Langmuir 1991, 7, 1635-1641
1635
Electron-Transfer Thermodynamics, Valence-Electron Hybridization, and Bonding of the meso-Tetrakis(2,6-dichlorophenyl)porphinatoComplexes of Manganese, Iron, Cobalt, Nickel, Copper, Silver, and Zinc and of the P+Mn(O)and 'P+Fe(O) Oxene Adducts Hui-Chan Tung, Pipat Chooto, and Donald T. Sawyer' Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received October 3,1990 The electron-transferoxidation-reduction chemistryfor meso-tetrakis(2,6-dichlorophenyl)porphine[( C k TPP)Hz] and its metal complexes[(C&TPP)MM = Mn, Fe, Co, Ni, Cu, Ag, and Zn]has been characterized by cyclic voltammetry and controlled-potential electrolysis. Electron spin resonance and magnetic measurements confiim that the cation radicals of (C&TPP)Hz,(C&TPP)Zn,and (C&TPP)Niare ligandcentered, that the spin density for (C&TPP)Cuand (C&TPP)Agis mainly within the porphyrin ring with three metal-nitrogen covalent bonds, and that the spin density for (C&TPP)Co'is localized on the metal. On the basis of these measurements electron transfer occurs within the porphyrin ring and the metalporphyrin bonding consists of covalent u bonds between d%p valence electrons of neutral metal and pyrrole p electrons of the porphyrin. The (C&TPP)Fem(ClOJand (C&TPP)MnWlO,)complexes form oxygen-atom adducts ['P+FeW=O and P+MnV=O]with metal-xygen bond energies (-AGBF) of 46 and 37 kcal, respectively,which are reduced to PFew=O and PMnTV=Owith M=O bond energies of 67 and 87 kcal, respectively.
Introduction Although there have been numerous, extensive characterizations of the electron-transfer thermodynamicsfor metalloporphyrins (with and without axial a systematic comparison of the transition-metal porphyrins formed by a rugged, sterically hindered porphine [meso-tetrakis(2,6-dichlorophenyl)porphine,(CbTPP)Hz]S in noncoordinating solvents (HzCClZ or MeCN) has not been presented. With tetraphenylporphyrins and octaethylporphyrins the presence of dioxygen often leads to autoxidation of the porphyrin ring and formation of p-oxobridged dimer~.~JO The presence of eight chlorine atoms at the 2,6-positions of the four phenyl groups at the meso positions of the porphyrin forces the phenyl rings to be perpendicular to the plane of the porphyrin ring. In turn this limits the distance of closest face-to-face approach such that p-peroxo and pox0 bridges are precluded. This has made it possible to form and characterize models for compounds I and I1 of horseradish peroxidase." The present study is directed to a quantitative comparison of the electron-transfer thermodynamics for the (1) Kadkh, K. M. In Progrese in Inorganic Chemistry; Lippard, S . J.,
Ed.;Wiley-Interscience: New York, lSW, Vol. 34, pp 435-605. (2) Guilard, R.; Lecomte, C.; Kadkh, K. M.Struct. Bonding 1987,64,
206268. (3) Bottomley, L. A,; Olson, L.; Kadhh, K. M. Adu. Chem. Ser. 1982, No. 201,279-311. (4) Kadieh, K. M.; Lin,X. Q.;Ding, J. Q.; Wu, Y. T.;Arnullo, C. Znorg. Chem. 1986,26,3236. (5) C ~ t r oC. , E. In The Porphyrins; Dolphin,D., Ed.;Academic: New York, 1979; Vol. V, p 16-27, and referen- therein. (6) Hartzell, C. R.;&uoedl, N. A. InBiochemical and Clinical Aspects of Oxygen; Caughey, W. S., Ed.;Academic Prm: New York, 1979 pp 337-364. (7)Fsrgtmn-Miller, 5.; Brautigan, D. L.;Margoliuh, E. In The Porphyrinu; Dolphin, D., Ed.; Academic Pres: New York, 1979; Vol. VII, pp 149-240, and referencea therein. (8) Traylor, P. S.;Dolphin, D.; Traylor, T. G . J. Chem. SOC.,Chem. Commun. 1984,4279. (9) Chin, D.-H.;Balch, A. L.; La Mar, G. N. J. Am. Chem. SOC.1980, 102,1446. (10) Chin, D.-H.;La Mar, G. N.; Balch, A. L. J. Am. Chem. SOC.1980, 102,4344. (11) Sugimoto,H.;Tung,H.-C.;Sawyer,D.T. J. Am. Chem. SOC.1988, 110,2465.
