Inorg. Chem. 1982, 21, 3413-3419 region is more complex; so despite the variety of isotopic spectral shifts collected here, some of the low-frequency Raman features remain unassigned. Acknowledgment. This research was supported by a grant from the Petroleum Research Fund, administered by the
3413
American Chemical Society. Y.M. was the recipient of a NATO Postdoctoral Fellowship. Registry No. Cuz(OzCCH3)4(Hz0)z, 15523-07-6; 65Cu,141 1906-3; "0, 14797-71-8; Dz, 7782-39-0; C~2(02CCH3)4(py)z, 15227-71-1; C U ~ ( O ~ C C H ~ )51798-89-1. ~(~~Z),
Contribution from the Laboratoire de Chimie de Coordination Bioorganique (LA 255), Centre d'Orsay de 1'UniversitE Paris-Sud, 9 1405 Orsay, France, and Laboratoire de Chimie Analytique, Centre d'Etudes Pharmaceutique, UniversitE Paris-Sud, 92290 Chatenay-Malabry, France
Structural Studies of Metalloporphyrins. 7.' 'H NMR and Electrochemical Investigation of the (meso-5,10,15,20-Tetraarylporphine)cobalt(III) Complexes XCo"'(TPP-p -R)2 J. HUET,* A. GAUDEMER, C. BOUCLY-GOESTER, and P. BOUCLY Received October 22, 1980
'H NMR spectra of five-coordinatecobalt(II1) meso-5,10,15,20-tetraarylporphinesXCo"'(TPP-p-R) (X = I, Br, C1, PF6, C104;R = H, C1, CH3, OCH3) display abnormally broad line widths. The magnitude of this line broadening varies with X and R and generally increases with temperature. Analysis of the data obtained at various temperatures suggests that this phenomenon is due in part to the presence of a small amount of the *-cationic species XCo"'(TPP-p-R)+., which is formed by the following disproportionation reaction: 2XCo"'(TPP-p-R) is Co"(TPP-p-R) + XCo"'(TPP-p-R)+. + X-. Electrochemical data obtained in CH2C12solution fully support this assumption. A second, but less important, source of the line broadening appears to be the self-exchange reaction between cobalt(I1) and cobalt(II1) porphyrins, which results in an exchange of ligand X and of protonated sites. In the present work, we report the results of an 'H N M R The magnetic properties of cobalt(II1) complexes such as study of various complexes XCo"'(TPP-p-R) shown in Figure vitamin B12 and various model compounds (cobaloximes, 1, which indicates that the line broadening has multiple origins porphyrins) have been the subject of many studies leading to and that the paramagnetism of these compounds probably does contradictory results and interpretation^.^ Although most not arise from an equilibrium between low-spin and high-spin cobalt(II1) complexes are diamagnetic because of their d6 forms. Electrochemical data are also presented which, together low-spin configuration (S = 0), some of them having a highwith the N M R study, lead to other possible explanations for spin configuration are paramagnetic: apart from Co"'F2- (S this paramagnetism. = 2)4 and complexes of the structure Co"'(P(C2HS),)X3 (X = C1, Br) (S = l)S some macrocyclic cobalt(II1) complexes Results and Discussion exhibit a paramagnetism6 the extent of which varies with the I. 'HNMR Spectra of Complexes XCo"'(TPP-p-R). In macrocyclic ligand and increases with temperature due to an 0.02-0.03 M solution in CDCl,, most of the complexes shown equilibrium between low-spin and high-spin configuration^.