Ni(II) Ferromagnetic Chains Showing Coexistent ... - ACS Publications

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

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Random Co(II)−Ni(II) Ferromagnetic Chains Showing Coexistent Antiferromagnetism, Metamagnetism, and Single-Chain Magnetism Xiu-Bing Li,†,‡ Yu Ma,†,§ and En-Qing Gao*,† †

Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of Chemistry and Molecular Engineering, East China Normal University, Shanghai 200062, China ‡ Jiansu Graphene Reseach Insitute, Changzhou 213149, China § College of Chemistry, Chemical Engineering and Materials Science, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong Normal University, Jinan 250014, China S Supporting Information *

ABSTRACT: A series of isomorphous compounds of general formula [Co1−xNix(tzpo)(N3)(H2O)2]n·nH2O (x = 0, 0.19, 0.38, 0.53, 0.68, 0.84, and 1; tzpo = 4-(5-tetrazolate)pyridine-N-oxide) was prepared. The compounds consist of homometallic or heterometallic chains with simultaneous azide-tetrazolate bridges. The heterometallic systems feature random distribution of metal ions. All compounds across the series exhibit intrachain ferromagnetic coupling, interchain antiferromagnetic (AF) ordering, field-induced metamagnetic transition, and, except the Ni-only compound, single-chain magnetic dynamics. The AF ordering temperature, the metamagnetic critical field, and the relaxation parameters show different composition dependence. Notably, the blocking temperature for the Co-rich materials is higher than the Co-only compound, suggesting synergy between the randomly distributed Co(II) and Ni(II) ions in promoting slow relaxation. The results imply rich physics in the random mixed-metal systems and demonstrate the possibility of improving single-chain relaxation properties by blending metal ions.



INTRODUCTION Single-chain magnets (SCMs) are a frontier topic that has attracted considerable interest of both physicists and chemists for the law underlying the unique magnetic behaviors and also for their implications in applications.1−4 An SCM exhibits slow spin dynamics, because magnetization reversal of an individual chain is blocked by an energy barrier arising from the noncompensating magnetic interactions and the uniaxial magnetic anisotropy within the chain. Various strategies toward SCMs have been demonstrated.5,6 Generally speaking, the ferro- and ferrimagnetic spin alignment is relatively easy to manipulate in heterospin compounds, so various metalradical,7−11 bimetallic (3d-3d,12−16 3d-4d,17,18 3d-5d,19−24 or d-f25,26) mixed-valence,27−30 and even more complex multispin SCMs,31−35 with at least one anisotropic metal centers, have been explored. Many homospin SCMs have also been reported, making use of ferromagnetism,36−38 canted antiferromagnetism,39−43 and topological ferrimagnetism.44,45 Between homoand heterospin SCMs there are a unique and rarely explored type of systems, where different metal ions in a variable ratio are randomly distributed at equivalent sites along the chain. Such compounds can be viewed as solid-solution materials that are structurally similar to but compositionally different from the corresponding homometallic compounds. They are heterometallic but obviously different from the usual heterometallic © XXXX American Chemical Society

compounds in which different metal ions in stoichiometric ratios are ordered at different sites. After studying some ferromagnetic homospin SCMs with simultaneous azide/ carboxylate/tetrazolate bridges,46−53 we initiated the investigation on bimetallic solid solution SCMs with the simultaneous bridges.54−57 Distinct composition dependence of SCM behaviors has been observed for different metal ions. Most interesting is the synergetic effect between Co(II) and Ni(II):54 metal blending leads to higher blocking temperature compared with Ni(II) and Co(II) materials. In continuation of these studies, here we report a new series o f i s o m o r p h o u s c om p o u n d s o f ge n e r a l f o r m u l a [Co1−xNix(tzpo)(N3)(H2O)2]n·nH2O (x = 0 (1-Co), 0.19, 0.38, 0.53, 0.68, 0.84, and 1 (1-Ni); tzpo = 4-(5-tetrazolate)pyridine-N-oxide). The compounds consist of homometallic or random heterometallic chains with simultaneous azidetetrazolate bridges mediating ferromagnetic (FM) coupling. As observed in a previous Co1−xNix series, all compounds except 1-Ni exhibit SCM dynamics. Differently, the present series are all antiferromagnets showing field-induced metamagnetism, and the SCM behaviors occur in the antiferromagnetic (AF) phase. The composition dependence of the AF ordering Received: April 17, 2018

