Császár, József

Kiss, László

English

Császár József

Kiss László

University of Szeged

THE SPECTRA OF ESTERS OF O X O H Y D R O X O B I S -- ( 8 - H Y D R O X Y-QUINOLINE) V AN ADIUM( V ) (Short communication) J. CSÁSZÁR and L. KISS Institute of Physical Chemistry, Attila József University, P.O.Box 105, H-6701 Szeged, Hungary (Received November 16, 1990) The vanadate ion reacts with 8—hydroxyquinoline in aqueous solution at pH z 6 to give a stable complex compound, I - H [1] (Structure 1). BIELIG and B A Y E R [2] confirmed the presence of the OH group via the IR spectrum; this result, together with the analytical data, leads to the formula (CgHgON^VO.OH. This complex dissolves in alcohols to give hydrated esters, I1—R (Structure 2) in the first step; in an excess of boiling aliphatic alcohols I1—R next gives intense red solutions [3], from which red crystals of the anhydrous esters, I—R (Structure 3), separate out on cooling. 0 O-HOH 0 0, If N l - H OH O-R f - R I - R O - R Structure 1 Structure 2 Structure 3 83 J. CSASZAE a n d L. KISS I—H deposits slowly from concentrated solution of I—Ft; the solubility of I—Ft and its tendency to undergo decomposition increase with increasing C atom number in Ft. We have prepared the compounds I—H, I'—Ft and I—R ( R = methyl, ethyl, propyl, butyl, amyl, hexyl and octyl) and measured their visible and UV spectra in the corresponding parent n—alcohols and pyridine (py). The spectra were obtained on BECKMAN DU, SPECORD U V - V I S instruments, using spectroscopically pure s o l -vents. The central V5 + ion is a 3d 0 species, and the complex I—H is diamagnetic [2b]; no d—d bands are expected, and none are observed. The complexes I—H and I—R all give five bands of high intensities (Table I). The bands in the range 240-370 nm are modified bands of 8—hydroxyquinoline due to it* <— r transitions, while the visible band ( s 490 nm) may be assigned to ligajid—to—metal charge—transfer transitions from the phenolate oxygens to the empty d-orbitals of the vanadium (Fig. 1) [4]. Such ¿1 loge > i\ Figure 1: Absorption spectra of I—C2H5 in ethanol(l) and pyridine(2) c = 3.10-" mol /dm 3 , d = 0.1 cm. 300 400 * / n m S00 transitions are generally characteristic of phenolate coordination to easily reducible meted ions [5]. 84 THE SPECTRA OF ESTERS It has been reported [6] that the reactions between I—H and aliphatic alcohols result in hydrated esters, I'—Ft, containing a molecule of water attached by h y d -rogen—bonding. In excess of boiling alcohol, which may well act as a mild dehydrating agent, the compounds I1—R axe transformed to anhydrous esters I—R.as final products. In the I R spectra of I—Ft, no hydroxyl frequencies are observed [6]. For a discussion of the mechanism of ester formation, see e.g. [6]. The absorption spectra of the hydrated and the anhydrous compounds are practically the same. Table I shows that the s p e c t -ra of the red products dissolved in the parent alcohols vary slightly as group R changes from CH3 to CgHiz; the visible band shifts somewhat hypsochromically and the intensities increase in the sequence CH3 —* CsHi7. Table I Spectral data on the compounds I—R measured in the parent alcohols R O H and pyridine R = in a l c o h o / n m and ( l o g t ) in p y r i d i n e C H 3 C 2 H 6 C 3 H 7 C 4 H 9 C S H I I C « H 13 C 8H 1 7 490(2.95 1 485(3 .35 ' 485 (3 .49 ' 480 (3 .44 ' 480 (3 .52 ' 480 (3 .58 ' 478 (3 .60 ' 367 (3 .58 ] 366 (3 .56 ' 6 ( 3 . 6 6 3 7 0 ( 3 . 6 3 372 (3 .69 3 7 2 ( 3 . 6 5 3 1 5 ( 3 . 7 1 ' 3 1 0 ( 3 . 7 3 ' 3 1 2 ( 3 . 7 6 3 1 0 ( 3 . 7 5 3 1 0 ( 3 . 7 8 3 1 2 ( 3 . 8 0 2 5 6 ( 4 . 4 4 2 5 6 ( 4 . 3 6 2 6 0 ( 4 . 3 0 ' 2 6 0 ( 4 . 3 4 ' 2 6 0 ( 4 . 3 5 ' 2 6 0 ( 4 - 3 2 1240(4.72) (242(4 .72 243 (4 .76 242 (4 .68 242 (4 .71 242 (4 .68 3 8 8 ( 3 . 5 0 ' 3 9 0 ( 3 . 4 9 3 8 7 ( 3 . 4 9 ' 3 9 2 ( 3 . 5 1 3 9 6 ( 3 . 4 8 3 9 2 ( 3 . 4 5 3 7 4 ( 3 . 7 0 ) 3 1 2 ( 3 . 7 8 ) 2 6 2 ( 4 . 3 0 ) 2 4 2 ( 4 . 7 0 3 1 0 ( 3 . 7 3 ) 3 1 0 ( 3 . 7 2 ) 3 1 0 ( 3 . 7 1 ) 315 3 . 7 4 ) 1315(3.72) 1313(3.72) 3 9 2 ( 3 . 4 8 ) 3 1 2 ( 3 . 