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The novel [Ru(Acriphos)(PPh3)(Cl)(PhCO2)] [1; Acriphos=4,5‐bis(diphenylphosphino)acridine] is an excellent precatalyst for the hydrogenation of CO2 to give formic acid in dimethyl sulfoxide (DMSO) and DMSO/H2O without the need for amine bases as co‐reagents. Turnover numbers (TONs) of up to 4200 and turnover frequencies (TOFs) of up to 260 h−1 were achieved, thus rendering 1 one of the most active catalysts for CO2 hydrogenations under additive‐free conditions reported to date. The thermodynamic stabilization of the reaction product by the reaction medium, through hydrogen bonds between formic acid and clusters of solvent or water, were rationalized by DFT calculations. The relatively low final concentration of formic acid obtained experimentally under catalytic conditions (0.33 mol L−1) was shown to be limited by product‐dependent catalyst inhibition rather than thermodynamic limits, and could be overcome by addition of small amounts of acetate buffer, thus leading to a maximum concentration of free formic acid of 1.27 mol L−1, which corresponds to optimized values of TON=16×103 and TOFavg≈103 h−1.

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Twofold lithiation of dibenzofuran (6) and dibenzothiophene y1phosphino)dibenzofuran (11) and 4-(dipheny1phosphino)di-(12) with n-butyllithium/N,N,N',N'-tetramethylethylenedi-benzothiophene (15) were obtained analogously via the monoamine (TMEDA) in n-hexane or n-pentane followed by the lithium compounds 9 and 14 of the heteroarenes. In the case reaction with chlorodiphenylphosphine yielded the title com-of dibenzofuran (6), it was shown that mono-and dilithiation pounds 4 and 5, respectively. The monophosphines 4-(diphen-is controlled by temperature.In a previous communication3) we have reported on the synthesis of 1,8-bis(diphenylphosphino)anthracene (1) and preliminary studies of its behaviour as a ligand in metal complexation. By using the 1,8-disubstituted anthracene unit as a rigid carbon skeleton, it was intended that two phosphorus centres should be placed at a distance just too long to allow both to coordinate simultaneously to one metal nucleus. Alternatively, the formation of dimetallic complexes should bring two metal nuclei, each coordinated to one of the phosphorus centres, into close proximity, possibly introducing novel structural features and properties, especially with respect to catalytic processes. However, on reaction of 1 with nickel(I1) chloride and bis(benzonitrile)palladium(II) chloride the concomitant cyclometallation of the carbon-hydrogen bond in position 9 of anthracene led to the formation of the extremely stable, squareplanar monometallic chelat complexes 2 and 3, respectively3! In pursuing our concept of rigid bidentate phosphine ligands for the formation of binuclear metal complexes we have been interested in 4,6-bis(diphenylphosphino)dibenzofuran (4) and 4,6-bis(dipheny1phosphino)dibenzothiophene (5). The replacement of the 1,S-disubstituted anthracene skeleton of 1 by the 4,6-disubstituted heteroarenes dibenzofuran and dibenzothiophene in 4 and 5, respectively, not only causes a small increase of the intramolecular distance between the phosphorus centres, but more importantly, it also alters the angle between the phosphorus lone pairs in such a way that a formation of monometallic chelate complexes should be rendered less favoured. Furthermore, the cyclometallation reaction, wich in the case of 1 leads to thc formation of the very stable monometallic chelate complexes 2 and 3, cannot occur in 4 and 5, because the oxygen and sulphur heteroatoms are located in the position between the two phosphorus substituents.On the other hand, the dibenzofuran oxygen and the dibenzothiophene sulphur atoms possess two lone pairs which might contribute additional weak coordinative bonds to metal atoms complexed by the phosphorus centres. Depending on the hybridization of the ring heteroatom, two idealized alternatives might be considered for this additionid coordination: With sp2 hybridization one lone pair of the ring heteroatom is located in the p orbital perpendicular to the plane of the condensed ring system and contributes to the electron sextet of the five-membered heteroaromatic ring. Th...

synthesized from the corresponding disulfoncs 7 and 12, resp., by vapor phase pyrolysis (500"C, 0.1 Torr).