A

DOI: 10.1021/acs.inorgchem.8b01053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry temperature, the metamagnetic critical field, and the relaxation parameters are analyzed.

red-brown, brown, greenish-brown, to green (1-Ni), consistent with the gradual replacement of Co by Ni. The crystals of each mixed-metal compound are uniform in color, suggesting that the compounds are not physical mixtures of Co(II) and Ni(II) crystals but homogeneous phases with microscopically mixed metal ions. The homogeneity is further supported by magnetic measurements (see below). Powder X-ray diffraction patterns (PXRD) of the heterometallic and homometallic compounds are very similar, confirming the isomorphism of these compounds. The IR spectra are also very similar (Figure S1). Crystal Structures. The structures of 1-Ni and one (Co0.62Ni0.38) of the mixed-metal compounds were determined by single-crystal X-ray analysis. We include for comparsion the structure of 1-Co, which was briefly descibed in a previous communication by us.53 The compounds are isomorphous onedimensional (1D) coordination polymers (Figure 2). The asymmetric unit contains two independent metal ions. Each metal ion is located at a centrosymmetric trans-octahedral surrounding formed by two azide N atoms, two tetrazolate N atoms, and two water O atoms. As can be seen from Table 1, the M−N/O distances decrease in going from 1-Co through Co0.62Ni0.38 to 1-Ni, well consistent with the smaller size of Ni(II) than Co(II). In each compound, M1 and M2 are bridged by a μ-1,1-azide ion in the μ-1,1 (end-on, EO) mode and a tetrazolate group via the N2−N3 bond. The M1-N-M2 angle for azide is ∼119°, and the M1···M2 distance decreases from 3.6358(6) through 3.624(1) to 3.598(1) Å on M going from Co through Co0.62Ni0.38 to Ni. The mixed (μ-EO-N3)(μ-N-N) bridges connect alternating M1 and M2 ions to give rise to a chain parallel to the a axis. Besides the azide and tetrazolate bridges, a noncovalent bridge between M1 and M2 ions may also serve to reinforce the chain structure: the M2-coordinated water molecule donates a hydrogen atom to a M1-coordinated water oxygen atom to form an O−H···O hydrogen bond. The pyridyl N-oxide groups are not coordinated but stick out from the chain to form multiple interchain hydrogen bonds (Table S1). Each N-oxide atom from one chain forms three O− H···O hydrogen bonds with three coordinated water molecules of three metal spheres from two different chains. Through these hydrogen bonds, the chains are associated into a layer parallel to the ac plane. Thus, the ligands serve as pillars between layers to afford a three-dimensional (3D) structure. The structure possesses 1D channels along the c axis, in which water molecules are enclosed and hydrogen-bonded to uncoordinated tetrazolate N atoms (N1 and N4) Magnetic Properties of 1-Ni and 1-Co. The magnetic susceptibility of 1-Ni was measured under 1 kOe in the temperature range of 2−300 K and is shown as χT and χ versus T plots in Figure 3. The measured χT value at 300 K is ∼1.47 emu K mol−1, higher than the spin-only value (1.00 emu mol−1 K) for an isolated Ni(II) ion. As the temperature is lowered, χ and χT increase to maximum values at 6.2 and 7.0 K, respectively, and then decrease rapidly. Fitting the linear region (300−50 K) of the χ−1(T) plot to the Curie−Weiss law led to C = 1.33 emu mol−1 K and θ = 28.3 K. The high-temperature behaviors are clearly indicative of ferromagnetic coupling between Ni(II) ions, mediated through the mixed (μ-N3)(μN-N) bridge along the chains. To evaluate the interaction, an empirical polynomial expression for FM Ni(II) chains (H = −J∑SiSi+1 with J > 0) was applied.63



