Synthesis and Properties of a Novel Series of Organometallic Merocyanines Combining the Potent Electron-Donating [(CpFeCO)2(μ-CO)(μ-C=CH−)] Fragment with Tropylium-Type Acceptors

Abstract

The potent electron-donating group [(CpFeCO)2(µ-CO)(µ-C=CH−)] was combined with different tropylium-based acceptors yielding the push-pull complexes [(CpFeCO)2(µ-CO)(µ-C=CH−CH=A)][BF4] where =A represents [=CH−(η7-C7H6)Cr(CO)3]+ (5), [={C10H7}]+ (azulenylium) (7) [={C10H4Me2iPr}]+ (guaiazulenylium) (8). Crystal structures of the precursor complexes [(CpFeCO)2(µ-CO)(µ-C=CH−CH=CH−C7H7)] (3), [(CpFeCO)2(µ-CO){µ-C=CH−CH=CH−(η6-C7H7)Cr(CO)3}] (4) and the push-pull derivative 5 are presented. Whereas complexes 3 and 4 demonstrate localised carbon−carbon double and single bonds in the π-bridge between the diiron moiety and the seven-membered ring a small but distinct π-bond delocalisation can be observed in the π-linker of the cationic derivative 5. From the NMR spectroscopic results a correlation between the 13C NMR shift of the bridging carbon atom µ-C of the diiron unit and the coupling constant J(1H-1H) of the β- and γ-protons of the π-bridge was found. This correlation indicates an increased π-bond delocalisation throughout the conjugated bridge with respect to the electron-accepting capability in the order tricarbonyl(η7-cycloheptatrienylium)chromium < guaiazulenylium < azulenylium implying a predominantly charge-delocalised form in the case of the azulenylium complex 7. Considerably large first hyperpolarisabilities β were determined for 5 and 8 by means of hyper-Rayleigh scattering. Based on the two-level model β0 values were calculated and related to the bond-length alternation in the π-linker. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)