Cobalt(II

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Syntheses and Characterization of Diammine−Nickel/Cobalt(II)− Bisdicyanamide M(NH3)2[N(CN)2]2 with M = Ni and Co Markus Mann,† Damian Mroz,† Laura Henrich,† Andreas Houben,† Jan van Leusen,† and Richard Dronskowski*,†,‡ †

Chair of Solid State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, 52056 Aachen, Germany ‡ Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic, 7098 Liuxian Blvd, Nanshan District, Shenzhen, China

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S Supporting Information *

ABSTRACT: Crystals of M(NH3)2[N(CN)2]2 with M = Ni and Co were obtained from the reaction of stoichiometric amounts of Na[N(CN)2] with NiCl2·6H2O or CoCl2·6H2O in aqueous, ammoniacal solutions. X-ray single-crystal structure analyses show that M(NH3)2[N(CN)2]2 with M = Ni and Co crystallize isotypically to each other and adopt the monoclinic space group P21/c (no. 14). The lattice parameters of Ni(NH3)2[N(CN)2]2 are a = 5.8498(9) Å, b = 10.6739(12) Å, and c = 6.8089(17) Å, β = 98.037(3)° and Z = 2, while those of Co(NH3)2[N(CN)2]2 are a = 5.8303(11) Å, b = 10.746(2) Å, c = 6.7773(13) Å, and β = 96.422(3)°. In addition, the crystal structure of the nickel compound was refined from neutron powder diffraction, augmented by DFT calculations as regards atomic displacement parameters. The IR spectra of the title compounds exhibit modes typical for the dicyanamide anion and ammonia. The UV/vis spectrum of Ni(NH3)2[N(CN)2]2 shows that the dicyanamide moiety is a medium-field ligand. Additional superconducting quantum interference device (SQUID) magnetic susceptibility measurements of Ni(NH3)2[N(CN)2]2 and Co(NH3)2[N(CN)2]2 confirm not only significant high-spin moments of χmT = 1.24 cm3·K·mol−1 (μeff = 3.15 μB) and 2.89 cm3·K·mol−1 (μeff = 4.81 μB), respectively, at 290 K and 0.1 T but also an absence of magnetic ordering.

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INTRODUCTION

In addition, a special interest lies in the 3d transition metal compounds with the composition M[L]2[dca]2, wherein L are neutral ligands. Examples for these ligands are dimethylformamide,26−29 methanol,27,30 pyridine,27,31 pyrimidine,32 methylpyrazine,33 water,27,33 and ammonia.34,35 These compounds are known for manganese, iron, cobalt, nickel, and copper, and they usually form one-dimensional M[dca]2 strings. In the cases of water and ammonia, however, two-dimensional M[dca]2 frameworks are formed. In both cases, the metal atom exhibits octahedral coordination, and the neutral ligand takes at least one donor position. Cu(NH3)2[dca]2 differentiates from the other M[L]2[dca]2 compounds since it is the only one that is known to form a string34 and a framework35 modification. This is caused either by the Jahn−Teller effect or by the ligand ammonia, which can also be coordinated by the dicyanamide anion. In both modifications there are μ1,3 and μ1,5 bonds, while the μ1,3 bonds could allow for long-range magnetic ordering comparable to the transition metal carbodiimides.36−39 Very unfortunately, however, the magnetic properties were not determined at that time.

The search for new compounds and gaining control over their molecular structure has been a perpetual goal for chemists and material scientists. While crystal design is a classical field for coordination and organometallic chemistry, there are also highly interesting inorganic ligands and moieties that can be used to design and control molecular properties. Specifically, the design of magnetic compounds as a combination of paramagnetic transition metal cations with diamagnetic ligands and anions has always been a tremendous challenge.1 One particularly fascinating inorganic moiety is the boomerangshaped dicyanamide anion [N(CN)2]−, often abbreviated as [dca]. The [dca] species is capable of coordinating metal ions through its three nitrogen atoms, which immediately explains the large variety of pseudobinary dicyanamide compounds. Examples are known for ammonium,2 lead,3 thallium,4 alkali metals5−8 (except Fr), alkaline-earth metals9 (except Be and Ra), transition metals (Ag, Cd, Hg, and Cr−Zn except Fe),4,10−16 and rare-earth metals (La, Ce, Pr, Nd, Sm, Eu, Gd, and Tb).17−19 Ternary compounds such as KCs[dca]2,20 LiK[dca]2,21 LiRb[dca]2,21 NaRb2[dca]3·H2O,20 NaCs2[dca]3,22 and LiCs2[dca]323 are known as well. © XXXX American Chemical Society

