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)]TCNQ(1.5).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 Mn-III complex with a Schiff-base ligand, [Mn(5-Cl-sal-N-1,5,8,12)](+), acts as the magnetic unit, and the pi-electron acceptor 7,7,8,8-tetracyanoquinodimethane (TCNQ(-)) is the conducting unit. The title compound (I) exhibits semiconducting behavior with room temperature conductivity sigma(RT)approximate to 1x10(-4) ohm(-1) cm(-1) and activation energy Delta approximate to 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 Mn-III 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)]TCNQ(1.5).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.