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Compact Fourier transform infrared spectrometer inside a glove box: Fourier transform spectroelectrochemistry and other applications
101
Vibrational Spectroscopy, 2 (1991) 101-106 Elsevier Science Publishers B.V., Amsterdam
Compact Fourier transform infrared spectrometer inside a glove box: Fourier transform spectroelectrochemistry and other applications Anushuyadevi Saravanarnuthu, Alice E. Bruce * and Mitchell R.M. Bruce * Department of Chemistry, University of Maine, Orono, ME 04469 (USA) (Received 10th December 1990)
Abstract Use of a compact Fourier transform infrared (ET-IR) spectrometer inside a glove box to investigate air and moisture sensitive compounds is reported. The F’l-IR spectrometer, manufactured by Midac Co., measures 18 cm X 28 cm x 61 cm and can be conveniently placed inside a standard glove box (Vacuum Atmospheres). This match of a low cost, compact ET-IR spectrometer with widely available glove box technology affords researchers a simple, routine method for acquiring ET-IR data for air and moisture sensitive compounds. Several applications are illustrated, including reaction kinetics and ET-IR spectroelectrochemistry. A design for an IT-IR spectroelectrochemical cell is also described.
Keywork
Infrared spectrometry; Fourier transform; Spectroelectrochemistry
A Fourier transform infrared (FT-IR) spectrometer (18 cm X 28 cm X 61 cm), manufactured by Midac Co. (Costa Mesa, CA), was conveniently placed inside a standard glove box (Vacuum Atmospheres, Hawthorne, CA) by way of the antechamber. This match of a low cost, compact FT-IR spectrometer with widely available glove box technology affords researchers a simple, routine method for acquiring IR data for air and moisture sensitive compounds. Several applications are illustrated, including the technique of IR spectroelectrochemistry. A design for an FT-IR spectroelectrochemical cell is also described. Standard glove box and Schlenk techniques have made the manipulation of air and moisture sensitive compounds routine [1,2]. Air and/or moisture sensitive solutions are commonly prepared for FT-IR analysis in an inert atmosphere using Schlenk techniques and then syringed into an FT-IR cell fitted with septa. However, this 0924-2031/91/$03.50
technique may be undermined by diffusion of oxygen through punctured septa over the course of an experiment. Alternatively, air sensitive samples may be prepared inside a glove box, transferred to an FT-IR cell, and then removed from the glove box for analysis. Time is an important consideration for samples prepared in this way. Each refilling of an FT-IR cell is a significant interruption of an experiment because lo-15 min is typically required to reintroduce a cell into a glove box. Further, the sample compartment of the FT-IR spectrometer may need to be purged of atmospheric moisture and carbon dioxide each time a sample is introduced. Placing the FT-IR spectrometer in a glove box offers several advantages. Contamination by oxygen and moisture is greatly reduced, purging of the sample compartment is eliminated, solutions can be prepared, a cell filled, and a spectrum acquired in a minimum of time, and experiments can be repeated quickly.
0 1991 - Elsevier Science Publishers B.V. All rights reserved
102
However, until recently, placing an FT-IR spectrometer in a glove box was not feasible because of the large footprint of most commercially available FT-IR spectrometers. The Midac Co. FT-IR spectrometer has an unusually small footprint that allows it to be placed inside a glove box with room to spare.
EXPERIMENTAL
Placement of spectrometer in the glove box All compartment covers of the FT-IR are removed before the instrument is placed in the antechamber to ensure complete removal of 0,. The spectrometer is designed to operate at any angle of incline, to sustain mechanical shocks, and maintain its optical alignment to deliver 0.5 cm-’ resolution. The resolution can be adjusted from 0.5 to 32 cm- ‘. In the experiments, all spectra were recorded using 4 scans per spectrum at 4 cm-’ resolution. The spectrometer is interfaced to a personal computer and is controlled using Speu tra Calc software (Galactic Industries, Salem, NH). The RS-232 cable required for instrument control was passed into the glove box through a gas feed-through opening which was then sealed with caulking compound. The integrity of the optics and electronics of the FT-IR spectrometer is an important consideration. The glove box is thoroughly purged before and after the experiment to minimize exposure of the instrument to solvent vapor. As a further precaution, the instrument can be placed in the glove box when a series of experiments is planned and removed when the experiments are completed. Reaction of a rhodium hydride complex, sodium cation, and CO, 3.2 mg (3 x 10d6 mol) Rh(PP,)H [3] and 3.1 mg (9 x 10m6 mol) NaBPh, were dissolved in 4 ml THF in a nitrogen-filled glove box. THF was dried with sodium benzophenone and distilled under N,. Carbon dioxide (Airco, grade 4) was bubbled into the solution for lo-15 s with rapid stirring. The glove box is equipped with gas feed through lines for the introduction of substrate gases. An aliquot of the solution was then trans-
A. SARAVANAMUTHU
ET AL.
