Power of Three: Incremental Increase in the Ligand ... - ACS Publications

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

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Power of Three: Incremental Increase in the Ligand Field Strength of N‑Alkylated 2,2′-Biimidazoles Leads to Spin Crossover in Homoleptic Tris-Chelated Fe(II) Complexes Jeremy J. Hrudka,† Hoa Phan,† Jeff Lengyel,† Andrey Yu. Rogachev,*,‡ and Michael Shatruk*,† †

Department of Chemistry and Biochemistry, Florida State University, 95 Chieftan Way, Tallahassee, Florida 32306, United States Department of Chemistry, Illinois Institute of Technology, 3101 South Dearborn St, Chicago, Illinois 60616, United States

‡

S Supporting Information *

ABSTRACT: Homoleptic complexes [Fe(Ln)]X2 (L1 = 1,1′(α,α′-o-xylyl)-2,2′-biimidazole, L2 = 1,1′-(α,α′-3,4-dibromo-oxylyl)-2,2′-biimidazole, L3 = 1,1′-(α,α′-2,5-dimethoxy-o-xylyl)2,2′-biimidazole; X = BF4− or ClO4−) were synthesized by direct reactions of the Fe(II) precursor salts and bidentate ligands L1, L2, or L3. All mononuclear complexes undergo gradual temperature-driven spin-crossover (SCO) between the high-spin (HS, S = 2) and low-spin (LS, S = 0) states. Complexes with ligands L1 and L2 synthesized in methanol exhibit complete SCO with the midpoint of the LS↔HS conversion varying from 233 to 313 K, while complexes with ligand L3, crystallized from an ethanol/dichloromethane mixture, exhibit incomplete SCO with the residual HS/LS ratio of ∼1:4 for [Fe(L3)3](BF4)2 and ∼1:1 for [Fe(L3)3](ClO4)2. Complexes with L1 can also be recrystallized from ethanol/ dichloromethane, in which case they exhibit very gradual and incomplete SCO, similar to those of the complexes with L3. The differences in magnetic behavior have been traced back to peculiarities of molecular packing observed in the corresponding crystal structures. Density-functional theoretical calculations provide justification to the SCO behavior of these complexes, as compared to the HS-only behavior observed for the parent [Fe(bim)3]2+ complex with nonalkylated 2,2′-biimidazole (bim).

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state.5 We have also shown that such analysis can be applied successfully to modify the ligands and induce or suppress SCO in the Fe(II) complexes. The initial motivation to inspect the correlation between the N···N separation in the free ligand and the spin state of its Fe(II) complex came from our earlier observation of a slight increase in the ligand field strength in the heteroleptic complexes [Fe(tpma)(L)](ClO4)2, where L = 2,2′biimidazole (bim) or 1,1′-(α,α′-o-xylyl)-2,2′-biimidazole (xbim) and tpma = tris(2-pyridylmethyl)amine. Both complexes showed SCO behavior with similar transition temperatures, but the measurements of the ligand-field splitting by optical absorption spectroscopy suggested a slight increase in the strength of the N-alkylated derivative, xbim, in comparison to the original bim ligand.6 It is well-established that the homoleptic complex [Fe(bim)3]2+ exhibits only the HS state.7 Incited by the slightly stronger ligand field of xbim relative to that of bim, as judged by the properties of [Fe(tpma)(L)](ClO4)2 complexes, we hypothesized that the increase in the ligand field strength can be even larger when going from a set of three bim ligands to a set of three N-alkylated 2,2′-biimidazoles. Indeed, the strain

INTRODUCTION The design of molecular complexes that exhibit magnetic and structural transition between states with different electronic configurations essentially boils down to conceiving a set of ligands that provides comparable zero-point energies of the low-spin (LS) and high-spin (HS) states.1 If such condition can be achieved, then one observes spin crossover (SCO) between these electronic states, which is typically driven by changes in temperature due to the higher electronic and vibrational entropy of the HS state. SCO is typically observed for complexes of octahedrally coordinated transition metal ions with 3d4−3d7 electronic configurations, although the field is dominated by 3d6 Fe(II) complexes.2 Various homoleptic and heteroleptic ligand sets have been shown to lead to SCO behavior for the Fe(II) ion, in most cases furnishing a coordination environment of six nitrogen atoms.3 Discovering new combinations of ligands that might promote SCO is becoming a challenge, as many possibilities have already been explored, to a different degree of success.4 We have recently demonstrated that the likelihood of SCO in homoleptic Fe(II) complexes with diimine ligands can be predicted quite reliably based on a very simple metric parameter, namely, the separation between the N-donor atoms of the chelating ligand in its free, noncoordinated © XXXX American Chemical Society

Received: January 25, 2018

A

DOI: 10.1021/acs.inorgchem.8b00223 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

