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Journal of the Chilean Chemical Society

versión On-line ISSN 0717-9707

J. Chil. Chem. Soc. v.48 n.2 Concepción jun. 2003

http://dx.doi.org/10.4067/S0717-97072003000200003 

J. Chil. Chem. Soc., 48, N 2 (2003)

Synopsis

HOMOGENEOUS CATALYSIS OF THE WATER GAS SHIFT
REACTION : IR, 1H- AND 13C-NMR IN SITU STUDIES
ON CATALYTIC IRIDIUM SYSTEMS

A.J. Pardey1, M.A. Moreno1, M.C. Ortega1, B. Mendez1,
A.B. Rivas1, S.A. Moya2, D. Villagra2, , M. Lutz3

1 Escuela de Química, Facultad de Ciencias, Universidad Central de Venezuela, Caracas 1020-A, Venezuela
2 Departamento de Química Aplicada, Facultad de Química y Biología, Universidad de Santiago de Chile,
Santiago de Chile, Chile
3 Departament of Chemistry, University of Joensuu, Finland

( Received : October 20, 2002 ­ Accepted : June 11, 2003)

ABSTRACT

Reported here are the in situ FT-IR and 13C-NMR spectroscopic studies of the complex cis-[Ir(CO)2(py)2](PF6 ) (py = pyridine) dissolved in 80% aqueous pyridine used as a precursor in the homogeneous catalysis of the water gas shift reaction (WGSR, CO + H2O Æ CO2 + H2). These spectroscopy studies reveal the presence of aminocarbonyliridium complexes with linear and bridging carbonyls groups as reaction intermediates. The WGSR catalysis by Ir4(CO)12 complex dissolved in 80% of aqueous 4-picoline or pyridine under 1.9 atm of CO at 100 C and the FT-IR and 1H-NMR in situ studies of the pyridine promoted disproportionation of Ir4(CO)12, are also described.

Key Words: Pyridine, WGSR, iridium catalyst.

INTRODUCTION

The water gas shift reaction is catalyzed by several homogeneous metal complexes, which can work under mild conditions1-2). The homogeneous catalysts can also be supported on polymers in order to take advantages of the heterogeneous systems3-6).

Our current interest in the catalysis of the WGSR by transition metal complexes7-8) as well as the catalytic reduction of nitrobenzene under WGSR conditions9-11) led us to study the catalytic properties of cis-[Ir(CO)2(amine)2](PF 6) (amine = 4-picoline, 3-picoline, pyridine, 2-picoline, 3,5-lutidine, or 2,6-lutidine) dissolved in aqueous amine solution12).

The present study has three goals. The first is to compare the WGSR catalytic reactivity of the Ir(CO)12 complex dissolved in 80% aqueous amine (amine = pyridine or 4-picoline) to the well-studied cis-[Ir(CO)2(amine)2](PF 6)/aqueous amine system7,12). The second is to carry out the in situ characterization studies of the cis-[Ir(CO)2(py)2](PF6 )/aqueous pyridine system. The third goal is to study by FT-IR and 1H-NMR the disproportionation reaction of the Ir4(CO)12 complex by anhydrous pyridine in order to compare the nature of the species formed under this conditions with the species produced under WGSR catalytic conditions by the above described Ir systems.

EXPERIMENTAL

Materials: The pyridine (py) and 4-picoline (4-pic) from Aldrich were distilled in KOH. Water was doubly distilled. The gases and gas mixtures N2, He/H2 (91.4%/8.6%, v/v), CO/CH4 (95.8%/4.2%, v/v) and CO/CH4/CO2/H2 (84.8%/5.1%/5.3%/4.8%, v/v) were purchased from BOC Gases and were used as received. The gas 13CO (99%) was purchased from Cambridge Isotop Laboratories, Inc and used as received. The iridium cis-[Ir(CO)2(py)2](PF 6) (amine = pyridine or 4-picoline) complexes were prepared as reported by Pardey et al.7,9). The complex Ir4(CO)12 was prepared as reported by Della Pergola et al.13). Both products were identified by IR spectroscopy.

