ISOSTRUCTURAL COMPLEXES EXHIBITING DIFFERENT THERMOCHROMIC PHASE TRANSITIONS. Beena Narayanan, Mohan M. Bhadbhade, Central Salt and Marine Chemicals Research Inst., Bhavnagar-364002, India
Thermochromic materials are fascinating because of the visible color change during the transition. Much spectroscopic data are available on thermochromic complexes ML2X2, where M=Cu,Ni, L=N,N - Diethylethylenediamine(Dieten) and X=anion, but only Cu(Dieten)2(ClO4)2, is studied using variable temperature X-ray methods, revealing 'switching on' of dynamic disorder in the chelate ring at the phase transition (Tc~44deg.C). Variations in M and X influence Tc significantly; for example Tc for Cu(Dieten)2(BF4)2 (1) is ~24deg.C whereas for Ni(Dieten)2(ClO4)2 (2) is ~110deg.C. Here, we report structures of 1 and 2 to correlate them with the thermochromic behavior in solid state. Interestingly, 1 and 2 are isostructural with Cu(Dieten)2(ClO4)2. Crystal Data :1 Triclinic P1, a=8.005(1), b=8.619(1), c=9.707(2)Å, V=529.1(4.5)Å3,.[[alpha]]=65.73(1), [[beta]]=66.5(1), [[gamma]]=63.49(1)deg., Z=1; R=0.063 (Rw=0.094). 2:: Triclinic.P1, a=8.098(3), b=8.853(4), c=9.795(5)Å, [[alpha]]=63.73(4), [[beta]]=63.31(5), [[gamma]]=63.54(5)deg., V= 542.1(5.3)Å3, Z=1; R=0.094 (Rw=0.107).The M-ion is essentially square planar with weak axial interactions, Cu...BF4[3.617(3)Å] and Ni...ClO4[3.733(6)Å] as observed in case of Cu(Dieten)2(ClO4)2.
With remarkable structural similarities, temperature responses of these crystals will be explanied in terms of the differences in intermolecular forces.