Stepwise synthesis of mixed-metal assemblies using pre-formed Ru(II) 'complex ligands' as building blocks

Abstract

Two families of heteronuclear coordination complexes have been prepared in a stepwise manner using pre-formed, kinetically inert [RuL3]2+ building blocks, in which L is a bis-bidentate bridging ligand with two pyrazole-pyridyl termini, coordinated at one end to the Ru(ii) centre. These pre-formed 'complex ligands'-with three pendant binding sites-react with additional labile transition metal dications to complete the stepwise assembly of mixed-metal arrays in which labile [Co(ii)/Cd(ii)] or inert [Ru(ii)] ions strictly alternate around the framework. When L = the thiophene-2,5-diyl spaced ligand Lth, the complex [Ru(Lth)3]2+ is formed in the expected 31 merfac ratio: reaction with labile Co(ii) or Cd(ii) ions completes formation of a heteronuclear square [Ru2Co2(Lth)6]8+ or one-dimensional coordination polymer {[CdRu(Lth)3]4+}∞, respectively. In these only the mer isomer of [Ru(Lth)3]2+ is selected by the self-assembly process, whereas the fac isomer is not used. When L = a 1,3-benzene-diyl spaced ligand (Lph), the complex ligand [Ru(Lph)3]2+ formed in the initial step is enriched in mer isomer (80-87% mer, depending on reaction conditions). Two quite different products were isolated from reaction of [Ru(Lph)3]2+ with Co(ii) depending on the conditions. These are the rectangular, hexanuclear 'open-book' array [Ru3Co3(Lph)9]12+ which contains a 21 proportion of fac/mer Ru(ii) metal centres; and the octanuclear cubic [Ru4Co4(Lph)12{Na(BF4)4}]13+ cage which is a new structural type containing all mer Ru(ii) vertices and all fac Co(ii) vertices. The cavity of this cubic cage contains a tetrahedral array of fluoroborate anions which in turn coordinate to a central Na(i) ion-an unusual example of a metal complex [Na(BF4)4]3- acting as the guest inside the cage-like metal complex [Ru4Co4(Lph)12]16+.