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Hyperfine coupling constants predicted by DFT in the most highly distorted four-coordinate d3 (VII) ion exhibit exceptional agreement with EPR.

A dicopper nitrenoid complex was prepared by formal oxidative addition of the nitrenoid fragment to a dicopper(I) center by reaction with the iminoiodinane PhINTs (Ts = tosylate). This nitrenoid complex, (DPFN)Cu2(μ-NTs)[NTf2]2 (DPFN = 2,7-bis(fluorodi(2-pyridyl)methyl)-1,8-naphthyridine), is a powerful H atom abstractor that reacts with a range of strong C-H bonds to form a mixed-valence Cu(I)/Cu(II) μ-NHTs amido complex in the first example of a clean H atom transfer to a dicopper nitrenoid core. In line with this reactivity, DFT calculations reveal that the nitrenoid is best described as an iminyl (NR radical anion) complex. The nitrenoid was trapped by the addition of water to form a mixed-donor hydroxo/amido dicopper(II) complex, which was independently obtained by reaction of a Cu2(μ-OH)2 complex with an amine through a protonolysis pathway. This mixed-donor complex is an analogue for the proposed intermediate in copper-catalyzed Chan-Evans-Lam coupling, which proceeds via C-X (X = N or O) bond formation. Treatment of the dicopper(II) mixed donor complex with MgPh2(THF)2 resulted in generation of a mixture that includes both phenol and a previously reported dicopper(I) bridging phenyl complex, illustrating that both reduction of dicopper(II) to dicopper(I) and concomitant C-X bond formation are feasible.

Prices of lithium-ion battery technologies have fallen rapidly and substantially, by about 97%, since their commercialization three decades ago. Many efforts have contributed to the cost reduction underlying the observed price decline, but the contributions of these efforts and their relative importance remain unclear. Here we address this gap by developing a set of cost change models to disentangle these efforts and estimate their individual contributions to the cost decline of lithium-ion cells. We collect data on lithium-ion cell components and their prices, develop a cost equation and cost change equations for these cells, and estimate the contributions of different low-level mechanisms of cost reduction, such as the impacts of changes in energy capacity characteristics, reductions in material prices, and changes in non-material costs. We find that between the late 1990s and early 2010s, about 38% of the observed cost decline resulted from efforts to increase cell charge density. Meanwhile, reductions in cathode materials prices contributed 18% of the cost reduction, and changes in non-material costs accounted for 14% of the cost decline. We also consider the contributions of high-level mechanisms, including research and development (R&D), learning-by-doing, and economies of scale. We find that the largest share of cost change was driven by public and private research and development, which we estimate contributed a majority of the observed cost reduction, with a lower contribution from economies of scale. Moreover, we find that the majority of the R&D contribution can be attributed to advancements in chemistry and materials science. Looking to the future, these results suggest that the nature of electrochemical battery technology, which often allows for many different combinations of electrode materials and electrolyte chemistries, presents further opportunities for new approaches and cost decline in batteries. However, public policy may be needed to help avoid premature lock-in, which can result from market forces favoring incumbent technologies.

Author(s): Dombrowski, James P; Ziegler, Micah S; Phadke, Neelay M; Mansoor, Erum; Levine, Daniel S; Witzke, Ryan J; Head-Gordon, Martin; Bell, Alexis T; Tilley, T Don | Abstract: Anionic molecular models for nonhydrolyzed and partially hydrolyzed aluminum and gallium framework sites on silica, M[OSi(OtBu)3 ]4 - and HOM[OSi(OtBu)3 ]3 - (where M=Al or Ga), were synthesized from anionic chlorides Li{M[OSi(OtBu)3 ]3 Cl} in salt metathesis reactions. Sequestration of lithium cations with [12]crown-4 afforded charge-separated ion pairs composed of monomeric anions M[OSi(OtBu)3 ]4 - with outer-sphere [([12]crown-4)2 Li]+ cations, and hydroxides {HOM[OSi(OtBu)3 ]3 } with pendant [([12]crown-4)Li]+ cations. These molecular models were characterized by single-crystal X-ray diffraction, vibrational spectroscopy, mass spectrometry and NMR spectroscopy. Upon treatment of monomeric [([12]crown-4)Li]{HOM[OSi(OtBu)3 ]3 } complexes with benzyl alcohol, benzyloxide complexes were formed, modeling a possible pathway for the formation of active sites for Meerwin-Ponndorf-Verley (MPV) transfer hydrogenations with Al/Ga-doped silica catalysts.

