Selective Activation of Chalcogen Bonding: An Efficient Structuring Tool toward Crystal Engineering Strategies

ConspectusAmong the noncovalent interactions available in the toolbox of crystal engineering, chalcogen bonding (ChB) has recently entered the growing family of σ-hole interactions, following the strong developments based on the halogen bonding (XB) interaction over the last 30 years. The monovalent character of halogens provides halogen bonding directionality and strength. Combined with the extensive organic chemistry of Br and I derivatives, it has led to many applications of XB, in solution (organo-catalysis, anion recognition and transport), in the solid state (cocrystals, conducting materials, fluorescent materials, topochemical reactions, ...), in soft matter (liquid crystals, gels,···), and in biochemistry. The recognition of the presence of two σ-holes on divalent chalcogens and the ability to activate them, as in XB, with electron-withdrawing groups (EWG) has fueled more recent interest in chalcogen bonding. However, despite being identified for many years, ChB still struggles to make a mark due to (i) the underdeveloped synthetic chemistry of heavier Se and Te; (ii) the limited stability of organic chalcogenides, especially tellurides; and (iii) the poor predictability of ChB associated with the presence of two σ-holes. It therefore invites a great deal of attention of molecular chemists to design and develop selected ChB donors, for the scrutiny of fundamentals of ChB and their successful use in different applications. This Account aims to summarize our own contributions in this direction that extend from fundamental studies focused on addressing the lack of directionality/predictability in ChB to a systematic demonstration of its potential, specifically in crystal engineering, and particularly toward anionic networks on the one hand, topochemical reactions on the other hand.In this Account, we share our recent results aimed at recovering with ChB the same degree of strength and predictability found with XB, by focusing on divalent Se and Te systems with two different substituents, one of them with an EWG, to strongly unbalance both σ-holes. For that purpose, we explored this dissymmetrization concept within three chemical families, selenocyanates R–SeCN, alkynyl derivatives R–CC–(Se/Te)Me, and o-carborane derivatives. Such compounds were systematically engaged in cocrystals with either halides or neutral bipyridines as ChB acceptors, revealing their strong potential to chelate halides as well as their ability to organize reactive molecules such as alkenes and butadiynes toward [2+2] cycloadditions and polydiacetylene formation, respectively. This selective activation concept is not limited to ChB but can be effectively used on all other σ-hole interactions (pnictogen bond, tetrel bond, etc.) where one needs to control the directionality of the interaction

Electronic Effects of Ligand Substitution on Spin Crossover in a Series of Diiminoquinonoid-Bridged Fe^II₂ Complexes

A series of four isostructural Fe^II₂ complexes, [(TPyA)₂Fe₂(^XL)]²⁺ (TPyA = tris­(2-pyridylmethyl)­amine; ^XL^2– = doubly deprotonated form of 3,6-disubstituted-2,5-dianilino-1,4-benzoquinone; X = H, Br, Cl, and F), were synthesized to enable a systematic study of electronic effects on spin crossover behavior. Comparison of X-ray diffraction data for these complexes reveals the sole presence of high-spin Fe^II at 225 K and mixtures of high-spin and low-spin Fe^II at 100 K, which is indicative of incomplete spin crossover. In addition, crystal packing diagrams show that these complexes are well-isolated from one another in the solid state, owing primarily to the presence of bulky tetra­(aryl)­borate counteranions, such that spin crossover is likely not significantly affected by intermolecular interactions. Variable-temperature dc magnetic susceptibility data confirm the structural observations and reveal that 54(1), 56(1), 62(1), and 84(1)% of Fe^II centers remain high-spin even below 65 K. Moreover, fits to magnetic data provide crossover temperatures of <i>T</i>_1/2 = 160(1), 124(1), 121(1), and 110(1) K for X = H, Br, Cl, and F, respectively, along with enthalpies of Δ<i>H</i> = 11.4(3), 8.5(3), 8.3(3), and 7.5(2) kJ/mol, respectively. These parameters decrease with increasing electronegativity of X and thus increasing electron-withdrawing character of ^XL^2–, suggesting that the observed trends originate primarily from inductive effects of X. Moreover, when plotted as a function of the Pauling electronegativity of X, both <i>T</i>_1/2 and Δ<i>H</i> undergo a linear decrease. Further analyses of the low-temperature magnetic data and variable-temperature Mössbauer spectroscopy suggest that the incomplete spin crossover behavior in [(TPyA)₂Fe₂(^XL)]²⁺ is best described as a transition from purely [Fe_HS-Fe_HS] (HS = high-spin) complexes at high temperature to a mixture of [Fe_HS-Fe_HS] and [Fe_HS-Fe_LS] (LS = low-spin) complexes at low temperature, with the number of [Fe_HS-Fe_HS] species increasing with decreasing electron-withdrawing character of ^XL^2–