ferred to an FT-IR cell, placed in the sample compartment, and a spectrum was acquired. Reaction of a molybdenum(O) complex with 8azidoquinoline The rate of disappearance of the azide band, v”(N,), at 2118 cm-‘, was measured as a function of concentration of Mo(CO),(CH,CN),(PPh,) (0.056-4.5 mM) and 8azidoquinoline (O.ll4.5 mM). Spectra Calc software facilitates the acquisition of repetitive spectral scans. One of the products of the reaction is the complex Mo(CO)JN(PPh,)(C,H,N)] which is isolated as red brown crystals in 20% yield. Addition of CO to the reaction mixture at 0°C increases the yield of Mo(CO),[N(PPh,)(C,H,N)] to 60%. Selected spectroscopic data for Mo(CO)JN(PPh,&H,N)]: ‘H NMR (200 MHz in CD,Cl,): S 6.45 (dt, lH), 6.85 (t, lH), 7.10 (dd, lH), 7.32 (dd, lH), 7.85-7.47 (m, 15H, PPh,), 8.10 (dd, lH), 9.14 ppm (dd, 1H). “P{‘H} NMR (81 MHz in CD&l,): 27.8 ppm (s). IR (nujol) ~(Mo-CO): 2000, 1886, 1859, 1820 cm-’ [4]. Spectroelectrochemistry of niobium ketene complex A 10 mM solution of CpiNb(Cl)(ketene) [Cp’ = C,H,Si(CH,),; ketene = OCC(Ph)(Et)] in 0.8 M TBAH-THF (TBAH = tetra-N-butylammonium hexafluorophosphate) was prepared in the
100
g
E 6
e
50
E 6 ;
0
c Wavenumber
(cm-‘)
Fig. 1. FT-IR spectrum of Rh(PP,)H (0.1 mM) and NaBPh4 (0.2 mM) in THF before addition of CO, (1); -cl min after addition of CO* (2).
COMPACT
FT-IR
103
SPECTROMETBR
glove box [5]. Electrochemical experiments were carried out with a Princeton Applied Research EG&G Model 273 potentiostat/galvanostat. In a typical reduction experiment the cell potential was 250 mV more negative than that employed in a standard electrochemical cell. This difference is undoubtedly due to cell resistance. A low rate of diffusion is desirable when all three electrodes are in the same compartment because products generated at the auxiliary electrode may eventually interfere with observation of products generated at the working electrode. During the time scale of the experiments, no detrimental effects due to diffusion were observed.
Wavenumber IB
(cm-l)
A
.2
RESULTS AND DISCUSSION
Figure 1 (trace 1) illustrates an FT-IR spectrum of a THF solution of Rh(PP,)H [O.l n&I; PP, = tris(2-(diphenylphosphino)ethyl)phosphine] and sodium tetraphenylborate (NaBPh,) (0.2 mM) [6]. The spectrum obtained after bubbling carbon dioxide gas through the solution for lo-15 s (Fig. 1, trace 2) shows three bands at 1720,136O and 1220 cm-’ that are characteristic of a metal-CO, intermediate [3,7-131. The initial rhodium-CO, intermediate persists for several minutes, long enough to be observed using an external spectrometer. Reaction kinetics (measured in a separate experiment) establish that the initial intermediate disappears in about 6 min. However, as this example illustrates, less than 1 min was required to introduce carbon dioxide, mix the solution, fill an FT-IR cell, and acquire a spectrum using the Midac spectrometer inside a glove box. Therefore this technique offers a convenient way to obtain IR data for short lived reaction intermediates. Reaction kinetics are also conveniently obtained using a Midac FT-IR inside a glove box. We have obtained kinetic data for the reaction of Sazidoquinoline with the MO(O) complex, Mo(CO),(CH,CN),(PPh,), by monitoring changes in IR absorption bands over time [4]. An acetonitrile solution of 8-azidoquinoline (4.5 mM) was mixed with an acetonitrile solution of the MO complex (4.5 mM) in a glove box. The reaction was stirred for 1 min, and an aliquot was trans-
8 { 4”
.l 19’40
’
19’20
’
19’00
Wavenumber
(cm-l)
’
1 Eii30
Fig. 2. Time dependent FT-IR spectral changes in the reaction of I-azidoquinoline (4.5 mM) and Mo(CO),(CH,CN),(PPh,) (4.5 mM). The first spectrum was acquired at t = 146 s; spectra at 60-s intervals are plotted. Arrows indicate growth or disappearance of peaks with time. (A) Disappearance of v’(N,) band at 2118 cm-’ for I-azidoquinoline. (B) Disappearance of e(GO) band at 1925 cm-’ for Mo(CO),(CH,CN),(PPh,), and appearance of ;(C=O) band at 1888 cm-’ for
Mo(CO),[N(PPh,X~H,N)l. ferred to an IR cell. The time elapsed between mixing the reagents and recording the first spectrum was 146 s and spectra were acquired at 30 s intervals. The disappearance of the azide band, Y”(N3) at 2118 cm-’ (Fig. 2A), is concurrent with disappearance of the carbonyl bands of Mo(CO), (CH,CN),(PPh,) [only the F(CkG) stretch at 1925 cm-’ is shown in the left hand portion of Fig. 2B]. Analysis of the kinetic data suggests that the reaction is first order in both Mo(CO),(CH,CN), (PPh,) and azide. An absorption band due to a new carbonyl species grows in at 1888 cm-’ (right hand side of Fig. 2B). This product has been
A. SARAVANAMUTHU
104
Ph3P
CCH3
IIN OC-MO-NCCH3 oC’
+
co
I
