Spin crossover in polymeric and heterometallic FeII species containing polytopic dipyridylamino-substituted-triazine ligands

Abstract

We report the synthesis and characterisation of the new polytopic ligands, ddta (N,N-di(pyridin-2-yl)-4,6-di(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-1,3,5-triazin-2-amine) and tptd (N²,N²,N⁴,N⁴-tetra(pyridin-2-yl)-6-(1,4,7,10-tetraoxa-13-azacyclopentadecan-13-yl)-1,3,5-triazine-2,4-diamine). Each contains N-donor dipyridylamino binding sites as well as separate and distinct mono-aza-15-crown-5 binding sites. The ligand ddta has been used to synthesise the polymeric heterometallic SCO compound trans-[Fe^II(NCS)₂(ddta)₂Na₂](ClO₄)₂·4CH₃CH₂CH₂OH, 1, and tptd has been used to synthesise the polymeric SCO compound trans-[Fe^II(NCS)₂(tptd)]·CH₃OH, 2, and the dinuclear compound cis-[(Fe^II)₂(NCS)₄(tptd)₂], 3. Magnetic susceptibility measurements show that 1 and a desolvated sample of 2 each undergo a gradual, one-step spin transition with T_½ values of ∼240 K and ∼110 K, respectively. The paucity of inter-chain intermolecular interactions, as well as the flexible, covalent bridges between Fe^II spin crossover sites, are likely to contribute to the gradual nature of the spin transition observed in each case. Variable temperature powder X-ray diffraction studies on 1 show the anisotropic behaviour of the unit cell parameters, where c and the b-c plane are most affected by structural changes occurring as the temperature is lowered.