Magnetic Properties of a Dinuclear Nickel(II) Complex with 2,6-Bis[(2

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Magnetic Properties of a Dinuclear Nickel(II) Complex with 2,6-Bis[(2hydroxyethyl)methylaminomethyl]-4-methylphenolate Hiroshi Sakiyama,*,† Yukako Chiba,† Katsuya Tone,† Mikio Yamasaki,‡ Masahiro Mikuriya,§ J. Krzystek,⊥ and Andrew Ozarowski⊥ †

Department of Material and Biological Chemistry, Faculty of Science, Yamagata University, 1-4-12 Kojirakawa, Yamagata 990-8560, Japan ‡ X-ray Research Laboratory, Rigaku Corporation, Matsubara 3-9-12, Akishima, Tokyo 196-8666, Japan § Department of Applied Chemistry for Environment and Research Center for Coordination Molecule-based Devices, School of Science and Technology, Kwansei Gakuin University, Gakuen 2-1, Sanda 669-1337, Japan ⊥ National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310, United States S Supporting Information *

ABSTRACT: Magnetic properties of dinuclear nickel(II) complex [Ni2(sym-hmp)2](BPh4)2·3.5DMF·0.5(2-PrOH) (1), where (symhmp)− is 2,6-bis[(2-hydroxyethyl)methylaminomethyl]-4-methylphenolate anion and DMF indicates dimethylformamide, were investigated using high-frequency and -field electron paramagnetic resonance (HFEPR). To magnetically characterize the mononuclear nickel(II) species forming the dimer, its two dinuclear zinc(II) analogues, [Zn2(sym-hmp)2](BPh4)2·3.5DMF·0.5(2-PrOH) (2) and [Zn2(sym-hmp)2](BPh4)2·2acetone·2H2O (2′), were prepared. One of them (2′) was structurally characterized by X-ray diffractometry and doped with 5% mol nickel(II) ions to prepare a mixed crystal 3. From the HFEPR results on complex 1 obtained at 40 K, the spin Hamiltonian parameters of the first excited spin state (S = 1) of the dimer were accurately determined as |D1| = 9.99(2) cm−1, |E1| = 1.62(1) cm−1, and g1 = [2.25(1), 2.19(2), 2.27(2)], and for the second excited spin state (S = 2) at 150 K estimated as |D2| ≈ 3.5 cm−1. From these numbers, the single-ion zero-field splitting (ZFS) parameter of the Ni(II) ions forming the dimer was estimated as |DNi| ≈ 10−10.5 cm−1. The HFEPR spectra of 3 yielded directly the single-ion parameters for DNi = +10.1 cm−1, | ENi| = 3.1 cm−1, and giso = 2.2. On the basis of the HFEPR results, the previously obtained magnetic data (Sakiyama, H.; Tone, K.; Yamasaki, M.; Mikuriya, M. Inorg. Chim. Acta 2011, 365, 183) were reanalyzed, and the isotropic interaction parameter between the Ni(II) ions was determined as J = −70 cm−1 (Hex = −J SA·SB). Finally, density functional theory calculations yielded the J value of −90 cm−1 in a qualitative agreement with the experiment.

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INTRODUCTION Polynuclear clusters containing paramagnetic transition metal ions linked and magnetically coupled through organic ligands have attracted considerable attention in the last two decades due to their unusual properties qualifying some of them as socalled single-molecule magnets (SMMs).1−4 Such molecules have been proposed as prospective candidates as qubits in quantum computing5,6 or as magnetic memory elements7 in potential applications. Magnetic properties of SMMs depend primarily on (a) anisotropy of their building blocks, that is, paramagnetic metal ions involved, and (b) the nature of coupling between them. The latter can be extremely complex in systems containing a large number of metal ions, sometimes as many as 25,8 which makes it difficult to study the coupling mechanisms. It is thus desirable to investigate much simpler systems such as dimers to elucidate and understand the coupling mechanisms between those ions. © XXXX American Chemical Society