Expanded deployment of renewable energy technologies can help society mitigate climate change. However, solar and wind energy resources are inherently variable. In this issue of Joule, Hunter and colleagues quantitatively compare a diverse set of energy storage and backup power technologies that can help variable energy resources meet demand. They estimate current technology costs, consider improvement potential, and approximate the uncertainties.

Summary Deeply decarbonizing electricity production will likely require that low-carbon sources meet energy demand throughout days, years, and decades. Wind and solar energy are possible low-carbon options, but resource variability can limit their reliability. Storage can help address this challenge by shaping intermittent resources into desired output profiles. But can solar and wind energy with storage cost-competitively fulfill this role? How do diverse storage technologies compare? We address these questions by analyzing systems that combine wind and solar energy with storage to meet various demand profiles. We estimate that energy storage capacity costs below a roughly $20/kWh target would allow a wind-solar mix to provide cost-competitive baseload electricity in resource-abundant locations such as Texas and Arizona. Relaxing reliability constraints by allowing for a few percent of downtime hours raises storage cost targets considerably, but would require supplemental technologies. Finally, we discuss storage technologies that could reach the estimated cost targets.

A major challenge to the implementation of artificial photosynthesis (AP), in which fuels are produced from abundant materials (water and carbon dioxide) in an electrochemical cell through the action of sunlight, is the discovery of active, inexpensive, safe, and stable catalysts for the oxygen evolution reaction (OER). Multimetallic molecular catalysts, inspired by the natural photosynthetic enzyme, can provide important guidance for catalyst design, but the necessary mechanistic understanding has been elusive. In particular, fundamental transformations for reactive intermediates are difficult to observe, and well-defined molecular models of such species are highly prone to decomposition by intermolecular aggregation. Here, we present a general strategy for stabilization of the molecular cobalt-oxo cubane core (Co4O4) by immobilizing it as part of metal-organic frameworks, thus preventing intermolecular pathways of catalyst decomposition. These materials retain the OER activity and mechanism of the molecular Co4O4 analog yet demonstrate unprecedented long-term stability at pH 14. The organic linkers of the framework allow for chemical fine-tuning of activity and stability and, perhaps most importantly, provide "matrix isolation" that allows for observation and stabilization of intermediates in the water-splitting pathway.

Divalent complexes of vanadium were synthesized employing bulky silyl(aryl)amido ligands −N(SiiPr3)DIPP and −N(SitBu2Me)DIPP (DIPP = 2,6-iPr2C6H3). Solid-state structural characterization revealed that although the ligand −N(SiiPr3)DIPP supports a monomeric, bis(amido) complex of vanadium, its constitutional isomer −N(SitBu2Me)DIPP affords a homoleptic complex in which the vanadium center is sandwiched between the arene rings, an unusual binding mode for arylamido ligands. Magnetometry studies indicate that V[N(SiiPr3)DIPP]2 and V[(η5-DIPP)N(SitBu2Me)]2 have similar high-spin d3 electron configurations. However, spectroscopic methods, including electron paramagnetic resonance, nuclear magnetic resonance, infrared, and UV–visible spectroscopies, in addition to cyclic voltammetry and reactivity studies, suggest that V[N(SiiPr3)DIPP]2 is stereochemically nonrigid in solution, while V[(η5-DIPP)N(SitBu2Me)]2 is not. This nonrigidity explicitly impacts the reactivity of V[N(SiiPr3)DIPP]2, which can be used to a...

