of this new polymer as a stationary phase in gas-liquid chromatography. In addition, as the polymer contains no silicon, the normal ionization detector electrode contamination associated with high temperature operation with presently available phases is nonexistent. Chemical modification of this material such as addition of polar groups to the polymer skeleton appears as a logical extension of this work. Such
modifications would enhance selectivity but often such substitutions reduce the thermal stability of the polymers ( I ) .
RECEIVED for review May 12, 1969. Accepted July 29, 1969. Paper presented at 156th National Meeting, ACS, Atlantic City, N. J., September 1968.
Reactions of Some Low-Spin Nickel Chelates with Heterocyclic Nitrogen Bases Kumar S . Math and Henry Freiser Department of Chemistry, University of Arizona, Tucson, Ariz. 85721
IT WAS RECENTLY observed that the addition of phenanthroline to nickel diphenylthiocarbazonate (dithizonate) resulted in the formation of such an intensely pink adduct that it provided the basis of the development of the most sensitive colorimetric method for the determinaton of nickel ( I ) . The spectrum of nickel dithizonate is remarkably different from that of most metal dithizonates which, interestingly enough, closely resembles the spectra of the nickel dithizonate adducts with phenanthroline or other nitrogen bases. Earlier work had uncovered the tendency of nickel complexes of such sulfur-containing ligands as the diakyldithiophosphates, akylxanthates (2) [but not the dialkyldithiocarbamates ($1 to form adducts with pyridine and other nitrogen bases whose spectra also exhibited changes. In these cases, however, in the absence of a highly conjugated system such as that of dithizone, the spectra both of the original nickel chelate and of the adducts consisted of the relatively weak ( E lo2)absorptions characteristic of d-d transitions of the metal ion. Quite obviously, in nickel dithizonate the intense absorptions in the spectrum are characteristic of the (metal-perturbed) ligand itself. This must also be true of the adducts. Several interesting questions are now raised which have analytical relevance. First, what structural changes occurred in the nickel dithizonate upon adduct formation to give rise to such profound spectral changes? Can these changes shed any further light on the puzzling nature of the structure of nickel dithizonate (4)? Finally, if instead of dithizone other sulfur-containing ligands having a well-developed conjugated electronic system are tested for the effect of adduct formation, will other highly sensitive colorimetric reactions for nickel be found? The present work addresses itself to these questions.
-
EXPERIMENTAL
Apparatus. A Cary Model 14 spectrophotometer was used for absorbance measurements. Molecular weight determinations were carried out in chloroform solutions using a Mechrolab vapor pressure osmometer Model 301A. Magnetic susceptibility measurements were carried out on the solids by a Faraday method. The observed molar susceptibility measurements were corrected for the diamagnetic contribution
