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Cent. Eur. J. Chem. • 12(6) • 2014 • 652-658 DOI: 10.2478/s11532-014-0517-3 Central European Journal of Chemistry Unusual potassium-oxalate coordination in the two-dimensional trimetallic [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O complex Short Communication Grzegorz Wrzeszcz*, Andrzej Wojtczak, Magdalena Zawadzka Faculty of Chemistry, Nicolaus Copernicus University, 87-100 Toruń, Poland Received 20 June 2013; Accepted 2 January 2014 Abstract: A new heterometallic compound, [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1), has been synthesized and characterized by elemental analysis, IR and electronic spectra, thermal analysis, variable temperature magnetic susceptibility measurements, and single crystal X-ray diffraction. Compound 1 consists of two-dimensional [{KCr(C2O4)3}n]2n− layers, [CoCl(NH3)5]2+ ions and water molecules. Within the 2-D layer, three different types of oxalate coordination modes are present. Each K cation is coordinated by eight oxygen atoms from oxalate groups and also weakly interacts with the ninth oxygen atom. The extensive network of hydrogen bond is formed between the [KCr(C2O4)3]2– layer and the [CoCl(NH3)5]2+ ions. These interactions involve all hydrogen atoms of ammonia ligands and water molecule. Keywords: Crystal structure • Oxalate bridge • Coordination mode • Spectroscopy • Hydrogen bonding © Versita Sp. z o.o. 1. Introduction The tris(oxalato) complexes [MIII(C2O4)3]3− (M = Cr, Fe, Co, ...) have received considerable interest in the last two decades. They have been used in the fields of (i) molecular organic/inorganic hybrid materials with organic π-donors [1-3], (ii) molecular based magnetic materials as building block to prepare of poly- and bimetallic compounds [4-6], (iii) magneto- and non-linear optics [7,8], and (iv) microporous materials [9]. Special attention is dedicated to polyfunctional oxalate bridged materials [1-3,7,10]. The oxalate anion is a very versatile ligand that can adopt many kinds of coordination modes: unidentate, bidentate chelate to one metal centre and bridging [11-13]. The remarkable ability of the oxalate as a bridging ligand has played a key role in the development of new coordination compounds, which have been of high interest not only due to their properties and potential applications but also due to their fascinating topologies and intriguing structure features. As a continuation of our studies on oxalate complexes [14-16], in this paper, we describe the synthesis, X-ray structure, IR and electronic spectra, thermal, and magnetic characterization of new two-dimensional heterometallic K(I)-Cr(III)-Co(III) complex derived from tris(oxalato) chromate(III), namely [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O. 2. Experimental procedure 2.1. Materials The compounds K3[Cr(C2O4)3]•3H2O [17] and [CoCl(NH3)5] Cl2 [18] were prepared according to the literature methods. Other reagents used in the syntheses were of analytical grade and used without further purification. 