Kinetic data are presented for the reaction of alkylchromium complexes (H2O)([14]aneN4)CrR2+ ([14]aneN4 = 1,4,8,11-tetraazacyclo-tetradecane) with Hg2+ via electrophilic substitution. The rate constants decrease in the order CH3 (1.1 × 106 mol-1 dm3 s-1) > C2H5 (7.7 × 102) > C3H7 (59) > 2-C3H7 (0.12) > CH2C(CH3)3 (0.0086). A series of meta and para substituted benzyl complexes obey a Ham-mett relationship with the reaction constant &rho; = -1.66, consistent with electrophilic substitution at &alpha;-carbon

Andreja Bakač

James H. Espenson

Vjera Butković

English

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ISSN-0011-1643CCA-2762 Original Scientific PaperKinetics and Mechanism of the Reactionbetween Mercury(II) and a Series of MacrocyclicOrganochromium Cations*Vjera Butkovi},a,** Andreja Baka~,b and James H. Espensonb,caRu|er Boskovi} Institute, P.O. Box 180, 10002 Zagreb, CroatiabAmes Laboratory, Iowa State University, Ames, Iowa 50011, USAcDepartment of Chemistry, Iowa State University, Ames, Iowa 50011, USAReceived February 23, 2001; revised June 19, 2001; accepted June 20, 2001Kinetic data are presented for the reaction of alkylchromium com-plexes (H2O)(14aneN4)CrR2+ (14aneN4 = 1,4,8,11-tetraazacyclo-tetradecane) with Hg2+ via electrophilic substitution. The rate con-stants decrease in the order CH3 (1.1  106 mol–1 dm3 s–1) > C2H5(7.7  102) > C3H7 (59) > 2-C3H7 (0.12) > CH2C(CH3)3 (0.0086). Aseries of meta and para substituted benzyl complexes obey a Ham-mett relationship with the reaction constant  = –1.66, consistentwith electrophilic substitution at -carbon.Key words: alkylchromium, electrophilic substitution, mercury(II),kinetics.INTRODUCTIONElectrophilic cleavage of metal-carbon bonds is a well-studied area inorganometallic chemistry.1 Mercury(II) ion and other commonly studiedelectrophiles are believed to react by direct displacement (SE2) of the alkylgroup by a back-side attack at -carbon to yield alkylmercury products. ForHg2+ as electrophile, this stereochemistry is strongly indicated in the case ofCROATICA CHEMICA ACTA CCACAA 74 (3) 735¿741 (2001)* Dedicated to Professor Smiljko A{perger on the occasion of his 80th birthday.** Author to whom correspondence should be addressed. (butka@joker.irb.hr)alkylcobaloximes where the reaction results in the inversion of configura-tion at -carbon.2Two series of organochromium complexes, (H2O)5CrR2+ and (15aneN4)-(H2O)CrR2+ (15aneN4 = 1,4,8,12-tetraazacyclopentadecane), have been ex-amined earlier3,4 for their reactivity towards Hg2+. For a given R group, es-pecially a bulky one, the complex in the aquachromium series is more reac-tive than its macrocyclic counterpart. The ratio of the rate constants for theaqua and macrocyclic compounds, kaq / k15ane, increased from about 3 for R =methyl to over 300 for R = isopropyl, suggesting that electron donation fromthe macrocycle, which would be expected to push the rate constant in theopposite direction, is overridden by the steric crowding that it adds. This ef-fect of the macrocycle is superimposed on the already strong steric effect ex-hibited by the alkyl group itself. Note that the electron donation (i.e. CH3 <C2H5 < 2-C3H7) and the steric bulk run in opposite directions.We have measured the kinetics of the reaction between Hg2+ and anotherseries of organochromium complexes, (H2O)(14aneN4)CrR2+, (14aneN4 =1,4,8,11-tetraazacyclotetradecane) Eq. (1).(H2O)(14aneN4)CrR2+ + Hg2+H O2  RHg+ + (14aneN4)Cr(H2O)23+ (1)The complexes chosen differ in subtle ways from those in the 15ane se-ries. The macrocycle is now somewhat smaller, which may make the approachto the alkyl group even more sensitive to steric effects. Also, the stereo-chemistry around the nitrogens is probably different in the two macrocycles.The 14ane complexes are almost certainly of the R, R, S, S (»two up, twodown«) kind, the same as the starting (14aneN4)Cr(H2O)R2+ and the reac-tion product, (14aneN4)Cr(H2O)23+. The (15aneN4)Cr complexes, on theother hand, have a tendency towards the less symmetrical »four up«5,6 or»three up, one down« stereochemistries, as observed for R = 4-BrC6H4CH2.7 Itis