1089
a uniunivalent electrolyte. Spectrophotometric and potentiometric titrations yielded an approximate pK, of 3.0 for the complex at 1.0 M ionic strength and 25”. [Co(en)z(SCH2COO)]C104was synthesized by a pro(14) Research Fellow of the Alfred P. Sloan Foundation, 1969-1971. cedure similar to that used t o produce CrSH2+in soluMaurice M. Bursey,14 Fred E. Tibbetts, 111, William F. Little tion.5 Cobalt(I1) perchlorate, ethylenediamine, and the Venable Chemical Laboratory, The University of North Carolina disulfide of mercaptoacetic acid react in 2:4:1 molar Chapel Hill,North Carolina 27514 ratios in deaerated water to yield the desired compound Received October 16, 1969 which owas recrystallized several times from water (A 5160 A (E 149)). Anal. Calcd: S , 8.67; C, 19.5; H, 4.88; N, 15.2. Found: S,8.53; C, 19.5; H,4.84; The Role of Coordinated Sulfur in Oxidation-Reduction. N, 15.13. Infrared absorption (KBr) at 1635 and 1340 I. The Chromium(I1) Reduction of cm-l is assigned to coordinated carboxylate, and the Mercaptoacetatobis(ethylenediamine)cobalt(III) and Its absence of S-H absorption at -2500 cm-l is indicative Glycollato Analog of thiolate coordination. This complex is not detectably basic under the conditions studied and, after allowSir: ing for the change in sulfur substituent from hydrogen Recent research has revealed a widespread utilization to alkyl, seems comparable to CrSH2+ in this sense.5 of nonheme iron-sulfur proteins in biological oxidationNo appreciable spectral change occurs between IO-’ reduction processes. Iron, acid-labile sulfur, and and 1 M HC104. Further, when sufficient complex is thiolate sulfur from cysteine are stoichiometrically and brought into 0.1 M HCl solution (by reaction with functionally related, implying coordination by sulfur. equivalent (C6H5)4A~C1 followed by centrifugation of Esr studies of isotopically substituted samples of a rep(C6H5)4AsC1O4) to be 0.1 M in complex, the pH is unresentative two-iron protein indicate that both iron changed from its original value. atoms, both acid-labile sulfur atoms, and at least some Reaction rates with chromium(I1) were determined of the cysteinyl sulfur participate in the redox site.2 25 O and 1.O M ionic strength (LiC10rHC104) using a t These developments serve to underscore the lack of a Durrum-Gibson stopped-flow spectrophotometer at quantitative information regarding the influence of COthe low-energy cobalt absorption peak. Co(en)z(HOordinated thiolate functions on the rates and mechaCHzCOO)*+ is consumed under pseudo-first-order connisms of oxidation-reduction reactions. We wish to 0.99[H30+]-l)ditions according to the rate law, (38 describe a markedly enhanced rate of inner-sphere re[Cr(II)][Co(III)] (time in seconds), over the ranges 1.O duction of a mononuclear cobalt(II1) complex arising X 1W2M < [Cr(II)] < 5.0 X M and 1.0 X from replacement of a single coordinated alcoholate M < [H30+] < 1.0 X lo-’ M . Elimination of the acid oxygen by thiolate sulfur. These represent the first dissociation constant from the value for the inverse acid studies of this type of the thiolate and alcoholate funcpath yields ko2 = 990 M-l sec-I for reaction of the detions. protonated species. [Co(en)~0CHzCOO]C104 was prepared by a method ko2 analogous to that used for [ C O ( ~ ~ ) ~ ( N H ~ C H ~ C O O ) ] C ~ Z . ~ ~ Co(en)i(0CH2COO)+ + Cr(I1) +products (1) Addition of aqueous sodium perchlorate to a solution of the initial product yielded the perchlorate salt which Chrominum(I1) was found, by spectrophotometry, to was recrystallized from water. Anal. Calcd: C, consume Co(en)z(HOCHzC00)2+in a 1 : 1 