Langmuir 2002, 18, 3241-3246
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Attachment of Cobalt “Picket Fence” Porphyrin to the Surface of Gold Electrodes Coated with 1-(10-Mercaptodecyl)imidazole Shouzhong Zou, Robert S. Clegg, and Fred C. Anson* Laboratory for Molecular Sciences, Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125 Received September 17, 2001 Self-assembled monolayers of 1-(10-mercaptodecyl)imidazole on Au electrodes were used to bind cobalt “picket fence” porphyrin (cobalt 5,10,15,20-tetrakis(R,R,R,R-2-pivalamidophenyl)porphyrin) to the electrode surface. The binding involved coordination of the cobalt center of the porphyrin to the pendant imidazole groups in the monolayer coating. Attempts to coordinate the Co(II) oxidation state of the porphyrin to the coatings were not successful. However, with the Co(III) oxidation state, substantial binding was achieved which persisted even when the Co(III) was reduced to Co(II). Absorption spectra of the attached porphyrin were obtained for both oxidation states of the cobalt center. The remaining axial coordination site on the attached cobalt porphyrin is accessible to ligands, for example, imidazole, in aqueous solution.
Introduction In a set of recent studies, the relaxation dynamics of photoexcited cobalt porphyrins and of a dioxygen-cobalt porphyrin complex in hydrocarbon solvents were examined and correlated with the catalytic activities of the porphyrins toward the electroreduction of O2.1-3 With the goal of extending such studies to porphyrins confined to electrode surfaces where they exhibit their electrocatalytic activities, we sought to attach porphyrins to the surface of an optically transparent electrode. Of particular interest to us was cobalt “picket fence” porphyrin (cobalt 5,10,15,20-tetrakis(R,R,R,R-2-pivalamidophenyl)porphyrin), CoTpivPP, because its complex with O2 is unusually stable at room temperature (in nonaqueous solvents).4 The high affinity of cobalt picket fence porphyrin for O2 depends on the coordination of a suitable base, for example, imidazole, to the unshielded axial coordination site on the cobalt(II) center.4 We therefore sought a scheme for the attachment of the porphyrin to the electrode that would provide the needed axial base as part of the attachment process. On the basis of reports of Offord et al.5 and Zhang et al.,6 we prepared a self-assembled monolayer (SAM) of 1-(10mercaptodecyl)imidazole on gold electrodes with the hope that coordination of CoTpivPP to the exposed imidazole ligand could be used to attach the porphyrin to the surface. This general approach proved to be successful if special care was taken in the preparation of the gold surfaces and, especially, in controlling the potential of the Au electrode as the attachment proceeded. In this report, the * Corresponding author. E-mail:
[email protected]. Tel: (626) 395-6000. Fax: (626) 577-4088. (1) Yu, H.-Z.; Baskin, J. S.; Steiger, B.; Wan, C. Z.; Anson, F. C.; Zewail, A. H. Chem. Phys. Lett. 1998, 293, 1-8. (2) Yu, H.-Z.; Baskin, J. S.; Steiger, B.; Anson, F. C.; Zewail, A. H. J. Am. Chem. Soc. 1999, 121, 484-485. (3) Steiger, B.; Baskin, J. S.; Anson, F. C.; Zewail, A. H. Angew. Chem., Int. Ed. 2000, 39, 257-260. (4) Collman, J. P.; Braumann, J. I.; Doxsee, K. M.; Halbert, T. R.; Hayes, S. E.; Suslick, K. S. J. Am. Chem. Soc. 1978, 100, 2761-2766. (5) Offord, D. A.; Sachs, S. B.; Ennis, M. S.; Eberspacher, T. A.; Griffin, J. H.; Chidsey, C. E. D.; Collman, J. P. J. Am. Chem. Soc. 1998, 120, 4478-4487. (6) Zhang, Z.; Hou, S.; Zhu, Z.; Liu, Z. Langmuir 2000, 16, 537-540.