2.2. Physical measurements Elemental analyses (C, H, N) were carried out with a Perkin Elmer Analyzer Model 240. Chromium content was determined spectrophotometrically as CrO42− at λ = 372 nm. IR spectra were recorded on a Perkin Elmer FT-IR 2000 spectrophotometer in the 4000–400 cm−1 region using the KBr discs and from 700 to 30 cm−1 using polyethylene plates techniques. Electronic spectra were measured on a SPECORD M-40 (Carl Zeiss, * E-mail: [email protected] 652 Unauthenticated Download Date | 6/16/17 8:30 AM G. Wrzeszcz, A. Wojtczak, M. Zawadzka Table 1. Crystal data and structure refinement parameters for [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1). Empirical formula Formula weight C6H16N5O12.5ClCoCrK 543.72 2192 F(000) 0.32×0.27×0.14 mm Crystal size Theta range for data collection Temperature 293(2) K Wavelength 0.71073 Å Index ranges Monoclinic Reflections collected / unique Crystal system Space group C2/c −33<=h<=34, −10<=k<=10, −28<=l<=23 Completeness to 2θ = 52 Unit cell dimensions Absorption correction A 23.9750(10) Å Max. and min. transmission B 7.4020(10) Å Refinement method C 20.0460(10) Å Data / restraints / parameters β 93.85(1) deg Goodness-of-fit on F2 Volume 3549.4(5) Å Z Calculated density Absorption coefficient 3 8 2.035 Mg m –3 2.57 to 31.26 deg 16647 / 5399 [R(int) = 0.0418] 99.9% Analytical 0.7665 and 0.5661 Full-matrix least-squares on F2 5399 / 0 / 257 1.083 Final R indices [I>2σ(I)] R1 = 0.0393, wR2 = 0.0927 R indices (all data) R1 = 0.0511, wR2 = 0.0983 Largest diff. peak and hole 0.725 and −0.661 e Å−3 2.006 mm−1 Jena) spectrophotometer. The magnetic susceptibility measurements were performed between 78–300 K by Faraday method on the balance constructed in our laboratory at field strength of 1.0 T. The magnetic field was calibrated with Hg[Co(NCS)4] [19]. The molar susceptibilities were corrected for diamagnetism using the Pascal’s constants (–215×10−6 cm3 mol−1) [20] and temperature independent paramagnetism for Co(III) (100×10−6 cm3 mol−1) [21]. The effective magnetic moments were calculated from the equation: µeff = 2.828(χMcorr ·T)1/2. 2.3. [CoCl(NH Synthesis )] ) ][KCr(C2O4of )3]•0.5H[CoCl(NH O synthesis 3 5 3 5 2 [KCr(C (1)2O4)3]•0.5H2O (1) Pentaamminechlorocobalt(III) chloride (0.25 g, 1 mmol) was dissolved in water (60 mL). In another beaker, potassium tris(oxalato)chromate(III) trihydrate (0.49 g, 1 mmol) was dissolved in water (20 mL) at room temperature. Both solutions were mixed, and the resulting solution was acidified by adding a few drops of 1 M HCl and left in refrigerator. After several days the dark red crystals suitable for the X-ray structure analysis were obtained. The crystals were filtered, washed with cold water and air-dried. Yield: 0.36 g (67%). Anal. Calcd. for C6H16N5O12.5ClCoCrK (1): C, 13.25; H, 2.97; N, 12.88; Cl, 6,52; Cr, 9.56. Found: C, 13.20; H, 3.37; N, 12.82; Cl, 6.32; Cr, 9.27%. IR (cm−1): νas(OH) 3609m, νs(OH) 3538m, νas(NH3) 3299s, νs(NH3) 3205s, νas(OCO) 1706s, 1685sh, 1659vs, 1626sh, νs(CO) + ν(CC) 1396vs, δs(NH3) 1334s, νs(CO) + δ(OCO) 1263s, ρr(NH3) 848m, ν(Cr-O) + δ(OCO) 800s, ν(Cr-O) + ν(CC) 542s, ring def. + δ(OCO) 484s, ν(Co-N) 470sh, ν(Cr-O) + ring def. 412s, δ(N-Co-N) 323s, ν(Co-Cl) 283s. λmax (nm (ε) in water): 698 (6), 553 (118), 421 (110), 364sh. 2.4. X-ray crystallographic data The diffraction data were measured for single crystal of a complex [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1) on an Oxford Sapphire CCD diffractometer using ω-2θ method. The complex crystallized in the monoclinic C2/c space group. The analytical absorption correction was applied [22]. The structure was solved by direct methods and refined with the full-matrix least-squares procedure using SHELX-97 package [23]. The hydrogen atoms were constrained during refinement. The details of data collection and refinement are presented in Table 1. CCDC 739373 contains the supplementary crystallographic data for [CoCl(NH3)5][KCr(C2O4)3]•0.5 H2O (1). These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223336-033; or e-mail: [email protected]. 