Solvent Polarity Predictably Tunes Spin Crossover - ACS Publications

ences should, and do, influence the stabilization of the LS ...... H RT. S R. ( ). (max). 1 exp(. /. / ). M. M. (1). Two clear trends can be observed ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Solvent Polarity Predictably Tunes Spin Crossover T1/2 in Isomeric Iron(II) Pyrimidine Triazoles Santiago Rodríguez-Jiménez, Alexis S. Barltrop, Nicholas G. White,† Humphrey L. C. Feltham, and Sally Brooker* Department of Chemistry and MacDiarmid Institute for Advanced Materials and Nanotechnology, University of Otago, PO Box 56, Dunedin 9054, New Zealand S Supporting Information *

ABSTRACT: Two isomeric pyrimidine-based Rdpt-type triazole ligands were made: 4-(4-methylphenyl)-3-(2-pyrimidyl)-5-phenyl4H-1,2,4-triazole (L2pyrimidine) and 4-(4-methylphenyl)-3-(4-pyrimidyl)-5-phenyl-4H-1,2,4-triazole (L4pyrimidine). When reacted with [FeII(pyridine)4(NCE)2], where E = S, Se, or BH3, two families of mononuclear iron(II) complexes are obtained, including six solvatomorphs, giving a total of 12 compounds: [FeII(L2pyrimidine)2(NCS) 2 ] (1), [Fe II (L 2pyrimidine ) 2 (NCSe) 2 ] (2), 2·1.5H 2 O, [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3), 3 and 3·2H2O, [FeII(L4pyrimidine)2(NCS)2] (4), 4·H2O, [FeII(L4pyrimidine)2(NCSe)2] (5), 5·2CH3OH, 5·1.5H2O, and [FeII(L4pyrimidine)2(NCBH3)2]· 2.5H2O (6·2.5H2O). Single-crystal X-ray diffraction reveals that the N6-coordinated iron(II) centers in 1, 2, 3·2CHCl3, 4, 5, and 5· 2CH3OH have two bidentate triazole ligands equatorially bound and two axial NCE co-ligands trans-coordinated. All structures are high spin (HS) at 100 K, except 3·2CHCl3, which is low spin (LS). Solid-state magnetic measurements show that only 3·2CHCl3 (T1/2 above 400 K) and 5·1.5H2O (T1/2 = 110 K) undergo spin crossover (SCO); the others remain HS at 300−50 K. When 3·2CHCl3 is heated at 400 K it desorbs CHCl3 becoming 3, which remains HS at 400−50 K. UV−Vis studies in CH2Cl2, CHCl3, (CH3)2CO, CH3CN, and CH3NO2 solutions for the BH3 analogues 3 and 6 led to a 6:1 ratio of Lnpyrimidine/Fe(II) being employed for the solution studies. These revealed SCO activity in all five solvents, with T1/2 values for the 2-pyrimidine complex (247−396 K) that were consistently higher than for the 4pyrimidine complex (216−367 K), regardless of solvent choice, consistent with the 2-pyrimidine ring providing a stronger ligand field than the 4-pyrimidine ring. Strong correlations of solvent polarity index with the T1/2 values in those solvents are observed for each complex, enabling predictable T1/2 tuning by up to 150 K. While this correlation is tantalizing, here it may also be reflecting solvent-dependent speciationso future tests of this concept should employ more stable complexes. Differences between solid-state (ligand field; crystal packing; solvent content) and solution (ligand field; solvation; speciation) effects on SCO are highlighted.

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monobidentate Rdpt (Figure 1) and dpt‑ (R absent) types of ligands.21,23,26−31 The SCO behavior of the resulting iron(II) complex can be tuned by modifying the three substituents attached to the central triazole ring of these ligands.21,23 Of interest here are the Rdpt-type ligands (Figure 1).21,23 Rdpt itself has two pyridyl rings as attached arms at 3C and 5C (Figure 1, middle) making it potentially ditopic (bis-bidentate); however, it normally coordinates to iron(II) centers in a monotopic bidentate manner when used together with NCEtype co-ligands (i.e., NCS, NCSe, NCBH3, TCNQ, etc.) that bind axially, generating mononuclear [FeII(Rdpt)2(NCE)2] complexes, many of which are SCO-active.21,23,34,36 When one pyridyl ring is replaced by a diazine ring, the resulting potentially ditopic RR′pt ligands (Figure 1, bottom left) have

INTRODUCTION The spin crossover (SCO) phenomenon can be observed in d4−d7 transition-metal ions when in an appropriate, intermediate, ligand field.1,2 The majority of reported systems that undergo SCO are based on iron(II) metal ions. While a range of environments have been employed for iron(II), the most often reported is an octahedral N6−donor environment. To trigger SCO it is necessary to apply an external stimulus to induce switching between the high-spin (HS) and low-spin (LS) states. These stimuli can be temperature,1−4 pressure,5−8 external magnetic field,9 light irradiation,10−14 or guest molecules.15−18 Ligands based on 1,2,4-triazoles can be easily functionalized in positions 3C, 4N, and 5C of the triazole ring (Figure 1) and have been successfully employed to generate a wide range of interesting iron(II) SCO behaviors.19−23 Such ligands include the bis-terdentate PMRT and PSRT ligands,24,25 and the bis- or © XXXX American Chemical Society

Received: January 14, 2018

A

DOI: 10.1021/acs.inorgchem.8b00128 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Existing bis-terdentate triazole ligands PMRT24 and thioether analogues PRST,25 and potentially bis-bidentate Rdpt ligands 21,23 (R = amino, methyl, phenyl, tolyl, hexadecyl, etc.;18,21,23,28,30−36 R′ = pyrazinyl,25,34 pyridazinyl,18 phenyl,36 quinolinyl, etc.),23,30,37,38 as well as the nonsymmetrically functionalized RR′pt-type (Lpz and Lpdz)18,33 and RR′pht-type (Lpyridazine, Lpyridine, and Lpyrazine)34,36,39 ligands, which have produced many SCOactive iron(II) materials. The [FeII(Lnpyrimidine)2(NCE)2]-type complexes of the RR′pht-type ligands L2pyrimidine and L4pyrimidine (in box) are explored herein.

facilitating tuning of SCO. To observe the different impacts of these distinct diazines on the ligand field strength of SCOactive iron(II) complexes, solution studies are far superior to solid-state studies, because then the SCO is not affected by the crystal lattice. In solution the electronic effects induced by the diazine-containing ligands are the predominant influence, together with solvation effects,42−52 on ligand field strength and hence T1/2.53 In the case of the [FeII(Lazine)2(NCBH3)2] complexes (Figure 1 bottom right), a strong correlation was found between the 15N NMR signal (either observed or calculated by density functional theory (DFT)) for the coordinating azine N atom of the ligand and the T1/2 of the resulting complex. With the correlation established for a particular family of complexes, the T1/2 for proposed new complexes in the family, varying in the nature of the substituents on aromatic rings, can be easily predicted before synthesis by a simple DFT calculation of the 15N NMR signals for the intended ligand. Note that a range of pyrimidine-based ligands, including bipyrimidine, has been used to generate many interesting SCOactive iron(II) materials.3,23,54−62 Here we report the synthesis and characterization of a pair of isomeric pyrimidine-based monotopic RR′pht ligands (R = tolyl and R′ = n-pyrimidine) that differ only in the pyrimidine N atom positions, 4-(4methylphenyl)-3-(4-pyrimidinyl)-5-phenyl-1,2,4-triazole (L2pyrimidine) and 4-(4-methylphenyl)-3-(4-pyrimidinyl)-5-phenyl-1,2,4-triazole (L4pyrimidine) (Scheme 1, bottom right), as well as a set of six new mononuclear iron(II) [FeII(L)2(NCE)2]-type complexes, where E = S, Se, and BH3, along with six solvatomorphs. As well as solid-state magnetic characterization of all six complexes, extensive solution studies on the pair of NCBH3 analogues are also presented, providing insights into the influence over the SCO caused by the (a) choice of isomeric pyrimidine ligand (ligand field effects) and (b) choice of solvent (solvation effects).

