Dinuclear bis-β-diketonato ligand derivatives of iron(iii) and copper(ii) and use of the latter as components for the assembly of extended metallo-supramolecular structures

Abstract

A range of 1,3-aryl linked, bis-beta-diketone derivatives (LH2) has been employed to synthesise neutral bis(ligand), dinuclear complexes incorporating square-planar copper(II) and tris(ligand) dinuclear helical derivatives containing octahedral iron(III). The 1H NMR spectra of the free ligands contain singlet peaks at ca. 16.2 ppm, indicative of enolic protons, confirming that the (bis) enol tautomer is present in solution. An X-ray structure of a ligand from the series incorporating tert-butyl terminal substituents confirms that the same tautomer persists in the solid and that the relative orientation of the bis-beta-diketone fragments is such that the coordination vectors lie at approximately 120 degrees to each other. The planar, dinuclear copper complexes form 1 : 2 adducts with pyridine and 4-(dimethylamino)pyridine, confirmed by X-ray structures, that incorporate five-coordinate metal centres. Based on this behaviour, the prospect of linking copper centres in the dinuclear complexes using the difunctional heterocyclic bases, 4,4'-bipyridine, 4,4'-trans-azopyridine and pyrazine as co-ligands has been probed. However, 4,4'-bipyridine was observed to coordinate through only one of its heterocyclic nitrogen atoms in the solid state to form a 1 : 2 ([Cu2(L)2]: 4,4'-bipyridine) adduct, analogous to the structures obtained with the above mono-functional nitrogen bases. Nevertheless, an X-ray structure determination shows that the related difunctional base, 4,4'-trans-azopyridine, coordinates in a bridging fashion via both its heterocyclic nitrogen atoms on alternate sides of each planar [Cu2(L)2] unit to produce an infinite one dimensional metallo chain. In contrast, with pyrazine, a new neutral, discrete assembly of type [Cu4(L)4(pyrazine)2] is formed. The X-ray structure shows that two planar dinuclear complexes are linked by two pyrazine molecules in a sandwich arrangement such that the coordination environment of each copper ion is approximately square pyramidal with the overall tetranuclear structure thus taking the form of a 'dimer of dimers'.

Hierarchical assembly of discrete copper(ii) metallo-structures from pre-assembled dinuclear (bis-β-diketonato)metallocycles and flexible difunctional co-ligands

“…2) has five-coordinate copper(II) centres with two planar dinuclear metallocyclic units bridged by two dmpip ligands such that the overall structure is a discrete tetranuclear architecture similar to those previously reported by us using dabco and pyrazine as bridging ligands. 13,20 However, in contrast to these previous structures, the present structure is somewhat 'stepped' resulting from the presence of the chair conformation of the respective bound dmpip ligands. This results in the mean planes of the two dinuclear platforms being separated by 6.35 Å -considerably less than occurs in the tetranuclear analogues incorporating 'linear' dabco (7.73 Å) or pyrazine (7.5 Å) as the bridging co-ligands.…”

Extended three-dimensional supramolecular architectures derived from trinuclear (bis-β-diketonato)copper(ii) metallocycles

“…Hence the use of an excess of linker might be expected to help promote formation of a prismatic arrangement rather than, for example, the step-like polymer observed for the structure described above and the structure of [Cu 2 (L 1 ) 2 (azpy)] n reported previously. 6 Clearly prismatic arrangements require more linker moieties per triangular building block than occurs in the above stepped polymeric structure, with its 1 : 1 (building block: linker) stoichiometry. In view of this the synthetic procedure outlined for {[Cu 3 (L 2 ) 3 (bipy)(THF)]·2.75THF} n was repeated using excess 4,4 -bipyridine.…”

“…Thus, the 1,3-aryl linked ligands tend to form dinuclear complexes on interaction with divalent metal ions; as is exemplified by the preparation of the copper(II) 'dimer' complex 1. 5,6 On reaction with trivalent metal ions, the 1,3-linked derivatives have been demonstrated to yield neutral dinuclear triple helical structures 9 while their 1,4-linked analogues yield tetranuclear, tetrahedral-shaped (neutral) products. 7 We have previously demonstrated that 1 can act as a dinuclear building block for the formation of a discrete tetranuclear 'dimer of dimers' on reaction with the ditopic linker unit pyrazine, while a one dimensional coordination polymer of stoichiometry [Cu 2 (L 1 ) 2 (azpy) 2 ] n was obtained when the 'offset' linear linker, 4,4 -trans-azopyridine (azpy) was employed.…”

Neutral (bis-β-diketonato) iron(iii), cobalt(ii), nickel(ii), copper(ii) and zinc(ii) metallocycles: structural, electrochemical and solvent extraction studies

“…4 In prior studies we have shown using X-ray diffraction that neutral species of stoichiometry Cu 2 L 1 2 (solvent) n contain planar Cu 2 L 1 2 units, with tetrahydrofuran or pyridine solvent molecules weakly bound in an axial position on each copper(II) (to yield square pyramidal coordination geometries). 5 In contrast to the situation for copper, the metal ion chemistry of L 1 with other divalent metals has received little attention, although two zinc(II) derivatives of this ligand (with R = Ph) have recently been reported.…”