Chelate alkyne complexes of divalent and trivalent ruthenium stabilized by N-donor ligation

Zinc amalgam reduction of tris(acetylacetonato)ruthenium(III), [Ru(acac)(3)], in the presence of the chelate alkyne N-donor ligands o-RC=CC6H4NMe2 gives the corresponding bis(acetylacetonato)ruthenium(II) complexes [Ru(acac)(2)(o-RC=CC6H4NMe2)] (R = Ph (1)1 SiMe3 (2)).Treatment of 2 with K2CO3/CH3OH gives the corresponding complex of 2-ethynyl-N,N-dimethylaniline (R-. = H (3)). Complexes 1-3 undergo reversible one-electron oxidation to the corresponding ruthenium(III) cations [Ru(acac)(2)(o-RC=CC6H4NMe2)](+) (R = Ph (1(+)), SiMe3 (2(+)), H (3(+))) by cyclic voltammetry in CH2Cl2 at -60 degrees C. The E-1/2(Ru3+/2+) values for this process are about 200 mV less positive than that for the corresponding pair of alkene complexes [Ru(acac)(2)(o-CH2=CHC6H4NMe2)](0,+). Treatment of complexes 1 and 2 with [FeCp2]PF6 gives the deep blue-violet PF6 salts of 1+ and 2+, whose magnetic moments (1.92 and 1.95 mu(B), respectively, at room temperature) and ESR spectra are typical of monomeric ruthenium(III) complexes. In both oxidation states, as a consequence of coordination, the bands due to C=C stretching in the IR spectra appear at 170-250 cm-l to low frequency of those for the free alkynes. The X-ray structures of 1 and 1+ establish that the metal ion is coordinated in an octahedral arrangement by two bidentate acac ligands and bidentate o-PhC=CC6H4NMe2. In contrast to the alkene o-CH2=CHC6H4NMe2, the alkyne,binds somewhat more strongly to ruthenium(III) (4d(5)) than to ruthenium(II) (4d(6)), as shown by the metal-carbon distances [2.113(5), 2.183(5) Angstrom and 2.107(5), 2.172(5) Angstrom for two independent molecules of 1; 2.080(3) Angstrom, 2.133(4) Angstrom for 1+], probably because the electron removed on oxidation comes from an antibonding orbital arising from the orthogonal pi(perpendicular to) orbital of the alkyne. The corresponding C=C distances [1.224(6), 1.240(6) Angstrom in 1; 1.245(4) Angstrom in 1(+)] are equal within experimental error, both being lengthened relative to that in free o-PhC=CC6H4NMe2; (1.190 Angstrom).