MAGNETIC EXCHANGE THROUGH HYDROGEN-BONDS - STRUCTURAL AND MAGNETIC CHARACTERIZATION OF CIS-HYDROXOAQUACHROMIUM(III) COMPLEXES OF TETRADENTATE AND MONODENTATE LIGANDS

The syntheses and characterization of a series of chromium(III) complexes of the general type cis-[CrA(4)(OH)(OH2)](2+) art: described. The ligands A(4) used include 1,4,8,11-tetraazacyclotetradecane (cyclam), N,N'-bis(2-pyridylmethyl)- 1,3-propanediamine (bispictn), N,N-bis(2-pyridylmethyl)-1,2-ethanediamine (bispicen), N,N'-bis(2 pyridylmethyl)-N,N'-dimethyl-1,2-ethanediamine (bispicMe(2)en),and ammonia. The binuclear complex [Cr(bispicMe(2)en)(OH)(OH2)] (ClO4)(2), [CrC16H25N4O2](ClO4)(2) (3), crystallizes in;space group P2(1)/n of the monoclinic system with eight mononuclear formula units in a cell of dimensions a = 14.381(4) Angstrom, b = 15.054(4) Angstrom, c = 21.547(6) Angstrom, and beta = 105.94(2)degrees. The structure has been refined to a final R factor of 0.0606 based on 3346 observed independent reflections. The central chromium(III) atom is pseudooctahedrally bonded to the four nitrogen atoms of the ligand and to two cis oxygen atoms from the nominal water and hydroxo ligands. The complex adopts the cis-or geometry in which the pyridine nitrogen atoms of the ligand are mutually trans. The complex [Cr(cyclam)(OH)(OH2)](ClO4)(2).0.5H(2)O (4) and the corresponding dithionate (4a), thiosulfate (4b), and iodide chloride salt (4c) have also been isolated, and the structure of the iodide chloride salt (4c) has been determined. The complex, [Cr(cyclam)(OH)(OH2)](Cl)(I), [CrC10H27N4O2](Cl)(I) (4c), crystallizes in space group P2(1)/e of the monoclinic system with four formula units in a cell of dimensions a = 10.361(2) Angstrom, b = 11.125(2) Angstrom, c = 15.211(2) Angstrom, and beta = 104.880(10)degrees. The structure has been refined to a final R factor of 0.0379 based on 2519 observed independent reflections. Both of these complexes 3 and 4c are binuclear in the solid state, the two adjacent chromium(III) centers being doubly bridged by hydrogen bonds between hydroxo and aqua ligands. The Cr...Cr separations are 4.999 and 4.925 Angstrom in 3 and 4c, respectively. The complexes exhibit surprisingly strong antiferromagnetic interactions, with triplet energies in the range 2-7 cm-L, with the exception of the tetraammine complexes 5 and complex 4c, which show only very weak antiferromagnetic coupling with triplet energies of 0-7 cm(-1). The bispicen complex (2) is known from other work to adopt a chain structure in the solid state, while the other complexes are binuclear, but magnetic susceptibility measurements alone are hard pressed to distinguish between these two structures. The EPR spectra of the complexes, however, can be used to readily distinguish between the chain and dimer forms of aggregation.