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Organometallic Electrodes: Modification of Electrode Surfaces through Cathodic Reduction of Cyclopentadienyldiazonium Complexes of Cobalt and Manganese Derek R. Laws,†, John Sheats,‡ Arnold L. Rheingold,§ and William E. Geiger*,† Department of Chemistry, University of Vermont, Burlington, Vermont 05405, ‡Department of Chemistry, Rider University, Lawrenceville, New Jersey, 08648, and §Department of Chemistry and Biochemistry, University of California at San Diego, San Diego, California 92093. Present address: Siemens Water Technologies Corp., 10 Technology Drive, Lowell, Massachusetts 01851. )

†

Received June 25, 2010. Revised Manuscript Received July 28, 2010 Two organometallic complexes having cyclopentadienyldiazonium ligands have been isolated and characterized by spectroscopy, X-ray crystallography, and electrochemistry. Both CoCp(η5-C5H4N2)2þ (22þ) and Mn(CO)3(η5-C5H4N2)þ (3þ) undergo facile cyclopentadienyldiazonium ligand-based one-electron reductions which liberate dinitrogen and result in strong binding of the cyclopentadienyl ligand to a glassy carbon surface, similar to the processes well established for organic aryldiazonium salts. The organometallic-modified electrodes are robust and have a thickness of approximately one monolayer (Γ = (2-4)  10-10 mol cm-2). Their voltammetric responses are as expected for a cobaltocenium-modified electrode, [CoCp(η5-C5H4-E)]þ, where Cp = cyclopentadienyl and E = electrode, and a “cymantrene”-modified electrode Mn(CO)3(η5-C5H4-E). The cobaltocenium electrode has two cathodic surface waves. The first (E1/2 = -1.34 V vs ferrocene) is highly reversible, whereas the second (Epc = -2.4 V) is not, consistent with the known behavior of cobaltocenium. The cymantrene-substituted electrode has a partially chemically reversible anodic wave at E1/2 = 0.96 V, also consistent with the behavior of its Mn(CO)3Cp parent. Many of the properties of aryl-modified electrodes, such as “blockage” of the voltammetric responses of test analytes, are also seen for the organometallic-modified electrodes. Surface-based substitution of a carbonyl group by a phosphite ligand, P(OR)3, R = Ph or Me, was observed when the cymantrene-modified electrode was anodically oxidized in the presence of a phosphite ligand. The successful grafting of organometallic moieties by direct bonding of a cyclopentadienyl ligand to electrode surfaces expands the chemical and electrochemical dimensions of diazonium-based modified electrodes.

Introduction Electrode surfaces to which aryl groups are strongly attached have attracted great interest since the original report in which their grafting onto glassy carbon (GC) was accomplished by cathodic reduction of the corresponding aryl diazonium cation.1 Driven in part by the attraction of preparing “permanent” electrode surfaces chemically modified by the properties of substituted aryls, a sophisticated understanding of aryl-modified electrodes has ensued, as described in a number of comprehensive reviews.2-4 Although a number of attachment strategies have been described, they usually involve the reaction of eq 1 (written for the benzene diazonium ion), in which one-electron reduction of the aryl diazonium ion is accompanied by loss of dinitrogen, providing an aryl radical which forms a strong (arguably covalent) bond with an atom on the electrode surface. ½C6 H5 N2 þ þ e - f C6 H5 • þ N2

ð1Þ

Some of the limitations that arise from the high oxidative reactivity and variable longevity of aryl diazonium ions have been overcome by employing in situ diazotization of an aryl *To whom correspondence should be addressed. E-mail: william.geiger@ uvm.edu. (1) Delamar, M.; Hitmi, R.; Pinson, J.; Sav
eant, J.-M. J. Am. Chem. Soc. 1992, 114, 5883. (2) Downard, A. J. Electroanalysis 2000, 12, 1085. (3) Pinson, J.; Podvorica, F. Chem. Soc. Rev. 2005, 34, 429. (4) McCreery, R. Chem. Rev. 2008, 108, 2646. (5) Morita, K.; Yamaguchi, A.; Teramae, N. J. J. Electroanal. Chem. 2004, 563, 249.

15010 DOI: 10.1021/la102579t

amine, followed by the cathodic deposition process, under either aqueous5-9 or nonaqueous10,11 conditions. In order to take advantage of how different functionalities might affect the chemical interactions between the modified interface and the solution substrates, effort has gone into expanding the types of aryl moieties that are fixed to the electrode,3,4 including, for example, anthraquinone-type systems.12 With the goal of providing a more fundamental alteration in the chemical makeup of the covalently attached layer, we have investigated the possibility of grafting organometallic cyclopentadienyl complexes by direct bonding of the ligand five-membered ring to the electrode. This approach would combine the durability of the modified layer with the rich13 and well-developed14 (6) Blankespoor, R.; Limoges, B.; Sch€ollhorn, B.; Syssa-Magal
e, J.-L.; Yazidi, D. Langmuir 2005, 21, 3362. (7) Corgier, B. P.; Marquette, C. A.; Blum, L. J. J. Am. Chem. Soc. 2005, 127, 18328. (8) Corgier, B. P.; Laurent, A.; Perriat, P.; Blum, L. J.; Marquette, C. A. Angew. Chem., Int. Ed. 2007, 46, 4108. (9) (a) Baranton, S.; B
elanger, D. J. Phys. Chem. B 2005, 109, 24401. (b) Lyskawa, J.; B
elanger, D. Chem. Mater. 2006, 18, 4755. (c) Breton, T.; B
elanger, D. Langmuir 2008, 24, 8711. (10) (a) Toupin, M.; Belanger, D. J. Phys. Chem. C 2007, 111, 5394. (b) Baranton, S.; Belanger, D. Electrochim. Acta 2008, 53, 6961. (c) Kullapere, M.; Seinberg, J.-M.; M€aeorg, U.; Maia, G.; Schiffrin, D. J.; Tammeveski, K. Electrochim. Acta 2009, 54, 1961. (11) Cline, K. K.; Baxter, L.; Lockwood, D.; Saylor, R.; Stalzer, A. J. Electroanal. Chem. 2009, 633, 283. (12) (a) Wildgoose, G. G.; Banks, C. E.; Leventis, H. C.; Compton, R. G. Microchim. Acta 2006, 152, 187. (b) Jurmann, G.; Schiffrin, D. J.; Tammeveski, K. Electrochim. Acta 2007, 53, 390. (c) Smith, R. D. L.; Pickup, P. G. Electrochim. Acta 2009, 54, 2305. (13) Astruc, D. Electron Transfer and Radical Processes in Transition-Metal Chemistry; VCH Publishers: New York, 1995. (14) Geiger, W. E. Organometallics 2007, 26, 5738.

Published on Web 08/20/2010

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redox chemistry of sandwich and half-sandwich transition metal complexes. An intrinsic limitation of any diazonium-based method is the requirement that the diazonium ion [R-N2þ] not be subject to intramolecular oxidation of the organic or organometallic “R” moiety by the highly electron-deficient N2þ group. This would seem to rule out the possibility of going through a diazoniumderivatized ferrocenene [FeCp(C5H4N2)]þ (Cp = η5-C5H5), 1þ,15 to attach the highly desirable16 ferrocenyl group. A more promising strategy was to focus on complexes in which the cyclopentadienyl-diazonium ligand is bonded to a metal which is already in a high oxidation state, or at least one that is relatively difficult to oxidize. An obvious candidate was the cobaltocenium complex [CoCp(η5-C5H4N2)]2þ, 22þ, which had been generated in situ for coupling reactions with phenols.17 It is extremely difficult to remove an electron from the formal Co(III) center in a cobaltocenium ion,18 with the E1/2 of the Co(III)/Co(IV) couple falling at ca. 2.7 V vs ferrocene.19 Once attached, the cobaltocenium moiety could be electrochemically monitored through its Nernstian Co(III)/Co(II) couple expected to fall at ca. -1.33 V vs ferrocene.20