Among many SMMs reported there have been several examples of polynuclear clusters consisting of nickel(II) ions, both in homonuclear9−11 and heteronuclear12−14 environments. With regard to nickel(II) dimers, however, only a few have been studied so far.15−17 With this in mind, some of us previously synthesized and reported on magnetic properties of a dinuclear nickel(II) complex [Ni2(sym-hmp)2](BPh4)2· 3.5DMF·0.5(2-PrOH) (1), where (sym-hmp)− is 2,6-bis[(2hydroxyethyl)methylaminomethyl]-4-methylphenolate anion and DMF indicates dimethylformamide (Scheme 1).18 Temperature dependence of magnetic susceptibility in 1 indicated an anti-ferromagnetic interaction between two nickel(II) ions with the interaction parameter J ≈ −70 cm−1 (Hex = −J SA·SB). This complex contains a bis(μ-phenolato)Received: July 19, 2016

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DOI: 10.1021/acs.inorgchem.6b01671 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Structure of the (sym-hmp)− Anion, the Ligand in Complexes 1, 2, 2′, and 3, and the Structure of [Ni2(symhmp)2]2+ Complex Cation

Table 1. Crystallographic Data and Refinement Parameters of 2′ empirical formula formula weight crystal system space group a/Å b/Å c/Å β/deg V/Å3 Z crystal dimensions/mm T/K λ/Å ρcalcd/g cm−3 μ/cm−1 F(000) 2θmax/deg No. of reflections measured No. of independent reflections data/variables R1a (I > 2.00σ(I)) wR2b (all reflections) goodness of fit indicator highest peak, deepest hole/e Å−3

dinickel(II) core structure. Magnetic properties19−21 of the phenolato-bridged complexes as well as their relation to metalloenzymes22−24 were studied recently. What follows is a critical re-examination of previous results using the technique of high-frequency and -field electron paramagnetic resonance (HFEPR). HFEPR allows us to study the magnetic properties of dimers similar to 1 even in the case of anti-ferromagnetic coupling in a given complex leading to a diamagnetic ground spin state (S = 0). This is so because of the presence of excited paramagnetic spin states (S = 1, 2, ...) that in propitious circumstances are thermally accessible and amenable to HFEPR studies.17,25−27 In particular, the aim of the current study was to determine experimentally the zerofield splitting (ZFS) characterizing the excited state(s) of the dimer, as well as the ZFS of the individual Ni(II) ions forming it, to resolve the role of the latter in the magnetic properties of the dimer. The nickel(II) ion is one of the simplest ions that show ZFS; the Ni(II) dimer is thus well-suited for studying the relationship between the individual ion and the coupled ions. As will be discussed later, the single-ion ZFS is the principal contribution to the ZFS of the excited dimeric states. To extract the single-ion ZFS of Ni(II), we also synthesized two structural analogues of 1, namely, the dimeric zinc(II) complexes [Zn2(sym-hmp)2](BPh4)2·3.5DMF·0.5(2-PrOH) (2) and [Zn2(sym-hmp)2](BPh4)2·2acetone·2H2O (2′). The latter was characterized structurally via X-ray diffraction (complex 2 did not result in suitable single crystals). Finally, we doped complex 2′ with 5% mol nickel(II) preparing complex 3, which we subjected to HFEPR studies.

R1 = ∑∥Fo| - |Fc∥/∑|Fo|. ∑w(Fo2)2]1/2. a

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RESULTS AND DISCUSSION Crystal Structure of [Zn2(sym-hmp)2](BPh4)2·2acetone· 2H2O (2′). Crystal data are included in Table 1. The crystal structure of 2′ is isomorphous to that of a previous cobalt(II) complex.28,29 The crystal consists of [Zn2(sym-hmp)2]2+ complex cations, tetraphenylborate anions, acetone molecules, and water molecules in a 1:2:2:2 molar ratio. The structure of the complex cation is depicted in Figure 1. Selected distances and angles with their estimated standard deviations are listed in Table 2. In the complex cation, a pair of sym-hmp− ligands incorporates two zinc(II) ions; the two zinc(II) ions are bridged by two phenolate oxygen atoms of sym-hmp−, forming a bis(μ-phenolato)dizinc(II) core structure. The Zn(1)···Zn(2)

C84H106B2N4O10Zn2 1484.16 monoclinic C2/c 22.8866(5) 19.2321(4) 17.2553(4) 95.991(7) 7553.5(3) 4 0.20 × 0.17 × 0.15 93 0.71075 1.305 6.975 3152.00 55.0 60492 8662 (Rint = 0.0159) 8662/477 0.0246 0.0642 1.040 0.36, −0.23 b

wR2 = [∑(w(Fo 2 − Fc2)2)/

Figure 1. ORTEP drawing of [Zn2(sym-hmp)2]2+ in 2′ with the atomic numbering scheme. The thermal ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity.