details of the successful attachment procedure are given and the electrochemical and spectroelectrochemical behavior of the attached CoTpivPP are described. Experimental Section Materials. Reagents were used as received from commercial sources (Aldrich, Fluka, Lancaster) except for tert-butylammonium perchlorate, which was recrystallized from benzene. Solvents were used as received from Aldrich, EM, and Burdick and Jackson except for diethyl ether, which was distilled from calcium hydride, and dichloroethane, chloroform, and benzene, which were passed through a column of basic alumina immediately before they were used. 1-(10-Mercaptodecyl)imidazole was prepared according to previously described procedures.7,8 Samples of cobalt 5,10,15,20-tetrakis(R,R,R,R-2-pivalamidophenyl)porphyrin were available from a recently synthesized batch.9 Apparatus and Procedures. Prepurified argon was passed through a column (Oxiclear) to remove residual O2 and, when necessary, through a Drierite column to remove residual H2O. Transparent gold film electrodes were prepared by vapor deposition of gold on chromium-coated glass microscope slides. Just prior to coating, the slides were cut to the desired size (1 × 5 cm), cleaned with a solution of Alconox, rinsed, immersed in freshly prepared piranha solution (5:1 H2SO4/30% H2O2; caution: this solution reacts violently with organic materials), rinsed with Nanopure water and ethanol, and blown dry with prepurified nitrogen. The depositions of Cr and Au were carried out in an evaporation chamber (LCI-14B, Consolidated Vacuum) having a base pressure of 3 × 10-6 Torr and a maximum pressure during deposition of 5 × 10-5 Torr. Au films with thicknesses of 5-20 nm were deposited on top of 2 nm undercoats of Cr. Film thicknesses were monitored with a quartz crystal microbalance (TM-100, R. D. Mathias). Gold sphere electrodes (0.15-0.20 cm2) were obtained by melting a 99.999% gold wire as described by Creager and Rowe.10 The resulting spheres were rinsed with Nanopure water and cycled between -0.3 and 1.5 V in 0.1 M H2SO4 until a stable voltammetric response was obtained. Before each experimental run, the electrodes were heated briefly to restore their luster. (7) Lee, T. R.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741-749. (8) Clegg, R. S.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 53195327. (9) Steiger, B.; Anson, F. C. Inorg. Chem. 2000, 39, 4579-4585. (10) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1994, 370, 203-211.
10.1021/la011444r CCC: $22.00 © 2002 American Chemical Society Published on Web 03/22/2002
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SAMs were formed by soaking Au electrodes in 1 mM solutions of 1-(10-mercaptodecyl)imidazole in ethanol for 10 min (longer soaking times produced no difference in the results) followed by thorough rinsing with ethanol. The immediate precursor to the thiol was the S-acetyl protected molecule, which was hydrolytically deprotected to obtain the free thiol used to prepare the SAMs. NMR spectra of the thiol solutions showed that the thiol was partially oxidized to the corresponding disulfide even though efforts were taken to avoid the presence of O2. SAMs obtained from solutions containing mixtures of the free thiol and the disulfide exhibited somewhat different behavior than those prepared from solutions containing only the thiol (vide infra). Spectra were obtained with a Varian-Cary spectrophotometer. Spectroelectrochemical data were obtained with the transparent gold film electrode mounted in a standard cuvette (1 × 1 × 5 cm) that contained the aqueous supporting electrolyte solution with the gold surface facing the solution. The effective area of the electrode, determined by the level of the solution in the cuvette, was typically 2-3 cm2. A Teflon cap held the electrode in position against the back face of the cuvette and provided mounts for auxiliary and reference electrodes and gas inlet and outlet ports. Conventional electrochemical instrumentation was employed. Potentials are reported with respect to a Ag/AgCl (3 M NaCl) reference electrode that was separated from the test solution by a Vycor-tipped salt bridge. Experiments were conducted at the ambient laboratory temperature of 20 ( 2 °C.
