Synthesis of a Chiral, Polydentate Ligand System Setting Out from L ‐Cysteine and First Nickel Complexes Thereof

Setting out from protected L ‐cysteine a 2, 5‐diketopiperazine V can be synthesized, the reduction of which with NaBH 4 /TiCl 4 leads to (6 R ,8 aR )‐7‐methyl‐6‐(sulfanylmethyl)‐thiazolidine [3,4‐ a ] piperazine, L 1 H as well as N , N ′‐dimethyl‐(2 R ,5 R )‐bis(sulfanylmethyl) piperazine, L 2 H 2 , which were separated and characterized. L 2 H 2 can be obtained selectively, if V is reduced by NaBH 4 /TiCl 4 in the presence of DIEA · HCl, and it represents a precursor for a novel, chiral ligand, as after deprotonation it provides two thiolato and two amino donor functions for the coordination of a metal atom. Deprotonation of L 1 H and L 2 H 2 with NaOMe followed by treatment with NiBr 2 (dme) led to the isolation of the dimeric complexes [L 1 NiBr] 2 ( 1 ) and [L 2 Ni] 2 ( 2 ), respectively. Both were fully characterized, and cyclic voltammetry indicated the possibility of Ni II → Ni III oxidations for complex 2 . 2 can be regarded as a structural model for the A clusters of the acetyl coenzyme A synthase.