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Langmuir 2006, 22, 9304-9312
Olefin Cross-Metathesis of a Vinyl-Terminated Self-Assembled Monolayer (SAM) on Au(111): Electrochemical Study Using a Ferrocenyl Redox Center Paula A. Brooksby,* Kelly H. Anderson, Alison J. Downard,* and Andrew D. Abell* MacDiarmid Institute for AdVanced Materials and Nanotechnology, Department of Chemistry, UniVersity of Canterbury, PriVate Bag 4800, Christchurch, New Zealand ReceiVed July 30, 2006 Self-assembled thiol monolayers bound to single-crystal Au(111) surfaces containing a terminal olefin have been prepared and used to monitor electrochemically the cross-metathesis (CM) between the surface and an olefin-terminated ferrocenyl (Fc) derivative from solution over time. Mixed SAM surfaces were prepared by first adsorbing a diluent for 2 days followed by the olefinic alkanethiol for known adsorption time intervals; three diluents of varying length were used. The oxidation peak areas from the voltammetry show the CM reaction yields a maximum amount of product at 100-150 min. Beyond this time, thiol desorption is apparent and the Fc oxidation peaks diminished. A kinetic simulation of the interfacial reactions involving CM and desorption reactions are described and aided in the interpretation of the voltammetric responses. The length of the diluent and the coverage of surface olefins were important factors in limiting undesirable self-CM reactions on the surface, and a model of the relationship between the diluent and surface concentration of olefin is described. This study shows that attention to monolayer formation and reaction conditions are important parameters when maximizing CM yields on surfaces.
Introduction Self-assembled monolayers (SAMs) are ordered molecular assemblies that chemisorb spontaneously onto suitable substrates, such as gold and silicon.1 SAMs, particularly thiolates adsorbed onto gold surfaces, are used extensively to investigate an endless array of interfacial phenomena, including fundamental electrontransfer studies, molecular wires, adhesion, corrosion, and sensors and for the fabrication of interfacial architectures comprising more than one chemical functionality on a surface.2 Functionality at the terminus of a SAM is typically incorporated into the thiolate prior to adsorption. Many groups have been introduced into SAMs in this way, but synthetic difficulties and destabilizing interactions from neighboring adsorbates are often associated with these methods.3 An alternative approach is to construct a SAM with a versatile “end group” that can be functionalized by chemical,1,4-7 electrochemical,8 or spectroscopic9 methods after adsorption. The advantage in fabricating a universal interface of this type is that it can be specifically modified to generate an array of desirable functionality. However, this approach presents some potential problems, including Au-S bond rupture and subsequent desorption under the reaction conditions and steric hindrance at the interface that can slow reaction rates or change mechanistic pathways and in some cases it may be difficult to remove bound intermediate molecular species. Knowing whether the functionalization reaction has worked can be problematic when the chemical groups involved are not easily detected. * To whom correspondence should be addressed. Phone: 64-3-3667001. Fax: 64-3-3642110. E-mail:
[email protected],
[email protected],
[email protected]. (1) Li, X.-M.; Huskens, J.; Reinhoudt, D. N. Nanotechnology 2003, 14, 1064. (2) Gooding, J. J.; Mearns, F.; Yang, W.; Liu, J. Electroanalysis 2003, 15, 81. (3) Lee, J. K.; Lee, K.-B.; Kim, D. J.; Choi, I. S. Langmuir 2003, 19, 8141. (4) Fabre, B.; Hauquier, F. J. Phys. Chem B 2006, 110, 6848. (5) Collman, J. P.; Devaraj, N. K.; Chidsey, C. E. D. Langmuir 2004, 20, 1051. (6) Yousaf, M. N.; Chan, E. W. L.; Mrksich, M. Angew. Chem., Int. Ed. 2000, 39, 1943. (7) Collman, J. P.; Devaraj, N. K.; Eberspacher, T. P. A.; Chidsey, C. E. D. Langmuir 2006, 22, 2457. (8) Kra¨mer, S.; Fuierer, R. R.; Gorman, C. B. Chem. ReV. 2003, 103, 4367. (9) Monsathaporn, S.; Effenberger, F. Langmuir 2004, 20, 10375.