using tabulated atomic and constitutional Pascal constants (5). Reagents. Dithizone (Fisher Scientific Co.) was purified by the method of Cowling and Miller (6). The di-p-tolylthiocarbazone and di-2-naphthylthiocarbazone were synthesized by previously described methods (7). 8-Mercaptoquinoline (K and K Laboratories, Inc.) was converted into its sodium salt which was recrystallized from water. Pyridine (Mallinckrodt), a,P,y-picolines, 2,6-lutidine (Eastman) and ethylenediamine (Eastman) were dried over potassium hydroxide and a constant boiling fraction was collected and used. Chloroform (Mallinckrodt Analytical Grade), CC14 (Mallinckrodt Analytical Grade), dioxane (Fisher Certified Chemical), 1,lO-phenanthroline, 5-nitrophenanthroline (G. Frederick Smith Co.), a,a’-dipyridyl (Eastman) and Ni(ClO& (Fisher Scientific Co. Reagent Grade) were used without further purification. Preparation of Nickel Dithizonate. A slight excess of 0.01M nickel perchlorate solution was added to a 0.01M dithizone solution in dioxane. The mixture was diluted with water and the precipitate obtained was filtered, washed with water several times, and finally twice with carbon tetrachloride. The nickel dithizonate thus obtained was dried in a vacuum oven at 40 “C for 12 hours. The nickel content of the complex was analyzed by using the dimethylglyoxime (8) and phenanthroline-dithizone methods (1). Found 10.1% Ni, Calcd. for NiDzz = 1 0 . 3 z Ni. The molecular weight in CHC1, was found by vapor pressure osmometry to be 532, 510 (theor. 569.1). From the magnetic susceptibility measurements, the moment of the complex was found to be perf = 0.73 Bohr magnetons. Preparation of Di-p-Tolylthiocarbazone and Di-2-Naphthylthiocarbazone Nickel Complexes. A 100-ml portion of 1.0 X 10-3M solution of di-p-tolylthiocarbazone or di-2-naphthylthiocarbazone in chloroform was shaken for one hour with 100 ml of 1 x 10-2M aqueous solution of nickel perchlorate using a phosphate buffer of pH 6.0. The separated organic phase was scrubbed with aqueous 1.0 X lO-,M NaOH to remove unreacted reagent, washed twice with water, then separated and filtered. These chloroform solutions of nickel ditolylthiocarbazonate and di-2-naphthylthiocarbazonate were used for measurement of equilibrium constants. ( 5 ) Landolt-Bornstein,
(1) K. S. Math, K. S. Bhatki, and H. Freiser, Talanta, 16, in press (1969). (2) . . R . L. Carlin, J. S. Dubnoff. and W. I. Huntress. Proc. Chem. SOC.,1964,228. (3) C. K. Jorgensen, J. Inorg. Nudear Chem., 24, 1571 (1962). (4) K. S. Math, Q. Fernando, and H. Freiser, ANAL.CHEM.,36, 1762 (1964). 1682
ANALYTICAL C H E M I S T R Y
“Zahlenwerte und Funktionen,” Vol.
11/10 (Magnetic properties 11), Springer-Verlag, Berlin, 1965. (6) H. Cowling and E. J. Miller, IND. ENG. CHEM.,ANAL.ED., 13, 165 (1941). (7) D. M. Hubbard and E. W. Scott, J. Amer. Chem. SOC.,65, 2390 (1953).
(8) E. B. Sandell, “Colorimetric Determination of Traces of Metals,” Third ed., Interscience Publishers, New York, 1959, p. 669.
1.0
20 h
-
c
0.9
0.8
0.7
0.6
w
0
a
0.5
0.4
d: 0.3
o'21
350
0.1
I
I
I
I
I
I
O400
4K)
100
aso
600
650
1
550
450
WAVE LENGTH
in
rn,u
Figure 2. Absorption spectra of Ni8MQ2-ethylenediamine mixtures in chloroform [Ni8MQ210 = 1.6 X 10-4 M
700
WAVELENGTH (mgj
[EN10 X 10' = (a) 0.0, (b) 9.5, (c) 19.1, (d) 38.1, (e) 61.9, (f) 95.4, (g) 190.7, (h) 999.6
Figure 1. Absorption spectra of NiDz~5-nitrophenanthroline M mixtures in chloroform [NiDz& = 1.88 X [S-NOe-~henl~ X lo6 = (a) 0.0, (b) 8.0, (c) 16.0, (d) 32.0, (e) 56.0, (f) 80.0, and (8) 160.0
Preparation of Nickel 8-mercaptoquinolinate [Ni(C9H6NS)J. A slight excess of O.OlMnicke1perchlorate solution was added to a 0.01M solution of the sodium salt of 8-mercaptoquinoline in water. The precipitate obtained was filtered, washed several times with water and then with carbon tetrachloride. The nickel 8-mercaptoquinolinate thus obtained was dried in a vacuum oven at 40 "C for about 12 hours. Preparation of Nickel Phenanthroline Dithizonate (NiDz2. phen). A volume of 50 ml of 10-3M phenanthroline in chloroform was added to an equimolar