Exposing the inadequacy of redox formalisms by resolving redox inequivalence within isovalent clusters.

In this report we examine a family of trinuclear iron complexes by multiple-wavelength, anomalous diffraction (MAD) to explore the redox load distribution within cluster materials by the free refinement of atomic scattering factors. Several effects were explored that can impact atomic scattering factors within clusters, including 1) metal atom primary coordination sphere, 2) M-M bonding, and 3) redox delocalization in formally mixed-valent species. Complexes were investigated which vary from highly symmetric to fully asymmetric by 57Fe Mössbauer and X-ray diffraction to explore the relationship between MAD-derived data and the data available from these widely used characterization techniques. The compounds examined include the all-ferrous clusters [ n Bu4N][(tbsL)Fe3(µ3-Cl)] (1) ([tbsL]6- = [1,3,5-C6H9(NC6H4-o-NSi t BuMe2)3]6-]), (tbsL)Fe3(py) (2), [K(C222)]2[(tbsL)Fe3(µ3-NPh)] (4) (C222 = 2,2,2-cryptand), and the mixed-valent (tbsL)Fe3(µ3-NPh) (3). Redox delocalization in mixed-valent 3 was explored with cyclic voltammetry (CV), zero-field 57Fe Mössbauer, near-infrared (NIR) spectroscopy, and X-ray crystallography techniques. We find that the MAD results show an excellent correspondence to 57Fe Mössbauer data; yet also can distinguish between subtle changes in local coordination geometries where Mössbauer cannot. Differences within aggregate oxidation levels are evident by systematic shifts of scattering factor envelopes to increasingly higher energies. However, distinguishing local oxidation levels in iso- or mixed-valent materials can be dramatically obscured by the degree of covalent intracore bonding. MAD-derived atomic scattering factor data emphasize in-edge features that are often difficult to analyze by X-ray absorption near edge spectroscopy (XANES). Thus, relative oxidation levels within the cluster were most reliably ascertained from comparing the entire envelope of the atomic scattering factor data.

Synthesis of Square Planar Cu4 Clusters.

Template-assisted synthesis of well-defined polynuclear clusters remains a challenge for [M4 ] square planar topologies. Herein, we present a tetraamine scaffold R L(NH2 )4 , where L is a rigidified resorcin[4]arene, to direct the formation of C4 -symmetric R L(NH)4 Cu4 clusters with Cu-Cu distances around 2.7 Å, suggesting metal-metal direct interactions are operative since the sum of copper's van der Waals radii is 2.8 Å. DFT calculations display HOMO to HOMO-3 residing all within a 0.1 eV gap. These four orbitals display significant electron density contribution from the Cu centers suggesting a delocalized electronic structure. The one-electron oxidized [Cu4 ]+ species was probed by variable temperature X-band continuous wave-electron paramagnetic resonance (CW-EPR), which displays a multiline spectrum at room temperature. This work presents a novel synthetic strategy for [M4 ] clusters and a new platform to investigate activation of small molecules.

Conjugated Molecular Nanotubes.

Molecular compounds with permanent tubular architectures displaying radial π-conjugation are exceedingly rare. Their radial and axial delocalization presents them with unique optical and electronic properties, such as remarkable tuning of their Stokes shifts, and redox switching between global and local aromaticity. Although these tubular compounds display large internal void spaces, these attributes have not been extensively explored, thus presenting future opportunities in the development of materials. By using cutting-edge synthetic methodologies to bend aromatic surfaces, large opportunities in synthesis, property discovery, and applications are expected in new members of this family of conjugated molecular nanotubes.

Intermolecular Resonance Correlates Electron Pairs Down a Supermolecular Chain: Antiferromagnetism in K-Doped p-Terphenyl.

Recent interest in potassium-doped p-terphenyl has been fueled by reports of superconductivity at Tc values surprisingly high for organic compounds. Despite these interesting properties, studies of the structure-function relationships within these materials have been scarce. Here, we isolate a phase-pure crystal of potassium-doped p-terphenyl: [K(222)]2[p-terphenyl3]. Emerging antiferromagnetism in the anisotropic structure is studied in depth by magnetometry and electron spin resonance. Combining these experimental results with density functional theory calculations, we describe the antiferromagnetic coupling in this system that occurs in all 3 crystallographic directions. The strongest coupling was found along the ends of the terphenyls, where the additional electron on neighboring p-terphenyls antiferromagnetically couple. This delocalized bonding interaction is reminiscent of the doubly degenerate resonance structure depiction of polyacetylene. These findings hint toward magnetic fluctuation-induced superconductivity in potassium-doped p-terphenyl, which has a close analogy with high Tc cuprate superconductors. The new approach described here is very versatile as shown by the preparation of two additional salts through systematic changing of the building blocks.

