was used as the quenching reagent, the column was eluted initially with 1 m perchloric acid for the chromic sulphate species and then with 2 m perchloric acid for, fist, the hexaquochromium(iii) and, later, the benzylchromium(ii1) ion. In the case of ferric perchlorate quenching, 2 m perchloric acid was used throughout. Exchange between Benzylchromium(III) Ion and p-chlorobenzylchromium(iii) Ion Unsubstituted benzylchromium(i1i) ion was prepared from labelled chromium(i1) ion and an aliquot of the unlabelled ELUTION TlME (h) ELUTION TIME (h) FIG. 1. (A) Elution of 51Cr-containing species in the exchange between benzylchromium(ii1) and chromium(i1) ions. (B) Elution of 51Cr-containing species in the reaction between benzylchromium(1ii) ion and benzylchloride. (C) Elution of 51Cr-containing species in the exchange between benzylchromiurn(ii1) and p-chlorobenzylchromium(111) ions. species injected into the labelled solution. After the required exchange time, an aliquot of the mixture was treated as before. Exchange between Benzylchromium(II1) Ion and Hexaquochromium (111) Ion The same general procedure was used, the labelled species being hexaaquochromium(111) ion. 3. Oxidation of Cr(II), Ion-exchange Method In many cases it proved convenient to use an adaptation of the isotope exchange method in order to study the reaction

3 PARRIS AND ASHBROOK TABLE 1. Cr exchange between benzylchromium(iii) and chromiurn(i1) in 70 vo1.z methanol - water Exchange time (min) Temperat ure [Cr(II)] ("C) x lo2 athe benzylchromium(iii) was prepared from C6H5CH2C(CH3)200H and thus there was present also an equivalent amount of Cr(lII)(H20)6. bcontains additionally 0.1 M LiCI. between benzylchromiurn(111) ion and the benzyl halides. : Labelled benzylchromiurn(ii1) ion was prepared from 1,ldimethyl-2-phenylethyl hydroperoxide and labelled Cr(II), mixed with the benzyl halide, and used in ion-exchange! chromatography as before. Results and Discussion Exchange Reactions The rate R at which a chromium(111) species exchanges with chromium(i1) follows the McKay exponential function Table 1 contains data for the exchange between benzylchromium(iii), B,Cr(II1),,2 +, and chromium- (11), Cr,;', ions at various temperatures, acid and salt concentrations. The exchange could perhaps result from the successive reactions i F is the extent to which distribution of the label between chromium species approaches the equilibrium value of 1 and, where the Cr(II1) is originally inactive, is given by S,' being the specific activity of species x at time t. For a second-order exchange rate since hexaaquochromium(11i) ion, Cr(III),;+, is present, formed coincidentally with the benzylchromium(111) species. However, the rate constant for reaction [5] is I 2 x lo-' M-' s-i. For reaction [6] the exchange rate in 70 vo1.x methanol - water at PC, [B,Cr(111)~+] = 2.0 x lop2 m, [Cr(II1),,3 +] = 5.0 x lop2 m was found to be very slow; after 1.5 h practically no exchange had occurred and an upper limit for the rate constant is calculable,

