Anal. Chem. 2004, 76, 1603-1610
In Situ Electrochemical-NMR Spectroscopy. Reduction of Aromatic Halides Richard D. Webster*
Research School of Chemistry, Australian National University, Canberra, ACT 0200, Australia
A three-electrode, two-compartment cylindrical electrochemical cell has been constructed for in situ operation in a 500 MHz 1H NMR spectrometer. The cell was designed around a standard 10-mm sample tube holder and is lowered into position within the magnet by heightadjustable aluminum rods that support the wires connecting the potentiostat to the working, auxiliary and reference electrodes. The electrochemical cell functions under diffusion-controlled conditions in nonspinning mode and is able to operate under variable temperature and oxygen-free regimes. The working electrode consists of a 10-nm-thick gold film deposited onto the surface of a 7.49-mm-o.d. glass NMR tube that is located directly within the radio frequency coils of the NMR probe. The NMR spectra were collected simultaneously to the current flowing during the electrolysis with typical NMR line widths being approximately 2 Hz. The highly symmetrical nature of the cell design meant that probe tuning and gradient shimming changed minimally between experiments, which allowed standard shim sets to be saved and recalled for later use, thereby greatly reducing the time taken for tuning processes. Insertion and removal of the cell from within the magnet is achieved within a few seconds, which combined with the rapid shimming process makes the cell suitable for routine operation. The cell was used to study the in situ reduction of 9-bromoanthracene, 9-chloroanthracene, and 4-bromobenzophenone in deuterated acetonitrile with tetrabutylammonium hexafluorophosphate as the supporting electrolyte. There have been several reports over the last 30 years of studies that employ electrolysis procedures to generate species in the bulk solution that are detected simultaneously using NMR spectroscopy.1-6 However, despite these promising pioneering studies on in situ electrochemical-NMR spectroscopy, the tech* E-mail:
[email protected]. (1) Richards, J. A.; Evans, D. H. Anal. Chem. 1975, 47, 964-966. (2) Mairanovskii, V. G.; Yuzefovich, L. Y.; Filippova, T. M. Russ. J. Phys. Chem. 1977, 51, 1058-1059. Mairanovsky, V. G.; Yusefovich, L. Y.; Filippova, T. M. J. Magn. Reson. 1983, 54, 19-35. (3) Albert, K.; Dreher, E.-L.; Straub, H.; Rieker, A. Magn. Reson. Chem. 1987, 25, 919-922. (4) Mincey, D. W.; Popovich, M. J.; Faustino, P. J.; Hurst, M. M.; Caruso, J. A. Anal. Chem. 1990, 62, 1197-1200. (5) Sandifer, M. E.; Zhao, M.; Kim, S.; Scherson, D. A. Anal. Chem. 1993, 65, 2093-2095. (6) Prenzler, P. D.; Bramley, R.; Downing, S. R.; Heath, G. A. Electrochem. Commun. 2000, 2, 516-521. 10.1021/ac0351724 CCC: $27.50 Published on Web 02/05/2004
© 2004 American Chemical Society
nique remains very under utilized compared to other commonly used spectroelectrochemical techniques, such as EPR, UV-vis, and IR spectroscopy.7,8 The bulk of studies that have used NMR spectroscopy to monitor solution-phase electrochemical processes have constructed their cells to fit into standard 5-mm NMR probes.1,4,5 Modern NMR spectrometers are able to function with probes of various sizes that are easily interchangeable (within a few minutes) with the standard 5-mm probe. Two studies have taken advantage of wider probe sizes available (10 mm and 16 mm) to design larger electrochemical-NMR cells that allow more convenient positioning of the electrodes and enable the use of two or more high-precision NMR tubes that act as separate compartments of the electrochemical cell.3,6 Some studies1,3 utilized a flow-cell-type arrangement so that the electrochemical reaction occurred outside of the portion of the magnet housing the detector and subsequently flowed into the area housing the radio frequency (RF) coils. The advantages of a flow cell arrangement are that the electrolysis progresses quickly due to improved mass transfer conditions, and the working electrode can be conveniently located away from the RF coils, which improves the tuning process of the NMR spectrometer. The disadvantages are the larger solution volumes that are used and the difficulty in maintaining an oxygen-free environment in flow cell systems. Other studies developed static or diffusioncontrolled electrochemical cells,3-6 in which the electrodes were located directly within the RF coils of the magnet/probe, but with various degrees of success (in terms of the NMR line width). It has been found that very thin metallic films coated on symmetrical glass NMR tubes offer reasonable electrochemical performance while minimizing the NMR line widths.4,6 In keeping with the shift toward higher field NMR spectrometers with improvements in technology, the more recent studies have used higher frequency spectrometers, 300 (1H)6 or 400 MHz (1H),3,5 while the earlier studies used relatively low field NMR spectrometers between 60 and 90 MHz (1H).1,2,4 Table S-5.3 summarizes several important properties of the existing electrochemical-NMR cells and their modes of operation. Reports that use solid-state electrochemical-NMR spectroscopy to study metal-adsorbates at electrochemical interfaces,9 includ(7) Spectroelectrochemistry: Theory and Practice; Gale, R. J., Ed.; Plenum Press: New York, 1988. (8) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (9) See, for example: Slezak, P. J.; Wieckowski, A. J. Magn. Reson., A 1993, 102, 166-172. Tong, Y. Y.; Oldfield, E.; Wieckowski, A. Anal.Chem. 1998, 70, 518A-527A. Babu, P. K.; Tong, Y. Y.; Kim, H. S.; Wieckowski, A. J. Electroanal. Chem. 2002, 524-525, 157-167.