molar ratio 20.45; H, 5.15; N, 15.9; Co, 16.7. Found: C, over a range of [ C O I ~ ~ ] : [from C ~ ~0.5 ~ ]to 2.5 at the 5 X 20.36; H, 5.12; N, 15.7; Co, 16.6 (asCo(tripyridine)z2+ l W 3 M level. The product spectrum excludes Crafter liquid-fire (HN03-HC104)decompcsition). Spec(H20)63+ as an appreciable product, indicating instead tral data were (A ( E ) : 05180 (132), 3600 A (140) in water an inner-sphere reaction with the bridging glycollate and 4990 (113), 3480 A (121) in 0.10 M HC104. Intense ligand captured in the inert Cr(Il1) coordination sphere. infrared absorption (in KBr) at 1635 and 1360 cm-l is A molecular model of the cobalt reactant indicates that characteristic of coordinated carboxylate. The failure the chelate link makes a doubly bridged reaction exto incorporate HzO on recrystallization together with tremely unlikely for steric reasons. The similar rate ) ( C H ~ - in the acid-independent path to the cis-Cospectral comparison with C ~ ~ - [ C O ~ ’ ~ ( N H B ) ~ ( H ~ O constant COO)](C104)3(A ( E ) : 5100 (67), 3550 A (50) in water)4 (NH3)4(0Ac)(HzO)2+-Cr(II)reaction (see Table I), provides assurance that the alcohol function remains where CrOAc2+is the dominant p r o d u ~ t , ‘and ~ the speccoordinated for reasonable periods in acidic or neutral tral properties of the products6 are consistent with carsolution. The molar conductance is that expected for boxylate bridging for this path although an alcohol bridge has not yet been rigorously excluded. The in(1) (a) R. Malkin and J. C. Rabinowitz, Ann. Reu. Biochem., 36, 113 (1967); (b) A. San Pietro in “Biological Oxidations,” T. P. Singer, Ed., verse acid dependence and high rate constant for the Interscience Publishers, New York, N. Y . , 1968, p 515. second path virtually demand alkoxide bridging. ( 2 ) (a) J. C. M. Tsibris, R. L. Tsai, I. C. Gunsalus, W. H. OrmeReaction of Cr(I1) with Co(en)z(SCHzCOO)+ at 0.1 Johnson, R. E. Hansen, and H. Beinert, Proc. Nutl. Acud. Sci. Li. S., 59, 959 (1968); (b) W . H. Orme-Johnson, R. E. Hansen, H.Beinert, 3. C.M. M HC10, is complete at the lowest observable equiTsibris, R. C. Bartholomaus, and I. C. Gunsalus, ibid., 60, 368 (1968). M) within the mixing molar concentrations ( 5 X (3) (a) M. D. Alexander and D. H. Busch, Inorg. Chem., 5, 1590 (1966); (b) K . Nakamoto, “Spectroscopy and Structure of Metal Chelate “dead time” of our apparatus. We conservatively esCompounds,” K. Nakamoto and P. J. McCarthy, Ed., John Wiley timate t ~ 2 X lo6 M-’ sec-’ (25’) for (4) (a) R. R. Miller, Ph.D. Thesis, University of Chicago, 1963. Acknowledgment. Support of this work by the National Science Foundation (GP 8096, GU 2059) is gratefully acknowledged.
+
We are indebted to Dr. E. Deutsch for forwarding this information. (b) See M. Linhard and M. Weigel, 2.Anorg. Chem., 264, 321 (1951), for spectral consequences of changing amine functions. (c) K. D. Kopple and R. R. Miller, Proc. Chem. Soc., 306 (1962).
( 5 ) M. Ardon and H. Taube, J . Amer. Chem. SOC.,89, 3661 (1967). (6) R. D. Butler and H. Taube, ibid., 87, 5597 (1965), present spectral
characteristics of Cr(II1)-glycollate complexes.
Communications to the Editor
1090 Table I. Rate of Reduction of Cobalt(II1) Complexes by Chromium(II)a Complex I. 11. 111. IV. V. VI. VII. VIII. IX. X. a
Rate, M-l sec-l
Ref
Co(NH3)6F2+ 9 X106 Co(NH3)6C12+ 2.6 X 106 C0(NH3)sOH2+ 1.7 X CO(NH~)~(OAC)~+ 3.5 x lo-' Co(NHs)a(OAc)z+ 15 50[HsO+] CO(NH~)~(HZO)(OAC)~+47 2.8[H30+]-1 CO(NH~)~(OOCCH~OH)~+3.1 C O ( ~ ~ ) ~ ( H O C H ~ C O O ) ~38+ Co(en)2(OCH2COO)+ 9.9 x 102 Co(en)2(SCH2C00)+ >2 x 105
+ +
b =
1.0 M , 25".
b b C
d e e
f g g
g
* Reference 9.
