Inorganica Chimica Acta 359 (2006) 4386–4392 www.elsevier.com/locate/ica
Note
Crystal structures, spectral and magnetic properties of cobalt(II) pyridinecarboxylates: A novel polymeric chain in {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n Dusˇan Miklosˇ a
a,*
, Jana Jasˇkova´ a, Peter Segl’a a, Maria Korabik b, Jerzy Mrozinski b, Reijo Sillanpa¨a¨ c, Masahiro Mikuriya d, Milan Melnı´k a
Department of Inorganic Chemistry, Slovak Technical University, Radlinske´ho 9, SK 812 37 Bratislava, Slovakia b Faculty of Chemistry, University of Wroclaw, F. Joliot-Curie 14, 50 383 Wroclaw, Poland c Department of Chemistry, University of Jyva¨skyla¨, FIN-40351 Jyva¨skyla¨, Finland d School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda 669-1337, Japan Received 31 January 2006; received in revised form 16 May 2006; accepted 5 July 2006 Available online 11 July 2006
Abstract Synthesis and characterization of two new cobalt(II) complexes, namely monomeric [Co(2-MeSnic)2(H2O)4] Æ 4H2O (2-MeSnic is 2methylthionicotinate) and polymeric {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n (2,6-(MeO)2nic is 2,6-dimethoxynicotinate), are reported. The characterizations were based on elemental analysis, infrared and electronic spectra as well as magnetic measurements. Crystal structures of both complexes have been determined. In both of them – ([Co(2-MeSnic)2(H2O)4] Æ 4H2O and {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n) – the CoII atom is sixcoordinated. In the 2nd complex, there are two nonequivalent CoII central atoms, involved in forming a linear polymeric chain with alternating cationic and neutral part. One of them is octahedrally coordinated by a carboxyl oxygen atom of 2,6-(MeO)2nic, two water molecules and the corresponding centrosymmetrically located atoms. The second CoII atom is also octahedrally coordinated by six water molecules. Both coordination polyhedra are bridged by a water molecule. The charge of the cationic part is compensated for by two independent anionic 2,6-(MeO)2nic units. The structure is held together by a complicated system of hydrogen bonds. 2006 Elsevier B.V. All rights reserved. Keywords: Crystal structure; Pyridinecarboxylate complexes; Cobalt(II) complexes; Polymeric complexes; Spectral and magnetic properties
1. Introduction Copper(II) and cobalt(II) complexes of carboxylato ligands have been the subject of a large number of research studies as pyridinecarboxylic acids and their derivatives play a significant role in medicine and their copper(II) and cobalt(II) complexes are of interest both from chemical and biological point of view. A carboxylate ion, RCOO,
*
Corresponding author. Tel.: +421 2 59325189; fax: +421 2 52493198. E-mail address:
[email protected] (D. Miklosˇ).
0020-1693/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.07.002
can coordinate to metals in a number of ways, namely as a unidentate ligand, as a chelating ligand, as a bridging bidentate ligand in syn–syn, syn–anti or anti–anti configuration, or as a monoatomic bridging ligand, either alone, with additional bridging, or in arrangements involving chelation and bridging [1,2]. Preparation, spectral properties, magnetic properties and crystal structure of CuII and CoII complexes [MX2(H2O)4] (X = nicotinate – nic or isonicotinate – isonic), [Cu(2-MeSnic)2(H2O)]2 (2-MeSnic = 2-methylthionicotinate) and [Cu{2,6-(MeO)2nic}2(H2O)]2 (2,6-(MeO)2nic = 2,6-dimethoxynicotinate) as well as of their adducts with chelating
D. Miklosˇ et al. / Inorganica Chimica Acta 359 (2006) 4386–4392
and N-heterocyclic ligands have been described in several previous papers [3–8]. In this paper, we describe the synthesis, spectral properties and crystal structures as well as magnetic measurements of mononuclear [Co(2-MeSnic)2(H2O)4] Æ 4H2O and polynuclear {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(lH2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n.
