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-C≡C-(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.

Carborane-based heteromolecular extended networks driven by directional C-Te⋯N chalcogen bonding interactions.

We demonstrate that o-closo-(TeMe)2carborane directs, in the presence of linear ditopic neutral Lewis bases, the formation of co-crystals with 1D extended supramolecular networks. Specifically, the network formation is systematically stabilized by short and quasi-linear C-Te⋯N chalcogen-bonding (ChB) interactions. In sum, we report efficient carborane-based tectons to rationally design high-dimensional neutral heteromolecular networks.

Cluster-Spin-Glass Magnetic Behavior and Morphology in the Coordination Polymer Alloys FeyCo1-yBTT.

We report the synthesis of a series of pseudo-1D coordination polymer (CP) materials with the formula FeyCo1-yBTT (BTT = 1,3,5-benzenetrithiolate). These materials were structurally characterized by PXRD Rietveld, EXAFS, and PDF analyses, revealing that the CP superstructure enables a continuous and isomorphous alloy between the two homometallic compounds. Lower Fe loadings exhibit emergent spin glass magnetic behavior, such as memory effects and composition-dependent spin glass response time constants ranging from 6.9 × 10-9 s to 1.8 × 10-6 s. These data are consistent with the formation of spin clusters within the lattice. The magnetic behavior in these materials was modeled via replica exchange Monte Carlo simulation, which provides a good match for the experimentally measured spin glassing and magnetic phase transitions. These findings underscore how the rigid superstructure of CP and MOF scaffolds can enable the systematic tuning of physical properties, such as the spin glass behavior described here.

Structure and Magnetic Properties of Pseudo-1D Chromium Thiolate Coordination Polymers.

The synthesis, structure, and magnetic properties of two novel, pseudo-one-dimensional (1D) chromium thiolate coordination polymers (CPs), CrBTT and Cr2BDT3, are reported. The structures of these materials were determined using X-ray powder diffraction revealing highly symmetric 1D chains embedded within a CP framework. The magnetic coupling of this chain system was measured by SQUID magnetometry, revealing a switch from antiferromagnetic to ferromagnetic behavior dictated by the angular geometrical constraints within the CP scaffold consistent with the Goodenough-Kanamori-Anderson rules. Intrachain magnetic coupling constants JNN of -32.0 and +5.7 K were found for CrBTT and Cr2BDT3, respectively, using the 1D Bonner-Fisher model of magnetism. The band structure of these materials has also been examined by optical spectroscopy and density functional theory calculations revealing semiconducting behavior. Our findings here demonstrate how CP scaffolds can support idealized low-dimensional structural motifs and dictate magnetic interactions through tuning of geometry and inter-spin couplings.

N-Chlorobenzimidazoles as efficient and structurally diverse amphoteric halogen bond donors in crystal engineering.

The ability of amphoteric N-chlorobenzimidazoles to self-associate into 1D chains through strong and linear N-Cl⋯N halogen bond interactions is demonstrated. The less polarisable Cl atom is strongly activated thanks to the intramolecular amphoteric character and the intermolecular cooperativity effect. The obtained family of compounds featuring different substitution patterns provides opportunities toward the elaboration of macroscopic polar structures.

Topochemical Polymerization of a Diacetylene in a Chalcogen-Bonded (ChB) Assembly.

The successful topochemical polymerization of bis(selenocyanatomethyl)butadyine 1 is achieved upon association in a 1 : 1 co-crystal with 1,2-bis(2-pyridyl)ethylene (2-bpen) through strong and linear (NC)-Se⋅⋅⋅NPy chalcogen bonding (ChB) interactions, allowing for an appropriate parallel alignment of the diacetylene moieties toward the solid-state reaction. Co-crystal 1⋅(2-bpen) undergoes polymerization upon heating at 100 °C. The reaction progress was monitored by IR, DSC and PXRD. An enhancement of the polymer conductivity by 8 orders of magnitude is observed upon iodine doping. Strikingly, the course of polymerization is accompanied with sublimation of the ChB acceptor molecules 2-bpen, providing the polymer in a pure form with full recovery of the co-former, at variance with the usual hydrogen-bonded co-crystal strategies toward polydiacetylenes.

