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‘<sub>4</sub>] have been prepared by reaction of NaBAr‘<sub>4</sub> with either [Cp*RuCl(PP)] (PP = (PEt<sub>3</sub>)<sub>2</sub>, <sup>i</sup>Pr<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>P<sup>i</sup>Pr<sub>2</sub> (dippe), (PPh<sub>3</sub>)<sub>2</sub>) or [Cp*RuCl(PR<sub>3</sub>)] plus PR<sub>3</sub> (PR<sub>3</sub> = PMe<sup>i</sup>Pr<sub>2</sub>, PPh<sup>i</sup>Pr<sub>2</sub>) in fluorobenzene under argon. The complexes [Cp*Ru(PEt<sub>3</sub>)<sub>2</sub>][BAr‘<sub>4</sub>], [Cp*Ru(dippe)][BAr‘<sub>4</sub>], and [Cp*Ru(PMe<sup>i</sup>Pr<sub>2</sub>)<sub>2</sub>][BAr‘<sub>4</sub>] have been structurally characterized by X-ray crystallography. Attempts to isolate analogous species containing other phosphine ligands such as P<sup>i</sup>Pr<sub>3</sub>, PCy<sub>3</sub>, and PMe<sub>3</sub> led to the sandwich derivative [Cp*Ru(η<sup>6</sup>-FPh)][BAr‘<sub>4</sub>], which was also structurally characterized. Both [Cp*Ru(PPh<sub>3</sub>)<sub>2</sub>][BAr‘<sub>4</sub>] and [Cp*Ru(PPh<sup>i</sup>Pr<sub>2</sub>)<sub>2</sub>][BAr‘<sub>4</sub>] are unstable and rearrange to the 18e sandwich species [Cp*Ru(η<sup>6</sup>-C<sub>6</sub>H<sub>5</sub>PR<sub>2</sub>)][BAr‘<sub>4</sub>] and to [Cp*Ru(η<sup>6</sup>-C<sub>6</sub>H<sub>5</sub>POR<sub>2</sub>)][BAr‘<sub>4</sub>] (R = Ph, <sup>i</sup>Pr) 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 N<sub>2</sub>, PR<sub>3</sub>, O<sub>2</sub>, H<sub>2</sub>, 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)]<sup>+</sup> (PP = (PH<sub>3</sub>)<sub>2</sub>, H<sub>2</sub>PCH<sub>2</sub>CH<sub>2</sub>PH<sub>2</sub>) adopt typically pyramidal structures (i.e. in the absence of bulky and rigid substituents on P) versus planar structures of [CpRu(NN)]<sup>+</sup> (NN = (NH<sub>3</sub>)<sub>2</sub>, H<sub>2</sub>NCH<sub>2</sub>CH<sub>2</sub>NH<sub>2</sub>). [CpRu(PP)]<sup>+</sup> is more stable but has nevertheless a higher affinity of adding a σ ligand than [Cp*Ru(NN)]<sup>+</sup>.