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

There is considerable interest in MnOHx moieties, particularly in the stepwise changes in those O-H bonds in tandem with Mn oxidation state changes. The reactivity of aquo-derived ligands, {MOHx}, 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, K-2[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 Delta E-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 K-2[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 O-2 at low temperature effects the formation of a new chromophore proposed to be a Mn(IV) species. The related complexes K(18C6)[Mn(III)(F-pin)(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.

Automated summary

There is a strong interest in MnOHx moieties due to the stepwise changes in O-H bonds with Mn oxidation state changes. The reactivity of aquo-derived ligands, {MOHx}, is influenced by the electronic character of other ligands. Preparation of mononuclear Mn complexes with all O-donors is rare despite oxygen coordination being prevalent in biological systems.