Above Room Temperature Spin Transition in Thermally Stable

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

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Above Room Temperature Spin Transition in Thermally Stable Mononuclear Fe(III) Complexes Bijoy Dey,† Subhadip Roy,† Jań Titiš,*,‡ Roman Bocǎ ,‡ Siba Prasad Bera,† Arpan Mondal,† and Sanjit Konar*,† †

Department of Chemistry, IISER Bhopal, Bhopal Bypass Road, Bhauri, Bhopal 462066, Madhya Pradesh, India Department of Chemistry, Faculty of Natural Sciences, University of SS Cyril and Methodius, 917 01 Trnava, Slovakia

‡

Inorg. Chem. Downloaded from pubs.acs.org by DURHAM UNIV on 01/02/19. For personal use only.

S Supporting Information *

ABSTRACT: Two solvent-free mononuclear Fe(III) complexes [Fe(L)2]NO3 (1) and [Fe(L)2]ClO4 (2) have been synthesized by employing a new πconjugated azo-phenyl substituted ligand, 2-((E)-((2-(ethylamino)ethyl)imino)methyl)-4-(2-phenyldiazenyl)phenol (HL). The noncoordinated azophenyl part of the ligand adopts two different conformations which can exert a varied local distortion around the metal center affecting the spin crossover behavior. The magnetic data (2−450 K) reveal that complex 1 displays spin crossover above room temperature where the ligand is in linear form, while complex 2 shows an incomplete spin transition where the ligand adopts a skew form in the solid state. These complexes represent rare examples of high-temperature spin transition for mononuclear Fe(III) complexes with T1/2 > 350 K with very high thermal stability. Presence of strong intermolecular interactions and solvent-free nature of the complexes leads to exceptional thermal stability up to 485 K (for 1) and 496 K (for 2) as revealed by thermogravimetric analysis. The magnetic data for complex 1 have been analyzed by employing an Ising-like model with vibrations yielding the enthalpy change ΔH and entropy change ΔS of the spin transition along with the critical temperature T1/2 and the solid-state cooperativeness Γ. Spin crossover behavior of complex 1 has also been characterized by differential scanning calorimetry and electron paramagnetic resonance measurements. Ab initio calculations have been performed to analyze the difference in energies of the ground state and excited states of the complexes.

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INTRODUCTION

In literature, numerous Fe(III) complexes have been reported for the N-SalEen ligand family where the complexes show hydrogen bonding between the counteranion and the amine hydrogen present in the ligand which is responsible for the spin transition at room temperature.11 The introduction of azo-phenyl group in N-salEen moiety may give rise to cooperative spin transition as this moiety can provide very effective π−π and C−H···π interactions. The ligand has multiple aspects to serve (Scheme 1). First, the coordinating part provides a N4O2 coordination environment which is favorable to show SCO with Fe(III).12 Second, the extended noncoordinated azo-phenyl part can exert various noncovalent interactions (H-bonding, π···π, C−H···π interactions, etc.) in crystal packing. Finally, azo-phenyl moiety is known for its photoisomerization behavior under irradiation of ultraviolet (UV) light, which can provide a way to control the magnetic property of these complexes in liquid state by the use of light as external stimuli.13 After synthesizing the ligand (HL), it was employed to different Fe(III) metal precursors to obtain [Fe(L)2]·X, where X = NO3 (1) and ClO4 (2). Complex 1 shows a very high-temperature spin transition, whereas

Spin crossover (SCO) is the most widely studied phenomenon within the magnetically bistable systems which is achievable by the use of external stimuli such as light, heat and pressure.1,2 Magnetically bistable materials have been shown to be deserving candidates for application in magnetic switches and memory devices.3 Fe(III) systems are relatively less explored as compared to Fe(II) counterparts4,5 holding the potential to be used in data storage device applications because of their air stability.6 Over the past decades, SCO compounds have been extensively investigated, but there are still very limited examples showing spin transition temperature (T1/2) around room temperature or above.7 One of the problems for obtaining high-temperature spin transition lies in the poor thermal stability of most of the mononuclear iron complexes.8,9 Weak intermolecular interactions in the solid state have enormous impact on SCO behavior as it is accompanied by metal and ligand bond length change.10 These interactions can also improve the thermal stability of the complexes. Hence, considering the importance of weak interactions in solid-state packing, a noncoordinated part attached with a chelating moiety can be effective in improving the thermal stability of the complexes by increasing intermolecular interactions. © XXXX American Chemical Society