separation is 3.2465(3) Å; a very similar separation was found in the previously studied dinuclear nickel(II) complex 118 [Ni 2 (sym-hmp) 2 ](BPh 4 ) 2 ·3.5DMF·0.5(2-PrOH) [Ni(1)··· Ni(2) = 3.2376(4) Å]. The Zn(1)···Zn(2) axis is a crystallographic C2 axis parallel to the b axis. The dihedral angle between two aromatic rings is ∼75°, and Zn−O−Zn bridging angle is 105.16(4)°. Each zinc(II) ion has a distorted octahedral geometry with a cis-N2O4 donor set, similar to the nickel(II) B


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Inorganic Chemistry Table 2. Selected Intramolecular Distances (Å) and Angles (deg) Zn(1)−O(1) Zn(1)−N(1) Zn(2)−O(3) Zn(1)···Zn(2) O(1)−Zn(1)−O(1)′ O(1)−Zn(1)−O(2)′ O(1)−Zn(1)−N(1)′ O(2)−Zn(1)−N(1) N(1)−Zn(1)−N(1)′ O(1)−Zn(2)−O(3) O(1)−Zn(2)−N(2) O(3)−Zn(2)−O(3)′ O(3)−Zn(2)−N(2)′ Zn(1)−O(1)−Zn(2)

2.0350(9) 2.2121(10) 2.1680(10) 3.2465(3) 75.23(4) 90.04(4) 155.44(4) 76.41(4) 110.59(4) 113.23(4) 89.53(4) 153.86(3) 89.65(4) 105.16(4)

Zn(1)−O(2) Zn(2)−O(1) Zn(2)−N(2)

2.1545(9) 2.0529(9) 2.1929(10)

O(1)−Zn(1)−O(2) O(1)−Zn(1)−N(1) O(2)−Zn(1)−O(2)′ O(2)−Zn(1)−N(1)′ O(1)−Zn(2)−O(1)′ O(1)−Zn(2)−O(3)′ O(1)−Zn(2)−N(2)′ O(3)−Zn(2)−N(2) N(2)−Zn(2)−N(2)′

108.46(4) 89.57(4) 156.90(4) 90.40(4) 74.46(4) 88.03(4) 151.02(4) 76.11(4) 114.06(4)

ion in complex 1. The two zinc(II) ions in [Zn2(sym-hmp)2]2+ are almost equivalent, and the symmetry of the dizinc core is approximated as D2 (in a precise sense C2); the dizinc core is twisted along the Zn···Zn axis. The dihedral angle between the two terminal N···N edges is ∼54°, and the dihedral angle between the two alcoholic O···O axes is ∼37°. These values are close to those for the nickel derivative (∼52° and ∼35°, respectively). The coordination geometries of both zinc(II) ions are C2-twisted octahedral, and the local symmetry around each zinc(II) ion can be approximated as C2 along the Zn···Zn axis. The Zn−O(phenolato) distances [2.0350(9)−2.0529(9) Å], the Zn−O(hydroxy) bond distances [2.1545(9)− 2.1680(10) Å], and the Zn−N bond distances [2.1929(10)− 2.2121(10) Å] are very close to those of the dinickel complex. Since the geometries around zinc(II) ions in 2′ are very similar to those around nickel(II) ions in the dinickel derivative, the coordination geometry of nickel(II) ion in the nickel(II) iondoped complex 3 is expected to be quite similar to that in the dinickel complex 1. HFEPR on [Ni2(sym-hmp)2](BPh4)2·3.5DMF·0.5(2-PrOH) (1). Complex 1 is EPR-silent at 5 K being diamagnetic as expected from magnetometric data. When the temperature was raised, a set of EPR signals appears that can be attributed to an excited triplet state (S = 1; Figure 2). These resonances achieve maximum intensity at ∼40 K. At above 100 K, another set of weak EPR signals can be observed (Figure S1 in the Supporting Information) that most probably originates from the next higher excited spin state, which is a quintet (S = 2). From the J value (ca. −70 cm−1),18 the Boltzmann populations of the excited triplet and quintet states are calculated as 7.5% and 0.05%, respectively, at 40 K, and 26% and 3.4%, respectively, at 100 K. Complex 1 strongly torques in field; it thus needed grinding and constraining in a pellet to produce interpretable spectra. Figure 3 shows a spectrum of a pellet. Although still not an ideal powder pattern, it could be interpreted using the following spin Hamiltonian:

{

HS = β(B ·g ·S) + DS Sz 2 − + ES(Sx 2 − Sy 2)

1 S(S + 1) 3

Figure 2. The low-field fragment of a 432 GHz EPR spectrum of 1 measured “as is” at indicated temperatures. (inset) A temperature dependence of the amplitude of the dominant feature in the spectrum.

