930
Langmuir 2007, 23, 930-935
Adsorption of Tentacled Tetragonal Star Connectors, C4R4-Co-C5(HgX)5, on Mercury Lubomı´r Pospı´sˇil,† Natalia Varaksa,‡ Thomas F. Magnera,‡ Thierry Brotin,‡ and Josef Michl*,†,‡ J. HeyroVsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejsˇkoVa 3, 18223 Prague, Czech Republic, and Department of Chemistry and Biochemistry, UniVersity of Colorado, Boulder, Colorado 80309-0215 ReceiVed August 15, 2006. In Final Form: October 10, 2006 For future use in self-assembly of surface structures, the adsorption on the surface of mercury of a series of tetraphenylcyclobutadienecyclopentadienylcobalt double-decker sandwich complexes with five mercury and sulfur containing “tentacles” on the cyclopentadienyl deck has been examined by combined electrochemical and Langmuir trough techniques.
Introduction Double-decker metal sandwich complexes 1 could be useful for the construction of surface structures. The lower deck can carry groups with surface affinity, ensuring good surface adhesion, while the upper deck remains unencumbered and is available for construction purposes. The synthesis and use of such doubledecker modules will be facilitated if the reactivities of the two decks are clearly differentiated. One viable possibility is the use of functionalized tetraphenylcyclobutadienecyclopentadienylcobalt sandwiches of the general structure 2.1 The cyclopentadienyl (Cp) ring with its five mercury- and sulfur-carrying “tentacles” serves as a pedestal meant to anchor the monomeric module to the metal surface, while the cyclobutadiene (Cb) ring with its four tetragonally arranged substituents is free for construction purposes. Although the affinity of this particular moiety for the surfaces of mercury and gold has been briefly communicated and two applications of the concept have been outlined, it has not been documented in detail, and the present paper represents an effort to do so. In the first application, only local structure mattered and submonomolecular coverage was used: two pedestals of type 2 were covalently attached to the axle of a dipolar altitudinal molecular rotor and held it above and parallel to a gold surface. Differential barrier height imaging was used to demonstrate control of the rotator orientation in a single-molecule experiment.2 This communication described one of the first if not the first demonstration of reversible external control of the conformation of a single molecule. The second publication3 described a motivation for the synthesis of extended regular two-dimensional covalent grid polymers from star-shaped monomers constrained to and oriented on a mercury surface. Before use, the grids were transferred to a substrate, either solid or perforated, by standard vertical or horizontal transfer techniques. Our work on two-dimensional grid assemblies on * Corresponding author. † Heyrovsky ´ Institute. ‡ University of Colorado. (1) Harrison, R. M.; Brotin, T.; Noll, B. C.; Michl, J. Organometallics 1997, 16, 3401. (2) Zheng, X.; Mulcahy, M. E.; Horinek, D.; Galeotti, F.; Magnera, T. F.; Michl, J. J. Am. Chem. Soc. 2004, 126, 4540. (3) Magnera, T. F.; Michl, J. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 4788.