4 1236 CAN. J. CHEM. VOL x lo-' M-l s-l. The average rate constant for reaction [4] at 0 C is 1.2, x lo-' M-' s-l, AS* = f 1.1 eu, and AH* = 10.3 f 0.3 kcal mol-l. Figure 1 B illustrates how two apparently distinct chromium(111) species (CrA3+ and Cr,3+) are produced in the reaction between benzylchromium(iii) ion and various benzyl halides. The rate of increase of [CrB3+] followed the rate of increase of [CrX2+], the initial product of reduction of the benzyl halide, C6H5CH2X, by the chromous ion. When the reaction was allowed to proceed after complete decomposition of the benzylchromium(111) ion, [CrB3+] increased, [CrX2+] decreased, and [CrA3+] remained virtually unchanged. It is concluded that CrB3+ results from aquation of CrX2+ and CrA3+ from the original oxidation of chromium(i1) ion by 1,l-dimethyl-2-phenylethyl hydroperoxide. The CrA3+ was identified as Cr(III)(H20)$+ by its absorption spectrum, A, 407 and 573 nm (5). Determination of bound ethanol in the Cr;+ species on the other hand showed 0.5 to 0.8 mol ethanol per atom Cr. Rate constants for exchange between two different benzylchromium species (see Fig. 1C and eq. [3], *benzyl = p-chlorobenzyl) were found to vary significantly with the ion elutionrate owing to that reaction which had occurred after injection into the ion-exchange column yet prior to separation thereon. There is no obvious way to arrest this exchange; however, at constant elutionrate the log (1 - F) plots are linear with a t = 0 intercept less than 1. The unit intercept is restored by shifting the plot ca. 112 h, indicative of a reasonably constant separation time. At 0 C the half-time for exchange was typically ca. 4 h and the rate constant for reaction [7] was found to be 1.2 x lop3 M-l s-l, AS* = f 1.6 eu and AH* = 10.9 f 0.3 kcal mol-l. The reactions between benzylchromium and benzyl halides were studied at temperatures between 0 and 25"C, Table2, and the rate of disappearance of benzylchromium monitored spectrophotometrically at 360 nm. For example [81 PhCH2Craq2 + + PhCH2Br -t Products [8A] PhCH2Craq2+ + PhCH2CI -+ Products [91 PhCH2Cra,2 + + p-brc6h4ch2br -+ Products Reaction [8] implies and includes the sequence: TABLE 2. Exchange rates between some substituted benzylchromium(i11) and chromium(i1) ions at O Ca [Species] x lo2 [Cr2+] x 10' k(m-l S-l) x lo2 C6H,CH2Cr " P-BrC6H4CH2Cr p-mec6h4ch2cr2 + ' p-meoc6h4ch2cr 'ILiC1041 = 0.4, IH+l = 0.3, 70 vo1.z methanol -water. *From p-brbenzylbromide. CFrom p-mebenzylbromide. dfrom p-meobenzylbromide. 'Data from Table 1. k4 [I31 CrBr,:+--+CrB3+ + Br- PhCH2CH2Ph + CrBraq2+ Reaction [9] implies and includes in addition the sequence : ks [14] PhCH2Cr,,2+ + p-brc6h4ch2br -4 The differential equations associated with these reaction schemes cannot be solved in closed form. Using known or estimated values for the rate constants k,-k,, a numerical integration by the Runge- Kutta method (8) confirmed that the observed spectrophotometric and concentration data are in agreement. The rate constants used were obtained as follows : k,: lo3 M-' s-' (1, 9); k,: calculated from experimental data for both the decrease in benzylchromium and the increase in CrA3+, 2.3 x lo-, s-i (2); k,, k,, k,, k7, k,: calculated from the initial rate of disappearance of the appropriate benzylchromium. These initial rates were practically the same for reactions [8] and [9]. Furthermore, organic product

5 1 PARRIS AND ASHBROOK 1237 rate constants used (at 0 C) were all 5.0 x lop5 M-' s-l. For the chlorides the value of 1.0 x lop7 was used. k,: aquation rate constants were calculated from available data.. For chloropentaaquochromium(jii) at O0C, [H'] = 0.5, a value 1.0 x s-' was used (7) and for the bromo species, 2.0 x lop6 s-l (10); k,: this was determined in the same way as k,, 1.0 x lop6 s-l. The values for k, and k, used here, 2.3 x and 1 x s-l, are comparablein ratio to the values at 28"C, 1.0 x and 4.6 x spl from ref. 2. Figure 2 shows the experimental concentrations of reactants, intermediates and products for the reaction between benzylchromium(lii) ion and p-bromobenzylbromide at O"C, together with values calculated using these rate constants. 1. J. K. KOCHI and D. DAVIS. J. Am. Chem. Soc. 86,5264 (1964). 2. J. K. KOCHI and D. BUCHANAN. J. Am. Chem. Soc. 87,853 (1965). 3. E. DEUTSCH and H. TAUBE. Inorg. Chem. 8,1532 (1968). 4. G. W. HAUPT. J. Res. Natl. Bur. Stand (1952).... REACTION TIME (h) 5. D. W. KEMP and E. L. KING. J. Am. Chem. Soc. 89,3433 (1967). I FIG. 2. Experimental (0) and calculated (-) concentrations 6, N, F ~ J, M, ~ J. TAI, ~ and ~ Y, YARBOROUGH,, J. Am. for the reaction between benzylchromium(i1i) and p-bromo- Chem. Soc. 84, 1145 (1962). benzyl bromide at O C in 70 ~01% methanollwater, P = 0.5, 1 7, T. W. SW~DDLE and E. L. KING. Inorg, Chem. 4, 532 [H+] = (IQAO \.,".,. 8. proportions from reactions between benzylchromium T. H. ESpENSON and 0. J. PARKER. J. Am. Chem. '0~ (1968). and several substituted benzyl halides were similar, 9, F, A. L. ANET and E. LEBLANC. J. Am. Chem. Sot, 79, : indicating that reaction proportions, and so rates, 2649 ( 1957). are essentially the same. For the benzyl bromides, the lo. F. A. GUTHRIE and E. L. KING. Inorg. Chem. 3,916(1964). I