Among half-sandwich complexes, our attention turned to [Mn(CO)3(η5-C5H4N2)]þ, 3þ, the diazotized derivative of Mn(CO)3Cp (“cymantrene”), which had been reported over four decades earlier.21 Although the metal center in a cymantrenyl complex is in the relatively low þ1 oxidation state, removal of an electron from it is thermodynamically relatively difficult (ca. 0.9 V vs ferrocene for cymantrene)22 owing to the strong electronwithdrawing power of the carbonyl ligands. Once the cymantrenyl moiety is attached, however, this anodic redox process could be used as an electrochemical monitor. The chemical reversibility of the [Mn(CO)3Cp]0/þ couple has been the subject of a recent paper.23 (15) Arguments have been made for the presence of a ferrocenediazonium reaction intermediate. See (a) Nesmeyanov, A. N.; Drozd, V. N.; Sazonova, V. A. Dokl. Akad. Nauk SSSR 1963, 150, 102. (b) Little, W. F.; Clark, A. K. J. Org. Chem. 1960, 25, 1979. (c) Weinmayr, V. J. Am. Chem. Soc. 1955, 77, 3012. (16) Connelly, N. G.; Geiger, W. E. Chem. Rev. 1996, 96, 877. (17) (a) Sheats, J. E.; Rausch, M. D. J. Org. Chem. 1970, 35, 3245–3249. (b) Sheats, J. E. In Organometallic Chemistry Reviews; Seyferth, D., Ed.; J. Organomet. Chem. Libr. 7; Elsevier Scientific: Amsterdam, 1979, p 498. (18) Sheats, J. E.; Miller, W.; Kirsch, T. J. Organomet. Chem. 1975, 91, 97. (19) Bard, A. J.; Garcia, E.; Kukharenko, S.; Strelets, V. V. Inorg. Chem. 1993, 32, 3528. (20) (a) Gritzner, G.; Kuta, J. Pure Appl. Chem. 1982, 54, 1527. (b) Stojanovic, R. S.; Bond, A. M. Anal. Chem. 1993, 65, 56. (21) Cais, M.; Narkis, N. J. Organomet. Chem. 1965, 3, 269. (22) (a) Huang, Y.; Carpenter, G. B.; Sweigart, D. A.; Chung, Y. K.; Lee, B. Y. Organometallics 1995, 14, 1423. (b) Atwood, C. G.; Geiger, W. E.; Bitterwolf, T. E. J. Electroanal. Chem. 1995, 397, 279. (c) Hershberger, J. W.; Amatore, C.; Kochi, J. K. J. Organomet. Chem. 1983, 250, 345. (d) Kochi, J. K. J. Organomet. Chem. 1986, 300, 139. (e) Pickett, C. J.; Pletcher, D. J. Chem. Soc., Dalton Trans. 1976, 636. (23) Laws, D. R.; Chong, D.; Nash, K.; Rheingold, A. L.; Geiger, W. E. J. Am. Chem. Soc. 2008, 130, 9859. (24) For examples and leading references, see (a) Merz, A.; Bard, A. J. J. Am. Chem. Soc. 1978, 100, 3222. (b) Oyama, N.; Yap, K. B.; Anson, F. C. J. Electroanal. Chem. 1979, 100, 233. (c) Willman, K. W.; Rocklin, R. D.; Nowak, R.; Kuo, K.-N.; Schultz, F. A.; Murray, R. W. J. Am. Chem. Soc. 1980, 102, 7629. (d) Roullier, L.; Waldner, E.; Laviron, E. J. Electroanal. Chem. 1982, 139, 199. (e) Simon, R. A.; Mallouk, T. E.; Daube, K. A.; Wrighton, M. S. Inorg. Chem. 1985, 24, 3119. (f) Nishihara, H.; Noguchi, M.; Aramaki, K. Inorg. Chem. 1987, 26, 2862. (g) Chidsey, C. E. D. Science 1991, 251, 919. (h) Takada, K.; Diaz, D. J.; Abru~na, H. D.; Cuadrado, I.; Gonz
alez, B.; Casado, C. M.; Alonso, B.; Mor
an, M.; Losada, J. Chem.;Eur. J. 2001, 7, 1109. (i) Hudson, R. D. A. J. Organomet. Chem. 2001, 637-639, 47.

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As described in the present work, both the cobaltocenyl and cymantrenyl moieties have been successfully grafted onto glassy carbon (GC) electrodes by the diazonium-based attachment method. Although a number of other approaches have been reported involving linkage of a metallocenyl group to an electrode,24 the present results appear to be the first examples involving direct bonding of the cyclopentadienyl ligand to an electrode surface. A preliminary description of the cobalt system has been published.25

Experimental Section Materials. Reagent-grade dichloromethane, 1,2-dichloroethane, acetonitrile, and nitromethane used in electrochemical experiments were twice distilled from CaH2, with the second distillation being carried out, bulb-to-bulb, under static vacuum. Tetrahydrofuran (THF) was dried and purified in a similar manner using potassium. Before use, these solvents were subjected to three consecutive freeze-pump-thaw cycles in order to remove oxygen. Water was of nanograde purity. Glassware was heated at 120 °C for 24 h before use. FeCp2 (Cp = η5-C5H5), FeCp*2 (Cp* = η5-C5Me5), K4Fe(CN)6, Mn(CO)3Cp, and [CoCp2][PF6] were purchased from Strem Chemical Co. and used as received. p-Dinitrobenzene was used as received from Acros. [NBu4][PF6] was purchased from Acros, recrystallized three times from ethanol, and dried at 120 °C under vacuum for 24 h. [NBu4][B(C6F5)4]26 was prepared by metathesis using alkali metal salts obtained from Boulder Scientific Co. and thrice recrystallized from dicholoromethane/diethyl ether before being dried at 120 °C under vacuum for 24 h. [NBu4][BPh4] was purchased from Acros and recrystallized three times from dichloromethane/diethyl ether before vacuum drying at 120 °C. [NBu4][ClO4] (Aldrich) was recrystallized three times from ethyl acetate and dried at 80 °C under vacuum for 24 h. [CoCp(η5-C5H4N2)][PF6]2, [2][PF6]2. This compound was generated by oxidation of the corresponding amine, [CoCp(η5C5H4NH2)][PF6], as described by Sheats and Rausch.17 The diazonium species was isolated for the first time in the present work, which was carried out under ambient conditions. A total of 0.10 g (0.29 mmol) of [CoCp(η5-C5H4NH2)][PF6] was dissolved in 10 mL of 6 M HCl and cooled to 0 °C. To this was added 0.030 g (0.44 mmol) of NaNO2 dissolved in 1 mL of H2O at 0 °C, at which time a color change from yellow to dark orange was observed. After stirring the solution for 10 min, 0.20 g (1.23 mmol) of [NH4][PF6] dissolved in 2 mL of H2O was added dropwise, producing a yellow precipitate. After 10 min, the solid was filtered through paper, washed with cold H2O, and allowed to air-dry for 1 h, yielding 0.070 g (0.14 mmol, 48%) of the desired diazonium salt. Although caution should be exercised when handling all diazonium salts, there were no indications that this compound is particularly hazardous. The product was determined to be moderately air sensitive, decomposing to a black species over several weeks when exposed to air. Kept at 0 °C under nitrogen, [2][PF6]2 was stable for several months. The yellow solid was recrystallized from CH3NO2/CH2Cl2, and X-ray quality crystals were obtained in the same fashion. 1H NMR (CD3NO2): δ (ppm) 7.22 (t, 2H), 6.52 (t, 2H), 6.50 (s, 5H). IR (KBr): νNdNþ 2298 cm-1 (w), νPF6830 cm-1 (s). Anal. Calcd for [2][PF6]2: C, 23.73; H, 1.79; N, 5.54. Found: C, 24.27; H, 2.05; N, 5.52. [Mn(η5-C5H4N2)(CO)3][A] (A = PF6, or BF4), [3][PF6 or BF4]. Both salts were prepared from the corresponding amine

Mn(η5-C5H4NH2)(CO)3, which was synthesized according to a previously published procedure.27 The diazonium species had been previously reported as well,21 although the published procedure did not give a pure compound in our hands. For this (25) Swarts, J. C.; Laws, D.; Geiger, W. E. Organometallics 2005, 24, 341. (26) LeSuer, R. J.; Buttolph, C.; Geiger, W. E. Anal. Chem. 2004, 76, 6395. (27) Holovics, T. C.; Deplazes, S. F.; Toriyama, M.; Powell, D. R.; Lushington, G. H.; Barybin, M. V. Organometallics 2004, 23, 2927.