mercury3-8 parallels the work of others on water-air interfaces.9-21 The use of a liquid/liquid interface for the assembly entails a potentially risky transfer step, but would permit the deposition of the same grid on many different substrates. The absence of steps, terraces, and any other permanent irregularities or periodic structure, and the high lateral mobility at the interface should provide optimal opportunities for the adsorbate structure alone to be in control of regular grid formation by reversible adsorbateadsorbate bonding. This is essential for the ultimate goal of access to large domains of grids with an arbitrarily chosen geometry and lattice constant. The metallic conductivity and reflectivity of mercury facilitate the control and monitoring of the assembly process. In the case of structures 2, the four phenyl groups of the Cb deck are to be p-substituted, permitting end-to-end linking into (4) Magnera, T. F.; Peslherbe, L. M.; Ko¨rblova´, E.; Michl, J. J. Organomet. Chem. 1997, 548, 83. (5) Magnera, T. F.; Pecka, J.; Vacek, J.; Michl, J. In Nanostructural Materials: Clusters, Composites, and Thin Films, Moskovits, M., Shalaev, V., Eds.; ACS Symposium Series 679, American Chemical Society: Washington, DC, 1997; p 213. (6) Magnera, T. F.; Pecka, J.; Michl, J. In Science and Technology of Polymers & AdV. Mater.; Prasad, P. N., Mark, J. E., Kandil, S. H., Kafafi, Z. H., Eds.; Plenum: New York, 1998; p 385. (7) Magnera, T. F.; Michl, J. Atualidades de Fı´sico-Quı´mica Orgaˆ nica 1998, 50. (8) Varaksa, N.; Pospı´sˇil, L.; Magnera, T. F.; Michl, J. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 5012. (9) Palacin, S.; Porteu, F.; Ruaudel-Teixier, A. Thin Films 1995, 20, 69. (10) Weissbuch, I.; Baxter, P. N. W.; Cohen, S.; Cohen, H.; Kjaer, K.; Howes, P. B.; Als-Nielsen, J.; Hanan, G. S.; Schubert, U. S.; Lehn, J.-M.; Leiserowitz, L.; Lahav, M. J. Am. Chem. Soc. 1998, 120, 4850. (11) Mingotaud, C.; Lafuente, C.; Amiell, J.; Delhaes, P. Langmuir 1999, 15, 289. (12) Weissbuch, I.; Baxter, P. N. W.; Kuzmenko, I.; Cohen, H.; Cohen, S.; Kjaer, K.; Howes, P. B.; Als-Nielsen, J.; Lehn, J.-M.; Leiserowitz, L.; Lahav, M. Chem.sEur. J. 2000, 6, 725. (13) Rapaport, H.; Kuzmenko, I.; Berfeld, M.; Kjaer, K.; Als-Nielsen, J.; Popovitz-Biro, R.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. J. Phys. Chem. B. 2000, 104, 1399. (14) Kuzmenko, I.; Rapaport, H.; Kjaer, K.; Als-Nielsen, J.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Chem. ReV. 2001, 101, 1659. (15) Armand, F.; Albouy, P.-A.; Da Cruz, F.; Normand, F.; Huc, V.; Goron, E. Langmuir 2001, 17, 3431. (16) Culp, J. T.; Park, J.-H.; Stratakis, D.; Meisel, M. W.; Talham, D. R. J. Am. Chem. Soc. 2002, 124, 10083. (17) Torres, G. R.; Agricole, B.; Delhaes, P.; Mingotaud, C. Chem. Mater. 2002, 14, 4012. (18) Romualdo-Torres, G.; Agricole, B.; Mingotaud, C.; Ravaine, S.; Delhaes, P. Langmuir 2003, 19, 4688. (19) Plaut, D. J.; Martin, S. M.; Kjaer, K.; Weygand, M. J.; Lahav, M.; Leiserowitz, L.; Weissbuch, I.; Ward, M. D. J. Am. Chem. Soc. 2003, 125, 15922.
10.1021/la062416h CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006
Adsorption of C4R4-CoC5(HgX)5 on Mercury
Langmuir, Vol. 23, No. 2, 2007 931
Table 1. Potentials of the Voltammetric Peak A (V vs Normal Ag/AgCl Electrode)
in the anticipated grid synthesis and by the requirements of a large potential window, good solubility, and high dielectric permittivity. We found tetrahydrofuran (THF) to be optimal for the assessment of redox properties in a wide range of potentials, whereas acetonitrile is better suited for the examination of adsorption properties, due to its higher dielectric permittivity and higher conductivity in impedance measurements. Figures S1-S9 shown in the Supporting Information provide additional illustrations. Experimental Section
compd
R
X
MeCN
THF
3 4 5 6 7 8 9 10 11 12
H H H H COOEt COOEt COOEt COOEt H COOEt
HgSMe HgSCOMe HgSCH2CH2SMe HgSCH2CH2SCOMe HgSEt HgSCOMe HgSCH2CH2SMe HgSCH2CH2SCOMe H H
-0.79 -0.72 -1.13 -1.14 -0.95 -0.88 -1.27 -1.19
-0.80 -0.69 -1.14a -1.05a -1.02 -0.75 -1.10 -1.08
a
The second of two overlapping peaks.