been used to generate new families of SCO-active compounds,18,33,34 including a dinuclear “three-guest three color” sensor when R′ = pyridazine and R = tolyl.18 However, when complexes of RR′pt are mononuclear, there can be ambiguity as to whether the iron(II) center is bound by the pyridine or the diazine arm.33 Hence interest has turned to the analogous monotopic RR′pht ligands (Figure 1, bottom right), in which this uncertainty is removed by replacing the remaining pyridine ring by a phenyl ring. This strategy, use of monotopic RR′pht ligands, has already proven interesting with the: (a) realization of the first quantitative guest-sensing SCO-active complex, [FeII(Lpyrazine)2(NCS)2] (R′ = pyrazine and R = tolyl);34 (b) synthesis of a family of [FeII(Lpyridine)2(NCE)2] complexes (R′ = pyridine and R = tolyl; E = S, Se, and BH3), which are SCOactive in CDCl3 solution, with the NCBH3 analogue showing a more complete and abrupt SCO,36 and (c) realization of easy prediction of spin-state tuning in advance of synthesis, illustrated using the family of [FeII(RR′pht)2(NCBH3)2] complexes (R = tolyl and R′ = pyridine or diazine, Figure 1 bottom right) in CDCl3 solutions.39 Indeed, the introduction of diazines,18,23,30,33,34,37,38 that is, pyridazine, pyrazine, and pyrimidine, is of considerable interest, because they are known to have poorer σ-donor and better πacceptor character than pyridine.40,41 These electronic differences should, and do, influence the stabilization of the LS versus HS state and hence the switching temperature (T1/2),

RESULTS AND DISCUSSION Ligand Synthesis. The synthetic procedure used to access the pair of isomeric monotopic L2pyrimidine and L4pyrimidine ligands (Scheme 1) is built on the general method described in 2004 by Klingele and Brooker.63 The four-step synthesis of L2pyrimidine (Figures S1 and S2) from relatively cheap commercially available starting materials starts by converting the commercially available reactant, 2cyanopyrimidine, to the carboxylic acid (Scheme 1, step (i).64,65 The carboxylic acid is then reacted with SOCl2 and dry ethanol to form the ester (Scheme 1, step (ii)) and then with hydrazine hydrate to afford the desired hydrazide (Scheme 1, step iii, A).66 The overall yield of pyrimidine-2-carbohydrazide (A) was 45% over three steps from 2-cyanopyrimidine. The five-step synthesis of L4pyrimidine (Figures S3 and S4) from relatively inexpensive commercially available starting materials begins with a ring-closing condensation reaction of formamide and 4,4′-dimethoxy-2-butanone to give 4-methylpyrimidine (Scheme 1, step (vii)) in 36% yield (Caution! Toxic CO evolved!).67 The methyl group can then be oxidized to the carboxylic acid, without degradation of the aromatic ring, using selenium dioxide (Scheme 1, step (viii)).68 Then the ester and hydrazide are generated in the same manner as described for the 2-isomer (Scheme 1, step (ix); and (x), B). The overall yield of pyrimidine-4-carbohydrazide (B) was 28% over four steps from 4,4′-dimethoxy-2-butanone.

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B

DOI: 10.1021/acs.inorgchem.8b00128 Inorg. Chem. XXXX, XXX, XXX−XXX

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(Scheme 1, bottom). These ligands are sparingly soluble in CH3OH or CH3CN but readily dissolve in chlorinated solvents, so CH3OH/CHCl3 and CH3CN/CHCl3 solvent mixtures were employed for the complexations. In all cases, combining solutions of [FeII(pyridine)4(NCE)2], where E = S, Se, and BH3, with the chosen ligand in a 1:2 ratio gives an immediate color change. In the case of L2pyrimidine the complexes with NCS or NCSe co-ligands are orange in both the reaction solution and the solid state, while with NCBH3 co-ligands the resulting color is red in both the reaction solution and the solid state. In contrast, regardless of the NCE co-ligand, the complexes of L4pyrimidine produce dark purple reaction solutions, and the solids range from dark purple powder to orange or light red crystals (see details below). Clearly these isomeric ligands, differing only in pyrimidine N positions, have distinctly different impacts on the color of the complexes derived from them. Complexes of the 2-Isomer. Vapor diffusion of diethyl ether into a 2:1 CH3OH/CHCl3 reaction solution gives analytically pure dark orange single crystals of [FeII(L2pyrimidine)2(NCS)2] (1) suitable for X-ray crystallography, in 23% yield. Similarly, [FeII(L2pyrimidine)2(NCSe)2] (2) was obtained as an analytically pure orange powder in 37% yield. Small orange single crystals of this complex with the same molecular formula, suitable for X-ray crystallography, were obtained by performing the reaction in 3:2 CH3CN/CHCl3 followed by vapor diffusion of tetrahydrofuran (THF) into the reaction solution; however, after these crystals are filtered off they absorb atmospheric moisture, giving [FeII(L2pyrimidine)2 (NCSe) 2]·1.5H 2O (2· 1.5H2O) in 40% yield. For the NCBH3 analogue, powder samples of solvatomorph [FeII(L2pyrimidine)2(NCBH3)2]·2H2O (3·2H2O) were obtained, as an intense red powder, by immediate precipitation from a red 2:1 CH3OH/CHCl3 reaction solution by addition of pentane, in 75% yield. When diethyl ether was vapor-diffused into a dark red CHCl3 solution of 3·2H2O dark red needlelike single crystals of the solvatomorph [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3) were obtained in 89% yield. Holding a sample of 3·2CHCl3 in the Versalab magnetometer at 400 K for 1 h (conditions that thermogravimetric analysis (TGA) results previously showed should result in loss of the CHCl3 of solvation), gave desolvated 3, which had a completely different magnetic response to that of 3·2CHCl3 (see below). Complexes of the 4-Isomer. Dark purple single crystals of [FeII(L4pyrimidine)2(NCS)2] (4), suitable for X-ray diffraction, were obtained by vapor diffusion of diethyl ether into a 2:1 CH3OH/CHCl3 reaction mixture. Upon isolation and air exposure, they become the monohydrate [FeII(L4pyrimidine)2(NCS)2]·H2O (4·H2O), obtained in 44% yield. In the case of [FeII(L4pyrimidine)2(NCSe)2], three different solvatomorphs were isolated. Small orange single crystals of the solvent-f ree solvatomorph [FeII(L4pyrimidine)2(NCSe)2] (5), suitable for X-ray diffraction, were obtained in 51% yield by vapor diffusion of THF into a dark purple CH3CN/CHCl3 (3:2) reaction mixture. Solvatomorph [FeII(L4pyrimidine)2(NCSe)2]·2CH3OH (5·2CH3OH) was obtained by reacting [FeII(pyridine)4(NCSe)2] and L4pyrimidine in a 2:1 CH3OH/CHCl3 reaction solution. Overnight a mixture of purple crystalline solid and brown powder precipitated out of the reaction solution; this solid was filtered off and air-dried. The purple solid was sparingly soluble in CH3OH, and the

Scheme 1. Synthesis of the Isomeric Pair of PyrimidineBased Triazole Ligands, L2pyrimidine (left), L4pyrimidine (right) (annotated with NMR numbering), and the [FeII(npymt)2(NCE)2] Complexes

The required thioamide (Scheme 1, C) was prepared in a one-pot reaction by refluxing 4-methylaniline, benzaldehyde, elemental sulfur, and sodium sulfide nonahydrate in dimethylformamide (DMF) in 41% yield (Scheme 1, step (v))69 and was ethylated (Scheme 1, step (vi)) just before reaction with the desired hydrazide to form the respective triazole ligand (Scheme 1, bottom). The ring-forming reaction to produce the desired triazole ligands typically requires a multiday reflux in a high-boilingpoint solvent, usually 1-butanol.63 This is also the case here, with yields of 40% for L2pyrimidine and 68% for L4pyrimidine, obtained analytically pure, after 3 d at reflux in 1-butanol. Inorganic Synthesis. Two families of mononuclear iron(II) complexes of the form [FeII(Lnpyrimidine)2(NCE)2], with E = S, Se, and BH3, were synthesized from the pair of isomeric pyrimidine-based ligands L2pyrimidine and L4pyrimidine C

