Structural Characterization of Thermochromic and Spin Equilibria in

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Structural Characterization of Thermochromic and Spin Equilibria in Solid-State Ni(detu)4Cl2 (detu = N,N′‑Diethylthiourea) Ibrahim A. Alfurayj,† Victor G. Young, Jr.,‡ and Michael P. Jensen*,† †

Department of Chemistry and Biochemistry, Ohio University, Athens, Ohio 45701, United States X-ray Crystallographic Facility, Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, United States

‡

S Supporting Information *

ABSTRACT: Consecutive thermochromic lattice distortional and spin crossover equilibria in solid-state Ni(detu)4Cl2 (detu = N,N′-diethylthiourea) are investigated by variable-temperature X-ray crystallography (173−333 K), DFT calculations, and differential scanning calorimetry. Thermochromism and anomalous magnetism were reported previously (S. L. Holt, Jr., et al. J. Am. Chem. Soc. 1964, 86, 519−520); the latter was attributed to equilibration of a singlet ground state and a thermally accessible triplet state, but structural data were not obtained. A crystal structure at 173(2) K revealed [Ni(detu)4]2+ centers with distorted planar ligation of nickel(II) to the four sulfur atoms, with an average Ni−S bond length of 2.226(3) Å. The nickel ion was displaced out-of-plane by 0.334 Å toward a proximal apical chloride at a nonbonding distance of 3.134(1) Å. Asymmetry in the trans S−Ni−S angles was coupled to a monoclinic ↔ tetragonal lattice distortion (T1/2 = 254 ± 11 K), resulting in thermochromism. Spin crossover occurs by tetragonal modulation of nickel(II) with approach of the proximal chloride at higher temperatures (T1/2 = 383 ± 18 K), which is consistent with a contraction of −0.096(4) Å in the Ni···Cl separation observed at 293 K. A high-spin (S = 1) square-pyramidal [Ni(dmtu)4Cl]+ model (dmtu = N,N′-dimethylthiourea) was optimized by DFT calculations, which estimated limiting equatorial Ni−S bond lengths of 2.45 Å and an apical Ni−Cl bond of 2.43 Å. Electronic spectra of the spin isomers were calculated by TD-DFT methods. Assignment of the FTIR spectrum was assisted by frequency calculations and isotope substitution.

I. INTRODUCTION Spin crossover is defined as the phenomenon wherein a molecule can exist in more than one physical state with differing electronic configurations accessible by external stimuli such as heat, light, or pressure.1 For example, many bistable iron(II) complexes can adopt metastable low-spin (1A1g, t2g6eg0, S = 0) and high-spin (5T2g, t2g4eg2, S = 2) states connected by abrupt spin transitions with thermal hysteresis and photochemical switching,1 which may be exploited for nanoscale computing and display devices.2 Spin crossover also raises fundamental questions with respect to chemical bonding and reactivity.3−5 This phenomenon is usually associated with octahedral complexes of first-row transition metals (i.e., 3d4− d7), in which promotion of paired electrons from σ-nonbonding t2g orbitals to σ-antibonding eg orbitals leads to isotropic ligand field expansion and increased electron spin (Scheme 1, left).1 In addition, the relative energy of the t2g orbitals can be strongly affected by π overlap with the ligands. Spin crossover is also possible for complexes of the 3d8 nickel(II) ion (Scheme 1, right), which involves tetragonal distortion of an intrinsically paramagnetic octahedral geometry (3A2g, t2g6eg2, S = 1) toward a diamagnetic, square-planar conformation (1A1g, b2g2eg4a1g2b1g0 under ideal D4h symmetry, S = 0),6−18 as demonstrated historically by the so-called Lifschitz complexes.15−18 A triplet (S = 1, 3B1g) ground state can be © XXXX American Chemical Society

retained in a square-planar geometry under reduced tetragonality arising from a sufficiently weak equatorial ligand field.6,7,19,20 The tetragonal distortion otherwise stabilizes the axial dσ* orbital (dz2, a1g) to the extent that promotion of an electron into the higher equatorial dσ* orbital (dx2−y2, b1g) becomes unfavorable. The resulting spin crossover (3B1g ↔ 1 A1g) is coincident with contraction of the equatorial coordinate bonds as well as expansion of the axial bonds, to the point that either or both axial ligands are displaced from the primary coordination sphere. In addition to the anisotropic ligand field rearrangement, the spin crossover of nickel(II) is also distinctive because the remaining orbitals of t2g parentage remain filled and stabilized. Spin crossover of nickel(II) is otherwise obtained by divergent distortion of a diamagnetic square-planar complex toward an intrinsically paramagnetic tetrahedral “allogon” (3T1, e4t24, S = 1).10,21 An inherent challenge to elaboration of spin crossover for nickel(II) is the requisite change in coordination number and/ or ligand field geometry within a solid-state matrix in order to obtain an effectively unimolecular spin equilibrium akin to the classical definition (Scheme 1). Nonetheless, a few examples are known. In one notable instance, a spin equilibrium was Received: September 24, 2015

A

DOI: 10.1021/acs.inorgchem.5b02203 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1