The combined organic fractions were dried with MgSO4, filtered, and dried under vacuum to yield a brown crystalline solid. Yield = 3.82 g (71%). 1H NMR (600 MHz, CDCl3) δ, ppm: 7.55 (s, 2H), 7.25 (d, J = 1.25 Hz, 2H), 7.08 (d, J = 1.26 Hz, 2H), 4.84 (s, 4H). HR-MS (ESI+) Calcd (Found) for Br 2 N 4 C 14 H 11 + [(M + H) + ]: 394.93300 (394.93227). 1,1′-(α,α′-2,5-Dimethoxy-o-xylyl)-2,2′-biimidazole (L3). A mixture of bim (114 mg, 0.850 mmol) and NaH (39 mg, 1.7 mmol) was dissolved in 6 mL of N,N-dimethylformamide (DMF) and stirred for 30 min. A solution of 2,3-bis(bromomethyl)-1,4dimethoxybenzene (250 mg, 0.771 mmol) in 3 mL of DMF was added. After stirring for 20 h at room temperature, the reaction mixture was concentrated under reduced pressure, diluted with 50 mL of water, and extracted with CH2Cl2 (3 × 50 mL). The combined organic fractions were dried with MgSO4, filtered, and dried under vacuum to yield a tan solid. Yield = 195 mg (86%). 1H NMR (600 MHz, CDCl3) δ, ppm: 7.16 (d, J = 1.23 Hz, 2H), 7.03 (d, J = 1.11 Hz, 2H), 6.85 (s, 2H), 5.19 (s, 4H), 3.82 (s, 6H). HR-MS (ESI+) Calcd (Found) for O2N4C16H17+ [(M + H)+]: 297.13515 (297.13604). [Fe(L1)3](BF4)2 (1a). Ligand L1 (150 mg, 0.635 mmol) was dissolved in 12 mL of methanol (MeOH). To this solution Fe(BF4)2·6H2O (71 mg, 0.214 mmol) was added with stirring, which caused immediate formation of a yellow precipitate. The suspension was stirred for 12 h, and yellow powder was isolated by filtration, washed successively with MeOH (3 × 5 mL) and diethyl ether (Et2O, 3 × 5 mL), and dried by suction. Yield = 134 mg (67%). Anal. Calcd (Found) for FeF8ON12C42B2H38 (1a·H2O), %: C, 52.75 (52.81); H, 4.01 (3.93); N, 17.58 (17.71). HR-MS (ESI+) Calcd (Found) for FeC42H36N122+ [M2+]: 764.25353 (764.24869). X-ray quality single crystals (Table 1) were grown in an H-tube by slow diffusion of either methanolic solutions of both reactants (sample 1a′) or solutions of Fe(BF4)2·6H2O in ethanol (EtOH) and L1 in CH2Cl2 (sample 1a″). [Fe(L1)3](ClO4)2 (1b). Complex 1b was prepared in a manner analogous to that described for complex 1a, starting with 300 mg (1.27 mmol) of L1 and 127 mg (0.423 mmol) of Fe(ClO4)2·6H2O in 22 mL of MeOH. Yield = 240 mg (59%). Anal. Calcd (Found) for FeCl2O9N12C42H38 (1b·H2O): C, 51.39 (51.63); H, 3.90 (3.86); N, 17.12 (17.17). HR-MS (ESI+) Calcd (Found) for FeC42H36N122+ [M2+]: 764.25353 (764.24703). X-ray quality single crystals of 1b· 3CH2Cl2 were grown by slow diffusion of solutions of Fe(ClO4)2· 6H2O in EtOH and L1 in CH2Cl2 in an H-tube. [Fe(L2)3](BF4)2 (2a). Complex 2a was prepared in a manner analogous to that described for complex 1a, starting with 149 mg (0.378 mmol) of L2 and 42 mg (0.124 mmol) of Fe(BF4)2·6H2O in 15 mL of MeOH. Yield = 80 mg (46%). Anal. Calcd (Found) for FeBr6F8N12C42B2H30 (2a): C, 35.74 (35.71); H, 2.14 (2.15); N, 11.91 (11.68). HR-MS (ESI+) Calcd (Found) for FeBr6N12C42H302+ [M2+]: 1237.71047 (1237.70443). X-ray quality single crystals of 2a·3MeCN were obtained by recrystallization from MeCN/Et2O. [Fe(L2)3](ClO4)2 (2b). Complex 2b was prepared in a manner analogous to that described for complex 1a, starting with 146 mg (0.371 mmol) of L2 and 34 mg (0.122 mmol) of Fe(ClO4)2·6H2O in 15 mL of MeOH. Yield = 95 mg (54%). Anal. Calcd (Found) for FeBr6Cl2O9N12C43H34 (2b·MeOH): C, 35.16 (35.19); H, 2.33 (2.23); N, 11.44 (11.53). HR-MS (ESI+) Calcd (Found) for FeBr6N12C42H302+ [M2+]: 1237.71047 (1237.70397). X-ray quality single crystals of 2b·3MeCN were obtained by recrystallization from MeCN/Et2O. [Fe(L3)3](BF4)2 (3a). Ligand L3 (50 mg, 0.17 mmol) was added to one side of a small H-tube, and Fe(BF4)2·6H2O (17 mg, 0.056 mmol) was added to the other side. To the side of the H-tube containing the iron salt was slowly added 3 mL of EtOH, while 1 mL of CH2Cl2 was added to the side containing the ligand. Then, both sides of the tube were filled with EtOH until the solutions from both sides barely came together in the joining middle bar. The H-tube was left undisturbed for 10 days, after which time yellow crystals were observed in one side of the tube. The crystals were collected on a glass-frit filter, washed successively with EtOH (3 × 1 mL) and Et2O (3 × 1 mL), and dried under suction. Yield = 31 mg (49%). Anal. Calcd (Found) for

induced by the presence of the eight-member ring in xbim (Scheme 1) leads to the decrease in the N···N separation, Scheme 1. Ligands Used in This Work

which should lead to the higher ligand field splitting in the transition metal complex.5 Herein, we report a study of a series of homoleptic Fe(II) complexes with three different N-alkylated 2,2′-biimidazoles. All these complexes exhibit temperaturedriven SCO, although its abruptness and completeness is strongly dependent on the molecular packing observed in the crystal structures. This study provides an example of successful implementation of the “chelating N···N distance” rule5 in order to arrive at new SCO materials via judicious ligand design. At the same time, it underscores the importance of more subtle crystal packing effects in the character of observed SCO behavior.

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MATERIALS AND METHODS

Caution! The complexes between metal ions and organic ligands with perchlorate counterion are potentially explosive. The compounds should be prepared in small amounts and handled with great care! Synthesis. All reactions were performed under an inert N2 atmosphere using standard Schlenk techniques. All reagents were purchased from Aldrich, except for 2,3-dimethylbenzene-1,4-diol (TCI). Commercial reagents were used as received. 2,2′-Biimidazole (bim),8 1,1′-(α,α′-o-xylyl)-2,2′-biimidazole (xbim, L1),9 1,2-dibromo4,5-bis(bromomethyl)benzene,10 and 2,3-bis(bromomethyl)-1,4-dimethoxybenzene11 were prepared by previously published procedures. Anhydrous commercial solvents were additionally purified by passing through a double-stage drying/purification system (Glass Contour Inc.). Elemental analyses were performed by Atlantic Microlab, Inc. (Norcross, GA, USA). 1H and 13C NMR data was collected on a 600 MHz Bruker NMR spectrometer in commercial deuterated solvents (Aldrich or Cambridge Isotopes). High-resolution electrospray ionization mass spectra in the positive ion mode (ESI+) were acquired on a JEOL Model T100 Accutof by indirect infusion of sample solutions in acetonitrile with formic acid. PPG425 or PEG600 with NaI were used as reference masses. An orifice voltage of 50 V, ring voltage of 12 V, and needle voltage of 2000 V were used. Thermogravimetric analyses (TGA) were performed under a continuous flow of Ar gas, at a heating rate of 10 K/min, using a Q600 SDT Analyzer (TA Instruments). 1,1′-(α,α′-3,4-Dibromo-o-xylyl)-2,2′-biimidazole (L2). 1,2-Dibromo-4,5-bis(bromomethyl)benzene (5.89 g, 13.61 mmol) and bim (2.19 g, 16.33 mmol) were combined in 180 mL of acetonitrile, MeCN. After addition of 35 mL of a 35% aqueous solution of NaOH, the suspension was heated to 90 °C and refluxed for 20 h. After cooling to room temperature, the mixture was diluted with 200 mL of water and extracted with dichloromethane, CH2Cl2 (3 × 150 mL). B


