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Mar 14, 2016 - ABSTRACT: Metal dilution effects on reverse spin transition (rST) in ... In the mixed crystals, the Zn complexes increased rST temperat...
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Metal Dilution Effects on the Reverse Spin Transition in Mixed Crystals of Type [Co1−xZnx(C16-terpy)2](BF4)2 (x = 0.1−0.7) Ryo Ohtani,† Saki Egawa,† Manabu Nakaya,† Hitomi Ohmagari,† Masaaki Nakamura,† Leonard F. Lindoy,‡ and Shinya Hayami*,†,§ †

Department of Chemistry, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555 Japan ‡ School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia § Institute of Pulsed Power Science (IPPS), Kumamoto University, 2-39-1 Kurokami, Chuo-ku, Kumamoto 860-8555, Japan S Supporting Information *

ABSTRACT: Metal dilution effects on reverse spin transition (rST) in mixed crystals of type [Co1−xZnx(C16-terpy)2](BF4)2 (x = 0.1−0.7) were investigated by comparison with behavior of [Co1−xFex(C16-terpy)2](BF4)2 (x = 0.1−0.4). In the mixed crystals, the Zn complexes increased rST temperatures linearly with increasing values of x, without changing the hysteresis width, while the Fe complexes decreased rST temperatures. Moreover, the strength of the metal dilution effects in the CoZn mixed crystals is weaker than what occurs in the CoFe mixed crystals.

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INTRODUCTION Spin transition (ST) materials show bistabilities in their magnetic, optical, and structural properties that can be utilized for sensors, displays, and switching devices.1−16 The ability to control the ST temperature is an important requirement in order to incorporate ST materials into such devices. The ST temperature is sensitive to various factors, for example, the applied ligand field strength,17,18 the packing structure adopted, 19,20 the presence of guest molecules in the structure,21−24 the crystal size,25,26 and the external pressure.27−30 Many kinds of chemical and physical modifications to ST molecules have been carried out in an attempt to investigate the relationship between the above factors and the ST temperature. An outcome of this research has been the design and synthesis of advanced ST compounds. In this study, we have focused on a metal dilution effect that was shown to influence the ST behavior of mixed crystals of ST materials. The metal dilution effect is generated by replacing some of the metal ions in the crystalline product with different metal ions to yield a mixed crystal.31−40 A slight change in the packing structure occurs in the mixed crystal that reflects a difference in the ion radii, leading to a metal dilution effect on the ST metal centers. As a consequence, these show different ST behaviors from those of the precursor compound. This effect differs from the effect of external mechanical pressure which may inhibit a structural change occurring in response to spin state switching from low spin (LS) to high spin (HS) (and involving bond length expansion); this is reflected by an increase in the ST temperature.27−30 On the other hand, metal dilution results in a © XXXX American Chemical Society

modulation of the intermolecular interactions in the crystal as a result of ion percolation leading to perturbation of the ST behavior. Létard et al. reported an increase in the hysteresis width to around 90 K on introducing Ni and Zn ions into [Fe(PM-PEA)2](NCS)2 (PM-PEA = (N-2′-pyridylmethylene)4-(phenylethynyl)aniline).31 Gütlich et al. reported a lightinduced excited spin-state trapping of LS [Fe0.02Mn0.98(terpy)2](ClO4)2 (terpy = 2,2′:6′,2″-terpyridine).32 Recently, Ohba et al. reported that incorporating guest molecules in the ST porous coordination polymer, [Fe(pz)Pt(CN)4] (pz = pyrazine), resulted in an effect similar to that caused by chemical pressure modulation of the ST temperature.41 Generally, the effect of percolation in mixed crystal systems is to decrease the size of the domain and result in weakened cooperativity.42 Our group has previously reported a metal dilution effect on reverse spin transition (rST) behavior in [Co(C16-terpy)2](BF4)2 (1; where C16-terpy = 4′-hexadecyloxy-2,2′:6′,2″terpyridine) induced by forming a mixed crystal with diamagnetic LS [Fe(C16-terpy)2](BF4)2.34 Pristine compound 1 exhibited an rST at T1/2↓ = 217 K and T1/2↑ = 260 K with thermal hysteresis loop of 43 K,43 while the mixed crystal [Co0.8Fe0.2(C16-terpy)2](BF4)2 exhibited a similar rST at T1/2↓ = 196 K and T1/2↑ = 240 K with thermal hysteresis loop of 44 K.34 This decrease of the T1/2 value demonstrates that a negative metal dilution effect had occurred in the CoFe mixed Received: November 8, 2015

