Ternary complexes of copper(II) with N-protected amino-acids and N-methylimidazole. Crystal and molecular structures of bis(N-acetyl-α-alaninato)bis(N-methylimidazole)copper(II) dihydrate

Abstract

Ternary complexes of the type [CuL₂(mim)₂][L =N-acetyl-α-, N-benzoyl-α-, N-acetyl-β-, or N-benzoyl-β-alaninate(1–); mim =N-methylimidazole], [CuL₂(mim)₂]·H₂O (L = benzoylglycinate, N-acetyl- or N-benzoyl-DL-leucinate), and [CuL₂(mim)₂]·2H₂O (L =N-acetyl-α-alaninate) have been synthesized, and characterized by electronic, i.r., and e.s.r. spectroscopy. The crystal structure of the title complex has also been determined by single-crystal X-ray diffraction. The compound crystallizes in the triclinic space group P with one molecule in a unit cell of dimensions a= 7.666(1), b= 8.430(1), c= 9.955(1)Å, α= 106.33(1), β= 86.27(1), and γ= 109.34(1)°. Least-squares refinement of the 215 variables gave a value of R 0.056 for 1 822 independent reflections having F_o > 4σ(F_o). The square-planar structure consists of a copper atom lying on the centre of symmetry surrounded by two carboxylic oxygen and two N-methylimidazole nitrogen atoms. Electronic and e.s.r. spectroscopic data are consistent with this type of structure (CuO₂N₂ chromophore) for the hydrated complexes and with a tetragonally elongated octahedral stereochemistry (CuO₄N₂ chromophore) for the anhydrous complexes. It is suggested that the amino-acid acts as an ‘ asymmetric ’ bidentate ligand in the latter complexes.