solution of nickel dithizonate in chloroform. The characteristic red color of the complex appeared immediately upon mixing. The mixture was evaporated under reduced pressure to a volume of approximately 10 ml. The solid crystalline substance that separated was filtered and dried in DUCUO. The moment of the complex was found to be peff = 2.79 Bohr Magnetons. Determination of Equilibrium Absorbance Measurements. Specific amounts of chloroform solutions of the appropriate nickel complex were pipetted into volumetric flasks containing solutions of varying amounts of adducting base in chloroform, and the volumes were adjusted to the mark with solvent. The absorption spectra of the solutions were obtained with the Cary 14 in the range from 500-700 nm using optical path lengths of 1-, lo-, and 100-mm thickness, depending on the concentrations employed. Absorbance measurements in the range of 520 nm and 680 nm were employed in the equilibrium calculations. The absorbance values were found to be constant for at least 4 hours. A similar procedure was used to obtain equilibrium absorbance measurements in carbon tetrachloride. RESULTS AND DISCUSSION The spectrum of nickel dithizonate differs from that of most of the other metal dithizonates. Whereas most of the other metal dithizonates (Hg, Pb, Zn, etc.) and the dithizonate anion have a single absorption band in the entire visible region (at about 500-530 nm with emol- 40 X lo3), the spectrum of nickel dithizonate has four characteristic absorption bands
= 340, 475, 565, and 675 nm with emo1 = 20.0, 27.0, 23.6, and 19.9 X loa, respectively). The spectrum of nickel dithizonate undergoes a profound change upon the addition of pyridine or other N-bases, collapsing to a single absorption band in the visible range between 520-535 nm with a emo1 = 4850 X loa, (see Figure 1 for representative spectra), thus resembling closely the spectra of the other dithizonates. The spectral behavior of the nickel chelates of di-p-tolylthiocarbazone and di-2-naphthylthiocarbazone is closely analogous to that of the nickel dithizonate system. Similarly, with 8-mercaptoquinoline, the spectrum of the nickel chelate has multiple absorption bands in the visible = 332, 390, and 538 nm with emo1 = 11.6, 6.4, region ,,A,( and 5.1 X lo3, respectively) in contrast to most of the other metal 8-mercaptoquinolinates-e.g., Hg, Pb, Zn, and the reagent anion (8 MQ-) which have a single absorption band in visible region at 410-440 nm with emol 7000. As with the dithizones, on the addition of pyridine or other Nbases to nickel 8-mercaptoquinolinate the spectrum simplifies to a single band in the range of 420-435 nm with emol 8000 (see Figure 2 for representative spectra). From the behavior of the sulfur containing ligands studied here, it would appear that, although adduct formation of the nickel chelate results in the appearance of an absorption band which is significantly stronger than that of the original chelate, the most strongly absorbing adduct band is obtained with the ligand having the strongest absorption. The change in the spectrum of the nickel complexes (NiL2) under investigation that accompanies the addition of pyridine or other bases can be used to determine the equilibrium constant of adduct formation. Because of the very low solubility of nickel chelates of dithizones and the other ligands studied here in most organic solvents it is not entirely unlikely that it might exist as a polymer. Thus, in addition to the simple case of adduct formation from a monomeric chelate
,,X(,
-
-
VOL. 41, NO. 12, OCTOBER 1969
1683
I
00
I
t
2.0
!