Transition-metal-catalyzed cross-metathesis (CM) is widely used in solution chemistry to couple two terminal olefins with the release of ethene.3,10-12 In addition, CM between an olefinterminated SAM and a solution-phase olefin is known to proceed under relatively mild reaction conditions.3,10-13 Thus, surfacesolution CM provides an extremely versatile methodology with which to control interfacial functionality on monolayer-protected gold nanoparticles and to a lesser extent flat surfaces, particularly gold. Realization of the enormous potential of surface metathesis chemistry to construct interfacial architecture requires a fundamental understanding of the olefin-catalyst behavior at the interface. Some limited studies have appeared in this area,1,14-16 but detailed examinations of reactions at planar gold surfaces have not been reported. Existing work simply aims to identify the final surface structure after a defined time in which it is assumed metathesis is complete, typically in excess of 24 h of reaction time.12 The use of an electroactive redox center incorporated into the olefin-terminated solution reactant provides a potentially convenient method of monitoring the progress of surface CM. A ferrocene (Fc) center provides such a probe since its immediate environment provides characteristic potential-dependent behavior. The Fc/Fc+ redox potential in aqueous solution is typically observed between 0.2 and 0.5 V (SCE), the exact value being dependent on other functionalities at the SAM interface, concentration of Fc groups on the surface, and identity and concentration of the solution electrolyte.17 Specific electrochemi(10) Connon, S. J.; Blechert, S. Angew. Chem., Int. Ed. 2003, 42, 1900. (11) Samanta, D.; Faure, N.; Rondelez, F.; Sarkar, A. Chem. Commun.2003, 1186. (12) Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J. Am. Chem. Soc. 2003, 125, 11360. (13) Blackwell, H. E.; O’Leary, D. J.; Chatterjee, A. K.; Washenfelder, R. A.; Bussmann, D. A.; Grubbs, R. H. J. Am. Chem. Soc. 2000, 122, 58. (14) Dutta, S.; Perring, M.; Barrett, S.; Mitchell, M.; Kenis, P. J. A.; Bowden, N. B. Langmuir 2006, 22, 2146. (15) Perring, M.; Dutta, S.; Arafat, S.; Mitchell, M.; Kenis, P. J. A.; Bowden, N. B. Langmuir 2005, 21, 10537. (16) Liu, X.; Guo, S.; Mirkin, C. A. Angew. Chem., Int. Ed. 2003, 42, 4785. (17) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307.
10.1021/la062244o CCC: $33.50 © 2006 American Chemical Society Published on Web 09/22/2006
Olefin Cross-Metathesis of a Vinyl-Terminated SAM
cal studies of Grubb’s-catalyzed CM reactions at SAMs over time are not known aside from some work noting the appearance of a ferrocenyl peak at the conclusion of a metathesis procedure.11,16 It is clear that current understanding of CM at monolayers adsorbed onto gold is inadequate. Details relating to appropriate reaction time, surface concentration of olefin, and monolayer stability remain unknown. Much of the literature on surface CM reactions and gold relates to nanoparticle interfaces, which have only partial applicability to planar surfaces. Studies on planar gold surfaces are predominantly concerned with examining the final surface structure rather than identifying intermediates. To address this problem we designed and prepared an olefinterminated thiol and an olefin-terminated Fc derivative to study CM reactions on Au(111) over time and as a function of monolayer composition. The Fc functionality allows for reliable electrochemical quantification of the CM product on the monolayer. It is important to note that CM is not expected to yield significant product if the interface is sterically congested, that is, the ruthenium catalyst would be unable to approach and bind to the SAM olefin.14 We use mixed monolayers of olefin-terminated molecules combined with shorter chain thiols (the diluent) to alleviate this problem. This also allows the effect of changing the surface concentration of the terminal olefin, relative to the diluent, to be investigated.
Langmuir, Vol. 22, No. 22, 2006 9305 Scheme 1a
Experimental Section Chemicals. NaClO4‚H2O (Scharlau Chemie S. A.), HClO4 (70%, AnalaR), and sodium thiomethoxide (Aldrich) were used as received. Dichloromethane (DCM) was dried over CaH2 and then freshly distilled under nitrogen. Milli-Q water, >18 MΩ cm-1, was used for all aqueous solutions. Methanol (HPLC, Aldrich), ethanol, (HPLC, Aldrich), N,N-dimethylformamide (DMF,