Received: February 14, 2019

A

DOI: 10.1021/acs.inorgchem.9b00448 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. TOF neutron Rietveld refinement of a powdery sample of Ni(NH3)[dca]2 at 10 K with measured data in red, calculated data in black, theoretical reflection positions in green, and the difference plot in blue. can be found in Figure S1 and Table S1. While the nickel compound is storable in air, the cobalt compound starts to oxidize after a few seconds and must be kept under an inert atmosphere. C/H/N elemental analysis was done with an Elementar varioEL (Elementar Analysensysteme GmbH, Langenselbold, Germany). Ni content was determined by gravimetric analysis as follows: 47.8 mg (0.213 mmol) of Ni(NH3)2[dca]2 was dissolved in 2 droplets of concentrated hydrochloric acid. The solution was then diluted with 100 mL of water. Dimethylglyoxime (ca. 150 mg, 1.29 mmol) was added, and after addition of some pellets of NaOH, the pink nickel bis(dimethylglyoximate) was formed. Ni(dmgH)2 was washed, dried, and retained as 63.7 mg (0.220 mmol), resulting in 12.9 mg (27.1 wt %) of nickel. Co content was determined by complexation titration with ethylenediaminetetraacetic acid (EDTA). First, 57.1 mg (0.254 mmol) of the sample was dissolved in 2 droplets of concentrated hydrochloric acid. The solution was then diluted with water to a total volume of 100 mL and separated in three samples. Murexide was used as an indicator, and a diluted ammonia solution was added until a yellow color was reached. Shortly after the addition, a blue solid was formed. With the addition of the EDTA solution (c = 0.01 mol·L−1), the solid started to dissolve, and the solution changed to violet. The titration was carried out until the blue solid was completely dissolved. On average, 6.28 mL of the EDTA solution was used for a 25 mL sample, resulting in 14.8 mg (25.9 wt %) of cobalt. Single-Crystal Diffraction. Small single crystals were mounted on glass fibers. Intensity data were collected with a Bruker SMART APEX CCD detector equipped with an Incoatec microsource (Mo Kα1 radiation, λ = 0.71073 Å, multilayer optics). Temperature control was achieved using an Oxford Cryostream 700 at 100 K. Collected data were integrated with SAINT+,40 and multiscan absorption corrections were applied with SADABS.41 The structure was solved by charge-flipping methods (Superflip)42 and refined on F 2 as implemented in Jana2006.43 Non-hydrogen atoms were assigned anisotropic displacement parameters. Hydrogen atoms of the ammonia group were found in the Fourier maps and constrained at Uiso(H) = 1.2 Ueq(N4). More crystallographic details can be found in Table S2. Spatial and isotropic displacement parameters of the Ni and Co compounds are found in Tables S3 and S5, whereas anisotropic parameters are given in Tables S4 and S6.

Transition metal dicyanamides have also gained attention as nonoxidic compounds for the electrochemical water oxidation. For example, Ni[dca]2 and Co[dca]2 were successfully used as cocatalyst in combination with BiVO4.24 Other working systems are α-Fe 2 O 3 /Co[dca] 2 /TiO 2 25 or simply M(DMF)2[dca]2 (with M = Fe, Co, and Ni) applied on FTO glass.26 We here present two new representative M(NH3)2[N(CN)2]2 (with M = Ni and Co) of this compound group which were synthesized and physically characterized.

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EXPERIMENTAL SECTION

Syntheses. Ni(NH3)2[dca]2 was synthesized by dissolving stoichiometric amounts of NiCl2·6H2O (2.886 g, 12.14 mmol) and Na[dca] (2.159 g, 24.25 mmol) in concentrated ammonia (25 wt %, 60 mL). Upon vaporization of ammonia, dark blue plates started to crystallize. The product (2.01 g, 8.94 mmol, 59.6%) was washed with ethanol and dried in a desiccator. The elemental analysis (wt %: Ni 27.1, C 21.1, H 2.5, N 50.3) is in good agreement with the theoretical values (wt %: Ni 26.1, C 21.4, H 2.7, N 49.8). Crystals of Ni(NH3)2[dca]2 suitable for single-crystal X-ray diffraction were selected and measured. Co(NH3)2[dca]2 was made under strict oxygen-free conditions with argon as protective gas. CoCl2·6H2O (1.784 g, 7.50 mmol) and Na[dca] (1.339 g, 15.04 mmol) were dissolved in 15 mL of water and the solution was degassed afterward. Upon addition of gaseous ammonia, a green solid was formed and dissolved in the solution as the ammonia concentration in the solution increased. Afterward, the excess ammonia was removed by blowing a slow but steady flow of argon over the solution. The green, amorphous solid formed once again and was filtered off. Upon removal of water and ammonia under vacuum, orange-pink crystals of Co(NH3)2[dca]2 started to grow. Crystals were taken out of the solution in an argon stream and stored under perfluoro polyalkyether. Suitable crystals for single-crystal X-ray diffraction were selected from the stored sample. The elemental analysis (wt %: Co 25.9, C 22.1, H 2.8, N 49.2) is in good agreement with the theoretical values (wt %: Co 26.2, C 21.3, H 2.7, N 49.8). PXRD of a powdery sample of Co(NH3)2[dca]2 (0.8223 g, 3.65 mmol, 48.5%) was done in a glass capillary to show its purity and confirm the crystal structure. The XRD and Rietveld refinement data B


