In Situ Spectroscopic Studies in Homogeneous Catalysis - Advances

2 In Situ Spectroscopic Studies in Homogeneous Catalysis Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch002

Robin Whyman Research and Technology Department, ICI Chemicals and Polymers Ltd., P.O. Box 8, The Heath, Runcorn, Cheshire WA7 4QD, United Kingdom

This chapter describes the various methods that have been developed for in situ spectroscopic studies on homogeneously catalyzed reactions requiring the use of elevated pressures and temperatures for optimum performance. The advantages and disadvantages of the two most commonly used techniques, vibrational and nuclear magnetic resonance spectroscopies, are discussed. A critical evaluation of the types of spectroscopic cells that have been developed for high-pressure infrared studies is presented. Examples of applications of the latter illustrate the ease with which (under favorable circumstances) unstable intermediates can be identified, the level of information that can be extracted from the spectra of relatively complex mixtures by using advanced instrumentation techniques, and the correlations between spectroscopic data and catalytic performance that can be obtained even with multicomponent catalyst systems, harsh and highly IR-absorbing reaction media, and extreme conditions of pressure and temperature.

MLANY I N D U S T R I A L L Y I M P O R T A N T C A T A L Y Z E D R E A C T I O N S require, for optimum performance, the use of high pressures and high temperatures. To provide as much insight as possible into the nature of the species present under such reaction conditions, it is highly desirable to have available, by the use of appropriate physical techniques, in situ methods of measurement. Clearly the use of such methods provides a significant advance over the traditional method of withdrawal of samples from high-pressure-high-temperature reactions followed by analysis under ambient conditions. Even if the catalyst precursor and the nature of the species obtained after withdrawal 0065-2393/92/0230-0019$06.00/0

© 1992 American Chemical Society

In Homogeneous Transition Metal Catalyzed Reactions; Moser, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1992.

20

H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS

Downloaded by MICHIGAN STATE UNIV on February 18, 2015 | http://pubs.acs.org Publication Date: March 1, 1992 | doi: 10.1021/ba-1992-0230.ch002

and depressurization are identical, it is highly likely that they exist in quite different forms under operating conditions. However, the use of in situ methods per se is extremely unlikely to lead directly to the identification of the actual catalytically active species, particularly with highly active catalysts. As Rooney (I) and others have pointed out, true catalytically active intermediates in solutions are very unstable and transient. Therefore, in most cases it is extremely difficult (by using spectroscopic techniques) to identify them among the much more numerous and more stable complexes that are also frequently present. These more stable complexes only transform, if at all, with a much lower frequency than those that form an intrinsic part of the catalytic cycle. In situ techniques can provide only a detailed insight into the most stable species present under a particular set of reaction conditions. Nevertheless, even if the results of in situ spectroscopic measurements taken in isolation are less than ideal, more meaningful information can emerge when they are assessed in parallel with kinetic studies and measurements of product distributions. Such a course should therefore provide the maximum benefit from in situ spectroscopic studies on catalyzed reactions.

Techniques for In Situ Studies For homogeneously catalyzed reactions an assessment of the information provided by appropriate physical techniques, together with factors such as general applicability and ease of experimentation, leads to the conclusion that vibrational spectroscopy and nuclear magnetic resonance spectroscopy are the techniques of choice. O f these, high-pressure infrared (HPIR) spectroscopy has become well established during the past 20 years (2), and the technique has become part of the traditional armory of physical methods, particularly in industrial laboratories. The application of nuclear magnetic resonance (NMR) spectroscopy is of rather more recent vintage, largely as a result of the generally greater degree of difficulty in the experimentation required (3-5). The two techniques are in many respects complementary. Thus, infrared (IR) spectroscopy has the particular advantages of speed and high sensitivity. The high sensitivity is compatible with the low catalyst concentrations typically used in many catalyzed processes. In contrast, N M R spectroscopy is characterized by slow time scale and low sensitivity. It frequently requires more concentrated solutions than those typically used in catalyzed reactions. In addition, instrumental limitations impose severe restrictions on the upper temperature limits that can be tolerated. O n the positive side, however, N M R data are typified by highly dispersed spectra from which, by the measurement of chemical shifts and couplings and by multinuclear operation, detailed structural information can be derived. This aspect is in sharp contrast to the data obtained from IR


2.

