ALTERNATING FERROMAGNETIC-ANTIFERROMAGNETIC INTERACTIONS IN A MANGANESE(II)-AZIDO ONE-DIMENSIONAL COMPOUND - [MN(BIPY)(N-3)(2)]

An alternating di-(mu-(end-on)azido)-di-(mu-(end-to-end)azido manganes e(II) one-dimensional compound, with formula [Mn(bipy)(N-3)(2)] (bipy= 2,2'-bipyridine), has been synthesized and characterized. Its crystal structure has been solved at room temperature. The complex crystallize s in the triclinic <P(1)over bar> space group, with a=7.547(2) Angstro m, b=9.137(4) Angstrom, c=9.960(4) Angstrom, alpha=110.76(4)degrees be ta=104.43(2)degrees, gamma=100.41(3)degrees, and Z=2. The structure co nsists of manganese chains in which the Mn-II ions are alternatively b ridged by two end-on (EO) and two end-to-end (EE) azido bridges. Each Mn-II ion has an octahedral coordination, completed by the two nitroge n atoms of the bipy ligand. The EO and EE bridges are arranged cis. Th is constitutes the first example of such an azido bridge chain for any metallic ion. ESR measurements show signals corresponding to Delta Ms =1 and Delta Ms=2 transitions, with no significant variations by modif ying the temperature. The thermal variation of molar susceptibility re veals the existence of alternating ferro- and antiferromagnetic intera ctions, through alternating EO and EE azido bridges, in the compound. A theoretical model has been developed for an S=5/2 alternating ferrom agnetic-antiferromagnetic coupled 1D system: the exchange parameters o btained with this model, considering the spin Hamiltonian H=-J(1) Sigm a S2iS2i+1-J(2) Sigma S2i+1S2i+2, are J(1)=13.8 K, J(2)=-17.01 K with g fixed at 2.0. Extended Huckel calculations are discussed to model th e end-on and end-to-end bridged systems.