2664
Coordination of Oxygen by Cobalt (11) Complexes in Aqueous Solution. A Calorimetric Study' H. K. J. Powell and G . H. Nancollas* Contribution from the Chemistry Department, State University of New York at Bufalo, Bufalo, New York 14214. Received August 12, 1971 Abstract: The oxygenation of cobalt(I1)-bis(histidinate), -bis(histamine), and -bis(ethylenediamine) complexes has been studied in aqueous solutions at 25.0'. Enthalpy changes have been determined calorimetrically and equilibrium data obtained by use of a polarographic oxygen analyzer. The reactions 2CoL,(aq) Oz(aq)e (CoL,J2.0z(aq)are characterized by large negative enthalphy and entropy changes. For L = histidinate and ethyl: enediamine, AH = -30.1 f 1.3 and -29.4 + 0.6 kcal mol-', A S = -70 * 5 and -49 * 4 eu, respectively. These data are discussed in relation to the oxidation and coordination reactions involved.
+
In
biological systems the reversible oxygen-carrying planar (cf. H2O2),but bond lengths are not fully consiscomplexes responsible for transport and storage of tent with a peroxy structure and the complexes have a oxygen are complex species of iron(I1) (e.g., hemoglogreater lability toward decomposition with a ~ i d , ~ , ~ . ~ bin and myoglobin) or copper(1) (hemocyanin) in which EDTA,7j9,10dithionite,? and temperature or pressure ariat ti on'^^^^^^^ than expected for a cobalt(II1) species. the metal ion and coordinated prosthetic group are emThe oxygen-carrying capacity of these complexes debedded within a hydrophobic protein matrixa2 In creases with time as irreversible oxidation to monosimple low molecular weight complexes, these ions seldom show reversible oxygenation properties. Hownuclear cobalt(II1) complexes occurs. For some ligands this is very rapid (e.g., diethylenetriamine, glycylever, the cobalt(I1) ion forms many simple complexes which react reversibly with oxygen to form 1 : 1 or 2 : 1 glycinamide), while for others it is quite slow (e.g., hiscomplex oxygen adducts, and following the pioneering tamine, ethylenediamine).& Recent studies have characterized 1 : 1 adducts with work of Calvin, et al.,4 and Hearon, et a/.,&these have been widely studied as model compounds for the bio"coboglobin" and the dimethyl ester of cobalt(I1) protological systems. porphyrin IX,21 Co(acacen)B22(acacen = bis(acety1Early studies established the formation of 2 : 1 adducts acetonato)ethylenediimine, B = N,N-dimethylformawith cobalt(I1) complexes containing (a) amino mide, pyridine, etc), N,N'-ethylenebis(3-methoxysaliand N,N'-iminodi-n-pro(b) dipeptides,6s8 (c) polyamines and a m m ~ n i a , ~ * ~ * ~cylidineiminato)cobalt(II),14 -l pylbis(salicylideneiminato)cobalt(II) in the solid state4 and (d) bis(salicyla1dehyde)ethylenediamine and reand certain solvents, 2 2 dimethylglyoxime 2 3 and vitamin lated ligand^.^^^^,^^ Oxygen affinity was observed for solids, e.g., (d), and aqueous solutions, although for BIZ,.2 4 With the exception of coboglobin and vitamin BlZr,the adducts form only in nonaqueous solution and complexes with few polarizable donor atoms in the ligmay be rapidly decomposed and oxidized by traces of and a nonaqueous electron-donor solvent is required to water. 2 2 , 2 These compounds are paramagnetic, and enhance the electron-donor properties of the cobalt(I1) esr studies suggest that 80-90% of the odd-electron Lewis base.I4N15 On the basis of magnetic susceptibildensity is transferred to the oxygen to give a cobalt(II1) it^,^ electronic absorption, C D ~ p e c t r a ,and ~ proton superoxo species with a bent Co-0-0 link.21*25From affinity of the 0-0 bridge,16 these complexes are genkinetic studies, a 1 : 1 adduct has been postulated as a eralized as essentially cobalt(II1) species linked by a reactive intermediate in the formation of 2 : 1 adperoxide group, CO"'L,-(O~~-)-CO"'L,. X-Ray studd u c t ~ and ~ ~ this ~ - has ~ ~ been confirmed by esr for COies17*18 have shown the bridge to be nonlinear and nonbalt(I1)-polyamine systems. 