observed for a planar macrocyclic nickel(II) complex after intercalation into a preorganized aromatic host, which destabilized the nonbonding axial lone pair through dipolar interaction.22 Another example is the complex [(Me5C5)Ni(acac)] (acac = acetylacetonato), which exhibits a diamagnetic ground state and a thermally populated triplet state, as observed by solid-state susceptibilities and in solution by paramagnetic shifting of 1H NMR resonances.23 An X-ray structure performed at 170 K revealed that the diamagnetic ground state is obtained by ene−allyl distortion of the cyclopentadienyl ring. Analogous spin equilibria were demonstrated in solution for the [(Cp)Ni(SR)2] (Cp = C5H5) sites of sulfur-bridged bimetallic complexes [(Cp)Ni{μ-P(S)R′2}2Ni(Cp)] (R′ = Me, OMe) and [(Cp)2Mo(μ-StBu)2Ni(Cp)],24,25 and for isoelectronic [(Cp)Co(L)(CO)].26 We subsequently reported a unimolecular solid-state spin equilibrium for [(κ3-TpPh,Me)NiS2CNMe2]27 (TpPh,Me = hydrotris{pyrazol-1-yl}borate),28 which involves modulation of the apical Ni−N bond length in a square-pyramidal geometry. This complex was intended to model the N3S2 active site ligand field of the nickel-dependent superoxide dismutase enzyme (NiSOD), for which spin crossover involving tetragonal modulation by an axial His-1 imidazole donor has been suggested.29,30 In another example, a terminal nickel(II) in a trimetallic chain complex was found to exhibit spin crossover by tetragonal interaction with a single axially disposed chloride or hexafluorophosphate ion.31 An early review by Barefield et al.10 highlighted five other examples of nickel(II) complexes that exhibit “anomalous magnetism”, particularly temperature-dependent solid-state susceptibilities attributed to thermal spin equilibria.32−42 None of the relevant spin isomers were structurally characterized, although X-ray structures of derivative complexes were consistent with tetragonal distortion as a basis for spin equilibria (Scheme 1).43−53 Our particular attention was drawn to a family of N,N′-dialkylthiourea halide complexes Ni(R2tu)4X2 that display spin equilibria over a range of temperatures depending on the ligand substituents (R = nBu < Et < Me) and the halide (X = Cl < Br < I).39−42 The lead complex of this type, Ni(detu)4Cl2 was reported by Holt, Bouchard, and Carlin as the prototypical example of solid-state spin crossover for nickel(II), with T1/2 = 383 K (Figure 1).39 Spin crossover in response to applied pressure was also reported for Ni(detu)4X2 in 1990 by Bray and Drickamer.40

Figure 1. van’t Hoff plot of the temperature-dependent magnetic susceptibility data previously reported by S. L. Holt, Jr., et al. for the spin equilibrium of Ni(detu)4Cl2.39

However, the structural basis of the anomalous magnetism in Ni(detu)4Cl2 remains to be elucidated. We accordingly report herein further investigation of the anomalous magnetism of Ni(detu)4Cl2, using differential scanning calorimetry, variable-temperature X-ray crystallography, and DFT calculations. Our results are consistent with a spin equilibrium that results from axial interaction of squareplanar [Ni(detu)4]2+ with a single proximal chloride ion. We also assigned electronic and vibrational spectra of Ni(detu)4Cl2, and identified a low-temperature lattice distortion that induces the previously reported thermochromism of the low-spin isomer.39

II. EXPERIMENTAL SECTION All materials were obtained from commercial vendors as ACS reagentgrade or better and used as received. FTIR spectra were recorded on a Thermo-Electron Nicolet 380 spectrophotometer. A UV−vis−NIR spectrum was recorded on an Agilent HP-8453 spectrophotometer. TGA and DSC data were obtained using TA Instruments SDT Q600 and Q2000, respectively. Data plotting and least-squares regressions were done with SigmaPlot 8.02.54 Analytical data were obtained by Atlantic Microlabs, Inc. (Norcross, GA). Ni(detu)4Cl2 was obtained according to literature syntheses.39−42,55,56 Under ordinary air at room temperature, N,N′B


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Inorganic Chemistry Table 1. Summary of the X-ray Crystallography for Ni(detu)4Cl2 radiation λ (Å) cryst syst space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z density (calcd, g cm−3) abs coeff (mm−1) reflns collected indep reflns (Rint) obsd reflns data/restraints/params GoF R1, wR2 (I > 2σ{I}) R1, wR2 (all data) diff peak, hole (e Å−3)

173(2) K

223(2) K

253(2) K

293(2) K

333(2) K

Mo Kα 0.71073 monoclinic P21/n 10.377(2) 29.722(6) 10.547(2) 90.405(2) 3253(1) 4 1.345 1.042 25784 6636 (0.0367) 5640 6636/238/348 1.275 0.0612, 0.1247 0.0721, 0.1281 0.760, −1.014

Cu Kα 1.54178 monoclinic P21/n 10.4078(4) 29.763(1) 10.5579(4) 90.308(3) 3270.4(2) 4 1.337 4.933 28868 6515 (0.0795) 4658 6515/238/348 1.042 0.0768, 0.1473 0.1057, 0.1614 0.704, −0.479



Cu Kα 1.54178 monoclinic P21/n 10.6070(7) 29.988(2) 10.6112(7) 90.100(4) 3375.2(5) 4 1.296

Table 2. Summary of Metrical Parameters for Nickel Coordination Sphere vs Temperature 173(2) K Ni1−S1 Ni1−S2 Ni1−S3 Ni1−S4 Ni−S (mean) Ni···(S)4 Ni1···Cl1 Ni1···Cl2

2.225(1) 2.229(1) 2.225(1) 2.223(1) 2.226(3) 0.334 3.134(1) 7.222(2)

S1−Ni1−S2 S2−Ni1−S3 S3−Ni1−S4 S4−Ni1−S1 S−Ni−S, cis (mean) S1−Ni1−S3 S2−Ni1−S4 S−Ni−S, trans (mean) Cl2···Ni1···Cl1

88.39(5) 87.51(5) 88.34(5) 90.70(5) 89(1) 160.05(5) 165.22(6) 162.6 177.94(3)

223(2) K bond lengths (Å) 2.221(2) 2.225(2) 2.221(2) 2.224(2) 2.223(2) 0.340 3.105(1) 7.212(2) bond angles (deg) 88.51(6) 87.66(6) 88.38(6) 90.19(6) 89(1) 160.23(7) 164.52(7) 162.4 178.29(3)

diethylthiourea (8.82 g, 66.7 mmol) was dissolved in n-butanol (100 mL), and NiCl2·6H2O (3.99 g, 16.8 mmol) was added to the solution with stirring. After 30 min, a green precipitate was recovered by filtration and recrystallized by vapor diffusion of diethyl ether into an ethanol solution. Yield: 6.26 g (9.51 mmol, 57.0%). Anal. Calcd (found) for C20H48Cl2N8NiS4: C, 36.48 (36.42); H, 7.35 (7.11); N, 17.02 (16.91); S, 19.48 (19.43). X - r ay C ry s t al l og ra p h y. A c r y s ta l o f N i ( d e t u ) 4 C l 2 (C20H48Cl2N8NiS4, 658.51 g mol−1) was placed on a glass capillary and mounted on a Bruker APEX-II CCD diffractometer.57 The data collection was carried out at 173(2) K using Mo Kα radiation and a graphite monochromator. Intensity data were corrected for absorption and decay (SADABS).58,59 Final cell constants were calculated from 2903 strong reflections from the actual data collection after integration (SAINT).60 The space group P21/n (No. 14) was determined on the basis of systematic absences and intensity statistics. Observed reflections were often split, suggestive of twinning; however, no twin law could be detected manually, or by CheckCIF.61 The structure was

