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

Abstract

[EN] The newly synthesized dinuclear complex [Fe-2(III)(mu-OH)(2)(bik)(4)](NO3)(4) (1) (bik, bis(1-methylimidazol-2-yl)ketone) shows rather short Fe center dot center dot center dot Fe (3.0723(6) angstrom) and Fe-O distances (1.941(2)/1.949(2) angstrom) compared to other unsupported Fe-2(III)(mu-OH)(2) complexes. The bridging hydroxide groups of 1 are strongly hydrogen-bonded to a nitrate anion. The Fe-57 isomer shift (delta = 0.45 mm s(-1)) and quadrupole splitting (Delta E-Q = 0.26 mm s(-1)) obtained from Mossbauer 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 = JS(1).S-2) 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 center dot center dot center dot Fe and Fe-OH distances are overestimated (3.281 and 2.034 angstrom, respectively). The agreement in these parameters improves dramatically (3.074 and 1.966 angstrom) 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 [Fe-II(bik)(3)](2+) species. Variable-temperature magnetic susceptibility measurements of [Fe-II(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. [Fe-II(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 degrees/2 degrees value of 29.6); low alcohol/ketone ratios in cyclohexane and ethylbenzene oxidation, a strong dependence of total turnover number on the presence of O-2, 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.