(1-Ferrocenyl-eta(6)-borabenzene)(eta(5)-cyclopentadienyl)cobalt(1+): A new heterobimetallic basic NLO chromophore

The heterobimetallic cationic NLO chromophore (1-ferrocenyl-eta(6)-borabenzene)(eta(5)-cyclopentadienyl)cobalt(1+) ([{1-((eta(5)-C5H5)Fe(eta(5)-C5H4))-eta(6)-BC5H5}Co(eta(5)-C5H5)](+), 1(+)) has been synthesized by reaction of FcBBr(2) and cobaltocene and subsequent oxidation with FeCl3. The neutral bimetallic compound 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 with a = 10.541(3) Angstrom, b = 17.647(5) Angstrom, c = 10.965(3) Angstrom, 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 positive charge is predominantly localized on the (borabenzene)cobalt moiety. The X-ray structure determination of 1-PF6 shows the two sandwich units in mutually trans positions, The structural data denote an increased electronic interaction between the donating ferrocenyl group and the borabenzene unit. A cyclic voltammetric study of 1-PF6 or 1 reveals three reversible redox couples (-1/0, 0/+1, +1/+2), resembling those of the mononuclear parent compounds, although the redox potentials are slightly shifted due to the mutual influence of the electron-donating and -accepting properties of the different sandwich units. The UV-vis spectrum 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 face of the directly coupled donor-acceptor pair.