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DOI: 10.1021/acs.inorgchem.5b02582 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Crystal structures of (a) 2·MeOH and (b) 4·EtOH·0.25H2O. Counter anions and solvent molecules are omitted for clarity. Found: C, 62.82; N, 7.10; H, 7.20. XRF for 7: Co:Zn = 48:52. Anal. Calcd for 8: C, 62.90; N, 7.08; H, 7.32. Found: C, 62.80; N, 7.12; H, 7.15. XRF for 8: Co:Zn = 38:62. Anal. Calcd for 9: C, 62.87; N, 7.07; H, 7.31. Found: C, 62.75; N, 7.10; H, 7.27. XRF for 9: Co:Zn = 35:65 Physical Measurements. Temperature-dependent magnetic susceptibilities were measured by a superconducting quantum interference device (SQUID) magnetometer at field strengths of 1 T with a sweep mode of 2 K/min. X-ray powder diffraction (XRPD) patterns were performed on a Rigaku SmartLab X-ray difractometer (RAD-2A with a 2.0 kW Cu Kα X-ray). X-ray diffraction data for the single crystals were collected with a Rigaku R-AXIS RAPID 191R diffractometer. Crystal evaluation and data collection were performed using Cu Kα r = 1.54187 Å radiation with a detector-to-crystal distance of 1.91 cm. The structures were solved by direct methods (Sir 2004) and refined by full-matrix least-squares refinement using the SHELXL-2013 computer program.44 The hydrogen atoms were refined geometrically by using a riding model. Elemental analyses (C,H,N) were carried out on a J-SCIENCE LAB JM10 analyzer at the Instrumental Analysis Centre of Kumamoto University. X-ray fluorescence spectroscopy was carried out on a Seiko instruments Inc. SEA2001. Differential scanning calorimetry (DSC) thermal analysis was carried out at 10 K min−1 on a SHIMADZU DSC50.

crystals, arising from the introduction of the smaller [Fe(C16terpy)2](BF4)2 complex into 1, with the LS Fe molecule being smaller than complex 1. On this basis, it was anticipated that a larger metal complex than 1, such as [Zn(C16-terpy)2](BF4)2 (2), would increase the T1/2 of the rST. Herein, we report the synthesis of mixed crystals of type [Co1−xZnx(C16-terpy)2](BF4)2 (x = 0.1−0.7) using [Zn(C16-terpy)2](BF4)2 together with an investigation of the corresponding metal dilution effects on the rST behavior by comparison with behavior of the [Co1−xFex(C16-terpy)2](BF4)2 (x = 0.1−0.4).

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EXPERIMENTAL SECTION

Synthesis. All reagents were commercially available and used without further purification. [Co(C16-terpy)2](BF4)2 (1). Compound 1 was prepared according to the methods described previously.34,43 [Zn(C16-terpy)2](BF4)2 (2). Zn(NO3)2·6H2O (53 mg, 0.18 mmol) was mixed with NaBF4 (40 mg, 0.36 mmol) in MeOH (10 mL). The precipitates were removed by filtration. The filtrate was mixed with C16-terpy ligands (170 mg, 0.36 mmol) in CHCl3 (10 mL), and the mixture was stirred for 1 day. The solution was then dried by evaporation. The obtained pink powder was recrystallized from MeOH. Single crystals of 2·MeOH were isolated following slow evaporation of the methanol solution. Pink crystals, yield 98 mg (45%). Anal. Calcd for 2: C, 62.77; N, 7.08; H, 7.31. Found: C, 62.87; N, 7.09; H, 7.02. Mixed Crystals [Co1−xZnx(C16-terpy)2](BF4)2 (x = 0.1 (3), 0.2 (4), 0.3 (5), 0.4 (6), 0.5 (7), 0.6 (8), 0.7 (9)). These compounds were synthesized by mixing stoichiometric amounts of 1 and 2 in ethanol. Single crystals of 4·EtOH·0.25H2O were isolated following slow evaporation of the ethanol solution. The other compounds of 3, 5, 6, 7, 8, and 9 were obtained as red-brown powders by slow evaporation of the ethanol solution. These samples were dried by annealing at 400 K for 1 h before characterizations to clarify the metal dilution effects by removing solvent effects on the rST. The accurate ratios of x in these samples were determined by X-ray fluorescent spectroscopy (XRF). Anal. Calcd for 3: C, 63.08; N, 7.12; H, 7.34. Found: C, 62.86; N, 7.16; H, 7.40. XRF for 3: Co:Zn = 92:8. Anal. Calcd for 4: C, 63.04; N, 7.11; H, 7.34. Found: C, 62.55; N, 7.12; H, 7.27. XRF for 4: Co:Zn = 83:17. Anal. Calcd for 5: C, 63.01; N, 7.08; H, 7.33. Found: C, 63.16; N, 7.14; H, 7.25. XRF for 5: Co:Zn = 71:29. Anal. Calcd for 6: C, 62.97; N, 7.11; H, 7.33. Found: C, 63.02; N, 7.12; H, 7.20. XRF for 6: Co:Zn = 61:39. Anal. Calcd for 7: C, 62.93; N, 7.09; H, 7.32.

