Steric versus Electronic Effects in the Structure of Heteroatom (S and O)-Substituted Free and Metal (Cr and W)-Complexed Carbenes

Abstract

DFT computational energies for free alkoxy-and thio-substitued free carbenes correlate with experimental steric data (Taft parameters) showing the importance of the steric volume at the expense of the electronic interaction in determining the conformation of free carbenes 1-14. The shorter C-O bond distances in the alkoxy derivatives render alkoxycarbene more sensitive to steric effects than thiocarbenes. This strong dependence of the conformation with the steric hindrance is not applicable to metal-complexed carbenes. Thio-substituted carbene complexes exist exclusively as syn-isomers due to the combination of an interaction of the sulfur lone pair with two CO ligands and the steric repulsion with the "CO-wall". The stronger C carbene -S bond compared to C carbene -O is responsible for the increased syn-anti rotation barrier observed for the alkylthio-substituted metal carbene complexes compared to their oxygen analogues. The differences of polarity between syn-and anti-isomers of alkylthio-substituted metal carbene complexes explain also the increment of the isomerization barrier with the polarity of the solvent. The effect of the substituent attached to the carbene carbon is not decisive in the conformation of these compounds.

Rhenium ethoxy- and hydroxycarbene complexes with thiophene substituents

“…Sierra reported a favoured positioning of oxygen lone pairs away from the metal carbonyl fragment in monocarbene complexes to minimise interactions of the lone pairs with the "carbonyl wall". 29 A large deviation from coplanarity between the carbene and thiophene planes is found, with Re-C carbene -C 2,thienyl -C 3,thienyl torsion angles of -44.2 (14) • and -36.7(14)…”

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

“…The ethyl groups (C7 and C6) and metal moiety are on the same side of the ((CO) 4 M)C carb -O(Et) bond (C7 carb -O7(Et) and C6 carb -O6(Et) respectively), ensuing the favoured anti-isomer orientation. [38][39][40][41] To determine if the thienylene spacer and metallacyclic biscarbene component (carbene carbon atoms, metal and ethoxy fragments) are in the same plane, a mean plane is drawn through C2, C3, C4 and C5, and a second through M, C7 carb , O7, C6 carb and O6. All of the crystal structures exhibited a near planar arrangement, as indicated by the angle measured between the two planes ( Table 3).…”

“…Ethoxycarbene complexes have characteristic carbene carbon angles of 130°(M-C carb -O), 125 (M-C carb -C5) and 105°( O-C carb -C5). [38][39][40][41] In the case of chelated ethoxycarbene complexes the M-C carb -O bond angle is increased to an average magnitude of 137.8(3)°, the M-C7 carb /C6 carb -C5/C4 bond angle significantly decreased to 113.2°(3) and the O-C carb -C5/C4 bond angle increased to 109.1(4)°because of the metallacycle ring strain effect on coordination (Table 3).…”

Recent advances in the field of multicarbene and multimetal carbene complexes of the Fischer-type

“…Some attention is given to spectral data in support of the carbene ligand character, while specific structural aspects are emphasized. Theoretical calculations in the past have focused mainly on the donor/acceptor nature of the carbene substituents of simple monocarbene complexes [12][13][14][15][16][17] or the steric and electronic effects of the heteroatom on the carbene ligand [18].…”

“…the "carbonyl wall" [18,210] and thus the alkyl group is orientated towards the carbonyls and normally nestles in a staggered position between two carbonyls (see, for example: [65][66][67]140]). This orientation may lead to an interaction between the alkoxy oxygen atom and the hetero atom (Y) of a hetero-aryl ring substituent.…”