Anal. Chem. 1983, 55, 1025-1029
1025
Size-Ex:clusion Chromatographic Behavior of Metal Complexes of ,&Diketones Masanorl Salts and Rokuro Kurode" Laboratory for Analytical Chemlstry, Faculty of Engineering, Universlty of Chiba, Yayoi-cho, Chiba 260, Japan
Masaml Shlbukawa St. Marianna University School of Medicine, Sugao, Mlyamae-ku, Kawasaki 213, Japan
Size-exclusion chromatographic behavior of six @-diketones and thelr Be( 11) and Cr(II1) complexes In pofy(vlny1 acetate) gel-organic solvent systems has been investlgated on the basis of the Ogston-Laurent model. The relationship between the dlstrlbutloni coefflclent and the molar volume Is strongly dependent on the type of substltuent In Pdlketone and eluant solvent and cannot be explained merely by the slze excluslon mechanism for all the systems studled, However, the llnear relatlonshlps between (-In K,,)1'2 and Vm"' In the Fractogel PVA 2000-p-dloxane system allow the effective size of a glven metal chielate dlssolved In p-dioxane to be estimated from K,, values of related free ligand and metal chelates.
Size-exclusion chromatography is a technique for separating and characterizing solute compounds primarily on the basis of their sizes in solution, although other secondary effects related to the mutual interactions between the solute, solvent, and gel are also frequently involved. This technique has been applied extensively to investigations involving organic polymers or biological molecules, while the application t o low molecular weight compounds or inorganic compounds has been limited partly because the behavior of such compounds is much more suibject to the secondary effects. In recent yeam, however, the application of size-exclusion chromatographly to metal complexes has been reported. The size-exclusionchromatographic behavior of metal P-diketonato complexes has been systematically investigated in various organic solvenit systems (1-9). Saitoh and Suzuki (7) have proposed a new theoretical treatment of the solute distribution in practical size-exclusion chromatography, taking metal acetylacetonates and normal alkanes as model compounds. They have applied the concept of a regular solution to the estimation of the contribution of the secondary effects to the separation mechanism in size-exclusion chromatography. Their view is such that p-dioxane is the nearest to the ideal eluant solvent for the poly(viny1 acetate) gel column; that is, the poly(viny1acetate) gel-p-dioxane system permits the solute compounds t o be differentiated only by the size exclusion effect (7). This means that the effective volumes of the solutes in p-dioxane can be estimated from their distribution coefficients obtained in size-exclusion chromatography. On the basis of these (observations,they have explained the chromatographic behavior of metal thenoyltrifluoroacetonates in the poly(viny1 acetate) gel-p-dioxane system in terms of the difference in effective volumes of these complexes reflected by the adduct formation with p-dioxane (9). The theoretical results presented by Saitoh and Suzuki, however, have been supported only by the chromatographic data for Be(I1) ,and Cr(II1) complexes with acetylacetone and normal alkanes (7). Therefore, the validity of the theoretical approach for the distribution of other metal complexes in
size-exclusion chromatography has not been evaluated. The present study was undertaken to investigate a correlation between the distribution coefficient and the molar volume for six @-diketonesand their metal complexes, based on the theory proposed by Ogston (10) and Laurent and Killander ( I I ) , in order to examine the utility of the poly(viny1 acetate) gel-pdioxane system for the size estimation of unknown species by size-exclusion chromatography.