a two-dimensional square grid structure on the mercury surface. Adhesion to the surface needs to be irreversible, or the linking agent must be available only at the surface. Otherwise, covalent cross-linking could occur not only on the surface but also in the bulk solution, producing multilayers and jumbled structures. It is not clear whether conditions can be found that ensure perfectly irreversible adsorption of the molecules on a mercury surface while leaving each monomer laterally mobile, with only limited lateral affinity for its neighbors. This appears to be desirable in order to avoid the formation of an immobilized tightly packed compact surface layer,22 in which organization into a square grid array would probably be hindered. Alternatively, the formation of a sturdy compact surface layer could be the goal, if its structure is that of an open regular grid. Both applications of structures 2 outlined above, and other applications that can be envisaged, require a knowledge of their adsorption properties as a function of the electrochemical potential. Presently, we describe the results of an examination of the adsorption of these compounds on mercury, with the expectation that their properties on gold may be similar. We build on prior studies such as that of Muscal and Mandler,23 who examined the formation of thiol self-assembled monolayers on mercury under potential control and summarized earlier work. The compounds used (Table 1) are assigned the same numbers (3-12) used in the previous study24 of their redox behavior, even though some of them are not examined presently. We describe primarily our results for compounds 4 and 5, carrying one and two sulfur atoms in each tentacle, respectively. We complement them by data for the others, especially 8-10, whose large upper deck would be expected to affect packing at the interface. The choice of solvent was dictated by its likely utility (20) Culp, J. T.; Park, J.-H.; Meisel, M. W.; Talham, D. R. Inorg. Chem. 2003, 42, 2842. (21) Culp, J. T.; Park, J.-H.; Benitez, I. O.; Huh, Y.-D.; Meisel, M. W.; Talham, D. R. Chem. Mater. 2003, 15, 3431. (22) De Levie, R. In AdVances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C., Eds.; Wiley: New York, Vol. 13, p 1. (23) Muskal, N.; Mandler, D. Electrochim. Acta 1999, 45, 537.
Electrochemical Instrumentation. A home-built system for cyclic voltammetry, phase-sensitive ac polarography, and dc polarography consisted of a fast rise-time potentiostat and a lock-in amplifier (EG&G model 5209) interfaced to a personal computer via an IEEE-interface card (PC-Lab, AdvanTech model PCL-848) and a 12-bit data acquisition card (AdvanTech PCL-818) using 12bit precision. The 160 Hz ac signal for the cell was derived from the internal oscillator of the lock-in amplifier. The amplitude was 5 mV (p-p). Impedance spectra were measured in the range of 1 Hz to 100 kHz using a frequency spectrum analyzer (Stanford Research, model SR780). A three-electrode electrochemical cell was used. The reference electrode, Ag|AgCl|1 M LiCl, was separated from the test solution by a salt bridge, and the half-wave potential of ferrocene against it was +0.562 V. The working electrode was a valve-operated static mercury electrode (SMDE2, Laboratornı´ Prˇ´ıstroje, Prague) with an area of 1.32 × 10-2 cm-2. The auxiliary electrode was a cylindrical platinum net. Oxygen was removed from the solution with a stream of argon. Electrochemical Mercury Langmuir-Blodgett (LB) Trough.8 A home-built Hg trough used a computer-controlled Teflon trolley fitted with a Cu block that functioned as an impenetrable barrier to molecules on the Hg surface and a Viton O-ring as a seal between the Cu barrier and the wall of the trough. Surface pressures were measured with a platinum Wilhelmy plate. The trough was situated in a dry N2 atmosphere and connected to a potentiostat through hermetically sealed leads. Since the mercury was in contact with copper, it became contaminated with it after some time. This did not matter for the present purposes, but in a more recent version of the instrument the Hg-metal contact is limited to platinum. The superphase was a 10-3 M solution of tetrabutylammonium hexafluorophosphate (TBA) in acetonitrile. The potential of zero charge on the Hg surface was determined from the location of the minimum in an electrocapillarity (surface pressure vs electrical potential) plot (Figure S1, Supporting Information). A plot obtained on a clean Hg surface was used to normalize the baselines of adsorbate isotherms measured at different potentials. Materials. The synthesis, purification, and characterization of the cobalt complexes used have been published.1 TBA (Aldrich) was vacuum-dried. Acetonitrile (Fluka) was used as received in electrochemical measurements and was dried over molecular sieves for Hg trough measurements. THF (Fluka) was freshly distilled under argon. Calculations. Molecular footprints were calculated from molecular dimensions derived from structures generated the Sibyl25 molecular mechanics procedure as implemented in the Spartan26 computer program. The molecular radius r of a deck was defined as the distance from the center of the Cp or Cb ring to the outermost atom of a substituent plus the van der Waals radius of that atom. The surface area S occupied by a deck assuming close hexagonal packing of circles was obtained from S ) 6r2 tan(π/6). For close square packing of circles, the area would be about 15% larger. These areas are the upper limits in that they assume radially stretched tentacles and no interlacing. (24) Brotin, T.; Pospı´sˇil, L.; Fiedler, J.; King, B. T.; Michl, J. J. Phys. Chem. B 1998, 102, 10062. (25) Clark, M.; Cramer III, R. D.; van Opdensch, N. J. Comput. Chem. 1989, 10, 982. (26) Spartan SGI version 4.1.1, Wavefunction, Inc., Irvine, CA.