^^ in Figure 1, except those with X = I, show abnormally broad According to some author^,^ cobalt(II1) porphyrins should be N M R signals. The extent of broadening varies with the nature diamagnetic due to the strong ligand field created by the of X and R and with temperature. macrocycle. The weak magnetic susceptibility observed for ClCo"'(HP)(H20) in the solid state could arise from cobalt(I1) Part 6: J. Huet and A. Gaudemer, Org. Magn. Reson., 15,347 (1981). impurities or from the temperature-independent paramagAbbreviations used in this work for wrDhvrin comolexes: HP. hemanetism (TIP).l0 toporphyrin; TPP, meso-5,10,15,2O-tek~phenylp&phine; TPP-p-R, meso-5,10,15,2O-tetraarylporphine;OEP, octaethylporphine; H, refers ' H N M R spectroscopy is very sensitive to the magnetic to the proton of the pyrrole ring. characteristics of compounds. Various features of the For a detailed review on the magnetic properties of vitamin BI2see S. species-chemical shifts,* line widths," H-H and H-Co C. Agarwal, Bull. SOC.Chim. Fr., 1-100(1979). F.A. Cotton and G. Wilkinson, "Advanced Inorganic Chemistry", 2nd coupling constants' 'J2-have been repeatedly used to study ed., Interscience, New York, 1966,p 671. the properties of cobalt(II1) complexes and also to obtain K.A. Jensen, B. Nygaard, and C. Th. Pedersen, Acta Chem. Scand., structural evidence for the existence of .Ir-cationic species 17, 1126 (1963). derived from metal10porphyrins.l~ R.Williams, E. Billig, J. H. Waters, and H. B. Gray, J . Am. Chem. Soc., 88, 43 (1966). Two groups of i n v e s t i g a t ~ r s ' ~recently J~ observed that, in M. Gerloch, B. M.Higson, and E. D. McKenzie, J. Chem. Sm., Chem. solution in a noncoordinating solvent, the cobalt(II1) porCommun.,1149 (1971). phyrins CICO"'(TPP),'~ [ C O " ' ( T P P ) ( H ~ O ) ~ ] C ~ O [Co"'~, V.L. Goedken and Shie-Ming Peng, J. Chem. Soc., Chem. Commun., 258 (1975). (OEP)(HzO)2]C104,and [CO~~~(OEP)(THF)~]C~O,~~ exhibit M. Lundeen, R.L. Firor, and K. Seff, Inorg. Chem., 17,701 (1978). abnormally broad lines in their 'HN M R spectra, suggesting W. P. Hambright, A. N. Thorpe, and C. C. Alexander, J. Inorg. Nucl. a paramagnetic character for these compounds. On the basis Chem., 30,3139 (1968). J. L. Sudmeier and G. L. Blackmer, J . Am. Chem. Soc., 92, 5238 of variable-temperature measurements using Evans' method,16 (1970). it was concluded that ClCo"'(TPP) showed a paramagnetism J. L. Sudmeier, A. J. Senzel, and G. L. Blackmer, Inorg. Chem., 10, that increased with t e m p e r a t ~ r e . ' ~ 90 (1971). J. K. M. Sanders and I. Baxter, Tetrahedron Lett., 4543 (1974). K.Yamamoto, J. Uzawa, and T . Chijimatsu, Chem. Lett., 89 (1979). H.Sugimoto, M. Nagano, Z. Yoshida, and H. Ogashi, Chem. Letf.,521
*Towhom correspondence should addressed at the Laboratoire de Chimie de Coordination Bioorganique.
(1980).
D.F. Evans, J . Chem. Soc., 2003 (1959).
0020-1669/82/1321-3413$01.25/0 0 1982 American Chemical Society
Huet et al.
3414 Inorganic Chemistry, Vol. 21, No. 9, 1982
X
R
:'
3r
Z Li
:t
FF, and
:H,CP?
31
C C,
-50
-3
-10
+ 10
* 50
*30
L
T'OC)
3:H
Figure 1. meso-5,10,15,20-Tetraarylporphinesstudied in this work.
Figure 3. Line widths for the BrCo"'(TPP-p-OCH3) complex as a function of temperature in CDCI,.