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2. Experimental
Quantum Design SQUID Magnetometer (type MPMSXL5). Corrections for diamagnetism of the constituting atoms were calculated using the Pascal constants [9], the value of 150 · 106 cm3 mol1 was used as the temperature-independent paramagnetism of cobalt(II) ion. The effective magnetic moments were calculated from the expression pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi leff ¼ 2:83 vcorr m T ðBMÞ:
2.1. Chemical reagents, analysis and physical measurements
2.2. Crystallography
All the chemicals used were of reagent grade (Aldrich or Sigma) and used without further purification. All organic reagents were purchased from Aldrich; their purity was checked by IR spectra. Cobalt was determined by electrolysis of water solutions obtained by the sample mineralisation (acid digestion) with a mixture of sulfuric acid and potassium peroxodisulfate; carbon, hydrogen, nitrogen and sulfur were determined by microanalytical methods (Thermo Electron Flash EA 1112). Electronic spectra (9000–50 000 cm1) of the powdered samples in nujol mulls were recorded at room temperature (r.t.) on a Specord 200. IR spectra in the region of 400– 4000 cm1 were recorded on a Magna 750 spectrometer at r.t. Spectra of the solid samples were obtained in nujol mulls and KBr pellets (1 wt%). Magnetic susceptibility measurements in the temperature range of 1.8–300 K were carried out on powdered samples of the complexes, at the magnetic field of 5 kG, using a
Crystal data collection procedures, and refinement results for the complexes [Co(2-MeSnic)2(H2O)4] Æ 4H2O and {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(lH2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n are given in Table 1. Data collection and cell refinement were carried out using Nonius CAD-4 diffractometer. The diffraction intensities were corrected for Lorentz, polarization and absorption effects. The structures of the complexes were solved with SHELXS-97 [10] using direct methods, while further refinement with full-matrix least-squares on F2 was carried out with SHELXL-97 [11]. Geometrical analysis was performed using SHELXL-97 [11]. The structure of the complexes was drawn by ORTEP-3 [12] (Figs. 1 and 2). Selected bond distances and angles for [Co(2-MeSnic)2(H2O)4] Æ 4H2O and {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n are given in Tables 2 and 3, respectively.
Table 1 Crystal data and structure (MeO)2nic}2 Æ 6H2O}n
refinement
for
[Co(2-MeSnic)2(H2O)4] Æ 4H2O
and
{[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-
Compound
[Co(2-MeSnic)2(H2O)4] Æ 4H2O
{[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n
Empirical formula Formula weight Crystal system, space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) Z, volume (A 3 Dcalc (Mg m ) l (mm1) F(0 0 0) Diffractometer ˚) Radiation type Mo Ka, k (A Temperature (K) Reflections collected/unique Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices (I > 2r(I)) ˚ 3) Largest difference peak and hole (e A
C14H28CoN2O12S2 539.44 triclinic, P 1
C32H60Co2N4O30 1098.70 triclinic, P 1
7.3182(2) 7.4967(4) 11.2936(5) 79.080(2) 73.418(3) 68.903(2) 1, 551.41(4) 1.624 1.030 281 Nonius CAD-4 0.71073 293(2) 5132/1879 full matrix, least-squares on F2 1879/0/198 1.102 R = 0.0273, Rw = 0.0664 0.319 and 0.396
7.6184(2) 8.2028(2) 19.1793(7) 95.049(1) 95.899(1) 107.527(2) 1, 1127.89(6) 1.618 0.840 574 Nonius CAD-4 0.71073 173(2) 10 335/3838 full-matrix, least-squares on F2 3838/0/422 1.096 R1 = 0.0309, wR2 = 0.0768 0.603 and 0.584
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D. Miklosˇ et al. / Inorganica Chimica Acta 359 (2006) 4386–4392 Table 2 ˚ ) and angles () for [Co(2-MeSnic)2(H2O)4] Æ Selected bond lengths (A 4H2O
Fig. 1. Molecular structure of [Co(2-MeSnic)2(H2O)4] Æ 4H2O (only two independent four uncoordinated water molecules are shown).