WHYMAN

In Situ Spectroscopic Studies in Homogeneous Catalysis

21

spectroscopy, in which overlapping spectra from closely related species are commonly observed. Such spectra are frequently difficult to separate, even with access to Fourier transform instrumentation.


The quantitative nature of N M R spectra also provides a contrast with vibrational spectroscopy because quantitative IR data can be generated only in favorable circumstances. The ability to operate these two complementary techniques in tandem can provide a distinct advantage. Following this introduction to the subject of in situ spectroscopic measurements, I shall concentrate on high-pressure IR studies. This chapter describes, in general terms, the key design criteria and the advantages and disadvantages of the various types of cells. I shall use examples taken from our work to illustrate various points such as the level (and limitations) of information that can be extracted by using current instrumentation and the harsh and corrosive chemical environments under which such cells can be used to provide meaningful data.

Design of High-Pressure Infrared Cells Some key items in the design of a high-pressure cell for in situ spectroscopic measurements are

• the material of construction, • the nature of the window material, • the design of the window-sealing arrangement, and • the ability to operate in a safe manner.

Key requirements for construction material are high mechanical strength and corrosion resistance toward reactants and products. Suitable materials are austenitic stainless steels such as type 316, nickel-molybdenumchromium alloys such as Hastelloy C-276, and various titanium alloys. Stainless steel is satisfactory for fairly mild pressure and temperature conditions. However, under certain conditions it reacts slowly with carbon monoxide to form small amounts of F e ( C O ) and Ni(CO) , which give rise to spurious bands in the IR spectra. Titanium alloys are strong, tough, and inert toward carbon monoxide. However, they are attacked at high temperatures by, for example, primary alcohols such as methanol and related molecules that contain active hydrogen atoms. Hastelloy C has the advantages of high strength and exceptional resistance to a wide variety of chemical process environments including strong oxidizing agents, mineral acids, carboxylic acids, and acetic anhydride. For reactions involving high hydrogen pressures the question of hydrogen embrittlement must also be considered, although this phenomenon generally becomes a problem only at temperatures higher than 200 °C. 5

4


22



The primary requirements for window materials are high mechanical strength and corrosion resistance, together with transparency over the maximum infrared range. The strength of the window material frequently limits the maximum operating pressure of the cell because the window materials available for long-wavelength transmission are generally mechanically weak. A compromise has to be reached. A summary of the properties of a variety of materials suitable for different pressure, temperature, and wavelength ranges is given in Table I. For much of the work with the ICI cells, we have found calcium fluoride windows to be very suitable.

Table I. Window Materials for High-Pressure IR Transmission Studies Material

Spectral Range (cm' )

Pressure (atm)

Temperature (°C)

50 14 11 5.8

4000-2000 4000-700 4000-1200 4000-650

10,000 1,000 650 200

>25() 250 250 100

1

6

Sapphire ZnS (Irtran 2) CaF NaCl 2

Youngs Modulus (psi x JO )

The window-sealing arrangement is also very important and a range of designs has been considered. These are best illustrated by reference to the various cell designs.

Types of High-Pressure Cells.

