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1. Modular Design of Multifunctionality in TCNQ-Based Fe(II) Spin Crossover Complexes

   Üngör, Ö.; Choi, E. S.; Shatruk, M. (April 2021, National Meeting of the Anerican Chemical Society)

   null (Ed.)

   Fe(II) coordination complexes with ligands of an intermediate field strength often show witching between the high-spin (HS) and low-spin (LS) electronic configurations, known as spin crossover (SCO). This spin-state conversion is achieved by changes in temperature, pressure, or photoexcitation, which make SCO complexes promising materials for various applications that rely on bistable systems. Multifunctional materials that exhibit both spin-state switching and conductivity can be created by combining Fe(II) SCO complexes with organic TCNQ-type electron acceptors. In such complexes, TCNQ●d– radical anions are typically arranged in layers of one-dimensional stacks that provide conducting pathways (Fig. 1). The stacking distance can be affected by structural changes induced by the alteration in the electronic configuration and, thus, bond lengths at the Fe(II) center, resulting in synergy between SCO and conductivity. The synthesis of such materials can be approached in two ways: (1) by coordinating TCNQ●d– ligands directly to the Fe(II) center, which is partially protected by blocking ligands that limit the growth of extended structures or (2) by co-crystallizing completely blocked Fe(II) centers with free TCNQ●d– radicals. We will discuss several examples of the second approach, in which homoleptic Fe(II) cationic SCO complexes with tridentate 2,6-bispyrazolyl-pyridine (bpp) type ligands have been co-crystallized with fractionally-charged TCNQ●d– radical anions. The temperature- and solvent-dependent magnetic behavior and transport properties of these materials will be discussed. We will also present new pathways to improve the design of such molecule-based conductors with spin-state switching properties. To the best of out knowledge, we report the first examples of Fe(II) based conducting molecular materials with abrupt temperature-driven spin transitions. 

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2. Iron(II) Complexes Featuring a Redox‐Active Dihydrazonopyrrole Ligand

   https://doi.org/10.1002/zaac.202100097  

   Jesse, Kate A.; Chang, Mu‐Chieh; Filatov, Alexander S.; Anderson, John S. (June 2021, Zeitschrift für anorganische und allgemeine Chemie)

   Abstract Nature uses control of the secondary coordination sphere to facilitate an astounding variety of transformations. Similarly, synthetic chemists have found metal‐ligand cooperativity to be a powerful strategy for designing complexes that can mediate challenging reactivity. In particular, this strategy has been used to facilitate two electron reactions with first row transition metals that more typically engage in one electron redox processes. While NNN pincer ligands feature prominently in this area, examples which can potentially engage in both proton and electron transfer are less common. Dihydrazonopyrrole (DHP) ligands have been isolated in a variety of redox and protonation states when complexed to Ni. However, the redox‐state of this ligand scaffold is less obvious when complexed to metal centers with more accessible redox couples. Here, we synthesize a new series of Fe‐DHP complexes in two distinct oxidation states. Detailed characterization supports that the redox‐chemistry in this set is still primarily ligand based. Finally, these complexes exist as 5‐coordinate species with an open coordination site offering the possibility of enhanced reactivity. 

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3. Multicenter interactions and ligand field effects in platinum( ii ) tripyrrindione radicals

   https://doi.org/10.1039/D3DT00894K  

   Tomat, Elisa; Curtis, Clayton J.; Astashkin, Andrei V.; Conradie, Jeanet; Ghosh, Abhik (May 2023, Dalton Transactions)

   The tripyrrin-1,14-dione biopyrrin, which shares the scaffold of several naturally occurring heme metabolites, is a redox-active platform for metal coordination. We report the synthesis of square planar platinum( ii ) tripyrrindiones, in which the biopyrrin binds as a tridentate radical and the fourth coordination position is occupied by either aqua or tert -butyl isocyanide ligands. These complexes are stable through chromatographic purification and exposure to air. Electron paramagnetic resonance (EPR) data and density functional theory (DFT) analysis confirm that the spin density is located predominantly on the tripyrrindione ligand. Pancake bonding in solution between the Pt( ii ) tripyrrindione radicals leads to the formation of diamagnetic π dimers at low temperatures. The identity of the monodentate ligand ( i.e. , aqua vs . isocyanide) affects both the thermodynamic parameters of dimerization and the tripyrrindione-based redox processes in these complexes. Isolation and structural characterization of the oxidized complexes revealed stacking of the diamagnetic tripyrrindiones in the solid state as well as a metallophilic Pt( ii )−Pt( ii ) contact in the case of the aqua complex. Overall, the properties of Pt( ii ) tripyrrindiones, including redox potentials and intermolecular interactions in solution and in the solid state, are modulated through easily accessible changes in the redox state of the biopyrrin ligand or the nature of the monodentate ligand. 

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4. Bimetallic, Silylene‐Mediated Multielectron Reductions of Carbon Dioxide and Ethylene

   https://doi.org/10.1002/anie.202011489  

   Whited, Matthew T.; Zhang, Jia; Conley, Anna M.; Ma, Senjie; Janzen, Daron E.; Kohen, Daniela (November 2020, Angewandte Chemie International Edition)

   Abstract A metal/ligand cooperative approach to the reduction of small molecules by metal silylene complexes (R₂Si=M) is demonstrated, whereby silicon activates the incoming substrate and mediates net two‐electron transformations by one‐electron redox processes at two metal centers. An appropriately tuned cationic pincer cobalt(I) complex, featuring a central silylene donor, reacts with CO₂to afford a bimetallic siloxane, featuring two Co^IIcenters, with liberation of CO; reaction of the silylene complex with ethylene yields a similar bimetallic product with an ethylene bridge. Experimental and computational studies suggest a plausible mechanism proceeding by [2+2] cycloaddition to the silylene complex, which is quite sensitive to the steric environment. The Co^II/Co^IIproducts are reactive to oxidation and reduction. Taken together, these findings demonstrate a strategy for metal/ligand cooperative small‐molecule activation that is well‐suited to 3dmetals. 

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5. Dynamics of Spin Crossover Molecular Complexes

   https://doi.org/10.3390/nano12101742  

   Ekanayaka, Thilini K.; Maity, Krishna Prasad; Doudin, Bernard; Dowben, Peter A. (May 2022, Nanomaterials)

   We review the current understanding of the time scale and mechanisms associated with the change in spin state in transition metal-based spin crossover (SCO) molecular complexes. Most time resolved experiments, performed by optical techniques, rely on the intrinsic light-induced switching properties of this class of materials. The optically driven spin state transition can be mediated by a rich interplay of complexities including intermediate states in the spin state transition process, as well as intermolecular interactions, temperature, and strain. We emphasize here that the size reduction down to the nanoscale is essential for designing SCO systems that switch quickly as well as possibly retaining the memory of the light-driven state. We argue that SCO nano-sized systems are the key to device applications where the “write” speed is an important criterion. 

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