Uncommon Coordination Modes of a Potential Heptadentate Aminophenol Donor

This work describes the synthesis, characterization and reactivity towards HoIII of a potential heptadentate N4O3 aminophenol donor. The crystal structure of the [Ho(1,1,4-H3L)(1,1,4-H6L)] complex (1,1,4-H6L = 6,6’-(2-(5-bromo-2-hydroxy -3-nitrobenzyl)-2,5,8,11-tetraazadodecane-1,12-diyl)bis(4-bromo-2-nitrophenol)) shows that the holmium atom binds two aminophenol ligands, one acting as trianionic hexadentate, and the other one as neutral monodentate. As far as we know, both coordination modes of the aminophenol are hitherto unknown for this kind of scarcely reported ligand. This leads to coordination number 7 for the HoIII ion, which is in a capped trigonal prism environment.


Introduction
Since the discovery of the first single-ion magnet (SIM) in 2003 [1], the bis-phthalocyanine terbium complex [Tb(Pc)2], the field of molecular magnetism began to focus on the coordination chemistry of lanthanoid elements. These elements, by themselves, fulfil two of the necessary requirements for a molecule to behave like a magnet: they present intrinsic anisotropy, and, usually, they have a high spin ground state. However, according to Reinhart and Long [2], the anisotropy of the molecule is modulated by the interaction between the single-ion electron density and the crystal field environment in which it is placed. In this sense, for oblate ions, like Dy III or Ho III , a strong axial crystal field should maximize the uniaxial anisotropy. In this way, it has been demonstrated that an axial pentagonal bipyramidal (pbp) environment usually increases the anisotropy of the complexes, improving their magnetic properties. Accordingly, the blocking temperature record for an air-stable molecular magnet (20 K) is held by a dysprosium(III) complex with pbp geometry [3]. Nevertheless, this temperature is still very low and, consequently, more research in the coordination chemistry of lanthanoid complexes with ligands that can lead to pbp geometries is still needed, in order to improve the magnetic behaviour of this kind of complex. With these considerations in mind, in this study we describe the synthesis of a new potentially heptadentate ligand, which could predetermine a pbp geometry by itself, and its reactivity towards holmium(III).

Materials and General Methods
All chemical reagents and solvents were purchased from commercial sources and used as received without further purification. Elemental analyses of C, H and N were performed on a THERMOSCIENTIFC FLASH SMART analyzer. 1 H-NMR spectrum of 3NO2,5Br-H3L and 3NO2,5Br-H6L 1,1,4 were recorded on a Varian Inova 400 spectrometer, using DMSO-d6 as solvent. Infrared spectrum of 3NO2,5Br-H3L was recorded in the ATR mode on a Varian 670 FT/IR spectrophotometer in the range 4000-500 cm −1 .

Single X-Ray Difraction Studies
Single crystals of [Ho(3NO2,5Br-H3L 1,1,4 )(3NO2,5Br-H6L 1,1,4 )]·1.5CH3C6H5 (2·1.5CH3C6H5) were obtained as detailed above. An ellipsoid diagram for 2 is shown in Figure 2 and main distances and angles are recorded in Table 1.  The crystal structure shows that the unit cell is composed of neutral [Ho (3NO2,5Br-H3L 1,1,4 )(3NO2,5Br-H6L 1,1,4 )] complexes, and toluene as solvate. In the complex, there are two aminophenol ligands joined to the holmium(III) ion. One of them acts as a trianionic hexadentate donor, using all its oxygen atoms and three of the four nitrogen atoms to coordinate to the metal centre. The distance Ho···N11 of 2.759(10) Å seems too long to be a real coordinated bond, and it should be best considered as a secondary intramolecular interaction [9]. Thus, this ligand provides an N3O3 environment to the Ho III centre. The coordination sphere of the metal ion is completed by an oxygen atom (O23) coming from the second aminophenol ligand, which acts as neutral monodentate.
Curiously, in this second ligand, the coordinated phenol oxygen atom is deprotonated, and the nitrogen (N21) with two benzyl substituents is protonated. Thus, this second neutral aminophenol ligand is a zwitterion, with the charge distribution shown in Scheme 3. As a result of the described features, Ho III reaches coordination number 7. Calculations of the distortion from an ideal HoN3O4 core with the SHAPE program [10] indicate that the geometry is closer to a capped trigonal prism.
The main distances and angles about the metal centres agree with those expected for holmium complexes with polydentate N,O donors [9], and this aspect does not deserve further consideration. Nevertheless, it should be noted once again that in this complex one of the aminophenol ligands acts as trianionic hexadentate, and the other one as neutral monodentate. None of these coordination modes have been previously described for this kind of scarcely related aminophenol ligand, which, as far as we know, in the only three previous examples crystallographically characterised [7,8], behaves as trianionic heptadentate. Therefore, this works contributes to increase the knowledge of the coordination chemistry of lanthanoids with a barely reported potentially heptadentate aminophenol ligand.