A dicopper nitrenoid complex was prepared by formal oxidative addition of the nitrenoid fragment to a dicopper(I) center by reaction with the iminoiodinane PhINTs (Ts = tosylate). This nitrenoid complex, (DPFN)Cu

Bimetallic motifs are a structural feature common to some of the most effective and synthetically useful catalysts known, including in the active sites of many metalloenzymes and on the surfaces of industrially relevant heterogeneous materials. However, the complexity of these systems often hampers detailed studies of their fundamental properties. To glean valuable mechanistic insight into how these catalysts function, this research group has prepared a family of dinucleating 1,8-naphthyridine ligands that bind two first-row transition metals in close proximity, originally designed to help mimic the proposed active site of metal oxide surfaces. Of the various bimetallic combinations examined, dicopper(I) is particularly versatile, as neutral bridging ligands adopt a variety of different binding modes depending on the configuration of frontier orbitals available to interact with the Cu centers. Organodicopper complexes are readily accessible, either through the traditional route of salt metathesis or via the activation of tetraarylborate anions through aryl group abstraction by a dicopper(I) unit. The resulting bridging aryl complexes engage in C-H bond activations, notably with terminal alkynes to afford bridging alkynyl species. The μ-hydrocarbyl complexes are surprisingly tolerant of water and elevated temperatures. This stability was leveraged to isolate a species that typically represents a fleeting intermediate in Cu-catalyzed azide-alkyne coupling (CuAAC); reaction of a bridging alkynyl complex with an organic azide afforded the first example of a well-defined, symmetrically bridged dicopper triazolide. This complex was shown to be an intermediate during CuAAC, providing support for a proposed bimetallic mechanism. These platforms are not limited to formally low oxidation states; chemical oxidation of the hydrocarbyl complexes cleanly results in formation of mixed valence CuICuII complexes with varying degrees of distortion in both the bridging moiety and the dicopper core. Higher oxidation states, e.g., dicopper(II), are easily accessed via oxidation of a dicopper(I) compound with air to give a CuII2(μ-OH)2 complex. Reduction of this compound with silanes resulted in the unexpected formation of pentametallic copper(I) dihydride clusters or trimetallic monohydride complexes, depending on the nature of the silane. Finally, development of an unsymmetrical naphthyridine ligand with mixed donor side-arms enables selective synthesis of an isostructural series of six heterobimetallic complexes, demonstrating the power of ligand design in the preparation of heterometallic assemblies.

A series of tri-tin complexes (LPhSn)3X with triple-decker structures (LPh = 2,5-di(o-pyridyl)-3,4-diphenylpyrrolate; X = Cl, AlCl4, OTf, and PF6) was synthesized by reducing LPhSnCl with LiBsBu3H and subsequent reactions. Structural characterization of (LPhSn)3Cl revealed a Sn-Sn-Sn core, and DFT calculations suggest that its HOMO is primarily σ-bonding along the tri-tin framework. (LPhSn)3Cl reacts with W(CO)5THF to afford (LPhSn)2(W(CO)5)2 and LPhSnCl, implying that (LPhSn)3Cl may exhibit dynamic behavior in solution.

Reduction of a dicopper(II) bis(hydroxide) complex with silanes in the presence of external copper or silver cations results in the formation of multinuclear hydride clusters, which were characterized by a variety of NMR spectroscopic experiments and X-ray crystallography. In particular, the pentanuclear complexes adopt an unusual planar "bow tie" configuration. The copper hydride complexes are efficient catalysts for the dehydrogenation of formic acid to H2 and CO2 .

Cationic μ-alkyl dicopper complexes [Cu2(μ-η1:η1-R)DPFN]NTf2 (R = CH3, CH2CH3, CH2C(CH3)3; DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine NTf2– = N(SO2CF3)2–) were synthesized by treatment of the acetonitrile-bridged dicopper complex [Cu2(μ-η1:η1-NCCH3)DPFN](NTf2)2 with LiR or MgR2. Structural characterization by X-ray crystallography and NMR spectroscopy revealed that the alkyl ligands symmetrically bridge the two copper centers, and the complexes persist in room-temperature solution. Notably, the μ-methyl complex showed less than 20% decomposition after 34 days in room-temperature THF solution. Treatment of the μ-methyl complex with acids allows installation of a range of monoanionic bridging ligands. However, surprisingly insertion into the dicopper–carbon bond was not observed upon addition of a variety of reagents, suggesting that these complexes exhibit a fundamentally new reactivity profile for alkylcopper species. Electrochemical characterization revealed oxidation–reduction events t...

