Synthesis, Structures, and Redox Properties of Octa(&mgr;(3)-sulfido)hexarhenium(III) Complexes Having Terminal Pyridine Ligands

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.

Octahedral Rhenium Cluster Complexes with 1,2-Bis(4-pyridyl)ethylene and 1,3-Bis(4-pyridyl)propane as Apical Ligands

A series of eight new octahedral rhenium cluster complexes with the general formula trans-[{Re₆Q₈}L₄X₂] (Q = S or Se; L = 1,2-Bis(4-pyridyl)ethylene (bpe) or 1,3-Bis(4-pyridyl)propane (bpp); X = Cl or Br) was synthesized and investigated. While bpe is a ligand with a conjugated aromatic system, bpp represents a molecule of opposite type and has independent aromatic systems of the two pyridine rings. It was shown that this difference in the electronic structure of the ligands has a fundamental effect on the electronic structure, electrochemical and luminescent properties of the corresponding cluster complexes. Specifically, the [{Re₆Q₈}(bpe)₄X₂] complexes in solutions show multiple quasi-reversible electrochemical transitions associated with reduction of the organic ligands. At the same time, the trans-[{Re₆Q₈}(bpp)₄X₂] complexes show multielectron quasi-irreversible reduction processes, which correlate with the mixed character of the lowest unoccupied molecular orbitals of these complexes. All the obtained new compounds exhibit red photoluminescence. The photophysical parameters (emission lifetimes and quantum yields) measured for the bpp complexes exceed those revealed for bpe complexes by more than an order of magnitude.

Tuning the Ground- and Excited-State Redox Potentials of Octahedral Hexanuclear Rhenium(III) Complexes by the Combination of Terminal Halide and N-Heteroaromatic Ligands

The present study reports that the ground- and excited-state Re₆(23e)/Re₆(24e) redox potentials of an octahedral hexanuclear rhenium(III) complex can be controlled by systematically changing the number and type of the N-heteroaromatic ligand (L) and the number of chloride ions at the six terminal positions. Photoirradiation of [Re₆(μ₃-S)₈Cl₆]^4- with an excess amount of L afforded a mono-L-substituted hexanuclear rhenium(III) complex, [Re₆(μ₃-S)₈Cl₅(L)]^3- (L = 4-dimethylaminopyridine (dmap), 3,5-lutidine (lut), 4-methylpyridine (mpy), pyridine (py), 4,4'-bipyridine (bpy), 4-cyanopyridine (cpy), and pyrazine (pz)). The bis- and tris-lut-substituted complexes, trans- and cis-[Re₆(μ₃-S)₈Cl₄(lut)₂]^2- and mer-[Re₆(μ₃-S)₈Cl₃(lut)₃]^-, were synthesized by the reaction of [Re₆(μ₃-S)₈Cl₆]^3- with an excess amount of lut in refluxed N,N-dimethylformamide. The mono-L-substituted complexes showed one-electron redox processes assignable to E _1/2[Re₆(23e)/Re₆(24e)] = 0.49-0.58 V versus Ag/AgCl. The ground-state oxidation potentials were linearly correlated with the pK _a of the N-heteroaromatic ligand [pK _a(L)], the ¹H NMR chemical shift of the ortho proton on the coordinating ligand, and the Hammett constant (σ) of the pyridyl-ligand substituent. The series of [Re₆(μ₃-S)₈X_6-n (L) _n ] ^n-4 complexes (n = 0, X = Cl, Br, I, or NCS; n = 1-3, X = Cl) showed a linear correlation with the sum of the Lever electrochemical parameters at the six terminal ligands (ΣE _L). The cyclic voltammograms of the mono-L-substituted complexes (L = bpy, cpy, and pz) showed one-electron redox waves assignable to E _1/2(L⁰/L^-) = -1.28 to -1.48 V versus Ag/AgCl. Two types of photoluminescences were observed for the complexes, originating from the cluster core-centered excited triplet state (³CC) for L = dmap, lut, mpy, and py and from the metal-to-ligand charge-transfer excited triplet state (³MLCT) for L = bpy, cpy, and pz. The complexes with the ³CC character exhibited emission features and photophysical properties similar to those of ordinary hexanuclear rhenium complexes. The emission maximum wavelength of the complexes with ³MLCT shifted to the longer wavelength in the order L = 4-phenylpyridine (ppy), bpy, pz, and cpy, which agreed with the difference between E _1/2[Re₆(23e)/Re₆(24e)] and E _1/2(L⁰/L^-). The calculated oxidation potential of the excited hexanuclear rhenium complex with the ³CC character was linearly correlated with pK _a(L), σ, and ΣE _L. The ground- and excited-state oxidation potentials were finely tuned by the combination of halide and L ligands at the terminal positions.