Article

Inorganic Chemistry Full details concerning the structure determination including all intensity data have been deposited under CCDC 1896186 (Ni phase) and 1896187 (Co phase). Copies of the data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: [email protected]). Computational Details. DFT calculations were carried out with the Vienna ab initio simulation package (VASP 5.4.4)44−47 for Ni(NH3)2[dca]2 by employing the PBE functional48 together with the projector-augmented wave method.49 Additionally, the D3 dispersion correction of Grimme and co-workers in conjunction with Becke− Johnson damping (D3-BJ) was used to account for van der Waals interactions.50 The kinetic energy cutoff of the plane-wave expansion was set to 500 eV. The structural optimization was performed with a convergence criterion of 10−6 eV, while a more strict 10−8 eV was used as the criterion for the electronic optimization and assuming a ferromagnetic ordering. After checking the k-mesh energetic convergence, a 4 × 2 × 3 supercell of the relaxed structure was created51,52 for subsequent phonon calculations with the finite displacement method, as implemented in Phonopy.53 A displacement of 0.01 Å was chosen, and only the Γ point was taken to calculate the forces. Phonopy54,55 and a custom-made MATLAB script56,57 were then utilized to obtain anisotropic displacement parameters (ADPs) by using a frequency cutoff of 0.1 THz and 50 × 30 × 45 q-points. It has been previously shown that theoretically predicted ADPs can be successfully incorporated for structure refinements, such as refining the hydrogen positions whenever facing experimental issues.58 Furthermore, it was already demonstrated that theoretical ADPs of neutron quality correlate extremely well with the experimental counterparts, e.g., in the case of urea.59 Neutron Powder Diffraction. Time-of-flight (TOF) neutron powder diffraction (NPD) experiments were performed with the POWGEN powder diffractometer at the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory (ORNL). Powder samples of Ni(NH3)2[dca]2 were filled into vanadium cans of 8 mm diameter. Vanadium and background corrections were applied using POWGEN’s standard data-reduction algorithm. TOF diffraction patterns were collected at 10 K with a neutron center-wavelength, λ = 0.800 Å, covering reflections with d-spacings in the range of 0.709−3.631 Å. The TOF data were refined with the FullProf suite60 taking profilefunction parameters from the instrument resolution file. Theoretically calculated anisotropic displacement parameters were applied to freely refine all hydrogen positions despite strong incoherent scattering (for further information see the “Computational Details” section). The sample exhibits a microstrain, so a broadening model in the quartic form was applied according to Laue class 2/m.61 More details about the latter parameters are given in Table S7. The neutron crystallographic parameters are found in Table 1, while Tables S8 and S9 contain spatial, isotropic, and anisotropic displacement parameters. Figure 1 provides an overview of the neutron Rietveld refinement. Infrared Spectra. IR spectra were recorded within the range of 4000−400 cm−1 using an ALPHA II FT-IR-spectrometer from Bruker equipped with an ATR Platinum Diamond measuring cell. All measurements were performed in an argon filled glovebox. The numerical data are found in Table S10. UV/Vis Spectra. The UV/vis spectrum of Ni(NH3)2[dca]2 was recorded within the 300−1400 nm range using an UV-2600 spectrometer from Shimadzu. A powdery sample was placed and pressed onto a BaSO4 pallet and measured in reflection geometry. Figure S2 shows the corresponding spectrum. Because of its oxygen sensitivity, measuring the cobalt sample was impossible given our apparatus. Magnetic Measurement. The magnetic properties of the title compounds were measured with a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-5XL). The polycrystalline samples were compacted and immobilized into a cylindrical polytetrafluoroethylene capsule. Measurements included temperature-dependent molar magnetic susceptibilities (2.0−290 K) at 0.1 T and field-dependent molar

Table 1. Crystallographic Data of Ni(NH3)2[dca]2 Obtained from Neutron Powder Diffraction source chemical formula formula weight (g·mol−1) temperature (K) λ (Å) crystal system space group a (Å) b (Å) c (Å) β (°) V (Å3) Z pattern d-range (Å) excluded d-region (Å) χ2 Rpa Rwpb

neutron time-of-flight (TOF) Ni(NH3)2[N(CN)2]2 224.8 10 0.800 monoclinic P21/c (no. 14) 5.75865(8) 10.69307(14) 6.72971(9) 96.4116(9) 411.808(10) 2 0.709−3.631 0−0.709 3.631−8.1869 7.22 4.97 5.04

Rp = 100·∑i(yoi − yci)/∑i(yoi). bRwp = 100·[∑iw(yoi − yci)2/∑i w(yoi)2]1/2. a

magnetizations (0−5.0 T) at 2.0 K. The data were corrected for diamagnetic contributions of the sample holders and the intrinsic contributions of the compounds (χdia,Ni = −1.124 × 10−4 cm3·mol−1, χdia,Co = −1.125 × 10−4 cm3·mol−1). Chemicals and Reagents. The following chemicals were used without further purification: NiCl2·6H2O (Merck, ≥98%), CoCl2· 6H2O (Riedel-de Haën, 99%), Na[N(CN)2] (Alfa Aesar, 96%) NH3 solution (25 wt %, Chemsolute, 99.998%), NH3 gas (Westfalen AG, 99.9999%).