DOI: 10.1021/la102579t

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Article reason, a diazotization procedure similar to that described above for the cobaltocenium complex was adopted. A total of 0.10 g (0.46 mmol) of Mn(η5-C5H4NH2)(CO)3 was added to 12 mL of 6 M HCl, a medium in which it is only sparingly soluble. The mixture was cooled to 0 °C, and 0.065 g (0.94 mmol) of NaNO2 dissolved in 1 mL of H2O at 0 °C was added. The color immediately changed from yellow to dark red, with some gas evolution. After 5 min, the small amount of undissolved starting material was removed by filtration through glass wool. To the filtrate, 0.150 g (0.92 mmol) of [NH4][PF6], dissolved in 1 mL of H2O, was added dropwise, resulting in an orange precipitate. After 5 min, the product was filtered through paper, giving 0.103 g (0.27 mmol, 59%) of the [PF6-] salt. Although this salt appeared to be relatively stable in air as a solid, attempts at recrystallization (even under N2) resulted in its decomposition. 1H NMR (CD3NO2): δ (ppm) 6.60 (t, 2H), 5.63 (t, 2H). IR (CH3NO2): νCO 2058 cm-1 (s), 1992 cm-1 (s). Anal. Calcd for [3][PF6]: C, 25.55; H, 1.07; N, 7.45. Found: C, 26.43; H, 1.31; N, 6.84. For the preparation of [3][BF4], the same procedure was followed up to and including filtration with glass wool. At this point, 0.20 g of Na[BF4] was added (no precipitation was observed), and the solvent was evaporated to dryness on a rotary evaporator. The resulting orange solid was dissolved in 10 mL of nitromethane, leaving solid NaCl which was removed by filtration through glass wool. The desired [BF4-] salt was obtained by precipitation following addition of diethyl ether or simply by evaporation of the nitromethane. This salt is evidently more stable than the [PF6-] salt, as X-ray quality crystals were grown from a CH3NO2/CH2Cl2 solution kept at about 250 K under nitrogen over the course of 2 days. 1H NMR (CD3NO2): δ (ppm) 6.60 (t, 2H), 5.63 (t, 2H). IR (CH3NO2): νCO 2058 cm-1 (s), 1992 cm-1 (s). IR (KBr): νNdNþ 2261 cm-1 (w), νCO 2055 cm-1 (s), 1984 cm-1 (s), νBF4- 1085 cm-1 (s), 1050 cm-1 (s). Anal. Calcd for [Mn(η5-C5H4N2)(CO)3][BF4]: C, 30.23; H, 1.27; N, 8.82. Found: C, 27.90; H, 1.91; N, 7.81. The reason for the deviation in % C in the elemental analysis was not probed, but the integrity of the assigned structure was confirmed by the X-ray data. No explosive behavior was encountered with either [3][PF6] or [3][BF4], although we should note that no more than 100 mg was generated at any one time. Electrochemistry. All electrochemistry except that involving aqueous solutions was conducted in a Vacuum Atmospheres drybox under nitrogen, using a Princeton Applied Research model 273A potentiostat interfaced to a personal computer. Most experiments used glassy carbon (GC) working electrodes supplied by Bioanalytical Systems. For these electrodes, surfaces were pretreated using a standard sequence of polishing with diamond paste (Buehler) of decreasing sizes (3-0.25 μm) interspersed by washings with nanopure water. Electrodes to be used for surface modification experiments were next sonicated in water, acetone, and last isopropanol each for 3 min. The experimental reference electrode was a Ag/AgCl wire separated from solution by a fine frit, but all potentials are reported versus the FeCp20/þ redox couple obtained by using ferrocene as an internal standard. To obtain reference potentials for modified surfaces, electrodes were polished after an experiment and the potential of the FeCp20/þ couple was measured at the unmodified surface. Pyrolized photoresist film (PPF) electrodes were prepared at the Ohio State University by spin coating AZ4330 photoresist on silicon wafers, followed by pyrolysis at 1000 °C for 60 min under 5% H2 in N2. Electrodes were then sonicated as above for GC electrodes. Following surface modification procedures, electrodes were always rinsed and sonicated for 3 min. The rinsing solvent was acetone in the case of cobaltocenium modifications, whereas dichloroethane was so employed for cymantrene modifications. This part of the procedure was conducted under ambient air. Instrumentation. 1H NMR spectra were acquired on a Bruker ARX 500 MHz spectrometer, and solution IR data were acquired on an ATI-Mattson infinity series FT-IR instrument interfaced to a computer employing Winfirst software at a resolution of 4 cm-1. Attenuated total reflectance Fourier transform infrared 15012 DOI: 10.1021/la102579t

Laws et al. (ATR-FTIR) was carried out by Dr. Franklin Anariba using a Bruker Tensor 27 FT-IR instrument with a spectral resolution of 4 cm-1.28 Accessories used were Ge total reflection (GATR; 65° incident angle relative to surface normal, Harrick Scientific) and a liquid-N2-cooled MCT detector. In the GATR accessory, the pyrolized photoresist film (PPF)29 substrate was positioned in contact with the flat surface of the hemispherical Ge crystal that functions as the ATR element. A PPF spectrum was obtained from a clean, unmodified surface and subtracted from the modified PPF spectrum. Survey and regional XPS spectra were acquired with a VG Scientific Escalab MKII spectrometer using a Mg anode. Samples were placed in the loading chamber, under vacuum, for at least 12 h prior to testing. Atomic ratios were calculated from peak areas and were corrected for elemental sensitivity factors using the software provided with the instrument. We thank Richard L. McCreery for the use of his laboratory at the Ohio State University to carry out these experiments. X-ray Crystallography. Data were collected on a Bruker D8 platform diffractometer equipped with an APEX CCD detector. The structures were solved by Patterson projections. All nonhydrogen atoms were refined anisotropically, and the hydrogen atoms were placed in idealized locations. All software was contained in the SMART, SAINT, and SHELXTL libraries distributed by Bruker AXS, Madison, WI.

