Phosphine oxide functionalised with two bipyridine subunits: a novel ligand for the engineering of sterically hindered complexes

Ligand L, containing two 6-methyl-6'-methyl-yl-2,2'-bipyridine moieties lin ked by a "PhP=O'' spacer, has been prepared. Subsequent complexation with [ Cu(CH3CN)(4)](BF4) led to a unique complex of formula [CuL](BF4). The molec ular structure has been determined by X-ray crystallography in the solid st ate and NMR studies in solution. The complex displays an intense absorption band in the visible wavelength range with maxima at 418 and 509 nm, and is oxidised reversibly at +0.66 V (DeltaE(p) = 80 mV) to the corresponding Cu (II) species, while two successive reductions of the coordinated bipyridine fragments occur at -1.60 (DeltaE(p) = 66) and -1.84 V (DeltaE(p) = 70 mV) vs. SCE. Direct spectrophotometric titrations with global analysis allowed us to elicit stability constants for the emergent complex and to highlight the formation of a dinuclear complex in the presence of excess copper.