Molecular Spin Crossover in Slow Motion: Light-Induced Spin-State

Article pubs.acs.org/IC

Molecular Spin Crossover in Slow Motion: Light-Induced Spin-State Transitions in Trigonal Prismatic Iron(II) Complexes Philipp Stock,†,⊥ Eva Deck,‡ Silvia Hohnstein,‡ Jana Korzekwa,§ Karsten Meyer,§ Frank W. Heinemann,§ Frank Breher,*,‡ and Gerald Hörner*,† †

Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 135, 10623 Berlin, Germany Institute of Inorganic Chemistry, Karlsruhe Institute of Technology (KIT), Engesserstrasse 15, 76131 Karlsruhe, Germany § Department Chemie und Pharmazie, Anorganische Chemie, Friedrich-Alexander-Universität Erlangen-Nürnberg, Egerlandstraße 1, 91058 Erlangen, Germany ‡

S Supporting Information *

ABSTRACT: A straightforward access is provided to iron(II) complexes showing exceedingly slow spin-state interconversion by utilizing trigonal-prismatic directing ligands (Ln) of the extended-tripod type. A detailed analysis of the interrelations between complex structure (X-ray diffraction, density functional theory) and electronic character (SQUID magnetometry, Mössbauer spectroscopy, UV/vis spectroscopy) of the iron(II) center in mononuclear complexes [FeLn] reveals spin crossover to occur along a coupled breathing/torsion reaction coordinate, shuttling the complex between the octahedral low-spin state and the trigonal-prismatic high-spin state along Bailar’s trigonal twist pathway. We associate both the long spin-state lifetimes in the millisecond domain close to room temperature and the substantial barriers against thermal scrambling (Ea ≈ 33 kJ mol−1, from Arrhenius analysis) with stereochemical constraints. In particular, the topology of the κ6N ligands controls the temporary and structural dynamics during spin crossover.

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state trapping, LIESST6−9), whereas typical spin-state lifetimes of 100 ns and below pertain at room temperature.10−17 Both of the expressed kinetic lability and the small thermal barriers are attributable to the generally small extent of angular rearrangement during SCO that accompanies the changes in molecular dimensions; that is, (de)population of antibonding eg-type d orbitals results in isotropic (contraction) elongation of Fe−D bonds (with D = donor of the ligand; Scheme 1).18−20 This view, coined as the single configurational coordinate (scc) model,21,22 implies structure-conservative SCO, rendering the spin states invariably labile in systems that obey the scc model. Although the scc model is nowadays recognized as a severe oversimplification of the SCO-related structure dynamics, sketches of the potential-energy surfaces of the spin states in terms of intersecting parabolas, along a radial mode, are still emblematic for SCO research. While the effects of subtle secondary SCO-related structural changes are currently intensively discussed,19 significant deviations from the working hypothesis of a fully symmetric mode have remained scarce. In part, the current interest in deviations from the scc model may be traced to fundamental insights into the control parameters of spin-state dynamics: According to arguments given by Toftlund and McGarvey,23 kinetic stabilization must be sought where the scc model is not obeyed to for reasons of

INTRODUCTION Transition metal complexes with energetically close-lying spin states qualify as highly potent building blocks of functional molecular materials and as multiresponsive molecular switches.1 Transitions among the spin states of suitable complexes can be driven and controlled by external stimuli such as light irradiation. To date, however, any real-world utilization is hindered by competing randomization via rapid thermal scrambling. For broadly applicable approaches, complexes with kinetically stabilized spin states are urgently needed. Recently, spin states of iron(II) and nickel(II) complexes could be trapped through light-driven isomerization within the ligand sphere. Here the spin state is altered via variation of the ligand-field strength (ligand-driven light-induced spin change)2,3 or variation of the ligand-field symmetry (lightdriven coordination-induced spin-state switching).4,5 Central to both concepts is the making/breaking of chemical bonds to trigger the spin switch, rendering the ligand-borne switching process a transition between two chemically different species in their respective electronic ground states. No comparable kinetic stabilization of spin states is, as yet, available for the closely related molecular spin crossover (SCO) between low spin (ls) and high spin (hs) complexes, which is most prominent for iron(II) complexes (ls: 1A1/t2g6eg0, S = 0; hs: 5T2/t2g4eg2, S = 2). In particular, it is only at cryogenic temperatures, where thermal barriers can effectively inhibit metal-centered spin-state exchange (light-induced excited spin© XXXX American Chemical Society

Received: January 29, 2016

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DOI: 10.1021/acs.inorgchem.6b00238 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Scheme 1. Paradigmatic scc Model of Spin Crossover in Hexacoordinate d6 Complexes in Terms of Isotropic Breathing without Significant Angular Components

Scheme 2. Extended Tripodal κ6N Polyimine Ligands Ln Investigated in This Study

ligand structure (a striking example is discussed in ref 8a). Specifically, deviations in terms of a trigonal torsion are emerging in experimental work, ligand-field considerations, and molecular modeling,24,25 which was started by Fleischer, Wentworth, Purcell, Holm, and others in the 1970s as a reaction to the first occurrences of trigonal-prismatic complexes. The authors invoke coupling of angular modes to the radial motion to an extent, which is defined by ligand structure and ionic radii of the central ions. More recent experimental studies give evidence of the effects of angular motion on the kinetics but do not provide a congruent picture. On the one hand, a number of synthetic studies that aimed at coupling of angular motion to the breathing mode in iron(II) complexes by design of rigid polydentate ligands, yielded in very rapid spin-state dynamics.14,26 The high kinetic lability in these systems in solution has been associated with enhanced spin-orbit coupling to an intermediate-spin 3T state along the emerging trigonal twist coordinate.14,16 By contrast, a correlation of SCO-induced trigonal torsion with the LIESST temperatures of solid iron(II) complexes by Guionneau et al. suggests a major role of trigonal torsion with respect to kinetic stabilization.8b In the light of this correlation we reasoned, whether our recent observation27,28 of long-lived hs states of ls iron(II) complexes may be associated with deviations from the scc models, imposed by their rigidly capped extended-tripod ligands. The present work now fully confirms this hypothesis and further substantiates the view of Guionneau et al. in that we can clearly identify major kinetic stabilization of the spin states in solution of iron(II) complexes by virtue of massive trigonal torsion. Our strategy to maximize the angular motion during SCO rests on simple reasoning: iron(II) complexes with trigonalprismatic hs states cannot be expected to remain prismatic in the ls state; that is, a stereochemical shuttle of the complexes between trigonal prismatic hs states and octahedral ls states is highly probable. Substantial trigonal torsion along the SCO coordinate was expected to emerge through the combination of octahedraldirecting iron(II) and trigonal-prismatic directing N6 ligands L1−3 (Scheme 2). Ligands of this type actually have shown structural flexibility in response to the preference of the metal ion: While the ls iron(II) complex of the thiophosphorylcapped ligand L3 retains octahedral coordination,28 L1 and its pyridine analogues previously provided broad access to trigonal-prismatic coordination.29−33 The multimode character of SCO in the iron(II) complexes [1]2+, [2]2+, and [3]2+ of the rigidly capped tripodal hexadentate κ6N ligands34 L1, L2, and L3 is established experimentally (X-ray-diffraction single-crystal X-ray structure

analysis; zero-field 57Fe Mössbauer spectroscopy) and theoretically (density functional theory, DFT), whereas the related and well-studied35−39 reference system [4]2+ shows structure conservative SCO. Laser-flash photolysis studies on solutions of 2(BF4)2 and the reference system 4(BF4)2 that is based on ligand L4 show that the additional SCO-induced torsion motion, which is imposed by ligand structure, massively hinders the spin dynamics in [2]2+; that is, it is only the coupling of radial and angular modes in the case of [2]2+ that translates into extraordinarily slow spin-state interconversion. Spin-state lifetimes in the millisecond range are found to be accessible at temperatures of ca. −20 °C; that is, way above the cryogenic regime, which is typically necessary to harbor the LIESST effect.

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RESULTS AND DISCUSSION Synthesis and Characterization of 1(BF4)2, 1(OTf)2, and 2(BF4)2. Synthesis. Iron(II) complexes of the ligands L1 and L2 were obtained by the methods outlined in Scheme 3. Reaction of Fe(BF4)2·6H2O with ligand L1, formed in situ via Schiff base condensation in ethanol as published by Chandrasekhar et al.,29 afforded the iron(II) compound 1(BF4)2 as an ethanol solvate in form of a yellow powder in good yields (57%). The analogous compound 1(OTf)2 was obtained by reacting the independently synthesized and isolated ligand L1 with anhydrous [Fe(MeCN)2](OTf)2 in tetrahydrofuran as yellow powder. After crystallization from ethanol, 1(OTf)2 gave correct elemental analysis when formulated as ethanol solvate. 2(BF4)2 was obtained by reacting the independently synthesized and isolated ligand L2 with anhydrous [Fe(MeCN)6](BF4)2 in tetrahydrofuran as ocherous powder. Single crystals for X-ray crystallography were obtained by isothermal diffusion techniques (see Experimental Section for details). Orange-colored 4(BF4)2, which serves as a reference compound throughout this study, was obtained via modified literature methods.38 B