An unusual cyclometalation reaction results from a C-C bond activation in Cp*(IPr)RuCl to give Cp*(IPr')Ru(L) featuring a NHC-C(sp2) chelating ligand (5-L; L = propene, N2; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene; IPr' = 1-(6-isopropylphenyl)-3-(2,6-diisopropylphenyl)imidazol-2-ylidene). DFT calculations were employed to elucidate the C-C bond activation pathway. Reactions of cyclometalated ruthenium complexes bearing NHC-C(sp2) and NHC-C(sp3) ligands (5-L and Cp*(IXy-H)Ru(N2), 1a, respectively where IXy = 1,3-bis(2,6-dimethylphenyl)-imidazol-2-ylidene; IXy-H is the deprotonated form of (IXy)) are reported. Deprotonation of 1a by an equimolar mixture of benzyl potassium and 18-crown-6 afforded a doubly-cyclometalated complex [Cp*(IXy-2H)Ru][K(18-crown-6)] (7). A lower CO stretching frequency in Cp*(IXy-H)Ru(CO) (8) vs. Cp*(IPr')Ru(CO) (9) suggests that the NHC-C(sp3) ligand is more electron donating. Complexes 5-L reacted with H2 to give the dihydride Cp*(IPr')RuH2 (11). In comparison, after an initial oxidative addition of H2, complex 1a with its more reactive Ru-C(sp3) bond underwent C-H bond reductive elimination, and a second oxidative addition of H2 afforded the trihydride Cp*(IXy)RuH3 (10). Reaction of 1a with B(C6F5)3 resulted in a zwitterionic complex Cp*Ru(IXy'') (12; IXy'' = 1-[2-((C6F5)3BCH2)C6H3-6-methyl]-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl) by the formation of a new C-B bond. In contrast, B(C6F5)3 abstracted a hydride from 5-L and promoted a very unusual C-C bond formation involving insertion of an allyl ligand into a Ru-C bond to form [Cp*Ru(IPr'')][HB(C6F5)3] (IPr'' = 1-[2-(CH2[double bond, length as m-dash]CHCH2)C6H3-6-isopropyl]-3-(2,6-diisopropyl)imidazol-2-ylidene-1-yl) (13).

A discrete, dicopper μ-alkynyl complex, [Cu2(μ-η1:η1-C≡C(C6H4)CH3)DPFN]NTf2 (DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine; NTf2– = N(SO2CF3)2–), reacts with p-tolylazide to yield a dicopper complex with a symmetrically bridging 1,2,3-triazolide, [Cu2(μ-η1:η1-(1,4-bis(4-tolyl)-1,2,3-triazolide))DPFN]NTf2. This transformation exhibits bimolecular reaction kinetics and represents a key step in a proposed, bimetallic mechanism for copper-catalyzed azide–alkyne cycloaddition (CuAAC). The μ-alkynyl and μ-triazolide complexes undergo reversible redox events (by cyclic voltammetry), suggesting that a cycloaddition pathway involving mixed-valence dicopper species might also be possible. Synthesis and characterization of the mixed-valence μ-alkynyl dicopper complex, [Cu2(μ-η1:η1-C≡C(C6H4)CH3)DPFN](NTf2)2, revealed an electronic structure with an unexpected partially delocalized spin, as evidenced by electron paramagnetic resonance spectroscopy. Studies of the mixed-valence μ-alkynyl complex’s reacti...

Syntheses of large polycyclic aromatic hydrocarbons (PAHs) and graphene nanostructures demand methods that are capable of selectively and efficiently fusing large numbers of aromatic rings, yet such methods remain scarce. Herein, we report a new approach that is based on the quantitative intramolecular reductive cyclization of an oligo(diyne) with a low-valent zirconocene reagent, which gives a PAH with one or more annulated zirconacyclopentadienes (ZrPAHs). The efficiency of this process is demonstrated by a high-yielding fivefold intramolecular coupling to form a helical ZrPAH with 16 fused rings (from a precursor with no fused rings). Several other PAH topologies are also reported. Protodemetalation of the ZrPAHs allowed full characterization (including by X-ray crystallography) of PAHs containing one or more appended dienes with the ortho-quinodimethane (o-QDM) structure, which are usually too reactive for isolation and are potentially valuable for the fusion of additional rings by Diels–Alder reactions.