2 6 (1) Work supported by National Science Foundation Grant No. The object of this research has been to determine GP9231. thermodynamic data for selected oxygenation reactions. (2) A. R. Fanelli, E. Antonini, and A. Caputo, Adcan.Protein Chem., 19,73 (1964). Thermodynamic data have been determined for the re(3) R. G. Wilkins, Adcan. Chem. S e r . , N o . 100, 111 (1971). action of 02(aq) with the cobalt(I1)-bis(histidinato), (4) A. E. Martell and M. Calvin, "Chemistry of Metal Chelate -bis(ethylenediamine), and -bis(histamine) (log K only) Compounds," Prentice-Hall, Englewood Cliffs, N. J., 1952. ( 5 ) J. 2. Hearon, D. Burk and A. L. Schade, J . Nut. Cancer Inst., complexes in aqueous solution at 25'. Hitherto, the 9,337 (1949), and references therein. (6) M. S. Michailidis and R. B. Martin, J . Amer. Chem. Soc., 91, 4683 (1969). (7) J. Simplicio and R. G. Wilkins, ibid., 89, 6092 (1967). (8) C. Tanford, D. C. Kirk, and M. K. Chantooni, ibid., 76, 5325 (1954). (9) F. Miller, J. Simplicio, and R. G. Wilkins, ibid., 91, 1962 (1969). (10) J. Simplicio and R. G. Wilkins, ibid., 91,1325 (1969). (1 1) F. Miller and R. G. Wilkins, ibid., 92,2687 (1970). (12) S. Fallab, Angew. Chem., I n t . Ed. Engl., 6,496 (1967). (13) S. Fallab, Chimia, 21,538 (1967); 23,177 (1969); 24,76 (1970). (14) C. Floriani and F. Calderazzo, J . Chem. Soc. A , 946 (1969). (15) C. Floriani, F. Calderazzo, and J. J. Salzmann, Inorg. Nucl. Chem. L e f t . , 2,379 (1966). (16) J. Simplicio, Ph.D. Dissertation, State University of New York at Buffalo, 1969. (17) W. P. Schaeffer,Inorg. Chem., 7,725 (1968).
Journal of the American Chemical Society
94:8
1 April 19, 1972
(18) M. Calligaris, G. Nardin, and L. Randaccio, J . Chem. S O C .A , 1069 (1970). (19) M. Calvin, R. H. Bailes, and W. K. Wilmarth, J . Amer. Chem. Soc., 68,2254 (1946). (20) W. I99 %) and a titrasaturated histidine solution (as above) in an O2 atmotion procedure was adopted, using the buffer solutions sphere. The cooling curve was followed for ca. 5 min; described above, Data are given in Table 111. then the calorimeter contents were quickly removed and Discussion the dissolved oxygen was determined. Transfer of oxEquilibrium Constants. Ko,(histidinate), (4.24 f ygen to or from the solution was negligible in the time 0.5) X lo6 hP2,is smaller than the value 6.0 X lo6 interval from the addition of CoClz to measurement of extrapolated for 25" from the data of Simplicia oxygen concentration. A manometric analysis indiand Wilkins.7 In a manometric study, Hearon, e t u Z . , ~ cated that complete equilibration between the gas and reported KO, = 2.30 X 106 at 26' using a graphical analysis of data. However, these results are question(31) D. D. Wagman, W. H. Evans, V . B. Parker, I. Halow, S . M. Bailey, and R. H. Shumm, Nat. Bur. Stand., U. S . , Tech. Note, No. able, since the number of moles of Ozabsorbed is greater 270-3,270-4(1969). than one-half the number of moles of cobalt at PO?= ( 3 2 ) J. L. Meyer and J. E. Bauman, J . Amer. Chem. SOC.,92, 4210 0.974 and 0.195 atm and 14". From their data we cal(1970).
-
Journal of the American Chemical Society
1 94:8 1 April 19, 1972
2667 Table IV. Comparison of Thermodynamic Data for Oxygenation and Nitrogenation Reactions, Substrate Oz(or N2) Adduct, 25.0" (1 M standard state for gas)
+
AS, cal deg-I Substrate
Physical state
COLZ( L = histidinate) CoLz(H20)22+ ( L = histamine) CoLz(HzO)z+ ( L = ethylenediaminey CoLz(HzO)z2+ (L = ethy1enediamine)d Co(Sa1en)c Co( 3-MeO-Salen)' Co( 3-EtO-Salen)"
A ; a I = 0.02 M A ; I = 0.13 M K C l
AG, kcal mol-'
AH, kcal mol-'
2 : l Adducts, (MLn)z-Oz 6.63 f 0.05' -9.05 i.0.07 8.47 dz 0.05 -11.55 f 0.07
mol-' 5
-30.1 f 1 . 3
-7Oi.