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RESULTS AND DISCUSSION Mixed crystals of type [Co1−xZnx(C16-terpy)2](BF4)2 (x = 0.1 (3), 0.2 (4), 0.3 (5), 0.4 (6), 0.5 (7), 0.6 (8), 0.7 (9)) were synthesized by a literature method using [Zn(C16-terpy)2](BF4)2 (2) instead of [Fe(C16-terpy)2](BF4)2 (10).34 The accurate ratios of x in mixed crystals 3−9 were determined by X-ray fluorescent spectroscopy (see Experimental Section). Crystal structures of 2·MeOH and 4·EtOH·0.25H2O were determined at 93 and 200 K, respectively, by single crystal Xray structural analyses (Figure 1 and Tables S1−S3). Both compounds crystallized in the triclinic P1̅ space group. 2· MeOH is a compound related to 1·MeOH, with much the same molecular structure as 1·MeOH but with one alkyl chain straight and the other bent. The Zn(II) ions are octahedrally coordinated by six nitrogen atoms from two C16-terpy ligands. The Zn−N distances are Zn−N(1) = 2.168(3) Å, Zn−N(2) = 2.061(2) Å, Zn−N(3) = 2.210(3) Å, Zn−N(4) = 2.181(3) Å, Zn−N(5) = 2.068(2) Å, and Zn−N(6) = 2.187(3) Å, respectively, demonstrating that 2 is a larger molecule than 1 B

DOI: 10.1021/acs.inorgchem.5b02582 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry in their solvated phases. 4·EtOH·0.25H2O is a mixed crystal incorporating Co and Zn ions in a ratio of 0.8:0.2. The Co−N distances are Co−N(1) = 2.142(2) Å, Co−N(2) = 2.023(3) Å, Co−N(3) = 2.151(2) Å, Co−N(4) = 2.213(2) Å, Co−N(5) = 2.022(3) Å, and Co−N(6) = 2.098(2) Å, respectively. These values indicate that Co(II) ions are in the HS state.45 The comparisons of M−N distances and bond angles among 1· MeOH, 4·EtOH·0.25H2O, and [Co(terpy)2](BF4)245 are demonstrated in Tables S3−S5. Unfortunately, crystal structures of their desolvated samples have not been obtained because of a loss of single crystallinities after removing lattice solvents. The isolated mixed crystals 3−9 were investigated by XRPD measurements (Figure S1). Slight peak shifts were observed in the respective diffraction patterns, with no evidence for mixed phases composed of 1 or 2 being present. These results demonstrated that 3−9 undoubtedly formed the respective mixed crystals. The magnetic behavior of the mixed crystals was measured using SQUID (Figures 2 and 3 and Figure S2). As-synthesized

compounds incorporating solvent molecules in their crystals showed no rST (Figure S2d,e), indicating that both solvent effects43,46 and metal dilution effects were developed in these compounds. Therefore, thermal treatments for the samples were carried out to remove the solvent effects and to investigate “undiluted” metal dilution effects in the mixed crystals. We confirmed no residual lattice solvents in the treated samples by elemental analyses. However, a TGA result of a desolvated sample indicated tiny amounts of the residual solvents that were removed up to 400 K (Figure S3); therefore, the magnetic behaviors of nonsolvated compounds were measured by a sequence of cooling mode starting from 400 K and subsequent heating mode. All nonsolvated compounds showed similar rST with hysteresis loops of around 40 K (Table 1 and Figure 4a). Table 1. rST Temperatures and Hysteresis Width of 1 (x = 0) and the Nonsolvated CoZn Mixed Crystals 3−8 Co1−xZnx x x x x x x x