RESULTS AND DISCUSSION Synthesis. The single-metal compounds were synthesized by the one-pot hydrothermal reactions of 4-cyanopyridine Noxide, sodium azide, and cobalt(II) or nickel(II) nitrate (Scheme 1). The reactions involve the in situ formation of Scheme 1. Synthesis of the Compounds Involving in Situ Ligand Formation

the tetrazolate group of the tzpo ligand via the [2 + 3] cycloaddition of the cyano group and the azide ion. The metalcatalyzed [2 + 3] cycloaddition in water is a safe, easy, and green routine to 5-substituted 1H-tetrazoles,58,59 and the onepot reactions of azide with various nitriles and metal sources have also been established as convenient methods for the synthesis of metal-tetrazolate coordination compounds.60−62 Considering the isomorphism of the Co and Ni compounds (vide infra) and the similarity of the two metal ions in size, we expected that isomorphous replacement with variable metal ratios is possible. Indeed, the Co1−xNix mixed-metal compounds have successfully been prepared by simply using various ratios of Co and Ni salts (Co/Ni = 5:1, 2:1, 1:1, 1:2, and 1:5) as starting materials. According to inductively coupled plasma atomic emission spectroscopy (ICP-AES) analyses, the metal compositions of the products correspond to Co0.81Ni0.19, Co0.62Ni0.38, Co0.47Ni0.53, Co0.32Ni0.68, and Co0.16Ni0.84, in good correlation with the starting Co-to-Ni molar ratios. There is a general trend that the Co fraction in the product is slightly less than that in the starting mixture. The color of these compounds (Figure 1) shows a gradual evolution from red (1-Co) through

Figure 1. Photographs (above) and PXRD profiles of the Co1−xNix compounds.

χ = (2Nβ 2g 2 /T )(AX3 + BX2 + CX + 1)/3k(DX2 + EX + 1) B

DOI: 10.1021/acs.inorgchem.8b01053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Structure of 1-Ni. (a) A fragment of the chain with mixed azide and tetrazolate bridges (ellipsoids in 50% probability). The hydrogen bond between coordinated waters is also shown. (b) A two-dimensional hydrogen-bonded layer. (c) A view of the 3D network showing the wateroccupied channels.

hand, the χ(T) plot at a low field of 20 Oe shows the maximum at 6.8 K, confirming the AF order. On the other hand, the χ(T) maximum disappears when the applied field is lifted to 2000 Oe or higher. The behaviors suggest field-induced metamagnetism: a sufficiently high field can overcome the interchain AF interactions to cause a transition from AF to FM states. The metamagnetism is confirmed by the isothermal magnetization measured at 2 K (Figure 4). The rather slow

Table 1. Comparison of Selected Bond Parameters compound

1-Co

Co0.62Ni0.38

1-Ni

M1-N8 M1-N2 M1-O2 M2-O3 M2-N8 M2-N3 M1-N8-M2

2.111(5) 2.122(5) 2.129(5) 2.095(5) 2.103(5) 2.149(5) 119.3(2)

2.109(3) 2.094(3) 2.115(3) 2.093(3) 2.095(3) 2.130(3) 119.1(1)

2.100(2) 2.062(2) 2.101(2) 2.078(2) 2.080(2) 2.097(2) 118.79(9)

Figure 3. Temperature dependence of χT of 1-Ni at 1 k Oe. The red line is the best fit (see text). (inset) The χ−T plots under different fields.

where X = J/kT, A = 0.147 09, B = −0.788 967, C = 0.866 426, D = 0.096 573, and E = −0.624 929. The best fit of the data above 20 K gave J = 22.4 cm−1 with g = 2.25. We also attempted to fit the data using de Neef’s empirical expression for FM Ni(II) chains, which includes an axial zero-field splitting parameter (D).64 The best fit using the data above 20 K led to J = 18.9 cm−1, D = 1.3 cm−1, with g = 2.30. The parameters obtained should be taken as rough estimations, since the fit is insensitive to the variation of the D parameter. The sharp decrease of χ below 6.2 K indicates that the ferromagnetic chains are AF ordered at low temperature. Further variable-temperature magnetic measurements were performed at different fields (Figure 3, inset). On the one

Figure 4. Field dependence of magnetization (M) of 1-Ni at 2 K. (a) The M(H) curve and the derivative plot (inset). (b) hysteresis loop.

