Alcohol Addition to Acetonitrile Activated by the [Re6(μ3-Se)8]2+ Cluster Core

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.

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.

Water-soluble Re6-clusters with aromatic phosphine ligands – from synthesis to potential biomedical applications

New hexarhenium clusters exhibit radio- and photoluminescence, have low cytotoxicity, are capable of penetrating into cells and exhibit photodynamic toxicity.

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).

Small molecule activation of nitriles coordinated to the [Re6Se8]2+ core: formation of oxazine, oxazoline and carboxamide complexes

Novel oxazine, oxazoline and carboxamide cluster complexes were prepared when different nucleophilic oxygen species reacted with nitriles coordinated to the Lewis acidic [Re6Se8]2+ cluster core. Reaction of ICH2CH2O- (generated in situ) with [Re6Se8(PEt3)5(NCR)]A2 (1A2 (R = Me) and 2A2 (R = Ph) where A = BF4-), leads to the formation of [Re6Se8(PEt3)5(2-methyloxazoline)]2+ (32+) and [Re6Se8(PEt3)5(2-phenyloxazoline)]2+ (42+). Similarly, reaction of BrCH2CH2CH2O- with the same nitrile complexes, 1A2 and 2A2 (where A = BF4- or SbF6-) leads to the corresponding oxazine complexes, [Re6Se8(PEt3)5(2-methyloxazine)]2+ (52+) and [Re6Se8(PEt3)5(2-phenyloxazine)]2+ (62+). In addition, reaction of 2(BF4)2 with KOH leads to the formation of the carboxamide complex, [Re6Se8(PEt3)5(phenylcarboxamide)](BF4) (7(BF4)). The neutral oxazine and oxazoline ligands can be removed using either heat or UV irradiation; UV irradiation was found to be more efficient at ligand removal as indicated by the shorter reaction times. The relative coordination strength of the neutral N-donor ligands was determined by these reaction times. X-ray structure determinations of 5(BF4)2 and 7(BF4) are also reported.

Amidine Production by the Addition of NH3 to Nitrile(s) Bound to and Activated by the Lewis Acidic [Re6(μ3-Se)8]2+ Cluster Core

Acetonitrile bound to and activated by the Lewis acidic [Re₆(μ₃-Se)₈]²⁺ cluster core was transformed into acetamidine in quantitative yield using NH₃ as the nucleophile at room temperature. The amidine ligand was removed by treating the cluster-acetamidine complexes with trifluoroacetic acid in CH₃CN, affording amidinium trifluoroacetate and the starting acetonitrile complexes.

Crystal structure of octa-μ3-selenido-(p-toluene-sulfonato-κO)penta-kis-(tri-ethyl-phosphane-κP)-octa-hedro-hexa-rhenium(III) p-toluene-sulfonate di-chloro-methane disolvate

The title compound, [Re6Se8{O3SC6H4(CH3)}{P(C2H5)3}5](CH3C6H4SO3)·2CH2Cl2, contains the face-capped hexa-nuclear [Re6(μ3-Se)8](2+) cluster core. The [Re6Se8](2+) cluster core displays a non-crystallographic center of symmetry and is bonded through the Re(III) atoms to five tri-ethyl-phosphane ligands and one p-toluene-sulfonate ligand. One p-toluene-sulfonate counter-ion and two di-chloro-methane solvent mol-ecules are also present in the asymmetric unit. One of the ethyl chains of one triethylphos-phane ligand and one of the CH2Cl2 solvent molecules are disordered over two sets of sites (occupancy ratios 0.65:0.35 and 0.5:0.5, respectively). The Re-O(sulfon-ate) bond length of 2.123 (5) Å is similar to other Re-O bond lengths of hexa-nuclear rhenium chalcogenide clusters containing other O-donor ligands such as dimethyl sulfoxide (DMSO), di-methyl-formamide (DMF) and hydroxide.

