NeSTES
Gas-Chromatographic Investigation on Clathrate-Forming Transition Metal Complexes A. C. Bhattacharyyal and Asit Bhattachaejee, Central Fuel Research Institute, P.0.-F.R.I., Dhanbad, Bihar, India THE USE of two clathrate-forming Werner complexes-e.g., Ni(4-methylpyridine)4(NCS)z and Ni(l-phenylethylamine)h(NCS)2 as stationary phases in gas chromatography has been recently reported in brief ( I ) . The first compound was found to separate molecules depending on their shapes, while for the second electronic interactions appeared to govern retentions in a G C column. The object of the present work was to study the influence of various substituents in the aromatic rings of these complexes on the elution order of molecules of varying shapes and structures ; for the substituted pyridine complexes the effect of replacing nickel by cobalt and iron has also been examined. Seven complexes of the type dithiocyanatotetrakis(1-arylalkylamine)nickel(II) having different substituents in the aromatic ring and the alkyl chain and six complexes of the general composition MBd(CNS)* where B is either 4-methyl- or 4-ethylpyridine and M i s Fe, Co, or Ni have been studied. All these Werner complexes are known for their ability to clathrate various organic molecules (2-6). On the basis of the present work an attempt has been made to gain an insight in the mechanism of the clathration process by all these various complexes.
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EXPERIMENTAL
Chemicals. 4-Methylpyridine (4-MePy), B.D.H., L.R., and 4-ethylpyridine (CEtPy), Fluka, were used as received. The 1-arylalkylamines were synthesized from the corresponding ketones by following suitable procedures of Leuckart reaction (7-9), the ketones themselves being obtained through general Friedel-Crafts reactions (IO). Boiling points of the amines and the melting points of their hydrochlorides agreed well with those in literature (8, 9). Werner complexes were prepared from these amines and pyridines as described in (3) and (3, respectively. Columns. 180-cm stainless steel columns (i.d., 4 mm) were used in each case. Packing materials were prepared by 1 Present address, SocikttB D’Application de Produits, Industriels et Chimiques, 32, Rue AndrC Cayron, 92 Asnieres, France.
(1) A. C. Bhattacharyya and Asit Bhattacharjee, J . Chromatogr., 41, 446 (1969). (2) P. De Radzitzky, J. Hanotier, J. Brandli, and M. HanotierBridoux, Rev. Inst. Franc. Petrole, 16, 886 (1961). (3) P. De Radzitzky and J. Hanotier, Ind. Eng. Chem., Process Des. Develop., 1, 10 (1962). (4) P. De Radzitzky and J. Hanotier, Erdoel KoAle, 15, 892 (1962). (5) W. D. Schaeffer, W. S. Dorsey, D. A. Skinner, and C . G. Christian, J. Amer. Chem. SOC.,p 5870. (6) F. V. Williams, ibid., p 5876. (7) M. L. Moore, “Organic Reactions,” Vol. V, R. Adams, Ed., Wiley, New York, 1952, p 301. (8) A. De Roocker and P:De Radzitzky, Bull. SOC. Chim. Belges, 72, 195 (1963). (9) A. De Roocker and P. De Radzitzky, ibid., 73, 181 (1964). (10) P. H. Gore, “Friedel-Crafts and Related Reactions,” G. A. Olah, Ed., Vol 111, Part I, Wiley, New York, 1964, p 1.
Figure 1. Retention times of some aromatic hydrocarbons on M(4-MePyI4(NCS)2 slurrying the solution of the complex in chloroform with Chromosorb-P (-60 80 mesh), removing most of the solvent on the water-bath. Prolonged heating at this stage must be avoided. Concentration of the phase on the support was 30% wt/wt in each case. Experiments were done with a Perkin-Elmer Model 810 gas chromatograph using nitrogen as carrier gas. For base-line compensation dual column were used. Nitrogen flow rate for all the Figures 1-4 were 60 ml per minute; because of inordinately long retentions on these columns when freshly packed, these were preheated at 80 ”C under nitrogen flow of 45 ml per minute for 120, 45, and 30 minutes for the Ni-, Co-, and Fe-complexes, respectively. Retentions on Apiezon-L were measured on a 2-m column (Ld., 3mm) packed with 12% of the phase on Chromosorb-W (-60 80 mesh).