3. Results and discussion 3.1. Synthesis and properties Pentaamminechlorocobalt(III) chloride and potassium tris(oxalato)chromate(III) were reacted in 1:1 molar ratio in aqueous medium forming a new two dimensional 653 Unauthenticated Download Date | 6/16/17 8:30 AM Unusual potassium-oxalate coordination in the two-dimensional trimetallic [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O complex complex (1), [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O. Addition of HCl to the reaction mixture is not crucial but increases yield a little. Although, the complex is a stable crystalline solid and can be stored in a dry atmosphere for extended periods of time, water molecules can be lost. Thermal decomposition of 1 was followed by combined DTA-TG under an air atmosphere. The liberation of ammonia molecules was accompanied by endothermic effects on the DTA curve (min.) at 270°C. Further heating above 300°C caused exothermic decomposition in one step similarly as in the parent complex, K3[Cr(C2O4)3] [24]. No attempt was made, however, to study the details of the pyrolysis process. Total decomposition occurs at ca. 450°C and leads to a mixture of KCl, Co3O4 and Cr2O3. 3.2. Spectroscopy In the electronic spectrum of 1, the strong absorption maxima were observed at 553 and 421 nm showing d-d transitions (4A2g→4T2g and 4A2g→4T1g) typical for Cr(III) octahedral complexes [25]. Position, especially of the first maximum is perturbed and molar absorption coefficients are somewhat higher than in parent [Cr(C2O4)3]3− [26]. It is an effect of superposition of Cr(III) and Co(III) bands. The former have molar absorption coefficients ca. 50% higher than the later, therefore dominate. Thus, 1A1→1E(a) and 1A1→1A2 Co(III) transitions (C4v symmetry) [25,27] overlap with 4A2g→4T2g Cr(III) transition and 1A1→1E(b) Co(III) transition occurring at 364 nm as a shoulder. Low intensity sharp band at 698 nm can be attributed to spin forbidden Cr(III) transitions (4A2g→2T2g,2Eg). The IR spectrum of 1 obviously present the characteristic absorptions of the oxalato bridged group, the coordinated ammonia, the metal-ligand, and the lattice water. For the oxalate ligand, various coordination modes via one, two, three or four oxygen atoms have been previously characterized by IR method [12,28,29]. The absorption bands which appear on the IR spectrum of the complex 1 near 1650 cm−1 and at 1396, 1263, 800, and 484 cm−1 are consistent with the presence of three different coordination modes of the oxalate ligand, in agreement with the X-ray diffraction study results (vide infra). For the coordinated ammonia, the bands at 3299, 3205, 1334 and 848 cm−1 are identified with antisymmetric and symmetric stretching, symmetric bending, and rocking vibrations, respectively [28]. The expected NH3 asymmetric deformation vibration bands are coincided on strong bands from oxalate ligand. It is worthy to notice that many other absorption bands are observed in the 3100–2100 cm−1 region and could be assigned to the ν(NH), (N-H•••O) vibrations. Figure 1. Asymmetric part of the structure of 1 with the atom numbering scheme. Thermal ellipsoids are plotted at 30% probability level. 