Instrumentation: Gas sample analyses were accomplished on a Hewlett-Packard 5890 Series II programmable gas chromatograph fitted with Carbosieve-B (mesh 80-100) column and a thermal conductivity detector. The column was programmed from 60 to 175 C at the He/H2 carrier-gas flow rate of 50 mL/min. Infrared spectra were recorded on a Perkin-Elmer 1760X-FT and on a Nicolet 750 spectrophotometers. 13C-NMR spectra were recorded in a Jeol Eclipse 270 spectrometer under the following operation conditions: resonance frequency of 68 MHz; repetition time of 1s in all cases; scan number of (500-3000). 1H-NMR spectra were recorded in a Brucker Avance 250 spectrometer under the following operation conditions: resonance frequency of 250 MHz; scan number of 16. Internal reference was TMS.

Batch reactor procedure: The batch reactor techniques used here were describ ed previously14). The catalysis solutions were prepared by dissolution of given amount of the iridium complexes (0.1 mmol) in 10 mL of the solvent system and placed in a reactor flask fitted with a Teflon stopcock plus a ground-glass joint for attachment to a vacuum line equipped with a manometer and gas inlet. The solution was then degassed by three freeze-pump-thaw cycles. The flask was charged with the CO/CH4 mixture such that P(CO) equaled 1.9 atm at 100 C (CH4 was present as internal standard). The reactor vessel was stirred magnetically in a thermally equilibrated glycerin bath, and a magnetic bar stirred the reaction solution continuously.

The progress of the reaction was followed by taking samples (1.0 mL) with a gas microsyringe (Precision Sampling Corp.) which can be inserted into a small T-tube section of the vacuum line capped with a rubber septum, at appropriate times for about four hours intervals, and analyzing such samples for H2, CO, CH4 and CO2 by GC techniques. After the gas phase above the reaction mixture was sampled, the solution was subjected to one freeze-pump-thaw cycle to remove the excess of CO and the H2/CO2 previously generated. The reaction mixture was recharged with fresh CO/CH4, and the glass vessel was returned to the glycerin bath until next reading. Quantities of H2, CO2 and CO in the sample were determined from GC calibrations made by using various sized samples (0.1 - 5.0 mL) of the standardized gas mixture CO/CH4/H2/CO2. Carbon monoxide consumption was determined by subtracting the amount of CO remaining in the catalysis vessel from the amount of initial CO present as calculated by the ideal gas law.

RESULTS AND DISCUSSION

WGSR catalysis studies. We have previously described an example of homogeneous catalysis of the WGSR by cis-[Ir(CO)2(py)2](PF 6) (amine = 4-picoline, 3-picoline, pyridine, 2-picoline, 3,5-lutidine, or 2,6-lutidine) dissolved in aqueous amine solution12). The WGSR catalytic activity for this system, defined as turnover frequency of hydrogen {TF = moles of H2/(g-atom. Wt. of Ir x 24)-1 }, followed the order 4-picoline {12} > 3-picoline {10} = pyridine {10} > 3,5-lutidine {5} > 2-picoline {3} = 2,6-lutidine {3} for [Ir] = 10 mM, P(CO) = 1.9 atm at 100 C. Electronic and steric effects both apparently influence this trend.

Further, the kinetics studies of the more active 4-picoline system, shows that the catalytic activity proved to be nonlinear in iridium total concentration, [Ir]tot, over the range 5 to 80 mM. This result was interpreted in terms of the presence of active mononuclear and polynuclear iridium species in the system, being the mononuclear the more active. The rate displays a first order dependence on CO pressure, P(CO), over the range 0.7 to 1.9 atm. The kinetics behavior with respect to P(CO) leads to propose the CO addition to the catalytic species prior to the rate-limiting step7).

