Oxidative addition of allylic substrates to coordinatively unsaturated ruthenium compounds, [Ru(η ⁵-C ₅Me ₅)(η-amidinate)]: Preparation, structure elucidation, and catalysis of novel ruthenium (IV)-η ³-allyl complexes

Oxidative addition reactions of allylic halides, acetates, and carbonates with [Ru(η ⁵-C ₅Me ₅)(η-amidinate)] [amidinate: ⁱPrNC(Me)=N ⁱPr (1a), ¹BuNC(Ph)=N ¹Bu (1b)], which shows signs of coordinative unsaturation, gave novel cationic π-allyl ruthenium(IV) species. The compounds [Ru(η ³-allyl)(η ⁵-C ⁵Mes)(η ²-amidinate)] ⁺X ^- were isolated by anion exchange of the products (X = PF ₆, BF ₄, BPh ₄), and were characterized by spectroscopic analysis. The crystallography of two of the [Ru(η ³-allyl)(η ⁵-C ₅Mes)(η ²-amidinate)] ⁺X ^- revealed a four-legged piano stool structure in which two nitrogen atoms in the amidinate ligand and two carbon atoms in the η ³-allyl ligands occupy the positions of four legs; the orientation of the η ³-allyl ligand was endo. Although cyclic voltammograms of the precursor, [Ru(η ⁵-C ₅Me ₅)(η-amidinate)], indicated possible oxidative addition of organic halides other than allylic halides to [Ru(η ⁵-C ₅Me ₅)(η-amidinate)], only allylic halides gave the corresponding Ru(IV) products. The importance of prior coordination of the carbon-carbon double bond of allylic substrates was evidenced by NMR observation of the intermediate in the reaction of 1a or 1b with allyl acetate. Addition of nucleophiles such as PhLi, dimethyl methylsodiomalonate, and piperidine to the [Ru(η ³-allyl)(η ⁵-C ₅Me ₅)(η ²-amidinate)] ⁺X ^- gave rise to allylation of these nucleophiles and regeneration of [Ru(η ⁵-C ₅Me ₅)(η-amidinate)]. The reactions of allyl methyl carbonate with nucleophiles were also achieved by catalysis of either [Ru(η ⁵C ₅Me ₅)(η-amidinate)] or [Ru(η ³-allyl)(η ⁵-C ₅Me ₅)(η ²-amidinate ^)]+X ^-.