DOI: 10.1021/acs.inorgchem.8b00128 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry brown solid was insoluble, so CH3OH was added (15 mL), and the insoluble solid was filtered off (later identified as elemental selenium. Caution! Toxic!). Vapor diffusion of diethyl ether into the orange filtrate gave light red block-shaped single crystals, suitable for X-ray diffraction, of [FeII(L4pyrimidine)2(NCSe)2]·2CH3OH (5·2CH3OH), in 46% yield. To obtain the third solvatomorph, namely, [FeII(L4pyrimidine)2(NCSe)2]·1.5H2O (5·1.5H2O), a purple 2:1 CH3OH/CHCl3 reaction solution was left stirring overnight to allow any elemental selenium to precipitate (Caution! Toxic!). After filtration, vapor diffusion of diethyl ether into the dark purple filtrate yielded a dark purple powder, which was filtered off and air-dried (73%). For the NCBH3 analogue, all attempts to generate a crystalline sample (using a wide range of crystallization methods70) were unsuccessful; however, when a large volume of pentane (100 mL) was added to a dark purple 2:1 CH3OH/ CHCl3 reaction solution (9 mL), precipitation of an intense fuchsia powder occurred. After air drying, the sample analyzed as [FeII(L4pyrimidine)2(NCBH3)2]·2.5H2O (6·2.5H2O), obtained in 53% yield. Crystal Structures of Complexes. Single crystals suitable for X-ray diffraction were obtained, and structures were determined at 100 K, for the ligand L4pyrimidine (Figures S5 and S6) and the four solvent-f ree complexes: [Fe I I (L 2 p y r i m i di n e ) 2 (NCS) 2 ] (1, Figures 2 and S7),

Figure 3. Perspective view of [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3). Hydrogen atoms are omitted for clarity, except those that belong to NCBH3 co-ligands (green). The disordered lattice CHCl3 solvents are not shown either, as they could not be modeled satisfactorily, so the SQUEEZE routine of PLATON71 was applied. Symmetry operation A is −x, 1 − y, 1 − z.

per complex (see experimental and Supporting Information). In all six cases the complexes are isostructural: the octahedral N6coordinated iron(II) center has four nitrogen atoms provided by two equatorially coordinated ligands, L2pyrimidine or L4pyrimidine, two from the triazole rings, and two more from the 2- or 4-pyrimidine rings; the remaining other two nitrogen atoms are from two trans-coordinated anionic NCE co-ligands (E = S, Se, or BH3) (Figures 1−5, S7−S12). The average Fe−N bond lengths at 100 K for complexes 1, 2, 4, 5, and 5·2CH3OH, 2.157−2.186 Å, are as expected for a HS iron(II) center, whereas for complex 3·2CHCl3 this average is 1.953 Å, consistent with an LS iron(II) center.1,21,23 The values of the octahedral distortion parameters (∑) for the isomorphous HS complexes 1, 2, and 5 are 112.2, 107.2 and 107.2°, respectively (Table 1). On the one hand, these values are higher than for any of the previously structurally characterized HS [Fe II (Rdpt) 2 (NCE) 2 ]-type complexes (range: 77.9−101.1°).21,23 On the other hand, the ∑ values for the other two HS complexes, 4 and 5·2CH3OH, 77.7 and 84.8° respectively (Table 1), fall at the low end of that literature range, but are still well-above the range observed for LS complexes of this type (42.5−65.7°).21,23 As expected, the ∑ value for the LS complex, 3·2CHCl3 (43.0°), is much lower and falls within the literature range for such complexes. For four of the structures the axial NCE co-ligands are significantly bent away from being linear with the iron(II) centers, with Fe−NC angles of 153−156° (Figure 6, Table 1), whereas for the remaining two structures, 3·2CHCl3 and 4, the Fe−NC angles (177−178°) are nearly linear (Figure 6, Table 1). The Fe-NC angle plays an important role in influencing the spin state observed in the solid state.72−74 A recent literature survey of 53 distinct crystal structures of related [FeII(Rdpt)2(NCE)2] complexes showed that the majority of structures (31 of 34) with an Fe-NC angle close to linear (162−178°) are SCO-active, while the majority of structures (15 of 19) with an Fe-NC bent away from linear (142−159°) are non-SCO.36 The SCO activity or inactivity of each of these complexes (see later), except for 4, is consistent with these correlations. The two distinct ranges of Fe-NC angles observed herein are likely due to them crystallizing in two different space groups: P1̅ for 1, 2, 5, and 5·2CH3OH versus P21/n for 3· 2CHCl3 and 4.36,73−75 As a result of this the weak intermolecular crystal packing interactions between the complex molecules differ (Tables S3−S8, Figures S13−S20). This is discussed in more detail in the following section.

Figure 2. Perspective view of the complex [FeII(L2pyrimidine)2(NCS)2] (1); note that complexes [Fe II (L 2pyrimidine ) 2 (NCSe) 2 ] (2), [FeII(L4pyrimidine)2(NCS)2] (4) and [FeII(L4pyrimidine)2(NCSe)2] (5) are isostructural with complex 1. Hydrogen atoms are omitted for clarity. Symmetry operation A is 1 − x, 1 − y, 1 − z.

[FeII(L2pyrimidine)2(NCSe)2] (2, Figure S8), [FeII(L4pyrimidine )2(NCS) 2] (4, Figures 4 and S9), and [FeII(L4pyrimidine)2(NCSe)2] (5, Figure S10), as well as for the solvates [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3, Figure 3 and S11, treated with the SQUEEZE routine of PLATON,71 as the disordered lattice CHCl3 molecules could not be modeled satisfactorily) and [FeII(L4pyrimidine)2(NCSe)2]· 2CH3OH (5·2CH3OH, Figures 5 and S12). All four solvent-f ree complexes crystallize in the triclinic space group P1̅, while 3·2CHCl3 and 5·2CH3OH crystallize in the monoclinic space group P21/n (Table 1). In all cases the asymmetric unit comprises half of the complex, with the other half generated by a center of inversion located at the iron(II) center. For the solvates, solvent molecules are also present in the asymmetric unit: for 5·2CH3OH this comprises one disordered molecule of CH3OH over two positions (50:50) per asymmetric unit, whereas for 3·2CHCl3 the microanalysis, TGA, and SQUEEZE results are consistent with this comprising approximately two disordered CHCl3 molecules D

DOI: 10.1021/acs.inorgchem.8b00128 Inorg. Chem. XXXX, XXX, XXX−XXX

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Table 1. Summary of Selected Parametersa of the Iron(II) Centers, and of the Pyrimidine, Phenyl and Tolyl Rings, Found in Complexes 1, 2, 3·2CHCl3, 4, 5, and 5·2CH3OH at 100 K space group spin state average Fe−N ∑ pyrimidine twisth phenyl twisth tolyl twisth FeII−N−C

1b

2c

3·2CHCl3d

4e

5f

5·2CH3OHg

P1̅ HS 2.169 112.2 9.1 7.6 89.8 155

P1̅ HS 2.166 107.2 9.7 10.8 88.3 155

P21/n LS 1.962 43.0 5.2 42.3 86.7 177

P21/n HS 2.164 77.7 6.8 15.2 74.5 178

P1̅ HS 2.160 107.2 9.3 7.7 87.0 153

P1̅ HS 2.160 84.8 27.1 42.3 68.3 156

Including iron(II) bond lengths (Å), octahedral distortion (i.e., ∑, sum of the deviations of the 12 cis angles from 90°), and torsion angles (deg). FeII(L2pyrimidine)2(NCS)2. cFeII(L2pyrimidine)2(NCSe)2. d[FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3. Lattice CHCl3 was treated by SQUEEZE71, as it could not be modeled satisfactorily. eFeII(L4pyrimidine)2(NCS)2. fFeII(L4pyrimidine)2(NCSe)2. g[FeII(L4pyrimidine)2(NCSe)2]·2CH3OH. hAngle between the mean plane of the triazole ring and the mean plane of the coordinated pyrimidine ring, and not coordinated phenyl ring, or tolyl ring, respectively. a b