EXPERIMENTAL SECTION Materials. The p-diketones and their metal chelates used are listed in Table I. HAA (see Table I for abbreviations)was washed with 1 M ammonia solution and then purified by distillation. HBA, HBFA, and HTTA were purified by sublimation. HDBM was recrystallized from diethyl ether. HTFA of reagent grade (Kanto Chemical Co., Tokyo, Japan) was used without purification. Beryllium(I1) chelates,except for Be(BA)*and Be(DBM)2, were prepared by using the following general method: a methanolic solution of each p-diketone was added to the aqueous solution of beryllium sulfate buffered at pH 6.7 with sodium acetate to precipitate the chelates. Be(BA), was prepared by adding a methanolic solution of HBA to an aqueous solution of beryllium sulfate. Be(DBM)2 was prepared by combining an acetone solution of HDBM and the aqueous solution of beryllium sulfate and then making the mixture strongly basic with sodium hydroxide. The chromium(II1) chelates, except for Cr(DBM)3 and Cr(TFA),, were prepared by a method analogous to that for Cr(AA), (12), while Cr(DBM)3 (13) and Cr(TFA)3 (14) were prepared according to the respective methods by Charles. The composition of each product, after purification by recrystallization, was confirmed by carbon and hydrogen elemental analysis. 'The results are listed in Table I. Benzene, acetone, and p-dioxane were used after purification of their reagent-grade materials by appropriate chemical treatment, drying, and distillation. The purity of each final material was tested by UV absorption spectrometry and/or gas chromatography. High-purity chloroform and tetrahydrofuran for liquid chromatography (HLC-SOL grade) were purchased from Kanto Chemical Co. (Tokyo, Japan) and used without further purifications. Fractogel PVA 2000 (cross-linked poly(viny1 acetate), dry particle size 32-63 pm, E. Merck, Darmstadt, G.F.R.) was successively washed with acetone, water, and methanol, in this order. The gel beads were then dried overnight at 60 OC. Standard polystyrenes of molecular weight 9000 and 233 000 (Pressure Chemical Co., Pittsburgh, PA) were employed as internal standards to calculate the distribution coefficients of the sample compounds. Chromatographic Apparatus and Procedure. Most of the parts coming into contact with the liquids were made of Teflon or Pyrex as the P-diketones used have high reactivity with metals. A Kyowa Seimitsu (Tokyo, Japan) Model KHU-26 chemically inert reciprocating pump was used in conjunction with a Model KU-1 plunger-type damper. A sample injection valve Model NV-508-6MP was obtained from Nihon Seimitsu Co. (Tokyo, Japan) and the sample volume was calibrated, turning out 0.074 cm3. A 5 mm X 550 mm Pyrex column, of which the inner wall had been treated with dimethyldichlorosilane, was packed with
0003-2'700/83/0355-1025$01.50/00 1983 Amerlcan Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
Table I. P-Diketones and Their Metal Complexes Studieda
c, % compound
abbreviation
R,
R,
acetylacetone benzoylacetone di benzoylmethane trifluoroacetylacetone benzoyltrifluoroacetone thenoyltrifluoroacetone bis( acetylacetonato)beryllium(II) bis( benzoylacetonato)beryllium(II)
a
HAA CH, CH, HBA Cd-4 CH3 HDBM C6H5 C6H5 HTFA CH, CF, HBFA C,H, CF, HTTA C,H3S CF, Be (AA 12 Be(BA), bis(dibenzoylmethanato)beryllium(II) Be(DBM), bis(trifluoroacetylacetonato )beryllium(II) Be(TFA), bis( benzoyltrifluoroacetonato)beryllium(II) Be(BFA), bis( thenoyltrifluoroacetonato)beryllium(II) Be (TTA), tris(acetylacetonato)chromium(III) Cr (AA ), tris( benzoylacetonato )chromium(III) Cr (BA ) 3 tris(dibenzoylmethanato)chromium(III) Cr ( DBM )B tris(trifluoroacety1acetonato )chromium(III) Cr(TFA), tris(benzoyltrifluoroacetonato)chromium(III) Cr (BFA), tris(thenoyltrifluoroacetonato)chromium(III) Cr (TTA), The 0-diketones have the following general formula (keto form): R,-COCH,CO-R,.