The oxo-cobalt cubane unit [Co4O4] is of interest as a homogeneous oxygen-evolution reaction (OER) catalyst, and as a functional mimic of heterogeneous cobalt oxide OER catalysts. The synthesis of several new cubanes allows evaluation of redox potentials for the [Co4O4] cluster, which are highly sensitive to the ligand environment and span a remarkable range of 1.42 V. The [CoIII4O4]4+/[CoIII3CoIVO4]5+ and [CoIII3CoIVO4]5+/[CoIII2CoIV2O4]6+ redox potentials are reliably predicted by the pKas of the ligands. Hydrogen bonding is also shown to significantly raise the redox potentials, by ∼500 mV. The potential-pKa correlation is used to evaluate the feasibility of various proposed OER catalytic intermediates, including high-valent Co-oxo species. The synthetic methods and structure-reactivity relationships developed by these studies should better guide the design of new cubane-based OER catalysts.

A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand was treated with phosphaalkynes to yield dicopper complexes that exhibit phosphaalkynes in rare μ-η2:η2 binding modes., A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu2(μ-η1:η1-MeCN)DPFN](NTf2)2, was treated with phosphaalkynes (RC 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 P, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare μ-η2:η2 binding coordination modes. X-ray crystallography revealed that these unusual “tilted” structures exist in two isomeric forms (R “up” vs. R “sideways”), depending on the steric profile of the phosphaalkyne's alkyl group (R = Me, Ad, or tBu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and tBu (up). With intermediate steric bulk (R = Ad), the energy difference between the two geometries is small enough that both are observed in solution, and NMR spectroscopy and computations indicate that the solid-state structure corresponds to the minor isomer observed in solution. Meanwhile, treatment of [Cu2(μ-η1:η1-MeCN)DPFN](NTf2)2 with 2-butyne affords [Cu2(μ-η2:η2-(MeC 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 1111111111111111111111111111111111 1111111111111111111111111111111111 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 0000000000000000000000000000000000 CMe))DPFN](NTf2)2: its similar ligand geometry demonstrates that the tilted μ-η2:η2 binding mode is not limited to phosphaalkynes but reflects a more general trend, which can be rationalized via an NBO analysis showing maximization of π-backbonding.

A major challenge to the implementation of artificial photosynthesis (AP), in which fuels are produced from abundant materials (water and carbon dioxide) in an electrochemical cell through the action of sunlight, is the discovery of active, inexpensive, safe, and stable catalysts for the oxygen evolution reaction (OER). Multimetallic molecular catalysts, inspired by the natural photosynthetic enzyme, can provide important guidance for catalyst design, but the necessary mechanistic understanding has been elusive. In particular, fundamental transformations for reactive intermediates are difficult to observe, and well-defined molecular models of such species are highly prone to decomposition by intermolecular aggregation. Here, we present a general strategy for stabilization of the molecular cobalt-oxo cubane core (Co

Low-coordinate organoCr(III) complexes supported by the silylamido ligand –N(SiMe3)DIPP (DIPP = 2,6-diisopropylphenyl) are ethylene polymerization catalyst precursors without the need of additional cocatalyst. The reaction of CrCl3(THF)3 with 3 or 2 equiv. of LiN(SiMe3)DIPP yields either a four-membered cyclometalated Cr complex or Cr[N(SiMe3)DIPP]2Cl, respectively, with no trace of Cr[N(SiMe3)DIPP]3. Addition of 1 equiv. of LiN(SiMe3)DIPP to Cr[N(SiMe3)DIPP]2Cl also leads to the four-membered metallacycle, which upon heating transforms to a new six-membered Cr metallacycle, likely via a σ-bond metathesis step. Cr[N(SiMe3)DIPP]2Cl can be readily converted to bis(amido)Cr(III) vinyl and alkyl complexes Cr[N(SiMe3)DIPP]2R (R = vinyl, Bn, and Me). All of these structurally characterized low-coordinate Cr(III) complexes with a Cr–C bond initiate the polymerization of ethylene in the absence of activators or cocatalysts, producing ultra-high-molecular weight polyethylene.

