The reaction of [B12H11NH3](-), the amino derivative of the closo-dodecaborate anion, with aldehydes in methanol in the presence of catalytic amounts of alkali gives N-protonated Schiff bases [B12H11NH=CHR](-) (R = C6H5, 2-C6H4OMe, 4-C6H4OMe, 4-C6H4SMe, 4-C6H4NMe2, 4-C6H4NHCOMe, 4-C6H4CN, 4-C6H4Br, 4-C6H4Cl, 3,4-C6H3O2CH2, 1-C10H7, 2-C10H7, CH=CHMe, CH=CHPh). Reduction of the Schiff bases with NaBH4 in aqueous 2-propanol gives quantitatively the corresponding monoalkylamino derivatives [B12H11NH2CH2R](-) (R = 2-C6H4OMe, 3,4-C6H3O2CH2, 4-C6H4NHCOMe, 4-C6H4CN). closo-Dodecaborate derivatives containing amino, carboxy, and isothiocyanate functions on aromatic rings, [B12H11NH2CH2C6H4-4-NH2](-), [B12H11NH2CH2C6H4-4-COOH](-), and [B12H11NH2CH2C6H4-4-NCS](-) were prepared. Functionalized derivatives of the [B12H12](2-) anion can be used in BNCT and as a linker for iodination and astatination of biomolecules for radioimmunodetection and for radioimmunotherapy. The structures of Schiff bases (Bu4N)[B12H11NH=CHC6H4-4-NMe2]*CHCl3 (I) and (Bu4N)[B12H11NH=CHC6H4-2-OMe] (II) were determined by single-crystal X-ray diffraction. I crystallizes in the monoclinic space group P2(1)/c with Z = 4 and unit cell dimensions a = 11.465(6) Angstrom, b = 21.314(7) Angstrom, c = 16.625(6) Angstrom, beta = 101.38(3)degrees. Crystals II belong to the monoclinic space group P2(1)/c system, a = 10.282(2) Angstrom, b = 20.272(5) a, c = 17.052(3) Angstrom, beta = 105.31(2)degrees, Z = 4. The trans configuration of the protonated imino group was established (B-N = 1.517(7) and 1.525(4) Angstrom, N-C-im = 1.274(6) and 1.274(3) Angstrom, C-im-C-ar = 1.421(7) and 1.437(4) Angstrom, for I and II, respectively). A strong intramolecular hydrogen bond between the imine hydrogen and the methoxy oxygen in the ortho position of the benzene ring (N ... O = 2.714(4) Angstrom, N-H = 0.93(2) Angstrom, H-N... O = 1.98(2) Angstrom, N-H ... O = 135(2)degrees) was found in II.