Received: August 26, 2018

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DOI: 10.1021/acs.inorgchem.8b02405 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

crystals of complex 1 were obtained from the filtrate after 2 days. Yield: 60% (based on Fe). Elemental analysis: calcd (%) for C34H38FeN9O5: C 57.63, N 17.79, H 5.41; found C 57.69, N 17.72, H 5.36. Selected IR data (KBr pellet; cm−1): 3438 ν(N−H), 3233 ν(C−Harom), 2943 ν(C−Haliph), 1628 ν(CN), 1531 and 1468 ν(CC), 1379 ν(NO3). Synthesis of [Fe(L)2]·ClO4 (2). Similar synthetic method as for complex 1 was employed for the synthesis of complex 2, except Fe(ClO4)3·6H2O (177 mg, 0.5 mmol) salt was used. Black colored crystals of 2 were obtained from the filtrate after slow evaporation over 2 days. Yield: 55% (based on Fe). Elemental analysis: calcd (%) for C34H38ClFeN8O6: C 54.74, N 15.02, H 5.13; found C 54.71, N 15.09, H 5.18. Selected IR data (KBr pellet; cm−1): 3438 ν(N−H), 3233 ν(C−Harom), 3058−2942 ν(C−Haliph), 1631 ν(CN), 1530 and 1468 ν(CC), 1107 ν(ClO4). Crystal Data Collection and Structure Determination. Intensity data were collected on a Brüker APEX-II diffractometer using a graphite monochromated Mo−Kα radiation (α = 0.71073 Å) at 120 K. Data collections were performed using φ and ω scan. Olex216a was used as the graphical interface, and the structures were solved with the ShelXT16 structure solution program using intrinsic phasing. The models were refined with ShelXL16 with full matrix leastsquares minimization on F.2 All non-hydrogen atoms were refined anisotropically. Crystallographic data for complexes 1 and 2 have been summarized in Table S1−S2 in the Supporting Information. Hirshfield Surface Analysis. Crystal Explorer package ver. 3.1 was used for hirshfeld surface and 2D fingerprint calculations. Crystal structures were imported from CIF files. Hirshfeld surfaces for both the complex molecules were generated using very high resolution and mapped with the dnorm function.17 Magnetic Susceptibility Measurements. Magnetic measurements were performed on polycrystalline samples using SQUID VSM magnetometer (MPMS 3) and MPMS-XL. In all cases, the temperature dependence of the magnetic moment was recorded with an applied field of 0.1 T. For high-temperature measurement, the samples were sealed in a homemade aluminum foil capsule, mounted inside another homemade aluminum foil bar, and then fixed to the end of the standard sample transport rod. The data were corrected for the diamagnetic contribution from the sample holder and for the ligand atoms using Pascal’s table.18 Theoretical Calculations. ORCA 4.0.0 computational package were employed for ab initio calculations at the truncated experimental geometries (azo-phenyl groups were replaced by −NH2 groups).19 Zero-order regular approximation (ZORA) along with the scalar relativistic contracted version of def2-TZVPP basis functions for the Fe atom and def2-SV(P) basis functions for other elements were taken to include the relativistic effects in the calculation. The calculations were based on state average complete active space selfconsistent field (SA-CASSCF) wave functions complemented by Nelectron valence second-order perturbation theory (NEVPT2).20 The active space comprised of five electrons in five metal-based d-orbitals. The state averaged approach was used, in which 1 sextet, 24 quartet, and 75 doublet states were equally weighted. The calculations utilized the RI approximation with appropriate decontracted auxiliary basis set and the chain-of-spheres (RIJCOSX) approximation to exact exchange. Increased integration grids (Grid4 and GridX5) and tight SCF convergence criteria were used. Energies of multiplets were calculated through quasi-degenerate perturbation theory in which an approximation to the Breit−Pauli form of the spin−orbit coupling operator (SOMF) was utilized.21 The components of g-factor were calculated using the effective Hamiltonian theory.

Scheme 1. Representation of the Ligand 2-((E)-((2(Ethylamino)ethyl)imino)methyl)-4-(2phenyldiazenyl)phenol (HL) and Its Various Aspects

complex 2 displays an incomplete spin transition in the temperature interval of 2−450 K. Both the complexes have pronounced thermal stability up to 485 and 496 K for complexes 1 and 2, respectively. To confirm the SCO behavior of complex 1, differential scanning calorimetry (DSC) study and electron paramagnetic resonance (EPR) measurements were carried out along with the magnetic susceptibility measurements. Complex 1 exhibits spin transition above room temperature with T1/2 = 410 K which is one of the highest reported spin-crossover transition temperatures for any mononuclear Fe(III) complex.14 Experimental findings are also supported by ab initio theoretical calculations which show that the energy difference of the ground state and first excited state is affected by the distortion around the metal center which is caused by the different crystallized form of the ligands in complexes. UV−vis spectroscopic studies showed that the azobenzene moiety of the ligand can photoisomerize in liquid state upon irradiation of UV light, which indicates potential application of these molecules as photomagnetic switches in liquid state.