increase of magnetization in the low-field region (0−1.0 kOe) is consistent with the occurrence of AF order. The magnetization experiences an abrupt jump on increasing the field from 1.0 to 3.5 kOe, which is indicative of the AF-to-FM transition. From the derivative plot of the magnetization curve, the critical field for the metamagnetic transition is estimated to be HC ≈ C

DOI: 10.1021/acs.inorgchem.8b01053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry 1.5 kOe. The magnetization loop (Figure 4b) shows the double sigmoidal shape typical of metamagnetism. There is no significant hysteresis in the low-field AF state (as expected for typical antiferromagnets), but irreversibility is obvious in the 1.5−3.5 kOe region for the field-induced FM state. Temperature-variable alternating-current (ac) measurements at zero direct-current (dc) field and different ac frequencies were performed (Figure S2). The real susceptibility (χ′) shows a frequency-independent maximum at 6.8 K, while the imaginary component (χ″) is zero. The behaviors indicate a typical antiferromagnetic state below TN = 6.8 K, with no signal of slow relaxation. The magnetic properties of 1-Co have been communicated earlier,53 and we summarize here the main features for comparison. The compound is quite similar to 1-Ni in static magnetic properties: it also shows intrachain ferromagnetic coupling, AF order, and field-induced metamagnetism, with higher TN (7.8 K) and lower HC (400 Oe). However, the strong magnetic anisotropy of Co(II) imparts slow dynamic character to 1-Co. It shows single-chain-based slow relaxation in the AF phases. Static Magnetic Properties of Mixed-Metal Compounds. The χT−T plots of the Co1−xNix series were compared in Figure 5. For all materials, χT increases with

Figure 6. (a) Isothermal magnetization curves of the Co1−xNix series at 2 K. (inset) The composition dependence of the critical field for metamagnetic transition. (b) ZFC (open symbols) and FC (solid symbols) magnetization of the Co1−xNix series under 20 Oe, the dashed line is a guide to the variation of TN.

content (Figure 6a, inset). Magnetization loop measurements (Figure S3) revealed that the materials all show negligible remnant magnetization at zero field, but different degree of irreversibility (hysteresis) was observed in nonzero field regions. For the mixed-metal materials (and 1-Co), the magnetization irreversibility occurs in the low-field region corresponding to the AF phase and becomes more significant in the Co-rich materials. The irreversibility could be due to the occurrence of slow relaxation of magnetization in the AF phase (vide infra). For comparison, 1-Ni shows no slow relaxation and no hysteresis in the low-field AF phase (vide supra). Zero-field-cooled (ZFC) and field-cooled (FC) magnetization were measured with the mixed-metal compounds at the low field of 20 Oe (Figure 6b). Each compound shows a maximum shared by the FC and ZFC curves, as expected for AF ordering. The temperature for the maximum was assumed to be the Neel temperature (TN). As shown by the dashed line in Figure 6b, TN shows rather irregular dependence on metal composition. Starting with the Co(II) compound (TN = 7.8 K), the temperature increases with a small dose of Ni(II) doping (8.1 K for Co0.81Ni0.19), then decreases steadily upon further decreasing the Co(II) fraction to the Ni-rich regime (5.9 K for Co0.16Ni0.84), and finally increases significantly to 6.8 K for the Ni(II) compound. The observations demonstrate that, unexpectedly, a small dose of metal doping into homometallic

Figure 5. Temperature dependence of χT of the Co1−xNix series at 1 k Oe. (inset) The composition dependence of the Curie and Weiss constants.

decreasing temperature and reaches a maximum in the range of 5.7−7.5 K. The Curie−Weiss fit of the data in the range of 50− 300 K led to positive Weiss temperature (Table S2), so all of the bimetallic materials inherit intrachain FO coupling. The variation of the Curie and Weiss constants with the Ni fraction is shown in the inset of Figure 5. As is expected from the meanfield theory, the Curie constant C decreases quasi-linearly as Co(II) is replaced by Ni(II). However, the Weiss temperature varies irregularly with metal composition, with a maximum value at about half replacement. This could indicate an irregular variation of magnetic correlation along the chain. Isothermal magnetization curves (Figure 6a) of the mixedmetal materials at 2 K all show the sigmoidal shape typical of the field-induced metamagnetic transition from AF to FM phases. The critical field (490−1000 Oe) lies between those of 1-Co (400 Oe) and 1-Ni (1500 Oe), increasing with the Ni(II) D