not clear how this change in stereochemistry may affect the reaction ra-tes, but one would expect the less symmetrical 15aneN4 complexes to allowa better approach to the reaction center. Our findings are presented below.EXPERIMENTALThe organochromium complexes were prepared in solution from (14aneN4)-Cr(H2O)22+ and the desired organic bromide or iodide in weakly acidic solution underan atmosphere of purified argon as described previously.4,8,9 The complexes were pu-rified by ion-exchange on a column of Sephadex C-25 cation exchange resin andstored in a refrigerator. Stock solutions of mercury(II) perchlorate were prepared byboiling the oxide in a slight excess of perchloric acid. These solutions were standard-ized as described previously.3736 V. BUTKOVI] ET AL.(Caution. Solutions of metal perchlorates are potentially explosive and should behandled with care.)Doubly distilled water, additionally purified by a passage through a Milli-Q sys-tem, was used throughout. All the experiments with air-sensitive complexes werecarried out in an atmosphere of Cr2+-scrubbed argon.Spectral and kinetic data were obtained using a Cary 219 spectrophotometerand a Durrum D-110 stopped-flow instrument. The kinetics was monitored at an ab-sorption maximum of organochromium complexes near 290 or 360 nm. All kinetic ex-periments were carried out at 25 °C in 0.25 mol dm–3 perchloric acid, at an ionicstrength of 0.50 mol dm–3 (HClO4 + LiClO4).RESULTS AND DISCUSSIONThe kinetics was determined under pseudo-first order conditions using alarge excess of Hg2+ over the concentration of organochromium complexes.The data obeyed the rate law of Eq. (2), as shown by the linearity of the firstorder plots and the linear dependence of the pseudo-first order rate constantson Hg2+ concentration, see Figure 1. The 1:1 stoichiometry and the forma-tion of HgR+, which were determined in earlier works3,4 with (H2O)5CrR2+REACTION OF ORGANOCHROMIUM CATIONS 737Figure 1. Plot of kobs vs. the concentrations of Hg2+ ions for the reaction with (H2O)-(14aneN4)CrR2+ complexes, where R = (a) C2H5; (b) n-C3H7; (c) n-C4H9; (d) n-C5H11.and (H2O)(15aneN4)CrR2+, are assumed to apply here as well. All the rateconstants are shown in Table I, along with selected data for the other twoseries.–d(H2O)(14aneN4)CrR2+ / dt = k14ane(H2O)(14aneN4)CrR2+Hg2+ (2)As expected, the kinetics is not acid dependent. This was confirmed inseveral experiments for R = C2H5 at H+ = 0.5 mol dm–3, which gave thesame rate constant as that obtained in 0.25 mol dm–3 HClO4. Similar obser-vations were made earlier in analogous reactions of other organochromiumcomplexes.The reactivity order methyl > ethyl > n-propyl demonstrates the over-whelming importance of steric bulk at -carbon and is typical of SE2 reac-tions. The effect is stronger for the macrocycles, whose methyl complexeslag behind (H2O)5CrCH32+ by less than an order of magnitude, but the dif-ference increases for the larger alkyls – ethyl and propyl. Further increasein the chain length to butyl and pentyl has little additional effect on the val-ues of the rate constants, Table I.738 V. BUTKOVI] ET AL.TABLE IKinetic data for the reaction of organochromium cations with Hg2+ ak / mol–1 dm3 s–1R (H2O)(14aneN4)CrR2+ (H2O)(15aneN4)CrR2+ b (H2O)5CrR2+ cCH3 (1.1  0.1)  106 3.1  106 1.0  107CH3CH2 (7.7  0.2)  102 2.53  103 1.40  105CH3CH2CH2 (5.9  0.4)  101 8.21  101 3.50  104CH3CH2CH2CH2 (3.1  0.1)  101CH3CH2CH2CH2CH2 (2.7  0.3)  101(CH3)2CH (1.15  0.08)  10–1 4.3  10–3 1.56(CH3)3CCH2 (8.6  0.4)  10–3 4.9  102c-C5H9 (1.49  0.09)  10–3c-C6H11 (1.26  0.15)  10–2 1.6  10–3 5.22  1044-CH3C6H4CH2 (2.29  0.11)  1033-CH3C6H4CH2 (1.66  0.15)  103C6H5CH2 (1.43  0.05)  103 1.14  103 4.87  1043-ClC6H4CH2 (2.97  0.09)  102a 25 °C, H+ = 0.25 mol dm–3,  = 0.5 mol dm–3 (HClO4 + LiClO4).b Ref. 4. c Ref. 3.The isopropyl group in the 14ane series causes a further drop in reac-tivity, as expected. Surprisingly, however, the neopentyl complex is evenslower by a factor of 13. This is different from the aqua series where thetrend is reversed and the neopentyl complex reacts more than 300 timesfaster than the isopropyl