Co-crystallization of the prominent Fe( ii ) spin-crossover (SCO) cation, Fe(3-bpp) 2 2+ (3-bpp = 2,6-bis(pyrazol-3-yl)pyridine), with a fractionally charged TCNQ δ − radical anion has afforded a ...hybrid complex Fe(3-bpp) 2 (TCNQ) 3 ·5MeCN (1·5MeCN, where δ = −0.67). The partially desolvated material shows semiconducting behavior, with the room temperature conductivity σ RT = 3.1 × 10 −3 S cm −1 , and weak modulation of conducting properties in the region of the spin transition. The complete desolvation, however, results in the loss of hysteretic behavior and a very gradual SCO that spans the temperature range of 200 K. A related complex with integer-charged TCNQ − anions, Fe(3-bpp) 2 (TCNQ) 2 ·3MeCN (2·3MeCN), readily loses the interstitial solvent to afford desolvated complex 2 that undergoes an abrupt and hysteretic spin transition centered at 106 K, with an 11 K thermal hysteresis. Complex 2 also exhibits a temperature-induced excited spin-state trapping (TIESST) effect, upon which a metastable high-spin state is trapped by flash-cooling from room temperature to 10 K. Heating above 85 K restores the ground-state low-spin configuration. An approach to improve the structural stability of such complexes is demonstrated by using a related ligand 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) to obtain Fe(bzimpy) 2 (TCNQ) 6 ·2Me 2 CO (4) and Fe(bzimpy) 2 (TCNQ) 5 ·5MeCN (5), both of which exist as LS complexes up to 400 K and exhibit semiconducting behavior, with σ RT = 9.1 × 10 −2 S cm −1 and 1.8 × 10 −3 S cm −1 , respectively.
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This work introduces an approach to uncoupling electrons via maximum utilization of localized aromatic units, i.e., the Clar’s π-sextets. To illustrate the utility of this concept to the design of ...Kekulé diradicaloids, we have synthesized a tridecacyclic polyaromatic system where a gain of five Clar’s sextets in the open-shell form overcomes electron pairing and leads to the emergence of a high degree of diradical character. According to unrestricted symmetry-broken UCAM-B3LYP calculations, the singlet diradical character in this core system is characterized by the y 0 value of 0.98 (y 0 = 0 for a closed-shell molecule, y 0 = 1 for pure diradical). The efficiency of the new design strategy was evaluated by comparing the Kekulé system with an isomeric non-Kekulé diradical of identical size, i.e., a system where the radical centers cannot couple via resonance. The calculated singlet–triplet gap, i.e., the ΔE ST values, in both of these systems approaches zero: −0.3 kcal/mol for the Kekulé and +0.2 kcal/mol for the non-Kekulé diradicaloids. The target isomeric Kekulé and non-Kekulé systems were assembled using a sequence of radical periannulations, cross-coupling, and C–H activation. The diradicals are kinetically stabilized by six tert-butyl substituents and (triisopropylsilyl)acetylene groups. Both molecules are NMR-inactive but electron paramagnetic resonance (EPR)-active at room temperature. Cyclic voltammetry revealed quasi-reversible oxidation and reduction processes, consistent with the presence of two nearly degenerate partially occupied molecular orbitals. The experimentally measured ΔE ST value of −0.14 kcal/mol confirms that K is, indeed, a nearly perfect singlet diradical.
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Incorporation of a five‐membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl ...unit, switches on conjugation, leads to a >200 nm red‐shift in the absorbance spectrum and injects a charge into a helical conjugated π‐system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co‐crystallized with {K+(18‐crown‐6)(THF)} and {Cs+2(18‐crown‐6)3}. UV/Vis‐monitoring of these systems shows a time‐dependent formation of mono‐ and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion.
Flat as a pancake: Relief of helicene strain, avoidance of antiaromaticity, and increase in charge delocalization assist in dehydrogenative ring closures that convert an anionic fused bis‐5helicene into a planarized decacyclic dianion where nine hexagonal units are fused to a single pentagon.
