86 (12) (13) (14) (15) (16) (17) (18) (19) (20)
W. Keiier-Schierlein and V. Preiog, Helv. Cbim. Acta, 44, 1981 (1961). V. Preiog and A. Waiser, He&. Chim. Acta, 45, 631 (1962). W. Keller-Schieriein and V. Prelog, Helv. Chim. Acta, 45, 1732 (1962). W. Keiler-Schieriein, P. Mertens, V. Preiog, and A. Walser, tie&. Chim. Acta, 48, 710 (1965). T. F. Emery, Sbchmistry, 6, 3858 (1967). M.Liinas, M. P. Klein, and J. 8 . Neilands, J. Mol. 8b/.,52, 399 (1970). A. Zaikin, J. D. Forrester, and D. H. Templeton, J. Am. Chem. SOC.,88, 1810 (1966). H. J. Llndner and S. Oattlicher, Acta Crystallogr., Sect. 8, 25, 832 (1969). P. Coppens, L. Leiserowitz'and D. Rabinovich. Acta Crystallogr., 18,
1035 (1965). (21) F. R. Ahmed. SFLS program. NCR-10, National Research Council. Otta. wa, 1966. (22) A. L. Patterson, Acta Crystallogr.,16, 1255 (1963). (23) "International Tables for X-Ray Crystallography", Vol. Ill, Kynoch Press, Birmingham, 1962, pp 202,214. (24) D.van der Helm and H. B. Nicholas. Acta Crystallogr., Sect. 8.26, 1858 (1970). (25) See paragraph at the end of paper regarding supplementary material. (26) R . E. Marsh and J. Donohue. Adv. Protein Chem., 22, 235 (1967). (27) C. K. Johnson, ORTEP Report ORNL-3794, Oak Ridge National Laboratory, Oak Ridge, Tenn.
The Reactivity of Cobalt( I) Complexes Containing Unsaturated Macrocyclic Ligands in Aqueous Solution A. Martin Tait,2a Morton Z. Hoffman,*Zaand E. Hayon*2b Contribution from the Department of Chemistry, Boston University. Boston, Massachusetts 0221 5, and the Pioneering Research Laboratory, U S . Army Natick Laboratories, Natick, Massachusetts 01 760. Received January 27, 1975
Abstract: The fast kinetics technique of pulse radiolysis has been used to generate and characterize, in aqueous solution,
Co(1) complexes containing the tetradentate 14-membered macrocyclic ligands 5,7,7,12,14,14-hexamethyl1,4,8,1 l-tetraazacyclotetradeca-4,ll-diene (4,11 -dieneNd), 5,7,7,12,12,14-hexamethyl1,4,8,11-tetraazacy~lotetradeca-4,14-diene (4,14dieneNr), and 2,3,9,10-tetramethyl-l,4,8,1l-tetraazacyclotetradeca-l,3,8,10-tetraene (1,3,8,10-tetraeneNd). The reaction of the hydrated electron, eaq-, with the Co(I1) complexes ( k 5 X 1Olo M - ' set;') produces the corresponding Co(1) species, Co1(1,3,8,10-tetraeneN4) is also generated by the action of C02- and (CH3)zCOH radicals on Co(1I). The absorption spectra of these monovalent complexes have been determined and their decay kinetics studied as a function of pH and added scavengers. The Co(1) species behave as bases, reacting with proton donors such as H@+, CH,C02H, HzPOd-, NH4+, HP042-, and H20. They also behave as powerful reducing agents, transferring an electron (k lo9 to lo6 M-I sec-I) to a variety of organic acceptors and one-electron metal complex oxidants such as Fe(III), Co(III), Cr(III), and Ru(II1) amine, bipyridyl, and macrocyclic complexes. The Co(1) species react rapidly with N20, CHd, and 0 2 .
