Microsoft Word - 10_Lomjansky et al Nova Biotechnologica et Chimica 15-2 (2016) 200 DOI 10.1515/nbec-2016-0020 © University of SS. Cyril and Methodius in Trnava REDETERMINATION OF ZERO-FIELD SPLITTING IN [Co(qu)2Br2] AND [Ni(PPh3)2Cl2] COMPLEXES DOMINIK LOMJANSKÝ1, FILIP VARGA1, CYRIL RAJNÁK1, JÁN MONCOĽ2, ROMAN BOČA1, JÁN TITIŠ1 1 Department of Chemistry, Faculty of Natural Sciences, University of SS. Cyril and Methodius in Trnava, Nám. J. Herdu 2, Trnava, SK-917 01, Slovak Republic (lomjanskyd@gmail.com) 2 Institute of Inorganic Chemistry, FCHPT, Slovak University of Technology, Bratislava, SK-812 37, Slovak Republic Abstract: A mononuclear CoII complex, [Co(qu)2Br2], and NiII complex, [Ni(PPh3)2Cl2], (qu = quinoline, PPh3 = triphenylphosphine) have been reinvestigated. Their crystal and molecular structures are reported along with IR and UV-Vis spectra. Magnetism of both complexes has been studied by using the DC SQUID magnetometry. These complexes exhibit a moderate magnetic anisotropy expressed by zero-field splitting parameter D. The D-value is positive for both complexes with D/hc = +5.94 cm-1 and D/hc = +12.76 cm-1, that is also confirmed by ab initio calculations. Key words: tetracoordinate complexes, electronic spectra, magnetic susceptibility, magnetization, zero-field splitting 1. Introduction Detection of static and dynamic magnetic properties of mononuclear 3d metal complexes is an important task in the emerging field of molecular magnetism. Single-molecule magnets (SMMs), as the main objects of interest, show slow magnetic relaxation as a consequence of the energy barrier (Eb ≈ |D|S 2, where D is axial zero- field splitting (ZFS) parameter) for the magnetization reversal between the two lowest MS = ±S states (GATTESCHI et al., 2006; FROST et al., 2016). SMMs are a major scientific target because of their potential applications in high-density magnetic memories and quantum-computing devices. The zero-field splitting represents a fine structure of the electronic energy levels that appears as a result of the splitting of the crystal-field terms into crystal-field multiplets. It is characterized by the axial ZFS parameter D that is related to the energy gap between the ground and excited states (BOČA, 2004; BOČA, 2006). Despite the relatively wide range of knowledge, a rational design of the D-parameter is still far from the routine, though there is a need of this when tuning the properties of the SMMs. The sign of the D-parameter is either negative or positive and it adopts values between 10-2 – 102 cm-1. Some insights to this target brought the magnetostructural D-correlations that bring a linear relationship between the axial distortion of the octahedron and the D-parameter in a series of NiII hexacoordinate complexes (TITIŠ and BOČA, 2010). For hexacoordinate CoII complexes the magnetostructural D-correlation has also been outlined (TITIŠ and BOČA, 2011). Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC 201 Lomjanský, D. et al. The case of tetracoordinate complexes is no less interesting. Tetrahedral ligand environments produce weaker crystal-fields in comparison with its octahedral counterparts. This results to the enhanced second-order perturbation (as a consequence of the spin-orbit coupling) of the ground crystal-field term, and thus to the significantly larger D-values (IDEŠICOVÁ et al., 2013). The present communication compares magnetic behavior of two different systems, the tetracoordinate CoII (Kramers ion, S = 3/2) and tetracoordinate NiII (non-Kramers ion, S = 1). Reinvestigation of complexes [Co(qu)2Br2] and [Ni(PPh3)2Cl2] (qu = quinoline, PPh3 = triphenylphosphine), hereafter 1 and 2, using the DC magnetometry and contemporary methods of quantum chemistry represents the aim of this study. In tetracoordinate CoII complexes the ground