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MATERIALS AND METHODS

Caution! The complexes between metal ions and organic ligands with perchlorate counteranion are potentially explosive. The compounds should be prepared in small amounts and handled with care! Materials and General Procedure. Salicylaldehyde, aniline, sodium nitrite, sodium hydroxide, hydrochloric acid (35%), Nethylethylenediamine, and metal salts, Fe(NO3)3·9H2O, and Fe(ClO4)3·6H2O were purchased from Merck. 2-Hydroxy-5-phenylazobenzaldehyde was prepared by a previously reported procedure.15 Ligand HL was synthesized by Schiff-base condensation reaction between 2-hydroxy-5-phenylazobenzaldehyde and N-ethylethylenediamine in situ while performing metalation. FT-IR spectra (4000−400 cm−1) were recorded on KBr pellets with a PerkinElmer Spectrum BX spectrometer. The elemental analyses were carried out on Elementar Microvario Cube Elemental Analyzer. Thermogravimetric analysis was recorded on a PerkinElmer TGA 4000 instrument. The variabletemperature powder X-ray diffraction (PXRD) data were collected on a PANalytical EMPYREAN instrument using Cu−Kα radiation. DSC experiments were performed on a PerkinElmer DSC 6000 instrument under nitrogen atmosphere with a scan rate of 2 K min−1 in both cooling and heating modes. Solid-state EPR spectra were recorded using Bruker EMXplus instrument. Synthesis of [Fe(L)2]NO3 (1) [HL = 2-((E)-((2-(Ethylamino)ethyl)imino)methyl)-4-(2-phenyldiazenyl)phenol]. 2-Hydroxy5-phenylazobenzaldehyde (296 mg, 1 mmol) was added to a solution of N-ethylethylenediamine (88 mg, 1 mmol, 0.1 mL) in 10 mL acetonitrile and left stirring for 30 min. Fe(NO3)3·9H2O (120 mg, 0.5 mmol) taken in 5 mL acetonitrile was then added dropwise to the previous mixture and stirred for another 30 min. The resulting mixture was filtered and kept for slow evaporation. Black colored

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RESULTS AND DISCUSSION Structural and Topological Description. Complexes 1 and 2 are analogous (Figure 1) except for the different spatial arrangements of the cationic part, counteranions, and supramolecular packings. Complex 1 crystallizes in monoclinic space group Pc, and asymmetric unit is composed of complex cation [Fe(L)2]+ and a nitrate anion where Fe(III) is in N4O2 B


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Inorganic Chemistry

Figure 1. Overlay diagram of complex 1 (blue) and complex 2 (yellow).

coordination environment due to the binding of two tridentate HL ligands in meridonial fashion (Figure 1). The metal center is octahedrally coordinated to two nitrogen (imine), two nitrogen (amine), and two oxygen atoms (phenolate) from the ligand. The Fe−O bond lengths (1.874(5) Å and 1.881(4) Å) are shorter than the Fe−N (imine) (1.926(5) Å and 1.927(5) Å) distances, which are substantially shorter than the Fe−N (amine) ones (2.037(6) Å and 2.027(5) Å). These values suggest a LS structure when compared to literature.11c,h The set of bond lengths and distortion parameters, ∑ and ξ are calculated for the two complexes and represented in Table 1 and related bond angles around the Fe(III) center are given in Table S3 in the Supporting Information.

Figure 2. Supramolecular tetrameric unit of complex 1 due to hydrogen bonding, C−H···O interactions, and anion···π interaction.

van der Waals (vdW) radius (3.22 Å) of a oxygen and carbon atom which confirms the presence of a anion···π interaction.22 These above-mentioned interactions form a tetrameric supramolecular unit (Figure 2) where one anion is surrounded by four counter cations. The cations are also interconnected to each other by various weak interactions such as C−H···π and π···π interactions. This tetrameric supramolecular unit propagates itself to form a supramolecular network along crystallographic bc plane where the anion and cations are strongly bound to each other by C−H···O and NH···O bondings (Figure 2). Each cation is connected with two other cations by the means of C−H···π interaction where the ethylene hydrogen (C4−H4A, C21−H21A) atom of one cation interacts with the phenyl ring of the other cations (Figure S2 in the Supporting Information). Strong π···π interaction is also present along crystallographic a axis (Figure S3 in the Supporting Information). The 3D packing diagram for complex 1 is shown in Figure 3 where it can be seen that each cation is surrounded by four anions and vice versa. Complex 2 crystallizes in orthorhombic Aba2 space group, and the asymmetric unit of is composed of half of the molecule which is crystallographically independent and related to the other half by inversion symmetry. Similar hydrogen bonding is present with the −NH of the cation with the perchlorate anion (N1−H1···O2, dD‑A = 3.062 Å). Imine hydrogens form a C− H···O interaction with the anion (C5−H5···O3); these interactions form a trimeric supramolecular unit (Figure 4) which is extended along crystallographic c axis forming a linear arrangement. These basic trimeric unit propagates itself to form 3D supramolecular arrangement where the cations and anions are having a linear arrangement (Figure 5). Each column of chain is connected with each other by C−H···π interactions. There are two types of C−H···π interactions, one with aromatic hydrogen (C13−H13) which extends in a crisscross manner and connects the chains. This C−H···π interaction is also accompanied by C−H···N interaction (C16−H16···N4) (Figure S4a in the Supporting Information). Another set of C−H···π interactions is with the ethylene hydrogens as shown

Table 1. Bond Lengths and Distortion Parameters for Complexes 1 and 2a complex 1

bond length

complex 2

bond length

Fe1−O1 Fe1−O2 Fe1−N1 (amine) Fe1−N3 (amine) Fe1−N2 (imine) Fe1−N4 (imine) Fe−L (Å)b ξ (Å)c ∑ (°)d

