One-Dimensional Palladium Wires: Influence of Molecular Changes on Supramolecular Structure

Nanostructured materials based on one-dimensional (1D) metal wires are of potential utility; however, to date, there is a lack of synthetic methods that allow for variation of structure and therefore properties. Here we report the use of molecular control elements to alter the solid-state structures of 1D palladium wires, including Pd–Pd bond distances and the porosity of the supramolecular framework

One-Dimensional Palladium Wires: Influence of Molecular Changes on Supramolecular Structure

Nanostructured materials based on one-dimensional (1D) metal wires are of potential utility; however, to date, there is a lack of synthetic methods that allow for variation of structure and therefore properties. Here we report the use of molecular control elements to alter the solid-state structures of 1D palladium wires, including Pd–Pd bond distances and the porosity of the supramolecular framework

Ground State and Excited State Tuning in Ferric Dipyrrin Complexes Promoted by Ancillary Ligand Exchange

Three ferric dipyrromethene complexes featuring different ancillary ligands were synthesized by one electron oxidation of ferrous precursors. Four-coordinate iron complexes of the type (^ArL)­FeX₂ [^ArL = 1,9-(2,4,6-Ph₃C₆H₂)₂-5-mesityldipyrromethene] with X = Cl or ^<i>t</i>BuO were prepared and found to be high-spin (<i>S</i> = ⁵/₂), as determined by superconducting quantum interference device magnetometry, electron paramagnetic resonance, and ⁵⁷Fe Mössbauer spectroscopy. The ancillary ligand substitution was found to affect both ground state and excited properties of the ferric complexes examined. While each ferric complex displays reversible reduction and oxidation events, each alkoxide for chloride substitution results in a nearly 600 mV cathodic shift of the Fe^III/II couple. The oxidation event remains largely unaffected by the ancillary ligand substitution and is likely dipyrrin-centered. While the alkoxide substituted ferric species largely retain the color of their ferrous precursors, characteristic of dipyrrin-based ligand-to-ligand charge transfer (LLCT), the dichloride ferric complex loses the prominent dipyrrin chromophore, taking on a deep green color. Time-dependent density functional theory analyses indicate the weaker-field chloride ligands allow substantial configuration mixing of ligand-to-metal charge transfer into the LLCT bands, giving rise to the color changes observed. Furthermore, the higher degree of covalency between the alkoxide ferric centers is manifest in the observed reactivity. Delocalization of spin density onto the <i>tert</i>-butoxide ligand in (^ArL)­FeCl­(O^<i>t</i>Bu) is evidenced by hydrogen atom abstraction to yield (^ArL)­FeCl and HO^<i>t</i>Bu in the presence of substrates containing weak C–H bonds, whereas the chloride (^ArL)­FeCl₂ analogue does not react under these conditions

Ground State and Excited State Tuning in Ferric Dipyrrin Complexes Promoted by Ancillary Ligand Exchange

Three ferric dipyrromethene complexes featuring different ancillary ligands were synthesized by one electron oxidation of ferrous precursors. Four-coordinate iron complexes of the type (^ArL)­FeX₂ [^ArL = 1,9-(2,4,6-Ph₃C₆H₂)₂-5-mesityldipyrromethene] with X = Cl or ^<i>t</i>BuO were prepared and found to be high-spin (<i>S</i> = ⁵/₂), as determined by superconducting quantum interference device magnetometry, electron paramagnetic resonance, and ⁵⁷Fe Mössbauer spectroscopy. The ancillary ligand substitution was found to affect both ground state and excited properties of the ferric complexes examined. While each ferric complex displays reversible reduction and oxidation events, each alkoxide for chloride substitution results in a nearly 600 mV cathodic shift of the Fe^III/II couple. The oxidation event remains largely unaffected by the ancillary ligand substitution and is likely dipyrrin-centered. While the alkoxide substituted ferric species largely retain the color of their ferrous precursors, characteristic of dipyrrin-based ligand-to-ligand charge transfer (LLCT), the dichloride ferric complex loses the prominent dipyrrin chromophore, taking on a deep green color. Time-dependent density functional theory analyses indicate the weaker-field chloride ligands allow substantial configuration mixing of ligand-to-metal charge transfer into the LLCT bands, giving rise to the color changes observed. Furthermore, the higher degree of covalency between the alkoxide ferric centers is manifest in the observed reactivity. Delocalization of spin density onto the <i>tert</i>-butoxide ligand in (^ArL)­FeCl­(O^<i>t</i>Bu) is evidenced by hydrogen atom abstraction to yield (^ArL)­FeCl and HO^<i>t</i>Bu in the presence of substrates containing weak C–H bonds, whereas the chloride (^ArL)­FeCl₂ analogue does not react under these conditions

One-Dimensional Palladium Wires: Influence of Molecular Changes on Supramolecular Structure

