CONFORMATIONAL ISOMERISM AND EFFECT OF COMPLEXATION ON CARBOXYLATE GROUP IN 2 CRYSTALLOGRAPHICALLY INDEPENDENT COORDINATION UNITS OF BENZOATO-O)BIS(1,3-DIAMINOPROPANE-N,N')COBALT(III) 4-CHLOROBENZOATE DIHYDRATE

The crystal and molecular structure of the title compound was determin ed by single crystal X-ray methods. The compound crystallises in the t riclinic system, space group P (1) over bar (No. 2), a = 10.477(3), b = 12.309(2), c = 13.420(3) Angstrom, alpha = 85.40(2), beta = 78.39(2) , gamma = 71.46(2)degrees Z = 2, D-c = 1.47, D-m = 1.46(1) g cm(-3). T he final R value for 4415 reflections was 0.038 (R(w) = 0.033, 1/sigma (2) weights). The structure consists of two crystallographically indep endent complex units, a non-coordinated 4-chlorobenzoate anion and two crystal water molecules. Both complex units possess similar C-i symme try with 1,3-diaminopropane ligands forming chelate rings and the axia l sites occupied by 4-chlorobenzoate anions. The central low-spin coba lt cations have the oxidation state +3. The Co-N distances are in the range 1.963(2)-1.972(2) Angstrom. The main differences in the two comp lex units are seen in the axial bond lengths and bite angles. In the c omplex unit a the axial bond length is 1.926(2) Angstrom and the total puckering of the chelate ring is 0.510 Angstrom, whereas in the unit b the corresponding values are 1.909(2) and 0.585 Angstrom. The corres ponding N1-Co-N2 bite angles are 93.03(9) and 88.76(9)degrees. The two conformational isomers arise from differences in hydrogen bonding and stacking of the benzoate anions. Ab initio calculations with the 6-31 G basis set for 4-chlorobenzoic acid and its anion reveal elongation of the bond between the carboxylate group and aromatic ring upon depro tonation of the acid. The complexation of the anion results in elongat ion in the corresponding O(coordinated)-C bond. There is strong correl ation between the axial bond length M-O and the angle O(non-coordinate d)-(carboxylate)-C(aromatic) in the structures containing a fragment s imilar to the title compound. The charge densities obtained by the STO -3G basis set show that the oxygen coordinated to the cobalt(III) cat ion carries more negative charge than the non-coordinated oxygen. In t he non-coordinated anion the charge densities are more evenly distribu ted.