1.874(6) 1.881(5) 2.037(6) 2.028(6) 1.927(5) 1.927(5) 1.945 0.346 44.76

Fe1−O1 Fe1−O11 Fe1−N1 (amine) Fe1−N11 (amine) Fe1−N2 (imine) Fe1−N21 (imine) Fe-L (Å)b ξ (Å) ∑ (°)

1.866(3) 1.866(3) 2.037(3) 2.037(3) 1.927(2) 1.927(2) 1.943 0.374 44.54

Structures determined at 120 K. bFe−L = (1/6)∑dFe−L. cξ = i=1 i=1 ∑6 dFe−L − dFe−Li|. d∑ = ∑12 90 − φi|11 − x, 1 − y, +z. a

There are presence of strong hydrogen bonding between −NH amino groups of the cation and the oxygen atoms of nitrate anion (N1−H1···O4, dD‑A = 2.97 Å and N3−H3···O5, dD‑A = 2.99 Å) (Figure 2). There are also presence of C−H···O interactions which incorporate the imino group (C5−H5 and C22−H22), ethylene hydrogens of the N-ethylethylenediamine moiety (C4−H4A, C4−H4B, C21−H21A, and C21− H21B) with the anion (hydrogen-bonding parameters are given in Tables S4−S5 in the Supporting Information). In addition, anion (NO3−)···π interaction is present between the oxygen atom of nitrate (O3) and C8arom of the phenyl ring of the cation (dO3-C8arom = 3.196 Å) (Figure S1 in the Supporting Information). The distance of 3.196 Å is less than the sum of C


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Inorganic Chemistry

Figure 5. Supramolecular packing for complex 2.

Figure 3. Supramolecular packing of complex 1.

angle between the two phenyl rings (ring I: formed by C6, C7, C8, C9, C10 and C11; ring II: formed by C12, C13, C14, C15, C16 and C17) is 5.4(3)° for complex 1, and for complex 2 the angle between two phenyl rings is 29.73(12)° (Figure 6c). Notable difference due to the linear and skew form of the ligand is the different weak intermolecular interactions between complexes 1 and 2; complex 2 has more C−H···π interaction due to the skew form of the azo phenyl group. The overlay diagram of complexes 1 and 2 (Figure 1) shows the different spatial arrangement of the bound ligand. Both the complexes 1 and 2 have been analyzed and classified from the topological viewpoint performed with the ToposPro program package and the TTD collection of periodic network topologies.23 For the purpose of topological analysis, the complex cation [Fe(L)2]+ and the anion (nitrate/perchlorate) have been considered together as one complete molecular unit, and vdW interactions among the molecular units have been taken into account. Underlying nets were obtained after performing a standard simplification procedure of the Coulomb or vdW-bonded molecular structures. The topology is determined for a net of molecular centers of mass, which are bonded on the basis of vdW interactions. In order to obtain a simpler underlying net, we used the procedure “Generate Representations” of the adjacency matrix of this net and considered the contacts that vary in solid angles.24 This procedure subsequently removes from the adjacency matrix the groups of contacts with the smaller solid angles. As a result, the following set of underlying nets were obtained (the topology type and solid angles are indicated in brackets). Below the point symbols are shown for all nets with new topological types. Point symbol lists the numbers (amount) and sizes of shortest circuits (closed chains of connected nodes) starting from any non-equivalent node in the net: For complex 1 (11 representations) 22-c (38141295206, 1.16%) → 20-c(3694985226, 1.73%) →18-c(3574805156, 2.18%) →16-c (3364675166, 2.20%) →14-c (3184555176, 2,54%) →12-c (3944251263, 2.56%) →10-c (364305762, 3.20%) → 8-c (bcu-x, 3.50%) → 6-c (hxl, 5.89%) → 4-c (sql, 11.58%) → 2-c (2C1, 13.47%). For complex 2 (6 representations) 20-c (3724104514, 0.33%) →18-c (360480513, 1.98%) →16-c(336472512, 3.31%) →12-c (312436518, 3.46%) → 8-c (bcu, 4.93%) → 4-c (sql, 12.15%).

Figure 4. Supramolecular trimerc unit of complex 2.

in Figure S4b in the Supporting Information. This aliphatic C− H···π interaction connects one cation with four neighboring cations. Structural comparison of complexes 1 and 2 reveals that the cationic part of the two complexes differs notably in their spatial arrangement due to the flexible nature of azo (−N N−) bond. The ligand attains a linear form in complex 1, while a skew form is foreseen in complex 2. The linear form of the ligand in complex 1 is stabilized by the π···π stacking interactions (Figure S3 in the Supporting Information), while C−H···π interactions (Figure S4a in the Supporting Information) stabilize the skew form of the ligand in 2. The distorted nature of the ligand is also realized from the torsion angle of the azo phenyl ring which is 179.07° (C10− N5−N6−C12) (Figure 6a) and 172.01° (C10−N3−N4− C12) (Figure 6b) for complexes 1 and 2, respectively. The azo phenyl group displays an E conformation, and the dihedral D


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Inorganic Chemistry

Figure 6. Cationic part of complex 1 (a), complex 2 (b), and linear (c, top) and skew (c, down) of complexes 1 and 2 respectively.

