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

Abstract

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 = PF(6), SbF(6)), 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)MCl(2)]X(2) (M = Pt, 2; Pd, 3; X = PF(6), SbF(6)) dissolved in CH(3)CN with visible light, produces the unique heterodimetallic compounds [Ru(bpy)(CH(3)CN)(2)(dppcb)MCl(2)]X(2) (M = Pt, 7; Pd, 8; X = PF(6), SbF(6)). In an analogous reaction the separable diastereoisomers (ΔΛ/ΛΔ)- and (ΔΔ/ΛΛ)-[Ru(bpy)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (5/6) lead to [Ru(bpy)(CH(3)CN)(2)(dppcb)Os(bpy)(2)](PF(6))(4) (9), where only the RuP(2)N(4) moiety of 5/6 is photochemically reactive. By contrast, in the case of [Ru(bpy)(2)(dppcb)NiCl(2)]X(2) (4; X = PF(6), SbF(6)) 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 CH(3)CN, present only in 7-9, cannot be directly influenced by "steric pressure". This has dramatic consequences for their photophysical parameters. The trans-[Ru(bpy)(CH(3)CN)(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)](PF(6))(4) (M = Os, Ru) dissolved in 1:1 (v/v) H(2)O-CH(3)CN produce H(2) during photolysis with visible light.

Versatile Supramolecular Coordination Behaviour of a Bis(bidentate) Tetraphosphane

“…[13] Therefore, the study of the synthetic protocol for these dyadic and related triadic assemblies in good yield is an important task. [12,15] The monometallic precursor complex [Ru(bpy) 2 -(dppcb)](PF 6 ) 2 [2] smoothly reacts with [NiBr 2 (DME)] to give the heterodimetallic species [Ru(bpy) 2 (dppcb)-NiBr 2 ](PF 6 ) 2 (1) in good yield (Scheme 1). [14] For tetraphosphane bridging ligands, different coordination modes containing more than one tetraphosphane ligand come into play (Schemes 1 and 2).…”

“…[2,6] (3) The design of selfassembling systems is a field with much still to explore. [2,6] (3) The design of selfassembling systems is a field with much still to explore.…”

“…Thus, the bis(bidentate) tetraphosphane ligand cis,trans,cis-1,2,3,4tetrakis(diphenylphosphanyl)cyclobutane (dppcb), designed in our laboratory, [8] forms supramolecular variants, which are important for hydrogen-producing catalysts. [2,10] Nevertheless, this synthetic protocol is often accompanied by detrimental effects, which lead to mixtures of oligonuclear systems. [2,10] Nevertheless, this synthetic protocol is often accompanied by detrimental effects, which lead to mixtures of oligonuclear systems.…”

“…[9] The combination of [Ru(bpy) 2 ] 2+ (bpy = 2,2Ј-bipyridine) chromophores with reactive metal centres usually occurs step by step. [10] However, Scheme 1 shows that the monometallic precursor complex [Ru(bpy) 2 -(dppcb)](PF 6 ) 2 smoothly reacts with [NiBr 2 (DME)] (DME = dimethoxyethane) to yield the heterodimetallic species [Ru(bpy) 2 (dppcb)NiBr 2 ](PF 6 ) 2 (1) in pure form. [10] However, Scheme 1 shows that the monometallic precursor complex [Ru(bpy) 2 -(dppcb)](PF 6 ) 2 smoothly reacts with [NiBr 2 (DME)] (DME = dimethoxyethane) to yield the heterodimetallic species [Ru(bpy) 2 (dppcb)NiBr 2 ](PF 6 ) 2 (1) in pure form.…”

“…[2,10] Nevertheless, this synthetic protocol is often accompanied by detrimental effects, which lead to mixtures of oligonuclear systems. Formation of the dyad [Ru(bpy) 2 (dppcb)NiBr 2 ](PF 6 ) 2 (1) and the triad ΔΛ/ΛΔ-[{Ru(bpy) 2 (cis-dppcbO 2 )} 2 NiBr](PF 6 ) 5 (2). In contrast, the oxidized form of the precursor complex, [Ru(bpy) 2 (cis-dppcbO 2 )](PF 6 ) 2 [cis-dppcbO 2 = cis,trans,cis-1,2-bis(diphenylphosphanoyl)-3,4-bis(diphenylphosphanyl)cyclobutane], reacts with [NiBr 2 (DME)] in a completely different way.…”