= = = = = = =

0 0.08 0.17 0.29 0.39 0.52 0.62

number

T1/2↓ (K)

T1/2↑ (K)

ΔT (K)

1 3 4 5 6 7 8

217 224 226 231 232 237 242

260 266 267 269 273 280 284

43 42 41 38 41 43 42

At this stage, we have not obtained crystal structures of magnetically “active” solvent free phase of 1 and 4; therefore, the correlation between molecular structure involving conformation of alkyl chains and rST has been unclear. The rST temperatures of the CoZn mixed crystals 3−8 increased linearly with increasing value of x according to the following expressions: T1/2↑ = 260 + 37.1x

(R2 = 0.974, SD = 7.96)

T1/2↓ = 219 + 36.1x

(R2 = 0.970, SD = 7.76)

This observed trend in the rST temperatures is opposite to that observed for CoFe mixed crystals [Co1−xFex(C16-terpy)2]-

Figure 2. rST behavior of 1 (△) and 6 (▲).

Figure 3. Magnetic behavior of 1 and the mixed crystals (a) in the cooling and (b) in the heating processes (▽; 1, ▼; 3, □; 4, ■; 5, ○; 6, ●; 7, ×; 8). C

DOI: 10.1021/acs.inorgchem.5b02582 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Relationships between T1/2↓ (●), T1/2↑ (○) and x value in (a) CoZn and (b) CoFe mixed crystals. x = 0 corresponds to pure [Co(C16terpy)2](BF4)2 (1).

ratios of 0.7 (9) and 0.4 (14)34 (Figure 5). At these threshold values corresponding to a high content of the dopants, PXRD

(BF4)2 (x = 0.1 (11), 0.2 (12), 0.3 (13), 0.4 (14)) (Table 2 and Figure 4b).34 In the cases of 11−13, the rST temperatures Table 2. rST Temperatures and Hysteresis Width of 1 (x = 0) and the Nonsolvated CoFe Mixed Crystals 11−13a Co1−xFex x x x x a

= = = =

0 0.1 0.2 0.3

number

T1/2↓ (K)

T1/2↑ (K)

ΔT (K)

1 11 12 13

217 210 192 180

260 251 237 229

43 41 45 49

These data were obtained from ref 34.

decreased linearly as the value of x increased in accord with the following expressions: T1/2↑ = 260 − 107x

(R2 = 0.989, SD = 12.0)

T1/2↓ = 219 − 129x

(R2 = 0.976, SD = 14.6)

These results clearly demonstrate that a positive metal dilution effect occurs in the CoZn mixed crystals 3−8; on the other hand, the 10 molecules produce a negative metal dilution effect in the CoFe mixed crystals 11−13. Moreover, we found that the “strength” of the metal dilution effect in the CoZn mixed crystals is weaker than what occurs in the CoFe mixed crystals. These differences would reflect from size differences among 1, 2, and 10, even though the sizes of magnetically active nonsolvated compounds are unclear at this stage. In the case of their solvated phases of 1·MeOH, 2·MeOH, and 10· acetone, average M−N bond lengths in the coordination spheres were obtained by the single crystal X-ray structural analyses (Table 3), and the difference between 1 (2.068 Å)43 and 2 (2.146 Å) is 0.078 Å, which is smaller than 0.121 Å, the difference between 1 and 10 (1.947 Å).47 The hysteretic behavior for the CoZn and CoFe mixed crystals was no longer present for the systems with mixing

Figure 5. Magnetic behavior of 9.