Fractogel PVA 2000 swollen by the eluant solvent to be used. The solvent reservoir was a commercially available glass syringe with the 200 cm3 capacity (9). A Hitachi (Tokyo, Japan) Model 139 spectrophotometer modified to accommodate a flow-throughcell (path length of 20 mm and volume of 0.063 cm3)or a Model RI-2 differential refractometer (Japan Analytical Industry Co., Tokyo, Japan) was used as a detector. The elution of HAA, HTFA, Be(AA),, Be(TFA),, and standard polystyrene in acetone system and of standard polystyrene in benzene system was monitored with a differential refractometer, while for the other compounds in these systems the UV-VIS spectrometric detection was carried out at an optimum wavelength for each compound. In other solvent systems, the spectrophotometer operating at 260 nm was used for the monitoring. The detection signal was fed into a Shimadzu (Kyoto, Japan) Chromatopac C-R1A data processor. Sample solutions were prepared by dissolving a desired compound in eluant to yield 0.1-8.0 mg of sample/lO an3of the eluant solvent used. Standard polystyrene of molecular weight 233 000 was added to each sample solution at a concentration of 0.05% (w/v) unless otherwise stated, except for acetone, as an internal reference. With acetone, a standard polystyrene of molecular weight 9000 was used. Elution was carried out at a constant flow rate of ca. 0.3 cm3/min and column temperature of 25.0 & 0.1 "C. The sensitivity of standard polystyrene by refractometric detection was so low in benzene that the concentration of 0.4% was adopted for this system. Density Measurements. The density of the solution of a sample compound was measured at 25.00 & 0.03 "C with a capped bicapillary-type pycnometer of volume about 2.0 cm3. Sample solutions were prepared by mass. For the systems studied, the molar volume of the solution (derived from its density) was a linear function of the mole fraction.
RESULTS AND DISCUSSION Molar Volume. For small molecules having the molecular weights less than 1000, it is known that the molar volume is an effective size parameter in size-exclusion chromatography (15). The partial molar volumes of the P-diketones and their metal chelates were thus determined by the density measurements on the solutions in p-dioxane or benzene in a similar manner as described by Irving and Smith (16). The density of the solution of a known mole fraction was converted into the molar volume, Vm,M,by the relationship
where X, and X2are the mole fractions of solvent and sample compound, respectively, M I and Mz are the corresponding molecular weights, and p is the density of the solution. The molar volume of the solution is also expressed in terms of the
H, %
calcd
found
calcd
found
57.96 72.49 79.11 38.11 54.68 42.58 51.58 67.28 74.89 35.24 51.66 40.29
58.07 72.54 79.05 38.37 54.69 42.64 51.73 68.25 74.67 35.36 52.18 40.67
6.81 5.48 4.87 2.56 2.75 1.79 6.06 5.08 4.61 2.37 2.60 1.69
6.95 5.55 4.98 2.59 2.69 1.84 6.12 5.26 4.69 2.30 2.74 1.81
partial molar volume of the solvent, Vm,,, and that of the as the following general equation: sample compound,
vm,2,
(2) = v m , l + (vm,Z - v m , l ) X 2 A plot of V,,M against X z has been confirmed by least-squares analysis to be linear within experimental error for all the values can be determined from the systems studied. slopes and the intercepts of the plots. The results are summarized in Table 11. The molar volumes of Be(TFA)z,of which the solubility in p-dioxane or benzene is so low that the density measurement is restricted in a very limited concentration range, and of reagent-grade HTFA were estimated by using the semiempirical relation (17) Vm,M(TFA), = 0*9nVm,HTFA (3) vm,M
vIm,2
where Vm,,A and Vm,M(mA), are the molar volumes of HTFA and of its metal chelate containing an integral number n of TFA- anions, respectively. As the partial molar volume of Cr(TFAI3 was determined as 347.6 cm3/mol, Vm,+p,A and Vm,Be(TF~)2 are calculated to be 128.7 and 231.7 cm3/mol, respectively. Irving and Smith have shown that the partial molar volume of Cr(AA), is nearly constant in various organic solvents (17) and is not very different from that for the solid state (18). Wakahayashi et al. (19)have shown that the partition behavior of HAA, HTFA, and HTTA between various organic solvents and an aqueous solution of sodium perchlorate can be explained satisfactorily by assuming that the molar volumes of HAA, HTFA, and HTTA are 102, 121, and 160 cm3/mol, respectively, which are very close to the values determined or estimated in this work. It is thus considered that the molar volumes of the p-diketones and their Be(I1) and Cr(II1) chelates used can be regarded as substantially constant regardless of the type of the organic solvents. Although P-diketones exist as a mixture of keto and enol tautomers, we may treat the molar volumes of both species as being approximately identical in the present study. Distribution Coefficient. The parameter K,, is frequently adopted for the characterization of a solute molecule in size-exclusion chromatographic elution not only for practical purposes but also in theoretical work. It is derived from the following equation and corresponds to the distribution coefficient of the solute between the interstitial solution phase and the swollen gel phase (4) V, = Vo K,,V,
+
ANALYTICAL CHEMISTRY, VOL.
