Table 1. Elemental analyses data and some physical properties of the La(III) and Pr(III) complexes of dithiosemicarbazones
Compound Color Yield, % Contents (found/calcd), %
C H N M
[La(PPDT)Cl(H2O)] (I) Brown 58 39.8/39.9 3.6/3.7 13.8/13.9 23.1/23.1
[Pr(PPDT)Cl(H2O)] (II) Brown 55 39.7/39.8 3.5/3.7 13.8/13.9 23.3/23.4
[La(PNDT)Cl(H2)O] (III) Brown 57 34.7/34.8 2.7/2.9 16.1/16.2 20.0/20.1
[Pr(PNDT)Cl(H2O)] (IV) Yellow 64 34.5/34.6 2.7/2.9 16.0/16.1 20.2/20.3
[La(PMDT)Cl(H2O)] (V) Yellow 65 39.8/39.9 3.8/3.9 12.6/12.7 20.9/21.0
[Pr(PMDT)Cl(H2O)] (VI) Yellow 60 39.7/39.8 3.8/3.9 12.5/12.6 21.2/21.3
washed with cold methanol and ether, and dried in vacuo. The yield was 55-65%.
RESULTS AND DISCUSSION
The reactions of La(III) and Pr(III) chlorides with piperazine dithiosemicarbazones (molar ratio 1 : 1) in methanol in the presence of a sodium hydroxide solution can be represented by the following equation:
LnCl3 + LH2
Methanol
[Ln(L)Cl(H2O)]2
where Ln = La(III) or Pr(III); LH2 = PPDTH2 (for I, II), PNDTH2 (for III, IV), PMDTH2 (for V, VI), respectively; refluxing time for I, II, III, IV, V, and VI is 12, 15, 8, 10, 12, and 10.
Elemental analyses data of the complexes are given in Table 1. The complexes are yellow- to brown-colored solids. The complexes are only soluble in dimeth-ylformamide and dimethylsulfoxide. Osmometric molecular weight measurements show the complexes to be dimers.
Magnetic moments and electronic spectra. The
lanthanum (III) complexes, as expected, have no resultant magnetic moment, whereas the magnetic moment of the praseodymium (III) complexes lie in the range of 3.58-3.65 which shows little deviation from Van Vleck values, i.e., 3.62 for the Pr3+ ion.
The electronic spectra of the paramagnetic lanthanide complexes have been the subject of recent extensive reviews [12-15], which deal particularly with the physical and theoretical aspects. The Pr3+ ion has the outer configuration 4f25s2p6 and follows the Russell-Saunders L, S, J coupling scheme but with a certain amount of configuration interaction. The 5 s and 5p subshells have a radial dispersion, which is greater than the 4f electrons, and, hence, shield the latter from the effect of coordinated ligands to a very large extent but not completely. Thus, the electronic spectra of Pr(III) can be considered [16] as derived from the spectra of the
gaseous ions by a fairly small perturbation. This perturbation can be of two general types. The first is a general naphelauxetic effect owing to a drift of ligand or 5s, 5p electron density into the metal ion, slightly expanding the 4f subshell and reducing the 4f-4f interactions. The second is a crystal field splitting effect, which removes the degeneracy of the free ion levels and which can be represented with fair success by a point negative charge model having the appropriate molecular symmetry.
The spectra of La3+ ions are composed of closely spaced sharp lines. The group of lines is normally spread over an energy interval of several hundred cm-1. These spectra arise from transitions among levels offn configuration. The 4f electrons of lanthanides are more or less protected from the influence of lattice by the polarization of 5s2 and 5p6 closed shell. The crystal field splitting effect in the La3+ ion is small, 200-300 cm-1 at the most. Thus, to a good approximation, the levels agree well with that for the free ion.
The f2 configuration of a free Pr3+ ion involves 13 energy levels. Their location is found experimentally by the analyses of the spectra for a gaseous the Pr3+ ion. The ground term appears to be 3H4. The absorption spectra of Pr3+ ion solution in the visible region involve four bands due to the transitions from the ground level to 3P0, 3P1 + 1I6, 3P0, and D2 levels. The f-f transitions are weakly allowed due to some mixing of the excited state of the opposite parity into the ground state and a marked enhancement in the intensity of the band upon complexation is observed. This is due to an increase in the configuration interaction upon complexation [17]. The Pr(III) complexes show bands around 1660016780, 20400-20600, 21100-21400, and 2320023500 cm-1 corresponding to the transitions from 3H4 to D2, 3P0, 3P1, and 3P2 energy levels, respectively.
The naphelauxetic effect is quantitatively described
by a naphelauxetic parameter (b) equal to the ratio of the interelectron repulsion parameter (either Slater's in-
NaOH
tegrals Fk, or Racah's parameter Ek) in the complex and in the free ion:
ß =
( Fk )
complex
( Fk )
free ion
ß(E )complex = -—J---.
(E )free ion
As the naphelauxetic effect value (1 - P) is small in the lanthanide complex, it can be, to a fairly good approximation, defined from the ratio of the wave numbers off-f transitions in the spectra of the complex and the free ion.
