Complexes of {W6I8}4+Clusters with Carboxylates: Preparation, Electrochemistry, and Luminescence

Octahedral Hexanuclear Rhenium Cluster Dimers Bridged by Pyrazine or 4,4'-Bipyridine with 23- and 24-Electron Configurations

New pyrazine (pz)- and 4,4'-bipyridine (4,4'-bpy)-bridged octahedral hexanuclear rhenium(III) cluster dimers, [{Re6(μ3-S)8Cl5}2(μ-L)]6- (L = pz, [1]6-; L = 4,4'-bpy, [2]6-), with 2 × 24 d-electrons {Re6(24e)}2 were obtained in a single-step reaction via photoirradiation of [Re6(μ3-S)8Cl6]4- with L in a 2:1 ratio at room temperature. The {Re6(23e)}2 dimers, [{Re6(μ3-S)8Cl5}2(μ-L)]4- (L = pz, [1']4-; L = 4,4'-bpy, [2']4-), were synthesized through two-electron oxidation of [1]6- and [2]6-, respectively. The single-crystal X-ray structures of [1]6- and [1']4- were determined, revealing structural distortion of the Re6 core of [1']4- due to the Jahn-Teller effect. The cyclic voltammograms of [1]6- and [2]6- showed two steps of one-electron redox processes attributable to Re6(23e)Re6(24e)/{Re6(24e)}2 and {Re6(23e)}2/Re6(23e)Re6(24e), respectively. The separation between the two redox potentials is small (0.056 V for [1]6- and 0.039 V for [2]6-). The magnetic susceptibilities of [1']4- and [2']4- were almost temperature-independent, with values of 2.98 and 2.85 μB, respectively, indicating paramagnetism. These results suggest weak electronic interaction between two cluster units bridged by pz or 4,4'-bpy in the intercluster mixed valence state. The compounds [1]6- and [2]6- show photoluminescence in the near-infrared region at 296 K in the solid state.

Evolution of the Electronic Structure of the trans-[Re6S8bipy4Cl2] Octahedral Rhenium Cluster during Reduction

Understanding the processes that occur during the redox transformations of complexes coordinated by redox-active apical ligands is important for the design of electrochemically active compounds with functional properties. In this work, a detailed analysis of the interaction energy and electronic structure was performed for cluster complexes trans-[Re₆S₈bipy₄Cl₂]ⁿ (n = 2-, 4-, 6-, 8-), which can be obtained by stepwise electrochemical reduction of a neutral cluster trans-[Re₆S₈bipy₄Cl₂] in DMSO solution. It was shown that the formation of open-shell paramagnetic ions with S = 1, 2 and 1 is the most energetically favorable for n = 2-, 4- and 6-, respectively.

Preparation, photoluminescence and excited state properties of the homoleptic cluster cation [(W6I8)(CH3CN)6]4

Solvated tungsten iodide cluster compounds are presented with the homoleptic cluster cation [(W₆I₈)(CH₃CN)₆]⁴⁺ and the heteroleptic [(W₆I₈)I(CH₃CN)₅]³⁺, synthesized from W₆I₂₂ in acetonitrile. Crystal structures were solved and refined on deep red single-crystals of [(W₆I₈)(CH₃CN)₆](I₃)(BF₄)₃·H₂O, [(W₆I₈)I(CH₃CN)₅](I₃)₂(BF₄), and on a yellow single-crystal of [W₆I₈(CH₃CN)₆](BF₄)₄·2(CH₃CN) on the basis of X-ray diffraction data. The structure of the homoleptic [(W₆I₈)(CH₃CN)₆]⁴⁺ cluster is based on the octahedral [W₆I₈]⁴⁺ tungsten iodide cluster core, coordinated by six apical acetonitrile ligands. The electron localisation function of [(W₆I₈)(CH₃CN)₆]⁴⁺ is calculated and solid-state photoluminescence and its temperature depedence are reported. Additionally, photoluminescence and transient absorption measurements in acetonitrile are shown. Results of the obtained data are compared to compounds containing [(M₆I₈)I₆]^2- and [(M₆I₈)L₆]^2- (M = Mo or W; L = ligand) clusters.