Stringing the Perylene Diimide Bow.

This study explores a new mode of contortion in perylene diimides where the molecule is bent, like a bow, along its long axis. These bowed PDIs were synthesized through a facile fourfold Suzuki macrocyclization with aromatic linkers and a tetraborylated perylene diimide that introduces strain and results in a bowed structure. By altering the strings of the bow, the degree of bending can be controlled from flat to highly bent. Through spectroscopy and quantum chemical calculations, it is demonstrated that the energy of the lowest unoccupied orbital can be controlled by the degree of bending in the structures and that the energy of the highest occupied orbital can be controlled to a large extent by the constitution of the aromatic linkers. The important finding is that the bowing results not only in red-shifted absorptions but also more facile reductions.

A Helicene Nanoribbon with Greatly Amplified Chirality.

We report the synthesis and characterization of a chiral, shape-persistent, perylene-diimide-based nanoribbon. Specifically, the fusion of three perylene-diimide monomers with intervening naphthalene subunits resulted in a helical superstructure with two [6]helicene subcomponents. This π-helix-of-helicenes exhibits very intense electronic circular dichroism, including one of the largest Cotton effects ever observed in the visible range. It also displays more than an order of magnitude increase in circular dichroism for select wavelengths relative to its smaller homologue. These impressive chiroptical properties underscore the potential of this new nanoribbon architecture in the context of chiral electronic materials.

Single-Walled Carbon Nanotubes: Mimics of Biological Ion Channels.

Here we report on the ion conductance through individual, small diameter single-walled carbon nanotubes. We find that they are mimics of ion channels found in natural systems. We explore the factors governing the ion selectivity and permeation through single-walled carbon nanotubes by considering an electrostatic mechanism built around a simplified version of the Gouy-Chapman theory. We find that the single-walled carbon nanotubes preferentially transported cations and that the cation permeability is size-dependent. The ionic conductance increases as the absolute hydration enthalpy decreases for monovalent cations with similar solid-state radii, hydrated radii, and bulk mobility. Charge screening experiments using either the addition of cationic or anionic polymers, divalent metal cations, or changes in pH reveal the enormous impact of the negatively charged carboxylates at the entrance of the single-walled carbon nanotubes. These observations were modeled in the low-to-medium concentration range (0.1-2.0 M) by an electrostatic mechanism that mimics the behavior observed in many biological ion channel-forming proteins. Moreover, multi-ion conduction in the high concentration range (>2.0 M) further reinforces the similarity between single-walled carbon nanotubes and protein ion channels.

Molecular Materials for Nonaqueous Flow Batteries with a High Coulombic Efficiency and Stable Cycling.

This manuscript presents a working redox battery in organic media that possesses remarkable cycling stability. The redox molecules have a solubility over 1 mol electrons/liter, and a cell with 0.4 M electron concentration is demonstrated with steady performance >450 cycles (>74 days). Such a concentration is among the highest values reported in redox flow batteries with organic electrolytes. The average Coulombic efficiency of this cell during cycling is 99.868%. The stability of the cell approaches the level necessary for a long lifetime nonaqueous redox flow battery. For the membrane, we employ a low cost size exclusion cellulose membrane. With this membrane, we couple the preparation of nanoscale macromolecular electrolytes to successfully avoid active material crossover. We show that this cellulose-based membrane can support high voltages in excess of 3 V and extreme temperatures (-20 to 110 °C). These extremes in temperature and voltage are not possible with aqueous systems. Most importantly, the nanoscale macromolecular platforms we present here for our electrolytes can be readily tuned through derivatization to realize the promise of organic redox flow batteries.

Electron Cartography in Clusters.

