1
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Kulmaczewski R, Armstrong IT, Catchpole P, Ratcliffe ESJ, Vasili HB, Warriner SL, Cespedes O, Halcrow MA. Di-Iron(II) [2+2] Helicates of Bis-(Dipyrazolylpyridine) Ligands: The Influence of the Ligand Linker Group on Spin State Properties. Chemistry 2023; 29:e202202578. [PMID: 36382594 PMCID: PMC10108139 DOI: 10.1002/chem.202202578] [Citation(s) in RCA: 2] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2022] [Revised: 11/15/2022] [Accepted: 11/16/2022] [Indexed: 11/18/2022]
Abstract
Four bis[2-{pyrazol-1-yl}-6-{pyrazol-3-yl}pyridine] ligands have been synthesized, with butane-1,4-diyl (L1 ), pyrid-2,6-diyl (L2 ), benzene-1,2-dimethylenyl (L3 ) and propane-1,3-diyl (L4 ) linkers between the tridentate metal-binding domains. L1 and L2 form [Fe2 (μ-L)2 ]X4 (X- =BF4 - or ClO4 - ) helicate complexes when treated with the appropriate iron(II) precursor. Solvate crystals of [Fe2 (μ-L1 )2 ][BF4 ]4 exhibit three different helicate conformations, which differ in the torsions of their butanediyl linker groups. The solvates exhibit gradual thermal spin-crossover, with examples of stepwise switching and partial spin-crossover to a low-temperature mixed-spin form. Salts of [Fe2 (μ-L2 )2 ]4+ are high-spin, which reflects their highly twisted iron coordination geometry. The composition and dynamics of assembly structures formed by iron(II) with L1 -L3 vary with the ligand linker group, by mass spectrometry and 1 H NMR spectroscopy. Gas-phase DFT calculations imply the butanediyl linker conformation in [Fe2 (μ-L1 )2 ]4+ influences its spin state properties, but show anomalies attributed to intramolecular electrostatic repulsion between the iron atoms.
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Affiliation(s)
- Rafal Kulmaczewski
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
| | - Isaac T Armstrong
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
| | - Pip Catchpole
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK.,Department of Chemistry, Lancaster University, Lancaster, LA1 4YB, UK
| | - Emily S J Ratcliffe
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
| | - Hari Babu Vasili
- School of Physics and Astronomy W. H. Bragg Building, University of Leeds, Leeds, LS2 9JT, UK
| | - Stuart L Warriner
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
| | - Oscar Cespedes
- School of Physics and Astronomy W. H. Bragg Building, University of Leeds, Leeds, LS2 9JT, UK
| | - Malcolm A Halcrow
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, UK
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2
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Ahmed M, Arachchige KSA, Xie Z, Price JR, Cruddas J, Clegg JK, Powell BJ, Kepert CJ, Neville SM. Guest-Induced Multistep to Single-Step Spin-Crossover Switching in a 2-D Hofmann-Like Framework with an Amide-Appended Ligand. Inorg Chem 2022; 61:11667-11674. [PMID: 35862437 DOI: 10.1021/acs.inorgchem.2c01253] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
A detailed study of the two-dimensional (2-D) Hofmann-like framework [Fe(furpy)2Pd(CN)4]·nG (furpy: N-(pyridin-4-yl)furan-2-carboxamide, G = H2O,EtOH (A·H2O,Et), and H2O (A·H2O)) is presented, including the structural and spin-crossover (SCO) implications of subtle guest modification. This 2-D framework is characterized by undulating Hofmann layers and an array of interlayer spacing environments─this is a strategic approach that we achieve by the inclusion of a ligand with multiple host-host and host-guest interaction sites. Variable-temperature magnetic susceptibility studies reveal an asymmetric multistep SCO for A·H2O,Et and an abrupt single-step SCO for A·H2O with an upshift in transition temperature of ∼75 K. Single-crystal analyses show a primitive orthorhombic symmetry for A·H2O,Et characterized by a unique FeII center─the multistep SCO character is attributed to local ligand orientation. Counterintuitively, A·H2O shows a triclinic symmetry with two inequivalent FeII centers that undergo a cooperative single-step high-spin (HS)-to-low-spin (LS) transition. We conduct detailed structure-function analyses to understand how the guest ethanol influences the delicate balance between framework communication and, therefore, the local structure and spin-state transition mechanism.