Figure 6. Two perspective views of the same overlay of the six structures of complexes [Fe II (L 2pyrimidine ) 2 (NCS) 2 ] (1, red), [FeII(L2pyrimidine)2(NCSe)2] (2, dark green), [FeII(L2pyrimidine)2(NCBH3)2] (3·2CHCl3, blue), [FeII(L4pyrimidine)2(NCS)2] (4, orange), [FeII(L4pyrimidine)2(NCSe)2] (5, light green), and [FeII(L4pyrimidine)2(NCSe)2]·2CH3OH (5· 2CH3OH, green) at 100 K. The structures were overlaid by fitting the iron(II) centers and the atoms in the triazole rings. These representations highlight the different angle of rotation between the coordinated pyridine and triazole rings and uncoordinated phenyl and tolyl rings and, more importantly, the significant differences between the Fe-NC angles. Two sets of Fe-NC angles can be clearly distinguished: nearly linear for 3·2CHCl3 and 4 (177−178°) vs 153−156° for the others. Hydrogen atoms and lattice solvent molecules are omitted for clarity.

Figure 4. Perspective view of the complex [FeII(L4pyrimidine)2(NCS)2] (4). Hydrogen atoms are omitted for clarity. Symmetry operation A is 1 − x, 1 − y, 1 − z.

complexes 3·2CHCl3 and 5·2CH3OH the phenyl/triazole dihedral angles are much larger (both 42.3°), while the tolyl/ triazole dihedral angles are 86.7° (within the usual range) and 68.3° (low), respectively (Figure 6, Table 1). Crystal Packing Comparisons. The three isomorphous complexes [FeII(L2pyrimidine)2(NCS)2] (1), [FeII(L2pyrimidine)2(NCSe)2] (2), and [FeII(L4pyrimidine)2(NCSe)2] (5) have the same packing (Figure 7) and interactions (Table S3, S4, and S6, Figures S13−S20), so the following description is applicable to all of them. The small deviation from having perfectly coplanar triazole, pyrimidine, and phenyl rings in these structures produces a wavelike layer of complexes (Figure 7, purple shaded layers). All of these layers are equivalent, but each layer is offset by half a molecule from the next layer. The molecules within each layer interact with one another through relatively long-range π···π contacts, between the relatively electrondeficient pyrimidine ring and the electron-rich phenyl ring of adjacent complexes, and hydrogen-bonding interactions. The tolyl 4N-substituent of the triazole ligands and the NCE coligand both penetrate between layers and interact with adjacent layers via hydrogen bonding, C−H···π interactions, and

Figure 5. Perspective view of [FeII(L4pyrimidine)2(NCSe)2]·2CH3OH (5·2CH3OH). Only one of the two half-occupancy CH3OH molecules per asymmetric unit is shown. Hydrogen atoms are omitted for clarity, except those in the lattice CH3OH molecules. Symmetry operation A is 1 − x, 2 − y, 1 − z.

The restrictions imposed by coordination of the triazole and pyrimidine rings to the iron(II) center usually result in them being close to being coplanar,21,23 and this is the case for 1, 2, 3·2CHCl3, 4, and 5 (5.2−9.7°). However, in 5·2CH3OH these two rings are relatively highly rotated away from coplanarity (27.1°; Figure 6, Table 1). The dihedral angles between the other pairs of aromatic rings, phenyl/triazole, and tolyl/triazole (Figure 6, Table 1) in complexes 1, 2, 4, and 5 have ranges of 7.6−15.2° and 74.5−89.8°, respectively, features observed before in related Rdpt-based complexes.21,23 In contrast, for E

DOI: 10.1021/acs.inorgchem.8b00128 Inorg. Chem. XXXX, XXX, XXX−XXX

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around the tolyl rings, between alternating layers (Figure S16). The accommodation of solvent molecules of crystallization in the tolyl region of the lattice is a recurring feature of this type of iron(II) system.18,34 Solvent-f ree [FeII(L4pyrimidine)2(NCS)2] (4) crystallized in the same P21/n space group as 3·2CHCl3, and a similar crystal packing arrangement is found. Within each flat layer (Figure 9)

Figure 7. Edge-on view of the wavelike layers of [FeII(L2pyrimidine)2(NCS)2] (1) with alternate layers highlighted in purple. Complexes [Fe I I (L 2 p y r i m i d i n e ) 2 (NCSe) 2 ] (2) and [FeII(L4pyrimidine)2(NCSe)2] (5) are isomorphous, so they pack in the same manner. Hydrogen atoms are omitted for clarity.

relatively long S···S or Se···Se interactions (Table S3 for 1, Table S4 for 2, and Table S6 for 5; and Figure S13). Complex [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3) has a distinct crystal packing organization from complexes 1, 2, 5, and 5·2CH3OH, as it crystallizes in the P21/n space group (Table 1). This produces alternating orientations of layers, ABAB (Figure 8).

Figure 9. View of [FeII(L4pyrimidine)2(NCS)2] (4) looking down the aaxis showing the alternating ABAB arrangement of the layers. Hydrogen atoms are omitted for clarity.

there are relatively long π···π interactions occurring between 4pyrimidine and phenyl rings as well as C−H···π and relatively long NCS···π contacts of adjacent complexes (Table S6, Figure S17). No interactions are found between alternating ABAB layers of the complex 4. For the CH 3 OH-solvate, [Fe II (L 4pyrimidine ) 2 (NCSe) 2 ]· 2CH3OH (5·2CH3OH), the complexes self-organize in onedimensional wavelike layers (Figure 10, purple shaded wavelike layers, Figure S18). Unlike for complexes 1, 2, and 5, in the case of 5·2CH3OH the complexes within each layer do not interact with one another, and the neighboring layers are offset

Figure 8. View of [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3), looking down the a-axis, showing the alternating ABAB arrangement of the layers A (blue) and B (red). Hydrogen atoms are omitted for clarity. SQUEEZE71 was applied, as the disordered lattice solvent could not be modeled satisfactorily.

Complexes within each layer interact through relatively longrange π···π contacts between the coordinated pyrimidine and the phenyl or triazole rings; also, there are relatively short-range π···π contacts between almost parallel phenyl rings and CH···B contacts between NCBH3 co-ligands and phenyl and tolyl rings (Figure S14, Table S5). Between the layers there are relatively long-range π···π contacts between adjacent tolyl rings and CH···N contacts between the uncoordinated N(2) atom (Figure 3) and adjacent tolyl rings (Figure S15, Table S5). Note that complex 3·2CHCl3 was treated by the SQUEEZE routine of PLATON,71 as the highly disordered lattice CHCl3 molecules could not be satisfactorily modeled. Those, now absent, solvent molecules were accommodated in the region

Figure 10. Perspective view of the complex [FeII(L4pyrimidine)2(NCSe)2]·2CH3OH (5·2CH3OH) showing the repeating wavelike layers of molecules (highlighted in purple). Each layer is offset from the next by approximately half a complex. The CH3OH solvent molecules occupy the space between the layers. Only one of the two half-occupancy CH3OH molecules is shown for clarity. Hydrogen atoms are omitted for clarity. Note that all wavelike layers are simply a visualization aid. F

