Hydrogénation of phosphido bridged ruthenium clusters. A view of P—C bond activation and cleavage in the coordinatively unsaturated molecule Ru3(CO)9(μ2-PPh2)(μ2-H): conversion of μ2-PPh2 to μ3-PPh

Treatment of Ru3(CO)8(μ3-η2-C≡CR)(μ2-PPh2)(R = But, Pri) with dihydrogen liberates olefin (CH2=CHR) and generates the coordinatively unsaturated cluster Ru3(CO)9(μ2-PPh2)(μ2-H) which has been shown by X-ray diffraction to have an unprecedented μ2-phosphido bridge in which a P—phenyl group interacts with an electron deficient ruthenium atom. This molecule provides an insight into P—C bond activation leading to cleavage. Further reaction of Ru3(CO)9(μ2-PPh2)(μ2-H) with H2 leads to the phosphinidene cluster Ru3(CO)9(μ3-PPh)(μ2-H)2 via elimination of benzene. Conversion of Ru3(CO)9(μ2-PPh2)(μ2-H) to another phosphinidene cluster Ru4(CO)13(μ3-PPh) occurs on pyrolysis. Hydrogénation of multi-site bound acetylides with H2 is a potentially useful synthetic route to unsaturated clusters. Hydrogenative conversion of μ2-PPh2 to μ3-PPh groups has significance for the synthesis of phosphinidene capped molecules as well as the chemistry of phosphido bridged clusters.

The use of electron spin resonance to measure reaction rates

At Cardiff, we have used electron spin resonance measurements to study the reactions of radical anions formed by the addition of electrons to unsaturated molecules. In certain cases the unsaturated molecule is such that the radical anion formed by the addition of the electron undergoes no reaction except that of disproportionation. For example, when the radical anion of tetraphenylethylene is made in tetrahydrofuran as solvent, disproportionation occurs: 2( Ph 2 C = C Ph 2 )· - Na + ⇌ Ph 2 C̅—C̅ Ph 2 + Ph 2 C = C Ph 2 . Na + Na + The presence of the four phenyl groups prevents the radical anion from dimerizing and electron transfer from one radical anion to another is the only possible reaction (Bennett, Evans, Evans, Owen & Tabner 1963; Evans & Evans 1963; Evans & Tabner 1963). In other cases, the radical anion formed by electron addition undergoes reaction and this reaction can be studied by the change with time of the electron spin resonance signal.