Nova Biotechnol Chim (2017) 16(2): 138-146 DOI: 10.1515/nbec-2017-0019  Corresponding author: cyril.rajnak@ucm.sk  Nova Biotechnologica et Chimica Diamagnetic cobalt(III)tris(o-ethylxanthate) and nickel(II)bis(o-ethylxanthate) Filip Varga1, Ján Titiš1, Cyril Rajnák1,, Ján Moncoľ2 and Roman Boča1 1 Department of Chemistry, Faculty of Natural Sciences, University of SS. Cyril and Methodius in Trnva, Nám. J. Herdu 2, Trnava, SK-917 01, Slovak Republic 2 Institute of Inorganic Chemistry, FCHPT, Slovak University of Technology, Bratislava, SK-812 37, Slovak Republic Article info Article history: Received: 11th October 2017 Accepted: 16th November 2017 Keywords: Ab initio calculations Co(III) complex Magnetism Ni(II) complex O-ethyl xanthate UV-VIS spectra Abstract Diamagnetic [Co(xanth)3] and [Ni(xanth)2] complexes have been prepared by reaction of Co(II) and Ni(II) salts with potassium O-ethyl xanthate (Kxanth). The isolated Co(III) and Ni(II) complexes have been characterized by single-crystal X-ray crystallography, UV-VIS and IR spectroscopy, computational methods, and magnetic measurements.  University of SS. Cyril and Methodius in Trnava       Introduction Transition metal complexes containing chelating ligands are the subjects of extensive research for a long time due to the broad range of their structural, spectral and magnetic properties (Gerloch and Constable 1994). In the past few decades, such coordination compounds have attracted increased attention of chemists because of their potential applications in various fields as chemical sensors, nonlinear optical materials and/or molecular magnetic materials (Noro et al. 2009). Over the past ten years, an extraordinary interest on single-molecule magnets (SMMs) has been registered. Single-molecule magnets, attractive for high-density information storage and quantum computing, are metal complexes, that beside other interesting quantum phenomena, exhibit slow relaxation of the magnetization. The earlier type of SMMs was based on high-spin polynuclear complexes, such as a dodecanuclear plaquette-type complex known as Mn12ac. In course of time a plethora 3d-metal complexes were identified as SMMs, including simple mononuclear systems containing Mn(III) (Grigoropoulos et al. 2013), Fe(III) (Mossin et al. 2012), Fe(II) (Feng et al. 2013), Fe(I) (Samuel et al. 2014), Co(II) (Rajnák et al. 2017), Ni(II) (Lomjansky et al. 2017), Ni(I) (Lin et al. 2016) and Cu(II) (Boča et al. 2017) central atoms (Frost et al. 2016). Among these systems, the S-donor ligands combined with the cobalt(II) centres attract a considerable attention because of the pronounced magnetic anisotropy that supports the SMM behaviour (Zadrozny and Long 2011). However, Co(II) can be readily oxidized to Co(III) in course of the complexation reaction on air giving rise the diamagnetic complex. Tetracoordinate Ni(II) complexes in their nearly tetrahedral geometry are also promising SMMs. Some ligands prefer the square planar arrangement around the Ni(II) central atom and these complexes are diamagnetic as well. Some relationships between the structure and magnetic properties are compared in Table 1. Herein we are Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:17 UTC Nova Biotechnol Chim (2017) 16(2): 138-146 139 Table 1. Comparison of structure-magnetism relationships