2.3. Preparation of the complexes The aqueous solutions of 2-MeSnic (or 2,6-(MeO)2nic) and equimolar quantity of sodium hydroxide (4 mmol) were slowly added to aqueous solutions (40 cm3) of cobalt(II) sulfate and refluxed for 5 h. Well-shaped pink crystals of [Co(2-MeSnic)2(H2O)4] Æ 4H2O or {[{2,6-(MeO)2nic}2(H 2O) 2Co(l-H 2O)Co(H 2O) 4(l-H 2 O)]{2,6-(MeO) 2nic} 2 Æ 6H2O}n, suitable for X-ray structure analysis, were collected after two weeks by filtration, washed with ethanol and finally dried in vacuo. Anal. Calc. for [Co(2-MeSnic)2(H2O)4] Æ 4H2O (C14H28CoN2O12S2): C, 31.17; H, 4.86; N, 5.19; Co, 10.92. Found: C, 31.54; H, 5.26; N, 5.24; Co, 10.77%. Selected IR data (cm1): 1588s (mas(COO) + m(C@N)), 1380vs (ms(COO), D = mas(COO) ms(COO = 208), 599m (d(py), pyridine ring in-plane bending). Electronic data (cm1): 20 800sh, 19 300 (m3), 8300br (m1). Anal. Calc. for {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O) Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n (C32H60Co2N4O30): C, 34.98; H, 5.50; N, 5.10; Co, 10.73. Found: C, 35.31; H, 5.40; N, 4.89; Co, 10.78%.
Co1–O3 Co1–O4 Co1–O6
2.0663(14) 2.1846(15) 2.0866(16)
O3–C16 O7–C16 C16–C10 C10–C12 C12–C11 C11–C15 C15–N9 N9–C13 C13–C10 C13–S2 S2–C14
1.264(3) 1.258(3) 1.505(3) 1.388(3) 1.387(3) 1.375(3) 1.342(3) 1.340(3) 1.414(3) 1.764(2) 1.801(2)
O3–Co1–O4 O3–Co1–O6 O4–Co1–O6 O3–Co1–O4#1 O3–Co1–O6#1 O3–Co1–O3#1
93.42(6) 90.16(7) 89.86(6) 86.58(6) 89.84(7) 180.0
O3–C16–O7 C10–C16–O3 C10–C16–O7 C10–C13–S2 N9–C13–S2 C13–S2–C14
125.36(19) 116.86(18) 117.73(18) 119.39(15) 118.41(15) 102.15(10)
Symmetry transformations used to generate equivalent atoms: #1 x + 1, y + 1, z + 1.
Selected IR data (cm1): 1635 s, 1601s (mas(COO) + m(C@N)), 1398vs (ms(COO), D = mas(COO) ms(COO ) = 203 and 237), 590m (d(py), pyridine ring inplane bending). Electronic data (cm1): 22 200sh, 18 700 (m3), 8500br (m1). 3. Results and discussion 3.1. Crystal structure of [Co(2-MeSnic)2(H2O)4] Æ 4H2O [Co(2-MeSnic)2(H2O)4] Æ 4H2O (Fig. 1) crystallizes in the triclinic system, space group P 1, unit cell dimensions ˚, a= a = 7.3182(2), b = 7.4967(4), c = 11.2936(5) A 79.080(2), b = 73.418(3), c = 68.903(2), Z = 1. In the molecular complex, the Co(II) central atom, located at a
Fig. 2. Molecular structure of {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n, including the hydrogen bond system from symmetry independent atoms. For clarity, only one of two uncoordinated {2,6-(MeO)2nic} is shown.
D. Miklosˇ et al. / Inorganica Chimica Acta 359 (2006) 4386–4392 Table 3 ˚ ) and angles () for {[{2,6-(MeO)2nic}2(H2O)2Selected bond lengths (A Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n Co1–O1 Co1–O5 Co1–O7
2.2590(17) 2.0243(17) 2.0691(16)
Co2–O1 Co2–O2 Co2–O3
2.2411(17) 2.0748(19) 2.0241(18)
C18–O7 C18–O9 C18–C25 C25–C27 C27–C26 C26–C23 C23–N16 N16–C24 C24–O13 O13–C32 C23–O14 O14–C31
1.276(3) 1.257(3) 1.488(3) 1.395(4) 1.376(4) 1.388(4) 1.325(3) 1.336(3) 1.354(3) 1.432(3) 1.358(3) 1.437(3)
C20–O4 C20–O10 C20–C17 C17–C29 C29–C21 C21–C22 C22–N15 N15–C19 C19–O8 O8–C28 C22–O12 O12–C30
1.260(3) 1.273(3) 1.497(4) 1.397(4) 1.380(4) 1.380(4) 1.329(3) 1.332(3) 1.355(3) 1.433(3) 1.358(3) 1.436(3)
Co1–O1–Co2
131.40(8)
O1–Co1–O5 O1–Co1–O7 O5–Co1–O7 O1–Co1–O5#1 O1–Co1–O7#1 O5–Co1–O7#1
86.39(7) 94.51(7) 87.80(7) 93.61(7) 85.49(7) 92.20(7)
O1–Co2–O2 O1–Co2–O3 O2–Co2–O3 O1–Co2–O2#2 O1–Co2–O3#2 O2–Co2–O3#2
95.96(7) 95.30(8) 90.34(8) 84.04(7) 84.70(7) 89.66(8)
O7–C18–O9 O7–C18–C25 O9–C18–C25 C24–O13–C32 C23–O14–C31
123.6(2) 115.6(2) 120.7(2) 118.1(2) 116.8(2)
O4–C20–O10 O4–C20–C17 O10–C20–C17 C19–O8–C28 C22–O12–C30
122.5(2) 120.4(2) 117.0(2) 117.8(2) 116.9(2)
Symmetry transformations used to generate equivalent atoms: #1 x, y, z + 1; #2 x, y + 1, z + 1.