7 2 When I—CH3 is dissolved in propanol, for example, and I—C3H7 in methanol, we obtain the same spectra as when I—CsH? is dissolved in propanol or I—CH3 in methanol; consequently, in the presence of a large excess of the other alcohol, a transesterification takes place. When the complexes I—CH3 — I—CgHi7 are dissolved 85 J. CSÁSZÁR and L. KISS in mixtures of two different alcohols, an equilibrium system forms; the calculated equilibrium constants are between 2.10*2 and 9.10"2, and A H varies between 8.5 and 17.5 k j mol- ' [7]. Dissolution of the hydrated esters, I1—R, in py results in yellow solutions. The final spectra are the same for all the complexes, with two characteristic bands at 390 nm (log £ = 3.50) and 310 nm (log e = 3.72). The red —• yellow transformation of the solutions may be followed well spectrophotometrically. From py solutions, yellow crystals can be isolated, the analytical data on which suggest the presence of two py molecules; on heating, two py molecules are released. The IR spectra of the yellow final products contain PV=0 and J/OH frequencies at 948 and 3440 cm"1, r e s -pectively. The compounds I—H and I'—R yield the same final products. The spectrophotometric measurements indicate that in py solution, where the py is both a solvent and a reactant, a pseudo—first—order reaction takes place. The rate constants and the calculated thermodynamic parameters are presented in Table II. Table II The pseudo—first—order rate constants and the calculated thermodynamic parameters on the reaction of I1—R and pyridine R = k ' / 1 0 s -1 E a A H * - A S * 298 K 308 K 318 K kJ m o P i k j • m o l " i J - K ' m o l " 1 C H 3 5 . 2 2 1 1 . 3 1 2 1 . 4 8 55. 7 9 53 . 2 3 129 . 0 0 C 2 H 5 3 .91 8 . 1 9 1 7 . 1 6 58. 3 0 55 . 74 123 . 1 6 C 3 H 7 1 . 9 0 4 . 8 4 1 1 . 2 4 70. 0 3 67 . 4 7 8 9 . 6 8 C 4 H 9 1.98 4 . 8 9 1 0 . 3 4 65. 1 7 62 . 6 1 105 . 5 5 C 5 H I I 1 .75 4 . 6 1 9 . 7 4 67. 7 2 65 . 16 9 7 . 9 4 C 6H 13 1 .72 4 . 5 3 9 . 3 9 66. 9 8 64 . 4 2 100 5 4 C 8H 1 7 1 .67 4 . 5 3 9 . 1 5 67. 11 64 . 5 5 100 2 7 86 THE SPECTRA OF ESTERS The data show that the rate constants decrease in the sequence Ci —> C5; for higher alcohols, no further significant change in the reaction rate is observed. The ARRHENIUS activation energy, E a , and the enthalpy of activation, AH*, increase from I—CH3 to i—C5HH. The parameters for I—C3H7 differ strongly from the expected ones. The high negative values of the activation entropy, AS*, are noteworthy. The activation parameters vary with the number of carbon atoms, n, according to a zig-zag shape, similarly to the changes in other parameters in the homologous series. On the other hand, a plot of AH* vs. AS* results in a linear correlation (r = 0.997). The mechanism of the reaction is problematic. We consider the following mechanism to be possible. On dissolution in py, the HOH molecule and thé R group split off to result in I—H. Then, one of the V—0 bonds opens and one py molecule, which has a high coordinating ability, is coordinated directly to the central V5 + via its lone pair electrons. This step is very fast and results in a positive charge on the central ion. The intermediate reacts with a further py molecule more slowly, because the charge on the central ion makes further V—0 bond—breaking more difficult. This second step takes place with measurable rate, according to the equation rate = k[I'-R][py]. Since [py] in py solution is about 12.573 mol dm - 3 , which is much higher than [I1—R], the reaction takes place as a pseudo—first—order reaction, and we may use the equation rate = k'[I'—R]. Table II presents the k' = k/12.573 values. If we consider that the charged activated complex is formed from two molecules (I1—R and one py molecule), that the final product has a still higher charge, and that the py molecules have a high polarization effect, the highly negative activation entropy values are obvious [8]. 87 J. CSÁSZÁR and h. KISS References [1] Monteqvi, R.t M. Gallego: Ann. Soc.esp.Fis.Quim., 32, 134 (1934). [2] (a) Bielig, H.J., E. Bayer. Liebigs Ann., 584, 96 (1953). (b ) Bayer, E., H.J. Bielig: Liebigs Ann., 584, H 6 (1953). [3] Bach, J.M., R.A. Trelles: An.Assoc.quim.argen., 28, 111 (1940). [4] Bonadies, J.A., C.J. Carrano: J.Amer.Chem.Soc., 108, 4088 (1986). [5] Patch, M.G., C.J. Carrano: Inorg.Chim.Acta, 56, 171 (1981). [6] Blair, A.J., D.A. Pantony, G.J. Minkoff. J.Inorg.Nucl.Chem., 5, 316 (1958). [7] Császár, J., J. Balog: Acta Phys. et Chem. Szeged, 6, 33 (1959). [8] Pearson, R.G.: J.Chem.Phys., 20, 1478 (1952). 88 

The spectra of esters of oxohydroxobis-(8-hydroxy-quinoline)vanadium(V) : Short communication

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The spectra of esters of oxohydroxobis-(8-hydroxy-quinoline)vanadium(V) : Short communication

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