Synthesis and Crystal Structure of the Bis(μ-hydroxo)diiron(II) Complex with Tridentate Ligands Having a Sterically Bulky Imidazolyl Group

The Fe2/O2-mediated arene hydroxylation is of current interest for understanding the reaction mechanism of a dioxygen activating non-heme dinuclear iron enzyme, such as toluene monooxygenase (TMO).1–3 Previously, we reported on intramolecular aromatic ligand hydroxylation via the decay of (μ-peroxo)diiron(III) complexes with the dinucleating ligands having a phenyl group as a reaction probe.4–6 In this study, we synthesized the bis(μ-hydroxo)diiron(II) complex [Fe2(OH)2(L)2](ClO4)2 (1) with a tridentate ligand (L) having 2-phenylimidazolyl groups in order to examine the Fe2/O2-mediated arene hydroxylation (Fig. 1). In this paper, we report on the crystal structure of 1 together with the reactivity toward dioxygen. The tridentate ligand L·H2O was synthesized according to a literature method.7 The diiron(II) complex with L was prepared under a N2 atmosphere. A mixture of Fe(CF3SO3)2·2CH3CN (0.436 g, 1.0 mmol) and L·H2O (0.456 g, 1.0 mmol) in dryTHF/CH2Cl2 (1:1, 20 mL) was added a THF solution (3 mL) containing H2O (18 μL, 1.0 mmol) and triethylamine (138 μL, 1.0 mmol) to give a yellow-green solution. Diethyl ether (20 mL) was added to the resulting yellow-green solution to give a yellow-green powder, which was filtered and washed with diethyl ether. Yield: 0.32 g, 47%. Anal. Found: C, 52.51; H, 4.43; N, 10.18%. Calcd for [Fe2(OH)2(L)2](CF3SO3)2·2H2O, C60H64F6Fe2N10O10S2: C, 52.41; H, 4.69; N, 10.19%. Single crystals of [Fe2(OH)2(L)2](ClO4)2 (1) suitable for X-ray crystallography were obtained by slow diffusion of diethyl ether onto a dry-CH2Cl2/MeCN (1:1) solution of a yellow-green powder containing (n-Bu)4NClO4. X-ray diffraction measurements were made on a Rigaku CCD Mercury diffractometer with graphite-monochromated Mo Kα radiation at 123 K. The structure was solved by a direct method (SHELXS 97)9 and expanded using a Fourier technique. The structure was refined by a full-matrix least-squares method by using a SHELXL 201410 (Yadokari-XG).11 All non-hydrogen atoms were refined with anisotropic displacement parameters. The μ-hydroxo hydrogen atoms were refined using distance restraints with Uiso constrained to 1.2-times the Ueq of the parent oxygen atoms. Other hydrogen atoms were included using a riding model. The crystal data are summarized in Table 1. X-ray crystallography of 1 reveals that the asymmetric unit contains a half of the complex cation [Fe2(OH)2(L)2] and a perchlorate anion. The molecular structure of the complex cation [Fe2(OH)2(L)2] of 1 is shown in Fig. 2. Selected bond distances (Å) and angles (°) are given in Table 2. The complex cation [Fe2(OH)2(L)2] of 1 has a centrosymmetric bis(μhydroxo)diiron(II) diamond core, as found for a closely related five-coordinated bis(μ-hydroxo)diiron(II) complex [Fe2(OH)22019 © The Japan Society for Analytical Chemistry