Synthesis and properties of mono- and dimetal Fischer multicarbene complexes derived from thiophene and thieno[2,3-b]thiophene

Access to multicarbene complexes of a fused thienothiophene substrate was obtained by the use of the tetrabrominated thieno[2,3-b]thiophene precursor in a lithium-bromide exchange reaction, followed by nucleophilic attack on metal hexacarbonyls (M = Cr, W). Subsequent alkylation afforded unique triscarbene complexes [M(CO)(4){{C(OEt)}(2)C6H1S2C(OEt)}M(CO)(5)] (M = Cr 12, W 13) featuring three non-equivalent carbene ligands on a single thiophene linker, as well as the bischelated tetracarbene complexes [M(CO)(4){{C(OEt)}(2)C6S2{C(OEt)}(2)}M(CO)(4)] (M = Cr 14, W 15). The triscarbene complexes 12 and 13 are the first examples of multi-alkoxycarbene complexes featuring three non-equivalent carbene ligands. The reaction also afforded the chelated mononuclear biscarbene complexes [M(CO)(4){C(OEt)}(2)C6H2S2] (M = Cr 10, W 11) in low yields. Similarly, employing tetrabromothiophene as precursor yielded the mononuclear chelate biscarbene complexes [M(CO)(4){C(OEt)}(2)C4H2S] (M = Cr 6, W 7) and the dinuclear tetracarbene complexes [M(CO)(4){{C(OEt)}(2)C4S{C(OEt)}(2)}M(CO)(4)] (M = Cr 8, W 9). Modification of the classic Fischer carbene synthetic methodology to a process of stepwise additions of lithiating agent and metal carbonyls to thieno[2,3-b]thiophene, facilitates the formation of the mixed metal biscarbene complex [W(CO)(5)C(OEt){C6H2S2}C(OEt)Cr(CO)(5)] 5, as analogue of the homonuclear biscarbene complexes [M(CO)(5)C(OEt){C6H2S2}C(OEt)M(CO)(5)], (M = Cr 3, W 4). The monocarbene complexes [M(CO)(5){C(OEt)C6H3S2}], (M = Cr 1, W 2) were also obtained in high yields, and the molecular structures of the tungsten complexes, with the exception of 9 and 11, were confirmed by single crystal X-ray diffraction studies.