term 4A2(Td) is split into the ground and excited multiplets (each referring to a Kramers doublet) separated by 2D. In reference Td symmetry the ground term of the Ni II ion is the orbitally degenerate 3T2. This term in lower symmetry (D2d) is split into the ground 3A2 and excited 3E terms. Spin-orbit coupling further split the 3A2 ground term into MS = 0 and MS = ±1 multiplets. Difference between them is equal to D. 2. Materials and Methods 2.1. Synthesis The compound 1, [Co(qu)2Br2], has been prepared as follows. A solution of 0.218 g (1 mmol) CoBr2 in 10 cm 3 of acetonitrile was added to the solution of 0.258 g (2 mmol) of qu in 10 cm3 of acetonitrile under an intense stirring (the molar ratio qu:CoBr2 = 2:1). A mixture was stirred for 3 h at room temperature. The filtrate was left for 3 days for a spontaneous evaporation. The dark blue crystals were separated on Büchner funel. The compound 2, [Ni(PPh3)2Cl2], has been prepared as follows. A solution of 0.238 g (1 mmol) of NiCl2⋅6H2O in 20 cm 3 of distilled water and 5 cm3 of acetic acid was added. A solution of 0.525 g (2 mmol) of PPh3 in 2.5 cm 3 of acetic acid was slowly added in solution of NiCl2⋅6H2O using a Pasteur pipette. Deep blue crystals were obtained after 20 hours standing. 2.2. Physical measurements Elemental analysis was carried out on FlashEA 1112 (ThermoFinnigan). IR spectra were measured using the ATR method (Magna FTIR 750. Nicolet) in the 400 – 4 000 cm−1 region. Electronic spectra were measured in Nujol mull (Specord 200, Analytical Jena) in the range 9 000 – 50 000 cm−1. Magnetic data were taken with the SQUID magnetometer (MPMS-XL7. Quantum Design) using the RSO mode of detection. The susceptibility data were scanned in the temperature range 2 – 300 K at the applied field of B = 0.1 T. The magnetization has been measured at T = 2.0 and 4.6 K. Raw data were corrected for the underlying diamagnetism using estimate of χdia/(10 −12 m3 mol−1) = −5M[g mol−1]. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC Nova Biotechnologica et Chimica 15-2 (2016) 202 2.3. Quantum-chemical calculations Ab initio calculations were performed with ORCA 3.0.3 computational package at the experimental geometry determined by the X-ray diffraction for the mononuclear entity (NEESE 2012). Relativistic effects were included in the calculations by zero order regular approximation (ZORA) together with the scalar relativistic contracted version of TZVP basis functions. The calculations of ZFS parameters were based on state average complete active space self-consistent field (SA-CASSCF) wave functions complemented by NEVPT2 theory (ANGELI et al., 2001; ANGELI et al., 2002; ANGELI et al., 2004; ATANASOV et al., 2011). The calculations utilized the RI approximation with appropriate decontracted auxiliary basis set and the RIJCOSX approximation to exact exchange. Increased integration grids (Grid4) and tight SCF convergence criteria were used. The ZFS parameters were calculated through quasi-degenerate perturbation theory (QDPT) in which an approximation to the Breit-Pauli form of the spin-orbit coupling operator (SOMF) and the effective Hamiltonian theory was utilized (NEESE, 2005; GANYUSHIN and NEESE, 2006; NEESE, 2007). Table 1. Selected crystal data. Compound 1, [Co(qu)2Br2] 2, [Ni(PPh3)2Cl2] Chemical formula C18H14Br2CoN2 C36H30Cl2NiP2 M(g mol−1) 477.06 654.15 Crystal system Space group Triclinic P -1 monoclinic P21/c T (K) 293(2) 298(2) a (Å) 8.685(4) 11.620(4) b (Å) 9.637(5) 8.202(2) c (Å) 11.232(5) 17.387(7) α (°) 80.671(4) 90 β (°) 74.592(4) 107.009(4) γ (°) 73.296(4) 90 V (Å3) 864.45(8) 1583.80(10) Z 2 2 Radiation type Mo Κα (λ=0.71073) Mo Κα (λ=0.71073) µ (mm−1) 5.614 0.907 F(000) 466 676 Index ranges -11≤h≤11,-12≤k≤12, -14≤l≤14 -8≤h≤14,-10≤k≤10, -21≤l≤19 ρ calc.