Tuning a single ligand system to stabilize multiple spin states of manganese: A first example of a hydrazone-based manganese(III) spin-crossover complex

Musa S. Shongwe*, Kaltham S. Al-Barhi, Masahiro Mikuriya, Harry Adams, Michael J. Morris, Eckhard Bill, Kieran C. Molloy

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

31 Citations (Scopus)


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., H 2(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,R 2)-L}2] and ionic [MnII{H2(3,5- R1,R2)-L}{H(3,5-R1,R2)-L}]ClO 4, 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. Made for each other! Manganese exists in variable oxidation states under ambient conditions. By employing versatile hydrazones, it was possible to stabilize multiple spin states of manganese with a single ligand system. Modulation of the ligand properties, coupled with crystal engineering, afforded the first example of a hydrazone-based MnIII spin-crossover complex, namely [Mn III{H(3,5-tBu2)L}2]ClO4, which exhibits a complete thermally induced spin transition.

Original languageEnglish
Pages (from-to)9693-9701
Number of pages9
JournalChemistry - A European Journal
Issue number31
Publication statusPublished - Jul 28 2014


  • hydrazones
  • ligand design
  • manganese
  • spin crossover
  • spin states
  • substituent effects

ASJC Scopus subject areas

  • Catalysis
  • Organic Chemistry

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