Guest Programmable Multistep Spin Crossover in a Porous 2‑D Hofmann-Type Material

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e., competing ferro- and antiferro-elastic interactions) drive concerted lattice distortions. To this end, a new SCO framework scaffold, [Fe^II(bztrz)₂(Pd^II(CN)₄)]·<i>n</i>(guest) (bztrz = (<i>E</i>)-1-phenyl-<i>N</i>-(1,2,4-triazol-4-yl)­methanimine, <b>1·</b><i><b>n</b></i><b>(guest)</b>), has been prepared that supports a variety of antagonistic solid state interactions alongside a distinct dual guest pore system. In this 2-D Hofmann-type material we find that inbuilt competition between ferro- and antiferro-elastic interactions provides a SCO behavior that is intrinsically frustrated. This frustration is harnessed by guest exchange to yield a very broad array of spin transition characters in the one framework lattice (one- (<b>1·(H</b>_<b>2</b><b>O,EtOH)</b>), two- (<b>1·3H</b>_<b>2</b><b>O</b>) and three-stepped (<b>1·∼2H</b>_<b>2</b><b>O</b>) transitions and SCO-deactivation (<b>1</b>)). This variety of behaviors illustrates that the degree of elastic frustration can be manipulated by molecular guests, which suggests that the structural features that contribute to multistep switching may be more subtle than previously anticipated

Guest Programmable Multistep Spin Crossover in a Porous 2‑D Hofmann-Type Material

The spin crossover (SCO) phenomenon defines an elegant class of switchable materials that can show cooperative transitions when long-range elastic interactions are present. Such materials can show multistepped transitions, targeted both fundamentally and for expanded data storage applications, when antagonistic interactions (i.e., competing ferro- and antiferro-elastic interactions) drive concerted lattice distortions. To this end, a new SCO framework scaffold, [Fe^II(bztrz)₂(Pd^II(CN)₄)]·<i>n</i>(guest) (bztrz = (<i>E</i>)-1-phenyl-<i>N</i>-(1,2,4-triazol-4-yl)­methanimine, <b>1·</b><i><b>n</b></i><b>(guest)</b>), has been prepared that supports a variety of antagonistic solid state interactions alongside a distinct dual guest pore system. In this 2-D Hofmann-type material we find that inbuilt competition between ferro- and antiferro-elastic interactions provides a SCO behavior that is intrinsically frustrated. This frustration is harnessed by guest exchange to yield a very broad array of spin transition characters in the one framework lattice (one- (<b>1·(H</b>_<b>2</b><b>O,EtOH)</b>), two- (<b>1·3H</b>_<b>2</b><b>O</b>) and three-stepped (<b>1·∼2H</b>_<b>2</b><b>O</b>) transitions and SCO-deactivation (<b>1</b>)). This variety of behaviors illustrates that the degree of elastic frustration can be manipulated by molecular guests, which suggests that the structural features that contribute to multistep switching may be more subtle than previously anticipated

Exploiting Pressure To Induce a “Guest-Blocked” Spin Transition in a Framework Material

A new functionalized 1,2,4-triazole ligand, 4-[(<i>E</i>)-2-(5-methyl-2-thienyl)­vinyl]-1,2,4-triazole (thiome), was prepared to assess the broad applicability of strategically producing multistep spin transitions in two-dimensional Hofmann-type materials of the type [Fe^IIPd­(CN)₄(R-1,2,4-trz)₂]·<i>n</i>H₂O (R-1,2,4-trz = a 4-functionalized 1,2,4-triazole ligand). A variety of structural and magnetic investigations on the resultant framework material [Fe^IIPd­(CN)₄(thiome)₂]·2H₂O (<b>A·2H</b>_<b>2</b><b>O</b>) reveal that a high-spin (HS) to low-spin (LS) transition is inhibited in <b>A·2H</b>_<b>2</b><b>O</b> due to a combination of guest and ligand steric bulk effects. The water molecules can be reversibly removed with retention of the porous host framework and result in the emergence of an abrupt and hysteretic one-step spin transition due to the removal of guest internal pressure. A spin transition can, furthermore, be induced in <b>A·2H</b>_<b>2</b><b>O</b> (0–0.68 GPa) under hydrostatic pressure, as evidenced by variable-pressure structure and magnetic studies, resulting in a two-step spin transition at ambient temperatures at 0.68 GPa. The presence of a two-step spin crossover (SCO) in <b>A·2H</b>_<b>2</b><b>O</b> under hydrostatic pressure compared to a one-step SCO in <b>A</b> at ambient pressure is discussed in terms of the relative ability of each phase to accommodate mixed HS/LS states according to differing lattice flexibilities