932 Langmuir, Vol. 23, No. 2, 2007
Pospı´sˇil et al. Table 2. Molecular Surface Area S in Å2 and van der Waals Radius r in Åa compd
SLB
Sel
Sth(Cp)
Sth(Cb)
rth(Cp)
rth(Cb)
4 5 6 8 9 10
small,b 220c 325,d 465e
220 345 375 230 385 380
290 520 700 290 520 700
200 200 200 500 500 500
9.2 12.3 14.2 9.2 12.3 14.2
7.6 7.6 7.6 12.1 12.1 12.1
a In acetonitrile; SLB is from compression isotherms; Sel is from the integration of peak A and double-step chronocoulometry; the upper limit Sth and rth values for the Cp and Cb decks are from molecular modeling (hexagonal array; values for a square array are ∼15% higher). b At potentials E < ∼ -0.95 V, SLB is ill defined and approaches zero. c At E > ∼ -0.95 V. d At E < ∼ -1.1 V. e At E > ∼ -1.1 V.
Figure 1. Phase-sensitive ac polarograms of 10-4 M solutions of 11 (a), 4 (b), 6 (c), and 5 (d) and 0.1 M TBAPF6 in acetonitrile. The real and imaginary admittance components are the full lines and dot-dashed lines, respectively. The blank is represented by a dotted line.
Results Redox Properties. The redox behavior has been described previously24 and displays the following major features (a representative voltammogram is shown in Figure S2, Supporting Information): an adsorption-confined reduction (A, to be discussed below), a diffusion controlled “metal-centered” reduction (B), a reductive cleavage of the tentacles (C), an anodic oxidation of the cleaved tentacles (D), and a reduction of the ester groups (E, absent in complexes 3-6). The peak denoted globally as “D” contains two anodic maxima. Also, peak E is composed of two redox steps, well separated in 12 and overlapped in the other compounds. In the following, we deal with the adsorption phenomena occurring from +0.1 to -1.5 V, where no diffusion-controlled primary redox reactions are observed. Thus, we are concerned only with the voltammetric peak A, whose adsorption-related nature follows conclusively from measurements of double-layer capacitance, and with its anodic counterpart. The position of peak A is listed in Table 1. In the compounds with two sulfur atoms per tentacle (5, 6, 9, and 10), it lies at more negative potentials (by 0.3-0.4 V) than those in compounds with only one (3, 4, 7, 8). The presence of the four ester groups on the Cb deck shifts it by 0.1-0.2 V to more negative values, except in the pair 6 and 10, whose A peaks appear at similar potentials. Peak A is absent in the voltammograms of the tentaclefree compounds 11 and 12. Double Layer Capacitance. A series of phase-sensitive ac polarograms yielded the vector components of the electrode admittance Y. The change in the imaginary component Y′′ is proportional to the double-layer capacitance C, which is dictated by an adsorption isotherm of an a priori unknown form.27 Timedependent phenomena related to adsorption kinetics are described in the Supporting Information. Experimental conditions (concentration and an equilibration time of 4-20 s) ensured the establishment of an initial adsorption equilibrium28 and at some electrode potentials may have permitted the start of phase transitions in the adsorbed layer. Figure 1 shows the C-E curves of the compounds with substituent-free phenyl groups on the Cb deck in acetonitrile at (27) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, Wiley: New York, 1980; p 338. (28) Koryta, J. Collect. Czech. Chem. Commun. 1953, 18, 206.