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Affiliation(s)
- Manan Ahmed
- School of Chemistry, The University of New South Wales, Sydney 2052, Australia
| | - Kasun S A Arachchige
- School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, Queensland 4072, Australia
| | - Zixi Xie
- The School of Chemistry, The University of Sydney, Camperdown, New South Wales 2006, Australia
| | - Jason R Price
- Australian Synchrotron, ANSTO Clayton, Victoria 3800, Australia
| | - Jace Cruddas
- School of Mathematics and Physics, The University of Queensland, St. Lucia, Queensland 4072, Australia
| | - Jack K Clegg
- School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, Queensland 4072, Australia
| | - Benjamin J Powell
- School of Mathematics and Physics, The University of Queensland, St. Lucia, Queensland 4072, Australia
| | - Cameron J Kepert
- The School of Chemistry, The University of Sydney, Camperdown, New South Wales 2006, Australia
| | - Suzanne M Neville
- School of Chemistry, The University of New South Wales, Sydney 2052, Australia
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3
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Kuppusamy SK, Mizuno A, García-Fuente A, van der Poel S, Heinrich B, Ferrer J, van der Zant HSJ, Ruben M. Spin-Crossover in Supramolecular Iron(II)-2,6-bis(1 H-Pyrazol-1-yl)pyridine Complexes: Toward Spin-State Switchable Single-Molecule Junctions. ACS OMEGA 2022; 7:13654-13666. [PMID: 35559184 PMCID: PMC9088905 DOI: 10.1021/acsomega.1c07217] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 12/22/2021] [Accepted: 03/21/2022] [Indexed: 05/27/2023]
Abstract
Spin-crossover (SCO) active iron(II) complexes are an integral class of switchable and bistable molecular materials. Spin-state switching properties of the SCO complexes have been studied in the bulk and single-molecule levels to progress toward fabricating molecule-based switching and memory elements. Supramolecular SCO complexes featuring anchoring groups for metallic electrodes, for example, gold (Au), are ideal candidates to study spin-state switching at the single-molecule level. In this study, we report on the spin-state switching characteristics of supramolecular iron(II) complexes 1 and 2 composed of functional 4-([2,2'-bithiophen]-5-ylethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L1) and 4-(2-(5-(5-hexylthiophen-2-yl)thiophen-2-yl)ethynyl)-2,6-di(1H-pyrazol-1-yl)pyridine (L2) ligands, respectively. Density functional theory (DFT) studies revealed stretching-induced spin-state switching in a molecular junction composed of complex 1, taken as a representative example, and gold electrodes. Single-molecule conductance traces revealed the unfavorable orientation of the complexes in the junctions to demonstrate the spin-state dependence of the conductance.
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Affiliation(s)
- Senthil Kumar Kuppusamy
- Institute
for Quantum Materials and Technologies (IQMT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
| | - Asato Mizuno
- Institute
of Nanotechnology, Karlsruhe Institute of
Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
| | - Amador García-Fuente
- Departamento
de Física, Universidad de Oviedo, ES-33007 Oviedo, Spain
- Centro
de Investigación en Nanotecnología y Nanomateriales
(CINN, CSIC), El Entrego ES-33940, Spain
| | - Sebastiaan van der Poel
- Kavli
Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Benoît Heinrich
- Institut
de Physique et Chimie des Matériaux de Strasbourg (IPCMS), CNRS-Université de Strasbourg,
23, rue du Loess, BP 43, 67034 cedex
2 Strasbourg, France
| | - Jaime Ferrer
- Departamento
de Física, Universidad de Oviedo, ES-33007 Oviedo, Spain
- Centro
de Investigación en Nanotecnología y Nanomateriales
(CINN, CSIC), El Entrego ES-33940, Spain
| | - Herre S. J. van der Zant
