Gas Chromatographic Study of n-Complexes of Hexenes with Palladium Dichloride and Silver(1) Milan Kraitr
Pedagogical lnstitufe, Plzeh, 306 79, Czechoslovakia Radko Komers
Institute of Chemical Process Fundamentals, Czechoslovak Academy of Sciences, Prague-Suchdol, 765 02, Czechoslovakia Frantiiek Cdta
Institute of Chemical Technology, Prague, 766 28, Czechoslovakia A comparison of the stability of complexes of isomer hexenes with PdC12 and Ag+ by gas chromatography has been carried out. The comparison is based on the retention data of eight hexenes measured on the solutions of both PdC12 and AgN03 in N-methylacetamide. The effective stability constants have been calculated for both series of the complexes in question. Their stability is discussed in terms of the differences in the structure. The decrease of the .rr-complex stability caused by steric hindrance of the double bond is much more significant with the alkene-PdCIp complexes. Thus, with these complexes, a wider range of stability constants can be observed, which is reflected in higher selectivity of the stationary phases consisting of PdC12-N-methylacetamide compared with those of AgN03-N-methylacetamide.
The stationary phases containing metal compounds capable of formation of *-complexes with unsaturated substances can be recognized as the most appreciated phases separating on the basis of chemisorption. For the separation of alkenes, silver nitrate solutions were most frequently used as reported for the first time by Bradford, Harvey, and Chalkley (I). The principle of reversible formation of complexes of a low stability in the run of the chromatographic procedure was utilized later by many other authors for the separation of alkenes. According to paper ( 2 ) , compounds of various metals were also tested as stationary phases; however, with the exception of the thallium compounds, they were unsuccessful in most cases. Recently, Gil-Av and Schurig (3) described a new stationary phase containing rhodium compounds. In our previous work ( 4 ) , we studied the separation of alkenes on stationary phases containing compounds of different metals, especially those of the platinum group. We succeeded in using the solutions of PdC12 in N-methylacetamide (NMA). For monoalkene-silver ion complexes which have not been isolated yet, the ratio a1kene:metal = 1:l (5-8) is generally anticipated. On the contrary, it was found (9,
(1) B. W. Bradford, D. Harvey, and D. E. Chalkley, J. lnst. Petrol.. 41, 80 (1955). (2) M. Kraitr, Sb. Pedagogicke Fak. v Plzni. Chemie, I X , 31 (1973). (3) E. Gil-Av and V. Schurig, Anal. Chem., 43, 2030 (1971). (4) M. Kraitr, Thesis, Institute of Chemical Technology, Prague, 1971 ( 5 ) S. Winstein and H. J. Lucas. J. Amer. Chem. SOC.,60, 836 (1938). (6) M. A. Muhsand F. T. Weiss, J . Amer. Chem. Soc., 84, 4697 (1962). (7) E. Gil-Av and J. Herling, J. Phys. Chem., 66, 1208 (1962). (8) R. J. Cvetanovit. F. J. Duncan, W. E. Falconer, and R. S. Irwin. J , Amer. Chem. SOC.,87, 1827 (1965). (9) J. L. Kondakov. F. Balas, and L. Vit, Chem. Listy. 24, 26 (1930).
