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

Abstract

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)}₂C₆H₁S₂C(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)}₂C₆S₂{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)}₂C₆H₂S₂] (M = Cr 10, W 11) in low yields. Similarly, employing tetrabromothiophene as precursor yielded the mononuclear chelate biscarbene complexes [M(CO)₄{C(OEt)}₂C₄H₂S] (M = Cr 6, W 7) and the dinuclear tetracarbene complexes [M(CO)₄{{C(OEt)}₂C₄S{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)₅C(OEt){C₆H₂S₂}C(OEt)Cr(CO)₅] 5, as analogue of the homonuclear biscarbene complexes [M(CO)₅C(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.