Gas Chromatography of Monoolefins with Stationary Phases Containing Rhodium Coordination Compounds E. Gil-Av and V. Schurig Department of Chemistry, The Weizmann Institute of Science, Rehovot, Israel
In a search for new complex-forming reagents suitable for gas chromatography, a series of RhI(C0)2 p-diketonates (in squalane solution) were found to interact rapidly and reversibly with olefins. I n particular, the compound derived from acetylacetone and, still more so, that from 3-trifluoroacetylcamphor (IV) showed high affinity, and were stable for prolonged periods when in constant use. Compound IV was examined in detail and its complexation with 27 CTCG n-alkenes and 12 cyclic olefins with four- to seven-membered rings was determined. The interaction with I V is in general far stronger than with silver. Furthermore, the effect of substituents is usually more pronounced, and in some cases has a different sense than for silver. These features make IV a most promising reagent for the solution of specially difficult analytical problems involving olefins. Milligram quantities of rhodium are sufficient to make an efficient column. THE HIGH SELECTIVITY of stationary phases containing silver nitrate for the gas chromatography of olefins had been recognized already in 1955 (I). Difficult analytical problems, as, e.g., the accurate determination of the equilibrium concentrations of isomeric cycloolefins ( 2 , 3 ) , have been solved with such stationary phases. Some palladium and platinum derivatives, capable of complexation with olefins, have also been studied (4). However, the results obtained did not appear to warrant further efforts. We wish now to report on a number of dicarbonyl-Rh'-@-diketonates, which show marked selectivity in their interaction with olefins (3, and which are of interest as reagents for difficult separations. EXPERIMENTAL
Apparatus. A Barber-Colman IDS Chromatograph Model 20, combined with a YEW Laboratory Recorder, was used. The detector was a tritium @-ionization cell, operated at 75 "C with argon as carrier and scavenger gas. The detector was run at highest sensitivity, since a very small sample size was used to prevent overloading conditions and the resulting peak tailing and lowering of retention. The chart was set in accordance with the expected emergence time of the peaks. Materials. Olefins were purchased from Chemical Samples Co. and Fluka AG. Squalane was obtained from Applied Science Lab., Inc., and Chromosorb P, AW-DMCS, 80-100 mesh, from Varian Aerograph Co. (C0)4Rh2C12 was purchased from Johnson, Matthey Ltd., London, England. Preparation of Complexes. Compounds 1-111 were prepared from (C0)4Rh2C12and the corresponding P-diketones ~
~~
~~
(1) B. W. Bradford, D. Harvey, and D. E. Chalkey, J . Znst. Petrol., London, 41, 80 (1955). (2) A. C. Cope, D. Ambros, E. Ciganek, C. F. Howell, and Z. Jacura, J. Amer. Chem. SOC.,82, 1750 (1960). (3) E. Gil-Av and J. Shabtai, Chem. Ind., London, 1959, 1630. (4) G. P. Cartoni, R. S. Lowrie, C. S. G. Phillips, and L. M. Venanzi, "Gas Chromatography 1960," R. P. W. Scott, Ed.
Butterworth Scientific Publications, London, 1960, p 273. V. Schurig and E. Gil-Av, Chem. Commun., 1971,650.