"ruggedized" porphine [(CbTPP)Hz] and its porphyrin complexes [ (CbTPP)M)] with manganese, iron, cobalt, nickel, copper, silver, and zinc in methylene chloride (HzCClZ), acetonitrile (MeCN), and dimethylformamide (DMF). In addition, the oxene adducts of the manganese and iron porphyrins have been characterized. Magnetic and electron spin resonance (ESR) measurements have been made of the porphyrins and their redox products to gain additional insights to the valence-electron hybridization and bonding for the metal centers. Although most contemporary discussions of oxygen activation and of the redox behavior of metalloporphyrins implicate high-oxidation states for the metals,l2-lDa more reasonable interpretation is the formation of species via covalent bonding between unpaired valence electrons of the metal and those of oxygen (or of oxidized oxygen and nitrogen ligands).N In the case of metal-oxygen intermediates they provide an effective path for oxygenatom transfer to olefinic substrates.11*2*The reversible binding of 02and CO by reduced iron porphyrins requires the presence of an axial base (histidine in the case of myoglobin and hemoglobin) to make the iron center nucleophilic.2z-a Such basicity is inconsistent with represen(12) D a m n , J. H.; Sono, M. Chem. Rev. 1987,87,12561276. (13) Ortiz de Montellano, P. R., Ed.Cytochrome P-450;Plenum: New York, 1986. (14) Unger, B. P.; Sligar, S. G.; Gunnalm, I. C. In The Bacteria; Sokaatch, J. R., Ed.;Academic P w : Orlando, FL, 1986; Vol. 10, p 657. (15) Groves, J. T.J. Chem. Educ. 1986,62,928. (16) Mmuy, D. Pure Appl. Chem. 1987,59,769. (17) Penner-Hahn, J. E.; Eble, K. S.; McMurry, T. J.; Renner, M.; Balch, A. L.; Groves, J. T.; D a m n , J. H.; Hcdgnon, K.0.J. Am. Chem. SOC.1986,108,7819. (18) Lee. W.A.:Yuan.L.-C.:Bruice.T.C. J.Am. Chem.Soc. 1988.110. 4x1 1.
(19) Traylor,T.G.;Nalrsno,T.;Dunlap,B.E.;Traylor,P.S.;Dolphin, D. J. Am. Chem. SOC.1986,108,2782. (20) Richert, 5. A.; Toaug, P. K. 5.; Sawyer, D. T.Znorg. Chem. 1989, 28,2471. (21) Sawyer, D. T.; Spencer, L.;Sugimoto, H.Zsr. J . Chem. 1987/1988, ~. , 3. --
28.
(22) Kau, L.-S.; Svaetita, E. W.; D a m n , J. H.; Hcdgnon, K. 0.Znorg. Chem. 1986,25, 4307.
0743-7463/91/2407-1635$02.50/0 0 1991 American Chemical Society
Tung et al.
1636 Langmuir, Vol. 7,No. 8, 1991 tations of positive-charged oxidation states for the iron (Por2-Fe2+and Por2-Fe3+). The importance of metalloporphyrins has prompted extensive physical characterization by molecular spectroscopy,2ba structural methods (X-ray, EXAFS, and NMR),W32and electrochemical measurements.33-98 The latter provide insight to the electron-transfer thermodynamics (via redox potentials) and to the inductive effects of (a) porphyrin substituents, (b) axial bases (ligands) of the metal center, and (c) the solvent matrix. Unfortunately, most interpretations of electrochemical data have focused on metal-centered electron transfer with associated comparisons of porphyrins, axial ligands, and solvent media. The most popular iron porphyrins [(TPPIFeCl and (OEP)FeCl] have ligand-centered (Cl) redox chemistry,m and their reduced forms [(TPP)Feand (0EP)Fel undergo autoxidationto form binuclear pox0 species [(TPP)FeOFe(TPP) and (OEP)FeOFe(OEP)].gJo With an inert solvent (MeCN or HzCC12), an almost inert counterion (ClO4-), and a "rugged" (protected) porphyrin [ (CbTPP)M] that cannot undergo metal-metal contact and pox0 formation, electrochemical measurementa can provide quantitative -menta of the electrontransfer thermodynamicsfor transition-metal porphyrins relative to those for the parent porphine [(CbTPP)H2] and (ChTPP)Zn" [closed-shellZn(dlOsp)porphyrin, 