Magnetism of the lanthanides(III) complexes with some polihydroxyflavones

Journal of Alloys and Compounds 425 (2006) 59–63

Magnetism of the lanthanides(III) complexes with some polihydroxyflavones Dorota Nowak ∗ , El˙zbieta Wo´znicka, Anna Ku´zniar, Maria Kopacz Department of Inorganic and Analytical Chemistry, Faculty of Chemistry, Rzesz´ow University of Technology, Powsta´nc´ow Warszawy 6 Aven., 35-959 Rzesz´ow, Poland Received 8 September 2005; received in revised form 15 November 2005; accepted 17 November 2005 Available online 21 February 2006

Abstract The La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III), Er(III), Tm(III), Yb(III) and Lu(III) ions form solid complexes with morin and quercetin-5 -sulfonic acid (QSA). The composition and some physicochemical properties of these compounds were described previously. In order to estimate magnetic properties, the oxidation number of metal ions and the nature of the metal–ligand bonding in the investigated rare earth element complexes the magnetic measurements were carried out. On the basis of the magnetic susceptibility (χ) versus T and the magnetic moments (μeff ) versus T the values of Curie (C) and Weiss (Θ) constants for all examined compounds were determined. Moreover, for Sm(III) and Eu(III) complexes the value of spin-orbit coupling parameter (λ) was calculated. It was stated that the obtained complexes are paramagnetic, except La(III) and Lu(III), which are diamagnetic. Furthermore, the magnetic properties of Sm(III) and Eu(III) compounds significantly differ from the Curie and the Curie–Weiss laws. The metal–ligand bonding in the investigated rare earth element complexes are most probably electrostatic with small participation of covalent bond. © 2006 Elsevier B.V. All rights reserved. Keywords: Rare earth compounds; Flavonoids; Magnetic properties

1. Introduction Quercetin (3,5,7,3 ,4 -pentahydroxyflavone) and morin (3,5,7,2 ,4 -pentahydroxyflavone) (Fig. 1), are the most widespread natural plant dyes, called flavonoids. Due to their properties these compounds find application in therapy as viral antigens and bactericide [1]. As antioxidants, flavonoids scavenge free radicals such as HO• , HOO• , NO• and stabilize the lower oxidation numbers of metal ions [2,3]. Quercetin and morin are known as analytical reagents used for the qualitative and quantitative determination of some metals [4–6]. Due to a convenient position of oxygen in the C(5)–OH and C(4) O as well as C(3)–OH and C(4) O groups, morin and quercetin form chelate complexes with ions of p-, d- and f-electron metals. Complexation reactions are sensitive and the mole absorption coefficients are of the order of 104 . In some

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papers, morin [7,8], quercetin [9,10] and sulfonic derivative of quercetin, quercetin-5 -sulfonic acid (QSA) [11] were applied in analytical practice to spectrophotometric and fluorimetric metal ions determination and as extraction reagents [12–16]. In [17], a complex of La(III) with morin was used for fluorescence determination of DNA. Furthermore, the complexes of Gd(III) and Eu(III) ions with quercetin show therapeutic (blood cancer) and antioxidant properties [18,19]. Earlier [20–24], complexes of some lanthanides with flavonoids in solid were studied and their nature was described. In [22–24], the measurements of the magnetic moments were carried out in order to estimate the oxidation numbers of lanthanide ions in complexes with QSA. The magnetic properties of the rare earth metal complexes are still badly understood, in contrast to the d-electron metals compounds. Moreover, no data on magnetic properties of complexes of all lanthanides with flavonoids have been found. So we decided to undertake this subject. This paper is a continuation and a completion of the research on magnetic properties of complexes of morin and QSA with lanthanides(III) [22–24]. The

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D. Nowak et al. / Journal of Alloys and Compounds 425 (2006) 59–63

Fig. 1. Structure of quercetin (R1 = H, R2 = OH, R3 = H); morin (R1 = OH, R2 = H, R3 = H) and quercetin-5 -sulfonic acid (R1 = H, R2 = OH, R3 = SO3 H).

obtained results were analyzed and compared with the literature data for the lanthanide(III) complexes with other ligands.

−1 Fig. 2. Temperature dependence of χM for complexes of Ce(III), Tm(III) and Yb(III) ions with morin and QSA.

2. Experimental

For the investigated compounds the Weiss constants (Θ) were calculated from the least squares fitting of the 1/χM versus T curves. The Curie constants (C) were determined from χM ·T = f(T) dependence for T equals 300 K. The obtained results were listed in Table 1.

