Pyrrolyl substituted allenylidene complexes of ruthenium

Abstract

Pyrrolyl and indolyl substituted allenylidene complexes of ruthenium have been prepared from the trapping of cationic trans-[Cl(dppm)₂RuCCCCH₂]⁺ with various pyrroles or N-methylindole. The reaction is rationalized as involving regioselective attack of the organometallic electrophile on the electron-rich heterocycle followed by proton migration to the terminal CH₂ entity of the intermediate butenynyl substituted σ-complex. Pyrrolyl substituted allenylidene complexes have spectroscopic and electrochemical properties intermediate between those of amino and aryl substituted congeners and can thus be regarded as vinylogous aminoallenylidene complexes. We present spectroscopic evidence that the pyrrole π-system is efficiently incorporated into the metallabutatriene chromophore including resonance Raman spectroscopy. According to our results, the respective frontier orbitals are delocalized across the entire ClRuC₃(pyrrolyl) entity which defies any classification of the individual redox events as metal or ligand centered redox processes. This issue has been specifically addressed by spectroelectrochemistry. The structure of the 1-methylindole-3-yl complex has been determined by X-ray crystallography. Bond parameters along the ruthenium–allenylidene chain are intermediate between those of amino and aryl substituted congeners and support our conclusions drawn from the spectroscopic results. While still electron rich, pyrrolyl substituted allenylidene complexes are easily deprotonated to their conjugate bases, which are substituted butenynyl complexes. This has been exemplified with the tetrahydroindole derived complex 3f.