Neutral and anionic duality of 1,2,4-triazole α-amino acid scaffold in 1D coordination polymers

A tiny supramolecular synthon, 4H-1,2,4-triazol-4-yl acetic acid (HGlytrz) which is bifunctional by design having an electronic asymmetry and conformational flexibility has been introduced to synthesize iron(II) complexes. Having 1,2,4-triazole or carboxylic extremities on the same framework HGlytrz could display dual functionality by acting as a neutral as well as anionic ligand based on the possibility of deprotonation of carboxylic group. Four new iron(II) HGlytrz complexes with ClO 4 - (1), NO 3 - (2), BF 4 - (3) and CF 3SO 3 - (4) anions were prepared. Formulation of their composition which is complicated due to ligand deprotonation is discussed. Unlike its ester protected counterpart ethyl-4H-1,2,4-triazol-4-yl-acetate (αGlytrz) which show hysteretic room temperature spin crossover, 1-4 remain in the high-spin state as revealed by 57Mössbauer spectroscopy. Prospects of such 1D coordination polymers with dangling unbounded carboxylic entities in the realm of self-assembled monolayer (SAM) are discussed. © 2011 Springer Science+Business Media B.V

Coordination preference and magnetic properties of FeII assemblies with a bis-azole bearing 1,2,4-triazole and tetrazole

With a new bis-azole molecular fragment (Htt) bearing 1,2,4-triazole and tetrazole, a mononuclear complex [Fe(tt) 2(H 2O) 4]·2H 2O (1), a trinuclear complex [Fe 3(tt) 6(H 2O) 6]·2H 2O (2) and a 1D coordination polymer [Fe(tt)(Htt) 2]BF 4·2CH 3OH (3) were obtained by varying reaction conditions. Htt acts either as an anionic or neutral ligand depending upon the reaction medium and pH. Thermal variation of spin states of 1-3 were investigated in the range 77-300 K by 57Fe Mössbauer spectroscopy. 1 totally remains in high-spin state over the entire temperature range whereas no spin crossover was evidenced in 2. Nearly 1:1 high-spin and low-spin population ratio is found in 3, which remains constant over the entire temperature range investigated. © 2011 Springer Science+Business Media B.V

Weak cooperativity in selected iron(II) 1D coordination polymers

The spin crossover behaviour of a new class of Fe II coordination polymers [Fe(phtptrz) 3]I 2 (1), [Fe(phtptrz) 3](ReO 4) 2{bullet operator}CH 3OH (2) and [Fe(phtptrz) 3]TaF 7{bullet operator}6H 2O (3) based on a novel ligand 4-(3 ′ -N-phtalimido-propyl)-1,2,4-triazole (phtptrz), were investigated by temperature dependent 57Fe Mössbauer spectroscopy and magnetic susceptibility measurements. The adverse effect of bulky substituent on 1,2,4-triazole, favorable supramolecular interactions and influence of increasing anion size on spin crossover profile is discussed. 1 and 2 show thermally induced spin conversions of gradual and incomplete nature with associated thermochromism, and transition temperatures T 1/2 ~ 163 K and 137 K, respectively. A spin state crossover is also identified for 3. © 2011 Springer Science+Business Media B.V

Fe(II) spin transition including an amino-ester 1,2,4-triazole derivative, operating at, below and above room temperature

[Fe(αEtGlytrz)3](NO3)2 displays a hysteresis loop of 60 K width, centered at room temperature along with a thermochromic effect. Such bistability domain is the widest for Fe(II) chains with 4R-1,2,4-triazole as ligands. Structure−properties relationships are discussed for this new family of spin-crossover chains with the ethyl-4H-1,2,4-triazol-4-yl-acetate (αEtGlytrz) ligand

Impact of ligand spacer and counter-anion in selected 1D iron(II) spin crossover coordination polymers

A new 1D coordination polymer [Fe(βAlatrz) 3](ClO 4) 2 {bullet operator} H 2O (1) with a neutral bidentate ligand, βAlatrz = 4H-1,2,4-triazol-4-yl-propionate, was prepared and its magnetic behavior was investigated by temperature dependent magnetic susceptibility measurements and 57Fe Mössbauer spectroscopy. The temperature dependence of the high-spin molar fraction derived from 57Fe Mössbauer spectroscopy recorded on cooling below room temperature reveals a gradual single step transition with T 1/2 = 173 K between high-spin and low-spin states in agreement with magnetic susceptibility measurements. 1 presents striking reversible thermochromism from white, at room temperature, to pink on quench cooling to liquid nitrogen. The phase transition is of first order as deduced from differential scanning calorimetry, with T 1/2 matching the one determined by both SQUID and 57Fe Mössbauer spectroscopy. A brief assessment has been made among closely related 1D coordination polymers to perceive the effect of ligand spacer length and anion effect on the spin crossover behavior of these new materials. © 2011 Springer Science+Business Media B.V