Mono-, di- and tetrarhenium Fischer carbene complexes with thienothiophene substituents

The oxidative cleaving of a rhenium-rhenium bond by bromine in binuclear Fischer carbene complexes proves to be an effective method to prepare mononuclear bromido-carbene complexes. The reaction of mono- and dilithiated thieno[2,3-b]thiophene (2,3-b-TTH₂) and thieno[3,2-b]thiophene (3,2-b-TTH₂) with [Re₂(CO)₁₀] affords dirhenium nonacarbonyl ethoxycarbene complexes, [Re₂(CO)₉{C(OEt)2,3-b-TTH}] (1a) and [Re₂(CO)₉{C(OEt)3,2-b-TTH}] (1b), and the tetrarhenium bis(ethoxycarbene) complexes from the dilithiated thiophene substrates, [Re₂(CO)₉-μ-{C(OEt)-2,3-b-TT-C(OEt)}Re₂(CO)₉] (2a) and [Re₂(CO)₉-μ-{C(OEt)-3,2-b-TT-C(OEt)}Re₂(CO)₉] (2b) featuring bridging thiophene linkers. Rhenium-rhenium bond cleavage by bromine of the monocarbene complexes yielded the scarce class of monorhenium bromido-carbene complexes, cis-[Re(CO)₄{C(OEt)2,3-b-TTH}Br] (3a) and cis-[Re(CO)₄{C(OEt)3,2-b-TTH}Br] (3b), while the corresponding reaction of the biscarbene tetrarhenium carbonyl complex of thieno[2,3-b]thiophene afforded the cleaving of both metal-metal bonds to give the novel dirhenium biscarbene dibromido complex with a thienothiophene spacer, [Re(CO)₄Br-μ-{C(OEt)-2,3-b-TT-C(OEt)}Re(CO)₄Br] (4a). A new indirect aminolysis route is described to prepare the chlorido dimethylaminocarbene complex 5acis-[Re(CO)₄{C(NMe₂)2,3-b-TTH}Cl], with unexpected cleavage of the Re-Re bond. Single crystal X-ray diffraction studies were performed on 1a, 2a, 3a, 5a, 1b and 3b. Spectroscopic and electrochemical methods are employed to investigate the electronic effect of the different conjugation pathways in the different thienothiophenyl carbene substituents, and the replacement of the rhenium pentacarbonyl fragment with a bromido ligand.