Communication pubs.acs.org/IC
A Mixed-Valence Fluorido-Bridged FeIIFeIII Complex Susanne Dammers,† Thomas Philipp Zimmermann,† Stephan Walleck,† Anja Stammler,† Hartmut Bögge,† Eckhard Bill,‡ and Thorsten Glaser*,† †
Lehrstuhl für Anorganische Chemie I, Fakultät für Chemie, Universität Bielefeld,D-33615 Bielefeld, Germany Max-Planck-Institute for Chemical Energy Conversion, D-45470 Mülheim an der Ruhr, Germany
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S Supporting Information *
ABSTRACT: The reaction of the new dinucleating ligand susan6‑Me with Fe(BF4)2·6H2O results in formation of the homovalent Fe II Fe II complex [(susan 6‑Me ){Fe II (μF)2FeII}]2+ and the mixed-valence FeIIFeIII complex [(susan6‑Me){FeIIF(μ-F)FeIIIF}]2+ depending on the absence or presence of dioxygen, respectively. Complex [(susan6‑Me){FeIIF(μ-F)FeIIIF}]2+ is the first molecular mixed-valence complex with a fluorido bridge. The short FeIII−μ-F bond of 1.87 Å causes a large reorganization energy, resulting in a localized class II system with an intervalence charge-transfer band of high energy at 10000 cm−1. erein, we report the first molecular fluorido-bridged mixed-valence FeIIFeIII complex. Mixed-valence complexes have been extensively investigated in the last decades for the fundamental study of electron transfer and electronic structure in general.1−3 Mixed-valence compounds of iron, in particular, have attracted interest because of their presence in the active site of metalloproteins4 and their interesting properties like double exchange.5,6 The coordination chemistry of complexes with Fe−F bonds is less well explored than that with other donor atoms, but complexes with Fe−F bonds have recently received increased attention in the field of magnetic materials7−9 and catalysis.10 Three-dimensional (3D) network structures of mixed-valence iron fluorides have been explored11−14 and are of relevance for cathode materials for rechargeable lithium batteries.15−17 We have developed a new class of dinucleating ligands that possess various terminal donor groups but provide no bridging donors to stabilize dinuclear complexes with external bridging ligands.18,19 The ligand susan was prepared by the reductive amination of pyridine aldehyde 2 with tetramine 1 (Figure 1a). As a slight variation of the ligand susan, we have prepared the ligand susan6‑Me by using the methyl-substituted pyridine carbaldehyde 3 (Figure 1a). The reaction of susan6‑Me with Fe(BF4)2·6H2O and NEt3 under an argon-blanketing atmosphere resulted in the formation of green crystals of [(susan6‑Me){FeII(μ-F)2FeII}](BF4)2 (4), indicating fluoride abstraction from BF4−.20 Upon crystallization under oxygen, green crystals of 4 deposited, followed by brown crystals, which were analyzed by single-crystal X-ray diffraction as [(susan6‑Me){FeIIF(μ-F)FeIIIF}](BF4)2 (5). Optimization of the reaction conditions allowed the isolation of pure samples of both complexes 4 and 5 (see the Supporting Information for details).
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© XXXX American Chemical Society
Figure 1. (a) Synthesis of the ligand susan6‑Me. Molecular structures of (b) [(susan6‑Me){FeII(μ-F)2FeII}]2+ in crystals of 4 and (c) [(susan6‑Me){FeIIF(μ-F)FeIIIF}]2+ in crystals of 5. Selected interatomic distances for 4 [5]: Fe1−N1 2.281(5) [2.2944(16)], Fe1−N2 2.172(5) [2.2343(16)], Fe1−N3 2.157(4) [2.2279(17)], Fe1−N4 2.208(5) [2.2276(17)], Fe1−F1 2.287(3) [1.8939(11)], Fe1−F2 1.930(3), Fe1− F3 [2.1624(10)], Fe2−N41 2.340(5) [2.2078(16)], Fe2−N42 2.163(5) [2.1988(16)], Fe2−N43 2.192(5) [2.1639(15)], Fe2−N44 2.220(5) [2.2196(16)], Fe2−F1 1.917(3), Fe2−F2 2.241(3) [1.8175(11)], Fe2− F3 [1.8704(10)], Fe1···Fe2 3.2757(10) [4.0031(4)].