-
-
The chemistry of cobalt complexes containing macrocyclic or tetradentate ligands with delocalized electronic structures may be taken as an approximate model for the reactions of vitamin Bl2 in biochemical p r o c e ~ s e sAmong .~~~ the oxidation states of cobalt, the Co(1) state has been proposed as a reaction intermediate in the reduction of cobaloxime and aquocobalamin by C05,6and in biological processes as vitamin B12s.4,7 Co(1) species display strong nucleophilic character, readily displacing halide ions from alkyl halides producing cobalt-alkyl complexes.8-'0 The catalytic role of some Co(1) complexes has recently been demonstrated in the reduction of several alkyl ammonium ionsI0 and in the conversion of nitrogen and acetylene to ammonia and ethylene.' I It has previously been r e p ~ r t e d ' ~that , ' ~ Co(1) complexes containing the macrocyclic ligands 4 , l l -dieneNd and aneN4I4 are exceptionally reactive and powerful reducing agents and cannot be handled in protic solvents when generated electrochemically from the corresponding Co(I1) complexes. Because of their short lifetimes, these species, or any other Co( I) macrocyclic species, have not been characterized in aqueous solution. The technique of pulse radiolysis is well suited to the study of such highly reactive low-valent species. The radiolysis of water provides a means of selectively generating one-electron reducing or oxidizing agents. The hydrated electron, eaq-, is produced directly by the ionizing radiation and is a powerful reducing agent (E,,O = 2.8 V)." Other reductants, such as C02- and (CH,)2COH radicals, can be generated by the reaction of O H radicals, produced in the radiation pulse, with HC02- and (CH&CHOH, respect i ~ e l y . ' * *The ' ~ reactions of the radicals with solutes, such Journal of the American Chemical Society
/
98:l
/
as coordination complexes, can be monitored using fast kinetics absorption spectrophotometry with a time resolution of -0.1 psec enabling the spectra of short-lived transient intermediates to be observed and their kinetics characterized. In this paper we describe the reactions of Co(I1) complexes containing the 4,1l-dieneN4, 4,14-dieneN4, and 1,3,8,10-tetraeneN4 macrocyclic ligands with reducing rad-
&
Me, [14] 4,ll-diene N4
Me6 [1414, 14-dieneN4
n vN Nw icals to generate the corresponding Co( I) species in aqueous solution. The spectra and reactivities of these low-valent complexes are examined in detail. It is important to note that the tetradentate equatorial ligand renders macrocyclic complexes of this type stable towards displacement of the ligand from the metal center so that at least the macrocyclic structure maintains its integrity upon change of the oxidation state of the metal.
January 7, 1976
87
Experimental Section Apparatus and Techniques. The pulse radiolysis apparatus has already been described in detai1.20-22 Single pulses of 2.3 MeV electrons of -30 nsec duration were generated (Febetron 705) and absorbed by the solution contained in a quartz cell (optical path = 2 cm). A 450-W xenon lamp which could be pulsed, thereby increasing the light output by a factor of 20-25, was used as the monitoring light source. Glass filters and a synchronized shutter were used to minimize the photolytic effects of the monitoring light. The pH of the solutions was adjusted by use of HCIO4, KOH, phosphate ( I mM) or tetraborate ( 1 m M ) buffer, except as otherwise indicated. Solutions were prepared from the solid salt just prior to use and were deoxygenated (except where indicated) by an Ar purge or saturated with N 2 0 at 1 atm partial pressure (2.5 X IO-* M).Extinction coefficients were evaluated by means of SCN- dosimetry taking €500 = 7.6 X IO3 M-' cm-l for the (SCN)2- radical.23 Rate constants were determined by a leastsquares computer fit of the kinetics data to the usual first- and second-order equations. Preparation of Complexes. The 4,l I-dieneN4 ligand as the c l o d - salt was preparedz4 from acetone and ethylenediamine by the method of CurtisZ5and used in reaction with cobaltous acetate inmethanol toprepare26[co(4,1 I-dieneN4)(C104)]CIO4. [co(4,14dieneN4)(C104)]C104 was similarly prepared from 4,14-dieneN4 in ethanol,z6 the free base having been obtained by the addition of cyanidez7 to the analogous Ni(I1) complex.z8 Dissolution of these low-spin five-coordinate Co(I1)-perchlorato complexes in water results in the displacement of the coordinated perchlorate group by HzO giving complexes of the form [co(4,1 1-dieneN4)(H20)2l2+ in aqueous s o l ~ t i o n .A~ ~sample ~ ~ ~ of [Co( 1,3,8,10-tetraeneN ~ ) ( H ~ O ) Z ] ( C Iwas O ~ kindly ) ~ ~ ~supplied to us by Professor J. F. Endicott (Wayne State University). The other complexes were available from our laboratory reserves. The sample of C r ( b ~ y ) 3 ~was + also supplied to us by Professor Endicott and the Ru(II1) complexes by Professor J. N . Armor (Boston University).
Table I. Rate Constants for the Reaction of Co(I1) Macrocyclic Complexes with eaa-, CO,; and (CHJJOH Radicals Reductant, R eaq-'
c0,- b .
(CH,),COHc
pH of CoII(4,11experiment dieneN,) 6-10 6.5 1.25,6.5
4.4
X
-
10"
CoII(4,14- CoII(1,3,8,10dieneN,) tetraeneN,) 3.4
X
-
10"
4.9 X 10" 4.7 x 109 5.5 x 109
UDetermined in the presence of (1.25-2.5) X 10-'M Co(I1) substrate, 1 M rert-butyl alcohol, 1 mM phosphate or borate buffer in was deterAr-purged solutions. Value for Co11(1,3,8,10-tetraeneN,) mined at pH 6.5 only. bDetermined in the presence of 1 x 1 0 - 4 M Co(I1) substrate, 0.1 M HC0,- in Ar-purged solutions. CDetermined in the presence of 1 X 1 0 - 3 MCo(I1) substrate, 2 M 2-propanol, perchloric acid, or 1 mM phosphate buffer in Ar-purged solutions.