complex. This reversal in reactivity seems to becaused by geometric constraints of the macrocycle. The increase in stericbulk at the -position in the neopentyl complex severely impedes the entryof Hg2+ into the pocket defined by the rigid backbone of the macrocycle andsecondary N–H bonds, as illustrated below.Another surprising finding is the effect of the nature of the macrocycleon the reactivity of the isopropyl complex. Contrary to what one might expecton the basis of the larger size and possibly more favorable stereochemistryin the 15ane complex, (H2O)(14aneN4)CrCH(CH3)22+ is almost 30 timesREACTION OF ORGANOCHROMIUM CATIONS 7392.02.53.03.5-0.2 0.0 0.2 0.4 0.6logk4-Me3-MeH3-ClFigure 2. Plot of the logarithm of rate constants against the Hammett sigma valuesfor the reaction of substituted benzyl complexes with Hg2+. The slope gives the reac-tion constant  = –1.66.more reactive. Similar results were observed in the reaction of another sec-ondary alkyl, cyclohexyl. Thus, the fine differences in the geometry of thetwo macrocycles cause the 15ane complexes to be more reactive for R = pri-mary alkyl, but less reactive for R = secondary alkyl.In the series of m- and p-substituted benzyl complexes, the rate increasesas the electron donating properties of the substituents increase. The plot oflog k vs. the Hammett  value (Figure 2) is linear and yields a negative reac-tion constant,  = –1.66, consistent with the proposed electrophilic substitutionat carbon. The absolute value is higher than that obtained in the aqua series,–0.62. The higher sensitivity to the substituent of the macrocycles is proba-bly a result of the lower reactivity, which in turn may signal stronger Cr–Cbonds. Thermodynamic bond dissociation energies (BDE) are not available,but the homolysis rates of (H2O)(15aneN4)Cr(CH2C6H4R)2+,10 and probablythose of the analogous 14ane complexes, are lower than the rates of thecorresponding complexes in the aqua series,11 as it would be expected if theBDEs were higher for the macrocycles.Acknowledgment. – This work was supported by the Croatian Ministry of Sci-ence under Contract 00980604 and the US Department of Energy, Office of Basic En-ergy Sciences, Division of Chemical Sciences under Contract W-7405-Eng-82.REFERENCES1. J. K. Kochi, Organometallic Mechanisms and Catalysis, Academic Press, NewYork, 1978.2. H. L. Fritz, J. H. Espenson, D. A. Williams, and G. A. Molander, J. Am. Chem.Soc. 96 (1974) 2378–2381.3. J. P. I. Leslie and J. H. Espenson, J. Am. Chem. Soc. 98 (1976) 4839–4844.4. G. J. Samuels and J. H. Espenson, J. Am. Chem. Soc. 19 (1980) 233–235.5. W. Clegg, P. Leupin, D. T. Richens, A. G. Sykes, and E. S. Raper, Acta Crystallogr.,Sect. C 41 (1985) 530–532.6. R. W. Hay and M. T. Tarafder, Transition Met. Chem. (London) 18 (1993) 604–608.7. S. Shi, J. H. Espenson, and A. Baka~, J. Am. Chem. Soc. 112 (1990) 1841–1846.8. A. Baka~, V. Butkovi}, J. H. Espenson, and M. Orhanovi}, Inorg. Chem. 32 (1993)5886–5888.9. A. Baka~, V. Butkovi}, J. H. Espenson, and M. Orhanovi}, Inorg. Chim. Acta300–302 (2000) 280–284.10. S. Shi, J. H. Espenson, and A. Baka~, Inorg. Chem. 29 (1990) 4318–4322.11. R. S. Nohr and J. H. Espenson, J. Am. Chem. Soc. 97 (1975) 3392–3396.740 V. BUTKOVI] ET AL.SA@ETAKKinetika i mehanizam reakcije izme|u `ive(II) i serije makrocikli~kihorganokromovih kationaVjera Butkovi}, Andreja Baka~ i James H. EspensonPrikazani su kineti~ki podaci za reakciju organokromovih kompleksa (H2O)(14-aneN4)CrR2+ s ionima Hg2+ (14aneN4 = 1,4,8,11-tetraazaciklotetradekan), koja seodvija elektrofilnom supstitucijom. Konstante brzine reakcije smanjuju se u nizuCH3 (1,1  106 dm3 mol–1 s–1) > C2H5 (7,7  102) > C3H7 (59) > 2-C3H7 (0,12) >CH2C(CH3)3 (0,0086). Kompleksi sa supstituiranim meta- i para- benzil ligandimazadovoljavaju Hammett-ovu korelaciju s reakcijskom konstantom  = –1,66, {to je uskladu s pretpostavkom o elektrofilnoj supstituciji na -ugljikovu atomu.REACTION OF ORGANOCHROMIUM CATIONS 741

Kinetics and Mechanism of the Reaction between Mercury(II) and a Series of Macrocyclic Organochromium Cations

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