JOURNAL OF RAMAN SPECTROSCOPY, VOL. zyxwvutsr 22, 301-305 (1991) Surface-Enhanced Raman Scattering of 4-Picolylamine and Its [Fe(CN)5 zyx 1”- Complexes M. L. A. Temperini, J. C. Rubim and 0. Sala Instituto de Quimica da Universidadede Sao Paulo, C.P. 20780,01498, zyxwvu SBo Paulo, Brazil A. H. Jubert,* M. E. Chacoo Villalba and P. J. Aymonino Quimica Inorganica (QUINOR), Facultad de Ciencias Exactas, Universidad Nacional de La Plata, C.C. 962, zyx (1900) La Plata, Argentina The zyxwvutsr SER spectra of the complexes [ Fe(CN),(Qpicolylamine)I 3- and [ Fe2(CN),,(4-picolylamine)l zyx 6- adsorbed on a silver electrode were investigated. The analysis of the frequency shift of the CN stretching mode indicated that both molecules adsorb via one or more CN ligand groups. For potentials more positive than 0.0 V the CN stretch- ing frequency for both complexes moves towards higher wavenumbers characterizing the presence of Fe(II1) over the surface. The SER spectra of Qpicolylamine adsorbed on a silver electrode in solutions of pH 2.8, 6.5 and 10.3 were also investigated, From the data obtained it was concluded that 4-picolylamine in solution at pH 6.5 and 10.3 adsorbs on the electrode through the nitrogen of the pyridine ring. INTRODUCTION Recent interest in the chemical and physical properties of chemically modified electrodes stems from the concept that a molecular species with desirable proper- ties can be immobilized on a solid surface, conferring those properties on the new surface. Surface modifi- cation techniques can thus be utilized to convert rela- tively inexpensive, poor electrode materials into useful electrodes. Anodization of metal or semiconductor elec- trodes in the presence of cyanometallate complexes such as hexacyanoferrate(II1) produces mixed metal cyan- ometallate complexes of the general formula [M(NC)M’(CN), LJn- (where M and M’ are transition metals and L is a donor ligand).1-3 These types of deri- vatized surface reactions can stabilize an electrode against considerable surface reactions such as oxide for- mation, without inhibiting desirable interfacial charge- transfer processes. They are also useful as electrocatalysts and in sensor applications. Electro- chemical and spectroscopic investigations of these metal-metallocyanide surface complexes are therefore desirable. Complexes of the pentacyanoferrate(I1) moiety with aromatic N-heterocycles have been extensively studied. The isolation and characterization of 4-picolylamine (4-picam) derivatives of pentacyanoferrate(I1) as the sodium salts of mononuclear [Fe(CN),(4-picam)] - and binuclear, 4-picam-bridged [Fe,(CN),,,(4-pi~am)]~ has been reported4 The dissociative behaviour and the redox properties of these complexes have been reported, in addition to the correlation properties with similar complexes of related ligand~.,-~ Surface-enhanced Raman scattering (SERS) has been used in adsorption studies of metallic complexes on silver electrodes.’-” This technique also provides valu- * Researcher From the Comision de InvestigacionesCientificas de la Provincia de Buenos Aires. 03774486/91/05030145 $05.00 Q 1991 by John Wiley & Sons, Ltd. able information on redox reactions of complexes adsorbed on a silver In this work, using the SERS technique, it was possible to monitor the electrochemical processes of adsorption and oxidation-reduction processes of the [Fe(CN),(4-picarn)l3 - and [Fe,(CN),,(4-picam)16- complexes. In order to obtain additional information, the SER spectra of the ligand were also obtained at several pH values. EXPERIMENTAL The instruments used to obtain the Raman and SER spectra and electrochemical measurements have been described elsewhere.’ 4-Picam (Reily Tar and Chemical) was used after dis- tillation. Mononuclear [Na,Fe(CN),(Cpicam)] * 5H,O and binuclear [Fe2(CN),,(4-picarn)] . 5HzO were pre- pared according to the reported m e t h ~ d . ~ All the SER spectra were obtained for a concentra- tion of the complexes of M. Working solutions were prepared with doubly distilled water. 4-Picam sol- utions at pH 2.8, 6.5 and 10.3 were prepared by the careful addition of 1 M HCI and 1 M NaOH to an aqueous solution of the ligand. Solutions of the com- plexes were prepared by the dissolution of the sub- stances in borate buffer solution (pH 8.4). The working electrode was a silver rod (99.99% pure) with a geometric area of 0.20 cmz inserted in acrylic resin. A platinum foil of large area was used as an aux- iliary electrode and a saturated calomel electrode (SCE) as the reference electrode. Before each experiment the working electrode was polished with 600-mesh sand- paper and rinsed with doubly distilled water and the working solution was dearated for 10 min with nitro- gen. The same solutions were used to obtain cyclic volt- ammograms. Received 14 December 1990 Accepted 13 January 1991