Rational synthesis of hexanuclear metallacycles by alkylation reactions of an S-bridged CoIIIPdIICoIII trinuclear complex containing non-binding thiolato groups

The reaction of an S-bridged CoIIIPdIICoIII trinuclear complex containing two non-bridging thiolato groups, [Pd{Co(aet)3}2]2+ (aet = 2-aminoethanethiolate), with o-dibromoxylene (o-xylBr2) in water produced a cyclic CoIII4PdII2 hexanuclear complex, [{Co2Pd(aet)4}2(o-L)2]8+ ([1]8+; o-L = o-bis(2-aminoethylthiomethyl)benzene), in which two CoIIIPdIICoIII trinuclear units are linked by two o-xyl2+ moieties through C–S bonds. A similar cyclic CoIII4PdII2 complex, [{Co2Pd(aet)4}2(m-L)2]8+ ([2]8+; m-L = m-bis(2-aminoethylthiomethyl)benzene), bearing a relatively large cavity that accommodates water molecule(s), was synthesized by the reaction of [Pd{Co(aet)3}2]2+ with m-dibromoxylene (m-xylBr2) in water. While [1]8+ afforded only the racemic (Δ4/Λ4) isomer, both the racemic ([2a]8+; Δ4/Λ4) and the meso ([2b]8+; Δ2Λ2) isomers were formed for [2]8+. In addition, the meso [2b]8+ was found to exist as a mixture of two diastereomers, (ΔS)2(ΛR)2 and (ΔSΔR)(ΛRΛS), which arise from the difference in chiral configurations (R and S) of asymmetric sulfide S atoms, while the racemic [1]8+ and [2a]8+ existed as a pair of enantiomers, (ΔS)4 and(ΛR)4, which were optically resolved. The complexes obtained were characterized on the basis of electronic absorption, CD, and NMR spectroscopies, along with single crystal X-ray analyses.