these radicals also reduce the corresponding Co(II1) complexes to C O ( I I ) . ~ ~ Transient Spectra. The rapid reaction of eaq- with the macrocyclic complexes assured that transient species would be fully formed 1 0 . 5 psec after the pulse and their absorption spectra determined before any appreciable decay had occurred. The transient species were observed in Ar-purged solutions containing 5 X M Co(II), 1 M tert-butyl alcohol, and l mM buffer. All spectra were corrected for the depletion of the substrate and for the absorption of the radical derived from tert- butyl alcohol,20 whenever necessary. The transient optical absorption spectrum obtained from the reaction of eaq- with Co11(4,11-dieneN4) at pH 7 is shown in Figure 1. This spectrum, with bands at 630 (e 9100 M-' cm-I), 450 (1060), 330 (3230), and 290 (3675) nm, is independent of pH from 3.5 to 10. Vasilevskis and Olson13 have investigated the electrochemical reduction of this Co(I1) complex in CH3CN and report that the resultant Co(1) species has a spectrum with maxima at 679 (e 15950 M - ' cm-I), 450 (AllOO), 350 (3300), 307 (3600), Results and 245 (2950) nm. Despite the small differences in Xmax and 6 between the two solvents, there is little doubt that the Reactions of Co(11) with eaq- and Other Reducing Radiaction of eaq- on the Co(I1) complex generates the Co(1) cals. Kinetics. The reaction rate constants for the various species. The transient spectrum obtained from the reaction Co(I1) macrocyclic complexes (as their perchlorate salts) of eaq- with Co11(4,14-dieneN4) at pH 7 (Figure 2) shows with eaq- were determined, in the presence of 1 M tertfive bands of similar intensities (c 2500-3500 M-l cm-I) in butyl alcohol as an O H radical scavenger,34 from the the uv-visible region. The spectrum of the Co1(4,14-dipseudo-first-order decay of eaq- monitored at 700 and 500 eneN4) species has not been previously reported. nm. From the dependence of these rate constants on the The reaction of eaq- with Co"( 1,3,8,10-tetraeneN4) at substrate concentration (1.25-2.50 X M ) , the values pH 6.5 produced a transient spectrum with a broad band of k(Co(I1) eaq-) were calculated and are recorded in centered at 700 nm (e 8400 M-l cm-I) shown in Figure 3. Table I. Decays were monitored over three-four half-lives. A similar spectrum has been obtained by Endicott upon the No reaction was observed between the Co(I1) complexes electrochemical reduction of this Co(I1) complex in aqueof 4 , l l -dieneN4 and 4,14-dieneN4 with the reducing radious solution.37 The reduction of this complex by CO2- at cals C02- and (CH3)ztOH by monitoring the wavelengths pH 6.5 and by (CH3)2COH at pH 1.25 and 6.5 produced at which the Co(1) species absorb (see following section) implying that k 4.2 and -0.002 M at pH 7. With Organic Electron Acceptors. The rate constants for electron transfer from co1(4,1 1-dieneN4) and Col( 1,3,8,10tetraeneN4) to various organic acceptors, A, were determined by monitoring the decay of the Co(1) band, the disappearance of A, or the formation of the corresponding reduced species, .A- or .AH, at a suitable wavelength.)* Because of the high reactivity of the acceptors with eag- to form .A- or -AH, the concentrations of Co(I1) (1 X lo-) M ) and A (55 X M )were chosen such that no more than 5% of esq- would be lost through direct reaction with A. The experiments were carried out at pH 9.2 for co'(4,11dieneN4) and pH 6.5 for Col( 1,3,8,1O-tetraeneN4) in order to minimize the reaction of Co(1) with H+. Solutions contained 1 m M buffer and 1 M tert-butyl alcohol. The values of k for some organic acceptors are given in Table I11 with all but one acceptor (3-benzoylpyridine) showing 100% electron transfer. The spectrum of the .A- species from the reaction of menaquinone with Co1(4,1l-dieneN4) is shown
+
+
Tait, Hoffman, Hayon
\
[N&]
XI$,
M
I
I
1
1
1
I
I
1
0
2
4
6
8
10
12
14
16
PKa
+
Figure 5. Dependence of log k (Co' H A ) for Co'(4,I I-dieneN4) on the pK, values of HA; experimental points taken from Table 11. The dashed lines represent slopes of 0 and -1. Insert: Dependence of firstorder rate constant for the decay of Co'(4,l I-dieneN4) on [CHICOIH] ( 0 )and [NH4+] (m).