(1) isolated, and spectroscopically and structurally characterized (by x-ray crystallography) as the phosphinimine complex, Mo(CO),[N(PPh,)(C,
H&)1 (Eqn. 1).
Experiments that involve monitoring the initial stages of an air sensitive reaction are made more convenient by having the spectrometer in a glove box. (Approximately 5 min is required to prepare a solution in a glove box, fill an IR cell, transfer the cell to an external spectrometer, purge the sample compartment, and acquire a spectrum.) Spectroelectrochernistry is a technique that combines electrochemistry with a number of specSpacer Auxiliary
Electrode Pt Wire
Reference
Fig. 3. Schematic of IT-IR
Electrode
troscopic methods. These methods include examples from absorption (Miissbauer, x-ray, ellipsometry), scattering (x-ray fluorescence, photoemission, Raman fluorescence), resonance (NMR, ESR) and fragmentation (mass) spectroscopy [14]. FT-IR spectroelectrochemistry is especially useful because it can provide detailed structural information about electrochemically generated species. Many IR spectroelectrochemical cells have been developed based on either transmission or reflectance of the infrared beam. These cells differ widely in sensitivity, path length, solvent interference, and internal cell resistance [15-201. The transmission spectroelectrochemical cell, illustrated in Fig. 3, is constructed from a commercially available demountable FT’-IR cell (Spectra Tech, Stamford, CT). The major modification is that the standard back plate has been replaced by one measuring 19 mm in depth (easily constructed in a machine shop) to accommodate the thickness of a second needle plate. The working electrode is Pt gauze (52 mesh, Aldrich) centered between NaCl windows. Both windows are drilled and separated by a 0.5~mm PTFE spacer; the path length of the cell measures 0.48 f 0.2 mm and the cell volume is approximately 0.1 ml. The cell path length was calculated using the 850 cm-’ band of
(0.5 mm)
Luer Lock Fittings
\
Back Plate
Ag Wire
spectroelectrochemical
cell.
ET AL.
Knurled
End Cap
COMPAm
Fl--IR
SPECTROMETER
benzene. {The cell design presented is a hybrid between an optically transparent cell which typically has separate electrode compartments, relatively long path lengths (e.g. 10 mm), and low iR drop and the optically transparent thin layer cell which typically has one compartment, short path lengths (e.g. 0.1 mm), and high iR drop [15].} Electrode wires (Pt working, Pt auxiliary, and Ag reference) are introduced through septa inserted in three of the Luer lock fittings of the needle plates and are passed through the drilled windows. Electrical contact between the Pt working electrode wire and the Pt gauze is established by threading the Pt wire through the gauze. The remaining electrode wires are positioned so that there is no contact between electrodes. Glass (or PTFE) sleeves insulate the wires from metal fittings. The Pt mesh is essentially transparent to the infrared beam and, as described below, strongly absorbing chromophores such as metal carbonyls or ketenes can be observed with little interference (A coarser Pt mesh working electrode can also be used.) The sample is injected into the assembled cell through a septum inserted in the fourth Luer lock fitting (a thin exit needle is placed in one of the other septa). As previously mentioned, punctured septa are prone to leakage of small amounts of oxygen and/or water. Spectroelectrochemistry inside a glove box minimizes this problem. A background spectrum of the cell, filled with electrolyte solution, is acquired in the transmission mode and is automatically subtracted from sample spectra. The results of an FT-IR spectroelectrochemical experiment are shown in Fig. 4. Trace 1 illustrates the spectrum in the carbonyl region of a 10 mM solution of CpiNb(Cl)(ketene) before reduction [5]. Upon reduction at -3.0 V (vs. Ag wire), the strong v(C=O) stretch for the ketene ligand in the starting complex disappears (at 1628 cm-‘) and a broad band grows in at 1535 cm-‘, assigned as the one electron reduction product, [Cp;Nb(Cl)(enolate)]- (Fig. 4, trace 2 and Eqn. 2). The applied potential is removed after 10 min (between traces 2 and 3). The enolate complex, [CpiNb(Cl)(enolate)]-, reacts via loss of chloride to produce a carbonyl complex with a band at 1900 cm-’ (Fig. 4, traces 3 and 4). With this experimental setup, high quality spectra were obtained in a reproduci-
48 1840
1640
1740 Wavenumber
1540
(cm-‘)
Fig. 4. FT-IR spectral changes during spectroelectrochemical experiment of 10 mM Cp;Nb(Cl)(lcetene) in 0.8 M TBAHTHF: (1) before reduction, (2) after 8 min at -3.0 V (the cell was turned off at t = 10 min), (3) t = 14 min, (4) t = 20 min.
