Surprising photochemical reactivity and visible light-driven energy transfer in heterodimetallic complexes

The bis(bidentate) phosphine cis, trans, cis-1,2,3,4-tetrakis(diphenylphosphino) cyclobutane (dppcb) has been used for the synthesis of a series of novel heterodimetallic complexes starting from [Ru(bpy)(2)(dppcb)]X-2 (1; X = PF6, SbF6), so-called dyads, showing surprising photochemical reactivity. They consist of [Ru(bpy)(2)](2+) "antenna" sites absorbing light combined with reactive square-planar metal centres. Thus, irradiating [Ru(bpy)(2)(dppcb)MCl2]X-2 (M = Pt, 2; Pd, 3; X = PF6, SbF6) dissolved in CH3CN with visible light, produces the unique heterodimetallic compounds [Ru(bpy)(CH3CN)(2)(dppcb)MCl2]X-2 (M = Pt, 7; Pd, 8; X = PF6, SbF6). In an analogous reaction the separable diastereoisomers (Delta Delta/Delta Delta)-and (Delta Delta/Delta Delta)-[Ru(bpy)(2)(dppcb)Os(bpy)(2)](PF6)(4) (5/6) lead to [Ru(bpy)(CH3CN)(2)(dppcb)Os(bpy)(2)](PF6)(4) (9), where only the RuP2N4 moiety of 5/6 is photochemically reactive. By contrast, in the case of [Ru(bpy)(2)(dppcb) NiCl2]X-2 (4; X = PF6, SbF6) no clean photoreaction is observed. Interestingly, this difference in photochemical behaviour is completely in line with the related photophysical parameters, where 2, 3, and 5/6, but not 4, show long-lived excited states at ambient temperature necessary for this type of photoreaction. Furthermore, the photochemical as well as the photophysical properties of 2-4 are also in accordance with their single crystal X-ray structures presented in this work. It seems likely that differences in "steric pressure" play a major role for these properties. The unique complexes 7-9 are also fully characterized by single-crystal X-ray structure analyses, clearly showing that the stretching vibration modes of the ligand CH3CN, present only in 7-9, cannot be directly influenced by "steric pressure". This has dramatic consequences for their photophysical parameters. The trans-[Ru(bpy)(CH3CN)(2)](2+) chromophore of 9 acts as efficient "antenna" for visible light-driven energy transfer to the Os-centred "trap" site, resulting in k(en) >= 2 10(9) s(-1) for the energy transfer. Since electron transfer is made possible by the use of this intervening energy transfer, in dyads like 2-4 highly reactive M(0) species (M = Pt, Pd, Ni) could be generated. These species are not stable in water and M(II) hydride intermediates are usually formed, further reacting with H+ to give H-2. Thus, derivatives of 3, namely [M(bpy)(2)(dppcb) Pd(bpy)](PF6)(4) (M = Os, Ru) dissolved in 1 : 1 (v/v) H2O-CH3CN produce H-2 during photolysis with visible light.