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.