| Abstract
| - Ruthenium-stabilized nucleophilic phosphinidene complexes (η6-Ar)(PCy3)Ru(PMes*) (Ar = p-cymene, Ar = benzene) are versatile reagents, which react either with electrophiles yielding unprecedented phosphenium ligands or with electron-rich alkyne, provided in the latter case the presence of p-cymene as arene ligand. A detailed kinetic study provides insight into the mechanism and rationalizes the role of p-cymene as a reactivity switch.
- A detailed comparative study of the structural and spectroscopic features and of the reactivity ofruthenium phosphinidene complexes (η6-Ar)(PCy3)Ru(PMes*) (2a, Ar = p-cymene; 2b, Ar = benzene)has been undertaken. The structures of complexes 2a and 2b have been determined by single-crystalX-ray diffraction and display similar features. Both compounds possess identical chemical behavior towardBrönsted acids such as HBF4: protonation of the phosphinidene ligand yields the new cationic complexes[(η6-Ar)(PCy3)Ru(PHMes*)]BF4 (3aBF4, Ar = p-cymene; 3bBF4, Ar = benzene), which exhibit anunprecedented phosphenium-bearing hydrogen substituent. 3aBF4 has been characterized using X-raydiffraction techniques. The lone pair of the phosphorus atom of the phosphinidene ligand remains alsoaccessible to the Lewis acid BH3: the reactions of 2a and 2b with borane give the adducts (η6-Ar)(PCy3)Ru[P(BH3)Mes*] (4a, Ar = p-cymene; 4b, Ar = benzene). In the presence of the larger boraneBPh3, no reaction occurs until water is introduced in the reaction vessel. This results in the generation of[(η6-Ar)(PCy3)Ru(PHMes*)]BPh3OH (3aBPh3OH, Ar = p-cymene; 3bBPh3OH, Ar = benzene)presumably through protonation of 2a and 2b by the previously unknown adduct H2O·BPh3. Phosphinidenecomplexes react also with electrophilic alkylating reagents such as organic iodides provided the alkylsubstituent is small. Treatment of 2a and 2b with 1 equiv of methyliodide leads to the alkylation at thephosphinidene center and yields the phosphenium complexes [(η6-Ar)(PCy3)Ru(PMeMes*)]I (5a, Ar =p-cymene; 5b, Ar = benzene). Examination of the reactivity toward electron-rich reagents such as thealkynes RCCH (R = Me3Si, Ph) yields unexpected results: 2a instantaneously reacts to generatephosphaindane complexes 6 and 7, whereas no reaction occurs when using 2b. A detailed kinetic studyprovides evidence for a dissociative mechanism involving the release of the phosphine ligand in 2a andexplains its specificity. The p-cymene ligand in 2a acts as a reactivity switch due to the higher sterichindrance of this arene.
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