Fischer mono- and biscarbene complexes of tungsten with mono- and dimeric heteroaromatic substituents

Abstract

An electrochemical study of a series of mono and biscarbene complexes of tungsten pentacarbonyl with mono-and dimeric heteroarene substituents are reported and compared in CH 3 CN and DCM.Results revealed that the order of oxidation (reduction) depends largely on the aryl substituent attached to the carbene carbon (2-thienyl, 2-furyl or 2-(N-methylpyrrolyl)). The order of oxidation (reduction) for monocarbene complexes containing a monomeric heteroarene substituent ((1) -(3)), a dimeric heteroarene substituent ((4) -(6)) or biscarbene complexes connected with a heteroarene substituent ( (7) - (9)) is the same, namely 2-thienyl > 2-furyl > 2-(N-methylpyrrolyl). Carbene complexes containing a larger conjugated heteroarene substituent attached to the carbene carbon reduce more easily than the monomeric analogues. Tungsten biscarbene complexes exhibit two separate oxidation potentials for the two metal centres or one large oxidation peak, consistent with the simultaneous oxidation of the two W metal centres.

Effect of CO substitution on the redox properties of Fischer Mo(0) carbene complexes Mo(CO)5=C(Y)(2-Furyl), Y=OEt, NHCy or NH2

“…It is well established that both the carbene ligand substituents, i.e. the heteroarene substituent, the heteroatom substituent as well as modification of the metal ligand sphere, influence the electrochemical behaviour of Cr(0) [6,14,15] and W(0) [16,17,18] Fischer carbene complexes. Although the synthesis of a couple of Mo(0) Fischer carbene complexes has been described in literature, their electrochemical behaviour has been neglected to date.…”

Electrochemistry of Triphenylarsine-substituted Tungsten(0) Fischer carbene complexes

“…It is not possible to determine reliable peak current ratios, i pa,ox /i pc,red , for complex 3 due to the overlapping of the oxidation of the cis and trans isomers. The oxidation potential of the AsPh 3 -containing complexes 1-3 is ca 0.300 V lower than that of the related pentacarbonyl complexes [(CO) 5 WC(OEt)(Ar)] (Ar = 2-thienyl (1), 2-furyl (2), 2-(N-methyl)pyrrolyl (3)) [8], see Figure 5 and Table 1. This is expected, since the AsPh 3 is a weaker -acceptor ligand than CO [30], see Figure 2.…”

“…Altering the R-groups of the phosphine ligand (PR 3 ) from Ph to OPh, showed to have a negligible effect on the electrochemical behaviour, while substitution of the ethoxy-group on the carbene carbon with various amine substituents [6] showed a marked effect on both the metal oxidation potential (0.08 -0.19 V) as well as the carbene ligand reduction potential (0.30 -0.52 V). It was also found that the order of oxidation (and reduction) for monocarbene complexes containing a monomeric heteroarene substituent, a dimeric heteroarene substituent or biscarbene complexes connected with a heteroarene spacer substituent is the same, namely 2-thienyl > 2-furyl > 2-(Nmethylpyrrolyl) [8]. To this end we were interested to see whether the electrochemical properties and reactivity patterns of phosphine-and arsine-substituted Fischer carbene complexes of W (0) are similar and also if arsine-substituted Fischer carbene complexes of W(0) containing different heteroarene substituents show the same order of oxidation as previously reported.…”

Carbene Radicals in Transition-Metal-Catalyzed Reactions

“…In general, the more electron-donating the substituents on the carbene atom, the more negative the reduction potential of the carbene ligand needed to form its radical anionic form. This applies to both aromatic and heteroatom substituents . The relation is similar for ancillary ligands on the pendant metal, where greater electron-donating capabilities or decreased π-accepting properties decrease the reduction potential of the carbene .…”

“…This applies to both aromatic and heteroatom substituents. 32 The relation is similar for ancillary ligands on the pendant metal, where greater electron-donating capabilities or decreased π-accepting properties decrease the reduction potential of the carbene. 33 Biscarbene derivatives were also investigated.…”