Bis( l- N, N-dipropylalaninato)copper(II) : X-ray crystal structure, the crystal structure prediction and conformational analysis with a new force field

Abstract

The structure of red monoclinic crystals of bis( l- N, N-dipropylalaninato)copper(II) was determined by the X-ray diffraction method. Coordination around the copper atom is distorted square-planar, with oxygen and nitrogen atoms in trans positions. The crystal structure consists of discrete molecules held by van der Waals forces. The main conformational feature of the chelate ring systems is the axial position of β-carbon alanine atoms. This paper proposes a new molecular mechanics model and a new force field derived with the aim to simulate and predict properties of both cis and trans copper(II) amino acid complexes. The force field was optimised on copper(II) coordination geometry of three molecules obtained in vacuo by a molecular quantum mechanics method, and on X-ray experimental data of five molecules. Respective equilibrium structures were calculated in the vacuum and condensed phase approximations. This paper examines the efficacy of the new force field FF1 to reproduce structural properties of 12 tetra-coordinated copper(II) amino acidates (including the title compound) with various copper(II) coordination polyhedron geometry, crystal symmetries, and intermolecular interactions. The FF1 force field was used to investigate the influence of N, N-dialkyl chain lengthening on stereochemistry of the copper(II) alaninato complexes. Molecular mechanics calculations suggest that reasons for the prevalence of equatorial and/or axial positions of the alanine C βH 3 residue in the crystal state should be more readily attributed to the crystal packing preferences than to intramolecular steric hindrances caused by N-dialkyl groups.