2.1. Synthesis The synthesis of complexes of lanthanide(III) ions with morin and QSA was carried out according to methods described in [21–24]. On the basis of the elementary analysis, thermogravimetric determination and UV–vis spectrophotometry it was stated that the obtained compounds have a composition and structure consistent with the literature data:

3. Results and discussion In order to estimate magnetic properties, the oxidation number of metal ions and the nature of the metal–ligand bonding in the rare earth element complexes with morin and QSA the magnetic measurements were carried out. The obtained values of magnetic moments indicate that the La3+ (f0 ) and Lu3+ (f14 ) complexes are diamagnetic, as may be expected from their closed-shell electronic configuration and the absence of unpaired electrons [37]. The complexes of the other lanthanides(III) ions with morin and QSA obey the Curie–Weiss law (Fig. 2). For all the complexes the values of the Weiss constant (Θ) are different from zero. This is probably the consequence of antiferromagnetic or ferromagnetic spin interaction or crystal field (CF) splitting of the paramagnetic spin state [27]. From the obtained results it appears that the deviations from the Curie–Weiss law are probably the consequence of crystal

- Compounds of morin: Ln(C15 H9 O7 )3 ·nH2 O, where n = 6 or 8. - Compounds of: Ln(C15 H9 O10 S)3 ·nH2 O (n = 12–15).

2.2. Magnetic measurements Measurements of magnetic susceptibility of the complexes of Ln(III) with morin and La(III), Ce(III), Tm(III), Yb(III) and Lu(III) with QSA were made with a Quantum Desigon SQUID magnetometer (type MPMS-5) in the temperature range from 2 to 300 K and magnetic field of 5 kOe. The contribution of the support cell was independently measured and subtracted. Measurements of magnetic susceptibility of the complexes of Pr(III), Nd(III), Sm(III), Eu(III), Gd(III), Tb(III), Dy(III), Ho(III) and Er(III) with QSA were made by Gouy’s method within the 70–300 K. The diamagnetic contribution of the compound was estimated using Pascal’s coefficients. The effective magnetic moments were calculated using the formula: μeff = 2.83(χM ·T)1/2 B.M., where χM is the magnetic susceptibility of the appropriate lanthanide with an allowance for diamagnetism [25], T is the temperature in K. The values of effective magnetic moments are listed in Table 1.

Table 1 The values of Curie constants (C), Weiss constants (Θ) and magnetic moments (μeff ) for the investigated complexes and the free lanthanide ions Ln

The value for isolated ions Ctheor

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

– 0.80 1.94 1.64 – – 7.87 11.81 14.10 14.06 11.47 6.94 2.56 –

(cm3

K/mol)

Complexes of QSA

μeff,calc. (M.B.)

μeff,observed (M.B.) [26]

C

0.0 2.54 3.58 3.62 0.84 0.0 7.94 9.72 10.63 10.6 9.59 7.57 4.54 0.0

0.0 2.5 3.5 3.6 1.5 3.4 8.0 9.3 10.6 10.4 9.5 7.4 4.5 0.0

– 0.83 1.64 1.51 – – 7.66 11.15 13.00 12.13 9.68 6.26 2.33 –

(cm3

K/mol)

Complexes of morin

Θ

μeff (M.B.)

C (cm3 K/mol)

Θ

μeff (M.B.)

