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Interplay between spin crossover and proton migration along short strong hydrogen bonds

Jornet-Mollá, Verónica; Giménez-Saiz, Carlos; Cañadillas-Delgado, Laura; Yufit, Dmitry S.; Howard, Judith A.K.; Romero, Francisco M.

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Authors

Verónica Jornet-Mollá

Carlos Giménez-Saiz

Laura Cañadillas-Delgado

Francisco M. Romero



Abstract

The iron(II) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can also be detected in the mixed-spin phase. 1 undergoes a light-induced excited-state spin trapping (LIESST) of the 100% HS phase, with breaking of the O4W⋯O5W HB and the onset of proton static disorder in the SSHB, indicating the presence of a light-induced activation energy barrier for proton motion. This excited state shows a stepped relaxation at T1(LIESST) = 68 K and T2(LIESST) = 76 K. Photocrystallography measurements after the first relaxation step reveal a single Fe site with an intermediate geometry, resulting from the random distribution of the HS and LS sites throughout the lattice.

Citation

Jornet-Mollá, V., Giménez-Saiz, C., Cañadillas-Delgado, L., Yufit, D. S., Howard, J. A., & Romero, F. M. (2021). Interplay between spin crossover and proton migration along short strong hydrogen bonds. Chemical Science, 12(3), 1038-1053. https://doi.org/10.1039/d0sc04918b

Journal Article Type Article
Acceptance Date Nov 16, 2020
Online Publication Date Nov 16, 2020
Publication Date Jan 21, 2021
Deposit Date Jun 15, 2021
Publicly Available Date Jun 15, 2021
Journal Chemical Science
Print ISSN 2041-6520
Electronic ISSN 2041-6539
Publisher Royal Society of Chemistry
Peer Reviewed Peer Reviewed
Volume 12
Issue 3
Pages 1038-1053
DOI https://doi.org/10.1039/d0sc04918b

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