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 155 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 T-1/2(up arrow) = T-1/2(down arrow) = 176 K for perchlorate and T-1/2(up arrow) = 193.8 and T-1/2(down arrow) = 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) angstrom (293 K) to 2.002(1) angstrom (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) angstrom, 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.