18-esystem]. Reduction potentials provide a direct measure of the relative electron affinity of (Cl8TPP)MXsystems, and oxidation potentials provide a direct measure of the relative ease of electron removal from the porphyrin ring, the metal, or the ligand anion [e.g., (CbTPP)Fe(Cl-1-1. When the removal of an electron from M(CbTPP)is easier than from Zn(Cl8TPP) [or H2(CbTPP)], it must be the result of the stabilization of the CbTPP'+ radical via (Cl8TPP'+)-(OM) covalent bond formation. Likewise, when removal of an electron from (Cl8TPP)M(Cl-)is easier than from H2(Cl8TPP)(Cl-), it must be the result of the stabilization of the CP atom radical via [ (CbTPP)M' 'Cl] covalent bond formation. If the presence of C1- does not have an effect on the metal porphyrin electrochemistry, the electron must come from the porphyrin or the solvent. Previous arguments and examples have established that the approximate covalent bond energy (-A&) for the (CbTPP)Fe-Cl bond is given by the relationm ~~
(23) Dawson, J. H.; Kau, L.-S.;Penner-Hahn,J. E.; Sono, M.; Eble, K. S.; Bruce, 0.5.;Hager, L.P.; Hodgmn, K. 0. J. Am. Chem. SOC.1986,
108,8114. (24) Goff, H. M. In Iron Por hyrins; Lever, A. B. P., Gray, H. B., E&.; Addifion-Weeley Publishing Ebmpany: Reading, MA, 1982; Part I, pp 261-267, and referenm therein. (26) Coban, J. P.; Sorrell,T.N.; Hoffman, B. M. J. Am. Chem. SOC. 1976, 97, 913. (26) Tang,S. C.; Koch, S.; Papaefthymiou, G. C.; Foner, S.; Frankel, R. B.; Ibere, J. A.; Holm, R. H. J. Am. Chem. SOC.1976,98,2414. (27) Haneon, L. K.; Eaton, W.A.; Sligar, S. G.;G d w , I. C.; Goutarman, M.; Connell, C. R. J. Am. Chem. SOC.1976,98, 2672. (28) Schappacher, M.; Richard, L.;Web, R.; Montiel-Montoya, R.; Bill, E.; G o ~ e rU.; , 'Rautwein, A. J. Am. Chem. SOC.1981,103,7646. (29) Gaul, E. M.; Kaaaner, R. J. Inor Chem. 1986,25,3734.
.
(30)Collman,J. P.;Sorrell,T.N.;Ho&on,K. 0.; Kulshrestha,A. K.; Strow, C. E. J. Am. Chem. SOC.1977,99,6180. (31) Koch, S.;Tang,5.C.; Holm, R. H.; Frankel, R.€3.; Ibere, J. A. J. Am. Chem. SOC.1976,97,916. (32) Ricard,L.;Schappacher,M.; Web, R.;Montiel-Montoya,R.; Bill, E.; Gonser, U.; Trautwein, A. Nouu. J. Chim. 1983,7,405. (33) Felton, R. H.; Linachitz, H. J. Am. Chem. SOC.1966,88, 1113. (34) Wolberg, A.; Manwen, J. J. Am. Chem. SOC.1970,92,2982. (35) Stanienda, A.;Biebl, G.Z . Phye. Chem. (Munich) 1967,52,254. (36)Fuhrhop, J.-H.; Kadiih, K. M.;D a h , D.G. J. Am. Chem. SOC. 1973,96, 5140. (37) Boucher, L. J.; Garber, H. K. Inorg. Chem. 1970,9,2644. (38) Felton, R.; Owen, G. 5.;Dolphin, D.; Fonnan, A.; Borg, D. C.; Fajer, J. Ann. N.Y.Acud. Sci. 1979,206,604.
(-AGBF) = (EO'cI.p-- E0'p~~I/pFe(~I-$ 23.1 kcal = [2.00 - (-2.9)] 23.1 = 53 kcal (1)
The present electrochemical study of (CbTPP)M complexes has been conducted such that the variation of conditions is limited to the transition metal (M) and its adducts in inert base-free solvents (MeCN and H2CC12). Magnetic and ESR data provide a measure of the spin state of the metalloporphyrins and their redox products as well as of the spin-density distribution between the metal and the CbTPP porphyrin. Previous structural assessments for related (C1eTPP)M systemsl1tmprovide additional perspective and insight to the bonding for the present group of metalloporphyrins. In combination these data and results provide the basis (a) for assigning the valence-electron hybridization of the metals in the family of metalloporphyrins and their adducts and (b) for assigning their covalent bond order, and (c) for asessing the approximate free energy of bond formation (-AGBF) for the metal center with the porphyrin nitrogen or the axial ligand (Cl, HO, or 0).