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In contrast, the crystalline sample 3·2CHCl3 has low χMT values, 0.20−0.30 cm3·K·mol−1, from 50 to 300 K (Figure 10) consistent with this CHCl3-solvated iron(II) complex being in the LS state1and consistent with the structure determination performed on 3·2CHCl3 at 100 K (Table 1). However, when this CHCl3 solvate is heated at 2 K·min−1 to 400 K, the beginning of a gradual and incomplete SCO is observed, with a T1/2 > 400 K (Figure 10), perhaps associated with partial solvent loss at these elevated temperatures. Holding the temperature at 400 K for 1 h results in the initial χMT value of 1.56 cm3·K·mol−1 slowly increasing to 3.43 cm3·K·mol−1, which is consistent with a fully HS iron(II) complex (Figure 10, solid black circles). TGA data obtained on 3·2CHCl3 indicate that these conditions induce complete loss of the CHCl3 molecules of solvation, leaving anhydrous 3. Subsequent cooling of 3 from 400 to 50 K (hollow black circles) causes only a slight decrease in χMT, to 3.07 cm3·K·mol−1, so 3 is not SCO-active and instead remains in the HS state.1 However, the powder sample 5·1.5H2O undergoes SCO over a range of 50 K (Figures 10 and S21, left). The χMT value is 3.75 cm3·K·mol−1 at 300 K, consistent with a HS iron(II) center. The magnetic susceptibility decreases gradually to 2.91 cm3·K·mol−1 on cooling to 130 K; then, SCO takes place (T1/2 = 110 K) producing a reduction of the χMT value to 0.87 cm3· mol−1 K at 50 K. When the sample is heated to 300 K the magnetic moment follows the same path. Two more cooling− heating cycles show that this SCO is reproducible34 and does not show a thermal hysteresis loop (Figure S21). The HS state observed by crystallography at 100 K for the solvatomorphs 5 and 5·2CH3OH strongly contrasts with the SCO observed for this powder sample of solvatomorph 5·1.5H2O. This is again unsurprising, as the critical importance of crystal packing and solvent content on magnetic behavior is well-established.1,4,73−75 The expectation that the NCE− co-ligands should provide an increasing ligand field strength, in the order NCS− < NCSe− < NCBH3−,76−80 is more or less borne out here, with the 2pyrimidine-cyanoborohydride complex 3·2CHCl3 having a particularly high T1/2 (>400 K), even higher than that recently reported for the pyridine-cyanoborohydride analogue [FeII(Lpyridine)2(NCBH3)2]·H2O (T1/2 = 309 K).36 But it is also clear that the different crystallization conditions and solvent contents of the samples studied herein are also impacting the magnetic responses observed. This is particularly marked in the case of 6·2.5H2O, the 4-pyrimidine-cyanoborohydride analogue of the high-temperature SCO-active 3· 2CHCl3, as it remains HS to 50 K, yet could have been expected to have a high tendency to undergo SCO to the LS state. The only other complex studied herein that is SCO-active in the solid state is the 4-pyrimidine-selenocyanate complex 5· 2H2O (T1/2 = 110 K). These findings illustrate the critical importance of crystal packing (including space group and Fe-NCE angle) and solvent content on the magnetic behavior in the solid state, something which is well-known.4,73−75,81−84 The impact of solvent on magnetic response is particularly nicely demonstrated with the related pyrazine-thiocyanate complex, [FeII(Lpyrazine)2(NCS)2], which was reported to show solvent dependence of the switching temperature: T1/2 = 212 K for violet solvent-free form versus T1/2 = 255 for the forest green THF solvate.34 UV−Vis Spectroscopy and Speciation. The ligands L2pyrimidine and L4pyrimidine (Figure S22) and a pair of isomeric NCBH3 complexes, [FeII(L2pyrimidine)2(NCBH3)2] (3) and

further from each other due to the solvent CH3OH molecules occupying the space between them (Figures 10 and S18); hence, there are no significant π···π or Se···Se interactions. Instead, the Se atoms interact weakly with tolyl rings (Table S8). The CH3OH molecule forms a hydrogen-bond contact with the uncoordinated N(2) atom of the 4-pyrimidine ring (Figure 5, Table S8, Figures S19 and S20), which may explain the greater rotation found for the coordinated ring (Table 1). Solid-State Magnetic Measurements. Variable-temperature (VT) magnetic susceptibility measurements, from 300− 50 K, or in the case of 3·2CHCl3 from 50 to 400−50 K (Figures 11 and S21), were performed on crystalline samples of

Figure 11. χMT versus T plot for compounds [FeII(L2pyrimidine)2(NCS)2] (1, dark cyan), [FeII(L2pyrimidine)2(NCSe)2] (2, red), [FeII(L2pyrimidine)2(NCSe)2]·1.5H2O (2·1.5H2O, orange), [Fe I I (L 2 p y r i m i d i n e ) 2 (NCBH 3 ) 2 ] ·2H 2 O (3·2H 2 O, fuchsia), [Fe I I (L 4 p y r i m i d i n e ) 2 (NCBH 3 ) 2 ]·2CHCl 3 (3·2CHCl 3 , black), [FeII(L4pyrimidine)2(NCBH3)2] (3, open black, data collected after holding at 400 K for 1 h), [FeII(L4pyrimidine)2(NCS)2]·H2O (4·H2O, light green), [Fe I I (L 4 p y r i m i d i n e ) 2 (NCSe) 2 ] (5, purple ), [FeII(L4pyrimidine)2(NCSe)2]·1.5H2O (5·1.5H2O, cooling mode as blue triangles, and heating mode as red triangles), and [FeII(L4pyrimidine)2(NCBH3)2]·2.5H2O (6·2.5H2O, dark green). To confirm T1/2 = 110 K, the derivatives of the SCO curves in the heating and cooling modes for 5·1.5H2O were examined (Figure S21, right).

[FeII(L2pyrimidine)2(NCS)2] (1), [FeII(L2pyrimidine)2(NCSe)2]· 1.5H2O (2·1.5H2O), [FeII(L2pyrimidine)2(NCBH3)2]·2CHCl3 (3·2CHCl3), [FeII(L4pyrimidine)2(NCS)2]·H2O (4·H2O), [FeII(L4pyrimidine)2(NCSe)2] (5), and [FeII(L4pyrimidine)2(NCSe)2]· 2CH3OH (5·2CH3OH); and on powder samples of [FeII(L2pyrimidine)2(NCSe)2] (2), [FeII(L2pyrimidine)2(NCBH3)2]· 2H2O (3·2H2O), [FeII(L4pyrimidine)2(NCSe)2]·1.5H2O (5· 1.5H 2 O), and [Fe II (L 4pyrimidine) 2 (NCBH 3 ) 2 ]·2.5H 2 O (6· 2.5H2O). To confirm the purity of each compound, all solid samples were characterized by elemental analysis, TGA, IR, and mass spectrometry (MS; see Experimental Section). The crystalline samples of 1, 2·1.5H2O, 4·H2O, 5, and 5· 2CH3OH, as well as the powder samples 2, 3·2H2O, and 6· 2.5H2O, have similar magnetic behavior (Figure 10). Their χMT values at 300 K range from 3.32 to 3.71 cm3·K·mol−1 and gradually decrease to 3.02−3.37 cm3·K·mol−1 at 50 K (Figure 10). These values are consistent with these eight iron(II) complexes being in the HS state across the temperature range studied.1 G

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Inorganic Chemistry [FeII(L4pyrimidine)2(NCBH3)2] (6), were studied by UV−vis spectroscopy at room temperature in five different solvents: CH2Cl2, CHCl3, (CH3)2CO, CH3CN, and CH3NO2 (Figures 12 and S23−S33).