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RESULTS Structural Description and Discussion. The crystalstructure details for Ni(NH3)2[dca]2 and Co(NH3)2[dca]2 presented here stem from the single crystals. The isotypism of both crystal structures was immediately obvious, and the structure type found manifests a new archetype which belongs to the monoclinic system with space group P21/c (no. 14) and Z = 2. The single-crystal lattice parameters are a = 5.7679(10) Å, b = 10.6906(19) Å, c = 6.7464(11) Å, and β = 96.719(3)° for Ni(NH3)2[dca]2. The boomerang-shaped dicyanamide anion of the nickel compound, exemplarily discussed in what follows, exhibits bond lengths and angles consistent with those given in the literature:4,23 We find d(C1−N1) = 1.151(4) and d(C2−N3) = 1.149(4) Å of the terminal C−N pairs which indicates a triple bond, while the central C−N distance with d(C1−N2) = 1.317(4) and d(C2−N2) = 1.312(4) Å corresponds to the expected distance of a C−N single bond. The angles of the [dca] anion are also typical for such a moiety with ∠(N1−C1− N2) = 173.1(4)°, ∠(N2−C2−N3) = 173.1(3)° and ∠(C1− N2−C2) = 120.5(3)°. Ni coordinates to nitrile N atoms of four dicyanamide ligands in the equatorial plane with d(Ni−N1) = 2.081(3) and d(Ni−N3) = 2.092(3) Å and to two ammonia-type N atoms in the axial position with d(Ni−N4) = 2.090(3) Å to form an almost perfect octahedron (Figure 2). These distances correspond well to those in Ni[dca]2 (d(Ni−N) = 2.05 and 2.14 Å)11,62 and Ni(NH3)6Cl2 (d(Ni−N) = 2.12 Å).63 Each nickel atom is connected to four other nickel atoms by end-toC


Article

Inorganic Chemistry

Figure 2. Crystal structure of Ni(NH3)2[dca]2 showing (a) the coordination of the nickel atom and (b) the side view on two Ni[dca]2 layers. The ammonia molecules were removed to allow better visibility of the layers in red and blue. The atomic displacement ellipsoids correspond to 50% probability using the refined anisotropic displacement parameters (ADPs) of the single-crystal structure refinement.

Figure 3. Hydrogen bond network between ammonia and the dicyanamide moiety in Ni(NH3)2[dca]2. The two red nickel atoms belong to two different Ni[dca]2 layers, while all blue nickel belong to one. The atomic displacement ellipsoids correspond to 90% probability using the refined (C, N, and Ni) and calculated (H) anisotropic displacement parameters (ADPs), respectively.

hydrogen to coordinate, while the terminal nitrogen atoms primarily coordinate to the metal atom.

end dicyanamide bridges forming corrugated layers of octahedra. These μ1,5 bonds differentiate the structure from the one of Cu(NH3)2[dca]235 with μ1,3 and μ1,5 dicyanamide bonds. While the nickel atoms lie within the layers, the dicyanamide moieties are found above or below. The layers are arranged in a parallel fashion along the [102] direction with a distance of d = 3.0516(2) Å (defined as the orthogonal vector between the layers) to each other (Figure 2). These layers are connected via hydrogen bridges formed by all hydrogen atoms of the ammonia molecule to all three nitrogen atoms of the dicyanamide anion. The bond lengths and angles for the cobalt compound are in the same range, as found in Table 2. They also fit well to the data of Co[dca]2 (d(Co−N) = 2.10 and 2.16 Å)11 and Co(NH3)6Cl2 (d(Co−N) = 2.16 Å).64

Table 3. Hydrogen Bond Lengths (Å) in Ni(NH3)2[dca]2

Ni(NH3)2[dca]2

Co(NH3)2[dca]2

120.5(3) 173.1(4) 173.1(3) 1.152(4) 1.148(4) 1.317(4) 1.312(3) 2.081(3) 2.092(3) 2.090(3)

120.6(2) 173.0(3) 173.1(3) 1.158(3) 1.153(3) 1.307(3) 1.311(3) 2.120(3) 2.135(3) 2.137(3)

D−H

(Å)

H···A

(Å)