(5)
2030
according to the literature (6). For I. however. sodium acetylacetonate (from acetylacetone and sodium metal in dry benzene) was used, giving a quantitative yield in 30 min. IV was prepared from (C0)4Rh2C12and barium-bis-3-trifluoroacetylcamphorate in chloroform (7). The barium chelate was obtained as follows: Equimolar amounts of 3-bromocamphor, ethyl trifluoroacetate and magnesium were refluxed in dry ether for 90 min (8). The reaction mixture was then treated with cold 10% HCl and the ether layer separated. Without isolating the 34rifluoroacetylcamphor the ether solution was extracted with 5 % NaOH and the ether and alkaline water phases treated simultaneously with BaCl2 solution. Barium-bis-3-trifluoroacetylcamphorate precipitated and was recrystallized from ethanol (7). All complexes were fully characterized by their spectral and analytical data. The usual care in handling metal carbonyl complexes should be observed (hood, gloves), although no disturbing effects have been noticed during preparation and use of the column materials, when not taking the above precautions. Preparation of the Column. Complex solutions in squalane of 0.01-0.1 molality were used as stationary phases. In a typical experiment, 10 mg of IV and 490 mg of squalane (0.05 molal) were diluted with 2.5 ml of chloroform (or n-hexane) and 2.48 grams of Chromosorb (85 %), as specified above, was added. The mixture was manually agitated until it became homogeneous. The packing was then exposed to a vacuum in a rotary evaporator for 30 min at room temperature and subsequently filled into a glass column of 2 m X 1.75 mm. A vibrator was used to achieve a uniform bed. Approximately 2.55 grams of packing was required for each experiment. The reference column was prepared in essentially the same way, except that no rhodium complex was added. After conditioning for 1 hr at 25 OC, operation could be started. To reach maximum activity, however, conditioning has to be extended to periods of up to 10 days, depending on complex concentration and gas flow. The highest operating temperature used was 50 "C and no serious bleeding of the complex IV was observed, when the Ar pressure did not exceed 25 psi. Columns of I and IV can be operated for extensive periods, when kept under Ar, without serious deterioration of activity. After storage of a column with ends left open, reconditioning as above is necessary. Complexes I1 and 111, on the other hand, are less stable. With IV, the number of theoretical plates reached a maximum at 25 psi under the experimental conditions (1200/m for n-pentane and 850/m for cis-2-butene). The retention times were flow-independent for all olefins showing that complexation is fast and reversible. Sampling. Depending on volatility 1-50 p1 of the olefin vapor (about 20-40 pg) was injected together with methylcyclohexane using a split ratio of 1 :30.
(6) F. Bonati and G. Wilkinson, J . Chem. SOC.,1964, 3156. (7) V. Schurig, Znorg. Chem., in press. (8) B. Feibush, R. E. Sievers, and C. S. Springer, Jr., 158th National Meeting of the Chemical Society, New York, N.Y., Sept 1969, No. INOR 66; B. Feibush, M. F. R. Richardson, R. E. Sievers and C. S. Springer, Jr., submitted for publication.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
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Table I. Retention Time. of Gaseous Olefins with 0.1 Molal Rhodium (I)-Dicarbonyl-0-diketonate Solutions in Squalane as Stationary Phases at 30 "C Squalane I IIb IIIb IV 0.58 0.01 1.13 0.23 10.0 Ethene 0.07 0.79 0.58 0.325 7.95 Propene 0.31 Iso-Butene 0.25 0.315 0.30 1.08 0.64 0.67 trans-2-Butene 0.33 0.56 4.7 1.36 1.20 0.90 12.0 cis-2-Butene 0.37 3.40 2.31 1.31 30.4 1-Butene 0.25 a Relative to n-pentane (corrected retention time 5 min at 25 psi argon) measured at highest activity of the columns. b Column deteriorates with time (measured after 20 hr).
Quantitative Analysis. In order t o demonstrate that no olefin is retained irreversibly, and that the method is thus applicable to quantitative analysis, a 1 : 1 mixture of 3-methyl-lpentene/cyclohexane was chromatographed with pure squalane and with a solution of IV in squalane as the stationary phase. Complete recovery of the olefin at the column exit was observed, as judged by the relative peak areas of the components under the two sets of conditions. Correction of Retention Times. Changes in activity of the column between measurements were taken into account by multiplying the retention volumes found by the ratio of the change of the relative retention volume of 1-butene (with respect to the non-interacting methylcyclohexane). RESULTS AND DISCUSSION
The present study is concerned with the separation of Cz-Cs alkenes and some cyclic olefins with the Rh' compounds I-IV.
IV The data, listed in Table I, indicate that all four RhI-Pdiketonates are capable of separating olefins. Compound IV is obviously superior to the rest, because of its high interaction with alkenes and its stability. The following discussion will, therefore, be essentially devoted to the complex containing the 3-trifluoroacetylcamphor ligand (8). Dicarbonyl-rhodium(I)-3-trifluoroacetylcamphorate (IV). Data on the influence of the structure of the olefins on their interaction with IV and corresponding data for complexes with silver ( 9 ) are listed in Table 11. With a few exceptions, discussed below, the equilibrium constants for Rh' (&h, in squalane, see Table 11, footnote 6)are always considerably higher than for Ag' ( K A ~in, ethylene glycol, see Table 11, footnote e). It should be emphasized that the values available for the silver-olefin equilibrium constants ( 9 ) were measured at a temperature 10 "C lower than the rhodium data. Hence, the actual difference between the two sets of (9) M. A. Muhs and F. T. Weiss, J . Amer. Chem. Soc., 84, 4697 (1962).
