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

Abstract

The bis(bidentate) phosphinecis,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)]X₂ (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]X₂ (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]X₂ (M = Pt, 7; Pd, 8; X = PF₆, SbF₆). In an analogous reaction the separable diastereoisomers (ΔΛ/ΛΔ)- and (ΔΔ/ΛΛ)-[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]X₂ (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 × 10⁹ 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 H₂. 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 H₂ during photolysis with visible light.