Crown-linked dipyridylamino-triazineligands and their spin-crossover iron(ii) derivatives: magnetism, photomagnetism and cooperativity

Abstract

The syntheses, crystallography and magnetic properties of a series of compounds of formula trans-[Fe^II(L¹)₂(NCX)₂] (X = S, Se, BH₃ (1–3)), cis-[Fe^II(L²)(NCX)₂]·CH₂Cl₂ (X = S, Se, BH₃ (4–6)) and trans-[Fe^II(L³)(NCX)₂]_n (X = S, Se (7–8)) are described (L¹ = 6-chloro-N²,N²-diethyl-N⁴,N⁴-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, L² = 6,6′-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(N²,N²-diethyl-N⁴,N⁴-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, L³ = 6,6′-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(N²,N²,N⁴,N⁴-tetra(pyridin-2-yl)-1,3,5-triazine-2,4-diamine)). The magnetostructural properties of 1–8 have been probed in detail by variable temperature magnetic measurements and crystallographic methods. 1–6 display mononuclear structures while 7 and 8 form 1-D chain structures. Complexes 4–6 have the potential to form 1D-chains via L² bridging, but instead form mononuclear complexes. Magnetic studies show that complexes 1, 2, and 4 remain in the high-spin (HS) state at all temperatures. An aged, dry, powdered sample of 3 gives an abrupt HS to LS transition (T_1/2 = 200 K), while a freshly prepared, powdered sample of 3·1.5H₂O displays thermal hysteresis (Δ = 7 K). Complexes 5, 6 and 7 undergo a gradual spin transition with T_1/2 values of 100 K, 150 K and 130 K, respectively. Cooperativity parameters are compared, with 3 showing cooperativity (positive C) and 5 and 6 showing anticooperativity. Photomagnetic LIESST (light induced excited spin state trapping) studies were performed on complexes 5 and 6 and reveal T(LIESST) values lower than 60 K. An attempt has been made to understand the electronic structure of complex 3 and its cooperativity behaviour using density functional methods, the calculations reproducing the sign and, in part, the magnitude of the cooperativity.