Nanostructured materials based on one-dimensional (1D) metal wires are of potential utility; however, to date, there is a lack of synthetic methods that allow for variation of structure and therefore properties. Here we report the use of molecular control elements to alter the solid-state structures of 1D palladium wires, including Pd–Pd bond distances and the porosity of the supramolecular framework

Ligand Field Strength Mediates Electron Delocalization in Octahedral [(^HL)₂Fe₆(L′)_<i>m</i>]^<i>n</i>+ Clusters

To assess the impact of terminal ligand binding on a variety of cluster properties (redox delocalization, ground-state stabilization, and breadth of redox state accessibility), we prepared three electron-transfer series based on the hexanuclear iron cluster [(^HL)₂Fe₆(L′)_<i>m</i>]^<i>n+</i> in which the terminal ligand field strength was modulated from weak to strong (L′ = DMF, MeCN, CN). The extent of intracore M–M interactions is gauged by M–M distances, spin ground state persistence, and preference for mixed-valence states as determined by electrochemical comproportionation constants. Coordination of DMF to the [(^HL)₂Fe₆] core leads to weaker Fe–Fe interactions, as manifested by the observation of ground states populated only at lower temperatures (<100 K) and by the greater evidence of valence trapping within the mixed-valence states. Comproportionation constants determined electrochemically (<i>K</i>_c = 10⁴–10⁸) indicate that the redox series exhibits electronic delocalization (class II–III), yet no intervalence charge transfer (IVCT) bands are observable in the near-IR spectra. Ligation of the stronger σ donor acetonitrile results in stabilization of spin ground states to higher temperatures (∼300 K) and a high degree of valence delocalization (<i>K</i>_c = 10²–10⁸) with observable IVCT bands. Finally, the anionic cyanide-bound series reveals the highest degree of valence delocalization with the most intense IVCT bands (<i>K</i>_c = 10¹²–10²⁰) and spin ground state population beyond room temperature. Across the series, at a given formal oxidation level, the capping ligand on the hexairon cluster dictates the overall properties of the aggregate, modulating the redox delocalization and the persistence of the intracore coupling of the metal sites

Selenium as a Structural Surrogate of Sulfur: Template-Assisted Assembly of Five Types of Tungsten–Iron–Sulfur/Selenium Clusters and the Structural Fate of Chalcogenide Reactants

Syntheses of five types of tungsten–iron–sulfur/selenium clusters, namely, incomplete cubanes, single cubanes, edge-bridged double cubanes (EBDCs), P^N-type clusters, and double-cuboidal clusters, have been devised using the concept of template-assisted assembly. The template reactant is six-coordinate [(Tp*)­W^VIS₃]^1– [Tp* = tris­(3,5-dimethylpyrazolyl)­hydroborate(1−)], which in the assembly systems organizes Fe^2+/3+ and sulfide/selenide into cuboidal [(Tp*)­WFe₂S₃] or cubane [(Tp*)­WFe₃S₃Q] (Q = S, Se) units. With appropriate terminal iron ligation, these units are capable of independent existence or may be transformed into higher-nuclearity species. Selenide is used as a surrogate for sulfide in cluster assembly in order to determine by X-ray structures the position occupied by an external chalcogenide nucleophile or an internal chalcogenide atom in the product clusters. Specific incorporation of selenide is demonstrated by the formation of [WFe₃S₃Se]^2+/3+ cubane cores. Reductive dimerization of the cubane leads to the EBDC core [W₂Fe₆S₆Se₂]²⁺ containing μ₄-Se sites. Reaction of these species with HSe^– affords the P^N-type cores [W₂Fe₆S₆Se₃]¹⁺, in which selenide occupies μ₆-Se and μ₂-Se sites. The reaction of [(Tp*)­WS₃]^1–, FeCl₂, and Na₂Se yields the double-cuboidal [W₂Fe₄S₆Se₃]^2+/0 core with μ₂-Se and μ₄-Se bridges. It is highly probable that in analogous sulfide-only assembly systems, external and internal sulfide reactants occupy corresponding positions in the cluster products. The results further demonstrate the viability of template-assisted cluster synthesis inasmuch as the reduced (Tp*)­WS₃ unit is present in all of the clusters. Structures, zero-field Mössbauer data, and redox potentials are presented for each cluster type

Cubane-Type Fe₄S₄ Clusters with Chiral Thiolate Ligation: Formation by Ligand Substitution, Detection of Intermediates by ¹H NMR, and Solid State Structures Including Spontaneous Resolution Upon Crystallization