-14.8 i: 0 . 4
-29.4 f 0 . 6
-49 i.4
A ; I = 1.0 M K C l
- 10
-29.4
- 65
Solid Solid Solid
-1.6 -1.45 -3.5
-19.1 -19.1 -18.9
- 59 - 59 - 52
-21.2 i 2 . 8
-33 i. 10
-15.0 -15.3 i 0 . 4 -13.2 -13.4
- 29 -37.5 i 1 . 0
-10.1 zt 1 . 4
-13 i 5
A; I = 1 . O M K C l
'
Log K
Ru(NH&OZO)~'
A ; Z = 0.1 M N a C l
Co(acacen)p y g Myoglobinh Hemoglobin (ox)" Hemoglobin (human)j
Pyridine solvent A; p H 8 . 5 A; p H 9 . 1 A; p H 7
R U ( N H ~ ) ~ ( H ~ fO ) ~ +
A; Z = 0.1 MNaCl
10.84 f 0 . 3
2 : l Adduct (MLn)z.Nz -11.4
1 :1 Adducts, MLn.Oz -6.45
1:1 Adduct, MLn*Nz a Oz(aq) product is
-6.2
+ substrate in aqueous solution. * All values from this work are the mean f standard deviation from the mean.
c
The reaction
0 2
kC0
/ \
'0'
Cok
H
Calculated for the product Lz(H20)Co. OZCO(H~O)LZ (see Discussion). e Reference 4; Salen = bis(salicyla1dehyde)ethylenediimine. J. N. Armor and H. Taube, J. Amer. Clzem. Soc., 92, 6170 (1970), substrate N Z S dinitrogen adduct. 0 From M. H. Keyes, M. Falley, and R. Lumry, ibid., 93, 2035 (19711, and G. Amiconi. M. Brunori, E. Antonini, G. Tauzher, and G. Costa, Nature (Londotr), 228, 549 (1970). Keyes, Falley, and Lumry; see footnote g. Calculated for 1 M standard state for oxygen.31 F. W. Roughton, Biochem. J., 29, 2604 (1935). j A. J. Splittgerber, Ph.D. Dissertation, University of Colorado, 1968. For hemoglobin, AH is the mean value for oxygenation of the four heme groups. d
+
f
culate KO, = (3.7 f 1.5) X lo6 at 26". Koz(histamine), (2.95 f 0.35) X los M-2 (Table 111), agrees with the value (2.9 X IO6) from kinetic data. Enthalpy and Entropy Data. AH(COL)~compares with Williams' value33 of - 12.25 + 0.08 kcal mol-' in 3 M NaC104, and falls into the expected sequence with values for Ni, Cu, Zn(II), uiz., -16.6 f 0.5, -21.3 f 0.6, and - 11.7 f 0.3 kcal mol-', respectively, 0.1 M KN03.34 AHo,(histidinate) contrasts with the temperature coefficient value, -38 kcal m01-l.~ A recalculation from Simplicio's temperature coefficient datal6 yields AHo2 = -30.6 =k 3.0 kcal mol-'. For L = ethylenediamine, the measured AHo2 corresponds to the following reactions
+
+
2CoL~(H20)2~+ 02(aq) COLZ(HZO)-OZ-COL(H~O)2H20 AH'o, A
+
0 2
A
4Co
/ \
O \' LH+
AH,,
COLz f H30+
H
+ H30++LHz2+ + Hz0
=
AH'of
AH01
+ AH,, + AHLEI,
AHLH~
(33) D. R . Williams, J . Chem. SOC.A , 1550 (1970). (34) W . F. Stack and H. A . Skinner, Trans. Faraday Soc., 63, 1136 (1967).
For ligand protonation, AH,,, = - 11.1 kcal mol-' and A S L H , = - 5.8 cal deg-1 mol-'. 35 Data for the olation reaction are not readily estimated, but reference to somewhat analogous reactions CO(NH,);(HZO)~'+ HzO 5 Co(NH3);,(0H)'+ + H,O+ (proton dissociation, A H = +9.0 kcal mol-', A S +2.9 cal deg-l mol-')36 and 2Fe3+
+ 4H2O =$=
=
+
(Fe(OH)2Fe)4+ 2 H 3 0 +
(diolation, AH = +10.0 kcal mol-', A S = +20 cal deg' suggests that AH,, and AS,, will be positive and about +IO kcal mol-' and + I O t o +20 cal deg-' mol-', respectively. If this is valid, then for the AHO2 29 kcal mol-' and first reaction AHlO, ASLO? ASoz - 15 cal deg-' mol-' = -65 cal deg-I mol-', values which are close to those for the histidine system. The observed enthalphy and entropy values are compared in Table IV with data for similar reactions, involving the formation of dinitrogen and p-dinitrogenruthenium(I1) complexes, 38 the formation of p-dioxygen-cobalt(I1) complexes in the solid ~ t a t eand , ~ the for-
-
-
-
(35) D. H . Everett and B . W. Pinsent, Proc. RoJ.. Soc., S e r . A , 215, 416 (1952); T. Davies, S . S. Singer, and L. A. I
