Spin Crossover in Three Mononuclear Iron (III) Schiff Base Complexes

metals Article Spin Crossover in Three Mononuclear Iron (III) Schiff Base Complexes Ivan Nemec 1,2,* , Ingrid Svoboda 3and Radovan Herchel 1 1Department of Inorganic Chemistry, Faculty of Science, PalackýUniversity, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic 2Central European Institute of Technology, CEITEC BUT, Technická3058/10, 61600 Brno, Czech Republic 3Material sciences, Darmstadt University of Technology, D-64287 Darmstadt, Germany *Correspondence: [email protected]; Tel.: +420-585-634-354 Received: 10 July 2019; Accepted: 31 July 2019; Published: 2 August 2019   Abstract: The synthesis, crystal structure, and magnetic properties of three new mononuclear complexes [Fe(R-LA)(L1)](BPh4), where R-LA 2− is a doubly deprotonated pentadentate Schiffbase ligand and L1 is a monodentate benzimidazole or furopyridine ligand, are reported. Ligandand anion-driven changes in crystal structures and magnetic behavior were investigated in terms of the magnetic susceptibility measurements and theoretical calculations. Keywords: spin crossover; Schiffbase ligands; iron (III) complex; structure; magnetism 1. Introduction Spin crossover (SCO) of (pseudo)octahedral complexes of iron(III) manifests as spin transition between S=1/2 (low-spin, LS) and S=5/2 (high-spin, HS) states [ 1 ]. Typically, the iron(III) complexes exhibitSCOforseveraltypesofcoordinationenvironments suchas{FeO 6 }, {FeO 3 S 3 }, {FeS 6 }, {FeN 2 O 2 S 2 }, {FeN 3 O 3 }, or {FeN 6 }, but the most explored are the {FeN 4 O 2 } complexes with tri-, tetra-, penta-, or hexadentate Schiffbase ligands [ 2 ]. For the last decade our attention focused predominantly on the Fe(III) complexes with pentadentate Schiffbases originating from reactions between derivatives of various ortho-hydroxy salycilaldehydes and aliphatic triamines [ 3 – 7 ]. In these compounds, two basic types of SCO complexes can be recognized due to different kinds of amine used in the synthesis of Schiff base ligands: so-called symmetric ones in which derivatives of bis(3-aminopropyl)amine are used (H 2 R-LA, Rsubstituted compounds of 4-azaheptamethylene-1,7-bis(salicylideneiminate), H 2 LA) and asymmetric ones with N-(2-aminoethyl)-1,3-propanediamine (H 2 R-LB, Rsubstituted compounds of 4-azahexamethylene-1,7-bis(salicylideneiminate), H 2 LB Scheme 1). Initially, SCO was observed only for compounds with the general formula [Fe(R-LA)(L1)](BPh 4 ) [ 8 ] or [{Fe(R-LA)} 2 ( µ -L2)](BPh 4 ) [ 9 ], and the observedtransitionswereofgradualorevenspin equilibrium character. Oneexceptionwas found in the case of a mononuclear compound with R-LA =bis(3-methoxysalicylideneiminopropyl)methylamine and L1 =4-aminopyridine, which exhibited rather cooperative SCO, possibly due to its crystal packing involving N–H ··· O and N–H ···π non-covalent interactions between the complex cations [ 10 ]. Other interesting results were obtained for compounds [Fe(LA)(L1A)][M(dmit) 2 ] · CH 3 CN, M = Ni, Pd, Pt, in which L1A (1-(pyridin-4-yl)-2-(N-methylpyrrol-2-yl) ethane) is a photoisomerable ligand and [M(dmit) 2 ] − anions can act as molecular conductors [ 11 ]. Compounds of the general formula [Fe(LA)(L1B)](BPh 4 ) also contain photoisomerable ligands L1B =3-phenylazopyridine or 4-phenylazopyridine [ 12 ]. Nevertheless, these complexes are not very interesting from a magnetic point of view because they exhibit only gradual spin transitions. This changed with the introduction of the shorter aliphatic chain in Schiffbase ligands by substitution of the derivatives of bis(3-aminopropyl)amine by N-(2-aminoethyl)-1,3-propanediamine. Increased rigidity of the resulting Metals 2019,9, 849; doi:10.3390/met9080849 www.mdpi.com/journal/metals Metals 2019,9, 849 2 of 11 ligands (supported also by the introduction of naphthyl instead of benzene rings) led to preparation of the [Fe(LB)(LP)] complexes (where LP stands for (1) pseduhalido ligand), which exhibited cooperative spin transitions, in some cases even accompanied by thermal hysteresis [3]. Metals 2019, 9, x FOR PEER REVIEW 2 of 11 naphthyl instead of benzene rings) led to preparation of the [Fe(LB)(LP)] complexes (where LP stands for (1) pseduhalido ligand), which exhibited cooperative spin transitions, in some cases even accompanied by thermal hysteresis [Error! Bookmark not defined.]