+
+
RESULTS AND DISCUSSION
Figure 1 illustrates the order of elution of some aromatic hydrocarbons from columns containing M ( ~ - M ~ P Y ) ~ ( Nas CS)~ stationary phases ( M = Fe, Co, Ni). Retentions measured on all other columns are given in Table I. It will be seen that retentions in Figure 1 are strictly governed by the molecular shape of the guest molecules (except for 1,2,3-trimethylbenzene from Co and Fe columns) and it appears that clathrate
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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Table I. Relative Retentions of Some Aromatic Compoundsa on
CH-R
NL
]
H, (NC s ) ~and M(4-Ethyl pyridine), (NCS)2
R2
4
Column materials at 80 "C
R1 =
R1
R1
R1
43,4,5trimethylM Ra = pheny!) Ni Co Fe Compounds nCB7 CHI cthylamine (84 "C) (70 "C) (80 "C) Apiezon Benzene 1 .oo 1.00 1.00 1.00 1.00 1.00 1.00 Toluene 1.75 1.58 1.78 1.41 1.80 1.55 1.69 1.36 1.75 1.57 2.12 3.30 2.88 3.15 ,.. 3.12 3.28 3.90 Chlorobenzene 3.16 2.95 4.22 2.21 Ethylbenzene 2.92 2.82 3.11 2.10 3.69 2.50 2.79 1.91 2.75 2.00 4.25 p-Xylene 3.24 2.59 3.51 2.14 1.87 2.61 2.86 2.72 3.12 2.28 4.88 m-Xylene 3.24 2.59 3.51 2.14 1.89 2.56 2.76 2.72 3.12 2.28 4.88 o-Xylene 3.36 3.08 4.41 2.52 4.61 2.88 3.41 2.82 3.50 2.72 5.93 Cumene 4.20 3.82 4.44 2.82 6.15 3.12 3.13 2.72 3.50 2.57 6.70 Anisole 6.34 5.81 9.40 4.33 6.30 6.38 4.44 6.27 7.40 7.70 6.00 o-Chlorotohene 5.24 4.30 7.70 3.84 6.80 5.75 5.21 4.72 5.37 4.28 9.20 p-Chlorotoluene 6.65 5.60 8.60 4.07 6.20 5.77 6.16 5.81 6.62 5.00 9.60 m-Chlorotoluene 6.34 5.40 8.45 3.92 6.60 5.45 5.38 5.81 6.62 5.00 9.60 1,3,5-Trimethyl benzene 7.27 4.15 6.64 3.56 4.20 3.48 4.76 3.82 5.37 3.00 11.50 m-Dichlorobenzene 8.95 8.00 14.50 5.70 10.00 10.25 8.75 7.45 ... 7.30 15.20 p-Dichlorobenzene 10.30 8.00 14.95 5.85 9.38 11.10 10.27 8.00 .,. 8.70 15.20 1,2,3-Trimethylbenzene 10.10 8.44 11.80 5.48 10.90 7.31 7.22 6.10 8.25 4.57 16.50 ... 17.30 o-Dichlorobenzene 12.00 9.65 21.50 7.92 14.0 13.50 11.53 ... ... Elution order of aliphatic hydrocarbons from Ce to CSwhich were studied, was found to follow increasing order of boiling point. =
i=
No2 (m) Cl(p) OCH3(p) R1 = H Rn = Rz = Rz Rz = CHI CHa H CHa 1.00 1.00 1.00 1.00
R1 = H Rs =
=
CHdp)
a
M
Ni CO
Fe
W
Table II. Paralmeta Separation Factors on M ( ~ - M ~ P Y ) ~ ( N Columns CS)~ Xylenes Chlorotoluenes Column Column temp a: temp a: 1.24 2.42 98 "C 80 "C 1.29 2.10 100 "C 90 "C 1.69 2.50 80 "C 80 "C
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formation itself is not dependent to any tangible extent on electronic interactions as is believed to be by some authors (11, 12). That the shape-discriminating feature of the 4MePy-complexes is not restricted to aromatic molecules alone is demonstrated with aliphatic compounds in Figures 2 and 3. Such behavior is common to all the three 4-MePy-complexes and stands in sharp contrast with that of other complexes studied. Most impressive is the large separation of the mand p-disubstituted benzenes; separation factors (p/m) for the xylenes (Table 11) are the highest reported so far. Due to thermal instabilities of the complexes these factors are dependent on the actual amount of the complexes present during analysis. Under comparable conditions the rate of deterioration of the columns follows the order Fe > Co > Ni. With proper calibration the Ni-column has been found to give reproducible separations of m- and p-xylenes for over a week. The order of elution from these columns may be explained through the presence of permanent clathration "holes" in the host lattices. Linear molecules which fit in such holes are subjected to strong van der Waals forces inside leading to their greater retentions over the nonlinear molecules. This high selectivity toward molecular shape is primarily responsible for the asymmetrical and broad peak of the p-isomer; mxylene peak is of normal width. Degree of asymmetry and (11) G. Gawalek and H. G. Konnecke, Chem. Tech. (Berlin), 15, 609 (1963). (12) F. Castellato and B. Casu, Riu. Combust., 20, 563 (1966). 2056