In the present section we report the reactive properties of dodecacarbonyltetrairidium(0) (Ir4(CO)12) complex in the catalytic activation of the WGSR. Dissolution of a solid yellow sample of Ir4(CO)12 (0.110 5 g, 1x10-4 mol) in 10 mL of 80% aq. 4-picoline or pyridine at room temperature leads to the gradual formation of a yellow solution. After charging with CO and heating the reaction round bottom flask at 100 C (P(CO) = 1.9 atm) in a glycerin bath, this solution changed to a permanent red-brown color solution in few minutes. Similar color changes were observed in the previous reported iridium(I),

cis-[Ir(CO)2(4-pic) 2](PF6)/aq. 4-picolina WGSR catalytic system7,12). In addition, the catalytic activity defined as H2 turnover frequencies (TF(H2) = mol H2(mol complex)-1(24 h)-1) for both iridium(0) and iridium(I) precursors dissolved in 4-piricone is similar (TF(H2) = 14 day-1) under the same catalytic conditions. This result suggests that the above-described catalytic systems could form catalytic species of similar nature. Further, Fachinetti et al.15,16) demonstrated that the analogous rhodium(0) and rhodium(I) WGSR catalytic systems, namely, Rh4(CO)12 and cis-[Rh(CO)2(py)2](PF 6), promoted by aqueous amine solutions under CO atmosphere formed the same catalytic species.

Characterization studies. The FT-IR spectrum in CHCl3 of recently prepared cis-[Ir(CO)2(py)2](PF 6) complex shows two strong bands in the n co region at 2080 and 1999 cm-1 before being used as catalyst12). On the contrary, the FT-IR spectrum of the active red-brown Ir/py catalytic solution, after being exposed to CO (1.9 atm) and heated at 100 C, displays five bands in the dCO region, at 1988(w), 1986(s), 1966(m) (characteristic of COs terminally coordinated to Ir) and at 1850(s) and 1767 cm-1 (characteristic of bridging COs), suggesting the presence of carbonyl iridium compounds as reaction intermediates.

Further, when a solid sample of cis-[Ir(CO)2(py)2](PF 6) (1.0X10-4 mol) was dissolved in 5 mL of pyridine-d5/H2O mixture, 4/1,v/v under 13CO (99 %) atmosphere and heated for 24 h at 80 °C, the resulting red-brown solution gave a 13C-NMR spectrum at 25 °C where, seven carbonyl resonance signals were observed at dC = 211.2 (d), 209.6 (t), 195.2 (d), 194.2 (d), 183.9 (s), 183.2 (s) and 176.2(s) ppm. The first four signals are assigned to bridging COs and the last three are assigned to terminally COs. It also was observed a singlet at dC =192.5 ppm (free CO in solution). The chemical shifts values for the first four signal are consistent with those reported previously for carbonyls bridging two Ir centers, while the last three signal are consistent with data for terminal COs coordinated to an Ir center17,18). Those data confirm the above FT-IR conclusion that the dominant species under the WGSR catalytic conditions must contain both terminally and bridging coordinated carbonyls.

Disproportionation of Ir4(CO)12. It has been reported that carbonyls of metal of Groups 8-10 undergo disproportionation in the presence of nitrogen bases. Also the ionic products have been characterized19). Among the metal carbonyls disproportionated by pyridine are Co2(CO)820), Rh4(CO)1215) and Rh6(CO)1615). However, there is not information available related to the reaction with pyridine of the known, stable iridium carbonyl, Ir4(CO)12.

So, we report here those studies. Dissolution a 0.5 g (4.5x10-4 mol) of Ir4(CO)12 in 5 mL of anhydrous pyridine, under reflux by 24 hours in nitrogen atmosphere, forms an orange solution in one hour and later changes to a final dark-brown color solution. The FT-IR spectrum (CaF2 cells) of the dark-brown solution shows seven bands in the nco region, at 2094.5(w), 2068.4(sh), 2048.4(s), 2031.5(s), 2013.1(s), 1822.6(m) and 1785.7(m) cm-1. The highest energy band at 2094.5 cm-1 is presumably due to a cationic iridium complex21). On the other hand, the two bands at 1822.6, and 1785.7 cm-1 can be assigned to bridging carbonyl CO groups of unknown anionic iridium clusters. As it happened in the disproportionation of Rh4(CO)1215) it was not possible to precipitate neither the cation nor the anion arising from the pyridine promoted Ir4(CO)12 disproportionation.