symmetry center, is octahedrally coordinated by an oxygen atom of the unidentate 2-MeSnic carboxyl group at a dis˚ , two water molecules (Co1–O4 tance Co1–O3 of 2.066 A ˚ and Co1–O6 2.087 A ˚ ) and the corresponding cen2.185 A trosymmetrically located atoms. The second carboxyl oxygen atom O7 at a considerably greater distance from Co1 ˚ is involved in a system of hydrogen bonds of 3.329 A including uncoordinated water molecules. 3.2. Crystal structure of {[{2,6-(MeO)2nic}2(H2O)2Co(lH2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n The complex {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n (Fig. 2) crystallizes also in the triclinic system, group P 1, unit cell dimensions a = 7.6184(2), b = 8.2028(2), c = 19.1793(7) ˚ , a = 95.0490(10), b = 95.8990(10), c = 107.527(2), A Z = 1. There are two nonequivalent CoII central atoms, both located at nonequivalent symmetry centers. One of them (Co1) is octahedrally coordinated by an oxygen atom of the carboxyl group of 2,6-(MeO)2nic, two water molecules (O1 and O5) and the corresponding centrosymmetrically located atoms. The second CoII atom (Co2) is also
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octahedrally coordinated, in this case by six water molecules (O1, O2, O3 and centrosymmetricals). Both CoII central atoms are bridged by a water molecule (O1), giving rise to a polymeric chain, in which a neutral part (around Co1) alternates with a cationic part (around Co2). The charge of the cationic part is compensated for by two independent anionic 2,6-(MeO)2nic units (only one of them is shown in Fig. 2). The presence of hydrogen atoms at the bridging O1 and thus the presence of bridging water has been confirmed by the corresponding maxima found in the difference Fourier map, while the existence of deprotonated 2,6-(MeO)2nic moieties has been confirmed by non-presence of the corresponding maxima at the relevant carboxyl oxygen atom. These facts have also been supported by spectral measurements (see Section 3.3). There are several polymeric Co(II) pyridinecarboxylate complexes known which contain bridging water in their structure. Cobalt complexes of the composition {[L2(H2O)2Co(l-H2O)CoL2(H2O)2(l-H2O)] Æ xH2O}n (where L is 3-hydroxy-4-methoxybenzoate [13], 2,6-dimethoxybenzoate [14], as well as hippurate [15], and x = 0–2) exist as one-dimensional polymers in which the cobalt atoms are linked to each other by bridging water molecules (the ˚, intra-chain Co–Co distances are 4.082, 3.97 and 3.996 A respectively) in a linear infinite chain. The Co(II) atom is six-coordinated, the equatorial ligands being two mutually trans water molecules and two unidentate carboxylate groups of the above carboxylate anions. Water-bridged polymeric structures have also been found in ternary Cu(II) complexes of acetate or N-protected amino acids with nitrogen basis [16] and Ni(II) and Fe(II) hippurate complexes [15,17]. In bis-(4-methoxybenzoate)Co(II) trihydrate [18], there are even two bridging water molecules between the central Co atoms, but also a bidentate carboxylic group acts as a bridge between the same Co atoms. While in the latter complexes the same complex moieties bridged by a water molecule are repeated in the polymeric chain, in pale-rose adipate compound {[(l-C6H8O4)4/2 Co(l-H2O)Co(H2O)4)(l-H2O)]}n [19], there are Co(lC6H8O4)4/2 complex units alternating with (l-H2O)Co(H2O)4)(l-H2O) complex units, forming a linear chain. Both central Co atoms (Co1 and Co2) are octahedrally coordinated, Co1 by the O atoms of two bridging trans-H2O molecules and four bidentate adipate anions, and Co2 by the O atoms of two bridging trans-H2O