Mono- and Dinuclear Iron Complexes of Bis(1-methylimidazol-2-yl)ketone (bik): Structure, Magnetic Properties, and Catalytic Oxidation Studies

The newly synthesized dinuclear complex [FeIII2(μ-OH)2(bik)4](NO3)4 (1) (bik, bis(1-methylimidazol-2-yl)ketone) shows rather short Fe···Fe (3.0723(6) Å) and Fe–O distances (1.941(2)/1.949(2) Å) compared to other unsupported FeIII2(μ-OH)2 complexes. The bridging hydroxide groups of 1 are strongly hydrogen-bonded to a nitrate anion. The 57Fe isomer shift (δ = 0.45 mm s–1) and quadrupole splitting (ΔEQ = 0.26 mm s–1) obtained from Mössbauer 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·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···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.