Cyclopentadienyl ruthenium(II) complexes with bridging alkynylphosphine ligands: synthesis and electrochemical studies

The reaction of [CpRuCl(PPh3)2] (Cp=cyclopentadienyl) and [CpRuCl(dppe)] (dppe=Ph2PCH2CH2PPh2) with bis- and tris-phosphine ligands 1,4-(Ph2PC≡C)2C6H4 (1) and 1,3,5-(Ph2PC≡C)3C6H3 (2), prepared by Ni-catalysed cross-coupling reactions between terminal alkynes and diphenylchlorophosphine, has been investigated. Using metal-directed self-assembly methodologies, two linear bimetallic complexes, [{CpRuCl(PPh3)}2(μ-dppab)] (3) and [{CpRu(dppe)}2(μ-dppab)](PF6)2 (4), and the mononuclear complex [CpRuCl(PPh3)(η1-dppab)] (6), which contains a “dangling arm” ligand, were prepared (dppab=1,4-bis[(diphenylphosphino)ethynyl]benzene). Moreover, by using the triphosphine 1,3,5-tris[(diphenylphosphino)ethynyl]benzene (tppab), the trimetallic [{CpRuCl(PPh3)}3(μ3-tppab)] (5) species was synthesised, which is the first example of a chiral-at-ruthenium complex containing three different stereogenic centres. Besides these open-chain complexes, the neutral cyclic species [{CpRuCl(μ-dppab)}2] (7) was also obtained under different experimental conditions. The coordination chemistry of such systems towards supramolecular assemblies was tested by reaction of the bimetallic precursor 3 with additional equivalents of ligand 2. Two rigid macrocycles based on cis coordination of dppab to [CpRu(PPh3)] were obtained, that is, the dinuclear complex [{CpRu(PPh3)(μ-dppab)}2](PF6)2 (8) and the tetranuclear square [{CpRu(PPh3)(μ-dppab)}4](PF6)4 (9). The solid-state structures of 7 and 8 have been determined by X-ray diffraction analysis and show a different arrangement of the two parallel dppab ligands. All compounds were characterised by various methods including ESIMS, electrochemistry and by X-band ESR spectroscopy in the case of the electrogenerated paramagnetic species.