Syntheses, characterization, and photochemical properties of amidate-bridged Pt(bpy) dimers tethered to Ru(bpy)3(2+) derivatives

Amidate-bridged diplatinum(II) entities Pt(2)(bpy)(2)(μ-amidato)(2)](2+) (amidate = pivalamidate and/or benzamidate; bpy = 2,2'-bipyridine) were covalently linked to one or two Ru(bpy)(3)(2+)-type derivatives. An amide group was introduced at the periphery of Ru(bpy)(3)(2+) derivatives to give metalloamide precursors Ru(bpy)(2)(BnH)](2+) (abbreviated as RuBnH, n = 1 and 2), where deprotonation of amide BnH 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 Pt(2)(bpy)(2)(μ-OH)(2)](NO(3))(2), RuBnH, and pivalamide, trinuclear complexes Pt(2)(bpy)(2)(μ-RuBn)(μ-pivalamidato)](4+) (abbreviated as RuBn-Pt(2)) were isolated and characterized. Tetranuclear complexes Pt(2)(bpy)(2)(μ-RuBn)(2)](6+) (abbreviated as (RuBn)(2)-Pt(2)) were separately prepared and characterized in detail. The quenching of the triplet excited state of the Ru(bpy)(3)(2+) derivative (i.e., Ru*(bpy)(3)(2+)) upon tethering the Pt(2)(bpy)(2)(μ-amidato)(2)(2+) moiety is strongly enhanced in RuB1-Pt(2) and (RuB1)(2)-Pt(2), while it is only slightly enhanced in RuB2-Pt(2) and (RuB2)(2)-Pt(2). These are partly explained by the driving forces for the electron transfer from the Ru*(bpy)(3)(2+) moiety to the Pt(2)(bpy)(2)(μ-amidato)(2)(2+) moiety (ΔG°(ET)); the ΔG°(ET) values for RuB1-Pt(2), (RuB1)(2)-Pt(2), RuB2-Pt(2), and (RuB2)(2)-Pt(2) 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.