PYRIDINE N-OXIDE COMPLEXES OF ZIRCONYL, THORIUM, AND URANYL PERCHLORATES

Transcription

1 PYRIDINE N-OXIDE COMPLEXES OF ZIRCONYL, THORIUM, AND URANYL PERCHLORATES P. RAIIA MURTHY AND C. C. PATEL Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, India Received October 2, 1963 ABSTRACT Pyridine N-oxide complexes having the composition ZrO(Py. O)~(C104)z, Th(Py. 0)8(C101)4, and U02(Py.0)6(C104)2 have been prepared. The infrared and electronic absorption spectra show that the bonding between the metal and pyridine N-oxide in the complexes has occurred by donation of the lone pair of p-electrons on oxygen to the metal, and that the T-bond character of NO group increases in the complexes as uranyl < thorium < zirconyl. The decrease in the vibrational structure of the U0z2+ spectrum in the visible region indicates strong coordination of pyridine N-oxide to the uranyl group. The decomposition temperatures of zirconyl, thorium, and uranyl complexes are 307, 350, and 319" C respectively. Recently Quagliano (I) et al. have prepared a number of metal complexes of pyridine N-oxide and determined their conductivity and magnetic susceptibilities. The same workers (2, 3) have also investigated the infrared spectra of these complexes. Carlin (4) has also prepared a number of pyridine N-oxide complexes of metal perchlorates and investigated their visible spectra. The survey of the literature shows that the pyridine N-oxide complexes of IV A group metal perchlorates and of uranyl perchlorate have not been prepared before. The pyridine N-oxide complexes of zirconyl, thorium, and uranyl perchlorates were, therefore, prepared for the first time and some of their properties studied. EXPERIMENTAL Materials ~ kplo~ed Pyridine N-oxide was prepared according to the method of Ochiai (5), purified by repeated vacuum distillations, and finally recrystallized from chloroform. Zirconyl perchlorate dihydrate and thorium perchlorate tetrahydrate were prepared as reported earlier ('37 7). Uranyl perchlorate hexahydrate was prepared by digesting uranyl nitrate hexahydrate with the calculated amount of perchloric acid and evaporating the mixture to dryness. Uranyl perchlorate formed was crystallized until free from nitrate ions. Preparation of the Complexes To 1 g of zirconyl perchlorate dihydrate in 20 ml of anhydrous ethanol, 2.5 g of pyridine N-oxide in 20 ml of ethanol was added. The solution was allowed to stand for 15 hours in the refrigerator and the crystals obtained were filtered, washed with ethanol, dried over phosphorus pentoxide under vacuum, and analyzed. Zirconium was estimated gravimetrically by precipitating the hydroxide with ammonium hydroxide and weighed as ignited oxide. Perchlorate was estimated by Kurz's method (8) by reducing the perchlorate with sodium nitrite at about 500" C and estimating the chloride formed by Volhard's method. Nitrogen was estimated by Duma's semimicro method. (Zr = 10.32, Clod = 22.38, and NZ = 9.48%. Calc. for ZrO(Py.O)6(C104)z: Zr = 10.40, Clod = 22.69, and Nz = 9.58Y0.) Crystals of the thorium complex were obtained immediately by mixing the reactants as above. The crystals were ~urified, dried as before, and analyzed in the same way as in the above case. (Th = 16.79, C104 = 27.86, and Nz = 7.99%. Calc. for Th(Py,O)s(ClO4)4: Th = 16.68, C104 = 28.45, and N2 = 8.067".) In the case of uranyl complex, it was necessary to evaporate the solution almost to dryness to get the crystals of the complex. The crystals were purified, dried, and analyzed as before. (U = 24.85, C104 = 20.85, and Nz = 7.40%. Calc. for UOz(Py.O)j(C104)~: U = 25.10, C104 = 21.06, and Nz = 7.42%.) Properties Zirconyl and thorium pyridine N-oxide complexes were white while uranyl complex was bright yellow in color. All the complexes were non-hygroscopic, insoluble in non-polar solvents, and soluble in water, Canadian Journal of Chemistry. Volume 42 (1864) 856