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Inorganic Chemistry Scheme 3. Synthesis of 1(BF4)2, 1(OTf)2, and 2(BF4)2

slightly increasing, both for 1(BF4)2 (3.2−3.3 cm3 K mol−1) and 2(BF4)2 (3.1−3.2 cm3 K mol−1) between 20 and 300 K (due to temperature-independent paramagnetism), revealing both compounds to be paramagnetic over this temperature range.40,41 The decrease of χMT below 20 K is due to the zerofield splitting. The χMT plot of 1(OTf)2 is consistent with these results, although the presence of a minor contaminant is identified here and also in the Mössbauer spectra (see Discussion of Figure S1 in the Supporting Information). Particularly, at T ≥ 120 K, a slight increase of the magnetization is observed, which may be attributed to an ls iron(II) impurity of (yet unknown) composition. 57 Fe Mössbauer spectra were thus recorded of compound 2(BF4)2 (Figure 1b) and of compound 1(OTf)2 (Figure S1, Supporting Information) at T = 77 K to obtain structure information besides the spin-state status. Both spectra were fitted to one quadrupole-split doublet (asymmetric for 2(BF4)2)42 with merely indistinguishable fit parameters. Isomeric shifts of δ = 1.12(1) mm s−1 and δ = 1.13(1) mm s−1 for 2(BF4)2 and 1(OTf)2, respectively, are unexceptional for hs iron(II) centers and corroborate our assignment of the spin state. By contrast, the substantial quadrupole splitting of the doublet by ΔEQ = 4.03(1) mm s−1 and ΔEQ = 4.04(1) mm s−1 is exceptional even for hs iron(II) complexes; the splitting is situated among the highest values ever observed.40 As quadrupole splitting reflects the degree of deviation of the coordination environment from cubic symmetry, massively distorted coordination of the iron(II) is expected for 1(OTf)2 and 2(BF4)2. This conclusion could be confirmed by means of X-ray crystallography (see below). Crystal Structures. Solid-state structures of 1(BF4)2, 1(OTf)2, and 2(BF4)2 were obtained by single-crystal X-ray

Spin-State in the Bulk. As magnetometry and Mössbauer spectroscopy revealed, the iron(II) centers in 1(BF4)2, 1(OTf)2, and 2(BF4)2 invariably possess hs character in the solid state. The molar magnetic susceptibility χMT of microcrystalline samples of the three compounds was recorded in a SQUID magnetometer over the temperature range of 2− 300 K (applied magnetic field: 1.0 T). Data of 1(BF4)2 and 2(BF4)2 are shown in Figure 1a (data of 1(OTf)2 are shown in Figure S1 of the Supporting Information). χMT was found to be

Figure 1. (a) Solid-state χMT/T diagrams (2−300 K) for 1(BF4)2 (●) and 2(BF4)2 (○); lines: fit of the data with g = 2.1, |D| = 4.738 cm−1, E/D = 0.0, TIP = 229.7 × 10−6 emu and g = 2.0, |D| = 4.962 cm−1, E/ D = 0.0, TIP = 374.4 × 10−6 emu for 1(BF4)2 and 2(BF4)2, respectively; (b) 57Fe Mössbauer spectrum of 2(BF4)2 at T = 77 K (symbols: measured data; line: fit) δ = 1.12(1) mm s−1; ΔEQ = 4.03(1) mm s−1; ΓFWHM = 0.63(1) mm s−1. C


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Inorganic Chemistry

Figure 2. Molecular structures of 1(BF4)2 (a) and 2(BF4)2 (b). Only the dicationic complexes are shown; [BF4]− counterions were omitted for clarity. (c) Central coordination polyhedra of the five independent cations of 1(BF4)2 in the asymmetric unit, illustrating the degree of trigonal distortion.

crystallography. Both complexes of L1 crystallize in the monoclinic space group P21/n. While 1(OTf)2 is identified as an ethanol solvate with two independent cations in the asymmetric unit, 1(BF4)2 crystallizes unsolvated with five independent cations in the asymmetric unit (Fe1−Fe5). 2(BF4)2 crystallizes in the hexagonal space group P63 with a single unsolvated cation in the asymmetric unit. All cation structures are found to be very similar, irrespective of the ligand, the counterion, and the status of solvation (Figure 2; selected structure details in Table 1; more extensive data in Table S1). The core structures of the independent cations of 1(BF4)2 are shown in Figure 2c. The most striking feature of the complex structures is clearly the shape of the [FeN6] coordination polyhedron. Evidently, the structure of [1]2+ is only slightly distorted from an ideal prism with a trigonal twist angle θ = 0° (graphical definition in Figure S2, Supporting Information),33,43,44 an observation that holds also for the complex ion [2]2+. In full agreement with the results of magnetometry and Mössbauer spectroscopy, the Fe−N bond lengths are typical of hs iron(II) complexes (210 < d̅Fe−N < 225 pm).45 Thereby, the Fe−N bond lengths involving the imidazole N atoms (Nhet) are generally 10−15 pm shorter than the Fe−N bond lengths involving the hydrazimine nitrogen atoms (Nimin). Accordingly, the iron ion is significantly displaced from the center of the N6 core toward the “open” (Nhet)3 end of the coordination sphere of L1; that is, h1 ≫ h2 (cf. Figure S2, Supporting Information). This displacement reads as a pushing out of the metal ion, both in hs-[1]2+and hs-[2]2+. Similar observations have been made before for Cu(I) complexes of a related extended-tripodal ligand.30 It is contrasted by the behavior of the hs iron(II) center in the related compound 4(PF6)2 with a tren-based coordination cap.35−39 Distorted-octahedral coordination prevails in the crystal structures of the latter (θ = 43°), accompanied by significant pulling in of the iron(II) center toward the “closed” (Nimin)3 end of the ligand; that is, h1 ≪ h2.

This observation indicates that L1−2 and L4 constitute members of qualitatively dif ferent ligand families.46 The trigonal twist angle θ varies between 5° and 15° in the five symmetry-independent cations of 1(BF4)2 (Figure 2c) and amounts to θ = 4.2° and 6.6° for the cations in 1(OTf)2 and 2(BF4)2 (Figure 2b), respectively. As expected, the rigidity of the capping group in L1−2 induces some additional deviation of the N6 core in 1(X)2 and 2(BF4)2 from ideal prismatic shape. For instance, the nonbonded N−N distances a in the equilateral (Nimin)3 triangle are somewhat smaller than those of the equilateral (Nhet)3 triangle, c, so that the polyhedron may be interpreted as a truncated trigonal pyramid (for a definition of a and c, see Figure S2 of the Supporting Information).30 As implied by the trigonal-prismatic structure of 1(X)2 and 2(BF4)2, both the trans N−Fe−N and the cis N−Fe−N angles deviate substantially from the octahedral values of 180° and 90°, respectively. The cis-angle distortion ∑cis, which sums the deviations from 90° for 12 cis N−Fe−N angles, amounts to 180−190°. These values should be compared to the data compiled by Guionneau et al. for an array of 18 N6-coordinate hs iron(II) complexes, where the cis distortion did not exceed ∑cis = 93°.47 As has been recognized in several studies,24a,25d,48 trigonal torsion and metal−ligand bond length are correlated in complexes of multidentate ligands. In other words, the chemical stimulus of M−N bond-length variation translates into torsional motion; a motion that is mediated by the rigid backbone of the ligand. A qualitatively similar correlation of trigonal torsion and metal−ligand bond length may be deduced from the crystal structures of the complexes [ML1](X)2 (●: M = Fe (this work); ○: Co, Ni, Zn, Cd (all from ref 29); Figure 3). It is found, however, that lattice effects significantly convolute with the θ-d̅M‑N correlation. Within our set of six independent cations of the iron(II) complexes 1(X)2, in particular, θ varies by as much as 10.5°, whereas the experimental average bond length d̅Fe−N varies by only 2 pm (blue circles in Figure 3). D


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Inorganic Chemistry Table 1. Selected Structural Detailsa for 1(X)2, 2(BF4)2, and DFT-Derived Structures of [FeLn] 1(BF4)2 b XRD (mean) distances d̅(Fe−Nimin) d̅(Fe−Nhet) angles (N−Fe−N)trans ∑cis c θd S(Oh)e S(TP)e DFTf (mean) distances d̅(Fe−Nimin) d̅(Fe−Nhet) angles (N−Fe−N)trans ∑cis

c

θd S(Oh)e S(TP)e

1(OTf)2

2(BF4)2

Fe1

Fe5

223.9(2) 213.8(4)

224.5(5) 214.2(8)

225.5(20) 213.1(6)

226.2(1) 212.0(1)

143.2(11) 180.2 14.7(5) 10.069 1.260 [1]2+

137.0(11) 191.7 6.0(12) 13.945 0.433 [2]2+

135.6(20) 190.4 4.2(8) 14.749 0.364 [3]2+

137.2(5) 188.6 6.6(1) 13.578 0.453 [4]2+

224.8(18) 198.0(4) 216.8(14) 199.0(2)

230.3(20) 198.2(4) 212.8(11) 197.5(3)

221.7(10) 196.0(1) 218.8(12) 200.8(2)

218.5(7) 200.5(1) 225.9(12) 199.0(1)

149.6(18) 166.8(5) 155.4 62.0 24.0(15) 44.1(2) 6.562 1.313 3.068 9.590

145.4(22) 165.4(10) 175.8 58.3 18.9(21) 42.1(8) 7.025 1.627 3.186 8.724

148.0(18) 166.6(3) 156.6 63.3 21.2(20) 43.7(4) 7.548 1.384 2.497 9.439

162.4(7) 173.9(1) 103.7 60.6 46.6(6) 53.1(1) 2.036 0.679 12.122 14.549

We cannot identify a significant dependence of θ on the bond lengths of the iron(II) complex ions in 1(X)2 (R2 < 0.6), but we associate the range of trigonal twist angles with lattice effects pertinent to the crystal structures of 1(X)2. In agreement with this notion, a DFT-derived torsional coordinate reveals a very shallow energy minimum for the hs surface of [1]2+ along θ (see below). The shaded area in the θ-d̅M‑N plot thus denotes the dynamic range of θ with d̅M‑N, biased by lattice effects. Nevertheless, the θ-d̅M‑N plot can be safely extrapolated to small values of d̅M‑N by including the crystal structure of compound 3(BF4)2 (i.e., [Fe(L3)](BF4)2; green in Figure 3).28 This latter compound with an ls iron(II) center compensates the contraction of the d̅Fe−N bonds through a significant trigonal torsion toward the regular octahedron (i.e., θ = 44.3°; additional structure parameters in Table 2). The largely Table 2. Spin Crossover Energiesa of the Iron(II) Complexes [FeLn]2+ complex ΔSCOEel, kJ mol−1