ble fashion, and the time required to repeat experiments was dramatically shortened.
P
Cp;NL@
t
R’ ‘R’
a
F’
Cp~Nt+! 'C-R
(2)
k,
Conclusion The compact Midac FT-IR placed inside a glove box allows for routine sampling and characterization of air and moisture sensitive compounds. This arrangement makes FT-JR spectroelectrochemical experiments especially convenient. The FT-IR electrochemical cell, designed primarily from commercially available parts, is easy to assemble and makes the technique of spectroelectrochemistry more accessible to non-specialists. Acknowledgement is made to the University of Maine Faculty Research Fund, the National Science Foundation, and the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Tom Jenks for helpful discussions and technical assistance.
REFERENCES 1 D.F. Shriver and M.A. Drezdzon, The Manipulation of Air-Sensitive Compounds, Wiley, New York, 1986.
106 2 A.L. Wayda and MY. Darensbourg (Eds.), Experimental GrganometaIIic Chemistry (ACS Symp. Series, Vol. 357), American Chemical Society, Washington, DC, 1987. 3 C. Bianchini and A. Meli, 3. Am. Chem. Sot., 106 (1984) 2698. 4 A. Saravanamuthu, J.-L. Fourquet, M. Leblanc, M.R.M. Bruce and A.E. Bruce, Inorg. Chem., 30 (1991) 3241. 5 A. Saravanamuthu, A.E. Bruce, M.R.M. Bruce, M.C. Fermin, AS. Hneihen and J.W. Bnmo, in preparation. 6 F. Destefano and M.R.M. Bruce, unpublished results. 7 E.G. Lundquist, J.C. Huffmann, K. Folting, B.E. Mann and K.G. Cat&on, Inorg. Chem., 29 (1990) 128. 8 E. Fujita, D.J. SzaIda, C. Creutz and N. Sutin, J. Am. Chem. Sot., 110 (1988) 4870. 9 E.G. Lundquist, J.C. Huffman and K.G. Caulton, J. Am. Chem. Sot., 108 (1986) 8309. 10 J.M. Maher, G.R. Lee and N.J. Cooper, J. Am. Chem. Sot., 104 (1982) 6797. 11 S. Gambarotta, F. Arena, C. Floriani and P.F. Zanazzi, J. Am. Chem. Sot., 104 (1982) 5082.
A.SARAVANAMlJTHlJETAL.
12 J.M. Maher and N.J. Cooper, J. Am. Chem. Sot., 102 (1980) 7604. 13 G. Fachinetti, C. Floriani and P.F. Zanarzi, J. Am. Chem. Sot., 100 (1978) 7405. 14 R.J. Gale (Ed.), Spectroelectrochemistry, Theory and Practice, Plenum Press, New York, 1988. 15 R. Greef, R. Peat, L.M. Peter, D. Pletcher and J. Robinson, Instrumental Methods of Electrochemistry, Wiley, New York, 1985, Ch. 10. 16 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 1980. 17 S.P. Best, R.J.H. Clark, R.P. Cooney and R.C.S. McQueen, Rev. Sci. Instrum., 57 (1987) 2071. 18 K. Ashley and S. Pons, Chem. Rev., 88 (1988) 673. 19 P.D. Boyle, D.C. Boyd, A.M. Mueting and L.H. Pignolet, Inorg. Chem., 27 (1988) 4424. 20 J.P. BuIIock and K.R. Mann, Inorg. Chem., 28 (1989) 4006.
