Towards robust alkane oxidation catalysts: electronic variations in non-heme iron(ii) complexes and their effect in catalytic alkane oxidation

Abstract

A series of non-heme iron(II) bis(triflate) complexes containing linear and tripodal tetradentate ligands has been prepared. Electron withdrawing and electron donating substituents in the para position of the pyridine ligands as well the effect of pyrazine versus pyridine and sulphur or oxygen donors instead of nitrogen donors have been investigated. The electronic effects induced by these substituents influence the strength of the ligand field. UV-vis spectroscopy and magnetic susceptibility studies have been used to quantify these effects and VT 1 H and 19 F NMR spectroscopy as well as X-ray diffraction have been used to elucidate structural and geometrical aspects of these complexes. The catalytic properties of the iron(II) complexes as catalysts for the oxidation of cyclohexane with hydrogen peroxide have been evaluated. In the strongly oxidising environment required to oxidise alkanes, catalyst stability determines the overall catalytic efficiency of a given catalyst, which can be related to the ligand field strength and the basicity of the ligand and its propensity to undergo oxidation.2

Molecular iron complexes as catalysts for selective C–H bond oxygenation reactions

“…62 These findings were supported by further topological variations of the BPMEN ligand system. [64][65][66] It was found that catalyst stability under oxidising conditions and can be altered by ligand tuning. Despite these efforts the best catalytic results in cyclohexane oxidation remained those achieved with the Please do not adjust margins Please do not adjust margins parent BPMEN ligand.…”

“…63 Substitution of the bridging nitrogen donors by oxygen or sulphur atoms (20a-d, Figure 11) results in a complete lack of catalytic activity. 65 In this context Britovsek et al 60,65 proposed that a strong ligand field inducing primarily low spin iron complexes positively influences catalytic activity and selectivity. A milestone was reached by Chen and White in 2007.…”

“…In the presence of PCA as cocatalyst cyclohexanol and cyclohexanone are produced from cyclohexane in yields up to 16.3% based on the substrate while the selectivity varies significantly depending on the reaction time and temperature (Table 1). Although Britovsek et al 65 reported [Fe II (20c)(OTf)2] to be catalytically inactive, Pombeiro and co-workers demonstrated the opposite for the analogous complexes bearing chloride and acetonitrile instead of triflate ligands (Table 1). 156 High selectivity towards alcohol formation and up to 1450 turnovers can be achieved by use of acid additives, in particular trifluoroacetic acid (TFA) and triflic acid (HOTf).…”

Aquasoluble iron(III)-arylhydrazone-β-diketone complexes: Structure and catalytic activity for the peroxidative oxidation of C5–C8 cycloalkanes

“…86-92% (Table S1, entries 31 and 34). Thus, the pronounced yield drops verified for cyclohexane oxidations when containing free radical scavengers corroborate the involvement of radical pathways that can compete with metal-based oxidations catalyzed by other Fe III complexes with N,O\ or O,O\ ligands [18,[32][33][34].…”

“…Peroxidative oxidation of alkanes, in particular cyclohexane, is a widely used model to check and compare the activity of the complexes when biomimicking active sites of various oxygenases [32][33][34]. Moreover, alkane functionalization stands as a challenge for the establishment of hydrocarbon feedstocks, and new methodologies for C\H bond oxidation can result in synthetic strategies toward more complex and valuable organic products [35][36][37][38][39][40].…”