Bis(acetylacetonato)ruthenium(II) complexes containing bulky tertiary phosphines. Formation and redox behaviour of Ru(acac) 2 (PR 3) (R = iPr, Cy) complexes with ethene, carbon monoxide, and bridging dinitrogen

Abstract

Reaction of cis-[Ru(acac) 2(η 2-C 8H 14) 2] ( 1) (acac = acetylacetonato) with two equivalents of P i Pr 3 in THF at −25 °C gives trans-[Ru(acac) 2(P i Pr 3) 2], trans- 3, which rapidly isomerizes to cis- 3 at room temperature. The poorly soluble complex [Ru(acac) 2(PCy 3) 2] ( 4), which is isolated similarly from cis-[Ru(acac) 2(η 2-C 2H 4) 2] ( 2) and PCy 3, 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 P i Pr 3 or PCy 3 to give exclusively cis-[Ru(acac) 2(η 2-C 2H 4)(L)] [L = P i Pr 3 ( 5), PCy 3 ( 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 = P i Pr 3 ( trans- 8), PCy 3 ( 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(P i Pr 3)} 2(μ-N 2)] ( 10). The structures of cis- 3, trans- 4, 5, 6 and 10 · C 6H 14 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 CH 2Cl 2 at −50 °C with E 1/2(Ru 3+/2+) values spanning −0.14 to +0.92 V (versus Ag/AgCl), whereas for the vinylidene complexes [Ru(acac) 2 (C CHR)(P i Pr 3)] [R = SiMe 3 ( 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: P i Pr 3, PCy 3 <C 2H 4 < C CHR < CO. The UV–Vis spectrum of the thermally unstable, electrogenerated Ru III–ethene cation 6 + has been observed at −50 °C. Cyclic voltammetry of the μ-dinitrogen complex 10 shows two, fully reversible processes in CH 2Cl 2 at −50 °C at +0.30 and +0.90 V (versus Ag/AgCl) corresponding to the formation of 10 + (Ru II,III) and 10 2+ (Ru III,III). The former, generated electrochemically at −50 °C, shows a band in the near IR at ca. 8900 cm −1 ( w 1/2 ca. 3700 cm −1) consistent with the presence of a valence delocalized system. The comproportionation constant for the equilibrium 10 + 10 2+ ⇌ 2 10 + at 223 K is estimated as 10 13.6.