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ing those under potential control, have been excluded from Table S-5.3, since the aims of those experiments are different from the solution phase experiments outlined in this work. The purpose of this study is to design an electrochemical-NMR cell that is suitable for routine operation in a modern high-frequency NMR spectrometer. Several criteria of operation were considered to be critical in designing the electrochemical-NMR cell, including its ability to (i) function in a moisture- and oxygen-free environment and at variable temperatures, (ii) be assembled and set up quickly and not require a dedicated NMR instrument, (iii) operate with low solution volumes to minimize the amount of deuterated solvents, and (iv) minimally interfere with the NMR line widths. EXPERIMENTAL SECTION Apparatus. Voltammetric experiments were conducted with a computer-controlled Eco Chemie µAutolab III potentiostat. NMR experiments were performed with a Varian INOVA 500 MHz 1H NMR spectrometer utilizing a 10-mm probe. Gold coating was performed on a Dynavac CS300 coating unit. Reagents. Anthracene (99%), 9-bromoanthracene (94%), 9-chloroanthracene (96%), benzophenone (99%), and 4-bromobenzophenone (98%) were obtained from Aldrich and used as received. Dimethyl benzene-1,3-dicarboxylate and dimethyl pyridine-2,5dicarboxylate were prepared as described previously.10 Bu4NPF6 was prepared and purified by standard methods11 and dried under vacuum at 413 K for 72 h and stored under nitrogen. Et4NPF6 (Aldrich) was dried at 433 K for 72 h and stored under nitrogen. Analytical grade LiClO4 (99.99%, Aldrich) was dried at 433 K for 120 h. HPLC grade acetonitrile (EM Science) was purified and dried according to a published procedure,12 stored over calcium hydride (under nitrogen), and distilled immediately prior to use. Acetonitrile-d3, (99.8% D, Aldrich) was used for all NMR experiments and stored over 4 Å molecular sieves under nitrogen. Design of the Electrochemical-NMR Cell. The main body of the electrolysis cell was manufactured from Delrin (acetal homopolymer) (Figure 1a) with dimensions similar to a conventional 10-mm NMR sample tube holder, with the upper portion machined to incorporate a lid (Figure 1b) that was sealed in place with an O-ring (Figure 1c). Delrin was chosen because it can be easily machined while still maintaining a very rigid structure. Three holes were drilled into the lid to enable gold plated electrical connectors to be glued in place, which served as connections to the working (Figure 1d), auxiliary (Figure 1e), and reference (Figure 1f) electrodes. A fourth hole was drilled into the lid to allow the liquid sample to be injected into the cell (Figure 1g). The compartments that housed the three electrodes were constructed from several glass tubes. The outermost compartment was a 10-mm-o.d. (0.46-mm wall thickness) high-precision NMR tube (Wilmad, 513-7PP) (Figure 1i) that was held in place inside the Delrin sleeve with an O-ring (Figure 1j), which also prevented oxygen from permeating into the cell. Inside the 10-mm-o.d. tube was placed a 7.49-mm-o.d. (0.51-mm wall thickness) high-precision NMR tube (Wilmad, 513B-7PP) that had been modified so that it consisted of two sections. The upper section contained several (10) Webster, R. D.; Bond, A. M. J. Org. Chem. 1997, 62, 1779-1787. (11) Fry, A. J.; Britton, W. E. Laboratory Techniques in Electroanalytical Chemistry; Kissinger, P. T.; Heineman, W. R., Eds.; Marcel Dekker: New York, 1984; Chapter 13. (12) Walter, M.; Ramaley, L. Anal. Chem. 1973, 45, 165-166.
