Mono- and dinuclear iron complexes of bis(1-methylimidazol-2-yl)ketone (bik): Structure, magnetic properties and catalytic oxidation studies

Abstract

The newly synthesized dinuclear complex [FeIII 2(μ-OH)2(bik)4](NO3)4 (1) (bik, bis(1-methylimidazol-2-yl)ketone) shows rather short Fe 3 3 3 Fe (3.0723(6) Å) and Fe O distances (1.941(2)/1.949(2) Å) compared to other unsupported FeIII 2(μ-OH)2 complexes. The bridging hydroxide groups of 1 are strongly hydrogen-bonded to a nitrate anion. The 57Fe isomer shift (δ=0.45mms 1) and quadrupole splitting (ΔEQ = 0.26 mm s 1) obtained from M€ossbauer spectroscopy are consistent with the presence of two identical high-spin iron(III) sites. Variable-temperature magnetic susceptibility studies revealed antiferromagnetic exchange (J = 35.9 cm 1 and H = JS1 3 S2) of the metal ions. The optimized DFT geometry of the cation of 1 in the gas phase agrees well with the crystal structure, but both the Fe 3 3 3 Fe and Fe OH distances are overestimated (3.281 and 2.034 Å, respectively). The agreement in these parameters improves dramatically (3.074 and 1.966 Å) when the hydrogen-bonded nitrate groups are included, reducing the value calculated for J by 35%. Spontaneous reduction of 1 was observed in methanol, yielding a blue [FeII(bik)3]2+ species. Variable-temperature magnetic susceptibility measurements of [FeII(bik)3](OTf)2 (2) revealed spin-crossover behavior. Thermal hysteresis was observed with 2, due to a loss of cocrystallized solvent molecules, as monitored by thermogravimetric analysis. The hysteresis disappears once the solvent is fully depleted by thermal cycling. [FeII(bik)3](OTf)2 (2) catalyzes the oxidation of alkanes with t-BuOOH. High selectivity for tertiary C H bond oxidation was observed with adamantane (3 /2 value of 29.6); low alcohol/ketone ratios in cyclohexane and ethylbenzene oxidation, a strong dependence of total turnover number on the presence of O2, and a low retention of configuration in cis-1,2-dimethylcyclohexane oxidation were observed. Stereoselective oxidation of olefins with dihydrogen peroxide yielding epoxides was observed under both limiting oxidant and substrate conditions.