[1]2+

[2]2+

[3]2+

[4]2+

28.5 (21.2)

32.8 (26.7)

49.7 (47.1)

32.1 (20.5)

a Differences in electronic energy, ΔSCOEel = Eel(ls) − Eel(hs), from optimized structures (B3LYP*/TZVP/COSMO(MeCN); data in parentheses: B3LYP*/TZVP.

undisturbed octahedral environment in 3(BF4)2 can be read from both the small extent of the cis-angle distortion, ∑cis = 51.0°, and the isotropic Fe−N bond lengths, which do not diverge significantly with donor nature. Clearly, the structural details of ls-3(BF4)2 are well-reproduced by the computed ls structures of [1]2+ and [2]2+ (stars in Figure 3). Thus, we conclude that 3(BF4)2 and 1-2(X)2 are both structural and spin-state antipodes. In particular, we correlate the diverging structures with the diverging spin states; that is, SCO in these compounds implies trigonal-prismatic hs states and octahedral ls states of the iron(II) centers. The following XRD-calibrated DFT study shows that this correlation holds true for complexes of ligands L1−3 but not for the tren-based ligand L4. As becomes evident both from inspection of the crystal structures (black in Figure 3) and the DFT-optimized structures (see Table 1), the structures of 4 are greatly insensitive toward switching of the spin state. Density Functional Theory-Derived Structures: Spin Crossover-Induced Trigonal Twist. The structural effects that derive from spin-state changes of [1]2+−[3]2+ were studied by DFT methods. The results are compiled in the lower part of Table 1, together with respective data for the complex [4] 2+ . Benchmarking of the DFT results with experimental data from X-ray single-crystal crystallography was performed for hs[1]2+ and hs-[2]2+ (this work), ls-[3]2+ (from ref 28), and hs[4]2+ and ls-[4]2+ (from refs 35 and 37). In the case of hs-[4]2+ and ls-[4]2+ previous DFT-derived data were available for comparison.35 Comparison shows that the chosen DFT routines are well-reproducing the experimental structures. A slight but systematic overestimation of the Fe−N bond lengths, typically by ΔdFe−N ≤ 4 pm, is an inherent feature of the B3LYP* functional. We find excellent agreement between experiment and theory in the case of the trigonal twist angle θ of the low-spin complexes ls-[3]2+ and ls-[4]2+ and the high-spin complex hs[4]2+. More significant is the deviation between experiment and DFT modeling in the case of hs-[1]2+ and hs-[2]2+. DFT

a

Bond lengths, nonbonded distances (pm), and angles (deg). Representative data for complex ions with Fe1 and Fe5. cSummed deviation from 90° of 12 N−Fe−N cis angles. dSee Figure S2 for definitions. eContinuous shape measures S(Oh) and S(TP) with reference to the octahedron and the trigonal prism, respectively (refs 25c and e). fFully optimized structures of hs-[FeLn] (B3LYP*/ TZVP); data for the ls states in italics. b

Figure 3. Correlation of radial and angular measures in the XRD structures of complexes of ligand L1−4: [FeLn](X)2 (blue: 1(X)2; red: 2(BF4)2; green: 3(BF4)2 from ref 28; black: 4(PF6)2 from ref 35. (○) [ML1](NO3)2 (M = Co, Zn, Cd) and [NiL1](ClO4)2 from ref 29). (★) DFT-optimized structures of ls-[1]2+ and ls-[2]2+.

E


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Inorganic Chemistry consistently computes θ ≈ 20° for the hs states at the B3LYP*/ TZVP level of theory, whereas much smaller values prevail in their crystal structures, that is, 4° < θ < 15° (cf. Figures 2 and 3). It is noted that embedding of the complexes in a dielectric continuum somewhat reduces this discrepancy. With a view to the significant susceptibility of hs-[1]2+ toward lattice effects on θ discussed above (see Figure 3), we associate the deviation of the DFT results with nonaccounted lattice effects. Irrespective of the slight divergence of theory and experiment, the complexes [1]2+−[3]2+ give rise to structural changes that go far beyond isotropic breathing (see [FeN6] cores of [1]2+ in Figure 4). With the SCO-related variation in the trigonal twist

Figure 5. Relaxed surface scans of [FeLn]2+ along the trigonal twist coordinate (θ = 0°: trigonal prism; θ = 60°: octahedron). (left) hs states. (right) ls states. Energy is given relative to the globally optimized ls structure of each complex.

torsional-energy range ΔEel of only 10 kJ mol−1, a substantial range of torsional angles is energetically accessible; that is, the experimentally observed values of 4° < θ < 15° are covered without significant energy costs. This corroborates our notion that the observed scatter in experimental torsion angles is a consequence of lattice-induced structural bias and further suggests ready racemization of the hs forms of [1]2+−[3]2+ along the Bailar trigonal twist (note that C3-symmetric structures are chiral; racemization occurs at θ = 0°). Comparison with the energy profiles of ls-[1]2+ and ls-[4]2+ (Figure 5, right) corroborates ideas of Vanquickenborne et al., who reasoned that racemization generally occurs on the hs energy surface, requiring ls complexes to undergo SCO en route.24d The divergence in racemization barrier heights for hs-[1]2+− [3]2+ can be largely ascribed to steric effects. In particular, the slightly enhanced barrier in the case of hs-[3]2+ reflects the higher degree of steric crowding, pertinent to the ortho-protons of the pyridine residues in the prismatic transition state (dH−H ≈ 200 pm). This situation is found less demanding in the case of the imidazole residues of L1 and L2 (dH−H ≈ 260 pm). By contrast, the substantially higher barriers that are recorded for hs-[4]2+ are ascribable to (i) the position of its torsional minimum at θ = 45°, being located very early on the torsion coordinate and (ii) to the substantially higher slope of the energy surface (Figure 5, black). Accordingly, a much smaller area on the torsional coordinate appears accessible for complexes of the tren-based ligand L4, if in the hs state. We conclude that the floppy alkyl-chain headgroup that connects the ligand arms of L4 accommodates much of the steric stress that is brought about by the “breathing” central ion. By contrast, the stiff architecture pertinent to L1−3 excludes major structural rearrangements in the headgroup of the ligand. The stimulus of the “breathing” metal ion cannot be compensated within the ligand headgroup but is relieved within the [FeN6] core via torsion of the ligands’ “loose ends”. A more complete modeling of the potential energy surfaces of the complexes [1]2+−[4]2+ is currently being performed, the results of which will be reported in due course. Density Functional Theory-Derived Spin Crossover Energies. Besides their diverging structural effects, the ligands

2+

Figure 4. Views on the [FeN6] core of [1] along the threefold axis, as defined by the vector P−S−Fe (DFT-optimized structures): (a) in the quintet-spin state and (b) in the singlet-spin state.

angle in the range of ΔSCOθ ≈ 20°, SCO of [1]2+−[3]2+ is best described in terms of a correlated radial and angular motion, shuttling the structures between the (octahedrally distorted) trigonal prism and the (trigonally distorted) octahedron, when in the hs state and the ls state, respectively. This notion is corroborated by a continuous shape map that finds the shape measures of the [FeN6] cores in [1]2+−[3]2+ aligned along the ideal trigonal-twist pathway by virtue of the spin state (Figure S3). Thus, the distortion pattern pertinent to SCO in [1]2+− [3]2+ parallels the low-energy pathway of racemization of trischelate iron(II) complexes, which was coined by Bailar in his famous hypothesis of trigonal distortion.49 By contrast, much smaller structure differences emerge for [4]2+, with ΔSCOθ ≈ 7−8°. Further inquiry in the torsional behavior of the hs iron(II) complexes [FeLn]2+ shows that this qualitative difference in the SCO-related structure response can be directly associated with the topology of the ligands Ln (Figure 5). Interestingly, it is on the hs surface, where ligandtopology related divergence is predominantly located. Relaxed-surface scans along the trigonal twist angle θ (interpodal variation in θ was neglected, nominally corresponding to C3 symmetry; it is noted that the actual symmetry is lower than C3 due to Fe−N bond-length variation) were performed for the complexes [FeLn]2+ interpolating between the structure extremes, θ = 60° and θ = 0°. The qualitative difference between the ligands L1−3 on the one hand and L4 on the other reflects the different topology of the ligands, which allows for the formation of six-membered interpodal chelates for L1−3, whereas more floppy eight-membered interpodal chelates prevail for L4. The torsional-energy profiles of hs-[FeLn]2+ are similar in shape and differ only slightly in slopes (Figure 5, left panel) for the ligands L1−3 (the significant energy offset of hs-[3]2+ reflects its inherently more stable ls state; see below). It is noted that particularly the profiles of hs-[1]2+−[2]2+ have very little curvature toward small trigonal twist angles. Within a limited F


