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The reaction of cyclohexene and 1,3-cyclohexadiene with tin(II)-coordinated Pt(II) complexes
hum1
of Molwular
Catalysis,
66 (1991) 321-327
321
The reaction of cyclohexene and 1,3-cyclohexadiene tin(II)-coordinated Pt(II) complexes Tetsu Yamakawa, Institute of Induwial Tokyo 106 (Japan]
Toshio Fujita Science,
University
with
and Sumio Shinoda* of Tokyo, 22-l Roppongi,
7-Ch4mu3, Minato-ku,
(Eeceived August 21, 1990; accepted December 28, 1990)
Abstract The reaction of cyclohexene and 1,3cyclohexadiene with a series of soluble tin(II)coordinated platinum(B) complexes (cti-[PtCLa(SnC1&..2-, [Pt(SnC1,).J2-, [Pt(SnC1,),]3’-) has been investigated under thermal condition. For cyclohexene, dehydrogenation took place catalytically with [Pt(SnC13),]2- and [R(SnC13)J3-, while only stoichiometic oxidation (-CH2-CHa+Bt(II)+ -CH=CH+pt(O) + 2H+) occurred with cti[FWl,(SnC13),]2-, showing the effect of the number of SnC13- ligand. For 1,3-cyclohexadiene, however, all three complexes were catalytically active (dehydrogenation and intermolecular hydrogen transfer reactions). The observed order of catalytic activity for both reactants ([Bt(SnC1,>,]2- > [Pt(SnCl,),]“-) is reconciled with the presence of ratecontrolling pre-equilibrium of substrate r-coordination, which would be more favorable for the former complex. Based on the kinetic analysis, a dehydrogenation path is suggested in which benzene is formed from cyclohexene without liberating 1,3_cyclohexadiene intermediate from the coordination sphere.
Introduction
The SnCl,- ligand has a unique ability to prevent the reduction of central transition-metal ions to metal due to its high rr-acceptor capacity [ 11, and it also tends to provide a vacant coordination site via its strong truns influence and/or its high lability [ 21. It is well known that the catalytic properties of Pt(II) complexes are altered remarkably by the addition of SnCla [ 2 I. However, the type of reaction is limited (e.g., hydrogenation, isomer&&ion and hydroformylation of alkenes), and further there are few cases that use a Ptn-SnCl,- complex in the definite form [3]. In the course of our study of dehydrogenation and hydrogen transfer reactions of hydrocarbons with soluble tin(U)-coordinated platinum(I1) complexes [4], we have found that three kinds of complexes (cti-[PtCl,(SnCl&J2-, [Pt(SnCl,),]2-, [Pt(SnCl&]3-) show quite specific characteristics themselves. In the present paper, results for their reaction with cyclohexene and 1,3-cyclohexadiene will be presented in detail. *Author to whom correspondence should be addressed.
0304-5102/91/$3.50
0 Elsevier Sequoia/Printedin The Netherlands
322
Experimental
Synthesis of tinCII)-coordinated platinum(X) complexes All procedures were performed under argon atmosphere. Tetraphenylphosphonium (PPh,+) salts of cis-[PtClz(SnClJ2]2- and [Pt(SnC1,),]3- complexes synthesized the were following literature method (51. [PPh,],[Pt(SnCl,),] was isolated here for the first time as follows: [PPh412[PtC14]and SnC12were dissolved in CH&lz at a ratio of 1:4 and the solution was stirred for 48 h at ambient temperature. The resulting orange solid was filtered, washed with benzene, and dried under vacuum. The purity of the isolated salt was satisfactory ( < 3% of [PPhl],[Pt(SnC13),] impurity as confirmed by “‘Sn NMR).
Reaction with tin(7I)coordinated platinum(71) complexes All the reagents and solvents were reagent grade. The reaction solution was prepared by dissolving the platinum(I1) complex (0.185 mmol) and substrate (cyclohexene or 1,3cyclohexadiene; 7.4-14.8 mmol) in 1,1,2,2tetrachloroethane solvent (150 ml) under argon atmosphere. 1,1,2,2-Tetrachloroethane was purified by washing with cont. H2S04 and then N&C03dissolved deionized water, followed by drying over CaC12 and fractional distillation. The reaction was carried out under dry nitrogen atmosphere (1 atm) at 80 “C. The reactor was equipped with a reflux condenser to which a gas buret was attached, and was shielded from light with ahuninum foil. Products formed in the liquid and gas phases were analyzed quantitatively by gas chromatography with TCEP (1,2,3-tris(2-cyanoethoxy)propane) and active-carbon columns, respectively. All the reactions were carried out at least twice to ensure reproducibility.