. Inspired by the abovementioned results we decided to attempt preparation of the [Fe(LA)(L1)](BPh4) complexes exhibiting cooperative SCO by using monodentate ligands, which could increase the rigidity and the number of significant non-covalent interactions in the resulting complexes. Therefore, we decided to use two different bulky monodentate ligands, benzimidazole (L1a) and 1-benzofuro[3,2-c]pyridine (L1b), together with two slightly different pentadentate ligands, H2LA and H23EtO-LA (4-azaheptamethylene-1,7-bis(3-thoxy-salicylideneiminate, Scheme 1). We were successful in the preparation of three compounds: [Fe(3EtO-LA)(L1a)](BPh4) (1a), [Fe(3EtOLA)(L1b)](BPh4)·CH3OH (2a), and [Fe(LA)(L1b)](BPh4) (2b). Here, we report their crystal structure and magnetic properties. O N N O N Fe H R1 N O R1 N N H N O N Fe H R1 N O R1 N OH N H R1 N OH R1 (a) (b) (c) Scheme 1. Structural formulas of (a) ligands H2LA (R1 = H) and H23EtO-LA (R1 = O–CH2–CH3) and (b) complex cations [Fe(3EtO-LA)(L1a)]+ in 1a (R1 = O–CH2–CH3) and (c) [Fe(3EtO-LA)(L1b)]+ in 2a (R1 = O–CH2–CH3) or [Fe(LA)(L1b)]+ in 2b (R1 = H). 2. Materials and Methods Chemicals were purchased from commercial sources (Sigma-Aldrich) and used as received. 1benzofuro[3,2-c]pyridine was prepared according to a previously reported procedure [13,14]. Elemental analysis was carried out on a Flash 2000 (ThermoFisher Scientific, Waltham, MA, USA). Magnetic susceptibility and magnetization measurements were done using a SQUID magnetometer (Quantum Design Inc., San Diego, CA, USA) from T = 2 K at B = 0.1 T. The magnetization data were taken at T = 2.0 and 4.6 K, respectively. Raw susceptibility was corrected, and diamagnetic corrections of the constituent atoms were estimated from Pascal constants. The effective magnetic moment was calculated as usual: μeff/μB = 798( χ ’T)1/2 when SI units are employed. Single-crystal X-ray diffraction data were collected on an Oxford diffractometer Xcalibur2 (Oxford Diffraction Ltd., Oxford, UK) with a Sapphire CCD detector and fine-focused sealed tube (Mo Kα radiation, λ = 0.71073 Å) source and equipped with an Oxford Cryosystem nitrogen gas-flow apparatus. All structures were solved and refined (full-matrix least-squares on Fo2−Fc2) by using SHELXS2014 software [15]. 2.1. Synthesis 2.1.1. Ligands Neutral, pentadentate ligands H2LA and H23EtO-LA were prepared by mixing the appropriate aldehyde and amine in a 2:1 molar ratio. Synthesis of all Schiff base ligands is analogous; therefore, only synthesis of H2LA is presented in detail [Error! Bookmark not defined.]. A methanol solution of salicylaldehyde (2.44 g, 20 mmol in 50 cm3) was combined with di(3-aminopropyl)amine (1.31 g, 10 mmol) and the mixture was refluxed for 60 min. The ligand was obtained as a yellow solution ready for subsequent use. Scheme 1. Structural formulas of ( a ) ligands H 2 LA (R 1 =H) and H 2 3EtO-LA (R 1 =O–CH 2 –CH 3 ) and ( b ) complex cations [Fe(3EtO-LA)(L1 a )] + in 1a (R 1 =O–CH 2 –CH 3 ) and ( c ) [Fe(3EtO-LA)(L1 b )] + in 2a (R1=O–CH2–CH3) or [Fe(LA)(L1b)]+in 2b (R1=H). Inspired by the abovementioned results we decided to attempt preparation of the [Fe(LA)(L1)](BPh 4 ) complexes exhibiting cooperative SCO by using monodentate ligands, which could increase the rigidity and the number of significant non-covalent interactions in the resulting complexes. Therefore, we decided to use two different bulky monodentate ligands, benzimidazole (L1 a ) and 1-benzofuro[3,2-c]pyridine (L1 b ), together with two slightly different pentadentate ligands, H 2 LA and H 2 3EtO-LA (4-azaheptamethylene-1,7-bis(3-thoxy-salicylideneiminate, Scheme 1). We were successful in the preparation of three compounds: [Fe(3EtO-LA)(L1 a )](BPh 4 ) ( 1a ), [Fe(3EtO-LA)(L1 b )](BPh 4 ) · CH 3 OH ( 2a ), and [Fe(LA)(L1 b )](BPh 4 ) ( 2b ). Here, we report their crystal structure and magnetic properties. 2. Materials and Methods Chemicals were purchased from commercial sources (Sigma-Aldrich) and used as received. 