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 L^th, the complex [Ru(L^th)₃]²⁺ is formed in the expected 3 : 1 mer : fac ratio: reaction with labile Co(II) or Cd(II) ions completes formation of a heteronuclear square [Ru₂Co₂(L^th)₆]⁸⁺ or one-dimensional coordination polymer {[CdRu(L^th)₃]⁴⁺}_∞, respectively. In these only the mer isomer of [Ru(L^th)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 (L^ph), the complex ligand [Ru(L^ph)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(L^ph)₃]²⁺ with Co(II) depending on the conditions. These are the rectangular, hexanuclear ‘open-book’ array [Ru₃Co₃(L^ph)₉]¹²⁺ which contains a 2 : 1 proportion of fac/mer Ru(II) metal centres; and the octanuclear cubic [Ru₄Co₄(L^ph)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(L^ph)₁₂]¹⁶⁺.