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The role of niobium(V) chloride in the catalytic cyclotrimerization of phenylacetylene
Journal of Molecular Catalysis, 42 (1987)
151 - 159
151
THE ROLE OF NIOBIUM(V) CHLORIDE IN THE CATALYTIC CYCLOTRIMERIZATION OF PHENYLACETYLENE G. LACHMANN, J. A. K. DU PLESSIS*
and C. J. DU TOIT
Department of Chemistry, Potchefstroom Potchefstroom 2520 (South Africa)
University for Christian Higher Education,
(Received October 28, 1986; accepted March 9, 1987)
Summary Phenylacetylene can be cyclotrimerized to 1,2,4- and 1,3,5-triphenylbenzene in 75% yield by niobium(V) chloride in carbon tetrachloride solvent at room temperature and atmospheric pressure. Oxygen and water must be excluded from the system. During the course of the reaction, three separate phases can be clearly distinguished. First a niobium(V) acetylene complex is formed which is slowly reduced by phenylacetylene. An autocatalytic reaction accelerates this reduction step and a catalytically active niobium(II1) complex is formed. A reaction mechanism is proposed which enables us to match the observed reaction course with a computersimulated reaction course. Introduction Acetylenes can be cyclotrimerized and polymerized in the presence of several transition metal compounds. It is shown that some of the halides and also other compounds of niobium, tantalum, molybdenum, tungsten and nickel can act as catalysts [ 1 - 51. Phenylacetylene is cyclotrimerized in the presence of niobium(V) chloride or tantalum(V) chloride to form 1,3,5- and 1,2,4-triphenylbenzene [ 31. With the halides of niobium, tantalum and tungsten [ 31 it is found that the halide, as well as the group and period of the metal, influence the activity and selectivity of the catalyst. Relatively more of the 1,2,4-triphenylbenzene isomer is formed in the presence of niobium(V) chloride and bromide than in the presence of tantalum(V) chloride and bromide, while in the case of molybdenum(V) chloride and tungsten(VI) chloride, polymers are the main products formed. Metallocycles have been proposed as possible reaction intermediates in catalytic transformations such as olefin metathesis and cyclotrimerization of alkynes. *Author to whom correspondence 0304-5102/87/$3.50
should be addressed. 0 Elsevier Sequoia/Printed
in The Netherlands
152
Metallocyclopentadienes are usually formed by the reaction of certain metal compounds with alkynes. Collman [6] reported the formation of metallocyclopentadiene complexes of iridium(II1) and rhodium(II1) from acetylenes. These cyclopentadiene compounds catalyze the cyclotrimerization of disubstituted acetylenes. The metallocyclopentadiene prepared from CpCo(P(C,H,)s), [7] and alkynes also catalyzes the cyclotrimerization reaction. Studies of the reaction of the cobalt cyclopentadiene [8] CpCoP(C,H,)s[C,(CH,),] with 2-butyne showed evidence that it proceeds first by a rate-determining loss of P(C,H,),, then coordination of the alkyne, and finally the elimination to form products. Two different paths, one for the highly active alkynes and the other for less active alkynes to produce hexasubstituted benzenes has been proposed. This is outlined in the scheme below.
R
tJ0
RC!=CR
L
RR
/=CC cp Ao,& k
R-CFC-R k
RC=CR
cp-co.
t
I RTC=C-R I
&C=C-R
-
.7=,-,
cpco 7” \ /’ JF R R--‘C=CLR
The cyclotrimerization of phenylacetylene in the presence of Niobium(V) chloride in carbon tetrachloride [9 - 111 exhibits different reaction phases. Initially 2 mol phenylacetylene react with a mole of niobium(V) chloride to form a niobium(V) chloride phenylacetylene compound. The reaction exhibits then an induction period after which the remaining phenylacetylene is rapidly trimerized. Cotton [ 121 reported on organo-niobium(II1) and -tantalum(III) complexes that act as polymerization catalysts and the indications are that the active intermediate in this study may also contain a niobium(II1) species. The mechanism of the cyclotrimerization of acetylenes is still not completely known. The purpose of this work was to obtain further information about the course of the cyclotrimerization of phenylacetylene in the presence of niobium(V) chloride.