55, NO. 7, JUNE 1983
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Table 11. Molar Volumes for P-Diketones and Their Beryllium(11) and Chromium(II1) Chelates in p-Dioxane
-
compd HAA HB,4 HDBM HBlFA HT'I'A Be( AA 12 Be(B-41, B~(DBM), WBFA), Be('TTA), Cr (,4A), a( IBA 13 Cr(lDBM),b Cr(TFA), Cr(lBFA), Cr(TTA),
molar vol eq Vm,M 85.718 + 17.14X, 85.700 + 63.48X, 85.712 t 106.2X2 85.698 + 82.99X2 85.699 + 74.41X2 85.710 + 106.9X2 85.712 t 202.3Xa 89.422 t 273.2X2 85.717 t 232.4X2 85.694 t 228.1X, 85.694 + 183.9X2 85.701 t 338.1X1 89.419 + 470.8Xa 85.695 t 261.9Xa 85.703 t 406.0Xa 85.700 + 373.7X1
highest mole fraction of compd studied 0.005 191 0.003 290 0.002 627 0.002 195 0.001 692 0.002 969 0.001 336 0.000 770 2 0.000 710 9 0.000 663 7 0.000 997 0 0.000 448 7 0.000 437 4 0.000 838 1 0.000 667 4 0.000 606 8
Vrn,zi cm3/mol 102.9 149.2 191.9 168.7 160.1 192.6 288.0 362.6 318.1 313.8 269.6 423.8 560.2 347.6 491.7 459.4
AVltl,Ma
k0.006 0.004
0.005 0.001
0.004 0.004 0.002 0.002 0.002 0.003 0.002 0.005 0.001
0.003 0.004 0.001
a Standard deviation with respect to Vrn,M of experimental points from the best straight line derived from least-squares Result on the benzene solution. analysis.
Table 111. K,, for 0-Diketones and Their Beryllium(I1) and Chromium(II1) Chelates on Fractogel PVA 2000 with Organic Solvent Systems Kav
compd
p-dioxane
benzene
chloroform
acetone
tetrahydrofuran
HAA H EIA HDBM HTFA HBFA HTTA Be I[ A A ), Be I( BA 11 BeI( DBM 12 Bel(TFA), Be 1( BFA) WTTA), Cr(AA), &/BA), fi( DBM ) j Cr(TFA), &(BFA)3 WTT-41,
0.71 2 0.654 0,596 0.663 0.586 0.646 0.509 0.439 0.363 0.51 2 0.396 0.491
0.762 0.746 0.727 0.760 0.689 0.864 0.503 0.469 0.412 0.581 0.427 0.603 0.304 0.276 0.227 0.398 0.314 0.501
0.496 0.455 0.415 0.586 0.504 0.567 0.297 0.258 0.219 0.423 0.319 0.409 0.243
0.669 0.726 0.769 0.567 0.610 0.674 0.589 0.653 0.668 0.406 0.414 0.463 0.569 0.532 0.435 0.340 0.303 0.360
0.649 0.587 0.516 0.559 0.485 0.520 0.564 0.445 0.330 0.402 0.298 0.354 0.466 0.321 0.210 0.316 0.216 0.280
0.578
0.295 0.22!2 0.41 2 0.305 0.405
where V,, V,, and V , are the elution volume of a sample compound, the column void volume, and the volume of the swollen gel phage, respectively (11). If the total volume of the gel bed, V , ( V , = V , f VJ, is introduced, eq 4 can be rearranged to
v,- v, = v, - v, K,, = v,
v, -- v,
The K,, values of the sample compounds were calculated according to eq 5. The V , value of each column used in this work was 10.82 cm3. 'The V , value was determined from measurements of the elution volume of standard polystyrene (molecular weight 9000 or 233 000), which can be regarded as excluded completely from the network of Fractogel PVA 2000. The dead volume relating to the spaces between the sample injector and the' column inlet and between the column outlet and the detector cell was corrected for in the calculation of the K,, value. No compounds gave an elution profile with excessive distortion, and the reproducibility of the Kavvalues obtained in the present work was satisfactory. The relative standard deviation in each case is less than 1.0%. The values given in Table I11 are the mean values from triplicate determinations. The column efficiency was dependent on the eluant solvent used. The numbers of theoretical plates for HAA in
0.188
0.135 0.372 0.241 0.345
-.