However, by reduction with CO/H2O of the totally disproportionated pyridine solution formed during the pyridine induced disproportionation of Rh4(CO)12 it was possible to isolate and structurally characterize the cluster anion, [(py)2H][Rh5(CO)13 (py)2]15). The molecular structure reported for this complex presents five Rh atoms located at the vertices of a trigonal bipyramid. There are seven terminal and six bridging carbonyls. In addition, the IR spectrum of this anionic cluster is the same in the solid state and in pyridine solution (nco = 2040(m), 2011(m), 1980(vs), 1818(m) and 1756(m) cm-1), showing that the anion [Rh5(CO)13(py)2] - retains its structure in both phases.

Further, all the n co bands assigned to this rhod ium(-I) anion were observed in the totally disproportionated solution of Rh4(CO)12 in anhydrous pyridine, together with bands at n co = 2095 and 2100 cm-1, which were assigned to the rhodium(I) cation, cis-[Rh(CO)2(py)2] +.

Thus, based on the analysis of the number as well as the position of the CO stretching bands of the Ir4(CO)12/py disproportionate solution, we suggest the presence of mononuclear cationic cis-[Ir(CO)2(py)2] + (bands at 2094.5 and 2013.1 cm-1), [Ir(CO)(py)1]- (band at 2068.4 cm-1) and polynuclear anionic iridium(-I) complexes (bands at 2048.4, 2031.5, 2013.1, 1822.6, and 1785.7 cm-1) with linear and bridging carbonyls as disproportionate products. These assignments are consistent with the reported results of Fachinetti et al. 15) discussed above.

In addition, the 1H-NMR spectrum (benzene-d6) of the dark-brown color solution formed by Ir4(CO)12 dissolved in 5 mL of anhydrous pyridine, under reflux by 24 hours in nitrogen atmosphere shows three multiplets at 5.9 - 6.0, 6.4 ­ 6.5 and 8.0 ­ 8.1 ppm corresponding to the protons of the pyridine rings. On the other hand, the 1H-NMR spectrum (benzene-d6) of the free pyridine shows three multiplets at 6.6 - 6.8, 6.9 ­ 7.1 and 8.5 ­ 8.6 ppm. The observed change on the chemical shift of these signals to higher field values of the former system can be attributed to the coordination of pyridine molecules to the Ir center under the reaction conditions.

Both FT-IR and 1H-NMR results suggest the formation of pyridine-carbonyl iridium compounds during the disproportionation of Ir4(CO)12 by anhydrous pyridine under nitrogen atmosphere.

Despite of some differences observed in the number and the position of the n co bands in the FT-IR spectra of the cis-[Ir(CO)2(py)2](PF 6) catalytic solution and the Ir4(CO)12 disproportionation solution, maybe due to solvent shift, some qualitative similarity between them suggests the formation in both solutions of mono and polynuclear iridium carbonyl complexes, the latter bearing terminal and bridging carbonyls.

CONCLUSIONS

The following conclusions can be drawn from the catalytic and characterization studies of the WGSR by Ir(0) and Ir(I) complexes dissolved in aqueous pyridine under CO.

The Ir4(CO)12 dissolved in aqueous 4-picoline or pyridine forms a catalytic system for the WGSR which is similar to the cis-[Ir(CO)2(Py)2](PF 6) (amine = 4-picoline or pyridine) under the same catalytic conditions.

Also, the FT-IR and 13C-NMR results from the cis-[Ir(CO)2(py)2](PF 6) catalytic system suggest the presence of carbonyl iridium compounds as reaction intermediates with linear and bridging carbonyls and with different nuclearities.

The FT-IR and 1H-NMR results from the pyridine promoted disproportionation of Ir4(CO)12 suggest the formation of cationic cis-[Ir(CO)2(py)2]+ and [Ir(CO)(py)3]+ and polynuclear anionic iridium(-I) complexes.

ACKNOWLEDGMENTS

The present work was carried out under the research programs CDCH-UCV (Project 03.12.3799.96), CONICIT-Venezuela (Project S1-2435), and FONDECYT-Chile (Project 1020076, SAM), andDicyt-usach (RL).

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