The many types of high-pressure

cells for vibrational spectroscopic studies in homogeneously catalyzed reactions fall into two basic categories. The first includes self-contained units, such as autoclaves fitted with windows, that can be stirred and heated. Thus chemical reactions can be monitored continuously from start to finish without perturbing the system (i.e., a truly in situ experiment). Typical of this category are the cells we have developed in ICI (6) and the Moser cells (7), which take advantage of the data-acquisition method involving cylindrical internal reflectance (the C I R C L E cell). High-pressure cells comprising the second category are those in which the cell and autoclave are separate components operated in conjunction with one another. Reacting solutions from an autoclave are circulated through the IR cell by means of either gravity or a pump. Typical of these flow cells are the Monsanto cell (8), which can also be used for UV-visible spectroscopic measurements, and the Penninger cell (9), which can be used for the study of slurries and of the gas-solid systems typical of heterogeneous catalysts. The Penninger cell was originally designed for studies of the liquid-phase hydroformylation reaction using C o ( C O ) immobilized on cross-linked polystyrene. A modification of the Monsanto cell has been developed by Union Carbide (10) to allow operation under more severe reaction conditions. The operating ranges of these cells are summarized in Table II. 2

H


2.

WHYMAN

In Situ Spectroscopic Studies in Homogeneous Catalysis Table II. High-Pressure IR Cells Maximum Operating Pressure (atm)

Reactors


Batch reactors ICI cell Moser CIR cell Flow type reactors Monsanto cell Penninger cell

Maximum Operating Temperature (°C)

1000

250

80

160

100

200

600

450

Advantages of Specific Cell Designs.

Windows CaF , ZnS ZnSe crystal 2

CaF CaF

2 2

The various cell designs have

their advantages and disadvantages. Although the details are described else where (2, 6-10), a few general comments are appropriate. ICI and Moser cells are complete reaction vessels with integral stirrers. Thus, chemical reactions can be followed from start to finish in a truly in situ manner. In both cell designs the windows are surrounded on all sides by the reaction solution. This arrangement minimizes the possibility of a "stagnant" area between the cell windows that would not be representative of the composition of the bulk solution. The importance of effective stirring, which is not necessarily a feature of all the reported cell designs, may be easily demonstrated by monitoring the spectrum of a solvent (e.g., η-heptane) under pressures of gaseous carbon monoxide. After initial pressurization to about 100 atm, followed by acti vation of the stirrer, strong absorptions are observed immediately at ca. 2140 cm" , showing dissolved carbon monoxide. The intensity of these ab sorptions does not increase significantly with time. In contrast, only very weak peaks are observed when the system is not stirred. The intensity of these peaks increases only slowly with time, and the system requires many hours in which to establish the equilibrium concentration of dissolved gas (6). 1

The advantage of the Monsanto-Penninger-type flow cells is that they are separate small-volume units that can be isolated from the autoclave, which contains the bulk of the reacting fluids. Thus any damage to the spectrometer in the event of a window failure is minimized. Continuousflow reactions can be monitored easily. Thus these cells can be very useful adjuncts as side stream reactors to monitor, for example, the progressive changes in reacting species or catalyst change-decay on chemical plants. However, they are not truly in situ experiments. The contents of the autoclave must be circulated through the cell and back again. This process inevitably leads to a pressure drop across the system, which can have two significant consequences. First, dissolved gases may be released. These gases usually tend to accumulate at the narrowest part of the system, namely the gap between the windows of the cell. Such accumulations lead to experi mental difficulties, particularly if quantitative measurements are required.


23

24

HOMOGENEOUS

TRANSITION M E T A L C A T A L Y Z E D REACTIONS

Second, and more seriously, the equilibria between different species in solution can be very sensitive to changes in pressure. This condition is particularly true for reactions involving C O - H and carbonylmetals. A very clear example of this disadvantage is shown in data obtained on the Co-Ru-catalyzed conversion of methanol and C O to methyl acetate (7). A comparison of the spectra obtained under nominally identical conditions shows that the major species, [Ru(CO) I ]~, observed in the in situ cell apparently partially decomposes during the time taken to pump the sample from an autoclave through a flow cell. This observation indicates that caution is required in interpreting data obtained from flow cells where a true instantaneous view of the reacting species is not available. 2


3

3

Identification and Characterization of Unstable Molecules and Reactive Intermediates Reaction of Ru(CO) PPh with Dihydrogen. 4