Experimental Section Instrumentation. Cyclic voltammetric measurements were made with a Bioanalytical Systems (BAS) Model CV-27 voltammograph, a BAS XY Recorder Model MF-8050, and a 15-mL electrochemical cell that included a working electrode,a platbumwire auxiliary electrode, and a Ag/AgCl reference electrode. The reference electrode was contained in a Pyrex tube with a softglass cracked tip, filled with aqueous tetramethylammonium chloride at a concentration to give a potential of 0.00V vs SCE, and placed inside a luggin capillary.= The platinum-wire auxiliary electrode was placed inside a glass tube closed with a medium-porosity glass frit. Glassy carbon (3.0-mm disk, BAS/ MF-2012) and platinum (1.6-mm disk, BAS/MF-2013) voltammetric working electrodes were obtained from Bioanalytical Systems, Inc. The electrode surfaces were polished with Beuhler No. 3 (0.05 pm) alumina immediately prior to each experiment. Controlled-potential electrolyses were accomplished with a PAR Model 173potentiostat/galvanostatequipped witha Model 179digitalcoulometer. The working electrodewas aglassy-carbon plate or platinumgauze in variousdimensions. The glassy-carbon electrode was cleaned by polishing with alumina, and the platinum-gauzeelectrode was cleaned by soakingin concentrated nitric acid and then water. The auxiliary electrode consisted of a piece of folded platinum gauze. UV-vis spectra were recorded on a Hewlett-Packard Model 8450A diode array spectrophotometer. The magnetic susceptibilites of the metal complexes were determined by the Evans method@ with a Varian XL-200 NMR spectrometer [DCClr solvent for (C&TPP)Cuand DCCla for (TPP)Ag]. The low solublity of (C&TPP)Agin MeCN-ds, C12CD2, DCCl2, and DCC& precluded accurate measurements. ESR spectra were recorded on a Varian Model E6S spectrometer. Chemicalsand Reagents. The reagents for the investigations and syntheses were the highest purity commercially available and were used without further purification. [CU(H~O)~](C~O~) was prepared by reduction of [Cu(H20),] (C104)2with hydrogen peroxide. The resulting salt was recrystallized from dry MeCN 4 times to give [Cu(MeCN)4]C104. Ag(ClO4) was obtained from Strem. High-purity argon gas was used to deareate the solutions. Burdick and Jackson Distilled in Glass grade dichloromethane (HCC12,0.005% H20),dimethylformamide(DMF,0.012% HaO), and acetonitrile (MeCN,0.002 7% H2O) were used without further purification for the electrochemical experiments. for
(39) Sawyer, D.T.; Robert,J. L.,Jr. Experimentol Electrochemistry Chemistr; Wiley-Interscience: New York, 1974; p 144. (40) Evans, D. F. J. Chem. SOC.1969, 2003.
Langmuir, Vol. 7, No. 8, 1991 1637
Oxidation-Reduction Chemistry for (Cl8TPP)M
(a) 0 4 mM (CbTPP)H*
8
' I I ,--.I
I
-1
(b) Zn(CI.TPP)
(b) 0 2 mM (CbTPP)Zn
fi
(c) CO(ClaTPP)
(c) 0.2 mM (CbTPP)Cu
V
(d) 0.5 mM (CbTPP)Ag
I
+10
00
-1 0
-2.0
E, V vs. SCE I
Q.0
I
I
t1.0
0.0
I
-1 .o
I
-2.0
E , V vs SCE
Figure 1. Cyclic voltammograms [at a glassy carbon electrode
Figure 2. Cyclic voltammograms in DMF (0.1 M TEAP) of (CbTPP)Hz, (CbTPP)Zn, (CbTPP)Cu, and (CbTPP)Ag at a glassy carbon electrode (area, 0.07 cm2). Scan rate was 0.1 V e-'.
in methylene chloride (0.1 M tetrabutylammonium perchlorate) at room temperature] for (a) 0.6 mM (CbTPP)Ha, (b) 0.6 mM (CbTPP)Zn, (c) 0.6 mM (C&TPP)Co,(d) 0.4 mM (CbTPP)Ni, and (e) 0.6 mM (CbTPP)Cu. Scan rate was 0.1 V 8-1.