Figure 3. A 406 GHz spectrum of 1 at 40 K as a pellet (black) accompanied by a powder-pattern simulation for S = 1 (red). Simulation parameters as in text.

|E1| = 1.6 cm−1, gx = 2.2, gy = 2.15, gz = 2.2 (the subscript 1 denotes the first excited spin state S = 1 of the dimer; in these conditions, particularly at the elevated temperature it was not possible to determine the sign of D1). Although the simulations using the above parameters could convincingly reproduce single-frequency spectra, we resorted to the tunable-frequency methodology30 plotting the turning points as a function of the sub-THz wave frequency (or energy) and fitting the parameters to thus obtain twodimensional (2-D) map (Figure 4). The parameters obtained in this way, at 40 K, are |D1| = 9.99(2) cm−1, |E1| = 1.62(1) cm−1, and g1 = [2.25(1), 2.19(2), 2.27(2)]. The spin Hamiltonian (eq 1) above is in the present context a “giant spin Hamiltonian”. Its parameters depend on the individual ion parameters as well as on the interactions between the ions. A spin Hamiltonian suitable for each single Ni(II) ion is

} (1)

where S arises from a giant spin approximation, and index S in DS can be 1 or 2 for the two lowest excited spin states of the dimer, with the following parameters: S = 1, |D1| = 10.1 cm−1, C


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Inorganic Chemistry

cm−1, respectively, and thus insignificant compared to the experimentally observed values of DS (10 and 3.5 cm−1 for S = 1 and 2, respectively; see below). Thus, according to eq 4a, |DNi1| = |DNi2| ≈ 10 cm−1. Since it is known from the magnetically diluted compound (see below) that DNi is positive, D1 should be negative, that is, ca. −10 cm−1 according to eq 4a. In addition to resonances originating from the first excited spin state S = 1, EPR signals attributed to the second excited (S = 2) state were also observed at T ≥ 100 K (Figure S1), and a very gross evaluation of the spin Hamiltonian parameters of that state yields |D2| ≈ 3.5 cm−1 (the subscript 2 denotes the second excited spin state S = 2 of the dimer). Ignoring the anisotropic interactions as above, the single-ion ZFS parameter is approximated as |DNi| ≈ 10.5 cm−1, which is consistent with the result obtained from the first excited triplet state of the dimer and confirms that D12 does not contribute significantly to D1 or D2. To obtain a yet more direct and possibly accurate estimate of the single-ion ZFS, we synthesized and measured a sample of polycrystalline mixed compound 3, in which paramagnetic Ni(II) ions were doped into a diamagnetic zinc isomorph 2′. HFEPR on Nickel(II) Ion-Doped [Zn2(sym-hmp)2](BPh4)2·2acetone·2H2O (3). The nickel(II) ion-doped zinc complex 3 showed strong EPR resonances at liquid-helium temperatures, as opposed to the magnetically undiluted Ni(II) complex 1 (Figure 5).

Figure 4. A 2-D map (magnetic field vs frequency or energy of the sub-THz wave quantum) of 1 at 40 K (squares) and its simulations using best-fitted spin Hamiltonian parameters as in the text. (red) Turning points with B0 parallel to the x axis of the ZFS tensor; (blue) B0∥y; (black) B0∥z. 2 Ĥ = μB B{g Ni}Sî + D Ni {Szî − Si(Si + 1)/3} 2

2

+ E Ni(Sxî − Syî )

(2)

with index i = 1 or 2 denoting the two Ni(II) ions. The Ni−Ni interactions in the dimer can be described by Ĥ = −JS1̂ ·S2̂ + D12{Sz1̂ Sz2̂ − S1̂ ·S2̂ /3} ̂ Sx2 ̂ − Sy1 ̂ Sy2 ̂ ) + E12(Sx1