A classic example of

3

the use of high-pressure infrared spectroscopy for the identification of unstable species derives from a study of the reaction of dihydrogen with hydrocarbon solutions of R u ( C O ) P P h (11). Convincing evidence was obtained for the reversible formation of R u ( C O ) ( H ) P P h according to the equilibrium in reaction 1. 4

3

3

Ru(CO) PPh 4

3

2

3

+ H «± Ru(CO) (H) PPh + C O 2

3

2

(1)

3

The reasons for the unequivocal nature of this example are as follows: • both ruthenium-containing species displayed relatively simple well-resolved spectra that were spatially separated and therefore easily distinguishable, • the spectra were not complicated by strong absorption bands caused by dissolved gases (e.g., C O ) , and • the stable osmium analog had been isolated and characterized previously, and spectroscopic data were therefore available for comparison. Intense interest has recently arisen in the characterization of complexes containing molecular dihydrogen. This system was therefore reexamined, as part of a collaborative exercise with the University of Nottingham, to seek spectroscopic evidence for the formation of R u ( C O ) ( H ) P P h as a possible precursor state to the dihydride. Supercritical xenon was used as the solvent. Although the dihydride was readily obtained by photolysis at ca. 50 atm of hydrogen pressure, no spectroscopic evidence for the intermediacy of the proposed molecular dihydrogen complex was obtained (12). 3

2

3


2.

WHYMAN


25

Reactions of Rh (CO) with Carbon Monoxide and Dihydrogen. 4

12

B y analogy w i t h the k n o w n c h e m i s t r y of the carbonylcobalts, t w o k e y species, i n t e r r e l a t e d t h r o u g h reactions of R h ( C O ) w i t h carbon m o n o x i d e a n d d i h y d r o g e n , are l i k e l y reactive intermediates. 4

1 2


(2)

T h e r e v e r s i b l e f o r m a t i o n of R h ( C O ) d u r i n g the reaction of R h ( C O ) u n d e r pressures o f carbon m o n o x i d e was first r e p o r t e d 20 years ago (13). F u r t h e r spectroscopic e v i d e n c e i n support of the b r i d g e d f o r m of R h ( C O ) was p r o v i d e d b y the matrix isolation experiments of H a n l a n a n d O z i n (14) a n d later b y the m u c h m o r e d e t a i l e d quantitative w o r k of O l d a n i a n d B o r (IS). 2

8

4

1 2

2

8

Rh (CO) has b e e n u s e d for m a n y years as a catalyst p r e c u r s o r for reactions i n v o l v i n g the a d d i t i o n of h y d r o g e n to organic substrates. H o w e v e r , d e f i n i t i v e e v i d e n c e for the second species, H R h ( C O ) , or its c o o r d i n a t i v e l y unsaturated d e r i v a t i v e , H R h ( C O ) , is v i r t u a l l y nonexistent. I n spite of i n tensive w o r l d w i d e research efforts, there is o n l y a single report of the d e tection of H R h ( C O ) (16) u n d e r e x t r e m e reaction conditions (at 1542 atm pressure). T h e v e r y h i g h pressure r e q u i r e m e n t is somewhat s u r p r i s i n g i n v i e w of the k n o w n catalytic activity of R h ( C O ) u n d e r reaction c o n d i t i o n s that are not far r e m o v e d f r o m a m b i e n t . It o c c u r r e d to us that the apparent n o n o b s e r v a t i o n of H R h ( C O ) u n d e r m i l d e r reaction c o n d i t i o n s m i g h t s i m p l y b e a c o n s e q u e n c e of c o i n c i d e n t absorption bands. 4