(g cm−3) 1.833 1.372 Final R indexes [F2 > 2σ(F2)], wR2(F2) R1=0.0321 wR2=0.0738 R1=0.0322 wR2=0.0742 R indices (all data) R1=0.0453 wR2=0.0804 R1=0.0453 wR2=0.0812 Data / restrains / parameters 3814/0/208 3232/0/186 Goodness of fit 1.029 1.033 Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC 203 Lomjanský, D. et al. 2.4. X-ray crystallography Data collection and cell refinement were carried out using a κ-axis diffractometer Gemini R CCD (OXFORD DIFFRACTION, 2010) with graphite monochromated Mo Kα radiation. The diffraction intensities were corrected for Lorentz and polarization factors. The structures were solved by direct methods using SIR-97 (ALTOMARE, 1999) or SHELXS-97 (SHELDRICK, 2008) and refined by the full-matrix least- squares procedure with SHELXL-97 (CLARK, 1995). The analytical or multi-scan absorption corrections were made by using CRYSALIS-RED (Oxford Diffraction 2010). Geometrical analyses were performed with SHELXL-97. Crystal data and conditions of data collection and refinement are reported in Table 1. 3. Results and Discussion The molecular structures with thermal ellipsoids of 1 and 2 are presented in Fig. 1. The coordination environment of 1 consists of two donor nitrogen atoms of two quinoline ligands completed by two bromide anions. The coordination sphere of 2 consists of two donor phosphorus atoms of triphenylphospine ligands completed by two chloride anions. In both cases the chromophore, {CoN2Br2} and {CoP2Cl2}, refers to a distorted tetrahedron. Important structural parameters, metal-ligand bond distances and bond angles, for the studied complexes are collected in Table 2. Fig. 1. Molecular structure of complexes 1 and 2 (thermal ellipsoids shown at 50 % of probability level, hydrogens omitted for clarity). We note that compounds under study have already been structurally characterized and are deposited in structural database (CCDC, 2016) under refcodes TUSQAB (KORCHAGIN et al., 2015) and CLTPNI (GARTON et al., 1963). By comparing the structural characteristics of these structures with complexes studied in this work we note marked similarities especially for the CoII complex. For the NiII complex the differences are more pronounced. One can found a shorter Ni-P distances (2.277 Å) and longer Ni-Cl distances (2.274 Å) in the CLTPNI structure. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC Nova Biotechnologica et Chimica 15-2 (2016) 204 Furthermore, the bond angles in CLTPNI (P-Ni-P, 116.6° and Cl-Ni-Cl, 123.2°) also shown significant deviations from those in 2. Table 2. Selected bond lengths (Å) and angles (°) in 1 and 2. bond lengths angles 1, [Co(qu)2Br2] Co(1)-Br(1) 2.377(5) Co(1)-Br(2) 2.382(6) Co(1)-N(1) 2.032(2) Co(1)-N(2) 2.059(2) Br(1)-Co(1)-Br(2) 113.27(2) N(1)-Co(1)-Br(1) 109.39(7) N(1)-Co(1)-Br(2) 107.75(7) N(1)-Co(1)-N(2) 111.90(10) N(2)-Co(1)-Br(1) 107.98(7) N(2)-Co(1)-Br(2) 106.57(7) 2, [Ni(PPh3)2Cl2] Ni(1)-P(1) 2.366(5) P(1)-Ni(1) 2.366(5) Ni(1)-Cl(1) 2.202(6) Cl(1)-Ni(1) 2.202(6) P(1)-Ni(1)-P(1) 112.00(3) Cl(1)-Ni(1)-P(1) 104.95(2) Cl(1)-Ni(1)-P(1) 103.82(2) Cl(1)-Ni(1)-P(1) 103.82(2) Cl(1)-Ni(1)-P(1) 104.95(2) Cl(1)-Ni(1)-Cl(1) 127.25(4) Important intermolecular contacts have been identified in the crystal structure of 1 (Fig. 2). These contacts (C7-H7…Br1, 3.006 Å and C1…Br2, 3.545 Å) interconnect the adjacent molecular units that create linear chains along the a crystallographic axis. In case of the crystal structure of 2, no intermolecular contacts shorter than the sum of the van der Waals radii have been found. Fig. 2. Intermolecular contacts in 1. The solid state electronic spectra of 1 and 2 are shown in Fig. 3. For complex 1 a number of d-d transitions can be identified in the range of 9 000 – 24 000 cm−1 which are followed by an intense charge transfer band. The first visible transition is a broad band at ca 9 000 cm−1, the second is split into three peaks between 15 000 – 17 000 cm−1. Furthermore, a weak band at ca 20 000 cm−1 is also visible. In the spectrum of 2, three relatively intense bands are present at ca 11 000, 18 000 and 24 000 cm−1. The second band is broad and asymmetric suggesting the presence of a number of d-d transitions. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC 205 Lomjanský, D. et al. Since the structure of the complexes is derived from a tetrahedron, the interpretation of the spectra can be done in the Td group of symmetry in the first approximation. The estimate for the crystal-field strength for tetrahedral systems is Dq(Td) = (4/9)Dq(Oh) = (4/9)(1/6)F4(R) (BOČA, 2006). The first transition in 1 is 4A2 → 4T2 that lies in the NIR region. It has not been detected by the used hardware. The first observed band refers to the 4A2 → 4T1(F) transition and its energy is about 18Dq = 9 000 cm−1. An estimate for the next allowed d-d transition 4A2 → 4T1(P) is 12Dq + 15B. Using Dq = 500 cm −1 the Racah parameter of CoII is estimated as B = 733 cm−1. This value is lowered relative to the free-ion value (B0 = 980 cm −1) owing to the nephelauxetic effect (LEVER, 1984). The assignment of the individual transitions within the band in the 15 000 – 17 000 cm−1 region can be done in the more realistic D2d group of symmetry (or C2v) where the 4T1g(P) mother term is split into { 4A2, 4E} daughter terms. In such a case also a spin-forbidden transitions can be considered since the close-lying 2E(G), 2A1(G), 2T1(G) and 2T2(G) terms might borrow the intensity from the spin-allowed transitions. The spin-orbit coupling is also in the play and it further modifies the term scheme. wavenumber/cm-1 9000 12000 15000 18000 21000 24000 27000 In te ns ity /a .u . 0.0 0.3 0.6 0.9 1.2 1.5 Ni(II) complex Co(II) complex Gaussian deconvolution Fig. 3. Solid state electronic spectra (nujol mull) of complexes under study. Deconvolution of the second band of complex 2 using three Gaussian primitives (dot-dashed lines). In complex 2, we expect three principal d-d transitions considering the Td group of symmetry. The first transition, 3T1 → 3T2 (3 500 cm −1), lies in the IR region (using the electronic structure parameters 10Dq = 3 850 cm−1, B = 725 cm−1). The second one appears at 6 550 cm−1, 3T1 → 3A2. The third transition is located in 14 250 – 15 240 cm−1 region, 3T1 → 3T1(P). This assignment is, however, hypothetical since the real symmetry of 2 is lower than Td, most likely C2v. Thus the 3T1(F,P) and 3T2 terms are split into { 3A2, 3B1, 3B2} and { 3A1, 3B1, 3B2} terms, respectively. Furthermore, the configuration interactions between ground and excited Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC Nova Biotechnologica et Chimica 15-2 (2016) 206 terms are effective causing the shift of the transitions. To elucidate the nature of detected spectral bands we have decompose the second one into Gaussian primitives. It was found that in this region three transitions are hidden (see Fig. 3). These are the transitions to 3A2, 3B1 and 3B2 (C2v) terms. According to these results, we assume that the first visible band refers to the 3T1 → 3A2 (Td) transition shifted to a higher energy. The third most intense band at ~24 000 cm−1 hides the charge-transfer transitions. The molar magnetic susceptibility for 1 and 2, corrected for the underlying diamagnetism, has been converted to the effective magnetic moment μeff that is displayed in Fig. 4. T/K 0 50 100 150 200 250 300 μ e ff/ μ B 0 1 2 3 4 5 6 B/T 0 1 2 3 4 5 6 7 M m ol /( N A μ B ) 0 1 2 3 0 10 20 30 40 χ m ol /( 10 -6 m 3 m ol -1 ) 0 2 4 6 8 10 T = 2.0 K B = 0.1 T T = 4.6 K T/K 0 50 100 150 200 250 300 μ e ff/ μ B 0 1 2 3 4 B/T 0 1 2 3 4 5 6 7 M m ol /( N A μ B ) 0 1 2 0 10 20 30 40 χ m ol /( 10 -6 m 3 m ol -1 ) 0 1 2 3 4 5 6 T = 2.0 K B = 0.1 T T = 4.6 K Fig. 4. Magnetic functions for complexes 1 (upper panel) and 2 (bottom panel). Left – temperature dependence of the effective magnetic moment (molar susceptibility as inset), right – field dependence of the magnetization. Circles – experimental data. Lines – fitted. Dashed lines for 2 refer to simulation with the use of negative D value (D = −12 cm−1). Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC 207 Lomjanský, D. et al. For complex 1, at the room temperature the value of μeff = 4.85 μB matches the S = 3/2 spin system with some orbital contribution to the g-factor (g ~ 2.5). On cooling the effective magnetic moment slightly decreases along a straight line which is a fingerprint of some temperature-independent paramagnetism. Below 10 K the decrease is more rapid until 3.17 μB at 1.9 K; this reflects some zero-field splitting (the splitting of the ground 4B1(D2d) term into two Kramers doublets). The magnetization data taken at T = 2.0 K indicate a saturation at B > 7.0 T. The magnetization per formula unit at B = 7 T is only M1 = Mmol/(NAμB) = 2.6 which again reflects the zero-field splitting. Magnetic data for complex 2 are different. At the room temperature the value of μeff = 3.32 μB. This value corresponds to the S = 1 spin system with some orbital contribution. Estimated value of the g-factor is g ~ 2.3. On cooling the effective magnetic moment decreases until ~10 K, than the decrease is more rapid until 1.41 μB at 1.9 K. Here again we expect the zero-field splitting. Susceptibility data show a clear plateau at low temperatures, thus we expect positive D value in this case. With increasing field the magnetization increases rapidly. At both temperatures the behaviour is almost identical. At B = 7.0 T the magnetization per formula unit is M1 = Mmol/(NAμB) = 1.39 and does not show a saturation feature. The experimental magnetic data have been fitted by assuming the spin Hamiltonian in the following form: 2 2 2 2 2 2 , 1 B ˆ ˆ ˆˆ ( / 3) ( ) ˆ ˆ ˆ( sin cos sin sin cos ) k l z x y m x k l x y k l y z k z H D S S E S S B g S g S g Sμ ϑ ϕ ϑ ϕ ϑ − − − = − + − + + + r h h h (1) where D is the axial and E is the rhombic zero-field splitting parameter, respectively. The second contribution is the spin Zeeman term. The minor improvements refer to the molecular-field correction zj and the uncompensated temperature-independent magnetism χTIM: molcorr TIM2 A 0 B mol1 ( / )zj N χ χ χ μ μ χ = + − (2) Table 3. Fitted magnetic parameters for complexes under study. Compound 1, [Co(qu)2Br2] 2, [Ni(PPh3)2Cl2] gx 2.643 2.284 gy 2.544 – gz 2.427 2.203 D (cm−1) +5.94 +12.76 E (cm−1) 1.24 – χTIM (10 −9 m3 mol−1) 5.23 9.98 zj (cm−1) −0.01 – R (χ) 0.022 0.021 R (M) 0.034 0.067 An advanced fitting procedure accounts simultaneously for the two data-sets: χ = f(T, B0 = 0.1 T) and M = f(B, T0) with T0 = 2.0 and 4.6 K, respectively. The powder Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC Nova Biotechnologica et Chimica 15-2 (2016) 208 average has been done by averaging 210 points of polar coordinates ( , )i iϑ ϕ distributed over the top hemisphere. It converged to a set of magnetic parameters listed in Table 3. The quality of the fits is good as expressed by discrepancy factors for the susceptibility R(χ) and magnetization R(M). For both complexes the D value is positive. The sign of the D-parameter arises from the assignment of the lowest crystal-field multiplet. For 1, the Kramers doublet 1 / 2SM = ± is ground state and 3 / 2SM = ± refers to the excited state, then 2D > 0 holds true. In case of 2, the ground state is the crystal-field multiplet with MS = 0 and the excited one with MS = ±1. Thus, the energy difference between these multiplets refers to D > 0. Positive D value has a specific effect on the progress of magnetic functions of the NiII complexes. The susceptibility