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

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)}(2)spiro]-(PF6)(2) (1), [{Fe-III(tpa)}(2)Br(4)spiro](PF6)(2) (2), and [{Fe-III(tpa)}(2)thea](PF6)(2) (3) (tpa = tris(2-pyridylmethyl)amine, spiroH(4) = 3,3,3',3'-tetramethyl-1,1'-spirobi(indan)-5,5',6,6'-tet-raol, Br(4)spiroH(4) = 3,3,3',3'-tetram ethyl-1, 1'-spirob i (indan)4,4',7,7'-tetrabromo-5,5',6,6'-tetraol, theaH(4) = 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 Br(4)spiro(4-) increase the ligand field strength relative to spiro(4-), while the stronger ligand field provided by thea4- arises from extension of the overlapping p-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.

Automated summary

Spin crossover complexes with the ability to switch between low spin and high spin states have potential applications in sensing, displays, and molecular electronics. This study focuses on dinuclear spin crossover complexes and investigates the influence of bridging ligands on the spin crossover behavior. The synthesis and characterization of three complexes are reported, and both solid-state and solution measurements are performed to gain insights into the interconversion between different spin states.