Modification of rhenium carbonyls with thienyl nucleophiles

Abstract

In the reaction between [Re(CO)5Br] and 2–lithiumthienyl, X–ligand substitution was expected. Li+{C4H3S}¯did not substitute Br¯, but an intermediate negatively charged complex was obtained (non–mobile on silica gel) and it was found that the thienyl had bonded to a carbonyl ligand, producing a dirhenium acylate complex. Such complexes are the precursors to neutral Fischer carbene complexes. After alkylation with Et3OBF4, [Re2 (CO) 9C(OEt)C4H3S] (1) was obtained, instead of a monorhenium monocarbene complex. Greater yields of 1 could be obtained, from reactions with [Re2(CO) 10] instead of [Re(CO) 5Br]. [Re2 (CO) 10] reacted with 5–lithium–2,2'–bithienyl and 2–lithium–3,6– dimethylthieno[3,2–b]thienyl and was then alkylated with Et3OBF4. The reactions proceeded smoothly and [Re2 (CO) 9C(OEt)C8H5S2] (2) and [Re2 (CO) 9C(OEt)C8H7S2] (3) were obtained. The substrates thiophene, 2,2'–bithiophene and 3,6–dimethylthieno[3,2–b]thiophene, can all be doubly lithiated under appropriate reaction conditions. These lithiated species were reacted with two equivalents of [Re2 (CO) 10]. In the case of bithiophene this produced, in good yield, the tetrametal biscarbene complex [Re2 (CO) 9C(OEt)C8H4S2C(OEt)Re2 (CO) 9] (8). In the thiophene and dimethylthieno[3,2–b]thiophene cases [Re2 (CO) 9C(OEt)C4H2SC(OEt)Re2 (CO) 9] (7) and [Re2 (CO) 9C(OEt)C8H6S2C(OEt)Re2 (CO) 9] (9) could be isolated in meagre quantities. This was ascribed to poor double lithiation (also steric hindrance in the case of 7). The carbene ligands reacted with water on the silica gel during column chromatography or in a control experiment with degassed water to produce aldehydes by reductive elimination from the metal. Protonation of the acylrhenate afforded rhenium hydrides which is also a potential precursor to aldehyde formation. This is believed to be a facile process for especially complex 9, isolated in very small quantity. Complexes 7–9 produced monocarbene aldehyde complexes [Re2(CO) 9C(OEt)C4H2SC(O)H] (12), [Re2 (CO) 9C(OEt)C8H4S2C(O)H] (13) and [Re2 (CO) 9C(OEt)C8H6S2C(O)H] (14), as well as dialdehyde compounds. Complexes 2 and 3 also produced aldehyde compounds. The formation of aldehydes from ethoxycarbene complexes is believed to involve hydroxycarbene intermediate species. Experiments were performed on [Re2 (CO) 10] and [Re(CO) 5Br]. They were reacted with 2–lithiumthienyl and then protonated. In the case of [Re2(CO) 10], hydride signals were observed on the 1H NMR spectrum, as well as aldehyde signals. In the case of [Re(CO) 5Br] there was strong NMR evidence indicating the formation of a hydroxycarbene complex. Complexes 1, 2, and 3 were reacted with Br2 (l). The metal–metal bonds were cleaved by the bromine to produce monorhenium carbene complexes [Re(CO) 4{C(OEt)C4H3S}Br] (4), [Re(CO) 4{C(OEt)C8H5S2}Br] (5), and [Re(CO) 4{C(OEt)C8H7S2}Br] 6) and [Re(CO) 5Br]. Complex 8 reacted with bromine to produce a monocleaved complex [Re2 (CO) 9C(OEt)C8H4S2C(OEt)Re(CO) 4Br] (11) and a biscleaved complex [Re(CO) 4Br{C(OEt)C8H4S2C(OEt)}Re2 (CO) 4Br] (10). Unique complexes [Re(CO) 4{C(OH)C4H3S}{μ–H}Re(CO) 4{C(O)C4H3S}] (15) and [Re(CO) 4{C(OH)C8H5S2}{μ–H}Re(CO) 4{C(O) C8H5S2}] (16) were obtained by starting with [Re(CO) 5Br] or [Re2 (CO) 10] and reacting them with 2–lithiumthienyl and 5–lithium–2,2'– bithienyl. These complexes were isolated from the column as very polar compounds after eluation of the aldehyde complexes. The dirhenium complex was obtained with a carbonyl– modified ligand (hydroxycarbene/acyl) on each of the metals. The complexes consist of two fragments held together by a hydrogen atom that bridges the two rhenium atoms (hydrido) and one that bridges the oxygen atoms of the carbene/acyl ligands (protonic).