C

Br6FeF8N15C48B2H39 (2a·3MeCN)

230(2)

formula

T (K)

1556127 1534.87 P21/c 14.123(1) 20.282(2) 25.365(2) 124.763(2) 5969(8) 4 yellow-orange 1.708 4.340 0.71073 30.71 108624

7250(3) 6 red-orange 1.051 0.350 0.71073 24.87 67594 0.039 8338 497 1 0.072, 0.178 0.096, 0.194 1.023 1.81, −0.31

7070(3) 6 red-orange 1.078 0.359 0.71073 24.74 48914 0.052 8027 497 1 0.082, 0.218 0.105, 0.236 1.047 2.46, −0.50

CCDC number molar mass (g/mol) space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z crystal color dcalc (g cm−3) μ (mm−1) λ (Å) 2θmax (deg) total reflections

18.496(4)

18.288(4)

1556128 1560.15 P21/c 14.065(1) 20.263(2) 25.222(2) 124.819(3) 5901(1) 4 red-orange 1.756 4.472 0.71073 28.42 59752

110(2)

4950(1) 4 red 1.601 0.708 0.71073 25.24 39263 0.027 4081 224 1 0.104, 0.112 0.313, 0.324 1.508 3.48, −1.92

1813100 1193.07 P213 17.043(1)

110(2)

FeF8O5N12C47B2H56 (1a·3CH2Cl2)

1556129 1539.62 P21/c 14.232(5) 20.614(7) 25.586(7) 125.03(1) 6147(3) 4 yellow 1.664 4.292 0.71073 28.40 52154

230(2)

Br6FeCl2O8N14.5C47H37.5 (2b·2.5MeCN)

230(2) 1556123 938.29 P63 21.275(4)

110(2)

1556122 938.29 P63 21.128(5)

T (K)

CCDC number molar mass (g/mol) space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z crystal color dcalc (g cm−3) μ (mm−1) λ (Å) 2θmax (deg) total reflections Rint unique reflections parameters refined restraints used R1, wR2 [I > 2σ(I)]a R1, wR2 (all data) goodness of fitb diff. peak/hole (e Å−3) Br6FeCl2O8N15C48H39 (2b·3MeCN)

FeF8O7N12C47B2H60 (1a·5MeOH·2H2O)

FeF8O5N12C47B2H56 (1a·5MeOH)

Formula

Table 1. Data Collection and Crystal Structure Refinement Parameters for 1a, 1b, 2a, 2b, 3a, and 3b

100(2) 1556124 1218.35 P213 17.1459(8)

8508(2) 6 yellow 1.157 0.325 0.71073 28.51 26123

22.899(3)

22.536(1) 8244(1) 6 red 1.194 0.335 0.71073 28.58 25287

1556131 988.24 R3̅ 20.713(2)

230(2) 1556130 988.24 R3̅ 20.552(1)

110(2)

5115(2) 5040(7) 4 4 red red-orange 1.549 1.605 0.685 0.790 0.71073 0.71073 25.24 25.24 55566 44951 0.020 0.026 4240 4183 224 251 0 3 0.083, 0.098 0.060, 0.168 0.253, 0.272 0.069, 0.176 1.187 1.045 1.25, −1.30 2.00, −0.73 FeF8O6.5N12C50B2H53 (3a·0.5EtOH·CH2Cl2)

1813101 1193.07 P213 17.230(2)

230(2)

FeCl8O8N12C45H42 (1b·3CH2Cl2)

110(2)

8097(1) 6 red 1.224 0.364 0.71073 28.54 25842

22.102(1)

1556132 994.56 R3̅ 20.567(1)

100(2)

8340(2) 6 yellow 1.188 0.353 0.71073 28.55 24995

22.447(3)

1556133 994.56 R3̅ 20.713(2)

230(2)

1556126 1543.88 P21/c 13.925(5) 20.066(8) 25.018(9) 124.359(7) 5223(2) 5771(4) 4 4 orange red 1.549 1.777 0.762 4.492 0.71073 0.71073 25.24 30.87 46913 64769 0.041 0.065 4347 18043 224 754 0 15 0.080, 0.241 0.067, 0.155 0.102, 0.257 0.128, 0.180 1.049 1.017 1.30, −1.09 2.04, −1.35 FeCl4.5O15.25N12C51.75H58 (3b·1.25EtOH·1.25CH2Cl2)

1556125 1218.35 P213 17.351(2)

230(2)

Br6FeF8O0.5N15C48B2H40 (2a·3MeCN·0.5H2O)

Inorganic Chemistry Article


Article

R1 = Σ∥F0| − |Fc∥/Σ|F0|; wR2 = [Σ[w(F02 − Fc2)2]/Σ[w(F02)2]]1/2 bGoodness-of-fit = [Σ[w(F02 − Fc2)2]/(Nobs − Nparams)]1/2, based on all data

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RESULTS AND DISCUSSION Synthesis. Preparations of ligands and complexes reported in this work were conducted under an inert atmosphere using standard Schlenk techniques. All complexes were synthesized by reacting precursor salts [Fe(H2O)6](BF4)2 (a) or [Fe(H2O)6](ClO4)2 (b) with ligands L1, L2, or L3 in the 1:3 stoichiometric ratio. Complexes 1a, 1b, 2a, and 2b, containing ligands L1 or L2, could be isolated in analytically pure form as

a

230(2)

0.027 4506 218 0 0.062, 0.178 0.074, 0.183 1.126 0.30, −0.28 0.028 4379 218 0 0.086, 0.177 0.092, 0.180 1.136 0.39, −0.68

100(2)

