Angle-dependent electronic effects in 4,4′-bipyridine-bridged Ru3 triangle and Ru4 square complexes

Use of 1,4,7,10-tetraazacyclododecane (cyclen) as a capping ligand and 4,4'-bipyridine (4,4'-bpy) as a bridging ligand enables assembly of redox-active Ru-3 triangle and Ru-4 square complexes. The former is produced by reacting [(cyclen)Ru(DMSO)Cl]Cl with 4,4'-bpy in a 3:1 ethanol: water mixture to precipitate [(cyclen)(3)Ru-3(4,4'-bpy)(3)]Cl-6 center dot 18H(2)O center dot THF (4), whereas the latter is generated as [(cyclen)(4)Ru-4(4,4'-bpy)(4)](CF3SO3)(8)center dot 2CF(3)SO(3)H center dot 5MeOH (7) by reacting (cyclen)Ru(CF3SO3)(3) with 4,4'-bpy in methanol. The crystal structure of 4 center dot 11H(2)O reveals an equilateral triangle in which the 4,4'-bpy bridges are bowed outward, such that the pyridine rings are all forced to be perpendicular to the Ru3 triangle. Consequently, adjacent pyridine rings are essentially coplanar, and the cyclic voltammogram of [(cyclen)(3)Ru-3(4,4'-bpy)(3)](6+) in acetonitrile displays three distinct one-electron oxidation events. Cyclic voltammetry measurements reveal redox processes centered at E-1/2 = 0.207, 0.342, and 0.434 V versus Cp2Fe0/+ that are assigned to 6+/7+, 7+/8+, and 8+/9+ couples of the [(cyclen)(3)Ru-3(4,4'-bpy)(3)](n+) triangle, respectively. In contrast, the structure of [(cyclen)(4)Ru-4(4,4'-bpy)(4)](8+) features a regular square geometry wherein the rings of the bridging 4,4'-bpy ligands are free to rotate, leading to just one four-electron oxidation couple centered at 0.430 V. Density functional theory calculations performed on [(cyclen)(3)Ru-3(4,4'-bpy)(3)](6+) reveal metal-based orbitals with contributions from the pi system of the bridging 4,4(')-bpy ligands, providing a likely pathway for electron transfer.