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(mu-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)(mu-OH)(2)](NO3)(2), RuBnH, and pivalamide, trinuclear complexes [Pt-2(bpy)(2)(mu-RuBn)(mu-pivalamidato)](4+) (abbreviated as RuBn-Pt-2) were isolated and characterized. Tetranuclear complexes [Pt-2(bpy)(2)(mu-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)(mu-amidato)(2)(2+) moiety is strongly enhanced in RuB1-Pt-2 and (RuB1) 2-Pt2, while it is only slightly enhanced in RuB2-Pt2 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)(mu-amidato)(2)(2+) moiety (Delta G degrees(ET)); the Delta G degrees(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.