FeCl2F8O6.5N12C50B2H53 (3a·0.5EtOH·CH2Cl2): C, 48.97 (48.94); H, 4.36 (4.59); N, 13.71 (13.94). HR-MS (ESI+) Calcd (Found) for FeN12O6C48H482+ [M2+]: 944.31692 (944.30926). [Fe(L3)3](ClO4)2 (3b). Complex 3b was prepared in the manner analogous to that described for complex 3a, starting with 50 mg (0.17 mmol) of L3 and 16 mg (0.056 mmol) of Fe(ClO4)2·6H2O (16 mg, 0.056 mmol). Yield = 38 mg (59%). Anal. Calcd (Found) for FeCl3.5O14.75N12C50.25H54 (3b·0.75EtOH·0.75CH2Cl2): C, 48.60 (48.50); H, 4.38 (4.63); N, 13.53 (13.70). HR-MS (ESI+) Calcd (Found) for FeN12O6C48H482+ [M2+]: 944.31692 (944.30588). Magnetic Measurements. Magnetic measurements were carried out on polycrystalline samples, using a superconducting quantum interference device (SQUID) magnetometer MPMS-XL (Quantum Design). Measurements were performed in a direct-current applied magnetic field of 0.1 T in the 1.8−400 K temperature range at the scan rate of 2 K/min. The data were corrected for the diamagnetic contribution from the sample holder and for the intrinsic diamagnetism using tabulated constants.12 X-ray Crystallography. Single-crystal X-ray diffraction was performed on a Bruker APEX-II diffractometer equipped with a CCD detector and a graphite-monochromated Mo Kα radiation source (λ = 0.71073 Å). In a typical experiment, a single crystal was suspended in Paratone-N oil (Hampton Research) and mounted on a cryoloop which was cooled to the desired temperature in an N2 cold stream. The data sets were recorded as ω-scans at 0.3° step width and integrated with the Bruker SAINT software package.13 In all experiments, a multiscan adsorption correction was applied based on multiple equivalent measurements (SADABS).14 The space group was determined with XPREP,15 and the crystal structure solution and refinement were carried out using the SHELX software.16 The final refinement was performed with anisotropic atomic displacement parameters for all non-hydrogen atoms, with the exception of some strongly disordered anions or solvent molecules, which were refined isotropically. All H atoms were placed in calculated positions and refined in the riding model. In the case of structures 1a, 3a, and 3b, the disorder of the interstitial solvent was too severe, which necessitated the use of the SQUEEZE procedure17 to account for the electron density of the disordered part. Full details of the crystal structure refinements and the final structural parameters have been deposited with the Cambridge Crystallographic Data Centre (CCDC), while the CCDC numbers and brief summary of data collection and refinements are provided in Table 1. Theoretical Calculations. Electronic structure calculations were performed at the density-functional level of theory, with the ORCA software package,18 starting with molecular geometries of complexes [Fe(bim)3]2+ and [Fe(L1)3]2+ obtained from X-ray crystal structure determination. Molecular geometry optimizations were carried out without symmetry constraints, using the TZVP basis set of triple-ζ quality. The atomic core orbitals were kept frozen up to the 1s level for the C and N atoms and up to the 2p level for the Fe atom. During the calculations, scalar relativistic effects were included by performing calculations within the zero-order regular approximation (ZORA). The initial optimization was performed with the PBEh-3c functional.18 The vibrational analyses performed after the geometry optimizations confirmed that the located extrema corresponded to true minima (no imaginary frequencies). Consequently, a more rigorous optimization procedure was carried out, using the PBE0 functional. Finally, zero-point energies of the HS and LS states were calculated using the B2PLYP-D functional with a chain of spheres approximation (RIJCOSX)19 and tight SCF convergence criteria.

0.020 4567 211 0 0.049, 0.142 0.059, 0.148 1.073 0.53, −0.57

230(2) 110(2)

0.022 4437 211 0 0.042, 0.114 0.050, 0.118 1.076 0.48, −0.53

230(2)

0.056 14285 696 9 0.085, 0.249 0.168, 0.297 1.058 1.69, −1.43

110(2)

0.047 13872 815 30 0.060, 0.134 0.095, 0.153 1.058 1.51, −1.62

230(2)

0.076 18477 725 12 0.081, 0.222 0.172, 0.272 1.029 1.58, −1.59 Rint unique reflections parameters refined restraints used R1, wR2 [I > 2σ(I)]a R1, wR2 (all data) goodness of fitb diff. peak/hole (e Å−3)

T (K)

Br6FeCl2O8N14.5C47H37.5 (2b·2.5MeCN) formula

Table 1. continued

Br6FeF8N15C48B2H39 (2a·3MeCN)

Br6FeCl2O8N15C48H39 (2b·3MeCN)

FeF8O6.5N12C50B2H53 (3a·0.5EtOH·CH2Cl2)

FeCl4.5O15.25N12C51.75H58 (3b·1.25EtOH·1.25CH2Cl2)

Inorganic Chemistry

D


Article

Inorganic Chemistry

Figure 1. Cationic complexes in the crystal structures of 1a (a), 2a (b), and 3a (c), with the thermal ellipsoids at 50% probability level. Only the atoms that belong to the asymmetric unit have been labeled. The H atoms have been omitted for the sake of clarity.

group P213). As will be shown below, the change in the interstitial solvent has a pronounced effect on the magnetic behavior of 1a and 1b. The noncentrosymmetric nature of the space groups found for the crystals of 1a and 1b indicates the presence of [Fe(L1)3]2+ cations of only one chirality (Δ or Λ) in any given single crystal of these complexes, but crystals of different chiralities are present in the product. The structures of 2a and 2b and of 3a and 3b are centrosymmetric, which means that the crystals contain a 1:1 mixture of Δ- and Λ-[Fe(Ln)3]2+ cations. No symmetry change was observed between the highand low-temperature structures for any of these complexes. Tetrafluoroborate complex 1a crystallizes in the chiral monoclinic space group P63 with a substantial amount of interstitial methanol molecules in the structure. Although the crystallinity of the complex was preserved upon handling of the crystals, severe solvent and anion disorder rendered modeling of the interstitial solvent positions nearly impossible, which necessitated the use of the SQUEEZE procedure.17 The count of the integrated electron density located in the void space of structure 1a allowed formal assignment of the interstitial solvent content in the crystal (Table 1). While we could not obtain X-ray quality crystals of 1b from MeOH, crystals of both 1a and 1b were grown from the EtOH/CH2Cl2 solvent mixture, which led to incorporation of CH2Cl2 molecules into the crystal lattice. The change in the nature of interstitial solvent results in drastically different crystal packing (see below). Due to the insolubility of ligand L2 in MeOH, complexes 2a and 2b were obtained in elementally pure powder form and subjected to recrystallization from MeCN/Et2O. The singlecrystal diffraction study revealed that both 2a and 2b crystallize in the same monoclinic space group, P21/c, with similar unit cell parameters and packing motifs. Complexes 3a and 3b, which were crystallized from EtOH/CH2Cl2, crystallize in the centrosymmetric space group R3.̅ Similar to the structure of 1a, the interstitial solvent molecules and some counteranions were severely disordered, and the SQUEEZE procedure had to be employed to achieve satisfactory crystal structure refinements for 3a and 3b. Only one-half of a BF4− or ClO4− anion, located on the 3-fold axis, could be refined in these structures, while the remaining anion and solvent content were calculated by integrating the excess electron density in the void space by the SQUEEZE routine17 and taking into account the results of elemental analysis. For both 3a and 3b, the integrated electron density from the disordered part of the structure matched