A. Zwickel and H. Taube, Reference 7b. Reference This work. h At 20", I.( = 1.20 M .
J. Amer. Chem. Soc., 81, 1288 (1959).
4a,c.
f
Reference 6.
Q
+
kS2
Co(en)2(SCH2COO)+ Cr(I1) ----f products
at present except that the steric component of the latter influence should contribute in the observed direction. Less dramatic effects than those we present have previously been reported for Co(II1) complexes with ligands containing uncoordinated thio ether functions. l 2 We are extending our studies to other reductants (particularly of the outer-sphere class), sulfur functions, and oxidizing centers. Acknowledgment. We acknowledge with thanks support for this research provided by the National Science Foundation (Grants GP-6848 and GP- 12223), the Petroleum Research Fund (Grant G-821), and the Public Health Service (National Institutes of Health Fellowship (GM 44691) to R. H. L. 1969-1970). We appreciate the support of the University of Florida for the stopped-flow spectrophotometer, an assistantship to R. H. L., and a summer research appointment to L. E. B. Our report has benefitted from comments by Professors Henry Taube and John Endicott and the referees.
This reaction is also characterized by 1 : 1 stoichiometry (spectrally determined) yielding a red chromium(II1) (12) E. S . Gould, J . Amer. Chem. Soc., 88, 2983 (1966). product which is indicative of a thiolate bridged reaction. Subsequent discussion will focus on the mechRobert H. Lane, Larry E. Bennett anistically comparable kp paths. Department of Chemistry, University of Florida GainesMe, Florida 32601 Table I summarizes our observations and relevant Received January 16,1970 results from previous work on inner-sphere reductions. The glycollate complex appears unreactive relative to C O ( N H ~ ) ~ O H ~Steric + . hindrance of the alkoxide oxygen by the methylene group probably contributes subIntramolecular Oxidative Phenol Coupling. 11. stantially to this effect. A decreased stability of the A Biogenetic-Type Synthesis of ( *)-Maritidinel precursor complex' is one likely consequence of the hindrance. We are studying the C~(II)-CO(NH~)~-Sir: Radioactive tracer experiments have verified that (HOCH3)3+* reaction to further evaluate the influence most, and probably all, of the Amaryllidaceae alkaloids of alkoxide ligands. are biosynthesized by way of intramolecular oxidative The most significant reactivity influence evident in coupling of either 0-methylnorbelladine (1) or 0,N-dithe results is that conferred by the thiolate ligand. The methylnorbelladine (2). Although it has been recogmercaptoacetate complex is more reactive toward renized that execution of this scheme in the laboratory duction by Cr(l1) than its oxygen analog by over three would provide an exceedingly simple synthetic route to orders of magnitude. In spite of its potentially hinthese alkaloids, Barton and Kirby's4 synthesis of galdering methylene substituent, it approaches the highest anthamine (3) remains the only reported biogeneticreactivities observed for this class of reactants and may type synthesis of an Amaryllidaceae alkaloid ; these approach rate-limiting substitution on Cr(II).7!9 An important barrier to activation for the class of reactions under consideration is the apparent necessity, OH I impressively supported e~perimentally,'~ to stretch the cobalt-bridging ligand bond prior to electron transfer. A further influential factor may be the extent to which the ligand u orbital overlaps the cobalt eg orbital" although experimental evidence on this point is less convincing. The much greater reactivity which we have observed for the sulfur over the oxygen complex corre1,R-H 3 lates nicely with a diminished steric hindrance of the 2, R = CH, larger thiolate sulfur by the methylene group and a lower bond strength and greater covalency expected for the workers were able to effect the intramolecular orthocobalt-sulfur bond, The potential influences of the para coupling of 2 in 1.4% yield. We recently dethermodynamic driving forces? and precursor complex scribed a new method for carrying out intramolecular stabilities' on the rate difference are difficult to assess oxidative phenol coupling' and wish now to report its use in the biogenetic-type synthesis of ( j=))-maritidine (7) (a) N. Sutin, Accounts Chew. Res., 1, 225 (1968); (b) H. Taube and E. Could, ibid., 2, 321 (1969); (c) J. C. Patel, R. E. Ball, J. F. Endicott, and R. G. Hughes, Inorg. Chem., 9, 23 (1970). (8) R. B. Jordan, A. M. Sargeson, and H. Taube, ibid., 5 , 1091 (1966). (9) J. P. Candlin and J. Halpern, ibid., 4, 766 (1965). (10) (a) R. I
