Calix[4]arene-supported rare earth octahedra

Sergio Sanz, Ruaraidh D. McIntosh, Christine M. Beavers, Simon J. Teat, Marco Evangelisti, Euan K. Brechin, Scott J. Dalgarno
2012 Chemical Communications  
A series of calix[4]arene-supported Ln III 6 clusters have been synthesised under facile bench top conditions. The magnetic and structural properties of these clusters are reported, the latter suggesting that the Ln III -calix[4]arene moiety may be used for the construction of other assemblies in a manner 10 akin to that for the Mn III -calix[4]arene analogue. p-tert-Butylcalix[4]arene (TBC[4]) is a readily accessible bowlshaped molecule composed of four phenol units linked by methylene
more » ... y methylene bridges. 1 The poly-phenolic nature of this ligand presents it as an excellent candidate for the complexation of 15 transition or lanthanide metal centres (TM and Ln respectively) at what is termed the lower-rim. Despite this feature, methylene bridged calix[4]arenes, unlike their thia and sulfonyl bridged analogues, 2 have received relatively little attention in the context of constructing polynuclear metal clusters. 3,4 In this regard we 20 have recently begun to map out TM and 3d-4f cluster motifs formed with TBC[4] under facile bench top conditions. We first used TBC[4] to synthesise Mn III 2 Mn II 2 clusters that are based around a general wing-tip butterfly core, and that behave as Single-Molecule Magnets (SMMs). 3b,c Within this structure the 25 tetra-anionic lower-rim of the calixarene is occupied by one Mn III ion. This moiety can subsequently be exploited in the synthesis of Mn III 4 Ln III 4 clusters by simple addition of Ln III ions to the reaction mixture used in the formation of the aforementioned Mn III 2 Mn II 2 SMMs. 3f,g Depending on the Ln employed in these 30 3d-4f systems, the resulting materials behave either as a molecular refrigerant (where Ln = Gd) or SMMs (where Ln = Dy or Tb). We also extended our 3d-4f studies to include Fe-Ln mixtures, the result of which was a series of Fe III 2 Ln III 2 clusters that are markedly different to the Mn III 4 Ln III 4 motif. 3h The TBC[4] 35 lower-rim is occupied by one Fe III centre, as has also recently been observed by Redshaw and co-workers in their investigations into calixarene-supported Fe III complexes as procatalysts for εcaprolactone polymerisation. 4e Although we recently used TBC[4] as a support in the 40 formation of a small library of clusters, we had not investigated the possibility of forming polynuclear lanthanide clusters with this ligand. The recent re-ignition of interest in the synthesis of polymetallic 4f clusters 5 originates from two major sources: single-molecule magnetism 6 and molecular cooling 7 in which the 45 large magnetic moments, weak exchange and (an)isotropy can be exploited as desired. Here we report the formation of a series of calix[4]arene-supported Ln III 6 clusters, synthesised under facile bench top conditions. Single crystal X-ray structure analysis shows that the metals are arranged at the vertices of an 50 octahedron, with the complexes forming an inter-digitated layer motif which differs markedly to those known for the solvates of TBC[4], 8 and the layered structures formed with the TBC[4]supported clusters described above. 3 This also represents the first polynuclear lanthanide cluster to be formed with TBC[4], and an 55 important entry into this new system. The reaction of LnCl 3 ·6H 2 O and TBC[4] in a basic DMF / ROH (R = Me, Et) solution produces colourless crystals of [Ln 6 (TBC[4]) 2 O 2 (OH) 3.32 Cl 0.68 (HCO 2 ) 2 (DMF) 8 (H 2 O) 0.5 ] (1 = Gd; 2 = Tb; 3 = Dy; 4 = Ho; Figure 1) after slow evaporation of the 60 mother liquor. ‡ Figure 1. Single crystal X-ray structure of compound 2. Additional solvent molecules and hydrogen atoms are omitted for clarity. Colour code: Ln = green, O = red, N = dark blue, C = grey, Cl = yellow. The 65 complex is shown in one of two disordered positions. Crystals of 1 -4 are in the monoclinic space group C2/m and all are found to be isostructural. Problems with poor diffraction hampered full structure solutions for compounds 1, 3 and 4, but full analysis was possible with synchrotron diffraction data 70 collected for 2. As the four TBC[4]-supported clusters are structurally analogous we provide a generic description. In each case the structures are disordered over two closely related positions, with additional disorder in some ligands (see CIF file for details), and thus we avoid giving detailed bond lengths and 75 angles (although major connectivities are describable). The metallic core (Figure 2) describes a [Ln III 6 ] octahedron [Ln1...Ln2,3 ~3.4 Å; Ln2...Ln3, ~4.4 Å; Ln1...Ln1', ~3.4 Å] in which the six metal ions are connected internally by two μ 4 -O 2 ions (O1 and symmetry equivalent, s.e; Ln...O, ~2.2 Å). The 80 fully deprotonated TBC[4] ligands house the apical Ln ions (Ln1 [journal], [year], [vol], 00-00 | 1
doi:10.1039/c1cc14603c pmid:21904753 fatcat:iwo64wbis5e43pen62do62mkbe