Crystal structure of an eight-coordinate terbium(III) ion chelated by N,N′-bis(2-hydroxybenzyl)-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine (bbpen2−) and nitrate

The reaction of terbium(III) nitrate pentahydrate in acetonitrile with N,N′-bis(2-hydroxybenzyl)-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine (H2bbpen), previously deprotonated with triethylamine, produced the mononuclear compound [Tb(Cbbpen)(NO3)]. The molecule lies on a twofold rotation axis and the TbIII ion is eight-coordinate with a slightly distorted dodecahedral coordination geometry.


Chemical context
As far as biological and biomedical applications are concerned, complexes of polydentate ligands with a range of metal ions in different oxidation states have been synthesized to model active sites of metalloproteins and to shed light on the consequences of heavy-metal chelation in living organisms, among many other applications (Colotti et al., 2013;Nurchi et al., 2013;Sears, 2013;Happe & Hemschemeier, 2014). Pyridyl and phenolate groups have been incorporated into these ligands because of their potential to mimic the coordination environments provided by the amino acids histidine and tyrosine, respectively (Hancock, 2013;Lenze et al., 2013). In this context, the heterotrifunctional Lewis base N,N 0 -bis(2hydroxybenzyl)-N,N 0 -bis(pyridin-2-ylmethyl)ethylenediamine (H 2 bbpen) is suitable for the coordination of a range of p-, dand f-block ions because of its versatile soft donor atoms in the pyridine rings and hard donors in the amine and phenolate groups (Neves et al., 1992;Schwingel et al., 1996). Electrochemical studies of the mononuclear [Mn(bbpen)]PF 6 , for example, revealed that this complex mimics some of the redox features of the photosystem II (PSII) (Neves et al., 1992). Complexes of bbpen 2with vanadium(III) and oxidovanadium(IV) have been obtained as models of the vanadium-modified transferrin, the probable vanadium-transporting protein in higher organisms (Neves et al., 1991(Neves et al., , 1993. Iron complexes of bbpen 2modified with electron-donating and -withdrawing groups (Me, Br, NO 2 ), in turn, have been ISSN 2056-9890 synthesized to provide detailed chemical information on the enzymatic activity of iron-tyrosinate proteins (Lanznaster et al., 2006). This ligand has also been employed to prepare lanthanide(III), gallium(III) and indium(III) complexes for medicinal applications such as the development of new contrast agents for magnetic resonance imaging, MRI (Wong et al., 1995(Wong et al., , 1996Setyawati et al., 2000).
More recently, lanthanide(III) chelate complexes have also attracted attention in the field of molecular magnetism due to their highly significant single-ion magnetic anisotropy (Sessoli & Powell, 2009;Luzon & Sessoli, 2012). Accordingly, a number of examples of mononuclear lanthanide complexes that exhibit single-molecule magnet (SMM) behaviour have been reported (Rinehart & Long, 2011;Chilton et al., 2013;Ungur et al., 2014;Zhang et al., 2014). Our interest in the class of lanthanide complexes in which two coordination sites are occupied by relatively labile ligands, as in the title complex, comes from the possibility of using them as starting materials for the preparation of heteronuclear aggregates of d-and fblock ions that present SMM features. In this case, the replacement of the labile ligands by specific bidentate metalloligands can give rise to heteronuclear metal aggregates in which desirable ferromagnetic or ferrimagnetic exchange interactions are favoured (Totaro et al., 2013;Westrup et al., 2014).

Structural commentary
The molecular structure of the title compound is shown in Fig. 1. The Tb III ion is eight-coordinate with a dodecahedral array of N and O atoms (Table 1); the four N atoms of the O 2 N 4 -ligand (bbpen) form one plane, the four O atoms the other, with the phenolic O atoms in the B-sites (roughly equatorial) and the nitrate group O atoms in the A-sites (above and below the equatorial plane). The normals to the two planes are essentially perpendicular. A twofold rotation axis passes through O3 and N1 of the nitrate group, the terbium(III) atom and the mid-point of the C7-C7 i bond [symmetry code (i) 1 À x, y, Àz + 1 2 ]. In the symmetry-unique part of the molecule, the pyridine and benzene rings are both essentially planar and form a dihedral angle of 61.42 (7) . The eightfold coordination pattern might also be described as a distorted bicapped trigonal prism with O1 and N2 as the capping atoms. However, this ignores the symmetry of the coordination, e.g. O1 and O1 i would occupy different sites in the coordination polyhedron. Also, some of the rectangular faces of the prism are difficult to identify. In contrast, the dodecahedral pattern incorporates the twofold symmetry and the distortion from the ideal geometry is minimal.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were included in idealized positions (with C-H distances set at 0.97 and 0.93 Å for the methylene and trigonal-planar groups, respectively) and their U iso values were set to ride (1.2Â) on the U eq values of the parent carbon atoms. Computer programs: APEX2 and SAINT (Bruker, 2010), SHELXS97 and SHELXL2013 (Sheldrick, 2008), ORTEPII (Johnson, 1976), ORTEP-3 for Windows and WinGX (Farrugia, 2012).

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.