Local Coordination and Electronic Structure Ramifications of Guest-Dependent Spin Crossover in a Metal–Organic Framework: A Combined X-ray Absorption and Emission Spectroscopy Study

Abstract

The porous Hoffman-type 3D lattice Fe­(pz)­[NiII(CN)4] exhibits thermally induced spin-crossover (SCO) behavior that is dependent on the solvent guest species occupying the pores. Here, in situ Fe K-edge X-ray absorption spectroscopy (XAS) and both non-resonant and resonant Kβ X-ray emission spectroscopy (XES) methods are used to probe this framework under two solvent environments that yield different extremes of spin crossover temperature: acetonitrile and toluene. While the acetonitrile pore environment engenders an SCO response around room temperature, toluene guests stabilize the high spin state and effectively suppress SCO behavior throughout the ambient temperature range. The multipronged X-ray spectroscopy approach simultaneously confirmed this spin crossover behavior and provided new local coordination and electronic structural insights of the framework under these two solvent environments. Extended X-ray absorption fine structure analysis revealed spin state and solvent guest-dependent differences in coordination bond lengths and structural disorder. Resonant XES measurements produced high-resolution XAS spectra with distinct pre-edge and edge features, whose assignment was established using both simple ligand field theory and time-dependent density-functional theory calculations and further supported by their observed resonance behavior in the 2D RXES plane. Edge feature variation with the Fe spin state was interpreted to reveal changes in specific metal-linker bond covalency.

X-ray Emission Spectroscopy of Single Protein Crystals Yields Insights into Heme Enzyme Intermediates

“…Upon addition of substrate, the spin state equilibrium shifts toward greater LS character, theorized to correspond to hydroxide-bound heme that is hydrogen-bonded to the pyrrolic nitrogen of l -Trp (Figure C) . This ligand-dependent spin crossover is supported further by previous XES studies on metal–organic frameworks . Papadopoulou et al estimated a ligand distribution of approximately 33/7/60 (X/water/hydroxide) in the presence of l -Trp at pH 8, where X represents an active site residue, such as Ala264 .…”

“…44 This ligand-dependent spin crossover is supported further by previous XES studies on metal−organic frameworks. 45 Papadopoulou et al estimated a ligand distribution of approximately 33/7/60 (X/water/hydroxide) in the presence of L-Trp at pH 8, where X represents an active site residue, such as Ala264. 42 Persistence of alanine coordination, and the associated steric constraints on the distal face of the heme cofactor, may help to explain ferric IDO's relatively low affinity for L-Trp.…”

X-ray Emission Spectroscopy of Single Protein Crystals Yields Insights into Heme Enzyme Intermediates

“…44 This ligand-dependent spin crossover is supported further by previous XES studies on metal-organic frameworks. 45 Papadopoulou et al estimated a ligand distribution of approximately 33/7/60 (X/water/hydroxide) in the presence of L-Trp at pH 8, where X represents an active site residue, such as Ala264. 41 Persistence of alanine coordination, and the associated steric constraints on the distal face of the heme cofactor, may help to explain ferric IDO's relatively low affinity for L-Trp.…”

Guest-Induced Reversible Single-Crystal-to-Single-Crystal Transformation Involving Displacement of 2D Layers and Spin Crossover Behavior Change in a Hofmann-Type Coordination Polymer

“…Over the last two decades, Hofmann-like Fe II SCO CPs based on anionic metallo-ligands ([M II (CN) 4 2– ], M II = Ni, Pd, Pt) have gained attention for their well-defined crystalline structures, flexible porous architectures, and guest-dependent SCO behavior, which provide a particularly suitable platform for directing SCO behavior by exploring host–guest interactions. The guest-dependent SCO behavior is clearly demonstrated in the three-dimensional (3D) Hofmann-type framework material [Fe II (pyrazine)M II (CN) 4 · n (guest)], in which SCO properties are dramatically influenced by the guest molecules, such as water, methanol, ethanol, acetone, acetonitrile, toluene . Interestingly, a guest-dependent memory effect was observed in this system .…”