The First Conducting Spin‐Crossover Compound Combining a Mn^III Cation Complex with Electroactive TCNQ Demonstrating an Abrupt Spin Transition with a Hysteresis of 50 K

Abstract

We present herein the synthesis, crystal structure, and electric and magnetic properties of the spin‐crossover salt [Mn(5‐Cl‐sal‐N‐1,5,8,12)]TCNQ1.5⋅2 CH3CN (I), where 5‐Cl‐sal‐N‐1,5,8,12=N,N′‐bis(3‐(2‐oxy‐5‐chlorobenzylideneamino)propyl)‐ethylenediamine, containing distinct conductive and magnetic blocks along with acetonitrile solvent molecules. The MnIII complex with a Schiff‐base ligand, [Mn(5‐Cl‐sal‐N‐1,5,8,12)]+, acts as the magnetic unit, and the π‐electron acceptor 7,7,8,8‐tetracyanoquinodimethane (TCNQ−) is the conducting unit. The title compound (I) exhibits semiconducting behavior with room temperature conductivity σRT≈1×10−4 ohm−1 cm−1 and activation energy Δ ≈0.20 eV. In the temperature range 73–123 K, it experiences a hysteretic phase transition accompanied by a crossover between the low‐spin S=1 and high‐spin S=2 states of MnIII and changes in bond lengths within the MnN4O2 octahedra. The pronounced shrinkage of the basal Mn−N bonds in I at the spin crossover suggests that the dx2-y2 orbital is occupied/deoccupied in this transition. Interestingly, the bromo isomorphic counterpart [Mn(5‐Br‐sal‐N‐1,5,8,12)]TCNQ1.5⋅2 CH3CN (II) of the title compound evidences no spin‐crossover phenomena and remains in the high‐spin state in the temperature range 2–300 K. Comparison of the chloro and bromo compounds allows the thermal and spin‐crossover contributions to the overall variation in bond lengths to be distinguished. The difference in magnetic behavior of these two salts has been ascribed to intermolecular supramolecular effects on the spin transition. Discrete hydrogen bonding exists between cations and cations and anions in both compounds. However, the hydrogen bonding in the crystals of II is much stronger than in I. The relatively close packing arrangement of the [Mn(5‐Br‐sal‐N‐1,5,8,12)]+ cations probably precludes their spin transformation.

Simultaneous Spin‐Crossover Transition and Conductivity Switching in a Dinuclear Iron(II) Coordination Compound Based on 7,7′,8,8′‐Tetracyano‐p‐quinodimethane

“…At 300 K, the σ value is 0.77 S cm −1 , which decreases exponentially with decreasing temperature, indicating thermally activated semiconducting behavior between 50 and 330 K. The activation energy ( E a ) is extracted from the slope of the Arrhenius‐type plot shown in the inset of Figure . The E a value of 57 meV is the lowest yet reported for a semiconducting SCO material . In contrast, above 330 K, the σ turns down sharply in conjunction with the thermal spin transition, and further decreases to reach 0.074 S cm −1 at 380 K. Therefore, the σ behavior strongly correlates with the spin state.…”

“…The E a value of 57 meV is the lowest yet reported for as emiconducting SCO material. [39][40][41][42][43][44][45][46][47][48][49][50][51][52] In contrast, above 330 K, the s turns down sharplyi nc onjunction with the thermal spin transition, and further decreases to reach 0.074 Scm À1 at 380 K. Therefore, the s behaviors trongly correlates with the spin state. On cooling the sample from 380 to 50 K, the s values do not coincide with the trend of the first heating process ( Figure S5, Supporting Information).…”

“…Indeed, such temperature dependence of dramatic lattice changes associated with p-p stacking distances are observed for many conducting compounds with 1D p-stacked TCNQ columns, whichc an be interpretedi nt erms of Peierls distortion. [46][47][48][49][50][51][52]73] At the same time, the [Fe 2 (bpypz) 2 ] 2 + units bridged by TCNQ Bm olecules also cause large lattice changes associated with markedlya ltered coordination bond lengths aroundt he Fe II centers upon SCO. [55][56][57][58][59][60][61][62][63][64][65] Therefore, the selfgrinding effect is likely ar esult of relatively large changes in the lattice caused by thermalS CO andP eierls transitions (see above).…”

“…[1][2][3][4][5][6][7][8][9][10][11] To date, hybrid materials of co-crystallized SCO complexes and molecular conductors have been reported; these were usually designed in Coulombs ets combining ac ationic SCO complex anda na nionic molecular conductor,n amely,i onic crystalline materials. [39][40][41][42][43][44][45][46][47][48][49][50][51][52] Amongt hese SCO conductors, the most common combinations are between cationic Fe III SCO complexes and anionic molecular conductors. However, the combination of Fe II SCO complexes and molecular conductors is still rare.…”

“…To date, hybrid materials of co‐crystallized SCO complexes and molecular conductors have been reported; these were usually designed in Coulomb sets combining a cationic SCO complex and an anionic molecular conductor, namely, ionic crystalline materials . Among these SCO conductors, the most common combinations are between cationic Fe III SCO complexes and anionic molecular conductors.…”