Towards the Molecular Design of Spin‐Crossover Complexes of 2,6‐Bis(pyrazol‐3‐yl)pyridines

Abstract

The molecular design of spin‐crossover complexes relies on controlling the spin state of a transition metal ion by proper chemical modifications of the ligands. Herein, the first N,N’‐disubstituted 2,6‐bis(pyrazol‐3‐yl)pyridines (3‐bpp) are reported that, against the common wisdom, induce a spin‐crossover in otherwise high‐spin iron(II) complexes by increasing the steric demand of a bulky substituent, an ortho‐functionalized phenyl group. As N,N’‐disubstituted 3‐bpp complexes have no pendant NH groups that make their spin state extremely sensitive to the environment, the proposed ligand design, which may be applicable to isomeric 1‐bpp or other families of popular bi‐, tri‐ and higher denticity ligands, opens the way for their molecular design as spin‐crossover compounds for future breakthrough applications.

Correlations between ligand field Δo, spin crossover T1/2 and redox potential Epa in a family of five dinuclear helicates

“…S41, ESI † ), at room temperature 446 nm ( ε = 5646 L mol −1 cm −1 ) obscured this d–d transition, so this charge transfer transition was monitored instead, as such bands have also been used by others to monitor SCO. 79 …”

Spin-Crossover in Iron(II) Complexes of N,N′-Disubstituted 2,6-Bis(Pyrazol-3-yl)Pyridines: An Effect of a Distal Substituent in the 2,6-Dibromophenyl Group

“…Recognizing them for the 'truly molecular' design [12] of SCO compounds (free from crystal packing [23,24] or substate [25,26] effects) relies on structure-function relations [12][13][14][15][16]18] identified for selected series of metal complexes in their systematic studies by an available solution-state technique such as NMR spectroscopy [12]. The results of these studies agree on a rather [27] general tendency of bulky groups close to the donor nitrogen atoms to stabilize the HS state of the metal ion through the steric demand that makes its LS state more unlikely, with a less straightforward role for other, more remote substituents [12].…”

“…We have proposed [27] a ligand design to obtain the first SCO-active iron(II) (and cobalt(II) [40]) complexes of N,N -disubstituted 3-bpp by introducing bulky ortho-groups into N-phenyl substituents. Following this research, here we report a series of 3-bpp ligands with 2,6-dibromophenyl groups functionalized in its para-position by another bromine atom and a methyl group with different electronic and steric properties (Scheme 1).…”

Manipulating metal spin states for biomimetic, catalytic and molecular materials chemistry

“…85 A low-spin stabilisation in a stereochemically similar complex with distal 2,6-dichlorophenyl substituents may have a similar explanation. 86…”