a 10-4 M bulk concentration (results in THF are similar, and a few selected ac polarograms are shown in Figure S3, Supporting Information). The tentacle-free complex 11 adsorbs at 0 to -0.5 V, but not at more negative potentials. The presence of five acetylthio tentacles in 4 extends the adsorption range all the way to -1.7 V, close to the potential of the diffusion-controlled reduction peak B. The double-layer capacitance is almost equally low on both sides of the potential of the voltammetric adsorption peak A, seen on the ac polarogram as a needlelike spike at -0.65 V. When the mercury atom carries an alkylthio instead of an acylthio group and two sulfur atoms are present in each tentacle (5 and 6), the double layer capacitance is again suppressed up to the main reduction process B at -1.85 V, but the adsorption peak A is shifted to about -1.0 or -1.1 V. Moreover, a new feature appears in the C-E curves: a narrow “pit”22 located at about -0.1 V and -0.6 V for 5 and 6, respectively. For 5, the C-E curve is particularly complicated between +0.1 and -0.5 V. The introduction of ethoxycarbonyl substituents into the para positions of the phenyl groups on the Cb deck (8-10) has little effect, except that the pit observed for 5 and 6 is absent in 9 and 10 (Figure S4, Supporting Information). Surface Area per Molecule from Compression Isotherms. The electrochemical Langmuir trough was used to determine the surface areas per molecule SLB for compound 4, each of whose tentacles contain a single sulfur atom, and compound 5, each of whose tentacles contain two (Table 2). For 4, Figure 2 shows representative compression isotherms, the linear dependence of the extrapolated area on the number of molecules spread initially on the surface, and the mean molecular areas deduced from the slope of the linear plot for a series of applied potentials E between -0.3 and -1.6 V. At values less negative than approximately -0.9 V, the molecular area equals ∼220 Å2, and the surface pressure remains nearly constant for many minutes (Figure 3); a stable compact surface film is apparently present. Between E ) -0.9 and -1.0 V, the area per molecule drops sharply, and, at more negative potentials, it has an ill-defined and much smaller value, which continues to approach zero over a period of several minutes (Figure 3). The behavior of 5 is different (Figure 4). The molecular area is ∼465 Å2 at potentials E less negative than approximately -1.1 V and changes abruptly to ∼325 Å2 at more negative applied potentials. In this instance, both values are stable and nearly constant in time. Surface Area per Redox Center from Electrochemical Measurements. The surface concentration of redox centers Γ is a function of the bulk concentration c and the applied potential, and was measured in two independent ways: the integration of the adsorption peak A29 and double-step chronocoulometry.30 (29) Laviron, E. J. Electroanal. Chem. 1974, 52, 395.
Adsorption of C4R4-CoC5(HgX)5 on Mercury
Langmuir, Vol. 23, No. 2, 2007 933
Figure 2. Molecular area SLB of 4 at an acetonitrile/mercury interface as a function of electrode potential E. Insets: lower right, Langmuir isotherms (surface pressure Π vs surface area S) at E ) -0.3 to -0.9 V (full lines) and E ) -1.0 to -1.6 V (dashed lines); upper left, plots of extrapolated covered area A against the number of molecules N at E ) -0.5, -0.7, and -0.9 V.
Figure 3. Time dependence of surface pressure Π after compression of a monolayer of 4 at an acetonitrile/mercury interface at potentials E ) -0.7 V (9), -1.3 V (b), and -1.6 V (2).
The charge Q obtained by the integration of a baseline-corrected adsorption peak is proportional to nΓ:
Q)
∫iV-1 dE ) nFΓ
(1)
where V is the voltage scan rate, n is the number of exchanged electrons, and F is the Faraday constant. Double-step chronocoulometry yields the value of nΓ at the initial potential of the step. The step is made to a potential where the compound is reduced under diffusion-controlled conditions (typically, the potential of the dc polarographic limiting diffusion current). The second potential step returns the potential to the initial value. The difference of the intercepts of linear plots of Q versus t1/2 for the sample and for a blank yields the amount of faradaic charge corresponding to the reduction of the surface amount Γ accumulated at the initial potential. Since both methods yielded identical values within experimental error, we mostly evaluated nΓ by the integration of voltammograms. Figure 5 shows typical voltammograms for 4 (30) Anson, F. C.; Christie, J. H.; Osteryoung, R. A. J. Electroanal. Chem. 1967, 13, 343.