p-dioxane, chloroform, and acetone systems, for example, were 460, 780, and 1900, respectively. Correlation between K,, Values a n d Molar Volumes. In order to calculate the available volume for a solute in a gel phase, Laurent and Killander (11) have designed a physical model of the gel network by assuming that the polymer chains of the gel are straight rigid rods, which are infinitely long and distributed a t random in the gel phase. On the basis of the theoretical expression by Ogston (IO),concerning the distribution of spaces in a random network of straight fibers, they gave the following equation:
K,, = exp[-rL(r,
+ rJ2]
where L is the concentration of rods in the solution, expressed as cm of rod per cm3, r, is the radius of a spherical particle, and rr is the radius of the rod. Because L and rr can be regarded as constant for a given column system, eq 6 can be rewritten by assuming r, to be proportional to the cube root of V,, as follows (6): (-ln K,,)1/2= k l
+ k2Vm1I3
(7)
where k l and k 2 are constants in a given column. In the present study, the correlations between K,, values and molar volumes for the model compounds are investigated using eq 7 . The (-ln K,,)1/2vs. V,,,lI3 plots for six P-diketones
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
1028
1.1
b
Cr(TTA), 0.9'
T TA)
/''Be(
HDBM
0.7
I
2
.--
~
HTTA HAA/
" t1 1.3
c r ( B Y
07'
"//HTTA
0 7. 5.0
7.0
6.0
Vrn
5.0
70
BO
1/3
Figure 3. Relationship between K, and V, for Pdiketones and their III) complexes. Solvent was chloroform. For other details metal(1I, see Figure 1.
a)
a)
7 0.9
6 0
Vrn
Figure 1. Relationship between K, and V, for Pdiketones and thelr metal(I1,III) complexes: (a) nonfluorinated compounds: (b) fluorinated compounds. Solvent was p -dioxane.
'
I
I
8.0
113
1 1
Cr(TTA),
,,,_
I
0.7 07
HAA
. --
N
05
>
,,'HTTA
HDBM
YL
-
b)
I
Y
1.1
'
HTTA 50
60
70
80
v m1'3
50
60
70
Vrn
80
113
Figure 2. Relationship between K,, and V, for @diketonesand their metai(I1,III) complexes. Solvent was benzene. For other detalis see Figure 1.
Flgure 4. Relationship between K, and V, for Pdlketones and their metal(I1,III) complexes. Solvent was acetone. For other details see Figure 1.
and their Be(I1) and Cr(II1) chelates in the Fractogel PVA 2000-p-dioxane system are shown in Figure 1. If the chromatographic elutions of solute compounds are based only on the size exclusion mechanism, all the plots should form a
straight line. However, the plots for each P-diketone and its metal(I1,III) chelates give a straight line which do not coincide with each other. The result shown in Figure 1 implies that some secondary effects other than the size exclusion effect
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
cr(DBMY 11.
O g:
Cr(BFA)
I( Be(TFA)
Be(T TA),
HTFA ,,,,e4'
Be(AA),
_.-I
_ . . . a -
systems as those studied in this work are linear (6), these curvatures of the plots may be attributed to the difference in the solutegel and/or solute-solvent interactions. However, details of such interactions have not so far been elucidated. In all instances, K,, values of a P-diketone and its IUe(I1) and Cr(II1) chelates decrease in that order, which indicates that the size exclusion effect is a dominant factor in the separation mechanism, although the behavior of these compounds cannot be explained only by the exclusion effect. From the observations described above, it is obvious that a plot for a given set of standard compounds, such as normal alkanes, cannot be used as a universal calibration curve for the size estimation. However, the linear relationships between (-ln Kav)1/2 and Vm1i3in the Fractogel PVA 2000-p-dioxane system suggest that the effective size of a given metal chelate dissolved in p-dioxane can be estimated from the K,, value by using the plot for the free ligand and a series of its metal chelates which do not form the solvated complexes. Registry No. Be(AA)z, 10210-64-7; Be(BA),, 14128-75-7; Be(DBM),, 19368-60-6; Be(TFA)2, 13939-10-1; Be(BFA)2, 14052-07-4;Be(TTA)2,13928-05-7;Cr(AA),, 21679-31-2;Cr(BA)3, 16432-36-3;Cr(DBM),, 21679-35-6; Cr(TFA),, 14592-89-3;Cr(BFA),, 280963-65-3;Cr(TTA),, 15488-08-1;HAA, 123-54-6;HBA, 93-91-4; HDBM, 120-46-7; HTFA, 367-57-7; HBFA, 326-06-7; HTTA, 326-91-0.