Results
and discussion
Reaction of cgcl~hecr;ene Figure 1 shows the time-courses of product formation for the reactions of cyclohexene with [Pt(SnC13)J2- (a) and [Pt(SnC13)J3- @I). Formation of benzene is catalytic in both cases because the turnover number exceeded 0.5 at 48 h ([Pt(SnC13),]2-) AND 96 H ([pT(sNcL3)J3-). The initial turnover rate of 1,3-cyclohexadiene formation is faster than that of benzene formation, and is much greater for [Pt(SnC13)_,12- (0.26 h-l) than for [Pt(SnC13),13(0.032 h-l). With respect to benzene formation, the initial turnover rate is slightly faster for [Pt(SnC13),]2- (1.1 X 10T2 h-‘) than for [Pt(SnC13),]3(7.3X low3 h-l), and the same tendency exists for the total amount of benzene formation (0.251 mm01 for [Pt(SnC13),12- and 0.203 mm01 for [Pt(SnC13)s]3- at 192 h). The observed higher reactivity of [Pt(SnC13),]2over [Pt(SnC13)J3- would be reconciled with the presence of rate-controlling pre-equilibrium of cyclohexene r-coordination, which may be more favorable for the former complex [ 61.
0.3
0 0 l
5
I >“,
O.Z-
0
8
0 0
k t
z % P@
l
0
Oo O.l-
P
lo
0
0
0
0 0 0 0
I
50
100
9
0
150
0,
0
200
time/h
(4
,k’I
0
I
(b)
50
,L
100
.Oot
150
200
time/h
F’ig. 1. Time-courses of product formation for the reactions of cyclohexene with [Pt(SnCl&]a(a) and [Pt(SnC&]“@> catalysts; catalyst 0.185 mmol, cyclohexene 14.8 mmol, solvent (1,1,2,2-tetrachloroethane) 150 ml, 80 “C; 1,3-cyclohexadiene (0), benzene (0).
It is interesting that cyclohexadiene was not detectable at all during benzene formation with cti-[PtC1,(SnC1,),]2- (41. Moreover, since only a trace amount of dihydrogen was detected, the contribution of the redox reaction (eqn. (1)) is large. -CH2-CH2-
+Pt(II) -
-CH=CH-
+Pt(O)+2H+
(I)
These differences between cis-[ PtClz(SnC1,)2]2- and [ Pt(SnCl& I”- (n = 4, n=5 m = 3) clearly show the effect of the number of SnCl,- ligands. Since cyclohexane was not detected, hydrogen transfer reactions of cyclohexene (2CBHI0--) C6H8+ C6H i2 and 3C6H,0+ C&H6+2&H,,) are neg-
m=2;
ligible . Reaction of 1,3-cgclohexadiene Figure 2 gives the time-courses of product formation (benzene and
cyclohexene) for the reactions of 1,3-cyclohexadiene with [Pt(SnC1a),]2- (a) and [Pt(SnC1,),]3- (b). Benzene is considered to be formed by either dehydrogenation (eqn. (2)) or intermolecular hydrogen transfer (eqn. (3)) reaction, while cyclohexene should be formed only by the latter reaction; since cyclohexane was not detected, another type of hydrogen transfer reaction, 3C6H8--) CBHIZ+ 2C6HB,can be neglected. C&Ha2C6HB-
C6H6+ H2 &HI0 + C6H6
(2) (3)
324
%
3 ii
o.3
z
k t
0 0.2-
z a
I P
e ro
0 l
O.l-
0
0
o.,
0
0
time/h
@>
20
40
60
time/h
F’ig. 2. Tie-courses for product formation for the reactions of 1,3_cyclohexadiene with [Pt(SnCla),]2- (a) and [F%(SnCl&]“- (b) catalysts; catalyst 0.185 mmol, 1,3_cyclohexadiene 14.8 mmol, solvent (1,1,2,2-tetrachloroethane) 150 ml, 80 “C; cyclohexene (0) benzene(0). TABLE 1 Dehydrogenation and hydrogen transfer reactions of 1,3cyclohexadiene with tin(D)-coordinated platinum(II) complex catalystsa Catalyst
Turnover number” 12 h
24 h
48 h
72 h
cis-[PtCle(SnCl&J2-
0.741 (1.10)
1.06 (1.68)
1.54 (2.55)
1.84 (3.15)
IWSnC1&12-
0.832 (0.054)
1.23 (0.079)
1.64 (0.112)
1.86 (0.126)
lNsnCM613-
0.312 (0.019)
0.534 (0.030)
0.962 (0.057)
1.42 (0.080)
‘Catalyst0.185 mmol, substrate 14.8 mmol, solvent (1,1,2,2-tetrachloroethane) 150 ml, reaction temperature 80 “C. ?‘urnover numbers for the hydrogen transfer reaction are in parentheses.