Figure 13. Solution-phase magnetic data presented as χMT vs T for complexes [FeII(L2pyrimidine)2(NCBH3)2] (3, stars and dotted lines) and [FeII(L4pyrimidine)2(NCBH3)2] (6, circles and solid lines) in five different deuterated solvents, CD 2Cl2 (pink), CDCl3 (blue), (CD3)2CO (orange), CD3CN (green), CD3NO2 (red). Note that each solution was prepared using a 6:1 L/Fe ratio to ensure it is present as [FeII(Lnpyrimidine)2(NCBH3)2]. The lines correspond to the best fit found for each compound using the regular solution model, eq 1.25,94,95

Figure 12. UV−Vis spectra for complexes [FeII(L2pyrimidine)2(NCBH3)2] (3, dotted line) and [FeII(L4pyrimidine)2(NCBH3)2] (6, solid lines) at room temperature in five different solvents: CH2Cl2 (pink), CHCl3 (blue), (CH3)2CO (orange), CH3CN (green), CH3NO2 (red). Note that each solution was generated using an iron(II)/Lnpyrimidine ratio of 1:6 (see Section S4).

Speciation in solution has been previously observed for analogous systems,36,85 so in an attempt to find the Lnpyrimidineto-[FeII(pyridine)4(NCBH3)2] ratio required to ensure that all i r o n ( I I ) i n s o l u t io n i s p r e s e n t a s t h e d e s i r e d [FeII(Lnpyrimidine)2(NCBH3)2] complex, the UV−vis spectra of a series of Fe/L ratios, from 1:2 to 1:10, were performed for each complex, in each of the five solvents (Figures S23−S32). In all cases it was found that a 1:6 Fe/L ratio was sufficient to lead to an unchanging UV−vis spectrum (further addition of L did not change the spectrum significantly). But the absence of isosbestic points in these titrations (Figures S23−S33), and a less than perfect match of the bands in the final spectra with the bands in the solid state reflectance spectra of these complexes (Table S9, Figure S34), indicates that we cannot rule out the presence of other species. It is difficult to conclusively prove that the species in each of these Fe/L 1:6 solutions is the desired [FeII(Lnpyrimidine)2(NCBH3)2] complex, but efforts to probe this point have been made. In particular, comparisons between the data on these cyanoborohydride solutions and those of the analogous tetrafluoroborate tris-ligand-type species [FeII(Lnpyrimidine)3]2+ (ref 88) were performed (all on solutions containing an Fe/L ratio of 1:6). While the comparisons of the UV−vis spectra are not clear-cut (Figure S33), the tris-ligand complexes are clearly shown to be diamagnetic (χMT always below 0.24 cm3 K mol−1) and 2:1 conductors, whereas in contrast the [FeII(Lnpyrimidine)2(NCBH3)2] complexes are shown to be SCO-active (see next section, Figure 13) and almost non-conductors, regardless of whether they are studied in (CH3)2CO or CH3CN solutions (Table S15, Figure S51). Taken together, it seems reasonable to propose that the predominant species present in these 1:6 Fe/L solutions differs for the cyanoborohydride versus tetrafluoroborate. The decision was made to use this 1:6 ratio in all of the subsequent solution studies (the solution-phase magnetic measurements

are presented in the next section). Nevertheless, note that caution must be taken with regard to the generality of the tantalizing conclusions reached in this solvent dependence of T1/2 study (see next section), as the extent that the [FeII(Lnpyrimidine)2(NCBH3)2] species dominates in solution could well be solvent-dependent too. The observed bands in the visible for both NCBH3-based complexes, in all solvents, are charge transfer (CT) in origin, as the molar extinction coefficient (ε) values are all larger than 915 L·mol−1·cm (please note they are calculated per Fe without consideration of the ratio of HS/LS present), so are higher than the expected values for octahedral iron(II) d−d bands (Table S9).1 The highest ε values for 3 and 6 are observed in CH3CN solution (8754 L·mol−1·cm at 490 nm and 6848 L·mol−1·cm at 525 nm, respectively). The lowest ε values for both compounds are observed in (CH3)2CO and CH2Cl2 solutions (3: 1026 L· mol−1·cm at 482 nm and 986 L·mol−1·cm at 461 nm, respectively; 6: 915 L·mol−1·cm at 477 nm and 1059 L· mol−1·cm at 517 nm, respectively) (Table S9), which is consistent with these solvents containing the highest HS fractions (see next section). Solution-Phase Magnetic Measurements. In recent studies, the solution-phase magnetic behavior in CDCl3 of a set of three SCO-active NCE (E = S, Se, and BH3) complexes of Lpyridine, [FeII(Lpyridine)2(NCE)2] (Figure 1)36 and of a set of five SCO-active NCBH3 complexes of Lazine ligands (Figure 1, bottom right)39 was reported. The latter study of NCBH3 complexes showed that, in CDCl3 solution, based on the observed T1/2 values in CDCl3, the relative ligand field strength for these five Lazine ligands, in increasing order, is L4pyrimidine < L2pyrimidine < Lpyridine < Lpyrazine < Lpyridazine. In the former study, of the three complexes of Lpyridine, the NCBH3 analogue is the most soluble, and it also undergoes a more abrupt and H

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CD2Cl2 < CDCl3 < (CD3)2CO < CH3CN < CH3NO2 (Figure 10 and Tables S10 and S11). Of these five solvents, the use of CH3NO2 maximizes the overall ligand field experienced by the complex, favoring the LS state, resulting in the highest T1/2. But for completeness, we also note here that it is not possible to rule out that the solvent choice is also modifying the speciation of [FeII(Lnpyrimidine)2(NCBH3)2], and that in turn this could be affecting the observed T1/2 values. The values of enthalpy (ΔH) and entropy (ΔS) obtained from the data modeling (Table 2) for compound 3 (ΔH = 19.2−49.2 kJ·mol−1 and ΔS = 62−139 J·mol−1·K−1) and compound 6 (ΔH = 19.1−21.6 kJ·mol−1 and ΔS = 52−108 J· mol−1·K−1) fall close to the expected range of values for other reported solution studies using iron(II) complexes (ΔH = 4− 41 kJ·mol−1 and ΔS = 22−146 J·mol−1·K−1).53,96 The values of T1/2 can be calculated by dividing the respective ΔH by ΔS (Table 2). Importantly, in all solvents, the T1/2 value for compound 3 (247−396 K) is always higher than for compound 6 (216−367 K), consistent with the earlier comment that L2pyrimidine provides a stronger ligand field than L4pyrimidine. The influence of the solvent over the spin state of SCOactive iron(II) complexes in solution has been described previously.47,48,50,53 Specifically, Halcrow and co-workers have investigated the effect that different solvents and solvent mixtures, via hydrogen bonding, have over T1/2 for the [FeII(3bpp)2](BF4)2 family.48,50 They observed that the more a solvent participates in hydrogen bonding, the more it stabilizes the LS state, so the lower the γHS and higher T1/2 it gives.48 This trend has also been reported for other systems.44,45,47 In the present case we observed that solvents with higher polarity index (P′)97,98 gave higher T1/2 values (Figure 14, Tables 2 and S12). Specifically, the five solvents used in this study, ordered in increasing P′, are

complete SCO, possibly due to enhanced hydrogen-bonding interactions with the CDCl3 solvent due to the presence of the H-bond donor in the BH3 (cf. S or Se, which lack this) moiety.36 The influences of H-bonding between complex and solvent47,48,50,53 and of solution pH87 on the SCO phenomenon have been observed for other solution SCO-active systems. For completeness, it is also helpful to note that, as expected,88 the tris ligand-type species [FeII(Lnpyrimidine)3]2+ are practically diamagnetic in solution (Figure S51).86 The initial solution-phase magnetic study, also in CDCl3, on the pair of isomeric NCBH3 analogues 3 and 6 (Figures S35 and S36), also revealed relatively abrupt SCO.39 Hence, to probe the influence on the SCO of the interaction between these complexes and the solvent utilized, 3 and 6 were also studied in four more deuterated solvents (Figure 10 and Figures S37−S44: CD2Cl2 pink, CDCl3 blue, (CD3)2CO orange, CD3CN green, and CD3NO2 red), using the VT Evans 1H NMR method,53,89−93 with an L/Fe ratio of 6:1, as described in the previous section. In all cases, 3 and 6 undergo a gradual SCO in the limited temperature range able to be studied (Figure 10, Tables S10 and S11), which ranged from 20 K below the boiling point of the target solvent down to 243 or 248 K (Tables S10 and S11). Each data set was modeled as a gradual and complete SCO using the regular solution model [eq 1; see Section S5 for further details] with good fits obtained (Figure 10 and Table 2).25,94,95 Table 2. Summary of the Thermodynamic Parametersa Obtained from Fitting the Datasetsb Obtained for Complexes 3c and 6d with equation 1,25,94,95 and the T1/2 (K) Calculated from These Values, as well as the R2 for the Fit compound