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Figure 2. Separation of two dialkylethers on Ni(4MePyh (NCSh column at 80 "C
Q
-
4
2
0
MINUTES.
broadness of the p-isomer peak increase in the order Fe > Co > Ni. Thus with the Ni-complex the p-xylene peak is nearly symmetrical ( I ) , with the Co-complex it is much broader (Figure 4) and it is almost semicircular with the Fecomplex. This is consistent with the fact that the ferrous complex has the highest capacity to accept foreign molecules into its lattice (6). The shape of this peak does not change to any great extent with higher flow rates; with increasing column temperatures this becomes slightly sharper but then at the cost of column efficiency. Increasing the length of the alkyl chain by one CH2 group in passing from 4-methyl- to 4-ethylpyridine results in total
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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i Figure 3. Separation of isooctane from n-parafflns on Fe(4MePyh (NCSh column at 70 OC
4
2
0
-MINUTES
loss of selectivity toward steric features of the guests under GC-conditions. This is seen from the relative retentions on M(~-E~PY)~(NC columns S ) ~ in Table I. The difference in retentions between m- andp-dichlorobenzenes is only apparent because on injecting a mixture Qf equal amounts of these compounds one broad peak is obtained. This loss of selectivity is difficult to explain. X-ray studies on the clathrate Ni(CN)nNH3.C6H6have proved conclusively that the benzene molecule is placed in the cavity of the two-dimensional complex (in contrast to three-dimensional encaging in case of quinol host) and that steric requirements are such that while benzene, pyrrole, thiophene, furan, and pyridine are trapped; toluene and xylenes are not trapped at all (13). Increasing the length of the coordinating molecule leads to more flexible steric requirements for the guests so that 4-MePy-complexes clathrate p-xylene extremely well. Still greater increase in the one-dimension of the cavity by placing an ethyl group in the 4-position of the pyridine ring leads possibly to an even larger hole-diameter of the host lattice so that only molecules having cross-sectional diameter larger than 1,3,5-trimethylbenzene would be preferentially retained on a GC-column. On the other hand far infrared spectra of the substituted pyridine complexes of Co and Ni reveal that except for the 4-methyl substitution, mass of the substituent alkyl group does not have any effect on metal-pyridine stretching frequency (14). When, however, there is a methyl group in the 4-position, this frequency is lowered as explained by hyperconjugative electron release by the methyl group which discourages metal tZp+. pyridine-pi* transfer. This reduces the overall bond strength and infrared frequency. With the 4-ethyl substitution, where hyperconjugation is negligible there is greater overlap of metal tzO pyridine-pi*. The increased M-N bond strength in this case may result in an overall stabilization and in greater density of the pi-electron cloud of the pyridine ring so that it may behave as a weak electron donor. Hence, a weak type of charge-transfer (CT) interaction may be significant during clathration. However G C is an insensitive technique to measure such effects. + .