On the basis of this postulation, turnover numbers for dehydrogenation and hydrogen transfer reactions are calculated for these two complexes as well as cis-[PtClz(SnC1&]2(Table 1). It is obvious from Table 1 that the turnover number for dehydrogenation exceeded unity for every complex. Similar to the cyclohexene dehydrogenation, the initial turnover rate of reaction(2)ishigherfor [Pt(SnC1,),]2- (9.3 X 10e2 h-‘) than for [Pt(SnC1,),]3-
(5.2 X lo-’ h-l). An analogous trend is observable for reaction (3) (6.8 x 10e3 h-l for [Pt(SnC13),]2- and 1.8X 10U3 h-’ for (Pt(SnC13),]3-). The reasons for this order of activity would be the same as for the case of cyclohexene (vi& supra). The dehydrogenation activity of cis-[PtCl,(SnC13),]2is comparable to that of [Pt(SnC13)4]2-, and notably the complex exhibits exceptionally high activity for the hydrogen transfer reaction. As far as the selectivity of catalyst is concerned, dehydrogenation occurs more frequently than hydrogen transfer for [Pt(SnC13),12- and [Pt(SnC13),]3-, while hydrogen transfer is much preferred by cti-[PtC12(SnC1,)2]2-. The ratios of the turnover numbers (dehydrogenation/hydrogen transfer) is u 17([Pt(SnCl&]3->,, 15 ([Pt(SnCl&12-> and 0.6 (cti-[PtC12(SnC13),]2-) during the period of observation (Table l), which suggests that the kind of operating catalyst species is unchanged in each reaction. These values further suggest that dehydrogenation becomes more advantageous with increasing numbers of SnCla- ligand. If this ratio is taken as a criterion, [Pt(SnCl,),]3and [Pt(SnCl,),]2resemble the [M(C,H,),]-K system (M= Cr(O), MO(O)) [ 71, while c&s-[PtC12(SnC13)2]2- shows the same tendency as [Fe(CsHs)(C,H,)] [7] and the RhCl,-quaternary onium salt system [8]. Interestingly, dehydrogenation does not occur at all with [Rh(C,Me,)(C,H,)], [ZrH(q5CGH7)(dmpe)] and [M(acac)2]/AlR3 (M= Co, Ni; R= C2H5, iso-&Ha) [9]. Consideraticm of the reaction mechanisms Alkene coordination in the square-planar Pt(II) complex is very common [lo], while few examples are reported for the trigonal-bipyramid complex [ll]. As the trigonal-bipyramid structure of [Pt(SnC13)5]“- is rather stable 121, it seems unlikely that the reactions of ]Pt(SnC13)5]3- occur only after it dissociates a SnCl:,- ligand to form [Pt(SnCl,),12-. Actually, no evidence of such dissociation was observed in the ’ “Sn NMR spectra, and the rate data seem incompatible with this view. Because (Pt(SnCl,),]“can cleave dihydrogen heterolytically to form [HPt(SnC13)4]3- [ 121, an alternative path may be possible as follows. In a trigonal-bipyramid (ML,) d* system such as [Pt(SnC13),]3-, the metal d,z orbital is vacant (LIMO) [ 13) and thereby some electrophilic character can be expected for the central metal atom. Since the SnCl,- ligand tends to remove electron density from the central Pt(II) ion, thus enhancing its cationic character and lowering the energy level of the vacant d,z orbital, an electrophilic mechanism [ 141 could be considered not only for dihydrogen activation but also for hydrocarbon activation (eqn. (4)). -CH2-CH2-+Pt2+
-
(-CH,-TH-I%]+
+H+
(4)
As seen in Fig. 2b, the concentration of 1,3-cyclohexadiene reaches its maximum at 48 h (1.28 mM), whereas the concentration of benzene increases almost linearly with time to cu. 100 h (9.0X lop3 mM h-l). These time profiles (no point of inflection for benzene formation at 48 h) indicate that the formation of benzene from cyclohexene does not mainly proceed in a consecutive process such that the primary product from cyclohexene (1,3-
326
6-
-
50
0 concentration
Fig. 3. Dependence 1,3-cyclohexadiene;
100 of
substrate/mM
of the initial rate for benzene formation on the concentration of substrate catalyst 0.185 mmol, solvent (1,1,2,2-tetrachloroethane) 150 ml, 80 “C.
