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Electronic control of spin-crossover properties in four-coordinate bis(formazanate) iron(II) complexes

Francesca Milocco, Folkert de Vries, Imke M. A. Bartels, Remco Havenith (UGent) , Jordi Cirera, Serhiy Demeshko, Franc Meyer and Edwin Otten
(2020) JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. 142(47). p.20170-20181
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Abstract
The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (Delta(O)) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T-1/2 above room temperature (300-368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the pi-acceptor properties of the formazanate ligand, resulting in an "inverted" ligand field, with an approximate "two-over-three" splitting of the d-orbitals and a high degree of metal-ligand covalency due to metal -> ligand pi-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C-Ar or N-Ar rings, which is ascribed to the opposing effect on metal-ligand sigma- and pi-bonding.
Keywords
SINGLE-CRYSTAL TRANSFORMATION, 2-STATE REACTIVITY, MOLECULAR MATERIALS, STATE, LIGAND, SERIES, TRANSITIONS, SOLVENT, FAMILY, COUNTERANIONS

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MLA
Milocco, Francesca, et al. “Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(Formazanate) Iron(II) Complexes.” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, vol. 142, no. 47, 2020, pp. 20170–81, doi:10.1021/jacs.0c10010.
APA
Milocco, F., de Vries, F., Bartels, I. M. A., Havenith, R., Cirera, J., Demeshko, S., … Otten, E. (2020). Electronic control of spin-crossover properties in four-coordinate bis(formazanate) iron(II) complexes. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 142(47), 20170–20181. https://doi.org/10.1021/jacs.0c10010
Chicago author-date
Milocco, Francesca, Folkert de Vries, Imke M. A. Bartels, Remco Havenith, Jordi Cirera, Serhiy Demeshko, Franc Meyer, and Edwin Otten. 2020. “Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(Formazanate) Iron(II) Complexes.” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 142 (47): 20170–81. https://doi.org/10.1021/jacs.0c10010.
Chicago author-date (all authors)
Milocco, Francesca, Folkert de Vries, Imke M. A. Bartels, Remco Havenith, Jordi Cirera, Serhiy Demeshko, Franc Meyer, and Edwin Otten. 2020. “Electronic Control of Spin-Crossover Properties in Four-Coordinate Bis(Formazanate) Iron(II) Complexes.” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 142 (47): 20170–20181. doi:10.1021/jacs.0c10010.
Vancouver
1.
Milocco F, de Vries F, Bartels IMA, Havenith R, Cirera J, Demeshko S, et al. Electronic control of spin-crossover properties in four-coordinate bis(formazanate) iron(II) complexes. JOURNAL OF THE AMERICAN CHEMICAL SOCIETY. 2020;142(47):20170–81.
IEEE
[1]
F. Milocco et al., “Electronic control of spin-crossover properties in four-coordinate bis(formazanate) iron(II) complexes,” JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, vol. 142, no. 47, pp. 20170–20181, 2020.
@article{8700258,
  abstract     = {{The transition between spin states in d-block metal complexes has important ramifications for their structure and reactivity, with applications ranging from information storage materials to understanding catalytic activity of metalloenzymes. Tuning the ligand field (Delta(O)) by steric and/or electronic effects has provided spin-crossover compounds for several transition metals in the periodic table, but this has mostly been limited to coordinatively saturated metal centers in octahedral ligand environments. Spin-crossover complexes with low coordination numbers are much rarer. Here we report a series of four-coordinate, (pseudo)tetrahedral Fe(II) complexes with formazanate ligands and demonstrate how electronic substituent effects can be used to modulate the thermally induced transition between S = 0 and S = 2 spin states in solution. All six compounds undergo spin-crossover in solution with T-1/2 above room temperature (300-368 K). While structural analysis by X-ray crystallography shows that the majority of these compounds are low-spin in the solid state (and remain unchanged upon heating), we find that packing effects can override this preference and give rise to either rigorously high-spin (6) or gradual spin-crossover behavior (5) also in the solid state. Density functional theory calculations are used to delineate the empirical trends in solution spin-crossover thermodynamics. In all cases, the stabilization of the low-spin state is due to the pi-acceptor properties of the formazanate ligand, resulting in an "inverted" ligand field, with an approximate "two-over-three" splitting of the d-orbitals and a high degree of metal-ligand covalency due to metal -> ligand pi-backdonation. The computational data indicate that the electronic nature of the para-substituent has a different influence depending on whether it is present at the C-Ar or N-Ar rings, which is ascribed to the opposing effect on metal-ligand sigma- and pi-bonding.}},
  author       = {{Milocco, Francesca and de Vries, Folkert and Bartels, Imke M. A. and Havenith, Remco and Cirera, Jordi and Demeshko, Serhiy and Meyer, Franc and Otten, Edwin}},
  issn         = {{0002-7863}},
  journal      = {{JOURNAL OF THE AMERICAN CHEMICAL SOCIETY}},
  keywords     = {{SINGLE-CRYSTAL TRANSFORMATION,2-STATE REACTIVITY,MOLECULAR MATERIALS,STATE,LIGAND,SERIES,TRANSITIONS,SOLVENT,FAMILY,COUNTERANIONS}},
  language     = {{eng}},
  number       = {{47}},
  pages        = {{20170--20181}},
  title        = {{Electronic control of spin-crossover properties in four-coordinate bis(formazanate) iron(II) complexes}},
  url          = {{http://doi.org/10.1021/jacs.0c10010}},
  volume       = {{142}},
  year         = {{2020}},
}
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