N4−H1 N4−H2 N4−H3

0.981(5) 1.008(5) 0.986(5)

H1···N1 H2···N2 H3···N3

2.769(5) 2.499(5) 2.808(5)

Regarding volume chemistry, we calculated the incremental volume of the [dca]− moiety using the method of Biltz.65 For ammonia, the incremental volume of a formula unit was derived from its crystal structure.66 The calculated incrementals of V([dca]−, Ni phase) = 40.9 cm3·mol−1 and V([dca]−, Co phase) = 41.7 cm3·mol−1 fit quite well to the reported average volume of dicyanamide4 V̅ ([dca]−) = 44.3(17) cm3· mol−1, in particular regarding the fact that the structures presented here were determined at a lower temperature (Table 4). Magnetism. Ni(NH3)2[dca]2 shows the typical magnetic behavior of a high-spin Ni2+ center with S = 1, as depicted in Figure 4. At 290 K and 0.1 T, the product of molar magnetic susceptibility and temperature χmT reaches a value of 1.24 cm3· K·mol−1 (μeff = 3.15 μB), which is well within the expected range of an isolated Ni2+ cation (0.98−1.53 cm3·K·mol−1).67 Upon cooling the compound, χmT is almost constant, linearly decreasing to 1.20 cm3·K·mol−1 at 100 K for all measured batches. Below that temperature, variously pronounced maxima are observed at about 14 K from one batch to the other. In general, such variation of the data between batches of the same compound is indicative of small impurities. In particular, the shape of the χmT versus T curve and the temperature at which the maximum occurs are most likely due to a very small amount of α-Ni[dca]2 characterized by a Curie temperature TC of 21 K.11 The formation of α-Ni[dca]2 is likely because it could be caused by outgassing of NH3 since neither storage nor measurements of the compounds are under an NH3 atmosphere. Although we observed a correlation between the age of the sample and the magnitude of the maximum, we could not fully eliminate the latter. To estimate the contribution of Ni(NH3)2[dca]2, we linearly extrapolate χmT from the higher temperatures yielding a value of 1.18 cm3·K·mol−1 (μeff = 3.07 μB) at 14 K. Further decrease of the temperature eventually results in a distinct drop of the χmT values below 6 K for all batches. Therefore, the χmT versus T curve with respect to the whole temperature range is in

Table 2. Selected Angles (deg) and Bond Lengths (Å) of Ni(NH3)2[dca]2 and Co(NH3)2[dca]2. ∠(C1−N2−C2) ∠(N1−C1−N2) ∠(N2−C2−N3) C1−N1 C2−N3 C1−N2 C2−N2 M−N1 (×2) M−N3 (×2) M−N4 (×2)

D−H···A N4−H1···N1 N4−H2···N2 N4−H3···N3

To allow for better accuracy, the information about the hydrogen network was extracted from the neutron powder data augmented by theoretical hydrogen ADPs as calculated from first principles. The neutron structure refinement with freely refined hydrogen positions shows that the layers are connected by three different hydrogen bonds between the ammonia molecule and all three nitrogen atoms of the dicyanamide moiety. Two hydrogen atoms coordinate to both terminal nitrogens of one dicyanamide with d(N1−H1) = 2.769(5) and d(N3−H3) = 2.808(5) Å and stem from the same ammonia molecule. The third one, however, coordinates the bridging nitrogen of a second dicyanamide with d(N2−H2) = 2.499(5) Å. Thus, we are facing two very weak and one stronger hydrogen bridge per ammonia between two layers, as depicted in Figure 3 and numerically given in Table 3. This is not surprising because the bridging nitrogen has only one D


Article

Inorganic Chemistry Table 4. Calculated Molar Volumes of [dca]− Based on M(NH3)2[dca]2 with M = Ni and Co compound

cell volume (Å3), Z

molar volume (cm3·mol−1)

M2+ volume (cm3·mol−1)65

NH3 volume (cm3·mol−1)66

[dca]− volume (cm3·mol−1)

Ni(NH3)2[dca]2 Co(NH3)2[dca]2

413.14, 2 421.95, 2

124.4 127.0

2 3

20.3 20.3

40.9 41.7

Figure 4. Temperature dependence of χmT at 0.1 T and molar magnetization Mm versus applied magnetic field B at 2.0 K (insets); (a) two different batches of Ni(NH3)2[dca]2 (χmT starts at 0.8 cm3·K·mol−1) and (b) Co(NH3)2[dca]2.