cyclohexadiene) is first liberated from the coordination sphere to the solution, and then dehydrogenated to benzene. In order to verify this, the rate of benzene formation at the same free 1,3+yclohexadiene concentration (1.28 mM) was evaluated from kinetic ana.lysis. It was found that the rate of benzene formation is approximately first order with respect to the concentration of charged 1,3cyclohexadiene (Fig. 3; /cl= 6.5 x 10m4 h-l). Since coordinated 1,3cyclohexadiene was barely detected in the solution (13C NMR), the charged amount should be virtually equivalent to the amount of free 1,3_cyclohexadiene. The rate at 1.28 mM concentration expected from Fig. 3 is only 7 X 10m4 mM h-l, which is apparently contradictory to the ‘consecutive mechanism’ postulated above. The same argument is valid for the [Pt(SnC13)4]2- catalyst (k, = 1.2 x 10m3 h-l).
Acknowledgement A part of this work was done at Seikei University, and we thank Professor Makoto Morita of that University for helpful discussions. Financial support by a Corning Research Grant is gratefully ackowledged.
References 1 H. B. Charman, J. Cha. 2 M. S. Holt, W. L. Wilson
Sot., B, (1970) 584. and J. H. Nelson, Chem.
Rev.,
89 (1989)
11.
327 3 (a) C.-Y. Hsu and M. Orchin, J. Am. (1975) 3553; (b) L. KoUar, G. Consigilio and P. Pino, J. Organomelall. Chem., 330 (1987) 305; (c) L. KoIlar, J. Bakos, I. Toth and B. Heil, J. 0rgarwme taU. Chem., 350 (1988) 277. 4 T. Yamakawa, T. Fujita and S. Shinoda, J. Mol. Catal., 52 (1989) L41. 5 (a) N. W. Alcock and J. H. Nelson, J. Chem. Sot., Dal&n Trans., (1982) 2415; (b) P. S. Pregosin and H. Riiegger, Inorg. Chim. Acta, 86 (1984) 55. 6 R. Pietropaolo, M. Graziani and U. BeIIuco, Inorg. Chem., 8 (1968) 1506. 7 G. Fochi, Organametallics, 7 (1988) 2255. 8 I. Amer, V. Orshav and J. Blum, J. Mol. Catal., 45 (1988) 207. 9 (a) K. Moseley and P. M. MaitIis, J. &em. Sot., A, (1970) 2884; (b) S. Datta, S. S. Wreford, R. P. Beatty and T. J. Neese, J. Am. Chem. Sot., IO1 (1979) 1053; (c) C. B. Hanson, Tetrahedron Lett., 21 (1980) 1581; (d) J. L. Costa, A. F. Noels, A. J. Hubert and P. Teyssie, Tetrahedron I&t., 25 (1984) 649. 10 U. BeIluco, Organometallic and Coordinated Chemistry of Platinum, Academic Press, New York, 1974. 11 (a) L. E. Manzer, Znorg. Chem, IS (1976) 2354; (b) L. Maresca, G. NatiIe, M. CaIligaris, P. Delise and L. Randaccio, J. Chem. Sot., Dalton Trans., (1976) 2386. 12 R. D. Cramer, R. V. Lindsey, Jr., C. T. Prewitt and U. G. Stolberg, J. Am. Chem. Sot., 87 (1965) 658. 13 A. R. Rossi and R. Hoffmann, Iwg. Chern., 14 (1975) 365. 14 E. Gretz, T. F. Oliver and A. Sen, J. Am. Chem. Sot., 109 (1987) 8109.
