Synthesis and properties of a novel series of organometallic merocyanines combining the potent electron-donating [(CpFeCO)(2)(mu-CO)(mu-C=CH-)] fragment with tropylium-type acceptors

The potent electron-donating group [(CpFeCO)(2)(mu-CO)(mu-C= CH-)] was combined with different tropylium-based acceptors yielding the push-pull complexes [(CpFeCO)(2)(mu-CO)(mu-C=CH-CH=A)][BF4] where =A represents [=CH-(eta(7)-C7H6)Cr(CO)(3)](+) (5), [={C10H7}](+) (azulenylium) (7) [= {C(10)H(4)Me(2)iPr}](+) (guaiazulenylium) (8). Crystal structures of the precursor complexes [(CpFeCO)(2)(mu-CO)(mu-C=CH-CH= CH-C7H7)] (3), [(CpFeCO)(2)(mu-CO){mu-C=CH-CH=CH-(eta(6)-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 pi-bridge between the diiron moiety and the seven-membered ring a small but distinct pi-bond delocalisation can be observed in the pi-linker of the cationic derivative 5. From the NMR spectroscopic results a correlation between the C-13 NMR shift of the bridging carbon atom mu-C of the diiron unit and the coupling constant J(H-1-H-1) of the beta- and gamma-protons of the pi-bridge was found, This correlation indicates an increased pi-bond delocalisation throughout the conjugated bridge with respect to the electron-accepting capability in the order tricarbonyl(eta(7)-cycloheptatrienylium)chromium guaiazulenylium azulenylium implying a predominantly charge-delocalised form in the case of the azulenylium complex 7. Considerably large first hyperpolarisabilities beta were determined for 5 and 8 by means of hyper-Rayleigh scattering. Based on the two-level model beta(o) values were calculated and related to the bond-length alternation in the pi-linker.