Flexible Syntheses of Tripodal Phosphine Ligands 1,1,2-Tris(diarylphosphino)ethane and Their Ruthenium η5‑C5Me5 Complexes

A series of four tripodal polyphosphine ligands (PAr2)2CHCH2PAr2 (1a–d) were synthesized and coordinated to ruthenium to produce the complexes [Ru­(η5-C5Me5)­((PAr2)2CHCH2PAr2)]­PF6 (2a–d) (Ar = phenyl (a), p-tolyl (b), o-tolyl (c), m-xylyl (d)). The 1,1,2-tris­(diarylphosphino)­ethane compounds were generated in a novel trisubstitution reaction of diarylphosphorylacetaldehyde diethyl acetal, a useful synthetic precursor that was used previously for the synthesis of phosphonium dimers. The tridentate ligands were subsequently combined with the ruthenium­(II) precursor Ru­(η5-C5Me5)­(1,5-COD)­Cl, in order to probe the coordination geometry of 1a–d. The resulting complexes 2a–d displayed piano-stool type structures where the phosphine ligands were coordinated in a fac geometry. Despite the large steric bulk of the ligands, as well as the ring strain caused by the formation of a four-membered ring and two five-membered rings with the ruthenium center, complexes 2a–d were quite stable. The ruthenium salts were air and moisture stable and did not react with CO, H2, or NaBH4, even at elevated temperatures.