of Molwular
Catalysis,
66 (1991) 321-327
321
The reaction of cyclohexene and 1,3-cyclohexadiene tin(II)-coordinated Pt(II) complexes Tetsu Yamakawa, Institute of Induwial Tokyo 106 (Japan]
Toshio Fujita Science,
University
with
and Sumio Shinoda* of Tokyo, 22-l Roppongi,
7-Ch4mu3, Minato-ku,
(Eeceived August 21, 1990; accepted December 28, 1990)
Abstract The reaction of cyclohexene and 1,3cyclohexadiene with a series of soluble tin(II)coordinated platinum(B) complexes (cti-[PtCLa(SnC1&..2-, [Pt(SnC1,).J2-, [Pt(SnC1,),]3’-) has been investigated under thermal condition. For cyclohexene, dehydrogenation took place catalytically with [Pt(SnC13),]2- and [R(SnC13)J3-, while only stoichiometic oxidation (-CH2-CHa+Bt(II)+ -CH=CH+pt(O) + 2H+) occurred with cti[FWl,(SnC13),]2-, showing the effect of the number of SnC13- ligand. For 1,3-cyclohexadiene, however, all three complexes were catalytically active (dehydrogenation and intermolecular hydrogen transfer reactions). The observed order of catalytic activity for both reactants ([Bt(SnC1,>,]2- > [Pt(SnCl,),]“-) is reconciled with the presence of ratecontrolling pre-equilibrium of substrate r-coordination, which would be more favorable for the former complex. Based on the kinetic analysis, a dehydrogenation path is suggested in which benzene is formed from cyclohexene without liberating 1,3_cyclohexadiene intermediate from the coordination sphere.
Introduction
The SnCl,- ligand has a unique ability to prevent the reduction of central transition-metal ions to metal due to its high rr-acceptor capacity [ 11, and it also tends to provide a vacant coordination site via its strong truns influence and/or its high lability [ 21. It is well known that the catalytic properties of Pt(II) complexes are altered remarkably by the addition of SnCla [ 2 I. However, the type of reaction is limited (e.g., hydrogenation, isomer&&ion and hydroformylation of alkenes), and further there are few cases that use a Ptn-SnCl,- complex in the definite form [3]. In the course of our study of dehydrogenation and hydrogen transfer reactions of hydrocarbons with soluble tin(U)-coordinated platinum(I1) complexes [4], we have found that three kinds of complexes (cti-[PtCl,(SnCl&J2-, [Pt(SnCl,),]2-, [Pt(SnCl&]3-) show quite specific characteristics themselves. In the present paper, results for their reaction with cyclohexene and 1,3-cyclohexadiene will be presented in detail. *Author to whom correspondence should be addressed.
0304-5102/91/$3.50
0 Elsevier Sequoia/Printedin The Netherlands
322
Experimental
Synthesis of tinCII)-coordinated platinum(X) complexes All procedures were performed under argon atmosphere. Tetraphenylphosphonium (PPh,+) salts of cis-[PtClz(SnClJ2]2- and [Pt(SnC1,),]3- complexes synthesized the were following literature method (51. [PPh,],[Pt(SnCl,),] was isolated here for the first time as follows: [PPh412[PtC14]and SnC12were dissolved in CH&lz at a ratio of 1:4 and the solution was stirred for 48 h at ambient temperature. The resulting orange solid was filtered, washed with benzene, and dried under vacuum. The purity of the isolated salt was satisfactory ( < 3% of [PPhl],[Pt(SnC13),] impurity as confirmed by “‘Sn NMR).
Reaction with tin(7I)coordinated platinum(71) complexes All the reagents and solvents were reagent grade. The reaction solution was prepared by dissolving the platinum(I1) complex (0.185 mmol) and substrate (cyclohexene or 1,3cyclohexadiene; 7.4-14.8 mmol) in 1,1,2,2tetrachloroethane solvent (150 ml) under argon atmosphere. 1,1,2,2-Tetrachloroethane was purified by washing with cont. H2S04 and then N&C03dissolved deionized water, followed by drying over CaC12 and fractional distillation. The reaction was carried out under dry nitrogen atmosphere (1 atm) at 80 “C. The reactor was equipped with a reflux condenser to which a gas buret was attached, and was shielded from light with ahuninum foil. Products formed in the liquid and gas phases were analyzed quantitatively by gas chromatography with TCEP (1,2,3-tris(2-cyanoethoxy)propane) and active-carbon columns, respectively. All the reactions were carried out at least twice to ensure reproducibility.