Zou et al. Scheme 1. Attachment of CoTpivPP to the Surface of Au Electrodesa
Results Attachment of CoTpivPP to Coated Au Electrodes. Initially, we attempted to attach CoIITpivPP to Au electrodes coated with SAMs prepared from 1-(10-mercaptodecyl)imidazole, 1-MDIm, by coordination of the porphyrin to the terminal imidazole ligands. The coated electrodes were exposed to solutions of the porphyrin (0.20.5 mM) in 1,2-dichloroethane for up to 12 h, removed, and washed with pure solvent. This procedure led to the attachment of only small quantities of the porphyrin. The affinity of the Co(II) form of the porphyrin for the imidazole sites of the SAM appeared to be insufficient to produce stable attachment of significant quantities of the porphyrin. This result was not surprising because the equilibrium constant for the coordination of 1-methylimidazole to CoIITpivPP has been estimated as 1.7 × 104 and 2.9 × 103 M-1 in toluene4 and benzonitrile,9 respectively. If comparable values applied in dichloroethane, 0.5 mM solutions of CoIITpivPP would not be expected to produce quantitative coordination of the porphyrin to the alkylimidazole groups on the Au electrodes. In addition, the typical substitutional lability of Co(II) would be expected to lead to the loss of the coordinated porphyrin when the electrodes were washed with pure solvent. The affinity of 1-methylimidazole for cobalt picket fence porphyrin is much higher when the cobalt center is oxidized to Co(III).9 The attachment of CoTpivPP to Au electrodes coated with 1-MDIm was therefore attempted with the oxidized porphyrin using the procedure depicted in Scheme 1: 1-MDIm-coated Au sphere electrodes were placed in a 0.5 mM solution of CoIITpivPP in 1,2dichloroethane also containing 0.2 M tetrabutylammonium perchlorate. The Au electrode and an auxiliary and reference electrode were connected to a potentiostat. The potential of the Au electrode was adjusted to 0.4 V where (1-MeIm)CoIITpivPP (1-MeIm, 1-methylimidazole) is known to be oxidized to the Co(III) state but CoIITpivPP is not.9 After 5 min, the electrode was removed, washed with pure solvent, transferred to aqueous 1 M NaClO4, and examined by cyclic voltammetry. Typical cyclic voltammograms that resulted from this procedure are shown in Figure 1. The first scan of the coated electrode from 0.45 V toward more negative potentials typically exhibited a clear, reversible response near 0.0 V and a
a (i) Freshly cleaned electrode dipped in a 1 mM solution of 1-(10-mercaptodecyl)imidazole in ethanol for 5 min. (ii) SAMcoated electrode placed in 0.5 mM CoTpivPP in 1,2-dichloroethane-0.2 M TBAP and adjusted to a potential of 0.4 V for 5 min.
second irreversible response near -0.45 V (Figure 1A). As the electrode potential was cycled several times between 0.45 and -0.6 V, the reversible response persisted as the irreversible response almost disappeared (Figure 1A). The persistent, reversible couple centered at 0.0 V can be attributed to the Co(III/II) couple of CoTpivPP coordinated to the pendant imidazole ligand. This assignment is based on the potential where the response appears, which is much less positive than that expected for the Co(III/II) couple without an axial base coordinated to the cobalt center.9,11 The initial transient response near -0.45 V in Figure 1A is most likely to originate from cobalt centers to which two imidazole ligands are coordinated because (in benzonitrile solutions) [(Im)2CoIIITpivPP]+ is known to be reduced to (Im)CoIITpivPP at considerably more negative potentials than those where the latter complex is oxidized to [(Im)CoIIITpivPP]+.9 The association of a second Im ligand to [(Im)CoIIITpivPP]+ is thermodynamically favored, but the rate of the coordination reaction is too slow to keep up with the electro-oxidation reaction at scan rates of 100 mV s-1.9 Thus, as the cobalt center of the coordinated porphyrin is cycled between its less (+3) and more (+2) labile oxidation states, any initially doubly ligated porphyrin is converted into its singly ligated counterpart. The coordinative properties toward cobalt porphyrins of SAMs containing pendant imidazole ligands are sensitive to the composition of the solutions used to deposit the SAMs on the Au electrodes. For example, SAMs prepared from mixtures of 1-MDIm and the disulfide resulting from its oxidation (by adventitious air) also coordinate (CoIIITpivPP)+ in both singly and doubly ligated forms, but (11) Song, E.; Shi, C.; Anson, F. C. Langmuir, 1998, 14, 4315-4321.
