Syntheses, Structural Variation, and Characterization of a Series of Crystalline Coordination Compounds with 4-Benzene-1,2,4-triazole: Polymorph, Incomplete Spin Transition, and Single Crystal-to-Single Crystal Transformation

Based on the 4-substituted 1,2,4-triazole derivate ligand 4-benzene-1,2,4-triazole (L), a series of crystalline coordination complexes varying from mononuclear to trinuclear species, namely, [Zn(L) 2 Br 2 ] ( 1 ), [Zn(L) 2 Br 2 ] ( 2 ), [Fe(L) 4 (NCS) 2 ] 2 ( 3 ), [Fe 2 (μ 2 -L) 3 (L) 2 (NCS) 4 ]·CH 3 OH·CH 3 CH 2 OH ( 4 ), [Fe 2 (μ 2 -L) 3 (L) 2 (NCS) 4 ]·2CH 3 CH 2 OH ( 5 ), [Fe 2 (μ 2 -L) 3 (L) 2 (NCS) 4 ]·2CH 3 CH 2 OH·1.5H 2 O ( 6 ), and [Ni 3 (μ 2 -L) 6 (L) 4 (H 2 O) 2 ](NO 3 ) 6 ·15.5H 2 O ( 7 ), have been isolated. 1 and 2 present a temperature-induced polymorphic phenomenon of two zinc(II) coordination complexes with L. The solvent effect plays the key role for the self-assembly of these Fe(II) complexes 3 – 6: 3 contains mononuclear Fe(L) 4 (NCS) 2 units without spin-transition behavior, whereas both 4 and 5 present binuclear Fe(II) complexes with three N 1,N 2 -1,2,4-triazole bridges exhibiting incomplete spin-transition behavior. The low-temperature X-ray structural analysis (100 K) of 4 also confirms that one of the Fe(II) centers is located at the low-spin (LS) state and the other Fe(II) center is located at the high-spin (HS) state. Interestingly, when the binuclear Fe(II) complex 5 was exposed in the water atmosphere, solvent-induced single crystal-to-single crystal transformation can be observed, and the binuclear Fe(II) complex 6 exhibiting antiferromagnetic interactions can be isolated. Further, a trinuclear crystalline compound is isolated when Ni(II) salts were used to react with L. Variable-temperature magnetic susceptibility measurement (2–300 K) reveals antiferromagnetic interactions in 7 . The polymorphic phenomenon ( 1 and 2 ), incomplete spin-transition phenomenon ( 4 and 5 ), and single crystal-to-single crystal transformation (from 5 to 6 ) also reveal great potential in the construction of these novel functional materials with L.