Results
and discussion
Reaction of cgcl~hecr;ene Figure 1 shows the time-courses of product formation for the reactions of cyclohexene with [Pt(SnC13)J2- (a) and [Pt(SnC13)J3- @I). Formation of benzene is catalytic in both cases because the turnover number exceeded 0.5 at 48 h ([Pt(SnC13),]2-) AND 96 H ([pT(sNcL3)J3-). The initial turnover rate of 1,3-cyclohexadiene formation is faster than that of benzene formation, and is much greater for [Pt(SnC13)_,12- (0.26 h-l) than for [Pt(SnC13),13(0.032 h-l). With respect to benzene formation, the initial turnover rate is slightly faster for [Pt(SnC13),]2- (1.1 X 10T2 h-‘) than for [Pt(SnC13),]3(7.3X low3 h-l), and the same tendency exists for the total amount of benzene formation (0.251 mm01 for [Pt(SnC13),12- and 0.203 mm01 for [Pt(SnC13)s]3- at 192 h). The observed higher reactivity of [Pt(SnC13),]2over [Pt(SnC13)J3- would be reconciled with the presence of rate-controlling pre-equilibrium of cyclohexene r-coordination, which may be more favorable for the former complex [ 61.
0.3
0 0 l
5
I >“,
O.Z-
0
8
0 0
k t
z % P@
l
0
Oo O.l-
P
lo
0
0
0
0 0 0 0
I
50
100
9
0
150
0,
0
200
time/h
(4
,k’I
0
I
(b)
50
,L
100
.Oot
150
200
time/h
F’ig. 1. Time-courses of product formation for the reactions of cyclohexene with [Pt(SnCl&]a(a) and [Pt(SnC&]“@> catalysts; catalyst 0.185 mmol, cyclohexene 14.8 mmol, solvent (1,1,2,2-tetrachloroethane) 150 ml, 80 “C; 1,3-cyclohexadiene (0), benzene (0).
It is interesting that cyclohexadiene was not detectable at all during benzene formation with cti-[PtC1,(SnC1,),]2- (41. Moreover, since only a trace amount of dihydrogen was detected, the contribution of the redox reaction (eqn. (1)) is large. -CH2-CH2-
+Pt(II) -
-CH=CH-
+Pt(O)+2H+
(I)
These differences between cis-[ PtClz(SnC1,)2]2- and [ Pt(SnCl& I”- (n = 4, n=5 m = 3) clearly show the effect of the number of SnCl,- ligands. Since cyclohexane was not detected, hydrogen transfer reactions of cyclohexene (2CBHI0--) C6H8+ C6H i2 and 3C6H,0+ C&H6+2&H,,) are neg-
m=2;
ligible . Reaction of 1,3-cgclohexadiene Figure 2 gives the time-courses of product formation (benzene and
cyclohexene) for the reactions of 1,3-cyclohexadiene with [Pt(SnC1a),]2- (a) and [Pt(SnC1,),]3- (b). Benzene is considered to be formed by either dehydrogenation (eqn. (2)) or intermolecular hydrogen transfer (eqn. (3)) reaction, while cyclohexene should be formed only by the latter reaction; since cyclohexane was not detected, another type of hydrogen transfer reaction, 3C6H8--) CBHIZ+ 2C6HB,can be neglected. C&Ha2C6HB-
C6H6+ H2 &HI0 + C6H6
(2) (3)
324
%
3 ii
o.3
z
k t
0 0.2-
z a
I P
e ro
0 l
O.l-
0
0
o.,
0
0
time/h
@>
20
40
60
time/h
F’ig. 2. Tie-courses for product formation for the reactions of 1,3_cyclohexadiene with [Pt(SnCla),]2- (a) and [F%(SnCl&]“- (b) catalysts; catalyst 0.185 mmol, 1,3_cyclohexadiene 14.8 mmol, solvent (1,1,2,2-tetrachloroethane) 150 ml, 80 “C; cyclohexene (0) benzene(0). TABLE 1 Dehydrogenation and hydrogen transfer reactions of 1,3cyclohexadiene with tin(D)-coordinated platinum(II) complex catalystsa Catalyst
Turnover number” 12 h
24 h
48 h
72 h
cis-[PtCle(SnCl&J2-
0.741 (1.10)
1.06 (1.68)
1.54 (2.55)
1.84 (3.15)
IWSnC1&12-
0.832 (0.054)
1.23 (0.079)
1.64 (0.112)
1.86 (0.126)
lNsnCM613-
0.312 (0.019)
0.534 (0.030)
0.962 (0.057)
1.42 (0.080)
‘Catalyst0.185 mmol, substrate 14.8 mmol, solvent (1,1,2,2-tetrachloroethane) 150 ml, reaction temperature 80 “C. ?‘urnover numbers for the hydrogen transfer reaction are in parentheses.
