In Situ IR Study of the Reaction Behavior of [Cr30z3-O)(#-O2CH)6(H20)2(O2CH)I'xH20 and Its Active Fragments for the Addition Reaction of Acetylene

XIE W E I W E N , CAI Y U K U I , Z H A N G L I N N A , * Z H E N G YU, H E L I N G J I E , and CAI S U H U A State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fujian, China

The reaction behavior of the complex [Cr3(#3-O)(/~-O2CH) 6- (H20)2(O2CH)I'xH20 [shown as (Cr3FF)] has been investigated in the system of acetylene with water vapor by means of in situ IR and TG analyses. It has been observed that some active fragments obtained by the loss of part of the ligands of (Cr3FF) demonstrated activity for the additional reaction of acetylene with water or hydrogen, forming either acetaldehyde or ethylene. On the basis of the study of the thermo-de- composition process, as well as the FAB-MS data, the nature of these active fragments has been elucidated. Index Headings: In situ IR; Reaction behavior; Chromium complex.

INTRODUCTION

As has been outlined in Ref. 1, a family of carboxylato complexes with the general formula [M3(#3-O)(#- 02CR)oL3] "+ (where n = 0, 1, 4, 7; M = some transition metals; and L = donor ligands) has received considerable attention from chemists because of its unusual physical and chemical properties, 2-5 which have not been satis- factorily explained so far. Moreover, in the few studies 0-1° concerning applications, this class of complexes already appears promising. In order to provide a scientific basis for the exploitation of these complexes, we have thus begun a program of systematic studies of their physical and chemical properties. This paper deals with the study of the react ion behavior of [Cr3(#3-O)(tt-O2CH)6- (H20)2(O2CH)] 'xH20 [shown as (Cr3FF)] in the system of acetylene with water vapor by means of IR and ther- mogravimetric (TG) analysis. The single crystal struc- ture of the complex has been determined, 11 and its struc- ture is schematically depicted in Fig. 1.

EXPERIMENTAL

Sample. (Cr3FF) was prepared according to the meth- od described in the literature. 12 A sample with 1.4 H20 was taken for analysis. The components found were: C, 15.3; H, 2.5; and Cr, 28.5%. The components calculated for C7H13.sOls.4Cr 3 w e r e : C, 15.3; H, 3.2; and Cr, 28.2%.

IR Measurements. The experiments of in si tu IR spec- tra were performed as follows: The (Cr3FF) (~ 40 mg) was placed in a variable-temperature cell, 13 which was then evacuated by an injector and purged with acetylene saturated with water vapor; this process was repeated twelve times. Finally, the cell was filled with about 20 mL of mixed gases and sealed; then the IR spectra of

Received 23 July 1992. * Author to whom correspondence should be sent.

gaseous products obtained in si tu were followed in a series of pre-assigned temperature increments up to 410°C on a Perkin-Elmer 577 spectrophotometer over the range 4000 to 200 cm -1.

The IR spectra of the solid samples run as KBr pellets were measured with the same spectrophotometer.

TG A n a l y s i s . TGA experiments with a heating rate of 5°C/min from room temperature to 500°C have been done on a TGA-2 thermogravimeter (Perkin-Elmer). The tem- perature was calibrated by a System-4 Controller. Data were collected and treated by a computer data station.

RESULTS AND DISCUSSION

React ion B e h a v i o r o f (Cr3FF) in the S y s t e m o f Ace ty - l ene w i t h W a t e r Vapor . The reaction path of (Cr3FF) in the system of C~H2 + H20 has been identified by in si tu variable-temperature IR spectra. The changes in the spectra of the products in gaseous phase alone were re- corded at elevated temperatures up to 304°C (Fig. 2).

As shown, with a rise in temperature the bands of water vapor (3450 and 1630 cm -1) can be detected (Fig. 2, 2 - 3). This pattern shows the process by which (Cr3FF) loses its adsorbed and crystallized water. Starting from about 150°C, the peaks of formic acid vapor (1100 and 1790- 1740 cm -1 are observed; its intensities increase as the temperature rises, then gradually decrease and disappear completely at about 250°C (Fig. 2, 4-7). This observation indicates that the terminal formic ligand of (Cr3FF) has been lost, creating the formic acid which then decom- poses due to heating. When the temperature reaches about 200°C, carbon monoxide and carbon dioxide appear (Fig. 2, 5-9); their intensities increase with a rise in temper- ature. This result seems to imply that from 200°C the following reactions take place:

HCOOH -~ CO2 + H2 ~- CO + H20. (1)

It is therefore understandable that CO and CO2 always appear together. In the meanwhile, it is worth empha- sizing the interesting fact that ethylene and acetaldehyde clearly appear at 220°C and 250°C and are temperature and time dependent (Fig. 2, 6-9). [The acetaldehyde might be produced at lower temperature; because its peak (1760-1730 cm -1) is very close to the formic acid peak, they are probably somewhat overlapping.] This experiment demonstrates that when the temperature reaches 220°C (or higher), some active fragments ob- tained by the loss of part of the ligands of (Cr3FF) show

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FIG. 1. Stereoscopic illustration of the molecule (Cr3FF).

an activity for the addition reaction of acetylene with hydrogen or water, forming either ethylene or acetal- dehyde, i.e.,

C2H2 + H2 --~ C2H4 (2)

C2H2 + H20 ~- CH3CHO. (3)

The source of hydrogen in reaction 2 comes from reaction 1, which is why ethylene is always observed after detec- tion of CO and CO2 in the system.