C1complexesm(M = Mn, Fe, Co, Ni, Cu, and Ag) in MeCN and in H2CC12 are summarized in Tables I1 and 111. In addition, the effect of excess C1- or H3O+ upon the redox chemistry of (CbTPP)Mcomplexesis included. The effect of CO (1atm) and of 0 2 (1atm) on the electrochemistry (C18TPP)M(C104). 5,10,15,20-Tetrakis(2,6-dichlorophenyl)of (CbTPP)M (M = Fe, Co, Mn) in DMF is illustrated in porphine (CbTPPH2)wassynthesized from 2,4,6-~0llidinea~~ and Figure 3 and summarized in Table IV. was used to prepare (C&TPP)MnCl,"(CbTPP)FeC1,Q*@ (CbFigure 4 illustrates the cyclic voltammograms in aceTPP)Co," (CbTPP)Zn,"l ( C ~ T P P ) C Uand , ~ (CbTPP)Ag,UThe perchlorate ealts [(CbTPP)Mn(ClOd and (C~~TPP)F~(CIOI)] tonitrile at -35 OC for (CbTPP)Mn(C104),(CbTPP)Fewere prepared by the addition of &clod in toluene.& Metal(C104),and (CleTPP)Co(ClO4) and for the oxene adduct bipyridyl complexes of copper(I1)and silver(1)were prepared by of the iron(II1) porphyrin [ (*CbTPP+)Fe(0)].ll The the addition of 2 equiv of bipyridine to the solution of metal potentials of the redox couples for these porphyrins and cation in acetonitrile. for the oxene adduct of (CbTPP)Mn(C104)are summarized in Table I. Results Magnetic and ESR Measurements. The magnetic Electrochemistry. The cyclic voltammogramsof (Cbmoments for (C&TPP)Cu(in 1:2 D&C12/DCCla) and (TPTPP)H2 and of the (CbTPP)M (M = Zn, Co, Ni, and Cu) P)Ag (in DCC13) are 1.94 f 0.07 and 1.92 f 0.02 pg, complexes in HzCC12 are illustrated in Figure 1. Similar respectively. The ESR spectra for (CbTPP)Cu and (Cbdata for (CbTPP)Hz,(CleTPP)Zn,(CleTPP)Cu,and (CbTPP)Ag in H2CC12 at 298 K and at 77 K are illustrated TPP)Ag in dimethylformamide (DMF) are presented in in Figure 5 and are similar to those of (TPP)Cuand (TPP)Figure 2. The (CleTPP)Zn,(CbTPP)Cu, and (C1aTPP)Ag.4 The ESR spectra for the oxidation products of CuAg complexes also have a reversible oxidation couple. In (bpy)z+and Ag(bpy)z+ in MeCN at 77 K (Figure 5) are H2CC12 (CbTPP)Hz,(CbTPPIZn,and (CbTPP)Cuexhibit characteristic of metal-centered radicals. Table V sumfour reversible redox couples, and (CbTPP)Ag has the marizes the ESR parameters for the copper and silver same electrochemicalbehavior as in DMF. The Ell2 values porphyrins and for the cation radicals of (CbTPP)Hz, for the oxidation and reduction couples of these porphy(TPP)H2, (CbTPP)Zn, (TPP)Zn, and (CbTPP)Ni. rin complexes in HzCClz (and in MeCN and DMF) are Reactivity with 0 2 and HO-.Cyclic voltammograms summarized in Table I. for (CbTPP)Cu and (CbTPP)Ag in the presence of 02 The redox potentials for the (CleTPP)Mm(OH2)2+, and HO- are shown in Figure 6. A recent study' has characterized the effect of (CbTPP)Mn,(CbTPP)Fe,and (CbTPP)Mm(py)2+, (ChTPP)MmOH,and (CbTPP)Mm(CbTPP)Coon the electrochemical behavior of HO- and (41) Badger, G. M.; Jones, R. A.; h l e t t , R. L. A u t . J. Chem. 1964, 02. With (CbTPP)Cu and (CbTPP)Ag and 0 2 or HO-, 17, 1028. the electrochemistry indicates that they and their products (42) Alder, A. D.;Longo, F. R.; Varadi, V. Inorg. Synth. 1976,113,213. do not interact. The UV-vis spectra of (CbTPP)H2,(Cb(43) Kobayaehi, H.; Higuchi, T.; Kaizu, Y.; Osada, H.; Aoki, M. Bull. Chem. SOC.Jpn. 1976,48,3137. (44) Baeolo, F., Ed. Inorganic Synthesis; McGraw-Hilk New York, 1986; Volume XVI, p 214. (46) Hill, C. L.; Williamson, M. M. Inorg. Chem. 1986,24, 2836.