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Inorganic Chemistry Ln were found to affect the SCO energy. Electronic energy differences ΔSCOEel were computed for the dications [FeLn]2+, both in the isolated form and embedded in a dielectric continuum parametrized for acetonitrile (Table 2). Although numerical frequencies have been computed for all complexes and spin states, we refrained from extracting SCO entropies, which would be necessary to derive free energies of SCO, ΔSCOG. The derived order along the computed SCO energy including continuum solvent effects, [1]2+ < [2]2+ ≈ [4]2+ ≪ [3]2+, is in qualitative agreement with the experimental phenomenology of the SCO transition temperatures T1/2, [1]2+ ≪ [2]2+ ≈ [4]2+ ≪ [3]2+. In particular, [3]2+ has ls character at and fairly above room temperature,18 whereas all the other complexes are mainly hs at room temperature but experience SCO at lower temperature (at least in solution for [1]2+ and [2]2+; see below). Interestingly, we find that the DFT-derived SCO energies coincide for the complexes [2]2+ and [4]2+; this somewhat unexpected finding (that is nicely reflected by the spectroscopic behavior of these complexes in solution) points to compensation among the electronic and steric effects that the diverging capping topology and the altered aldimine-donor environment are expected to exert on the SCO energies. Spin Crossover in Solution. Consistent with the results of the solid-state studies, room-temperature spectroscopic studies of 1(BF4)2 and 2(BF4)2 are indicative of conserved paramagnetic character also in solution. For instance, 1H NMR spectra of all complexes of L1−2 in CD3CN at room temperature reveal broad, unstructured resonances spread out to δ > 60 ppm. The proportions of the peak areas are in agreement with the anticipated complex constitution. The small number of individual resonances points to a C3-symmetric structure of the complex in solution. Complementary studies of UV/vis-spectra of 1(BF4)2 and 2(BF4)2 (in MeOH and 1,1,1-trifluoroethanol, respectively) revealed the iron(II) center to undergo SCO to the ls state at low temperature (eq 1). ls‐[FeLn]2 + ⇌ hs‐[FeLn]2 +

Figure 6. Variable-temperature UV/vis spectra; (a) 1(BF4)2 in ethanol (c = 2 × 10−4 mol L−1; d = 0.1 cm); (b) 2(BF4)2 in 1,1,1trifluoroethanol (c = 4 × 10−4 mol L−1; d = 0.1 cm); (c) thermochromism of the MLCT-bands of 1(BF4)2 (gray; λ = 440 nm) and 2(BF4)2 (black; λ = 470 nm); lines: phenomenological fits to a Boltzmann model with εmax (T → 0 K) = 8000 L mol−1 cm−1.

metal-to-ligand charge transfer (MLCT) transitions of the ls component present in solution. Most interestingly, concentrated solutions of 1(BF4)2 and 2(BF4)2 were visually identified to be thermochromic. To quantify the thermochromism, UV/vis spectra were measured at variable temperatures (Figure 6a,b). In both cases, we observe continuous spectral changes, proceeding with clean isosbestic points (1(BF4)2: λip [nm] = 360, 380; 2(BF4)2: λip [nm] = 250, 283, 340, 398). The observations are well in accord with the implications of a thermal SCO equilibrium. On cooling the solutions of 1(BF4)2 and 2(BF4), both the near-UV absorption at λ = 350 nm and the vis absorption around λ = 450 nm gain intensity. Most significant to this study is the continuous growth of the diagnostic absorption bands in the vis region that accompanies the temperature decrease. As becomes evident from inspection of Figure 6b, the spectra of compound 2(BF4)2 are highly susceptible to temperature variation. Between T = 353 and 224 K, we record a rapid increase in the molar absorption coefficient of 2(BF4)2 up to ε470 nm ≈ 5.000 L mol−1 cm−1 at λmax = 470 nm (Figure 6c). This value should be compared with values of ε490 nm ≈ 8.000 L mol−1 cm−1,28 observed for the topologically closely related ls compound 3(BF4)2 in acetonitrile. Using the data of 3(BF4)2 as an educated guess of the limiting absorption coefficient of ls-2(BF4)2 (fit in Figure 6c), the transition temperature of 2(BF4)2 is estimated as T1/2 ≈ 230 K. The enthalpy and the entropy of the SCO are accordingly estimated as ΔSCOH ≈ 19 kJ mol−1 and ΔSCOS ≈ 81 J K−1 mol−1, respectively (see Figure S4; given the assumptions made, we refrain from reporting fit statistics). These values are well within the range expected for thermal SCO complexes.45,50 The coincidence of the transition temperature with the reported37 behavior of 4(BF4)2 indicates very similar ligandfield strengths of the iron(II) centers in both compounds; a finding that is fully confirmed by coinciding SCO energies of both complexes, derived from DFT computation (see Table 2). By contrast, the ligand-field strength in 1(BF4)2 is found to be significantly smaller. Its transition temperature is certainly below 100 K but cannot be extracted from the temperature

(1)

The UV response at room temperature of both compounds is unexceptional for (predominantly) hs iron(II) complexes. Intense ligand-centered transitions dominate the near-UV region (εmax > 28.000 L mol−1 cm−1; λmax = 269 and 290 nm for 1(BF4)2 and 2(BF4)2, respectively) with low-energy shoulder(s) (εmax > 7.000 L mol−1 cm−1; λmax = 339 nm and 320/364 nm for 1(BF4)2 and 2(BF4)2, respectively), which are extending toward the vis range. It is noted that up to the longwavelength limit of our setup (λ ≤ 1100 nm), we find no indication of absorption bands (ε ≥ 2 L mol−1 cm−1) that could be assigned as a spin-allowed ligand field transition of a hs iron(II) center. Vis features of 1(BF4)2 (Figure 6a) and 2(BF4)2 (Figure 6b), however, are reminiscent of UV/vis spectral data reported for methanol solutions of compound 4(BF4)2 (λmax = 445 nm; ε445 nm ≈ 1500 L mol−1 cm−1), which has been identified previously as a thermal SCO compound, both in solution and in the solid state (transition temperature T1/2 ≈ 220 K).37,38 In particular, the vis regime of 1(BF4) features a single weak absorption band (λmax = 410 nm; εmax ≈ 200 L mol−1 cm−1) situated at the tailing of more intense near-UV bands, while a moderately intense (λmax = 470 nm; εmax ≈ 1200 L mol−1 cm−1) absorption band is clearly discernible in the vis spectral regime of 2(BF4). The vis response of both compounds is attributed to G


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Inorganic Chemistry dependence of ε440 nm with reliability (fit in Figure 6c). Between T = 293 and 193 K, we record a fivefold increase in the molar absorption coefficient of 1(BF4)2 up to ε440 nm ≈ 500 L mol−1 cm−1. Both the band position and the intensity are at odds with an assignment as LF-absorption bands, but are indicative of ingrowing MLCT transitions of admixed ls components. On the basis of the close spectral analogies between 1(BF4)2 and 4(BF4)2, we suggest that also 1(BF4)2 adopts a low-spin configuration upon lowering the temperature in solution, to some extent at least. Spin Crossover in Slow Motion. The above analysis of the SCO-related structural changes in [1]2+−[3]2+ has unequivocally identified massive trigonal torsion as a second important configurational coordinate; that is, these compounds define a class of complexes that do not obey to the scc model. Alluding to the ideas of Hauser, high thermal barriers toward SCO must arise as an inevitable consequence of the breakdown of the scc model (an instructive example is discussed in refs 8a, 51, and 52). Accordingly, a laser-flash photolysis (LFP) study of 2(BF4)2 in 1,1,1-trifluoroethanol reveals unprecedented spinstate lifetimes at room temperature, which are accompanied by a massive increase in lifetime at lower temperature (Figure 7).

reversibility of the photoinduced processes are also in full agreement with the implications of light-induced SCO. The lifetimes of transient decay, τobs (τobs−1 = kobs = kHL + kLH; kHL and kLH denote the rate constants of the hs → ls and the ls → hs transitions, respectively), as obtained from monoexponential fits of the experimental data at variable temperature, are compiled in Figure 7b. An impressive lifetime τobs = 36 μs is recorded for 2(BF4)2 already at room temperature. This value should be compared with the lifetimes τobs = 65 ns and τobs = 480 ns, which are recorded for 4(BF4)2 (1,1,1trifluoroethanol solution at T = 293 K) and for 3(BF4)2 (MeOH solution at T = 293 K),27,28 respectively. Strikingly, the lifetimes of 2(BF4)2 experience a dramatic further increase on cooling of the solutions (Figure 7b); for sake of comparison, data for 4(BF4)2 are also given. In particular, at T = 246 K, the lifetime amounts to τobs > 1.2 ms, exceeding by a factor greater than 40 the longest lifetime previously recorded.27 Both the lifetime and its temperature dependence break with the general characteristics of iron(II) complexes of multidentate ligands, which show spin-state lifetimes in the range of 50−200 ns and minor thermal activation barriers.10−17 Utilizing the aforementioned preliminary thermodynamics parameters of the SCO process (see Figure S4), the experimentally observed rate constants kobs were split into their constituent rate constants kHL + kLH. It is noted that in the temperature range under study, the contribution of kLH dominates the overall kinetics; that is, the ls → hs transition is the limiting factor of the signal lifetime in the case of the SCO compound 2(BF4)2, as opposed to ls compounds such as 3(BF4)2, where only the hs → ls transition contributes. Arrhenius treatment of the temperature dependence of kHL and kLH yields the activation parameters for both directions (see Figure S7); such-derived thermal barriers Ea are substantial and amount to ∼32 kJ mol−1 and ∼51 kJ mol−1 for the hs → ls and the ls → hs transitions, respectively. We associate the extraordinarily slow spin dynamics and the exceedingly high thermal activation barriers in 2(BF4)2 with the coupling of angular and radial modes on the reaction coordinate of SCO in [2]2+, rendering its thermal barriers substantially larger than in the case of scc-obeying compounds. As becomes evident from the comparison of 2(BF4)2 and 4(BF4)2, who share very similar SCO energetics, but obviously differ in their stereomobility, the kinetics effect of the additional torsional mode can be substantial. In consequence, slow spin dynamics are generally expected for complexes of ligands that conserve the capping topology of L1−2. In agreement with this notion, we have previously recorded long lifetimes and high thermal barriers for a number of members of this ligand family,27,28 although we note that variation in the barrier heights within this family is significant; cf., Ea (HL) ≈ 32 kJ mol−1 and Ea (HL) ≈ 18 kJ mol−1, for 2(BF4)2 and 3(BF4)2, respectively. We associate the variation in the thermal barriers within the homotopological family with significant variation in the driving force of SCO; such variation can be actually read from the electronic energy changes ΔEel compiled in Table 2.

