Synthesis, reactivity and complexation studies of N,S exo-heterodisubstituted o-carborane ligands. Carborane as a platform to produce the uncommon bidentate chelating (pyridine)N-C-C-C-S(H) motif

Abstract

The synthesis of N,S-heterodisubstituted 1-(2′-pyridyl)-2-SR-1,2-closo-C₂B₁₀H₁₀ compounds (R = Et, 2; R = ⁱPr, 3) has been accomplished starting from 1-(2′-pyridyl)-1,2-closo-C₂B₁₀H₁₁ (1), and their partial deboronation reaction leading to the structurally chiral [7-(2′-pyridyl)-8-SR-7,8-nido-C₂B₉H₁₀]^– derivatives (R = Et, [4]^–; R = ⁱPr, [5]^–) has been studied. Capillary electrophoresis combined with the chiral selector α-cyclodextrin has permitted the separation of the electrophoretically pure racemic [7-(2′-pyridyl)-8-SR-7,8-nido-C₂B₉H₁₁]^– ions into two peaks each one corresponding to the interaction of one enantiomer with the α-cyclodextrin. The N,S-heterodisubstituted o-carborane containing a mercapto group, 1-(2′-pyridyl)-2-SH-1,2-closo-C₂B₁₀H₁₀, 1, is one of the two examples of a rigid bidentate chelating (pyridine)N-C-C-C-S(H) motif having been structurally fully characterized. To study the potential of such a binding site, 1 has been tested as a ligand with metal ions requiring different coordination numbers, two (Au⁺) and four (Pd²⁺ and Rh⁺). The crystal structures of the Pd(II) and Au(I) complexes are reported. For the Pd(II) complex, 1 acts as a bidentate ligand whereas for Au(I), 1 acts as a monodentate ligand through the thiolate.