(13) J. H. Rayner and H. M. Powell, J . Chem. SOC.,1952 319. (14) J. Burgess, Spectrochim. Acta, 24A, 277 (1968).
1
V I I u
-
6
4 MINUTES
I
0
Figure 4. Separation of m- and p-xylenes on c0(4-MePy)~ (NCS)2 column at 90 "C
On the basis of exhaustive chemical and physical studies De Radzitzky and coworkers have firmly established that CTinteractions between the aromatic nuclei of the host and the guest are primarily responsible for clathrate formation itself in case of complexes of the type Ni( l-arylalkylamine)4(NCS)I although steric factors are not always negligible (IS, 16). Pi-donor-acceptor systems have of late been studied by GLC but the difficulty of isolating this effect even with strong donoracceptor systems like polymcthylbenzenes-trinitroaromatics has been pointed out (17, 18). This difficulty is all the more greater in case of arylalkylamine complexes where CT-interactions are very weak indeed as evident from failure to detect the CT-band (15). That is why retention times in Table I do not reveal any system consistent with such effect. Thus for example, based on pure electronic interactions ethylbenzene and p-xylene (pi-donors) should have emerged later than chlorobenzene and p-dichlorobenzene (pi-acceptors), respectively, from the column containing the complex with R1 = NOz, Rz = CHI (pi-acceptor). Obviously, non-CT factors like vapor pressure, diffusion override the weak CTeffect in the G C column. In the columns having pi-donor systems in the host complex (Table I, RI = Rz = CH, and
(15) J. Hanotier, M. Hanotier-Bridoux, and P. De Radzitzky, Bull. SOC.Chim.Belges, 74, 381 (1965). (16) J. Hanotier, W. Brandli, and P. De Radzitzky, ibid., 75, 265 (1966). (17) A. R. Cooper, C. W. P. Crowne, and P. G . Farrell, Trans. Faraday Soc., 63,447 (1967). (18) A. R. Cooper, C. W. P. Crowne, and P. G . Farrell, J. Chrornatogr., 27, 362 (1967).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
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the trimethylphenylethylamine complex) the general order of elution of various guests although seeming to conform to CTeffect (p-xylene is eluted earlier than p-dichlorobenzene) can not be taken as an unambiguous proof of this effect, because here vapor pressure of the guest and electronic interaction operate in the same direction. Based on these arguments and on the additional data of Table I our previous interpretation of the elution order of various molecules from the Ni(1phenylethylamine)4(NCS)2column must be modified ( I ) . In contrast, gas chromatography can be a very diagnostic tool for shape-selective stationary phases like liquid crystals (19), tri-o-thymotide (20), desoxycholic acid (21), and M(4MePy)4(NCS)2as shown in the present work.
ACKNOWLEDGMENT
Authors are grateful to Pierre De Radzitzky for his kind help. Thanks are due to A. N. Basu for his encouragement throughout this work and to A. Lahiri for permission to publish this paper.
RECEIVED for review May 6, 1969. Accepted August 7, 1969. (19) M. J. S. Dewar and J. P. Schroeder, J . Amer. Chem. SOC.,86, 5235 (1964). (20) A. 0.S. Maczek and C. S.G. Phillips,~"GasChromatography," 1960, R. P. W. Scott, Ed., Butterworths, London, 1961, p 284. (21) A. 0. S. Maczek and C . S. G. Phillips, .I. Chromatogr., 29, 7 (1967).