(3)

where J represents isotropic interactions, and D12 and E12 indicate anisotropic interactions. The ZFS parameters of eq 1 are related to DNi, ENi, D12, and E12 through the coefficients αS and βS: DS = αSD12 + βS(DNi1 + D Ni2)

(4a)

ES = αSE12 + βS(E Ni1 + E Ni2)

(4b)

Formulas for αS and βS can be found in many textbooks.31,32 They depend on the total spin of the dimer, and accordingly, the DS and ES parameters of the giant-spin Hamiltonian are different in different spin states. For a dinuclear Ni(II) system, α1 = 1, β1 = −1/2, α2 = 1/3, β2 = 1/6 (Table 3.3 in Bencini31). The parameters D12 and E12 contain contributions related to the magnetic dipole−dipole interactions and to the anisotropic exchange interactions. D12 = D12

dipolar

+ D12

exchange

Figure 5. A 203.2 GHz spectrum of 3 “as is” at 5 K (black) accompanied by simulations using spin Hamiltonian parameters as in the text. Although the ZFS tensor is nearly rhombic, a better agreement between simulation and experiment obtains for positive D. Attempts to grind the sample and create a more perfect powder pattern resulted in disappearance of the resonances, presumably due to destroying the crystal structure and changing the coordination pattern of Ni(II).

(5a)

E12 = E12 dipolar + E12 exchange

(5b)

dipolar

D12 can be estimated from the structure of a compound according to Ddipolar = −3g 2 aveμB 2 /R3 Ni − Ni

(6) −1

resulting for RNi−Ni = 3.24 Å in D = −0.18 cm . The anisotropic exchange contribution (represented by D12exchange and E12exchange) is very difficult to estimate from theory (such as density functional theory (DFT)). To our knowledge this has not been attempted in the case of Ni(II) dimers, but the anisotropic exchange contribution is not expected to be large. We assume it to be zero. The contribution of the anisotropic interactions to the D1 and D2 parameters characterizing the S = 1 and S = 2 states of the dimer is therefore −0.18 and −0.06 dipolar

The resonances could be interpreted as originating from a triplet state (S = 1) with the following parameters: DNi = +10.1 cm−1, |ENi| = 3.1 cm−1, giso = 2.2. These spin Hamiltonian parameters thus represent the ground (S = 1) spin state of the single-ion Ni(II) ligated by sym-hmp. The tunable-frequency EPR methodology did not bring about much improvement in determining these parameters, since an attempt to produce a powder pattern from 3 failed, presumably due to an increased lability of the zinc crystal compared to its nickel isomorph. D


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Inorganic Chemistry

the Ni(II) ion. Negative DNi from −3 to −5.8 cm−1 was found in both cis-40 and trans-41N4O2 environments. Contrary to this, positive D Ni = +7.596 cm −1 was found in trans-N 4 S 2 coordination,42 which was attributed to strong delocalization of the nickel dz2 orbital due to the presence of two sulfur ligands on the z axis, resulting in lowering of the orbital reduction factor kz. Magnetic Reanalysis of 1. On the basis of the HFEPR results, the previously obtained magnetic data18 were reanalyzed using the Ginsberg equation43 (Hex = −J SA·SB). The magnetic susceptibility (χ) versus temperature (T) and χT versus T plots are shown in Figure 6. Fixing the giso and DNi

From the 2-D field versus frequency map (Figure S2), the magnetic parameters were confirmed as |DNi| = 10.0(1) cm−1, |ENi| = 3.2(1) cm−1, and giso = 2.20(5). Single-frequency spectra of 3 indicate a positive sign for DNi, which is consistent with the expected tetragonally elongated coordination geometry around the nickel(II) ion; the coordination of the alcoholic oxygen atoms is expected to be weaker than the average of the coordination of the bridging phenolate oxygen atoms and the amine nitrogen atoms.18 Moreover, the observed magnitude of DNi is in excellent agreement with the prediction based on the ZFS of the thermally excited triplet state of the dinuclear [Ni2(symhmp)2](BPh4)2 complex reported above. In this case like for dimeric complexes of other multielectron ions, the ZFS is dominated by the contribution due to the monomeric fragments.16,17,26,33 In the idealized D2h symmetry (distorted elongated octahedron with the z axis along the longest bonds), the formulas for the spin Hamiltonian parameters of Ni(II) are34,35 gx = ge −

gy = ge − gz = ge −

D= +

E=

8λkx 2 E(3B3g ) − E(3A g ) 8λk y 2 E(3B2g ) − E(3A g ) 8λkz

Figure 6. χ versus T and χT vs T (insertion) plots of 1 in the temperature ranges of 0−300 K (left) and 0−20 K (right). Observed data (○), the best-fit theoretical curves (−), theoretical χ vs T curves without considering impurity (···), without considering both impurity and D (---), and without considering both impurity, D, and TIP (-·-·-).