molecules and four monodentate H2O molecules (Co2), respectively. Thus, this kind of polymeric chain is similar to that of {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n. Nevertheless, in the adipate complex, there are alternating two neutral complex moieties in the chain and, moreover, these chains are mutually bridged by the adipates forming a supramolecular layer. Therefore, {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n is the first Co(II) complex known to the authors containing polymeric chains, in which cationic and neutral
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complex moieties are alternating and the charge of the chain is compensated for by deprotonated pyridinecarboxylate units. The structure of {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n is stabilized by an extensive and complicated system of hydrogen bonds (Fig. 2). There is just one ‘‘intramolecular’’ hydrogen bond between the bridging water (oxygen atom O1) and uncoordinated carboxyl oxygen atom (O9) of the coordinated 2,6dimethoxynicotinate. With the distance O1. . .O9 of ˚ it is the strongest hydrogen bond in the structure. 2.594 A The second hydrogen atom of the bridging water is bonded ˚. to a free water molecule O11 with O1. . .O11 of 2.875 A Oxygen O9 is also H-bonded to free water molecules O15 ˚ and O6 with O9. . .O6 of with O9. . .O15 of 2.833 A ˚ 2.890 A. The uncoordinated 2,6-dimethoxynicotinate is anchored in the structure by strong hydrogen bonds between its carboxyl oxygen atoms (O4 and O10) and coordinated water ˚ and two molecules O3 at Co2 with O4. . .O3 of 2.665 A ˚ and O10. . .O5 of O5 at Co1 with O10. . .O5 of 2.678 A ˚ for O5 from two neighboring chains. Correspond2.787 A ingly, the O5 water molecule, coordinated on Co1, acts as ‘‘bridging’’ by hydrogen bonding between two uncoordinated 2,6-dimethoxynicotinates, fixing them relative to the polymeric chain. Carboxyl oxygen O4 is also hydrogen-bonded to a free water molecule O6 with O4. . .O6 of ˚ and to O15 with O4. . .O15 of 2.828 A ˚ , and O10 2.822 A ˚. to a free water molecule O11 with O10. . .O11 of 2.771 A Also methoxy oxygen O8 is hydrogen-bonded to a free ˚. water molecule O15 with O8. . .O15 of 2.907 A Water molecules O2, O3, coordinated to Co2, are also involved in hydrogen bonds: O3, besides the above mentioned O4. . .O3 bond, in an H-bond to O11 with ˚ , and O2 in an H-bond to the carboxyl O3. . .O11 of 2.799 A oxygen O7, coordinated to Co1 within the same chain, with ˚ , and in a strong H-bond to the free O2. . .O7 of 2.903 A ˚. molecule O15 with O2. . .O15 of 2.662 A There is also a hydrogen bond between free water mole˚ and relatively cules O6 and O11 with O6. . .O11 of 2.823 A weak H-bonds of the methoxy oxygen O13 to free water ˚ and molecules O6 and O15 with O13. . .O15 of 3.097 A ˚. O13. . .O6 of 3.257 A 3.3. Spectroscopic data All typical features of IR spectra are clearly compatible with the structural characteristics of the complexes under study. The IR spectra of both complexes show one or two absorption bands in the region from 3200 to 3500 cm1. These bands correspond to the antisymmetric and symmetric OH stretch and confirm the presence of water in the compounds. The absence of the carbonyl peaks (at about 1680 cm1) and other characteristic peaks for free acids indicates that 2-methylthionicotinic acid (2-MeSnicH) or 2,6-dimethoxynicotinic acid (2,6-(MeO)2-