Temperature Dependence of the Reaction Products. As is very well known, a chemical reaction is strongly de- pendent upon the reaction temperature and time; there- fore, there was a need to determine the optimum reaction conditions. For this purpose an IR experiment was con- ducted as follows: Samples of the same weight were kept constantly apart at different temperatures in the same volume of C2H2 + H20, with increases in the IR inten- sities of the reaction products with time measured in s i tu until a constant value had been reached. As a criterion, the last value was then taken for the relative total content of the products at that temperature. The results obtained as a function of temperature are displayed in Fig. 3.

As shown, the activity of the addition reactions is very apparent at 220°C; as a matter of fact, the highest yield for CH3CHO is at about 300°C.

It is worth pointing out that the same trend has been obtained by GC analysis as well.

Nature of the Active Fragments. It has already been mentioned that (Cr3FF) shows an activity for the addi- tion reaction of acetylene at some steps in its thermo- decomposition process. There is sufficient reason, there- fore, to speculate that, if this active component could be prepared earlier, the addition reaction might take place at lower temperature. In order to verify this assumption, it is worthwhile to identify the IR spectral characteristics of the active component. With this aim in mind, the IR spectra were recorded for the sample showing obvious activity of the addition reaction, i.e., the sample which had been kept constantly at 220°C for 300 min in the system of C2H2 + H20 (Fig. 4) (denoted as sample re- acted). By comparing its spectrum with that of (Cr3FF),

• ~2~/~/~ v~ 9 5o4%

8 27t'c

7 ~ # ~-,-.v-.,,~. w---- 254"c

6 ~ 5 4 22o ° C

2o3"c z

,7,0-,74~HCOOH ~ j k f ~ - ~ , , f

101 "C

69"C

~ ~ 1 R.T.

l,Ha

4000 3000 2000 1600 1200 800 400 200 WAVENUMBER (cm-')

FIG. 2. In situ variable-temperature IR spectra of the gaseous prod- ucts obtained in the system of (Cr3FF) with acetylene and water vapor.

we find that the stretching frequencies of the carboxylic group have shifted from 1635 (s), 1555 (s), 1370 (s), and 1310 (w) cm -1 for (Cr3FF) (Fig. 4, 1) to 1545 (s) and 1370 (s) cm -1 for the reacted component (Fig. 4, 2); in the meanwhile an intensive and broad band with a peak value at about 555 cm -1 is observed as well. Bearing this in mind, we have therefore tentatively assigned the last three bands as the characteristic frequencies of the active component and used them as a criterion to control the pretreatment of (Cr3FF). The spectrum illustrated in Fig. 4, 3 is that of (Cr~FF) pretreated in Ar for 120 min

lOmin

CH3CHO

C2H 4

150 200 250 300 350 400 TEMPERATURE (°C}

FIG. 3. Relative content of the products plotted as a function of the reaction temperature. (The figures in the parentheses are of the reaction time.)

1184 Volume 47, Number 8, 1993

1370

4000 3000 2000 1600 1200 800 400 200

WAVE NUMBER (cm-')

FIG. 4. IR spectra: (1) (Cr3FF); (2) (Cr~FF) reacted in C2H~ + H20 for 300 min at 220°C; (3) (Cr3FF) pretreated in Ar for 120 min at 236°C.

a t 236°C. As shown, it a l ready possesses the character is t ic frequencies of the active component . Tak ing this sample as a catalyst , we followed up the s tudy of its act ivi ty for the addi t ion react ion of C2H2 with H20 by in situ IR analysis with respect to t empera tu re . The results are demons t r a t ed in Fig. 5 in compar ison with those for (Cr3FF) and Cr203 as a b lank tes t for react ion conditions. As is clearly shown, the active componen t can act ivate C2H2 to react with H20, forming CH3CHO at about 150°C, whereas (Cr3FF) and Cr203 at abou t 230°C demons t r a t e much less producing capaci ty under the same condit ion (Fig. 5). This fact comple te ly suppor ts the assumpt ion men t ioned earlier. By the way, it is wor th point ing out t ha t the active c o m p o n e n t was ra ther stable, because it was exposed in air for a long t ime before being used.

