Crown-linked dipyridylamino-triazine ligands and their spin-crossover iron(II) derivatives: magnetism, photomagnetism and cooperativity.

The syntheses, crystallography and magnetic properties of a series of compounds of formula trans-[Fe(II)(L(1))2(NCX)2] (X = S, Se, BH3 (1-3)), cis-[Fe(II)(L(2))(NCX)2]·CH2Cl2 (X = S, Se, BH3 (4-6)) and trans-[Fe(II)(L(3))(NCX)2]n (X = S, Se (7-8)) are described (L(1) = 6-chloro-N(2),N(2)-diethyl-N(4),N(4)-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, L(2) = 6,6'-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(N(2),N(2)-diethyl-N(4),N(4)-di(pyridin-2-yl)-1,3,5-triazine-2,4-diamine, L(3) = 6,6'-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-diyl)bis(N(2),N(2),N(4),N(4)-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 L(2) 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 (T1/2 = 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 T1/2 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.