yellow microcrystalline powders from simple mixing of the starting materials in MeOH for 12 h. Attempts to arrive at analytically pure microcrystalline samples of complexes 3a and 3b, which contain ligand L3, using reactions in MeOH and some other solvents were unsuccessful. Therefore, these complexes were prepared by slow diffusion of reactant solutions in an H-shaped tube, which afforded X-ray quality single crystals. Single crystals of 1a and 1b were grown by a similar method, although 1a was crystallized from a pure MeOH solution while 1b was crystallized from a CH2Cl2/EtOH mixture. Crystals of 1a could also be obtained from CH2Cl2/ EtOH. Due to the lack of solubility of ligand L2 in MeOH, single crystals of 2a and 2b were grown by recrystallization of the analytically pure powder from a MeCN/Et2O solvent mixture. Elemental analyses and high-resolution mass spectrometry confirmed the identity of the complexes. Properties discussed hereafter are of crystalline or recrystallized samples, as summarized in Table 1. Examination of the complexes by TGA revealed that except for the minor solvent loss all tetrafluoroborate complexes remain stable at least to 550 K while all perchlorate complexes undergo explosive decomposition above 470 K (see the Supporting Information). Crystal Structures. All compounds were characterized by single crystal X-ray diffraction, which revealed the crystallization of homoleptic mononuclear complexes (Figure 1) with significantly different unit cell parameters refined from the data collected at 100/110 and 230 K. In addition, in the cases of 2a, 2b, 3a, and 3b, a color change from yellow/orange to darkred was observed visually when the crystals were cooled from room temperature to 110 K. These observations suggest that the SCO takes place in these compounds as the temperature is lowered. All crystal structures contained interstitial solvent, the content of which for each specific case is given in Table 1. In the following discussion we will mention the solvent content whenever necessary but omit it from the formulas of the complexes for brevity. The crystal structure determination revealed that for the same ligand isomorphous structures are formed in the pairs 2a and 2b (space group P21/c) and 3a and 3b (R3̅). However, the structures of 1a and 1b were initially determined using crystals obtained from different solvents and thus showed different symmetries (hexagonal P63 and cubic P213, respectively). Subsequently, we were able to recrystallize 1a from the mixture of EtOH and CH2Cl2, and the crystal structure of 1a·3CH2Cl2 turned out to be isomorphous to that of 1b·3CH2Cl2 (space E


Article

Inorganic Chemistry

Table 2. Selected Bond Lengths and Angular Distortion Parameter Σ90 in the Crystal Structures of 1a, 1b, 2a, 2b, 3a, and 3b complex

1a′

temperature, K

110

Fe−N1a Fe−N2 Fe−N3 Fe−N4 Fe−N5 Fe−N6 d(Fe−N)average Σ90(N−Fe−N) (deg) complex

a

1a″ 230

1.926(8) 1.95(1) 1.963(7) 1.97(1) 1.986(7) 1.995(6) 1.965(5) 61.1(3) 2a

1.957(6) 1.980(6) 1.981(8) 1.983(6) 1.991(6) 1.997(8) 1.982(7) 64.2(2)

temperature, K

110

230

Fe−N1a Fe−N2 Fe−N3 Fe−N4 Fe−N5 Fe−N6 d(Fe−N)average Σ90(N−Fe−N) (deg)

1.963(4) 1.972(4) 1.978(4) 1.980(4) 1.983(4) 2.000(4) 1.979(4) 64.1(2)

2.111(5) 2.115(5) 2.116(5) 2.121(5) 2.125(5) 2.127(5) 2.119(5) 93.6(2)

1b″

110 Bond Lengths (Å) 2.034(6) 2.034(6) 2.034(6) 2.037(7) 2.037(7) 2.037(7) 2.036(7) 70.8(3) 2b

110

230

Bond Lengths (Å) 1.964(5) 2.084(6) 1.970(4) 2.095(6) 1.975(4) 2.099(6) 1.976(4) 2.099(6) 1.984(4) 2.100(6) 2.001(4) 2.111(6) 1.978(4) 2.098(6) 61.6(2) 85.5(2)

230

100

230

2.150(5) 2.150(5) 2.150(5) 2.160(5) 2.160(5) 2.160(5) 2.155(5) 95.1(2) 3a

2.021(4) 2.021(4) 2.021(4) 2.025(4) 2.025(4) 2.025(4) 2.023(4) 69.3(2)

2.151(6) 2.151(6) 2.151(6) 2.156(6) 2.156(6) 2.156(6) 2.154(6) 96.6(2) 3b

110

230

100

230

2.101(1) 2.101(1) 2.101(1) 2.110(1) 2.110(1) 2.110(1) 2.106(1) 87.8(5)

2.177(2) 2.177(2) 2.177(2) 2.178(2) 2.178(2) 2.178(2) 2.178(2) 99.5(6)

2.106(3) 2.106(3) 2.106(3) 2.118(4) 2.118(4) 2.118(4) 2.112(4) 89.1(1)

2.179(3) 2.179(3) 2.179(2) 2.179(3) 2.179(2) 2.179(2) 2.179(2) 101.2(1)

The N atoms have been labeled arbitrarily, in the order of increasing Fe−N bond lengths.