solvent

T1/2

ΔHa

ΔSa

R2

c

CD2Cl2 CDCl3 (CD3)2CO CD3CN CD3NO2 CD2Cl2 CDCl3 (CD3)2CO CD3CN CD3NO2

247 262 312 353 396 216 232 291 334 367

31.1 23.5 19.2 49.2 35.3 23.4 19.8 20.2 21.6 19.1

126 90 62 139 89 108 86 69 65 52

0.99 0.99 0.99 0.99 0.99 0.98 0.99 0.99 0.97 0.99

3

6d

CH 2Cl 2 (3.1) < CHCl3 (4.1) (CH3)2 CO (5.1) < CH3CN (5.8) < CH3NO2 (6.8)

Indeed excellent linear correlations (R2 = 0.96) are observed between the T1/2 values for each of the complexes 3 and 6 and

ΔH (kJ·mol−1) and ΔS (J·mol−1·K−1) fall within the expected literature range of values for solution studies of iron(II) complexes (ΔH = 4−41 kJ·mol−1 and ΔS = 22−146 J·mol−1·K−1).53,96 bTables S10 and S11. cFeII(L2pyrimidine)2(NCBH3)2. d II 4pyrimidine Fe (L )2(NCBH3)2. a

χM T (T ) =

χM T (max) 1 + exp( −ΔH /RT + ΔS /R )

(1)

Two clear trends can be observed throughout all measurements. First, there is always a lower HS molar fraction (γHS) for 3 than for 6 in a given solvent at a given temperature (Tables S10 and S11). This indicates that, of this isomeric pair of ligands, L2pyrimidine provides the iron(II) center with a stronger ligand field than L4pyrimidine. Second, the five solvents can be ordered as a function of increasing ability to stabilize the LS state, with the same order obtained for both complexes:

Figure 14. Plot showing the linear fit of T1/2 for complexes [FeII(L2pyrimidine)2(NCBH3)2] (3, stars and dotted lines, slope = 42.45) and [FeII(L4pyrimidine)2(NCBH3)2] (6, circles and solid lines) vs polarity index (P′) of each of the solvent used in this study: CD2Cl2 (pink), CDCl3 (blue), (CD3)2CO (orange), CD3CN (green), CD3NO2 (red). I

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Inorganic Chemistry the polarity index P′ of the solvent used (Figure S45). Note that of course solvents can be described by many different parameters and properties: besides P′, it was also found that some of other solvent parameters (e.g., Reichardt’s ENT99 or Swain and co-workers’s Basity100,101) and solvent physical properties (e.g., Hildebrand’s solubility parameter,102 relative permittivity, and the dipole moment) also gave linear fits with good correlations (R2 = 0.93−0.71) when plotted against the T1/2 values of these two complexes (Tables S12 and S13, Figures S45−S50). In contrast, poor to no correlations were found for some other well-known solvent parameters, such as Taft and co-workers’s α and β,103 or Gutmann and co-workers’s AN and DN104,105 (see Section S5.4 for more details). Interestingly, the slopes of the linear fits for both complexes are quite similar (Figure 11; slopes for 3: 42.45 and 6: 44.11) indicating that the tuning of the ligand field by the solvent is occurring in a similar manner for both complexes. This is most likely mainly through hydrogen-bonding interactions between the solvent and NCBH3 co-ligands, rather than through the uncoordinated nitrogen atoms of the different n-pyrimidine rings (Figures 1−4), as that would tend to result in distinctly different slopes. These findings are also consistent with the earlier proposal that the distinct magnetic behavior of the NCBH3 analogues, as compared to the NCS and NCSe analogues, in the family of [FeII(Lpyridine)2(NCE)2] complexes is due to hydrogen-bonding interactions between the BH3 moiety and the solvent molecules.36 While in Halcrow’s system the maximum shift of T1/2 between solvents is 73 K, in our system it is double that, ∼150 K, for both complexes 3 and 6. This different degree of tuning of the T1/2 as a f unction of solvent choice for different SCO-systems is unsurprisingHalcrow and co-workers’ system and our system are quite different, that is, cationic versus neutral complexes of homoleptic versus heteroleptic ligands. Consistent with this we also reported significantly different degrees of tuning of T1/2 as a f unction of substituent choice for these two different types of SCO-active systems.39 The distinctly different magnetic behaviors of complexes 3 and 6 between the solid state (Figure 8) and solution phase (Figure 10) highlight the strong effect that crystal packing and solvent content has on the SCO behavior.2 In solution the electronic effects induced by the ligands and the solvation effects51 (i.e., hydrogen-bonding ability and polarity of the solvent) are the major influences tuning the ligand field strength,25,36,53 as is clearly illustrated herein. But as noted above: the influence of solvent choice on speciation, however small, must be examined in such solution-based studies, as it may well be a complicating factor in studies of this type.

(NCBH3)2]·2CHCl3 (3·2CHCl3) sample is LS from 50 to 300 K, before starting to undergo SCO with a T1/2 above 400 K. When heated for 1 h at 400 K, the CHCl3 is desorbed, and the resulting solvent-free sample 3 is HS from 400 to 50 K. The other SCO-active sample is [FeII(L4pyrimidine)2(NCSe)2]· 1.5H2O (5·1.5H2O), which undergoes SCO with a T1/2 of 110 K. UV−Vis studies in CH2Cl2, CHCl3, (CH3)2CO, CH3CN, and CH3NO2 solutions for the cyanoborohydride samples 3 and 6 led to a ratio of 6:1 Lnpyrimidine per iron(II) being employed in the solution-phase magnetic measurements in these five solvents. These were performed by the Evans NMR method, revealing that both compounds undergo gradual SCO in all solvents, with T1/2 values for the 2-pyrimidine complex (247−396 K) that were consistently higher than for the 4pyrimidine complex (216−367 K). Interestingly, the SCO is strongly influenced by the polarity of the solvent, enabling the T1/2 to be tuned by 150 K by the choice of solvent. Furthermore, a strong linear correlation is found between the polarity index (P′) of the solvent and the T1/2 of the complex in that solvent. This is a key finding, as it enables fine-tuning of T1/2 by choice of solvent, that is, by choice of P′. But it is important to note that it is quite possible that the extent to which [FeII(Lnpyrimidine)2(NCBH3)2] is the dominant species in solution could also be solvent-dependent, which would make speciation a confounding factor in this study. Hence it is important that future studies probing the tantalizing prospect that arises from the present study, of fine-tuning T1/2 by choice of solvent (more specifically by choice of solvent polarity index), should instead employ more robust complexes (either polydentate or macrocyclic) than those of the present bidentate Rdpt-type ligand family. The linear fits to the experimental data were similar (R2 and slope values) for both 3 and 6, indicating that the mechanism of tuning of the ligand field, and hence T1/2, is similar for both complexes, so the mechanism is most likely due to hydrogenbonding interactions between the BH3 moieties (rather than the uncoordinated nitrogen atoms of the 2- and 4-pyrimidine rings) with the solvent molecules. In summary, this study highlights the influence that (a) packing effects have on SCO in the solid state and (b) solvent polarity has over SCO in solution-phase studies. These findings highlight the need for solution-phase studies to be more commonly performed in tandem with the solid-state magnetic measurements that are standardly reported. Clearly the issue of speciation must also be carefully considered when working in solution, so it is recommended that future solution SCO studies employ complexes of polydentate or macrocyclic ligands for which it is possible to prove that the species present in solution is solely the expected one. Nevertheless the introduction of diazines, including the 2- and 4-pyrimidines described herein, as functional groups in Rdpt-type ligands is producing appealing examples of SCO-active iron(II) materials, so further work studying these and related systems is currently underway.