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1-mm-diameter holes that served to connect the working electrode to the auxiliary electrode compartments (Figure 1k). The lower section was joined to the upper section by bringing the two sections together in a lath and then carefully sealing the opposite end of the lower section to create an air pocket that reduced the dead volume of the cell (Figure 1l). The 7.49-mm-o.d. tube was held symmetrically in place inside the 10-mm-o.d. tube with two Teflon spacers located at the top and bottom of the glass tubes (Figure 1m). Inside the 7.49-mm-o.d. NMR tube was a 6-mm-o.d. (1-mm wall thickness) low-precision Pyrex tube (Figure 1n) that contained a porosity no. 2 frit in its base (Figure 1o) and rested on two small glass legs on the glass intersection within the 7.49-mm-o.d. NMR tube. The 6-mm-o.d. tube acted as an additional compartment to house the platinum mesh auxiliary electrode (Figure 1p) that was linked to the gold-plated connector (Figure 1e) with a 0.5-mmdiameter platinum wire. Prior to assembling the cell, the 7.49-mm-o.d. NMR tube was soaked in HNO3/water (50/50) for 30 min, rinsed in water, then further cleaned in a Piranha solution (70% H2SO4/30% H2O2) for 30 min before rinsing with ethanol and warm water (60 °C). The cleaning procedure removed organic contaminants that affected the adhesion of gold onto the glass surface.13 The outer surface of a 30-mm length of the 7.49-mm-o.d. NMR tube that resided inside the RF coils within the magnet/probe was then uniformly coated with a thin film of gold metal by rotating the tube with a motor-driven assembly in a vacuum evaporator (Figure 1q). A weighed amount of gold was placed in a tungsten filament and positioned a measured distance (r) above the center of the NMR tube. The mass of gold (m) needed to produce the desired average thickness (l) of gold film was approximated by using eq 1 and assuming that the gold radiated in all directions from the tungsten filament and that the upper half of the surface of the NMR tube was equidistant from the gold source.
0.5m ) 4πr2lD
(1)
The 0.5 factor in eq 1 is due to only half the NMR tube being coated per unit time, and D is the density of gold (18.9 g cm-3). The connection between the working electrode and gold-coated connector (Figure 1d) was made via a 100-µm-diameter platinum wire (Johnson Matthey). A 5-mm length of the platinum wire was held in contact with the gold film working electrode with Teflon tape wrapped tightly about the 7.49-mm-o.d. NMR tube. The remaining length of the platinum wire that was in contact with the solution was shielded with heat shrink tubing that had been flattened in a vice to allow it to fit between the gap between the 10-mm-o.d. and 7.49-mm-o.d. NMR tubes. This ensured that the electrolysis occurred predominantly at the gold electrode and not at the platinum wire. The reference electrode consisted of a 150-µm-diameter enamel-coated copper wire that was placed between the 10-mmo.d. and 7.49-mm-o.d. NMR tubes. The end 1 mm of the wire was stripped of enamel, and the exposed copper metal was electroplated with silver to produce a pseudo reference electrode that was positioned ∼5 mm from the top of the gold working electrode (Figure 1r). (13) Hilmi, A.; Luong, J. H. T. Anal. Chem. 2000, 72, 4677-4682.