in Figure 6a and it agrees well with the previously published ~pectrum.)~ The extent of electron transfer from Co(1) to A was determined by comparing the quantitative absorbance due to -A- formed from the reaction of Co(1) with A with that obtained directly from the reaction of eaq- with A. Considering the very rapid latter reaction to be 100% efficient in generating .A-,38 the efficiency of the electron transfer reactions of Co(1) with A could be determined. In Figure 6b a plot is made of the efficiency of electron transfer from Co'(4,ll-dieneNd) as a function of the standard reduction potential of A at pH 9.2, E,.38 It can be seen that for acceptors with E, values more negative than -0.88 V, no electron transfer was observed; for E, more positive than -0.85 V, transfer was quantitative. The apparent oxidation potential for Co1(4,11-dieneN4) is evaluated from the midpoint of this "titration" curve to be +0.86 V. In the same way, transfer from Col( 1,3,8,10-tetraeneN4) was quantitative to acceptors with E, values at pH 6.5 more positive than -0.24 V; for acceptors with E, values more negative than -0.35, no transfer was observed. From this we estimate the apparent oxidation potential of Col( 1,3,8,10tetraeneN4) in aqueous solution to be ca. +0.28 V. The reaction of Co(1) with A is not reversible under these experimental conditions. Indeed, the .A- radicals themselves undergo irreversible bimolecular Thus, the
/ Co(I) Complexes Containing Unsaturated Macrocyclic Ligands
90 Table 111. Rate Constants for the Reaction of Co(1) with Oxidants k(Co(1) + S), M - I sec-'
1
B 6ot
401 20
g
71,
, 8- ,
-1 2
I
I
I
,
-06
-0.8
-10
,
, -02
-04
Em. V
0051
I
A
" O003 o4I
01
,
1
-14
I
350
A'
I
1
I
I
1
I
/ 'hi,h I
I
400
I
I
1
1
i
I
550
500
450
I
600
A , nm
Figure 6. (A) Optical absorption spectrum of the semiquinone radical anion of menaquinone (SA-) produced from the reaction of co1(4,11dieneN4) with menaquinone (5 X IO-' M )in the presence of 1 M rerrbutyl alcohol and 1 mM tetraborate at pH 9.2. (9) Dependence of the efficiency of electron transfer (expressed as percent) in the reaction of cot(4,Il-dieneN4)with organic electron acceptors, as a function of the standard reduction potential of the acceptors at pH 9.2 in 1 M rerrbutyl alcohol and 1 mM tetraborate. Acceptors used: 1, menaquinone; 2, 9,1O-anthraquinone-2-sulfonate;3, eosin Y;4, fluorenone; 5, fluorescein; 6, 3-benzoylpyridine;7, dimethyl maleate; 8, benzophenone; 9, acetophenone.
apparent oxidation potentials for the Co(I) species differ from the thermodynamic potential.38f With Metal Complexes. Co'(4,l 1-dieneN4) was generated by the eaq- reduction of the corresponding Co(I1) complex (5 X M) at pH 9.2 in the presence of (0.1-2) X M metal complex oxidants, 1 M tert-butyl alcohol, and 1 m M tetraborate buffer in Ar-purged solutions. Experiments were performed as rapidly as possible in order to ensure that the M(II1) complexes were in the mildly alkaline solution for 0, a plot of log k vs. ApK gives a slope of -1 with the rate constant decreasing with increasing values of ApK. In the vicinity of ApK = 0, a
-
Tait, Hoffman, Hayon
sharp transition from a slope of zero to -1 is theoretically predicted. In practice, the limiting slopes are seldom achieved and the transition point usually shows decided curvature. The data in Table I1 for Co'(4,ll-dieneNp) are plotted in the form of log k vs. pK, in Figure 5, inasmuch as pKb for Co(I) is not known. The values of k were not derived at constant ionic strengths. However, inasmuch as the rate constant is a relatively weak function of ionic strength under these condition^,^^ we feel that the narrow range of ionic strengths of the solutions did not introduce any error of magnitude into the plot shown in Figure 5. The values of p (and especially p1/2)did not vary greatly for the individual donors used and did not significantly affect the linearity of the plots of k vs. [HA]. Thus, despite the limited number of points and the uncertainties involved, the trend depicted in Figure 5 supports our contention that the Co(1) species reacts with these acids via proton transfer reactions. Deviations from ideality, also seen in even simpler systems,47have been reconciled in terms of the asymmetry of charge, molecular structure and steric restriction, hydrogen bonding, and electron distribution in the donors and acceptors. From the general region of the transition shown in Figure 5 , we estimate pKb for Co1(4,11-dieneN4) to be in the range 6-10.
