Ruthenium-Stabilized Low-Coordinate Phosphorus Atoms. <i>p</i>-Cymene Ligand as Reactivity Switch

Abstract

A detailed comparative study of the structural and spectroscopic features and of the reactivity of ruthenium 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-crystal X-ray diffraction and display similar features. Both compounds possess identical chemical behavior toward Brönsted 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 an unprecedented phosphenium-bearing hydrogen substituent. 3aBF4 has been characterized using X-ray diffraction techniques. The lone pair of the phosphorus atom of the phosphinidene ligand remains also accessible 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 borane BPh3, 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. Phosphinidene complexes react also with electrophilic alkylating reagents such as organic iodides provided the alkyl substituent is small. Treatment of 2a and 2b with 1 equiv of methyliodide leads to the alkylation at the phosphinidene 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 the alkynes RCCH (R = Me3Si, Ph) yields unexpected results: 2a instantaneously reacts to generate phosphaindane complexes 6 and 7, whereas no reaction occurs when using 2b. A detailed kinetic study provides evidence for a dissociative mechanism involving the release of the phosphine ligand in 2a and explains its specificity. The p-cymene ligand in 2a acts as a reactivity switch due to the higher steric hindrance of this arene.