Too Many Cooks Spoil the Broth – Variable Potencies of Oxidizing Mn Complexes of a Hexadentate Carboxylato Ligand

Abstract

Given the hexadenticity of the monoanionic ligand in the procatalyst [Mn(tpena)(H2O)](ClO4) {tpena– = N,N,N′‐tris(2‐pyridylmethyl)ethylenediamine‐N′‐acetate}, it is perhaps surprising that this complex can catalyze the epoxidation of alkenes. When peracetic acid is used as terminal oxidant, the selectivity and rates of reactions are comparable with those reported for the manganese complexes of the commonly employed neutral tetradentate N4 ligands under analogous conditions. Cyclooctene conversion rates are similar when tert‐butyl hydroperoxide (TBHP) is used; however, the selectivity is greatly diminished. In the absence of organic substrates, [MnII(tpena)]+ catalyzes water oxidation by TBHP (initial rate ca. 23 mmol/h when [Mn] = 0.1 mM, at room temp.). To explain the variations in the selectivity of catalytic epoxidations and the observation of competing water oxidation, we propose that several metal‐based oxidants (the “cooks”) can be generated from [MnII(tpena)]+. These embody different potencies. The most powerful, and hence least selective, is proposed to be the isobaric isomer of [MnIV2(O)2(tpena)2]2+, namely an oxylic radical complex, [(tpena)MnIII(μ2‐O)MnIV(O·)(tpena)]2+. The formation of this species depends on the catalyst concentration, and it is favoured when TBHP is used as the terminal oxidant. The generation of the less potent [MnIV(O)(tpena)]+, which we propose as the direct oxidant in epoxidation reactions, is favoured in non‐aqueous solutions when peracetic acid is used as the terminal oxidant.

A coordinatively flexible hexadentate ligand gives structurally isomeric complexesM2(L)X3(M= Cu, Zn;X= Br, Cl)

“…1). The presence of a dominant [M(tpena)] + ion in these spectra was entirely comparable to the spectra for the simple Cr III and Co III salts containing the [M(tpena)] + cation (Vad et al, 2011;de Sousa et al, 2015). Thus, the usual characterization methods gave no indication of the significant structural differences between the copper research papers 70 Wegeberg et al Three Zn and Cu complexes of tpena Acta Cryst.…”

“…2:1 metal-tpena stoichiometry and availability of relatively strongly coordinating halide anions provide the conditions required for the isolation of neutral dinuclear compounds. In zinc complexes (I) and (II), tpena acts as a hexadentate ligand and the encapsulated metal binding site is essentially the same as that reported previously for the mononuclear Co III and Cr III complexes (Vad et al, 2011;de Sousa et al, 2015). The tetrahedral OX 3 coordination of the second zinc ion is quite common (Cambridge Structural Database, Version 5.36; Groom & Allen, 2014).…”

“…7). All previously published complexes containing tpena have been ionic species with a 1:1 metal-tpena ratio in which the metal ion binds either all six tpena donors in six or sevencoordinated complexes (Vad et al, 2011;de Sousa et al, 2015) or releases one pyridine donor to bind an exogenous ligand (Vad et al, 2012;de Sousa et al, 2015). Carboxylate bridging to another metal ion has also been observed previously in related systems, but in those cases the link has resulted in polynuclear manganese(II) ionic assemblies with all the metal ions in similar environments (Berggren et al, 2009;Egdal et al, 2011).…”

“…We have introduced derivatives of this class of ligand which incorporate a single amino acid group and employed them to prepare model complexes for redox-active non-heme sites in metalloenzymes. Functional biomimetic and catalytic reactivity, for example, in the activation of O 2 and H 2 O, has been demonstrated (Poulsen et al, 2005;Nielsen et al, 2006;Vad et al, 2011Vad et al, , 2012de Sousa et al, 2015;Deville et al, 2015). Dangling pyridines and the carboxylate groups are proposed to assist in proton transfer and the carboxylate group can be either terminal or bridge between metal ions to form oligomers and polymers (Berggren et al, 2009;Egdal et al, 2011).…”

Directing a Non‐Heme Iron(III)‐Hydroperoxide Species on a Trifurcated Reactivity Pathway

“…[25] The pendant pyridinium moiety of the tpenaH ligand is as econd site availablef or deprotonation by ab ase, and [Fe III (OO)(tpena)] is ap lausible product from the reactiono f[ Fe III (OOH)(tpenaH)] 2 + with two equivalents of base (Scheme5). [26] The relativelyo pen face presented by tpena in these structures suggestst hat formationo faheteroleptic complex with an h 2diatomic ligand is also ar easonable structure for the peroxido complex, especially since h 2 -OO 2À ligands are no more sterically demanding than monodentate oxide (O 2À )l igands. Indeed,h eptacoordination has been structurally characterized in the high-spind 5 metal ion complexes [Fe(OIPh)(tpena)](ClO 4 ) 2 [20] and [Mn(OH 2 )(tpena)](ClO 4 ) 2 .…”