Thermally Controlled Synthesis of Octahedral Rhenium Clusters with 4,4′-Bipyridine and CN− Apical Ligands

The selective preparation, structural and spectroscopic study of two new rhenium cluster complexes trans-[Re6S8(bpy)4(CN)2] and trans-[Re6S8(bpy)2(CN)4]2− (bpy = 4,4′-bipyridine) obtained by reactions of corresponding hexarhenium cyanohalides with molten bpy are reported. The complexes were crystallized as solvates, displaying supramolecular structures based on cluster units linked by numerous weak interactions with bpy molecules. The molecular compound trans-[Re6S8(bpy)4(CN)2] (1) is insoluble in water and common organic solvents, while the ionic compound trans-Cs1.7K0.3[Re6S8(bpy)2(CN)4] (2) is somewhat soluble in DMSO, DMF and N-methylpyrrolidone. The presence of the redox-active ligand bpy leads to the occurrence of multi-electron reduction transitions in a solution of 2 at moderate potential values. The ambidentate CN− ligand is the secondary functional group, which has potential for the synthesis of coordination polymers based on the new cluster complexes. In addition, both new compounds show a weak red luminescence, which is characteristic of complexes with a {Re6S8}2+ cluster core.

Synthesis, Structure, and Spectroscopic Study of Redox-Active Heterometallic Cluster-Based Complexes [Re5MoSe8(CN)6]n

The heterometallic cluster-based compound K₅[Re₅MoSe₈(CN)₆] was obtained by high-temperature reaction from a mixture of ReSe₂ and MoSe₂ in molten potassium cyanide. The redox behavior of the [Re₅MoSe₈(CN)₆]^5- cluster anion was studied by cyclic voltammetry in aqueous and organic media showing two reversible one-electron-redox transitions with E_1/2 of -0.462 and 0.357 V versus Ag/AgCl in CH₃CN. Aqueous media potentials were found to be noticeably shifted to higher values because of solvation. Chemically accessible potentials allowed us to structurally isolate and characterize the [Re₅MoSe₈(CN)₆]ⁿ (n = 3-, 4-, and 5-) cluster complex in several charge states with corresponding cluster skeletal electron (CSE) numbers ranging from 24 to 22. The electronic absorption of the [Re₅MoSe₈(CN)₆]ⁿ cluster complex varies significantly upon a change of the CSE number, especially in the visible and near-IR regions. The local cluster core distortion upon electron removal was confirmed by density functional theory calculation, while the overall geometry of the cluster anion remained practically unaltered.

Redox Potential and Crystal Chemistry of Hexanuclear Cluster Compounds

Most of TM₆-cluster compounds (TM = transition metal) are soluble in polar solvents, in which the cluster units commonly remain intact, preserving the same atomic arrangement as in solids. Consequently, the redox potential is often used to characterize structural and electronic features of respective solids. Although a high lability and variety of ligands allow for tuning of redox potential and of the related spectroscopic properties in wide ranges, the mechanism of this tuning is still unclear. Crystal chemistry approach was applied for the first time to clarify this mechanism. It was shown that there are two factors affecting redox potential of a given metal couple: Lever’s electrochemical parameters of the ligands and the effective ionic charge of TM, which in cluster compounds differs effectively from the formal value due to the bond strains around TM atoms. Calculations of the effective ionic charge of TMs were performed in the framework of bond valence model, which relates the valence of a bond to its length by simple Pauling relationship. It was also shown that due to the bond strains the charge depends mainly on the atomic size of the inner ligands.