~
Table 11. Comparison of Complexation of Aliphatic and Cyclic Monoolefins with Rh(C0)2(3-trifluoroacetylcamphorate)in Squalanea at 50 "Cand AgN08 in Ethylene Glycol Olefin Ethene Propene 1-Butene cis-2-Butene trans-2-Butene iso-Butene 1-Pent ene cis-2-Pentene trans-2-Pentene 2-Methyl-1-butene 3-Methyl-1-butene 2-Methyl-2-butene 1-Hexene 3-Methyl-1-pentene 4-Methyl- 1-pentene 3,3-Dimethyl-1butene cis-2-Hexene trans- 2-Hexene cis-3-Hexene trans-3-Hexene cis-3-Methyl-2pentene trans-3-Methyl-2pentene cis-4-Methyl-2pentene trans-4-Methyl-2pentene 2-Methyl-Zpentene 2-Ethyl-1-butene 2,3-Dimethyl-2butene Cyclopentene Cyclohexene Cycloheptene Methylenecyclopentane 1-Methyl-cyclopentene 3-Methyl-cyclopentene 1,2-Dimethyl-lcyclopentene Methylenecyclohexane 1-Methylcyclohexene 3-Methylcyclohexene 4-Methylcyclohexene Methylenecyclobutane
-103.7 -47.7 -6.3 3.7 0.9 -6.9 30.0 36.9 36.4 31.2 20.1 38.6 63.5 54.1 53.9
0.002 0.01 0.03 0.045 0.035 0.03 0.085 0.11 0.105 0.09 0.06 0.12 0.24 0.18 0.17
0.16 1600 0.125 223 0.425 264 68 0.195 46 0.115 0.045 10 1.105 240 113 0.73 39 0.31 0.125 8 190 0.63 2.5 0.135 256 3.32 138 1.42 215 2.00
41.2 68.8 67.9 66.4 67.1
0.115 0.295 0.28 0.27 0.27
0.165 1.655 0.84 2.605 0.925
22.3 9.1 7.7 5.4
1.4 3.9 4.9 4.3 1.1 3.0 5.1 0.8 4.3 3.4 2.8
9 92 40 173 48
3.6 0.8 3.9 1.0
3.1
67.6 0.295
0.34
3
0.7
70.5 0.325
0.36
2.2
0.7
56.3 0.185
1.44
58.6 0.195 67.3 0.28 64.7 0.265
0.475 0.335 0.31
29 4 3.4
0.375 0.305 0.615 7.045
15 1 .o 67
75.7 0.43
1.235
38
75.8 0.45
0.46
0.4
65.0 0.295
0.51
14.5
73.2 44.2 83.0 115.0
0.375 0.175 0.585 1.625
135
0
3.1 0.7 0.6 3.5 0.1 7.3 3.6 12.8 4.0 1.9
105.8 1.19
1.19
0
103.3 1.055
3.845
53
6.0
110.0 1.42
1.42
0
0.5
104.0 1.05
1.095
0.8
3.5
102.7 1.075
1.13
1.0
3.8
41.2 0.14
10.5
1460
5.8
Measured after 90 hr of conditioning at 25 "C. Relative retention volume on squalanewith reference to methylcyclohexane, which had a corrected retention volume of 35 min at 25 psi argon pressure. Relative retention volume with reference to methylcyclohexane on a 0.05 molal solution of IV in squalane. Equilibrium constant for the formation of a 1 : 1 complex of r - ro the olefin with IV, calculated from the relationship K = -- X a
0.05 '
At maximum activity (10 days of conditioning), the Kp.t,
values would have been about 20% higher. e Equilibrium constant for the formation of a 1 : 1 complex of the olefin with silver, calculated as in footnote d, taken from reference (9).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
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I
I
I
I I
-
-
I
2-methyl I -butene frons2- pentene
I
I I
-
3-methylI-butene t
cis ‘2-pentene
-
-
I
I
6
9
I
3
0
-4I
T IME(min)
Figure 1. Separation of butenes with a 0.1 molal solution of RhI(CO), (acetylacetonate) (I) in squalane at 30 “C For chromatographic conditions, see Experimental; argon pressure
25 psi. C,, Ca,and C612-paraffins emerged at 2.7, 6.9, and 19.6 min,
respectively values should be in general still higher. An important decrease of with rise of the temperature has been reported for many olefins (10). Aliphatic Monoolefins. SUBSTITUTION AT THE DOUBLE BOND. Substitution at the double bond considerably decreases the interaction with IV. This is readily seen when comparing the values of the equilibrium constant of ethylene with those for straight chain 1-n-alkenes. The effect is still larger when 2 substituents are present with the decrease being more pronounced for the type RR’C=CH2, than for symmetrically substituted olefins, RCH=CHR’. Parallel observations have been made for silver, however, the reduction of interaction is far stronger in the case of IV. Thus, for instance, the drop of the equilibrium constants for isobutene, as compared with ethylene is 1/160 for rhodium and 115.7 for silver. Trisubstituted alkenes have still lower equilibrium constants than compounds of the isobutene type, and tetrasubstituted olefins, such as for instance 2,3-dimethyl-2-butene, d o not interact at all with IV. GEOMETRY