Properties of homo- and heteronuclear mixed biscarbene complexes with conjugated bithiophene units

The syntheses and structures of homo- and heteronuclear biscarbene complexes with bithiophene spacers were investigated. The complexes were synthesized by lithiation of bithiophene followed by metallation using combinations of the metal precursors MnMeCp(CO)3, W(CO)6, Mo(CO)6 and Cr(CO)6, after which the reaction was quenched with triethyloxonium tetrafluoroborate. This classical Fischer method yielded monocarbene complexes, [MLnC(OEt)C4H2S–C4H3S], ([MLn] = Cr(CO)5 1a, W(CO)5 2a or MnMeCp(CO)2 3a), homonuclear biscarbene complexes, [MLnC(OEt)C4H2S–C4H2SC(OEt)MLn], ([MLn] = Cr(CO)5 1b, W(CO)5 2b or MnMeCp(CO)2 3b) and heteronuclear biscarbene complexes, [MLnC(OEt)C4H2S–C4H2SC(OEt)M′Ln] (1d: [MLn] = Cr(CO)5 and [M′Ln] = W(CO)5; 1e: [MLn] = MnMeCp(CO)2 and [M′Ln] = Cr(CO)5; 1f: [MLn] = Cr(CO)5 and [M′] = Mo(CO)5); 2d: [MLn] = MnMeCp(CO)2 and [M′Ln] = W(CO)5; 3c: [MLn] = MnMeCp(CO)2 and [M′Ln] = Mo(CO)5). Electron density calculations with the gaussian03 software package of 1e revealed a polar rod with the negative pole towards the chromium carbene side, whereas the biscarbenes 1d and 1b showed very little polarity. By-products resulting from activation of the carbene moieties in homonuclear biscarbene complexes included (i) ester-type complexes of the form [MLnC(OEt)C4H2S–C4H2SC(O)OEt], ([MLn] = Cr(CO)5 1c or W(CO)5 2c), formed in situ in the reaction of 1b and 2b, (ii) the organic bis-ester compound [EtOC(O)C4H2S–C4H2SC(O)OEt] 4, where both metal moieties had been substituted by oxygen and (iii) the carbon–carbon coupled dimeric bithienyl compound [C4H3S–C4H2SC(O)C(O)C4H2S–C4H3S] 5. By-products obtained from heteronuclear biscarbene reactions contain the former diketo compound (or a derivative) as spacer between two metal carbonyl fragments and have the general formula [MLnC(OEt)C4H2S–C4H2SCR–CR′C4H2S–C4H2SC(OEt)MLn] (5a: [M] = Cr(CO)5, R = OH, R′ = OEt; 5b: [M] = W(CO)5, R = R′ = O; 5c: [M] = Mo(CO)5, R = R′ = O). Reaction of 1d, 1e and 1f with hex-3-yne resulted in the formation of benzannulated products 6a, 6b and 6c. All novel complexes were fully characterized using various spectroscopic techniques. The crystal structures of 1b, 2a and 5 are reported.