Results and Discussion Crystal Structures of [2][PF6]2 and [3][BF4]. To our knowledge, X-ray structures of organometallic diazonium complexes have not been previously reported. Crystalline [2][PF6]2 is a slightly air-sensitive orange solid. The structure of the dication is shown in Figure 1a, and Table 1 lists relevant bond distances and angles (with collection parameters and a complete data set located in the Supporting Information). The two Cp rings are almost completely eclipsed, in contrast to those in ferrocene,30a cobaltocene,30b carboxycobaltocenium,30c and other cobaltocenium derivatives,30d,e all of which exhibit a nearly perfectly staggered conformation in the solid state. At least two examples do exist for an eclipsed conformation, in a pyrrole-derivatized cobaltocenium analogue and in 1,10 -diaminocobaltocenium.31b Distances of Co-C bonds in [2][PF6]2 vary from 1.996 to 2.065 A˚, within the range observed for cobaltocenium analogues and slightly shorter than those observed for cobaltocene (2.079 to 2.111 A˚).32 C-C distances are between 1.412 and 1.433 A˚, within the range previously determined for cobaltocene and cobaltocenium species. C-C distances, as well as intraring C-C-C angles, do not differ significantly in going from the substituted to the unsubstituted ring. The two Cp rings in [2][PF6]2 are nonparallel, having a dihedral angle of approximately 3.20°, which likely results from the effect of the diazonium group. The carbons of a particular ring are in approximately the same plane. (28) Anariba, F.; Viswanathan, U.; Bocian, D. F.; McCreery, R. L. Anal. Chem. 2006, 78, 3104. (29) Ranganathan, S.; McCreery, R. L. Anal. Chem. 2001, 73, 893. (30) (a) Dunitz, D., J.; Orgel, L. E.; Rich, A. Acta Crystallogr. 1956, 9, 373. (b) Bunder, W.; Weiss, E. J. Organomet. Chem. 1975, 92, 65. (c) Riley, P. E.; Davis, R. J. Organomet. Chem. 1978, 152, 209. (d) Komatsuzaki, N.; Mitsunari, U.; Shirai, K.; Tanaka, T.; Sawada, M.; Takahashi, S. J. Organomet. Chem. 1995, 498, 53. (e) Beer, P. D.; Hesek, D.; Kingston, J. E.; Smith, D. K.; Stokes, S. E.; Drew, M. G. B. Organometallics 1995, 14, 3288. (31) (a) Cuadrado, I.; Casado, C.; Lobete, F.; Alonso, B.; Gonzalez, B.; Losada, J.; Amador, U. Organometallics 1999, 18, 4960. (b) Inyushin, S.; Shafir, A.; Sheats, J. E.; Minihane, M.; Whitten, C. E.; Arnold, J. Polyhedron 2004, 23, 2937. (32) (a) Bunder, W.; Weiss, E. J. Organomet. Chem. 1975, 92, 65. (b) Riley, P. E.; Davis, R. J. Organomet. Chem. 1978, 152, 209. (c) Komatsuzaki, N.; Mitsunari, U.; Shirai, K.; Tanaka, T.; Sawada, M.; Takahashi, S. J. Organomet. Chem. 1995, 498, 53. (d) Beer, P. D.; Hesek, D.; Kingston, J. E.; Smith, D. K.; Stokes, S. E. Organometallics 1995, 14, 3288. (e) Cuadrado, I.; Casado, C.; Lobete, F.; Alonso, B.; Gonzalez, B.; Losada, J.; Amador, U. Organometallics 1999, 18, 4960. (f) Inyushin, S.; Shafir, A.; Sheats, J. E.; Minihane, M.; Whitten, C. E.; Arnold, J. Polyhedron 2004, 23, 2937.

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Figure 1. Diagrams of the molecular structures of [2][PF6]2 and [3][BF4] with 50% probability ellipsoids. Hydrogen atoms and counteranions omitted for clarity. (a) [2][PF6]2, (b) [3][BF4] side view, and (c) [3][BF4] top view.

Table 1. Selected Bond Lengths (A˚) and Angles (deg) for [2][PF6]2

Table 2. Selected Bond Lengths (A˚) and Angles (deg) for [3][BF4]

Bond Lengths (A˚)

Bond Lengths (A˚)

Co-C(1) Co-C(2) Co-C(3) Co-C(4) Co-C(5) Co-C(6) Co-C(7) Co-C(8) Co-C(9) Co-C(10) N(1)-N(2) N(1)-C(1) C(1)-C(2) C(1)-C(5) C(2)-C(3) C(3)-C(4) C(4)-C(5) C(6)-C(7) C(6)-C(10) C(7)-C(8) C(8)-C(9) C(9)-C(10) Co-to-center C5N2 Co-to-center C5

1.9964(19) 2.058(2) 2.065(2) 2.053(2) 2.045(2) 2.037(2) 2.040(2) 2.032(2) 2.028(2) 2.030(2) 1.094(3) 1.389(3) 1.429(3)

1.412(3) 1.424(3) 1.425(3) 1.421(3) 1.646 1.633

Bond Angles (deg) N(2)-N(1)-C(1) Co-C(1)-N(1) N(1) out of C4 plane N(2) out of C4 plane

176.2(2) 127.85 3.67 5.22

Crystalline [3][BF4], the structure of which is shown in Figure 1b and c, is a mildly air-sensitive orange solid. Table 2 lists relevant bond distances and angles, with collection parameters and a complete data set located in the Supporting Information. An interesting feature of this structure is the high degree of symmetry that is present, which is striking in comparison to members of the cymantrene family such as Mn(CO)3Cp and Mn(CO)3(η5-C5H4NH2).23 In the present case, the C(2)-C(1)-N(1) and C(5)-C(1)-N(1) angles are identical (unlike 22þ), and perfect mirror symmetry is Langmuir 2010, 26(18), 15010–15021

Mn-C(1) Mn-C(2) Mn-C(3) Mn-C(4) Mn-C(5) Mn-C(6) Mn-C(7) Mn-C(8) C(6)-O(1) C(7)-O(2) C(8)-O(3) N(1)-N(2) N(1)-C(1) C(1)-C(2) C(1)-C(5) C(2)-C(3) C(3)-C(4) C(4)-C(5) Mn-to-center C4

2.078(3) 2.1286(18) 2.1493(18) 2.1493(18) 2.1286(18) 1.8195(19) 1.8195(19) 1.801(3) 1.128(2) 1.128(2) 1.143(4) 1.094(4) 1.380(4) 1.423(2)

1.390(3) 1.757

Bond Angles (deg) N(2)-N(1)-C(1) Mn-C(1)-N(1) Mn-C(6)-O(1) Mn-C(7)-O(2) Mn-C(8)-O(3) C(6)-Mn-C(7) C(7)-Mn-C(8) C(8)-Mn-C(6) N(1) out of C4 plane N(2) out of C4 plane

180.0(3) 125.65(19) 178.84(17) 178.84(17) 178.3(3) 91.67(11) 90.76(8) 90.76(8) 3.91 3.80

observed about Mn, the C(8)-O(3) carbonyl group, and both N(1) and N(2) of the diazonium group. Mn(CO)3Cp* reportedly exhibits mirror symmetry as well, though acquisition conditions were poor for the structure determination.33 In cymantrene, the overall orientation is similar, but the carbonyl bond is shifted 5.93° (33) Fortier, S.; Baird, M. C.; Preston, K. F.; Morton, J. R.; Ziegler, T.; Jaegar, T. J.; Watkins, W. C.; MacNeil, J. H.; Watson, K. A.; Hensel, K.; Le Page, Y.; Charland, J.; Williams, A. J. J. Am. Chem. Soc. 1991, 113, 542.

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out of a perfect eclipsed arrangement.34 Mn(CO)3(η5-C5H4NH2) does not possess this degree of symmetry.23 The three Mn-C(O) bonds of 3þ are close to the expected 180° for a perfect piano stool orientation, and C(O)-Mn-C(O) bonds are all close to the expected 90°. Mn-C(O) bond lengths range from 1.801 to 1.8195 A˚, slightly longer than distances observed in Mn (CO)3Cp (average Mn-C(O) = 1.780 A˚), Mn(CO)3(η5-C5H4NH2) (average Mn-C(O) = 1.793 A˚), or Mn(CO)3Cp* (average Mn-C(O) = 1.729 A˚). This may be due to the electropositive effect of the N2 group. The metal-to-ring distance of 1.757 A˚ is shorter than distances observed for Mn(CO)3Cp, Mn(CO)3Cp*, [Mn(CO)3Cp*]þ, Mn(CO)3(η5-C5H4NH2), or [Mn(CO)3(η5-C5H4NH2)]þ, so it is unlikely that a weakening of the η5 bond occurs as a result of the diazonium functionality. All ring carbons lie in approximately the same plane. In both 22þ and 3þ, nitrogen N(1) is bent away from the metal center, out of the Cp plane by essentially the same distance, at an angle of 3.91°. C-N and N-N bond distances in both structures are similar to those observed for aryl diazonium salts.35 Electrochemistry of [2][PF6]2 and [3][PF6]. Dozens of publications have reported the loss of dinitrogen that is coupled to the facile one-electron reduction of aryl diazonium species.1-4 The resulting aryl radical is available to react with an electrode atom to create a rigidly bonded modified surface that is often referred to as having a covalent linkage.3 Here we present evidence for analogous behavior of cyclopentadienyl diazonium reductions. The relevant electrochemical and chemical reactions as they relate to these complexes are denoted in eqs 2 and 3, where ML is either CoCpþ or Mn(CO)3. MLðη5 -C5 H4 N2 Þþ þ e - f MLðη5 -C5 H4 Þ• þ N2