ThelimiteddatafortheotherCo(I)speciesshowsCo'(4,14dieneN4) to be only slightly less reactive than its isomer but Col( 1,3,8,1O-tetraeneN4) is 3-4 orders of magnitude less reactive than the diene complexes. This implies that the value of k for the reaction of Co1(l,3,8,10-tetraeneN4)with H2O may be of the order of IO-) M-'sec-' (or less) and so the Co(1) species may show modest stability in alkaline sol ~ t i o nbarring ~ ~ any hydrolysis of the ligand. At pH 6.5, however, the residual absorption 20 sec after the pulse resembles the original Co(I1) substrate (Figure 3) which may be the end product of the reaction of Co1(l,3,8,10-tetraeneN4) with proton donors via some hydrido intermediate. Electron Transfer Reactions. The Co(1) species are powerful reducing agents, reacting rapidly with a wide range of electron acceptors. The apparent oxidation potentials of these species in aqueous solution can be compared with existing data in the literature despite the nonthermodynamic character of the data presented in this paper and the nonaqueous solvent system employed by other workers. Ell2 values for Co'(4,l I-dieneN4) and Col( 1,3,8,10-tetraeneN4) in CH3CN have been extrapolated to the aqueous system, and through an estimation of C H ~ C H - H Z O junction potential, Eoxovalues of +0.7 and +0.48 V, respectively, have been obtained.49 Considering all the factors involved, the similarity of both sets of data is, perhaps, remarkable and the dieneN4 complex is established as the stronger reducing agent. Even in their limited form, these data demonstrate the profound effect of the degree and position of unsaturation of the macrocyclic ligand on the redox potential of the complex. The Co(1) species react rapidly with metal complex oxidants with rate constants that cover a range of three orders of magnitude (106-109M - ' sec-I). Values of k < IO6 M-' sec-' cannot be determined by the pulse technique due to the high concentration of oxidant required to scavenge Co(1); the trivalent metal complexes used themselves react very rapidly with %q- (k = 4-9 X 1Olo M-I ~ e c - ' ) . ' ~It . ~ ~ can probably be safely assumed that these reactions occur via an outer-sphere mechanism inasmuch as the electrontransfer reactions of the oxidants employed are well established to be of that type.so*51Because of the lack of any information concerning the self-exchange rate constants for the Co(1) species and the lack of certainty about the exact values of their redox potentials in aqueous solution, we
/ Co(I)Complexes Containing Unsaturated Macrocyclic Ligands
92 made no attempt to fit the data in Table 111 to the Marcus relationship in a quantitative manner.52 However, certain trends are apparent from the data. If one assumes that the electrostatic effects for Co'(4,l l-dieneN4) and Co'( 1,3,8,1O-tetraeneN4) are similar, then the relative rate constants for their reactions will be dependent upon their relative reorganizational energies and redox potentials. In reaction with C r ( b ~ y ) 3 ~and + C0(bpy)3~+,the rate constants for Co'( 1,3,8,10-tetraeneN4) are approximately an order of magnitude lower than are those for Col(4,11-dieneN4). It must be pointed out that the differences in the ionic strength of the solutions cannot account for the differences in k shown by the two Co(1) species. Because of the lower reducing ability of the Co'(tetraeneN4) species, its reactivity would be expected to be lower than that of the Co'(dieneN4) by approximately two orders of magnitude if the self-exchange rate constants of the two species were equal. From the very limited data it appears that the selfexchange rate constant for Co'(4,ll -dieneN4) would be an order of magnitude or so greater than that of C0'(1,3,8,10tetraeneN4) if the free energy correlation holds for these reactions. Indeed, Endicott has concluded5' that this correlation does not hold true for electron transfer reactions of Co( 111) complexes containing these macrocyclic ligands and that the self-exchange rate constant for the [Co(4,11dieneN4)(0H2)2l3+y2+ couple is about seven orders of magnitude lower than that of [Co( 1,3,8,10-tetraeneN4)(OH2)2]3+*2+.Finally, it should be mentioned that the Cu1(4,11-dieneN4) complex, the redox potential of which is similar to that of Co'( 1,3,8,1O-tetraeneNs), undergoes electron transfer to the metal complex oxidants with rates similar to those of the latter complex; Ni'(4,ll-dieneNd) and Ni'(aneN4) tend to parallel Co'(4,ll -dieneN4) in both rates and redox proper tie^.