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A mononuclear complex Fe(tBu2qsal)2 has been obtained by a reaction between an Fe(II) precursor salt and a tridentate ligand 2,4-di(tert-butyl)-6-((quinoline-8-ylimino)methyl)phenol (tBu2qsalH) ...in the presence of triethylamine. The complex exhibits a hysteretic spin transition at 117 K upon cooling and 129 K upon warming, as well as light-induced excited spin-state trapping at lower temperatures. Although the strongly cooperative spin transition suggests substantial intermolecular interactions, the complex is readily sublimable, as evidenced by the growth of its single crystals by sublimation at 573 → 373 K and ∼10–3 mbar. This seemingly antagonistic behavior is explained by the asymmetric coordination environment, in which the tBu substituents and quinoline moieties appear on opposite sides of the complex. As a result, the structure is partitioned in well-defined layers separated by van der Waals interactions between the tBu groups, while the efficient cooperative interactions within the layer are provided by the quinoline-based moieties. The abrupt spin transition is preserved in a 20 nm thin film prepared by sublimation, as evidenced by abrupt and hysteretic changes in the dielectric properties in the temperature range comparable to the one around which the spin transition is observed for the bulk material. The changes in the dielectric response are in excellent agreement with differences in the dielectric tensor of the low-spin and high-spin crystal structures evaluated by density functional theory calculations. The substantially higher volatility of Fe(tBu2qsal)2, as compared to a similar complex without tBu substituents, suggests that asymmetric molecular shapes offer an efficient design strategy to achieve sublimable complexes with strongly cooperative spin transitions.
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5.
Quantum Mimicry With Inorganic Chemistry Campanella, Anthony J.; Üngör, Ökten; Zadrozny, Joseph M.
Comments on modern chemistry. Part A, Comments on inorganic chemistry,
2024, Volume:
44, Issue:
1
Journal Article
Peer reviewed
Quantum objects, such as atoms, spins, and subatomic particles, haveunique physical properties that could be useful for many different applications, ranging from quantum information processing to ...magnetic resonance imaging. Molecular species also exhibit these quantum properties, and, importantly, these properties are fundamentally tunable by synthetic design, unlike ions isolated in a quadrupolar trap, for example. In this comment, we distill multiple, distinct, scientific efforts into an emergent field that is devoted to designing molecules that mimic the quantum properties of objects like trapped atoms or defects in solids. Mimicry is endemic in inorganic chemistry and featured heavily in the research interests of groups across the world. We describe this new field of using molecular inorganic chemistry to mimic the quantum properties (e.g. the lifetime of spin superpositions, or the resonant frequencies thereof) of other quantum objects as "quantum mimicry." In this comment, we describe the philosophical design strategies and recent exciting results from the application of these strategies.
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Mononuclear heteroleptic complexes Fe(tpma)(bimz)(ClO4)2 (1a), Fe(tpma)(bimz)(BF4)2 (1b), Fe(bpte)(bimz)(ClO4)2 (2a), and Fe(bpte)(bimz)(BF4)2 (2b) (tpma = tris(2-pyridylmethyl)amine, bpte = ...S,S′-bis(2-pyridylmethyl)-1,2-thioethane, bimz = 2,2′-biimidazoline) were prepared by reacting the corresponding Fe(II) salts with stoichiometric amounts of the ligands. All complexes exhibit temperature-induced spin crossover (SCO), but the SCO temperature is substantially lower for complexes 1a and 1b as compared to 2a and 2b, indicating the stronger ligand field afforded by the N2S2-coordinating bpte ligand relative to the N4-coordinating tpma. Our findings suggest that ligands with mixed N/S coordination can be employed to discover new SCO complexes and to tune the transition temperature of known SCO compounds by substituting for purely N-coordinating ligands.
Incorporation of a five‐membered ring into a helicene framework disrupts aromatic conjugation and provides a site for selective deprotonation. The deprotonation creates an anionic cyclopentadienyl ...unit, switches on conjugation, leads to a >200 nm red‐shift in the absorbance spectrum and injects a charge into a helical conjugated π‐system without injecting a spin. Structural consequences of deprotonation were revealed via analysis of a monoanionic helicene co‐crystallized with {K+(18‐crown‐6)(THF)} and {Cs+2(18‐crown‐6)3}. UV/Vis‐monitoring of these systems shows a time‐dependent formation of mono‐ and dianionic species, and the latter was isolated and crystallographically characterized. The ability of the twisted helicene frame to delocalize the negative charge was probed as a perturbation of aromaticity using NICS scans. Relief of strain, avoidance of antiaromaticity, and increase in charge delocalization assist in the additional dehydrogenative ring closures that yield a new planarized decacyclic dianion.
Kein Flachwitz: Verringerung von Helicen‐Spannung, Verhinderung von Antiaromatizität und Erhöhung von Ladungsdelokalisierung unterstützen eine dehydrierende Ringschlussreaktion, die ein anionisches anelliertes Bis‐5Helicen in ein planares dekacyclisches Dianion mit neun Sechsringen und einem Fünfring umwandelt.