Attachment of Cobalt Porphyrin to Gold Electrodes
Figure 1. (A) Cyclic voltammograms for CoTpivPP coordinated to pendant imidazole groups of a SAM of 1-MDIm on a gold sphere electrode (0.20 cm2). The CoTpivPP was attached to the coated electrode by the procedure depicted in Scheme 1. The continuously recorded voltammograms (outermost to innermost) are the 1st, 2nd, 4th, and 8th scans. The dashed curve is the response from the same coated electrode before exposure to CoTpivPP. Supporting electrolyte: 1 M NaClO4. Scan rate: 0.1 V s-1. (B) Steady-state cyclic voltammogram obtained as in (A) except that the SAM was deposited from a solution containing a mixture of 1-MDIm and its corresponding disulfide. The response is unaffected by repetitive scanning. (C) Outermost curve: first scan cyclic voltammogram as in (B) except that the initial electrode potential was -0.55 V. The dashed and dotted curves are steady-state voltammograms obtained when the repetitive potential scans were confined to the region between 0.45 and -0.25 V or between -0.15 and -0.55 V.
both forms persist as the coordinated cobalt porphyrin is cycled between its +3 and +2 oxidation states (Figure 1B). We infer that the SAMs formed from mixtures of the thiol and disulfide produce structures that are less uniform and contain porphyrin and imidazole groups positioned so that the coordination and dissociation of a second imidazole is able to proceed as the cobalt center in the porphyrin is oxidized and reduced, respectively. However, the lack of an anodic response near the cathodic peak at -0.45 V indicates that the rate of the coordination of the second imidazole ligand to the cobalt center is not adequate for coordination equilibrium to be achieved as the voltammogram is recorded. The same slow rate of ligand coordination was also observed when the cyclic voltammetry was examined in homogeneous solution.9 This interpretation is supported by the results shown in Figure 1C: When the cyclic voltammetric scan is commenced from -0.55 V, where the coating contains only (1-MDIm)CoIITpivPP, the anodic peak near 0.0 V is larger than its cathodic counterpart (because the [(1-MDIm)CoIIITpivPP]+ produced near 0.0 V is partially converted to [(1-MDIm)2CoIIITpivPP]+ by the time the potential reaches 0.0 V on the return scan) and the cathodic response near -0.45 V corresponds to the reduction of the doubly
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ligated porphyrin. In addition, if the potential scan is limited to the range between -0.25 and 0.45 V, the reversible response at 0 V gradually diminishes to produce steady-state peak currents with equal magnitudes as expected for the [(1-MDIm)CoIIITpivPP]+/(1-MDIm)CoIITpivPP couple. For potential scans limited to the range between -0.15 and -0.55 V, a cathodic response from the reduction of [(1-MDIm)2CoIIITpivPP]+ (to yield (1-MDIm)CoIITpivPP + 1-MDIm) appears only in the first few scans of the potential from -0.15 V to more negative values because the rate of reattachment of the second 1-MDIm to the cobalt center is inadequate to achieve coordination equilibrium at the scan rate employed. With SAMs prepared from 1-MDIm, the steady-state voltammetric response obtained after several repetitive cycles (Figure 1A) is considerably simpler than that observed with homogeneous solutions of the same porphyrin in the presence of 1-MeIm because of the more extensive changes in coordination that accompany the oxidation in the latter case.9 The response is also much better defined than that for CoTpivPP irreversibly adsorbed on the surface of pyrolytic graphite electrodes.9 Attachment via the pendant imidazole ligands apparently produces a more uniform array of electroactive sites than results when the same porphyrin is adsorbed directly on the surface of graphite electrodes. The area under the anodic peak of the steady voltammogram in Figure 1A corresponds to ca. 2 × 10-11 mol cm-2 of CoTpivPP on the electrode. This quantity is about 1/ as large as that reported by Offord et al.5 for the 3 coordinative attachment of a Ru porphyrin to a SAM prepared from the same imidazole-terminated thiol. The smaller value obtained with CoTpivPP probably reflects both the larger size of this porphyrin and the higher affinity of Ru(II) than of Co(III) for the imidazole attachment site. The large difference in the cross-sectional areas of a terminal imidazole group