^^ The values of k for co1(4,11-dieneN4) in Table 111, measured at low and constant ionic strength at pH 9.2, provide some insight into the details of these electron transfer reactions. For Co(en)s3+ and Co(NH3)b3+ (reduction potentials of -0.3053 and +0.10 V,54 respectively) the rate constants are virtually identical but, assuming the general validity of the Marcus relationship, can be attributed to the compensating effects of higher reducibility but lower exchange reactivitySoCshown by C O ( N H ~ ) ~The ~ + Co(II1)-macrocy. clic complexes, which have very similar redox potentials5s and as +1 charged species will have a different electrostatic effect in their reaction with the +1 charged reductants compared to the other Co(II1) complexes, show reactivity of the same magnitude as C0(en)3~+and Co(NH3)b3+. Perhaps the anomalous variations5' in the Marcus reorganizational parameter are in effect here. R u ( N H ~ ) ~ ~which +,'~ has a reduction potential (+0.05 V)57 similar to that of C O ( N H ~ ) and ~ ~ +an electron exchange rate 1 O l 2 times faster,50 reacts at a rate that is about two orders of magnitude faster; the Marcus correlation would predict a difference of about five-six orders of magnitude. The lower rate shown by the Ru(III)-NOS8 complex may be due to its lower reducibilityS9 or may reflect electron delocalization in that complex allowing its behavior to be ascribed in part to its valence tautomeric Ru(I1)-NO+ form. C r ( b ~ y ) 3 ~( E +o = -0.40 V)56 and C0(bpy)3~+( E o = +0.37 V)60 show equal and very high rates while the more powerfully oxidizing Fe(b~y)j~+ (Eo ~ l= +1.1 V)60 reacts almost two orders of magnitude more slowly. The involvement of the ligand orbitals in the electron transfer process must be considered inasmuch as the ?r system of the aromatic ligand could provide a facile pathway as has been inferred from the rate constants for the reaction of C02- radicals with complexes containing aromatic ligands.62 The behavior of F e ( b ~ y ) , ~ + , Journal of the American Chemical Society
/ 98:i /
with its high reducibility and exchange reactivity, can only be viewed as anomolous at the present time. Other Reactions. The reaction of 0 2 with all three Co(1) species is very rapid. Cobalt complexes can behave as oxygen c a r r i e d 3 and the reaction of Co(1) with 0 2 can be viewed as proceeding via electron transfer with the formation of 0 2 - (Co(1) 0 2 Co(I1) 02-) or a coordination mechanism involving incorporation of 0 2 into the coordination sphere of the Co(1) center:
-
+
CO(1)
+0 2
-
[Co(I)-O2]
-
+
[Co(II)-02-]
-
[Co( III)-022-]
+
Inasmuch as the Co(1) 0 2 reaction did not regenerate the spectra of the original Co(I1) species, it is possible that the reaction proceeds via the coordination route. We have also found that the reaction of Cu'(4,l 1-dieneN4) with 0 2 does not generate free 0 2 - radicals but the Ni(1) macrocyclic species do.46 In contrast, Sellers and S i m i ~ have ~ ~found that COaq+ 0 2 generates 0 2 - . Pratt and c o - w o r k e r ~have ~ ~ shown that N 2 0 reacts rapidly with a number of Co(1) complexes of dimethylglyoxime, 2,2'-bipyridyl, 1,2-bis(diphenylphosphine)ethane, and with vitamin B I ~ according S to the overall stoichiometry: 2Co(I) N20 2Co(II) N2. All these reactions have been proposed to occur via the simple two-electron reduction of N20 to N2 with the concomitant formation of Co(II1) which is then itself reduced by Co(1) to yield the Co(I1) final product. No evidence has been obtained for the addition of N20 into the coordination sphere of the metal. In our experiments, the rapid reaction of Co(1) with excess N 2 0 would deplete the supply of Co(1) prohibiting the further loss of Co(II1). The residual spectrum after the decay of Co(1) did not resemble the Co(1I) substrate but could be that of Co"'(4,l I - d i e r ~ e N d ) . ~ ~ The reaction of nucleophilic low-valent complexes with alkyl halides is one of the more general routes to the preparation of Co(II1)-alkyl The rate constant for the reaction of vitamin Bl2s with CH31 to form methylcobalamine is 3.4 X lo4 M-l ~ e c - l .We ~ find the rate constant for the reaction of Co1(4,11-dieneN4) with CH31 to be about four orders of magnitude higher suggesting, as Endicott has indicated,I2 that the Co'(4,l I-dieneN4) species does not follow the classical s N 2 mechanism found for other Co(1) nucleophiles, including vitamin BI2s, but rather reacts via an electron transfer mechanism generating Co(II), CH3 radicals, and I- in the first instance. Because of the high concentration of the Co(1I) substrate we were required to use we were unable to make spectral observations at X C370 nm. It could not be established by this experiment whether the initial