AT THE DOUBLE BOND. I n common with silver, IV shows throughout higher interaction with the cis than with the trans isomers. However, whereas with silver the characteristic feature is an almost invariable ratio between the constants for isomers of the two series, with little influence of the bulk of the substituents, the findings for IV are entirely different, as an “inverse” steric effect is observed. For IV, in fact, the constants of the cis isomers increase with the bulk of the substituting alkyl residues, whereas for the corresponding trans isomers either no influence or the normal decrease of interaction with buik has been observed. INFLUENCE OF THE CHAIN LENGTHAND OF BRANCHING.F o r all normal chain 1-olefins (RCH=CH2) studied, the K values for IV are practically constant, contrary to the results with silver, for which the interaction decreases with increase of R. On the other hand, mono-substitution a to the double bond has the same effect as for silver, that is, a comparatively small lowering of the complexation constant is observed. The relative decrease is, however, larger, and becomes dramatic, (10) E. Gil-Av and J. Herling, J . Phys. Chem., 66, 1208 (1962). 2032
when two groups are substituted in the a-position. Thus, for 3,3-dimethyl-l-butene the ratios of the complexation constants compared with 1-butene are 1/29 for rhodium and 1/2 for silver. Substitution in the /3 position, as in 4-methyl-lpentene, showed little effect on complexation, contrary to observations with silver. Cyclic Olefins. The interaction of the parent ring compounds decreases in the order C, > Cg >> Cg, as for silver (9) and copper (11). However, the differences are considerably emphasized with rhodium and, in particular, for cyclohexene almost no interaction takes place with IV. No satisfactory explanation has thus far been given for the dependence of complexation with metals on the ring size of cyclenes (9). A methyl group reduces interaction considerably, when placed at the double bond. I-Methylcyclohexene does not interact at all with IV at 50 “C, and the same is true for the tetrasubstituted 1,2-dimethyl-l-cyclopentene.Substitution at other ring positions has relatively little influence. The complexities introduced by the ring structure are further illustrated by the data for the methylenecyclanes. These compounds d o not behave like their aliphatic counterparts RR’C=CH2, but show high interaction, particularly methylenecyclobutane, which has a K value almost equal t o that of ethylene with rhodium. The remarkable difference between the affinity of isobutene and methylenecyclobutane for IV contrasts sharply with their almost equal argentation constants. NATURE OF THE INTERACTION Square planar rhodiurn(1) complexes are electronically and coordinatively unsaturated and therefore undergo readily nucleophilic exchange reactions (12, 13). Five-coordinate trigonal bipyramidai transition states are generally involved in such reactions. In some cases, five-coordinate inter-
(11) J. M. Harvilchuck, D. A. Aikens, and R. C. Murray, Jr.. Znorg. Chem., 8, 539 (1969). (12) R. Cramer, J. Amer. Cltem. Soc., 86, 217 (1964). (13) A. Wojcicky and F. Basolo, ibid., 83, 525 (1961).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 14, DECEMBER 1971
mediates have been assumed (14, 15) and recently, stable trigonal bipyramidal complexes were isolated (16, 17). We, therefore, proposed (5)that olefin interaction with I-IV occurs via a five-coordinate intermediate. A 1 :1 complex seems, in fact, t o be present, since a plot of r - ro/rous. Rh’ concentrations is linear in the range of 0.01-0.05M solutions in squalane. However the observed large increase of the K values during conditioning is not yet fully understood and will have t o be accounted for by further experiments. The influence of the structure of the olefins on the equilibrium constants wjth silver has been explained in terms of strain, steric, and electronic effects (9, 10). Though