(46) Manoharan,P. T.;Rogers, M. T.In Electron Spin Resononce of Metal Compleres;Yen, T.F., Ed.;Plenum Prese: New York, 1988, p 143. (47) Tsang, P.K. S.; Sawyer, D. T.Inorg. Chem. 1990,29, W .
Tung et al.
1638 Langmuir, Vol. 7,No. 8, 1991
Table I. Redox Potentidr for Ha(CIJ'PP), for Ita M d Com lesm M P and M P C10 ] and for the Oxono Adduetr of [(C~TPP)Fe](ClO~) and [(ClLPP)Mnl(Cl84)In b l a (and &Cd{ [and DMP] V VB SCE +MQ+Mnp
Elis
Mp
+MPP++
+1.63 +1.33 +1.41
HQ
Zn"P (d%p) NiQ (d%p) 'CoQ (d'ep)
Mnpn++Mop'+
+1.52 +1.26 (+1.40) +1.37 ( + F a + )
+'PFewO (d%p) (+1.72)
(+1.59) (+'PFd"O +) +1.46 (+1.57) (d%p,coval4)
(Mn"P)+ (d%p) (+I.=)
(+La)
+PMnvO (d%p)
C U T (d%p*)
+1.40 (CuV2+)
A g m P (d%pz)
m-M"Pc
M V P - + W -1.54 [-1.301 -1.72 [-1.641 -1.72
-0.88
-1.10 [-0.93] -1.26 [-1.221 -1.20 -1.30 (COW-)
-0.86 (-0.68)
-1.34 (CO'"-)
-0.93 (-1.00)
-1.17 (-Few-)
(-0.54) (PFewO
-
(-0.80)(PMnwO
(+0.52) (+ PMnwO) (d%p) +1.20 (CUT+) [+1.17] (d%p2) +0.77 (AgmP+) [ +0.75] (d$p*)
-1.61 (-Fenpt)
P F e W ) (d%p9
-1.27 (-1.17)
(-1.69)
P M n W ) (d%p)
-1.09 ( C U P ) [-1.081 (d%4)
-1.70 [-1.561 (CUTS)
-1.01 (Agmp)
[-0.811 ( d W )
r u p d p t Roman numerale indicate the covalence (number of covalent bonds) for the metal,not the oxidation state.
Elin, V ~8 SCE"
PMnm(OH2)2+
+ HsO+
+1.57
-0.03 -0.02
[+1.66]
-0.08
+1.45 +1.34 +1.64 +1.36 [+I.(%] [+0.80] PCbn(H&Cln) + HsO+ PNin(H&Cls) + HsO+ *PCum(H&Clp)
+ HaO+
a
Mnp+-MQ
+1.24 +1.03 [+l.Ol] +1.23
(Cdnp)+(d%p*) +1.M (+1.68) (d%p,coval 2) (F#P)+ (d%p) +1.52 (+1.72)
a The
p+-MQ
-1.16 [-0.861 . . -1.17
-1.68 1-1.181 --1.69-
[-0.95] (+PFe-) [-0.87] -1.18 -0.80 -1.31 -0.45 -0.75 (+ PCO-)
[-1.601
-0.4 +0.31 +0.25 +0.23
[-0.85] [-0.75] -1.19 (-0.121 [-0.961 -1.20 [-0.91]
[-1.621 -1.63 [-1.311
PCO-) -1.26
(4
-1.29 [-1.781 -1.70
[ 1, irreversible proceee; peak potantid.
TPP)Zn, (CbTPP)Cu,and (ChTPP)Agalsoare unaffected by the presence of 02 and/or HO-.The addition of HOor 02 does not alter the ESR spectrum of (ChTPP)Ag at 77 K.
