SYNTHESIS AND MAGNETIC-PROPERTIES OF A TETRANUCLEAR COPPER(II) COMPLEX WITH A MU-1,2,3,4-SQUARATO COORDINATION MODE - CRYSTAL-STRUCTURE OF (MU-1,2,3,4-SQUARATO)TETRAKIS[(TRIS(2-AMINOETHYL)AMINE)COPPER(II)] PERCHLORATE

The crystal and molecular structure of [Cu-4(tren)(4)(C4O4)](ClO4)(6) (where tren and C4O42- are tris(a-aminoethyl)amine and the dianion of 3,4-dihydroxycyclobutenedione, respectively) has been determined by single-crystal X-ray analysis. The compound crystallizes in the monoclinic system, space group C2, with a = 26972(4) Angstrom, b = 7.910(1) Angstrom, c = 18.293(2) Angstrom, beta = 132.39(1)degrees, and Z = 2. The structure consists of tetranuclear [Cu-4(tren)(4)(C4O4)](6)+ cationic units and uncoordinated perchlorate anions. The geometry df the metal ions is trigonal bipyramidal with tren terminal nitrogen atoms in the equatorial positions and the tren central nitrogen and a squarate oxygen in the axial positions. This complex is the first example of an X-ray structurally characterized tetrameric transition metal complex with the four metal cations being bridged by a single squarato ligand in a mu-1,2,3,4-coordination mode. The magnetic behavior of the complex has been studied over the 2.5-300 K temperature range. The fitting of the experimental magnetic susceptibilities vs temperature to the Heisenberg-Dirac-Van Vleck [H = -J(S-1 .(2) + S-1 . S-4 + S-2 . S-3 + S-3 . S-4) - j(S-1 . S-3 + S-2 . S-4)], S-1= S-2 = S-3 = S-4 = 1/2 spin exchange model yields J and j values of -19.0 and -0.8 cm(-1), respectively. The value of the antiferromagnetic interaction through the OCCO fragment, which is the largest found in structurally characterized squarato-bridged copper(II) complexes, has been interpreted through theoretical calculations in the light of its structural and electronic characteristics.