Abrupt versus Gradual Spin-Crossover in FeII(phen)2(NCS)2 and FeIII

Article pubs.acs.org/IC

Abrupt versus Gradual Spin-Crossover in FeII(phen)2(NCS)2 and FeIII(dedtc)3 Compared by X‑ray Absorption and Emission Spectroscopy and Quantum-Chemical Calculations Stefan Mebs,*,† Beatrice Braun,‡ Ramona Kositzki,† Christian Limberg,‡ and Michael Haumann*,† †

Institut für Experimentalphysik, Freie Universität Berlin, 14195 Berlin, Germany Institut für Chemie, Humboldt-Universität zu Berlin, 12489 Berlin, Germany

‡

S Supporting Information *

ABSTRACT: Molecular spin-crossover (SCO) compounds are attractive for information storage and photovoltaic technologies. We compared two prototypic SCO compounds with FeIIN6 (1, [Fe(phen)2(NCS)2], with phen = 1,10phenanthroline) or FeIIIS6 (2, [Fe(dedtc)3], with dedtc = N,N′-diethyldithiocarbamate) centers, which show abrupt (1) or gradual (2) thermally induced SCO, using K-edge X-ray absorption and Kβ emission spectroscopy (XAS/XES) in a 8− 315 K temperature range, single-crystal X-ray diffraction (XRD), and density functional theory (DFT). Core-to-valence and valence-to-core electronic transitions in the XAS/XES spectra and bond lengths change from XRD provided benchmark data, verifying the adequacy of the TPSSh/TZVP DFT approach for the description of low-spin (LS) and highspin (HS) species. Determination of the spin densities, charge distributions, bonding descriptors, and valence-level configurations, as well as similar experimental and calculated enthalpy changes (ΔH), suggested that the varying metal− ligand bonding properties and deviating electronic structures converge to similar enthalpic contributions to the free-energy change (ΔG) and thus presumably are not decisive for the differing SCO behavior of 1 and 2. Rather, SCO seems to be governed by vibrational contributions to the entropy changes (ΔS) in both complexes. Intra- and intermolecular interactions in crystals of 1 and 2 were identified by atoms-in-molecules analysis. Thermal excitation of individual dedtc ligand vibrations accompanies the gradual SCO in 2. In contrast, extensive inter- and intramolecular phen/NCS vibrational mode coupling may be an important factor in the cooperative SCO behavior of 1.

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INTRODUCTION Molecular spin-crossover (SCO) metal coordination compounds are attracting increasing research effort because of their potential applicability, for example, as reversible switches in information storage technologies or as dye sensitizers in solar cells.1−13 The metal spin state also is an important determinant for the functioning of metal cofactors in many biological enzymes.14−18 Comprehensive reviews covering all aspects of modern SCO research have been published recently.19−31 The SCO phenomenon, which is the transition from the lowspin (LS) to the high-spin (HS) electronic state in response to an external stimulus, is known for more than 80 years32 and typically observed in (pseudo)octahedrally coordinated transition-metal complexes with d4 to d7 valence electron configurations in solid-state structures. Particularly prominent in the field are iron complexes because of frequent observation of SCO in both iron(II) and iron(III) species.19,24,26,33 SCO is commonly induced by temperature variation, but pressure- or light-induced SCO systems also are well-known.34−41 SCO is further detected in polynuclear or polymeric compounds.42,43 Compounds showing thermally induced SCO can be classified © XXXX American Chemical Society

according to their actual transition temperature (Tc), sharpness of the transition (abrupt versus gradual SCO), and hysteresis effects (Tc variation for increasing or decreasing temperature). These properties may vary dramatically, for example, in response to ligand exchange and polarization changes of the metal−ligand bonding. Various theoretical models have been proposed to describe the temperature dependence of SCO, for example, in terms of enthalpy and entropy contributions to the free energy change, in particular emphasizing the crucial role of molecular vibrations and intermolecular interactions (cooperativity) for the SCO process.23,44−57 A unifying description or even prediction of the sophisticated interplay of the numerous parameters involved in SCO compounds, however, has turned out to be difficult. Determination of the relevant factors that govern the spin-transition properties is essential for the construction of tailored compounds with adapted SCO behavior for potential applications. Received: June 5, 2015

