Substituent dependence on the spin crossover behaviour of mononuclear Fe(ii) complexes with asymmetric tridentate ligands.

Ryo Saiki,Graham N Newton,Takuya Shiga,Reiji Kumai,Haruka Miyamoto,Hajime Sagayama,Hiroki Oshio

doi:10.1039/c9dt00204a

Abstract

Three mononuclear iron(ii) complexes of the formula [FeII(H2L1-3)2](BF4)2·x(solv.) (H2L1-3 = 2-[5-(R-phenyl)-1H-pyrazole-3-yl] 6-benzimidazole pyridine; H2L1: R = 4-methylphenyl, H2L2, R = 2,4,6-trimethylphenyl, H2L3, R = 2,3,4,5,6-pentamethylphenyl) (1, H2L1; 2, H2L2; 3, H2L3) with asymmetric tridentate ligands (H2L1-3) were synthesized and their structures and magnetic behaviour investigated. Significant structural distortions of the dihedral angles between phenyl and pyrazole groups were observed and found to depend on the nature of the substituent groups. Cryomagnetic studies reveal that 1 and 2 show gradual spin crossover behavior, while 3 remains in the high spin state between 1.8 and 300 K.

Highlights

Bistable molecules attract significant interest due to their potential applications as components in molecular electronic and nanoscale devices.1 Spin crossover (SCO) molecules are one such class of bistable material that lend themselves to potential molecular switching applications.2 SCO behaviour can be tuned by modifying ligand field strength, complex nuclearity and intermolecular interactions
Matsumoto et al reported a series of twodimensional SCO complexes, [FeIIH3LMe][FeIILMe]X (H3LMe = tris[2-(((2-methylimidazol-4-yl)methylidene)aminoethyl)amine), X= ClO4, BF4, PF6, AsF6, SbF6-), which show different SCO behaviour depending on the nature of the interlayer elastic interactions mediated by the anions
We recently developed a mononuclear Fe(II) spin crossover system with asymmetric tridentate ligands H2L (H2L = 2-[5-(phenyl)1H-pyrazole-3-yl] 6-benzimidazole pyridine), which shows spin transition around 260 K and found that the electronic state of the system could be effectively tuned by deprotonation of the pyrazole or benzimidazole moieties

Summary

Introduction

Bistable molecules attract significant interest due to their potential applications as components in molecular electronic and nanoscale devices. Spin crossover (SCO) molecules are one such class of bistable material that lend themselves to potential molecular switching applications. SCO behaviour can be tuned by modifying ligand field strength, complex nuclearity and intermolecular interactions. The trimethyl and pentamethyl groups are likely to cause significant steric hindrance of the neighbouring pyrazole moieties, resulting in large dihedral angles This twisting of the molecules affects the intermolecular interactions such as p stacking and hydrogen bonds. Structural analysis of 1’ at 270 K was performed, and confirmed that the iron ion has a high spin state based on coordination bond length (2.173 Å) and S value (140.0o). These data are consistent with partial spin crossover behaviour. Structural analyses after light irradiation were performed at the KEK synchrotron and the high spin spin state of iron(II) ion was confirmed based on average coordination bond lengths (2.127 Å) and S values (129.1o)

Conclusions

Application of Bistable molecules toward electronics and nanodevices

SCO review