3.3. Description of the structure The asymmetric unit of the structure contains two complex counterions, potassium cation and a water molecule positioned on the two-fold axis, therefore exhibiting the partial occupancy of 0.50. Therefore, only one water hydrogen atom was included in the final model, while the second H is related by the twofold symmetry. The asymmetric part of the structure with the atom numbering scheme is shown on Fig. 1. The selected bond distances and angles are listed in Table 2. The details of the H-bond network are presented in Table 3. The Co(III) coordination sphere in the complex cation is a slightly distorted octahedron with a chloride ion and five ammonia molecules. The Co-Cl distance is 2.2645(7) and the Co-N distances vary from 1.949(2) to 1.968(2) Å. The bond angles within the CoClN5 coordination sphere vary from 88.18(6) to 93.20(8) and from 177.77(11) to 178.44(7)o. Three oxalate ligands form an octahedral coordination sphere of Cr(III) in the complex anion. The Cr-O bond distances vary from 1.960(2) Å for Cr2-O21 to 1.980(2) Å for Cr2-O11. The O-Cr-O angles vary in a relatively broad range between 82.47(7) and 95.66(7)o while those between the oxygen atoms occupying the opposite corners of the polyhedron are from 172.44(7) to 173.98(7)o. The deviations from the expected 90/180 degrees reflect the tight five-membered chelate ring formed by each of the oxalate ligands. All three chelate rings are planar, the absolute values for the torsion angles varying between 0.1(2) and 0.6(3), 0.2(3) and 4.0(3), as well as 2.8(2) and 12.3(3)o for Cr2-O11-C1C2-O21, Cr2-O31-C3-C4-O41 and Cr2-O51-C5-C6O61 rings, respectively. The largest deviations from the ring planarity are detected for the ring formed by the C5-C6 oxalate ligand involved in five interactions to surrounding K1 ions. 654 Unauthenticated Download Date | 6/16/17 8:30 AM G. Wrzeszcz, A. Wojtczak, M. Zawadzka Table 2. Selected bond lengths and distances [Å], and angles [deg] for [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1). Co1-Cl1 2.2645(7) K1-O51 2.743(2) Co1-N2 1.968(2) K1-O22#2 2.751(2) Co1-N3 1.955(2) K1-O11#4 2.804(2) Co1-N4 1.965(2) K1-O52#5 2.965(2) Co1-N5 1.951(2) K1-O12#2 3.015(2) Co1-N6 1.949(2) K1-O61#4 3.133(2) K1-O62#5 3.182(2) Cr2-O11 1.980(2) K1-O31#4 3.370(2) Cr2-O21 1.960(2) K1-O52 3.187(2) Cr2-O31 1.977(2) Cr2-O41 1.969(2) Cr2-O11-K1#1 Cr2-O51 1.970(2) Cr2-O31-K1#1 Cr2-O61 1.979(2) Cr2-O51-K1 140.64(8) K1#3-O52-K1 149.97(7) 100.84(6) 84.01(6) N6-Co1-N5 89.64(11) N6-Co1-N3 90.69(11) O51-K1-O22#2 73.81(6) N5-Co1-N3 88.91(9) O51-K1-O11#4 160.51(5) N6-Co1-N4 177.77(11) O22#2-K1-O11#4 103.23(5) N5-Co1-N4 89.28(9) O51-K1-O52#5 123.71(6) N3-Co1-N4 91.24(10) O22#2-K1-O52#5 110.27(6) N6-Co1-N2 88.65(10) O11#4-K1-O52#5 N5-Co1-N2 93.20(8) O51-K1-O12#2 N3-Co1-N2 177.78(8) O22#2-K1-O12#2 N4-Co1-N2 89.47(9) O11#4-K1-O12#2 63.77(5) N6-Co1-Cl1 89.67(9) O52#5-K1-O12#2 130.43(6) N5-Co1-Cl1 178.44(7) O51-K1-O61#4 124.15(5) N3-Co1-Cl1 89.70(7) O22#2-K1-O61#4 160.43(5) N4-Co1-Cl1 