2 MURTHY AND PATEL: PYRIDINE N-OXIDE COMPLEXES 857 acetonitrile, and dimethyl formamide. When the complexes were heated, they exploded. To minimize the impact of explosion, the differential thermal analysis of the complexes were carried out by mixing the complexes with alumina in the ratio of 1:lO. The DTA for zirconyl, thorium, and uranyl complexes gave a big exothermic peak at 307', 350, and 319" C respectively. Infrared and Electronic Absorption Studies The infrared spectra of the complexes were taken in Nujol mull with Perkin-Elmer infrared spectrophotometer model 21, equipped with sodium chloride optics. The infrared absorption frequencies of these complexes together with those of pyridine N-oxide (2) are given in Table I. TABLE I Infrared frequencies (cm-l) of zirconyl, thorium, and uranyl complexes together with pyridine N-oxide Py.0 in ZrO(Py. O)a Th (PY. 0)s UO*(Py.0)6 solution (2) (clod2 (Clod4 (clod2 Assignments 3070 w 3125 w 3096 m 3106 m CH stretching w 2010 w 2000 m 2. vg v w 1888 v v w 1883 v w ~l~b+~lla v v w 1832 v v w vloa+~s w b - ~10af~l7e m 1686 w ~11+~10b 1631 m 1637 m Ring def 1610 m 1555 v w NOTE: v = very. s = strong, m = medium. w = weak, b = broad, sh = shoulder. Ring def KO str C-H in plane def Ring breathing V6 Vl7Z VlOb NO bending Electronic absorption spectra of the complexes in pure dry acetonitrile were taken with Hilger Uvispek spectrophotometer, model H 700, using I cm matched pair of quartz cells. The spectra are given in Figs. 1 and 2. DISCUSSION The three main vibration frequencies of pyridine N-oxide which are likely to be influenced on complex formation with metal ion are KO stretching vibration, NO bending vibration, and C-H out-of-plane vibration frequencies (3). Shindo (9-11) has assigned the strong band at 1265 cm-i to NO stretching vibration in the spectrum of pyridine N-oxide in carbon disulphide, while in the solid state the band occurs around 1243 cm-l. The NO stretching frequency of the complexes in mull is lowered to below 1225 cm-l (Table I). The decrease in the frequency is attributed to the change in the nature of the NO bond as a result of coordination. With the formation of h'i t 0 bond, there is a decrease in the n-bond character of NO. The NO stretching vibrational frequencies of zirconyl, thorium, and uranyl complexes are at 1222, 1218, and 1207 cm-l VlOa Vll -

3 858 CANADIAN JOURNAL OF CHEMISTRY. VOL. 42, FIG. 1. Ultraviolet absorption spectra of (1) 'Th(Py.O),(C104)c, (2) ZrO(Py.O)s(ClOa)z, and (3) UOn(Py.0)5(C10a)n in acetonitrile. FIG. 2. Absorption spectra of uranyl pyridine X-oxide complex in acetonitrile in the visible region. respectively, showing gradual decrease in the a-bond character of NO and consequent increase in the bond strength between the metal and the oxygen. Using these frequencies, the force constants calculated for the NO group in the complexes are 6.577, 6.531, and millidynes per A respectively. The decreasing order also follows the decreasing order of a-bond character of the NO group in the complexes. The 840 cm-i band in the spectrum of pyridine N-oxide has been assigned to NO bending vibration (2). Because of the coordination of oxygen atom of NO group to the metal atom, a noticeable shift of NO bending could be expected. However, a very slight shift of this vibration is observed in these complexes. The small shift is due to the two opposing effects. One, similar to that of coordinated water where bending vibration, 6013, is shifted to the higher frequency and the other, lowering in the frequency because of the decrease in a-bond character of the NO bond. If both these effects are practically equal, a very small shift in the frequency would result. Kakiuti (3) and co-workers have assigned C-H out-of-plane vibrations at 741 (vll), 935 (vlob), 993 (v5), 846 (vlo,), and 972 cm-i (~17,) for pyridine N-oxide. In the complexes studied almost all vibrations except (vll) and (vlob) appear as weak shoulders, and all are considerably shifted to higher frequencies owing to the decrease in the electron density of the pyridine ring. This observation is in agreement with the findings of Kross (12). All the remaining bands due to the pyridine ring are very little affected on complex formation and their assignments are made after the work of Kida (2). The in-plane ring vibration band of pyridine N-oxide around 1475 cm-i has merged with the Nujol band. In the uranyl complex, the strong band at 907 cm-i is due to the asymmetric stretching frequency and the 858 cm-i weak band due to the syininetric stretching frequency of uranyl ion (13). The asymmetric vibration band is shifted to the lower frequency. A similar shift was observed by Coinyns et al. (14) in uranyl acetylacetone complex. Such a shift is possible when the ligand groups are presumed to lie in or nearly in a plane perpendicular to the axial 0-U-0 direction (16). The force constant Keii, and ru-o(zp ru-ocl, distances calculated for U0S2+ group in the complex are 6.87 millidynes per A and 1.74 A and 1.72.t% respectively. The U-0 distances are calculated using equations of Jones (15) and McGlynn (16) respectively. The broad strong band at 1085 cm-i in all the three complexes is due to the degenerate stretching vibration of the perchlorate ion.