Water-Soluble Rhenium Clusters with Triazoles: The Effect of Chemical Structure on Cellular Internalization and the DNA Binding of the Complexes

Here we explore the effect of the nature of organic ligands in rhenium cluster complexes [Re₆ Q₈ L₆ ]^4- (where Q=S or Se, and L=benzotriazole, 1,2,3-triazole or 1,2,4-triazole) on the biological properties of the complexes, in particular on the cellular toxicity, cellular internalization and localization. Specifically, the study describes the synthesis and detailed characterization of the structure, luminescence and electrochemical properties of the four new Re₆ clusters with 1,2,3- and 1,2,4-triazoles. Biological assays of these complexes are also discussed in addition to those with benzotriazole using cervical cancer (HeLa) and immortalized human fibroblasts (CRL-4025) as model cell lines. Our study demonstrates that the presence of hydrophobic and π-bonding rich units such as the benzene ring in benzotriazole significantly enhances cellular internalization of rhenium clusters. These ligands facilitate binding of the clusters to DNA, which results in increased cytotoxicity of the complexes.

Soluble Molecular Rhenium Cluster Complexes Exhibiting Multistage Terminal Ligands Reduction

Substitution of terminal halide ligands of octahedral rhenium cluster complexes [Re₆Q₈X₆]^4- in a melt of 4,4'-bipyridine (bpy) led to us obtaining four new compounds with the general formula trans-[Re₆Q₈(bpy)₄X₂] (Q = S or Se; X = Cl or Br) in high yield. In contrast to most of the known molecular rhenium cluster complexes with heteroaromatic terminal ligands, compounds 1-4 are soluble in organic solvents. This made it possible to carry out a detailed characterization of the new compounds both in solids and in solutions. In particular, it was shown that compounds 1-4 in the DMSO solution exhibit four reversible reduction processes. A comparison of the obtained data with the results of DFT calculations of the electronic structure suggests that these processes correspond to two-electron reduction of all four bpy ligands. The reduction potentials are shifted to the positive region compared with the potential of free bipyridine, and the value of the shift depends on the composition of the cluster core. The presence of four transitions also suggests that electronic exchange between terminal ligands in the cis-position is possible. The study of the luminescence of the compounds showed that emission maxima of selenide clusters almost coincide with those of sulfide ones, while luminescence spectra of the complexes with chloride terminal ligands (1 and 3) are slightly blue-shifted relative to the spectra of the complexes with bromide ligands (2 and 4).

Electrochemical behaviors and relativistic DFT calculations to understand the terminal ligand influence on the [Re6(μ3-Q)8X6]4− clusters

A Born–Haber thermodynamic cycle was used to determine the redox potential in a series of rhenium(iii) clusters theoretical analysis at DFT level was considered to estimate the free energy of the reversible process ReIII6/ReIII5Re^IV.

Self-assembly of ambivalent organic/inorganic building blocks containing Re6 metal atom cluster: formation of a luminescent honeycomb, hollow, tubular metal-organic framework