1-benzofuro[3,2-c]pyridine was prepared according to a previously reported procedure [ 13 , 14 ]. Elemental analysis was carried out on a Flash 2000 (ThermoFisher Scientific, Waltham, MA, USA). MagneticsusceptibilityandmagnetizationmeasurementsweredoneusingaSQUIDmagnetometer (Quantum Design Inc., San Diego, CA, USA) from T=2 K at B=0.1 T. The magnetization data were taken at T=2.0 and 4.6 K, respectively. Raw susceptibility was corrected, and diamagnetic corrections of the constituent atoms were estimated from Pascal constants. The effective magnetic moment was calculated as usual: µeff/µB=798(χ’T)1/2when SI units are employed. Single-crystal X-ray diffraction data were collected on an Oxford diffractometer Xcalibur2 (Oxford Diffraction Ltd., Oxford, UK) with a Sapphire CCD detector and fine-focused sealed tube (Mo K α radiation, λ =0.71073 Å) source and equipped with an Oxford Cryosystem nitrogen gas-flow apparatus. All structures were solved and refined (full-matrix least-squares on F o2− F c2 ) by using SHELXS2014 software [15]. 2.1. Synthesis 2.1.1. Ligands Neutral, pentadentate ligands H 2 LA and H 2 3EtO-LA were prepared by mixing the appropriate aldehyde and amine in a 2:1 molar ratio. Synthesis of all Schiffbase ligands is analogous; therefore, only synthesis of H 2 LA is presented in detail [ 9 ]. A methanol solution of salicylaldehyde (2.44 g, Metals 2019,9, 849 3 of 11 20 mmol in 50 cm 3 ) was combined with di(3-aminopropyl)amine (1.31 g, 10 mmol) and the mixture was refluxed for 60 min. The ligand was obtained as a yellow solution ready for subsequent use. 2.1.2. Mononuclear Precursors Synthesis of all precursors was very similar [ 9 ]; therefore, only the synthesis of [Fe(LA)Cl] is described. The methanol solution of ligand H 2 LA (10 mmol in 50 cm 3 ) was combined with a solution of iron(III) chloride hexahydrate (2.70 g, 10 mmol) in 40 cm 3 of methanol. The mixture was refluxed for 20 min, and then triethylamine (2.22 g, 22 mmol) was added to complete deprotonation of the Schiffbase ligand. The resulting violet solution was refluxed for 30 min and left to cool slowly to room temperature when a dark violet micro-crystalline powder precipitated. This was filtered offusing a fritted funnel, washed with methanol and diethylether, and dried. 2.1.3. Mononuclear Complexes 1a,2a, and 2b The complexes were prepared in the same manner by mixing 100 mg of the [Fe(LA)Cl] or [Fe(3EtO-LA)Cl] (0.193 mmol in the preparation of 1a and 2a , 0.233 mmol in the preparation of 2b ) precursor complexes with a heterocyclic derivate (23 mg of L1afor the preparation of 1a, 33/38 mg of L1 b for the preparation of 2a / 2b ) in a molar ratio of 1:1. After 30 min of reflux, the solution was filtered through paper filter into an equimolar amount of solution of NaBPh4. As an example, the synthesis of [Fe(3EtO-LA)(L1 a )](BPh 4 ) is described in detail. To a solution of [Fe(3EtO-LA)Cl] (100 mg in 30 cm 3 of methanol), L1 a was added. The resulting violet solution was refluxed for 30 min and filtered into a solution of NaBPh 4 (66 mg in 5 cm 3 ). Black crystals precipitated overnight. They were collected, washed with methanol and diethyl ether, and dried. 2.1.4. Elemental Analysis 1a : calcd (%) for C 55 B 1 Fe 1 H 57 N 5 O 4 , M w =918.7 g · mol −1 , C, 71.9; H, 6.3; N, 7.6. Found: C, 71.5; H, 6.1; N, 7.3. Yield =67%. 2a : calcd (%) for C 60 B 1 Fe 1 H 62 N 4 O 6 , M w =1001.8 g.mol −1 , C, 71.9; H, 6.2; N, 5.6. Found: C, 71.6; H, 6.0; N, 5.2. Yield =48%. 2b : calcd (%) for C 55 B 1 Fe 1 H 50 N 4 O 3 , M w =881.7 g.mol −1 , C, 74.9; H, 5.7; N, 6.4. Found: C, 74.5; H, 5.8; N, 6.1. Yield =55%. 2.2. Theoretical Calculations The theoretical calculations were carried out using the ORCA 4.1 computational package [ 16 ]. Three density functional theory (DFT) functionals, B3LYP [ 17 – 19 ], OPBE [ 20 , 21 ], and TPSSh [ 22 , 23 ], were used to optimize the molecular structures together with the polarized tripleζ quality basis set def2-TZVP proposed by Ahlrichs and co-workers [ 24 ], where “verytightopt” optimization criteria were used in ORCA. The calculations utilized the RI approximation with the decontracted auxiliary def2/J Coulomb fitting basis set [ 25 ] and the chain-of-spheres (RIJCOSX) approximation to exact exchange [ 26 , 27 ] as implemented in ORCA. Increased integration grids (Grid5 and Gridx5 in ORCA convention) and tight SCF convergence criteria were used in all calculations. Moreover, the SCF stability test as implemented in ORCA was done for all final optimized geometries to verify whether the SCF solution was at a local minimum and not in a saddle point [28,29]. 