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IO) that the whole molecule of PdCl2 took part in forming the complex with olefin and the ratio was 1:l again. These complexes seem to be of relatively higher stability than similar complexes of silver ion. If they were isolated the dimeric structure (A) (11-13)was found alkene
I
Pld
c1
J '\
c1 Pd
/
If the r-complex-forming reaction of PdCl2 is to be utilized for the chromatographic separation of alkenes, the conditions favorable for side reactions occurrence have to be avoided. At temperatures above 50 "C, *-allyl complexes (14) of the general formula [ P ~ C ~ ( C , H Z ~ - ~ ) ] ~ threaten to be formed, the stability of which is considerably higher compared with that of metal-alkene *-cornplexes. The formation of *-allyl complexes is considered to be practically irreversible. The presence of water leads to a decomposition of the originating metal-alkene a-cornplexes and to the reduction on metallic palladium (13-15). Moreover, this could also give rise to disturbing isomerization reactions (16, 17). In the present paper, the comparison of the stability of isomeric hexenes-PdC12 or -Ag+ r-complexes has been carried out by gas chromatography. The stability constants of these complexes have been calculated from the retention data of individual hexenes measured on the stationary phases consisting of PdC12-NMA and AgN03NMA, respectively. They are discussed from the point of view of the structure differences of both complex types which are connected with the selectivity differences of both stationary phases compared. This paper starts from the previous study (18) concerning the evaluation of solute-solvent interactions of the same systems by calculating some thermodynamic quantities from the retention data. (10) K. S. Kharash, R. C. Seyler, and F. R. Mayo, J . Amer. Chem. Soc.. 60, 883 (1938). (11) N . Hagihara, M. Kamada, and R. Okawara, Ed., "Handbook of Organometallic Compounds," Interscience, London, 1970, p 907. (12) W. M. MacNevin and S. A . Giddings, Chem. lnd.. London. 1191 (1960) (13) G. F. Pregaglia, M. Conati, and F. Conti, Chim. Ind.. 46, 1923 (1966). (14) E. 0.Fischer and H. Werner, "Metal-*-complexes," Vol. I . , Elsevier, Amsterdam, 1966, p 210. (15) J. Smidt, R. Hafner, R. Jira, J. Sedlmeier, R. Sieber, R. Ruttinger, and H . Kojer, Angew. Chem.. 71, 176 (1959) (16) I . I . Moiseiev, A. A. Grigoriev, and S. V. Pestrikov. Zh. Obshch. Khim.. 4 , 354 (1968). (17) I . I. Moiseiev and A. A. Grigoriev, Dokl. Akad. Nauk SSSR. 178, 1090 (1968). (18) M . Kraitr, R. Komers, and F. Cuta, J. Chromatogr., 86, 1 (1973).
Table I. Effective Stability Constants ( K ) of Hexene-PdC1, Complexes in NMA Solutions at 30 "C (0.49M PdCI,) Boiling point oc (20)
Compound
1 4-Methyl-1-pentene
2 3 4 5 6 7
cis-4-Methyl-2-pentene trans-4-Methyl-2-pentene
2-Methyl-1-pentene 1-Hexene 2-Ethyl-1-butene trans-2-Hexene 8 cis-2-Hexene
53.9 56.3 58.6 60.7 63.5 64.7 67.9 68.3
v,
D
KO
K,1. mole-'
1.35 1.54
107
61.1 63.8 66.5 85.0 85.7 94.4 95.8 105
1.5 1.8
122 79.1 91.0 167
1.000 1.15
2.11 1.27 1.50 2.18
100
119 173
0.38 0.14 1.9 0.13 0.49 1.3
V, truns-4-methyl-Z-pentene, 69.2 ml.
Table 11. Effective Stability Constants ( K ) of Hexene-Ag+ Complexes in NMA Solutions at 30 "C 10.58M AgN03)and Stability Constants ( K ' ) of Hexene-Ag+ Complexes i n EG Solutions at 40 "C Boiling point, oc (20)
Compound
1 4-Methyl-1-pentene
2 3 4 5 6 7
cis-4-Methyl-2-pentene trans-4-Methyl-2-pentene
2-Methyl-1-pentene 1-Hexene 2-Ethyl-1-butene trans-2-Hexene 8 cis-2-Hexene
a
53.9 56.3 58.6 60.7 63.5 64.7 67.9 68.3
VO
D
KO
K, 1. mole-'
K' ( 6 ) ,1. mole-1
1.10
84.0 89.5 76.4
61.7 64.5 67.1 85.9 86.5 95.3 96.7 106
0.62 0.66 0.24 0.50 0.86 0.72 0.20 0.62
2.8
1.17 1.00"
1.45 1.70 1.71 1.41 1.89
111
130 135 108 144
3.1
0.7
...
4.3 3.5 0.8 3.1
V, trans-4-methyl-Z-pentene, 66.2 ml.