Discussion and Conclusions The data of Table I (and Figures 1 and 2) indicate that the oxidation and reduction couples for (ChTPP)H2,(ChTPP)Zn, and (C4TPP)Ni occur at similar potentials and are consistent with porphyrin-centered electron-transfer processes. In a recent report20 we presented arguments that neutral porphyrin complexes [M(por)] consist of uncharged metal centers [Zn(d%p), Mn(d6sp),Fe(desp),Co(d'sp), and Ni(d$p)] bonded via two metal-nitrogen covalent bonds (sp-p) with uncharged porphyrin (all four nitrogens equivalent via resonance). Thus, the removal and addition of electrons for (ChTPP)Hz, (CbTPP)Zn, and (ChTPP)Ni (Figure 1 and Table I) occur within the *-electron manifold of the porphyrin ring. The first reduction potential is directly porportional to the relative electron affinity of the metalloporphyrincenters with (CgTPP)H2having the greatest electron affinity for this group (Table I). The (Cl8TPP)Wo' (d7sp)complex is unique with two metal-nitrogen (sp-p) covalent bonds and a metalcentered unpaired electron, which facilitates the addition of an electron to give (ChTPP)"Co- (desp). (The Roman
numeral superscript represents the covalence (number of covalent bonds) rather than the oxidation number or state of the metal.) Removal of an electron occurs within the porphyrin and is facilitated [first oxidation at +0.82 V vs SCE in contrast to +1.23 V for (ChTPP)Ni, Figure 1 and Table I] via stabilization of the porphyrin cation radical by formation of a third cobalt-nitrogen covalent bond (cobalt radical-porphyrin radical coupling) to give a diamagnetic species, (ChTPP+)Com (d%p2). This difference in oxidation potentials for PNi and PCo', A E p is an approximatemeasureof the free-energy of formation for the third cobalt-nitrogen covalent bond (-AGBF) of +pCoIn*ll,20
= [(Ei/z)pNi- (El/Jpco.] X 23.1 kcal = (1.23- 0.82) X 23.1 = 9.5 kcal (2) The (CbTPP)nFe+ complex (metal valence-electronhybridization, d$p) also is unique with two metal-nitrogen (sp-p) covalent bonds. The first reduction is metalcentered to give (ChTPP)Fe" (desp),followed by a second metal-centered reduction to give (ChTPP)nFe- (d7sp).In contrast, oxidation of (CbTPP)nFe+ (d5ep) is ligandcentered to give the porphyrin cation radical, (CbTPP'+)Q Fe+ (dssp). Oxidation of (CkTPP)Fen in the presence of ligands (L= HzO, py, HO-, C1-, or DMF) yields (ChTPP)Fem(OH2)2+ (d%p2), (CbTPP)Fem(py)2+(d6sp2), (ClsTPP)FeWH, (ChTPP)FemCl,and (CbTPP)Fe(DMF)+, Tables I1 and IILm Reference to the data of Table I and the results from an earlier studym indicate that the (ChTPP+)Mnm complex has unique bonding with three manganesenitrogen (d5p-p) covalent bonds with the positive charge at the porphyrin ring (S= 4/2). Reference to Tables I1 and I11 confirms that the presence of py (or C1-) has no effect upon the reduction potential of (ChTPP+)Mnm. Additionof an electron occurs at the porphyrin ring (4.04 V vs SCE, Figure 4) to give (ChTPP)Mnn (d6sp, S = 6 / ~ ) with two metal-nitrogen sp-p covalent bonds [analogous to (C18TPP)Znn, (CbTPP)Nin, (CbTPP)COn, and (ChTPP)Fen]. Subsequent addition of electrons is porph rin centered to give (ChTPP*-)Mnnand (ChTPP2-)Mn , which is comparable to (CgTPP)Znn and (CbTPP)Nin. Oxidation of (CbTPP+)Mnmoccurs at the porphyrin to give (CbTPPz+)MnW(dCp)with four d % p pmanganeeenitrogen covalent bonds (two quaternized pyrrole nitrogens). As with (ChTPP)Con', oxidation of (CbTPP)Mnn occurs at the porphyrin ring to give a cation radical, which is stabilized via coupling with one of the five unpaired d -A&,
K-
Langmuir, Vol. 7, No. 8, 1991 1639
Oridation-Reduction Chemistry for (C1sTPP)M
MP HnP + C1PZnWl-) [+pMn*]Cl PF&l PCdnCl PNin C1' P C 9 + c1'PAgm + c1-
Table 111. Bedox Potentiah for HdClnTPP) and for Its MetnlKhloro Complexes in H&Cb Ella V w SCEo +*PMIILcl+ P M W l P M W l PMn(Cl-) P M W l + PMn(Cl-) PMn(Cl-) -WP -MnP - M V [+1.64] (HP+ +) +1.27 (HP' + HCl+ -1.06 (+ HP? -1.44 (+ Ha*) -1.22 (+ ZnnP*-) -1.64 (- zinpp-) [+1.191 (+PZnnCl+) +0.81 ('PZnnCl+) +1.49 (+PMnn'C1 +) -0.06 -1.34 (+ M n U P - ) +1.64 (*+PFeWl) +1.35 (+*PFeWl+) -0.29 -0.97 -1.63
-
[-0.14]
+
a
[
-0.81 -1.22 (+ NinP-) -1.20 (+ CumP-) -1.01 (+ Agmp)
+1.21 +1.22
+1.45 +1.!54
+
+
+0.77
-1.29 -1.64 (+ CumPl-)
1, irreversible redox process; peak potential. -T
I
I
I
(a) (CbTPP)Mn(CIOd (a) (ClaTPP)Fe
(b) (CI.TPP)Fe
i CO(1
alm)
__ -
/--0
/'
I
,/
;'a +
/---
'I
n
(c) (ClsTPP)Fe + 01( 1 a m )
-:l/ 1
,--
T
'.