for related compounds. Complex Chromophore Magnetic property Reference (Ph4P)2[Co(SPh)4] Tetrahedral SMM Zadrozny and Long 2011 [Co(PPh3)2Cl2] Tetrahedral SMM Yang et al. 2013 [Co(PPh3)2Br2] Tetrahedral SMM Boča et al. 2014 [Co(PPh3)2I2] Tetrahedral SMM Saber and Dunbar 2014 [Co(PPh3)2(SCN)2] Tetrahedral SMM Rajnák et al. 2016 [Ni(PPh3)2Cl2] Tetrahedral Paramagnet, no SMM Lomjanský et al. 2016 [Ni(PPh3)2(NCS)2] Planar Diamagnet, no SMM Rajnák, unpublished reporting about synthesis, characterization and structural properties of the [Co(xanth)3] and [Ni(xanth)2] complexes with chelating S-donor ligand which were found to be diamagnetic. Experimental Chemicals and handling All inorganic and organic reactants of reagent grade quality were purchased and used as received (Sigma-Aldrich). The solvents ethanol and acetonitrile were used without further purification. Synthesis of [Co(xanth)3] (1) The compound 1 has been prepared as follows. 0.16 g (1 mmol) of potassium O-ethyl xanthate, was dissolved in ethanol (10 cm3) under an intense stirring. 0.064 g of cobalt(II) chloride was also dissolved in 10 cm3 of ethanol under an intense stirring (the molar ratio of xanth: cobalt(II) chloride = 2:1). The two solutions were mixed and the final solution was refluxed for 4 hours. The filtrate was left for the duration of 5 days for a spontaneous evaporation. Deep dark green crystals were obtained by slow evaporation of the solution at room temperature. Yield: 0.085 g Anal. Calc. for 1, C9H15CoO3S6 (M = 422.57 g mol-1): C, 25.58; H, 3.58; S, 45.53. Found: C, 25.88; H, 3.56; S, 46.02. Selected IR bands (KBr) /cm-1: 2976(w), 2484(w), 1898(w), 1861(w), 1721(w), 1464(w), 1444(m), 1433(m), 1387(m), 1365(s), 1273(w), 1233(s), 1143(w), 1116(s), 1055(w), 1030(s), 1000(s), 861(m), 810(w), 657(w), 548(w), 438(s). UV/Vis (Nujol) νmax/103 cm-1 (relat. absorb.): 15.8 (0.435), 20.7 (0.526). Synthesis of [Ni(xanth)2] (2) The compound 2 was prepared as follows. A 100 cm3 round bottom flask was charged with potassium ethyl xanthogenate (0.150 g, 0.94 mmol) and acetonitrile (20 cm3) and slight heated. NiCl2·6H2O (0.111 g, 0.47 mmol) was added once. Yellowish solution changed colour immediately to green. Reaction was heated with an oil bath for four hours at 80°C. In process of cooling a mixture changed colour to brown. After 3 days dark red crystalline compound was filtered off and dried in vacuum. Yield: 0.026 g. Anal. Calc. for 1, C6H10NiO2S4 (M = 301.10 g mol-1): C, 23.93; H, 3.35; S, 42.60. Found: C, 24.14; H, 3.05; S, 41.13. Selected IR bands (KBr) /cm-1: 2982(w), 2935(w), 2891(w), 2503(w), 1464(m), 1429(w), 1388(w), 1366(s), 1325(w), 1244(s), 1144(w), 1113(s), 1058(w), 1020(s), 996(s), 855(m), 808(w), 662(w), 554(w), 435(s) (s – strong, m – medium, w-weak); S-C=S (554 cm-1), C=S (1058 – 1144 cm-1), C-O-C (855 cm-1), νCH3 (2935 cm-1), νCH2 (2891 cm-1) a δC-H (1325 – 1464 cm-1). UV/Vis (Nujol) νmax/103 cm-1 (relat. absorb.): ~15.0 (0.191), 20.7 (0.459), 23.8 (0.513). Physical measurements Elemental analyses were carried out by Flash 2000 CHNS apparatus (Thermo Scientific). The IR spectra were measured on ATR holder with highly effective diamond crystal in the region of 4000  400 cm−1 by Nicolet 5700 spectrometer (Thermo Electron) with DTGS/KBr detector. The solid sample for FT-IR measurements was not dried prior to its using and was used as freshly synthesized. Absorption UV-Vis spectra for solid sample (Nujol mull) were measured by Specord. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:17 UTC Nova Biotechnol Chim (2017) 16(2): 138-146 140 Table 2. Crystal data and structure refinement for 1, 2. Empirical formula C9H15CoO3S6 C6H10NiO2S4 Formula weight (g.mol-1) 422.50 301.09 Crystal system trigonal orthorhombic Space group R–3 Pbca T (K) 100(1) 100(1) a (Å) 14.8164(14) 7.4174(8) b (Å) 14.8164(14) 7.1003(6) c (Å) 12.8790(12) 20.760(2) α (°) 90 90 β (°) 90 90 γ (°) 120 90 V (Å3) 2 448.5(5) 1 093.36(19) ρcalc (g.cm−3) 1.719 1.829 Z 6 4 Radiation type Cu Kα Cu Kα µ (mm−1) 1.54186 1.54186 F(000) 1 296 616 Abs. coefficient 15.426 9.439 R1[F2 > 2σ(F2)], wR2(F2) 0.0328, 0.0755 0.0476, 0.1220 Δρmax, Δρmin (e Å−3) 0.45, -0.59 1.45, -0.45 CCDC no. 1569076 1569077 250 Plus (Analytica Jena) with the DAD detector in the range of 9 000 – 50 000 cm-1. Magnetic data was taken with the SQUID apparatus (Quantum Design, MPMS-XL7) in the RSO mode of the detection. The detected magnetic moment of the specimen has been converted to the molar magnetic susceptibility mol and the dimensionless product function T/C0 with the reduced Curie constant C0 = NA0B2/kB containing the fundamental physical constants in their usual meaning. X-ray crystal structure determination Data collection and cell refinement of 1, 2 were made by Stoe StadiVari diffractometer using PILATUS3R 300K HPAD detector and micro- focused source Xenocs FOX3D with CuKα at 100K. Corrections to Lorentz, polarization and multi-scan absorption effects were applied. The structure was solved by charge-flipping or direct methods and refined anisotropically by common least-squares methods. The programs SUPERFLIP (Palatinus and Chapuis 2007), SHELXT (Sheldrick 2015), SHELXL (ver. 2016/6) (Sheldrick 2015) and OLEX2 (Dolomanov et al. 2009) have been used for structure determination, refinement and drawing. The hydrogen atoms were refined with fixed distances from the parent carbon atoms. Crystal data on 1, 2 are presented in Table 2. Results and Discussion Structural data The compound 1, [Co(xanth)3], possesses a molecular structure. The central Co(III) atom is hexacoordinate by three bidentate xanth ligands (Fig. 1). The Co-S distances are almost identical (2.268 – 2.282 Å, see Table 3). As expected, the S-Co-S angles are acute (41 – 51 deg). The compound 1, [Co(xanth)3], crystallizes in trigonal system in space group R–3. The cobalt atom lies on 3-fold axis and is coordinated in distorted octahedron. The coordination octahedron around cobalt atom of 1 is formed by six sulfur atoms [Co1–S1 = 2.2693(7) Å and Co1–S2 = 2.2743(8) Å (Table 3)] of three bidentate carbamate group of xanth. The compound 2, [Ni(xanth)2] crystallize in orthorhombic system in space group Pbca (Table 2) and it possesses a molecular structure with the planar chromophore {NiIIS4} – see Fig. 2. The nickel atom lies in the centre of symmetry and is bonded by four sulfur atoms [Ni1–S distance in region 2.21– 2.22 Å (Table 4)] of two bidentate carbamate groups of xanth square-planar coordination. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:17 UTC Nova Biotechnol Chim (2017) 16(2): 138-146 141 Fig. 1. Molecular and crystal structure of complex [Co(xanth)3], 1. Table 3. Selected bond lengths (Å) and angles (°) in complex 1. Co1–S1 2.2693(7) Co1–S2 2.2743(8) Co1–S1i 2.2693(7) Co1–S2i 2.2743(8) Co1–S1ii 2.2693(7) Co1–S2ii 2.2744(8) S1–Co1–S1i 94.18(3) S1–Co1–S1ii 94.19(3) S1i–Co1–S1ii 94.19(3) S1ii–Co1–S2i 166.63(2) S1–Co1–S2 76.73(2) S1i–Co1–S2 166.63(2) S1–Co1–S2ii 166.63(2) S1i–Co1–S2ii 96.22(2) S1i–Co1–S2i 76.73(2) S1–Co1–S2i 96.22(2) S1ii–Co1–S2ii 76.73(2) S1ii–Co1–S2 96.22(2) S2i–Co1–S2 94.31(3) S2i–Co1–S2ii 94.31(3) S2–Co1–S2ii 94.31(3) Symmetry codes: (i) 1-y, x-y, z; (ii) 1+y-x, 1-x, z Fig. 2. Molecular and crystal structure of the complex [Ni(xanth)2], 2. Electronic spectra The solid state electronic spectra of 1 and 2 measured in Nujol mull are presented in Fig. 3. These complexes exhibit a similar spectral profile with a number of electronic transitions that have varying peak intensity. The first visible transitions are slightly intense and located in the range of 15 000 – 25 000 cm. This part of the spectrum is almost identical for both complexes. In this range Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:17 UTC Nova Biotechnol Chim (2017) 16(2): 138-146 142 we can identify two transitions at ~15 000 and 20 000 cm. Moreover, in the spectrum of the Ni(II) complex another peak is visible at ~24 000 cm. For the Co(III) complex this transition is missing, however, there is a broad asymmetric band in this location with a maximum at 28 000 cm. More intense and clearly identifiable peaks are located in the range of 30 000 – 50 000 cm. Table 4. Selected bond lengths (Å) and angles (°) in complex 2. Ni1–S1 2.2182(8) Ni1–S1iii 2.2183(8) Ni1–S2 2.2201(7) Ni1–S2iii 2.2201(7) S1–Ni1–S1iii 180.0 S1–Ni1–S2 79.43(3) S1iii–Ni1–S2iii 79.43(3) S1iii–Ni1–S2 100.57(3) S1–Ni1–S2iii 100.57(3) S2–Ni1–S2iii 180.0 Symmetry codes: (iii) 1-x, -y, 1-z; (iv) -x, 1-y, -z The hexacoordinate Co(III) complexes in the strong crystal field possess the electronic ground term 1A1g that is well separated from its excited counterparts. A modelling using the generalized crystal-field theory (GCFT) with the crystal field poles F4 = 20 000 cm-1 (10Dq = 33 333 cm-1) lifts the first spin-allowed d-d transition to E1 = 26 000 cm-1 (1A1g  1T1g) and the second one to E2 = 28 000 cm-1 (1A1g  1T2g). The square- planar Ni(II) complexes in the strong crystal field also possess the electronic ground term 1A1g. A modelling by the GCFT with the crystal field poles F2 = 20 000 and F4 = 10 000 cm-1 gave the lowest spin-allowed excitation energies at E1 = 14 900 cm-1 (1A1g  1A2g) and E2 = 21 300 cm-1 (1A1g  1B1g). Ab initio Wavenumber/cm1 10000 15000 20000 25000 30000 35000 40000 45000 50000 In te n si ty /a .u . 0.0 0.5 1.0 1.5 Ni(II) Co(III) Kxanth Fig. 3. Solid state electronic spectra for complexes 1 and 2. Spectrum of the Kxanth is added for comparison. calculations based on multireference CASSCF method improved by NEVPT2 approach have been performed to verify the crystal-field. theory results. To this end, the active space extended by the second d-shell (n electrons, where n = 6 for Co(III) and 8 for Ni(II), were correlated in 10 orbitals) and def2-TZVP basis set were used within these calculations. Averaging of 10 triplet and 15 singlet states of Ni(II) and 5 quintet, 45 triplet and 50 singlet states of Co(III) was utilized. The results obtained for the experimental geometry of complexes are collected in Table 5. It can be seen, the agreement between the GCFT and ab initio results is good (Table 6). Thus, estimation of the crystal field poles in the previous analysis and the conclusions that follow may be considered correct. Comparison of the calculated results with the experimental spectra also confirms this claim. Table 5. Assignment of transitions in 1 and 2. 