Deconvoluting the atom-specific electron density within polynuclear systems remains a challenge. A multiple-wavelength anomalous diffraction study on four clusters that share the same [Co6 Se8 ] core was performed. Two cluster types were designed, one having a symmetric ligand sphere and the other having an asymmetric ligand sphere. It was found that in the neutral, asymmetric, CO-bound cluster, the Co-CO site is more highly oxidized than the other five Co atoms; when an electron is removed, the hole is distributed among the Se atoms. In the neutral, symmetric cluster, the Co atoms divide by electron population into two sets of three, each set being meridional; upon removal of an electron, the hole is distributed among all the Co atoms. This ligand-dependent tuning of the electron/hole distribution relates directly to the performance of clusters in biological and synthetic systems.

Towards Catalytic Ammonia Oxidation to Dinitrogen: A Synthetic Cycle by Using a Simple Manganese Complex.

Oxidation of the nucleophilic nitride, (salen)Mn≡N (1) with stoichiometric [Ar3 N][X] initiated a nitride coupling reaction to N2 , a major step toward catalytic ammonia oxidation (salen=N,N'-bis(salicylidene)-ethylenediamine dianion; Ar=p-bromophenyl; X=[SbCl6 ]- or [B(C6 F5 )4 ]- ). N2 production was confirmed by mass spectral analysis of the isotopomer, 1-15 N, and the gas quantified. The metal products of oxidation were the reduced MnIII dimers, [(salen)MnCl]2 (2) or [(salen)Mn(OEt2 )]2 [B(C6 F5 )4 ]2 (3) for X=[SbCl6 ]- or [B(C6 F5 )4 ]- , respectively. The mechanism of nitride coupling was probed to distinguish a nitridyl from a nucleophilic/electrophilic coupling sequence. During these studies, a rare mixed-valent MnV /MnIII bridging nitride, [(salen)MnV (µ-N)MnIII (salen)][B(C6 F5 )4 ] (4), was isolated, and its oxidation-state assignment was confirmed by X-ray diffraction (XRD) studies, perpendicular and parallel-mode EPR and UV/Vis/NIR spectroscopies, as well as superconducting quantum interference device (SQUID) magnetometry. We found that 4 could subsequently be oxidized to 3. Furthermore, in view of generating a catalytic system, 2 can be re-oxidized to 1 in the presence of NH3 and NaOCl closing a pseudo-catalytic "synthetic" cycle. Together, the reduction of 1→2 followed by oxidation of 2→1 yield a genuine synthetic cycle for NH3 oxidation, paving the way to the development of a fully catalytic system by using abundant metal catalysis.

Maximizing Electron Exchange in a [Fe3] Cluster.

The one-electron reduction of ((tbs)L)Fe3(thf)¹ furnishes [M][((tbs)L)Fe3] ([M]⁺ = [(18-C-6)K(thf)2]⁺ (1, 76%) or [(crypt-222)K]⁺ (2, 54%)). Upon reduction, the ligand (tbs)L6⁻ rearranges around the triiron core to adopt an almost ideal C3-symmetry. Accompanying the ((tbs)L) ligand rearrangement, the THF bound to the neutral starting material is expelled, and the Fe-Fe distances within the trinuclear cluster contract by ∼0.13 Å in 1. Variable-temperature magnetic susceptibility data indicates a well-isolated S = 11/2 spin ground state that persists to room temperature. Slow magnetic relaxation is observed at low temperature as evidenced by the out-of-phase (χ(M)″) component of the alternating current (ac) magnetic susceptibility data and by the appearance of hyperfine splitting in the zero-field 57Fe Mössbauer spectra at 4.2 K. Analysis of the ac magnetic susceptibility yields an effective spin reversal barrier (U(eff)) of 22.6(2) cm⁻¹, nearly matching the theoretical barrier of 38.7 cm⁻¹ calculated from the axial zero-field splitting parameter (D = -1.29 cm⁻¹) extracted from the reduced magnetization data. A polycrystalline sample of 1 displays three sextets in the Mössbauer spectrum at 4.2 K (H(ext) = 0) which converge to a single six-line pattern in a frozen 2-MeTHF glass sample, indicating a unique iron environment and thus strong electron delocalization. The spin ground state and ligand rearrangement are discussed within the framework of a fully delocalized cluster exhibiting strong double and direct exchange interactions.

Meta-Atom Behavior in Clusters Revealing Large Spin Ground States.