A

DOI: 10.1021/acs.inorgchem.5b01822 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

decay from occupied valence MOs (valence-to-core, v2c, decay).104−107 The c2v and v2c spectra thus access the valence-level structure. DFT calculation of both c2v and v2c transitions, on the other hand, is well established.90,100,102−104,108−118 The XAS/XES−DFT combination thus should be ideal for studying SCO compounds, but relatively few studies in which the three methods were combined have been reported so far.98,119−121 This calls for further exploration of the method’s full potential for characterization of SCO compounds. Herein, we employed XAS/XES and DFT to characterize two classical iron SCO complexes, namely, FeII(phen)2(NCS)2 (1; phen = 1,10-phenanthroline)122 and FeIII(dedtc)3 (2; dedtc = N,N′-diethyldithiocarbamate).32 These complexes are wellknown and have been studied extensively by other methods (see, for example, refs 26, 32, 39, 44, 46, 47,59, 60, 63−65, and 123−128), and 2 was even among the first SCO compounds discovered in the early 1930s.32 However, a combined XAS/ XES−DFT analysis was not yet available. The complexes vary in their formal oxidation states of the (pseudo)octahedral metal centers [FeII (1A1g, LS ground state, and 5T2g, HS ground state) in 1 versus FeIII (2T2g, LS ground state, and 6A1g, HS ground state) in 2], in the coordination of iron [nitrogen ligands (Nphen/NCS) in 1 versus sulfur ligands (Sdedtc) in 2], and in the temperature dependence of the SCO (abrupt in 1 versus gradual in 2).44,60 The Fe−N and Fe−S bonds both represent significantly polarized−covalent interactions, which combine covalent with electrostatic attraction. However, the larger differences in the atomic electronegativity of the Fe−N couple (1.8 for Fe vs 3.0 for N) compared to the Fe−S couple (1.8 for Fe vs 2.5 for S) show that the former interaction is expected to be more polarized than the latter interaction. Following the bond classification method of Green,129 the neutral phen ligands in 1 are two-electron donors (L2-type ligand) and form dative covalent bonds, whereas the charged [NCS]− ligands form shared covalent bonds (X-type ligand) with the Fe center, leading to a FeL4X2 coordination in 1. The dedtc ([S2C(EtOH)2]−) ligands are denoted as LX because they formally exhibit one dative and one shared covalent bond, leading to a FeL3X3 coordination in 2. Their differences in metal bonding and SCO behavior make 1 and 2 suitable model systems for comparative characterization by XAS/XES−DFT. Kα-detected and narrow-band Kβ-detected K-edge XAS and Kβ XES spectra in the 8−315 K temperature range were collected and revealed pronounced spectral changes in the LS and HS states of 1 and 2. Calculation of c2v and v2c spectra assigning electronic transitions and MO configurations was complemented by electronic structure analysis including spin density, charge distribution, bonding descriptor, and valencelevel energy determination, as well as by calculation of enthalpies and vibrational entropies by DFT using models based on single-crystal structures of 1 and 2. We present a comprehensive picture of the main constraints for the different SCO behavior in the two complexes.

An arsenal of experimental methods in principle facilitates monitoring of changes of the molecular structure, electronic configuration, thermodynamics, and vibrational dynamics in response to SCO, including X-ray and neutron diffraction, optical, nuclear magnetic and electron paramagnetic resonance (NMR and EPR), and Mössbauer spectroscopy, electrosusceptibility and magnetic moment studies, calorimetry, and IR, Raman, and nuclear inelastic scattering (NIS) spectroscopy, to name the most applied techniques, and many SCO compounds have been thoroughly characterized by combinations of these methods (for example, see refs 20, 35, 36, 41, 46, 48, and 58−71). However, some of these methods do not provide direct structural and electronic information at the same time and cannot detect both the LS and HS states as occurring at cryogenic or elevated temperatures, or species discrimination suffers from the rapid kinetics of the SCO process. The oftenrequired cryogenic temperatures, in particular for studies on biological materials such as metalloproteins, could even lead to spin-state assignments that do not reflect the functional state under ambient conditions. Accordingly, experimental methods that can probe both the molecular and electronic properties over a wide temperature range and in the LS and HS states in solid or solution samples and can yield spectroscopic information that can be reliably evaluated and interpreted by theoretical approaches are interesting for characterization of SCO compounds. Computational quantum chemistry using density functional theory (DFT) plays an increasingly important role in the characterization of SCO compounds because it offers access to the electronic structure, as well as to thermodynamic and vibrational properties, and much progress has been made in the development of suitable DFT approaches.22,28,72−79 Adequate DFT methods for characterization of SCO compounds should provide reliable molecular geometries, spin densities, and enthalpy/entropy changes and should assign the LS energetic ground state correctly. Recent work has demonstrated that, for example, the combination of the TPSSh hybrid functional for low exact exchange admixtures with a triple-ζ valence plus polarization basis set meets these requirements and yields reasonable structural, electronic, and thermodynamic parameters for various SCO compounds of iron.28,79−85 However, the outcome of DFT calculations needs to be verified for each system to gain confidence in the underlying model structures and assumptions by quantitative comparison to experimental benchmark data such as crystallography and spectroscopy results, which was attempted in the present work. Synchrotron-based X-ray absorption and emission spectroscopy (XAS/XES) techniques provide both molecular and electronic information, are applicable to samples in all aggregate states at cryogenic or elevated temperatures, and detect all spin or oxidation states of transition-metal systems equally well.86−97 Extended X-ray absorption fine structure (EXAFS) analysis provides precise metal−ligand bond lengths and interatomic distance distributions. The main Kβ emission lines in XES spectra are a sensitive measure of the spin state due to spin polarization of the Kβ1,3 and Kβ′ features (3p → 1s decay) in response to strong metal(p−d) electron coupling.92,98−100 The low-energy region (preedge) of the K-edge absorption (X-ray absorption near-edge structure, XANES) selectively probes resonant excitation of core-level (1s) electrons into unoccupied valence molecular orbitals (MOs) with metal(d) or ligand(s,p) character (core-to-valence, c2v, transitions),101−103 whereas the complementary Kβ2,5 emission lines reflect radiative electronic