In order to clarify the componen t s of these active frag- men t s in detail, the t he rma l decomposi t ion process of (Cr3FF) has been s tudied by means of differential ther- mal analysis, the rmograv imet r i c analysis, x-ray powder diffraction, and infrared spectra; '4 the proposed process is as follows:

1 o f ~

J

, J /

f" / 2

j "

j /

100 150 200 250 300 TEMPERATURE (°C)

Fro. 5. Relative IR intensity of CH3CHO plotted as a function of the reaction temperature: (1) by active component; (2) by (CraFF); (3) by Cr203.

[Cr3(u3-O)(~-O2CH)6(H20)2(O2CH)].xH20

l xH20 (crystallized and 40-150°C adsorbed) was lost.

[Cr3(u~-O)(u-O2CH)6(H~O)~(O~CH)]

(O2CH) was 150-260°C lost.

[Cr3(u3-O) (t~-O2CH)6(H20)2]

260-350°C . (2H20 + 4u-O2CH (227-350°C for a | + 0.5t~-OCH)

crystal sample) '~ was lost.

1.5[Cr203CH]

1.5(CH) was 350-500oC lost.

1.5 Cr203

Incidentally, it was interesting to identify the form of the fragment 1.5(Cr203CH). With the fragmentation paths of (Cr3FF) in Ref. 15 in mind, it has been found that an ion such as [Cr30(O2CH)3] ÷ could sequentially eliminate OCH, forming:

TABLE I. Relationship between the component of the samples determined by T G A and their activity in the addition reaction."

Theoretical Activity of value of Experimental the addition

Sample Molecular formula MW* value of MW Lost ligands reaction

(Cr~FF) [Cr30(O2CH)6 (H20)2 (O2CH)]-3H20 577 . . . . . . No

1 8 6 ° C [Cr30(O2CH)6(H20)~ 0.502CH] 501 502 3H20(crystallized) + No (ll0 min) 0.40~CH

220°C 397 398 3H20(crystallized) + Lower (300 min) 2H20(coordinating) +

[Cr~O(OCH)5]

254°C [Cr30(O~CH)4] (110 min)

O2CH + u-O2CH 352 346 3H20(crystallized) + Higher

2H20(coordinating) + O2CH + 2tL-O2CH

352 344 3H20(crystallized) + Higher 2H20(coordinating) + O2CH + 2tt-O2CH

287°C [Cr30(O2CHL] (90 min)

a MW: Molecular weight.

APPLIED SPECTROSCOPY 1185

-OCH

[Cr30(O2CH)3] + • [Cr30(O2CH)2. O] + m / z = 307(24) m / z = 278(14)

-OCH

) [Cr30(O2CH)" O2]+. m / z = 249(14)

The figures in the parentheses show the relative abundance of the ions. Thus, because of the agreement of the "molecular weights" (247.5 for 1.5(Cr203CH) and 249 for [Cr30(O2CH)- 02] ÷ ), there is reason to believe that they are identical.

With understanding of the entire decomposition process, we have been able to more thoroughly investigate the nature of the active fragments. In this effort, four samples (cf. Fig. 3) with different degrees of activity were chosen for the TGA study. The first sample was the one kept at 186°C for 110 rain, which could hardly show the reaction activity; the second--a t 220°C for 300 min--showed low reaction activity (which could be observed from the long reaction time needed); the t h i rd - - at 254°C for 110 m i n - - a n d the four th - -a t 278°C for 90 m i n - - both revealed increased activity. For these four samples, the "molecular weights" were calculated from the TGA experi- mental values, and then their "molecular formula" was deter- mined according to the decomposition process mentioned. The results are listed in Table I in a comparison with (Cr3FF).

We have therefore been led to the following conclusions, valid for the results demonstrated in Table I and Fig. 3:

1. The skeleton of (Cr3FF) obtained by the loss of the crys- tallized water and the terminal formic ligand does not ac- tivate the acetylene to cause the addition reaction with either H2 or H20.

2. On this basis, the active fragment obtained by the loss of one more #-OzCH already possesses the activity of the ad- ditional reaction, but it is low.

3. The most active components probably are the fragments obtained by losing 2-3~-02CH, which are in an amorphous state ~4 and are stable in air. Intuitively, the existence of such fragments has been further verified by FAB-MS data 1'~ ex- hibiting the following fragmentation pattern:

-OUCH

[Cr30(O2CH)6] + ~ [Cr30(O2CH)~] + m / z 442(100) m / z ~ 397(26)

O~CH

• [Cr30(OzCH)4] + m / z = 352(14)

--O2CH

) [Cr30(O2CH):d + m / z = 307(24)

ACKNOWLEDGMENTS

This research was supported partly by the PNSF of Fujian and the SF of Fuzhou Laboratory for Structural Chemistry, Academia Sinica, Fuzhou.

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