Bis(acetylacetonato)ruthenium(II) complexes containing bulky tertiary phosphines. Formation and redox behaviour of Ru(acac)2 (PR3) (R = iPr, Cy) complexes with ethene, carbon monoxide, and bridging dinitrogen

Reaction of cis-[Ru(acac)2(η2-C8H14)2] (1) (acac = acetylacetonato) with two equivalents of PiPr3 in THF at −25 °C gives trans-[Ru(acac)2(PiPr3)2], trans-3, which rapidly isomerizes to cis-3 at room temperature. The poorly soluble complex [Ru(acac)2(PCy3)2] (4), which is isolated similarly from cis-[Ru(acac)2(η2-C2H4)2] (2) and PCy3, appears to exist in the cis-configuration in solution according to NMR data, although an X-ray diffraction study of a single crystal shows the presence of trans-4. In benzene or toluene 2 reacts with PiPr3 or PCy3 to give exclusively cis-[Ru(acac)2(η2-C2H4)(L)] [L = PiPr3 (5), PCy3 (6)], whereas in THF species believed to be either square pyramidal [Ru(acac)2L], with apical L, or the corresponding THF adducts, can be detected by 31P NMR spectroscopy. Complexes 3–6 react with CO (1 bar) giving trans-[Ru(acac)2(CO)(L)] [L = PiPr3 (trans-8), PCy3 (trans-9)], which are converted irreversibly into the cis-isomers in refluxing benzene. Complex 5 scavenges traces of dinitrogen from industrial grade dihydrogen giving a bridging dinitrogen complex, cis-[{Ru(acac)2(PiPr3)} 2(μ-N2)] (10). The structures of cis-3, trans-4, 5, 6 and 10 · C6H14 have been determined by single-crystal X-ray diffraction. Complexes trans- and cis-3, 5, 6, cis-8, and trans- and cis-9 each show fully reversible one-electron oxidation by cyclic voltammetry in CH2Cl2 at −50 °C with E1/2(Ru3+/2+) values spanning −0.14 to +0.92 V (versus Ag/AgCl), whereas for the vinylidene complexes [Ru(acac)2 (Cdouble bond; length as m-dashCHR)(PiPr3)] [R = SiMe3 (11), Ph (12)] the process is irreversible at potentials of +0.75 and +0.62 V, respectively. The trend in potentials reflects the order of expected π-acceptor ability of the ligands: PiPr3, PCy3