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MATERIALS AND METHODS

Sample Preparation. Polycrystalline powder of FeII(phen)2(NCS)2 (1; phen = 1,10-phenanthroline)122 was kindly provided by W. Kuch (Physics Department, Freie Universität Berlin) and synthesized in the group of F. Renz (Chemistry Department, Leibniz Universität Hannover). Polycrystalline powder of FeIII(dedtc)3 (2; dedtc = N,N′-diethyldithiocarbamate) was purchased from SigmaAldrich. All chemicals were at least analysis grade. For X-ray B

DOI: 10.1021/acs.inorgchem.5b01822 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Metric Parameters of 1 and 2 from XRD, XAS, and DFT bond lengths [Å] 1

1

2

spin

T [K]

method

Fe−Nphena

Fe−Cphen

LS LS LS HS HS HS

130 80 d 293 280 d

XRDb XAS DFTe XRDb XAS DFTe

2.01/2.01 2.00(2)c 1.98/1.99 2.20/2.21 2.19(2)c 2.21/2.29

2.83 2.83(3)c 2.80 3.03 3.02(3)c 3.09

spin

T [K]

method

Ncis−Fe−Ncis

LS LS HS HS

130 d 293 d

XRDb DFTe XRDb DFTe

170.7 178.9 160.2 161.4

NCS−Fe−Ntrans

Fe−N(CS)

Fe−(NC)S

NCS

NCS

4.70 4.57(9)c 4.72 4.81 4.74(9)c 4.80

1.14 nd 1.18 1.16 nd 1.19

1.64 nd 1.63 1.63 nd 1.62

1.96 1.92(2) 1.94 2.06 2.03(2) 2.01 angles [deg] Ncis−Fe−Ntrans

172.8 175.2 166.4 162.1 bond lengths [Å]

91.9 93.4 89.7 81.2

SCN−Fe−NCS 90.6 93.3 94.8 103.9

Fe−NC

165.7 165.8 167.1 171.7 angles [deg]

spin

T [K]

method

Fe−S

S−C

C−N

Fe−C

S−Fe−S

S−C−S

LS LS LS 30:70 50:50 HS HS

80 8 d 350 284 f d

XRD XAS DFTe XRD XAS XAS DFTe

2.30 2.30(2)c 2.33 2.37 2.34(2)c 2.47(2)c 2.49

1.72 nd 1.73 1.71 nd nd 1.74

1.33 nd 1.34 1.31 nd nd 1.34

2.80 2.84(3)c 2.79 2.85 2.79(3)c 3.03(3)c 2.91

163.8 nd 163.3 160.8 nd nd 159.5

111.0 nd 111.5 112.6 nd nd 116.0

a Values for Ntrans/Ncis refer to the phenanthroline N atoms opposite (trans) or in cis position to the NCS groups (Figure 1). bXRD data for 1 were taken from ref 122. cThe error (in parentheses) for the XAS data refers to a (±) distance variation of the bond lengths, which increases the fit error sum by 5% (Table S4(A,B)); the error in the bond lengths from XRD was