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 [Fe(III)(2)(μ-OH)(2)(bik)(4)](NO(3))(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 Fe(III)(2)(μ-OH)(2) complexes. The bridging hydroxide groups of 1 are strongly hydrogen-bonded to a nitrate anion. The (57)Fe isomer shift (δ = 0.45 mm s(-1)) and quadrupole splitting (ΔE(Q) = 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 = 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···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 [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°/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 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.

Figure 1

Some selected ligands used in mono- and dinuclear non-heme iron oxidation catalysis, including the bik ligand used in this study.

Figure 2

Molecular structure of the dinuclear [Fe^III₂(μ-OH)₂(bik)₄]⁴⁺ cation of 1 in the crystal. All C–H hydrogen atoms, nitrate anions and co-crystallized water molecules have been omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level. Symmetry operation a: 1–x, y, 1/2–z.

Figure 3

Hydrogen bonding interactions in [Fe^III₂(μ-OH)₂(bik)₄](NO₃)₄·2H₂O (1). Left: Hydrogen bonds between two nitrate anions and the [Fe^III₂(μ-OH)₂(bik)₄]⁴⁺ cation of 1. The Fe^III₂(μ-OH)₂ core with donor atoms and the two hydrogen bonded nitrate anions are shown. Right: Hydrogen bonds resulting in infinite linear chains. Symmetry operation i: 1/2 – x, 1/2 – y, 1 – z.

Figure 4

Plots of χ_MT versus T (A) and χ_M versus T (B) for 1 from 2 to 300 K in 0.08 T field.

Figure 5

Exchange-coupling constant J for {1_bare} (○) and {1(NO₃)₂} (□) calculated with relaxed DFT scans as a function of Fe···Fe distance. The solid circle and solid square labeled “opt” are the results for the optimized structures of {1_bare} and {1(NO₃)₂}. The experimental data point is labeled “exp”. The upper curve is obtained from the curve though the data points for {1_bare} by multiplication with 1.25.

Figure 6

UV-Vis spectral changes observed for 1 (0.34 mM) in methanol at room temperature showing the formation of an [Fe^II(bik)₃]²⁺ species (t = 120 min). The UV-Vis spectrum of independently synthesized [Fe^II(bik)₃](OTf)₂ (2) (0.3 mM) is included for comparison.

Figure 7

Molecular structure of the [Fe^II(bik)₃]²⁺ cation in the crystal of 2·MeOH at 150 K All hydrogen atoms, triflate anions and the co-crystallized methanol molecule have been omitted for clarity. Displacement ellipsoids are drawn at the 50% probability level.

Figure 8

¹H NMR spectral changes upon cooling a solution of [Fe^II(bik)₃](OTf)₂ (2) in methanol-d₄. The inset shows the changes in the solution magnetic moment at various temperatures.

Figure 9

Thermal variation of the χ_MT product of [Fe^II(bik)₃](OTf)₂·MeOH (2·MeOH). A: χ_MT versus T plot upon heating from 6 K to 400 K (◇), subsequent cooling (△), second heating (○). B: χ_MT versus T plot of 2·MeOH when cooling first from room temperature to 6 K (◇) and back up again (○).