Strong σ-Hole Activation on Icosahedral Carborane Derivatives for a Directional Halide Recognition.

Crystal engineering based on σ-hole interactions is an emerging approach for realization of new materials with higher complexity. Neutral inorganic clusters derived from 1,2-dicarba-closo-dodecaborane, substituted with -SeMe, -TeMe, and -I moieties on both skeletal carbon vertices are experimentally demonstrated herein as outstanding chalcogen- and halogen-bond donors. In particular, these new molecules strongly interact with halide anions in the solid-state. The halide ions are coordinated by one or two donor groups (µ1 - and µ2 -coordinations), to stabilize a discrete monomer or dimer motifs to 1D supramolecular zig-zag chains. Crucially, the observed chalcogen bond and halogen bond interactions feature remarkably short distances and high directionality. Electrostatic potential calculations further demonstrate the efficiency of the carborane derivatives, with Vs,max being similar or even superior to that of reference organic halogen-bond donors, such as iodopentafluorobenzene.

Activating Chalcogen Bonding (ChB) in Alkylseleno/Alkyltelluroacetylenes toward Chalcogen Bonding Directionality Control.

Activation of a deep electron-deficient area on chalcogen atoms (Ch=Se, Te) is demonstrated in alkynyl chalcogen derivatives, in the prolongation of the (C≡)C-Ch bond. The solid-state structures of 1,4-bis(methylselenoethynyl)perfluorobenzene (1Se) show the formation of recurrent chalcogen-bonded (ChB) motifs. Association of 1Se and the tellurium analogue 1Te with 4,4'-bipyridine and with the stronger Lewis base 1,4-di(4-pyridyl)piperazine gives 1:1 co-crystals with 1D extended structures linked by short and directional ChB interactions, comparable to those observed with the corresponding halogen bond (XB) donor, 1,4-bis(iodoethynyl)-perfluorobenzene. This "alkynyl" approach for chalcogen activation provides the crystal-engineering community with efficient, and neutral ChB donors for the elaboration of supramolecular 1D (and potentially 2D or 3D) architectures, with a degree of strength and predictability comparable to that of halogen bonding in iodoacetylene derivatives.

Activating both Halogen and Chalcogen Bonding Interactions in Cation Radical Salts of Iodinated Tetrathiafulavalene Derivatives.

Halogen bonding (XB) interactions are investigated in cation radical salts of bis(methylthio)-5,5'-diiodotetrathiafulvalene (1). Electrocrystallization of 1 in the presence of Bu4 NCl affords a 1 : 1 salt formulated as (E-1)Cl. Particularly strong I⋅⋅⋅Cl- XB interactions are observed around the Cl- anion with the distances at 78 % the sum of the van der Waals radii, a consequence of the XB charge activation in the cation radical. Moreover, the Cl- environment is complemented by two extra S⋅⋅⋅Cl- chalcogen bonding (ChB) interactions, an original feature among reported halide salts of TTF derivatives. Electrostatic potential calculations on the cation radical further demonstrate the efficient activation of the S atoms of the 1,3-dithiole rings (Vs,max =87.2 kcal/mol), as strong as with the iodine atoms (Vs,max =87.9 kcal/mol). The radical cations form weakly dimerized stacks, as confirmed by the variable-temperature magnetic susceptibility and the weak conductivity (4.8×10-5  S cm-1 ).

Electronic engineering of a tetrathiafulvalene charge-transfer salt via reduced symmetry induced by combined substituents.

A 1 : 1 metallic charge-transfer salt is obtained by cosublimation of (Z,E)-(SMe)2Me2TTF and TCNQ. X-ray diffraction studies confirm the formation of segregated stacks comprising donor and acceptor molecules in [(E)-(SMe)2Me2TTF](TCNQ). The crystal packing features lateral SS interactions between TTF stacks, which is in sharp contrast to that in (TTF)(TCNQ). Structural analysis and theoretical studies afford a partial charge-transfer (ρ ≈ 0.52), leading to a system with the electronic structure close to quarter-filled. Resistivity measurements reveal that this material behaves as a metal down to 56 K and 22 K at 1 bar and 14.9 kbar, respectively. The thermopower is negative in the metallic regime, indicating the dominant role of the acceptor stacks for the observed conducting behavior. Analysis of single-crystal EPR spectra shows the remaining spin susceptibility at 4.3 K, suggesting the importance of the Hubbard U correction. These results highlight the judicious engineering of electronic and geometrical effects on the TTF core; the combined use of methyl and thiomethyl groups has decreased the TCNQ bandwidth while maintaining the segregated stacks, converting the metal to insulator (M-I) transition to more 4kF like. In addition, the enhanced SS contacts between the TTF stacks lead to more rapidly decreasing M-I transition temperature under various pressures.

