Znorg. Chem. 1982,21, 2336-2344
2336
ranges: [Co"'] = (4.03-36.3) X lo4 M, [I-] = (1.0 - 20.0) X M, and [H+] = 0.1-1.8 M at 25 "C and ionic strength 2.0 M. Kinetics. Kinetic measurements were made under identical conditions used for stoichiometry measurements. The appearance of iodine (as 13-) was monitored at fixed wavelength in the range 388-460 nm with iodide in sufficient excess to ensure first-order conditions. Plots of In ( A , -At) vs. time were linear for at least 4 half-lives, indicating that the formation of iodine is first order in [CO~'']. Derived first-order rate constants koW at three different acidities at 25.0 "C and Z = 2.0 M are collected in Table IV. At each acidity the data are well described by rate law 18, where kobsd= k, + kf[I-]. d[Iz]/dt = -Yzd[Co"']/dt = kob~[CO''l] (18) Values of k, and kf from a linear regression fit are given in Table IV. The first-order rate constant k, increases with increasing acidity, while the rate constant kf is acid independent. The mechanism of eq 19-22 predicts the observed stoichiometry and rate law.
21.
fast
I2
The rate constant for reaction 21 (1.60 M-' si) at 25 OC and Z = 2.0 M)" is much higher than that for reaction 20, ensuring that reactions 19 and 20 are the rate-determining steps. For this mechanism, 2k, = k4 and 2kf = k5. The value of 2k, = k, kb[H30+] calculated from the data in Table I for decarboxylation of Co(py),(C03)(H20),+ at each experimental acidity used for the reaction with iodide is in agreement, within experimental error, with the observed value. This confirms (within the precision limits of the data) that the k, terms corresponds to decarboxylation reaction 19 and that the kf term arises from direct reduction of Co(py),(C03)(H20),+ by iodide (reaction 20). The rate constant k5 is about 40 times less than that for the corresponding reduction of C o ( ~ y ) ~ ( H ~by 0 )I-~(reaction ~+ 21) at 25 0C.2s The rate constants for reduction of Co(Py)z(C03)(Hz0)2+, Co(PY)z(Hz0)43+,25and CO(NHd2(H?O)> k,, so that k3 = 3 X lo3 M-' s-I at 25 OC, in good agreement with the value of -2 X lo3 M-' s-l recorded for several other uncharged entering ligands.Is The values deduced for k2 and k3 at various temperatures (neglecting k,) are recorded in Table
+
and (4)
The concentration of hydrogen ion was computed after taking into account the hydrolysis of Fe3+ (i.e., [H'] = [HC104] + [FeOH2+]).14 Values of kf and k, at specified acidities were obtained as the slope and intercept of the least-squares plots of kow vs. [Fe3+]using the data of Table I1 and eq 2 and the known values Of & I 4 Plots O f kd[H+] + Kd) VS. [H+]-' also yielded straight lines, indicating that k4[H+] (en),(NH,) > 5
Table VII. Rate and Activation Parameters for the Formation of FeLnf SpecieP
LH (NH,),CoSalHZ+ cis-(en),(NH,)CoSalH2+ (c@)-(tetren)CoSalHZ+ H,Sal HSal(NH, ) ,CoC,O,+ (NH,),CoC20,H2+
c,H,oH~~~ CH,CO, He H,Of
kll, s-l
M-I
5.6 5.1 5.6 smallb smallb 870 smallb 25 27 160
**
AH11 kcal mol-' 25 i 5 16.6 i 2.1 15.5 i 1.3
2 6 f 12 15.3 i 0.6
ASll*, cal deg-' mol-' + 2 8 t 16 Of 7 -3i5
35 t 36 2.9 i 1.6
AH2,#,
M-' sC1
kcal mol-'
ASz2*, cal deg-' mol-'
7.7 7.7 6.6 35 185 370 46 7.2 28 1400
8.4 i 0.9 6.4 i 1.4 9.1 i 3.2 6.0 * 0.9 13.3 i 3.4 14.5 i 0.7
-17 i 3 -24 i 5 -15 * 11 -22 i 3 + 6 f 11 +11.0 i 2.4
10-2k22,
11.1 f 2.2 10.1 i 0.4
-5.4 +1.3
f f
7.0 1.0
ref this work this work this work this work this work 7 7 21 22 15
At T = 25 OC;I= 1.0 M unless otherwise specified. Not possible to measure by our procedure but probably not greater than 10 M-' s-' At I = 0.1 M. Values of k , , for various substituted phenols differ very little from those for phenol itself.1g e At I = 0.5 M . Waterexchange rate constants (units s-]).