nicH) are present either coordinated or uncoordinated in the complexes in deprotonated form only. Moreover, the broad bands assigned to antisymmetric stretching vibration mas(COO) and symmetric stretching vibration ms(COO) of the carboxyl group for the complexes are in the expected regions at about 1600 and 1400 cm1, respectively [20] (see Section 2.3). The difference between the antisymmetric stretch and symmetric stretch gives information on carboxylic bonding mode for the complexes after comparison with D values of compounds with ionic carboxylic groups (for 2,6-(MeO)2nicNa, the D value is 203 cm1 and 197 cm1 for 2-MeSnicNa). The greater D values 237 cm1 for complex {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(lH2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n and 208 cm1 for complex [Co(2-MeSnic)2(H2O)4] Æ 4H2O, respectively, in comparison with 2,6-(MeO)2nicNa and 2-MeSnic alone are typical for unidentately bonded carboxylic groups of pyridinecarboxylate. On the other hand, the splitting of the band assigned to mas(COO) in IR spectra of complex {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2, 6-(MeO)2nic}2 Æ 6H2O}n (D value is 203 cm1) is rather typical for ionic bonded carboxylic groups of 2,6-(MeO)2nic. Non-coordination of the nitrogen atom of the pyridine ring of 2,6-(MeO)2nic or 2-MeSnic anions in the complexes is in agreement with the positions of absorption bands at about 600 cm1, which are due to in-plane deformation of the uncoordinated pyridine ring [20]. The suggested unidentate O-coordination of the carboxyl group of 2,6-(MeO)2nic or 2-MeSnic as well as ionic carboxylic groups of 2,6-(MeO)2nic in the complexes under study are also in agreement with the crystal structure determined by X-ray analysis for both complexes. The electronic spectra of both complexes in the solid state (see Section 2.3) are clearly consistent with an octahedral or a tetragonally distorted octahedral structure [21]. The electronic spectra consist of a band m1 assigned to the lowest energy transition 4T1g ! 4T2g in the near infrared and the band m3 in the visible region near 20 000 cm1 which is assigned to the 4T1g ! 4T2g(P) transition. The visible band m3 has a shoulder or fine structure to it. The expected band m2 in the visible region assigned to the 4T1g ! 4A2g transition (band m2 is completely overlapped by the multiple m3 band) has not been observed (neither as a shoulder). Whilst the first two transitions have the normal intensities of octahedral spin allowed Laporte forbidden bands, the latter transition involves a two-electron process for strong fields and is expected to be very much weaker and very often not observed at all [21,22].
3.4. Magnetic data The magnetic properties of both complexes ([Co(2-MeSnic)2(H2O)4] Æ 4H2O and {[{2,6-(MeO)2nic}2(H2O)2Co(lH2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n) in the form of vm and vmT (vm is the molar susceptibility for CoII center) are shown in Figs. 3–5.
D. Miklosˇ et al. / Inorganica Chimica Acta 359 (2006) 4386–4392 4.0
1.2
3.5
1.0
3.0
χ [cm3mol-1] m
2.5
0.6
2.0 1.5
0.4
1.0
χm T [cm3Kmol-1]
0.8
0.2 0.5 0.0
0.0 0
50
100
150
200
250
300
T [K]
Fig. 3. Temperature dependence of magnetic susceptibility vm (s) and vmT (d) for complex [Co(2-MeSnic)2(H2O)4] Æ 4H2O.
0.7 0.6
0.6
0.3
χ [cm3mol-1] M
-1
0.4
3
χ M [cm mol ]
0.5
0.2
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T [K] Fig. 4. Temperature dependence of magnetic susceptibility vm (d) and vm in the temperature range from 1.8 to 20 K (s) for complex {[{2,6(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n.
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χ T [cm Kmol ] m
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T [K]
Fig. 5. Temperature dependence of magnetic susceptibility vmT (d) for complex {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6(MeO)2nic}2 Æ 6H2O}n.