Figure 7. Formation of different topological motifs in complexes 1 (a) and 2 (b). Pink and green spheres represent the complex cation [Fe(L)2]+ with nitrate anion (in complex 1) and the complex cation [Fe(L)2]+ with perchlorate anion (in complex 2), respectively.

In complex 1, chains with 2C1 topology are first formed (Figure 7a). These chains are stitched together and form a plane net with sql topology. The distances between the nodes in the chain are smaller (7.977 Å, gray solid lines) than between the chains (8.298 Å, blue dashed lines). In complex 2, each unit is surrounded by equidistant neighboring units, and for this reason, in complex 2 chains are not formed, and a plane

net with sql topological motif immediately formed (Figure 7b). In complex 1, the formation of another type of plane net is observed. This plane net is obtained from net with sql topology by forming additional bonds in the layer. These layers have been classified with hxl topology (Figure 7a). 3D frameworks in both complexes are formed by linking layers with hxl and sql topology for complexes 1 and 2, respectively. E


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Inorganic Chemistry

contributions of respective contacts to the HS surface. As summarized in Figure 8c, the largest difference is observed for purely disperive contacts (H···H), where contribution is equal to 57.6% and 51.7% for 1 and 2, respectively. Other hydrogeninvolving contacts (H···C, H···O, H···N) follow the reverse trend, that is, have a systematically bigger contribution in structure 2 (Figure 8c). Since a conformation of ligand may be a crucial factor here, we have compared contact-decomposed dnorm HSs of 1 and 2, as shown in Figure S5 in the Supporting Information. The inspection of H···C, H···O, H···N mapped surfaces suggests that skewed forms of the ligand in 2 may be governed by weak hydrogen bonding. Indeed, it can be seen that for compound 2, part of the H···H areas are directly replaced by H···C contacts and to a lesser extent by H···O, H··· N ones; thus, it can be understood that tilting of ligand’s arms is a result of ligand adjustment to, for example, C−H···π stacking. Quite an interesting feature of 2D fingerprint plots is that they are featured by so-called “upper spikes” (Figure 8d,e). Contact-decomposed 2D FPs (Figures S6 and S7 in the Supporting Information) allowed us to unequivocally ascribe those spikes to H···O contacts as well as regions for the other contacts (see Supporting Information for detailed figures). Note that there is no “lower spike” in either case, because the complex molecule in both, 1 and 2, acts only as a donor of strong hydrogen bonding. According to earlier reports, this is a distinct feature of crystal structures encompassing more than one molecule, especially if a guest molecule contributes to strong hydrogen bonding.27 Magnetic Susceptibility Measurements. Phase purity of the complexes has been checked by powder XRD pattern which shows that the bulk phase is representative of the single crystal structure. The variable-temperature magnetic measurement for complexes 1 and 2 was carried out in temperature ranges 2−380 K and 350−450 K (high-temperature squid option) in both heating and cooling modes. The SCO for a mononuclear iron(III) compound proceeds between SL = 1/2 (low-spin, L) and SH = 5/2 (high-spin, H) states. The spinonly contribution to the effective magnetic moment is μeff(L)/ μB = gL[SL(SL + 1)]1/2 = 1.73 using gL ∼ 2.0. The value of μeff(H) amounts to 5.92 μB. A representative example is shown in Figure 9 for the complex 1. The low-temperature value of the effective magnetic moment amounts to μeff ∼ 1.93 μB at T = 5 K, which implies the value of the magnetogyric factor gL is ∼2.2 owing to some contribution of the orbital magnetic moment via the spin−orbit interaction. On heating, the effective magnetic moment stays almost constant until 200 K (1.97 μB), then it starts to increase. A gradual increase culminates at the highest temperature of the data taking when μeff ∼ 4.85 μB at T = 450 K. The last points do not show a signature of the saturation so that the SCO proceeds even above this temperature. The measurement in heating and cooling mode does not reveal any difference, and it shows a SCO without any evidence of thermal hysteresis loop. The LS to HS conversion is recovered by considering the balance of the involved species:

Thus, the 8-coordinated underlying nets of bcu-x and bcu topological type for complexes 1 and 2, respectively, were obtained. Hirshfield Surface Analysis. The analysis of Hirshfeld surfaces (HS)24 and 2D fingerprint plots nowadays has become an indispensable tool that provides a useful way to compare crystal structures.25 Probably the biggest advantage of this technique is that it allows to express the differences between crystal structures in question not only qualitatively but also quantitatively. Given its sensitivity, it has been widely used for characterization of various geometrical features of supramolecular adducts characterized by conformational flexibility.26 For this reason, we have applied HS analysis to complexes 1 and 2. The HS of title complexes have been mapped with dnorm function (Figure 8) to reveal the main differences between

Figure 8. (a and b) HS mapped with dnorm function for 1 and 2, respectively. (c) Graph presenting percentage contributions to the HS of respective contacts for both complexes. (d and e) 2D Fingerprint plots for 1 and 2, respectively. “Upper spikes” are denoted with black ovals, while the other characteristic contacts are denoted with an arrrow.

