A temperature-dependent order-disorder and crystallographic phase transition in a 0D Fe-II spin crossover compound and its non-spin crossover Co-II isomorph

The new dipyridylamino/triazine ligand DDE (N-2, N-2, N-4, N-4-tetraethyl-N-6, N-6-di(pyridin-2-yl)-1,3,5triazine- 2,4,6-triamine) has been incorporated into the mononuclear Fe-II SCO compounds cis-[Fe-II(NCSe)(2)(DDE)(2)] (1), cis-[Fe-II(NCBH3)(2)(DDE)(2)] (2), and cis-[Fe-II(NCS)(2)(DDE)(2)] (3). Magnetic susceptibility measurements reveal that each of 1, 2 and 3 undergoes a complete, continuous spin transition with a T-1/2 of similar to 260 K, similar to 300 K and similar to 205 K, respectively. An analogue and isomorph of 1, cis-[Co-II(NCSe)(2)(DDE)(2)] (4), remains high spin down to low temperatures. Variable temperature single crystal data reveal that 1 and 4 undergo a crystallographic phase transition (from orthorhombic Pbcn at high temperatures to monoclinic P2/c at low temperatures) accompanied by an order-disorder transition of ethyl moieties of the DDE ligand. In the Pbcn phase, the structures of 1 and 4 contain one crystallographically unique M-II centre, while in the P2/c phase, 1 and 4 contain two crystallographically unique M-II centres. Variable temperature powder X-ray diffraction experiments reveal that the crystallographic phase transition occurs at similar to 250 K for 1. The occurrence of the concomitant order-disorder and crystallographic phase transitions undergone by 1 and 4 is not directly apparent in their magnetic susceptibility measurements, and this is likely due to the local environment of the M-II centres remaining largely undisturbed as the transitions occur. The compound 2 is isostructural to 1 and 4 at low temperatures.