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 triscar- 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.

Chelated Fischer carbene complexes of annulated thiophenes: synthesis, structure and electrochemistry

“…The C7 carbene carbon resonance is more upfield in the chromium bischelated carbene complex of trans-TT (3), compared to the analogous cis-TT carbene complex. 32 The same effect is seen in comparing 16 (trans-DTT) with 17 (cis-DTT). The larger extent of π-conjugation in trans-TT and trans-DTT, compared to their constitutional isomers, permits more electron density to be delocalised to the C7 carbene carbons of 3 and 16.…”

“…0.20 (Cr) and 0.25 ppm (W) more upfield. 32 This is a result of the larger extent of π-conjugation and more delocalised π-orbital electrons in trans-TT compared to cis-TT, allowing electron density to be withdrawn from the 5′-position throughout the trans-TT spacer in a carbene complex. 36 The proton resonances are very similar when comparing the monochelated carbene complexes of trans-TT, (1 (Cr) and 5 (W)) to their analogous trans-DTT carbene complexes (9 (Cr) and 12 (W)).…”

“…Considering the triscarbene complex of trans-TT (4), the C6 and C6′ carbene carbon chemical shifts are more downfield compared to the analogous cis-TT carbene complex. 32 Comparing the metallacyclic tetracarbonyl biscarbene fragment of 4 to its intramolecular C5′ pentacarbonyl carbene fragment, the carbene signals are upfield and the carbonyl carbon signals downfield.…”

“…3). 32 The metal centers have an octahedral arrangement of ligands with small deviations in the bond angles caused by the chelate ring/s. The two cis carbonyl ligands axially arranged (and trans to each other) are slightly bent towards the inside of the chelated rings.…”

“…[15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][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.…”