Figure 7. (a) Variable-temperature transient recovery profiles of 2(BF4)2 in 1,1,1-trifluoroethanol (λobs = 470 nm; lines represent monoexponential fits; additional profiles at T > 273 K are compiled in Figure S5); (b) temperature dependence of the signal-relaxation lifetimes τobs of 2(BF4)2 (filled symbols) and 4(BF4)2 (open symbols).

In essence, the LFP experiment uses a short-lived photochemical stimulus to drive the SCO equilibrium toward the hs state and records the recovery of the equilibrium via transient absorption spectroscopy;10−17 this acquisition scheme has been shown previously to be applicable for 3(BF4)2 28 and for a number of closely related analogues.27 The ls component in the SCO equilibrium (eq 1) provides the optical channel for excitation via its intense MLCT transition. While the low level of ls contributions interferes with LFP studies in the case of 1(BF4)2, compound 2(BF4)2 could be studied (Figure 7a; additional profiles at T > 273 K in Figure S5). The spectral features observed after optical excitation of 2(BF4)2 are fully consistent with spin-state interconversion processes. That is, the transient absorption spectra, recorded several microseconds after pulsing a solution of 2(BF4)2, reflect the bleaching of the diagnostic MLCT band of the ls component in the lowtemperature spectra of 2(BF4)2 (Figure S6). The lack of spectral evolution during the course of transient decay and the

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CONCLUSIONS One of the central problems of SCO research and probably the single-most important problem that interferes with the transfer of the phenomenon to “real-world” applications is the limited kinetic stability of the underlying spin states on the molecular H


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Inorganic Chemistry Table 3. Crystallographic Data for 1(BF4)2, 1(OTf)2, and 2(BF4)2 empirical formula Mr [g mol−1] color crystal system space group a [Å] b [Å] c [Å] β [deg] Z V [Å3] ρcalcd [g cm−3] crystal size [mm] T [K] μ [mm−1] 2Θ(max) [deg] reflections measured unique observed R1 (I > 2σ(I)) ωR2 (all data) ρfin(max/min) [e Å−3] CCDC

1(BF4)2

1(OTf)2·2EtOH

2(BF4)2

C15H21B2F8FeN12PS 661.9 yellow monoclinic P21/n 14.1612(2) 52.6916(9) 17.1576(3) 90.134(10) 20 12 802.6(4) 1.717 0.45 × 0.25 × 0.20 100(2) 0.825 54.20 258 268 27 689 25 156 0.0569 0.1263 0.848/−0.543 1434184

C21H33F6FeN12O8PS3 878.59 yellow monoclinic P21/n 10.298(2) 13.959(3) 25.782(5) 97.13(3) 4 3677.5(13) 1.587 0.60 × 0.40 × 0.40 200(2) 0.714 52.00 52 015 7237 6518 0.0368 0.1072 0.794/−0.376 1434185

C18H27B2F8FeN12PS 704.01 ochre hexagonal P63 10.078(1)

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scale of matter. Widespread supramolecular approaches are able to stabilize the spin states by use of efficient concatenation in multicenter systems. By contrast, there has been little hope to stabilize spin states of isolated SCO complexes beyond the nanosecond time scale other than at cryogenic temperatures. The observation of long spin-state lifetimes and the exceptionally high thermal barriers against spin-state scrambling (Arrhenius energies Ea equal 32 and 51 kJ mol−1 for the hs → ls and the ls → hs transitions, respectively) that we detail in this work, coincides with peculiarities of the extended-tripod ligand structure; that is, rigid capping units with interpodal six-chelate topology render the ligands strongly poised toward a trigonalprismatic coordination. While the conceptual connection between the SCO reaction coordinate and ligand-imposed trigonal distortion has quite a history tracing back to, among others, Wentworth, Purcell, and Vanquickenborne,24 we feel that our approach marks a significant extension of their ideas in that the coupling among the radial and the angular motion is maximized in our complexes. This notion is fully confirmed by an experimentally calibrated DFT modeling study: Radial breathing is identified to be blended by a substantial torsional motion of the coordination sphere, shuttling the complexes between octahedral structure and trigonal-prismatic structure in the low spin and the high spin states, respectively. In conclusion, an exciting alternative option is added to the SCO curriculum: long-lived spin states of isolated iron(II) complexes in solution. In particular, the limitations in lifetime are weakened as we extend the time domain of spin-state interconversion to the millisecond regime at temperatures only slightly below room temperature. The generality and conceptual simplicity of the chosen approach should allow for ready access to further examples of molecular SCO in slow motion.

16.090(3) 2 1415.3(4) 1.652 0.10 × 0.10 × 0.10 200(2) 0.751 51.98 15 949 1871 1837 0.0429 0.1194 1.025/−0.446 1434186

EXPERIMENTAL AND COMPUTATIONAL DETAILS

All synthetic manipulations were performed using standard Schlenk line or drybox techniques under an atmosphere of argon or dinitrogen. Reagents were purchased from Aldrich or Acros and used without further purification. N-Methyl-1H-imidazole-2-carbaldehyde was synthesized according to published procedures.53 Reference compound 4(BF4)2 was obtained according to a modified published procedure38 via in situ Schiff base coupling of tris(2-aminoethyl)-amine and 1Himidazole-4-carbaldehyde in ethanol solution in the presence of iron(II) tetrafluoroborate hexahydrate. In agreement with the literature data, elemental analysis is consistent with the molecular structure when formulated as a hydrate. Additional analytical data: 1H NMR (200 MHz, 298 K, CD3CN): δ [ppm] = 184.8 (3 H), 153.6 (3 H), 129.8 (3 H), 95.7 (3 H), 80.6 (3 H), 39.0 (3 H), 36.1 (3 H), 28.7 (3 H). HR-MS (ESI) calc. for C18H24FeN10: [M]2+ 218.0766; found: [M]2+ 218.0762. IR spectra were measured on Nicolet Magna System 750 or Bruker Vertex 70 spectrometers using KBr disks or the attenuated total reflection (ATR) technique on solid samples. The intensity of the absorption band is indicated as very weak (vw), weak (w), medium (m), strong (s), very strong (vs), and broad (br). NMR spectra were recorded on Bruker ARX 200 and Bruker AV 300 and 400 spectrometers; chemical shifts are given relative to tetramethylsilane for 1H, to CFCl3 for 19F, and H3PO4 for 31P. For broad 1H NMR signals, the full width at half-maximum (fwhm) values (ω1/2) are given in hertz. Mass spectrometric analyses (ESI mode) were performed on an Orbitrap LTQ XL, Thermo Scientific, IonSpec-mass spectrometer. Elemental analyses (C, H, N, S) were performed by combustion analysis using a Thermo Finnigan EAGER 300 (Flash 1112) or an Elementar Varia EL apparatus. Note that no satisfactory elemental analyses could be obtained for L1, which can be attributed to the fact that the ligand is only hardly soluble in most organic solvents preventing a further purification by crystallization. Solid-state variabletemperature magnetic susceptibility measurements were performed using a Quantum Design MPMS-XL5 superconducting quantuminterference device (SQUID) magnetometer operating at 1.0 T. Diamagnetic correction for the sample and the sample holder was applied. 57Fe Mössbauer spectra were recorded on a WissEl Mössbauer spectrometer (MRG-500) at 77 K in constant acceleration mode. 57 Co/Rh was used as the radiation source. WinNormos for Igor Pro I