– −55.69 −20.43 −39.76 – – +17.76 −4.16 +12.43 +2.55 +5.68 −3.40 −42.67 –

0.0 2.58 3.5 3.48 2.38 3.25 7.83 9.48 10.21 9.89 8.80 7.08 4.06 0.0

– 0.70 1.60 1.26 – – 9.78 12.80 13.80 11.82 10.17 6.56 2.35 –

– −35.38 −26.98 −42.02 – – −0.57 −6.61 −2.92 −3.49 −8.07 −8.17 −32.68 –

0.0 2.28 3.45 3.01 1.74 3.23 8.91 10.11 10.51 9.70 8.92 7.19 4.14 0.0

D. Nowak et al. / Journal of Alloys and Compounds 425 (2006) 59–63

field interaction [27]. The investigated complex compounds are paramagnetic. In the first approximation a rare earth ion in a molecular compound behaves as a free-ion. The paramagnetic behavior of the tripositive lanthanide ions is due to the presence of unpaired 4f electrons. Since these electrons were shielded by outer closedshell electrons, the spin-orbit coupling is considered to play essential role in their magnetic properties. Hence, their total magnetic moments J, are expressed as J = L + S, where L denotes an angular moment and S is a spin moment. It follows, therefore, that the magnetic moment of a complex should indicate whether or not these 4f electrons are involved in bond formation. On the basis of the Curie law the value of magnetic moments were appointed for the complexes of lanthanides(III) with morin and QSA. They are similar to the theoretical values given by Hund (Eq. (1)) when the multiplet width are larger compared with kT. The agreement is quite good with except of Sm(III) and Eu(III) complexes. The higher value of μeff for this compounds suggests a possible interaction of the ligand field with the central ion or may be associated with a multiplet width comparable with kT. Taking into consideration the second case, the magnetic susceptibility Sm(III) and Eu(III) ions is given by the Van Vleck formula (Eq. (2)) [28].  μ = g J(J + 1) M.B., (1) 6 χM =

J=0 (2J

6

+ 1)χ(J) exp[−λJ(J + 1)/2kT ]

J=0 (2J

+ 1) exp[−λJ(J + 1)/2kT ]

,

(2)

where NgJ2 β2 J(J + 1) 2Nβ2 (gJ − 1)(gJ − 2) χ(J) = + , 3kT 3λ

(3)

3 S(S + 1) − L(L + 1) gJ = + . 2 2J(J + 1)

(4)

Eu3+ ion possesses six unpaired electrons. Spin (S) and orbital (L) moments are equal to 3 and total magnetic moment J = L − S = 0. The 7 F ground term is split by the spin-orbit coupling into seven states, 7 FJ , with J taking values from 0 to 6, and the energies: E(J) =

λJ(J + 1) , 2

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Fig. 3. Experimental magnetic data χM T vs. T for Sm(III) and Eu(III) complexes with morin.

the 7 F term is the sum of orbital and spin contributions:  2 Nβ 2 (χM T )HT = L(L + 1) + gS2 S(S + 1)], [gL 3k

(6)

where gL = 1 and gS = 2. The theoretical value of (χM T)HT should be equal to 12Nβ2 /k = 4.50 cm3 K/mol. In fact, this limit can never be reached because only the first three low-lying states with energies 0, λ and 3λ can be significantly populated. In our case (χM T)HT at 300 K is equal to 1.30 cm3 K/mol for Eu(III)–morin complex (Fig. 3) and 1.32 cm3 K/mol for complex of Eu(III) ions with QSA. The obtained values are in good agreement with this reported in [30] for isolated Eu(III) complex. At the low temperature limit (χM T)LT should be zero because the 7 F0 ground state is non-magnetic, but it is finite and nonzero owing to the term χ(0) arising from coupling between 7 F0 and 7 F1 states through the Zeeman perturbation. The value of (χM T)LT is simply related to λ through (7): (χM )LT =

8Nβ2 2.086 × 10−3 = . λ λ

(7)

The experimental magnetic susceptibilities χM versus temperature for complexes of europium(III) with morin and QSA are plotted in Fig. 4. As T is lowered, χM smoothly increases and then tends to a plateau at ca. 100 K. Then the χM values are 5.85 × 10−3 and 6.18 × 10−3 cm3 K/mol for Eu(III)–morin and Eu(III)–QSA, respectively. The obtained results compares fairly

(5)

where the energy of the ground state is taken as the origin [29]. The energy levels of the lowest J = 0 state and the exited J = 1 state are very close to each other thereby the exited state can be easily accessed by an external magnetic field. In this case, in calculation of a magnetic susceptibility the temperatureindependent unit in Eq. (3) becomes important. Therefore, the magnetic susceptibility of the isolated Eu3+ ion is giving by Eq. (2) with all gJ = 3/2 except g0 , which is equal to 2 + L = 2 + S = 5. The high-temperature limit (χT)HT for kT  λ, expressed as (6), is obtained by assuming that the magnetic susceptibility of

Fig. 4. Experimental data χM vs. T for Sm(III) and Eu(III) complexes with morin.

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D. Nowak et al. / Journal of Alloys and Compounds 425 (2006) 59–63

