Abstract
Two (thieno[3,2-b]thiophene) and three annulated thiophenes (dithieno[2,3-b;3',2'-d]thiophene and dithieno[3,2-b;2',3'-d]thiophene) were employed as building blocks to synthesize linear or semi-circular chelated mononuclear biscarbene and dinuclear tetracarbene complexes. The electronic properties of the annulated thienylene chelated carbene complexes were investigated by cyclic voltammetry experiments and compared to non-chelated Fischer-type monocarbene complexes. Density functional theory (DFT) calculations were used to assign the redox events and to probe the extent of electron delocalisation 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.
Highlights
Comparing the chelated biscarbene complex to a standard thiophene biscarbene complex, R1 of the latter is at −1.85 V (0.4 V more negative).42. This is in sharp contrast to the results found for chelated monocarbene complexes, where they are reduced at more negative potentials compared to non-chelated carbene complexes in the same study
Chelated carbene complexes were synthesized from using tetrabromothienylene starting materials
The reaction conditions chosen for the preparative route allowed for the targeted bischelated dinuclear tetracarbene complexes as major products
Summary
Thiophene oligomers (and polymers) are widely studied because of their electroconductive and photonic properties.1–14 Application of α-connected, conjugated thiophene units in chain stuctures1,5–7 and α,β-fused thiophene rings in band structures1,2,4,8–12 are finding wide application as organic conductors/superconductors4,7,8 and non-linear optical materials.1,5,6,9 In addition, oligothiophenes are appealing units to act as spacer-ligands between metal fragments for potential long distance metal–metal communication.3,13,14 Electron excessive thiophene donors and strong electron-withdrawing Fischer carbene carbon acceptors arranged in a ‘push–pull’ fashion via a conjugated pathway are conducive to electron transfer and polarisation processes with potentially diverse electronic applications.Unlike chelated N-heterocyclic biscarbene complexes, chelated Fischer carbene complexes (FCCs) reported in the literature are more rare.15–31 Dinuclear chelated tetracarbene complexes could be prepared utilising thiophene or thieno [2,3-b]thiophene if the tetrabrominated thienyl substrates were employed as starting materials.25,32 Both chelated mononuclear biscarbene and dinuclear tetracarbene complexes were isolated, as well as the intermediate dinuclear triscarbene complex (with a monocarbene ligand on one metal terminal, and a chelated biscarbene metal carbonyl group on the other side of the spacer thienothiophene with the sulphur atoms both orientated in the same (cis) direction, up-up), Fig. 1. The known chromium and tungsten biscarbene complexes of trans-DTT, with C5 and C5′ pentacarbonyl carbene fragments (10 and 13 respectively), formed in trace amounts (
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