3. Results and Discussion 3.1. Crystal Structures The crystal structures were determined by single-crystal X-ray diffraction for all three presented compounds 1a , 2a , and 2b , and these crystallized in triclinic (P-1 for 1a and 2a ) and monoclinic (P2 1 /c for 2b ) space groups (Table 1). All three compounds consist of complex Fe(III) cations charge balanced by BPh 4− anions. In 2a , the additional methanol molecule is in its asymmetric unit, which is heavily Metals 2019,9, 849 4 of 11 disordered. It was not possible to model it reasonably; therefore, the SQUEEZE procedure [ 30 ] was used to subtract the corresponding electronic density. Table 1. Crystal data and details of structure determination. Compound 1a 2a 2b Formula C55H57BFeN5O4C59H58BFeN4O5C55H50BFeN4O3 Formula weight 918.71 969.75 881.65 Crystal system Triclinic Triclinic Monoclinic Space group P-1 P-1 P21/c Cell parameters a/Å10.1648(7) 13.2869(4) 19.1978(10) b/Å15.2130(10) 13.6926(5) 11.7562(13) c/Å16.2320(10) 16.4083(6) 21.381(2) α/◦81.990(6) 70.732(3) 90 β/◦72.359(6) 78.232(3) 110.458(8) γ/◦86.049(6) 67.395(3) 90 V/Å32367.7(3) 2591.29(17) 4521.2(8) Z2 2 4 T/K 100(2) 190(2) 293(2) Density, Dc/g cm−31.289 1.243 1.295 Abs. coefficient/mm−10.371 0.344 0.384 Data/restraints/param 8326/0/601 6630/3/644 7931/0/577 R1a, wR2b(all data) 0.0660, 0.1073 0.0545, 0.0867 0.0618, 0.0810 R1a, wR2b[I >2σ(I)] 0.0379, 0.0978 0.0355, 0.0829 0.0340, 0.0763 Goodness of fit 1.049 1.028 0.954 CSD number 1939657 1939658 1939656 aR1=P(|Fo|–|Fc|)/P|Fo|.bwR2={P[w(F2o−F2c)2]/P[w(F2o)2]}1/2. The complex cations in 1a , 2a , and 2b consist of a pentadentate Schiffbase ligand (3EtO-LA 2− in 1a and 2a , LA 2− in 2b ) coordinated to the central Fe(III) atom, and the sixth coordination site is occupied by a monodentate N-donor heterocyclic ligand (L1 b in 2a and 2b , L1 b in 1a ). The pentadentate Schiff base ligands coordinate iron centers in a cis-conformation of the oxygen atoms, which is typical for compounds with [Fe(R-LA)] + cations (Figure 1) [ 31 ]. The monodentate ligand is in a trans-position to the secondary amine group of the pentadentate ligand. The metal–ligand bond lengths (Figure 1) are close to the values typical for the LS state with the longest bonds observed between the iron atoms and secondary nitrogen atoms of the pentadentate ligands (in Å, 2.0293(19) in 1a , 2.0539(16) in 2a , 2.0843(15) in 2b ) or the nitrogen atom of the heterocyclic monodentate ligand (in Å, 2.0105(18) in 1a , 2.0497(15) in 2a , 2.0990(16) in 2b ). The Fe–N bonds involving the imino nitrogen atoms are rather shorter and similar for all three structures, ranging between 1.98 to 2.01 Å. The Fe–O bonds are even shorter: 1.87–1.89 Å. The angular distortion parameter Σ is rather small [32]: 17.4◦(1a), 23.7◦(2a), and 30.3 (2b). The crystal structures of 1a, 2a , and 2b do not contain hydrogen bonding of significant strength. In 1a , the secondary amine group from the complex cation forms offset N–H ···π non-covalent contact with the aromatic ring of the BPh 4− anion. The shortest N ··· C distance is 3.446(3) Å (Figure S1 in supplementary materials). The N–H group from benzimidazole also forms N–H ···π interaction with the aromatic ring of the BPh 4− anion—the N ··· Cg distance is 3.227(4) Å (where Cg stands for the ring centroid). In 2a , the interaction between complex cations is provided by very offset ring–ring interactions with a shortest C ··· C distance of 3.192(3) Å. Other non-covalent interactions include weak C–H ··· O and C–H···πcontacts (Figure S2). In 2b , the secondary amine group from the complex cation forms weakly offset N–H ···π interaction with the aromatic ring of the BPh 4− anion (the shortest N ··· C distance is 3.713(3) Å). Other non-covalent interactions in 2b are weak C–H···O and C–H···πcontacts (Figure S3). Metals 2019,9, 849 5 of 11 Metals 2019, 9, x FOR PEER REVIEW 4 of 11 occupied by a monodentate N-donor heterocyclic ligand (L1b in 2a and 2b, L1b in 1a). The pentadentate Schiff base ligands coordinate iron centers in a cis-conformation of the oxygen atoms, which is typical for compounds with [Fe(R-LA)]+ cations (Figure 1) [31]. The monodentate ligand is in a trans-position to the secondary amine group of the pentadentate ligand. (a) (b) (c) Figure 1. Depiction of the molecular structures of [Fe(3EtO-LA)(L1a)]+ in 1a (a), [Fe(3EtO-LA)(L1b)]+ in 2a (b) and [Fe(LA)(L1b)]+ in 2b (c). Selected bond lengths (in Å): in 1a, Fe1–O1 = 1.8896(15), Fe1–O2 = 1.8951(15), Fe1–N1 = 1.9608(19), Fe1–N2 = 2.0293(19), Fe1–N3 = 1.971(2), Fe1–N4 = 2.0105(18); in 2a, Fe1–O1 = 1.8719(12), Fe–O2 = 1.8796(12), Fe1–N1 = 1.9787(15), Fe1–N2 = 2.0539(16), Fe1–N3 = 1.9840(15), Fe1–N4 = 2.0497(15); in 2b, Fe1–O1 = 1.8885(13), Fe1–O2 = 1.8892(13), Fe1–N1 = 1.9997(17), Fe1–N2 = 2.0843(15), Fe1–N3 = 2.0099(16), Fe1–N4 = 2.0990(16). Table 1. Crystal data and details of structure determination. Compound 1a 2a 2b Formula C55H57BFeN5O4 C59H58BFeN4O5 C55H50BFeN4O3 Formula weight 918.71 969.75 881.65 Crystal system Triclinic Triclinic Monoclinic Space group P-1 P-1 P21/c Cell parameters a /Å 10.1648(7) 13.2869(4) 19.1978(10) b /Å 15.2130(10) 13.6926(5) 11.7562(13) c /Å 16.2320(10) 16.4083(6) 21.381(2) α / ˚ 81.990(6) 70.732(3) 90 β / ˚ 72.359(6) 78.232(3) 110.458(8) γ / ˚ 86.049(6) 67.395(3) 90 V / Å3 2367.7(3) 2591.29(17) 4521.2(8) Z 2 2 4 T / K 100(2) 190(2) 293(2) Density, Dc /g cm−3 1.289 1.243 1.295 Abs. coefficient /mm 0.371 0.344 0.384 Data/restraints/param 8326/0/601 6630/3/644 7931/0/577 R1a , wR2b (all data) 0.0660, 0.1073 0.0545, 0.0867 0.0618, 0.0810 R1a , wR2b [I > 2σ(I)] 0.0379, 0.0978 0.0355, 0.0829 0.0340, 0.0763 Goodness of fit 1.049 1.028 0.954 CSD number 1939657 1939658 1939656 a R1 = ∑ (|Fo| – |Fc|)/∑|Fo|. b wR2 = {∑[w(F2o − F2c)2]/∑[w(F2o)2]}1/2. Figure 1. Depiction of the molecular structures of [Fe(3EtO-LA)(L1 a )] + in 1a ( a ), [Fe(3EtO-LA)(L1 b )] + in 2a ( b )and [Fe(LA)(L1 b )] + in 2b ( c ). Selected bond lengths (in Å): in 1a , Fe1–O1 =1.8896(15), Fe1–O2 =1.8951(15) , Fe1–N1 =1.9608(19), Fe1–N2 =2.0293(19), Fe1–N3 =1.971(2), Fe1–N4 =2.0105(18); in 2a , Fe1–O1 =1.8719(12), Fe–O2 =1.8796(12), Fe1–N1 =1.9787(15), Fe1–N2 =2.0539(16), Fe1–N3 = 1.9840(15), Fe1–N4 =2.0497(15); in 2b , Fe1–O1 =1.8885(13), Fe1–O2 =1.8892(13), Fe1–N1 =1.9997(17), Fe1–N2 =2.0843(15), Fe1–N3 =2.0099(16), Fe1–N4 =2.0990(16). 3.2. Magnetic Properties The temperature dependence of the effective magnetic moment for 1a , 2a , and 2b is shown in Figure 2. All three compounds undergo spin crossover from the LS to the HS state (S=1/2 → 5/2), which start above ca. 150 K. Evidently, the spin crossover is incomplete until 300 K, because room temperature values of the effective magnetic moment are less than the spin-only value for S=5/2 and g=2.0 (5.93 µB ). Moreover, the low temperature values of µeff vary in the range ≈ 2–3 µB , which suggests that a small portion of iron(III) complexes stay in the HS state. This is also supported by the field dependence of molar magnetization measurements (measured at 2 and 4.6 K, Figure S4), which unequivocally confirm the LS ground state with a larger contribution of non-converted HS molecules in 1a and 2a (M mol /N AµB =1.5 in 1a , 1.4 in 2a at 2 K and 7 T) than in 2b (M mol /N AµB =1.1 at 2 K and 7 T). Metals 2019, 9, x FOR PEER REVIEW 5 of 11 The metal–ligand bond lengths (Figure 1) are close to the values typical for the LS state with the longest bonds observed between the iron atoms and secondary nitrogen atoms of the pentadentate ligands (in Å, 2.0293(19) in 1a, 2.0539(16) in 2a, 2.0843(15) in 2b) or the nitrogen atom of the heterocyclic monodentate ligand (in Å, 2.0105(18) in 1a, 2.0497(15) in 2a, 2.0990(16) in 2b). The Fe–N bonds involving the imino nitrogen atoms are rather shorter and similar for all three structures, ranging between 1.98 to 2.01 Å. The Fe−O bonds are even shorter: 1.87–1.89 Å. The angular distortion parameter Σ is rather small [32]: 17.4˚ (1a), 23.7˚ (2a), and 30.3 (2b). The crystal structures of 1a, 2a, and 2b do not contain hydrogen bonding of significant strength. In 1a, the secondary amine group from the complex cation forms offset N−H··· π non-covalent contact with the aromatic ring of the BPh4− anion. The shortest N···C distance is 3.446(3) Å (Figure S1 in supplementary materials). The N–H group from benzimidazole also forms N−H··· π interaction with the aromatic ring of the BPh4− anion—the N···Cg distance is 3.227(4) Å (where Cg stands for the ring centroid). In 2a, the interaction between complex cations is provided by very offset ring–ring interactions with a shortest C···C distance of 3.192(3) Å. Other non-covalent interactions include weak C−H···O and C−H··· π contacts (Figure S2). In 2b, the secondary amine group from the complex cation forms weakly offset N−H··· π interaction with the aromatic ring of the BPh4− anion (the shortest N···C distance is 3.713(3) Å). Other non-covalent interactions in 2b are weak C−H···O and C−H··· π contacts (Figure S3). 