Electronic Effects of Ligand Substitution on Spin Crossover in a Series of Diiminoquinonoid-Bridged Fe^II₂ Complexes

A series of four isostructural Fe^II₂ complexes, [(TPyA)₂Fe₂(^XL)]²⁺ (TPyA = tris­(2-pyridylmethyl)­amine; ^XL^2– = doubly deprotonated form of 3,6-disubstituted-2,5-dianilino-1,4-benzoquinone; X = H, Br, Cl, and F), were synthesized to enable a systematic study of electronic effects on spin crossover behavior. Comparison of X-ray diffraction data for these complexes reveals the sole presence of high-spin Fe^II at 225 K and mixtures of high-spin and low-spin Fe^II at 100 K, which is indicative of incomplete spin crossover. In addition, crystal packing diagrams show that these complexes are well-isolated from one another in the solid state, owing primarily to the presence of bulky tetra­(aryl)­borate counteranions, such that spin crossover is likely not significantly affected by intermolecular interactions. Variable-temperature dc magnetic susceptibility data confirm the structural observations and reveal that 54(1), 56(1), 62(1), and 84(1)% of Fe^II centers remain high-spin even below 65 K. Moreover, fits to magnetic data provide crossover temperatures of <i>T</i>_1/2 = 160(1), 124(1), 121(1), and 110(1) K for X = H, Br, Cl, and F, respectively, along with enthalpies of Δ<i>H</i> = 11.4(3), 8.5(3), 8.3(3), and 7.5(2) kJ/mol, respectively. These parameters decrease with increasing electronegativity of X and thus increasing electron-withdrawing character of ^XL^2–, suggesting that the observed trends originate primarily from inductive effects of X. Moreover, when plotted as a function of the Pauling electronegativity of X, both <i>T</i>_1/2 and Δ<i>H</i> undergo a linear decrease. Further analyses of the low-temperature magnetic data and variable-temperature Mössbauer spectroscopy suggest that the incomplete spin crossover behavior in [(TPyA)₂Fe₂(^XL)]²⁺ is best described as a transition from purely [Fe_HS-Fe_HS] (HS = high-spin) complexes at high temperature to a mixture of [Fe_HS-Fe_HS] and [Fe_HS-Fe_LS] (LS = low-spin) complexes at low temperature, with the number of [Fe_HS-Fe_HS] species increasing with decreasing electron-withdrawing character of ^XL^2–

Mn^III–Fe^III Heterometallic Compounds within Hydrogen-Bonded Supramolecular Networks Promoted by an [Fe(CN)₅(CNH)]^2– Building Block: Structural and Magnetic Properties