I
/'
i
L-J I
+2.0
1
I
+1.o
0.0
I
-1 .o
E, V vs SCE
Fipure 4. Cvclic voltammograma lat a r r h v carbon electrode V vs. SCE
Figure 3. Cyclic voltammograms at a glassy carbon electrode (area,0.09 cm*) in DMF (0.1 M TEAP) of (a) 0.5 mM (CleTPP)Fe,(b)0.5 mM (CbTPP)Fe in the presence of CO (1atm), and (c) 0.5 mM (CleTPP)Fe in the presence of 02 (1atm). Scan rate was 0.1 v 8-1.
Table IV. Redox Potentiah in DMF (0.1 M TEAP) for (CbTPP)M(py) Complexes in the Presence of Ar, 0 2 , and
co
Ella, V w S C B
PMnn(py) gas
(-4
Ar
-0.16 -0.16 -0.16
(-4
PFG(PY) (4 (-4
PCdr(PY) (+)
+OB4 -1.04 [+0.10] On [4.06] [+0.13] +0.06 CO [+0.76] -0.87 [+0.10] a [ 1, irreversible redox process; peak potential. -1.18 -1.18 -1.18
(-4 -0.76 [-0.62] -0.76
electrons of manganese. This facilitated removal of an electron (-0.03 V vs SCE) relative to that for (Cl8TPP)Nix (+1.23 V) is an approximate measure of the bond energy for the third manganese-nitrogen bond (-AGBF, 29 kcal; eq 2). The uncharged (C18TPP)Cu (d%p2)and (CbTPP)Ag
Scan rate WaeO.1 V
8-1.
(d%p2)complexes contain an odd number of electrons, with magnetic momenta that are consistent with one unpaired electron, and ESR spectra that indicate that the spin density is concentrated within the porphyrin ring (Figure 5). The ESR spectrum for (.CbTPP)Agm should have nine hyperfine lines due to four equivalent porphyrin nitrogen atoms that are split by interaction of the unpaired electron with the silver nucleous (I = l/2). The expected l&line spectrum is exteneively overlappedsuch that only 11 lines are observed.& The one-electron reductions of (*CbTPP)CuIIIand ('C&TPP)Agm (Figure 2 and Table I) are facilitated [relative to thoee for ( C k TPP)ZnU]by the presence of an unpaired electron in the porphyrin. Hence, the results are in accord with d%p2 hybridization for the Cu and Ag atoms within the porphyrin ring and three metal-nitrogen (spLp) covalent bonds. The copper hyperfine coupling constanta (Al)cuand the unpaired electron densities (a2)at copper for a series of
1640 Langmuir, Vol. 7,No. 8, 1991
Tung et al.
Table V. ESR Parameters for MetallowrDhurh ComDlexes and Their Cation Radicals in H&Cb at 298 K and 77 K metal
complex0 (+*C&TPP)Hz (+*TPP)Hz
(+*C&TPP)Znn
(+*TPP)ZnII (+*C&TPP) Nin
(C&TPP')Cum (C&TPP)*)Cum (TPP*)Cum (TPP*)Cum (C&TPP*)Agm (C&TPP*)Agm (TPP*)Agm (TPP*)AgnI (bpy)zz+CU*" (bpy)zz+Ag'n
0
probe temp, K 77 77 77 77 77 298 77 298 77 298 77 298 77 77 77
porphyrin
(An), G
gr
2.1230 2.1930 2.1098 2.1754
92 90
81
2.0027 1.9998 2.0082 2.0077 2.0009 2.0240 1.9915 2.0130 2.0494 2.0620 2.0652 2.0628 2.0605
MIIN, G
porphyrin spin density, %
16 16 15 17 23 26 22 23
100 100 100 100 100 57 57 53 62 82 93 73 80