ð2Þ

MLðη5 -C5 H4 Þ• þ electrode ðEÞ f MLðη5 -C5 H4 -EÞ

ð3Þ

In principle, ML may be any 13-electron organometallic moiety that is not subject to self-oxidation by the diazonium group. It is to be expected that the radical-electrode reaction of eq 3 will be in competition with the radical-solvent (SH) reaction of eq 4 which would result in formation of an unsubstituted cyclopentadienyl complex, MLCp, in solution. The efficiency of the deposition process will depend on the relative rates of the reactions in eqs 3 and 4. MLðη5 -C5 H4 Þ• þ SH f MLðη5 -C5 H5 Þ þ S• 2þ

ð4Þ

þ

The 18-electron complexes 2 and 3 both possess only limited stability in CH3CN, the solvent most commonly employed for electrochemical work with aryl diazonium salts.2 Solutions containing millimolar amounts of these organometallic compounds undergo color changes within minutes of dissolution in room-temperature acetonitrile, and in CD3CN the decomposition of both compounds was evidenced by significant changes in their NMR spectra.36 Thus, the deposition processes were carried out in other solvents. Encouraged by NMR data which established the long-term stability of both 22þ and 3þ in CD3NO2, we turned (34) Fitzpatrick, P. J.; Le Page, Y.; Sedman, J.; Butler, I. S. Inorg. Chem. 1981, 20, 2852. (35) (a) Glaser, R.; Horan, C. J. Can. J. Chem. 1996, 74, 1200. (b) Wallis, J. D.; Easton, R. J. C.; Dunitz, J. D. Helv. Chim. Acta 1993, 76, 1411. (c) Glaser, R.; Chen, G. S.; Barnes, C. L. Angew. Chem., Int. Ed. Engl. 1992, 31, 740. (d) Glaser, R.; Horan, C. J.; Nelson, E. D.; Hall, M. K. J. Org. Chem. 1992, 57, 215. (36) For 22þ, the decomposition in CH3CN is not as rapid as that for 3þ, and the first several CV scans of it in acetonitrile are almost certainly representative of the pure compound.

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Figure 2. (a) CV of 3 mM [2][PF6]2 in CH3CN/0.1 M [NBu4][PF6]. 2 mm GCE. (b) CV of 1 mM [2][PF6]2 in CH3NO2/0.1 M [NBu4][PF6]. 2 mm GCE. (c) Same conditions as (b) but with five consecutive scans.

to nitromethane as a solvent for deposition of both metal systems. Dichloroethane also proved suitable for the manganese system. Owing to the historical importance of acetonitrile as an electrolysis solvent for aryl diazonium ions, we include some voltammetry data here for the organometallic diazonium ions in this solvent. The first CV scan of [2][PF6]2 at a GC electrode in a very fresh acetonitrile solution with [NBu4][PF6] as the supporting electrolyte showed two prominent reduction events (see Figure 2a). The first, at Epc ca. -0.2 V, is highly irreversible and is assigned to the reduction of the diazotized ligand in the complex (eq 2, ML = CoCpþ). The second, at E1/2 = -1.34 V, is reversible and can be Langmuir 2010, 26(18), 15010–15021

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Article Scheme 1

attributed to the cobaltocenium/cobaltocene couple both in the bulk of solution (formed through the sequence of eqs 2 and 4, ML = CoCpþ) and on the electrode surface (formed through the sequence of eqs 2 and 3, ML = CoCpþ). There is an additional minor cathodic feature at Epc ca. -1.1 V, the origin of which is unknown. For reasons already stated, quantitative deposition studies were carried out using nitromethane. A CV scan of [2][PF6]2 in CH3NO2/[NBu4][PF6] is shown in Figure 2b. In this case, the diazotized ligand reduction had an onset at ca. 0.0 V, with the cobaltocenium reduction again being observed at E1/2 = -1.35 V. Figure 2c shows consecutive CV scans in this medium. Following the first scan, a new feature grows in at ca. Ep = -1.0 V as the original broad cathodic wave of 22þ decreases. Owing to the fact that the species responsible for this feature is removed by postelectrolysis sonication, its identity was not investigated. The rising portion of the CV wave at ca. -1.5 V arises from reduction of the nitromethane solvent. The deposition process is shown graphically in Scheme 1. The cathodic electrochemistry of [3][PF6] is similar to that of [2][PF6]2 in that a facile but irreversible reduction is observed in nitromethane (Epc ca. -0.3 V), dichloroethane (Epc ca. -0.5 V), benzonitrile (Epc ca. -0.2 V), and acetonitrile (Epc ca. -0.2 V). Representative CVs in dichloroethane and acetonitrile are shown in Figure 3. Once again, the diazonium-based reduction gave rise to surface coating, as evidenced by electrode passivation observed upon successive CV scans. In this case, the redox process for the deposited group, the cymantrenyl moiety Mn(CO)3(η5-C5H4-E) (E = electrode), is not observed owing to the fact that it is not electroactive in this potential range (the reduction of cymantrene, Mn(CO)3Cp, occurs at ca. -2.8 V).37 As will be detailed below, however, an anodic wave for oxidation of the deposited cymantrenyl moiety was observed at E1/2 = 0.96 V, appropriate for the one-electron oxidation of a Mn(CO)3(η5-C5H4R) compound (E1/2 = 0.92 V for R = H).23 An anodic oxidation process was also observed for [3][PF6] in homogeneous solution. Initiating at 0.4 V and scanning positive in dichloroethane/[NBu4][B(C6F5)4], a chemically reversible anodic wave was observed at E1/2 = 1.02 V, attributed to the oneelectron oxidation of the organometallic center in 3þ. The positive shift in E1/2 compared to that of Mn(CO)3(η5-C5H5) is a consequence of the electron-withdrawing effect of the diazonium ion. The overall electron-transfer sequence for 3þ is given in eq 5. 1:02V ½MnðCOÞ3 ðC5 H4 N2 Þ2þ s F R -e

32þ

þ - 0:2 to - 0:5V

½MnðCOÞ3 ðC5 H4 N2 Þ s f 3þ

þe

MnðCOÞ3 ðC5 H4 -EÞ þ N2 ð5Þ

(37) (a) Sawtelle, S. M.; Johnston, R. F.; Cook, C. C. Inorg. Chim. Acta 1994, 221, 85. (b) Salmain, M.; Jaouen, G.; Fiedler, J.; Sokolov
a, R.; Pospísil, L. Collect. Czech. Chem. Commun. 2001, 66, 155.