Article

Inorganic Chemistry software was used for the quantitative evaluation of the spectral parameters (least-squares fitting to Lorentzian peaks). The minimum experimental line widths were 0.20 mm s−1. The temperature of the samples was controlled by an MBBC-HE0106 Mössbauer He/N2 cryostat within an accuracy of ±0.3 K. Isomer shifts were determined relative to α-iron at 298 K. UV/vis spectra in solution were measured using a Varian Cary 50 spectrometer equipped with a UV/vis quartz immersion probe (light path 1 mm, Hellma), in a home-built measuring cell. LFP experiments were performed with the 532 or 355 nm output of a Nd:YAG laser system.54 Transient decays were recorded at individual wavelengths by the step-scan method in the range from 370 to 700 nm and obtained as the mean signals of eight pulses. Spectral resolution was in the range of ±5 nm. The duration of the pulses (fwhm ca. 8 ns; 2−3 mJ per pulse) was generally much shorter than the decay lifetimes of the transient signals, so that deconvolution was not required for kinetics analysis. Solutions of 2(BF4)2 and 4(BF4)2 in high-purity 1,1,1-trifluoroethanol (concentration of [2]2+ ≈ 1 × 10−3 M) were rigorously deoxygenated by flushing with analytical-grade argon for 20 min prior to and kept under argon during measurement in sealed quartz cuvettes. For data acquisition at variable temperatures we used a temperature-controlled cell holder (Quantum Northwest, model TC 125). X-ray Crystallography. Crystal data and details concerning data collection and refinement are given in Table 3. Data collection was performed on a Bruker Smart APEX2 diffractometer or STOE IPDS II diffractometer using graphite-monochromated Mo Kα radiation with λ = 0.710 73 Å. The structures were solved by direct methods and refined by full-matrix least-squares procedures on F2 using SHELXTL (ver. 6.12) and SHELXL-97 program suites for 1(BF4)2.55 Structure solution for 1(OTf)2 and 2(BF4)2 was by dual-space direct methods with SHELXT56 with full-matrix least-squares refinement using SHELXL-2014/7.57 All non-hydrogen atoms were refined anisotropically. H atoms were placed in calculated positions using a riding model. One of the EtOH crystal lattice solvent molecules in 1(OTf)2 and both [BF4]− counteranions in 2(BF4)2 showed minor disorder but were successfully refined with suitable restraints. Upon convergence, the final Fourier difference map of the X-ray structures showed no significant peaks. The crystal under study of 1(BF4)2 turned out to be a twin; the twin matrix applied was [1 0 0 0−1 0 0 0−1], resulting in a batch scale factor of 0.2648(8). The asymmetric unit contained a total of 5 independent cations and 10 independent [BF4]− anions. One of the [BF4]− anions was disordered around the central boron atom (B8). Two alternative orientations were refined resulting in site occupancy factors of 88.3(4)% for F81−F84 and 11.7(4)% for F85− F88. Similarity and pseudoisotropic restraints were applied to the anisotropic displacement ellipsoids of the disordered atoms. Crystallographic data (excluding structure factors) for the structures in this paper were deposited with the Cambridge Crystallographic Data Centre as supplementary publication Nos. CCDC 1434184−1434186. Synthesis. L1. On the basis of the published synthesis of similar ligands,29,32 1H-imidazole-4-carbaldehyde (2.90 g, 30 mmol) was dissolved in ethanol (15 mL) and added dropwise to a solution of thiophosphoric acid tris(N1-methylhydrazide)58 (1.98 g, 10.00 mmol) in ethanol (10 mL). The reaction mixture was stirred under reflux for 10 h and then cooled to room temperature. After the volume of the solution was reduced to 10 mL, L1 precipitates at −20 °C. Colorless L1 (3.26 g; yield 75%) was collected by filtration and dried in vacuo. mp (Ar, sealed capillary) 182−195 °C. 1H NMR (300 MHz, 298 K, deuterated dimethyl sulfoxide (DMSO-d6)): δ [ppm] = 3.15 (9 H, d, 3 JPH = 9.3 Hz, N-CH3), 7.20 (3 H, d, 4JHH = 0.8 Hz, 5-Him), 7.68 (3 H, d, 4JHH = 1.7 Hz, NCH), 12.22 (3 H, s (br), ω1/2 = 70 Hz, NH). 13 C{1H} NMR (75 MHz, 298 K, DMSO-d6): δ [ppm] = 32.8 (d, 2JCP = 7.7 Hz, N-CH3), 119.5 (s, 5-Cim), 132.4 (d, 3JCP = 16.0 Hz, N CH), 134.3 (s, 4-Cim), 136.3 (s, 2-Cim). 31P{1H} NMR (121 MHz, 298 K, DMSO-d6): δ [ppm] = 72.1 (s). 15N NMR (41 MHz, 298 K, DMSO-d6): δ [ppm] = 325.0 (s, NCH), 221.1 (s, 3-Nim), 126.6 (s, N-CH3). FT-IR (solid, ATR): ν̃ = 3100 vw, 2823 vw, 2660 vw, 2612 vw, 1616 vw, 1553 vw, 1514 vw, 1465 w, 1404 vw, 1358 vw, 1335 vw, 1293 w, 1225 w, 1222 w, 1147 m, 1095 m, 1046 vw, 1001 vw, 934 s,

909 s, 842 s, 747 s, 719 s, 651 m, 627 s, 614 s, 512 vs, 470 m, 446 s, 410 w. L2. N-Methyl-1H-imidazole-2-carbaldehyde (0.50 g, 4.53 mmol) was dissolved in 5 mL of methanol containing 5 mg of MgSO4. Under stirring, thiophosphoric acid tris(N1-methylhydrazide) (0.30 g, 1.51 mmol), dissolved in methanol (5 mL), was added dropwise. The reaction mixture was heated to reflux and stirred for 10 h. The resulting solid was filtered, redissolved in 5 mL of methanol, and concentrated in vacuo producing a yellow oil. Addition of 10 mL of acetonitrile furnished crystalline L2 at room temperature (yield: 66%). mp (Ar, sealed capillary) 144−149 °C (decomp). Elemental Analysis: found (calcd) for C18H27N12PS·H2O: C 43.63 (43.89), H 5.45 (5.93), N 34.50 (34.13), S 6.63 (6.51)%. 1H NMR (300 MHz, 298 K, CD3CN): δ [ppm] = 3.26 (9 H, dd, 5JHH = 0.50 Hz, 3JPH = 9.02 Hz, PNCH3), 3.66 (9 H, s, Nim-CH3), 6.93 (3 H, m, MeNimCH), 6.95 (3 H, d, 3JHH = 1.13 Hz, NimCH), 7.71 (3 H, d, 4JPH = 1.32 Hz, NCH). 13 C{1H} NMR (75 MHz, 298 K, CD3CN)): δ [ppm] = 32.7 (d, 2JPC = 9.94 Hz, PN-CH3), 35.9 (s, NimCH3), 125.0 (s, MeNimCH), 129.4 (s, NimCH), 133.0 (d, 3JPC = 15.39 Hz, NCH), 143.5 (s, Cipso). 31P{1H} NMR (121 MHz, 298 K, CD3CN): δ [ppm] = 72.4. FT-IR (solid, ATR): ν̃ = 3105 vw, 2948 vw, 1598 vw, 1519 vw, 1461 w, 1442 w, 1412 w, 1288 w, 1245 w, 1211 w, 1161 m, 1135 m, 960 vs, 857 s, 768 s, 753 vs, 711 m, 684 s, 621 s, 517 s, 470 s, 419 m, 388 w. 1(BF4)2. Thiophosphoric acid tris(N1-methylhydrazide) (95 mg, 0.48 mmol) was dissolved in ethanol (5 mL), and the solution was added dropwise to a suspension of 1H-imidazole-5-carbaldehyde (139 mg, 1.45 mmol) in ethanol (3 mL). Stirring the mixture at room temperature for 1 h produced a colorless solution, to which was then added dropwise and slowly a solution of iron(II) tetrafluoroborate hexahydrate (162 mg, 0.48 mmol) in ethanol (1.5 mL). A yellow suspension resulted, which was filtered. The residue was washed with diethyl ether (5 mL) and dried in vacuo (yield: 57%). Single crystals for X-ray crystallography were obtained by isothermal diffusion of diethyl ether into a solution of 1(BF4)2 in acetonitrile at 3 °C. HR-MS (ESI) calcd for C15H21FeN12PS: [M]2+ 244.0404; found: [M] 2+ 244.0405; Elemental Analysis: Found (calcd) for C15H21FeN12PSB2F8·0.5EtOH: C 27.96 (28.06), H 3.05 (3.53), N 24.79 (24.54), S 4.27 (4.68). 1H NMR (200 MHz, CD3CN, 296 K): δ [ppm] = 1.1 (9H, ω1/2 = 29 Hz), 3.4 (3H, ω1/2 = 36 Hz), 19.6 (3H, ω1/2 = 80 Hz), 77.0 (3H, ω1/2 = 194 Hz), 82.0 (3H, ω1/2 = 248 Hz). IR (KBr): ν̃ = 3355 s (broad), 3133 m, 2917 m, 2847 w, 1608 m (broad), 1466 s, 1452 s, 1293 m, 1216 m, 1170 s, 1077 s, 971 s, 858 w, 768 s, 718 m, 619 m, 522 m, 496 m. 1(OTf)2. L1 (108 mg, 0.25 mmol) and dry Fe(MeCN)2(OTf)2 (101 mg, 0.25 mmol) were combined. Dry tetrahydrofuran (10 mL) was added, and the mixture was stirred overnight. The resulting yellow suspension was then filtered, and the residue was washed with dry diethyl ether (5 mL) and dried in vacuo (yield: 74%). Single crystals were grown by isothermal diffusion of hexane (15 mL) into a solution of 1(OTf)2 in ethanol (5 mL) at room temperature. mp (Ar, sealed capillary) 272−280 °C (decomp). HR-MS (ESI) calcd for C15H20FeN12PS: [M−H]+ 487.0742; found: [M−H]+ 487.0970; Elemental Analysis: Found (calcd) for C17H21F6FeN12O6PS3·0.25 EtOH: C 26.09 (26.34), H 2.42 (2.84), N 20.67 (21.06), S 11.95 (12.05). 1H NMR (300 MHz, 298 K, CD3CN): δ [ppm] = 1.4 (9 H, ω1/2 = 51 Hz), 3.6 (3 H, ω1/2 = 9 Hz), 19.9 (3 H, ω1/2 = 39 Hz), 78.2 (3 H, ω1/2 = 194 Hz), 83.2 (3 H, ω1/2 = 258 Hz). 31P{1H} NMR (121 MHz, 298 K, CD3CN): δ [ppm] = −257.1. 19F NMR (282 MHz, 298 K, CD3CN): δ [ppm] = −79.2. FT-IR (solid, ATR): ν̃ = 3128 w, 2916 w, 1611 vw, 1545 vw, 1447 vw, 1355 vw, 1280 m, 1239 s, 1222 s, 1163 s, 1086 m, 1028 s, 970 vs, 857 s, 768 s, 719 m, 634 s, 574 m, 516 vs, 490 s, 417 w. 2(BF4)2. L2 (490 mg, 1.04 mmol) and [Fe(MeCN)6](BF4)2 (490 mg, 1.04 mmol) were combined in a Schlenk tube. Dry tetrahydrofuran (10 mL) was added, and the mixture was stirred overnight. The resulting ocherous suspension was then filtered, and the residue was washed with dry tetrahydrofuran (5 mL) and dried in vacuo (yield: 93%). Single crystals were grown by diffusion of diethyl ether (30 mL) into a solution of 2(BF4)2 in acetonitrile (20 mL) at room temperature. mp (Ar, sealed capillary) 294−302 °C (decomp). J