Figure 2. Crystal packing of 1a′: side view of the layers of cations parallel to the ab plane (a) and top view of a fragment of one cationic layer, highlighting the intermolecular π−π and σ−π interactions (b). The anions and solvent molecules have been omitted for the sake of clarity. Color scheme: Fe = red, N = blue, C = gray, H = off-white.

reasonably well with the total number of electrons calculated for the mixture of interstitial CH2Cl2 and EtOH molecules (Table 1) as determined by elemental analysis on the same batch of crystalline material, from which the single crystals were selected for the X-ray studies. Table 2 provides the specific and average Fe−N bond lengths for all complexes at different temperatures, as well as the values of the angular distortion parameter (Σ90) calculated as the sum of deviations of the 12 cis-N−Fe−N bond angles from the ideal octahedral value of 90°. The changes in the bond lengths and Σ90 are diagnostic of the changes in the spin state at the transition metal center. The conversion from the LS to the HS state causes the elongation of the Fe−N bonds due to the population of the antibonding orbitals on the FeII ion and the increase in the distortion parameter Σ90 due to the smaller N− Fe−N angles in the HS state.5,20

Complexes 1a and 1b were obtained as either MeOH or CH2Cl2 solvates (Table 1). For the convenience of the following discussion, we will label the complex obtained from MeOH as 1a′ and those recrystallized from EtOH/CH2Cl2 as 1a″ and 1b″. The Fe−N bond lengths observed for the hexagonal structure of 1a′ at 110 and 230 K are typical of the LS FeII ion (Table 2), although a slight increase in both d(Fe− N)average and Σ90 values for the higher-temperature structure might indicate an onset of the LS → HS conversion. In contrast, complexes 1a″ and 1b″ exhibit substantial changes in their structural parameters between the low- and hightemperature structures. The increases in d(Fe−N)average from ∼2.03 to ∼2.15 Å and in Σ90 from ∼70 to ∼95° give a clear indication of the temperature-driven SCO taking place in these complexes, with a substantial HS fraction still being present at 110 K. The structural parameters of complexes 2a and 2b also change substantially with temperature (Table 2). Complexes 3a F


Article

Inorganic Chemistry

ring centroid is 3.63 Å for 2a and 3.64 Å for 2b. These are the only significant cation−cation interactions found within the layer, while the interlayer interactions are much weaker. Similar to 2a and 2b, the structures of complexes 3a and 3b are isomorphous, although the higher rhombohedral symmetry is observed as compared to the monoclinic symmetry of 2a and 2b. In the crystal packings of 3a and 3b, each [Fe(L3)3]2+ cation interacts with three neighboring cations by forming intermolecular σ−π interactions between the methoxy groups and the biimidazole units and π−π interactions between the xylene fragments of L3 ligands (Figure 5a). In 3a, the spacing between the carbon atom of the methoxy group and the centroid of the biimidazole ring is 3.60 Å, while the interplanar spacing between the xylene rings is 3.55 Å. Similar intermolecular distances are observed in 3b. Such intermolecular interactions create an overall van-der-Waals connected three-dimensional network of the [Fe(L3)3]2+ cations, with the anions and solvent molecules occupying large voids inside the network (Figure 5b). These interstitial species are heavily disordered, which necessitated the use of the SQUEEZE procedure in the crystal structure refinement. It is interesting that the extensive intermolecular σ−π and π−π interactions observed in the crystal structures of 3a and 3b do not lead to a more cooperative SCO behavior, as will be shown below. In contrast, the weaker interactions observed in the crystal structures of 2a and 2b make their SCO behavior comparable to that of 1a′, which nevertheless exhibits more efficient crystal packing. We will discuss this point further after presenting magnetic properties of these complexes. Magnetic Properties. Magnetic measurements performed on polycrystalline samples revealed that all complexes tend to exist in the HS state at elevated temperatures, where the product of magnetic susceptibility and temperature (χT) approaches the values of 3.2−3.5 emu·K/mol, typically observed for the HS Fe(II) ion (Figure 6).22 Complexes 1a and 1b were obtained either from MeOH or from EtOH/CH2Cl2; thus, the resulting crystal structures contained different interstitial solvents. We indicate this difference by referring to these samples as 1a′/1b′ and 1a″/ 1b″, respectively. The change in the crystallization solvent leads to drastic differences in the crystal packing (Figures 2 and 3), which also affects dramatically the magnetic behavior (Figure 6a). In the case of 1a′ and 1b′, a complete and gradual SCO is observed, with the HS and LS fractions being equal (γHS = γLS = 0.5) at T1/2 = 313 and 312 K, respectively. In contrast, for 1a″ and 1b″ we observe a much more gradual and incomplete SCO. The χT curves tend to a plateau at around 50−70 K but continue to decrease at lower temperatures due to zero-field splitting effects. The plateau-like region indicates kinetic trapping of the residual HS fraction at γHS ≈ 0.38 for 1a″ and 0.31 for 1b″. Such behavior, which is common for complexes with gradual SCO that extends into the 50−70 K temperature range, is explained by the lack of sufficiently energetic elastic interactions to propagate the HS → LS conversion below certain temperature.23−25 The difference in the magnetic behavior of the solvates of 1a and 1b reiterates the sensitivity of SCO to crystal packing effects that might depend on synthetic and crystallization methods.26−28 Complex 2a and 2b exhibit a gradual and complete SCO (Figure 6b), similar to the behavior observed for 1a′ and 1b′. The T1/2 values are equal to 233 K for 2a and 263 K for 2b. The SCO behavior observed for 3a and 3b is much more gradual and incomplete (Figure 6c), thus resembling the

and 3b, in contrast, exhibit Fe−N bond lengths typical of the HS state at 230 K. The values observed compare well to those reported for the HS complex [Fe(bim)3](SO4) (2.191−2.205 Å).21 At 100 K, the metric parameters observed for 3a and 3b are intermediate between those expected for the HS and LS FeII ions. Thus, the comparison of the d(Fe−N) and Σ90 values for all six complexes indicates the occurrence of SCO, which takes place at lower temperatures in the case of complexes 3a and 3b. This observation suggests that the change in the peripheral structure of the N-alkylated biimidazole ligands might have substantial impact on the SCO behavior by modifying the crystal packing of the complexes. The crystal structure of 1a′ exhibits drastically different packing motif as compared to those in the structures of 1a″ and 1b″. In the structure of 1a′, the [Fe(L1)3]2+ cations are packed in layers parallel to the ab plane, and the layers are completely separated from each other by BF4− anions and solvent molecules which fill the interlayer space (Figure 2a). Weak van der Waals contacts, namely, π−π interactions between biimidazole and xylene fragments and σ−π interactions between xylene fragments of adjacent cations within each layer (Figure 2b), allow for weak elastic coupling between SCO centers in the crystal lattice. In the structures of 1a″ and 1b″, however, the anions and solvent molecules are contained within small cavities between [Fe(L1)3]2+ cations so that the intermolecular cation−cation contacts are “diluted” by the interstitial species (Figure 3). Such arrangement leads to much less cooperative SCO behavior, as will be shown below.