them. The red areas, called also as “spots”, indicate regions in which intermolecular contacts are shorter than sum of vdW radii of participating atoms; those areas are indicative of strong hydrogen bonds. Weak hydrogen bonds and short dispersive contacts are detectable as faint-red spots. The first pronounced difference between complexes in HS is found in strong hydrogen bonding. As seen in Figure 8a, the close environment of nitrate anion is characterized by the presence of a pseudo centrosymmetrically alinged set of four red spots. In complex 2, in turn, there are presence of two red spots in the region of hydrogen-bond interaction with perchlorate anion (Figure 8b). This difference clearly emphasizes the bifurcated nature of HB interaction in the former complex (that is, one N−H hydrogen interacts with two nitrate oxygen atoms), while in the latter complex, N−H···O HBs are more directional. Calculation of 2D fingerprint plots of 1 and 2 provided percentage

χmol = (1 − x H)χL + x HχH

(1)

with the mole fractions χH + χL = 1. The individual susceptibility contributions are thought to obey the Curie− Weiss law: F


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Inorganic Chemistry

Figure 9. Temperature dependence of the effective magnetic moment for complexes 1 (a) and 2 (c). Solid line fitted. Temperature evolution of HS mole fraction and the Arrhenius-like plot (b).

χL =

2 NAμ0 μB2 gL SL(SL + 1) + αL kB 3 T − ΘL

ij x H yz z ln K = lnjjj j 1 − x zzz H{ k

(2)

Individual parameters of the above equations are “active” in the different parts of the χ (or χT or μeff) vs T dependence as follows: (i) The value of gL influences the temperature interval 10−200 K; ΘL refers to a small drop of μeff below 10 K; αL balances a small tangential between 10−200 K. (ii) The hightemperature part around 450 K is dominated by gH ∼ 2. (iii) The SCO region is mapped by Δeff which is proportional to the constant enthalpy change ΔH = RΔeff which principally influences the transition temperature since Tc ∼ ΔH; the abruptness (deviation from the pure Boltzmann statistics) is recovered by the solid-state cooperativeness Γ; finally, the averaged vibrational frequencies νH̅ and νL̅ determine the temperature-dependent entropic term since ΔS = R ln rT just at just at xL = xH = 0.5. The last term manifests itself in the asymmetric development of the Arrhenius plot lnK vs T−1. The results of the data fitting are involved in Figure 9;29 the discrepancy factor of the fit is R = 0.011. The obtained set of free parameters reads: gL = 2.312(8), ΘL = −0.50(12) K, αL = −2.4(3) × 10−9 m3 mol−1 (SI units), gH = 1.950(7), Δeff = 1050(65) K, Γ = 294(2), νL = 233 cm−1, and νL/νH = 1.14(1); standard deviations are in parentheses. To this end, the thermodynamic parameters are ΔH = 8.73 kJ mol−1 and ΔS = 21.27 J K−1 mol−1 at Tc = 410 K (when xL = xH = 0.5). These data span the higher edge of those typical for the Fe(III) containing SCO systems.30 The value of the enthalpy change is not too far from the experimental DSC determination (Figure S8 in the Supporting Information); however, the predicted transition temperature is by far higher than experimentally found Tc = 360 K. We only can say that this discrepancy is due to the approximate character of the applied Ising-like model of the SCO. The magnetic data for the complex 2 are essentially analogous to 1 (Figure 9c). However, the on-set of the SCO starts above 300 K, and the data fitting is rather problematic for such an incomplete transformation. The close packing of complex 1 is responsible for the more cooperative spin transition behavior, whereas the metal centers are loosely bound in complex 2. The distance among the metal centers are 8.05−8.22 Å for complex 1, while it is 9.399 Å for complex 2. Also, the C−H···π and π···π interaction present in complex 1 leads to better communication among the metal centers.

where the universal physical constants adopt their usual meaning. The free parameters for the LS species are gL, Weiss constant ΘL, and the temperature-independent term αL (this combines uncompensated underlying diamagnetism and the temperature-independent paramagnetism). For the HS molecules, ΘH and αH can be ignored and gH is close to 2.0. The SCO process can be viewed as a unimolecular reaction and can be described by several theoretical models. We utilized the Ising-like model (equivalent to the regular solution model)28 that is based upon a microscopic Hamiltonian: Ĥ = (Δ0 /2)σ ̂ − Γ⟨σ ⟩σ ̂

(3)

where σ̂ is a fictitious spin with eigenvalues ±1, xH = (1 + ⟨σ⟩)/2 is the mole fraction of the HS state, Δ0 is site formation energy [K] for the enthalpic term, and Γ is solid-state cooperativeness [K]. The HS mole fraction is obtained by an iterative solution of the master equation: ⟨σ ⟩T =

F−1 F+1

with the factor ÄÅ É ÅÅ −(Δeff − kBT ln rT − 2Γ⟨σ ⟩t ) ÑÑÑ Å ÑÑ Å F = expÅÅ ÑÑ ÅÅÇ ÑÑÖ kBT

