Crystal Structure of cis-Bis(2,2′-bipyridyl)bis(trifluoromethanesulfonato)cobalt(II)

Cobalt(II) complexes with polypyridyl ligands were used as photo-catalysts for hydrogen generation from water,1 sensitizers in dye-sensitized solar cells,2 or building blocks for magnetic supramoleculars,3 because of their redox and magnetic properties. When synthesizing a new functional cobalt(II) complex with polypyridyl ligands, such as cis-[Co(L)2(N-N)2] or cis-[Co(L-L)(N-N)2] (L = monodentate ligand, L-L = bidentate ligand, N-N = 2,2′-bipyridine (bpy) or 1,10-phenanthroline (phen) etc.), cis-[Co(solvent)2(N-N)2] or cis-[CoII(OTf )2(N-N)2] (OTf– = trifluoromethanesulfonato or CF3SO3) have been utilized as a precursor.3 This is a popular synthetic method for not only cobalt(II) complexes, but also many transition metal complexes. However, there are only four reports concerning the crystal structure of cis-[MII(OTf )2(bpy)2] (M = Mn, Ni, Cu, or Zn),3 because OTf– ligands might be too labile to be isolated from a reaction solution as single crystals. Smith and co-workers prepared cis-[MII(OTf )2(bpy)2] (M = Mn or Co) complexes from the reaction of cis-[MCl2(bpy)2] with Ag(OTf ) in acetonitrile.3 The cis-[MnII(OTf )2(bpy)2] complex was characterized by elemental analysis, IR spectra, and X-ray crystal structure, whereas the Co(II) complex was reported based on only IR spectroscopic data. Fortunately, we were able to determine the crystal structure of cis-[CoII(OTf )2(bpy)2] (Fig. 1). Here we report on it, and compare the lattice parameters of our Co2+ complex with those of other transition metal complexes, cis-[MII(OTf )2(bpy)2]. We obtained yellow crystals of cis-[CoII(OTf )2(bpy)2], which were suitable for X-ray crystallography, from the reaction mixture of [Co(CO)3(bpy)2](OTf )4 and di-2-pyridylamine (Hdpa) in EtOH–H2O containing a small amount of HOTf(aq). We expected that [Co(bpy)2(Hdpa)] would be obtained from the reaction, but the anticipated complex was not yielded. The 1H NMR spectrum of the product showed that [Co(bpy)3] and a CoIII-Hdpa complex, probably [Co(Hdpa)3] or trans[Co(OH2)2(Hdpa)2], existed in the NMR sample. We tried to crystalize the CoIII-Hdpa complex by the vapor diffusion of diethyl ether into an EtOH–MeOH (1:1) solution of the mixed product. However, this plan went wrong. From the solution, two kinds of yellow crystals, which had different shapes, for [Co(bpy)3](OTf )3 and cis-[CoII(OTf )2(bpy)2], and a yellowbrown oily product, which would be the CoIII-Hdpa complex, were obtained. The Co2+ complex, cis-[CoII(OTf )2(bpy)2], might be produced from a reduction reaction of the Co3+ complex, [Co(CO3)(bpy)2], by EtOH in the reaction solution. We thought that it would be important for coordination chemists, because it is the first report concerning the crystal structure of cis-[CoII(OTf )2(bpy)2]. In 2013, Kurahashi and Fujii reported on a very unique cobalt complex with a salen ligand (Fig. S1, Supporting Information) and an OTf– ligand, [Co(salen)(OTf )], which was characterized by X-ray crystallography, cyclic voltammetry, L-edge X-ray absorption spectroscopy, the temperature dependence of the magnetic moment, X-band EPR spectra, and NMR spectra.5 Based on their experiments, they concluded that the complex contained both [CoIII(salen)(OTf )] and [CoII(salen•+)(OTf )] (salen•+ = salen ligand radical) character, and in the crystal the complex had a significant [CoIII(salen)(OTf )] character compared with the [CoII(salen•+)(OTf )] character.5 This interesting and uncommon complex had been the only reported instance of the Co complex with an OTf– ligand, until our report concerning the common Co2+ complex, cis-[CoII(OTf )2(bpy)2]. X-ray crystallography was performed at 173 K on a Rigaku XtaLAB P200 diffractometer using multi-layer mirror monochromated Cu-Kα radiation. The structure was solved by 2019 © The Japan Society for Analytical Chemistry