Improved Synthesis of (TBA)2[W6Br14] Paving the Way to Further Study of Bromide Cluster Complexes

Octahedral cluster complexes of molybdenum and tungsten, [M₆X₈Y₆]^n- (M = Mo, W; X, Y = Cl, Br, I), are promising active components in various fields, including biomedicine and solar energy. Cluster complexes draw considerable attention due to their X-ray opacity, red/near-IR luminescence, and ability to convert triplet molecular oxygen to active singlet oxygen under UV and visible irradiation. Among the octahedral cluster complexes of molybdenum and tungsten, compounds with a {W₆Br₈}⁴⁺ core are the least studied. There are only a few examples of compounds with substituted terminal ligands, and their properties are not well understood. Among other things, this is due to more labor-intensive and expensive methods for obtaining the starting compounds in comparison with molybdenum counterparts. In this paper, we describe the synthesis of an octahedral cluster complex, (TBA)₂[W₆Br₁₄] (TBA⁺ = tetrabutylammonium), in gram quantities, starting from simple substances─W, Br₂, and Bi─in 70% yield. The formation of pentanuclear tungsten cluster complexes was recorded as a byproduct. Compounds with substituted terminal ligands (TBA)₂[W₆Br₈Y₆] (Y = NO₃, Cl, I) were obtained. We also discuss the instability of (TBA)₂[W₆Br₈(NO₃)₆] under light exposure, the optical properties of a series of compounds (TBA)₂[W₆Br₈Y₆] (Y = Cl, Br, I), and the effect of terminal ligands on the chemical shifts in ¹⁸³W NMR spectra in dimethyl sulfoxide-d₆. The presented approach to the synthesis of one of the main precursors of various bromide cluster complexes on a gram scale can stimulate the study of their properties and development of new functional materials based on them.

Evidencing ((n-C4H9)4N)2[W6I14] red–NIR emission and singlet oxygen generation by two photon absorption

Two photon absorption induced NIR emission has been observed for the first time for octahedral transition metal clusters.

Cyanide Complexes Based on {Mo6I8}4+ and {W6I8}4+ Cluster Cores

Compounds based on new cyanide cluster anions [{Mo₆I₈}(CN)₆]^2–, trans-[{Mo₆I₈}(CN)₄(MeO)₂]^2– and trans-[{W₆I₈}(CN)₂(MeO)₄]²⁻ were synthesized using mechanochemical or solvothermal synthesis. The crystal and electronic structures as well as spectroscopic properties of the anions were investigated. It was found that the new compounds exhibit red luminescence upon excitation by UV light in the solid state and solutions, as other cluster complexes based on {Mo₆I₈}⁴⁺ and {W₆I₈}⁴⁺ cores do. The compounds can be recrystallized from aqueous methanol solutions; besides this, it was shown using NMR and UV-Vis spectroscopy that anions did not undergo hydrolysis in the solutions for a long time. These facts indicate that hydrolytic stabilization of {Mo₆I₈} and {W₆I₈} cluster cores can be achieved by coordination of cyanide ligands.

Enhanced Photocatalytic Activity and Stability in Hydrogen Evolution of Mo6 Iodide Clusters Supported on Graphene Oxide

Catalytic properties of the cluster compound (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆] (TBA = tetrabutylammonium) and a new hybrid material (TBA)₂Mo₆Iⁱ₈@GO (GO = graphene oxide) in water photoreduction into molecular hydrogen were investigated. New hybrid material (TBA)₂Mo₆Iⁱ₈@GO was prepared by coordinative immobilization of the (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆] onto GO sheets and characterized by spectroscopic, analytical, and morphological techniques. Liquid and, for the first time, gas phase conditions were chosen for catalytic experiments under UV–Vis irradiation. In liquid water, optimal H₂ production yields were obtained after using (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆] and (TBA)₂Mo₆Iⁱ₈@GO) catalysts after 5 h of irradiation of liquid water. Despite these remarkable catalytic performances, “liquid-phase” catalytic systems have serious drawbacks: the cluster anion evolves to less active cluster species with partial hydrolytic decomposition, and the nanocomposite completely decays in the process. Vapor water photoreduction showed lower catalytic performance but offers more advantages in terms of cluster stability, even after longer radiation exposure times and recyclability of both catalysts. The turnover frequency (TOF) of (TBA)₂Mo₆Iⁱ₈@GO is three times higher than that of the microcrystalline (TBA)₂[Mo₆Iⁱ₈(O₂CCH₃)^a₆], in agreement with the better accessibility of catalytic cluster sites for water molecules in the gas phase. This bodes well for the possibility of creating {Mo₆I₈}⁴⁺-based materials as catalysts in hydrogen production technology from water vapor.