Reactions in a sealed glass tube between melted pyrazine (pyz) and a Cs(3)Re(6)Q(i)(7)Br(i)Br(a)(6).H(2)O inorganic rhenium cluster compound (Q = S, Se; "i" for inner and "a" for apical positions) containing [Re(6)Q(i)(7)Br(i)Br(a)(6)](3-) units led to the substitution of three apical bromine ligands by three pyrazine groups with the formation of 3 CsBr as a byproduct. The resulting fac-Re(6)Q(i)(7)Br(i)(pyz)(a)(3)Br(a)(3) building unit, based on a Re(6) metal atom cluster, is neutral and noncentrosymmetric and exhibits an ambivalent organic/inorganic nature owing to the opposite disposition of the three apical pyrazine groups versus the three apical bromine atoms. These compounds were characterized by single-crystal and powder X-ray diffraction, elemental and thermal analyses, and luminescence measurements. The crystal structure of fac-Re(6)Q(i)(7)Br(i)(pyz)(a)(3)Br(a)(3).xH(2)O (Q = S (1) and Se (2)) displays an original, neutral metal-organic framework based on the self-assembling of fac-Re(6)Q(i)(7)Br(i)(pyz)(a)(3)Br(a)(3) hybrid building units. The latter are held together by supramolecular interactions: pi-pi, hydrogen bonds (C-H...N, C-H...Br(a), and C-H...Br(i)), and van der Waals contacts. It gives rise to a honeycomb porous structure of parallel hollow open-ended channels wherein the water molecules are located. Their removal does not lead to the collapsing of the structural edifice. The channel walls are constituted by hydrogen atoms from pyrazine as well as apical bromine from the cluster unit. To our knowledge, the structures of 1 and 2 constitute with that of PTMTC (perchlorotriphenylmethyl functionalized by carboxylic group radicals) one of the rare examples of stable open frameworks stabilized by supramolecular interactions between neutral molecules. Moreover, 1 is the first example of luminescent Re(6) compound built up from a noncentrosymmetric Re(6)S(i)(7)Br(i) cluster core.

Excited-state properties of octahedral hexarhenium(III) complexes with redox-active N-heteroaromatic ligands

New sulfide-capped octahedral hexarhenium(III) complexes containing 4-phenylpyridine (ppy) or 1,2-bis(4-pyridyl)ethane (bpe) ((n-C(4)H(9))(4)N)[mer-{Re(6)S(8)Cl(3)(ppy)(3)}] ((Bu(4)N)[1]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(ppy)(2)}] ((Bu(4)N)(2)[2a]), ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(ppy)(2)}] ((Bu(4)N)(2)[2b]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(bpe)(2)}] ((Bu(4)N)(2)[3a]), and ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(bpe)(2)}] ((Bu(4)N)(2)[3b]) were prepared, and X-ray single-crystal structure determination was carried out for (Bu(4)N)(2)[2a] and (Bu(4)N)(2)[3a]. The photophysical properties of these complexes were studied both in acetonitrile at 298 K and in the solid state at 298 and 80 K, along with those of the known 4,4'-bipyridine (bpy) analogues ((n-C(4)H(9))(4)N)[mer-{Re(6)S(8)Cl(3)(bpy)(3)}] ((Bu(4)N)[4]), ((n-C(4)H(9))(4)N)(2)[trans-{Re(6)S(8)Cl(4)(bpy)(2)}] ((Bu(4)N)(2)[5a]), and ((n-C(4)H(9))(4)N)(2)[cis-{Re(6)S(8)Cl(4)(bpy)(2)}] ((Bu(4)N)(2)[5b]). The photophysical data of [5a](2-) and [5b](2-) in solution and in the solid state were significantly different from those of other complexes. On the basis of experimental observations of [2a](2-) and [5a](2-) and density-functional theory (DFT) calculations, it was concluded that [5a](2-) and [5b](2-) exhibited metal (Re(6)S(8) core)-to-ligand (bpy) charge transfer (MLCT) type emission. This is the first unambiguous demonstration of MLCT type emissions for the hexarhenium complexes. The MLCT components, where present, are only minor in the case of the emissions of [1](-), [2a](2-), [2b](2-), and [4](-); these can be explained primarily as the contributions of the intracore electronic transitions. The emissions of [3a](2-) and [3b](2-) can be assigned almost completely to the electronic transitions within the Re(6)S(8) core. The different emission characteristics of the bis(bpy) complexes ([5a](2-) and [5b](2-)) from the tris(bpy) complex ([4](-)) are a result of the increase in the number of nitrogen donors on the Re(6)S(8) core, which stabilizes the Re(6)S(8) core energy to a lower level than the energy of the bpy ligand pi* orbital. On the other hand, it has been shown that the emissions of the bis(ppy) ([2a](2-) and [2b](2-)) and bis(bpe) complexes ([3a](2-) and [3b](2-)) are best characterized by the higher pi* energy level of each N-heteroaromatic ligand, which lead to a stronger metal character in the emissive excited state of the complex.