EXPERIMENTAL Apparatus. The apparatus used for the present study was the same as described before ( 1 8 ) , with the katharometer and the column of 85-cm x 4-mm i.d. glass U-tube. The hydrocarbon samples were introduced as vapors by a syringe. The amount of samples varied from 10-4-10-3 gram. Materials. Besides materials given in the previous paper (18) the following chemicals were used: anhydrous barium chloride was prepared from BaC12 2H20 by recrystallization and calcination at 600 "C to the constant weight; sodium nitrate was recrystallized and dried at 110 "C; methanol was redistilled and dried over magnesium. These chemicals were supplied by Lachema (Brno, CSSR) and certified reagent grade. Preparation of Stationary Phases and Column Packings. The preparation of the stationary phases consisting either of PdC12-NMA or AgXO,-NMA was described previously ( 2 8 ) . An equivalent procedure was chosen also for BaC12-NMA and NaN03-NMA except that the preparation could be performed by daylight. The stationary phases contained 5-1770 PdC12, 9.6% &Nos, 5.9-11.8% BaC12, and 4.9% XaN03. The packings were prepared by dissolving the stationary phases in dried acetone, except that BaClZ-SMA was dissolved in methanol, and slurring them in the amount of 20% w/w with the support. Acid-treated Chromaton N of 0.25-0.43 mm (Lachema) was used as the support. Kitrogen was employed as the carrier gas at a flow rate of from 30 to 60 ml/min. Retention Data. The retention data of the investigated compounds were expressed relative t o trans-4-methyl-2-pentene as the standard. For this hydrocarbon, the specific retention volumes were calculated.
.
RESULTS The stability constants of the hexene-PdCl2 or Agcomplexes in NMA solutions a t 30 "C (anticipating the ratio alkene /(PdC12 or Ag+) = 1:l)were calculated from the specific retention volumes of hexenes taken on PdC12-NMA (0.49 mole of PdC12/1.) or AgN03-NMA(0.58 mole of Ag?;Oa/l.). As in the papers (6-8) concerning the stability constants of alkene-Agf or rhodium (3) complexes, the following equation was used in our calculation also
where K is the stability constant, D is the distribution ratio of the alkene between the PdC12-NMA or AgN03NMA and the gas phase, K D is the distribution constant of the alkene between the inert referential stationary phase (BaC12-NMA or NaN03-NMA) of the same salting out effect as with the complex-forming phases, and the ~ the PdCl2 or AgN03 concentration (mole/ gas phase, c , is 1.). Then, the salting out effect is compensated in the calculation. The D and K D values were calculated from the specific retention volumes V , in the ordinary way (19), the densities of the stationary phases were determined by the pycnometric method. The values of the constants in question corresponding to the hexene-PdCl2 complexes and BaC12-NMA system (0.29 mole of BaC12/1.) are summarized in Table I together with the boiling points of the individual hexenes. Similar data concerning the hexene-Ag+ complexes and NaN03-NMA system (0.57 mole of NaN03/1.) are given in Table 11. The stability constants of hexene-Ag+ complexes in ethylene glycol (EG) solutions a t 40 "C taken from the literature (6) are listed in Table I1 for comparison.