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CONCLUSIONS The pyrimidine-substituted Rdpt-type ligands L2pyrimidine and L4pyrimidine were synthesized by extending our multistep literature procedure for the related Rdpt-type ligands.63 These have been used to generate two new families of mononuclear iron(II) complexes [FeII(Lnpyrimidine)2(NCE)2], where n = 2 or 4 and E = S, Se, or BH3, as well as several solvatomorphs. Six complexes were structurally characterized at 100 K, revealing two triazole ligands coordinated equatorially in a bidentate manner and two axially trans-coordinated NCE coligands, with the iron(II) centers in the HS state in all cases. VT magnetic susceptibility measurements showed that all but two of the samples remain HS from 300−50 K, in the solid state. The other two samples are SCO-active. The [FeII(L2pyrimidine)2-

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EXPERIMENTAL SECTION

Instrumentation and Measurements. Elemental analyses (C, H, N) were measured at the Campbell Microanalytical Laboratory, University of Otago. 1H and 13C NMR spectra (Figures S1−S4) were recorded on a Varian 400 or 500 MHz NMR spectrometer at 298 K. Chemical shifts are reported in parts per million and referenced to the residual protonated solvent peak in the 1H NMR spectra and the residual solvent peak in the 13C NMR spectra [CDCl3: 7.26 ppm (1H) J

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Article

Inorganic Chemistry

SCO using the regular solution model [eq 1]25,94,95 with good fits, and they were performed with OriginPro version 9.1.0 from OriginLab Corporation. A diamagnetic correction for the sample95 and a correction for the variation of the density of the solvent with temperature112 were applied to each data set. General Procedures. Dry ethanol was prepared by distilling absolute ethanol from Mg/I2.63 2-Cyanopyrimidine, 4,4′dimethoxy-2butanone, hydrazine monohydrate (80% solution w/w), and all HPLC-grade solvents were bought commercially and used as received without further purification. The petrol ether used as an eluent had a bp range of 40−60 °C. The iron(II) starting complexes [Fe II (pyridine) 4 (NCS) 2 ], 77 [Fe II (pyridine) 4 (NCSe) 2 ], 77 and [FeII(pyridine)4(NCBH3)2]111 were prepared by mixing [FeII(H2O)6](BF4)2 and NH4(SCN), KSeCN, or NaBH3CN, respectively, in H2O containing excess pyridine following analogous reported methods.77,111 Other chemicals were bought commercially and used as received without further purification. Reactions were conducted in air unless otherwise stated. Caution! Hydrazine hydrate is potentially explosive! Perform any reaction with it behind a blast screen in a f umehood. Organic Synthesis. Pyrimidine-2-carboxylic Acid. 2-Cyanopyrimidine (1.1 g, 10 mmol) was added to a room-temperature (RT) solution of sodium hydroxide (0.9 g, 22 mmol) in H2O (10 mL), giving a pale yellow suspension. The mixture was heated to 55 °C for 3 h to give a light yellow solution. This was cooled to RT and then acidified with 2 mol L−1 HCl (aq) (∼20 mL) to pH 4. The solution was then taken to dryness to give the crude product as a pale yellow powder contaminated with NaCl (2.8 g), which was reacted without further purification. C5H4N2O2 (M = 124.10 g mol−1). 1H NMR (400 MHz, D2O): δ (ppm) = 8.72 (d, J = 5.0 Hz, 2H, H2), 7.46 (t, J = 5.0, 1H, H1). 13C NMR (126 MHz, d6-DMSO): δ (ppm) = 165.6, 158.6, 158.3, 123.5. IR (ATR) ν, cm−1: 3064, 2641 (br), 1741, 1608, 1405, 1271, 1190, 672. Ethyl Pyrimidine-2-carboxylate. All of the crude NaCl contaminated pyrimidine-2-carboxylic acid from the previous step (2.8 g, maximum 10 mmol) was suspended in dry ethanol and cooled in an ice bath. Thionyl chloride (5 mL, 8.2 g, 70 mmol) was added dropwise, and the resulting white suspension was refluxed at 80 °C for 4 h. The reaction solution was cooled to RT and taken to dryness. Saturated aqueous sodium bicarbonate (30 mL) was added to neutralize the reaction mixture, and the product was promptly extracted with CH2Cl2 (6 × 20 mL). The combined organic fractions were dried with magnesium sulfate and taken to dryness to give 3 as a colorless oil, which on drying under vacuum for 5 h became a yellow solid (1.1 g, 73% from 2-cyanopyrimidine). C7H8N2O2 (M = 152.15 g mol−1): calcd C 55.26, H 5.30, N 18.41%; found C 55.33, H 5.29, N 18.43%. 1H NMR (400 MHz, CDCl3): δ = 8.96 (d, J = 4.9 Hz, 2H, H2), 7.49 (t, J = 4.9 Hz, 1H, H1), 4.56 (q, J = 7.1 Hz, 2H, CH2), 1.48 (t, J = 7.1 Hz, 3H, CH3). 13C NMR (126 MHz, d6-DMSO): δ (ppm) = 163.3, 157.8, 156.7, 123.0, 62.8, 14.2. ESI-MS m/z: 175.0474 ([Na(C7H8N2O2)]+ calcd 175.0484). IR (ATR) ν, cm−1: 2995, 2940, 2900, 1727, 1477, 1439, 1413, 1390, 1362, 1310, 1276, 1173, 1156, 1083, 1020, 844, 806, 712. Pyrimidine-2-carbohydrazide (A). Caution! Hydrazine hydrate (80% solution w/w) is potentially explosive. Perform the reaction behind a blast screen in a fume hood. Dispose of hydrazine hydrate residues appropriately. Ethyl pyrimidine-2-carboxylate (1.1 g, 7 mmol) was dissolved in ethanol (5 mL). Hydrazine monohydrate (80% w/w) (5 mL) was added, and the resulting yellow solution was refluxed for 4 h behind a blast shield. When the reaction solution was cooled to −18 °C in a freezer a pale pink/pale brown precipitate formed. This was filtered off, washed with cold ethanol (10 mL), and air-dried for 1 h to give the desired product as a yellow solid (0.6 g, 61%). C5H6N4O (M = 138.13 g mol−1): calcd C 43.48, H 4.38, N 40.56%; found C 43.36, H 4.36, N 40.69%. 1H NMR (400 MHz, d6-DMSO): δ = 10.01 (s, 1H, NH), 8.91 (d, J = 4.9 Hz, 2H, H2), 7.62 (t, J = 4.9 Hz, 1H, H1), 4.59 (s, 1H, NH2). 13C NMR (101 MHz, d6-DMSO): δ = 162.0, 158.7, 158.1, 123.2. ESI-MS m/z: 139.0603 ([C5H6N4O+H]+ calcd 139.0614), 161.0434 ([Na(C5H6N4O)]+ calcd 161.0434). IR (ATR) ν, cm−1: 3172, 1665, 1584, 1525, 1384, 1338, 1291, 1130, 992, 865, 677, 660.

and 72.16 ppm (13C); deuterated dimethyl sulfoxide (d6-DMSO): 2.50 ppm (1H) and 39.52 ppm (13C)]. High-resolution electrospray ionization (ESI) MS spectra were recorded using a Bruker MicrOTOF-Q mass spectrometer in CH3OH, and m/z values have a standard error of ±10 ppm at 263 K. IR spectra were recorded on a Bruker ATR-IR spectrometer with a diamond anvil Alpha-P module, within the range between 400 and 4000 cm−1. TGA measurements were collected on a TA Instruments TGA Q50. In each case the temperature was ramped at 10 K·min−1 up to 393 K and then kept constant for a total elapsed time for 100 min, after which the mass was constant (Δmass