Selective functionalisation of Re6 cluster anionic units: from hexa-hydroxo [Re6Q8(OH)6]4- (Q = S, Se) to neutral trans-[Re6Q8L4L'2] hybrid building blocks

The reaction of [Re6Q(i)8(OH)(a)6]4- (Q = S, Se) with p-tert-butylpyridine (TBP) in water leads to neutral trans-[Re6Q8(TBP)4(OH)2] whose hydroxyl reactivity with carboxylic acid and TBP exchange reaction with functional pyridine have been investigated.

Synthesis and electrochemical study of the first tetrazolate hexanuclear rhenium cluster complex

The linkage isomers [Re(6)Se(8)(PEt(3))(5)(1,5-MeN(4)C)](+) and [Re(6)Se(8)(PEt(3))(5)(2,5-MeN(4)C)](+) were generated upon reaction of tetrabutylammonium azide with the corresponding acetonitrile complex, [Re(6)Se(8)(PEt(3))(5)(NCCH(3))](2+); these are the first (tetrazolato)rhenium complexes reported to date.

A family of octahedral rhenium cluster complexes [Re6Q8(H2O)n(OH)6-n]n-4 (Q=S, Se; n=0-6): structural and pH-dependent spectroscopic studies

The conversions of hexahydroxo rhenium cluster complexes [Re6Q8(OH)6]4- (Q=S, Se) in aqueous solutions in a wide pH range were investigated by chemical methods and spectroscopic measurements. Dependences of the spectroscopic and excited-state properties of the solutions on pH have been studied in detail. It has been found that a pH decrease of aqueous solutions of the potassium salts K4[Re6Q8(OH)6].8H2O (Q=S, Se) results in the formation of aquahydroxo and hexaaqua cluster complexes with the general formula [Re6Q8(H2O)n(OH)6-n]n-4 that could be considered as a result of the protonation of the terminal OH- ligands in the hexahydroxo complexes. The compounds K2[Re6S8(H2O)2(OH)4].2H2O (1), [Re6S8(H2O)4(OH)2].12H2O (2), [Re6S8(H2O)6][Re6S6Br8].10H2O (3), and [Re6Se8(H2O)4(OH)2] (4) have been isolated and characterized by X-ray single-crystal diffraction and elemental analyses and infrared (IR) spectroscopy. In crystal structures of the aquahydroxo complexes, the cluster units are connected to each other by an extensive system of very strong hydrogen bonds between terminal ligands.

Cluster core controlled reactions of substitution of terminal bromide ligands by triphenylphosphine in octahedral rhenium chalcobromide complexes

Reactions of rhenium chalcobromides Cs4[{Re6(mu3-S)8}Br6].2H2O, Cs3[{Re6(mu3-Se)8}Br6].2H2O, Cs3[{Re6(mu3-Q)7(mu3-Br)}Br6].H2O (Q = S, Se), and K2[{Re6(mu3-S)6(mu3-Br)2}Br6] with molten triphenylphosphine (PPh3) have resulted in a family of novel molecular hybrid inorganic-organic cluster compounds. Six octahedral rhenium cluster complexes containing PPh3 ligands with general formula [{Re6(mu3-Q)8-n(mu3-Br)n}(PPh3)4-nBrn+2] (Q = S, n = 0, 1, 2; Q = Se, n = 0, 1) have been synthesized and characterized by X-ray single-crystal diffraction and elemental analyses, 31P{1H} NMR, luminescent measurements, and quantum-chemical calculations. It was found that the number of terminal PPh3 ligands in the complexes is controlled by the composition and consequently by the charge of the cluster core {Re6Q8-nBrn}n+2. In crystal structures of the complexes with mixed chalcogen/bromine ligands in the cluster core all positions of a cube [Q8-nBrn] are ordered and occupied exclusively by Q or Br atoms. Luminescence characteristics of the compounds trans-[{Re6Q8}(PPh3)4Br2] and fac-[{Re6Se7Br}(PPh3)3Br3] (Q = S, Se) have been investigated in CH2Cl2 solution and the broad emission spectra in the range of 600-850 nm were observed.