153
Experimental The reactions were carried out at atmospheric pressure (- 85 kPa) in a dry nitrogen atmosphere by injecting phenylacetylene into a solution of niobium(V) chloride in carbon tetrachloride solvent. The reaction vessel was either a 50 cm3 flask fitted with a silicon rubber septum or a modified IR spectrophotometer cell with an adjustable path length fitted with quartz windows [ 131. Niobium(V) chloride was sublimed under vacuum, and carbon tetrachloride was dried over PZ05 and distilled in a nitrogen atmosphere. The niobium(V) chloride solution was prepared by saturating hot carbon tetrachloride with niobium(V) chloride and cooling the solution to a predetermined temperature to obtain the necessary niobium concentration. The niobium content was determined gravimetrically as NbzOs. Phenylacetylene was distilled under reduced pressure in a nitrogen stream and stored in a refrigerator. In situ measurement of the phenylacetylene during the reaction was carried out by recording the GC-H stretch vibration on a Beckmann IR 33 spectrophotometer. A Carlo-Erba 4100 gas chromatograph with a 3% SE-30 on ChromosorbWAW column was used to determine volatile components in the reaction mixture. Identification of products was done by GC/MS analysis. Proton NMR measurements were obtained on a Varian XL-100 NMR spectrometer at room temperature. Stopped flow measurements were done by mixing equal volumes of 12 X 1O-3 mol dmv3 NbCls and 110 X low3 mol dmd3 phenylacetylene in carbon tetrachloride in a Durrum stopped-flow spectrophotometer and the UV-visible spectra recorded on an Omarapid-scan monochromator. Solutions were degassed by a few cycles of freeze-th.awing in vacuum prior to use. An experiment was performed using radioactive niobium(V) bromide as catalyst. Niobium(V) bromide was prepared by the method reported by Fairbrother [14] and was converted to NbBr4*Qr by neutron activation in a nuclear reactor. A solution containing 3.2 X 10m3 mol dme3 radioactive niobium(V) bromide, 32 X 10e3 mol dmp3 phenylacetylene and 12.8 X 10e3 mol dmv3 ethylaluminium dichloride in dry o-dichlorobenzene was prepared. Sufficient time was allowed (5 min) for the total conversion of phenylacetylene. An excess of water was added to hydrolyze any niobium complexes, and the delivered bromide ions were precipitated with silver nitrate. The amount of precipitated bromide and the amount of organic bromine were measured separately with a NaI(T1) scintillation counter. Results and discussion Phenylacetylene oligomerizes spontaneously to 1,3,4_triphenylbenzene, 1,2,4_triphenylbenzene and higher polymers in the presence of dis-
154
solved niobium(V) chloride. In carbon tetrachloride as solvent and in the absence of water and oxygen, the trimerization reaction is dominant and up to 75% conversion to triphenylbenzene is obtained. In Fig. 1 the consumption of phenylacetylene uersus time (curve 1) and the accompanied formation of fl-chloroethylbenzene (curve 2) and triphenylbenzene (curve 3) are shown. These results were obtained by IR spectrophotometry and GC analysis.
0
I
05
I
15
2
TIME x MINUTES-’
Fig. 1. Consumption of phenylacetylene and formation of products. [NbCIS] = 16 X 10e3 mol dme3, solvent = CC14, T = 285 K; Curve 1 l C&I&SH, curve 2 o C&&H=CHCl, Curve 3 A triphenylbenzene.
Curve 1 shows that the reaction course can be divided into three distinct phases. Directly after mixing, a small amount of phenylacetylene reacts but no trimerization occurs. The P-chloroethenylbenzene is an artifact of the analysis, because it is formed when the reaction is stopped by adding water. After this initial reaction phase (A), an induction period (B) follows during which hardly any products are formed. During the induction period, an active cyclization catalyst forms which leads to the cyclization phase (C). This is characterized by a rapid formation of triphenylbenzene while the phenylacetylene is totally consumed. The three reaction phases will now be discussed separately. Fast initial reaction (A) Immediately after injecting phenylacetylene, 2 mol phenylacetylene react with each mol niobium(V) chloride present. The reaction is too fast to be followed by the IR techniques used. Rapid-scanning stopped-flow measurements in the UV/Visible region show a weak and short-lived absorption band between 500 nm and 550 nm, with a flat peak at 525 nm. The growth and decay of this peak is shown in Fig. 2. Although the nature of the complexes which form initially is not known, Fig. 2 shows that the initial reaction takes place in at least two steps and is completed after a few seconds. The set of intermediates suggested in Scheme 1 fulfills these requirements.
