New bio-inspired iron oxidation catalysts

Abstract

New derivatives of a tetra-amide macrocyclic ligand H4BJ (3,3,8,8-tetramethyl-3,4,7,8- tetrahydrobenzo[b] [ 1,4,7,10]tetraazacyclododecine-2,5,6,9( 1H, 1 OH)-tetraone) were synthesised and characterised, and the effect ofsubstituents on the aromatic ring ofthe macrocycle were explored. It was found that derivatives ofthe H4BJ macrocycle could be conveniently prepared via direct substitution ofthe aromatic ring in the preformed H4BJ macrocycle and this led to the formation ofthe dibrominated derivative H4BJBr2 and the mononitrated derivative H4BJNO2. Reduction ofthe nitro group in H4BJNO2 to an amine was achieved with the use of palladium on carbon catalyst and hydrazine hydrate to form H4BjNH2 in high yield. Spectroscopic and X-ray structure determination showed that these macrocycles exhibit a twisted macrocyclic ring conformations that result from the preferred trans arrangement ofthe two oxalamide carbonyl groups. The formation ofthe iron complexes ofthese ligands and the behaviour ofthese bio-inspired complexes as catalysts for the oxidation of organic substances by hydrogen peroxide were investigated. The molecular structure of(NEt4)2[Fe(OH)(BJBr2)] was determined by X-ray crystallography this showed a saddle shaped conformation for the macrocycle with the iron atom displaced by 0.543(8) A out ofthe Nav plane. This is significantly further out ofthe Nav plane when compared to the Fem'TAMLs (PPh4)[Fe(H2O)B *] and (PNP)[Fe(H2O)(D *Cl 2)]. The dependence ofthe rates ofthe catalytic hydrogen peroxide oxidations ofthe azo dye Orange II on the concentration ofthese iron catalysts, hydrogen peroxide, Orange II, the pH and temperature were investigated in detail. The optimum pH at which the maximum initial rate of oxidation of Orange II occurred was determined to be at pH 9.0 for Fe(BJ) with an initial rate of 2.06 xlO'7 mol L'1 s’1, pH 9.7 for Fe(BJBi'2) with an initial rate of 2.02 xlO'7 mol L'1 s4, pH 9.6 for Fe(BJNO2) with an initial rate of2.72 xlO’7 mol L'1 s’1 and pH 9.3 for Fe^NH2) with an initial rate of 7.27 xlO’8 mol L'1 s’\ Hydrolytic stability in perchloric and hydrochloric acid revealed that the Fe(BJX2) complexes were significantly more stable than the previously reported Fe]II-TAML complexes, including (Et4N)2[Fenl(Cl)(B *)] with the most stable complex Fe^Nf^) having a remarkable halflife of 4886 minutes (3.4 days) in hydrochloric acid solution of pH 1.0. The stability ofthe Fe(BJX2) complexes in the absence of a substrate that can be oxidised and in the presence of hydrogen peroxide was enhanced over that of(Et4N)2[Feni(Cl)(B *)] over the range of temperatures examined (25, 40 and 60 °C). Turn-over numbers ofthe complexes were investigated using a more slowly oxidised dye Safranine-O. It was calculated that Fe(BJNO2) performed the greatest minimum TONs of 255. A mechanism that is consistent with the kinetic data is shown below, and the rate constants kia and k/b were calculated. , Substrate K1a _ kjb \ LFe'" + H2O2 ------ LFe-intermediate---------~L[ev=O —---- ► OSubstrate + LFe111 k.1a Fast L = TAML ligand At 25 °C when the hydrogen peroxide concentration is very large relative to the catalyst concentration, the rate offormation ofthe LFe-intermediate species is no longer rate determining. The rate determining step becomes the formation ofthe active oxidant (which is represented for simplicity as LFev=O) species from the LFe-intermediate species and the rate constant k/u can be calculated.