9-Ferrocenyl-9-borafluorene (FcBC12H8; 3) is readily accessible from FcHgCl and BrBC12H8. The 9-borafluorenyl substituent is bent out of the plane of the cyclopentadienyl ring toward the iron center with corresponding dip angles of α* = 25.5° and 17.1° for the two crystallographically independent molecules in the asymmetric unit of 3. The degree of ligand bending is one of the highest ever measured for a ferrocenylborane derivative, thereby indicating a pronounced Fe−B interaction. In line with that, oxidation of the Fe center to its FeIII state using AgBF4 results in a substantial decrease of the dip angle in the ferricinium species [3]BF4 (α* = 6.3°). Compound 3 can therefore be considered a rare example of a redox-switchable main group Lewis acid. The stability of the molecular framework of 3 is sufficiently high to allow for the isolation and structural characterization of its pyridine-N-oxide adduct (4). Moreover, reaction of 3 with 0.5 equiv of 1,1′-dilithioferrocene (1,1′-fcLi2 × 2/3 tmeda) and subsequent limited access of O2 leads to the formation of the trinuclear mixed-valence complex Li[Fc-(BC12H8)-fc-(BC12H8)-Fc], which has been structurally characterized as 12-crown-4 adduct (Li(12-c-4)2[6]).
The di- and trinuclear ferrocene species Li[Fc-BPh(2)-Fc] (Li[]) and Li(2)[Fc-BPh(2)-fc-BPh(2)-Fc] (Li(2)[]) have been investigated with regard to their electrochemical properties and the degree of intervalence charge-transfer after partial oxidation. Li[] shows two distinct one-electron redox waves for its chemically equivalent ferrocenyl substituents in the cyclic voltammogram (E(1/2) = -0.38 V, -0.64 V; vs. FcH/FcH(+)). The corresponding values of Li(2)[] are E(1/2) = -0.45 V (two-electron process) and -1.18 V. All these redox events are reversible at r. t. on the time scale of cyclic voltammetry. X-ray crystallography on the mixed-valent Fe(II)(2)Fe(III) complex Li(12-c-4)(2)[] reveals the centroid-centroid distance between the cyclopentadienyl rings of each of the terminal ferrocenyl substituents (3.329 A) to be significantly smaller than in the central 1,1'-ferrocenediyl fragment (3.420 A). This points towards a charge-localized structure (on the time scale of X-ray crystallography) with the central iron atom being in the Fe(III) state. Mössbauer spectroscopic measurements on Li(12-c-4)(2)[] lend further support to this interpretation. Spectroelectrochemical measurements on Li[] and Li(2)[] in the wavelength range between 300-2800 nm do not show bands interpretable as intervalence charge-transfer absorptions for the mixed-valent states. All data accumulated so far lead to the conclusion that electronic interaction between the individual Fe atoms in Li[] and Li(2)[] occurs via a through-space pathway and/or is electrostatic in nature.
In order to explore whether benzene or ferrocene presents a more attractive π face to Li+ ions, the
lithium tetraorganylborate Li[BFc2Ph2] has been synthesized, which contains two phenyl rings as well as
two ferrocenyl substituents as potential coordination sites (Fc = (C5H5)Fe(C5H4)). The compound
crystallizes from toluene/dibutyl ether as the contact ion pair [Li(OBu2)][BFc2Ph2], with the Li+ ion
located between the two borylated ferrocenyl cyclopentadienyl rings. This finding indicates ferrocene to
be a stronger Li+ binder than benzene. In line with this conclusion, the hexaphenyl derivative [(Li(OBu2))2][1,1‘-fc(BPh3)2] was found to have each of its cyclopentadienyl substituents coordinated to one
Li+ ion, thereby forming a multiple-decker sandwich complex in the solid state (fc = (C5H4)2Fe).
The sodium pentaphosphide dimer [(tBu 3 Si) 3 P 5 Na 2 (THF)] 2 has been synthesized in high yield from the reaction of four equivalents of the sodium silanide tBu 3 SiNa with P 4 . X-ray quality crystals of the sodium pentaphosphide dimer [(tBu 3 -Si) 3 P 5 Na 2 (THF)] 2 (monoclinic, P2 1 /n) were grown from benzene. The sodium pentaphosphide (tBu 3 Si) 3 P 5 Na 2 can be oxidized with one equivalent of TCNE to give the bicyclo[2.1.0]-
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