Structure and magnetic properties of an unprecedented syn-anti μ-nitrito-1κO:2κO′ bridged Mn (III)-salen complex and its isoelectronic and isostructural formate analogue

Abstract

The preparation, crystal structures and magnetic properties of two new isoelectronic and isomorphous formate- and nitrite-bridged 1D chains of Mn(III)-salen complexes, [Mn(salen)(HCOO)]_n (1) and [Mn(salen)(NO₂)]_n (2), where salen is the dianion of N,N′-bis(salicylidene)-1,2-diaminoethane, are presented. The structures show that the salen ligand coordinates to the four equatorial sites of the metal ion and the formate or nitrite ions coordinate to the axial positions to bridge the Mn(III)-salen units through a syn-anti μ-1κO:2κO′ coordination mode. Such a bridging mode is unprecedented in Mn (III) for formate and in any transition metal ion for nitrite. Variable-temperature magnetic susceptibility measurements of complexes 1 and 2 indicate the presence of ferromagnetic exchange interactions with J values of 0.0607 cm⁻¹ (for 1) and 0.0883 cm⁻¹ (for 2). The AC measurements indicate negligible frequency dependence for 1 whereas compound 2 exhibits a decrease of χ_ac′ and a concomitant increase of χ _ac′′ on elevating frequency around 2 K. This finding is an indication of slow magnetization reversal characteristic of single-chain magnets or spin-glasses. The μ-nitrito-1κO:2κO′ bridge seems to be a potentially superior magnetic coupler to the formate bridge for the construction of single-molecule/-chain magnets as its coupling constant is greater and the χ_ac′ and χ_ac′′ show frequency dependence.