Figure 3. View of crystal packing of 1a″ down the b axis showing the solvent molecules contained within van der Waals framework of the SCO cations. The BF4− anions have been omitted for clarity. Color scheme: Fe = red, Cl = yellow, N = blue, C = gray, H = off-white.

Complexes 2a and 2b are isomorphous and reveal crystal packing with layered arrangement of the [Fe(L2)3]2+ cations (Figure 4a). An important feature in their crystal structures is intermolecular interactions between the peripheral Br atoms and aromatic xylene rings of L2 ligands on adjacent [Fe(L2)3]2+ cations (Figure 4b). The distance between the Br atom and the G


Article

Inorganic Chemistry

Figure 4. Intermolecular Br−π interactions (a) and crystal packing viewed down the a axis (b) in the structure of 2a. Hydrogen atoms have been omitted for clarity. Color scheme: Fe = red, Br = yellow, N = blue, C = gray, H = off-white.

Figure 5. Important intermolecular interactions (a) and crystal packing view showing the large voids (b) in the supramolecular cationic framework of 3a. Color scheme: Fe = red, N = blue, C = gray, H = off-white, O = bright-red.

Figure 6. Temperature dependences of the χT product for polycrystalline samples 1a′, 1a″, 1b′, and 1b″ (a), 2a and 2b (b), and 3a and 3b (c). All measurements were performed in the cooling mode at the scan rate of 2 K/min.

clearly reveals differences in the SCO behavior in the series 1a− 3a and 1b−3b. It appears that the T1/2 values tend to decrease as the size of the peripheral fragment on the N-alkylated bim derivative becomes larger, but the transitions also become more gradual when larger lattice voids are formed between the SCO cations. Both these factors contribute to the decrease in the

behavior of 1a″ and 1b″. The residual HS fraction can be estimated from the low-temperature plateau as 0.2 for 3a and 0.5 for 3b. To better visualize the comparison of the ligand and crystal-packing dependent SCO behavior in these complexes, we also represent the SCO as temperature-dependent γHS curves (Figure 7). An examination of these dependences H


Article

Inorganic Chemistry

Figure 7. Temperature dependences of the χT product for 1a′, 2a, and 3a (a) and 1b′, 2b, and 3b (b).

step approach which uses different levels of DFT to optimize the geometry of molecular species at a lower level of theory with certain approximations, then to improve the optimized geometries with a more accurate theory, and finally to calculate single-point energies for these geometries by using a higherlevel DFT method. More specifically, the initial geometry optimization was performed with the PBEh-3c functional.18 At the second stage, a more rigorous optimization procedure was carried out, using the PBE0 functional, with dispersion corrections and tight SCF convergence criteria. This was the most time-consuming step in our calculations. Finally, the single-point E°HS and E°LS values were calculated using the B2PLYPD functional, which would be prohibitively expensive to be used at the geometry optimization step. The DFT calculations resulted in the ΔE°HL values of 3.92 kcal/mol for [Fe(bim)3]2+ and −1.73 kcal/mol for [Fe(L1)3]2+, thus indicating the preference of these cationic complexes for the LS and HS states, respectively. These results are in excellent agreement with the experimental findings, according to which the complexes of [Fe(bim)3]2+ exist only in the HS state,7 while the complexes of [Fe(L1)3]2+, i.e., compounds 1a and 1b, show temperature-induced SCO, as shown in the present work. Moreover, increasing the cooperativity of interactions between the [Fe(L1)3]2+ cations in the solid state stabilizes the LS state and sharpens the SCO curve, moving the T1/2 value to the room temperature (Figure 6a). Concluding Remarks. According to the recently formulated empirical rule,5 N-alkylation of 2,2′-biimidazole with the formation of a strained eight-member ring should convert this relatively weak-field ligand to a significantly stronger ligand that is expected to furnish homoleptic Fe(II) complexes with SCO behavior. The experimental study reported herein is in excellent agreement with this prediction. A series of homoleptic Fe(II) complexes prepared with three different alkylated derivatives of bim shows consistent SCO properties, although the temperature and abruptness of SCO is strongly dependent on the crystal packing motifs. We find that complexes with smaller Nalkylating substituents can afford more effective molecular packing of the SCO cations which leads to more cooperative spin-state conversion manifested by more abrupt SCO curves centered around room temperature. This effect is clearly demonstrated by the decreasing temperature and lesser abruptness of SCO in the 1a′ → 2a → 3a and 1b′ → 2b → 3b series. At the same time, the cooperativity of the spin-state conversion can be dramatically diminished in the case of crystal

cooperativity of the spin-state conversion and stabilization of the HS state relative to the LS state. It also should be pointed out that the χT curves measured for polycrystalline complexes 1a′, 1b′, 2a, and 2b coincided when measured in the heating and cooling modes. While the structures determined from single-crystal X-ray diffraction experiments showed the presence of a substantial amount of interstitial solvent, the elemental analysis of microcrystalline samples showed that most of the solvent has been lost (see the Materials and Methods section). Furthermore, the TGA analysis revealed that these samples exhibit negligible mass loss and remain thermally stable above 400 K (Figures S5 and S6), the highest temperature of magnetic measurements. These observations justify the reversible magnetic behavior observed. It also could be of interest to study the magnetic behavior of these complexes in solution, where the cooperative effects are completely eliminated, or under mother liquor that prevents the loss of interstitial solvent. The latter method, however, typically leads to decreased cooperativity as the SCO cations only become more separated in the crystal lattice, thus weakening intermolecular interactions between SCO centers. Theoretical Studies. To gain further insight into the emergence of SCO in homoleptic Fe(II) complexes with alkylated and nonalkylated 2,2′-biimidazoles, density-functional theory (DFT) calculations were performed on both HS and LS configurations of the [Fe(bim)3]2+ and [Fe(L1)3]2+ cations. The analysis of the spin-state energetics was performed by evaluating the difference between the zero-point energies of the HS and LS states: ΔE°HL = E°HS − E°LS. Ideally, the negative and positive values of ΔE°HL should indicate the preference for the HS and LS states, respectively, but accurate evaluation of the zero-point energy differences between the HS and LS states in SCO complexes is challenging. In particular, when using DFT methods, the calculated value of ΔE°HL strongly depends on the functional used.29−31 Nevertheless, the majority of modern functionals can provide a reliable estimate of the change in the ΔE°HL value as a function of the coordination environment of the metal ion.32 The use of double-hybrid functionals is the preferred method to give the most accurate insight into the likelihood of SCO in Fe(II) complexes.30,33−35 The development of higher-level approaches to predict correctly the spin state of a particular Fe(II) complex can lead to extremely high-cost computations. To circumnavigate this problem and attain reasonably accurate theoretical results without a substantial loss of accuracy, we developed a threeI