155
C,H,CTCH NbCl, + C6H5C=CH -
C&b-Cl
c6HS
-
Clyb-C=CHCl
1
2 C&C=CH
C6H5 /
Nb,C=CHCl
c1 3
'T=CHCl
Cl,vb-C=CHCl -
i C6H5C=CH
c6HS
3
4
Scheme 1.
Either one of species 1, 2 or 3 could be responsible for the absorption band at 525 nm. If the reaction is stopped seconds after mixing the reagents by adding water, 2 mol fl-chloroethenylbenzene are formed for each mol niobium present. The reaction Cl,Nb( C,H,C=CHCl),
2
2C6H,CH=CHCl + 3HCl+ NbzO,*nH,O
4 is indicated by the experimental evidence. In the analogous experiment with tagged niobium(V) bromide, NbBr4%r only three of the five bromium atoms can be precipitated with aqueous silver nitrate. The remaining two are retained in the organic phase. This supports the evidence for an insertion reaction by phenylacetylene.
0
0.5
I
-
1.5 -
2
TIME x SECONDS-’
Fig. 2. The change of absorbance at 525 nm after mixing the reagents. [C&CXH] = 55 x 10e3 mol dmm3, [NbCIS] = 6 x 10e3 mol dmV3, solvent = CCL, T = 293 K, spectrophotometer cell thickness = 20 mm.
156
Induction period (B) During the induction period very little change can be detected. Phenylacetylene is converted very slowly to higher polymers. This accounts for the slow decrease in the phenylacetylene concentration, but no trimerization occurs. No change in the IR or UV/Visible spectra is found, except for a slow darkening of the solution because of insoluble high polymers. It is assumed that the spontaneous reduction of a niobium(V) complex occurs slowly during the induction period, forming a catalytically active niobium(II1) complex (5). C&C=CH
C1,Nb(C6H&=CHC1)2 w
[ Cl,Nb] + oligomers
K2
4
5
The formation of chloride-containing oligomers explains the decreasing amount of P-chloroethenylbenzene which is measured (Fig. 1). This reaction can be accelerated substantially by adding ethylaluminium dichloride to the system, as shown in curve 2 of Fig. 3.
Fig. 3. Influence of mulecular oxygen and C2HSAlC12 on the induction time. [NbClS] = 16 x 10e3 mol dmp3, [C&CZH] = 130 x 10e3 mol dmp3, solvent = CCl4, T = 285 K; curve 1 o control, curve 2 l [AI]/[Nb] = 0.18, curve 3 A [02]/[Nb] = 0.2.
Curve 3 of Fig. 3 shows that small amounts of oxygen lengthen the induction period greatly. This indicates that phenylacetylene is capable of reducing niobium(V) only very slowly. This slow reduction by phenylacetylene is not sufficient to explain the abrupt termination of the induction period. The kinetics of the reaction can be explained by an autocatalytic step which accelerates the production of the Nb(II1) catalyst (Scheme 2), similar to the rhodium system reported by Rund [15]. The overall reaction of Scheme 2 is much faster than the reduction of 4 by phenylacetylene. The production of [Cl,Nb] by ethylaluminium dichloride or the destruction of [ClsNb] by oxygen will therefore hasten or respectively retard the ending of the induction period by a large margin.
157
[ Cl,Nb] + Cl,Nb( C6H,C=CHC1)2 5
11
4
[Cl,Nb] - - - [Cl,Nb(C&C=CHCl),] C6HSC=CH
I 2 [ CIsNb] + oligomers 5 Scheme 2.
An entirely different way of shortening the induction period is by irradiating the solution with a tungsten light. In a typical experiment, the induction time is reduced from 40 s in the dark to less than 10 s. The part of the solution which is irradiated in such a experiment is blackened completely, while the rest of the solution is still clear. The product spectrum is not different from that of an unirradiated sample. This shows that irradiating the sample enhances the slower thermic reaction, and that no separate photochemically induced reaction occurs. Cycliza tion reaction (C) Once the cyclization has started, the reaction rate can be described as a first approximation, by a simple second-order rate law, such as in the case of the reaction of cobalt cyclopentadiene with 2-butyne [8] (Scheme 3). C6I-b
[Cl,Nb] + BC&I,C=CH -
,\C=C-H C1sNb,C_C_H
c6HA6H5 + C6H5
+ [Cl,Nb] Scheme 3.
