Key structural and optoelectronic properties of 9,10-dihydro-9,10-diboraanthracene (DBA) derivatives carrying mesityl (2a), 2-methylnaphthyl- (2b) and 9-phenyl-2,7-di-tert-butylanthryl (2c) substituents at the boron atoms have systematically been compared with the properties of their all-carbon congeners 4a-c. The experimental investigations have been augmented by quantum-chemical calculations. Steric repulsion leads to large dihedral angles between the aryl substituents and the DBA (2a-c) or anthrylene (4a-c) cores; as a result, the B-C bonds of 2a-c are kinetically shielded from hydrolysis and oxidative degradation. Lithium metal reduces the mesityl derivative 2a to the inverse sandwich complexes [Li(OR2)n]2[2a] (X-ray crystallography; OR2 = THF, n = 2; Et2O, n = 1). In line with the nodal structures of the LUMO of 2a/HOMO of [Li(THF)2]2[2a], the C-C bond lengths of the anionic fragment [2a](2-) show characteristic differences to those of 2a and come close to the C-C bond lengths of the isoelectronic species 4a. X-ray crystallography on anti-2b × 2 C6H6 and anti-4b × 2 C6H6 reveals an essentially identical packing of the main molecules. The benzene solvate molecules, however, interact in a very different manner with anti-2b or anti-4b, which can be traced down to subtle disparities between the electron density distributions of the two compounds. 2a-c undergo a photoinduced aryl-to-DBA charge transfer; the back electron transfer results in blue (2a), green (2b) and red (2c) emission, albeit with low quantum yields. 4a-c are characterised by a local π-π* photoexcitation of the central 9,10-anthrylene fragments and corresponding blue emission. Each of the compounds 2a-c gives rise to two reversible DBA-centred one-electron transitions in the cyclic voltammogram.

The reaction of 9,10-dibromo-9,10-dihydro-9,10-diboraanthracene (9,10-dibromo-DBA, 3) with two equivalents of 9-lithio-2,6- or 9-lithio-2,7-di-tert-butylanthracene gave the corresponding 9,10-dianthryl-DBAs featuring two (4) or four (5) inward-pointing tert-butyl groups. Compound 4 exists as two atropisomers, 4 and 4', due to hindered rotation about the exocyclic B-C bonds. X-ray crystallography of 5 suggests that the overall interactions between facing tert-butyl groups are attractive rather than repulsive. Even in solution, 4/4' and 5 are stable toward air and moisture for several hours. Treatment of 3 with 10-lithio-9-R-2,7-di-tert-butylanthracenes carrying phenyl (R=Ph), dimesitylboryl (R=Mes(2)B), or N,N-di(p-tolyl)amino (R=Tol(2)N) groups gave the corresponding 9,10-dianthryl-DBA derivatives 9-11 in moderate to good yields. In these molecules, all four solubilizing tert-butyl groups are outward pointing. The solid-state structures of 4, 5, 9, and 10 reveal twisted conformations about the exocyclic B-C bonds with dihedral angles of 70-90°. A significant electron-withdrawing character was proven for the Mes(2)B moiety, but no appreciable +M effect was evident for Tol(2)N. Compounds 5, 9, and 11 show two reversible DBA-centered reduction waves in the cyclic voltammogram. In the case of 10, a third reversible redox transition can be assigned to the Mes(2)B-anthryl substituents. The UV/Vis absorption spectrum of 5 is characterized by a very broad band at λ(max)=510 nm, attributable to a twisted intramolecular charge-transfer interaction from the anthryl donors to the DBA acceptor. The corresponding emission band shows pronounced positive solvatochromism (λ(em)=567 nm, C(6)H(12); 680 nm, CH(2)Cl(2)) in line with a highly polar excited state. The charge-transfer bands of 10 and 11, as well as the emission bands of 9 and 10, are redshifted relative to those of 5. The Tol(2)N derivative 11 is essentially nonfluorescent in solution, but emits bright wine-red light in the solid state.

Iron-containing ligands targeting the human histamine H(3) receptor (hH(3)R) were prepared. The compounds contain ferrocene sandwich complexes coupled via different linkers to a basic hH(3)R antagonist/inverse agonist pharmacophore. In a click chemistry approach, a triazole was successfully inserted as a new linking element. Two ferrocenylmethylamines and a ferrocenyltriazole were the most affine hH(3)R ligands within this series, showing receptor binding in the nano- and subnanomolar concentration range.

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