91.45(7) O11#4-K1-O61#4 57.21(4) N2-Co1-Cl1 88.18(6) O52#5-K1-O61#4 68.02(5) O12#2-K1-O61#4 107.85(5) 75.63(5) 100.03(5) 57.40(5) O21-Cr2-O41 92.49(7) O51-K1-O62#5 O21-Cr2-O51 94.28(7) O22#2-K1-O62#5 60.85(5) O41-Cr2-O51 95.66(7) O11#4-K1-O62#5 106.22(5) O21-Cr2-O31 92.06(8) O52#5-K1-O62#5 53.80(5) O41-Cr2-O31 82.61(7) O12#2-K1-O62#5 111.11(5) O51-Cr2-O31 173.49(7) O61#4-K1-O62#5 121.75(5) O21-Cr2-O61 173.98(7) O51-K1-O52 O41-Cr2-O61 92.94(8) O22#2-K1-O52 91.46(6) O51-Cr2-O61 82.59(7) O11#4-K1-O52 155.20(5) O31-Cr2-O61 91.21(7) O52#5-K1-O52 80.60(3) O21-Cr2-O11 82.47(7) O12#2-K1-O52 140.25(5) O41-Cr2-O11 172.44(7) O61#4-K1-O52 107.10(5) O51-Cr2-O11 90.35(7) O62#5-K1-O52 63.64(5) O31-Cr2-O11 91.91(7) O51-K1-O31#4 109.88(5) 118.61(5) O61-Cr2-O11 89.36(5) 43.32(5) 92.37(7) O22#2-K1-O31#4 O21-Cr2-K1#1 120.06(5) O11#4-K1-O31#4 53.93(4) O41-Cr2-K1#1 132.51(6) O52#5-K1-O31#4 114.76(5) O51-Cr2-K1#1 113.51(5) O12#2-K1-O31#4 61.87(4) O31-Cr2-K1#1 64.13(5) O61#4-K1-O31#4 51.38(4) O61-Cr2-K1#1 57.25(5) O62#5-K1-O31#4 160.13(5) O11-Cr2-K1#1 47.69(5) O52-K1-O31#4 134.43(5) Symmetry transformations used to generate equivalent atoms: #1 x,y-1,z #2 -x+1/2,-y+1/2,-z #3 -x+1/2,y-1/2,-z+1/2 #4 x,y+1,z #5 -x+1/2,y+1/2,-z+1/2 Figure 2. The nine-coordination environment of potassium in complex 1. Atoms are displayed as gray, red, pink and blue ellipsoids for carbon, oxygen, chromium and potassium, respectively. The potassium K1 cation is found 3.7243(8) Å from Cr2[x,y+1,z] and interacts with nine oxygen atoms of neighbouring oxalates (Fig. 2, Table 2), the distances ranging from K1-O51 2.743(2) to K1-O52 3.187(2) Å and also contact K1-O31[x,y+1,z] of 3.370(2) Å. The latest is close to the K-O distance of 3.5120(3) Å as reported by Nelyubina et al. [30]. Analysis of the electron-density distribution allowed the authors to conclude that such interaction is similar to the shorter bonds in that the unshared electron pairs of oxygen pointed towards the metal centre. Therefore the contact K1-O31[x,y+1,z] found in the structure reported here might be considered as an additional K-O bond, what results in the ninecoordination environment of potassium. There are numerous literature reports on the polyhedral sphere of potassium cations, some of them formed by oxalate ligands. Among those, most have eight K-O bonds. The polyhedron with eight K-O interactions formed by oxalate ions and water molecules was reported for the complexes containing yttrium and terbium [31] with the K-O distances ranging from 2.844 to 2.941 Å, while K-O(oxalate) bonds are 2.844 and 2.858 Å, for Y and Tb complex, respectively. The coordination sphere formed by oxalate ligands and water was reported [32] with nine K-O bonds ranging from 2.756 to 3.108 Å. In that structure the K-O bonds involving oxalate O are from 2.768 to 3.108 Å. The search with CDS [33] has revealed 58 structures with oxalate ligands forming K-O interactions, and the bond lengths ranging from 2.374 to 3.301 Å. Therefore, the structure reported here seems to be unique in that the oxalates are the only ligands bound to potassium, they as many as nine K-O interactions, and some of them are 655 Unauthenticated Download Date | 6/16/17 8:30 AM Unusual potassium-oxalate coordination in the two-dimensional trimetallic [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O complex