253(2) K

293(2) K

2.224(3) 2.222(3) 2.223(3) 2.222(3) 2.223(1) 0.342 3.090(3) 7.212(3)

2.231(2) 2.232(2) 2.230(3) 2.232(2) 2.231(1) 0.349 3.038(3) 7.221(3)

88.6(1) 87.9(1) 88.6(1) 89.5(1) 88.7(7) 161.0(1) 163.5(1) 162.3 178.76(6)

88.6(1) 87.8(1) 88.7(1) 89.3(1) 88.6(6) 161.2(1) 162.8(1) 162.0 179.11(6)

solved by direct methods using SHELXS-97 and refined using SHELXL-2014.62,63 All non-hydrogen atoms were located using difference Fourier techniques and refined with anisotropic displacement parameters. All hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. A thermal ellipsoid plot and packing and space-filling diagrams were rendered using Mercury.64,65 Additional data sets were acquired at 223, 253, 293, and 333 K on a Bruker-AXS VENTURE PHOTON-100 diffractometer using Cu Kα radiation. The 173 K model was applied and refined as already described, except at 333 K where the data quality and incipient disorder of the ethyl groups were intractable. All five data collections are summarized in Table 1. Relevant metrical parameters for the nickel coordination sphere versus temperature are compiled in Table 2. Density Functional Theory. A simplified model [Ni(dmtu)4]Cl+ (dmtu = N,N′-dimethylthiourea) was derived from the crystal structure of Ni(detu)4Cl2. Geometry optimizations were carried out using the Amsterdam Density Functional software package (version C


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Inorganic Chemistry

Figure 2. Molar heat capacity of Ni(detu)4Cl2, determined by differential scanning calorimetry as a function of increasing temperature starting at 185 K. Principal features are labeled as discussed in the text. Left inset shows the thermogram for the initial and second sweeps of the sample melt. Right inset shows color photos of a solid sample of Ni(detu)4Cl2 at 77 K (left) and at 295 K (right), with centerpoint of the equilibrium marked with a vertical arrow. Temperatures of X-ray data sets are denoted by red arrows. Approximate baselines (red) are empirically derived and intended for reference purposes only.

Scheme 2

Figure 3. Thermal ellipsoid plots (50% probability) for [Ni(detu)4]Cl2 at 173(2) K, showing views roughly parallel (left; the axial Cl2 site is out-offrame low) and perpendicular (right) to the (S)4 plane. Except for the amide protons, hydrogen atoms are omitted for clarity. 2008.01)66,67 under C2 symmetrythe software does not support C4 symmetrywith diamagnetic (S = 0) and paramagnetic (S = 1) electron configurations. All calculations utilized default convergence parameters, frozen atomic cores, the Vosko−Wilk−Nusair LDA local exchange-correlation functional,68 the Becke−Perdew GGA corrections,69,70 and the TZP basis set provided in the ADF library. A solvent dielectric was also applied using the COSMO method with default parameters consistent with acetone.71 Optimizations of the Cs and C2v isomers of free dmtu were also performed, as listed in Tables S1 and S2. Optimized coordinates for the spin-isomeric models are given in Tables S3 and S4. Energies and isocontour plots of select frontier orbitals for the free ligand and the complex spin isomers are also shown in Supporting Information. Spin-allowed electronic excitations were calculated for the optimized complex models by time-dependent density functional theory (TD-DFT, Tables S5 and S6);72−74 frequency calculations were performed on C2v- and Cs-dmtu (Table S7).75

III. RESULTS AND DISCUSSION Solid Ni(detu)4Cl2 was obtained in good yield as previously described.39,41 The previously reported susceptibility data were fit as an ordinary equilibrium, assuming μLS = 0 and μHS = 3.2 μB.39 Our present analysis (Figure 1) again yielded ΔH° = 4.2(1) kcal mol−1 and ΔS° = 11.0(3) cal mol−1 K−1 (r2 = 0.996), which corresponds to T1/2 = 383 ± 18 K. Also consistent with the earlier report,39 the solid complex was thermochromic at lower temperatures, appearing as dark forest green at room temperature and changing to sky blue on cooling (Figure 2 and Figure S1). The structural origin of the thermochromism was also elucidated herein. Thermochemistry. We obtained differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) data on solid samples of Ni(detu)4Cl2. DSC measurements were initiated at 185 K and recorded on heating to 422 K (Figure 2). Two broad, weak endothermic features were observed, one below and the other above ambient temperature. D