Figure 5. ATR-IR spectra of (a) Ni(NH3)2[dca]2 and (b) Co(NH3)2[dca]2.

agreement with Ni2+ centers characterized by small distortions of the octahedral ligand field causing zero-field splitting. Furthermore, weak ferromagnetic or antiferromagnetic exchange interactions of lower dimensionality (that is, 2D and less, e.g., between the Ni2+ cations in the (102) plane) cannot be excluded based on the SQUID data. Since the analysis of the neutron powder-diffraction data reveals no evidence for long-range order exchange interactions, however, we consider them as nonexistent. Because of the small extent of the impurities in all batches, the molar magnetization at 2.0 K (see Figure 4a) is only distinctly affected at fields up to 1 T. At larger fields, Mm increases with continuously decreasing slope and reaches a value of 2.0 NAμB (corresponding to a magnetic moment of 2.0 μB per Ni2+ center) at 5.0 T without being saturated. Taking into account the χmT versus T curve, we estimate a saturation value of about 2.2−2.3 NAμB for an

isolated Ni2+ center of Ni(NH3)2[dca]2. Since this value is close to the observed value at 5.0 T, the data are in agreement with the absence of exchange interactions, as alluded to before. For Co(NH3)2[dca]2, the SQUID measurements reveal high-spin Co2+ centers with S = 3/2 and distinct contributions from the partially unquenched orbital momentum, as presented in Figure 4b. At 290 K and 0.1 T, χmT reaches a value of 2.89 cm3·K·mol−1 (μeff = 4.81 μB), which is well within the expected range 2.31−3.51 cm3·K·mol−1 of an isolated Co2+ center.67 By decreasing the temperature, χmT continuously decreases, taking a value of 1.60 cm3·K·mol−1 at 2.0 K. This behavior is not due to antiferromagnetic exchange interactions but is caused by the single-ion effects of octahedrally coordinated Co2+ centers, namely electron interrepulsion, ligand-field effect, and spin−orbit coupling. The latter induces distinct mixing of the states, in particular between the 4T1g E


Article

Inorganic Chemistry

field theory allows the calculation of the ligand strength f([dca]−) in a heteroleptic complex if the octahedral shows little to no distortion.74 Using ΔO = f·g with the weighting of the ligand components and the reported g-value of g(Ni2+) = 8700 cm−1,74 the calculation yields that dicyanamide arrives at f([dca]−) = 1.13. It is a medium-field ligand located between thiocyanate and pyridine, as compared in Table 6.

states originating from the 4F ground term and the excited 4P term. All effects combined generate also magnetic anisotropy and result in the decrease of χmT upon cooling due to the thermal depopulation of the energy states. Therefore, the data must not be interpreted in terms of the Curie−Weiss law. The anisotropy is also reflected in the Mm versus B plot at 2.0 K. The magnetization sharply rises with rising fields up to 2 T, and marginally increases at higher fields approaching 2.0 NAμB at 5.0 T. The saturation magnetization is, however, expected to be about 3.6−4.0 NAμB estimated from the χmT value at room temperature. Both observations can be readily explained by the mixed nature of the energy states as well as the anisotropy of the Co2+ center. Besides mixing energy states with less magnetic moments into the ground state, the easy-axis magnetization is (almost) saturated at 5.0 T for anisotropic center, while the magnetizations in the direction of the other axes are smaller and slowly increase with the applied field. Since the data were taken from a powder sample, they show the mean magnetization which is, therefore, just about half of the saturation magnetization. Summarizing, both the Mm versus B curves and the χmT versus T curves of Ni(NH3)2[dca]2 and Co(NH3)2[dca]2 are consistent with slightly distorted octahedral Ni2+ or Co2+ centers, respectively, with zero or negligible exchange interactions between these centers. The distinct deviations of the magnetizations at fields below 1 T and the values of χmT below 100 K for different batches of Ni(NH3)2[dca]2 indicate the presence of an impurity, most likely α-Ni[dca]2. Moreover, the absence of any additional characteristic χmT or Mm signals confirms the nonexistence of long-range magnetic ordering for both compounds. IR Spectra. The frequencies as obtained from the IR spectra of both title compounds confirm the presence of the [dca] group and ammonia, as depicted in Figure 5. The typical vibrations of [dca] have been reported before, e.g., for Hg2[dca]2 and Tl[dca]4. The remaining bands were assigned to ammonia.34,68 In total, the IR spectra are in good agreement with the data of Cu(NH3)2[dca]2.34 The assignment of the vibrations can be found in Table S9. UV/Vis Spectrum. The UV/vis spectrum of Ni(NH3)2[dca]2 shows three absorption bands at ν1 = 10 200 cm−1, ν2 = 17 000 cm−1, and ν3 = 28 650 cm−1. These electronic transitions fit very well for an octahedral d8 complex and are comparable to Ni[dca]269 and [Ni(NH3)62+].70 The numerical entries are given in Table 5, and the spectrum can be found in Figure S2.

