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.

A Bimetallic Benzene-1,2,4,5-Tetrathiolate (btt) Molybdenocene Complex Cp2 Mo(btt)MoCp2 : Radical and Diradical States.

Benzene-1,2,4,5-tetrathiolate (btt) has been used as a bridging ligand to prepare a redox active (molybdenocene dithiolene)-based bimetallic complex Cp2 Mo(btt)MoCp2 , which exhibits four successive electron transfers up to the tetracation. Spectro-electrochemical investigations together with DFT and TD-DFT calculations evidence that the two electroactive MoS2 C2 metallacycles are electronically coupled in the monocationic as in the dicationic state. Two salts of the dication [Cp2 Mo(btt)MoCp2 ]2+ have been structurally characterized with PF6 - and HSO4 - counterions, showing different chair or boat conformations associated with variable folding angles of the two MoS2 C2 metallacycles along the S-S hinge. The bis-oxidized dicationic complex exhibits a diradical character, with both radicals essentially localized on the metallacycles and with antiferromagnetic coupling evidenced from magnetic susceptibility measurements.

A highly conducting mixed-valence nickel bis(dithiolene) salt [Et4N][Ni(Me-thiazSe-dt)2]2 with selone substitution.

The prototypical [Ni(dmit)2] complex (dmit: 1,3-dithiole-2-thione-4,5-dithiolate) is modified here by combining the N-R substitution found in [Ni(R-thiazdt)2] complexes (R-thiazdt: N-alkyl-thiazoline-2-thione-4,5-dithiolate) and the selone substitution found in [Ni(dmiSe)2] complex (dmiSe: 1,3-dithiole-2-selone-4,5-dithiolate) to give a novel N-methyl substituted, radical anionic complex formulated as [Ni(Me-thiazSe-dt)2]1- (Me-thiazSe-dt: N-methyl-thiazoline-2-selone-4,5-dithiolate). Both this anionic complex and its mixed-valence Et4N+ salt crystallize with a rare cis arrangement of the two dithiolene ligands around the Ni atom. In the 1 : 2 [Et4N][Ni(Me-thiazSe-dt)2]2 salt, the complexes organize into dimerized chains well isolated from each other, giving the salt a strong one-dimensional character. It shows however a high RT conductivity of 4.6 S cm-1 and small activation energy of 33 meV, indicating a possible Mott insulator behavior, which is not suppressed under pressures up to 10 GPa.

Mixed-Valence Conductors from Ni Bis(diselenolene) Complexes with a Thiazoline Backbone.

Highly conducting, mixed-valence, multi-component nickel bis(diselenolene) salts were obtained by electrocrystallization of the monoanionic species [Ni(Me-thiazds)2]-1 (Me-thiazds: N-methyl-1,3-thiazoline-2-thione-4,5-diselenolate), with 1:2 and 1:3 stoichiometries depending of the counter ion used (Et4N+ and nBu4N+ vs Ph4P+, respectively). This behavior strongly differs from that of the corresponding monoanionic dithiolene complexes whose oxidation afforded the single component neutral species. This provides additional rare examples of mixed-valence conducting salts of nickel diselenolene complexes, only known in two examples with the dsit (1,3-dithiole-2-thione-4,5-diselenolate) and dsise (1,3-dithiole-2-selone-4,5-diselenolate) ligands. The mixed-valence salts form highly dimerized or trimerized bi- and trimetallic units, rarely seen with such nickel complexes. Transport measurements under a high pressure (up to 10 GPa) and band structure calculations confirm the semiconducting character of [Ph4P][Ni(Me-thiazds)2]3 and the quasi metallic character of [Et4N][Ni(Me-thiazds)2]2 and [NBu4]x[Ni(Me-thiazds)2]2 salts (0 < x < 1).

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, Single-Crystal-to-Single-Crystal [2+2] Photocycloadditions Driven by Chalcogen-Bonding Interactions.