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and the distal chloride adopted a comparable interaction with one amide on each of four adjacent complexes, with an average NH···Cl2′ separation of 2.54(4) Å. The unit cell at 173(2) K was nearly tetragonal, but definitely monoclinic (Figure S3). The equatorial (S)4 plane was disposed along the basal ac plane, but with a pronounced tilt along the trans S2−Ni1−S4 bond vector roughly parallel to the shorter a axis. Every distal chloride was surrounded by a nearly square array of four complexes; conversely, every complex was surrounded by four distal chloride anions. In-plane Ni···Cl2′ distances ranged from 7.482(2) to 7.581(2) Å (the shorter Ni··· Cl2 axial separation is perpendicular into an adjacent layer). With respect to the thermochromism, an informative comparison can be made to the previously reported structure of green [Ni(datu) 4 ]I2 49 (datu = N,N′-diallylthiourea; ALTUNO.cif).81,82 A tetragonal unit cell (P4/n, No. 85) with 4-fold crystallographic symmetry at nickel was reported at room temperature. A C4-symmetric [Ni(datu)4]2+ cation was found: symmetry-equivalent Ni−S bond lengths were 2.221(4) Å; cis and trans S−Ni−S angles were 88.1(1)° and 159.1(1)°, respectively; the nickel atom was displaced 0.40 Å out of the equatorial (S)4 plane toward a proximal axial iodide at a Ni···I distance of 3.74 Å; and the second distal iodide was disposed linearly from the opposite face at a Ni···I distance of 6.64 Å. This structure is analogous to [Ni(detu)4]Cl2; however, spin equilibria were not reported for Ni(datu)4X2 (X = Cl, Br, I).42 Of related interest is blue [Ni(dmtu)4]Br2·2H2O50 (dmtu = N,N′-dimethylthiourea; MTUNIB.cif),81,82 which is apparently distinct from the blue-green anhydride reported to exhibit spin crossover.41 The nickel(II) ion occupied a center of symmetry in a rigorously planar (S)4 ligand field with a well-separated water molecule and bromide. The symmetry-related Ni−S bond lengths of 2.204(2) and 2.230(2) Å were similar to those of [Ni(datu)4]I2 and [Ni(detu)4]Cl2 at 173(2) K, and thus indicative of diamagnetism. On the other hand, longer Ni−S bond lengths of 2.45−2.49 Å, as well as short axial Ni−Cl bond lengths of 2.40−2.52 Å, were found in hexacoordinate complexes trans-[Ni(L)4Cl2] (L = thiourea,51 SURNIC.cif;81,82 ethylenethiourea,52 NIETUR.cif;81,82 and trimethylenethiourea,53 MTHUNI.cif);81,82 triplet ground states were demonstrated for the ethylenethiourea (3.3 μB at 298 K)56 and thiourea complexes (3.2 μB at 295 K).83 Compared with the tetragonal symmetry of [Ni(datu)4]I2, a small monoclinic distortion is evident in the unit cell of [Ni(detu)4]Cl2 at 173(2) K: the β angle of 90.405(2)° is only slightly obtuse, and the basal a and c axes differ by only one percent from their average (Table 1). The relative contraction in the a axis is accommodated by the aforementioned tilting of the equatorial ligand plane with respect to the basal ac cell plane, with particular orthogonal displacement of the nearly parallel Ni1−S4 bond vector in the same direction as Cl1, whereas the perpendicular S1−Ni1−S3 angle along the c axis is compressed in the opposite direction. The [Ni(detu)4]2+ complex is thus distorted from C4 point symmetry, such that the trans S1−Ni1−S3 and S2−Ni1−S4 angles differ slightly, 160.05(5)° and 165.22(6)°, respectively. This ligand field distortion is coupled to the monoclinic lattice distortion, which together elicit the low-temperature thermochromism of [Ni(detu)4]Cl2. Four additional data sets were acquired for Ni(detu)4Cl2 between 223 and 333(2) K. The accuracy of the derived crystallographic model was reduced at higher temperatures, and further discussion and interpretation are accordingly restricted

Qualitative experiments showed that the lower temperature feature was coincident with the thermochromism (Figure S1). The second feature was coincident with the reported spin equilibrium (i.e., T1/2 = 383 ± 18 K, Figure 1)39 and was truncated as a sharp endothermic peak at 385 K. A strong, sharp melting endotherm initiated at 398 K and maximized at 411 K (Figure 2 inset). A complex exotherm was observed at 390 K in the cooling sweep. During a second heating sweep, all features just enumerated were absent and a new melt appeared at 408 K. We suggest that the sharp feature at 385 K results from collapse of residual diamagnetic [Ni(detu)4]Cl2 (ca. 50 mol %, Figure 1) to paramagnetic [Ni(detu)4Cl]Cl as the melting point is approached. Irreversible formation of intrinsically paramagnetic trans-[Ni(detu)4Cl2] in the melt (Scheme 2) would quench the spin equilibrium and shift the melting point in the second sweep. TGA data showed negligible mass loss (≤0.5%) to the high-temperature limit of the DSC experiment (Figure S2). Decomposition was observed between 430 and 600 K; the residual mass of 95 g mol−1 is consistent with formation of NiS (90.7 g mol−1) as the major product. A melting point of 415 K was previously reported for Ni(detu)4Cl2, with decomposition observed at 480 K.76 X-ray Crystallography. In order to span the DSC data, five X-ray data sets were collected on Ni(detu)4Cl2 between 173(2) and 333(2) K. The structure was solved initially using the optimal low-temperature data set (Figure 3). This revealed a [Ni(detu)4]2+ center with nonbonded axial chlorides. Four equatorial Ni−S bonds averaged to 2.226(3) Å, the four S− Ni−S cis angles to 89(1)°, and the two trans angles to 163(3)° (Table 2). The nickel atom was displaced 0.334 Å above the least-squares (S)4 plane toward a proximal chloride disposed directly over an axial site at a Ni1···Cl1 distance of 3.134(1) Å; this separation is equivalent to the sum of van der Waals radii for chloride (1.75 Å)77 and low-spin nickel(II) (1.35 Å),78 so a Ni−Cl covalent bond is not indicated. The second distal chloride was positioned on the opposing axial vector at a Ni1··· Cl2 separation of 7.222(2) Å and a Cl1···Ni···Cl2 angle of 177.94(3)°. Taken alone, the structure of a given detu ligand was unremarkable, but taken together, the ligands support solidstate packing relevant to the material properties. The N2CS core was essentially planar, and average Ni−S−C angles of 112(1)° and Ni−S−C−N torsions of 29(7)° indicated coordination primarily through a sulfur lone pair rather than the CS bond. The CS bond lengths averaged 1.738(2) Å, compared with 1.706(3) and 1.698(4) Å observed in two structures of free detu,79,80 which is consistent with an enhanced contribution from zwitterionic resonance structures upon coordination (Scheme 3); however, the C−N bond Scheme 3

lengths averaged 1.327(6) Å, compared to 1.32(1) Å for free detu. The ligands were folded into the cis,trans conformation also found in free detu.79,80 The trans amide N−H bond was poised inward over the equatorial (S)4 plane, supporting a 4fold hydrogen-bonding interaction with the apical chloride with an average NH···Cl1 distance of 2.47(4) Å; the cis amide N−H bond pointed outward into the surrounding equatorial space, E