3.2. Magnetic Properties The temperature dependence of the effective magnetic moment for 1a, 2a, and 2b is shown in Figure 2. All three compounds undergo spin crossover from the LS to the HS state (S = 1/2 → 5/2), which start above ca. 150 K. Evidently, the spin crossover is incomplete until 300 K, because room temperature values of the effective magnetic moment are less than the spin-only value for S = 5/2 and g = 2.0 (5.93 μ Β). Moreover, the low temperature values of μ eff vary in the range ≈2–3 μ Β, which suggests that a small portion of iron(III) complexes stay in the HS state. This is also supported by the field dependence of molar magnetization measurements (measured at 2 and 4.6 K, Figure S4), which unequivocally confirm the LS ground state with a larger contribution of non-converted HS molecules in 1a and 2a (Mmol/NA μ Β = 1.5 in 1a, 1.4 in 2a at 2 K and 7 T) than in 2b (Mmol/NA μ Β = 1.1 at 2 K and 7 T). (a) (b) (c) Figure 2. The temperature dependence of the effective magnetic moment for 1a (a), 2a (b), and 2b (c). The experimental data are displayed as empty circles, calculated data are displayed as full lines. The experimental data were analyzed with the help of the Ising-like model [33,34] having following Hamiltonian ˆ垐 2 H σ γ σσ Δ =− (1) Figure 2. The temperature dependence of the effective magnetic moment for 1a ( a ) , 2a ( b ), and 2b ( c ). The experimental data are displayed as empty circles, calculated data are displayed as full lines. The experimental data were analyzed with the help of the Ising-like model [ 33 , 34 ] having following Hamiltonian ˆ H=∆ 2ˆ σ−γhσiˆ σ(1) Metals 2019,9, 849 6 of 11 where σ is fictitious spin with eigenvalues − 1 for LS and +1 for HS states, ∆ is the energy difference between HS and LS states, γ stands for the cooperativeness of the system ( γ >0), and < σ >is the thermal average of the fictitious spin, which is calculated by solving the implicit equation hσi=−1+reffexp[−(∆−2γhσi)/kT] +1+reffexp[−(∆−2γhσi)/kT](2) where r eff is the effective degeneracy ratio of HS and LS states, and it incorporates both the spin and vibrational degeneracies of the respective spin states [ 35 ]. Then, the molar fraction of HS species, x’ HS , is computed as x0HS =1 2(1+hσi). (3) In order to fit the experimental magnetic data, the overall susceptibility was calculated as χmol =(x00 HS +xrHS)χHS +(1−x00 HS −xrHS)χLS (4) where x rHS is the mole fraction of the residual high-spin state at low temperature, x” HS is the rescaled high-spin fraction calculated from the Ising-like model as x” HS =x 0HS (1 – x rHS ), and the molar susceptibility values for LS and HS states were calculated by the Curie–Weiss law as χLS =NAµ0µ2 BSLS(SLS +1) 3k g2 LS T−ΘLS (5) χHS =NAµ0µ2 BSHS(SHS +1) 3k g2 HS T−ΘHS . (6) To summarize, the variation of Ising-like model parameters ( ∆ , γ , ρεφφ ) leads to temperature variation of the HS mole fraction, which is subsequently utilized to calculate the temperature variation of the overall molar susceptibility and, hence, the effective magnetic moment. Due to the fact that the spin crossover is not finished until 300 K, the g HS value was fixed to 2.0, which is a typical value for HS octahedral iron(III) complexes. Also, the Weiss constant of the HS state was set to zero, because the low temperature data are dominated by the LS fraction. Then, we are left with these additional free parameters: g LS ,x rHS , and ΘLS . Finally, a fitting procedure was applied to find the best parameters describing the experimental magnetic data, and the values of the parameters are listed in Table 2. The spin transition temperatures T1/2are increasing in the following order: 2a <1a <2b. Table 2. The magnetic parameters for 1a,2a, and 2b a.. Compound gLS ΘLS xrHS ∆(K) γ(K) reff∆H (J mol−1)∆S (J K−1mol−1)T1/2(K) 1a 2.34 −4.3 0.15 930 116 35.0 7730 29.6 262 2a 2.01 −1.5 0.10 1200 95 150 9975 41.7 239 2b 2.01 −0.8 0.05 1033 127 17 8592 23.6 365 athe thermodynamic parameters were calculated as ∆H=NA∆,∆S=Rlnreff, and T1/2=∆H/∆S. 3.3. Theoretical Calculations In order to evaluate the impact