Table 6. Ligands and Their Respective Strength Parameter f after Jørgensen74 in Comparison to the Here Determined Strength Parameter of [dca]− ligand −

Br SCN− Cl− F− H2O NCS−

T2g(F) ← 3A2g

Ni(NH3)2[dca]2 [Ni(NH3)62+]70 Ni[dca]269

10200 10750 9700

3

T1g(F) ← 3A2g 17000 17500 16000

3

0.72 0.73 0.78 0.90 1.00 1.02

ligand −

[dca] py NH3 en bipy CN−

f 1.13 1.23 1.25 1.28 1.33 1.70

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CONCLUSIONS With Ni(NH3) 2 [dca] 2 and Co(NH3 )2 [dca]2 , two new heteroleptic nickel and cobalt complexes with dicyanamide ([N(CN)2]− and [dca]−) and ammonia as ligands were synthesized. Their crystal structures were determined, magnetic measurements performed, and their IR spectra and the UV/vis spectrum of the nickel compound measured. For Ni(NH3)2[dca]2, the structure model was improved by combining neutron powder diffraction with state-of-the-art DFT calculations, especially to determine the hydrogen positions and to derive meaningful anisotropic displacement parameters. Magnetic measurements are consistent with slightly distorted octahedral high-spin Ni2+ and Co2+ centers and the absence of exchange interactions between these centers. Therefore, long-range magnetic ordering is nonexistent in both compounds. The acquired data of the IR spectra are similar to the data of the previously reported dicyanamides and confirm the presence of the dicyanamide and ammonia moieties. The dicyanamide anion was characterized by its incremental volume in both compounds, and its medium-field ligand strength was determined from the UV/vis spectrum. The structures differ from the framework modification of Cu(NH3)2[dca]2 in the coordination of the [dca] moiety. While Ni/Co(NH3)2[dca]2 only have μ1,5 bonds, one finds μ1,3 and μ1,5 bonds in Cu(NH3)2[dca]2. The difference is most likely caused by the Jahn−Teller effect and should be investigated further. The next step toward new magnetic compounds with μ1,3 bonds could be a theoretical Cr(NH3)2[dca]2 compound, for example. Since the [dca] is a medium-field ligand, it should be a high-spin d4-complex and is affected by the Jahn−Teller effect just like Cu(NH3)2[dca]2.

Table 5. Absorption Bands of Ni(NH3)2[dca]2 in Comparison to Those of [Ni(NH3)62+]70 and Ni[dca]269 in Wavenumber (cm−1) and the Corresponding Electronic Transitions 3

f

T1g(P) ← 3A2g

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28650 28200 26140

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00448. PXRD of Co(NH3)2[dca]2, crystallographic data of M(NH3)2[dca]2 with M = Ni and Co including X-ray intensities, atomic coordinates, and isotropic displacement parameters of the X-ray and TOF neutron

The Racah parameter was calculated from the three absorption bands of Ni(NH3)2[dca]2, applying the reported equation for A2 ground-state ions,71 and it results in B′ = 1007 cm−1. The corresponding nephelauxetic ratio of β = 0.97 indicates a weak nephelauxetic effect and hence some delocalization of the d electrons.71−73 Additionally, ligandF