Article

Inorganic Chemistry

(5) Phan, H.; Hrudka, J. J.; Igimbayeva, D.; Lawson Daku, L. M.; Shatruk, M. A simple approach for predicting the spin state of homoleptic Fe(II) tris-diimine complexes. J. Am. Chem. Soc. 2017, 139, 6437−6447. (6) Phan, H. V.; Chakraborty, P.; Chen, M. M.; Calm, Y. M.; Kovnir, K.; Keniley, L. K.; Hoyt, J. M.; Knowles, E. S.; Besnard, C.; Meisel, M. W.; Hauser, A.; Achim, C.; Shatruk, M. Heteroleptic FeII complexes of 2,2′-biimidazole and its alkylated derivatives: spin-crossover and photomagnetic behavior. Chem. - Eur. J. 2012, 18, 15805−15815. (7) Abushamleh, A. S.; Goodwin, H. A. Coordination of 2,2′biimidazole with iron, cobalt, nickel and copper. Aust. J. Chem. 1979, 32, 513−518. (8) Xiao, J. C.; Shreeve, J. M. Synthesis of 2,2′-biimidazolium-based ionic liquids: use as a new reaction medium and ligand for palladiumcatalyzed Suzuki cross-coupling reactions. J. Org. Chem. 2005, 70, 3072−3078. (9) Thummel, R. P.; Goulle, V.; Chen, B. Bridged derivatives of 2,2′biimidazole. J. Org. Chem. 1989, 54, 3057−3061. (10) Rivera, J. M.; Martin, T.; Rebek, J. Chiral softballs: synthesis and molecular recognition properties. J. Am. Chem. Soc. 2001, 123, 5213− 5220. (11) Mazzini, F.; Galli, F.; Salvadori, P. Vitamin E metabolites: synthesis of [D2]- and [D3]-γ-CEHC. Eur. J. Org. Chem. 2006, 2006, 5588−5593. (12) Bain, G. A.; Berry, J. F. Diamagnetic corrections and Pascal’s constants. J. Chem. Educ. 2008, 85, 532−536. (13) SMART and SAINT; Bruker AXS Inc.: Madison, WI, 2007. (14) Sheldrick, G. M. SADABS. University of Gottingen: Gottingen, Germany, 1996. (15) Sheldrick, G. M. XPREP. Space group determination and reciprocal space plots; Siemens Analytical X-ray Instruments: Madison, WI, 1991. (16) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (17) Van der Sluis, P.; Spek, A. L. BYPASS: an effective method for the refinement of crystal structures containing disordered solvent regions. Acta Crystallogr., Sect. A: Found. Crystallogr. 1990, 46, 194− 201. (18) Grimme, S.; Brandenburg, J. G.; Bannwarth, C.; Hansen, A. Consistent structures and interactions by density functional theory with small atomic orbital basis sets. J. Chem. Phys. 2015, 143, 054107. (19) Neese, F.; Wennmohs, F.; Hansen, A.; Becker, U. Efficient, approximate and parallel Hartree-Fock and hybrid DFT calculations. A ’chain-of-spheres’ algorithm for the Hartree-Fock exchange. Chem. Phys. 2009, 356, 98−109. (20) Guionneau, P.; Marchivie, M.; Bravic, G.; Létard, J. F.; Chasseau, D. Structural aspects of spin crossover. Example of the [FeIILn(NCS)2] complexes. Top. Curr. Chem. 2004, 234, 97−128. (21) Tan, Y. H.; Wu, J. J.; Zhou, H. Y.; Yang, L. F.; Ye, B. H. Anion and pH induced spontaneous resolution of Δ- and Λ-[M(H2Biim)3]SO4 (M = Ru2+, Co2+, Ni2+, Mn2+, Fe2+, and Zn2+) enantiomers. CrystEngComm 2012, 14, 8117−8123. (22) Shatruk, M.; Dragulescu-Andrasi, A.; Chambers, K. E.; Stoian, S. A.; Bominaar, E. L.; Achim, C.; Dunbar, K. R. Properties of Prussian blue materials manifested in molecular complexes: observation of cyanide linkage isomerism and spin-crossover behavior in pentanuclear cyanide clusters. J. Am. Chem. Soc. 2007, 129, 6104−6116. (23) Chakraborty, P.; Enachescu, C.; Walder, C.; Bronisz, R.; Hauser, A. Thermal and light-induced spin switching dynamics in the 2D coordination network of {[Zn1‑xFex(bbtr)3](ClO4)2}∞: the role of cooperative effects. Inorg. Chem. 2012, 51, 9714−9722. (24) Arroyave, A.; Lennartson, A.; Dragulescu-Andrasi, A.; Pedersen, K. S.; Piligkos, S.; Stoian, S. A.; Greer, S. M.; Pak, C.; Hietsoi, O.; Phan, H.; Hill, S.; McKenzie, C. J.; Shatruk, M. Spin crossover in Fe(II) complexes with N4S2 coordination. Inorg. Chem. 2016, 55, 5904−5913. (25) Lennartson, A.; Southon, P.; Sciortino, N. F.; Kepert, C. J.; Frandsen, C.; Morup, S.; Piligkos, S.; McKenzie, C. J. Reversible guest

packing that leads to inefficient intermolecular interactions between the SCO cations. Thus, the lack of extensive intercationic contacts in the structures of 1a″ and 1b″ leads to much less abrupt and incomplete SCO as compared to the behavior of different solvates, 1a′ and 1b′, obtained with the same N-alkylated derivative of bim. The observed increase in the ligand-field strength of the strained N-alkylated bim derivatives relative to the parent bim ligand is well-justified by the theoretical calculations on the homoleptic Fe(II) complexes. The three-stage approach to the calculation of the zero-point energy differences between the HS and LS states, as described in this work, offers a cost-effective yet sufficiently accurate strategy for evaluating trends in the spin-state energetics and ligand-field splitting for a series of related transition metal complexes.

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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.8b00223. 1 H and 13C NMR spectra of ligands L2 and L3; TGA curves of complexes (PDF) Accession Codes

CCDC 1556122−1556133 and 1813100−1813101 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]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Jeff Lengyel: 0000-0002-5053-6263 Michael Shatruk: 0000-0002-2883-4694 Present Address

H.P.: Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The National Science Foundation is gratefully acknowledged for the support of this research via the Award CHE-1464955 to M.S.

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REFERENCES

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