2

3

E( B1g ) − E(3A g ) −4λ 2kz 2

E(3B1g ) − E(3A g ) 2λ 2kx 2

+

2λ 2k y 2 E(3B2g ) − E(3A g )

values to 2.20 and +10.1 cm−1, respectively, the data were first tried to be fitted using the J and the temperature-independent paramagnetism (per Ni) (TIP) parameters; however, the data in the low-temperature range could not be fitted, because the χ slightly increased on cooling below 10 K (Figure 6 right). When the paramagnetic impurity, assumed as a nickel(II) monomer, was next introduced to reproduce the increase in χ, the data could be fitted well with the parameters (J, TIP, ρ = −69.6(9) cm−1, 189(3) × 10−6 cm3 mol−1, 3.2(5) × 10−4) with good discrepancy factors (R(χ) = 7.3 × 10−5 and R(χT) = 4.8 × 10−5), and the resulting theoretical curves are shown in Figure 6. The amount of assumed impurity is very small (∼0.03%); however, it significantly affects the fit results. Now, the reason for the failure in the previous analysis seems to be due to not considering the impurity. Since the impurity and the TIP cause similar effect in the low-temperature range, the overestimation on the TIP occurred by not considering the impurity, and this led to the underestimation of the |DNi| value. However, this mistake is thought to be avoidable, because the temperature-dependent behaviors of ρ, DNi, and TIP are different from each other on the χ scale. The effect of TIP appears constantly in the whole-temperature range, while the effects of ρ and DNi appear only in the low-temperature range. In addition, only ρ makes the susceptibility rise with temperature lowering. Actually, the DNi value of +9.2(1.2) cm−1, obtained from careful magnetic data fitting, is within the error limits equal to that obtained from HFEPR. DFT Calculation of the Exchange Integrals. We attempted estimating the exchange integrals by “broken symmetry” DFT calculations44−49 using the software package ORCA.50,51

E(3B3g ) − E(3A g ) −2λ 2kx 2 E(3B3g ) − E(3A g )

+

2λ 2k y 2 E(3B2g ) − E(3A g )

(7)

where the free-ion Ni(II) spin−orbit coupling constant λ equals −315 cm−1,35 and kx, ky, kz are the covalency-related orbital reduction factors. In the elongated octahedron, the lowest states energy sequence is 3Ag < (3B3g ≈ 3B2g) < 3B1g. 3Ag is the ground state and in the present case the excited state energies are 6900, 8000, and 9500 cm−1.18 Low 3B3g and 3B2g energies compared to 3B1g result in positive D. From gz = 2.2, the kz2 value is estimated to be 0.75. Note that different notation is used than in the original paper18 (z along a different axis). If we relate the DNi value obtained in this paper with some other related Ni(II) complexes with N2O4 donor set, except for [Ni(pydc)(pydm)]·H2O36 [(pydc)− = pyridine-2,6-dicarboxylate anion; pydm = 2,6-bis(hydroxymethyl)pyridine] (DNi = −14 cm−1), the magnitude of the DNi values of the Ni(II) complexes with trans-N2O4 donor set is comparable or slightly smaller. The DNi value of [Ni(LNNOO)2(H2O)2]37 (LNNOO = C(CN)2NO·MeOH) was DNi = +9.47 cm−1, and the DNi value of [Ni(CMA)2(im)2(MeOH)2]38 (CMA = 9,10-dihydro-9-oxo10-acridineacetate ion; im = imidazole) was DNi = +5.6 cm−1. The DNi values of a related nickel(II) complex with cis-N2O4 donor set, [Ni(ox) (dmiz)2]39 [(ox)− = C2O42−; dmiz = 1,2dimethylimidazole], was DNi = +1.875 cm−1, which was far smaller than that of complex 3. The larger value of complex 3 is thought to be due to the twisted coordination geometry around E


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Inorganic Chemistry

Figure 7. Symmetric and antisymmetric combinations of the dz2-type and dx2−y2-type magnetic orbitals of the Ni(II) ions. The overlap integral is 0.02 and 0.14 for the dz2 and dx2−y2 orbitals, respectively.