of various coordinated heterocyclic ligands on the spin crossover properties of pentacoordinate Schiffbase iron(III) complexes, density functional theory (DFT) calculations were employed. Here, we tested three DFT functionals, B3LYP, OPBE, and TPSSh, which were selected by benchmark studies to be suitable functionals for the study of spin crossover phenomena [ 36 – 38 ] . The molecular geometries of the complex cations [Fe(R-LA)(L1)] + of 1a, 2a , and 2b were optimized for LS (doublet) and HS (sextet) states. Furthermore, we added another three analogous SCO complexes, namely, DAVCEJ, DETBEJ, and KISJUS, to address the robustness of this Metals 2019,9, 849 7 of 11 theoretical approach. The respective donor–acceptor distances calculated with the TPSSh functional are summarized in Table 3; for other functionals, please see Tables S1 and S2. The experimental X-ray data are available for the low-spin state of 1a , DAVCEJ, and KISJUS, whereas DAVCEJ01 is only available in a high-spin state structure because the reported room temperature X-ray data of DETBEJ and KISJUS01 correspond to incomplete spin transition at this temperature with respect to the magnetic data. Table 3. The interatomic donor–acceptor distances for density functional theory (DFT)-optimized geometries of [Fe(R-LA)(L1)] + of 1a, 2a, and 2b and DAVCEJ, DETBEJ, and KISJUS using the TPSSh functional a.. Method Compound Fe–O Fe–Nim Fe–Nam Fe–Nhetero X-ray analysis 1a (LS) 1.8896/1.8951 1.9608/1.971 2.0293 2.0105 DAVCEJ (LS) 1.869/1.880 1.961/1.974 2.091 2.000 DAVCEJ01 (HS) 1.908/1.920 2.072/2.096 2.257 2.146 KISJUS (LS, 100 K) 1.863/1.880 1.946/1.954 2.023 1.982 TPSSh (LS) 1a 1.875/1.892 1.949/1.972 2.047 2.010 2a 1.872/1.880 1.979/1.984 2.054 2.050 2b 1.888/1.889 2.000/2.010 2.084 2.099 DAVCEJ 1.875/1.890 1.953/1.968 2.105 2.008 DETBEJ 1.872/1.899 1.953/1.978 2.044 2.016 KISJUS 1.867/1.896 1.949/1.972 2.044 1.991 TPSSh (HS) 1a (HS) 1.931/1.950 2.104/2.113 2.282 2.201 2a (HS) 1.934/1.936 2.105/2.109 2.281 2.229 2b (HS) 1.939/1.942 2.104/2.107 2.276 2.225 DAVCEJ (HS) 1.938/1.945 2.095/2.098 2.345 2.202 DETBEJ (HS) 1.938/1.939 2.105/2.109 2.276 2.228 KISJUS (HS) 1.935/1.945 2.107/2.109 2.286 2.174 adistances in Å. Figure 3shows a comparison of DFT-optimized donor–acceptor distances with the experimental data. It is evident that none of these DFT functionals provided perfect results: both B3LYP and OPBE overestimatedFe–N am andFe–N het distances; Fe–N im wasoverestimatedbyB3LYP and underestimated by OPBE. Moreover, the Fe–N am distance of DAVCEJ (HS) is extremely long. It seems to us that the best results were provided by the TPSSh functional. Indeed, only this functional correctly predicted the LS ground state for the studied compounds according to the energy comparison of HS and LS isomers depicted in Figure 4. However, there is large variation in the energy difference, E HS –E LS , between 1.92 kcalmol −1 for 2a and 12.91 kcalmol −1 for DETBEJ. Obviously, the E HS –E LS energy difference does not correlate with the T 1/2 value of the studied compounds, and this inconsistency could also be ascribed to variation of the entropy within this series. However, all the studied compounds have a very similar composition of the Fe III complex cation and have BPh 4− as the counterion; thus, it can be anticipated that variation of the entropy for the whole series should be minute. Thus, it is most likely that the large variation in the E HS –E LS energy difference is due to imperfectness of the DFT functional and/or due to neglect of the crystal packing and non-covalent intermolecular interactions in the solid state, which cannot be reproduced with geometry optimization of [Fe(R-LA)(L1)] + cations in vacuum. Metals 2019,9, 849 8 of 11 Metals 2019, 9, x FOR PEER REVIEW 8 of 11 Figure 3. The variation of the donor–acceptor distances in DFT-optimized geometries (full lines + symbols) compared to the experimental X-ray data (dotted lines). Figure 4. The energy separation of HS and LS states of 1a, 2a, and 2b and DAVCEJ, DETBEJ, and KISJUS calculated using B3LYP, OPBE, and TPSSh with the def2-TZVP basis set. 4. Conclusions In this article we reported the synthesis, crystal structure, and magnetic properties of three new iron(III) complexes with two different pentadentate Schiff base ligands and monodentate heterocyclic Figure 3. The