Synthesis of Schiff Bases Derived from the Ammoniaundecahydro-closo-dodecaborate(1−) Anion, [B12H11NHCHR]-, and Their Reduction into Monosubstituted Amines [B12H11NH2CH2R]-: A New Route to Water Soluble Agents for BNCT

The reaction of [B 12 H 11 NH 3 ] -, 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 [B 12 H 11 NH CHR] - (R = C 6 H 5, 2-C 6 H 4 OMe, 4-C 6 H 4 OMe, 4-C 6 H 4 SMe, 4-C 6 H 4 NMe 2, 4-C 6 H 4 NHCOMe, 4-C 6 H 4 CN, 4-C 6 H 4 Br, 4-C 6 H 4 Cl, 3,4-C 6 H 3 O 2 CH 2, 1-C 10 H 7, 2-C 10 H 7, CH CHMe, CH CHPh). Reduction of the Schiff bases with NaBH 4 in aqueous 2-propanol gives quantitatively the corresponding monoalkylamino derivatives [B 12 H 11 NH 2 CH 2 R] - (R = 2-C 6 H 4 OMe, 3,4-C 6 H 3 O 2 CH 2, 4-C 6 H 4 NHCOMe, 4-C 6 H 4 CN). closo -Dodecaborate derivatives containing amino, carboxy, and isothiocyanate functions on aromatic rings, [B 12 H 11 NH 2 CH 2 C 6 H 4 -4-NH 2 ] -, [B 12 H 11 NH 2 CH 2 C 6 H 4 -4-COOH] -, and [B 12 H 11 NH 2 CH 2 C 6 H 4 -4-NCS] -, were prepared. Functionalized derivatives of the [B 12 H 12 ] 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 (Bu 4 N)[B 12 H 11 NH CHC 6 H 4 -4-NMe 2 ]*CHCl 3 ( I ) and (Bu 4 N)[B 12 H 11 NH CHC 6 H 4 -2-OMe] ( II ) were determined by single-crystal X-ray diffraction. I crystallizes in the monoclinic space group P 2 1 / c with Z = 4 and unit cell dimensions a = 11.465(6) Å, b = 21.314(7) Å, c = 16.625(6) Å, β = 101.38(3)°. Crystals II belong to the monoclinic space group P 2 1 / c system, a = 10.282(2) Å, b = 20.272(5) Å, c = 17.052(3) Å, β = 105.31(2)°, Z = 4. The trans configuration of the protonated imino group was established (B−N = 1.517(7) and 1.525(4) Å, N−C im = 1.274(6) and 1.274(3) Å, C im −C ar = 1.421(7) and 1.437(4) Å, 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) Å, N−H = 0.93(2) Å, H N ···O = 1.98(2) Å, N−H···O = 135(2)°) was found in II .