1 2

4

3

4

4

1 2

4

I n o u r o r i g i n a l w o r k , u s i n g a dispersive grating spectrometer, a d e t a i l e d study of this system was h a m p e r e d b y strong absorptions caused b y d i s s o l v e d c a r b o n m o n o x i d e . T h e situation has b e e n considerably i m p r o v e d w i t h access to F o u r i e r transform i n s t r u m e n t a t i o n , b u t p r o b l e m s still r e m a i n . F i r s t , the subtraction of i n f i n i t e absorbance f r o m i n f i n i t e absorbance gives meaningless i n f o r m a t i o n . S e c o n d , the subtraction of the s p e c t r u m of R h ( C O ) f r o m a composite R h ( C O ) - R h ( C O ) s p e c t r u m is not t r i v i a l because v e r y slight p r e s s u r e - d e p e n d e n t shifts i n the b a n d m a x i m a positions result i n the appearance of differential peaks i n the subtracted spectra. 4

4

1 2

2

1 2

8

N e v e r t h e l e s s w e d e c i d e d to investigate this system again b y i n i t i a l l y generating the R h ( C O ) - R h ( C O ) e q u i l i b r i u m m i x t u r e at 600 a t m of c a r b o n m o n o x i d e pressure a n d r o o m t e m p e r a t u r e . D i h y d r o g e n was t h e n i n t r o d u c e d a n d spectral changes carefully m o n i t o r e d . I n s p e c t i o n of the resultant spectra ( F i g u r e 1) reveals that the absorbances of the 2041- a n d 1884c m " bands characteristic of R h ( C O ) do not r e m a i n constant w i t h respect to each other u p o n the a d d i t i o n of d i h y d r o g e n . T h e relative absorbance of the 2 0 4 1 - c m " b a n d increases b y ca. 2 0 - 2 5 % . I n a d d i t i o n , t h e r e is a p a r a l l e l increase i n the half-height b a n d w i d t h of this peak (Table III). A s i m i l a r t r e n d , 4

1

1 2

2

8

4

1 2

1




26


2.

WIIYMAN

27

In Situ Spectroscopic Studies in Homogeneous Catalysis Table III. Addition of H to Rh (CO)i -Rh (CO)e-CO System in η-Heptane 2

4

Reaction Conditions

Absorbance Ratios"

250 atm C O 400 atm C O 600 atm C O 600 atm C O - H (1:1) 600 atm C O - H (1:2) 600 atm C O - H (1:3) 130 atm H

0.83 0.78 0.83 0.98 1.10 1.01 0.84

2

2

2


i

2

1

Half-Height Bandwidth " 1

1.00 1.00 1.04 1.18 1.18 1.15 1.04

"2041 cm" /1883 cm" maxima. ''2041-em- band. l

1

1

although much less marked (ca. 5-10% increase), is noted in the absorbance of the 2068-cm band. 1

Furthermore, when the C O - H pressure is slowly released and replaced by H alone, the absorbance ratios and half-height bandwidths return, within experimental error, to their initial values. During this operation, as expected from previous work (13), the spectrum reverts to that of R h ( C O ) alone. The addition of dihydrogen to the R h ( C O ) - R h ( C O ) equilibrium mixture thus appears to result in the generation of a transient species that is char acterized by absorption bands at ca. 2041 and 2068 cm" . O f these values, the former peak is significantly more intense. Carbonyl stretching frequencies for R h ( C O ) j , R h ( C O ) , and their co balt analogs, including H C o ( C O ) , are collected in Chart I. Comparison 2

2

4

4

12

2

12

8

1

4

2

2

8

4

Chart I. Metal Carbonyl Stretching Frequencies Co (CO) 2

Rh (CO) 4

l2

Co (CO) 4

l2

Rh (CO) 2

8

Bridged Isomer

8

Nonbridged Isomer

HCo(CO)

4

2118 vw 2084 s 2075 vs 2070 vs

2071 vs 2063 vs 2055 vs

2069 vs

2060 s 2052 m

2044 vs

2044 vs 2042 vs

2038 m 2028 m

2031 ms 2022 vs

2029 s 1996 vw

1884 s

1867 s

1861 mw 1845 s

1866 sh 1857 s

N O T E : Spectra measured in paraffin hydrocarbon solvents. Frequencies measured in wavenumbers (reciprocal centimeters). ABBREVIATIONS: s, strong; m, medium; w, weak; v, very; sh, shoulder.