variation of the donor–acceptor distances in DFT-optimized geometries (full lines + symbols) compared to the experimental X-ray data (dotted lines). Metals 2019, 9, x FOR PEER REVIEW 8 of 11 Figure 3. The variation of the donor–acceptor distances in DFT-optimized geometries (full lines + symbols) compared to the experimental X-ray data (dotted lines). Figure 4. The energy separation of HS and LS states of 1a, 2a, and 2b and DAVCEJ, DETBEJ, and KISJUS calculated using B3LYP, OPBE, and TPSSh with the def2-TZVP basis set. 4. Conclusions In this article we reported the synthesis, crystal structure, and magnetic properties of three new iron(III) complexes with two different pentadentate Schiff base ligands and monodentate heterocyclic Figure 4. The energy separation of HS and LS states of 1a , 2a , and 2b and DAVCEJ, DETBEJ, and KISJUS calculated using B3LYP, OPBE, and TPSSh with the def2-TZVP basis set. 4. Conclusions In this article we reported the synthesis, crystal structure, and magnetic properties of three new iron(III) complexes with two different pentadentate Schiffbase ligands and monodentate heterocyclic ligands. The main motivation for this research was to increase the rather low cooperativity of Metals 2019,9, 849 9 of 11 spin crossover behavior which was typically observed for this group of compounds previously. In general, it is well established that by introducing stacking interactions, the cooperativity of SCO systems might be enhanced. Therefore, relatively large, rigid, monodentate ligands (benzimidazole or 1-benzofuro[3,2-c]pyridine) capable of forming stacking interactions were deliberately introduced into the synthesis of the reported compounds. However, the prepared compounds did not exhibit any regular patterns of significant stacking interactions. The magnetic properties of the compounds showed that our attempt to increase SCO cooperativity was not successful, and the compounds exhibited weakly cooperative SCO without thermal hysteresis but with rather high critical SCO temperatures (T 1/2 =262 ( 1a ), 239 ( 2a ), 365 ( 2b ) K). DFT calculations were also employed in the study using three functionals, B3LYP, OPBE, and TPSSh. The best agreement with the experimental structures was found for the TPSSh functional, and also, only this functional identified LS isomers lower in energy than HS isomers. However, the general trend in the energy separation of HS and LS isomers and the spin crossover transition temperature T 1/2 found from the experimental data was not fully recovered, which most probably points to the importance of the intermolecular interactions. Supplementary Materials: The following are available online at http://www.mdpi.com/2075-4701/9/8/849/s1, Figure S1: The N–H ···π non-covalent interactions in 1a , Figure S2: The non-covalent interactions in 2a , Figure S3: The non-covalent interactions in 2b , Table S1: The interatomic donor–acceptor distances for DFT-optimized geometries, Table S2: The interatomic donor–acceptor distances for DFT-optimized geometries. Author Contributions: Conceptualization, I.N.; methodology, I.N. and R.H.; validation, I.N. and R.H.; formal analysis, R.H.; investigation, I.N., R.H. and I.S.; writing—original draft preparation, I.N. and R.H.; writing—review and editing, I.N. and R.H.; visualization, I.N. and R.H. Funding: This research was funded by the institutional sources of the Department of Inorganic Chemistry, Palack ý University Olomouc, Czech Republic (I.N. and R.H.), project CEITEC 2020 (LQ1601) with financial support from the Ministry of Education, Youth and Sports of the Czech Republic under the National Sustainability (I.N.). Acknowledgments: Ivan Nemec would like to acknowledge Jozef Mikloviˇc for providing a small amount of 1-benzofuro[3,2-c]pyridine. Conflicts of Interest: The authors declare no conflict of interest. References 1. Van Koningsbruggen, P.J.; Maeda, Y.; Oshio, H. Iron(III) spin crossover compounds. In Spin Crossover in Transition Metal Compounds I; Gütlich, P., Goodwin, H.A., Eds.; Springer: Berlin, Germany, 2004; pp. 259–324. 2. Harding, D.J.; Harding, P.; Phonsri, W. Spin crossover in iron(III) complexes. Coord. Chem. Rev. 2016 ,313, 38–61. [CrossRef] 3. Nemec, I.; Boˇca, R.; Herchel, R.; Tr á vn í ˇcek, Z.; Gembick ý , M.; Linert, W. 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