A Two-in-one Pincer Ligand and its Diiron(II) Complex Showing Spin State Switching in Solution through Reversible Ligand Exchange

A novel pyrazolate‐bridged ligand providing two {PNN} pincer‐type compartments has been synthesized. Its diiron(II) complex LFe2(OTf)3(CH3CN) (1; Tf=triflate) features, in solid state, two bridging triflate ligands, with a terminal triflate and a MeCN ligand completing the octahedral coordination spheres of the two high‐spin metal ions. In MeCN solution, 1 is shown to undergo a sequential, reversible, and complete spin transition to the low‐spin state upon cooling. Detailed UV/Vis and 19F NMR spectroscopic studies as well as magnetic measurements have unraveled that spin state switching correlates with a rapid multistep triflate/MeCN ligand exchange equilibrium. The spin transition temperature can be continuously tuned by varying the triflate concentration in solution. A pyrazolate‐bridged ligand with two {PNN} pincer‐type compartments has been developed. Its diiron(II) complex LFe2(OTf)3(NCMe) shows a sequential, reversible, and complete spin state switching between the all‐high‐spin and all‐low‐spin states in solution, which correlates with a rapid multistep triflate/MeCN ligand exchange equilibrium. The spin transition temperature can be continuously tuned by varying the triflate concentration.