Only one of the steps in Scheme 3 is rate-determining. The probability that a metal hydride could act as an intermediate was ruled out by a proton-NMR measurement. No indication of a metal hydride
158
peak is found for a filtered reaction mixture after the phenylacetylene is consumed. This solution is still catalytically active because addition of phenylacetylene leads to cyclization without the occurrence of the induction period. If the proposed reactions are a fair representation of the total reaction scheme, it must be possible to find a set of rate laws and rate constants which describe the observed reaction course. The following set of equations and constants is proposed: For Scheme 1, a single second-order rate law is proposed for the slowest step : -d[NbCls] dt
= k i[ NbCl,] [ C,H,C=CH]
k,-
1
The value for k, is an approximate minimum value which fits the experimental data for the reaction phase A. The production of the active Nb(II1) catalyst during the induction period is described by the reduction of niobium(V) by phenylacetylene (k,) and Scheme 2. The rate constants of Scheme 2 are combined into a single constant k, . This leads to an overall production rate for 5 of
d[51 = k,[4] dt
[C6H,C=CH] + k,[4]
[5] [C&HsC=CH]
k2k3
10-5, -
200
The values of k, and k, are selected to give the observed induction time and rate by which the induction period is terminated. The cyclization reaction is best described by a second-order rate law for the slowest step in Scheme 3. -d [ C&H,C=CH] dt
= k4[ 51 [ C,H,C=CH]
k,-
2
Taking k, - 2 gives the best fit for the trimerization phase (C). Using these equations, a numerical integration gives a reaction course which fits the measured course well. The fit stays acceptable at different niobium(V) chloride concentrations and different phenylacetylene concentrations.
Conclusion
In this paper it is shown how niobium(V) chloride can act as an efficient catalyst for the trimerization of phenylacetylene. Niobium(V) chloride by itself is not a trimerization catalyst, but two factors are responsible for the rapid development of catalytic activity in the presence of phenylacetylene. These are : (1) the ability of phenylacetyleme to reduce niobium(V) chloride spontaneously;
159
(2) the ability of niobium(II1) which is formed in the spontaneous reduction to enhance the reduction rate in an autocatalytic reaction. The cyclotrimerization is retarded by substances which oxidize niobium(II1) and prevent the autocatalytic reaction step. A ligand like water which forms strong bonds with niobium is also effective in destroying any catalytic activity by blocking the necessary reaction sites. Acknowledgement We thank Dr. Rudi van Eldik and the Institut fur Physikalische und Theoretische Chemie der UniversilSt Frankfurt am Main for their assistance and use of their laboratories. The financial assistance of the Potchefstroom University for Christian Higher Education is gratefully acknowledged.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
G. Dandliker, Helu. Chim. Acta, 52 (1969) 1482. T. Masuda, N. Susaki and T. Higashimura, Macromolecules, 8 (1975) 717. T. Masuda, T. Mouri and T. Higashimura, Bull. Chem. Sot. Jpn., 53 (1980) 1152. J. A. K. du Plessis and C. J. du Toit, S. Afr. J. Chem., 32 (1979) 147. J. A. K. du Plessis, S. Afr. J. Chem., 34 (1981) 87. J. P. Collman, J. W. Kang, W. F. Little and M. F. Sullivan, Inorg. Chem., 7 (1968) 1298. Y. Wakatsuki, T. Kiramitsu and H. Yamazaki, Tetrahedron Lett., (1974) 4549. D. R. McAllister, J. E. Bercaw and R. G. Bergman, J. Am. Chem. Sot., 99 (1977) 1666. C. J. du Toit, J. A. K. du Plessis and G. Lachmann, S. Afr. J. Chem., 38 (1985) 8. C. J. du Toit, J. A. K. du Plessis and G. Lachmann, S. Afr. J. Chem., 38 (1985) 188 C. J. du Toit, J. A. K. du Plessis and G. Lachmann, S. Afr. J. Chem., 38 (1985) 195. F. A. Cotton, W. T. Hall, K. J. Cann and F. J. Karol, Macromolecules, 14 (1981) 233. C. J. du Toit, J. A. K. du Plessis and G. Lachmann, CHEMSA, 1984,536. F. Fairbrother, The Chemistry of Niobium and Tantalum, Elsevier, Amsterdam, 1967, p. 86. J. C. Rund, F. Basolo and R. G. Pearson, Znorg. Chem., 3 (1964) 658.
