Substituent effect of the coordinated pyridine in a series of pyrazolato bridged dinuclear diiron(ii) complexes on the spin-crossover behavior

Abstract

Two series of pyrazolato bridged dinuclear NCS and NCBH3 diiron(II) complexes with various types of 3- or 4-substituted pyridines, [{Fe(NCS or NCBH3)(X-py)}2(mu-bpypz)2], were prepared and their variable-temperature magnetic susceptibilities were measured. There were found linear correlations of the spin-crossover temperatures Tc not only between the NCS and NCBH3 complexes with the corresponding substituted pyridines, but also between the Tc and the Hammett constants, supporting the electronic substituent effect of the coordinated pyridine rather than a steric effect. The ligand field and the interelectronic repulsion parameters together with the thermodynamic data and/or cooperativity factor were discussed in relation with their spin-crossover behavior.

A Unified Treatment of the Relationship Between Ligand Substituents and Spin State in a Family of Iron(II) Complexes

“…That is, more electron‐withdrawing pyridyl X substituents stabilize the low‐spin state of [Fe(bpp X,H ) 2 ] 2+ . This result is consistent with previous studies of complexes with pyridyl donor ligands,14, 15, 16 but it is the opposite trend to the electronegativity plot (Figure 2). An improved correlation is found when T ${}_{1/2}$ is plotted against σ p + , a modified Hammett parameter accounting for conjugation of the ligand substitutents with a positively charged reaction center (Figure 3, bottom) 25.…”

“…A sterically crowded ligand sphere generally leads to high‐spin complexes 12. However, the effect of ligand electronic character on metal‐ion spin state is less clear‐cut, with electron‐withdrawing substituents being reported to stabilize either the low‐spin13, 14, 15, 16 or the high‐spin state17, 18 in different series of compounds. While the literature includes data from solution and the solid‐state, these effects are best quantified by solution measurements which determine a complex's spin state in the absence of crystal‐packing effects or any other influences from a rigid solid lattice 19.…”

“…Electron‐withdrawing substitutents indeed stabilize either the low‐spin13, 14, 15, 16 or the high‐spin state17, 18 of a complex, depending on their position in the molecule and on which types of substituent are considered. The relationship between ligand design and metal‐ion spin state is a fine balance between opposing M−L σ‐ and π‐bonding effects.…”

Modification of Cooperativity and Critical Temperatures on a Hofmann-Like Template Structure by Modular Substituent

“…[18]. These orders of T c are opposite to the expected electronic effect according to the Hammet constants [23]. In a previous related study, Real and co-workers discussed the effect of substituted pyridines for the analogous series of [Fe(X-py) 2 Ag(CN) 2 ] (X = halogen atoms) [11].…”

The synthetic pathways to a family of pyridine-containing azoles - promising ligands for coordination chemistry

“…4 Recently, several research groups published papers on the coordination ability of polydentate ligands based on five-membered azoles as a central linker between two pyridines. 5,6,7,8 In this paper we describe our study on the synthesis and characterization of several groups of expanded bipyridines (Py-X-Py type molecules) based on a five-membered heterocyclic central fragment X. We focused our efforts on pyridine-containing pyrazoles, isoxazoles, imidazoles, oxazoles, thiazoles, oxadiazoles, triazoles, and 1,3,4-triazepines.…”