Representative orbitals are plotted in Figure 7. Various metal orbitals have very different ability, dictated by symmetry, to interact with the bridging atoms, and as a result their relative importance in transmitting the exchange interactions, measured by the overlap integral of the magnetic orbitals, is also very unequal. In the multielectron Ni(II) ions studied here, there are two contributions to the exchange interactions, which are associated with the symmetric and antisymmetric combinations involving the pairs of either dx2−y2 orbitals or dz2 orbitals localized on each of the interacting Ni(II) ions. The DFT calculations show that almost all exchange is transmitted through the dx2−y2 orbitals, as their overlap integral (0.14) is much larger than the overlap of the dz2 orbitals (0.02). The unpaired electron density on the Ni dz2 orbital is 0.88, compared to 0.80 on the dx2−y2 orbital thus showing more significant delocalization and participation in bonding of the latter. Also, the energy difference between the antisymmetric and symmetric combinations (see Figure 7) of the dz2 orbitals (0.107 eV) is much smaller than in the combinations of the dx2−y2 orbitals (0.743 eV), indicating again that the latter exchange pathway contributes more to the anti-ferromagnetic coupling.62 The calculated exchange integral is −134 cm−1 from the spin-projected formula, while the value of −90 cm−1 from the “nonprojected” one is still larger than the experimental value obtained from magnetometry (−70 cm−1). The previous

The X-ray structure was used in calculations. The system of coordinates was chosen so that the z axis was perpendicular to the least-squares equatorial plane O2N2 of one of the nickel atoms. The “broken symmetry” procedure applied to a system of two metal ions, A and B, each containing two unpaired spins, first performs a self-consistent field (SCF) calculation for a high-spin (HS) molecule with spin equal to 2. In the next stage, another SCF calculation is performed taking all spins on atom A “up” and alls spins on atom B “down”, which is referred to as the broken symmetry (BS) solution. Finally, the magnitude of J (for Hamiltonian, eq 1) is evaluated as J = −2(EHS − EBS)/ (⟨S2⟩HS − ⟨S2⟩BS), where E are the energies and ⟨S2⟩ are the expectation values of the spin-squared operator in the HS and BS states. The J value obtained from that formula is referred to in the literature as “spin-projected”.47,48,52 The “nonspinprojected” formula, J = −2(EHS − EBS)/⟨S2⟩HS, results in much smaller J values. There have been discussions among experts as to which approach should be used.47,48,52 Ahlrichstype basis set TZVPP53−55 for Ni(II) and all coordinated atoms and SVP53−55 for other atoms were used, combined with the B3LYP56−59 functional. Ahlrichs polarization functions60 from basis H−Kr and auxiliary bases from the TurboMole library were also used.61 The interactions that contribute to antiferromagnetism of dinuclear complexes involve pairs of overlapping “magnetic orbitals” localized on each metal ion. F


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Inorganic Chemistry experience17 of some of us is that the experimental J values were between the projected and unprojected J calculated by the BS approach; thus, the present result is somewhat disappointing.

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way we could measure the single-ion ZFS directly, obtain from HFEPR a value of +10.1 cm−1, confirming the previous estimates, and fix its sign as positive. The numerical values of the spin Hamiltonian parameters are remarkably consistent between HFEPR and magnetometry.