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DSC (Figure 2) and the thermochromism (Figure S1). Although the monoclinic ↔ tetragonal distortion equilibrium is endothermic, the variable-temperature X-ray structures suggest that a key factor in the favorable entropy is the sidechain flexibility within the tightly packed equatorial layers of the crystal lattice (Figure S3). Of particular interest to the spin equilibrium is a contraction of the proximal Ni1···Cl1 separation with increasing temperature, −0.096(4) Å observed between 173(2) and 293(2) K (Table 2). The temperature-dependent Ni1···Cl1 separations were fit (Figure 4) to the previously reported spin equilibrium (Figure 1),39 using limiting values of 3.135 Å for the low-spin isomer (extrapolated from the 173(2) K crystal structure) and 2.434 Å for the high-spin isomer (estimated by a DFT calculation, vide inf ra). This result further illustrates the independence of the spin and monoclinic distortion equilibria (Scheme 2), and demonstrates that the structural basis for spin crossover is tetragonal modulation of nickel(II) by the proximal chloride. Offsetting increases are expected in the Ni−S bond lengths and the Ni···(S)4 displacement, but the predicted shifts are of lesser magnitude, +0.03 Å over the same temperature range (vide inf ra), such that the experimental observations were not statistically significant, +0.008(5) Å and +0.015 Å, respectively. Nonetheless, the observed expansion of the ac basal plane would accommodate the predicted inflation of the equatorial ligand field. Although the spin equilibrium is endothermic (Figures 1 and 2), factors likely contributing to a favorable entropy are the higher spin multiplicity and elongation of Ni−S bonds in the high-spin isomer.10,39 As previously reported,39−42 the temperature range of the spin crossover is strongly affected by the N,N′-dialkyl substituents (i.e., nBu < Et < Me). The detu ligands also support a pattern of N−H···Cl hydrogen bonding that controls the disposition of the critical proximal chloride in the spin equilibrium and frames the lattice structure through equatorial interactions with the distal chloride. However, with respect to the latter point, it is noteworthy that the spin crossover behaves as an ordinary equilibrium, with no evidence of intermolecular cooperativity. DFT Calculations. In the absence of additional data over the full temperature range of the spin equilibrium, which in any case is truncated by melting (Figure 2), DFT calculations were used to model the associated structural and spectroscopic changes. Geometry optimizations were first performed for free dmtu in C2v−cis,cis and Cs−cis,trans conformations; energies of the minimized structures differed by less than 0.05 eV in favor of the latter. Frontier orbitals for the free ligand are shown in Figure S4; these can be compared to a previous calculation,84 reported UV absorption spectra,85 and photoelectron spec-

to the unit cell parameters and the structure of the (S)4Ni···Cl heavy-atom core. The monoclinic cell and ligand field distortions were attenuated at higher temperatures, coincident with a striking expansion of the cell volume (Tables 1 and 2). The following trends were taken as a measure of the distortion from 4-fold symmetry: the difference in trans S−Ni−S angles; the difference in a and c axis lengths from their mean; and the obtuseness of the β angle (Figure 4). Each of these were fit by

Figure 4. van’t Hoff plots for the spin and monoclinic ↔ tetragonal distortional equilibria for Ni(detu)4Cl2. The distortional equilibrium (left vertical axis, color) is circumscribed by three parameters, each scaled to the monoclinic fraction (α): obtuseness of the unit cell β angle (blue); divergence of a and c axis lengths (green); and divergence of the S−Ni−S trans angles (red). The observed temperature-dependent Ni1···Cl1 separation (right vertical axis, black) is fit separately to the previously reported spin equilibrium (Figure 1).39

nonlinear least-squares sigmoidal fits, y = c1/[1 + e−(x−x0)/c2] (where, according to the van’t Hoff equation, c1 is the amplitude, c2 = R/ΔH°, x = 1/T, and x0 = 1/T1/2). The difference in trans S−Ni−S angles gave c1 = 5.4(4)°, ΔH° = 4.0(9) kcal mol−1, and ΔS° = 16(4) cal mol−1 K−1 (r2 = 0.99). The difference in the a and c axis lengths gave c1 = 8.3(6) × 10−3 Å, ΔH° = 6(1) kcal mol−1, and ΔS° = 23(7) cal mol−1 K−1 (r2 = 0.99). The obtuseness of the β angle gave c1 = 0.45(4)°, ΔH° = 2.5(5) kcal mol−1, and ΔS° = 10(3) cal mol−1 K−1 (r2 = 0.99). Although some artifactual compensation is evident in the derived thermodynamic parameters, the respective fits give a coincident T 1/2 = 254 ± 11 K (Figure 4), thereby demonstrating that the observed distortions from an ideal tetragonal unit cell and C4 complex point symmetry are indeed coupled as a single ordinary equilibrium. The T1/2 value also corresponds to both the low-temperature feature observed by

Figure 5. DFT-optimized C2-symmetric models for low-spin [Ni(dmtu)4]Cl+ (S = 0, left) and high-spin [Ni(dmtu)4Cl]+ (S = 1, right), and a leastsquares (S)4 overlay (center, with high-spin isomer in gray). Hydrogen atoms are excluded for clarity, with the exception of the trans-amide protons proximal to chloride. F


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Inorganic Chemistry

Figure 6. Selected Ni 3d frontier molecular orbitals for low-spin [Ni(dmtu)4]Cl+ (S = 0, left) and high-spin [Ni(dmtu)4Cl]+ (S = 1, right; β-spin only) models under C2 point symmetry.