The molecular structures of both complexes are shown in Figure 1b,c. Thermal ellipsoid plots and more bond lengths and angles are provided in the Supporting Information. From charge considerations, the iron ions in 4 are both FeII, which is supported by bond-valence-sum calculations (Fe1, 2.27; Fe2, 2.32). The angles Fe1−F1−F2 and Fe1−F2−Fe2 are 102.0° and 103.3°, respectively. Although the ligand susan6‑Me prohibits inversion symmetry, the central {Fe2(μ-F)2} core of 4 is almost planar (torsion angles 10000 cm−1 because of the strong electronic coupling matrix element HAB,5,32−35 the high IVCT energy in the valencelocalized complex 5 arises from the strong reorganization energy λ.3 This can be assigned to the presence of the strong short FeIII−μ-F and weak long FeII−μ-F bonds, providing a strong localization force. In summary, the complex [(susan6‑Me){FeIIF(μ-F)FeIIIF}]2+ is the first mixed-valence FeIIFeIII complex with a fluorido bridge. The bond-length distribution in the {FeII(μ-F)FeIII} core is highly asymmetric with a short FeIII−μ-F bond and a long FeIII−μ-F bond. This results in a large reorganization energy barrier and a valence-localized nature. This study shows the ability of our new bis(tetradentate) dinucleating ligand system to support and stabilize unusual dinuclear core structures, and the flexibility of the ligand backbone allows rearrangement for monoand dibridged core structures.
Figure 3. (a) Temperature-dependent magnetic measurements of 4 (black) and 5 (red) at 1 T. Solid lines correspond to simulations using the parameters provided in the text. (b) X-band EPR spectrum of 5 recorded in frozen CH3CN at 7.9 K. Experimental conditions: ν = 9.42886 GHz; microwave power = 0.1 mW; field modulation = 10 G; powder simulation with S = 1/2; g = [1.868, 1.861, 1816]; Lorentzian lines with 16 mT full width at half-maximum with a 36% Gaussian contribution. (c) Electronic absorption spectra of solutions of complexes 4 and 5 and the free ligand susan6‑Me in CH3CN.
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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.6b03093. Experimental details, Figures S1−S4, and Tables S1 and S2 (PDF) CIF file (CIF) CIF file (CIF)
Because 5 is the first molecular mixed-valence FeIIFeIII complex with a fluorido bridge, a comparison of its coupling constant is not straightforward. In the class II mixed-valence 3D solid Fe2F5·2H2O, a ferrimagnetic order at TC = 48 K indicates also antiferromagnetic coupling.13 In a FeIIFeII complex of trigonal-bipyramidal coordination with a linear fluorido bridge, the exchange is even stronger (J = −16.3 cm−1) than that in 5 because of a shorter FeII−μ-F bond length of 2.02 Å.9 To the best of our knowledge, an analogous FeIIIFeIII with a single fluorido bridge has not been reported. Interestingly, in the triple-fluoridobridged complex [F3Fe(μ-F)3FeF3]3−, a weak ferromagnetic interaction (J = 0.7 cm−1) was reported.29 In this respect, the coupling constant of J = −10.1 cm−1 is relatively large for fluorido-bridged systems despite the long FeII−μ-F bond. The UV−vis spectra (Figure 3c) provide intense absorption features above 35000 cm−1 assigned mainly to π−π* transitions of the pyridine ligands. The diferrous complex 4 exhibits an
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Thorsten Glaser: 0000-0003-2056-7701 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by a grant from the Deutsche Forschungsgemeinschaft. C
DOI: 10.1021/acs.inorgchem.6b03093 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.6b03093 Inorg. Chem. XXXX, XXX, XXX−XXX
