Ligand additivity in 47-electron clusters. Electrochemistry of (μ-H)Ru₃(μ₃-η ³-XCCRCR′)(CO)_9-n(PPh₃)_n and reactions with electrophiles: One-electron oxidation and adduct formation

Reactions of clusters HRu₃(μ₃-η³-XCCRCR′)(CO) _9-n(PPh₃)_n (X=OMe, R=R′=Me, n=1, 2, 3; X=MeO, R=H, R′=EtO, n=2, 3; X=Et₂N, R=H, R′=Me, n=1, 2) with electrophilic reagents proceed either by 1-electron transfer or by Lewis acid-base adduct formation. The HOMO for the cluster series is Ru-Ru bonding with contributions from all three Ru atoms. Cyclic voltammograms of HRu₃(μ₃-η³-XCCRCR′)(CO) _9-n(PPh₃)_n (n=2, 3) display in each case an electrochemically reversible to quasi-reversible, 1-electron oxidation, followed by an irreversible, 1-electron oxidation at a significantly more positive potential. The potential for the first oxidation is lowered both by an increasing degree of PPh₃ substitution and an increasing pi donor capability of the allylidene substituents. The dependence of the oxidation potential upon substitution of the metal and carbon framework atoms is analyzed as an example of ligand additivity in cluster systems. Radical cations derived from the di- and tri-substituted 1,3-dimetalloallyl clusters can be generated by oxidation with tris(4-bromophenyl)aminium hexachloroantimonate. These radical cations decompose within a few minutes at room temperature but are stable for long periods at temperatures below -40°C. Electrophilic addition of E=H¹⁺, Ag¹⁺ or Au(PPh₃)¹⁺ to the Ru-Ru bonds is observed. Adducts [ERu₃H(μ₃-η³-XCCRCR′)(CO) _9-n(PPh₃)_n]¹⁺ have been characterized by IR and ¹H- and ³¹P-NMR spectroscopies. The Au(PPh₃) moiety is found to bridge two or three Ru-Ru bonds in these adducts. Substitutional isomerism is induced by electrophilic addition.