28


between the terminal carbonyl stretching frequencies of the various complexes leads to an estimate of the position of the absorption maxima for the purported H R h ( C O ) to occur at ca. 2043 (s) and 2067 (m) cm" . The analysis of the high-pressure spectroscopic data presented here is therefore consistent with the transient presence, at room temperature, of H R h ( C O ) upon addition of dihydrogen to the R h ( C O ) - R h ( C O ) equilibrium mixtures generated under carbon monoxide. 1

4

4

4

12

2

8


Synthesis Gas Chemistry Under Extreme Reaction Conditions During our work on synthesis gas chemistry, we discovered a range of composite homogeneous catalysts for the selective production of C oxygenate esters, particularly ethylene glycol diacetate, directly from synthesis gas (17). 2

CO + H

R u :

2

^

(

A

1 0 1 )

> CH OAc + C H OAc + CH OAc + CH OAc 3

2

5

2

tlvlAC

2

J

(3)

I

CH OAc

CH OH

2

2

In glacial acetic acid as solvent (eq 3), these complex catalyst combinations (which contain mixtures of ruthenium and rhodium as major and minor components, respectively) are promoted by both nitrogen-containing bases and alkali metal cations. Genuine synergistic effects on both catalytic activity and selectivity to C products were observed (Table IV). This behavior is unusual in the light of the commonly observed inverse relationship between catalytic activity and selectivity. Severe reactions conditions (1000 atm C O - H , 230 °C) were required for optimum selectivities to C products. To gain some insight into the nature of the species present in solution, we decided to make IR spectroscopic measurements on the various catalyst 2

2

2

Table IV. Activity-Selectivity Behavior of R u - R h - E t N - H O A c Catalyst Combinations 3

Catalyst Component Catalyst Combination

( ) Rh mmo1

Ru

1 2 3 4 5

—

2.0 2.0

-

—

0.2 0.2 0.2

-

2.0

N O T E : Reaction conditions were as pressure; C O - H (1:1); 2 3 0 °C; and CO consumed, moles per liter per Moles of C H O A c per mole of C H 2

fl

fc

2

3

Et N

Activity

Selectivity

_ 2.0

0.23 0.20 0.22 0.44 0.89

0.09 0.06 0.06 1.38 1.31

3

-

2.0 2.0

0

11

follows: 50 mL of glacial acetic acid solvent; 1000 atm of 2 h. hour. OAc.

CH OAc(H) 2


2.

WHYMAN


29

components at pressures and temperatures approaching those under which the catalytic reactions were investigated. These experiments were difficult, partly because the effective spectral range imposed by the strongly absorbing background spectrum of glacial acetic acid, even at extremely narrow path lengths of ca. 20-30 μπι between the cell windows, is only 2200-1900 cm and partly because of the corrosive nature of acetic acid toward O-rings and ancillary equipment.


1

Some interesting correlations between the spectroscopic data and the catalytic activities-selectivities have been identified. For the ruthenium cat alyst alone the addition of E t N has, within experimental error, no influence on either catalytic activity or product selectivity. Infrared spectra of both systems measured at 750 atm and 200 ° C are essentially identical and are consistent with the presence of predominantly [ Ru(CO) 0Ac] together with much smaller concentrations of Ru(CO) and R u ( C O ) . 3

3

5

3

2

12

The addition of E t N to the rhodium catalyst doubles the activity and dramatically increases the selectivity to C oxygenate esters. The IR spectra of the two systems show distinct differences. Whereas R h ( C O ) is the only detectable species with rhodium alone, the effect of the addition of E t N is reflected in the additional formation of [Rh (CO) X]~ (X is either Η or OAc). In the composite R u - R h - E t N (Ru:Rh = 10:1) catalyst, the spectrum (Figure 2) is dominated by an absorption at 2040 cm" . The most likely assignment of this peak is to a species H R u ( C O ) O A c (n is 3 or 4) formed by a homolytic cleavage reaction between [ R u ( C O ) O A c ] and H , presumably mediated by the presence of the rhodium complex in solution. In addition, minor amounts of [Rh (CO) X]~ may be present. Although it is not precisely clear how the spectroscopic data relate to the catalytically active species in these complex systems, direct correlations between the spectroscopic data and the catalytic performance are readily apparent. 3

