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

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 via methylene 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. [M-8(L-fur)(12)]X-16 (M = Co, Cu, X = BF4; and M = Zn, X = ClO4) are octanuclear cubes with approximate D-4 symmetry in which two cyclic tetranuclear helicate M4L4 units are connected by four additional 'pillar' ligands. In contrast [Ni-4(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 Ni-4 square are spanned by alternately two and one bridging ligands. [M-4(L-th)(6)](BF4)(8) (M = Co, Ni, Cu) are likewise molecular squares with similar structures to [Ni-4(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 {[Cd-2(L-th)(3)](BF4)(4)}(infinity), 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, Cd-2(L-th)(2) double helicate units.