HAA by the first {Mn(iii)OH} complex with all O-donor ligands

Abstract

There is considerable interest in MnOH_x moieties, particularly in the stepwise changes in those O–H bonds in tandem with Mn oxidation state changes. The reactivity of aquo-derived ligands, {MOH_x}, is also heavily influenced by the electronic character of the other ligands. Despite the prevalence of oxygen coordination in biological systems, preparation of mononuclear Mn complexes of this type with all O-donors is rare. Herein, we report several Mn complexes with perfluoropinacolate (pin^F)²⁻ including the first example of a crystallographically characterized mononuclear {Mn(III)OH} with all O-donors, K₂[Mn(OH)(pin^F)₂], 3. Complex 3 is prepared via deprotonation of K[Mn(OH₂)(pin^F)₂], 1, the pK_a of which is estimated to be 18.3 ± 0.3. Cyclic voltammetry reveals quasi-reversible redox behavior for both 1 and 3 with an unusually large ΔE_p, assigned to the Mn(III/II) couple. Using the Bordwell method, the bond dissociation free energy (BDFE) of the O–H bond in {Mn(II)–OH₂} is estimated to be 67–70 kcal mol⁻¹. Complex 3 abstracts H-atoms from 1,2-diphenylhydrazine, 2,4,6-TTBP, and TEMPOH, the latter of which supports a PCET mechanism. Under basic conditions in air, the synthesis of 1 results in K₂[Mn(OAc)(pin^F)₂], 2, proposed to result from the oxidation of Et₂O to EtOAc by a reactive Mn species, followed by ester hydrolysis. Complex 3 alone does not react with Et₂O, but addition of O₂ at low temperature effects the formation of a new chromophore proposed to be a Mn(IV) species. The related complexes K(18C6)[Mn(III)(pin^F)₂], 4, and (Me₄N)₂[Mn(II)(pin^F)₂], 5, have also been prepared and their properties discussed in relation to complexes 1–3.