Coordinatively Unsaturated Ruthenium Phosphine Half-Sandwich Complexes:  Correlations to Structure and Reactivity

A number of 16e two-legged piano-stool complexes [Cp*Ru(PP)][BAr‘₄] have been prepared by reaction of NaBAr‘₄ with either [Cp*RuCl(PP)] (PP = (PEt₃)₂, ⁱPr₂PCH₂CH₂PⁱPr₂ (dippe), (PPh₃)₂) or [Cp*RuCl(PR₃)] plus PR₃ (PR₃ = PMeⁱPr₂, PPhⁱPr₂) in fluorobenzene under argon. The complexes [Cp*Ru(PEt₃)₂][BAr‘₄], [Cp*Ru(dippe)][BAr‘₄], and [Cp*Ru(PMeⁱPr₂)₂][BAr‘₄] have been structurally characterized by X-ray crystallography. Attempts to isolate analogous species containing other phosphine ligands such as PⁱPr₃, PCy₃, and PMe₃ led to the sandwich derivative [Cp*Ru(η⁶-FPh)][BAr‘₄], which was also structurally characterized. Both [Cp*Ru(PPh₃)₂][BAr‘₄] and [Cp*Ru(PPhⁱPr₂)₂][BAr‘₄] are unstable and rearrange to the 18e sandwich species [Cp*Ru(η⁶-C₆H₅PR₂)][BAr‘₄] and to [Cp*Ru(η⁶-C₆H₅POR₂)][BAr‘₄] (R = Ph, ⁱPr) under trace amounts of oxygen. The geometry of the 16e complexes as well as their affinity for an additional ligand depend on the substituents on the phosphorus. The reactivity with respect to the addition of N₂, PR₃, O₂, H₂, and HCl to form 18e derivatives has been studied. Some model systems have been analyzed using density functional theory (DFT) calculations. Also included are comparative studies on the NN counterparts. The moieties [CpRu(PP)]⁺ (PP = (PH₃)₂, H₂PCH₂CH₂PH₂) adopt typically pyramidal structures (i.e. in the absence of bulky and rigid substituents on P) versus planar structures of [CpRu(NN)]⁺ (NN = (NH₃)₂, H₂NCH₂CH₂NH₂). [CpRu(PP)]⁺ is more stable but has nevertheless a higher affinity of adding a σ ligand than [Cp*Ru(NN)]⁺.