57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

The subject of this award is to contribute to an in-depth understanding of the initial steps in hydrogen peroxide activation for manganese catalysts by defining the physical and reactivity properties of peroxomanganese(III) intermediates, which are proposed to be among the first species formed when manganese complexes react with hydrogen peroxide. Such catalytic systems are of interest because manganese centers and hydrogen peroxide mediate important oxidative transformations, including those involved in the functionalization of petroleum feedstocks and fabric and paper bleaching. In year two of this project we have i) published a detailed study comparing geometric and electronic stuctures of peroxomanganese(III) complexes supported by tetradentate aminopyridyl and aminoquinolinyl ligands, ii) designed and synthesized Mn(II) complexes supported by new aminoquinoline ligands expected to be more oxidatively robust than their first-generation analogues, and iii) elucidated the reaction landscape of a manganese(II) complex with superoxide and hydrogen peroxide, which features the conversion of a peroxomanganese(III) adduct to a bis(m-oxo)dimanganese(III,IV) species.

In total, this two-year award has supported the training of three graduate students and has resulted in three publications to date, positively impacting my career as an assistant professor.

Figure 1. Ligands used to support peroxomanganese(III) complexes, representative structure of a [Mn^III(O₂)(L⁷py₂^R)]⁺ complex, and correlation of Mn-O(peroxo) distance with lowest-energy MCD transition.

In this year, we completed our comparison of Mn^III-O₂ adducts supported by tetradentate ligands with systematically varied steric and electronic properties (Geiger, R. A. et al. Eur. J. Inorg. Chem. 2012, 1598-1608.). Magnetic circular dichroism (MCD) and density functional theory (DFT) studies of these six complexes establish a correlation between the computed Mn-O(peroxo) bond length and the position of the lowest-energy MCD band (band 1; see Figure 1). Notably, this correlation is attributed to the steric properties of the supporting ligand; electronic perturbations among this series have a limited effect. For example, pyridine substituents (R = Cl, H, and Me) in the 4-position hardly affect the energy of band 1, whereas steric interactions between pyridine or quinoline groups (e.g., for the L⁷py₂^6-Me and L⁷q₂ ligands) lead to a red-shift of band 1 that is attributed to an elongation of one Mn-O(peroxo) bond (Figure 1, far right). We intend to exploit this tuning of the Mn-O(peroxo) interaction to activate Mn^III-O₂ species using sterically encumbered ligands. These results also suggest that enzyme active sites may prevent formation of symmetric Mn^III-O₂ adducts (i.e., with near equivalent Mn-O(peroxo) distances) by putting strict steric constraints on the peroxo binding pocket. Presumably, asymmetric Mn^III-O₂ spcies with an elongated Mn-O(peroxo) bond would be more reactive towards electrophilic substrates, as previously suggested (Annaraj, J. et al. Angew. Chem. Int. Ed. 2009, 48, 4150-4153).

Our previous work has established that the peroxomanganese(III) adducts supported by our L⁷py₂^R family of ligands are quite electrophilic (Geiger, R. A. et al. Dalton Trans. 2011, 40, 1707-1715.). However, we have been unable to trap any intermediates when these compounds are treated with acids or acyl chlorides. Instead we observe the immediate disappearance of the peroxomanganese(III) chromophore and the formation of manganese(II) species. Acids or acyl chloride reagents are expected to react with the Mn^III-O₂ unit to generate Mn^III-OOH or Mn^III-OOR species, respectively, which could subsequently undergo O-O cleavage to yield high-valent oxomanganese species. Although such reactions have been observed for corresponding iron(III) complexes, such chemistry has remained quite elusive for manganese(III) centers. For our complexes, we propose that the peroxomanganese(III) species reacts with acids or acyl chlorides to yield a strong oxidant that initiates decomposition of the L⁷py₂^R ligand. In support, Groni et al. have observed that the peroxomanganese(III) complex [Mn^III(O₂)(mL₅²)]⁺ (mL₅² = N-methyl-N,N¢,N¢-tris(2-pyridylmethyl)ethane-1,2-diamine) decays upon treatment with acid to yield a Mn(II) center bound to two pyridinecarbyoxylate ligands that are formed by oxidation of mL₅² (Groni, S. et al. Inorg. Chem. 2008, 47, 3166-3172). Thus, we reasonably suspect that the methyl linkers of the L⁷py₂^R ligands could be susceptible to oxidization by high-valent oxomanganese species. Accordingly, we have synthesized the L⁷BQ and L⁸BQ ligands, where the quinoline rings are attached directly to the 1,4-diazepane and 1,5-diazacyclooctane fragments, respectively (Figure 2). The crystal structure of the [Mn^II(L⁷BQ)(OTf)₂] complex (Figure 2) shows the tetradentate ligand bound in the trans fashion, similar to that of the L⁷py₂^R ligands. We are currently in the process of characterizing the peroxomanganese(III) complexes supported by the BQL⁷ and BQL⁸ ligands and exploring their activation by acids and acyl chlorides. We note that the putative [Mn^III(O₂)(L⁷BQ)]⁺ complex shows enhanced thermal stability relative to [Mn^III(O₂)(L⁷q₂)]⁺.

Figure 2. Structures of the L⁷BQ ligand (left) and an ORTEP diagram of the [Mn^II(L⁷BQ)(OTf)₂] complex (right).

In Year 1 of this project, we described the geometric and electronic structures of a peroxomanganese(III) unit supported by the pentadentate N4py ligand (N4py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine). This complex was generated by treatment of [Mn^II(N4py)(OTf)₂] with KO₂ in acetonitrile (Scheme 1). Unlike all other peroxomanganese(III) complexes reported to date, [Mn^III(O₂)(N4py)]⁺ can only be generated using KO₂. If [Mn^II(N4py)(OTf)₂] is treated with H₂O₂ and Et₃N in acetonitrile, the dinuclear [Mn^IIIMn^IV(m-O)₂(N4py)₂]³⁺ complex is formed even at low temperatures. When [Mn^III(O₂)(N4py)]⁺ is formed from [Mn^II(N4py)(OTf)₂] and KO₂ in high yields, it decays to a mixture of manganese(II) species. However, if it is formed in only 50% yield or less, it converts to the [Mn^IIIMn^IV(m-O)₂(N4py)₂]³⁺ complex. Separate experiments have shown that treatment of [Mn^III(O₂)(N4py)]⁺ with [Mn^II(N4py)(OTf)] also leads to the formation of [Mn^IIIMn^IV(m-O)₂(N4py)₂]³⁺. Thus, the Mn^II-N4py system shows a complex reaction landscape, featuring a variety of intermediates. Because dissociation of a pyridylmethyl arm is require for formation of [Mn^III(O₂)(N4py)]⁺, our current hypothesis is that the rate of pyridine dissociation influences whether or not the Mn^II center reacts with an oxidant to afford a Mn^III-O₂ or Mn^IIIMn^IV dimer. Understanding the complex factors that govern the relative rates of formation of these intermediates is important given that oxo-bridged Mn^IIIMn^IV dimers are often viewed as "dead-end" intermediates due to their high thermodynamic stability. Understanding how to disfavor the formation of such species, could be an important step in accessing more reactive intermediates.

Scheme 1. Experimentally observed reaction landscape for [Mn^II(N4py)(OTf)]⁺ in the presence of H₂O₂ or KO₂.