Synthesis, Structure, and Electronic Properties of Extended π‐Conjugated Group 6 Fischer Alkoxy–Bis(carbene) Complexes

Abstract

The synthesis, structure and electronic properties of novel Group 6 Fischer alkoxy-bis(carbene) complexes are reported. The UV/Vis spectra of these species display two main absorptions at approximately 350 and 550 nm attributable to a ligand-field (LF) and metal-to-ligand charge-transfer (MLCT) transitions, respectively. The planarity of the system and the cooperative effect of both pentacarbonyl metal moieties greatly enhance the conjugation between the group at the end of the spacer and the metal carbene fragment provoking dramatic changes in the LF and MLCT absorptions. This is in contrast to related push-pull Fischer monocarbenes, where the position of the MLCT band remains mostly unaltered regardless the substituent attached to the donor fragment. In addition, the MLCT maxima can be tuned with subtle modifications of the electronic nature of the central aryl fragment in the novel A-π-D-π-A (A = acceptor, D = donor) systems. DFT and time-dependent (TD) DFT quantum chemical calculations at the B3LYP/def2-SVP level have also been performed to determine the minimum-energy molecular structure of this family of compounds and to analyse the nature of the vertical one-electron excitations associated to the observed UV/Vis absorptions as well as to rationalise their electrochemical behaviour. The ability of tuning up the electronic properties of the compounds studied herein may be of future use in material chemistry.

Fischer-Type Carbene Complexes of Tris(1,4-phenylene)amines and Tri(2-furyl)phosphine

“…This is a direct consequence of the well-known strong electronwithdrawing ability of the (CO) 5 MC group, which behaves in a manner similar to that of a Lewis acid complexed carbonyl function. [33][34][35]42 Similar SOPT energies have been computed for the analogous complex 1a (ΔE (2) = −31.2 kcal/mol vs −14.8 and −15.1 kcal/mol).…”

“…This indicates that the delocalization of the LP of the nitrogen atom into the carbene moiety induces a strong bathochromic shift in the MLCT band (Δλ max = 50−75 nm), as found in related push−pull Fischer carbene complexes. 42,47 Time-dependent density functional theory (TD-DFT) calculations were effected on complexes A(a) and 1a to gain insight into the nature of the MLCT absorption. The computed vertical excitation energies concur reasonably well with the experimental data (see Table 2).…”

Tuning the Photophysical Properties of BODIPY Molecules by π‐Conjugation with Fischer Carbene Complexes

“…[14] A comparable red-shift has only been observed in biscarbene complexes, where the cooperative effect of both {(CO) 5 M = C(OR)} fragments greatly enhances the p-conjugation between the tether and the metal-carbene moieties. [18] Therefore, dyads 3 can be considered as new push-pull systems with extended p-conjugation, where the BODIPY fragment behaves as a p-donor substituent forced by the strong electron-withdrawing effect of the pentacarbonylmetal-carbene unit through the ethylenic spacer. [19] Differently, the bathochromic shift induced in the MLCT absorption band of aminocarbene complexes 4 is not that significant (l abs % 530 nm), reflecting the lower extension of the pconjugation between the BODIPY and the metal-carbene fragments in these complexes.…”

An Entry to Mixed NHC–Fischer Carbene Complexes and Zwitterionic Group 6 Metal Alkenyls

“…Commercially available reagents were used without further purification. Fischer carbene complexes 3 a , 3 b , 3 c , 3 d , 3 e , 3 f , 3 i , 3 j , 7 a , 7 d , 9 a , 9 b , and 9 c were prepared according to literature methods.…”