(c) Influence of Temperature T ("C). Measurements of the spectra from -50 to +50 OC revealed that, for a given complex, increasing the temperature generally enhances the broadening of the signals, the change of line width being reversible. However, there are exceptions to the above rule: for example, in the case of BrCo"'(TPP-p-OCH,), Avl12values for H, and OCH3 go through a maximum and then decrease (Figure 3). The existence of the maximum is not clearly understood, but two possible explanations can be put forward: (a) increase in temperature gives more a radical cation; (b) increased 'up um nd GROUP, electron spin relaxation rate gives narrower lines (suggestion of a reviewer). The range of temperatures which could be covered was too narrow to see whether this was also the case for the other compounds. 11. Possible Origins of the Line Broadenings. (a) MetalFigure 2. lH NMR study of the BrCo'"(TPP) complex as a function Centered Paramagnetism. Secondary effects such as quadof temperature and the spectrum (d) of the [CO"'(TPP)(~~)~]+B~-rupolar relaxation of the cobalt atom (nuclear spin 7 / 2 ) and complex (solvent CDC13). viscosity of the solutions being neglected, line broadening in solution-state N M R spectra appears to have two main ori(a) Influence of Axial Ligand X. The half-width, Av'/~,for gins:'* (a) the presence of paramagnetic entities; (b) chemical a given type of proton, varies considerably with X and increases exchange between magnetically nonequivalent sites. Our rein the order sults with cobalt(II1) porphyrins do not seem to agree with X = I < Br < C1 < PFs < C104 the first assumption for the following reasons: First, if paramagnetism were the main cause of line broadening, a which is the order of decreasing coordinating ability. For relationship should be observed between the Avlj2 values for examples, the following values of Au'/~(CH,)(Hz) were obthe various protons and the distances between these protons served: and the metal center.lg Such is not the case: in the spectrum X of BrCo"'(TPP), for example, AulI2increases in the order H, < Hortho,Hmcta,HWra(Figure 2) whereas in paramagnetic I Br C1 PF, C10, metallotetraphenylporphine,the H, signals, which are nearer XCd"(TPP-p-OCH,) 2 t 0.5 11 t 1 22 t 1 50 * 3 2100 to the metal than any other protons of the molecule, also at T = O " C undergo the largest broadening, whatever the mechanism reFrom the study of another set of samples we obtained the sponsible for line broadening, whether it be the Fermi-contact same values (within experimental error). Maximum broadmechanism (e.g., ClMn111(TPP-p-CH3))20or the dipolar inening is observed for X = PF6 and Clod, and in these cases teraction mechanism (e.g., CO"(TPP-~-CH,)).~~ Second, the only a qualitative explanation of the phenomenon can be put line broadening should be accompanied by the shifts of the forward. For X = Br, the extent of broadening is not so large signals either to high field or to low field depending on the and the resolution of the spectra allowed a more exhaustive relative importance of the two mechanisms mentioned search of the origins of the broadening to be made (Figure above.*23 In complexes X C O ' ~ ( T P P - ~ Rno) such shifts have been observed. Theory predicts that metal ions possessing d6 2). For X = I, the lines are not perceptibly broadened. (b) Influence of the Phenyl Para Substituent R. For the high-spin configurations must have electron relaxation times same X, Avl increases as the electron-donating character of T I short enough to allow a good resolution of the ' H N M R R increases but no linear correlation could be found between spectra of their complexes whatever their geometry.24 This Avl and the Hammett u parameters." For example, with prediction was fully supported by the observation of well-reBrdo"'(TPP-p-R) at T = 0 OC the following reproducible values were observed: PYRIDINE
PHENYL
(18)
T. J. Swift, 'NMR of Paramagnetic Molecules. Principles and
(19) (20) (21) (22) (23) (24)
Applications", G. N. La Mar, W. D. Horrocks, Jr., and R. H. Holm, Eds., Academic Press, New York and London, 1973, Chapter 11. J. Reuben, Prog. NMR Spectrosc., 9, 20 (1973). G. N. La Mar and F. A. Walker, J . Am. Chem. SOC.,97,5103 (1975). G. N. La Mar and F. A. Walker, J. Am. Chem. Soc.,95, 1790 (1973). G. N. La Mar and F. A. Walker, J . Am. Chem. Soc., 95, 1782 (1973). G. N . La Mar and F. A. Walker, J. Am. Chem. Soc., 95,6950 (1973). See ref 18, p 81.
R Av,,,(Hp), Hz
a
H
CH,
OCH,
3
8
14
18
(17) H. H. Jaffe, Chem. Reu., 53, 191 (1953).
Structural Studies of Metalloporphyrins
Inorganic Chemistry, Vol. 21, No. 9, 1982 3415
Table I. Line Widths in Partially Oxidized Mg11(TPP-p-R)13and in BrCo"'(TPP-p-R) (CDCI,)
re1 broadening: A u ~ , ~ ( H ~ ) / A ' ~ ,z(Hortho)
para
para
Mg(TPP-p-R)tP
1 5