A ( t Bu-DAC)Ir(COD)Cl complex ( t Bu-DAC = 1,3-bis(N- tert -butyl)diamidocarbene, COD = 1,5-cyclooctadiene) reacts with titanocene(II) and zirconocene(II) sources to form heterobimetallic complexes by metallocene coordination at the remote oxalamide binding site. Structural and spectroscopic data reveal the formation of a doubly-reduced [ t Bu-DAC] 2− ligand upon metallocene complexation. The metallocene-substituted complexes catalyze benzaldehyde hydrosilation at rates over an order of magnitude faster than those observed with the parent iridium complex.

The dianionic ligand TIPSDAX (2,7-di-tert-butyl-9,9-dimethyl-N4,N5-bis(triisopropylsilyl)-4,5-diamido-9H-xanthene) was synthesized and employed to support low-valent first-row transition metal complexes featuring iron(II) and cobalt(II). These neutral complexes were studied structurally and electrochemically. Cyclic voltammetry revealed a variety of oxidation features in the ligand and both metal complexes, while only the Co(II)(TIPSDAX)(THF) complex exhibited a reduction event. The cobalt(II) complex was chemically reduced to afford an isolable anionic Co(I) complex whose solid-state structure revealed a considerably more linear geometry at the metal center.

Artificial photosynthesis (AP) promises to replace society's dependence on fossil energy resources via conversion of sunlight into sustainable, carbon-neutral fuels. However, large-scale AP implementation remains impeded by a dearth of cheap, efficient catalysts for the oxygen evolution reaction (OER). Cobalt oxide materials can catalyze the OER and are potentially scalable due to the abundance of cobalt in the Earth's crust; unfortunately, the activity of these materials is insufficient for practical AP implementation. Attempts to improve cobalt oxide's activity have been stymied by limited mechanistic understanding that stems from the inherent difficulty of characterizing structure and reactivity at surfaces of heterogeneous materials. While previous studies on cobalt oxide revealed the intermediacy of the unusual Co(IV) oxidation state, much remains unknown, including whether bridging or terminal oxo ligands form O2 and what the relevant oxidation states are. We have addressed these issues by employing a homogeneous model for cobalt oxide, the [Co(III)4] cubane (Co4O4(OAc)4py4, py = pyridine, OAc = acetate), that can be oxidized to the [Co(IV)Co(III)3] state. Upon addition of 1 equiv of sodium hydroxide, the [Co(III)4] cubane is regenerated with stoichiometric formation of O2. Oxygen isotopic labeling experiments demonstrate that the cubane core remains intact during this stoichiometric OER, implying that terminal oxo ligands are responsible for forming O2. The OER is also examined with stopped-flow UV-visible spectroscopy, and its kinetic behavior is modeled, to surprisingly reveal that O2 formation requires disproportionation of the [Co(IV)Co(III)3] state to generate an even higher oxidation state, formally [Co(V)Co(III)3] or [Co(IV)2Co(III)2]. The mechanistic understanding provided by these results should accelerate the development of OER catalysts leading to increasingly efficient AP systems.

Reactivity studies of the thermally stable ruthenostannylene complex [Cp*(IXy)(H)2 Ru-Sn-Trip] (1; IXy=1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; Cp*=η(5) -C5 Me5 ; Trip=2,4,6-iPr3 C6 H2 ) with a variety of organic substrates are described. Complex 1 reacts with benzoin and an α,β-unsaturated ketone to undergo [1+4] cycloaddition reactions and afford [Cp*(IXy)(H)2 RuSn(κ(2) -O,O-OCPhCPhO)Trip] (2) and [Cp*(IXy)(H)2 RuSn(κ(2) -O,C-OCPhCHCHPh)Trip] (3), respectively. The reaction of 1 with ethyl diazoacetate resulted in a tin-substituted ketene complex [Cp*(IXy)(H)2 RuSn(OC2 H5 )(CHCO)Trip] (4), which is most likely a decomposition product from the putative ruthenium-substituted stannene complex. The isolation of a ruthenium-substituted stannene [Cp*(IXy)(H)2 RuSn(=Flu)Trip] (5) and stanna-imine [Cp*(IXy)(H)2 RuSn(κ(2) -N,O-NSO2 C6 H4 Me)Trip] (6) complexes was achieved by treatment of 1 with 9-diazofluorene and tosyl azide, respectively.

