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

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)][BF 4 ] where =A represents [=CH−(η 7 ‐C 7 H 6 )Cr(CO) 3 ] + ( 5 ), [={C 10 H 7 }] + (azulenylium) ( 7 ) [={C 10 H 4 Me 2 i Pr}] + (guaiazulenylium) ( 8 ). Crystal structures of the precursor complexes [(CpFeCO) 2 (µ‐CO)(µ‐C=CH−CH=CH−C 7 H 7 )] ( 3 ), [(CpFeCO) 2 (µ‐CO){µ‐C=CH−CH=CH−(η 6 ‐C 7 H 7 )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 13 C NMR shift of the bridging carbon atom µ‐C of the diiron unit and the coupling constant J ( 1 H‐ 1 H) 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(η 7 ‐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 β 0 values were calculated and related to the bond‐length alternation in the π‐linker. (© Wiley‐VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)