CO and CO2 fixation by Se-Ru-CO hydride clusters

The selective insertion of CO and CO2 into the C-O and O-H bonds of alcohols by the Se-Ru-CO hydride clusters [(μ-H)Ru4(CO)10Se2](-) (1) and [(μ3-H)Ru5(CO)14Se](-) (2) was demonstrated by a cooperative effect of the protonic hydride, the electron-rich Ru atom, and the electronegative Se atom as well as the symmetry of the clusters. These reactions generated the first examples of Se-containing ruthenium carboxylate and alkylcarbonate clusters [{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4(μ-η(1):η(1)-OOCR)}](3-) (R = Me, 3; Et, 4) and [{(μ-H)Ru4(CO)10Se2}2{Ru2(CO)4(μ-η(1):η(1)-OOCOR)}](3-) (R = Me, 5; Et, 6), respectively. These results disclosed herein provide a new avenue for the capture and storage of CO and CO2 and useful synthetic routes to novel RCOO(-)- and ROCOO(-)-bridged ruthenium selenide clusters.

Chemical transformations supported by the [Re6(μ3-Se)8]2+ cluster core

This Perspective article discusses three interesting chemical transformations promoted by the octahedral [Re(6)(μ(3)-Se)(8)](2+) cluster core. These include (1) nucleophilic addition of alcohols to cluster-bound nitrile(s) to produce imino ester complexes; (2) synthesis of cluster-imine complexes with geometric specificity by reacting cluster nitrile solvates with organic azides; and (3) preparation and reactivity studies of carbonyl complexes of the cluster. We found that cluster-bound nitrile ligands were activated toward nucleophilic attack by methanol or ethanol, affording predominantly the Z-configured cluster-imino ester complexes, for which a mechanism evoking bifurcated hydrogen bonding interactions involving both the alcohol OH group and two nearest Se atoms of the cluster core was proposed. When reacted with organic azides, cluster-bound nitrile ligands were displaced and cluster-imine complexes were obtained, presumably through the formation of the corresponding cluster-azide complexes, followed by their photodecomposition. Lastly, cluster complexes featuring all-terminal carbonyl ligands were synthesized. Back-bonding interactions were verified, both experimentally and by computational studies. Their thermal and photo-stabilities were also evaluated, so was their reactivity toward methyl lithium for the eventual making of cluster-carbene catalysts. These findings, together with those by others, portend an exciting, new direction in the chemistry of solid-state type transition metal clusters.

Direct observation of a {Re6(μ3-S)8} core-to-ligand charge-transfer excited state in an octahedral hexarhenium complex

A new complex, [Re(6)S(8)Cl(5)ppy](3-) (ppy = 4-phenylpyridine), was synthesized and characterized. The complex showed red emission at 296 K in both the crystalline phase and CH(3)CN. The transient absorption spectrum of [Re(6)S(8)Cl(5)ppy](3-) revealed that the emissive excited state involved the {Re(6)(μ(3)-S)(8)} core-to-ligand charge-transfer character.

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.

Cluster carbonyls of the [Re(6)(mu(3)-Se)(8)](2+) core: synthesis, structural characterization, and computational analysis

The reactions of the previously reported cluster complexes [Re(6)(mu(3)-Se)(8)(PEt(3))(5)I]I, trans-[Re(6)(mu(3)-Se)(8)(PEt(3))(4)I(2)], and cis-[Re(6)(mu(3)-Se)(8)(PEt(3))(4)I(2)] with the [Re(6)(mu(3)-Se)(8)](2+) core with CO in the presence of AgSbF(6) afforded the corresponding cluster carbonyls [Re(6)(mu(3)-Se)(8)(PEt(3))(5)(CO)][SbF(6)](2) (), trans-[Re(6)(mu(3)-Se)(8)(PEt(3))(4)(CO)(2)][SbF(6)](2) (), and cis-[Re(6)(mu(3)-Se)(8)(PEt(3))(4)(CO)(2)][SbF(6)](2) (). Infrared spectroscopy indicated weakening of the bond in CO, suggesting the existence of backbonding between the cluster core and the CO ligand(s). Electrochemical studies focusing on the reversible, one-electron oxidation of the cluster core revealed a large increase in the oxidation potential upon going from the acetonitrile derivatives to their carbonyl analogs, consistent with the depleted electron density of the cluster core upon CO ligation. Disparities between the IR spectra and oxidation potential between and indicate that electronic differences exist between sites trans and cis to the location of a ligand of interest. The active role played by the Se atoms in influencing the cluster-to-CO bonding interactions is suggested through this result and density functional (DF) computational analysis. The computations indicate that molecular orbitals near the HOMO account for backbonding interactions with a high percentage of participation of Se orbitals.

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.