151 - 159
151
THE ROLE OF NIOBIUM(V) CHLORIDE IN THE CATALYTIC CYCLOTRIMERIZATION OF PHENYLACETYLENE G. LACHMANN, J. A. K. DU PLESSIS*
and C. J. DU TOIT
Department of Chemistry, Potchefstroom Potchefstroom 2520 (South Africa)
University for Christian Higher Education,
(Received October 28, 1986; accepted March 9, 1987)
Summary Phenylacetylene can be cyclotrimerized to 1,2,4- and 1,3,5-triphenylbenzene in 75% yield by niobium(V) chloride in carbon tetrachloride solvent at room temperature and atmospheric pressure. Oxygen and water must be excluded from the system. During the course of the reaction, three separate phases can be clearly distinguished. First a niobium(V) acetylene complex is formed which is slowly reduced by phenylacetylene. An autocatalytic reaction accelerates this reduction step and a catalytically active niobium(II1) complex is formed. A reaction mechanism is proposed which enables us to match the observed reaction course with a computersimulated reaction course. Introduction Acetylenes can be cyclotrimerized and polymerized in the presence of several transition metal compounds. It is shown that some of the halides and also other compounds of niobium, tantalum, molybdenum, tungsten and nickel can act as catalysts [ 1 - 51. Phenylacetylene is cyclotrimerized in the presence of niobium(V) chloride or tantalum(V) chloride to form 1,3,5- and 1,2,4-triphenylbenzene [ 31. With the halides of niobium, tantalum and tungsten [ 31 it is found that the halide, as well as the group and period of the metal, influence the activity and selectivity of the catalyst. Relatively more of the 1,2,4-triphenylbenzene isomer is formed in the presence of niobium(V) chloride and bromide than in the presence of tantalum(V) chloride and bromide, while in the case of molybdenum(V) chloride and tungsten(VI) chloride, polymers are the main products formed. Metallocycles have been proposed as possible reaction intermediates in catalytic transformations such as olefin metathesis and cyclotrimerization of alkynes. *Author to whom correspondence 0304-5102/87/$3.50
should be addressed. 0 Elsevier Sequoia/Printed
in The Netherlands
152
Metallocyclopentadienes are usually formed by the reaction of certain metal compounds with alkynes. Collman [6] reported the formation of metallocyclopentadiene complexes of iridium(II1) and rhodium(II1) from acetylenes. These cyclopentadiene compounds catalyze the cyclotrimerization of disubstituted acetylenes. The metallocyclopentadiene prepared from CpCo(P(C,H,)s), [7] and alkynes also catalyzes the cyclotrimerization reaction. Studies of the reaction of the cobalt cyclopentadiene [8] CpCoP(C,H,)s[C,(CH,),] with 2-butyne showed evidence that it proceeds first by a rate-determining loss of P(C,H,),, then coordination of the alkyne, and finally the elimination to form products. Two different paths, one for the highly active alkynes and the other for less active alkynes to produce hexasubstituted benzenes has been proposed. This is outlined in the scheme below.
R
tJ0
RC!=CR
L
RR
/=CC cp Ao,& k
R-CFC-R k
RC=CR
cp-co.
t
I RTC=C-R I
&C=C-R
-
.7=,-,
cpco 7” \ /’ JF R R--‘C=CLR
The cyclotrimerization of phenylacetylene in the presence of Niobium(V) chloride in carbon tetrachloride [9 - 111 exhibits different reaction phases. Initially 2 mol phenylacetylene react with a mole of niobium(V) chloride to form a niobium(V) chloride phenylacetylene compound. The reaction exhibits then an induction period after which the remaining phenylacetylene is rapidly trimerized. Cotton [ 121 reported on organo-niobium(II1) and -tantalum(III) complexes that act as polymerization catalysts and the indications are that the active intermediate in this study may also contain a niobium(II1) species. The mechanism of the cyclotrimerization of acetylenes is still not completely known. The purpose of this work was to obtain further information about the course of the cyclotrimerization of phenylacetylene in the presence of niobium(V) chloride.