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Co‐crystallization of the spin‐crossover (SCO) cationic complex, Fe(1‐bpp)22+ (1‐bpp=2,6‐bis(pyrazol‐1‐yl)pyridine) with fractionally charged organic anion TCNQδ− (0<δ<1) afforded hybrid materials ...Fe(1‐bpp)2(TCNQ)3.5 ⋅ 3.5MeCN (1) and Fe(1‐bpp)2(TCNQ)4 ⋅ 4DCE (2), where TCNQ=7,7,8,8‐tetracyanoquinodimethane, MeCN=acetonitrile, and DCE=1,2‐dichloroethane. Both materials exhibit semiconducting behavior, with the room‐temperature conductivity values of 1.1×10−4 S/cm and 1.7×10−3 S/cm, respectively. The magnetic behavior of both complexes exhibits strong dependence on the content of the interstitial solvent. Complex 1 undergoes a gradual temperature‐driven SCO, with the midpoint temperature of T1/2=234 K. The partial solvent loss by 1 leads to the increase in the T1/2 value while complete desolvation renders the material high‐spin (HS) in the entire studied temperature range. In the case of 2, the solvated complex shows a gradual SCO with T1/2=166 K only when covered with a mother liquid, while the facile loss of interstitial solvent, even at room temperature, leads to the HS‐only behavior.
The cationic complex Fe(1‐bpp)22+ (1‐bpp=2,6‐bis(pyrazol‐1‐yl)pyridine), co‐crystallized with the fractionally charged organic anion TCNQδ− (0<δ<1), affords hybrid semiconducting materials in which the stacks of anions serve as charge‐transport pathways. The materials show gradual spin‐crossover behavior with high sensitivity to the content of interstitial solvent in the crystal lattice.
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Designing spins that exhibit long-lived coherence and strong temperature sensitivity is central to designing effective molecular thermometers and a fundamental challenge in the ...chemistry/quantum-information space. Herein, we provide a new pathway to both properties in the same molecule by designing a nuclear spin, which possesses a robust spin coherence, to mimic the strong temperature sensitivity of an electronic spin. This design strategy is demonstrated in the group of trinuclear Co(III) spin-crossover compounds (CpCo(OP(OR)2)3)2Co(SbCl6) where Cp = cyclopentadienyl and R = Me (1), Et (2), i-Pr (3), and t-Bu (4). Nuclear magnetic resonance analyses of the 59Co nuclear spins reveal 59Co chemical-shift temperature sensitivity (Δδ/ΔT) values that span from 101(1) ppm/°C in 1 to 149(1) ppm/°C in 2 and 150(2) ppm/°C in 4, where the latter two are record temperature sensitivities for any nuclear spin. Additionally, complexes 2 and 4 have T 2 * values of 74 and 78 μs in solution at ambient temperatures surpassing those from electron-spin-based complexes, which typically display long coherence times only at extremely low temperatures. Our results suggest that spin-crossover phenomena can enable electron-spin-like temperature sensitivities in nuclear spins while retaining robust coherence times at room temperature.
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Co-crystallization of the prominent Fe(
ii
) spin-crossover (SCO) cation, Fe(3-bpp)
2
2+
(3-bpp = 2,6-bis(pyrazol-3-yl)pyridine), with a fractionally charged TCNQ
δ
−
radical anion has afforded a ...hybrid complex Fe(3-bpp)
2
(TCNQ)
3
·5MeCN (
1
·5MeCN, where
δ
= −0.67). The partially desolvated material shows semiconducting behavior, with the room temperature conductivity
σ
RT
= 3.1 × 10
−3
S cm
−1
, and weak modulation of conducting properties in the region of the spin transition. The complete desolvation, however, results in the loss of hysteretic behavior and a very gradual SCO that spans the temperature range of 200 K. A related complex with integer-charged TCNQ
−
anions, Fe(3-bpp)
2
(TCNQ)
2
·3MeCN (
2
·3MeCN), readily loses the interstitial solvent to afford desolvated complex
2
that undergoes an abrupt and hysteretic spin transition centered at 106 K, with an 11 K thermal hysteresis. Complex
2
also exhibits a temperature-induced excited spin-state trapping (TIESST) effect, upon which a metastable high-spin state is trapped by flash-cooling from room temperature to 10 K. Heating above 85 K restores the ground-state low-spin configuration. An approach to improve the structural stability of such complexes is demonstrated by using a related ligand 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) to obtain Fe(bzimpy)
2
(TCNQ)
6
·2Me
2
CO (
4
) and Fe(bzimpy)
2
(TCNQ)
5
·5MeCN (
5
), both of which exist as LS complexes up to 400 K and exhibit semiconducting behavior, with
σ
RT
= 9.1 × 10
−2
S cm
−1
and 1.8 × 10
−3
S cm
−1
, respectively.
Co-crystallization of the cationic complex Fe(3-bpp)2
2+
with fractionally charged TCNQ
δ
−
anions (0 <
δ
< 1) affords semiconducting spin-crossover (SCO) materials. The abruptness of SCO is strongly dependent on the interstitial solvent content.
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