DISCUSSION The ethylene glycol solutions of AghT03 were mostly used as the stationary phases in the existing studies (6-8) of the equilibria of complex formation between silver ion and alkene performed by gas chromatography. It was assumed that the AgN03 was dissociated in these solutions and the interactions between the silver ions and the solvent could be neglected. Also in the AgN03-NMA system (19) E. Leibnitz and H . G . Struppe, "Handbuch d e r Gas-Chromatographie," Akad. Verlag, Gest u.Portig K-9.. Leipztg. 1966, p 739. (20) R . R . Dreisbach, "Physical Properties of Chemlcal Compounds." Vol I I , American Chemical Society, Washington, D.C., 1959. A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 8, J U L Y 1974
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the dissociation of AgN03 can be anticipated (21, 22). On the contrary, PdClz is not likely to be dissociated in the NMA solutions. This fact can be supported by the comparison of the salting out effect for the hydrocarbons in the PdC12-NMA and BaC12-NMA systems of the same molarity and the corresponding systems of AgN03-KMA and NaN03-NMA, respectively [BaC12 and NaN03 are dissociated (23, 24) in NMA]. As far as the nature of the interaction between Ag+ or PdClz and the solvent is concerned, a stronger solvation effect, possibly a formation of complex bond, may be assumed depending on the solvent structure. However, information in the literature is different. While the interactions between Ag+ and NMA are considered to be a solvation effect (22), similar systems are interpreted as the complex-forming reaction (21). Similarly, the formation of complex was found in the PdClZ-NMA system (25, 26). Therefore, the calculated stability constants have to be considered the effective constants. Calculating the stability constants, we supposed that the whole PdCl2 molecule or Ag+ took part in the complex-forming reaction with alkenes. With regard to the low ratio of the alkene concentrations to the concentrations of the complex forming salts in the stationary phase, the multistage complex equilibrium need not be taken into account in the gas chromatographic procedure. For the calculation, the ratio (alkene/metal) = 1:l is assumed in both cases. With the PdClz complexes, the formation of relatively stable dimeric compounds is not assumed but only the formation of intermediate monomer compounds. This is supported by the fact that the Equation 1 is satisfactory for the calculation of K and practically the same values are obtained in the concentration range of 0.491.11 mole of PdClZ/l. The complex-forming reactions are reversible with both stationary phases; however, with PdClz the necessary conditions concerning operating temperature (lower than 50 "C) and dry medium must be fulfilled. In the calculations of K of monoalkene-silver ion complexes (6-8) in ethylene glycol solutions of concentrations lower than lM, the salting o u t effect of the complex-forming compound was of less importance and it was usually neglected (6, 8). On the contrary, in the solutions of PdClz and AgN03 in NMA, the salting out effect was considerably significant and had to be compensated, For instance, the V, of 2-methylpentane measured on the 0.49M and 0.58M solutions of PdClz and AgN03, respectively were found t o be lower than that which would correspond to pure NMA by approximately 17%. The salting out effect was judged with regard to the inert hydrocarbon (2-methylpentane). The K D values were related to the reference solutions of the inert electrolytes with the same salting out effect as it was in the phases with complex-forming compounds. Then, it validates for the corresponding pairs of the stationary phases that K U of 2-methylpentane has approximately the same value as D of 2-methylpentane or K of 2-methylpentane = 0. In order to compensate for the salting out effect of AgN03, the solution of N a N 0 3 of the same molarity was used. Our results proved that no specific interactions took (21) P. A . Temussi. T. Tancredi, and F . Quadrifoglio, J. Phys. Chem.. 73, 4227 (1969) (22) A. E. Pucci. J. Vedel, and B. Tremiilon, J . Electroanai Chem. l n terfac. Electrochem.. 22, 253 (1969) (23) L. Weeda and G . Somsen. R e d Trav Chim. Pays-Bas. 86, 263 (1967) (24) Chandra Dinesh, Gopal Ram.. J. lndian Chem. Soc.. 45, 351 (19681, (251 H. Pivcovl and B. Schneider, Coll. Czech. Chem. Commun , 30, 2045 (1965) (26) B B Wayland and R . F. Schramm, Inorg. Chem.. 8,971 (1969)