153
Experimental The reactions were carried out at atmospheric pressure (- 85 kPa) in a dry nitrogen atmosphere by injecting phenylacetylene into a solution of niobium(V) chloride in carbon tetrachloride solvent. The reaction vessel was either a 50 cm3 flask fitted with a silicon rubber septum or a modified IR spectrophotometer cell with an adjustable path length fitted with quartz windows [ 131. Niobium(V) chloride was sublimed under vacuum, and carbon tetrachloride was dried over PZ05 and distilled in a nitrogen atmosphere. The niobium(V) chloride solution was prepared by saturating hot carbon tetrachloride with niobium(V) chloride and cooling the solution to a predetermined temperature to obtain the necessary niobium concentration. The niobium content was determined gravimetrically as NbzOs. Phenylacetylene was distilled under reduced pressure in a nitrogen stream and stored in a refrigerator. In situ measurement of the phenylacetylene during the reaction was carried out by recording the GC-H stretch vibration on a Beckmann IR 33 spectrophotometer. A Carlo-Erba 4100 gas chromatograph with a 3% SE-30 on ChromosorbWAW column was used to determine volatile components in the reaction mixture. Identification of products was done by GC/MS analysis. Proton NMR measurements were obtained on a Varian XL-100 NMR spectrometer at room temperature. Stopped flow measurements were done by mixing equal volumes of 12 X 1O-3 mol dmv3 NbCls and 110 X low3 mol dmd3 phenylacetylene in carbon tetrachloride in a Durrum stopped-flow spectrophotometer and the UV-visible spectra recorded on an Omarapid-scan monochromator. Solutions were degassed by a few cycles of freeze-th.awing in vacuum prior to use. An experiment was performed using radioactive niobium(V) bromide as catalyst. Niobium(V) bromide was prepared by the method reported by Fairbrother [14] and was converted to NbBr4*Qr by neutron activation in a nuclear reactor. A solution containing 3.2 X 10m3 mol dme3 radioactive niobium(V) bromide, 32 X 10e3 mol dmp3 phenylacetylene and 12.8 X 10e3 mol dmv3 ethylaluminium dichloride in dry o-dichlorobenzene was prepared. Sufficient time was allowed (5 min) for the total conversion of phenylacetylene. An excess of water was added to hydrolyze any niobium complexes, and the delivered bromide ions were precipitated with silver nitrate. The amount of precipitated bromide and the amount of organic bromine were measured separately with a NaI(T1) scintillation counter. Results and discussion Phenylacetylene oligomerizes spontaneously to 1,3,4_triphenylbenzene, 1,2,4_triphenylbenzene and higher polymers in the presence of dis-
154
solved niobium(V) chloride. In carbon tetrachloride as solvent and in the absence of water and oxygen, the trimerization reaction is dominant and up to 75% conversion to triphenylbenzene is obtained. In Fig. 1 the consumption of phenylacetylene uersus time (curve 1) and the accompanied formation of fl-chloroethylbenzene (curve 2) and triphenylbenzene (curve 3) are shown. These results were obtained by IR spectrophotometry and GC analysis.
0
I
05
I
15
2
TIME x MINUTES-’
Fig. 1. Consumption of phenylacetylene and formation of products. [NbCIS] = 16 X 10e3 mol dme3, solvent = CC14, T = 285 K; Curve 1 l C&I&SH, curve 2 o C&&H=CHCl, Curve 3 A triphenylbenzene.
Curve 1 shows that the reaction course can be divided into three distinct phases. Directly after mixing, a small amount of phenylacetylene reacts but no trimerization occurs. The P-chloroethenylbenzene is an artifact of the analysis, because it is formed when the reaction is stopped by adding water. After this initial reaction phase (A), an induction period (B) follows during which hardly any products are formed. During the induction period, an active cyclization catalyst forms which leads to the cyclization phase (C). This is characterized by a rapid formation of triphenylbenzene while the phenylacetylene is totally consumed. The three reaction phases will now be discussed separately. Fast initial reaction (A) Immediately after injecting phenylacetylene, 2 mol phenylacetylene react with each mol niobium(V) chloride present. The reaction is too fast to be followed by the IR techniques used. Rapid-scanning stopped-flow measurements in the UV/Visible region show a weak and short-lived absorption band between 500 nm and 550 nm, with a flat peak at 525 nm. The growth and decay of this peak is shown in Fig. 2. Although the nature of the complexes which form initially is not known, Fig. 2 shows that the initial reaction takes place in at least two steps and is completed after a few seconds. The set of intermediates suggested in Scheme 1 fulfills these requirements.