Figure 4. Molecular area SLB of 5 at an acetonitrile/mercury interface as a function of electrode potential E. Insets: lower right, Langmuir isotherms (surface pressure Π vs surface area S) at E ) -1.0 V (left scale) and E ) -1.6 V (right scale); upper left, plots of extrapolated covered area A against number of molecules N at E ) -0.5, -0.7, -1.0, and -1.1 V (upper lines) and E ) -1.2, -1.5, and -1.6 V (lower lines).
Figure 5. The dependence of the voltammetric peak A (Figure 2) on the bulk concentration of 9 (a) and 4 (b). Curves from bottom to top correspond to 6, 24, 48 and 100 µM concentrations, respectively. The supporting electrolyte was 0.1 M TBAPF6 in acetonitrile. A scan of 3.75 V/s started after 20 s waiting time at the initial potential -0.6 V (a) and 0 V (b) located in the region of maximum adsorption.
and 9 as a function of bulk concentration. The shape of the adsorption peak was not symmetrical, and compounds with a single sulfur atom in each tentacle had a more pronounced shoulder than those with two. In all cases, the peak height depends on the initial potential Ei of the voltage scan. The peak current obtained in 10-5 M solutions also depends on the equilibration time at Ei.28,31 The time necessary for achieving the adsorption equilibrium was estimated from the time dependence of the nΓ values, and was 0.1-10 s for 0.1-0.01 mM concentrations, respectively. All values reported were obtained at equilibration times of 4-20 s for the range of concentrations given above. The peak height remained unchanged at these times. The peak current depends on the bulk concentration in a nonlinear fashion. As the 10-4 M concentration is approached, saturation is reached, and the peak becomes independent of bulk concentration. We argue below that a redox process occurring in this potential range can only be associated with the reduction of Hg(I) cations (31) Delahay, P.; Trachtenberg, I. J. Am. Chem. Soc. 1957, 79, 2355.
934 Langmuir, Vol. 23, No. 2, 2007
Pospı´sˇil et al.
Figure 6. Adsorption isotherms of 4 (3), 5 (.), and 6 (b) in 0.1 M TBAPF6 in acetonitrile at -0.300 V and 25 °C. The point . at 1 mM concentration was obtained in THF.
available from anodic dissolution of the electrode and complexed to sulfur atoms in the tentacles. Then, n ) 1, and this is also compatible with the ∼90 mV width of peak A. In the following, we therefore simply use Γ instead of nΓ. The width of peak A alone does not prove that n ) 1 because the electrode process is irreversible and because the irregular shape of the peak suggests the presence of more than one type of environment for the redox center. The reverse voltage scan yields no obvious corresponding adsorptive anodic counterpart, and significant anodic dissolution of mercury during a backscan apparently occurs at much less negative potentials (Figure S5, Supporting Information). As can be seen from similar capacitance values on both sides of peak A, reductive electron transfer to the mercury cations does not cause an immediate destruction of the adsorbed layer. Preparative electrolysis showed24 that a reductive cleavage of the tentacles from the metal sandwich does not occur at any of these potentials. The dependence of Γ on the bulk concentration c (the adsorption isotherm) for compounds 4, 5, and 6 can be compared only in a narrow interval of potentials near -0.3 V, because at most other potentials the saturation coverage is established already at the lowest practically usable concentrations (Figure 6). Plots of c/Γ versus c are linear (not shown) and indicate that adsorption obeys the Langmuir isotherm27
βc ) Γ/(Γmax - Γ)
(2)
with the potential-dependent adsorption coefficients β ranging from 3 to 8 × 104 L‚mol-1. The maximum limiting surface concentration Γmax of Hg(I) redox centers can be obtained over a much wider potential range, from about -0.2 V to the onset of the reduction peak A, and a comparison of Γ values of complexes 4, 8, 9, and 10 is given in Figure 7. The experimental footprint area per Hg(I) redox center in the neutral state was obtained for 4-10 from the limiting surface concentration as 1/NΓmax, where N is Avogadro’s number (Sel in Table 2). The average number of Hg(I) redox centers associated with a molecule is not known, but theoretically expected footprint areas per molecule can be estimated from molecular size and orientation on the surface. For the anticipated orientation, with the sandwich decks parallel to the surface, the theoretical sizes of the Cp deck [Sth(Cp)] and the Cb deck [Sth(Cb)] were approximated as areas of regular hexagons centered at the projection of the cobalt atoms into the deck plane, with an internal radius equal to the van der Waals radius of the deck with tentacles fully extended, as obtained from molecular modeling (close hexagonal packing of circles, Table 2). In 8, the Cb deck is larger than the Cp deck and is likely to dictate the nature and density
Figure 7. The dependence of the surface concentration Γ on the potential for complexes 4 (1), 8 (O), 9 (4), and 10 ([).
