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)2­Br2] (1), [Zn­(L)2­Br2] (2), [Fe­(L)4­(NCS)2]2 (3), [Fe2­(μ2-L)3­(L)2­(NCS)4]·CH3OH·CH3CH2­OH (4), [Fe2­(μ2-L)3­(L)2­(NCS)4]·2CH3CH2­OH (5), [Fe2­(μ2-L)3­(L)2­(NCS)4]·2CH3CH2­OH·1.5H2O (6), and [Ni3­(μ2-L)6­(L)4­(H2O)2]­(NO3)6·15.5H2O (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 N1,N2-1,2,4-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.