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)(eta(2)-C8H14)(2)] (1) (acac = acetylacetonato) with two equivalents of (PPr3)-Pr-i in THF at -25 degrees C gives trans-[Ru(acac)(2)((PPr3)-Pr-i)(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)(eta-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 PPr3 or PCy3 to give exclusively cis-[Ru(acac)(2)(eta(2)-C2H4)(L)] [L = (PPr3)-Pr-i (5), PCy3 (6)], whereas in THF species believed to be either square pyramidal [Ru(acac)(2)L], with apical L, or the corresponding THF adducts, can be detected by P-31 NMR spectroscopy. Complexes 3-6 react with CO (1 bar) giving trans-[Ru(acac)(2)(CO)(L)] [L = (PPr3)-Pr-i (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)((PPr3)-Pr-i)} (2)(mu-N-2)] (10). The structures of cis-3, trans-4, 5, 6 and 10(.)C(6)H(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 CH2Cl2 at -50 degrees C with E-1/2(Ru3+/2+) values spanning -0.14 to +0.92 V (versus Ag/AgCl), whereas for the vinylidene complexes [Ru(acac)(2) (C=CHR)((PPr3)-Pr-i)] [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 pi-acceptor ability of the ligands: (PPr3)-Pr-i, PCy3 < C H-2(4) < C=CHR < CO. The UV-Vis spectrum of the thermally unstable, electrogene rated Ru-III-ethene cation 6(+) has been observed at -50 degrees C. Cyclic voltammetry of the mu-dinitrogen complex 10 shows two, fully reversible processes in CH2Cl2 at -50 degrees C at +0.30 and +0.90 V (versus Ag/AgCl) corresponding to the formation of 10+ (Ru-II.III) and 10(2+) (Ru-II,Ru-III). The former, generated electrochemically at -50 degrees 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+) reversible arrow 2 10(+) at 223 K is estimated as 10(1,3,6). (c) 2004 Elsevier B.V. All rights reserved.