Vibrational Spectroscopy, 2 (1991) 101-106 Elsevier Science Publishers B.V., Amsterdam
Compact Fourier transform infrared spectrometer inside a glove box: Fourier transform spectroelectrochemistry and other applications Anushuyadevi Saravanarnuthu, Alice E. Bruce * and Mitchell R.M. Bruce * Department of Chemistry, University of Maine, Orono, ME 04469 (USA) (Received 10th December 1990)
Abstract Use of a compact Fourier transform infrared (ET-IR) spectrometer inside a glove box to investigate air and moisture sensitive compounds is reported. The F’l-IR spectrometer, manufactured by Midac Co., measures 18 cm X 28 cm x 61 cm and can be conveniently placed inside a standard glove box (Vacuum Atmospheres). This match of a low cost, compact ET-IR spectrometer with widely available glove box technology affords researchers a simple, routine method for acquiring ET-IR data for air and moisture sensitive compounds. Several applications are illustrated, including reaction kinetics and ET-IR spectroelectrochemistry. A design for an IT-IR spectroelectrochemical cell is also described.
Keywork
Infrared spectrometry; Fourier transform; Spectroelectrochemistry
A Fourier transform infrared (FT-IR) spectrometer (18 cm X 28 cm X 61 cm), manufactured by Midac Co. (Costa Mesa, CA), was conveniently placed inside a standard glove box (Vacuum Atmospheres, Hawthorne, CA) by way of the antechamber. This match of a low cost, compact FT-IR spectrometer with widely available glove box technology affords researchers a simple, routine method for acquiring IR data for air and moisture sensitive compounds. Several applications are illustrated, including the technique of IR spectroelectrochemistry. A design for an FT-IR spectroelectrochemical cell is also described. Standard glove box and Schlenk techniques have made the manipulation of air and moisture sensitive compounds routine [1,2]. Air and/or moisture sensitive solutions are commonly prepared for FT-IR analysis in an inert atmosphere using Schlenk techniques and then syringed into an FT-IR cell fitted with septa. However, this 0924-2031/91/$03.50
technique may be undermined by diffusion of oxygen through punctured septa over the course of an experiment. Alternatively, air sensitive samples may be prepared inside a glove box, transferred to an FT-IR cell, and then removed from the glove box for analysis. Time is an important consideration for samples prepared in this way. Each refilling of an FT-IR cell is a significant interruption of an experiment because lo-15 min is typically required to reintroduce a cell into a glove box. Further, the sample compartment of the FT-IR spectrometer may need to be purged of atmospheric moisture and carbon dioxide each time a sample is introduced. Placing the FT-IR spectrometer in a glove box offers several advantages. Contamination by oxygen and moisture is greatly reduced, purging of the sample compartment is eliminated, solutions can be prepared, a cell filled, and a spectrum acquired in a minimum of time, and experiments can be repeated quickly.
0 1991 - Elsevier Science Publishers B.V. All rights reserved
102
However, until recently, placing an FT-IR spectrometer in a glove box was not feasible because of the large footprint of most commercially available FT-IR spectrometers. The Midac Co. FT-IR spectrometer has an unusually small footprint that allows it to be placed inside a glove box with room to spare.
EXPERIMENTAL
Placement of spectrometer in the glove box All compartment covers of the FT-IR are removed before the instrument is placed in the antechamber to ensure complete removal of 0,. The spectrometer is designed to operate at any angle of incline, to sustain mechanical shocks, and maintain its optical alignment to deliver 0.5 cm-’ resolution. The resolution can be adjusted from 0.5 to 32 cm- ‘. In the experiments, all spectra were recorded using 4 scans per spectrum at 4 cm-’ resolution. The spectrometer is interfaced to a personal computer and is controlled using Speu tra Calc software (Galactic Industries, Salem, NH). The RS-232 cable required for instrument control was passed into the glove box through a gas feed-through opening which was then sealed with caulking compound. The integrity of the optics and electronics of the FT-IR spectrometer is an important consideration. The glove box is thoroughly purged before and after the experiment to minimize exposure of the instrument to solvent vapor. As a further precaution, the instrument can be placed in the glove box when a series of experiments is planned and removed when the experiments are completed. Reaction of a rhodium hydride complex, sodium cation, and CO, 3.2 mg (3 x 10d6 mol) Rh(PP,)H [3] and 3.1 mg (9 x 10m6 mol) NaBPh, were dissolved in 4 ml THF in a nitrogen-filled glove box. THF was dried with sodium benzophenone and distilled under N,. Carbon dioxide (Airco, grade 4) was bubbled into the solution for lo-15 s with rapid stirring. The glove box is equipped with gas feed through lines for the introduction of substrate gases. An aliquot of the solution was then trans-
A. SARAVANAMUTHU
ET AL.
