Effect of Methoxy Ligand-Substituent and NCE Coligands in Tuning T_1/2 of Coordinatively Elastic Fe(II) Spin Crossover Complexes

Abstract

We report the multistep synthesis of a thiadiazole based ligand (L OMe-Nap ) with a methoxy substituent in the sixposition of the naphthalene ring, along with the corresponding iron(II) [Fe II (L OMe-Nap ) 2 (NCE) 2 ] 1−3 (E = S, Se, and BH 3 ) complexes. We also report the synthesis of the [Fe II (L Nap ) 2 (NCBH 3 ) 2 ] (4) complex with the previously reported L Nap ligand. The complexes obtained were characterized by single crystal X-ray crystallography and SQUID magnetometry in all four cases (1−4). Complex 1 shows a weak thermally induced excited spin state trapping (TIESST) effect up to about 60 K. The effects of the methoxy ligand substituent in altering the noncovalent intramolecular π−π interaction are discussed, as well as the tuning of the switching temperature (T 1/2 ) by the choice of NCE. The S < Se < BH 3 series is established in both families of compounds.

Synergetic spin singlet–quintet switching and luminescence in mononuclear Fe(ii) 1,3,4-oxadiazole tetradentate chelates with NCBH3 co-ligand

“…The magnetic behavior changes associated with the lattice solvent removal are well established in the SCO literature. 26,30,32…”

“…In the first step, 2-naphthoic acid was converted to methyl naphthalene-2-carboxylate (I) by Fischer esterification in high yields. 26,29 2-Naphthohydrazide (II) was synthesized in a subsequent nucleophilic reaction of methyl naphthalene-2carboxylate with hydrazine monohydrate in moderate yields. 26,29 The following condensation reaction with chloroacetyl chloride yielded N′-(2-chloroacetyl) naphthalene-2-carbohydrazide (III) in high yields.…”

“…6,8,19 For making the spin-switchable molecules useful for applications, one of the main hurdles faced by materials chemists is designing complexes with an appropriate ligand field that enables the spin switch close to ambient accessible temperature. In the process of tuning the ligand field strength through ligand design, three NCE co-ligands, where E = BH 3 , Se and S, are commonly studied to tune the ligand field strength and are more commonly used in the solid state [23][24][25][26] and also used rarely in solution. 27,28 Tuning of the T 1/2 in the solid state is often governed by anions, lattice solvent effects and packing effects due to π-π interactions and hydrogen bonding interactions (also mediated by anions or lattice solvents) which makes it hard to enable predictable tuning.…”

“…[29][30][31][32][33] These effects can be both advantageous (increase in cooperativity, thus more abrupt spin transition with hysteresis) and disadvantageous (SCO is dependent on these interactions and the complex is locked in one spin state) to the SCO phenomenon in the solid state. 26,30,34,35 In solution, packing effects are absent and the ligand field has the main influence, and some good linear correlations with different parameters such as the Hammett constants, 36 solvent polarity index (P′), 37,38 alkyl chain lengths, 37 choice of NCEs, and coordination ability parameter (a TM ) are achieved. 28 The main drawback with the solution spin crossover studies is the speciation.…”

“…7,39,43 Mononuclear complexes based on 1,3,4-thiadiazole with a luminophore such as naphthalene in the ligand design have been recently reported by some of us. 26,29 The extended aromatic ring also facilitates the cooperativity between the metal centres to induce an abrupt spin transition with a wide hysteresis up to 58 K. 29 Ligand systems based on other bis-bidentate ligands such as triazole are also commonly studied for SCO investigation in Fe(II), 19,26,32,44,45 whereas Fe(II) complexes based on 1,3,4-oxadiazole are rarely reported in the literature. Some of us reported the first dinuclear Fe(II) 1,3,4-oxadiazole complex with a wide hysteresis of 26 K. 33 The role of counterions in tuning the spin transition in the solid state was also investigated.…”