Slow Magnetic Relaxation of Co(II) Single Chains Embedded within Metal-Organic Superstructures.

Two coordination polymers of the type Co(BPDC)(N-ox), with BPDC being 4,4'-biphenyldicarboxylate and N-ox being pyridine N-oxide (PNO) or isoquinoline N-oxide (IQNO), have been synthesized and characterized. The compounds feature 2D and 3D metal-organic networks that encapsulate Co(II)-based chains in a rigid superstructure. The dc and ac magnetic properties of these Co(BPDC)(N-ox) materials have been investigated alongside those of a related Co(BDC)(PNO) compound (where BDC is 1,4-benzenedicarboxylate), which contains a smaller dicarboxylate linker. These Co(II)-containing coordination polymers exhibit slow magnetic relaxation, as observed by ac susceptibility measurements. The observed magnetic behavior of all compounds is consistent with an antiferromagnetic interaction between canted Co spins along the 1D skeleton, resulting in single-chain magnet behavior. In the case of Co(BPDC)(IQNO), weak interchain magnetic interactions yield 3D antiferromagnetic order while the inherent magnetic behavior stemming from the chain component is maintained. The combination of these effects in this material puts it at the frontier between single-chain magnets and classical bulk antiferromagnets. This work contributes to the limited group of materials featuring the organization of single-chain magnets within a coordination polymer superstructure.

MnIII-FeIII 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 = N, N'-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 (1) and [(Me-F-Anil)(18-crown-6)][Mn(acacen)(MeOH)Fe(CN)5(CNH)]·MeOH (2), respectively. Compound 1 exhibits a one-dimensional topology, while compound 2 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 FeIII moiety as [Fe(CN)5(CNH)]2-. Moreover, the resulting -CNH ligand induces hydrogen bonding interactions connecting the chains for 1 or the molecules for 2 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 1 and an antiferromagnetically interacting S = 3/2 spin ground state for 2.

Photoinduced reversible spin-state switching of an FeIII complex assisted by a halogen-bonded supramolecular network.

The organization of a molecular FeIII complex embedded in a halogen-bonded 2D network is chemically tuned to trigger temperature- and light-induced spin-state switching. We attribute the associated magnetic properties and the unprecedented photoswitching effect to the optimized structural confinement provided by the presence of the supramolecular host framework.

Spin-state modulation of molecular FeIII complexes via inclusion in halogen-bonded supramolecular networks.

The cationic complex [Fe(qsal)2]+ (Hqsal = N-(8-quinolyl)salicylaldimine) is encapsulated in anionic halogen-bonded 1D and 2D networks derived from sym-triiodotrifluorobenzene, [(C6F3I3)Cl]- and [(C6F3I3)I]-. Structural analysis and magnetic measurements show that the spin-state of this complex can be modulated by its inclusion in supramolecular host frameworks.

2D Conductive Iron-Quinoid Magnets Ordering up to Tc = 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 (Ea = 0.26(1) cm-1) and 1.0(3) × 10-3 S/cm (Ea = 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 [FeIII2(L3-•)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 Tc = 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 Ea = 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.

Solid-State Redox Switching of Magnetic Exchange and Electronic Conductivity in a Benzoquinoid-Bridged Mn(II) Chain Compound.