Multielectron Donors Based on TTF−Phosphine and Ferrocene−Phosphine Hybrid Complexes of a Hexarhenium(III) Octahedral Cluster Core

Electroactive molecular materials precursors are obtained through coordination chemistry of the hexarhenium cluster core [Re(6)Se(8)](2+) on the six available apical positions with redox-active phosphines bearing tetrathiafulvalene- or ferrocene-based moieties. Single-crystal X-ray diffraction study and electrospray mass spectrometry ascertain the synthesis of these hexasubstituted electroactive clusters, containing up to 12 redox active sites. Cyclic voltammetry experiments demonstrate that these compounds can be reversibly oxidized at rather low potentials, thus allowing an easy access to the corresponding radical species which should provide new conducting and/or magnetic molecular materials.

Ligand-ligand redox interaction through some metal-cluster units

The ligand-ligand redox interaction separated by di-, tri-, and hexanuclear cluster units is discussed in terms of the splitting of the redox waves (DeltaE(L)) and the comproportionation constants (Kcom(L)) of the ligand-based mixed valence state. Although two redox-active monodentate ligands in the mononuclear ruthenium(II) complexes (either cis or trans configurations) do not show appreciable splitting in their reduction waves, interestingly those separated by giant triruthenium and hexarhenium cluster units clearly show splitting. The molecular orbital considerations in the literature suggest that these units possess some pi-type molecular orbitals composed of metal dpi and possibly ligand ppi orbitals. Absence of the redox interactions in oxo-bridged diruthenium(II) and oxo-centered trirhodium(III) complexes where such pi molecular orbitals (including an antibonding one) are fully occupied, indicates the importance of empty pi* orbitals for the ligand redox communication.

Preparation, structures, and redox and emission characteristics of the isothiocyanate complexes of hexarhenium(III) clusters [Re6(mu3-E)8(NCS)6]4- (E = S, Se)

Hexarhenium(III) complexes with terminal isothiocyanate ligands, [(n-C(4)H(9))(4)N](4)[Re(6)(mu(3)-S)(8)(NCS)(6)] (1) and (L)(4)[Re(6)(mu(3)-Se)(8)(NCS)(6)] (L(+) = PPN(+) (2a), (n-C(4)H(9))(4)N(+) (2b)), have been prepared by three different methods. Complex 1 was prepared by the reaction of [(n-C(4)H(9))(4)N](4)[Re(6)(mu(3)-S)(8)Cl(6)] with molten KSCN at 200 degrees C, while 2b was obtained by refluxing the chlorobenzene-DMF (2:1 v/v) solution of [Re(6)(mu(3)-Se)(8)(CH(3)CN)(6)](SbF(6))(2) and [(n-C(4)H(9))(4)N]SCN. The [Re(6)(mu(3)-Se)(8)(NCS)(6)](4)(-) anion was also obtained from a mixture of Cs(2)[Re(6)(mu(3)-Se)(8)Br(4)] and KSCN in C(2)H(5)OH by a mechanochemical activation at room temperature for 20 h and isolated as 2a. The X-ray structures of 1 and 2a.4DMF have been determined (1, C(70)H(144)N(10)S(14)Re(6), monoclinic, space group P2(1)/n (No. 14), a = 14.464(7) A, b = 22.059(6) A, c = 16.642(8) A, beta = 113.62(3) degrees, V = 4864(3) A(3), Z = 2; 2a.4DMF, C(162)H(144)N(14)O(4)P(8)S(6)Se(8)Re(6), triclinic, space group P1 (No. 2), a = 15.263(2) A, b = 16.429(2) A, c = 17.111(3) A, alpha = 84.07(1) degrees, beta = 84.95(1) degrees, gamma = 74.21(1) degrees, V = 4098.3(8) A(3), Z = 1). All the NCS(-) ligands in both complexes are coordinated to the metal center via nitrogen site with the Re-N distances in the range of 2.07-2.13 A. The redox potentials of the reversible Re(III)(6)/Re(III)(5)Re(IV) process in acetonitrile are +0.84 and +0.70 V vs. Ag/AgCl for [Re(6)(mu(3)-S)(8)(NCS)(6)](4)(-) and [Re(6)(mu(3)-Se)(8)(NCS)(6)](4)(-), respectively, which are the most positive among the known hexarhenium complexes with six terminal anionic ligands. The complexes show strong red luminescence with the emission maxima (lambda(max)/nm), lifetimes (tau(em)/micros), and quantum yields (phi(em)) being 745 and 715, 10.4 and 11.8, and 0.091 and 0.15 for 1 and 2b, respectively, in acetonitrile. The data reasonably well fit in the energy-gap plots of other hexarhenium(III) complexes. The temperature dependence of the emission spectra and tau(em) of 1 and [(n-C(4)H(9))(4)N](4)[Re(6)(mu(3)-S)(8)Cl(6)] are also reported.

