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

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 [Mn^II{H(3,5-R¹,R²)-L}₂] and ionic [Mn^II{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 [Mn^III{H(3,5-tBu₂)-L}₂]ClO₄ complex is the first example of a hydrazone-based Mn^III complex to exhibit spin crossover. Formation of the tetravalent complexes [Mn^IV{(3,5-R¹,R²)-L}₂] (R¹=tBu, R²=H; R¹=R²=tBu) necessitates base-assisted abstraction of the hydrazinic proton.