Figure 3. The 2D supramolecular structure of the layer formed by [Cr(C2O4)3]3− anions and K+ cations, which are parallel to the Y and Z crystallographic axes. Atoms are displayed as gray, red, pink and blue ellipsoids for carbon, oxygen, chromium and potassium, respectively. Metal centres are labelled for clarity. Figure 5. Figure 4. A sketch of the unit cell of complex 1. Atoms are colourcoded: potassium ions are larger blue spheres, cobalt ions are medium size blue spheres, chromium is displayed as pink spheres, and chloride ions are green. Carbon, nitrogen and oxygen atoms are displayed gray, small blue and red spheres, respectively. Hydrogen atoms are omitted for clarity. among the longest K-O(ox) reported. The CSD search revealed also that some compounds with different ligands might have a potassium coordination number as large as eleven or twelve [34,35]. The K-O series of interactions, involving also oxygen atoms coordinated to Cr(2) central ion, results in the layer of [Cr(C2O4)3]3− complex ions bridged by K+ cations and parallel to the Y and Z crystallographic axes (Fig. 3). These layers are bridged by the complex [CoCl(NH3)5]2+ cations and water molecules. The coordination of a Temperature dependencies of corrected molar susceptibility (top) and reciprocal susceptibility (bottom) for 1. potassium cation to the oxalato oxygens results in short contacts K1•••C2[-x+1/2,-y+1/2,-z] of 3.514(2) Å and K1•••C5 of 3.326(2) Å. The structure analysis revealed that three oxalate anions have a different coordination mode. One oxalate (C1C2O42−), coordinates the metal centres via all four oxygen atoms, and three of them are involved in K binding. Only two oxygen atoms of other oxalate (C3C4O42−), are involved in coordination bonds to Cr and one of them forms an additional contact to K, while each of remaining two oxygens participate in three hydrogen bonds to the ammonia ligands. For the third oxalate (C5C6O42−), all oxygen atoms are involved in coordination of surrounding K ions while two of them form a chelate ring with Cr. The structure contains an extensive network of hydrogen bonds (Table 3, Fig. 4). All N-H group of ammonia ligands act as donors in these interactions, while acceptors are the oxalate oxygen atoms. The N•••O distances vary from N5•••O21[x,-y+1,z+1/2] 2.918 Å to 3.380 Å for N5•••O31[x,-y,z+1/2]. The single N3-H32•••Cl1[-x,y,-z+1/2] interaction is formed. The N•••Cl distance being 3.374 Å. Among these H-bonds, three bifurcated interactions are formed by N2-H23, N3-H33 and N5-H52 groups. Water molecule is involved in a pair of two-fold related O•••Cl H-bonds. The O3•••Cl1[x,y+1,z] distance being 3.473 Å. 656 Unauthenticated Download Date | 6/16/17 8:30 AM G. Wrzeszcz, A. Wojtczak, M. Zawadzka Table 3. Hydrogen bonds for [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O (1). D-H d(D-H) d(H×××A) <DHA d(D×××A) A N2-H23 0.890 2.454 136.22 3.157 O52 N2-H23 0.890 2.491 127.28 3.112 O11 [ -x+1/2, y+1/2, -z+1/2 ] N2-H22 0.890 2.124 164.51 2.991 O22 [ x, -y+1, z+1/2 ] N2-H21 0.890 2.484 123.43 3.066 O62 [ x, y+1, z ] N3-H33 0.890 2.200 175.70 3.088 O42 [ -x, y, -z+1/2 ] N3-H33 0.890 2.548 125.37 3.148 O41 [ -x, y, -z+1/2 ] N3-H32 0.890 2.516 162.31 3.374 Cl1 [ -x, y, -z+1/2 ] N3-H31 0.890 2.088 163.04 2.951 O32 [ -x, y+1, -z+1/2 ] N4-H43 0.890 2.253 155.65 3.086 O12 [ -x+1/2, y+1/2, -z+1/2 ] N4-H42 0.890 2.239 154.62 3.067 O62 N4-H41 0.890 2.283 170.61 3.164 O42 [ -x, y, -z+1/2 ] N5-H53 0.890 2.192 138.41 2.918 O21 [ x, -y+1, z+1/2 ] N5-H52 0.890 2.430 129.59 3.075 O32 [ x, -y, z+1/2 ] N5-H52 0.890 2.491 176.66 3.380 O31 [ x, -y, z+1/2 ] N5-H51 0.890 2.278 152.99 3.097 O32 [ -x, y+1, -z+1/2 ] N6-H63 0.890 2.155 154.98 2.985 O42 [ -x, y+1, -z+1/2 ] N6-H62 0.890 2.545 115.86 3.041 O3 N6-H61 0.890 2.201 170.65 3.082 O62 [ x, y+1, z ] O3-H1O3 0.919 2.591 160.95 3.473 Cl1 [ x, y+1, z ] 3.4. Magnetic properties of the complex Magnetic properties of complex 1 were studied within the range 78-300 K (Fig. 5). The temperature dependencies of magnetic susceptibilities obey the Curie-Weiss law, i.e., χMcorr = C/(T−θ). The best fit values of the Curie (C) and Weiss constant (θ) are 1.77 cm3 K mol−1 and −0.1 K, respectively. Although complex 1 shows a small negative Weiss constant, the magnetic moment is nearly constant down to liquid nitrogen temperature (3.79±0.05 B.M.), and from mathematical point of view (the same agreement factor), the complex equally obeys the Curie law with the same C parameter. It indicates that magnetic interactions even if operate are negligible, and complex 1 is essentially paramagnetic. The observed magnetic moment of 1 has approximately the expected spin-only value for isolated Cr(III) (S = 3/2), which is 3.87 B.M. In spite of the oxalate bridge ability to transmit exchange interactions effectively [36,37], any interaction between Cr(III) ions through the O-K-O, O-K-O-C-O and O-C-O-K-O-C-O paths has been observed, due to long K-O distances and presumably purely electrostatic K-O interactions within [{KCr(C2O4)3}n]2n− layers. Symmetry code 4. Conclusions A novel tris(oxalato)chromate(III)-based twodimensional three-metal complex [CoCl(NH3)5] [KCr(C2O4)3]•0.5 H2O (1) has been synthesized and structurally characterized. An interesting aspect to this structure is the simultaneous presence of three different coordination modes of the oxalate ligands to both, potassium and chromium, metal centres: tetradentate (C1C2O42−: µ3-ox and µ2-O11), bidentate (C3C4O42−: µ2-ox and µ2-O31) and tetradentate (C5C6O42−: µ4-ox and µ2-O51, µ2-O52, µ2-O61). A 2D structure is formed due to the charge incompatibility of initial complexes. The crystallisation of 1 depends on the stabilizing effect of the potassium cations through multiple K-O interactions. The structure of 1 seems to be unique in that the oxalates are the only ligands bound to potassium, they as many as nine K-O interactions. On the other hand, magnetic susceptibility measurements show no exchange interaction between Cr(III) ions. 657 Unauthenticated Download Date | 6/16/17 8:30 AM Unusual potassium-oxalate coordination in the two-dimensional trimetallic [CoCl(NH3)5][KCr(C2O4)3]•0.5H2O complex References [1] E. Coronado, J.R. Galán-Mascarós, C. Giménez-Saiz, C.J. Gomez-García, Synth. Met. 85, 1677 (1997) [2] E. Coronado, J.R. Galán-Mascarós, C.J. Gómez-García, V. Laukhin, Nature 408, 447 (2000) [3] H. Akutsu, A. Akutsu-Sato, S.S. Turner, P. Day, E. Canadell, S. Firth, R.J.H. Clark, J.-i. Yamada, S.i. Nakatsuji, Chem. Commun. 18 (2004) [4] H. Tamaki, J.Z. Zhong, N. Matsumoto, S. Kida, M. Koikawa, N. Achiwa, Y. Hashimoto, H. Okawa, J. Am. Chem. 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