Spin-state Switching and Single-molecule Conductance Studies of Iron(II) Complexes Composed of Functional 2,6-bis(pyrazol-1-yl)pyridine-based Ligands

Iron(II)-based spin-crossover (SCO) complexes are promising candidates for several applications—for example, as switching and memory elements in molecule-based information processing architectures. Consequently, the spin-state switching characteristics of such complexes have been studied at the bulk, thin-film, and single-molecule levels. In an effort to link the bulk and single-molecule switching regimes, we have studied three mononuclear iron(II) complexes composed of 2,6-bis(pyrazol-1-yl)pyridine (BPP)-based ligands L1-L3. Complex 1—[Fe(L1)2](BF4)2·CH3CN—underwent bistable spin-state switching with a transition temperature (T1/2) of 253 K and a thermal hysteresis width (ΔT1/2) of 5 K. Complex 2—[Fe(L2)2](BF4)2·CH3CN—showed two-step bistable switching—T1/2 = 258 K (step 1) and 206 K (step 2)—with wide ΔT1/2—40 K (step 1) and 56 K (step 2)—at a scan rate of 0.25 K/min. Remarkably, widening of ΔT1/2 with slow scan rates, below 1 K/min, is noted for 2. In contrast, 3—[Fe(L3)2](BF4)2·2CH3CN—showed lattice-solvent-dependent spin-state switching; loss of lattice solvent resulted in a high spin (HS) phase that underwent gradual and incomplete switching. ... mehrSingle-molecule transport studies of 2 and 3 functionalized with thiomethyl (SMe) anchoring groups indicate charge transport through segments of the molecules attributed to the partially stretched configurations of the complexes in the junctions.