ferred to an FT-IR cell, placed in the sample compartment, and a spectrum was acquired. Reaction of a molybdenum(O) complex with 8azidoquinoline The rate of disappearance of the azide band, v”(N,), at 2118 cm-‘, was measured as a function of concentration of Mo(CO),(CH,CN),(PPh,) (0.056-4.5 mM) and 8azidoquinoline (O.ll4.5 mM). Spectra Calc software facilitates the acquisition of repetitive spectral scans. One of the products of the reaction is the complex Mo(CO)JN(PPh,)(C,H,N)] which is isolated as red brown crystals in 20% yield. Addition of CO to the reaction mixture at 0°C increases the yield of Mo(CO),[N(PPh,)(C,H,N)] to 60%. Selected spectroscopic data for Mo(CO)JN(PPh,&H,N)]: ‘H NMR (200 MHz in CD,Cl,): S 6.45 (dt, lH), 6.85 (t, lH), 7.10 (dd, lH), 7.32 (dd, lH), 7.85-7.47 (m, 15H, PPh,), 8.10 (dd, lH), 9.14 ppm (dd, 1H). “P{‘H} NMR (81 MHz in CD&l,): 27.8 ppm (s). IR (nujol) ~(Mo-CO): 2000, 1886, 1859, 1820 cm-’ [4]. Spectroelectrochemistry of niobium ketene complex A 10 mM solution of CpiNb(Cl)(ketene) [Cp’ = C,H,Si(CH,),; ketene = OCC(Ph)(Et)] in 0.8 M TBAH-THF (TBAH = tetra-N-butylammonium hexafluorophosphate) was prepared in the
100
g
E 6
e
50
E 6 ;
0
c Wavenumber
(cm-‘)
Fig. 1. FT-IR spectrum of Rh(PP,)H (0.1 mM) and NaBPh4 (0.2 mM) in THF before addition of CO, (1); -cl min after addition of CO* (2).
COMPACT
FT-IR
103
SPECTROMETBR
glove box [5]. Electrochemical experiments were carried out with a Princeton Applied Research EG&G Model 273 potentiostat/galvanostat. In a typical reduction experiment the cell potential was 250 mV more negative than that employed in a standard electrochemical cell. This difference is undoubtedly due to cell resistance. A low rate of diffusion is desirable when all three electrodes are in the same compartment because products generated at the auxiliary electrode may eventually interfere with observation of products generated at the working electrode. During the time scale of the experiments, no detrimental effects due to diffusion were observed.
Wavenumber IB
(cm-l)
A
.2
RESULTS AND DISCUSSION
Figure 1 (trace 1) illustrates an FT-IR spectrum of a THF solution of Rh(PP,)H [O.l n&I; PP, = tris(2-(diphenylphosphino)ethyl)phosphine] and sodium tetraphenylborate (NaBPh,) (0.2 mM) [6]. The spectrum obtained after bubbling carbon dioxide gas through the solution for lo-15 s (Fig. 1, trace 2) shows three bands at 1720,136O and 1220 cm-’ that are characteristic of a metal-CO, intermediate [3,7-131. The initial rhodium-CO, intermediate persists for several minutes, long enough to be observed using an external spectrometer. Reaction kinetics (measured in a separate experiment) establish that the initial intermediate disappears in about 6 min. However, as this example illustrates, less than 1 min was required to introduce carbon dioxide, mix the solution, fill an FT-IR cell, and acquire a spectrum using the Midac spectrometer inside a glove box. Therefore this technique offers a convenient way to obtain IR data for short lived reaction intermediates. Reaction kinetics are also conveniently obtained using a Midac FT-IR inside a glove box. We have obtained kinetic data for the reaction of Sazidoquinoline with the MO(O) complex, Mo(CO),(CH,CN),(PPh,), by monitoring changes in IR absorption bands over time [4]. An acetonitrile solution of 8-azidoquinoline (4.5 mM) was mixed with an acetonitrile solution of the MO complex (4.5 mM) in a glove box. The reaction was stirred for 1 min, and an aliquot was trans-
8 { 4”
.l 19’40
’
19’20
’
19’00
Wavenumber
(cm-l)
’
1 Eii30
Fig. 2. Time dependent FT-IR spectral changes in the reaction of I-azidoquinoline (4.5 mM) and Mo(CO),(CH,CN),(PPh,) (4.5 mM). The first spectrum was acquired at t = 146 s; spectra at 60-s intervals are plotted. Arrows indicate growth or disappearance of peaks with time. (A) Disappearance of v’(N,) band at 2118 cm-’ for I-azidoquinoline. (B) Disappearance of e(GO) band at 1925 cm-’ for Mo(CO),(CH,CN),(PPh,), and appearance of ;(C=O) band at 1888 cm-’ for
Mo(CO),[N(PPh,X~H,N)l. ferred to an IR cell. The time elapsed between mixing the reagents and recording the first spectrum was 146 s and spectra were acquired at 30 s intervals. The disappearance of the azide band, Y”(N3) at 2118 cm-’ (Fig. 2A), is concurrent with disappearance of the carbonyl bands of Mo(CO), (CH,CN),(PPh,) [only the F(CkG) stretch at 1925 cm-’ is shown in the left hand portion of Fig. 2B]. Analysis of the kinetic data suggests that the reaction is first order in both Mo(CO),(CH,CN), (PPh,) and azide. An absorption band due to a new carbonyl species grows in at 1888 cm-’ (right hand side of Fig. 2B). This product has been
A. SARAVANAMUTHU
104
Ph3P
CCH3
IIN OC-MO-NCCH3 oC’
+
co
I
(1) isolated, and spectroscopically and structurally characterized (by x-ray crystallography) as the phosphinimine complex, Mo(CO),[N(PPh,)(C,
H&)1 (Eqn. 1).