The face-to-face association of (E)-1,2-di(4-pyridyl)ethylene (bpen) molecules into rectangular motifs stabilized for the first time by chalcogen bonding (ChB) interactions is shown to provide photoreactive systems leading to cyclobutane formation through single-crystal-to-single-crystal [2+2] photodimerizations. The chelating chalcogen bond donors are based on original aromatic, ortho-substituted bis(selenocyanato)benzene derivatives 1-3, prepared from ortho-diboronic acid bis(pinacol) ester precursors and SeO2 and malononitrile in 75-90 % yield. The very short intramolecular Se⋅⋅⋅Se distance in 1-3 (3.22-3.24 Å), a consequence of a strong intramolecular ChB interaction, expands to 3.52-3.54 Šin the chalcogen-bonded adducts with bpen, a distance (<4 Å) well adapted to the face-to-face association of the bpen molecules into the reactive position toward photochemical dimerization.

N-Iodosaccharin-pyridine co-crystal system under pressure: experimental evidence of reversible twinning.

This work presents a single-crystal X-ray diffraction study of an organic co-crystal composed of N-iodosaccharin and pyridine (NISac·py) under hydrostatic pressure ranging from 0.00 (5) GPa to 4.5 (2) GPa. NISac·py crystallizes in the monoclinic system (space group B21/e). The unconventional setting of the space group is adopted (the conventional setting is P21/c, No. 14) to emphasise the strongly pseudo-orthorhombic symmetry of the lattice, with a ß angle very close to 90°. The crystal structure contains one molecule each of N-iodosaccharin (NISac) and pyridine (py) in the asymmetric unit (Z' = 1), linked via an Nsac...I...N'py halogen-bonding motif. A gradual modification of this motif is observed under pressure as a result of changes in the crystalline environment. Mechanical twinning is observed under compression and the sample splits into two domains, spanning an unequal volume that is mapped by a twofold rotation about the [100] direction of the B21/e unit cell. The twinning is particularly significant at high pressure, being reversible when the pressure is released. The structure of the twinned sample reveals the continuity of a substantial substructure across the composition plane. The presence of this common substructure in the two orientations of the twinned individuals can be interpreted as a structural reason for the formation of the twin and is the first observed example in a molecular crystal. These results indicate that the anisotropy of intermolecular interactions in the crystal structure results in an anisotropic strain generated upon the action of hydrostatic compression. Periodic density functional theory calculations were carried out by considering an isotropic external pressure, the results showing good agreement with the experimental findings. The bulk modulus of the crystal was obtained from the equations of state, being 7 (1) GPa for experimental data and 6.8 (5) GPa for theoretical data.

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.

Chalcogen Bonding in Co-Crystals: Activation through 1,4-Perfluorophenylene vs. 4,4'-Perfluorobiphenylene Cores.

The ability of alkylseleno/alkyltelluroacetylenes such as bis(selenomethylethynyl)-perfluorobenzene (4F-Se) to act as a ditopic chalcogen bond (ChB) donor in co-crystals with ditopic Lewis bases such as 4,4'-bipyridine is extended here to the octafluorobiphenylene analog, 4,4'-bis(selenomethylethynyl)-perfluorobiphenyl (8F-Se), with the more electron-rich 4,4'-bipyridylethane (bpe), showing in the 1:1 (8F-Se)•(bpe) co-crystal a shorter and more linear C-Se•••N ChB interaction than in (4F-Se)•(bpe), with Se•••N distances down to 2.958(2) Šat 150 K, i.e., a reduction ratio of 0.85 vs. the van der Waals contact distance.

Supramolecular rectangles through directional chalcogen bonding.

Supramolecular rectangles are built from the 2+2 chalcogen bonding-based (ChB) association of 1,8-bis(telluromethylethynyl)-anthracene (BTMEA) and ditopic Lewis bases such as 4,4'-bipyridyl-ethane and analogs, demonstrating the strength and directionality of the ChB interaction in such alkynyl-telluroalkyl derivatives.

Introducing Selenium in Single-Component Molecular Conductors Based on Nickel Bis(dithiolene) Complexes.