The reaction of [Fe­(CN)₆]^3– and [Mn­(acacen)]⁺ (H₂acacen = <i>N</i>,<i>N</i>′-bis­(acetylacetone)­ethylenediamine) building units in the presence of supramolecular cations, [(F-Anil)­(18-crown-6)]⁺ (F-Anil⁺ = 3-fluoroanilinium) or [(Me-F-Anil)­(18-crown-6)]⁺ (Me-F-Anil⁺ = 3-fluoro-4-methylanilinium), affords two new bimetallic compounds, [(F-Anil)­(18-crown-6)]­[Mn­(acacen)­Fe­(CN)₅(CNH)]·MeOH (<b>1</b>) and [(Me-F-Anil)­(18-crown-6)]­[Mn­(acacen)­(MeOH)­Fe­(CN)₅(CNH)]·MeOH (<b>2</b>), respectively. Compound <b>1</b> exhibits a one-dimensional topology, while compound <b>2</b> is a dinuclear discrete system due to the coordination of a MeOH molecule at the axial position of the [Mn­(acacen)]⁻ unit. For both systems, the acidity of the corresponding supramolecular cation triggers the protonation of the Fe^III moiety as [Fe­(CN)₅(CNH)]^2–. Moreover, the resulting −CNH ligand induces hydrogen bonding interactions connecting the chains for <b>1</b> or the molecules for <b>2</b> into higher dimensional supramolecular networks. Magnetic properties of compounds incorporating these [Fe­(CN)₅(CNH)]^2– building blocks were, for the first time, thoroughly investigated, indicating a three-dimensional antiferromagnetic order of single-chain magnets for <b>1</b> and an antiferromagnetically interacting <i>S</i> = 3/2 spin ground state for <b>2</b>

An Azophenine Radical-Bridged Fe₂ Single-Molecule Magnet with Record Magnetic Exchange Coupling

One-electron reduction of the complex [(TPyA)₂Fe^II₂(^NPhL^2–)]²⁺ (TPyA = tris­(2-pyridylmethyl)­amine, ^NPhLH₂ = azophenine = <i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>‴-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone) affords the complex [(TPyA)₂Fe^II₂(^NPhL^3–•)]⁺. X-ray diffraction and Mössbauer spectroscopy confirm that the reduction occurs on ^NPhL^2– to give an <i>S</i> = 1/2 radical bridging ligand. Dc magnetic susceptibility measurements demonstrate the presence of extremely strong direct antiferromagnetic exchange between <i>S</i> = 2 Fe^II centers and ^NPhL^3–• in the reduced complex, giving an <i>S</i> = 7/2 ground state with an estimated coupling constant magnitude of |<i>J</i>| ≥ 900 cm^–1. Mössbauer spectroscopy and ac magnetic susceptibility reveal that this complex behaves as a single-molecule magnet with a spin relaxation barrier of <i>U</i>_eff = 50(1) cm^–1. To our knowledge, this complex exhibits by far the strongest magnetic exchange coupling ever to be observed in a single-molecule magnet

An Azophenine Radical-Bridged Fe₂ Single-Molecule Magnet with Record Magnetic Exchange Coupling

One-electron reduction of the complex [(TPyA)₂Fe^II₂(^NPhL^2–)]²⁺ (TPyA = tris­(2-pyridylmethyl)­amine, ^NPhLH₂ = azophenine = <i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>‴-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone) affords the complex [(TPyA)₂Fe^II₂(^NPhL^3–•)]⁺. X-ray diffraction and Mössbauer spectroscopy confirm that the reduction occurs on ^NPhL^2– to give an <i>S</i> = 1/2 radical bridging ligand. Dc magnetic susceptibility measurements demonstrate the presence of extremely strong direct antiferromagnetic exchange between <i>S</i> = 2 Fe^II centers and ^NPhL^3–• in the reduced complex, giving an <i>S</i> = 7/2 ground state with an estimated coupling constant magnitude of |<i>J</i>| ≥ 900 cm^–1. Mössbauer spectroscopy and ac magnetic susceptibility reveal that this complex behaves as a single-molecule magnet with a spin relaxation barrier of <i>U</i>_eff = 50(1) cm^–1. To our knowledge, this complex exhibits by far the strongest magnetic exchange coupling ever to be observed in a single-molecule magnet

A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from <i>T</i>_c = 26 to 80 K

