✍ Scribed by David D. Díaz; Sayam Sen Gupta; Jane Kuzelka; Marcin Cymborowski; Michal Sabat; M. G. Finn
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A novel chelating chiral bis(formamidine–urea) ligand was synthesized and used to prepare the corresponding Ni^II^ and Cu^II^ complexes. Spectroscopic and X‐ray crystallographic analysis of the former revealed that the urea moiety of the acyclic tetradentate ligand is deprotonated and binds to the square‐planar Ni^2+^ ion through amide nitrogen atoms. This complex showed quasi‐reversible redox behavior in cyclic voltammetry, with E~1/2~ = 640 mV vs. Cp~2~Fe^+^/Cp~2~Fe corresponding to the Ni^III^/Ni^II^ couple. The Cu complex was shown by EPR spectroscopy to also adopt a square‐planar geometry [g~∥~ = 2.25 (A~∥~ = 184 G), g~⟂~ = 2.06 at 6 K], and reduction was found to be irreversible, perhaps due to the rigid nature of the tetradentate ligand. The nickel complex was found to be a modestly active catalyst for epoxidation of electron‐richalkenes under Mukaiyama’s conditions, likely involving a radical mechanism. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)
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