Synthesis and Crystal Structure of a Mononuclear Cobalt(III) Complex with 2-[(2-Hydroxymethylphenyl)iminomethyl]phenol and Dimethylaminopyridine

Single-molecule magnets (SMMs) are intensively discussed as high-density storage, quantum computing, or spintronic technology.1–3 We have reported the weak SMM ability of a tetranuclear Co complex, [Co4(L1-H)4Cl2(CH3OH)2], but found that the stability of the complex depends on the coordinating solvent molecules.4 To improve the stability, we are working on the synthesis of new Co complexes in which the coordination solvents are replaced with various monodentate ligands. While an attempt was made to prepare a tetranuclear Co complex coordinated with dmpa, we obtained single crystals of [Co(L1-H)(HL1-H)dmpa]·CH3CN (1). Here, we report on the synthesis and the crystal structure of 1 (Fig. 1). The ligand H2L1-H was obtained by a literature method.5 An acetonitrile solution (10 mL) containing bis(acetylacetonato) diaquacobalt(II) (0.182 g, 0.5 mmol), H2L1-H (0.113 g, 0.5 mmol), and triethylamine (0.055 g, 0.5 mmol) was stirred at ca. 80°C for 30 min. To the obtained solution was added 4-dimethylaminopyridine (0.122 g, 1.0 mmol) with stirring. The orange precipitate was filtered off, and the filtrate was allowed to stand at room temperature. Reddish brown crystals suitable for X-ray analysis were obtained after 3 days. Yield, 0.009 g (3.0%). Measurements were made on a Rigaku VariMax Saturn CCD 724+ diffractometer with graphite-monochromated Mo-Kα radiation at 113 K. An empirical analytical absorption correction was applied. The structure was solved by direct methods (SIR92) and expanded using Fourier techniques. The nonhydrogen atoms were refined anisotropically. The alcoholic hydrogen atom, H3A, was found from Fourier peaks. The other hydrogen atoms were located at the calculated positions and refined using the riding model. The final cycle of a full-matrix least-squares refinement on F2 using SHELXL-2016 was based on observed reflections and variable parameters and converged with unweighted and weighted agreement factors of R1 (0.0483) and wR2 (0.1226).6 Details of the X-ray structure determination are listed in Table 1. An ORTEP drawing of the title complex 1 is shown in Fig. 2, and selected bond lengths and angles are provided in Table 2. The crystal structure of 1 consists of a Co atom, dianionic (L1-H)2–, monoanionic (HL1-H)–, and neutral dmpa ligands. The first ligand, (L1-H)2–, is coordinated to Co1 as a tridentate ligand with mer-manner. This coordination mode is similar to that of Co(III) units in the previously reported [Co4(L1-H)4Cl2(CH3OH)2]. The second ligand, (HL1-H)–, is bound to Co(III) as a bidentate chelate by phenoxo oxygen and imino nitrogen atoms. Only the tridentate coordination mode of (HL1-H)–, such as [Co(HL1-H)2]NO3, has been reported so far. This is the first example of the structurally analyzed bidentate coordination mode of H2L1-H. The remaining coordination site is occupied by the dmpa. The coordination geometry around the central Co atom is an octahedral with mer-N3O3 coordination environment. The bond distances between Co1 and the coordinating oxygen atoms are in the range of 1.8803(16) – 1.9083(16)Å and those for coordinating nitrogen atoms are in the range of 1.9352(19) – 1.9705(19)Å. These distances are common for Co(III) complexes.7 To confirm the oxidation state assignment, the bond valence sum (BVS) calculation was applied for Co1.9,1