(Salicylato)pentaaminecobalt(III)-Iron(II1)
Inorganic Chemistry, Vol. 21, No. 6, 1982 2343
Complexes
Table VIII. Rate Data for the Dissociation of N,CoSalFe4+ Specie?
5 NH,
0.025 0.15
0.163f 0.006 0.155It 0.008
20.0"c 0.128 0.147
k-" = 0.031f 0.013M-' s-'
0.025 0.275f 0.002 0.10 0.254f 0.007 0.20 0.251f 0.004 k l 1= 0.045f 0.011M-' s-' 0.025 0.404t 0.008 0.05 0.389t 0.010 0.10 0.392* 0.009 k l 1= 0.085f 0.034M-' s-' (en),(NH,)
0.025 0.214f 0.002 0.)5 0.207i 0.005 0.15 0.214f 0.005 0 40 0.219f 0.003 k = 0.064f 0.008M-' s-' 0.025 0.363f 0.005 0.050 0.342f 0.007 0.10 0.311f 0.011 0.20 0.340t 0.010 k-'' = 0.09f 0.02 0.025 0.545k 0.007 0.050 0.503i 0.011 0.10 0.498* 0.009 0.20 0.503f 0.009 k l 1= 0.157f 0.006M-' 8'
tetren
0.025 0.450f 0.009 0.050 0.453f 0.008 0.150 0.503f 0.009 0.250 0.517f 0,010 k l 1= 0.168f 0.030M - I s-' 0.025 0.724f 0.010 0.150 0.794f 0.010 0.300 0.841f 0.012 = 0.288f 0.048M-' s-'
25.0'C 0.216 0.236 0.239 30.0"C 0.297 0.331 0.358
0.35 0.159f 0.004 0.60 0.1562 0.002 0.80 0.162f 0.006 k Z= 20.136f 0.006s-'
0.155 0.152 0.159
0.40 0.70 0.90
0.265i 0.007 0.262f 0.007 0.272* 0.005 k Z =20.227f 0.006s?
0.256 0.254 0.265
0.50 0.90
0.388 0.390
0.402f 0.013 0.402f 0.012
k 2 ,= 0.326f 0.016s-' 20.0"C 0.180 0.196 0.206 0.214
0.60 0.80 0.90
0.228i 0.004 0.237f 0.006 0.256f 0.004
0.223 0.233 0.252
k Z =20.188f 0.004s-' 25.0"C 0.305 0.310 0.312 0.328
0.40 0.80 0.90
0.354f 0.010 0.387f 0.006 0.397f 0.005
0.345 0.380 0.390
k 2 ,= 0.301f 0.011s-' 30.0"C 0.455 0.454 0.470 0.485
0.40 0.60 0.80
0.533f 0.013 0.551i 0.006 0.588i 0.003
0.520 0.540 0.578
k-2z = 0.452f 0.003s-'
20.0"C 0.424 0.438 0.496 0.511 25.0"C 0.672 0.780 0.831
0.35 0.533t 0.017 0.45 0,537f 0.015 0.60 0.547f 0.021 0.80 0.570f 0.008 k Z =20.448f 0.012s-I
0.528 0.532 0.543 0.566
0.40 0.869f 0.010 0.70 0.910f 0.025 0.90 0.969f 0.032 k Z= z 0.716f 0.025 s"
0.860 0.902 0.962
30.0"C 0.025 1.07f 0.01 0.99 0.40 1.28f 0.03 0.150 1.22f 0.04 1.20 1.31 t 0.03 0.60 1.27f 0.07 0.300 1.25 0.90 1.47t 0.07 k l 1= 0.44f 0.09M - I s'' k-2z= 1.07f 0.04s-' [ F e 3 + ]=~ 1.05X lo+; [complex] = 2.6X M (N, = 5 NH,, (en),(NH,)), 1.56X M (N, = tetren). k , =k&&
[H+])[Fe"].
1.27 1.30 1.46
- (kl, + k,,Kh/
Table IX. Rate and Activation Parameters for the Spontaneous and AcidCatalyzed Dissociation of Iron(ll1)-Mono(salicy1ato) Specie?
k-,,(25"C),
complex
M-1 s-l
(NH,) ,CoSalFe4+ czs-(en),(NH,)CoSalFe4+ (or@)-( tetren)CoSalFe4+ FeSal+ FeSalH
,+
a Z=
1.0M (C104-).
