Conformational flexibility of 1,3-diaminopropane in chelate rings. Redetermination of the structure of trans-dinitratobis(1,3-diaminopropane)copper(II)

Abstract

The flexibility of tn (tn=1,3-diaminopropane) in pseudo-octahedral MO 2tn 2 complexes (where M=Co, Ni, Cu; O is in axial position and tn displays chair conformation) manifests itself mainly as variation in the bite angle (NMN), bite size (interring NN distance) and puckering values of the chelate ring. Because the structure of trans-dinitratobis(1,3-diaminopropane)copper(II) determined earlier by X-ray methods had unusual values for the flexibility and a rather modest R value, it was used as a model compound in CFF calculations and redetermined by X-ray methods. The refinement improved from the earlier R index of 0.122 to 0.039. The current parameters are: C 6H 20CuN 6O 6, M=337.81, orthorhombic, a=8.1888(9), b=10.234(2), c=16.349(2) Å, V=1370.2(3) Å 3, space group Pcab (No. 61), Z=4. The site symmetry around the central Cu(II) ion is near to D 4 h . An important deviation from the exact D 4 h symmetry is the widening of 3.2(2)° in the bite angle from the ideal 90°. The puckering values describing the distance of the three carbon atoms of tn from the CuN 4 plane are 0.513(11), −0.263(12) and 0.429(11) Å, which are exceptionally low values indicating a flattened ring. The optimized structure came more up to the expected statistical average bite angle value. To explain the difference between the optimized and the observed value of the bite angle, extended Hückel calculations were carried out. Both hydrogen bonding and mutual interaction of the ligands were investigated based upon the structures found from the Cambridge Structural Database. The most probable explanation for the variation in the bite angle is hydrogen bonding.