155
C,H,CTCH NbCl, + C6H5C=CH -
C&b-Cl
c6HS
-
Clyb-C=CHCl
1
2 C&C=CH
C6H5 /
Nb,C=CHCl
c1 3
'T=CHCl
Cl,vb-C=CHCl -
i C6H5C=CH
c6HS
3
4
Scheme 1.
Either one of species 1, 2 or 3 could be responsible for the absorption band at 525 nm. If the reaction is stopped seconds after mixing the reagents by adding water, 2 mol fl-chloroethenylbenzene are formed for each mol niobium present. The reaction Cl,Nb( C,H,C=CHCl),
2
2C6H,CH=CHCl + 3HCl+ NbzO,*nH,O
4 is indicated by the experimental evidence. In the analogous experiment with tagged niobium(V) bromide, NbBr4%r only three of the five bromium atoms can be precipitated with aqueous silver nitrate. The remaining two are retained in the organic phase. This supports the evidence for an insertion reaction by phenylacetylene.
0
0.5
I
-
1.5 -
2
TIME x SECONDS-’
Fig. 2. The change of absorbance at 525 nm after mixing the reagents. [C&CXH] = 55 x 10e3 mol dmm3, [NbCIS] = 6 x 10e3 mol dmV3, solvent = CCL, T = 293 K, spectrophotometer cell thickness = 20 mm.
156
Induction period (B) During the induction period very little change can be detected. Phenylacetylene is converted very slowly to higher polymers. This accounts for the slow decrease in the phenylacetylene concentration, but no trimerization occurs. No change in the IR or UV/Visible spectra is found, except for a slow darkening of the solution because of insoluble high polymers. It is assumed that the spontaneous reduction of a niobium(V) complex occurs slowly during the induction period, forming a catalytically active niobium(II1) complex (5). C&C=CH
C1,Nb(C6H&=CHC1)2 w
[ Cl,Nb] + oligomers
K2
4
5
The formation of chloride-containing oligomers explains the decreasing amount of P-chloroethenylbenzene which is measured (Fig. 1). This reaction can be accelerated substantially by adding ethylaluminium dichloride to the system, as shown in curve 2 of Fig. 3.
Fig. 3. Influence of mulecular oxygen and C2HSAlC12 on the induction time. [NbClS] = 16 x 10e3 mol dmp3, [C&CZH] = 130 x 10e3 mol dmp3, solvent = CCl4, T = 285 K; curve 1 o control, curve 2 l [AI]/[Nb] = 0.18, curve 3 A [02]/[Nb] = 0.2.
Curve 3 of Fig. 3 shows that small amounts of oxygen lengthen the induction period greatly. This indicates that phenylacetylene is capable of reducing niobium(V) only very slowly. This slow reduction by phenylacetylene is not sufficient to explain the abrupt termination of the induction period. The kinetics of the reaction can be explained by an autocatalytic step which accelerates the production of the Nb(II1) catalyst (Scheme 2), similar to the rhodium system reported by Rund [15]. The overall reaction of Scheme 2 is much faster than the reduction of 4 by phenylacetylene. The production of [Cl,Nb] by ethylaluminium dichloride or the destruction of [ClsNb] by oxygen will therefore hasten or respectively retard the ending of the induction period by a large margin.
157
[ Cl,Nb] + Cl,Nb( C6H,C=CHC1)2 5
11
4
[Cl,Nb] - - - [Cl,Nb(C&C=CHCl),] C6HSC=CH
I 2 [ CIsNb] + oligomers 5 Scheme 2.
An entirely different way of shortening the induction period is by irradiating the solution with a tungsten light. In a typical experiment, the induction time is reduced from 40 s in the dark to less than 10 s. The part of the solution which is irradiated in such a experiment is blackened completely, while the rest of the solution is still clear. The product spectrum is not different from that of an unirradiated sample. This shows that irradiating the sample enhances the slower thermic reaction, and that no separate photochemically induced reaction occurs. Cycliza tion reaction (C) Once the cyclization has started, the reaction rate can be described as a first approximation, by a simple second-order rate law, such as in the case of the reaction of cobalt cyclopentadiene with 2-butyne [8] (Scheme 3). C6I-b
[Cl,Nb] + BC&I,C=CH -
,\C=C-H C1sNb,C_C_H
c6HA6H5 + C6H5
+ [Cl,Nb] Scheme 3.
