Temperature-Dependent in Situ Ligand Cyclization via CC Coupling and Formation of a Spin-Crossover Iron(II) Coordination Polymer

The reaction of N1,N2-bis(pyridin-4-ylmethylene)ethane-1,2-diamine (L) with Fe(NCS)2 under various temperatures gave rise to three iron(II) coordination polymers, namely, one-dimensional [Fe(L′)(NCS)2] (1), two-dimensional [Fe(L)2(NCS)2]·H2O (2), and one-dimensional [Fe(L)2(NCS)2]·2CH2Cl2·4MeOH (3). The formation of 1 involved an in situ CC coupling reaction, L to L′ [L′ = 5,6-di(pyridin-4-yl)-1,2,3,4-tetrahydropyrazine], which was catalyzed by cyanide ions decomposed from thiocyanates; the manganese(II) (1a) and zinc(II) (1b) analogues of 1 were also synthesized for comparison. Magnetic studies showed that complex 1 underwent a pressure-dependent one-step incomplete spin transition whereas complexes 2 and 3 were paramagnetic in the whole temperature range.