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place between the reference solution and hexenes and that both the solutions show almost identical salting out effects. Considering that the salting out effect of PdC12 could not be omitted, an attempt was made to compensate for it with barium dichloride of the same molarity. However, a significantly lower salting out effect in PdC12 solution proved the assumption that PdClz was not dissociated and that the compensation in question was unfeasible. Therefore, such concentrations of PdClz and BaC12 which possessed the same salting out effect for the inert hydrocarbon were tentatively searched for. The pair of 0.49M PdCl2 and 0.29M BaClz was found to be convenient. The fact that the K values calculated from the retention data for solutions containing different concentrations of PdClz show only small differences, can be used for the authorization of this compensation (for 0.49M and 1.11M PdClz the differences are larger than 5% rel. only with the complexes of 2-methyl-1-pentene and 2-ethyl-1-butene. This fact also points to the linearity of the dependence of ( D K D ) / K Dupon Cpd& in the mentioned concentration range. The K values were determined with the accuracy of about &20%. Since calculated K are considered to be the effective constants, the comparison of their values between the PdC12 and Ag+ complexes is not feasible. With negligible exceptions, the K values are higher for the complexes mentioned in the first place. Nevertheless, in agreement with other authors (21, 22, 25, 26), the coefficients of possible side reactions are likely to be higher in the phases containing PdC12. This fact would express the relation in question even more strongly. It is consistent with the experience that palladium forms a-complexes of higher stability compared with those of silver ion. The comparison of K with K' corresponding to the hexeneAg+ complexes in " M A and similar complexes in ethylene glycol (EG), respectively (Table 11), shows that side reaction coefficients are higher in NMA, but their ratio remains unchanged. With negligible exceptions, practically the same relations can be observed in both the series ( K and K ' ) regardless of the fact that K' were taken at a temperature higher by 10 "C. According to the K values obtained (Table I), the hexene-PdClz complexes can be divided approximately into the three groups: complexes of higher stability (1, 2, 5 , 8), complexes of lower stability (3, 7 ) and complexes of negligible stability (4, 6). Similarly, with Ag+ complexes (Table 11), the groups of stable (1, 2, 4, 5 , 6, 8) and unstable (3, 7 ) complexes can also be seen. The r-complex stability is markedly affected by the steric arrangement of the molecules of reacting components. Consistent with the existing notions ( 6 4 , the Ag' complexes of cis isomers are of a considerably higher stability as compared with those of trans isomers. With both the investigated isomer pairs (2 and 3, 8 and 7 ) , the K values corresponding to cis isomers are approximately three times higher (Table 11). A similar situation can also be seen with the PdCl2 complexes where K values are three or five times higher for the cis isomer complexes 7 and 2 (Table I). According to general considerations, the complexes in which the double bond is sterically unhindered are expected to be of the highest stability. In addition to cis isomers, similar properties are shown also with hexenes where the double bond is sterically unhindered in position 1 (4-methyl-1-pentene and 1-hexene). A very low stability is shown by 2-methyl-1-pentene ( 4 ) or 2-ethyl-1-butene (6)-PdClZ complexes as compared with that corresponding to the Agt complexes (Tables I and 11). This is very likely due to a steric effect again. The double bond of the ligand
coordinates with the silver ion itself. Therefore, the steric hindrance of the double bond caused by an alkyl group a t the double bond carbon atom is not strong enough to prevent the complex formation to a considerable degree. On the contrary, the whole PdCl2 molecule is combined with an alkene. This bulky grouping very likely prevents complexation of the alkenes with the screened double bond. The effect of the steric hindrance of the double bond is more significantly expressed with the complexes containing PdC12. Therefore, a wider range of the stability con-
stants can be seen with these complexes, and it is responsible for a wider range of the retention data of the individual alkenes. The higher selectivity of the chromatographic separation of alkenes by PdC12-NMA can be explained on the basis of the structure differences between the alkenePdC12 and Ag+ complexes. Received for review June 18, 1973. Accepted January 14, 1974.
Gas Chromatographic Determination at the Parts-per-Million Level of Aliphatic Amines in Aqueous Solution Antonio Di Corcia and Roberto Samperi lstituto di Chimica Analitica, Universita di Roma, 00785 Roma, Italy
Modifications of Sterling FT-G and Vulcan, which are two well-known examples of graphitized carbon blacks (GCB) with suitable amounts of KOH and polyethylene glycols (PEG), e.g., PEG-2OM and PEG-1500, allow the linear elution of free aliphatic amines from C1 to C16 to be performed. In particular, because of the use of such packing materials, the quantitative determination of C 1 - G aliphatic amines in aqueous solution is made possible even at the sub-ppm level. At these high sensitivities, the sole factor affecting in some measure the quality of the chromatographic profile is due to the water disturbance. By varying the liquid-to-solid ratio, gas-liquid-solid (GLS) columns have been evaluated in terms of selectivity and elution time.
