Molecular squares, cubes and chains from self-assembly of bis-bidentate bridging ligands with transition metal dications

Abstract

The two new ligands L^fur and L^th consist of two chelating pyrazolyl-pyridine termini connected to furan-2,5-diyl or thiophene-2,5-diyl spacers viamethylene groups. Reaction of these with a range of transition metal dications that prefer octahedral coordination affords a series of unusual structures which are all based on a 2M : 3L ratio. [M₈(L^fur)₁₂]X₁₆ (M = Co, Cu, X = BF₄; and M = Zn, X = ClO₄) are octanuclear cubes with approximate D₄ symmetry in which two cyclic tetranuclear helicate M₄L₄ units are connected by four additional ‘pillar’ ligands. In contrast [Ni₄(L^fur)₆](BF₄)₈ is a centrosymmetric molecular square consisting of two dinuclear Ni₂L₂ units of opposite chirality that are connected by a pair of additional L^fur ligands such that the four edges of the Ni₄ square are spanned by alternately two and one bridging ligands. [M₄(L^th)₆](BF₄)₈ (M = Co, Ni, Cu) are likewise molecular squares with similar structures to [Ni₄(L^fur)₆](BF₄)₈ with the significant difference that the two crosslinked double helicate M₂L₂ units are now homochiral. The Cd(II) complexes both behave quite differently to the first-row metal complexes, with [Cd(L^fur)(BF₄)](BF₄) being a simple mononuclear complex with a single ligand in which the furan oxygen atom is weakly interacting with the Cd(II) centre. In contrast, in {[Cd₂(L^th)₃](BF₄)₄}_∞, where this quasi-pentadentate coordination mode of the ligand is not possible because thiophene is too poor an electron donor, the ligand reverts to bis-bidentate bridging coordination to afford a one-dimensional chain consisting of an infinite sequence of crosslinked, homochiral, Cd₂(L^th)₂ double helicate units.