parentage show significant π overlap with ligand SALCS composed primarily of the out-of-plane 3b2 orbitals localized on sulfur (Figure S4), such that the effectively degenerate pair of orbitals (39b, 40b: 68% Ni 3dxz + 3dyz, 21% S 3p) is found 0.2 eV below the HOMO and the fifth orbital (39a: 48% Ni 3dxy + 3dx2−y2, 38% S 3p) is 0.6 eV below the HOMO. Also noteworthy with respect to electronic spectroscopy are the ligand-centered π* orbitals (4b2, Figure S4), which are found 3.1−3.6 eV above the HOMO of the low-spin isomer (Figure S5, left). To a good approximation, spin crossover results from tetragonal modulation of the equatorial Ni−S and axial Ni−Cl σ* overlap (Scheme 1). In the high-spin isomer (Figure 6, right), the α-spin equatorial orbital (41a) is stabilized by expansion of the Ni−S bond lengths, such that it becomes the α-spin HOMO and falls 0.16 eV (1300 cm−1) below the incipient axial hole in the β-spin LUMO (40a); the latter is destabilized by enhanced Ni−Cl σ* interaction (70% Ni 3dz2, 10% Cl 3pz). The total energy of the high-spin model was also destabilized relative to the diamagnetic isomer by 0.36 eV (2900 cm−1). Comparison can be drawn to the observed unimolecular spin equilibrium (i.e., ΔH° = 1400 cm−1 and ΔG° = 560 cm−1 at 298 K, Figure 1),39 with the caveat that the computational model omits noncovalent interactions with the distal chloride counterion and with flanking complex cations. Electronic Spectroscopy (TD-DFT Calculations). Solidstate UV−vis−NIR reflectance spectra of Ni(detu)4Cl2 were reported previously.39−41 At 298 K, a ligand field band assigned to the low-spin isomer was observed at 14,900 cm−1 (λmax = 673 nm, with a weak shoulder at 743 nm). Consistent with the thermochromism, this feature shifted to 15,500 cm−1 at 77 K (647 nm, with a shoulder at 612 nm). The tail of near-UV charge transfer bands extended to 20,000 cm−1 (500 nm). The high-spin isomer was correlated with an intermediate 20,400 cm−1 (490 nm) absorption band appearing at higher temperature or applied pressure.39,40 We also obtained a transmission spectrum from a Nujol mull of Ni(detu)4Cl2 (Figure 7).

tra86,87 of N,N′-dialkyl-substituted thioureas. A simplified [Ni(dmtu)4]Cl+ model was derived from the low-temperature X-ray structure, and the geometry optimizations were performed for diamagnetic and paramagnetic electron configurations (S = 0 and 1, respectively); for technical reasons, the calculations were carried out under applied C2 point symmetry, but the models converged to nearly ideal C4 symmetries (Figure 5). Calculated energies and isocontour plots of the metal-centered frontier orbitals are illustrated for both spin isomers in Figure 6 (an expanded range is shown in Figure S5). Compared with ideal tetragonal symmetry (i.e., D4h, Scheme 1), the reduction in symmetry relevant to the complex models circumscribes the following: D4h → C4v, loss of horizontal mirror and inversion symmetry plus perpendicular 2-fold rotations (e.g., B2g →B2); C4v → C4, loss of vertical mirror symmetry (i.e., A1, A2 → A and B1, B2 → B); C4 → C2, loss of 4-fold rotation (i.e., B → A and E → 2 × B). The low-spin model showed little departure from the experimental structure. The nickel atom was disposed 0.33 Å above the (S)4 plane toward the axial chloride, with averaged S−Ni−S angles of 88.8° (cis) and 163.2° (trans), a Ni···Cl separation of 3.36 Å, and Ni−S bond lengths of 2.26 Å. In the high-spin model, displacement of the nickel atom from the (S)4 plane increased to 0.55 Å with reduced S−Ni−S angles of 87.1° (cis) and 153.9° (trans); a short axial Ni−Cl bond length of 2.43 Å and longer equatorial Ni−S bond lengths of 2.45 Å were also obtained. A constant averaged distance of 2.23 Å for the NH···Cl hydrogen-bonding interaction was maintained in both spin isomers. The ligand field splitting of the low-spin isomer shows strong tetragonal distortion typical of a square-planar complex (Figure 6, left). The nonbonding axial lone pair (40a: 83% Ni 3dz2, 7% Ni 4s, 4% Cl 3pz, 3% S 3p) is the HOMO, above which the equatorial LUMO (41a: 40% Ni 3dx2−y2 + 3dxy, 47% S 3p) is destabilized 1.3 eV by strong covalent σ* interaction with the dmtu ligands, primarily with in-plane 6b1 donor orbitals localized on sulfur (Figure S4). The remaining orbitals of t2g G


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Inorganic Chemistry

cm−1 (452 nm), MLCT bands appeared above 25,600 cm−1 (391 nm), and intraligand π → π* transitions began at 34,100 cm−1 (290 nm). Taken together, the computed bands of the low-spin isomer appear to agree well with the experimental spectrum reported at room temperature.39,40 The high-spin electron configuration imposes a low-energy β-spin hole (40a, Figure 6) that supports a second set of redshifted ligand field and LMCT bands (Table 3). The lowest ligand field transition (dxz,yz → dz2, 3B1g → 3Eg) was calculated accordingly at 7200 cm−1 (1380 nm); the analogous transition to the higher empty orbital (dxz,yz → dx2−y2, 3B1g → 3Eg) was calculated at 16,100 cm−1 (621 nm). The other donor orbital gives rise to transitions at 7900 cm−1 (1260 nm) and at 14500 cm−1 (688 nm) with weaker intensities (dxy → dz2, 3B1g → 3A2g {orbitally forbidden} and d xy → dx2 −y 2, 3 B1g → 3B2g , respectively). Relatively intense low-energy LMCT bands were calculated at 12,000 cm−1 (832 nm), 16,900 cm−1 (592 nm), and 21,100 cm−1 (473 nm). The high-spin isomer was correlated to an experimental band at 20,400 cm−1 (490 nm),39,40 which was assigned as the unresolved ligand field transitions to the higher acceptor orbital, and a low-energy shoulder at 12,000 cm−1 (833 nm), which was assigned as the analogous transitions to the lower acceptor orbital. 40 Calculations herein indicate alternate assignment of these features as LMCT transitions; however, the neglect of twoelectron ligand field excitations and the lack of an experimental spectrum dominated by the high-spin isomer preclude assessment of the computational accuracy in this instance.72 Vibrational Spectroscopy. FTIR spectra were obtained for solid Ni(detu)4Cl2 (Figure 8, Figure S8, and Table 4). Only a partial listing of four “significant” bands was reported previously.41 For comparative purposes, spectra were recorded for free detu at normal isotopic abundance (Figures S9 and S10) and after exchange of the amide protons with D2O (i.e., N,N′-d2-detu, Figures S11 and S12). The spectra were similar, because the metal−ligand stretching modes of the complex fell below the experimental range.88 However, attenuation of zwitterionic resonance structures in free detu shifted core

