Synthesis of Anion-Mixed Crystals of Assembled Complexes Bridged by 1,2-Bis(4-pyridyl)ethane and the Ligand Field of Fe(NCS)(NCBH3) Unit

Anion-mixed iron(II) assembled complexes using NCS and NCBH3 as anionic ligand and bridged by 1,2-bis(4-pyridyl)ethane (bpe) are synthesized under several conditions for various ratios of NCS to NCBH3. Elemental analysis showed that the experimentally observed fractions of NCS tend to be more than those in the preparation. Some structures are determined by using single crystal X-ray structural analysis. Powder X-ray diffraction patterns are compared among the anion-mixed crystals. 57Fe Mössbauer spectroscopy reveals that a new doublet appears and it is different from the doublets of Fe(NCS)2 and Fe(NCBH3)2 units and the relative area of this new doublet reaches maximum when the fraction of NCS is around 1.0 per each iron, suggesting that the new doublet is ascribed to an Fe(NCS)(NCBH3) unit. Corresponding anion-mixed complexes enclathrating p-dichlorobenzene (p-DCB) are synthesized. Superconducting quantum interference device (SQUID) measurement and Mössbauer spectroscopy reveal that the complexes show spin-crossover (SCO) phenomenon and the relative area of the low-spin state is more than the expected value. The existence of Fe(NCS)(NCBH3) is supported by the results of SQUID measurement and density functional theory (DFT) calculation. Ligand field splitting strength of Fe(NCS)(NCBH3) unit is estimated to be intermediate between Fe(NCS)2 and Fe(NCBH3)2 units. Complexes enclathrating p-DCB display propeller conformation of bpe around the iron atom, while complexes without a guest molecule have distorted propeller conformation of bpe around the iron atom. It is suggested that such difference determines whether the SCO is on or off.