Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

Jett T Janetzki; Maxim G Chegerev; Gemma K Gransbury; Robert W Gable; Jack K Clegg; Roger J Mulder; Guy N L Jameson; Alyona A Starikova; Colette Boskovic

doi:10.1021/acs.inorgchem.3c02598

Controlling Spin Crossover in a Family of Dinuclear Fe(III) Complexes via the Bis(catecholate) Bridging Ligand

Inorg Chem. 2023 Sep 25;62(38):15719-15735. doi: 10.1021/acs.inorgchem.3c02598. Epub 2023 Sep 10.

Authors

Jett T Janetzki¹, Maxim G Chegerev², Gemma K Gransbury³, Robert W Gable¹, Jack K Clegg⁴, Roger J Mulder⁵, Guy N L Jameson¹, Alyona A Starikova², Colette Boskovic¹

Affiliations

¹ School of Chemistry, University of Melbourne, Victoria 3010, Australia.
² Institute of Physical and Organic Chemistry, Southern Federal University, Rostov-on-Don 344090, Russian Federation.
³ Department of Chemistry, The University of Manchester, Manchester M13 9PL, U.K.
⁴ University of Queensland, St Lucia, Queensland 4072, Australia.
⁵ CSIRO Manufacturing, Clayton, Victoria 3168, Australia.

PMID: 37691232
DOI: 10.1021/acs.inorgchem.3c02598

Abstract

Spin crossover (SCO) complexes can reversibly switch between low spin (LS) and high spin (HS) states, affording possible applications in sensing, displays, and molecular electronics. Dinuclear SCO complexes with access to [LS-LS], [LS-HS], and [HS-HS] states may offer increased levels of functionality. The nature of the SCO interconversion in dinuclear complexes is influenced by the local electronic environment. We report the synthesis and characterization of [{Fe^III(tpa)}₂spiro](PF₆)₂ (1), [{Fe^III(tpa)}₂Br₄spiro](PF₆)₂ (2), and [{Fe^III(tpa)}₂thea](PF₆)₂ (3) (tpa = tris(2-pyridylmethyl)amine, spiroH₄ = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tetraol, Br₄spiroH₄ = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-4,4',7,7'-tetrabromo-5,5',6,6'-tetraol, theaH₄ = 2,3,6,7-tetrahydroxy-9,10-dimethyl-9,10-dihydro-9,10-ethanoanthracene), utilizing non-conjugated bis(catecholate) bridging ligands. In the solid state, magnetic and structural analysis shows that 1 remains in the [HS-HS] state, while 2 and 3 undergo a partial SCO interconversion upon cooling from room temperature involving the mixed [LS-HS] state. In solution, all complexes undergo SCO from [HS-HS] at room temperature, via [LS-HS] to mixtures including [LS-LS] at 77 K, with the extent of SCO increasing in the order 1 < 2 < 3. Gas phase density functional theory calculations suggest a [LS-LS] ground state for all complexes, with the [LS-HS] and [HS-HS] states successively destabilized. The relative energy separations indicate that ligand field strength increases following spiro^4- < Br₄spiro^4- < thea^4-, consistent with solid-state magnetic and EPR behavior. All three complexes show stabilization of the [LS-HS] state in relation to the midpoint energy between [LS-LS] and [HS-HS]. The relative stability of the [LS-HS] state increases with increasing ligand field strength of the bis(catecholate) bridging ligand in the order 1 < 2 < 3. The bromo substituents of Br₄spiro^4- increase the ligand field strength relative to spiro^4-, while the stronger ligand field provided by thea^4- arises from extension of the overlapping π-orbital system across the two catecholate units. This study highlights how SCO behavior in dinuclear complexes can be modulated by the bridging ligand, providing useful insights for the design of molecules that can be interconverted between more than two states.