The field of single molecule magnetism remains predicated on super- and double exchange mechanisms to engender large spin ground states. An alternative approach to achieving high-spin architectures involves synthesizing weak-field clusters featuring close M-M interactions to produce a single valence orbital manifold. Population of this orbital manifold in accordance with Hund's rules could potentially yield thermally persistent high-spin ground states under which the valence electrons remain coupled. We now demonstrate this effect with a reduced hexanuclear iron cluster that achieves an S = 19/2 (χ(M)T ≈ 53 cm(3) K/mol) ground state that persists to 300 K, representing the largest spin ground state persistent to room temperature reported to date. The reduced cluster displays single molecule magnet behavior manifest in both variable-temperature zero-field (57)Fe Mössbauer and magnetometry with a spin reversal barrier of 42.5(8) cm(-1) and a magnetic blocking temperature of 2.9 K (0.059 K/min).

Ligand Field Strength Mediates Electron Delocalization in Octahedral [((H)L)2Fe6(L')m](n+) 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 [((H)L)2Fe6(L')m](n+) 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 [((H)L)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 (Kc = 10(4)-10(8)) 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 (Kc = 10(2)-10(8)) with observable IVCT bands. Finally, the anionic cyanide-bound series reveals the highest degree of valence delocalization with the most intense IVCT bands (Kc = 10(12)-10(20)) 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.

Formation of heterobimetallic zirconium/cobalt diimido complexes via a four-electron transformation.

The reactivity of the reduced heterobimetallic complex Zr((i)PrNP(i)Pr2)3CoN2 (1) toward aryl azides was examined, revealing a four-electron redox transformation to afford unusual heterobimetallic zirconium/cobalt diimido complexes. In the case of p-tolyl azide, the diamagnetic C3-symmetric bis(terminal imido) complex 3 is formed, but mesityl azide instead leads to asymmetric complex 4 featuring a bridging imido fragment.

Reversible dioxygen uptake at [Cu4] clusters.

Dioxygen binding solely through non-covalent interactions is rare. In living systems, dioxygen transport takes place via iron or copper-containing biological cofactors. Specifically, a reversible covalent interaction is established when O2 binds to the mono or polynuclear metal center. However, O2 stabilization in the absence of covalent bond formation is challenging and rarely observed. Here, we demonstrate a unique example of reversible non-covalent binding of dioxygen within the cavity of a well-defined synthetic all-Cu(i) tetracopper cluster.

Synthesis of open-shell, bimetallic Mn/Fe trinuclear clusters.

Concomitant deprotonation and metalation of hexadentate ligand platform (tbs)LH6 ((tbs)LH6 = 1,3,5-C6H9(NHC6H4-o-NHSiMe2(t)Bu)3) with divalent transition metal starting materials Fe2(Mes)4 (Mes = mesityl) or Mn3(Mes)6 in the presence of tetrahydrofuran (THF) resulted in isolation of homotrinuclear complexes ((tbs)L)Fe3(THF) and ((tbs)L)Mn3(THF), respectively. In the absence of coordinating solvent (THF), the deprotonation and metalation exclusively afforded dinuclear complexes of the type ((tbs)LH2)M2 (M = Fe or Mn). The resulting dinuclear species were utilized as synthons to prepare bimetallic trinuclear clusters. Treatment of ((tbs)LH2)Fe2 complex with divalent Mn source (Mn2(N(SiMe3)2)4) afforded the bimetallic complex ((tbs)L)Fe2Mn(THF), which established the ability of hexamine ligand (tbs)LH6 to support mixed metal clusters. The substitutional homogeneity of ((tbs)L)Fe2Mn(THF) was determined by (1)H NMR, (57)Fe Mössbauer, and X-ray fluorescence. Anomalous scattering measurements were critical for the unambiguous assignment of the trinuclear core composition. Heating a solution of ((tbs)LH2)Mn2 with a stoichiometric amount of Fe2(Mes)4 (0.5 mol equiv) affords a mixture of both ((tbs)L)Mn2Fe(THF) and ((tbs)L)Fe2Mn(THF) as a result of the thermodynamic preference for heavier metal substitution within the hexa-anilido ligand framework. These results demonstrate for the first time the assembly of mixed metal cluster synthesis in an unbiased ligand platform.