1 2 experimental ab initio experimental ab initio 11000 w 13500 3T1g 2800 – 4300 16000 m 18100 5T2g 13500 sh 13500 3Eg 20500 m 19400 3T2g 16000 m 16700, 17800, 28000 s 21500 1T1g 20600 m 18200 31500 s 32800 1T2g 23700 m 23400 3B2g, 23400 35000 s 34400 3T2g 31500 s 36500 s 36500 3T1g 39500 s 36000 w – weak, m – medium, s – strong, sh – shoulder Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:17 UTC Nova Biotechnol Chim (2017) 16(2): 138-146 143 Table 6. Calculated transition energies below 50 000 cm-1 for 1 and 2a. 1, [Co(xanth)3] 2, [Ni(xanth)2] GCFT CASSCF-NEVPT2 GCFT CASSCF-NEVPT2 Term Energy Mult. Energy Term Energy Mult. Energy 1A1g 0 1 0 1A1g 0 1 0 5T2g 3T1g 3T2g 1T1g 1T2g 3T2g 3T1g 3Eg 3T1g 3T2g 5Eg 1T2g 15095 15095 15095 16003 16003 16003 16045 16045 16045 26046 26046 26046 28160 28160 28160 39708 39708 39708 39738 39738 39738 43430 43430 45803 45803 45803 46315 46315 46315 48429 48429 49973 49973 49973 3 3 3 5 5 5 3 3 3 1 1 1 1 1 1 3 3 3 3 3 3 5 5 3 3 3 3 3 3 3 3 1 1 1 1 1 1 13433 13471 13487 18097 18126 18158 19305 19454 19461 21442 21459 21479 32393 32891 32907 34281 34386 34406 36540 36548 36602 39661 39671 41659 41664 41710 41740 41742 41852 42923 42941 44395 44445 44597 45531 45586 47628 3B1g 3A2g 3Eg 1A2g 3B2g 1B1g 1Eg 3Eg 1A1g 1B2g 3A2g 1Eg 3Eg 4179 4554 7588 7588 14900 20846 21277 23443 23443 23970 23970 31317 37520 38958 41225 41225 45180 45180 3 3 3 3 1 1 1 3 1 3 3 3 3 1 3 1 1 1 2770 2967 4288 13502 16677 17813 18257 23399 23438 24332 25556 26762 36081 37096 37965 40728 40795 41733 a GCFT calculations in an idealized octahedral (square planar) geometry; ab initio CASSCF calculations in the experimental geometry. Magnetic data The measured magnetic susceptibility consists of the inherent temperature-dependent paramagnetic signal arising from the uncompensated spin and/or orbital angular momentum para, temperature- independent paramagnetism TIP arising from the close lying excited states, the underlying diamagnetism dia and the signal of the sample holder h (usually diamagnetic). Eventually some traces of the dioxygen  or other paramagnetic impurities PI might be also present so that  = para +TIP +dia +h +PI                  (Eq. 1)  On heating, the molar magnetic susceptibility of 1 decreases and passing through the zero it becomes almost constant (Fig. 4). The inherent paramagnetism of a hexacoordinate Co(III) complex possessing the ground electronic term 1A1g is thought to be absent, para ~ 0. The low- temperature data refer to the traces of dioxygen that is present in the powder sample and obeys the Curie law. Also, the feature round 50 K is typical for the solidus-solidus phase transition of O2(s). Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:17 UTC Nova Biotechnol Chim (2017) 16(2): 138-146 144 T/K 0 50 100 150 200 250 300  m o l/1 0 -9 m 3 m o l-1 -5 0 5 10 15 T/K 0 50 100 150 200 250 300  T / C 0 -0.2 0.0 0.2 0.4B = 0.1 T O2   Fig. 4. Magnetic data for 1. Left – molar magnetic susceptibility (SI units), right – dimensionless product function.  