of close packing in an adsorbed state. In this case, the packed surface layer may form a square grid, and the anticipated molecular footprint size would then be ∼15% larger than that shown in Table 2. However, in 4-6 and 10, the Cp deck is distinctly larger than the Cb deck, and in 9 the radii of the two decks are comparable. In these cases, the assumption of close hexagonal packing seems reasonable.
Discussion The Langmuir isotherms, the properties of peak A, and the measured capacitance all prove quite unequivocally that the tentacled sandwich complexes 3-10 adsorb strongly to the mercury surface. The tentacle-free complexes 11 and 12 do not exhibit the adsorption peak A, and their C-E curves show that they do not adsorb at all at potentials more negative than -0.5 V, and adsorb much less than the tentacled complexes even above -0.5 V. This weak adsorption is probably due to the tetraphenylcyclobutadiene deck, since, to our knowledge, unsubstituted Cp rings of molecules such as ferrocene or various Cp metal carbonyls have never been reported to adsorb significantly on mercury from good solvents. Although the details of adsorbate orientation remain unknown, the sensitivity of the results to tentacle presence and structure leaves no doubt that the tentacles are the part of the molecules that is turned toward the surface, at least at electrode potentials more negative than -0.5 V, but most likely at all potentials. Indeed, it is quite plausible that the intuitively reasonable expectation that the Cp and Cb decks align roughly parallel to the surface in order to enable all five tentacles to interact with it is actually correct. The surface area per molecule SLB is considerably smaller than the upper limit area Sth(Cp) deduced from molecular models for molecule 4, which has short and relatively inflexible tentacles, and even more so for molecule 5, which has longer and more flexible tentacles (Table 2). However, this does not imply that the molecules fail to lie flat on the mercury and instead stand on their edge. More likely, the assumption of rigidly stretched tentacles made in the derivation of Sth(Cp) is unrealistic, and the tentacles curl and interlace. There are two regimes of adsorption for the tentacled compounds: strong chemisorption in the form of a compact surface film at potentials less negative than peak A, and weak physisorption at potentials much more negative. This is reminiscent of the observations made earlier for self-assembled monolayers of thiols on mercury.23 The formation kinetics of the compact layer is slow and involves phase changes, which we address in the Supporting Information. When the electrode potential is moved from the less negative to the more negative region, it apparently takes a few minutes before all the vestiges of the compact film disappear.