ß
V,
complex
Vf
free i
Since, in the general case, the energy levels in the lanthanide free ions are unknown the relative naphelauxetic effects P and are usually determined from the experimental data using the spectra of the lanthanide aqua ions as standards.
n
ß = n I
V
complex
van„o ■
n = 1
From the mean P values, Sinha's covalency param -eter (5) and bonding parameter (b1/2) are calculated us -ing following formulas:
Ô = x 100 and bm = ß
2
1/2
Table 2. Electronic spectral parameters of the Pr(III) complexes of dithiosemicarbazones
Complex ß 5 b1/2
[Pr(PPDT)Cl(H2O)] 0.9890 1.1122 0.0741
[Pr(PNDT)Cl(H2O)] 0.9900 1.0101 0.0707
[Pr(PMDT)Cl(H2O)] 0.9896 1.0509 0.0721
Table 3. Significant IR spectral bands (cm 1) of the La(III) and Pr(III) complexes of dithiosemicarbazones
These parameters measure the amount of f-ligand mixing, i.e., covalency.
The values of parameters p, 5, and b1/2 of these complexes are given in Table 2. The values of P, which are less than unity, and positive values of 5 and b1/2 sup -port partial covalent nature of bonding between the metal and ligands. The dithiosemicarbazones also exhibit bands at ca. 30000 cm-1, which are assigned to the azomethine linkage and aromatic ring n-n* or n-n* transitions. Upon complexation these bands move to slightly lower energy.
IR spectra. These ligands can exist either as thione or thiol tautomeric forms or as an equilibrium mixture of both forms, since they have a thioamide, -NH-C(=S) function. The IR spectral data of La(III) and Pr(III) complexes are given in Table 3. The infrared spectra in the solid state do not show any v(S-H) band but exhibit a medium v(N-H) band at ca. 3150 cm-1, in -dicating that in the solid state, they remain mainly in the thione form. However, in solution they readily con -vert to the thiol tautomeric form with the concomitant formation of the La(III)/Pr(III) complexes of the proto-nated mercapto form of the ligands. This is indicated by the absence of -NH band in the complexes. The IR spectra of the complexes also show a new band at ca. 620 cm-1 owing to conversion of C=S to C-S. The new
Complex v(O-H) v(C=N) v(C-S) v(Ln-N) v(Ln-S)
I 3400 sb 1600 s, 1550 s 625 m 425 m 380 w
II 3420 sb 1600 s, 1540 s 620 m 415 m 370 w
III 3410 sb 1605 s, 1545 s 615 m 430 m 375 w
IV 3400 sb 1595 s, 1540 s 610 m 420 m 365 w
V 3425 sb 1610 s, 1535 s 620 m 420 m 385 w
VI 3400 sb 1600 s, 1530 s 615 m 410 m 375 w
band in the complexes at ca. 365-385 cm-1 assigned [18] to v(Ln-S) and shows that sulfur is bonded to the metal atom. The v(C=N) shift of the dithiosemicarbazones from 1570-1560 cm-1 to lower energy in the spectra of the complexes indicates [11] coordination of the imine nitrogens (1). However, the N(2)H from the two dithiosemicarbazone moieties, by thione thiol tautomerism, produces an additional carbon-nitrogen double bonds, N(2)=C(S), which is indicated by the appearance of a band at ca. 1600 cm-1 in the spectra of the complexes. Bands in the 430-410 cm-1 region are assigned [19, 20] to v(Ln-N) and support coordination of the imine nitrogens. In addition, all the complexes show a broad band at ca. 3400 cm1 due to v(OH) of coordinated water molecules.
Thus, the infrared spectra reveal that piperazine dithiosemicarbazones behave as dibasic tetradentate ligands.
Proton magnetic resonance spectra. The 1H NMR
spectra of the complexes have been recorded in deuter-ated dimethylsulfoxide. The different 1H NMR signals are given in Table 4. The intensities of all the resonance lines were determined by planimetric integration. A
Table 4. XH NMR data (5, ppm) of lanthanum(III) complexes of piperazine dithiosemicarbazones
Complex Phenyl ring Piperazine ring -NH N=CH
[La(PPDT)Cl(H2O)]2 7.40-7.68 m. 3.46 s. 12.60 s. 8.34 s.
[La(PNDT)Cl(H2O)]2 7.48-7.60 m. 3.50 s. 12.90 s. 8.38 s.
[La(PMDT)Cl(H2O)]2 7.45-7.62 m. 3.45 s. 12.78 s. 8.35 s.
comparison of the ligands with the complexes leads to the following conclusions:
(a) The signals of the N(2)H is seen at ca. ô 12.6012.90 ppm in the ligands. In the complexes, neither this signal nor a thiol SH signal is visible.
(b) The chemical shift at ca. ô 3.38 can be due to piperazine ring protons, which shifted slightly down-field in the complexes.
(c) The phenyl ring protons appear as a multiplet in the range ô 7.40-7.82 ppm.
Thus, the above studies suggest that two thiol sulfur atoms and two azomethine nitrogen atoms of the
ligands are involved in chelation. However, coordination of all the donor atoms with single metal atom seems unlikely due to steric reason. It can be possible that one sulfur atom and one azomethine nitrogen coordinate to one metal atom, while the second sulfur atom and second azomethine nitrogen of the same ligand coordinate to another metal atom, leading to a binuclear structure A. The molecular weight determination further supports the proposed structure. Attempts are being made to develop single crystals suitable for X-ray structure but so far no success has been achieved.
'C=N-N^f
R \ S
N N—
R
CI—L^-OH2 H2O Ln Cl
H
H
(A)
Thermal analysis. The thermal stability and decomposition ranges of the l
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.