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Thermal‐ and Photoinduced Spin‐State Switching in an Unprecedented Three‐Dimensional Bimetallic Coordination Polymer
Author(s) -
Niel Virginie,
Thompson Amber L.,
Goeta Andrés E.,
Enachescu Cristian,
Hauser Andreas,
Galet Ana,
Muñoz M. Carmen,
Real José A.
Publication year - 2005
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200400930
Subject(s) - monoclinic crystal system , crystallography , bimetallic strip , enthalpy , octahedron , spin transition , spin states , spin crossover , single crystal , chemistry , materials science , crystal structure , metal , thermodynamics , inorganic chemistry , physics , organic chemistry
The compound {Fe(pmd)[Ag(CN) 2 ][Ag 2 (CN) 3 ]} (pmd=pyrimidine) was synthesized and characterized. Magnetic, calorimetric and single crystal visible spectroscopic studies demonstrate the occurrence of a two‐step high‐spin (HS) ⇄ low‐spin (LS) transition. The critical temperatures are T c1 =185 and T c2 =148 K. Each step involves ∼50 % of the iron centers, with the low‐temperature step showing a hysteresis of 2.5 K. The enthalpy and entropy variations associated with the two steps are Δ H 1 =3.6±0.4 kJ mol −1 and Δ S 1 =19.5±3 J K −1 mol −1 ; Δ H 2 =4.8±0.4 kJ mol −1 and Δ S 2 =33.5±3 J K −1 mol −1 . Photomagnetic and visible spectroscopy experiments show that below 50 K, where the LS state is the thermodynamically stable state, the compound can be switched quantitatively to the HS state using green‐red light (550–650 nm). HS‐to‐LS relaxation experiments in the dark at temperatures between 15 and 55 K show that the relaxation takes place via a two‐step cooperative process, which was analyzed in the context of the mean field theory. The crystal structure has been studied at 290, 220, 170, 90 and 30 K together with 30 K after irradiation. The compound adopts monoclinic symmetry ( P 2 1 / c , Z =16) at all temperatures. There are five [FeN 6 ] pseudo‐octahedral sites linked by pmd, [Ag(CN) 2 ] − and [Ag 2 (CN) 3 ] − bridging ligands to form an unprecedented three‐dimensional (6,6) topology. The structural analysis allows for an understanding of the microscopic mechanism of the two‐step behavior of the thermally induced spin transition as well as the corresponding relaxation of the photoexcited compound based on the individual changes of the five sites. Synergy between metallophilic interactions and the spin transition is also shown by the variation of the Ag⋅⋅⋅Ag distances. Correlations between the variation of the unit‐cell volume and the change of Ag⋅⋅⋅Ag interactions within each step with the asymmetric change of the anomalous heat capacity have also been inferred.