Two selenated analogues of the all-sulfur single-component molecular conductor [Ni(Et-thiazdt)2] (Et-thiazdt = N-ethylthiazoline-2-thione-4,5-dithiolate) have been prepared from their precursor radical-anion complexes. Replacement of the thione by a selenone moiety gives the neutral [Ni(Et-thiazSedt)2] complex. It adopts an unprecedented solid-state organization (for neutral nickel complexes), with the formation of perfectly eclipsed dimers and very short intermolecular Se···Se contacts (81% of the van der Waals contact distance). Limited interactions between dimers leads to a large semiconducting gap and low conductivity (σRT = 1.7 × 10-5 S cm-1). On the other hand, going from the neutral [Ni(Et-thiazdt)2] dithiolene complex to the corresponding [Ni(Et-thiazds)2] diselenolene complex gives rise to a more conventional layered structure built out of uniform stacks of the diselenolene complexes, different, however, from the all-sulfur analogue [Ni(Et-thiazdt)2]. Band structure calculations show an essentially 1D electronic structure with large band dispersion and a small HOMO-LUMO gap. Under high pressures (up to 19 GPa), the conductivity increases by 4 orders of magnitude and the activation energy is decreased from 120 meV to only 13 meV, with an abrupt change observed around 10 GPa, suggesting a structural phase transition under pressure.

A radical mixed-ligand gold bis(dithiolene) complex.

A mixed-ligand bis(dithiolene) gold complex is isolated from scrambling reaction of two neutral radical symmetric complexes. Its asymmetric character translates into specific structural and electronic properties including larger electrochemical and optical band gaps than those of both precursors.

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 ).

Hydrogen bonding interactions in single component molecular conductors based on metal (Ni, Au) bis(dithiolene) complexes.

Introduction of hydrogen bonding (HB) interactions in single component conductors derived from nickel and gold bis(dithiolene) complexes is explored with the 2-alkylthio-1,3-thiazole-4,5-dithiolate (RS-tzdt) with R = CH2CH2OH through the preparation of the neutral [Ni(HOEtS-tzdt)2]0 (closed-shell) and [Au(HOEtS-tzdt)2]˙ (radical) complexes. At variance with many other radical gold dithiolene complexes which have a strong tendency to dimerize in the solid state, [Au(HOEtS-tzdt)2]˙ crystallizes into uniform stacks interconnected by strong O-HN HB involving the nitrogen atom of the thiazole ring. [Au(HOEtS-tzdt)2]˙ is isostructural with its neutral, closed-shell nickel analog [Ni(HOEtS-tzdt)2]0, a rare situation in this metal bis(dithiolene) chemistry. It demonstrates how the strength of the HB directing motif can control the overall structural arrangement to stabilize the same structure despite a different electron count. The nickel complex behaves as a band semiconductor with weak room temperature conductivity (1.6 × 10-5 S cm-1), while the gold complex is described as a Mott insulator with a three orders of magnitude improved conductivity (6 × 10-2 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.

Charge localization in 1D tetramerized organic conductors: the special case of (tTTF)2ClO4.

We report a detailed structural and spectroscopic study of the 1D 2:1 cation radical salt (tTTF)2ClO4, where tTTF = trimethylenetetrathiafulvalene, which exhibits a semiconductor-semiconductor phase transition at ca. T = 137 K. Crystal structures are determined above and below the transition; the tTTF molecules in stacks are grouped into weakly interacting tetramers. The reorganization of tTTF stacks is accompanied with an order-disorder transition in anion sublattice. Polarized infrared and Raman spectra of (tTTF)2ClO4 are measured in the broad frequency range as a function of the temperature (10-293 K). The structural and vibrational features are investigated to elucidate the origin of the semiconductor-semiconductor phase transition. We discuss the electron-intramolecular vibration coupling effects in the vibrational spectra of (tTTF)2ClO4 and identify signatures of high- and low-temperature states of charge localization in the tetramerized system. Both the C=C and C-S stretching modes of tTTF give evidence of strong charge distribution fluctuations in conducting stacks for T > 137 K, which are responsible for the appearance of molecules with charge +1e, and charge localization in tTTF tetramers for T < 137 K. The uniqueness of the salt (tTTF)2ClO4 in comparison with other tetramerized 1D systems is discussed.

Ambipolar Discotic Liquid Crystals Built Around Platinum Diimine-Dithiolene Cores.

Platinum diimine dithiolene complexes bearing mesogenic groups on one or both ligands have been prepared through an original ligand metathesis reaction to introduce the dithiolene ligand. The neutral diimine ligands, the intermediate platinum dichloride diimine complexes, and the target compounds were characterized by a combination of electronic (electrochemistry, absorption and emission spectroscopy, DFT calculations) and structural (SAXS, DSC) tools. Several novel liquid crystalline platinum diimine-dithiolene were identified over a large temperature range, and the systems were endowed with ambipolar properties, associated with the high reversibility of both oxidation and reduction processes.