DOI: 10.1021/acs.inorgchem.8b01053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry compounds can either increase (when doping 1-Co with Ni) or decrease (when doping 1-Ni with Co) the AF ordering temperature. The mechanism is open to further investigation. The FC and ZFC curves for each mixed-metal compound overlap at high temperature but begin to bifurcate at a certain temperature below TN. The bifurcation is more significant in the Co-rich materials. The ZFC-FC divergence is not typical of AF ordering and indicates some magnet-like mechanism in the AF phases. Dynamic Magnetic Properties of Mixed-Metal Compounds. The ac susceptibilities of the mixed-metal compounds were measured at zero dc field and different ac frequencies (Figures 7 and S4). The real component (χ′) for each

The energy barrier (Δτ = 41−61 K) for the thermal activation of magnetization reversal increases with the Co(II) content and lies in the physically reasonable range. Therefore, it can be concluded that the Co1−xNix materials inherit SCM behavior from the parent Co(II) material. It is worth noting that the SCM relaxation occurs in the AF phases and justifies the magnet-like irreversibility observed in the static behaviors (isothermal magnetization and ZFC/FC). Similar behaviors have been reported for a few compounds52,69−74 but not found in random systems. It is notable that the SCM behaviors in AF phase are often coexistent with metamagnetism. It could be because the interchain AF interactions must be weak for both SCM and metamagnetic behaviors. Strong interchain interactions would suppress slow relaxation of individual chains and meanwhile would be difficult to be overcome by applied field. The blocking temperature is a parameter of great concern for SCM-based materials. It is worthwhile to see how the metal composition influences the temperature in the Co1−xNix series. For convenience, the peak temperature (TP) of the χ″(T) plot at 1 kHz can be taken as a measure of blocking temperature. As shown in Figure 8, the temperature varies in an irregular way

Figure 7. ac susceptibilities of Co0.81Ni0.19 and Co0.62Ni0.38 at zero dc field and different ac frequencies. (inset) The χ′(T) maximum, which was plotted in a larger temperature range using the data at 1 Hz. The ac plots for other mixed-metal compounds are provided in the Supporting Information (Figure S4).

Figure 8. Composition dependence of TP (1 kHz), Δτ and τo).

with the metal composition. While the Ni(II) parent compound does not show slow relaxation of magnetization in the temperature range studied (above 1.8 K), a small amount of Co(II) doping (Co0.16Ni0.84) can evoke the slow relaxations. Further increasing the Co(II) content from 0.16 to 0.81, slow relaxation moves to higher temperature, with TP increasing from 2.0 to 3.8 K. The trend is “normal”, because it is wellknown that magnetic anisotropy is a key ingredient for SCM dynamics and that Co(II) possesses much larger magnetic anisotropy than Ni(II) for the orbital degeneracy of octahedral Co(II). However, the trend is not followed when going from the Co-rich mixed-metal compounds(Co 0.81 Ni 0.19 and Co0.62Ni0.38, TP = 3.7 K @ 1 kHz) to the parent Co-only compound (2, TP = 3.3 K @ 1 kHz). In other words, the incorporation of some amount of less anisotropic Ni(II) into the Co(II) material does not reduce but raise the temperature for slow relaxation. This is “abnormal”. The phenomenon is similar to that found in another series of Co1−xNix compounds with simultaneous azide and carboxylate bridges,57 confirming that the random mixing of Co(II) and Ni(II) ions can lead to a synergic effect to the benefit of slower relaxation. Although not very strong in these known series, the synergic effect suggests