Figure 7. UV−vis−NIR transmission spectrum of solid Ni(detu)4Cl2 (Nujol mull, 295 K), compared to calculated transitions of the lowspin (red; Figure S6 and Table S5) and high-spin isomers (green; Figure S7 and Table S6) weighted by their respective mole fractions (Figure 1).

The electronic spectra of the spin isomers were modeled using TD-DFT techniques to calculate spin-allowed singleelectron excitations. Significant transitions are listed in Table 3 and compared to the experimental mull spectrum in Figure 7 (calculated transitions for the spin isomers are compiled in Tables S5 and S6, and simulated spectra are shown in Figures S6 and S7). Calculations for the low-spin isomer were consistent with previous interpretations of the room-temperature data.39,40 A prominent ligand field band (dxz,yz → dx2−y2, 1 A1g → 1Eg under ideal D4h symmetry) was calculated at 14,700 cm−1 (680 nm). The two other single-electron ligand-field bands (dz2 → dx2−y2, 1A1g → 1B1g {orbitally forbidden} and dxy → dx2−y2, 1A1g → 1A2g) were calculated as weaker features flanking the first band, at 14,300 cm−1 (697 nm) and at 16,100 cm−1 (619 nm), respectively. Two-electron ligand-field excitations lying at higher energies were not calculated. Relatively intense LMCT bands commenced above 22,100 Table 3. Significant Spin-Allowed TD-DFT Transitions no.

E (cm−1)

λ (nm)

1A 1B/2B 2A 3B/4B 5B/6B 6A 7B/8B

14,300 14,700 16,100 22,100 24,500 25,600 27,000

697 680 619 452 409 391 370

1B/2B 1A 3B/4B 3A 5B/6B 7B/8B 9B/10B 11B/12B 13B/14B 12A 15B/16B

7,200 7,900 12,000 14,500 15,100 16,100 16,900 21,100 23,800 24,300 25,600

1380 1260 832 688 663 621 592 473 421 411 390

f low-spin 2.1 3.8 1.7 1.1 1.4 1.3 1.4 high-spin 2.5 7.2 9.8 2.3 6.4 2.2 1.2 3.6 6.6 5.5 6.8 H

orbitals

× × × × × × ×

10−8 10−3 10−4 10−2 10−2 10−3 10−2

40a → 41a 39b/40b → 39a → 41a 37b/38b → 35b/36b → 40a → 42a 39b/40b →

× × × × × × × × × × ×

10−3 10−12 10−3 10−5 10−4 10−3 10−2 10−2 10−3 10−8 10−2

39b/40b → 40a 39a → 40a β 37b/38b → 40a 39a → 41a β 35b/36b → 40a 39b/40b → 41a 33b/34b → 40a 37b/38b → 41a 35b/36b → 41a 41a → 42a α 33b/34b → 41a

assignment LF (dz2 → dx2−y2) LF (dxz,yz → dx2−y2) LF (dxy → dx2−y2) LMCT LMCT MLCT MLCT

41a 41a 41a 42a β β β β β β β β

LF (dxz,yz → dz2) LF (dxy → dz2) LMCT LF (dxy → dx2−y2) LMCT LF (dxz,yz → dx2−y2) LMCT LMCT LMCT MLCT LMCT DOI: 10.1021/acs.inorgchem.5b02203 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

cm−1), and two degenerate in-plane bends (E: KNO3, 695; BaCS3, 314 cm−1).93 Strong bands at 1591 and 1578 cm−1 in the spectrum of Ni(detu)4Cl2 were assigned as a single split δ(N−Hcis) mode. These peaks were blue-shifted +25 cm−1 from those of free detu, which is consistent with the significant admixture of νa(N−C−N) character predicted by the frequency calculation and normal coordinate analyses on Cs-dmtu.89,90 The next strong band at 1528 cm−1 was blue-shifted by only +7 cm−1 compared with free detu, and on the same basis was assigned as the δ(N−Htrans) mode with some admixture of νs(N−C−N) character.90 Coupling of these first two modes to energetically and structurally proximal N,N′-diethyl deformations also seems likely. Straightforward extrapolation of these assignments to the strongest peaks in the spectrum of N,N′-d2-detu gives small red shifts compared to detu (νD/νH = 0.92−0.96, Table 4) that are consistent with previous assignments for dmtu,89,90 but does not account for a unique additional peak at 1558 cm−1 in the spectrum of N,N′-d2-detu. The νa(N−C−N) mode was assigned to a weak band at 1339 cm−1 that was apparently blue-shifted +13 cm−1 from detu and +49 cm−1 from N,N′-d2detu, which is consistent with a δ(N−Hcis) component. The frequency calculation gave an intense {νs(N−C−N) − ν(C S)} mode for dmtu at 1226 cm−1; this core mode was assigned accordingly to a strong band at 1255 cm−1 in the complex spectrum that was blue-shifted +10 cm−1 from detu and was profoundly altered in the N,N′-d2-detu spectrum. The {ν(C S) + νs(N−C−N)} mode was assigned to a 788 cm−1 band that was red-shifted −9 cm−1 from detu. The π(CS) mode was assigned to a weak band at 543 cm−1 that was red-shifted −12 cm−1 with significant loss of intensity compared with the corresponding band of detu. The δs(N2CS) mode was assigned to a band at 424 cm−1 by analogy to a strongly polarized 448 cm−1 band in the Raman spectrum of dmtu,89 and comparable assignment in the normal coordinate analyses;89,90 the analogous δa(N2CS) mode likely falls below the experimental cutoff. The π(N−H) modes were assigned to weak bands at 652 and 612 cm−1, because these bands showed large shifts upon isotope substitution, from 645 and 611 cm−1 for detu to 532 and 472 cm−1 for N,N′-d2-detu (νD/νH = 0.77−0.82). All other peaks were considered to represent typical N,N′-diethyl deformations,92 as well as the δ(C−N{H}−Et) modes.

