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) comple xes 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-py ridylmethyl)-1,2-ethanediamine (bispicen), N,N'-bis(2 pyridylmethyl)-N ,N'-dimethyl-1,2-ethanediamine (bispicMe(2)en),and ammonia. The binucl ear complex [Cr(bispicMe(2)en)(OH)(OH2)] (ClO4)(2), [CrC16H25N4O2](ClO 4)(2) (3), crystallizes in;space group P2(1)/n of the monoclinic syste m 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 b eta = 105.94(2)degrees. The structure has been refined to a final R fa ctor of 0.0606 based on 3346 observed independent reflections. The cen tral chromium(III) atom is pseudooctahedrally bonded to the four nitro gen atoms of the ligand and to two cis oxygen atoms from the nominal w ater and hydroxo ligands. The complex adopts the cis-or geometry in wh ich 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 correspond ing dithionate (4a), thiosulfate (4b), and iodide chloride salt (4c) h ave 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, an d beta = 104.880(10)degrees. The structure has been refined to a final R factor of 0.0379 based on 2519 observed independent reflections. Bo th of these complexes 3 and 4c are binuclear in the solid state, the t wo adjacent chromium(III) centers being doubly bridged by hydrogen bon ds between hydroxo and aqua ligands. The Cr...Cr separations are 4.999 and 4.925 Angstrom in 3 and 4c, respectively. The complexes exhibit s urprisingly strong antiferromagnetic interactions, with triplet energi es in the range 2-7 cm-L, with the exception of the tetraammine comple xes 5 and complex 4c, which show only very weak antiferromagnetic coup ling 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, w hile the other complexes are binuclear, but magnetic susceptibility me asurements alone are hard pressed to distinguish between these two str uctures. The EPR spectra of the complexes, however, can be used to rea dily distinguish between the chain and dimer forms of aggregation.