Tuning a Single Ligand System to Stabilize Multiple Spin States of Manganese: A First Example of a Hydrazone‐Based Manganese(III) Spin‐Crossover Complex |
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| Authors: | Dr. Musa S. Shongwe Kaltham S. Al‐Barhi Prof. Masahiro Mikuriya Harry Adams Dr. Michael J. Morris Dr. Eckhard Bill Prof. Kieran C. Molloy |
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| Affiliation: | 1. Department of Chemistry, College of Science, Sultan Qaboos University, PO Box 36, Al‐Khod 123, Muscat (Sultanate of Oman);2. Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, 2–1 Gakuen, Sanda 669‐1337 (Japan);3. Department of Chemistry, University of Sheffield, Sheffield S3?7HF (UK);4. Max‐Planck‐Institut für Chemische Energiekonversion, Stiftstrasse 34‐36, D‐45470 Mülheim (Germany);5. Department of Chemistry, University of Bath, Claverton Down, Bath BA2 7AY (UK) |
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| Abstract: | A series of bis‐chelate pseudo‐octahedral mononuclear coordination complexes of manganese with the chromophore [MnN4O2]n+ (n=0, 1) have been generated in all three principal oxidation states of this transition‐metal center under ambient conditions by utilizing a readily tunable, versatile phenolic pyridylhydrazone ligand system (i.e., H2(3,5‐R1,R2)‐L; L=ligand). Strategic combinations of the nature and position of a variety of substituent groups afforded selective, spontaneous stabilization of multiple spin states of the manganese center, which, upon close crystallographic scrutiny, appears to be in part due to the occurrence or absence of hydrogen‐bonding interactions that involve the phenolate/phenolic oxygen atom. The divalent complexes are isolable in two forms, namely, molecular [MnII{H(3,5‐R1,R2)‐L}2] and ionic [MnII{H2(3,5‐R1,R2)‐L}{H(3,5‐R1,R2)‐L}]ClO4, with the latter complex converting easily into the former complex on deprotonation. Accessibility of the higher‐valent states is achievable only when the phenolate oxygen atom is sterically hindered from participation in hydrogen bonding. The [MnIII{H(3,5‐tBu2)‐L}2]ClO4 complex is the first example of a hydrazone‐based MnIII complex to exhibit spin crossover. Formation of the tetravalent complexes [MnIV{(3,5‐R1,R2)‐L}2] (R1=tBu, R2=H; R1=R2=tBu) necessitates base‐assisted abstraction of the hydrazinic proton. |
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| Keywords: | hydrazones ligand design manganese spin crossover spin states substituent effects |
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