Figure 8. FTIR spectrum of solid Ni(detu)4Cl2 (KBr pellet, 295 K).

ν(CN) and ν(CS) modes to lower and higher energies, respectively (Scheme 3);41,42 moreover, isotopic substitution of the amide protons affected the associated normal modes.89,90 To assist provisional assignment of the spectra, a frequency calculation was performed on Cs-dmtu (Figure S13 and Table S7). Normal coordinate analyses were reported previously for Cs−cis,trans and C2v−trans,trans conformations of dmtu;89,90 spectroscopic data were consistent with the former, as subsequently confirmed by X-ray crystallography.91 The spectrum of Ni(detu)4Cl2 displayed strong bands at 3199 and 3222 cm−1 that are obviously assigned as the ν(N− H) modes. The relatively low frequencies are consistent with N−H···Cl interactions observed in the crystal structure (Figure 3). Free detu showed a single peak at 3231 cm−1 that downshifted to 2411 and 2361 cm−1 for N,N′-d2-detu (νD/νH = 0.73−0.75, compared to 0.73 predicted by Hooke’s law). Weak bands were observed for ν(C−H) modes between 2872 and 3097 cm−1. Although the fingerprint region was more complicated, only three other limiting classes of normal modes can be expected:89,90 four amide bending vibrations, including two in-plane δ(N−H) modes at high energy and two out-of-plane π(N−H) modes at low energy; six normal modes of the N2CS core; and typical distortions of the N,N′-diethyl substituents.92 Isolated N2CS core modes can be compared to those of D3h-symmetric nitrate and trithiocarbonate: two degenerate asymmetric stretches (E: KNO3, 1370; BaCS3, 920 cm−1), one symmetric stretch (A1: KNO3, 1049; BaCS3, 510 cm−1), one out-of-plane bend (A2: KNO3, 828; BaCS3, 516

IV. CONCLUSIONS As previously reported,39 Ni(detu)4Cl2 exhibits separate solidstate thermochromic and spin equilibria. In the present work,

Table 4. Proposed FTIR Assignments νobs, cm−1 Ni(detu)4Cl2

detu

N,N′-d2-detu

assignment

3222, 3199 (vs) 3097−2872 (m−w) 1591, 1578 (vs) 1528 (vs) 1339 (m) 1255 (s) 788 (m) 652, 612 (m−w) 543 (vw) 424 (vw)

3231 3092−2868 1566, 1554 1521 1326 1245 797 645, 611 555 419

2411, 2361 3089−2871 1518, 1509 1399 1290 (?) 789 532, 472 552 420

ν(N−H) ν(C−H) δ(N−Hcis) δ(N−Htrans) νa(N−C−N) νs(N−C−N) − ν(CS) ν(CS) + νs(N−C−N) π(N−H) π(CS) δs(N2CS)

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these have been structurally defined by a combination of DSC, variable-temperature X-ray crystallography, and DFT calculations. A monoclinic ↔ tetragonal distortional equilibrium centered at T1/2 = 254 ± 11 K was observed, and this lattice distortion induced the observed thermochromism by perturbing the ideally C4-symmetric ligand field at diamagnetic [Ni(detu)4]2+. In contrast, the independent spin equilibrium was centered at T1/2 = 383 ± 18 K.39 An X-ray crystal structure at 173(2) K revealed a low-spin [Ni(detu)4]2+ complex with short equatorial Ni−S bonds averaging 2.226(3) Å and a long apical Ni···Cl separation of 3.134(1) Å to a proximal chloride. A monotonic contraction in the Ni···Cl separation of −0.096(4) Å was observed by X-ray crystallography upon increasing the temperature to 293 K. A paramagnetic high-spin [Ni(dmtu)4Cl]+ isomer was modeled by DFT calculations and exhibited longer equatorial Ni−S bond lengths of 2.45 Å and a shorter Ni−Cl apical bond of 2.43 Å. Taken together, these results are consistent with a spin equilibrium resulting from tetragonal ligand field modulation of [Ni(detu)4]2+ by a single proximal chloride. The room temperature electronic and FTIR spectra of Ni(detu)4Cl2 were assigned with the assistance of TD-DFT and frequency calculations. Consistent with the structural results, the spectra were dominated by low-spin [Ni(detu)4]2+. Several other nickel(II) complexes exhibit solid-state anomalous magnetism within a variety of donor sets, in as yet undefined geometries.10,32−38 Moreover, several examples of structurally characterized Ni(II) complexes show long apical Ni−Cl bond lengths in the range of 2.6−2.9 Å.14,94−97 We further conclude that the phenomenon of solid-state spin crossover of nickel(II) is likely to be more common than is generally realized.

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REFERENCES

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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.5b02203. X-ray crystallographic data (CIF) DFT models (XYZ) Computational output and FTIR spectroscopy (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Ohio University. The authors thank the Ohio University 1804 Fund for support in licensing the DFT software and acquiring the associated hardware (MPJ), and the Saudi Arabian Cultural Mission to the United States for a Graduate Scholarship (IAA). Purchase of the Bruker-AXS VENTURE PHOTON-100 diffractometer by the University of Minnesota was supported by the National Science Foundation (MRI-1229400). The authors thank Professor Sunggyu Lee for access to the TGA and DSC instrumentation and Amber R. Tupper for technical assistance with these experiments. The authors also thank Ahmed M. Aboelenen for other technical assistance. J


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