Gas chromatographic determination of strong organic bases is often unsatisfactory although this method is recommended and widely used. Errors accrue from loss of sample, ghosting phenomena and badly tailed elution peaks. The source of these unwelcome effects is always traced to abnormally strong solid-gas interactions, which take place either by using gas-liquid (GLC) or gas-solid chromatography (GSC). Therefore, efforts for improving the gas chromatographic method for the analysis of aliphatic amines are generally confined to the search for means of eliminating strong adsorption effects. In GLC, strong adsorption is the result of hydrogenbonding to the protonized silanol groups of the commercially available support materials, which are siliceous in nature. Among the numerous remedies offered to reduce the influence of the support, the treating of the substrates with an alkali hydroxide actually appears to be the most effective. Alkali coating enhances the performance of gasliquid columns; yet it is common enough to find large irregularities when quantitative determinations of basic nitrogen-containing compounds are attempted (1-4). With exception (j),works reported in literature concerning R. A . Simonaitis and G. C. Guvernator I l l , J . Gas Chromafogr.. 5 , 49 (1967). L . D Metcalfe and A . A . Schmitz, J. Gas Chromatogr.. 1, 15 (1964) J. J. Cincottaand R Feinland, Anai. Chem.. 3 4 , 774 (1962) S. Hantzsch, J. Gas Chromatogr.. 6, 228 (1968) G. R . Umbreit, R . E. Nygren. and A . J . Testa, J. Chromatogr.. 43, 25 (1969)
GLC of unaltered amines deal only with qualitative aspects or a t the best with quantitative determinations of concentrated solutions. In GSC, peak tailing, which sharply increases as the sample size of a basic eluate is decreased, is accounted for by the formation of some kind of chemical complex between the adsorbate and traces of oxygen complexes (6) or metal transition salts (7) contaminating the surfaces of the commonly used adsorbing materials. Thermal treatment a t 1000 "C of graphitized carbon black (GCB) with a stream of hydrogen was found very effective in removing either chemical (6) or geometrical irregularities (8), thus reducing greatly the peak tailing for hydrogen-bonding compounds (9). However, hydrogentreated carbon surfaces pick up carbon-oxygen complexes on exposure to water (10). This precludes the determination of amines in aqueous solution. The usual way of preempting surface heterogeneities is to add a suitable amount of a nonvolatile liquid phase itself containing basic nitrogen, so as to make high-energy sites unavailable for adsorption of eluates. Hollis (11) reported that linear elution for aliphatic amines was obtained by coating porous polymers with either polyethylene imine (PEI) or tetraethylene pentamine (TEPA). Also, the effect of increasing the PEI loading was to gradually reduce retention times of amines (7). TEPA modified GCB enabled us to achieve a well-defined elution peak for few nanograms of methylamine (12). In this work, by varying the liquid-to-solid ratio, gasliquid-solid (GLS) columns were evaluated in their ability to separate the first terms of aliphatic amines. It was pointed out that a large range of selectivity could be made available by changing the surface concentration of the liquid. However, because of the volatility of TEPA, aliphatic amines containing more than three carbon atoms could hardly be eluted. (6) A . Di Corciaand R Samperi, J . Chrornafogr., 77, 277 (1973). (7) J. R . Lindsay Smith and D. J. Waddington, Ana/ Chem.. 40, 522 (1968). (8) A . Di Corcia and R . Samperi, J. Phys. Chem., 77, 1301 (1973) (9) A . Di Corcia and F. Bruner, Anai. Chem.. 43, 1634 (1971) (10) R . Nelson Smith, J. Duffield, R. A . Pierotti, and J. Mooi, J. Phys. Chem.. 60, 495 (1956). (11) 0. L . Hollis,Ana/. Chem., 38, 309 (1966). (12) A. Di Corcia. D. Fritz, and F . Bruner, Anal. Chem 42, 1500 (1970) A N A L Y T I C A L C H E M I S T R Y , VOL. 46,
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