Acid-Base Chemistry of Transition-Metal-π-Thiophene Complexes

Studies on the acid-base properties of (arene)Ru(C₄R₄S) complexes are summarized. The reactivity of π-thiophene complexes towards H⁺ and OH^- depends on the oxidation state of the complex which in turn determines the hapticity of the thiophene ligand. Weak acids protonate (C₆Me₆)Ru(η⁴-C₄R₄S) to give monocationic η⁴-2-C₄R₄HS complexes. The S-C(sp³) bond in the C₄H₅S complex spontaneously cleaves in the solid state as well as in solution to give a 1-thiapentadienyl derivative. For other R groups the protonated thiophene complexes do not undergo C-S bond scission. The thiophene ring in other (C₆Me₆)Ru(η⁴-C₄R₄S) complexes can be cleaved using sources of (C₅R′₅)Ru⁺ (R′ = H, Me) to give products containing the Ru₂-(μ-η³:η⁴-SC₄R ₄) group. This demonstrates the superior ability of two metals to effect C-S bond activation. In the case of Ru(II) complexes, such as (arene)Ru(η⁵-C₄R′₄S)²⁺, their C-S bonds are easily cleaved using aqueous base to give acylthiolato complexes of the type (arene)Ru(η⁴-SC₃R′₃COR′). One of the curiosities of this chemistry is that the nucleophile, OH^-, initially attacks at sulfur. Amines and ammonia react with the dicationic complexes to give the iminium salts (arene)Ru(η⁴-SC₃R′ ₃C(NHR)R′)⁺.