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One-Electron Reduction of 22þ and 3þ. It was difficult to carry out a voltammetric mechanistic analysis of the cathodic reduction of the two diazonium salts, owing to the partial electrode deposition that is inherent to a single voltammetry scan. Nevertheless, the peak potentials of the waves arising from the diazonium reduction were recorded as a function of CV scan rate, mechanically cleaning the electrode between each scan. A negative shift of 111 mV in the plot of Epc vs log scan rate (Supporting Information, Figure S1) was observed for 22þ, consistent with a totally irreversible electron-transfer step in eq 2 and a transmission coefficient, R, of 0.27. However, the R2 value of the linear slope was only 0.91. A similar set of experiments on 3þ gave a linear slope of only 54 mV, consistent with R = 0.55. The diazonium charge-transfer reactions certainly appear to be electrochemically irreversible, consistent with the concomitant gain of an electron and loss of dinitrogen (eq 2), but the voltammetric data do not allow more specific conclusions about the fundamental one-electron process. Chemical reductions of 22þ and 3þ were also carried out in order to test the assumption that the primary homogeneous reactions of the neutral radicals ML(η5-C5H4)• would involve H-atom abstraction from solvent (eq 4). When an equivalent of decamethylferrocene was added to a solution of either 22þ or 3þ as a one-electron reductant, gas evolution occurred and the solution turned the green color of the oxidized byproduct, the decamethylferrocenium ion. When this experiment was carried out in CD3NO2 with 5 mM 22þ, 1H-NMR of the resulting solution showed that cobaltocenium was, indeed, the major product (δ = 5.78, singlet). When CH3NO2/0.1 M [NBu4][PF6] was used as the medium for the chemical reduction of 22þ, the cobaltocenium/ cobaltocene wave was observed in CV scans at an electrode inserted into a solution after the reaction. The reduction of 3þ by decamethylferrocene in either CH3NO2 or dichloroethane resulted in only one set of carbonyl IR bands at 2022 and 1934 cm-1, matching those of Mn(CO)3(η5-C5H5). These data support the idea that any neutral radical ML(η5-C5H4)• that does not form a bond with the electrode surface abstracts a hydrogen atom from the solvent to produce the unsubstituted Cp derivative, [CoCp2]þ in the case of 2þ and Mn(CO)3Cp in the case of 3þ. Electrode Modification with [2][PF6]2. The electrochemical reduction of [2][PF6]2 results in a chemically modified working electrode according to the reaction shown in Scheme 1. Only the results obtained using a nitromethane/[NBu4][PF6]-deposition medium are presented here, although surface films were successfully produced in acetonitrile (vide ante) and benzonitrile as well. Following electrode modification by either CV or controlled potential electrolysis (details below), electrodes were sonicated in acetone to remove any weakly bound species. Electrochemistry of the surface films was then studied in fresh solutions containing only solvent (acetonitrile, dichloroethane, dichloromethane, tetrahydrofuran, water) and supporting electrolyte ([NBu4][PF6], except for Li[ClO4] in water). In all cases, CV scans showed a reversible reduction at E1/2 values between -1.3 and -1.4 V (see Figure 4a). These are ascribed to the cobaltocenium/cobaltocene redox couple based on their potentials being close to those reported for [CoCp2][PF6] in nonaqueous solvents (E1/2 = -1.32 V in CH2Cl2/[NBu4][PF6], -1.35 V in CH3CN/[NBu4][BF4]).38,39 Cobaltocene is also known to undergo a one-electron reduction to the cobaltocene anion.39 In tetrahydrofuran, which has a more negative potential window than dichloromethane, the modified electrode exhibited the expected second reduction to the cobaltocene anion, Epc = -2.4 V (see Supporting Information, (38) Stojanovic, R. S.; Bond, A. M. Anal. Chem. 1993, 65, 56. (39) Geiger, W. E. J. Am. Chem. Soc. 1974, 96, 2632.

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Figure 3. (a) Successive CV scans of 1.1 mM [3][PF6] in dichloroethane/0.1 M [NBu4][PF6]. 1 mm GCE, ν = 0.1 V s-1. (b) Successive CV

scans of 0.2 mM [3][PF6] in CH3NO2/0.1 M [NBu4][PF6]. 1 mm GCE, ν = 0.1 V s-1.

Figure S2). However, this reduction was chemically irreversible, consistent with the high reactivity of the cobaltocene anion,39 so that scans were not generally carried to these more negative potentials. Film deposition was attempted at several different electrode surfaces, including Pt, Au, indium tin oxide, pyrolyzed photoresist films, and boron-doped diamond,40 with surface waves being observed in all cases. The quality of the surface wave was dependent on the electrode material, as demonstrated by a CV using a modified Pt electrode, shown in Figure 4b. (40) Boron-doped diamond electrodes were prepared by Dr. Heidi Martin, Case Western Reserve University, Department of Chemical Engineering. We thank Prof. Martin for these samples. (41) Harper, J. C.; Polsky, R.; Wheeler, D. R.; Lopez, D. M.; Arango, D. C.; Brozik, S. M. Langmuir 2009, 25, 3282. (42) (a) Kariuki, J. K.; McDermott, M. T. Langmuir 1999, 15, 6534. (b) Haccoun, J.; Vautrin-Ul, C.; Chausse, A.; Adenier, A. Prog. Org. Coat. 2008, 63, 18. (43) Paulik, M. G.; Brooksby, P. A.; Abell, A. D.; Downard, A. J. J. Phys. Chem. C 2007, 111, 7808. (44) Yu, S. S. C.; Tan, E. S. Q.; Jane, R. T.; Downard, A. J. Langmuir 2007, 23, 11074. For evidence of monolayer coverage of nitrophenyl-modified surfaces, see Brooksby, P. A.; Downard, A. J. Langmuir 2004, 20, 5038.

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A common occurrence in aryl diazonium surface modifications is multilayer film formation.28,41-44 In our work, the orientation and spacing of the films was not studied, so it is not known if they are close-packed. Therefore, measured surface coverages (Γ) of coated electrodes may not give an accurate measure of film thickness. Values of Γ were nonetheless measured using a CV integration method following deposition. The method required an inexact baseline (charging current) extrapolation, and therefore, the values are to be taken only as estimates. GC electrodes modified with a deposit from a single CV scan from 0.5 to -1.4 V at ν = 0.1 V s-1 gave Γ values ranging from 3  10-10 to 4  10-10 mol cm-2, close to the value of 4.3  10-10 mol cm-2 calculated for a “flat” close-packed monolayer of cobaltocenium ions in which the line between the two centroids of the cyclopentadienyl rings is roughly parallel to the electrode surface and separated by 3.4 A˚ (Scheme 1). The [PF6]- (or other) counterion needed to balance the charge of the [CoCp(η-C5H4-E)]þ cation is not included in this calculation, as its position is unknown. Langmuir 2010, 26(18), 15010–15021

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Figure 4. CVs of cobaltocenium modified electrodes in CH2Cl2/ 0.1 M [NBu4][PF6], ν = 0.1 V s-1. (a) GC electrode that was modified in CH3NO2 containing 1.0 mM [2][PF6]2 for 300 s at Eapp = -0.9 V. (b) Pt electrode that was modified in CH3NO2 containing 1.0 mM [2][PF6]2 for 100 s at Eapp = -0.7 V.

To study the extent of surface coverage as it relates to deposition time or potential, controlled potential electrolysis was used for deposition. According to Figure 5a, which shows Γint (surface coverage as determined by CV integration) to be consistent with the figure for electrodes coated for different electrolysis times at Eapp = -0.52 V, coverage increased over the first ∼200 s and then approached a maximum value close to 6  10-10 mol cm-2, slightly above the value expected for a flat monolayer. There was also some dependence of coverage on the electrolysis potential, a factor that had been noted earlier for nitrophenyl depositions.42-44 Maximum coverage was achieved with Eapp = -1.1 V (Γint ca. 7  10-10 mol cm-2), but apparently decreased coverage occurred at even more negative deposition potentials (Figure 5b). This latter effect is likely due to the fact that surface-bound cobaltocenium is reduced to the more reactive cobaltocene at such negative potentials, possibly resulting in an unknown follow-up reaction product terminating in a cobalt-cyclopentadienyl or cobalt-cyclopentadiene complex which is not electroactive in the potential range scanned. An analogous explanation involving follow-up products has been offered for nitrophenyl depositions performed at potentials negative of the E1/2 of nitrobenzene.42 The cobaltocenium films are robust, exhibiting little change in electrochemical behavior when stored and then reused after 4 weeks. Modified electrodes were typically stored under nitrogen, although no deleterious effects have been observed after handling them in air. However, because of the reactive nature of 19-electron Langmuir 2010, 26(18), 15010–15021

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Figure 5. (a) Γint vs deposition time for 1 mM [2][PF6]2 in CH3NO2/0.1 M [NBu4][PF6] using an Eapp of -0.52 V. (b) Γint vs Eapp for 1 mM [2][PF6]2 in CH3NO2/0.1 M [NBu4][PF6] at 298 K using a deposition time of 25 s. Each data point represents a different modified GC electrode.