0.05 0.09 0.29 8.8b
AHt, kcal mol-'
17.1 f 2.9 15.3 i 2.3 16.6f 0.9 11.3 i 1.0
cal K-' mol-'
-7 f 9 -12 t 8 -5
f
3
k-zz(25 "c), s-
",
AS',
kcal mol-'
cal K-' mol-'
0.23 0.30 0.72
14.9f 1.3 14.9i 0.5 14.8i 0.5
-12 i 4 -11 f 2 -loi 2
5.5 c 0.2
17.5i 0.4
-17 t 3.0
+3.4f 1.3
k-'' = k , K H ; spontaneous dissociation of FeSal+ was not observable.
NH3,suggesting that, as for the spontaneous dissociation path, the rate of dissociation of Fe3+ from the protonated binuclear species N5CoSalFe4+increases with steric crowding at the
cobalt(II1) center. Alternatively, the effect may be the result of mediation of the electrostatic repulsion between the Fe(II1) and Co(II1) centers by the solvation shells of the bridged
Inorg. Chem. 1982, 21, 2344-2348
2344
binuclear species, repulsion being least for the most highly solvated (NH3)5species and greatest to the least solvated tetren analogue.
Acknowledgment. The authors are grateful to the John D. and Francis H. Larkin Foundation of the State University of
New York at Buffalo for financial support and to Utkal University for a leave of absence to A.C.D. Registry No. Co(NH3)+3alH2+,30931-74-9; cis-C~(en)~(NH~)SalH2+, 59296-02-5; (aj3S)-Co(tetren)SalH2+, 78715-92-1; Fe, 7439-89-6; Fe(OH2)2+,15365-81-8; Fe(OH2),(OH)2+,15696-19-2; H2Sal, 69-72-7.
Contribution from the Gray Freshwater Biological Institute, College of Biological Sciences, University of Minnesota, Navarre, Minnesota 55392
Demethylation of Methylcobalamin by Tetrahaloaurates. Kinetics and Mechanism YUEH-TAI FANCHIANG* Received June 23, I981
The stoichiometries and kinetics of reactions of methylcobalamin (CH3-BI2)with AuX4- (X = C1 or Br) in acidic solution have been examined. Under anaerobic conditions, the reactions occur with a 2: 1 stoichiometry (AuX4-:CH3-BI2),producing aquocobalaminwith an oxidid corrin ring, CH3X,and metallic gold. The stoichiometryand reaction products are interpreted in terms of one-electron oxidative demethylation of CH3-BI2.Kinetic data support a mechanism which involves an equilibrium prior to the electron transfer between CH3-BI2and AuX4-. Effects of pH and ionic strength on the kinetics are also examined. The detailed mechanism for the Co-C bond cleavage is discussed. of CH3-BI2(reaction 1) were estimated spectrophotometrically ac-
Introduction Oxidation-reduction reactions and their related cleavage of the cobalt-carbon bond are important in our understanding of the biological roles of methylcobalamin (CH3-BI2).' They are also of considerable mechanistic interest.2 Several modes of cleavage of the Co-C bond of CH3-B12induced by metal ions have been described in detail. These include electrophilic demethylation by Hg2+ and PdC142-,4,reductive demethylation by Cr2+5and Sn(II),6 and redox switch by PtC162-/ PtC142-*' It has been briefly reported that CH3-BI2can be demethylated by AuC14-.* The mechanism for this reaction was suggested to be similar to that proposed for the methylation of platinum complexes by CH3-B12.' We undertook the study on the demethylation of CH3-B12by gold complexes to determine whether this mechanism occurs. In contrast to the previous report, our work provides results which support the single-electron oxidative demethylation of CH3-B12by gold complexes.