and of the porphyrin coordinated to it leads to a significant mismatch between the number of imidazole groups in the SAM coatings and the number of CoTpivPP molecules that can be coordinated to them.5 This mismatch is indicated schematically in Scheme 1, which also shows the coordination of the porphyrin to the coating on the unencumbered side of the CoTpivPP. This depiction is based on the known coordination geometry of (1-MeIm)CoIITpivPP4, but the presently available data do not exclude (the less likely) coordination to the encumbered side of [CoIIITpivPP]+. The steady-state response shown in the innermost solid curve in Figure 1A is quite stable: The magnitude of the peak currents decreased by less than 10% after 12 h of continuous scanning, and negligible decreases occurred even if the electrode potential was maintained for several minutes at -0.4 V where the cobalt center is present as Co(II). The persistence of the reduced porphyrin, (1MDIm)CoIITpivPP, on the electrode surface is remarkable considering the relatively small value (3 × 103 M-1) of the equilibrium constant for the coordination of 1-methylimidazole to CoIITpivPP (in benzonitrile).9 It may be that the high affinity of the Co(III) porphyrin for the imidazole ligand9 produces a porphyrin layer that inhibits access of alternative axial ligands (H2O or ClO4-) that presumably participate in the removal of the porphyrin from the electrode surface by attacking the coordination bond between the Co(II) center and the pendant imidazole groups. The insolubility of the porphyrin in aqueous solutions may also contribute to the stability of the porphyrin coordinated to the 1-MDIm SAM. This possibility is consistent with the observation that exposure of the coordinated porphyrin coatings to organic solvents
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Figure 2. (A) Steady-state cyclic voltammogram for CoTpivPP coordinated to a 1-MDIm coating on a gold film electrode. The dashed curve is the response before the coating was exposed to the cobalt porphyrin. (B) Spectroelectrochemical data for a coating like the one used to record (A). The electrode potential was stepped sequentially from 0.3 to -0.6 V in 1 M NaClO4 under argon. The spectra shown correspond to potentials of 0.30, 0.10, 0, -0.10, -0.30, -0.35, -0.40, -0.50, and -0.60 V (see text). The plotted absorbances are the differences between the absorbances measured with the attached CoTpivPP and the background spectrum that was recorded (and stored) before the cobalt porphyrin was attached. (C) Variation in the absorbance at 440 nm of a coated gold film electrode like the ones used in (A) and (B). The absorbances were recorded as the electrode potential was being scanned from 0.3 to -0.6 to 0.3 V at 0.1 V s-1. The potential changed by ∼30 mV during the recording of each absorbance value. The absorbance values were plotted at the potential corresponding to the center of each 30 mV interval. The solid curve was added to guide the eye.
in which CoIITpivPP is soluble, for example, benzene or tetrahydrofuran, leads to rapid loss of the porphyrin from the coatings. Whatever its origin, the stability of the layer of attached (1-MDIm)CoTpivPP is impressive. The final response shown in Figure 1A survives even if the electrode is exposed for several minutes to 1 M HClO4 where uncoordinated imidazole groups are protonated. Spectroelectrochemistry of the Attached CoTpivPP. To monitor the oxidation state of the cobalt center of the attached porphyrin, a transparent Au film electrode was employed instead of the Au sphere electrode. The procedures for coating the electrode with 1-MDIm and for coordinating the cobalt porphyrin to the coating were essentially the same as those that were used with the Au sphere electrodes (vide supra) except that after exposure to the solution of CoTpivPP, the uncoated sides of the glass slides were carefully cleaned with methanol-soaked cotton swabs to remove any porphyrin adhering to the glass surface. A cyclic voltammogram obtained for the
