HS⇆LS Transition in Iron(II) Two-Dimensional Coordination Networks Containing Tris(tetrazol-1-ylmethyl)methane As Triconnected Building Block

Novel tripodal ligand 1,1′,1″-tris­(tetrazol-1-ylmethyl)­methane (111tz) and products of its reactions with perchlorate as well as with tetrafluoroborate salts of iron­(II) are presented. The isostructural complexes, [Fe­(111tz)2]­(ClO4)2 and [Fe­(111tz)2]­(BF4)2, were isolated as two-dimensional (2D) coordination networks revealing a honeycomb-like pattern with cages occupied by disordered anions. 111tz molecules act as a tridentate ligand bridging three adjacent Fe­(II) ions, and the nitrogen N4 atom of six tetrazole rings (tz) is placed in octahedron vertices of FeN6 chromophores. The complexes, crystallizing in the P3̅ space group, were characterized by variable-temperature single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Variable-temperature magnetic susceptibility measurements show that both systems undergo abrupt and complete spin transition with T1/2↑ = T1/2↓ = 176 K for perchlorate and T1/2↑ = 193.8 and T1/2↓ = 192.8 K for the tetrafluoroborate analogue. Change of spin state in both complexes is accompanied by a thermochromic effect. The HS→LS transition in [Fe­(111tz)2]­(ClO4)2 involves shortening of the Fe–N4 bond lengths from 2.171(2) Å (293 K) to 2.002(1) Å (100 K). In [Fe­(111tz)2]­(BF4)2, lowering of temperature from 293 to 100 K is accompanied by shortening of the Fe–N4 distances from 2.179(2) to 1.987(2) Å, respectively. Perchlorate in [Fe­(111tz)2]­(ClO4)2 or tetrafluoroborate anions in [Fe­(111tz)2]­(BF4)2 are engaged in the formation of intermolecular contacts within as well as with the neighboring 2D layer.