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Versatile Supramolecular Coordination Behaviour of a Bis(bidentate) Tetraphosphane



The bis(bidentate) phosphane cis,trans,cis-1,2,3,4-tetrakis(diphenylphosphanyl)cyclobutane (dppcb) has been used for the synthesis of supramolecular complexes, so-called dyads and triads. Depending on the monometallic precursor compound [Ru(bpy)2(dppcb)](PF6)2 or [Ru(bpy)2(cis-dppcbO2)](PF6)2 [bpy = 2,2′-bipyridine, cis-dppcbO2 = cis,trans,cis-1,2-bis(diphenylphosphanoyl)-3,4-bis(diphenylphosphanyl)cyclobutane], [Ru(bpy)2(dppcb)NiBr2](PF6)2 (1) or ΔΛ/ΛΔ-[{Ru(bpy)2(cis-dppcbO2)}2NiBr](PF6)5 (2) is exclusively formed in good yield by reaction with [NiBr2(DME)] (DME = dimethoxyethane). The versatile coordination behaviour of dppcb compared with that of cis-dppcbO2 is confirmed by cis,trans,cis-2,3-bis(diphenylphosphanoyl)-1,4-bis(diphenylphosphanyl)cyclobutane (2,3-trans-dppcbO2, 4). Although two dppcb ligands coordinated simultaneously to a PdII centre cannot produce a square-planar PdP4 core, the reaction of two equivalents of 4 with [Pd(CH3CN)4](BF4)2 exclusively leads to meso-(MMMP/MPPP)-[Pd(2,3-trans-dppcbO2-P,P′)2](BF4)2 (3). This means that trans configurations of coordinating 2,3-trans-dppcbO2 with respect to the cyclobutane rings allow the formation of this PdP4 moiety. However, in the case of 4 it is also possible to obtain [PdCl2(2,3-trans-dppcbO2-P,P′)] (5) in excellent yield by using [PdCl2(COD)] (COD = cyclooctadiene). In 5, only one ligand 4 is attached to the PdII centre, and a trans configuration of 4 within the five-membered chelate ring can again be seen. All solid-state structures 15 have been determined by single-crystal X-ray structure analysis. The corresponding solution structures are in agreement with these solid-state structures and have been authenticated by multinuclear NMR spectroscopy, FAB and MALDI-TOF-MS measurements, UV/Vis spectroscopy and cyclic voltammetry. In the case of 2, the emission and lifetime properties of the excited state indicate that triads such as 2 could be suitable catalysts for the reductive part of water splitting.

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