HAA 50
1029
LITERATURE CITED 70
60
80
V m113 Figure 5. Relationship between K,, and V , for Pdlketones and their metal(I1,I11) complexes. Solvent was tetrahydrofuran. For other details see Figure 1.
are involved in the elution process even in the poly(viny1 acetate) gel-p-dioxane system. The slope and the intercept of the plot depend strongly on the type of the substituents in the @-diketone. For example, the slope of the plot for each fluorinated (3-diketone and its metal chelates is smaller than that for the nionfluorinated compounds. Shown in Figures 2-5, for comparison, are the plots of (-ln K,,)li2 vs. V,ILi3for other various organic solvent systems. A linear relation between (-ln K,,)1/2 anid Vm1i3,which should be common with all the sample compounds, is not observed in all the systems studied. The plots illustrated in Figures 2-5 are not linear, except for a few cases, e.g., the plot for HDBM and its metal chelates in thle benzene system, in contrast to those obtained in the p-dioxane system shown in Figure 1. Taking into account the fact that the (-ln K,,)li2 vs. Vm1i3plots for normal alkanes in the same gel-solvent
(1) Yamamoto, Y.; Yamamoto, M.; Ebisui, S.;Takagi, T.; Hashimoto, T.; Izuhara, M. Anal. Lett. 1973, 6 , 451-460. (2) Saitoh, K.; Satoh, M.; Suzuki, N. J . Chromafogr. 1974, 9 2 , 291-297. (3) Saitoh, K.; Suzuki, N. J . Chromafogr. 1975, 109, 333-339. (4) Yamamoto, M.; Yamamoto, Y. Anal. Chlm. Acta 1976, 87, 375-386. (5) Suzuki, N.; Saitoh, K.; Shibukawa, M. J . Chromatogr. 1977, 138, (6) (7) (8) (9)
(IO) (11) (12) (13)
(14) (15) (16) (17) (18) (19)
79-87. Suzuki, N.; Saltoh, K. Bull. Chem. SOC.Jpn. 1977, 5 0 , 2907-2910. Saitoh, K.; Suzuki, N. Bull. Chem. SOC. Jpn. 1978, 51, IlEi-120. Noda, H.; Saitoh, K.; Suzuki. N. J . Chromafogr. 1979, 168, 250-254. Saitoh, K ; Suzukl, N. Anal. Chem. 1980, 5 2 , 30-32. Ogston, A. G. Trans. Faraday SOC. 1958, 5 4 , 1754-1757. Laurent, T. C.; Klliander, J. J . Chromatogr. 1964, 14, 317-330. Fernelius, W. C.; Blanch, J. E. Inorg. Synth. 1957, 5 , 130-131. Charles, R. G. Inorg. Synth. 1966, 8 , 135-138. Charles, 13. G. Inorg. Synth. 1966, 8 , 138-140. Smith, W. 6 . ; Kollmansberger, A. J . fhys. Chem. 1985, 69, 4157-4161. Irving, H. M. N. H.; Smith, J. S. J . Inorg. Nucl. Chem. 19813, 30, 1873- 1883. Omori, T.; Wakahayashi, T.; Oki, S.;Suzuki, N. J Inorg. Nucl. Chem. 1964, 2 6 , 2265-2270. Irvlng, H. M. N. H. "Ion Exchange and Solvent Extraction", Marinsky, J. A,, Ed., Marcel Dekker: New York, 1970; pp 139-187. Wakahayashi, T.; Oki, S.; Omori, T.; Suzuki, N. J . Inorg. Nucl. Chem. 1984, 26, 2255-2264.
RECEIVED for review December 2.1982. AcceDted Februarv 25, 1983.
