A LINEAR OPEN-CHAIN PIPERAZINE-PYRIDINE LIGAND AND ITS MESO-HELICAL CO COMPLEX

An oligomeric ligand (HPPy)(2)P, thyl)piperazinyl)methyl)pyridylmethyl )]piperazine, was designed to resemble structurally open-chain aza-cro wns. Owing to the all-trans configuration of piperazine and pyridine f ree electron pairs, it should adopt a near linear overall structure in solvent. Theoretical calculations at ab initio level confirm the over all linear structure of free ligand. The crystal structure of the comp lex [(HPPy)(2)P] [Co(NO3)(2)](2) shows a contraction from similar to 3 to 2 nm structure. Each coordination site creates either a Lambda or Delta configuration around the metal ion, thus causing a ligand with a n even number of coordination centres to be meso-helical. The complex (C30H48N12O14Co2) crystallizes in the monoclinic space group P2(1)/n ( No. 14) with the cell dimensions a = 11.673 (4), b = 13.212 (8), c = 1 2.561 (7) Angstrom, beta = 90.33 ( 1)degrees, V= 1937 (2) Angstrom(3) and Z = 2. The structure is refined to a final agreement factor R(F-o( 2)) = 0.0576 using 2121 reflections with I > 4 sigma(I). The molecule is centrosymmetric with two identical coordination centres, in which t he regular octahedral geometry is tetragonally distorted. (C) 1998 Els evier Science S.A. All rights reserved.