Article

Inorganic Chemistry HR-MS (ESI) calc. for C18H26FeN12PS: [M−H]+ 529.1211; found: [M−H] + 529.1246; Elemental Analysis: Found (calcd) for C18H27B2F8FeN12PS: C 30.82 (30.71), H 3.78 (3.87), N 23.72 (23.88), S 4.76 (4.55). 1H NMR (300 MHz, 298 K, CD3CN): δ [ppm] = 7.1 (9 H, ω1/2 = 79 Hz), 18.9 (9 H, ω1/2 = 228 Hz), 44.5 (3 H, ω1/2 = 557 Hz), 50.7 (3 H, ω1/2 = 660 Hz), 63.4 (3 H, ω1/2 = 564 Hz). 31P{1H} NMR (121 MHz, 298 K, CD3CN): δ [ppm] = −245.6. 19 F NMR (282 MHz, 298 K, CD3CN): δ [ppm] = −151.1. FT-IR (solid, ATR): ν̃ = 3154 v, 1583 w, 1490 w, 1460 w, 1425 w, 1288 vw, 1233 w, 1181 w, 1158 w, 1053 vs, 972 vs, 885 s, 773 s, 733 m, 714 m, 684 w, 635 w, 502 vs, 407 w, 386 w. Computational Details. All DFT calculations were performed using ORCA2.9.1.59 TZVP basis sets60 were used throughout. B3LYP* (15% exact exchange) was used as a reparameterized version of the B3LYP61,62 functional (with 20% exact exchange) to account for the SCO energy in iron(II) complexes. The SCF energies were converged to 1 × 10−7 Hartree in energy. Dispersion contributions were approximated using Grimme’s DFT-D6 atom-pairwise dispersion corrections of the parent B3LYP functional.63 Cartesian coordinates of the optimized structures of [1]2+−[4]2+ (ls and hs) are compiled in the Tables S2−S9. Numerical frequency calculations revealed the stationary points to be minima on the potential surface. It is noted that both spin states of [2]2+ gave rise to one low-frequency imaginary mode, ascribable to rotation of a methyl substituent of an imidazole ring. Relaxed surface scans along the torsion coordinate θ (with 0° ≤ θ ≤ 60°) were performed with imposed threefold symmetry by constraining the three intrapodal torsions to θ. Solvent effects were taken into account at the polarizable continuum model level, using conductor-like screening model64 implemented in ORCA, with permittivity ε = 37.5 for acetonitrile.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.H. thanks Prof. Dr. A. Grohmann (Technische Univ. Berlin) for continuous and generous support and Dr. T. Pȩdziński (Adam-Mickiewicz Univ. Poznan) for assistance with laser flash photolysis. We thank E. Moos (KIT Karlsruhe) for technical assistance and N. Kroll (Technische Univ. Berlin) for providing a sample of 4(BF4)2.

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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.6b00238. Copies of the crystallographic data can be obtained, free of charge, on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K., or online at https:// summary.ccdc.cam.ac.uk/structure-summary-form. Additional SQUID and Mö ssbauer data, transient absorption spectra, Van’t Hoff and Arrhenius analyses, Cartesian coordinates of computed structures, and sketches of reference polyhedral. (PDF) X-ray crystallographic information of 1(BF4)2, 1(OTf)2, and 2(BF4)2. (CIF)

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REFERENCES

(1) (a) Kahn, O.; Martinez, C. J. Science 1998, 279, 44−48. (b) Létard, J.-F.; Guionneau, P.; Goux-Capes, L. Top. Curr. Chem. 2004, 235, 221−249. (c) Gütlich, P.; Gaspar, A. B.; Garcia, Y. Beilstein J. Org. Chem. 2013, 9, 342−391. (d) Bousseksou, A.; Molnar, G.; Salmon, L.; Nicolazzi, W. Chem. Soc. Rev. 2011, 40, 3313−3335. (2) Milek, M.; Heinemann, F. W.; Khusniyarov, M. M. Inorg. Chem. 2013, 52, 11585−11592. (3) Rösner, B.; Milek, M.; Witt, A.; Gobaut, B.; Torelli, P.; Fink, R. H.; Khusniyarov, M. M. Angew. Chem., Int. Ed. 2015, 54, 12976− 12980. (4) Venkataramani, S.; Jana, U.; Dommaschk, M.; Sönnichsen, F. D.; Tuczek, F.; Herges, R. Science 2011, 331, 445−448. (5) Dommaschk, M.; Peters, M.; Gutzeit, F.; Schütt, C.; Näther, C.; Sönnichsen, F. D.; Tiwari, S.; Riedel, C.; Boretius, S.; Herges, R. J. Am. Chem. Soc. 2015, 137, 7552−7555. (6) Decurtins, S.; Gütlich, P.; Köhler, C. P.; Spiering, H.; Hauser, A. Chem. Phys. Lett. 1984, 105, 1−4. (7) Letard, J.-F.; Capes, L.; Chastanet, G.; Moliner, N.; Letard, S.; Real, J.-A.; Kahn, O. Chem. Phys. Lett. 1999, 313, 115−120. (8) (a) Hauser, A.; Enachescu, C.; Lawson Daku, M.; Vargas, A.; Amstutz, N. Coord. Chem. Rev. 2006, 250, 1642−1652. (b) Marchivie, M.; Guionneau, P.; Letard, J.-F.; Chasseau, D. Acta Crystallogr., Sect. B: Struct. Sci. 2005, 61, 25−28. (9) Klingele, J.; Kaase, D.; Schmucker, M.; Lan, Y. H.; Chastanet, G.; Letard, J.-F. Inorg. Chem. 2013, 52, 6000−6010. (10) Dose, E. V.; Hoselton, M. A.; Sutin, N.; Tweedle, M. F.; Wilson, L. J. J. Am. Chem. Soc. 1978, 100, 1141−1147. (11) (a) McGravey, J. J.; Lawthers, I. J. Chem. Soc., Chem. Commun. 1982, 906−907. (b) Beattie, J. K. Adv. Inorg. Chem. 1988, 32, 1−53. (c) Toftlund, H. Coord. Chem. Rev. 1989, 94, 67−108. (12) Xie, C.-L.; Hendrickson, D. N. J. Am. Chem. Soc. 1987, 109, 6981−6988. (13) Al-Obaidi, A. H. R.; Jensen, K. B.; McGarvey, J. J.; Toftlund, H.; Jensen, B.; Bell, S. E. J.; Carroll, J. G. Inorg. Chem. 1996, 35, 5055− 5060. (14) (a) Chang, H.-R.; McCusker, J. K.; Toftlund, H.; Wilson, S. R.; Trautwein, A. X.; Winkler, H.; Hendrickson, D. N. J. Am. Chem. Soc. 1990, 112, 6814−6867. (b) McCusker, J. K.; Toftlund, H.; Rheingold, A. L.; Hendrickson, D. N. J. Am. Chem. Soc. 1993, 115, 1797−1804. (15) Brady, C.; McGarvey, J. J.; McCusker, J. K.; Toftlund, H.; Hendrickson, D. N. Top. Curr. Chem. 2004, 235, 1−22. (16) McCusker, J. K.; Rheingold, A. L.; Hendrickson, D. N. Inorg. Chem. 1996, 35, 2100−2112. (17) Hain, S. K.; Heinemann, F. W.; Gieb, K.; Müller, P.; Hörner, G.; Grohmann, A. Eur. J. Inorg. Chem. 2010, 2010, 221−232. (18) Spin-Crossover Materials: Properties and Applications; Halcrow, M. A., Ed.; John Wiley & Sons: Chichester, U.K, 2013. (19) Halcrow, M. A. Chem. Soc. Rev. 2011, 40, 4119−4142 and references therein.. (20) Gawelda, W.; Pham, V. T.; Benfatto, M.; Zaushitsyn, Y.; Kaiser, M.; Grolimund, D.; Johnson, S. L.; Abela, R.; Hauser, A.; Bressler, C.; Chergui, M. Phys. Rev. Lett. 2007, 98, 057401. (21) Buhks, E.; Navon, G.; Bixon, M.; Jortner, J. J. Am. Chem. Soc. 1980, 102, 2918−2923. (22) (a) Hauser, A.; Vef, A.; Adler, P. J. Chem. Phys. 1991, 95, 8710− 8717. (b) Gütlich, P.; Hauser, A.; Spiering, H. Angew. Chem., Int. Ed.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Present Address ⊥

Dr. Philipp Stock, Department of Interface Chemistry and Surface Engineering, Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Straße 1, 40237 Düsseldorf, Germany. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

Support of this work by the Deutsche Forschungsgemeinschaft (SFB 658, Elementary processes in molecular switches on surfaces) and of the Federal Ministry of Education and Research (BMBF Project Nos. 02NUK020B and 02NUK020C) is gratefully acknowledged. K