I
N i c k e l dithizonate
I
t
6.0
4.0
L O G . K,,
Figure 4. Plot of log KAD for various Ni chelates us. that of NiDzz 0 Nickel-di-p-tolyl-thiocarbazonate
A Nickel 8-mercaptoquinolinate
-LOG [DIPYRIWL]
0
Nickel-di-2-naphthyl-thiocarbazonate
Figure 3. Plot according to Equation 6 of dat for adduct formation between NiDzz and cy,cy’dipyridyl in chloroform
(1) 2,6-Lutidine, (2) 2-picoline, (3) pyridine, (4) 3-picoline, (5) 4-picoline, (6) ethyIenediamine, (7) ap’dipyridyl, (8) 1.10-phenanthroline
(a) x = 1, (b) x = 2, (c) x = 3
where A0 is the absorbance in the absence of adducting base, B, and A , the absorbance in the presence of B, whose concentration is denoted by its negative logarithm, pB. In all the systems studied here it was possible to measure the absorbance of the mixtures at wavelengths where the only significant absorption was that due to the simple chelate (680 nm for the nickel dithizonate and analogs, 540 nm for the nickel 8mercaptoquinolinate) so that the absorbance A could be considered to be directly proportional to the concentration of the simple chelate in Equations 4, 5, and 6. By plotting the data
two other possibilities can be considered : (a) the nickel complex is polymeric but the adduct is monomeric (NiL),
K‘AD + nxB e x N i L . nB
(2) (b) both the original complex and the adduct is polymeric (NiL),
+ nxB U(NiL K”AD
9
nB),
(3)
Expressions based on these formulations are: A0 - A log KAD= npB +:log _ _ A
log K‘AD= nxpB
+ x log (A0 - A ) log A
+ log-e,=-1
(5)
and log K ” A D = nxpB
-A + log Aorespectively, A t
as log
-A A)Z us. pB, as called for in Equation 5, it should
be possible to distinguish between Reactions 1, 2, and 3 , particularly if sufficiently precise absorbance measurements at low p B values can be obtained. In this study, A values at low p B were obtained with the zero to 0.1 absorbance scale of the Cary 14. As can be seen from Figure 3, in which x has been assigned values of 1,2, and 3, only the curve in which x = 1 is linear throughout. In contrast, the other two curves show significant deviation from linearity in the low p B range. Further means of discriminating between reactions l, 2, and 3 may be obtained by comparing the slopes of the linear portions of the curves in Figure 3. Because the value of n was indepen-
Table I. Adduct Formation Equilibrium Constants
Base Pyridine 2-Picoline 3-Picoline +Picoline 2,6-Lutidine a,a’-Dipyridyl 1 ,IC-Phenanthroline 5-Nitro-I ,IO-phenanthroline Ethylenediamine
1684
ANALYTICAL CHEMISTRY
Nickel dithizonate CHCla CC14 1 .08 1.46 -0.42 0.01 1.32 ... 1.45 1.92 -0.60 4.63 5.96 4.65 3.86
log KAD Ni-8 mercaptoquinolinate in CHC13 -0.54 very weak -0.44
-0.16 very weak 2.35 3.97
...
3.38
Ni-di-p-tolylthiocarbazonate in CHC13 0.88 -0.53 0.94 1.22 -0.82 3.80 5.16
...
3.74
Ni-di-2naphthylthiocarbazonate 1.29 0.04 1.59 1.72
...
4.45 5.49
... ...
dently found by a mole ratio method to be unity, the values of the slopes of these lines should be equal to x . The observed values of 1.01, 1.55, and 2.10 when x is assumed to be 1, 2, or 3, point strongly to the validity of Reaction 1. A further confirmation of the validity of Equations 1 and 4 was obtained from the observation that values of the equilibrium constant, KAD, were independent of the initial concentration of NiDzz over a hundredfold concentration range. Hence, it is possible to eliminate the possibility that NiDzz is polymeric in chloroform solution on the basis of these spectrophotometric measurements alone. Although the monomeric nature of NiDza in CHCla was independently proved by a molecular weight determination, the spectrophotometric approach outlined here can be of use in those cases where molecular weight determinations are not feasible. Values of KAD calculated according to Equation 4 for the systems studied are listed in Table I. These values exhibit the expected increase with increasing adducting ligand basicity. In the cases of pyridine, 3-, and 4-picolines there is a linear increase of log KAD with pKa values of these ligands. For 2picoline and 2,6-lutidine, however, steric hindrance results in weaker adducts. In all of these cases, the adducts contain only one mole of base, accounting for only five coordination sites around the nickel. Five-coordinate nickel has been observed in 1 :1 adducts of N-bases with nickel chelates of other S-containing ligands (9). Yet ligands such as dipyridyl, phenanthrolin 2, and ethylenediamine, are