Non-heme Iron(II) Complexes Containing Tripodal Tetradentate Nitrogen Ligands and Their Application in Alkane Oxidation Catalysis

Abstract

A series of iron(II) bis(triflate) complexes containing tripodal tetradentate nitrogen ligands with pyridine and dimethylamine donors of the type [N(CH(2)Pyr)(3-n)()(CH(2)CH(2)NMe(2))(n)] [n = 0 (tpa, 1), n = 1 (iso-bpmen, 3), n = 2 (Me(4)-benpa, 4), n = 3 (Me(6)-tren, 5)] and the linear tetradentate ligand [(CH(2)Pyr)MeN(CH(2)CH(2))NMe(CH(2)Pyr), (bpmen, 2)] has been prepared. The preferred coordination geometry of these complexes in the solid state and in CH(2)Cl(2) solution changes from six- to five-coordinate in the order from 1 to 5. In acetonitrile, the triflate ligands of all complexes are readily displaced by acetonitrile ligands. The complex [Fe(1)(CH(3)CN)(2)](2+) is essentially low spin at room temperature, whereas ligands with fewer pyridine donors increase the preference for high-spin Fe(II). Both the number of pyridine donors and the spin state of the metal center strongly affect the intensity of a characteristic MLCT band around 400 nm. The catalytic properties of the complexes for the oxidation of alkanes have been evaluated, using cyclohexane as the substrate. Complexes containing ligands 1-3 are more active and selective catalysts, possibly operating via a metal-based oxidation mechanism, whereas complexes containing ligands 4 and 5 give rise to Fenton-type chemistry.

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

“…24 For several complexes, the µ eff values are significantly lower than expected for a HS iron(II) centre, which is suggestive of the existence of spin crossover (SC) behaviour in these complexes. 68,[76][77][78] The magnetic behaviour of the iron(II) complexes in CD 3 …”

“…343K), shows only the pyridyl protons as sharp singlets, whereas the methylene protons are very broad, indicating considerable fluxional behaviour of the ligand backbone (see Figure S11). This indicates that these sulphur donors, despite their stronger coordination compared to the oxygen donors, do not enforce the same conformational rigidity as seen for the amine donors in the BPMEN complex [Fe (1) 24 The main difference between the spectra is the absence of the protons in the para position of the pyridine rings, which are typically observed at a chemical shift of 10-20 ppm. It was often observed for these complexes that insufficient numbers of resonances were found to account for all the methylene and ethylene protons or they were very broad due to fluxional behaviour, with the result that peak integration was unreliable.…”

“…24,53,54 The bis(pyrazyl) ligand BPZMEN 4 was prepared by methylation of the previously reported precursor. 55 The preparation of the novel oxygen-bridged ligands 5 and 7 was less straightforward.…”

“…21 In contrast, some non-heme iron complexes have shown a dramatically different behaviour. Much higher conversions of H 2 O 2 into oxidised products, combined with a more selective oxidation reactivity have been observed with certain non-heme iron(II) complexes containing tetradentate nitrogen-based ligands, which can be classified into the following categories, according to their ligand structure: tripodal tetradentate ligands such as TPA 22,23 and iso-BPMEN, 24 linear tetradentate ligands such as BPMEN, 25 BQEN 26 and (S,S-PDP) 27 and cyclic tetradentate ligands, for example Me2 PyTACN (see Figure 1). 28,29 Several non-heme iron catalysts have shown stereospecific and dioxygenindependent oxidation of alkanes with H 2 O 2 13, 17, 30 and the C-H regio-selectivities and kinetic isotope effects have indicated a more selective oxidant than those responsible for oxidation in Fenton-type systems.…”

Lithium, sodium and potassium picolyl complexes: syntheses, structures and bonding

“…Me6TREN was prepared by a literature method. 22 NMR spectra were recorded on a Bruker AV 400 MHz spectrometer operating at 400.13 MHz for 1 H, 155.47 products could not be obtained so 1 H NMR spectra for all products except 2, which was not sufficiently soluble, are included in Supporting Information as evidence of good bulk purity.…”