compound exhibits a frequency-independent maximum at certain temperature, around which the imaginary component (χ″) remains zero. The behaviors are typical of AF ordering. The temperature for the χ′ maximum corresponds to TN and is in good agreement with the value obtained from ZFC/FC data. The irregular dependence of TN on metal composition is also obvious from the χ′−T plots (Figure S5). Upon further lowering the temperature from TN, the χ′ component becomes frequency-dependent, and meanwhile χ″ becomes nonzero and shows frequency-dependent maxima. The parameter measuring frequency dependence, ϕ = (ΔTp/ Tp)/Δ(log f), was estimated from the peak temperature (Tp) of the χ″(T) plots of these compounds, except for Co0.16Ni0.84, which shows the χ″(T) maximum only at 997 Hz. The values ϕ = 0.12−0.14 are within the usual range (0.1 ≤ ϕ ≤ 0.3) for superparamagnets including SCMs.65−68 The τ(T) data derived from the χ″ data were fitted to the Arrhenius law τ = τo exp(Δτ/T) (Figure S6). The values of the attempt time (τ0) are from 9.6 × 10−12 to 7.6 × 10−11 s (Table S2), which are also within the range (1 ×10−5 to 1 × 10−13 s) reported for SCMs.5,6 E

DOI: 10.1021/acs.inorgchem.8b01053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

blending metal ions. We hope that our experimental results can stimulate theoretical and further experimental interest to disclose the underlying physics and to explore the potential of the metal-blending approach for the design of SCM-based materials.

an approach to increase the blocking temperature of magnetic relaxation. Phenomenologically, the irregular composition dependence of TP can be related to the variations of two parameters: the activation energy Δτ and the attempt time τo. As shown in Figure 8, Δτ decreases, but τo increases upon replacing Co(II) with Ni(II) across the Co1−xNix series from x = 0 to 1. According to the Arrhenius law, the decrease of Δτ tends to shift TP at a given frequency to lower temperature (or reduce the relaxation time τ at a given temperature), while the increase of τo tends to raise TP (or τ). The two competing effects could lead to a maximum TP at certain composition. When the Ni(II) fraction is increased from x = 0 to 0.19 and 0.38, the effect of the rapid increase in τo overcompensates the effect of the decrease in Δτ, so TP increases. For further replacement above x = 0.38, the increase of τo become slower, and the effect of Δτ dominates and thus leads to lower TP. According to the general theory for SCM dynamics,3,75 Δτ comes from the correlation barrier (Δξ) for the creation of a domain wall and the anisotropic barrier (ΔA) for spin flip inside a domain wall. In chains composed of ordered and uniform local spin units (single spins or polynuclear units), ΔA and Δξ are related to the anisotropy of the local units and magnetic exchange between the local units, respectively. Obviously, the local-spin model is not applicable to our systems featuring random distribution not only in spin size and anisotropy but also in spin exchange (JM‑M′/JM‑M/JM′‑M′). We are unaware of any theoretical study on spin dynamics in such complicated systems. Another formidable challenge is the control of the τ0 parameter. The dynamic theory uses τ0 to describe the intrinsic spin dynamics in the absence of any energy barrier.3 The parameter for known SCMs and SCM-based materials has been found in the wide range from 1 × 10−5 to 1 × 10−13,6 but the factors influencing it are still mysterious even for simple uniform chains. The plots of TP against Δτ and τo (Figure S7) can be drawn according to the Arrhenius law in the form TP = −Δτ/ln(2πfτo) with f = 1000 Hz. The plots illustrate the importance of not only Δτ but also τo for the design of SCMs with high blocking temperature. For example, to achieve a TP value of 20 K, one must have Δτ ≈ 425 K if τo is in the magnitude order of 1 × 10−13 s, but a small Δτ value of ∼50 K is enough if τo in the order of 1 × 10−5 s. However, for lack of knowledge of the underlying physics, it is still impossible to control the τo parameter for SCMs (and also for SMMs, singlemolecule magnets76−78).