bidentate as indicated by the substantial increase in log KAD so that in these complexes, nickel would appear to be six-coordinated. Of course, some of the increased stability arises from a more favorable entropy change as can be seen from Table 11. The enthalpy of formation of NiDzz. dipy is significantly more negative but far from twice the value for NiDz2 .Py which might be expected for 6-coordination. However, a comparison of adduct formation thermodynamics of these complexes with those involving diethyldithiophosphate (dtp), a ligand that has no extensive x-band conjugation shows that with Nidtpzpy,, a diadduct in which nickel is 6-coordinate is formed, the enthalpy is less than twice that observed for the monoadduct, Nidtpz .phen, which strongly indicates a 6-coordinate octahedral nickel configuration. It is interesting to note that for CCla, a poorer solvent for NiDzz than CHC13,the values of log KAD are uniformly greater which may reflect some (weak) interaction between NiDzz and CHC13. The reactions of pyridine, dipyridyl, phenanthroline, and ethylenediamine with the nickel complexes under investigation serve not only to give analytically more useful complexes-Le., more stable, more soluble, and more highly colored-but changes in their properties help improve our understanding about the structures of the low spin nickel chelates studied here. The behavior of these complexes is indicative of the square planar configuration of nickel. The multiple absorption bands observed in the spectra of the simple nickel chelates might be reasonably attributed to charge-transfer transitions. Gray and Ballhausen’s (/O) (9) A. Sgamelloti, C. Furlani, and F. Magrini, J. Inorg. Nucl. Chem., 30,2655-2660, (1965). (10) H. B. Gray and C. J. Ballhausen, J . Amer. Chem. SOC.,85, 260 (1963).
Table 11. Thermodynamics of Adduct Formation of Several Nickel Chelates at 25 OC Adduct - AG - AH Complex “kcal/male “kcal/male AS ‘, e.u. 4.1 -9.0 1.45 NiDzz . pya 6.4 -2.1 5.75 NiDzz . dipyo 7.0 -17.0 2.0 Nidtpz . pyb 11.6 - 30 2.7 Nidtpz . 2 pyb 11 .o - 23 4.1 Nixan2 . 2 pyb Thermodynamic quantities calculated from following data :
NiDzz py NiDzz . dipy From ref
0 “C
log k A D 20 “C
30 “C
1.25 4.49
1.09 4.29
1.04 4.15
( 2 ) dtp = diethyldithiophosphate
xan
=
ethylxanthate.
analysis of the spectra of a series of square planar nickel complexes with ligands having a x-orbital system predicted the existence of two types of charge-transfer transitions: (a) ligand + metal, which should give two bands spaced by 10,000 cm-1 and (b) metal + ligand, which will have three closely spaced bands. Although the nickel chelates studied here ~ the close resemblance of their have, at best, D z symmetry, spectra to those of nickel, platinum, and palladium cyanides strongly indicates that, as in the cyanide cases, metal + ligand transitions are involved here. The most important molecular orbital in square planar metal complexes is one consisting of a 4p, metal (nickel) orbital with four ligand “ring” x orbitals giving rise to a stable xbonding orbital ( l O , / l ) . Because the strongly coordinating nitrogen bases can form axial bands with nickel, they remove the 4p, orbital from the ring x systems and eliminate the charge transfer bands. Hence, the multiple absorption bands collapse to a single band in the presence of adduct forming bases. The “ring” x orbital stability can be considered to rise with stability constants of complexes which are in order as nickel dithizonate < nickel-di-p-tolylthiocarbazonate < nickel-8 mercaptoquinolinate. In this case, one would expect corresponding KAD values to be in the reverse order, as was observed (Table I). The plot of KAD values of nickel dithizone adducts US. nickel di-p-tolylthiocarbazonate and nickel 8-mercaptoquinolinate systems gave two parallel straight lines (Figure 4), which indicates that factors influencing adduct formation in all cases are similar in nature. ACKNOWLEDGMENT
The authors gratefully acknowledge the assistance of R. D. Feltham with the magnetic susceptibility measurements. RECEIVED for review May 15,1969. Accepted August 1,1969. The work reported here was carried out with the financial assistance of the U. S. Atomic Energy Commission, Grant NO.At (11-1)-676. (11) E. Billig, R. Williams, I. Bernal, J. H. Waters, and H. B. Gray, Inorg. Chem., 3,663 (1964).
VOL. 41, NO. 12, OCTOBER 1969
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