CRYSTAL, MOLECULAR, ELECTRONIC AND VIBRATIONAL-STRUCTURES OF DI-MU-THIOCYANATO-BIS-[DI-(3-AMINOPROPYL)AMINE]-DICOPPER(II) DIPERCHLORATE

The crystal and molecular structures of the dimeric [Cu(dpta)(NCS)(ClO4)](2) (dpta=di(3-aminopropyl)amine) have been reinvestigated by X-ray diffraction. The crystals are orthorhombic, space group Pbca with a=13.938(3), b=12.424(3), c=15.864(2) Angstrom, V=2747(1) Angstrom(3) and Z=4. The structure was refined to R=0.050 (R(w)=0.049) from 1742 observed unique reflections. Each Cu atom is coordinated to the 3 N atoms of a dtpa ligand and to the N atom of an NCS- group in an almost square planar arrangement. The Cu atom makes further axial contacts with the S atom of another [Cu(dpta)(NCS)](+) unit and with one O atom of a ClO4- group thus forming a centrosymmetric dimeric unit [(dpta)(CuSCNCu)-Cu-NCS(dpta)](2+). Such units are held together by the ClO4- ions which provide Cu-O contacts and H bonds to neighbouring dpta H atoms. The ultimate arrangement around Cu may be described as a strongly elongated octahedron. The bond lengths (in Angstrom) are: Cu-N(primary amine)=1.999(6) and 2.012(6), Cu-N(secondary amine)=2.052(5), Cu-N(NCS)=2.000(6), Cu-S=2.795(2), Cu-O=2.93(1). Considering the plane of the two bridging NCS groups as equatorial, the dpta ligand is in a meridional position with chair-chair conformation in full agreement with molecular mechanics calculations. Chair-skew boat conformers, however, are predicted to lie close in energy. The IR and Raman spectra of the compound were assigned by normal coordinate analysis and MNDO calculations. The bands 855 (R), 890 (IR) and 980 (IR+R) cm(-1) are typical for mer-coordinated dpta and can be used as a diagnostic tool to detect the ligand coordination mode.