Experiments that involve monitoring the initial stages of an air sensitive reaction are made more convenient by having the spectrometer in a glove box. (Approximately 5 min is required to prepare a solution in a glove box, fill an IR cell, transfer the cell to an external spectrometer, purge the sample compartment, and acquire a spectrum.) Spectroelectrochernistry is a technique that combines electrochemistry with a number of specSpacer Auxiliary
Electrode Pt Wire
Reference
Fig. 3. Schematic of IT-IR
Electrode
troscopic methods. These methods include examples from absorption (Miissbauer, x-ray, ellipsometry), scattering (x-ray fluorescence, photoemission, Raman fluorescence), resonance (NMR, ESR) and fragmentation (mass) spectroscopy [14]. FT-IR spectroelectrochemistry is especially useful because it can provide detailed structural information about electrochemically generated species. Many IR spectroelectrochemical cells have been developed based on either transmission or reflectance of the infrared beam. These cells differ widely in sensitivity, path length, solvent interference, and internal cell resistance [15-201. The transmission spectroelectrochemical cell, illustrated in Fig. 3, is constructed from a commercially available demountable FT’-IR cell (Spectra Tech, Stamford, CT). The major modification is that the standard back plate has been replaced by one measuring 19 mm in depth (easily constructed in a machine shop) to accommodate the thickness of a second needle plate. The working electrode is Pt gauze (52 mesh, Aldrich) centered between NaCl windows. Both windows are drilled and separated by a 0.5~mm PTFE spacer; the path length of the cell measures 0.48 f 0.2 mm and the cell volume is approximately 0.1 ml. The cell path length was calculated using the 850 cm-’ band of
(0.5 mm)
Luer Lock Fittings
\
Back Plate
Ag Wire
spectroelectrochemical
cell.
ET AL.
Knurled
End Cap
COMPAm
Fl--IR
SPECTROMETER
benzene. {The cell design presented is a hybrid between an optically transparent cell which typically has separate electrode compartments, relatively long path lengths (e.g. 10 mm), and low iR drop and the optically transparent thin layer cell which typically has one compartment, short path lengths (e.g. 0.1 mm), and high iR drop [15].} Electrode wires (Pt working, Pt auxiliary, and Ag reference) are introduced through septa inserted in three of the Luer lock fittings of the needle plates and are passed through the drilled windows. Electrical contact between the Pt working electrode wire and the Pt gauze is established by threading the Pt wire through the gauze. The remaining electrode wires are positioned so that there is no contact between electrodes. Glass (or PTFE) sleeves insulate the wires from metal fittings. The Pt mesh is essentially transparent to the infrared beam and, as described below, strongly absorbing chromophores such as metal carbonyls or ketenes can be observed with little interference (A coarser Pt mesh working electrode can also be used.) The sample is injected into the assembled cell through a septum inserted in the fourth Luer lock fitting (a thin exit needle is placed in one of the other septa). As previously mentioned, punctured septa are prone to leakage of small amounts of oxygen and/or water. Spectroelectrochemistry inside a glove box minimizes this problem. A background spectrum of the cell, filled with electrolyte solution, is acquired in the transmission mode and is automatically subtracted from sample spectra. The results of an FT-IR spectroelectrochemical experiment are shown in Fig. 4. Trace 1 illustrates the spectrum in the carbonyl region of a 10 mM solution of CpiNb(Cl)(ketene) before reduction [5]. Upon reduction at -3.0 V (vs. Ag wire), the strong v(C=O) stretch for the ketene ligand in the starting complex disappears (at 1628 cm-‘) and a broad band grows in at 1535 cm-‘, assigned as the one electron reduction product, [Cp;Nb(Cl)(enolate)]- (Fig. 4, trace 2 and Eqn. 2). The applied potential is removed after 10 min (between traces 2 and 3). The enolate complex, [CpiNb(Cl)(enolate)]-, reacts via loss of chloride to produce a carbonyl complex with a band at 1900 cm-’ (Fig. 4, traces 3 and 4). With this experimental setup, high quality spectra were obtained in a reproduci-
48 1840
1640
1740 Wavenumber
1540
(cm-‘)
Fig. 4. FT-IR spectral changes during spectroelectrochemical experiment of 10 mM Cp;Nb(Cl)(lcetene) in 0.8 M TBAHTHF: (1) before reduction, (2) after 8 min at -3.0 V (the cell was turned off at t = 10 min), (3) t = 14 min, (4) t = 20 min.