these factors cannot be readily separated from each other, it has been assumed that the electronic and steric effects oppose each other as a result of prevalent u bonding of the olefins t o silver (9,18). This view is supported by the observation of increased complex stability upon deuterium substitution (18). For the rhodium compounds studied, the effect of structure follows a similar pattern as for silver. In particular, a higher complexation constant was measured for CzD4than for CzH4 with IV (5). There are, however, quite a number of features of the interaction between olefins and IV which cannot be satisfactorily explained at the present time. ANALYTICAL APPLICATIONS The striking selectivity of silver nitrate solutions for olefin separations is well known. The group of Rhl compounds studied, and in particular IV, permits further enhancement of selectivity in many cases for the following reasons: I n general interaction is stronger than for silver; the effect of changes in structure is more pronounced; compound IV leads t o a considerable increase of retention time already in 0.01M solution, and by varying its concentration up t o 0.1 molal, retention can be changed drastically; the Rhl P-diketonates are soluble in apolar solvents. (14) L. Cattalini, R. Ugo, and A. Orio, J . Amer. Chem. Soc., 90,4800 (1968). (15) H. C. Volger, M. M. P. Graasbeek, H. Hogeven, and K. Vrieze, Inorg. Chim. Acta, 3, 145 (1969). (16) P. W. Clark and G. E. Hartwell, h o r g . Chem., 9, 1948 (1970). (17) D. I. Hall and R. S . Nyholm, Chem. Commurz., 1970,488. (18) R. J. Cvetanovic, F. J. Duncan, W. E. Falconer, and R. S . Irwin, J . Amer. Chem. Soc., 87, 1827 (1965).
Certain problems in qualitative and quantitative analysis and of peak identification should be more readily solved using, these reagents. Many olefins are far better separated with IV than with silver, and as a result accurate quantitative analysis and determination of trace amounts of impurities could be facilitated. Contrary t o the solvents used for silver nitrate, the apolar stationary phases which dissolve the Rh’ compound permit one t o separate simultaneously nonolefinic hydrocarbons according t o their boiling points. Since the non-olefinic components do not interact with the Rh’ 0-diketonates, they can be readily recognized by the lack of change of their retention with varying concentration of Rh’, or during conditioning of the column. Caution has, of course, t o be exercised not t o confuse such hydrocarbons with olefins which d o not show interaction (e.g., the type RR’C=CR’’R”’). Finally, it should be pointed out that the order of emergence of certain olefins, as, e . g . , the pairs ;sobutene/trans-2-butene (Figure l ) and l-pentene/cis-2-pentene (Figure 2), is inverted when going from a silver t o a Rh’ column. Such a behavior is very useful for peak identification. Though no attempt was made to optimize chromatographic conditions, high peak resolution was obtained, as can be seen in the Figures. It should also be pointed out that efficient columns can be obtained, using no more than milligram quantities of rhodium compounds. Only a limited number of Rh’ square planar coordination compounds have been studied thus far. It is likely that, by further modification of the ligands attached to rhodium, reagents of equal or better properties than IV could be found. Possibly, complexes stable at higher temperatures and still showing suitable interaction with olefins could be discovered. ACKNOWLEDGMENT Thanks are due t o Dr. B. Feibush for useful discussions and t o Miss Edna Greener and Mr. Frantiek Mike: for their invaluable assistance in carrying out some of the synthetic and gas chromatographic work, respectively. RECEIVED for review July 14, 1971. Accepted September 7, 1971. Thanks are due to the Stiftung Volkswagenwerk for a grant to one of us (V.S.) and to Johnson, Matthey Ltd., for a loan of rhodium.
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