product of the Co(1) CH3I reaction was Co(I1) (A,, , 330 nm) or Co(III)-CH3 (Amax. -280 nm). However, we have determined that co"(4,11dieneN4) reacts rapidly with CH3 radicals at pH 9.2 with k = (1-2) X lo8 M-I sec-' producing the spectrum of the alkyl complex,36 the rate constant being of the same magnitude as that determineds2 using CH3 radicals generated in the charge transfer photochemistry of Co(NH~)sO~CCH + 67 ~ in~highly acidic solution. Experimentally, alkyl complexes of co111(4,11-dieneN4) have not been isolated from the reaction of alkyl halides with the Co(1) species. This failure may be related to certain reaction conditions, such as acidity, allowing for competing reaction pathways (for example, direct electron transfer6*) in the reaction of Co(I1) with CH3 radicals. The reaction of CH3I with Co1(l,3,8,10-tetraeneN4) was too slow to monitor by the pulse technique. This reaction and the reaction of the Co(I1) species with CH3 radicals have been shown to produce the Co(III)-CH3 complex by other technique^.^*^^ Be-
January 7, 1976
+
+
-
+
+
93
+
cause of its slowness, the Col(1,3,8,1O-tetraeneN4) CH,I reaction presumably occurs via the s N 2 m e ~ h a n i s m . ~ ~
Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. The authors also wish to acknowledge their many conversations with Professor J. F. Endicott about this work. References and Notes (1) Presented in part at the Symposium on Electron Transfer Mechanisms, 167th National Meeting of the American Chemical Society, Los Angeles. Calif., April 1974, INOR 131. (2) (a) Boston University; (b) U.S. Army Natick Laboratories. (3) G. Costa, Pure Appl. Chem., 30, 335 (1972). (4) J. M. Pratt. "Inorganic Chemistry of Vitamin B12". Academic Press, New York, N.Y., 1972. (5) J. k. Bayston and M. E. Winfield, J. Catal.. 9, 217 (1967). (6) L. P. Lee and G. N. Schrauzer, J. Am. Chem. SOC.,90, 5274 (1968). (7) H. A. 0. Hill, J. M. Pratt. and R. J. P. Williams, Chem. Brit., 5, 156 (1969); G. N. Schrauzer, Adv. Chem. Ser., 100, l(1970). (8)E. Ochiai, K. M. Long, C. R. Sperati, and D. H. Busch, J. Am. Chem. SOC.,91, 3201 (1969). (9) K. Farmery and D. H. Busch. Inorg. Chem., 11,2901 (1972). (10) G. Costa, A. Puxeddu. and E. Reisenhofer, J. Chem. SOC., Dalton Trans., 2034 (1973). (11) E. B. Fleischer and M. Krishnamurthy, J. Am. Chem. Soc., 94, 1382 (1972). (12) D. P. Rillema, J. F. Endicott. and E. Papaconstantinou, horg. Chem., I O , 1739 (1971). (13) J. Vasilevskis and D. C. Olson, Inorg. Chem., 10, 1228 (1971); 8, 1611 (1969). (14) The abbreviations used for the macrocyclic ligands are based on the ~ ' ~will be used in their slmsuggestions of Busch and c o - w ~ r k e r s , ' ~and plest forms: aneN4 = Mee[14]aneN4 = 5,5,7,12,12,1Chexamethyl1,4,8.1l-tetraazacyclotetradecane: 4,l ldleneN4 Mee[ 14]4,1 ldleneN4 = 5,7.7,12,14,14-hexamethyl-l,4,8,1 l-tetraazacyclotetradeca4,l ldiene; 4,14dieneN4 Mea[ 14]4,14dieneN4 = 5,7,7,12,12,15hexamethyl-l.4,8.ll-tetraazacycbtetradeca-4,14diene: 1,3,6,10tetraeneN4 Me4[ 1411,3,8,10-tetraeneN4 = 2,3,9,10-tetramethyl1,4.8.11-tetraazacyclotetradeca-1,3,8,10-tetraene. (15) V. L. Goedken, P. H. Merrell, and D. H. Busch, J. Am. Chem. Soc.. 94, 3397 (1972). (16) J. C. Dabrowiak. P. H. Merrell, and D. H. Busch. Inorg. Chem.. 11, 1979 (1972). (17) E. J. Hart and M. Anbar, "The Hydrated Electron", Wiley-Interscience, New York, N.Y., 1970. (18) A. 0. Allen, "The Radiation Chemistry of Water and Aqueous Solutions", Van Nostrand, New York, N.Y., 1961. (19) M. Ebert, J. P. Keene, A. J. Swallow, and J. H. Baxendale. Ed.. "Pulse Radiolysis", Academic Press, New York, N.Y., 1965. (20) M. Simic, P. Neta, and E. Hayon. J. fhys. Chem., 73, 3794 (1969). (21) J. P. Keene, E. D. Black, and E. Hayon. Rev. Sci. Instrum., 40, 1199 ( 1969). (22) E. Hayon. J. Chem. fhys., 51, 488 (1967). (23) J. H. Baxendale, P. L. T. Bevan. and D. A. Scott, Trans. Faraday Soc., 64, 2398 (1968). (24) A. M. Tait and D. H. Busch, Inorg. Synth., in press. (25) N. F. Curtis and R. W. Hay, Chem. Commun., 524 (1966). (26) V. L. Goedken and D. H. Busch, horg. Chem., in press. (27) J. L. Love and H. K. J. Powell, horg. Nucl. Chem. Lett., 3, 133 (1967). (28) N. F. Curtis. Y. M. Curtis, and H. K. J. Powell, J. Chem. SOC.A, 1015 (1966). (29) ESR data have shown3' that these Co"(dieneN4) macrocyclic complexes are flve-coordinate in acetonlrile solutions. A recent x-ray crystallographic study31 indicates that In the solid state the 4.1 ldieneN4 comdex exists as six-coordinate with two H2O molecules OccuDving . . the axiai positions. (30) A. M. Tait. f . V. Covecchio, E. S. Gore, and D.H. Busch, unpublished results.