eparation of Aluminum from ther Elements by Chromatography in Oxalic-Hydrochloric Acid Mixtures and Its Application to Silicate Analysis F. W. E. Strelow, C. J. Liebenberg and F. von S. Toerien National Chemical Research Laboratory, Pretoria, South Africa
ONLYA FEW ion exchange procedures for the separation of aluminium from the silicate forming elements iron, titanium, zirconium, magnesium, calcium, manganese, sodium, and potassium have been described in the literature. Yosimura and Vaki ( I , 2) used ammonium acetate for the cation exchange separation of aluminium from calcium and magnesium, but hydrolysis of aluminium caused difficulties. In the method of Oki et a f . (3) the quantity of aluminium and other elements is limited because of the relatively small separation factor cy;: ,E 2 in 0.8M HC1. Maines (4) has adsorbed Fe(III), titanium and aluminium selectively on an anion exchange resin from sulfosalicylate solutions. However, aluminium is not very strongly adsorbed under the experimental conditions and large columns have to be used to avoid losses, Furthermore, the destruction of sulfosalicylate is rather tedious. Oxalic acid should have promise as an eluting reagent for the separation of the above elements because it forms considerably more stable complexes with tri- and quadrivalent elements than with divalent ones, and because it is a moderately strong acid. Its eluting properties therefore can be modified simply by selecting and mixing the right concentrations of oxalic and a strong mineral acid. It also is relatively easily destroyed. The cation exchange separation of aluminium from gallium reported by Tsintsevich et al. ( 5 ) apparently is the only procedure described for aluminium in oxalic acid, besides the investigation of the separation of aluminium from oxalate by Djurfelt et al. (6). (1) J. Yosimura and H. Vaki, Japan Analyst, 6, 362 (1957). (2) J. Yosimura and H. Vaki, 2. Anal. Chem., 161, 393 (1958). (3) Y. Oki, S. Oki, and S . Hidekata, Bull. Chem. Soc. Japan, 35, 273 (1962). (4) A. D. Maines, Anal. Chim. Acta, 32, 211 (1965). ( 5 ) E. P. Tsintsevich, I. P. Alimarin, and L. F. Marchenkova, Ve'estnik Moskoti. Uniu., Ser. Mat., Mekh., Astron., Fiz. Khim., 13, 221 (1958), Chem. Abstr., 53, 10898 (1959). (6) R. Djurfelt, J. Hansen, and 0. Samuelson, Stiensk Kem. Tidskr., 59, 14 (1947). 2058
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A systematic investigation of the anion exchange distribution coefficients of elements in oxalic-hydrochloric acid mixtures showed the expected differences in the adsorption of divalent and higher valent elements. Furthermore, it was shown that aluminium is less strongly adsorbed at higher hydrochloric acid concentrations than Fe(III), Ti(IV), and other multivalent elements. A method for the quantitative separation of aluminium from Fe(III), Ti(IV), Zr(IV), Mo(VI), V(V), Ca, Mg, Mn(II), K, Na, and other elements was developed which has definite advantages over other methods. The separation has been combined with a complexometric procedure which uses excess DCyTA, back-titration with standard zinc solution, and xylenol orange as indicator at pH 5.5 for determination (7, 8). The method was applied to synthetic mixtures of elements and standard silicates and was found to give accurate and precise results. EXPERIMENTAL Reagnets and Apparatus. Analytical grade reagents were used throughout, excepting DCyTA (1,Zdiaminocyclohexane-tetraacetic acid). DCyTA was obtained from E. Merck A.G., Darmstadt, Germany, and standardized against 99.99% pure aluminum foil Diu zinc sulfate. Borosilicate-glass tubes of 2.0-2.5 i.d., fitted with B19 groundglass joints at the top and fused-in No. 2 porosity sinterplates and taps at the bottom were used as columns. AGl-X8 anion exchange resin and AG50-X8 cation exchange resin (200-400 mesh) were supplied by Bio-Rad Laboratories of Richmond, Calif. Bromic acid was prepared by passing an aqueous solution of KBr03 through a cation exchange column. The main eluting agents were: Solution 1: 0.05M oxalic acid-0.10M HCl-0.92 HzOZ. Solution 2 : 0.05M oxalic acid-0.50M HC1-0.02x HzOz. Solution 3 : 0.25M oxalic acid-0.10M HC1.
x
(7) R. Pfibil and V. VeselY, Talanfa, 10, 1287 (1963). (8) K. E. Burke and C. M. Davies, ANAL.CHEM., 36,172 (1964).
ANALYTICAL CHEMISTRY, VOL. 41, NO. 14, DECEMBER 1969