The lower assembly shown in Figure 1 was degassed under vacuum in a Schlenk-type apparatus and then purged with nitrogen. The cell was filled with deoxygenated solvent/solute via an airtight syringe through the hole in the lid (Figure 1g) while being maintained under a nitrogen atmosphere. The sealed oxygen-free cell was then attached to the upper section of the assembly through the two hollow aluminum tubes (Figure 1h) that joined to two 3-mm-diameter solid aluminum poles (Figure 1s) and fastened with a stainless steel spring clip (Figure 1t). The solid aluminum rods were threaded through a cylindrical Delrin supporting block (Figure 1u) that rested on the upper entrance to the NMR magnet. The heights of the aluminum rods were adjustable by screws drilled into the side of the Delrin block (Figure 1v). For this work, adjustment to the height of the aluminum rods was required only the first time the cell was placed in the magnet of the spectrometer. Each of the wires connecting the electrochemical cell (Figure 1d-f) to the potentiostat were intersected with an RF choke (Figure 1w) that resided on top of the Delrin upper block. The RF chokes acted to filter out RF noise introduced from the potentiostat (and leads connecting the potentiostat to the cell) onto the deuterium lock signal. For an 11.7440-T NMR magnet, the 2H resonance frequency is 76.753 MHz. High frequencies between 10 and 1000 MHz require chokes with inductance values in the microhenries region,14 and experimentally, it was found that chokes with an inductance value between 100 and 500 µH were the most efficient in maintaining the D-lock signal. Spectroscopic Performance of the Electrochemical-NMR Cell. An important feature that affects the NMR line width (spectral resolution) is the uniformity of the magnetic field experienced by molecules in different parts of the sample being examined.15 Therefore, to acquire spectra with narrow line widths, it is very important that the portion of the cell that resides within the RF coils of the probe is highly symmetrical. Another important factor that has been highlighted in previous studies4,6 is the attenuation of the RF radiation due to the metallic working electrode. The depth of penetration (approximated to the “skin depth”, δ) is defined as the distance into the material at which the power flux has fallen by e-1 () 0.368) of its surface value and can be calculated from eq 2,
δ ) (F/πνµ0)1/2
Figure 1. Cross section through the electrochemical-NMR cell: (a) Delrin NMR tube holder, (b) Delrin lid, (c) O-ring, (d) connection to working electrode, (e) connection to auxiliary electrode, (f) connection to reference electrode, (g) Teflon stopper, (h) hollow aluminum rods, (i) 10-mm-o.d. NMR tube, (j) O-ring, (k) holes in 7.49-mm-o.d. NMR tube, (l) air pocket in 7.49-mm-o.d. NMR tube, (m) Teflon spacers, (n) 6-mm-o.d. Pyrex tube, (o) porosity no. 2 glass frit, (p) platinum mesh auxiliary electrode, (q) Au film (∼10 nm) working electrode, (r) Ag metal pseudo reference electrode, (s) Al rods, (t) stainless steel spring clip, (u) Delrin support block, (v) stainless steel screws, and (w) RF chokes.
(2)
where F is the electrical conductivity (2.44 × 10-8 Ω m for Au), ν is the RF field of the NMR spectrometer, and µ0 is the magnetic permeability (4π × 10-7 H m-1).16 The significance of the calculation is that in order to avoid loss of resolution or sensitivity, film thicknesses substantially less than the skin depth should be used. For a gold conductor and fields of 100, 300, 500, and 1000 MHz, δ can be calculated to be 7860, 4340, 3520, and 2490 nm, respectively, indicating that in situ measurements at very high frequencies (g500 MHz 1H) are possible. Figure 2a shows the 500-MHz 1H NMR spectrum of the (14) Brophy, J. J. Basic Electronics For Scientists, 5th ed.; McGraw-Hill: New York, 1990; p 42. (15) Kemp, W. NMR in Chemistry; Macmillan: Houndmills, 1986; p 32. (16) Seeger, J. A. Microwave Theory, Components, and Devices; Prentice Hall: Englewood Cliffs, New Jersey, 1986; p 24-25.
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Figure 2. 500-MHz 1H NMR spectra of the aromatic region of 25 mM dimethyl benzene-1,3-dicarboxylate obtained in the nonspinning electrochemical-NMR cell with a 10-mm probe in CD3CN: (a) 10-mm-o.d. NMR tube; (b) 7.49-mm-o.d. NMR tube inside 10-mm-o.d. NMR tube; (c) 2.3-nm Au; (d) 5.1-nm Au; (e) 10-nm Au; (f) 21-nm Au; and (g) 47-nm Au. (c-g are the same as b, except with variable thicknesses of Au films coated onto the 7.49-mm-o.d. NMR tube.)
aromatic region of dimethyl benzene-1,3-dicarboxylate obtained in the electrochemical-NMR cell (in nonspinning mode) containing only a 10-mm-o.d. tube in the region of the RF coils. The spectrum shows a singlet, a doublet, and a triplet due to Ha, 2Hb, and Hc, respectively. Figure 2b shows the spectrum recorded under the same instrumental conditions when a 7.49-mm-o.d. NMR tube was placed inside the 10-mm-o.d. tube. A decrease in signal intensity was observed following the addition of the inner tube due to the decrease in solution volume. Figure 2c-g shows the change in the NMR spectra as gold films of increasing thickness were coated onto the outer surface of the 7.49-mm-o.d. NMR tube (a photograph of the gold coated tubes is given in Figure S-1.6). The 10-mm probe was tuned before each measurement to optimize the sensitivity, and the shim coils were adjusted to obtain the narrowest line widths possible under the unusual conditions. For film thicknesses of
