Crystal Structure of Bis(μ-hydroxo)diiron(III) Complex with Tripodal Ligands Having a Terminal Carboxylate Group

Abstract

Hydroxoor oxo-bridged non-heme diiron centers with terminal carboxylates are structural motifs found in metalloproteins, such as soluble methane monooxygenase (sMMO) and ribonucleotide reductase (RNR).1–4 Synthetic diiron model complexes with Fe2(OH)2, Fe2(O)(OH), and Fe2(O)2 cores are of particular importance for obtaining fundamental insights into the structural and spectroscopic properties of the active centers in the metalloproteins mentioned above.2–4 Previously, we reported that the crystal structure of a (μ-oxo)(μ-hydroxo)diiron(III) complex, [Fe2(6Me2-BPP)2(O)(OH)] (2),5 with a tetradentate tripodal ligand (6Me2-BPP) having a terminal carboxylate, which was derived from deprotonation of its conjugate acid, [Fe2(6Me2-BPP)2(OH)2]. In this paper, we report on the crystal structure of the bis(μ-hydroxo)diiron(III) complex [Fe2(6Me2BPP)2(OH)2](NO3)1.9553·Br0.0447·8H2O (1) (Fig. 1). A single crystal of [Fe2(6Me2-BPP)2(OH)2](NO3)1.9553·Br0.0447· 8H2O (1) suitable for X-ray crystallography was obtained by the recrystallization of [Fe2(6Me2-BPP)2(OH)2](NO3)2·4.5H2O from water. It was picked up on a hand-made cold copper plate mounted inside a liquid N2 Dewar vessel and mounted on a glass rod at –80°C. 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)6 and expanded using a Fourier technique. The structure was refined by a full-matrix least-squares method by using the SHELXL 20147 (YadokariXG).8 The asymmetric unit contains two very similar molecules that could be nearly related by each other with a translation symmetry in the [0 1 1] direction. This could be interpreted as a structure with a half unit-cell volume. However, the counter anions (NO3, Br –) and crystallizing water molecules are located at different positions that could not be related by a translational symmetry. Also, the diffractions with k + l = odd showed significant intensities (around 1/3 of those with k + l = even). Therefore, we concluded that the structure should have two independent molecules in the asymmetric unit. All nonhydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms in the μ-hydroxo groups and water molecules were restrained to ensure reasonable distances (0.84 Å) by applying the default values for O–H DFIX restraints. 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 two halves of complex cations, [Fe2(6Me2-BPP)2(OH)2] (molecules A and B), 1.9553 nitrate ions, 0.0447 bromide ion, and eight water molecules. The partial content of the bromide ion appears to be due to the preparation route of the 6Me2BPPH ligand, which was obtained from the reaction of bis(6methyl-2-pyridylmethyl)amine with 3-bromopropionic acid in the presence of triethylamine.5 ORTEP drawing of the complex cation [Fe2(6Me2-BPP)2(OH)2] (molecule A) is shown in Fig. 2. Selected bond distances (Å) and angles (°) are given in Table 2. Only the 2019 © The Japan Society for Analytical Chemistry