This signal escapes at the room temperature when (300 K) = -2.5 × 10-9 m3 mol-1 (SI units) consisting only  = TIP +dia +h. The underlying diamagnetism can be calculated by means of the Pascal constants, however also a simple estimate using the molar mass Mr can be applied such as dia [10-12 m3 mol-1] = -5Mr [g mol-1]; this amounts to dia = -2.1 × 10-9 m3 mol-1. As the excited states of the same multiplicity are far away the ground state the temperature- independent paramagnetic term will be small. It can be hand-calculated by the spin-Hamiltonian formalism that offers the formula 2 TIP A 0 B2 ( ) / 3xx yy zzN         (Eq. 2) containing the -tensor components (restricted to the first excitation energy) 2 0 1 1 0 ˆ a aa L E E       (Eq. 3) The matrix element of the angular momentum operator for an octahedral Co(III) system is T/K 0 50 100 150 200 250 300  m o l/1 0 -9 m 3 m o l-1 -5 0 5 10 15 T/K 0 50 100 150 200 250 300  T / C 0 -0.2 0.0 0.2 0.4B = 0.1 T z x, y Fig. 5. Magnetic data for 2. Left – molar magnetic susceptibility (SI units), right – dimensionless product function. Lines – calculated with F2 = 20 000 and F4 = 10 000 cm-1. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 28.02.20 08:17 UTC Nova Biotechnol Chim (2017) 16(2): 138-146 145 1 1 1 , , 1g ˆA T 2x y zL  and using the first singlet excitation energy E/hc = 21 000 cm1 one gets TIP = +2.4  109 m3 mol1. The computer evaluation using the generalized crystal field theory yields TIP = +3.3  109 m3 mol1. The molar susceptibility for 2 at low temperature starts to decay on heating (Fig. 5), however it reaches almost constant value at the room temperature of mol = 6.7 × 10-9 m3 mol-1 (SI units). This behaviour clearly deviates from the Curie law. The product function develops according to a straight line and this feature is a fingerprint of a considerable temperature-independent paramagnetism TIP. Small anomaly around 50 K is attributed to the solidus-solidus phase transition of dioxygen traces which are present in the powder sample. Using the crystal field poles F2 = 20 000 and F4 = 10 000 cm-1, the GCFT calculations for a quadro-Ni(II) gave TIP = +3.1  109 m3 mol1. Conclusions Two novel low-spin Co(III) and Ni(II) complexes have been prepared and characterized by spectroscopic methods. X-ray structure analysis has shown that the complexes 1 and 2 possess with {CoS6} and {NiS4} chromophore, respectively. The study of bond angles and distances has revealed coordination polyhedron in octahedral (1) and square planar (2) arrangements. Magnetic studies confirmed a premise on diamagnetic behaviour of both complexes. Our primary goal was to prepare paramagnetic compounds of Co(II) and Ni(II). However, all attempts to prepare such compounds using the xanth ligand were unsuccessful. Acknowledgement Slovak grant agencies (APVV-14-0078, APVV-16-0039, VEGA 1/0919/17, VEGA 1/0534/16) are acknowledged for the financial support. This article was also created with the support of the MŠVVaŠ of the Slovak Republic within the Research and Development Operation Programme for the project University Science Park of STU Bratislava (ITMS project no. 26240220084) cofounded by the European Regional Development Fund.. 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