Phosphorescent complexes of {Mo6I8}4+ with triazolates: [2+3] cycloaddition of alkynes to [Mo6I8(N3)6]2−

“Click” reaction of activated alkynes with [Mo₆I₈(N₃)₆]²⁻ produces novel emissive triazolate complexes with the {Mo₆I₈}⁴⁺ cluster core.

Terminal Ligand (L) Effects on Zero-Magnetic-Field Splitting in the Excited Triplet States of [{Mo6Br8}L6]2- (L = Aromatic Carboxylates)

We report the emission properties of the octahedral hexamolybdenum(II) bromide-core ({Mo₆Br₈}⁴⁺) clusters having a series of terminal aromatic carboxylate ligands (RCOO), [{Mo₆Br₈}(RCOO)₆]^2-, in solution and crystalline phases. The acid dissociation constant of RCOOH (p K_a(L)) was shown to govern the redox and emission properties of the clusters. Temperature ( T)-controlled emission experiments (3-300 K) demonstrated that the clusters showed large T-dependent emission energies (ν̃^em) and lifetimes (τ^em) because of zero-magnetic-field splitting in the emissive excited triplet (T₁) states. The spin sublevel (Φ _n, n = 1-4) model in the T₁ state of the cluster explained very well the T-dependent emission characteristics (ν̃^em and τ^em), irrespective of the clusters studied. Furthermore, we revealed that the energy difference between the lowest-energy (Φ₁) and energetically upper-lying third (Φ₃) or fourth spin sublevels (Φ₄), Δ E₁₃ or Δ E₁₄, respectively, correlated very well with p K_a( L). The results are discussed in terms of the variation of the effective nuclear charge of the Mo metal center(s) in [{Mo₆Br₈}(RCOO)₆]^2- with that of p K_a(L).

Synthesis, structures, redox properties, and theoretical calculations of thiohalide capped octahedral hexanuclear technetium(iii) clusters

Thiohalide capped octahedral hexanuclear technetium(iii) clusters were synthesized and characterized.

Functionalization of [Re6Q8(CN)6]4− clusters by methylation of cyanide ligands

Methylation of anionic cluster complexes [Re₆Q₈(CN)₆]⁴⁻ with ((CH₃)₃O)BF₄ or CF₃SO₃CH₃ afforded homoleptic isonitrile cluster complexes [Re₆Q₈(CH₃NC)₆]²⁺ (Q = S, Se, Te).

Host-Guest Binding Hierarchy within Redox- and Luminescence-Responsive Supramolecular Self-Assembly Based on Chalcogenide Clusters and γ-Cyclodextrin

Water-soluble salts of anionic [Re₆ Q₈ (CN)₆ ]^4- (Q=S, Se, Te) chalcogenide octahedral rhenium clusters react with γ-cyclodextrin (γ-CD) producing a new type of inclusion compounds. Crystal structures determined through single-crystal X-ray diffraction analysis revealed supramolecular host-guest assemblies resulting from close encapsulations of the octahedral cluster within two γ-CDs. Interestingly, nature of the inner Q ligands influences strongly the host-guest conformation. The cluster [Re₆ S₈ (CN)₆ ]^4- interacts preferentially with the primary faces of the γ-CD while the bulkier clusters [Re₆ Se₈ (CN)₆ ]^4- and [Re₆ Te₈ (CN)₆ ]^4- exhibit specific interactions with the secondary faces of the cyclic host. Furthermore, analysis of the crystal packing reveals additional supramolecular interactions that lead to 2D infinite arrangements with [Re₆ S₈ (CN)₆ ]^4- or to 1D "bamboo-like" columns with [Re₆ Se₈ (CN)₆ ]^4- and [Re₆ Te₈ (CN)₆ ]^4- species. Solution studies, using multinuclear NMR methods, ESI-MS and Isothermal titration calorimetry (ITC) corroborates nicely the solid-state investigations showing that supramolecular pre-organization is retained in aqueous solution even in diluted conditions. Furthermore, ITC analysis showed that host-guest stability increases significantly ongoing from S to Te. At last, we report herein that deep inclusion alters significantly the intrinsic physical-chemical properties of the octahedral clusters, allowing redox tuning and near IR luminescence enhancement.