On the basis of this postulation, turnover numbers for dehydrogenation and hydrogen transfer reactions are calculated for these two complexes as well as cis-[PtClz(SnC1&]2(Table 1). It is obvious from Table 1 that the turnover number for dehydrogenation exceeded unity for every complex. Similar to the cyclohexene dehydrogenation, the initial turnover rate of reaction(2)ishigherfor [Pt(SnC1,),]2- (9.3 X 10e2 h-‘) than for [Pt(SnC1,),]3-
(5.2 X lo-’ h-l). An analogous trend is observable for reaction (3) (6.8 x 10e3 h-l for [Pt(SnC13),]2- and 1.8X 10U3 h-’ for (Pt(SnC13),]3-). The reasons for this order of activity would be the same as for the case of cyclohexene (vi& supra). The dehydrogenation activity of cis-[PtCl,(SnC13),]2is comparable to that of [Pt(SnC13)4]2-, and notably the complex exhibits exceptionally high activity for the hydrogen transfer reaction. As far as the selectivity of catalyst is concerned, dehydrogenation occurs more frequently than hydrogen transfer for [Pt(SnC13),12- and [Pt(SnC13),]3-, while hydrogen transfer is much preferred by cti-[PtC12(SnC1,)2]2-. The ratios of the turnover numbers (dehydrogenation/hydrogen transfer) is u 17([Pt(SnCl&]3->,, 15 ([Pt(SnCl&12-> and 0.6 (cti-[PtC12(SnC13),]2-) during the period of observation (Table l), which suggests that the kind of operating catalyst species is unchanged in each reaction. These values further suggest that dehydrogenation becomes more advantageous with increasing numbers of SnCla- ligand. If this ratio is taken as a criterion, [Pt(SnCl,),]3and [Pt(SnCl,),]2resemble the [M(C,H,),]-K system (M= Cr(O), MO(O)) [ 71, while c&s-[PtC12(SnC13)2]2- shows the same tendency as [Fe(CsHs)(C,H,)] [7] and the RhCl,-quaternary onium salt system [8]. Interestingly, dehydrogenation does not occur at all with [Rh(C,Me,)(C,H,)], [ZrH(q5CGH7)(dmpe)] and [M(acac)2]/AlR3 (M= Co, Ni; R= C2H5, iso-&Ha) [9]. Consideraticm of the reaction mechanisms Alkene coordination in the square-planar Pt(II) complex is very common [lo], while few examples are reported for the trigonal-bipyramid complex [ll]. As the trigonal-bipyramid structure of [Pt(SnC13)5]“- is rather stable 121, it seems unlikely that the reactions of ]Pt(SnC13)5]3- occur only after it dissociates a SnCl:,- ligand to form [Pt(SnCl,),12-. Actually, no evidence of such dissociation was observed in the ’ “Sn NMR spectra, and the rate data seem incompatible with this view. Because (Pt(SnCl,),]“can cleave dihydrogen heterolytically to form [HPt(SnC13)4]3- [ 121, an alternative path may be possible as follows. In a trigonal-bipyramid (ML,) d* system such as [Pt(SnC13),]3-, the metal d,z orbital is vacant (LIMO) [ 13) and thereby some electrophilic character can be expected for the central metal atom. Since the SnCl,- ligand tends to remove electron density from the central Pt(II) ion, thus enhancing its cationic character and lowering the energy level of the vacant d,z orbital, an electrophilic mechanism [ 141 could be considered not only for dihydrogen activation but also for hydrocarbon activation (eqn. (4)). -CH2-CH2-+Pt2+
-
(-CH,-TH-I%]+
+H+
(4)
As seen in Fig. 2b, the concentration of 1,3-cyclohexadiene reaches its maximum at 48 h (1.28 mM), whereas the concentration of benzene increases almost linearly with time to cu. 100 h (9.0X lop3 mM h-l). These time profiles (no point of inflection for benzene formation at 48 h) indicate that the formation of benzene from cyclohexene does not mainly proceed in a consecutive process such that the primary product from cyclohexene (1,3-
326
6-
-
50
0 concentration
Fig. 3. Dependence 1,3-cyclohexadiene;
100 of
substrate/mM
of the initial rate for benzene formation on the concentration of substrate catalyst 0.185 mmol, solvent (1,1,2,2-tetrachloroethane) 150 ml, 80 “C.