The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH₂) with Fe^II affords the solid (Me₂NH₂)₂­[Fe₂L₃]·​2H₂O·6DMF​. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of Fe^III centers with mixed-valence ligands of the form (L₃)^8– that result from a spontaneous electron transfer from Fe^II to L^2–. Upon removal of DMF and H₂O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me₂NH₂)₂­[Fe₂L₃], and a fit to N₂ adsorption data of this activated compound gives a BET surface area of 885(105) m²/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally characterized extended solid that contains tetraoxolene radical ligands

A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from <i>T</i>_c = 26 to 80 K

The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH₂) with Fe^II affords the solid (Me₂NH₂)₂­[Fe₂L₃]·​2H₂O·6DMF​. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of Fe^III centers with mixed-valence ligands of the form (L₃)^8– that result from a spontaneous electron transfer from Fe^II to L^2–. Upon removal of DMF and H₂O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me₂NH₂)₂­[Fe₂L₃], and a fit to N₂ adsorption data of this activated compound gives a BET surface area of 885(105) m²/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally characterized extended solid that contains tetraoxolene radical ligands

Low-Coordinate Iron(II) Complexes of a Bulky Bis(carbene)borate Ligand

The bulky bis­(carbene)­borate ligand H₂B­(^tBuIm)₂^– allows for the synthesis of three- and four-coordinate iron­(II) complexes, including heteroleptic H₂B­(^tBuIm)₂FeN­(TMS)₂ and homoleptic [H₂B­(^tBuIm)₂]₂Fe. The magnetic properties of these coordinatively unsaturated complexes have been characterized by SQUID magnetometry, but no evidence of single-molecule magnet behavior is observed, despite large negative uniaxial zero field splitting. The three-coordinate complex H₂B­(^tBuIm)₂FeN­(TMS)₂ serves as a precursor for the synthesis of the four-coordinate mixed carbene complex H₂B­(^tBuIm)₂(ⁱPr₂Im)­FeCl, which has a coordination environment similar to that found in tris­(carbene)­borate iron­(II) chloride complexes. Despite this similarity, attempts to prepare the corresponding iron­(IV) nitride were unsuccessful, suggesting that subtle structural factors are critical to stabilizing this species

2D Conductive Iron-Quinoid Magnets Ordering up to <i>T</i>_c = 105 K via Heterogenous Redox Chemistry

We report the magnetism and conductivity for a redox pair of iron-quinoid metal–organic frameworks (MOFs). The oxidized compound, (Me₂NH₂)₂[Fe₂L₃]·2H₂O·6DMF (LH₂ = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) was previously shown to magnetically order below 80 K in its solvated form, with the ordering temperature decreasing to 26 K upon desolvation. Here, we demonstrate this compound to exhibit electrical conductivity values up to σ = 1.4(7) × 10^–2 S/cm (<i>E</i>_a = 0.26(1) cm^–1) and 1.0(3) × 10^–3 S/cm (<i>E</i>_a = 0.19(1) cm^–1) in its solvated and desolvated forms, respectively. Upon soaking in a DMF solution of Cp₂Co, the compound undergoes a single-crystal-to-single-crystal one-electron reduction to give (Cp₂Co)_1.43(Me₂NH₂)_1.57[Fe₂L₃]·4.9DMF. Structural and spectroscopic analysis confirms this reduction to be ligand-based, and as such the trianionic framework is formulated as [Fe^III₂(L^3–•)₃]^3–. Magnetic measurements for this reduced compound reveal the presence of dominant intralayer metal–organic radical coupling to give a magnetically ordered phase below <i>T</i>_c = 105 K, one of the highest reported ordering temperatures for a MOF. This high ordering temperature is significantly increased relative to the oxidized compound, and stems from the overall increase in coupling strength afforded by an additional organic radical. In line with the high critical temperature, the new MOF exhibits magnetic hysteresis up to 100 K, as revealed by variable-field measurements. Finally, this compound is electrically conductive, with values up to σ = 5.1(3) × 10^–4 S/cm with <i>E</i>_a = 0.34(1) eV. Taken together, these results demonstrate the unique ability of metal-quinoid MOFs to simultaneously exhibit both high magnetic ordering temperatures and high electrical conductivity