Enhancement of Intermolecular Magnetic Exchange through Halogen...Halogen Interactions in Bisadeninium Rhenium(IV) Salts

Two novel Re-IV salts of general formula [H(2)ade](2)[(ReX6)-X-IV]X(2 center dot)4H(2)O [H(2)ade(2+) = 9.H-adenine-1,7-diium; X = Cl(1) and Br(2)] have been synthesized and magnetostructurally characterized. 1 and 2 are isostructural salts that crystallize in the orthorhombic system with space group Fdd2. Both compounds are made up of discrete mononuclear [(ReX6)-X-IV](2-) and X- anions and doubly protonated adenine cations. The six-coordinate rhenium(IV) ion is bonded to six halide ligands [X = Cl (1) and Br (2)] in an octahedral geometry. Short intermolecular Re-IV-X...X-Re-IV interactions, as well as Re-IV-X...H-N(H(2)ade) and Re-IV-X...H-O-w hydrogen bonds, are present in the crystal lattice of 1 and 2. Magnetic susceptibility measurements on polycrystalline samples of 1 and 2 in the temperature range 2.0-300 K show the occurrence of significant intermolecular antiferromagnetic interactions in both compounds, resulting in the observation of maxima in chi(M) at ca. 6.0 (1) and 12.0 K (2). The larger spin delocalization from the Re-IV ion onto the peripheral bromide ligands when compared to the chloride ligands accounts for the enhancement of the magnetic exchange observed in 2.