Article

Inorganic Chemistry

(7) Irran, E.; Jürgens, B.; Schnick, W. Trimerization of alkali dicyanamides M[N(CN)2] and formation of tricyanomelaminates M3[C6N9] (M = K, Rb) in the melt: crystal structure determination of three polymorphs of K[N(CN)2], two of Rb[N(CN)2], and one of K3[C6N9] and Rb3[C6N9] from X-ray powder diffractometry. Chem. Eur. J. 2001, 7, 5372−5381. (8) Starynowicz, P. Structure of caesium dicyanamide. Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1991, 47, 2198−2199. (9) Jürgens, B.; Irran, E.; Schnick, W. Syntheses, Vibrational Spectroscopy, and Crystal Structure Determination from X-Ray Powder Diffraction Data of Alkaline Earth Dicyanamides M[N(CN)2]2 with M = Mg, Ca, Sr, and Ba. J. Solid State Chem. 2001, 157, 241−249. (10) Manson, J. L.; Kmety, C. R.; Epstein, A. J.; Miller, J. S. Spontaneous Magnetization in the M[N(CN)2]2 (M = Cr, Mn) Weak Ferromagnets. Inorg. Chem. 1999, 38, 2552−2553. (11) Manson, J. L.; Kmety, C. R.; Huang, Q.-z.; Lynn, J. W.; Bendele, G. M.; Pagola, S.; Stephens, P. W.; Liable-Sands, L. M.; Rheingold, A. L.; Epstein, A. J.; Miller, J. S. Structure and Magnetic Ordering of MII[N(CN)2]2 (M = Co, Ni). Chem. Mater. 1998, 10, 2552−2560. (12) Reckeweg, O.; Dinnebier, R. E.; Schulz, A.; Blaschkowski, B.; Schneck, C.; Schleid, T. About the air- and water-stable copper(I) dicyanamide: synthesis, crystal structure, vibrational spectra and DSC/TG analysis of Cu[N(CN)2]. Z. Naturforsch., B: J. Chem. Sci. 2017, 72, 159−165. (13) Hodgson, S. A.; Hunt, S. J.; Sørensen, T. J.; Thompson, A. L.; Reynolds, E. M.; Faulkner, S.; Goodwin, A. L. Anomalous Thermal Expansion and Luminescence Thermochromism in Silver(I) Dicyanamide. Eur. J. Inorg. Chem. 2016, 2016, 4378−4381. (14) Reckeweg, O.; Schulz, A.; Schneck, C.; Lissner, F.; Schleid, T. Syntheses, single-crystal structures, vibrational spectra and DSC/TG analyses of orthorhombic and trigonal Ag[N(CN)2]. Z. Naturforsch., B: J. Chem. Sci. 2016, 71, 827−834. (15) Manson, J. L.; Lee, D. W.; Rheingold, A. L.; Miller, J. S. Buckled-layered Structure of Zinc Dicyanamide, ZnII[N(CN)2]2. Inorg. Chem. 1998, 37, 5966−5967. (16) Jürgens, B.; Irran, E.; Höppe, H. A.; Schnick, W. Phase Transition of a Dicyanamide with Rutile-like Structure: Syntheses and Crystal Structures of α- and β-Cd[N(CN)2]2. Z. Anorg. Allg. Chem. 2004, 630, 219−223. (17) Jürgens, B.; Irran, E.; Schnick, W. Synthesis and characterization of the rare-earth dicyanamides Ln[N(CN)2]3 with Ln = La, Ce, Pr, Nd, Sm, and Eu. J. Solid State Chem. 2005, 178, 72−78. (18) Nag, A.; Schmidt, P. J.; Schnick, W. Synthesis and Characterization of Tb[N(CN)2]3·2H2O and Eu[N(CN)2]3·2H2O: Two New Luminescent Rare-Earth Dicyanamides. Chem. Mater. 2006, 18, 5738−5745. (19) Nag, A.; Schnick, W. Synthesis, Crystal Structure and Thermal Behavior of Gadolinium Dicyanamide Dihydrate Gd[N(CN)2]3· 2H2O. Z. Anorg. Allg. Chem. 2006, 632, 609−614. (20) Reckeweg, O.; Wakabayashi, R. H.; DiSalvo, F. J.; Schulz, A.; Schneck, C.; Schleid, T. About alkali metal dicyanamides: syntheses, single-crystal structure determination, DSC/TG and vibrational spectra of KCs[N(CN)2]2 and NaRb2[N(CN)2]3·H2O. Z. Naturforsch., B: J. Chem. Sci. 2015, 70, 365−372. (21) Reckeweg, O.; DiSalvo, F. J. Synthesis and single-crystal structure of the pseudo-ternary compounds LiA[N(CN)2]2 (A = K or Rb). Z. Naturforsch., B: J. Chem. Sci. 2016, 71, 157−160. (22) Jürgens, B.; Milius, W.; Morys, P.; Schnick, W. Trimerisierung von Dicyanamid-Ionen C2N3− im Festkörper - Synthesen, Kristallstrukturen und Eigenschaften von NaCs2(C2N3)3 und Na3C6N9·3· H2O. Z. Anorg. Allg. Chem. 1998, 624, 91−97. (23) Mann, M.; Reckeweg, O.; Dronskowski, R. Synthesis and Characterization of the New Dicyanamide LiCs2[N(CN) 2]3. Inorganics 2018, 6, 108. (24) Shang, Y.; Niu, F.; Shen, S. Photocatalytic water oxidation over BiVO4 with interface energetics engineered by Co and Ni-metallated dicyanamides. Chin. J. Catal. 2018, 39, 502−509.

scattering, the microstrain and the Lorentzian-anisotropic-strain mixing parameter of the Rietveld refinement, and the assignment of the IR vibrations and the UV/vis spectrum (PDF) Accession Codes

CCDC 1896186 and 1896187 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 (0) 241 80-92642. Tel.: +49 (0) 241 80-93642. Email: [email protected]. ORCID

Jan van Leusen: 0000-0003-3688-631X Richard Dronskowski: 0000-0002-1925-9624 Author Contributions

M.M. performed the syntheses, determined the structures, and performed the UV/vis measurement. L.H. performed the ATRIR measurements with the support of M.M. The TOF neutron powder diffraction experiment was performed by M.M. and A.H. DTF calculations were performed by D.M. The magnetic measurements were performed and interpreted by J.v.L. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Melanie Kirkham and Cheng Li for assistance with the neutron diffraction experiments at POWGEN. We gratefully acknowledge the financial support provided by JCNS to perform the neutron scattering measurements at the Spallation Neutron Source (SNS), Oak Ridge, TN. Part of the research conducted at the SNS was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Computer time was provided by the Jülich−Aachen Research Alliance (Project No. jara0069).

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