- Kavli
Institute of Nanoscience, Delft University of Technology, PO Box 5046, 2600 GA Delft, The Netherlands
| | - Mario Ruben
- Institute
for Quantum Materials and Technologies (IQMT), Karlsruhe Institute of Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
- Institute
of Nanotechnology, Karlsruhe Institute of
Technology (KIT), Hermann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany
- Université
de Strasbourg (Unistra), Institute de Science et d′Ingénierie
Supramoléculaire (ISIS), Centre Européen de Science
Quantique (CESQ), 8,
Allée Gaspard Monge, F-67000 Strasbourg, France
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4
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Kulmaczewski R, Kershaw Cook LJ, Pask CM, Cespedes O, Halcrow MA. Iron(II) Complexes of 4-(Alkyldisulfanyl)-2,6-di(pyrazolyl)pyridine Derivatives. Correlation of Spin-Crossover Cooperativity with Molecular Structure Following Single-Crystal-to-Single-Crystal Desolvation. CRYSTAL GROWTH & DESIGN 2022; 22:1960-1971. [PMID: 35431660 PMCID: PMC9007408 DOI: 10.1021/acs.cgd.2c00005] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 01/03/2022] [Revised: 01/24/2022] [Indexed: 06/14/2023]
Abstract
The complex salts [Fe(L 1)2]X2 (1X 2 ; L 1 = 4-(isopropyldisulfanyl)-2,6-bis(pyrazolyl)pyridine; X- = BF4 -, ClO4 -) form solvated crystals from common organic solvents. Crystals of 1X 2 ·Me2CO show abrupt spin transitions near 160 K, with up to 22 K thermal hysteresis. 1X 2 ·Me2CO cocrystallizes with other, less cooperative acetone solvates, which all transform into the same solvent-free materials 1X 2 ·sf upon exposure to air, or mild heating. Conversion of 1X 2 ·Me2CO to 1X 2 ·sf proceeds in a single-crystal to single-crystal fashion. 1X 2 ·sf are not isomorphous with the acetone solvates, and exhibit abrupt spin transitions at low temperature with hysteresis loops of 30-38 K (X- = BF4 -) and 10-20 K (X- = ClO4 -), depending on the measurement method. Interestingly, the desolvation has an opposite effect on the SCO temperature and hysteresis in the two salts. The hysteretic spin transitions in 1X 2 ·Me2CO and 1X 2 ·sf do not involve a crystallographic phase change but are accompanied by a significant rearrangement of the metal coordination sphere. Other solvates 1X 2 ·MeNO2, 1X 2 ·MeCN, and 1X 2 ·H2O are mostly isomorphous with each other and show more gradual spin-crossover equilibria near room temperature. All three of these lattice types have similar unit cell dimensions and contain cations associated into chains through pairwise, intermolecular S···π interactions. Polycrystalline [Fe(L 2)2][BF4]2·MeNO2 (2[BF 4 ] 2 ·MeNO2; L 2 = 4-(methyldisulfanyl)-2,6-bis(pyrazolyl)pyridine) shows an abrupt spin transition just above room temperature, with an unsymmetrical and structured hysteresis loop, whose main features are reversible upon repeated thermal scanning.
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Affiliation(s)
- Rafal Kulmaczewski
- School
of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K.
| | | | - Christopher M. Pask
- School
of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K.
| | - Oscar Cespedes
- School
of Physics and Astronomy, University of
Leeds, E. C. Stoner
Building, Leeds LS2 9JT, U.K.
| | - Malcolm A. Halcrow
- School
of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K.
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5
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Liu S, Zhu Z, Li XL, Tang J. New iron( ii) spin-crossover metallomesogen with long aliphatic chain terminated by a CC bond. Inorg Chem Front 2022. [DOI: 10.1039/d1qi01181b] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/09/2023]
Abstract
A new Fe(ii) complex with long aliphatic chains and terminal CC bonds exhibits SCO behavior and liquid-crystalline properties.