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porphyrin attached to the 1-MDIm-coated Au film is shown in Figure 2A. As with coated Au sphere electrodes (Figure 1), voltammograms obtained with the Au films typically exhibited both a reversible and an irreversible response. However, in contrast with Au sphere electrodes coated with 1-MDIm (Figure 1A), the irreversible cathodic peak near -0.45 V persists during repeated cycling of the potential of the gold film electrode. The two responses in Figure 2A reflect the persistent presence of both singly and doubly ligated cobalt(III) centers in the attached porphyrin. Coatings such as the one that gave rise to the voltammogram in Figure 2A were examined spectroelectrochemically. A typical set of spectra is shown in Figure 2B. Each spectrum was recorded 1-2 min after the electrode potential had been adjusted to a new value. Longer waiting periods produced insignificant changes in the spectra. The absorbance values plotted in Figure 2B are the differences between the absorbances of the 1-MDIm-coated electrode before and after CoTpivPP was coordinated to the coating. Typical raw spectra of the type used to obtain the curves in Figure 2B are shown in Figure S1 (Supporting Information). The two absorption maxima at 416 and 440 nm correspond to the Co(II) and Co(III) oxidation states based on comparison with spectra measured in 1:1 benzene/methanol solutions (Figure S2, Supporting Information). The absorbance value at 416 nm in Figure 2B was used to estimate the quantity of porphyrin on the electrode surface using the molar absorbance of 2.3 × 105 M-1 cm-1 obtained from the solution spectra. The value obtained, 4.3 × 10-11 mol cm-2, is somewhat larger than that estimated from the area under the peak in Figure 1A (2 × 10-11 mol cm-2), which may indicate an increased molar absorbance of the attached porphyrin. To observe the reduction of the porphyrin coordinated to the 1-MDIm SAM from Co(III) to Co(II), the changes in the absorbance of the coating at 440 nm were monitored as the electrode potential was scanned from 0.3 to -0.6 to 0.3 V, as in Figure 2A. The results, shown in Figure 2C, are in accord with expectations based on the voltammogram in Figure 2A: The absorbance is steady at potentials positive of ∼0.1 V, but it decreases abruptly between 0.1 and -0.1 V as the [(1-MDIm)CoIIITpivPP]+ in the coating is reduced; between -0.1 and -0.35 V, the absorbance is relatively constant as expected from the absence of cathodic current in this potential range in Figure 2A and a second decrease in absorbance commences near -0.35 V where the [(1-MDIm)2CoIIITpivPP]+ in the coating is reduced. Following reversal of the direction of the potential scan at -0.6 V, there are only small changes in absorbance until about -0.1 V where the reoxidation of (1-MDIm)CoIITpivPP produces a notable increase in absorbance. The final absorbance matches its initial value showing that [(1-MDIm)2CoIIITpivPP]+ is also regenerated during the reverse scan. The increase in absorbance occurs less abruptly than did the absorbance decrease in the first half of the cycle because of the sluggish rate of coordination of the second 1-MDIm group to the cobalt center. When the absorbance was monitored at 416 nm to follow changes in the quantity of (1-MDIm)CoIITpivPP in the coating, the data conformed to the mirror image of Figure 2C. Additional Ligand Coordination to Attached CoTpivPP. The attachment of CoTpivPP to 1-MDIm coatings as depicted in Scheme 1 leaves the sixth coordination site on Co available for coordination of other ligands. In homogeneous solution, this site, which is surrounded by the four groups that constitute the “pickets”, does not coordinate 1-methylimidazole when the cobalt center is present as Co(II).4 However, when the center is oxidized
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phyrin, so the addition of pyrazine causes a smaller negative shift in the position of the cathodic peak (curve 2 in Figure 3B). The anodic current peaks observed after the direction of the scan is reversed are somewhat broader than their cathodic counterparts. This behavior reflects the sluggish rates of the coordination reactions that determine the positions of the anodic response. Discussion
Figure 3. (A) (1) Steady-state cyclic voltammogram for CoTpivPP coordinated to the pendant imidazole groups of a SAM prepared as in Figure 1B. Supporting electrolyte: 1 M NaClO4. Scan rate: 0.1 V s-1. (2) Repeat after the supporting electrolyte was made 1 mM in imidazole. (3) Repeat after the electrode used to record curve 2 was transferred to pure 1 M NaClO4 supporting electrolyte. (B) (1) As in (A) with a freshly prepared SAM coating. (2) Repeat after the supporting electrolyte was made 10 mM in pyrazine.