Metal atom lability in polynuclear complexes.

The asymmetric oxidation product [((Ph)L)Fe3(µ-Cl)]2 [(Ph)LH6 = MeC(CH2NHPh-o-NHPh)3], where each trinuclear core is comprised of an oxidized diiron unit [Fe2](5+) and an isolated trigonal pyramidal ferrous site, reacts with MCl2 salts to afford heptanuclear bridged structures of the type ((Ph)L)2Fe6M(µ-Cl)4(thf)2, where M = Fe or Co. Zero-field, (57)Fe Mössbauer analysis revealed the Co resides within the trinuclear core subunits, not at the octahedral, halide-bridged MCl4(thf)2 position indicating Co migration into the trinuclear subunits has occurred. Reaction of [((Ph)L)Fe3(µ-Cl)]2 with CoCl2 (2 or 5 equivalents) followed by precipitation via addition of acetonitrile afforded trinuclear products where one or two irons, respectively, can be substituted within the trinuclear core. Metal atom substitution was verified by (1)H NMR, (57)Fe Mossbauer, single crystal X-ray diffraction, X-ray fluorescence, and magnetometry analysis. Spectroscopic analysis revealed that the Co atom(s) substitute(s) into the oxidized dimetal unit ([M2](5+)), while the M(2+) site remains iron-substituted. Magnetic data acquired for the series are consistent with this analysis revealing the oxidized dimetal unit comprises a strongly coupled S = 1 unit ([FeCo](5+)) or S = 1/2 ([Co2](5+)) that is weakly antiferromagnetically coupled to the high spin (S = 2) ferrous site. The kinetic pathway for metal substitution was probed via reaction of [((Ph)L)Fe3(µ-Cl)]2 with isotopically enriched (57)FeCl2(thf)2, the results of which suggest rapid equilibration of (57)Fe into both the M(2+) site and oxidized diiron site, achieving a 1:1 mixture.

Nonspherical anion sequestration by C-H hydrogen bonding.

Macrocyclic arenes laid the foundations of supramolecular chemistry and their study established the fundamentals of noncovalent interactions. Advancing their frontier, here we designed rigidified resorcin[4]arenes that serve as hosts for large nonspherical anions. In one synthetic step, we vary the host's anion affinity properties by more than seven orders of magnitude. This is possible by engineering electropositive aromatic C-H bond donors in an idealized square planar geometry embedded within the host's inner cavity. The hydrogen atom's electropositivity is tuned by introducing fluorine atoms as electron withdrawing groups. These novel macrocycles, termed fluorocages, are engineered to sequester large anions. Indeed, experimental data shows an increase in the anion association constant (K a) as the number of F atoms increase. The observed trend is rationalized by DFT calculations of Hirshfeld Charges (HCs). Most importantly, fluorocages in solution showed weak-to-medium binding affinity for large anions like [PF6]- (102< K a <104 M-1), and high affinity for [MeSO3]- (K a >106).

Radially Oriented [n]Cyclo-meta-phenylenes.

Molecular compounds with zigzag carbon nanotube geometries are exceedingly rare. Here we report the synthesis and characterization of carbon-based nanotubes with zigzag geometry, best described as radially oriented [n]cyclo-meta-phenylenes, extending the tubularene family of compounds. By the incorporation of edge-sharing benzene rings into the tubularene's radial π-surface, we have uncovered the first step to give rise to the emergence of radial orbital distribution in zigzag nanorings.

Defying strain in the synthesis of an electroactive bilayer helicene.

We report the synthesis of a bilayer chiral nanographene incorporating a [7]helicene scaffold and two perylene-diimide (PDI) subunits. Twofold visible-light-induced oxidative cyclization of a phenanthrene framework selects for the desired PDI-helicene, despite the immense strain that distinguishes this helicene from two other accessible isomers. This strain arises from the extensive intramolecular overlap of the PDI subunits, which precludes racemization, even at elevated temperatures. Relative to a smaller homologue, this PDI-helicene exhibits amplified electronic circular dichroism. It also readily and reversibly accepts four electrons electrochemically. Modifications to the core phenanthrene subunit change the fluorescence and electrochemistry of the PDI-helicene without significantly impacting its electronic circular dichroism or UV-visible absorbance.