ble fashion, and the time required to repeat experiments was dramatically shortened.
P
Cp;NL@
t
R’ ‘R’
a
F’
Cp~Nt+! 'C-R
(2)
k,
Conclusion The compact Midac FT-IR placed inside a glove box allows for routine sampling and characterization of air and moisture sensitive compounds. This arrangement makes FT-JR spectroelectrochemical experiments especially convenient. The FT-IR electrochemical cell, designed primarily from commercially available parts, is easy to assemble and makes the technique of spectroelectrochemistry more accessible to non-specialists. Acknowledgement is made to the University of Maine Faculty Research Fund, the National Science Foundation, and the Donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this research. We thank Tom Jenks for helpful discussions and technical assistance.
REFERENCES 1 D.F. Shriver and M.A. Drezdzon, The Manipulation of Air-Sensitive Compounds, Wiley, New York, 1986.
106 2 A.L. Wayda and MY. Darensbourg (Eds.), Experimental GrganometaIIic Chemistry (ACS Symp. Series, Vol. 357), American Chemical Society, Washington, DC, 1987. 3 C. Bianchini and A. Meli, 3. Am. Chem. Sot., 106 (1984) 2698. 4 A. Saravanamuthu, J.-L. Fourquet, M. Leblanc, M.R.M. Bruce and A.E. Bruce, Inorg. Chem., 30 (1991) 3241. 5 A. Saravanamuthu, A.E. Bruce, M.R.M. Bruce, M.C. Fermin, AS. Hneihen and J.W. Bnmo, in preparation. 6 F. Destefano and M.R.M. Bruce, unpublished results. 7 E.G. Lundquist, J.C. Huffmann, K. Folting, B.E. Mann and K.G. Cat&on, Inorg. Chem., 29 (1990) 128. 8 E. Fujita, D.J. SzaIda, C. Creutz and N. Sutin, J. Am. Chem. Sot., 110 (1988) 4870. 9 E.G. Lundquist, J.C. Huffman and K.G. Caulton, J. Am. Chem. Sot., 108 (1986) 8309. 10 J.M. Maher, G.R. Lee and N.J. Cooper, J. Am. Chem. Sot., 104 (1982) 6797. 11 S. Gambarotta, F. Arena, C. Floriani and P.F. Zanazzi, J. Am. Chem. Sot., 104 (1982) 5082.
A.SARAVANAMlJTHlJETAL.
12 J.M. Maher and N.J. Cooper, J. Am. Chem. Sot., 102 (1980) 7604. 13 G. Fachinetti, C. Floriani and P.F. Zanarzi, J. Am. Chem. Sot., 100 (1978) 7405. 14 R.J. Gale (Ed.), Spectroelectrochemistry, Theory and Practice, Plenum Press, New York, 1988. 15 R. Greef, R. Peat, L.M. Peter, D. Pletcher and J. Robinson, Instrumental Methods of Electrochemistry, Wiley, New York, 1985, Ch. 10. 16 A.J. Bard and L.R. Faulkner, Electrochemical Methods, Fundamentals and Applications, Wiley, New York, 1980. 17 S.P. Best, R.J.H. Clark, R.P. Cooney and R.C.S. McQueen, Rev. Sci. Instrum., 57 (1987) 2071. 18 K. Ashley and S. Pons, Chem. Rev., 88 (1988) 673. 19 P.D. Boyle, D.C. Boyd, A.M. Mueting and L.H. Pignolet, Inorg. Chem., 27 (1988) 4424. 20 J.P. BuIIock and K.R. Mann, Inorg. Chem., 28 (1989) 4006.