to Co(III) the imidazole is strongly bound to this site.9 To determine if similar coordination occurs with the attached porphyrin, the effect of dissolved imidazole on the cyclic voltammetric response was inspected. Shown in Figure 3A are the results produced by the addition of imidazole, Im, to the supporting electrolyte solution. The response obtained in the absence of imidazole (curve 1) resembles that in Figure 1B. In the presence of 1 mM imidazole (curve 2), the cathodic peak near 0 V is much diminished while that near -0.5 is enhanced, as is the anodic peak near 0 V. This behavior is as expected if the added imidazole converts much of the singly ligated [(1-MDIm)CoIIITpivPP]+ complex into its doubly ligated form, [(1MDIm)(Im)CoIIITpivPP]+. Reduction of the latter complex near -0.5 V yields only [(1-MDIm)CoIITpivPP], most of which is oxidized to [(1-MDIm)CoIIITpivPP]+ to produce the anodic peak near 0 V in curve 2. The small increase in anodic current at potentials ahead of the main anodic peak is believed to reflect the much slower formation of [(1-MDIm)(Im)CoIIITpivPP]+. The dissolved imidazole not only affects the state of coordination of the attached Co(III) porphyrin, but it also causes gradual detachment of the porphyrin from the (1MDIm) SAM: As is shown in curve 3 in Figure 3A, transfer of the electrode used to record curve 2 to an imidazolefree supporting electrolyte leads to a diminished cyclic voltammetric response, as expected if a portion of the porphyrin initially present were lost from the electrode surface during the recording of curve 2. Similar changes in the cyclic voltammetric response resulted in the presence of pyrazine, as shown in Figure 3B. Pyrazine is bound less strongly than imidazole by the Co(III) por-
That the steps depicted in Scheme 1 can be used to accomplish the coordinative attachment of CoTpivPP to an imidazole-terminated SAM was not surprising because the approach was similar to that employed successfully by Offord et al. to bind Ru and Os porphyrins to Au electrodes.5 What was surprising was the stability of the resulting attached (1-MDIm)CoTpivPP in both the Co(II) and Co(III) oxidation states. The Ru and Os porphyrins of Offord et al.5 involve metal centers that are substitutionally inert, while Co(II) porphyrins typically exhibit labile coordination of axial ligands and even Co(III) porphyrins undergo moderately labile exchange reactions of axial ligands.12,13 The imperviousness of the layer of closely packed, hydrophobic cobalt porphyrins to potential bond-breaking reactants in the aqueous solution and the insolubility of CoTpivPP in water are probably responsible for the stability of the Co-imidazole bonds. The spectroelectrochemical data in Figure 2 are the first of their type for cobalt porphyrins attached to an electrode surface. They demonstrate the presence in the 1-MDIm coatings of two forms of the oxidized porphyrin which are reduced at well-separated potentials. 1-MDIm coatings prepared on Au sphere electrodes appeared to be freer of defects than those obtained on the thin Au films required for the spectroelectrochemical experiments. The latter coatings appeared to offer less uniform arrays of coordinated porphyrins than the former. Thus, cyclic voltammetric responses from CoTpivPP attached to 1-MDIm-coated Au sphere electrodes exhibited only a single reversible couple after the initial coating was subjected to a few potential cycles (Figure 1A) while a pair of well-separated responses was always present with coatings on Au films (Figure 2A). The susceptibility to axial ligation at the sixth coordination site of the [(1-MDIm)CoIIITpivPP]+ bound coordinatively to the SAM coating was demonstrated by the data in Figure 3. Thus, although the axial coordination site involved in the attachment of the CoTpivPP to the pendant imidazole group of the SAM is apparently not readily accessible to reactants (H2O, H+, ligands) in the solution, the trans axial coordination site is fully accessible. This feature could be useful in experiments directed at the coordination of O2 to the attached CoTpivPP. Conclusions Attachment of [Co TpivPP]+ to coatings of 1-MDIm on Au electrodes via CoIII-imidazole coordination was demonstrated. Once achieved, the attachment is remarkably stable as the (1-MDIm)CoTpivPP is cycled between the Co(III) and Co(II) oxidation states. The stability of saturated coatings was attributed to the inaccessibility of the Co-imidazole linkage to attack by potential alternative ligands from solution and to the insolubility of the porphyrin in aqueous media. The coordination position trans to the axial attachment site is accessible to ligands from solution. III
(12) Shi, C.; Anson, F. C. Inorg. Chem. 1998, 37, 1037-1043. (13) Fleishcher, E. B.; Jacobs, S.; Mestichelli, L. J. Am. Chem. Soc. 1968, 90, 2527-2531.
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Acknowledgment. This work was supported by the National Science Foundation via Grant CHE-9816329 and by the Laboratory for Molecular Science, an NSF-supported facility at Caltech. Helpful discussions with Dr. Beat Steiger, Professor Ahmed Zewail, and Professor Jim Hutchinson are acknowledged with pleasure as is the assistance of Dr. Robert Rossi and Joseph Nemanick in the preparation of gold film electrodes.
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Supporting Information Available: Two figures giving the raw spectra of 1-MDIm-coated Au film electrodes before and after the attachment of CoTpivPP to them and the solution spectra of CoIITpivPP, [CoIIITpivPP]+, (1-methylIm)CoIITpivPP, and [(1-methylIm)2CoIIITpivPP]+ in 1:1 benzene/methanol. This material is available free of charge via the Internet at http://pubs.acs.org. LA011444R