cobaltocene, films are sensitive to the amount of time the electrode is held at a reductive potential. Figure S3 in the Supporting Information contains a series of CVs of a modified electrode for which the scan was initiated after being held for different periods of time at a potential negative of the [CoCp(η-C5H4-E)]þ/0 wave. A decline in the surface wave was seen with an increasing period of application of the negative starting potential, with the apparent surface coverage decreasing by about one-third after application of the potential for 120 s. Reduced films are also highly sensitive to oxygen levels, as one CV scan to -1.8 V in an oxygen-containing dichloromethane solution resulted in complete loss of the surface wave on subsequent scans. The reaction of 19-electron cobaltocene with O2 has been documented.45 Electrochemical Function of Cobaltocenium-Modified Electrodes. The properties of cobaltocenium-modified GC electrodes were probed by observing the response of these electrodes in solutions containing well-known redox-active molecules. As expected, there was increased blockage of the ferrocene/ferrocenium couple with increased cobaltocenium deposition time (see Figure 6) (an exception is for CV scans in acetonitrile, which showed little if any blockage). It was also noticed that, if the cobaltocenium wave was scanned before recording the CV of (45) (a) Kojima, H.; Takahashi, S.; Hagihara, N. J. Chem. Soc., Chem. Commun. 1973, 230. (b) Kojima, H.; Takahashi, S.; Yamakazi, H.; Hagihara, N. Bull. Chem. Soc. Jpn. 1970, 43, 2272. (c) Hay-Motherwell, R. S.; Wilkinson, G.; Sweet, T. K. N.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1994, 2223. (d) Rapicault, S.; Paday, F.; Degrand, C. J. Organomet. Chem. 1996, 525, 139.

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Figure 6. CVs showing electrode passivation effect with increasing deposition time for 1.0 mM FeCp2 in CH2Cl2/0.1 M [NBu4][PF6]. 3 mm GCE, ν = 0.1 V s-1. Modification conditions: CH3NO2/ 1.0 mM [2][PF6]2 for 25, 50, 100, 200, and 300 s at -0.62 V.

ferrocene, less blockage occurred. It is possible that the original scan of the cobaltocenium wave rids the film of weakly held cobaltocenium moieties, thereby making more electrode sites available for electron transfer to homogeneous substrates. The electron-transfer blockage shown in Figure 6 is evidence that the modified electrodes do not possess an abundance of pinholes or other deformities.46 Mild blockage was also observed for the p-dinitrobenzene reductions in CH2Cl2/0.1 M [NBu4][PF6] and for the [Fe(CN)6]4-/3- couple in 1 M Li[ClO4]/H2O (Supporting Information Figure S4). XPS Analysis of Cobaltocenium-Modified Electrodes. X-ray photoelectron spectroscopy (XPS) analysis was performed on cobaltocenium-modified electrodes. For these experiments, pyrolyzed photoresist film (PPF) replaced glassy carbon as the electrode material owing to the lower surface roughness of the former.47 The XPS spectrum, shown in Figure 7, indicates the presence of Co, O, N, and C. Carbon and oxygen are abundant in the substrate itself, so the only meaningful signals are the Co 2p and N 1s peaks at 781.5 and 400.3 eV, respectively. The N 1s peak implies there may be a small amount of azo-bonding between film species, as has previously been observed for aryl diazonium modifications.48 Surprisingly, the spectrum contained no phosphorus signal and only a small fluorine signal. The character and location of the counterion in the modified film remains ambiguous. Electrode Modification with [3][PF6]. Depositions of the manganese compound were carried out in a dichloroethane/ [NBu4][PF6] electrolyte using [3][PF6]. After the usual deposition by either controlled potential electrolysis (ca. 100 s) or cathodic CV scans, GC electrodes were sonicated in dichloroethane and tested in solutions containing only [NBu4][B(C6F5)4] and either dichloroethane or dichloromethane. Note that in this case the electrolyte anion in the test solution was the weakly coordinating tetrakis(pentafluorophenyl)borate anion, which was chosen in order to promote chemical reversibility for the cymantrene/ cymantrenium cation couple23 of the modified electrode. The CVs of the modified electrode surface displayed a surface anodic wave in the expected potential range for oxidation of a member of the cymantrene family. It was chemically reversible at higher scan rates (see Figure 8, ν = 2 V s-1), allowing determination (46) (a) Amatore, C.; Sav
eant, J.-M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39. (b) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329. (47) Ranganathan, S.; McCreery, R. L. Anal. Chem. 2001, 73, 893. (48) Doppelt, P.; Hallais, G.; Pinson, J.; Podvorica, F.; Verneyre, S. Chem. Mater. 2007, 19, 4570.

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Figure 7. XPS spectrum of cobaltocenium-modified PPF electrode.

Figure 8. First three CV scans of cymantrene-modified electrode in CH2Cl2/0.05 M [NBu4][B(C6F5)4]. Modification conditions: dichloroethane/0.1 M [NBu4][PF6], 1.1 mM [3][PF6] for 100 s at -1.0 V. 3 mm GCE, ν = 2 V s-1.

of its E1/2 as 0.96 V. Since one-electron oxidation of the parental model Mn(CO)3Cp has E1/2 = 0.92 V in the same medium,23 this wave is attributed to the electrode-deposited cymantrenyl moiety, [Mn(CO)3(C5H4-E)]0/þ. The surface wave is less well-behaved at slower scan rates (see Supporting Information Figure S5), displaying a broader anodic character with less evidence of chemical reversibility. Loss of surface current was observed with repetitive scans at all scan rates, although this effect was diminished at faster scan rates. It appears as though the surface-based cymantrene radical cation is not as stable as it is in a homogeneous solution. It is probable that the surface holds ordinarily weak nucleophiles which react with the cymantrene radical cation. Even [PF6]- is known to react with such a radical cation on the CV time scale.23,49 Although the product of the follow-up reaction is not known at this point, it does not appear to involve cleavage of the cyclopentadienyl-manganese moiety from the electrode surface. One indication of this is that electrodes scanned an increasing number of times exhibited about the same blockage effects when transferred to a pure solution containing a test redox agent. An example of the blocking effect on the ferrocene oxidation using a freshly modified electrode is shown in Figure 9. A CV showing the ferrocene blockage on an electrode that had been scanned multiple times through the cymantrene wave is available in the Supporting Information (Figure S6). In terms of surface coverage, we calculate a value of Γ = 3.9  10-10 mol cm-2 for a “flat” monolayer with the cyclopentadienyl (49) Stone, N. J.; Sweigart, D. A.; Bond, A. M. Organometallics 1986, 5, 2553.

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Figure 9. CVs of 1.0 mM FeCp20/þ in CH2Cl2/0.05 M [NBu4][TFAB] at a bare and cymantrene-modified 3 mm GCE, ν = 0.1 V s-1. Modification conditions: dichloroethane/1.0 mM [3][PF6] for 100 s at -1.0 V.

