Syntheses, characterization, and photochemical properties of amidate-bridged Pt(bpy) dimers tethered to Ru(bpy)32+ derivatives

Abstract

Amidate-bridged diplatinum(II) entities [Pt₂(bpy)₂(μ-amidato)₂]²⁺ (amidate = pivalamidate and/or benzamidate; bpy = 2,2′-bipyridine) were covalently linked to one or two Ru(bpy)₃²⁺-type derivatives. An amide group was introduced at the periphery of Ru(bpy)₃²⁺ derivatives to give metalloamide precursors [Ru(bpy)₂(BnH)]²⁺ (abbreviated as RuBnH, n = 1 and 2), where deprotonation of amideBnH affords the corresponding amidate Bn, B1H = 4-(4-carbamoylphenyl)-2,2′-bipyridine, and B2H = ethyl 4′-[N-(4-carbamoylphenyl)carbamoyl]-2,2′-bipyridine-4-carboxylate. From a 1 : 1 : 1 reaction of [Pt₂(bpy)₂(μ-OH)₂](NO₃)₂, RuBnH, and pivalamide, trinuclear complexes [Pt₂(bpy)₂(μ-RuBn)(μ-pivalamidato)]⁴⁺ (abbreviated as RuBn-Pt₂) were isolated and characterized. Tetranuclear complexes [Pt₂(bpy)₂(μ-RuBn)₂]⁶⁺ (abbreviated as (RuBn)₂-Pt₂) were separately prepared and characterized in detail. The quenching of the triplet excited state of the Ru(bpy)₃²⁺ derivative (i.e., Ru*(bpy)₃²⁺) upon tethering the Pt₂(bpy)₂(μ-amidato)₂²⁺ moiety is strongly enhanced in RuB1-Pt₂ and (RuB1)₂-Pt₂, while it is only slightly enhanced in RuB2-Pt₂ and (RuB2)₂-Pt₂. These are partly explained by the driving forces for the electron transfer from the Ru*(bpy)₃²⁺ moiety to the Pt₂(bpy)₂(μ-amidato)₂²⁺ moiety (ΔG°_ET); the ΔG°_ET values for RuB1-Pt₂, (RuB1)₂-Pt₂, RuB2-Pt₂, and (RuB2)₂-Pt₂ are estimated as −0.01, 0.00, +0.22, and +0.28 eV, respectively. The considerable difference in the photochemical properties of the B1- and B2-bridged systems were further examined based on the emission decay and transient absorption measurements, which gave results consistent with the above conclusions.