patterns were obviously different from those of samples with mixing ratios below the threshold values (Figure S1), and the energy gained in the structural phase transition at the rST is no longer sufficient to match the generated metal dilution effects that occur in the mixed crystals.34 The different threshold values also corroborate the different strengths of the respective metal dilution effects. The weaker metal dilution effect in the CoZn complex does not impact on the hysteretic behavior of rST under x = 0.7 while, on the other hand, the stronger effect in the CoFe species does disrupt the behavior even at x = 0.4. In conventional ST compounds without alkyl chains, a Zn complex usually gives a negative metal dilution effect in mixed crystals, associated with extended intermolecular distances.31−40 On the other hand, the positive metal dilution effect was observed in the rST CoZn mixed crystals incorporating Zn complexes. This (opposite) positive effect in rST compounds is associated with synchronized phase transitions of spin states and alkyl chains in which the rST temperature is in accord with structural phase transition temperature of the alkyl chains.43 Doping of Zn complexes increases interchain interaction with the structural phase transition temperatures of the alkyl chains

Table 3. Differences in the Average M−N Bond Lengths in the Coordination Sphere among 1, 2, and 10

av bond lengths in the coordination sphere (Å) diff from value of 1 (Å)

1 (Co)

2 (Zn)

2.068

2.146

10 (Fe) 1.947

0.078

−0.121 D

DOI: 10.1021/acs.inorgchem.5b02582 Inorg. Chem. XXXX, XXX, XXX−XXX

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being increased (Figure S4), resulting in the positive effect to increase the rST temperatures. Moreover, the hysteresis width of the rST in the mixed crystals showed no change when the mixing ratio (x) was under 0.6 (in Zn) and under 0.3 (in Fe) (Figure 4). These results are also in accord with cooperative transitions leading to the rST in [Co(C16-terpy)2](BF4)2 being generated by the phase transition involving the alkyl chains and not due to the usual cooperativity that arises from intermolecular interaction between the metal complex units.43 Even though the Zn complexes of type 2 interrupt the intermolecular interaction in 1, the hysteretic behavior of the rST remains. Previously reported mixed crystal systems showed a strong influence of the metal ion mixing ratio on the corresponding hysteresis width because the intercalation of different metal ion types resulted in gradually reduced cooperativity in accord with fragmentation of the magnetic domain occurring, leading to disappearance of their ST hysteresis.33,42 The observed metal dilution effects on the hysteretic behavior of the CoZn and CoFe mixed crystals were concluded to result from the rST being synchronized with a structural transition involving the alkyl chains.

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by KAKENHI Grant-in-Aid for Scientific Research (B) 26288026 and Grant-in-Aid for Scientific Research on Innovative Areas 2506 [Science of Atomic Layers]. This work was supported by JSPS Grant-in-Aid for Young Scientists (B) 15K17833. This work was partially supported by the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02582. CCDC 1402810 and 1402811 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_ request/cif/. Tables of crystal and DSC results Crystallographic (CIF) Crystallographic 1402811) (CIF)

AUTHOR INFORMATION

Corresponding Author

CONCLUSIONS In conclusion, we have prepared mixed crystals of type [Co1−xZnx(C16-terpy) 2](BF4)2 (x = 0.1−0.7) and have investigated the resulting metal dilution effect on the rST behavior of these species by comparison with behavior of [Co1−xFex(C16-terpy)2](BF4)2 (x = 0.1−0.4). The metal dilution effects are switched between positive and negative by introducing Zn and Fe metal complexes into the [Co(C16terpy)2](BF4)2 structure, inducing a change in the latter’s rST temperature. In the case of the Zn complex, the rST temperature is increased, while the Fe complex leads to a decrease in the rST temperature. Moreover, the strength of the metal dilution effect in the CoZn mixed crystals is weaker than what occurs in the CoFe mixed crystals. On the other hand, their hysteretic behavior is not influenced by the formation of the mixed crystals because the hysteresis of the rST reflects a phase transition principally associated with the incorporated alkyl chains, rather than with intermolecular interactions involving the (attached) metal complex units. Clearly, the application of the metal dilution effect to tune the ST behavior in “soft” metal complexes incorporating alkyl chains provides a useful means for modulating the ST temperature of such systems without affecting the hysteretic behavior.

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Article

parameters and SQUID, TGA, XRPD, (PDF) data for 2·MeOH (CCDC 1402810) data for 4·EtOH·0.25H2O (CCDC E

DOI: 10.1021/acs.inorgchem.5b02582 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02582 Inorg. Chem. XXXX, XXX, XXX−XXX