High temperature spin crossover behaviour of mononuclear bis-(thiocyanato)iron(II) complexes with judiciously designed bidentate N-donor Schiff bases with varying substituents.

We present herein a family of molecular cis-[FeII(X-PPMA)2(NCS)2]·H2O [4-X-N-(phenyl(pyridin-2-yl)methylene)aniline; X-PPMA; X = -Cl (1), -Br (2), and -CH3 (3)] complexes that exhibit spin crossover behaviour above room temperature. Judiciously designed bidentate N-donor Schiff bases of 2-benzoylpyridine and para-substituted anilines in combination with Fe(NCS)2 were used for the synthesis of complexes 1-3. The relatively strong ligand field of the Schiff bases stabilises the low spin state of iron(II) up to 300 K which is evident from magnetic measurements, room temperature Mössbauer spectra and crystallographic bond/angle distortion parameters. Interestingly, complexes 1-3 crystallize in a tetragonal system with either a P43212 or P41212 chiral space group from achiral building units due to the supramolecular helical arrangements of molecules through intermolecular (pyridine)C-H⋯C(NCS) interactions in the crystalline state. Complexes 1 and 2 exhibit complete, gradual and slightly irreversible spin crossover behaviour in the temperature range of 300-500 K with equilibrium temperatures (T1/2) 375 K (1) and 380 K (2). The spin state evolution of iron(II) in complexes 1 and 2 is monitored between 150 K and 450 K through variable temperature crystallographic studies in the warming mode. The structural data are in good agreement with the 94% (1) and 87% (2) high spin conversion of iron(II) at 450 K. At a high temperature (450 K), some minor irreversible ligand motion is noticed in complexes 1 and 2, in addition to a complete solvent loss that may induce the slight irreversibility of the spin crossover. On the other hand, complex 3 shows a complete and gradual spin crossover in the temperature range of 10-475 K with strong irreversible features. The equilibrium temperatures obtained upon first warming (T1/2↑) and second cooling (T1/2↓) are 375 K and 200 K, respectively. In complex 3, the loss of a water molecule triggers strong deviations in the spin crossover behaviour. Moreover, dehydrated complex 3 exhibits photoswitching LIESST effect with a relaxation temperature T(LIESST) = 60 K.

Screening of biologically important Zn2+ by a chemosensor with fluorescent turn on-off mechanism.

Reported herein the synthesis, characterization and biologically important zinc ion binding propensity of a weakly fluorescent chemosensor, 4-methyl-2,6-bis((E)-(2-(4-phenylthiazol-2-yl)hydrazono)methyl)phenol (1). 1H NMR spectroscopic titration experiment reveals the binding knack of 1 to the essential Zn2+. The photo-physical studies of 1 exhibit an enhancement in the fluorescence by several folds upon binding with the zinc ions attributed to PET-off process, with a binding constant value of 5.22×103M-1. 1 exhibits an excellent detection range for Zn2+ with lower detection limit value of 2.31×10-8M. The selectivity of 1 was studied with various mono and divalent metal cations and it was observed that most cations either quenches the fluorescence or remains unchanged except for Cd2+, which shows a slight enhancement in fluorescence intensity of 1. The ratiometric displacement of Cd2+ ions by Zn2+ ions shows an excellent selectivity towards in-situ detection of Zn2+ ions. Photo-physical studies also support the reversible binding of 1 to Zn2+ ions having on and off mechanism in presence of EDTA. Such recognition of the biologically important zinc ions finds potential application in live cell imaging.