The development of N,N,N,N-tetradentate Fe(II) complexes for alkene and alcohol oxidation
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We developed a catalyst system capable of oxidising olefinic and alcohol substrates under ambient air and at low temperatures. Our catalyst system comprises of an iron precursor in combination with a bis-heterocyclic diamine ligand set. These ligands have different steric and electronic properties, by i) varying the substituents on the pyridyl ring, and ii) varying the N-heterocycle. Firstly, we prepared a series of chiral (R,R) bis-heterocyclic secondary diamine ligands (L3.6 to L3.10; (R,R)-L3.6, heterocycle = pyridine; (R,R)-L3.7, heterocycle = 6-methyl-2-pyridine; (R,R)-L3.8, heterocycle = 6-bromo-2-pyridine; (R,R)-L3.9, heterocycle = 1-methyl-imidazole; (R,R)-L3.10, heterocycle = quinoline) and the corresponding iron(II) complexes (C3.1 to C3.5; (R,R)-C3.1, heterocycle = pyridine; (R,R)-C3.2, heterocycle = 6-methyl-2-pyridine; (R,R)-C3.3, heterocycle = 6-bromo-2-pyridine; (R,R)-C3.4, heterocycle = 1-methyl-imidazole; (R,R)-C3.5, heterocycle = quinoline). Nuclear magnetic resonance (NMR, 1H, 13C) spectroscopy, mass spectrometry (MS) and ultraviolet-visible (UV-Vis) spectroscopy were used to characterise these ligands and complexes. These complexes were investigated as catalysts in the oxidation of cis-cyclooctene. All the complexes exhibited similar catalytic activity, with turnover numbers between 9.60 and 12.70, which led us to believe that the complexes have low stability. This was confirmed with electrospray ionisation (ESI) MS, which indicated the oxidative degradation of the catalysts, which leads to lower stability and subsequently lower catalytic activity. To improve the stability, a series of chiral (R,R) and (S,S) bis-heterocyclic tertiary diamine ligands (L4.1 to L4.4; (R,R) and (S,S)-L4.1, heterocycle = pyridine; (R,R) and (S,S)-L4.2, heterocycle = 6-methyl-2-pyridine; (R,R) and (S,S)-L4.3, heterocycle = 6-bromo-2-pyridine; (R,R) and (S,S)-L4.4, heterocycle = 1-methyl-imidazole) and their Fe(II)-triflate complexes (C4.1 to C4.4; (R,R) and (S,S)-C4.1, heterocycle = pyridine; (R,R) and (S,S)-C4.2, heterocycle = 6-methyl-2-pyridine; (R,R) and (S,S)-C4.3, heterocycle = 6-bromo-2-pyridine; (R,R) and (S,S)-C4.4, heterocycle = 1-methyl-imidazole) was prepared. These complexes were characterised by a variety of spectroscopic and analytical techniques. These included NMR (1H, 13C) spectroscopy, MS, UV-Vis spectroscopy, elemental analysis and magnetic susceptibility. With the desired complexes in hand, we commenced their evaluation as catalysts in the oxidation of cis-cyclooctene and benzyl alcohol. Of the series of complexes evaluated, (R,R)-C4.1 was able to convert 96% of cis-cyclooctene to cyclooctene epoxide with 100% selectivity. The addition of substituents in the 6-position of the pyridine ring had a pronounced steric effect that resulted in the complexes favouring a high-spin configuration, which led to lower catalytic activity. Replacing the pyridine donor with an imidazole donor also displayed a steric effect, which resulted in a complex that possesses a weaker ligand field and lower catalytic activity. The addition of acetic acid to the oxidation system resulted in up to a 30% increase in the conversion. For benzyl alcohol oxidation, the highest conversion of 73% was seen when employing (S,S)-C4.1. Different parameters of the oxidation reaction were optimised, which included oxidant concentration and catalyst loading. The catalytic activity increased as the amount of H2O2 increased, but also resulted in over-oxidation to benzoic acid. Using 25 μmol of catalyst resulted in the highest catalytic activity. The limitations and functional group tolerance of this catalyst system were investigated by extending the alcohol substrate scope and this system was able to oxidise allylic, benzylic as well as aliphatic primary and secondary alcohols to the corresponding aldehyde and ketone products.