We demonstrate that incorporation of a redox-active benzoquinoid ligand into a one-dimensional chain compound can give rise to a material that exhibits simultaneous solid-state redox switching of optical, magnetic, and electronic properties. Metalation of the ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol ((N,O)LH4) with Mn(III) affords the chain compound Mn((N,O)L)(DMSO). Structural and spectroscopic analysis of this compound show the presence of Mn(II) centers bridged by (N,O)L(2-) ligands, resulting partially from a spontaneous ligand-to-metal electron transfer. Upon soaking in a solution of the reductant Cp2Co, Mn((N,O)L)(DMSO) undergoes a ligand-centered solid-state reduction to [Mn((N,O)L)](-), as revealed by a suite of techniques, including Raman and X-ray absorption spectroscopy. The ligand-based reduction engenders a dramatic modulation of the physical properties of the chain compound. An electrochromic response, evidenced by a color change from dark green to dark purple is accompanied by a nearly 40-fold increase in magnetic coupling strength, from J = -0.38(1) to -15.6(2) cm(-1), and a 10,000-fold increase in electronic conductivity, from σ = 2.33(1) × 10(-12) S/cm (Ea = 0.64(1) eV) to 8.61(1) × 10(-8) S/cm (Ea = 0.39(1) eV). Importantly, the chemical reduction is reversible: treatment of the reduced compound with [Cp2Fe](+) regenerates the oxidized chain. Taken together, these results highlight the ability of benzoquinoid ligands to facilitate solid-state ligand-based redox reactions in nonporous coordination solids, giving rise to reversible switching of optical properties, magnetic exchange interactions, and electronic conductivity.

A new family of [Cu(II)Ln(III)M(V)] heterotrimetallic complexes (Ln = La, Gd, Tb; M = Mo, W): model systems to probe exchange interactions and single-molecule magnet properties.

Four isostructural trinuclear 3d-4f-4(5)d heterotrimetallic complexes, with the general formula [L(2)CuLn(H2O)5(µ-NC)M(CN)7], were obtained from the association of binuclear 3d-4f complexes and {M(V)(CN)8}(3-) metalloligands (M = Mo, Ln = La ; M = W, Ln = La ; M = Mo, Ln = Gd ; M = Mo; Ln = Tb , where H2L(2) = 1,2-ethanediylbis(2-iminomethylene-6-methoxy-phenol)). The metalloligand coordinates through a single-cyanido group at the apical position of the copper(ii) ion belonging to the {Cu(II)Ln(III)} binuclear complex. The analysis of the magnetic data for the La(iii) derivatives (compounds and ), in the 1.85-300 K temperature range, shows a weak ferromagnetic exchange interaction between Cu(II) and Mo(V)/W(V) ions across the cyanido bridge (JCuM/kB = 3.6(6) K; g = 2.23(5) for and JCuM/kB = 3.8(6) K, g = 2.21(5) for , with H = -2JCuMSCu·SM). These results were used to simulate the magnetic properties of compound , using the isotropic spin Hamiltonian H = -2JCuMoSCu·SMo - 2JCuGdSCu·SGd. The resulting magnetic interaction between Cu(II) and Gd(III) ions via the phenoxo-bridge was found to be weakly ferromagnetic (JCuGd/kB = +4.5(2) K with JCuMo/kB = +3.6(2) K, gGd = gCu = 2.00 and gMo = 1.98). The dc magnetic properties for compound also show a predominant ferromagnetic interaction, while the ac magnetic measurements indicate the presence of the slow relaxation of the magnetization below 3.5 K.

An S = 12 semiquinoid radical-bridged Mn6 wheel complex assembled from an asymmetric redox-active bridging ligand.

The asymmetric redox-active ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol ((N,O)LH4) is prepared and metalated to afford the hexanuclear complex [Mn6((N,O)L)6](6-). Structural analysis and magnetic measurements reveal this complex to feature Mn(II) ions bridged by (N,O)L(3-)˙ radicals, which are antiferromagnetically coupled to give an S = 12 ground state.

A [3Fe-3S](3+) cluster with exclusively µ-sulfide donors.

A [3Fe-3(µ-S)](3+) cluster is reported in which each ferric center has a distorted trigonal pyramidal geometry, with an S = 1/2 ground state for the cluster and unusually anisotropic hyperfine coupling constants as determined by variable temperature magnetometry and Mössbauer spectroscopy.

A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from Tc = 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) m(2)/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.