cyclohexadiene) is first liberated from the coordination sphere to the solution, and then dehydrogenated to benzene. In order to verify this, the rate of benzene formation at the same free 1,3+yclohexadiene concentration (1.28 mM) was evaluated from kinetic ana.lysis. It was found that the rate of benzene formation is approximately first order with respect to the concentration of charged 1,3cyclohexadiene (Fig. 3; /cl= 6.5 x 10m4 h-l). Since coordinated 1,3cyclohexadiene was barely detected in the solution (13C NMR), the charged amount should be virtually equivalent to the amount of free 1,3_cyclohexadiene. The rate at 1.28 mM concentration expected from Fig. 3 is only 7 X 10m4 mM h-l, which is apparently contradictory to the ‘consecutive mechanism’ postulated above. The same argument is valid for the [Pt(SnC13)4]2- catalyst (k, = 1.2 x 10m3 h-l).
Acknowledgement A part of this work was done at Seikei University, and we thank Professor Makoto Morita of that University for helpful discussions. Financial support by a Corning Research Grant is gratefully ackowledged.
References 1 H. B. Charman, J. Cha. 2 M. S. Holt, W. L. Wilson
Sot., B, (1970) 584. and J. H. Nelson, Chem.
Rev.,
89 (1989)
11.
327 3 (a) C.-Y. Hsu and M. Orchin, J. Am. (1975) 3553; (b) L. KoUar, G. Consigilio and P. Pino, J. Organomelall. Chem., 330 (1987) 305; (c) L. KoIlar, J. Bakos, I. Toth and B. Heil, J. 0rgarwme taU. Chem., 350 (1988) 277. 4 T. Yamakawa, T. Fujita and S. Shinoda, J. Mol. Catal., 52 (1989) L41. 5 (a) N. W. Alcock and J. H. Nelson, J. Chem. Sot., Dal&n Trans., (1982) 2415; (b) P. S. Pregosin and H. Riiegger, Inorg. Chim. Acta, 86 (1984) 55. 6 R. Pietropaolo, M. Graziani and U. BeIIuco, Inorg. Chem., 8 (1968) 1506. 7 G. Fochi, Organametallics, 7 (1988) 2255. 8 I. Amer, V. Orshav and J. Blum, J. Mol. Catal., 45 (1988) 207. 9 (a) K. Moseley and P. M. MaitIis, J. &em. Sot., A, (1970) 2884; (b) S. Datta, S. S. Wreford, R. P. Beatty and T. J. Neese, J. Am. Chem. Sot., IO1 (1979) 1053; (c) C. B. Hanson, Tetrahedron Lett., 21 (1980) 1581; (d) J. L. Costa, A. F. Noels, A. J. Hubert and P. Teyssie, Tetrahedron I&t., 25 (1984) 649. 10 U. BeIluco, Organometallic and Coordinated Chemistry of Platinum, Academic Press, New York, 1974. 11 (a) L. E. Manzer, Znorg. Chem, IS (1976) 2354; (b) L. Maresca, G. NatiIe, M. CaIligaris, P. Delise and L. Randaccio, J. Chem. Sot., Dalton Trans., (1976) 2386. 12 R. D. Cramer, R. V. Lindsey, Jr., C. T. Prewitt and U. G. Stolberg, J. Am. Chem. Sot., 87 (1965) 658. 13 A. R. Rossi and R. Hoffmann, Iwg. Chern., 14 (1975) 365. 14 E. Gretz, T. F. Oliver and A. Sen, J. Am. Chem. Sot., 109 (1987) 8109.