2

6

16

3

6

15

3

1

n

3

6

2

2

15

Conclusion In this overview of in situ spectroscopic studies, with emphasis on highpressure IR measurements, examples of applications have illustrated the following points: • the ease with which, under favorable circumstances, unstable intermediates can be identified, • the level of information that can be extracted from the spectra of relatively complex mixtures by using advanced instrumen tation techniques, and • the correlations between spectroscopic data and catalytic per formance that can be obtained even with multicomponent cat-



30


2200

2150

2100

2050 cm"

2000

1950

1900

1

Figure 2. Reactions of synthesis gas with a Ru-Rh-Et3N (10:1:10) composite catalyst in glacial acetic acid. Reaction conditions: a, 750 atm, 100 °C; b, 850 atm, 175 ° C , c, 880 atm, 200 °C.

alyst systems, harsh and highly IR-absorbing reaction media, and extreme conditions of pressure and temperature.

Acknowledgment I acknowledge numerous colleagues from the former Corporate Laboratory and New Science Group of ICI for their help with the design, construction, operation, and development of the ICI high-pressure IR cells.


2.

WIIYMAN


31


References

1. Rooney, J. J. J. Mol. Catal. 1985, 31, 147. 2. Whyman, R. In Laboratory Methods in Vibrational Spectroscopy; Willis, Η. Α van der Maas, J. H.; Miller, R. G. J., Eds.; Wiley: Chichester, England, 1987; p 281. 3. Jonas, J.; Hasha, D. L.; Lamb, W. J.; Hoffman, G. Α.; Eguchi, T.J.Magn. Reson. 1981, 42, 169. 4. Heaton, B. T.; Strona, L.; Jonas, J.; Eguchi, T.; Hoffman, G. A. J. Chem. Soc., Dalton Trans. 1982, 1159. 5. Roe, D. C. J. Magn. Reson. 1985, 63, 388. 6. Hunt, Κ. Α.; Page, R. W.; Rigby, S.; Whyman, R. J. Phys. Ε 1984, 17, 559. 7. Moser, W. R.; Cnossen, J. E.; Wang, A. W.; Krouse, S. A. J. Catal. 1985, 95, 21. 8. Morris, D. E.; Tinker, Η. B. Rev. Sci. Instrum. 1972, 43, 1024. 9. Penninger, J. M. L. J. Catal. 1979, 56, 287; High Pressure Sci. Technol., Proc. Int. AIRAPT Conf., 7th 1980, 2, 842. 10. Walker, W. E.; Cosby, L. Α.; Martin, S. T. U.S. Patent 3 886 364, 1975. 11. Whyman, R. J. Organomet. Chem. 1973, 56, 339. 12. Upmacis, R. K. Ph.D. Thesis, University of Nottingham, 1986. 13. Whyman, R. J. Chem. Soc., Chem. Commun. 1970, 1194. 14. Hanlan, L. Α.; Ozin, G. A. J. Am. Chem. Soc. 1974, 96, 6324. 15. Oldani, F.; Bor, G. J. Organomet. Chem. 1983, 246, 309. 16. Vidal, J. L.; Walker, W. E. Inorg. Chem. 1981, 20, 249. 17. Whyman, R. J. Chem. Soc., Chem. Commun. 1983, 1439; U.S. Patent 4 665 222, 1987. RECEIVED for review October 19, 1990. A C C E P T E D revised manuscript May 28, 1991.


In Situ Spectroscopic Studies in Homogeneous Catalysis - Advances

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