well with the earlier reported data (5.99 × 10−3 cm3 K/mol [30] and 6.00 × 10−3 cm3 K/mol [31]). At very low temperature χM increases again as T is lowered further due to the unavoidable presence of a few parts per million of a rare earth metal ion with a paramagnetic ground state in the sample [30]. At low temperature λ can be determined directly from relation (7). For the complex Eu(III)–morin λ is equal to 357 cm−1 , in the case of Eu(III)–QSA, λ = 338 cm−1 . The λ values reported for other complexes of Eu3+ are in the range 337–420 cm−1 [30,32,33]. The 6 H ground term for Sm(III) is split by spin-orbit coupling into six levels. The energies, E(J), increase from 6 H5/2 to 6 H15/2 [29]: λ[J(J + 1) − 35/4] . (8) 2 Since the spin-orbit coupling parameter is of the order of 200 cm−1 , the first excited state 6 H7/2 can be populated at room temperature. Taking into account the six states arising from 6 H for the calculation of magnetic susceptibilities (Eq. (2)) the (χM T)LT should tend to the value 0.089 cm3 K/mol. Moreover, the χM versus T should exhibit rounded minimum for a T value defined by kT/λ = 0.956. In our case, the χM does not show the minimum predicted by the theory. The experimental χM T versus T plots are nearly linear over the whole temperature range and for Sm(III)–morin compound is presented in Fig. 3. When T approaches absolute zero, χM T tends to 3.9 × 10−2 cm3 K/mol. For Sm(III)–morin complex λ is found equal to 223 cm−1 . The agreement between the calculated and the theoretical (200 cm−1 ) value is quite good. For both Eu(III) and Sm(III) several free-ion states may be populated at room temperature, which leads to magnetic properties deviating from the Curie law. If the free-ion approximation to fit the magnetic data is used, the conformity of theoretical and experimental values is good. This agreement does not mean that the free-ion states remain unperturbed by the crystal field. For the studied complexes the effective moment at room temperature is close to that of the free Ln3+ , complying with the trivalent state of the rare earths. As temperature decreases, μeff decreases reflecting the gradual depopulation of the crystal field (CF) split energy levels of Ln3+ ions or spin-orbit coupling [34,35]. A more pronounced decrement of μeff with T is observed for the complex of Pr(III) with morin (Fig. 5). The Pr3+ (3 H4 ) is non-Kramers ion and probably at very low temperatures due to the crystal field splitting only the lowest singlet level of the ground state is populated [36]. The free-ion ground state of Nd(III) is 4 I9/2 . The first excited state, 4 I11/2 , is located at about 2000 cm−1 above, such that it is fully depopulated, even at room temperature. In the case of Nd3+ the crystal field splits the 4 I9/2 free-ion ground state into five Kramers doublets. These doublets are almost equally populated at 300 K, such that χM T is equal to the value calculated from Eq. (3) in the free-ion approximation (1.64 cm3 K/mol). The values χM T at 300 K for Nd(III)–morin and Nd(III)–QSA complexes amount to 1.30 and 1.67 cm3 K/mol, respectively. On the other hand, as the temperature decreases, the Kramers doublets are E(J) =

Fig. 5. Experimental data μeff vs. T for Pr(III) and Nd(III) complexes with morin and QSA.

successively depopulated. In the case of complex Nd(III)–morin at 1.9 K (χM T) is equal to 0.55 cm3 K/mol. Hence, the χM T value is twice as small as that at room temperature, which is consistent with only the lowest Kramers doublet being populated [30]. For Sm(III) the 6 H5/2 ground state is split into three Kramers doublets, which can be a reason for the lower than expected value χM T at low temperature. In the case of Ce(III), Er(III) and Yb(III) ions deviation from the Curie–Weiss law may result from Kramers degeneration of lanthanides levels, too. At room temperature the free-ion approximation to fit the experimental data is fairly good. The values of the magnetic moments correspond to the number of unpaired electrons of free lanthanide ions. Hence, the oxidation state of metal ion in the complex does not change and equals +3. The metal–ligand bond in the obtained complexes is mainly electrostatic with some degree of covalency [27,37,38]. The deviation from the Curie–Weiss law at low temperature is consequence of weak influence of the crystalline field on the central ions of the studied compounds. 4. Conclusion This paper is devoted to the physicochemical properties of complexes of morin and QSA with lanthanides(III). One of the goals was to see to what extent the free-ion approximation is appropriate to interpret the magnetic properties of rare earth metal–flavonoid complexes. We found that at room temperature the obtained magnetic data have been satisfactory interpreted in the free-ion approximation. Moreover, the magnetic measurements permitted us to determine unequivocally the oxidation state of metal ion in the investigated compounds, which is the crucial problem in flavonoids complexes chemistry, for the sake of reduction properties of flavonoids. Furthermore, the obtained results confirmed that flavonoids are the ligands of weak field. References [1] Z. Jerzmanowska, Wiad. Chem. 27 (1973) 623 (in Polish). [2] M. Kopach, D. Novak, Zh. Obshch. Khim. 61 (1991) 1361. [3] M. Kopach, S. Kopach, E. Skuba, Zh. Obshch. Khim. 74 (2004) 1035.

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