Only one of the steps in Scheme 3 is rate-determining. The probability that a metal hydride could act as an intermediate was ruled out by a proton-NMR measurement. No indication of a metal hydride
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peak is found for a filtered reaction mixture after the phenylacetylene is consumed. This solution is still catalytically active because addition of phenylacetylene leads to cyclization without the occurrence of the induction period. If the proposed reactions are a fair representation of the total reaction scheme, it must be possible to find a set of rate laws and rate constants which describe the observed reaction course. The following set of equations and constants is proposed: For Scheme 1, a single second-order rate law is proposed for the slowest step : -d[NbCls] dt
= k i[ NbCl,] [ C,H,C=CH]
k,-
1
The value for k, is an approximate minimum value which fits the experimental data for the reaction phase A. The production of the active Nb(II1) catalyst during the induction period is described by the reduction of niobium(V) by phenylacetylene (k,) and Scheme 2. The rate constants of Scheme 2 are combined into a single constant k, . This leads to an overall production rate for 5 of
d[51 = k,[4] dt
[C6H,C=CH] + k,[4]
[5] [C&HsC=CH]
k2k3
10-5, -
200
The values of k, and k, are selected to give the observed induction time and rate by which the induction period is terminated. The cyclization reaction is best described by a second-order rate law for the slowest step in Scheme 3. -d [ C&H,C=CH] dt
= k4[ 51 [ C,H,C=CH]
k,-
2
Taking k, - 2 gives the best fit for the trimerization phase (C). Using these equations, a numerical integration gives a reaction course which fits the measured course well. The fit stays acceptable at different niobium(V) chloride concentrations and different phenylacetylene concentrations.
Conclusion
In this paper it is shown how niobium(V) chloride can act as an efficient catalyst for the trimerization of phenylacetylene. Niobium(V) chloride by itself is not a trimerization catalyst, but two factors are responsible for the rapid development of catalytic activity in the presence of phenylacetylene. These are : (1) the ability of phenylacetyleme to reduce niobium(V) chloride spontaneously;
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(2) the ability of niobium(II1) which is formed in the spontaneous reduction to enhance the reduction rate in an autocatalytic reaction. The cyclotrimerization is retarded by substances which oxidize niobium(II1) and prevent the autocatalytic reaction step. A ligand like water which forms strong bonds with niobium is also effective in destroying any catalytic activity by blocking the necessary reaction sites. Acknowledgement We thank Dr. Rudi van Eldik and the Institut fur Physikalische und Theoretische Chemie der UniversilSt Frankfurt am Main for their assistance and use of their laboratories. The financial assistance of the Potchefstroom University for Christian Higher Education is gratefully acknowledged.
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
G. Dandliker, Helu. Chim. Acta, 52 (1969) 1482. T. Masuda, N. Susaki and T. Higashimura, Macromolecules, 8 (1975) 717. T. Masuda, T. Mouri and T. Higashimura, Bull. Chem. Sot. Jpn., 53 (1980) 1152. J. A. K. du Plessis and C. J. du Toit, S. Afr. J. Chem., 32 (1979) 147. J. A. K. du Plessis, S. Afr. J. Chem., 34 (1981) 87. J. P. Collman, J. W. Kang, W. F. Little and M. F. Sullivan, Inorg. Chem., 7 (1968) 1298. Y. Wakatsuki, T. Kiramitsu and H. Yamazaki, Tetrahedron Lett., (1974) 4549. D. R. McAllister, J. E. Bercaw and R. G. Bergman, J. Am. Chem. Sot., 99 (1977) 1666. C. J. du Toit, J. A. K. du Plessis and G. Lachmann, S. Afr. J. Chem., 38 (1985) 8. C. J. du Toit, J. A. K. du Plessis and G. Lachmann, S. Afr. J. Chem., 38 (1985) 188 C. J. du Toit, J. A. K. du Plessis and G. Lachmann, S. Afr. J. Chem., 38 (1985) 195. F. A. Cotton, W. T. Hall, K. J. Cann and F. J. Karol, Macromolecules, 14 (1981) 233. C. J. du Toit, J. A. K. du Plessis and G. Lachmann, CHEMSA, 1984,536. F. Fairbrother, The Chemistry of Niobium and Tantalum, Elsevier, Amsterdam, 1967, p. 86. J. C. Rund, F. Basolo and R. G. Pearson, Znorg. Chem., 3 (1964) 658.