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

Abstract

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 E 1/2 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.

The fragment bis(acetylacetonato)ruthenium: a meeting-point of coordination and organometallic chemistry

“…The cis-arrangement of the ethene molecules in [Ru(acac) 2 (g 2 -alkene) 2 ] suggested by NMR spectroscopic data, and confirmed for the ethene complex 3 by single-crystal X-ray diffraction analysis [29], presumably maximizes back-bonding from Ru II . In contrast to the related rhodium(I) complex [Rh(acac)(g 2 -C 2 H 4 ) 2 ], however, the ethene molecules in the ruthenium(II) complex are mutually orthogonal, thus possibly reducing steric interference in the coordination sphere; in solution they are equivalent on the 1 H and 13 C NMR time-scales, even at À95°C, indicative of rapid rotation.…”

“…Cyclooctene gives the labile complex cis-[Ru(acac) 2 (g 2 - 1 Ligand abbreviations used in this paper: acac = acetylacetonate,2,4-pentanedionato, C 5 H 7 O 2 ; 1,5-COD = 1,5-cyclooctadiene; NBD = norbornadiene, [2,2,1]bicyclohepta-2,5-diene; phen = 1,10-phenanthroline; dppe = 1,2 bis(diphenylphosphino)ethane; dppp = 1,3-bis(diphenylphosphino)propane; S-BINAP = S-2,2 0 -bis(diphenylphosphino)-1,1 0 -binaphthyl; OEP = dianion of 2, 3,7,8,12,13,17, 18- (2), which is stable in solution only in the presence of an excess of cyclooctene [26], whereas the more stable ethene complex, cis-[Ru(acac) 2 (g 2 -C 2 H 4 ) 2 ] (3), can be isolated as a yellow-orange solid in ca. 60% yield [29]. At least for the mono-ene complexes, the presence of water in the THF seems to be beneficial, possibly because it increases the reducing power of the zinc and removes the by-product, Zn(acac) 2 .…”

“…Ammonia reacts with 3 at room temperature giving trans-[Ru(acac) 2 (g 2 -C 2 H 4 )(NH 3 )], which subsequently forms the cis-isomer, even at room temperature, while pyridine at room temperature gives a 3:2 mixture of cis-and trans-[Ru(acac) 2 (g 2 -C 2 H 4 )(py)] which is converted into pure cis-isomer in refluxing benzene [29]. Under the same conditions, triphenylstibine reacts with either 2 or 3 to give complexes of the type cis-[Ru(acac) 2 (g 2 -alkene)(SbPh 3 )], which reacts further with Ph 3 Sb to give cis-[Ru(acac) 2 (SbPh 3 ) 2 ]; in neither step can a trans-isomer be detected.…”

“…Although their potentials are too high to permit isolation of salts, the alkene-ruthenium(III) cations can be electrogenerated in an optically transparent thin layer electrode (OTTLE) cell, detected by their characteristic electronic spectra, and maintained in CH 2 Cl 2 solution for long periods at À50°C. For example, the bands in the electronic spectrum of cis-[Ru(acac) 2 (g 2 -C 2 H 4 ) 2 ] (3) at 32,700 cm À1 (e 6000 M À1 cm À1 ) and 38,800 cm À1 (e 8000 M À1 cm À1 ) are replaced, on electro-oxidation at À50°C with an applied potential of +1.20 V (vs. Ag/AgCl), by bands at 13,300 cm À1 (e 2100 M À1 cm À1 ), 16,900 cm À1 (e 1200 M À1 cm À1 ), and 34,100 cm À1 (e 8000 M À1 cm À1 ) due to the one-electron oxidation product [29]. The original spectrum is regenerated by application of a potential of +0.70 V and the observation of isosbestic points during both oxidation and reduction confirms the presence of just two absorbing species in solution.…”

Synthesis, characterization and catalytic studies of ruthenium(II) chalconate complexes

“…[32] The signal due to two alkene protons also appears in the region 6.9-7.1 ppm and hence is merged with the multiplet of aromatic protons. [33] The peak for methyl protons appears as a singlet at 1.53-1.56 ppm. [34] In addition, a peak corresponding to -OCH 3 was observed in the expected region.…”