ring roughly perpendicular to the electrode surface, similar to the approach taken above for the cobaltocenium-modified electrode. Integration of CV peaks such as those shown in Figure 8 gave Γ values between 2  10-10 and 3  10-10 mol cm-2, close to that expected for a monolayer. The ΔEp value of 50 mV at ν = 2 V s-1 shows that the cymantrene-to-electrode electron-transfer rate is very fast.50 For the sake of completeness, we add that the cymantrenyl attachment appeared to also work well on boron-doped diamond40 electrodes, but little if any coverage was observed on indium tin oxide or highly ordered pyrolytic graphite electrodes. To aid in identification of the immobilized species, XPS and IR spectra of films on PPF were collected. These spectra were taken prior to electrochemical scanning of modified electrodes but after sonication in an appropriate solvent. The XPS spectrum in Figure 10a contains the expected Mn 2p peak at 641 eV. As observed with the cobaltocenium deposition (vide ante), there is also an N 1s signal at 399.8 eV, perhaps from a small amount of azo-bonding or possibly from trapped, unreduced diazonium species. Although the latter is supported by the presence of an F 1s signal, the associated P signals are essentially absent. The IR spectrum, shown in Figure 10b, contains two peaks in the metal-carbonyl region, at ν = 1934 and 2019 cm-1, consistent with the presence of a neutral cymantrenyl group on the PPF surface (ν = 1934 and 2022 cm-1 for Mn(CO)3Cp in dichloromethane).23 The peaks at 843 and 745 cm-1 may arise from νCH(bend) motions of the cyclopentadienyl CH bonds, which show activity in this region.51 Arguing against the presence of unreacted diazonium complex, the strong absorption of [PF6]- expected at 786 cm-1 (ref 52) is not observed. Carbonyl Substitution at Modified Electrode. It is wellknown that cymantrene-type complexes exhibit significantly enhanced anodic chemical reversibility if one or more of its CO groups is replaced by a donor ligand.22 In an effort to produce a (50) k0 of ca. 100 s-1 based on treatment of Laviron, E. J. Electroanal. Chem. 1979, 101, 19. (51) (a) Kramer, J. A.; Hendrickson, D. N. Inorg. Chem. 1980, 19, 3330. (b) Trupia, S.; Nafady, A.; Geiger, W. E. Inorg. Chem. 2003, 42, 5480. (c) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Complexes, 5th ed.; John Wiley & Sons: New York; 1997, part B, pp 285-290. (52) Measured in our laboratory by attenuated total reflectance on [NH4][PF6] powder. For literature references, see Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Complexes, 5th ed.; John Wiley & Sons: New York; 1997, part A, pp 214-218.

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Figure 10. (a) XPS spectrum and (b) IR spectrum of cymantrenemodified PPF electrode.

more oxidatively stable film, and to probe the possibility of ligand substitution reactions at the electrode-bound Mn(CO)3 moiety, the electrochemistry of the modified electrode was studied in the presence of phosphite ligands. Redox-initiated substitution reactions, which take advantage of the accelerated donor-for-CO substitution rates for the 17-electron cation radical, have been reported for these types of complexes in homogeneous solutions.22a The CO-substitution products have less positive redox potentials and more stable radical cations compared to their tricarbonyl counterparts. In the present case, triphenyl- and trimethyl-phosphite, P(OPh)3 and P(OMe)3, respectively, were employed as substitution ligands, with the overall process being summarized in Scheme 2. A freshly modified cymantrenyl electrode was placed in a solution of CH2Cl2/[NBu4][TFAB] containing 6 mM P(OPh)3. As shown in Figure 11, successive CV scans gave diminution of the original surface wave at Epa ca. 1.0 V and appearance of a redox-reversible product wave at E1/2 = 0.60 V. The E1/2 shift of ca. -0.4 V is as expected for substitution of one CO by P(OPh)3 in this system (the homogeneous E1/2 values are 0.92 V for MnCp(CO)3 and 0.47 V for MnCp(CO)2(P(OPh)3) under these electrolyte conditions). Once the substitution has occurred, the film product is no longer significantly compromised by anodic scans, as no loss in surface signal occurred after 10 CV scans at ν = 0.2 V s-1. To confirm that the product wave arises from a surface species, the electrode was removed, sonicated in dichloroethane, and returned to a cell containing only CH2Cl2/[NBu4][TFAB]. CV scans again revealed the reversible product wave. The carbonyl substitution process depicted in Scheme 2 is thus confirmed, but with less than a one-third efficiency, as determined by CV current. The decomposition reaction of the 17 e- tricarbonyl complex apparently competes with the carbonyl substitution reaction on the CV time scale. A similar set of experiments was undertaken using P(OMe)3 as the substitution ligand at a cymantrenyl-modified electrode. DOI: 10.1021/la102579t

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Laws et al. Scheme 2

Figure 11. First three CV scans of cymantrene-modified electrode in CH2Cl2/[NBu4][TFAB] with 5 μL (6 mM) of P(OPh)3. 3 mm GC electrode, ν = 2 V s-1.

Figure 12. First five CV scans of cymantrene-modified electrode in dichloroethane/[NBu4][TFAB] with 10 μL (38 mM) of P(OMe)3. 2 mm GCE, ν = 0.2 V s-1. Modification conditions: five CV scans in dichloroethane/1.2 mM [3][PF6].

The observed reversible product wave at E1/2 = -0.20 V indicated formation of the disubstituted species, as MnCp(CO)(P(OMe)3)2 is oxidized at E1/2 = -0.29 V in CH2Cl2/[NBu4][TFAB]. As shown in Figure 12 and Supporting Information Figure S7 (cyclic and differential pulse voltammetry scans, respectively), the amount of substitution product increases with the number of scans, apparently as the reaction becomes more complete. Once it had reached a limit, the new surface wave obeyed the expected direct dependence of peak current on scan rate, confirming its assignment to a surface redox process. Some unexpected and interesting effects were seen, however, in this substitution reaction. When the substituted electrode was removed, rinsed, and immersed in a pure electrolyte solution, the surface wave was very small. However, if the same electrode was then reimmersed in the solution containing P(OMe)3, the 15020 DOI: 10.1021/la102579t

substitution wave reappeared, at roughly the same height as seen prior to removal from the original solution. It is not surprising that the monosubstituted complex MnCp(CO)2(P(OMe)3) was not observed. Although the second substitution of the tricarbonyl complex is expected to be slower than the first owing to increased electron-density at the metal center, both should be fast on the CV time scale,22a leading to rapid formation of [MnCp(CO)(P(OMe)3)2]þ from the tricarbonyl parent. However, the diminution of this wave in a solution not containing the free ligand is not presently understood. To our knowledge, redox-based substitution reactions of surface-bound metal-containing compounds have not been described in the literature, and more systematic work on such reactions is planned.

Conclusions The X-ray crystallographic and electrochemical characteristics of organometallic complexes containing a cyclopentadiazonium ligand have been studied for the first time. Both CoCp(η5-C5H4N2)2þ and Mn(CO)3(η5-C5H4N2)þ undergo irreversible one-electron reductions either by mild reducing agents or at modest electrode potentials. The redox site is localized on the cyclopentadienyldiazonium ligand, resulting in rapid loss of dinitrogen and formation of the radical ML(η5-C5H4)• (ML = CoCpþ or Mn(CO)3), which reacts either by abstracting a hydrogen atom from solvent or by forming a bond with an atom on the electrode. The latter results in a modified electrode in which the cyclopentadienyl ligand is strongly attached to the surface. The resulting organometallic modified electrodes have a surface coverage of approximately one monolayer. They can be stored indefinitely, and restoration of the original unmodified electrode requires mechanical polishing. The modified electrodes exhibited “blockage” of the voltammetric responses of test redox systems such as ferrocene in dichloromethane and [Fe(CN)6]4- in water. A redox-enhanced phosphite-forcarbonyl ligand substitution reaction occurs when the cymantrenyl-modified system Mn(CO)3(η5-C5H4-E) is anodically oxidized. The successful diazonium-based modification of electrode surfaces by direct bonding of a cyclopentadienyl ligand to the electrode broadens the chemical scope of the diazonium attachment method beyond that of organic aryls. The chemistry and electrochemistry of the organometallic systems studied in this work parallel those known for their homogeneous counterparts, raising the possibility of performing systematic reactions, including electron-transfer induced catalysis, with the organometallic electrodes. Acknowledgment. This work was sponsored by the National Science Foundation (CHE-0411703 and CHE-0808909). We gratefully acknowledge Dr. Alison Downard for helpful conversations Langmuir 2010, 26(18), 15010–15021

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and Dr. Franklin Anariba (University of California at Riverside) for experimental assistance, and thank Dr. Heidi Martin for furnishing boron-doped diamond electrodes. We are especially grateful to Dr. Richard L. McCreery for providing laboratory space, instrumentation, and advice to D.R.L. at the Ohio State University.

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Supporting Information Available: Tables of crystal data, collection and refinement parameters, bond distances, and angles for [2][PF6]2 and [3][BF4] in CIF format. Plot of Epc vs log of CV scan rate (Figure S1) and six voltammetric scans (Figures S2-S7). This material is available free of charge via the Internet at http://pubs.acs.org.

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