In the magnetic field 0.5 T, complex [Co(2-MeSnic)2(H2O)4] Æ 4H2O behaves as a paramagnetic one. The value of the vmT at 300 K is 3.16 cm3 K mol1 (leff per CoII center equals 5.03 BM), which is larger than that expected
4391
for the spin-only value of high-spin CoII, 3 1 vmT = 1.87 cm K mol S = 3/2 (leff = 3.87 BM). It indicates that an important orbital contribution is involved. The vmT values continuously, but weakly decrease with lowering temperature, to the value 2.00 cm3 K mol1 at 1.8 K (leff = 4.00 BM). The values of Curie (C) and Weiss (h) constants for the higher temperature range 50–300 K are equal to 3.27 cm3 K mol1 and 6.1 K, respectively. The absence of a maximum in vm curve indicates that the possible antiferromagnetic interaction is very weak. Crystal structure shows that molecules of this complex are isolated in the crystal structure, suggesting that weak decrease of magnetic moment with lowering temperature and negative Weiss constant are rather resulting from zero-field splitting and orbital contribution than antiferromagnetic coupling in this case [23]. The cobaltous ion is generally one of the more difficult of 3d transition metal ions to describe magnetism because of its unquenched orbital angular momentum [16]. At higher temperatures, population of the higher-energy Kramers doublets becomes significant, and the magnetic interpretation is more difficult [23,24]. In the case of polymeric complex {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n, the value of the vmT product at 300 K at a magnetic field 0.5 T (Fig. 5) is 2.82 cm3 K mol1 (leff per CoII center equals 4.75 BM). This value substantially exceeds the spin-only value for a high spin cobalt(II) (S = 3/2, 1.875 cm3 K mol1 with g = 2) but is close to the value expected when the spin momentum and the angular momentum exist independently (3.37 cm3 K mol1 [23–26]. This is indicative of an unquenched orbital contribution, typical of the 4T1g ground state. The vmT values gradually decrease with the temperature to the minimum value 1.49 cm3 K mol1 at 18 K (leff = 3.46 BM), then increase with lowering temperature and reach a maximum of 2.78 cm3 K mol1 (leff = 4.68 BM) at 6 K. In the lowest temperature region they again decrease to the value of 1.16 cm3 K mol1 at 1.8 K (leff = 3.04 BM). The high-temperature behavior (>18 K) is typical of a magnetically isolated six-coordinated high-spin cobalt (II) ion with spin orbit coupling in the 4T1g ground state [25]. Increase of magnetic moment below 18 K suggests intramolecular ferromagnetic interaction between cobalt ions in the polymeric chain. From the structural information, we know that the complex is polymeric and there are two nonequivalent CoII central atoms, both located at nonequivalent symmetry centers. There is no mirror plane between neighbours, the Dzialoshinsky–Moriya exchange mechanism operates and the structure is canted, giving rise to weak ferromagnetic behaviour [26]. However, the crucial factor determining ferromagnetic exchange is the orthogonality of adjacent magnetic orbitals [23,25]. The bridging water molecule (Co1–O1–Co2 angle equals 131.40(8) [] ) is situated to align CoII ions ( Co1–Co2 distance along the poly˚ ) in such a way that their meric chain is equal to 4.101 A magnetic orbitals are approximately orthogonal. These
D. Miklosˇ et al. / Inorganica Chimica Acta 359 (2006) 4386–4392
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www.ccdc.ac.uk. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2006.07.002.
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M 1 [βmol-1]
References
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H [Oe]
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Fig. 6. Field dependence of magnetization at 1.8 K (d) and 5 K (s) for complex {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6(MeO)2nic}2 Æ 6H2O}n.
two factors determine the observed ferromagnetic interactions. Test for long-range ferromagnetic order in very small fields of 50 Oe proved negative. High field magnetization isotherms, shown in Fig. 6, indicate that saturation does not occur till 2 K and the maximum value of magnetization at 2 K for magnetic field of 5 T is 1.29 Nb, which is smaller than the Ms expected for S = 3/2 system with g = 2 (3 Nb) , and smaller than the value usually observed for CoII complexes (2.20 Nb) too [25]. Rapid decrease of vmT below 6 K suggests the possibility of weak interchain antiferromagnetic interactions through H-bonds system in the crystal structure. Acknowledgements This work was financially supported by Grant 1/2452/05 of the Slovak Grant Agency for Science and Science and Technology Assistance Agency under the contract No. APVT-20-005504, by Grant 3T069A 039915 of the State Committee for Scientific Research (Poland) and by Grants-in-Aid for Scientific Research No. 14540516 (Japan). Appendix A. Supplementary material Crystallographic data for the structure analysis have been deposited with the Cambridge Crystallographic Data Center, CCDC No. 296403 for compound [Co(2-MeSnic)2(H2O)4] Æ 4H2O and CCDC No. 296402 for compound {[{2,6-(MeO)2nic}2(H2O)2Co(l-H2O)Co(H2O)4(l-H2O)]{2,6-(MeO)2nic}2 Æ 6H2O}n. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK, fax: +44 1223 336 033, e-mail:
[email protected]: or www.http:/
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