Mono- and binuclear complexes of iron(ii) and iron(iii) with an N4O ligand: synthesis, structures and catalytic properties in alkane oxidation

Abstract

Three mononuclear iron complexes and one binuclear iron complex, [Fe(tpoen)Cl]·0.5(Fe₂OCl₆) (1), [Fe(tpoen)Cl]PF₆ (2), Fe(tpoen)Cl₃ (3) and [{Fe(tpoen)}₂(µ-O)](ClO₄)₄ (4) (tpoen = N-(2-pyridylmethoxyethyl)-N,N-bis(2-pyridylmethyl)amine), were synthesized as functional models of non-heme iron oxygenases. Crystallographic studies revealed that the Fe(II) center of 1 is in a pseudooctahedral environment with a pentadentate N₄O ligand and a chloride ion trans to the oxygen atom. The Fe(III) center of 3 is ligated by three nitrogen atoms of tpoen and three chloride ions in a facial configuration. Each Fe(III) center of 4 is coordinated with four nitrogen atoms and an oxygen atom of tpoen with the Fe–O–Fe angle of 172.0(3) Å. Complexes 2, 3 and 4 catalysed the oxidation of cyclohexane with H₂O₂ in the total TNs of 24–36 with A/K ratios of 1.9–2.4. Under the same conditions they also catalysed both the oxidation of ethylbenzene to benzylic alcohol and acetobenzene with good activity (30–47 TN) and low selectivity (A/K 0.7), and the oxidation of adamantane with moderate activity (15–18 TN) and low regioselectivity (3°/2° 3.0–3.2). With mCPBA as oxidant the catalytic activities of 2, 3 and 4 increased 1.8 to 2.3-fold for the oxidation of cyclohexane and ethylbenzene and 6.3 to 7.5-fold for the oxidation of adamantane. Drastic enhancement of the regioselectivity was observed in the oxidation of adamantane (3°/2° 18.5–30.3).