Cubane-type clusters [Fe₄S₄(SR*)₄]^2– containing chiral thiolate ligands with R* = CH(Me)Ph (<b>1</b>), CH₂CH(Me)Et (<b>2</b>), and CH₂CH(OH)CH₂OH (<b>3</b>) have been prepared by ligand substitution in the reaction systems [Fe₄S₄(SEt)₄]/R*SH (<b>1</b>–<b>3</b>, acetonitrile) and [Fe₄S₄Cl₄]^2–/NaSR*(<b>3</b>, Me₂SO). Reactions with successive equivalents of thiol or thiolate generate the species [Fe₄S₄L_4–<i>n</i>(SR*)_<i>n</i>]^2– (L = SEt, Cl) with <i>n</i> = 1–4. Clusters <b>1</b> and <b>2</b> were prepared with racemic thiols leading to the possible formation of one enantiomeric pair (<i>n</i> = 1) and seven diastereomers and their enantiomers (<i>n =</i> 2–4). Reactions were monitored by isotropically shifted ¹H NMR spectra in acetonitrile or Me₂SO. In systems affording <b>1</b> and <b>2</b> as final products, individual mixed-ligand species could not be detected. However, crystallization of (Et₄N)₂[<b>1</b>] afforded <b>1</b>-[<i>SS</i>(<i>RS)(RS)</i>] in which two sites are disordered because of occupancy of <i>R</i> and <i>S</i> ligands. Similarly, (Et₄N)₂[<b>2</b>] led to <b>2</b>-[<i>SSSS</i>], a consequence of spontaneous resolution upon crystallization. The clusters <b>3</b>-[<i>RRRR</i>] and <b>3</b>-[<i>SSSS</i>] were obtained from enantiomerically pure thiols. Successive reactions lead to detection of species with <i>n =</i> 1–4 by appearance of four pairs of diastereotopic SC<i>H</i>₂ signals in both acetonitrile and Me₂SO reaction systems. Identical spectra were obtained with racemic, <i>R-</i>(−), and <i>S</i>-(+) thiols, indicating that ligand–ligand interactions are too weak to allow detection of diastereomers (e.g., [<i>SSSS</i>] vs [<i>SSRR</i>]). The stability of <b>3</b> in Me₂SO/H₂O media is described

Syntheses of α-Pyrones Using Gold-Catalyzed Coupling Reactions

Sequential alkyne activation of terminal alkynes and propiolic acids by gold(I) catalysts yields compounds having α-pyrone skeletons. Novel cascade reactions involving propiolic acids are reported that give rise to α-pyrones with different substitution patterns

Cubane-Type Fe₄S₄ Clusters with Chiral Thiolate Ligation: Formation by Ligand Substitution, Detection of Intermediates by ¹H NMR, and Solid State Structures Including Spontaneous Resolution Upon Crystallization

Cubane-type clusters [Fe₄S₄(SR*)₄]^2– containing chiral thiolate ligands with R* = CH(Me)Ph (<b>1</b>), CH₂CH(Me)Et (<b>2</b>), and CH₂CH(OH)CH₂OH (<b>3</b>) have been prepared by ligand substitution in the reaction systems [Fe₄S₄(SEt)₄]/R*SH (<b>1</b>–<b>3</b>, acetonitrile) and [Fe₄S₄Cl₄]^2–/NaSR*(<b>3</b>, Me₂SO). Reactions with successive equivalents of thiol or thiolate generate the species [Fe₄S₄L_4–<i>n</i>(SR*)_<i>n</i>]^2– (L = SEt, Cl) with <i>n</i> = 1–4. Clusters <b>1</b> and <b>2</b> were prepared with racemic thiols leading to the possible formation of one enantiomeric pair (<i>n</i> = 1) and seven diastereomers and their enantiomers (<i>n =</i> 2–4). Reactions were monitored by isotropically shifted ¹H NMR spectra in acetonitrile or Me₂SO. In systems affording <b>1</b> and <b>2</b> as final products, individual mixed-ligand species could not be detected. However, crystallization of (Et₄N)₂[<b>1</b>] afforded <b>1</b>-[<i>SS</i>(<i>RS)(RS)</i>] in which two sites are disordered because of occupancy of <i>R</i> and <i>S</i> ligands. Similarly, (Et₄N)₂[<b>2</b>] led to <b>2</b>-[<i>SSSS</i>], a consequence of spontaneous resolution upon crystallization. The clusters <b>3</b>-[<i>RRRR</i>] and <b>3</b>-[<i>SSSS</i>] were obtained from enantiomerically pure thiols. Successive reactions lead to detection of species with <i>n =</i> 1–4 by appearance of four pairs of diastereotopic SC<i>H</i>₂ signals in both acetonitrile and Me₂SO reaction systems. Identical spectra were obtained with racemic, <i>R-</i>(−), and <i>S</i>-(+) thiols, indicating that ligand–ligand interactions are too weak to allow detection of diastereomers (e.g., [<i>SSSS</i>] vs [<i>SSRR</i>]). The stability of <b>3</b> in Me₂SO/H₂O media is described