CONFORMATIONAL FLEXIBILITY OF 1,3-DIAMINOPROPANE IN CHELATE RINGS - REDETERMINATION OF THE STRUCTURE OF TRANS-DINITRATOBIS(1,3-DIAMINOPROPANE)COPPER(II)

The flexibility of tn (tn = 1,3-diaminopropane) in pseudo-octahedral M O2tn2 complexes (where M = Co, Ni, Cu; O is in axial position and tn d isplays chair conformation) manifests itself mainly as variation in th e bite angle (N-M-N), bite size (interring N-N distance) and puckering values of the chelate ring. Because the structure of trans-dinitratob is(1,3-diaminopropane)copper(II) determined earlier by X-ray methods h ad 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 0.12 2 to 0.039. The current parameters are: C6H20CuN6O6, M = 337.81, ortho rhombic, a = 8.1888(9), b = 10.234(2), c = 16.349(2) angstrom, V = 137 0.2(3) angstrom3, space group Pcab (No. 61), Z = 4. The site symmetry around the central Cu(II) ion is near to D4h. An important deviation f rom the exact D4h symmetry is the widening of 3.2(2)-degrees in the bi te angle from the ideal 90-degrees. The puckering values describing th e distance of the three carbon atoms of tn from the CuN4 plane are 0.5 13(11), -0.263(12) and 0.429(11) angstrom, 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 t he difference between the optimized and the observed value of the bite angle, extended Huckel calculations were carried out. Both hydrogen b onding and mutual interaction of the ligands were investigated based u pon the structures found from the Cambridge Structural Database. The m ost probable explanation for the variation in the bite angle is hydrog en bonding.