-
Tait, Hoffman, Hayon
/
(31) M. D. Glick. J. M. Kuszaj. and J. F. Endicott, J. Am. Chem. Soc.. 95, 5097 (1973). (32) The coordination number of the metal center of this complex in aqueous solution has not been determined although to be six-coordi- it aDDears .. nate in the solid ~ t a t e . 3 ~ (33) M. D. Glick, W. G. Schmonsees, and J. F. Endicott. J. Am. Chem. Soc., Q6.5661 (19741. .I (34) L - k D o r f m a n and G. E. Adams. "Reactivity of the Hydroxyl Radical in Aqueous Solutions", U S . Department of Commerce, National Bureau of Standards, NSRDS-NBS 46 (1973). (35) P. Neta, M. Simic, and E. Hayon, J. fhys. Chem., 73, 4207 (1969). (36) A. M. Tait, M. Z. Hoffman, and E. Hayon, J. fhys. Chem., submkted for publication. (37) J. F. Endicott. private communication. (38) (a) P. S. Rao and E. Hayon, J. fhys. Chem., 77, 2753 (1973): (b) /bid.., 77, 2274 (1973): (c) Biochem. Biophys. Res. Commun., 51, 467 (1973): (d) J. Am. Chem. SOC.,96, 1287 (1974): (e) bid., 96, 1295 (1974): (f) ibid., 97, 296 (1975). (39) P. S.Rao and E. Hayon, Biochim. Biophys. Acta, 292, 516 (1973); E. Hayon and M. Simic. J. Am. Chem. Soc., 95, 1029 (1973). (40) R. L. Willson, C. L. Greenstock. G. E. Adams, R. Wageman, and L. M. Dorfman, Inf. J. Radiat. fhys. Chem., 3, 211 (1971). (41) M. Anbar, M. Bambenek, and A. B. Ross, "Selected Specific Rates of Reactions of Transients from Water in Aqueous Solution", U S . Department of Commerce, National Bureau of Standards, NSRDS-NBS 43 (1972). (42) P. Neta, Chem. Rev.. 72, 533 (1972). (43) G. D. Venerable II, and J. Halpern, J. Am. Chem. SOC.,93, 2176 (1971). (44) G. N. Schrauzer and R. J. Holland, J. Am. Chem. SOC.,93, 1505 (1971): G. N. Schrauzer and R. J. Windgassen. /bid., 89, 1999 (1967). (45) R. G. S.Banks and J. M. Pratt, J. Chem. SOC.A, 654 (1968). (46) A. M. Tait, M. Z. Hoffman, and E. Hayon. horg. Chem., in press. (47) M. Eigen. Angew. Chem., h t . Ed. Engl., 3, l(1964). (48) S. W. Benson, "The Foundations of Chemicaf Kinetics", McGraw-Hill, New York. N.Y., 1960, p 525. (49) T. S.Roche and J. F. Endicott, J. Am. Chem. Soc., 94, 8622 (1972). (50) (a) H. Taube, "Electron Transfer Reactions of Complex Ions in Solution", Academic Press, New York, N.Y., 1970: (b) R. G. Linck, MTf h f . Rev. Sci.. Inorg. Chem., 9, 303 (1971); (c) N. Sutin in "Inorganic Biochemistry", Vol. 2, G. L. Eichorn, Ed., Elsevier, New York, N.Y., 1973, p 61 1: (d) L. E. Bennett, frog. Inorg. Chem., 16, 1 (1973). (51) D. P. Rillema and J. F. Endicott. Inorg. Chem., 11, 2361 (1972). (52) R. A. Marcus, Annu. Rev. fhys. Chem., 15, 155 (1964). (53) A. Zwickel and H. Taube, Discuss. Faraday SOC..29,42 (1960). (54) W. Latimer, "Oxidation Potentials", 2nd ed, Prentice-Hall, Englewood Cliffs, N.J., 1952. (55) D. P. Rillema, J. F. Endicott. and R. C. Patel, J. Am. Chem. SOC.,94, 394 (1972). (56) The pK. for Ru(NH3)e3+ is 12.5 (D. Waysbort and G. Navon, Chem. Commun., 1410 (1971)) thereby eliminating the possibility of the deprotonated complex being a major contributor to the reaction. (57) H. S.Lim, D. I. Barclay, and F. C. Anson, horg. Chem., 11, 1460 (1972). (58)The pK. of Ru(NH3)6NO3+ is probably >12 (F, Bottomley and J. R. Crawford, J. Chem. Soc., Dalton Trans., 2145 (1972)). (59) J. N. Armor and M. 2. Hoffman, horg. Chem., 14, 444 (1975). (60) D. A. Buckingham and A. M. Sargeson in "Chelating Agents and Metal Chelates", F. P. Owyer and D. P. Mellor. Ed., Academic Press, New York, N.Y., 1964. (81) Although Fe(bpyh3+ is sensitive to base, all experbents using the complex were performed as rapid1 as possible in less.than 30 mln. At pH 9.2. the half-life for Fe(bpy)a3Iis longer than 1 hr (G. Nord, J. Chem. Soc.. Dalton Trans., 866 (1972)). (62) M. Z. Hoffman and M. Simic, Inorg. Chem., 12, 2471 (1973). (63) R. G. Wilkins Adv. Chem. Ser., No. 100, 111 (1910). (64) R. M. Sellers and M. G. Simic, submltted for publication. (65) R. G. S.Banks. R. J. Henderson, and J. M. Pratt, J. Chem. SOC.A, 2886 (1968). (66) N. Sadasivan, J. A. Kernohan, and J. F. Endicott, Inorg. Chem., 6, 770 (1967). (67) E. R. Kantrowitz. M. 2. Hoffman, and J. F. Endlcott. J. Phys. Chem., 75, 1914 (1971). (68) f. S. Roche and J. F. Endicott, horg. Chem., 13, 1575 (1974). (69) D. G. Brown, Prog. Inorg. Chem., 18, 177 (1973). \
Co(1)Complexes Containing Unsaturated Macrocyclic Ligands
