Synthesis and properties of Fischer (multi)carbene complexes of condensed thiophenes

Abstract

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]-TT) and thieno[3,2-b]thiophene ([3,2-b]-TT) with Re2(CO)10 afforded dirhenium nonacarbonyl ethoxycarbene complexes (2.1 and 2.5) and the tetrarhenium bis(ethoxycarbene) complexes (2.2 and 2.6) from the dilithiated thiophene substrates featuring bridging thiophene linkers. Rhenium–rhenium bond cleavage by bromine of the monocarbene complexes yielded the scarce class of mono-rhenium bromido-carbene complexes (2.3 and 2.7), while the corresponding reaction of the biscarbene tetrarhenium carbonyl complex of [2,3-b]-TT afforded the cleaving of both metal–metal bonds to give the novel dirhenium biscarbene dibromido complex with a thienothiophene spacer (2.4). A new indirect aminolysis route is described to prepare the chlorido dimethyl- aminocarbene complex (2.8), with unexpected cleavage of the Re–Re bond. Spectroscopic, structural and electrochemical methods are employed to investigate the structural and 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.

Nucleophilic attack on the central sulphur of dithieno[2,3-b;3ˈ,2ˈ-d ]thiophene ([2,3-b;3ˈ,2ˈ-d]-DTT) by n-BuLi opened the central thiophene ring and afforded, after subsequent reaction with Cr(CO)6 and alkylation with [Et3O][BF4], a series of mono- and biscarbene complexes containing a 3,3ˈ- dithienyl backbone with a SBu substituent (3.1-3.4). Repeating the reaction with diisopropylamine as the nucleophile, led to a dihydrodesulphurization reaction with ring-opening of the central thiophene ring of [2,3-b;3ˈ,2ˈ-d]-DTT and elimination of the sulphur atom. Subsequent reaction with n-BuLi or LDA, Cr(CO)6 and [Et3O][BF4] afforded 3,3ˈ-dithienyl monoand biscarbene complexes (3.8 and 3.9). In both instances the α,αˈ-dithienothiophene biscarbene complex (3.5) was observed spectroscopically but not isolated. By using α,αˈ- dibromodithieno[2,3-b;3ˈ,2ˈ-d]thiophene as substrate, improved yields of the targeted mono- and biscarbene complexes of [2,3-b;3ˈ,2ˈ-d]-DTT (M = Cr, W) could be obtained. The biscarbene complexes were unstable in the reaction mixture but in the case of tungsten could be isolated after in situ aminolysis with dimethylamine. The use of KHMDS as base converted Cr(CO)6 to K[Cr(CO)5(CN)] and after reaction with [2,3-b;3ˈ,2ˈ-d]-DTT and subsequent alkylation with [Et3O][BF4], afforded the chromium tetracarbonyl carbene complex of [2,3-b;3ˈ,2ˈ-d]-DTT (3.7). Two ([3,2-b]-TT) and three annulated thiophenes ([2,3-b;3ˈ,2ˈ-d]-DTT and dithieno[3,2-b;2ˈ,3ˈ- d]thiophene ([3,2-b;2ˈ,3ˈ-d]-DTT)) were employed as building blocks to synthesize chelated mononuclear biscarbene and dinuclear tetracarbene complexes. The electronic properties of the annulated thienylene chelated carbene complexes are investigated by cyclic voltammetry experiments and compared to non-chelated mono-carbene complexes of the Fischer-type. Density functional theory (DFT) calculations are used to assign the redox events and to probe the extent of electron delocalization as well as the possibility of electronic (intramolecular metal-metal) communication as a result of intervalence. The differences of these electronic properties in the conjugated chelated carbene complexes are compared to chelated carbene compounds without a linear conjugated pathway.

The transfer of thiophene and [2,3-b]-TT Fischer carbene complexes into the coordination sphere of Pt(II) complexes by transmetallation was applied to prepare new cis-Pt(II) biscarbene complexes of the Fischer-type. Numerous reagents (two Pt(II) precursors and a range of varying monocarbene complexes), solvents and reaction conditions were studied for the transmetallation. Mostly neutral mononuclear Pt(II)-biscarbene complexes were obtained (5.2, 5.7, 5.9a/b and 5.12a/b). In the case of 5.9 and 5.12, two geometric stereoisomers of the cis-Pt-biscarbene complexes are observed, respectively. An insoluble cationic Pt triscarbene complex (5.12d) is indicated for the reaction with dimethylaminecarbene complexes. The stability and reactivity of the novel Pt(II) Fischer multicarbene complexes were investigated. The performance of 5.2 and 5.7 as (pre)catalysts in the model catalytic alkyne hydrosilylation reactions are evaluated as a new application for this class of compounds.