University of Groningen

Mechanisms in iron, nickel, and manganese, catalysis with small molecule oxidantsPadamati, Sandeep

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Summary

Oxidation plays a vital role in the metabolism of living organisms, manifested in a finely organized set of enzymes and proteins to selectively oxidize substrates in a controlled fashion.1 Biochemical oxidation has inspired many scientists to model the processes found in living organisms to synthetic reactions, so called bio-mimics.2 However, the bio-mimetic complexes developed are often quite different structurally to their biological counterparts due to limitations in the synthesis of large molecular structures to surround the active centers, and especially second co-ordination sphere effects. In this thesis, spectroscopic and electrochemical studies of oxidation of iron, nickel and manganese complexes are described with the terminal oxidants H2O2, NaOCl and O2. The aim of this thesis is to give insight into the complex nature of the transition metal centers and the ligands around it under oxidative conditions.

NaOCl is a widely used oxidant both industrially and for domestic purposes. It is made in nature by a haloperoxidases using atmospheric oxygen and chloride for the chlorination of substrates to produce chlorinated products, that finds use as antibiotics, dyes etc. NaOCl is also employed in the oxidation of alkanes, alkenes, and alcohols. However, there is also a limitation to the use of NaOCl due to solubility, high pH dependence, and selectivity. Therefore phase transfer catalysts (PTCs), pH, and metal catalysts have been applied to tame this reactive oxidant. In chapter 1, the electrochemical synthesis of

NaOCl, its pH dependence, and the use of PTCs, and metal complexes are discussed. Furthermore, the role of metal complexes in tuning the activity of NaOCl to oxidize or chlorinate substrates, particularly by manganese, iron and nickel complexes is discussed. The goal of this chapter is to provide an overview into how these complexes behave under oxidation conditions and also the role and mode of action of NaOCl.

The complex [(MeN3Py)Fe(II)(CH3CN)]2+ (1), a biomimic of Rieske dioxygenases, was synthesized a decade ago for use in oxidation catalysis. It was reported that with 1 a difference in chemoselectivity in the dihydroxylation of alkenes (syn/anti) upon switch of solvent from acetonitrile to acetone occurs.3 In chapter 2, the formation of high valent iron-oxo species upon reaction of 1 with H2O2 at room temperature in acetonitrile, and its reactivity with cyclooctene are discussed. In this chapter the role of ligand exchange, the concentration of water present under catalytic conditions, and the affect these have on the rate of formation/decay of the intermediates is discussed. Addition of water leads to rapid exchange of ligands coordinated to Fe(II) center, which accelerates coordination of H2O2 leading to formation of a transient Fe(III)-OOH species at room temperature. This Fe(III)-OOH species is spectroscopically similar to the Fe(III)-peroxy species found in nature. However, when excess water is present, formation of a thermodynamically stable Fe(III)-O-Fe(III) species was observed, which highlights the importance of controlling the amount of water present in the reaction mixture (scheme 1). Formation of the Fe(III)-OOH species, and its subsequent conversion to a relatively inert Fe(III)-O-Fe(III) species, was studied by a combination of UV-vis absorption, EPR, resonance Raman and NMR spectroscopies. Furthermore, the Fe(III)-OOH is shown not to react directly with cyclooctene, but instead decays to a reactive species (presumably an Fe(IV)=O species), this latter species discussed in detail in chapters 3 and 4.

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Scheme 1. Reaction of H2O2 with 1 in acetonitrile and equilibria involving water.

In chapter 3, the reaction of 1 with H2O2 at room temperature, -30 °C, and -80 °C in acetone, and the reactivity with cyclooctene is discussed. The goal of this chapter is to understand the differences in chemoselectivity by switching of solvent from acetonitrile to acetone on the oxidation of cyclooctene, and the role of the sixth coordination site on iron center. In acetone the labile ligands on Fe(II) are exchanged with acetone, or trace amounts of water to form a high spin complex. Furthermore, ligand exchange facilitates coordination of H2O2 to form an Fe(IV)=O species. The NIR absorption band of this Fe(IV)=O species is red shifted compared to acetonitrile, indicating a different co-ordination environment is achieved. In contrast to acetonitrile, formation of an Fe(III)-OOH species is not observed at room temperature, however, at -30 °C, and -80 °C the Fe(III)-OOH species is observed, indicating that ligand exchanges plays a crucial role in observing this intermediate. A key difference with acetonitrile is that acetone is oxidized in situ to acetic acid, which accelerates the formation of an Fe(III)-O-Fe(III) acetato dimer, which is not observed in acetonitrile. The Fe(IV)=O species formed in acetone reacts with cyclooctene to form cyclooctene epoxide, however, in contrast to earlier reports3 trans-diol is not observed. Instead an unidentified species similar to an alpha hydroxyl ketone is formed by oxidation of the diol product. Thus the difference in chemoselectivity between the solvents could be assigned to the role of the sixth coordination site on the Fe(II) center, and the acceleration of ligand exchange which leads to a different landscape of intermediates formed in the reaction mixture (scheme 2).

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Scheme 2 General overview of the reaction of H2O2 with 1 in acetone, and cyclooctene epoxidation.

