Architectural diversity via metal template-assisted polymer synthesis: A macroligand chelation approach to linear and star-shaped polymeric ruthenium tris(bipyridine) complexes

Polymeric metal complexes are constructed by combining living polymerization techniques with coordination chemistry. These metal-centered linear and star-shaped materials combine the film-forming properties of polymers with optical and other features of metal complexes. A metal template approach described herein offers a versatile alternative to the metalloinitiator method previously employed to generate Ru tris(bipyridine)-centered polystyrenes. Specifically, 4,4'-bis(chloromethyl)-2,2'-bipyridine and 4-chloromethyl-2,2'-bipyridine were utilized as initiators for both the bulk and solution polymerization of styrene using atom transfer radical polymerization (ATRP). Narrow dispersity polystyrenes with bipyridine (bpy) binding sites at the end (bpyPS) or center (bpyPS(2)) of the chains result. These bpyPS(n) macroligands were chelated to Ru precursor complexes, RuL(2)Cl(2) (L = bpy, phen) or Ru(DMSO)(4)Cl(2), to form complexes with one or three bpyPS(n) macroligands, respectively. Linear polymers, [RuL(2)(bpyPS(n))](2+), with Ru chromophores at the end or center of the chains, as well as Ru-centered star-shaped polymers, [Ru(bpyPS(n))(3)](2+), with three and six arms were produced. In all cases, dehalogenation with AgPF(6) was crucial for efficient macroligand chelation. The relative efficiency of these reactions was estimated by UV/vis spectroscopy. Molecular weight determination by GPC was coupled with in-line diode array UV/vis spectroscopy to confirm the presence of the Ru chromophores in the eluting polymer fractions. The convergent macroligand chelation approach to star-shaped polymeric metal complexes typically works best for polymers of low to moderate molecular weights (<similar to 65K), with higher molecular weights possible for systems with a single macroligand coordinated. Specific molecular weight thresholds encountered are determined by the number of macroligands, the position of the bpy on the polystyrene chain, and the total number of arms emanating from the metal core.