(Ethene)bis(acetylacetonato) Complexes of Divalent and Trivalent Ruthenium

Zinc dust reduction of tris(acetylacetonato)ruthenium(III), [Ru(acac)3], in hot aqueous THF
in the presence of ethene gives cis-[Ru(acac)2(η2-C2H4)2] (5), isolated as an orange solid in
ca. 60% yield. A single-crystal X-ray crystallographic study shows the metal atom to be
coordinated octahedrally by a pair of bidentate O-bonded acac groups and two mutually cis
orthogonal ethene ligands. In the NMR spectra of 5, the ethene carbon atoms remain
equivalent, while the protons are inequivalent in pairs, down to −95 °C; intermolecular
exchange with ethene is slow on the NMR time scale at room temperature but becomes fast
above ca. 80 °C. One of the ethene molecules of 5 is replaced by ligands at room temperature
to give [Ru(acac)2(η2-C2H4)(L)] [L = SbPh3 (6), MeCN (7), NH3 (8), and C5H5N (9)]. Complexes
6 and 7 are isolated as cis-isomers, 9 as a cis−trans mixture, and 8 in the form of the trans-isomer which transforms into the cis-isomer when heated or chromatographed on alumina.
The structures of cis-8 and trans-9 have been confirmed by X-ray crystallography. The results
indicate that ligand substitutions on 5 may occur via a square pyramidal intermediate [Ru(acac)2(η2-C2H4)] in which ethene occupies the apical site. All the complexes display either
a quasi-reversible or a fully reversible one-electron oxidation by cyclic voltammetry at −50
°C, with E1/2 values in the range +0.37−0.59 V for 6−9 and +0.95 V for 5 (in 0.5 M [Bu4N]PF6/CH2Cl2 vs Ag/AgCl/MeCN). The blue oxidized species 5+, 6+, and 8+ can be generated by
bulk electrolysis and are stable for hours at −50 °C. Their UV−vis spectra and the ESR
spectra of 6+ and 8+ indicate that the oxidized species are ruthenium(III)-ethene cations.
Oxidation of the known cyclooctene complex cis-[Ru(acac)2(η2-C8H14)(SbPh3)] (2) with AgPF6
at −50 °C gives an isolable, deep blue ruthenium(III) salt [2]+[PF6]-, which is stable at room
temperature.