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Modelling Sterical Interactions in bis(N, N-diethylglycinato)copper(II)

The blue crystals of bis(N, N-diethylglycinato)copper(II) deposited from red methylene chloride solution. It has been established that in the red form the coordination number of copper is four, while the blue colours arise from the coordination number of copper greater than four. Thus, it was obvious that the copper(II) coordination geometry changed upon crystallization, and indeed the X-ray determination and refinement of the crystal structure revealed that the coordination about the copper atom was trans distorted pyramid with the carbonyl oxygen (O22) from neighbouring molecule occupying the fifth copper(II) coordination position (see Figure below). Such type of coordination geometry has not yet been observed for anhydrous trans-bis copper(II) amino acidates. The molecular mechanics force field FFW developed for modelling copper(II) amino acid complexes in simulated crystalline surroundings and for isolated molecules1 reproduces well the unit cell dimensions (see compared numbers in the figure below). The right drawing shows the fit between the experimental X-ray crystal (dark) and theoretical FFW crystal (light) molecular structures. FFW was also used to study the strain energy distribution among the conformers of bis(N, N-diethylglycinato)copper(II) in vacuo without the influence of intermolecular interactions. The conformer found in the experimental crystal structure is predicted 9 kcal mol-1 higher in energy than the most stable conformer in vacuo. The most stable conformer has the ethyl chains oriented perpendicularly to the chelate ring planes, shielding the copper coordination site for axial bonding. Conversely, the crystalline conformer has the ethyl groups oriented to allow axial bonding to the copper(II) as well as forming intermolecular aliphatic-aliphatic interactions between the ethyl groups in the crystal lattice. Therefore, we may suppose that not only the changes in the copper(II) coordination geometry but also in the molecular conformation occured upon crystallization. 1 J. Sabolović, C. S. Tautermann, T. Loerting, K. R. Liedl, Inorg. Chem. 42 (2003) 2268-2279.

crystal structure; X-ray; crystal simulation; molecular mechanics; force field; conformational analysis

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