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Affiliation(s)
- Shuting Liu
- State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
- School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China
| | - Zhenhua Zhu
- State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
| | - Xiao-Lei Li
- State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
| | - Jinkui Tang
- State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
- School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China
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6
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Kuppusamy SK, Suryadevara N, Mizuno A, Spieker L, Salamon S, Sleziona S, Maas A, Pollmann E, Heinrich B, Schleberger M, Wende H, Ruben M. Structural Insights into Hysteretic Spin-Crossover in a Set of Iron(II)-2,6-bis(1H-pyrazol-1-yl)pyridine) Complexes. Chemistry 2021; 28:e202103853. [PMID: 34939670 PMCID: PMC9305185 DOI: 10.1002/chem.202103853] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/26/2021] [Indexed: 11/21/2022]
Abstract
Bistable spin‐crossover (SCO) complexes that undergo abrupt and hysteretic (ΔT1/2) spin‐state switching are desirable for molecule‐based switching and memory applications. In this study, we report on structural facets governing hysteretic SCO in a set of iron(II)‐2,6‐bis(1H‐pyrazol‐1‐yl)pyridine) (bpp) complexes – [Fe(bpp−COOEt)2](X)2⋅CH3NO2 (X=ClO4, 1; X=BF4, 2). Stable spin‐state switching – T1/2=288 K; ΔT1/2=62 K – is observed for 1, whereas 2 undergoes above‐room‐temperature lattice‐solvent content‐dependent SCO – T1/2=331 K; ΔT1/2=43 K. Variable‐temperature single‐crystal X‐ray diffraction studies of the complexes revealed pronounced molecular reorganizations – from the Jahn‐Teller‐distorted HS state to the less distorted LS state – and conformation switching of the ethyl group of the COOEt substituent upon SCO. Consequently, we propose that the large structural reorganizations rendered SCO hysteretic in 1 and 2. Such insights shedding light on the molecular origin of thermal hysteresis might enable the design of technologically relevant molecule‐based switching and memory elements.
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Affiliation(s)
- Senthil Kumar Kuppusamy
- Karlsruhe Institute of Technology: Karlsruher Institut fur Technologie, Institute for quantum materials and technologies, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen,, 76344, Karlsruhe, GERMANY
| | - Nithin Suryadevara
- Karlsruhe Institute of Technology: Karlsruher Institut fur Technologie, IQMT, GERMANY
| | - Asato Mizuno
- Karlsruhe Institute of Technology: Karlsruher Institut fur Technologie, INT, GERMANY
| | - Lea Spieker
- University of Duisburg-Essen: Universitat Duisburg-Essen, Faculty of Physics and CENIDE, GERMANY
| | - Soma Salamon
- University of Duisburg-Essen: Universitat Duisburg-Essen, Faculty of Physics and CENIDE, GERMANY
| | - Stephan Sleziona
- University of Duisburg-Essen: Universitat Duisburg-Essen, Faculty of Physics and CENIDE, GERMANY
| | - André Maas
- University of Duisburg-Essen: Universitat Duisburg-Essen, Faculty of Physics and CENIDE, GERMANY
| | - Erik Pollmann
- University of Duisburg-Essen: Universitat Duisburg-Essen, Faculty of Physics and CENIDE, GERMANY
| | - Benoît Heinrich
- IPCMS: Institut de physique et chimie des materiaux de Strasbourg, DMO, FRANCE
| | - Marika Schleberger
- University of Duisburg-Essen: Universitat Duisburg-Essen, Faculty of Physics and CENIDE, GERMANY
| | - Heiko Wende
- University of Duisburg-Essen: Universitat Duisburg-Essen, Faculty of Physics and CENIDE, GERMANY
| | - Mario Ruben
- Karlsruhe Institute of Technology: Karlsruher Institut fur Technologie, Institute of Nanotechnology, GERMANY
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7
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Yang Q, Meng YS, Liu T, Tang J. Thermal and photoinduced spin-crossover of mononuclear Fe II complexes based on bppCHO ligand. Dalton Trans 2021; 51:602-607. [PMID: 34904976 DOI: 10.1039/d1dt03664e] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/21/2022]
Abstract
Two new iron(II) complexes [Fe(bppCHO)2](ClO4)2 (1·ClO4) and [Fe(bppCHO)2](BF4)2·H2O (1·BF4) were synthesized by using 2,6-di(1H-pyrazol-1-yl)isonicotinaldehyde (bppCHO) as the ligand. The structures and spin states of the complexes were characterized by single-crystal X-ray diffraction and magnetic susceptibility measurements. Complex 1·ClO4 exhibits light-induced excited spin state trapping (LIESST) up to 53 K and two-step spin-crossover (SCO) over room temperature, in which the first step shows a thermal hysteresis of 26 K, whereas 1·BF4 exhibits a gradual SCO behavior. A magnetostructural study reveals that the difference in the SCO property is related to the supramolecular interactions and crystal packing effect.