4 MURTHY AND PATEL: PYRIDINE N-OXIDE COMPLEXES 859 The electronic absorption of spectrum of pyridine N-oxide in n-heptane gives one intense band at 280 mp and a weak band at about 320 mp. The former band is assigned to T-T* transition, and the latter to the n + T* transition by Ito (17) et al. On complex forination the weak band at 320 mp disappears while the band at 280 inp shifts to lower wavelength around 275 mp. This shows that the coinplex formation has taken place by donation of the lone pair of $-electrons on oxygen of the NO group to the metal atom, thus supporting the findings by the vibrational spectra. The absorption maximum in zirconyl, thorium, and uranyl complexes occurs at 276, 274, and 272 mp respectively. This shows that T-electrons in the NO group are held in the following increasing strength: uranyl < thorium < zirconyl complex. This observation is in agreement with the decrease in NO stretching frequency. The intensity of the band around 275 mp is roughly proportional to the number of pyridine N-oxide ligailds attached to the metal atom. In place of nearly 10 bands due to the free uranyl ion in solution, uranyl pyridine X-oxide complex gave only three weak bands in the visible region at 432 (r = 71.88), 484 (r = 15.07), and 505 mp (E = 6.955) (Fig. 2). From their position and low intensity, the 432 and 484 mp bands are the modified form of uranyl bands. The lowest intensity 505 mp band may also be due to the uranyl ion, although it falls just outside the range of absorption usually observed in the spectrum of uranyl ions in solution. The decrease in the number of bands indicates the strong coordination between pyridine N-oxide and uranyl group. From the number of pyridine N-oxide ~nolecules in the metal complexes, the coordination number for both zirconium and uranium is seven and that for thorium is eight. In general, compounds with coordination number of seven are rare. However, both zirconium and uranium (18) exhibit coordination number of seven in complex ions (21-F.i)~- and (UOzF5)3-. The zirconyl conlplex with coordination number seven nlay be represented either as a face-centered octahedron (d5sp hybridization) having the extra oxygen atom at the center of one face of the octahedron or as a trigonal prism with the extra oxygen at the center of one square face (face-centered trigonal prism with d4sp2 hybridization). The uranyl coinplex with coordination number seven may have pentagonal bipyramidal arrangement. In this case, two separate sets of hybrids are possible: d2sp2 to give five planar bonds and d,zp, or df to give the axial bonds (18). The arrangement may resemble that of (UOzFb)3- complex ion (19). Thorium complex with eight coordination number may have the square-antiprism structure with d5p3 hybridization as in the acetylacetonate complexes of Th(IV), U(IV), and Ce(1V) (20). ACKNOWLEDGMENT The authors are thankful to Professor M. R. A. Rao for his interest in the work. REFERENCES 1. J. V. QUAGLIANO, J. FUJITA, G. FRANZ, D. H. PHILLIPS, J. A. WALIMSLEY, and S. Y. TYREE. J. Am. Chem. Soc. 83, 3770 (1961). 2. S. KIDA, J. V. QUAGLIANO, J. A. WALDISLEY, and S. Y. TYREE. Spectrochim. Acta, 19, 189 (1963). 3. Y. KAKIUTI, S. KIDA, and J. V. QUAGLIANO. Spectrochim. Acta, 19, 201 (1963). 4. L. CARLIN. J. Am. Chem. Soc. 83, 3773 (1961). 5. E. OCHIAI. J. Org. Chem. 18, 534 (1953). 6. P. RAMA MURTHY and C. C. PATEL. Naturwiss (1961). 7. P. RAMA MURTHY and C. C. PATEL. 1. Inorg.. ~ucl. Chek (1963). \, 8. E. KURZ, G. KOBER, and M. BERL. Anal. ~lhem. 30, 1983 (1958). 9. H. SHINDO. Pharm. Bull. (Tokyo), 4, 460 (1960); Kagaku No Rydiki, ZGkam, 28, 163 (1958). 10. H. SHINDO. Chem. Pharm. Bull. (Tokyo), 6, 117 (1958). 11. H. SHINDO. Chem. Pharm. Bull. (Tokyo), 7, 791 (1959). 12. R. D. K~oss, V. A. FASSEL, and M. MARGOSHES. J. Am. Chem. Soc. 78, 1332 (1956).

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