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Inorganic Chemistry Engl. 1994, 33, 2024−2054. (c) Hauser, A. Comments Inorg. Chem. 1995, 17, 17−40. (d) Hauser, A. Top. Curr. Chem. 2004, 234, 155− 198. (23) Al-Obaidi, A. H. R.; McGarvey, J. J.; Taylor, K. P.; Bell, S. E. J.; Jensen, K. B.; Toftlund, H. J. Chem. Soc., Chem. Commun. 1993, 536− 538. (24) The phenomenological and conceptual connection between spin crossover and trigonal twisting motion has a history going back to the 1970s: (a) Fleischer, E. B.; Gebala, A. E.; Swift, D. R.; Tasker, P. A. Inorg. Chem. 1972, 11, 2775−2784. (b) Wentworth, R. A. D. Coord. Chem. Rev. 1972, 9, 171−187. (c) Purcell, K. F. J. Am. Chem. Soc. 1979, 101, 5147−5152. (d) Vanquickenborne, L. G.; Pierloot, K. Inorg. Chem. 1981, 20, 3673−3677. (f) Comba, P. Inorg. Chem. 1989, 28, 426−431. (25) (a) Larsen, E.; La Mar, G. N.; Wagner, B. E.; Parks, J. E.; Holm, R. H. Inorg. Chem. 1972, 11, 2652−2668. (b) Comba, P.; Sargeson, A. M.; Engelhardt, L. M.; Harrowfield, J. M. B.; White, A. H.; Horn, E.; Snow, M. R. Inorg. Chem. 1985, 24, 2325−2327. (c) Alvarez, S.; Avnir, D.; Llunell, M.; Pinsky, M. New J. Chem. 2002, 26, 996−1009. (d) Knight, J. C.; Alvarez, S.; Amoroso, A. J.; Edwards, P. G.; Singh, N. Dalton Trans. 2010, 39, 3870−3883. (e) Alvarez, S. Chem. Rev. 2015, 115, 13447−13483. (26) Schenker, S.; Stein, P. C.; Wolny, J. A.; Brady, C.; McGarvey, J. J.; Toftlund, H.; Hauser, A. Inorg. Chem. 2001, 40, 134−139. (27) Stock, P.; Pędziński, T.; Spintig, N.; Grohmann, A.; Hörner, G. Chem. - Eur. J. 2013, 19, 839−842. (28) Stock, P.; Spintig, N.; Scholz, J.; Epping, J. D.; Oelsner, C.; Wiedemann, D.; Grohmann, A.; Hörner, G. J. Coord. Chem. 2015, 68, 3099−3115. (29) Chandrasekhar, V.; Azhakar, R.; Pandian, B. M.; Boomishankar, R.; Steiner, A. Dalton Trans. 2008, 37, 5962−5969. (30) Trapp, I.; Löble, M. W.; Meyer, J.; Breher, F. Inorg. Chim. Acta 2011, 374, 373−384. (31) Löble, M. W.; Ona-Burgos, P.; Fernandez, I.; Apostolidis, C.; Morgenstern, A.; Walter, O.; Bruchertseifer, F.; Kaden, P.; Vitova, T.; Rothe, J.; Dardenne, K.; Banik, N. L.; Geist, A.; Denecke, M. A.; Breher, F. Chem. Sci. 2013, 4, 3717−3724. (32) Löble, M. W.; Casimiro, M.; Thielemann, D. T.; Ona-Burgos, P.; Fernandez, I.; Roesky, P. W.; Breher, F. Chem. - Eur. J. 2012, 18, 5325−5334. (33) Cremades, E.; Echeverria, J.; Alvarez, S. Chem. - Eur. J. 2010, 16, 10380−10396. (34) For a review on multifunctional ligands, see: (a) Kuzu, I.; Krummenacher, I.; Meyer, J.; Armbruster, F.; Breher, F. Dalton Trans. 2008, 37, 5836−5865. (35) Brewer, G.; Olida, M. J.; Schmiedekamp, A. M.; Viragh, C.; Zavalij, P. Y. Dalton Trans. 2006, 35, 5617−5629. (36) Sunatsuki, Y.; Ikuta, Y.; Matsumoto, N.; Ohta, H.; Kojima, M.; Iijima, S.; Hayami, S.; Maeda, Y.; Kaizaki, S.; Dahan, F.; Tuchagues, J.P. Angew. Chem., Int. Ed. 2003, 42, 1614−1618. (37) Ohta, H.; Sunatsuki, Y.; Kojima, M.; Iijima, S.; Akashi, H.; Matsumoto, N. Chem. Lett. 2004, 33, 350−351. (38) Sunatsuki, Y.; Ohta, H.; Kojima, M.; Ikuta, Y.; Goto, Y.; Matsumoto, N.; Iijima, S.; Akashi, H.; Kaizaki, S.; Dahan, F.; Tuchagues, J.-P. Inorg. Chem. 2004, 43, 4154−4171. (39) Brewer, C.; Brewer, G.; Luckett, C.; Marbury, G. S.; Viragh, C.; Beatty, A. M.; Scheidt, W. R. Inorg. Chem. 2004, 43, 2402−2415. (40) Seredyuk, M.; Gütlich, P. Z. Anorg. Allg. Chem. 2011, 637, 965− 976. (41) El Hajj, F.; Sebki, G.; Patinec, V.; Marchivie, M.; Triki, A.; Handel, H.; Yefsah, S.; Tripier, R.; Gómez-Garcia, C. J.; Coronado, E. Inorg. Chem. 2009, 48, 10416−10423. (42) Both of the signal parts of the doublet have the same integrated signal area. Sources of asymmetry in Mössbauer spectra: (a) Gütlich, P.; Bill, E.; Trautwein, A. X. Mössbauer Spectroscopy and Transistion Metal Chemistry; Springer-Verlag: Berlin, Germany, 2011. (b) Blume, M. Phys. Rev. Lett. 1965, 14, 96−98. (c) Long, G. J.; Robinson, W. T.; Tappmeyer, W. P.; Bridges, D. L. J. Chem. Soc., Dalton Trans. 1973, 573−579.

(43) Voloshin, Y. Z.; Varzatskii, O. A.; Stash, A. I.; Belsky, V. K.; Bubnov, Y. N.; Vorontsov, I. L.; Potekhin, K. A.; Antipin, M. Y.; Polshin, E. V. Polyhedron 2001, 20, 2721−2733. (44) Burdukov, A. B.; Vershinin, M. A.; Pervukhina, N. V.; Boguslvaskii, E. G.; Eltsov, I. V.; Shundrin, L. A.; Selector, S. L.; Shokurov, A. V.; Voloshin, Y. Z. Inorg. Chem. Commun. 2014, 44, 183− 187. (45) Gütlich, P.; Goodwin, H. A. Top. Curr. Chem. 2004, 233, 1−47. (46) The tren-derived hexadentate ligands have been previously described as “3 × 2” coordinating ligands: Lazar, H. Z.; Forestier, T.; Barrett, S. A.; Kilner, C. A.; Letard, J.-F.; Halcrow, M. A. Dalton Trans. 2007, 36, 4276−4285. (47) Guionneau, P.; Marchivie, M.; Bravic, G.; Letard, J.-F.; Chasseau, D. Top. Curr. Chem. 2004, 234, 97−128. (48) Comba, P. Coord. Chem. Rev. 1993, 123, 1−48. (49) Bailar, J. C., Jr. J. Inorg. Nucl. Chem. 1958, 8, 165−175. (50) (a) Gütlich, P.; Garcia, Y.; Goodwin, H. A. Chem. Soc. Rev. 2000, 29, 419−427. (b) Toftlund, H.; McGarvey, J. J. Top. Curr. Chem. 2004, 233, 151−166. (51) SCO-induced structure changes of this compound are subject to current experimental scrutiny: (a) Vanko, G.; Bordage, A.; Papai, M.; Haldrup, K.; Glatzel, P.; March, A. M.; Doumy, G.; Britz, A.; Galler, A.; Assefa, T.; Cabaret, D.; Juhin, A.; van Driel, T. B.; Kjaer, K. S.; Dohn, A.; Moller, K. B.; Lemke, H. T.; Gallo, E.; Rovezzi, M.; Nemeth, Z.; Rozsalyi, E.; Rozgonyi, T.; Uhlig, J.; Sundström, V.; Nielsen, M. M.; Young, L.; Southworth, H.; Bressler, C.; Gawelda, W. J. Phys. Chem. C 2015, 119, 5888−5902. (b) Zhang, X.; Lawson Daku, M. L.; Zhang, J.; Suarez-Alcantara, K.; Jennings, G.; Kurtz, C. A.; Canton, S. E. J. Phys. Chem. C 2015, 119, 3312−3321. (c) Canton, S. E.; Zhang, X.; Lawson Daku, M. L.; Liu, Y.; Zhang, J.; Alvarez, S. J. Phys. Chem. C 2015, 119, 3322−3330. (52) Renz, F.; Oshio, H.; Ksenofontov, V.; Waldeck, M.; Spiering, H.; Gütlich, P. Angew. Chem., Int. Ed. 2000, 39, 3699−3700. (53) Howson, S. E.; Allan, L. E. N.; Chmel, N. P.; Clarkson, G. J.; Deeth, R. J.; Faulkner, A. D.; Simpson, D. H.; Scott, P. Dalton Trans. 2011, 40, 10416−10433. (54) Pedziński, T.; Markiewicz, A.; Marciniak, B. Res. Chem. Intermed. 2009, 35, 497−506. (55) (a) SHELXTL, v6.12; Bruker AXS Inst. Inc: Madison, WI, 2000. (b) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (56) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Adv. 2015, 71, 3−8. (57) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (58) Majoral, J. P.; Kraemer, K.; Navech, J.; Mathias, F. Tetrahedron 1976, 32, 2633−2644. (59) Neese, F. WIREs Comput. Mol. Sci. 2012, 2, 73−78. (60) Schäfer, A.; Horn, H.; Ahlrichs, R. J. Chem. Phys. 1992, 97, 2571−2577. (61) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (62) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (63) Grimme, S. J. Comput. Chem. 2006, 27, 1787−1799. (64) Klamt, A.; Schüürmann, G. J. Chem. Soc., Perkin Trans. 2 1993, 5, 799−805.

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