The Dean-Evans relation in (31)P NMR spectroscopy and its application to the chemistry of octahedral tungsten sulfide clusters

One of the challenges in studying the chemistry of hexanuclear octahedral metal clusters is analyzing the many possible complexes, including stereoisomers, when these complexes consist of mixed axial ligands (two or more). In the case of W(6)S(8)L(6-n)(PR(3))(n)(n = 0-6; L = nonphosphine Lewis base ligands, PR(3) = phosphines) clusters, in situ identification of the 10 possible complexes is possible by (31)P NMR due to P-W-W-P coupling. A linear relation for (31)P NMR shifts (delta((31)P)) of these W(6)S(8)L(6-n)(PR(3))(n) complexes, analogous to the Dean-Evans relation for (19)F NMR shifts of octahedral tin complexes, is found and expressed as delta((31)P) = delta(ref) + pC + qT with two variables (p and q, the number of ligands L in the cis or trans position to PR(3), respectively) with two constants (C and T, characteristic of a given ligand L). (31)P NMR investigation of over 200 complexes in 26 W(6)S(8)L(6-n)(PR(3))(n) systems show that this relation is generally valid for W(6)S(8) clusters. Such a relation helps spectroscopic assignments and demonstrates the trans and cis influence on hexanuclear clusters. Large bulky ligands cause deviations from the linear behavior due to steric effects. With the help of 2-D (31)P NMR spectroscopy, mixtures of W(6)S(8)(PR(3))(6-n)(PR'(3))(n) (n = 0-6) complexes can also be unequivocally interpreted. The Dean-Evans relation is expanded to account for different phosphine ligands. Partial substitution reactions of these W(6)S(8) complexes by phosphines were investigated using (31)P NMR, and four single crystals of mixed-ligand clusters are characterized with X-ray diffraction. In summary, (31)P NMR and other NMR techniques, combined with Dean-Evans relations, are invaluable analytical tools for studying molecular W(6)S(8) cluster chemistry and are likely to be useful for studying other mixed-ligand metal clusters.

Electronic structure of diamagnetic and paramagnetic hexanuclear chalcohalide clusters of rhenium

Hexanuclear chalcohalide clusters of rhenium(III) of general formula [Re(6)S(4+x)Cl(10-x)](x-) with x = 1-4 have been studied using quantum chemical DFT calculations. The optimized structures reproduce the geometrical features found from X-ray data for the members of the series. The relative stability of different stereoisomers for the mono- and dianions has been estimated. The analysis of the tetraanion series [Re(6)Q(8)X(6)](4-) with Q = S, Se and X = Cl, Br, I, and CN demonstrates the influence of the mu(1)- and mu( 3)-ligands on the strength of Re-apical ligand bond. It is shown that the tetragonal distortion found for the stable oxidized paramagnetic species [Re(6)S(8)Cl(6)]*(3-) results from the Jahn-Teller effect for a doubly degenerate electronic state.