Experimental Section Meteripla. Fine gold powder and NaAuCQ2H20were purchased from either Goldsmith or Ventron, Inc., and were used as received. KAuBr4.2H20 was synthesized by the method of Block.9 AuX2solutions were generated by reducing AuX4- with zinc amalgam in acidic solution under an atmosphereof argon. CH3-BI2was synthesized by the method described by Dolphinloor purchased from Sigma, Inc. Cob(I1)alamin (B12,) solutions were generated by reducing aquocob(I1I)alamin (H20-B12+)with equimolar amounts of EuZ+under argon. Concentrations of CH3-BI2,H20-BI2+,and B12rin solution were determined from their published molar absorptivities.11m12All other chemicals were reagent grade and were used as received. Stoichiometric Studies and Reaction Roducts. Consumption ratios for the reactions between CH3-BI2and AuC14- or AuBr4- were determined spectrophotometrically at 351 or 537 nm with a GCA/ McPherson spectrophotometer in subdued light under an atmosphere of argon. B12products were identified spectrophotometrically. Demethylation products under various conditions were identified with a Becker gas chromatographModel 417 with a column (8 ft by 2 mm) of 5% FFAP on chromasorb W-AW-DMCS, 80-100 mesh at 45 "C, and with a Brilker 270 MHz proton NMR spectrometer. Equilibrium Constant Measurements. The equilibrium constants for 5,6-dimethylbenzimidazole*base-onnand 'base-off' conversion
*Correspondenceshould be addressed at the Department of Biochemistry, Medical School, University of Minnesota, Minneapolis, MN 55455.
0020-1669/82/1321-2344$01.25/0
cording to eq 2." Measurementswere made at 1.0 M ionic strength which was maintained with NaC1, NaBr, or NaC104 (23 "C).
pK2 can be considered as 4.7, which is the pK, for free 5,6-dimethylbenzimidazolein aqueous ~olution.'~ According to eq 2, pK, in 1.O M perchlorate, 1.O M chloride, and 1.O M bromide solutions were estimated to be 0.92, 1.7, and 2.1, respectively (at 23 "C). J. M. Poston and T. C. Stadtman, "Cobalamin, Biochemistry and Pathophysiology", B. M. Babior, Ed., Wiley, New York, 1975, p 111. J. K. Kochi, 'Organometals and Organometalloids, Occurrence and Fate in the Environment", ACS Symp. Ser. No. 82, F. E. Brickman and J. M. Bellama, Eds., American Chemical Society, Washington, D.C., 1978, p 205. (a) R. E. DeSimone, N. W. Penley, L. Charbonneau, S.G. Smith, J. M. Wood, H. A. 0. Hill, J. M. Ratt, S.Ridsdale, and R.J. P. Williams, Biochim. Biophys. Acta, 304, 851 (1973); (b) V. C. W. Chu and D. W. Gruenwedel, Bioinorg. Chem., 7, 169 (1977). W. H. Scovell, J. Am. Chem. SOC.,96, 3451 (1974). J. H. Espenson and T. D. Sellers, Jr., J. Am. Chem. Soc., %,94 (1974). Y.-T. Fanchiang and J. M. Wood, J . Am. Chem. Soc., 103, 5100 (198 1). (a) R.T. Taylor and M. L. Hanna, Bioinorg. Chem., 6,281 (1976); (b) Y.-T. Fanchiang, W. P. Ridley, and J. M. Wood, J. Am. Chem. Soc., 101, 1442 (1979). G. Agnes, S. Bendle, H. A. 0. Hill, F. R. Williams, and R. J. P. Williams, J. Chem. SOC.,Chem. Commun., 850 (1971). B. P. Block, Inorg. Synth., 4, 14 (1953). D. Dolphin, Merhods Enzymol., ISC, 34 (1971). W. H. Pailes and H. P. C. Hogenkamp, Biochemistry, 7,4160 (1978). J. M. Pratt, "Inorganic Chemistry of Vitamin BI2", Academic Press, London, 1972, Chapter 5, p 44. Equation 2 was derived from equilibria presented in cq 1. The molar absorptivitiu at 305 nm for the base-on and baseoff CH3-BI2are iven respectivelyby = 1.3 X lo' M-I cm-' and -e = 2.2 X 18M-I cm-I. It is assumed that protonated base-off CH3-BI2has the same molar absorptivity as unprotonated base-off CH3-B1p D. D. Perrin in 'Dissociation Constants in Organic Bases in Aqueous Solution", International Union of Pure and Applied Chemistry (1972). The assumption that pK2 is the same as that of free 5,6-dimethylbenzimidazole is justified on the basis that pK, of l-@-o-ribo-5,6-dimethylbenzimidazole has been estimated to be 4.68 (M. T. Davies, P. Mamalis, V. Petrow, and B. Sturgeon, J. Pharm. Pharmocol, 3, 420 (1951)).
0 1982 American Chemical Society