In chapter 4, the role of acetic acid and the increase in conversion to cyclooctene epoxide is discussed. Recently it was reported by Que, Costas and co-workers4,5 that the use of acetic acid with non-heme Fe(II) complexes, enhances the formation of epoxides, and this is mostly assigned to the reaction of a proposed Fe(III)-O2C(O)CH3 or Fe(V)=O intermediates formed in solution. However, only limited spectroscopic data for the formation and reactivity of these intermediate with alkenes is available. In chapter 4, the addition of H2O2 to 1 results in formation of Fe(IV)=O, which increases with concentration of acetic acid, however, with excess H2O2, an Fe(III)-OOH species reaches a high steady state concentration prior to formation of the Fe(IV)=O species. The presence of acetic acid mainly increases the rate and extent of oxidation of 1 with H2O2 and ultimately the rate of formation of an Fe(III) species and thereby an increase in the maximum concentration of the Fe(IV)=O species, however, ultimately formation of an Fe(III)-O-Fe(III) acetato bridged dimer occurs. The Fe(IV)=O species reacts rapidly with cyclooctene to yield the epoxide product and regenerate the Fe(II) species, and the yield of epoxide is increased selectively by the presence of excess acetic acid and slow addition of H2O2. Furthermore, the presence of acetic acid reduces the formation of cis-diol and side products.

Formation of a high valent Ni(IV)-oxo species was already postulated over two decades ago,6 and was proposed to be involved in the oxidation of alkanes and alkenes,7 however, spectroscopic data of these high valent intermediates is scarce. In chapter 5, characterization an unprecedented high valent [(TMTACN)2Ni(IV)2(μ-O)3]2+ (3) complex is

reported, which is formed at room temperature by reaction of [(TMTACN)2Ni(II)2(μ-X)3]X (X = Cl or Br) with NaOCl in methanol or acetonitrile (scheme 3). The reaction was studied by a combination of UV-vis absorption, resonance Raman, 1H NMR, and EPR spectroscopy, ESI mass spectrometry, and DFT methods. Formation of 3 by exchange of bridging chlorido/bromido ligands to form a oxido bridged species involves complex structural rearrangements, and various intermediates, therefore indicating a complex

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and dynamic system. 3 is active in C-H oxidation and thus formation of 3 with NaOCl opens further opportunities to tame the reactivity of NaOCl for selective oxidations.

Scheme 3. Formation of [Ni(IV)2(μ-O)3(TMTACN)2]2+ by reaction of 2 with NaOCl

Activation of molecular oxygen for the oxidation of organic substrates is challenging, but is achieved by several enzymes in nature such as methane monooxygenases.8 Controlled activation of molecular oxygen by transition metal complexes, especially by manganese complexes is challenging. In chapter 6, the first example of selective C-H oxidation of a manganese complex of the ligand (dpeo) is reported, where methylene group on the attached ligand is oxidized in stepwise manner to produce an alkoxide and ultimately a ketone group (scheme 4). A combination of resonance Raman, EPR, X-ray, ESI mass spectroscopic techniques were used to monitor the progress of the reaction and to characterize the intermediates formed. The rate of oxidation is highly dependent on the concentration of O2 and hence under dilute conditions the reaction proceeds more quickly and as the concentration is increased mass transfer of oxygen into the solvent limits the reaction rate. The ability to control the reaction rate in this way allowed the monitoring of the changes of the ligand over time primarily by resonance Raman spectroscopy. Labelling studies with 18O2 and H2

18O indicate that the inserted O atoms originate from molecular oxygen and from two different O2 molecules. The X-ray structure of L-O-Mn(III)-O-L intermediate (L = dpeo), shows the binding of two pyridyl groups and alkoxide from ligand indicating insertion of oxygen, and change in oxidation state from Mn(II) to Mn(III). Furthermore, EPR data shows that the initial decrease of Mn(II) signals occurs, and after two days recovery of Mn(II) signals indicating a catalytic system. Formation of the final ketone ligand results in bidentate coordination being resumed. Thus snapshots of the reaction indicate that Mn(II) dpeo activates oxygen, with presumably formation of high valent Mn(III)-O-O•, Mn(IV)=O intermediates which leads to the oxidation of ligand.

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Scheme 4. Stepwise oxidation of [MnII(dpeo)2(solvent)2]2+ to [MnIII(hdpeo)2]+ and finally

[MnII(hdipe)2(solvent)2]2+, together with the structures of the ligands dpeo, hdpeo- and hidpe.

In conclusion, the central theme of this thesis is the study of reaction mechanisms, and identification of intermediates to gain insight into several Fe, Ni and Mn systems. Central to these studies is the application of a broad range of spectroscopic, theoretical and electrochemical methods under conditions employed in catalysis.

References

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Yang, Y.-S.; Zhou, J. Chem. Rev. 2000, 100, 235-350. 3 Klopstra, M.; Roelfes, G.; Hage, R.; Kellogg, R. M.; Feringa, B. L. Eur. J. Inorg. Chem., 2004, 846-856. 4 Iyer, S. R.; Javadi, M. M.; Feng, Y.; Hyun, M. Y.; Oloo,W. N.; Kim, C.; Que, L. Jr., Chem. Commun. 2014, 50,

13777-13780.

5 Serrano-Plana, J.; Aguinaco, A.; Belda, R.; Garcia-Espana, E.; Basallote, M. G.; Company, A.; Costas, M. Angew. Chemie - Int. Ed. 2016, 55, 6310–6314.

6 Yoon, H.; Burrows, C. J. J. Am. Chem. Soc. 1988, 110, 4089– 4090. 7 Corona, T.; Draksharapu, A.; Padamati, S. K.; Gamba, I.; Martin-Diaconescu, V.; Acuña-Parés, F.; Browne,

W. R.; Company, A. J. Am. Chem. Soc. 2016, 138, 12987-12996. 8 Lieberman, R. L.; Rosenzweig, A. C. Nature, 2005, 434, 177-182.

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