Adsorption of C4R4-CoC5(HgX)5 on Mercury
As already stated above, the adsorption peak A, which corresponds to an irreversible surface redox process,32,33 occurs in a potential range where it can only be associated with the reduction of mercury cations available from anodic dissolution of the electrode and complexed to sulfur atoms in the tentacles, similarly as in our previous work with trigonal connectors.8 This conclusion is in accord with the absence of peak A in the tentaclefree compounds 11 and 12 and with its shift to more negative potentials in compounds whose tentacles contain two sulfur atoms and therefore offer better complexation possibilities for mercury cations than those whose tentacles contain a single sulfur atom. It also provides an easy explanation for the otherwise puzzling observation that the presence of four electron-withdrawing ethoxycarbonyl groups in the upper deck shifts peak A to more negative potentials and makes the reduction more difficult. If peak A corresponded to the addition of electron density to the double-decker sandwich structure, the opposite would be expected. When it is postulated to correspond to the reduction of a mercury ion attached to the sulfur atoms in tentacles belonging to neighboring sandwich molecules, it is quite believable that an increase in the size of the upper deck pushes the neighbors apart and makes it harder for the sulfur atoms to provide optimum stabilization to the mercury ions. A specific structure for the compact film containing sulfurcoordinated mercury ions and present at potentials less negative than that of peak A cannot be proposed, but it is certainly quite disordered. The multiplicity of environments available to the mercury cations is reflected in the complex shape of peak A. Plausible qualitative structural suggestions for the compact layer can be made based on the data of Table 2. The equality of the molecular footprint area SLB and the area per Hg(I) redox center Sel observed for 4 shows that, on the average, there is one mercury cation attached to a HgSCOMe tentacle per molecule, as assumed above. This would be compatible, for instance, with a compact film structure in which, on the average, each sandwich molecule uses the sulfur atom of four of its five tentacles to attach a bridging mercury cation to bind to a sulfur atom on one of the tentacles of four of its six neighbors in a hexagonal array, or of its four nearest neighbors in a square array. For 8, which has the same tentacles, Sel is the same as for 4, and they presumably have the same average structure. The much larger upper limit areas per molecule calculated under the assumption of a hexagonal lattice and rigidly stretched noninterlacing tentacles, Sth(Cp) and Sth(Cb), are clearly not needed in reality. At potentials that are at least ∼0.2 V more negative than peak A, 4 (and presumably also 8) appears to be only weakly physisorbed, and poses no detectable resistance to compression in an LB trough. At less negative potentials, the compact film is the stable form of the adsorbed species, whether the mercury cations are present or have been removed by reduction, and, although the two instances do not differ significantly in the area per molecule SLB, their structures around the mercury ion sites must be different enough to make the redox process irreversible. (32) Tender, L.; Carter, T. M.; Murray, R. W. Anal. Chem. 1994, 66, 3173. (33) Laviron, E. J. Electroanal. Chem. 1974, 52, 355.
Langmuir, Vol. 23, No. 2, 2007 935
The much larger area SLB taken up by a molecule of 5 is attributable to its longer tentacles, HgSCH2CH2SMe. Now, Sel is only about 3/4 the size of SLB, and the average number of the Hg(I) redox centers per molecule is close to one per tentacle, or one per two sulfur atoms. The Sel value is about the same for all molecules with two sulfur atoms in each of their tentacless5, 6, 9, and 10sand it is quite likely that their compact films have similar structures. The higher density of mercury cations in 5 compared with 4 is sensible, given the larger number of tentacle sulfur atoms eager to accommodate them. The better chelation of the mercury cations is also compatible with the more negative potentials at which peak A appears (Table 1). An interesting problem is posed by the behavior of 5 at potentials more negative than peak A (approximately -1.1 V), where the mercury cations have been reduced and have presumably disappeared into the mercury electrode. Still, the molecules remain strongly adsorbed, and the Langmuir isotherm yields a sizable area per molecule, close to 2/3 of that observed when the mercury ions were present. The stability of the film must be due to a combination of adhesion of the tentacles to the mercury surface and of lateral van der Waals attractions between the now tightly packed adsorbed molecules. This structural transformation then accounts for the irreversible nature of the redox process and for the absence of an oxidation counterpart to peak A. Qualitatively, the same result is obtained from measurement of the time dependence of capacitance, discussed in the Supporting Information. The other molecules with two sulfur atoms in a tentacle presumably behave similarly. The small anodic peaks with diffusion shape observed during a backscan, followed by a successive reduction wave (Figure S5, Supporting Information), are reasonable for a not quite completely blocked electrode.
Conclusions All the results are compatible with the initial expectation that molecules of type 2 will adsorb strongly on a mercury, with their Cp decks facing the surface. Although in some cases weak physisorption is observed at sufficiently negative electrode potentials, the formation of compact surface layers in several steps of structural reorganization into a progressively more tightly packed form seems to be the rule. It remains to be seen whether there is enough mobility at the earlier stages of the process to allow suitably chosen substituents on the Cb deck to dictate a self-assembly into a regular square grid. Acknowledgment. This work was supported by the USARO (W911NF-05-1-0535), the NSF (OISE 0532040), the Czech Ministry of Education (MSˇ MT LC510, OC140 COST and KONTAKT No. ME 857), and the Academy of Sciences of the Czech Republic (GAAV A400400505). Supporting Information Available: Supplement to Results Section, including general information and a discussion on nucleation and growth kinetics. This material is available free of charge via the Internet at http://pubs.acs.org. LA062416H