(4)

(5)

This contains the degeneracy ratio that accounts to the molecular vibrations in the form: ÄÅ ÉÑm 2SH + 1 ÅÅÅ 1 − exp(hνL̅ /kT ) ÑÑÑ Å ÑÑ rT = Å 2SL + 1 ÅÅÅÇ 1 − exp(hνH̅ /kT ) ÑÑÑÖ (6) where m is number of active modes (m = 15 for a hexacoordinate complex), and hν̅H and hν̅L are averaged (Einstein-type) vibrational energies for the respective spin species. Enthalpy of the spin transition is then ΔH = RΔeff with Δeff = Δ0 + m(hνHS ̅ − hνLS ̅ )/2

(8)

(7)

and the entropy of the spin transition reads ΔS = R ln rT just at xL = xH = 0.5, and Tc = ΔH/ΔS is the critical temperature when xL = xH = 0.5. The equilibrium constant is G


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Inorganic Chemistry Photoswitching Behavior. Presence of the azo-benzene moiety in the ligand system enables the compounds to behave as photo switches as this moiety can switch from trans to cis form in the presence of UV light irradiation.31 Trans conformation of the azo-benzene moiety is thermodynamically more stable32 than the cis conformation. So, the trans conformation is adopted by the azo-benzene moiety in the free ligand and complexes. To switch them into cis conformation, irradiation experiments were carried out in DMF solvent. Ligand in the free state has two bands in 300− 600 nm region between which the band at 370 nm is assigned to the π → π* and the band at 450 nm is due to n → π* transition (Figure S9 in the Supporting Information).33 When DMF solutions of the ligand and complexes were irradiated at 365 nm wavelength of light the π → π* band decreased in intensity, and the n → π* band increased in intensity which indicate the photo conversion of the trans isomer to cis isomer.34 Complexes 1 and 2 show one strong band at 380 nm which is attributed to π → π* transition as they have wavelength and intensities similar to the ligand.35 While lowering the temperature intensity of the transition at 380 nm increases, the transition can be assigned to MLCT transition of the LS state, and the absorption enhancement is due to the increase of the LS state by thermal crossover (Figure S10 in the Supporting Information).36 Aforementioned, upon irradiation there is a decrease in π → π* transition, and an increase in n → π* band was observed. Concomitantly MLCT absorption decreased suggesting that LS to HS state change is induced by photoisomerization. EPR Measurements. EPR spectroscopy can be used to distinguish the paramagnetic low spin (LS) and high spin (HS) states of iron(III) complexes.37 LS spectra have small x,y anisotropy (or rhombicity) with three g values in the range 2.5−1.7, while HS spectra are characterized by signals at g ∼ 6 and g ∼ 2 in the case of axial symmetry and at g ∼ 4.3 if the symmetry of the metal complex is rhombic.38 Therefore, EPR technique allows to follow the variation of the spin states of complexes 1 and 2 as a function of temperature. For this reason, X-band EPR measurements were performed on powdered samples of 1 and 2 from 150 to 380 K (Figure 10). For complex 1, the EPR spectrum at 200 K is characteristic of the LS state with one transition at geff 2.10, indicating an almost axial symmetry. At 380 K, the spectral pattern changes significantly and signals at geff 5.94 and 2.10 indicate the SCO and the transition from LS to HS (Figure 10a).1a,39 The values of g are comparable with those of other HS Fe(III) species with small rhombicity.40 Hagen has showed that, in most of cases, at the X-band, the zero-field interaction dominates the Zeeman interaction and that the EPR behavior of half-integer spin systems is readily described with rhombograms where the g factors are reported as a function of rhombicity, E/D.41 For complex 1, with geff close to 6 and 2, a very small value of the ratio E/D is expected. For complex 2, an anisotropic EPR spectrum with absorptions at g 2.23, 2.17, and 2.00 is detected (Figure 10b). These values are comparable with those of other LS Fe(III) complexes.42 The spectra do not change significantly with temperature. Overall, the pattern can be interpreted with a S = 1/2 spin state and is consistent with the slight rhombic symmetry of the X-ray structure.43 We have also calculated the g factors with ORCA 4.0.0 software package by using the effective Hamiltonian approach: The values of gxyz = (2.399, 2.384, 1.855), giso = 2.213 for 1 and gxyz = (2.349,

Figure 10. Solid-state variable-temperature EPR spectra for complexes 1 (a) and 2 (b) at indicated temperatures.