of the complex 2 appears a plateau at low temperatures which is often attributed to the presence of ferromagnetic interaction between the complex molecules. In case of 2, however, simulation of the magnetic functions (see Fig. 4) clearly confirmed that this behaviour is the result of the positive D-parameter. Also the almost identical progress of the magnetization at different temperatures is a consequence of the MS = 0 ground state. Table 4. Calculated magnetic parameters. Compound 1, [Co(qu)2Br2] 2, [Ni(PPh3)2Cl2] g1 2.222 2.243 g2 2.275 2.339 g3 2.284 2.379 D (cm−1) +5.45 +20.06 E/D 0.03 0.17 The zero-field splitting has also been studied by ab initio calculations. We have applied the CAS(n,5)/NEVPT2 (n = 7 for 1, n = 8 for 2) methodology that results to magnetic parameters collected in Table 4. Comparing the experimental and calculated values we conclude that the correlation between them is acceptable. For complex 1 the agreement between the fitted and calculated D is excellent. However, the g-factor components are quite low, possibly due to the formation of magnetic chains in the solid state. For complex 2 the calculated D is somewhat larger than the experimental value. Considering the earlier reported high-frequency and high-field electron paramagnetic resonance (HFEPR) spectroscopy results (D = +13.20 cm−1) (KRZYSTEK et al., 2002) it is evident that just the experimental D-value must be taken as the more realistic one. Overestimation of the calculated D may be associated with the omission of some important solid state effects that in such calculations cannot be included. Contributions to the D-values that come from individual electronic transitions are presented in Fig. 5. One can see that the most important contributions are those at low transition energies (< 10 000 cm−1). In complex 1, the largest contribution (D = −14.5 cm−1) comes from the transition at 4 818 cm−1. This sizeable negative value is compensated by significant positive contributions mainly at 3 870 and 4 507 cm−1. Bereitgestellt von Slovenská poľnohospodárska knižnica | Heruntergeladen 17.01.20 15:07 UTC 209 Lomjanský, D. et al. In 2 the situation is analogous, however, with more pronounced D contributions. The D for the transition at 8 365 cm-1 is negative and very large (−48 cm-1). However, the contributions from transitions at lower energies dominate at summation giving the moderate positive total D-value. By using this method, the origin of the positive D-parameter of the studied complexes can be explained. Transition energy/cm−1 5000 10000 15000 20000 C on tr ib ut io n to D /c m − 1 -60 -40 -20 0 20 40 complex 1 complex 2 Fig. 5. Calculated contributions to the D-parameter from individual electronic transitions. 4. Conclusions We have prepared two tetracoordinate complexes, namely [Co(qu)2Br2] and [Ni(PPh3)2Cl2] where qu = quinoline and PPh3 = triphenylphosphine are N-donor and P-donor ligands, respectively. From structurally point of view these complexes are not new, since such structures are in disposal in structural database. We have redetermined the structure of these complexes and studied their spectral and magnetic properties. Static magnetism has been studied by using the DC SQUID magnetometry. We confirmed that the studied complexes exhibit a moderate magnetic anisotropy expressed by zero-field splitting parameter D. The D-value is positive for both complexes with D/hc = +5.94 cm-1 for the CoII and D/hc = +12.76 cm-1 for the NiII complex. These results have been supported by ab initio calculations. Based on these results, we do not expect the slow magnetic relaxation for these complexes. Acknowledgments: Slovak grant agencies (APVV-14-0078, APVV-14-0073 and VEGA 1/0534/16) are acknowledged for the financial support. 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