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 manganese(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 crystallizes in the triclinic <P(1)over bar> space group, with a=7.547(2) Angstrom, b=9.137(4) Angstrom, c=9.960(4) Angstrom, alpha=110.76(4)degrees beta=104.43(2)degrees, gamma=100.41(3)degrees, and Z=2. The structure consists of manganese chains in which the Mn-II ions are alternatively bridged 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 nitrogen atoms of the bipy ligand. The EO and EE bridges are arranged cis. This 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 modifying the temperature. The thermal variation of molar susceptibility reveals the existence of alternating ferro- and antiferromagnetic interactions, through alternating EO and EE azido bridges, in the compound. A theoretical model has been developed for an S=5/2 alternating ferromagnetic-antiferromagnetic coupled 1D system: the exchange parameters obtained with this model, considering the spin Hamiltonian H=-J(1) Sigma 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 the end-on and end-to-end bridged systems.