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EXPERIMENTAL SECTION

General. All the chemicals were purchased and used without further purification. Elemental analyses (C, H, and N) were obtained at the Elemental Analysis Service Centre of Kyushu University. IR spectra were recorded on a Hitachi 270−50 spectrometer. Na(sym-hmp). This ligand was prepared according to the literature method.18 [Ni2(sym-hmp)2](BPh4)2·3.5DMF·0.5(2-PrOH) (1). This complex was prepared according to the literature method.18 [Zn2(sym-hmp)2](BPh4)2·3.5DMF·0.5(2-PrOH) (2). Zinc(II) chloride (0.27 g, 2.0 mmol) and Na (sym-hmp; 0.67 g, 2.2 mmol) were dissolved in methanol (20 mL). After the precipitating sodium chloride was removed by filtration, the resulting solution was refluxed for 1 h. The addition of sodium tetraphenylborate (0.68 g, 2.0 mmol) resulted in the precipitation of white microcrystals. The compound was recrystallized by slow diffusion of 2-propanol into a DMF solution. Yield: 1.16 g (36%). Anal. Calcd for C90H118.5B2N7.5O10Zn2: C, 66.90; H, 7.35; N, 6.45; Zn, 8.5. Found: C, 66.80; H, 7.40; N, 6.50; Zn, 8.0%. Selected IR data [ν/cm−1] using a KBr disk: 3100−3300, 2800−3050, 1640, 1580, 1468, 1424, 1372, 1316, 1256, 1056, 866, 788, 728, 700, 604, 564, 494, 446, 358. m/z 689−698, {[Zn(sym-hmp)]2+ − H+}+. Molar conductance in DMF [Λ/S cm2 mol−1]: 86. [Zn2(sym-hmp)2](BPh4)2·2acetone·2H2O (2′). Single crystals suitable for X-ray diffraction method were obtained by slow diffusion of 2-propanol into an acetone solution of 2. Selected IR data [ν/cm−1] using a KBr disk: 3584, 3500, 3290−3100, 3052−2861, 1685, 1476, 1320, 1267, 1076, 1060, 736, 709, 612. Nickel(II) Ion-Doped [Zn2(sym-hmp)2](BPh4)2·2acetone·2H2O (3). This complex was prepared as slightly greenish microcrystals by a method similar to that of 2 and 2′ using zinc(II) chloride including 5% of nickel(II) chloride hexahydrate. Selected IR data [ν/cm−1] using a KBr disk: 3585, 3498, 3292−3098, 3051−2832, 1685, 1477, 1321, 1267, 1076, 1060, 734, 708, 612. The nickel content of 3 was roughly estimated as ∼5% from the electronic spectra in DMF. HFEPR. HFEPR spectra were recorded a using a homodyne spectrometer associated with a 15/17-T superconducting magnet. Detection was provided with an InSb hot electron bolometer (QMC Ltd., Cardiff, UK). Magnetic field was modulated at 50 kHz for detection purposes. A Stanford Research Systems SR830 lock-in amplifier converted the modulated signal to dc voltage.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01671. Two figures showing HFEPR spectra of complex 1 at elevated temperatures, and the field versus frequency dependence of HFEPR resonances in complex 3 (PDF) Crystal structure data for complex 3 (Data deposited as CCDC No. 1490833) (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Hiroshi Sakiyama: 0000-0001-8285-3362 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Part of this work was conducted at the NHMFL, which is funded by the NSF through a Cooperative Agreement DMR 1157490, the State of Florida, and the U.S. Department of Energy. This work was supported by JSPS KAKENHI Grant No. 15K05445. Financial support by Yamagata Univ. is gratefully acknowledged.

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REFERENCES

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CONCLUSIONS After re-examining by HFEPR the magnetic properties of a dinuclear nickel(II) complex [Ni 2 (sym-hmp) 2 ](BPh 4 ) 2 · 3.5DMF·0.5(2-PrOH) (1), where (sym-hmp)− is 2,6-bis[(2hydroxyethyl)methylaminomethyl]-4-methylphenolate anion, we confirmed that its ground spin state S is zero due to a strong anti-ferromagnetic exchange of the two Ni(II) ions (J = −70 cm−1). The complex is correspondingly “EPR-silent” at liquid helium temperatures. Raising the temperature results in populating two excited spin states of the dimer, S = 1 and S = 2. Their properties, in particular, the spin Hamiltonian parameters, could be accurately measured (for S = 1) or roughly estimated (for S = 2) by HFEPR. From those parameters we conclude that the magnetic anisotropy in the excited spin states of the dimer is largely generated by the single-ion ZFS of the individual Ni(II) ions through the isotropic exchange between them. The single-ion ZFS was estimated from the ZFS of both excited spin states of the dimer as ca. ±10 cm−1. Independently, we prepared a magnetically diluted crystal of [(NixZn1−x)2(symhmp)2](BPh4)2·2acetone·2H2O (3), where x = 0.05. In this G


Article

Inorganic Chemistry

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Magnetic Properties of a Dinuclear Nickel(II) Complex with 2,6-Bis[(2

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