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 NaBH4/TiCl4 leads to (6R,8aR)-7-methyl-6-(sulfanylmethyl)-thiazolidine [3,4-a] piperazine, (LH)-H-1 as well as N,N'-dimethyl-(2R,5R)-bis(sulfanylmethyl)piperazine, (LH2)-H-2, which were separated and characterized. (LH2)-H-2 can be obtained selectively, if V is reduced by NaBH4/TiCl4 in the presence of DIEA center dot 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 (LH)-H-1 and (LH2)-H-2 with NaOMe followed by treatment with NiBr2(dme) led to the isolation of the dimeric complexes [(LNiBr)-Ni-1](2) (1) and [(LNi)-Ni-2](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.