2.306, 1.909), giso = 2.188 for 2 were predicted. These results indicate a rhombic g tensor with a slight anisotropy of the x and y axes, in agreement with EPR measurements. Thermogravimetric Analysis. The remarkable high thermal stabilities of mononuclear complexes 1 and 2 are revealed by TGA analysis (Figure 11). The samples were

Figure 11. TGA plots for complexes 1 (black) and 2 (red).

heated under N2 atmosphere and found to be thermally stable up to 485 and 496 K for 1 and 2, respectively. Compared to analogous mononuclear iron complexes, these complexes are exceptionally stable and decompose only at higher temperatures. It can be said that presence of strong intermolecular interactions in the solid state are responsible for improving their resistance to pyrolysis.7d The thermal stability is also supported by VT-PXRD patterns which show good crystalline nature up to 450 K (Figure S11 in the Supporting Information). The experimental PXRD patterns of 1 and 2 at 300 K are in good agreement with the simulated data which indicates their phase purity. Theoretical Studies. To support the magnetic results, we have performed ab initio calculations on experimental geometries of complexes 1 and 2. Our intention was above all to reveal the electronic structure of 1 and 2 and confirm the spin H


Article

Inorganic Chemistry

magnetic switch at extreme conditions. Considering the vast number of SCO complexes with salicylaldehyde as a basic building unit, this type of azo benzene moiety-based salicylaldehyde can be a new effective strategy to synthesize magnetic bistable molecules with photoswitching property.

state of their ground electronic term. The main results are listed in Supporting Information. At first glance it is obvious that the similarity in the molecular structure of the complexes under study is also reflected in the electronic structure. The calculations predict LS ground state (spin doublet) for both complexes that is a one component of the 2T2g term in idealized Oh symmetry. Remaining two components of this term lie at 3207, 3297 cm−1 for 1 and 3506, 3922 cm−1 for 2. First excited state is the spin sextet (HS state, 6A1g in Oh) lying at 1054 cm−1 for 1 and 2334 cm−1 for 2. For octahedral systems having regular ligand fields, the ground term is 6A1g. Far above this term lies the 2T2g term, which gradually decreases in the stronger fields. Subsequently, this term can be split in lower symmetry. We assume that this is also the case for 1 and 2, as we have what to do with strong fields (short metal−ligand bonds) and low symmetry (most likely C2v). In this context, it is useful to compare the structural distortion of the complexes. The overall distortion of the polyhedron can be quantified by the general procedure of Continuous Shape Measure (SHAPE program).44 This approach measures the similarity between the real and ideal polyhedra according to the agreement factor S (for ideal symmetry S = 0). For complexes under study, we got S(6-OC) = 0.320 (1) and 0.409 (2), from which it follows that 2 is more distorted. Thus, it is clear that in case 2 the term splitting is larger, causing a more progressive decrease of one component of the 2T2g below 6A1g term (larger HS-LS gap). Therefore, the smaller HS-LS gap of 1 results in an increase in the thermal population of the sextet state, which becomes more pronounced at temperatures above 300 K (Figure 12). An energy diagram of the d orbitals with their compositions has been shown in Figure S12 (Supporting Information).

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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.8b02405. Drawings of coordination polyhedral, magnetic plots, PXRD, and bond angle bond distance tables and hydrogen bonding tables (PDF) Accession Codes

CCDC 1845589−1845590 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +91-7556692392. Tel: +917556691313. *E-mail: [email protected]. ORCID

Bijoy Dey: 0000-0003-4185-4240 Subhadip Roy: 0000-0002-0097-6113 Roman Boča: 0000-0003-0222-9434 Siba Prasad Bera: 0000-0003-3633-8612 Arpan Mondal: 0000-0001-9192-7255 Sanjit Konar: 0000-0002-1584-6258 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K. thanks SERB, Government of India (project no.EMR/ SERB/2014000053) and IISER Bhopal for generous financial and support. B.D and A.M thank IISER Bhopal for a Ph.D. fellowship. S.R. acknowledges SERB, Government of India (file number PDF/2017/001188) for National Postdoctoral Fellowship. S.P.B. thanks UGC, India for fellowship. Grant agencies (Slovakia: APVV-14-0078, APVV-14-0073, APVV-160039, VEGA 1/0919/17, and VEGA 1/0534/16) are acknowledged for financial support. We are grateful to Professor MingLiang Tong and Dr. Yan-Cong Chen (Sun Yat-Sen University, P. R. China) for high-temperature (350-450 K) magnetic measurements. We are also thankful to Professor Eugenio Garribba (University of Sassari, Italy) and Jan K. Zareb̨ a Wroclaw (University of Science and Technology, Poland) for kindly extending their help in the analysis of EPR spectra and Hirshfeld surface, respectively. We thank three anonymous referees whose comments improved the paper.

Figure 12. Temperature variation of population of the HS (first excited 6A1g) state according to CASSCF/NEVPT2 results.

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CONCLUSIONS The magnetic susceptibility studies of complexes 1 and 2 confirm that the spin crossover occurs above 350 K. The experimental data of 1 were analyzed using a theoretical model based upon the Ising-like Hamiltonian giving the set of thermodynamic parameters: ΔH = 8.73 kJ mol−1, ΔS = 21.27 J K−1 mol−1, and Tc = 410 K. Ab initio calculations revealed the structure of energy levels according to which complexes 1 and 2 stabilize the LS (doublet) ground state. First, the excited state is the HS (sextet) state which is thermally accessible over 300 K. Our findings reveal that introduction of π-based ligands can introduce flexibility in the system and also make them highly thermal resistant, paving way for their potential use as a

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REFERENCES

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Inorganic Chemistry

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