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Affiliation(s)
- Qianqian Yang
- State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. .,School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefe 230026, P. R. China
| | - Yin-Shan Meng
- State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Rd., Dalian 116024, P. R. China
| | - Tao Liu
- State Key Laboratory of Fine Chemicals, Dalian University of Technology, 2 Linggong Rd., Dalian 116024, P. R. China
| | - Jinkui Tang
- State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China. .,School of Applied Chemistry and Engineering, University of Science and Technology of China, Hefe 230026, P. R. China
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8
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Capel Berdiell I, Davies DJ, Woodworth J, Kulmaczewski R, Cespedes O, Halcrow MA. Structures and Spin States of Iron(II) Complexes of Isomeric 2,6-Di(1,2,3-triazolyl)pyridine Ligands. Inorg Chem 2021; 60:14988-15000. [PMID: 34547208 DOI: 10.1021/acs.inorgchem.1c02404] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
Iron(II) complex salts of 2,6-di(1,2,3-triazol-1-yl)pyridine (L1) are unexpectedly unstable in undried solvent. This is explained by the isolation of [Fe(L1)4(H2O)2][ClO4]2 and [Fe(NCS)2(L1)2(H2O)2]·L1, containing L1 bound as a monodentate ligand rather than in the expected tridentate fashion. These complexes associate into 44 grid structures through O-H···N hydrogen bonding; a solvate of a related 44 coordination framework, catena-[Cu(μ-L1)2(H2O)2][BF4]2, is also presented. The isomeric ligands 2,6-di(1,2,3-triazol-2-yl)pyridine (L2) and 2,6-di(1H-1,2,3-triazol-4-yl)pyridine (L3) bind to iron(II) in a more typical tridentate fashion. Solvates of [Fe(L3)2][ClO4]2 are low-spin and diamagnetic in the solid state and in solution, while [Fe(L2)2][ClO4]2 and [Co(L3)2][BF4]2 are fully high-spin. Treatment of L3 with methyl iodide affords 2,6-di(2-methyl-1,2,3-triazol-4-yl)pyridine (L4) and 2-(1-methyl-1,2,3-triazol-4-yl)-6-(2-methyl-1,2,3-triazol-4-yl)pyridine (L5). While salts of [Fe(L5)2]2+ are low-spin in the solid state, [Fe(L4)2][ClO4]2·H2O is high-spin, and [Fe(L4)2][ClO4]2·3MeNO2 exhibits a hysteretic spin transition to 50% completeness at T1/2 = 128 K (ΔT1/2 = 6 K). This transition proceeds via a symmetry-breaking phase transition to an unusual low-temperature phase containing three unique cation sites with high-spin, low-spin, and 1:1 mixed-spin populations. The unusual distribution of the spin states in the low-temperature phase reflects "spin-state frustration" of the mixed-spin cation site by an equal number of high-spin and low-spin nearest neighbors. Gas-phase density functional theory calculations reproduce the spin-state preferences of these and some related complexes. These highlight the interplay between the σ-basicity and π-acidity of the heterocyclic donors in this ligand type, which have opposing influences on the molecular ligand field. The Brønsted basicities of L1-L3 are very sensitive to the linkage isomerism of their triazolyl donors, which explains why their iron complex spin states show more variation than the better-known iron(II)/2,6-dipyrazolylpyridine system.
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Affiliation(s)
- Izar Capel Berdiell
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K
| | - Daniel J Davies
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K
| | - Jack Woodworth
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K
| | - Rafal Kulmaczewski
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K
| | - Oscar Cespedes
- School of Physics and Astronomy, University of Leeds, E. C. Stoner Building, Leeds LS2 9JT, U.K
| | - Malcolm A Halcrow
- School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, U.K
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