(6)-BORABENZENE)(ETA(5)-CYCLOPENTADIENYL)COBALT(1- A NEW HETEROBIMETALLIC BASIC NLO CHROMOPHORE() )

The heterobimetallic cationic NLO chromophore 6)-borabenzene)(eta(5)-c yclopentadienyl)cobalt(1+) Fe(eta(5)-C5H4))-eta(6)-BC5H5}Co(eta(5)-C5H 5)](+), 1(+)) has been synthesized by reaction of FcBBr(2) and cobalto cene and subsequent oxidation with FeCl3. The neutral bimetallic compo und 1 is paramagnetic and exhibits an ESR spectrum similar to that of the mononuclear 19-valence-electron complex Co(1-Ph-eta(6)-borabenzene )(2). Complex 1-PF6 crystallizes in monoclinic space group P2(1)/c wit h a = 10.541(3) Angstrom, b = 17.647(5) Angstrom, c = 10.965(3) Angstr om, beta = 96.38(2)degrees, Z = 4, and V = 2027.0(10) Angstrom(3). NMR data for the oxidized diamagnetic cation If indicate that the positiv e charge is predominantly localized on the (borabenzene)cobalt moiety. The X-ray structure determination of 1-PF6 shows the two sandwich uni ts in mutually trans positions, The structural data denote an increase d electronic interaction between the donating ferrocenyl group and the borabenzene unit. A cyclic voltammetric study of 1-PF6 or 1 reveals t hree reversible redox couples (-1/0, 0/+1, +1/+2), resembling those of the mononuclear parent compounds, although the redox potentials are s lightly shifted due to the mutual influence of the electron-donating a nd -accepting properties of the different sandwich units. The UV-vis s pectrum of 1(+) confirms the enhanced donor-acceptor interaction by a strong, negative solvatochromic absorption about lambda = 600 nm which is assigned to a charge-transfer transition. The determination of the first hyperpolarizability beta by means of hyper Rayleigh scattering yields (90 +/- 30) x 10(-30) esu, which is unexpectedly high in the fa ce of the directly coupled donor-acceptor pair.