EXPERIMENTAL SECTION

Synthesis. Chemical reagents were obtained from commercial sources and used as received. Caution! Metal-azido compounds are potentially explosive and should be handled in a small quantity with care. [Ni(tzpo)(N3)(H2O)2]·H2O (1-Ni). A mixture of Ni(NO3)2·6H2O (0.10 mmol, 30 mg), 4-cyanopyridine N-oxide (0.20 mmol, 24 mg), NaN3 (1.0 mmol, 65 mg), and H2O (6 mL) was stirred for 3 min. The clear solution that resulted was sealed in a Teflon-lined autoclave, heated at 100 °C for 24 h, and then cooled to room temperature in 12 h. Red crystals of 1 were collected in 70% yield (based on Ni). Anal. Calcd for C6H10NiN8O4: C, 22.74; H, 3.18; N, 35.36%. Found: C, 22.48; H, 3.52; N, 35.56. IR (KBr cm−1): 3138(br,s), 2079(s), 1654(m), 1629(m), 1529(w), 1436(m), 1301(w), 1210(m), 1185(m), 863(m), 837(m), 657(m). [Co1−xNix(tzpo)(N3)(H2O)2]·H2O (Co1−xNix). The heterometallic compounds were synthesized by a similar procedure using a mixture of Co(NO3)2·6H2O and Ni(NO3)2·6H2O as metal sources. The total molar quantity of the metal salts used was fixed at 0.1 mmol, while the Co-to-Ni molar ratio is varied according to 5:1, 2:1, 1:1, 1:2, and 1:5. The yields were 60−72%. The C/H/N elemental analytic results (Table S3) and the IR data (Figure S1) of the products are very similar to those for 1-Ni. Single-Crystal X-ray Analysis. Data collection, structural solution, and refinements were performed according to the general procedures provided in the Supporting Information. For the mixedmetal compound, the metal sites were assumed to be occupied by a mixture of Co and Ni atoms, and the occupancies were fixed at 0.62 and 0.38, respectively, according to ICP-AES analysis. The crystallographic data and refinement parameters are collected in Table 2.

Table 2. Crystal Data and Refinements Parameters formula Fw crystal system space group a, Å b, Å c, Å β, deg V, Å3 Z Dc, g cm−3 μ, mm−1 unique refln Rint S on F2 R1 [I > 2σ(I)] wR2 (all data)



CONCLUSIONS We have described an isomorphous series of Co1−xNix chain compounds (x = 0−1) with simultaneous azide and tetrazolate bridges. All compounds exhibit intrachain FM interactions, AF ordering, and metamagnetism, with the ordering temperature and the metamagnetic critical field tuned by metal composition. The mixed-metal compounds studied (x = 0.19−0.84) and the Co(II)-only compound show slow relaxation of single-chain magnetization in AF phases. The blocking temperature varies with metal composition in an irregular way, which is related to competitive effects of Δτ and τo. Notably, the randomly distributed metal ions show some kind of synergetic effects on slow spin dynamics, so that the Co-rich systems can have higher blocking temperature than the Co-only compound. The work implies rich physics in the random mixed-metal systems and demonstrates an approach of improving SCM properties by



1-Ni

Co0.62Ni0.38

C6H10NiN8O4 316.93 monoclinic P21/c 7.195(2) 21.552(7) 7.319(2) 94.510(4) 1131.4(6) 4 1.861 1.744 2587 0.0212 1.054 0.0318 0.0730

C6H10Co0.62Ni0.38N8O4 317.07 monoclinic P21/c 7.2474(17 21.642(5) 7.3171(17) 94.174(3) 1144.7(5) 4 1.840 1.602 2615 0.0264 1.149 0.0537 0.1324

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b01053. Discussion of physical measurements, single-crystal X-ray analysis, tabulated parameters for hydrogen bonds and F

DOI: 10.1021/acs.inorgchem.8b01053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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selected magnetic data, tabulated analytic results of the mixed-metal materials, IR spectra, plot of ac susceptibility versus temperature, plot showing hysteretic behavior, plot of ln τ versus T−1, additional plots (PDF) Accession Codes

CCDC 1836778−1836779 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.



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

En-Qing Gao: 0000-0002-5631-2391 Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant Nos. 21471057, 21773070, and 21675103).

■ ■

DEDICATION This paper is dedicated to Professor Dai-Zheng Liao on the occasion of his 80th birthday. REFERENCES

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DOI: 10.1021/acs.inorgchem.8b01053 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b01053 Inorg. Chem. XXXX, XXX, XXX−XXX