An N-heterocylic carbene (NHC) ring-opening reaction was observed upon treatment of the silyl complex Cp∗(IXy-H)(H)RuSiH2Mes [1; IXy = 1,3-bis(2,6-dimethylphenyl)imidazol-2-ylidene; IXy-H = 1-(2-CH2C6H3-6-methyl)-3-(2,6-dimethylphenyl)imidazol-2-ylidene-1-yl (the deprotonated form of IXy); Cp∗ = η5-C5Me5] with LiCH2SiMe3. This reaction results in formation of a novel, anionic Fischer-type carbene complex Cp∗(H)Ru{κ2-CHN(Xyl)CH[Si(CH2SiMe3)Mes]N(Xyl)Li} (4). This ring-opening reaction demonstrates a new pathway for NHC degradation via the cooperation of ruthenium, silicon, and lithium. Additionally, treatment of the dinitrogen complex Cp∗(IXy-H)Ru(N2) with LiCH2SiMe3 led to the doubly-metallated complex [Cp∗(IXy-2H)Ru]Li (5), which exhibits a solid-state polymeric structure and metallation of both xylyl groups of the IXy ligand. Finally, removal of a hydride from 4, achieved with two equiv of B(C6F5)3, led to C–H activation of the Cp∗ ligand and formation of an unusual, formally dianionic η5-Me4C5CH2B(C6F5)3 ligand in a ruthenium carbene complex [η5-Me5C5CH2B(C6F5)3](H)Ru{κ3-CHN(Xyl)CH[SiH(CH2SiMe3)Mes]N(Xyl)} (7).

A strong σ-donating cyclic (alkyl)(amino) carbene (cAAC) triggers rearrangement of the silyl(aryl) amido ligand –N(SiMe3)Dipp (Dipp = 2,6-diisopropylphenyl) in the coordination sphere of titanium(III) to afford a novel zwitterionic titanium imido complex with a TiCH2SiMe2[cAAC] linkage. Reduction of this species produces a new DippN=Ti imido complex containing a cAAC-centered radical species, characterized by single-crystal diffraction analysis and electron paramagnetic resonance spectroscopy.

Incorporation of Mn into an established water oxidation catalyst based on a Co(III)4O4 cubane was achieved by a simple and efficient assembly of permanganate, cobalt(II) acetate, and pyridine to form the cubane oxo cluster MnCo3O4(OAc)5py3 (OAc = acetate, py = pyridine) (1-OAc) in good yield. This allows characterization of electronic and chemical properties for a manganese center in a cobalt oxide environment, and provides a molecular model for Mn-doped cobalt oxides. The electronic properties of the cubane are readily tuned by exchange of the OAc− ligand for Cl− (1-Cl), NO3− (1-NO3), and pyridine ([1-py]+). EPR spectroscopy, SQUID magnetometry, and DFT calculations thoroughly characterized the valence assignment of the cubane as [MnIVCoIII3]. These cubanes are redox-active, and calculations reveal that the Co ions behave as the reservoir for electrons, but their redox potentials are tuned by the choice of ligand at Mn. This MnCo3O4 cubane system represents a new class of easily prepared, versatile, and redox-active oxido clusters that should contribute to an understanding of mixed-metal, Mn-containing oxides.

The synthesis of discrete, cationic binuclear μ-aryl dicopper complexes [Cu2(μ-η(1):η(1)-Ar)DPFN]X (Ar = C6H5, 3,5-(CF3)2C6H3, and C6F5; DPFN = 2,7-bis(fluoro-di(2-pyridyl)methyl)-1,8-naphthyridine; X = BAr4(-) and NTf2(-); Tf = SO2CF3) was achieved by treatment of a dicopper complex [Cu2(μ-η(1):η(1)-NCCH3)DPFN]X2 (X = PF6(-) and NTf2(-)) with tetraarylborates. Structural characterization revealed symmetrically bridging aryl groups, and (1)H NMR spectroscopy evidenced the same structure in solution at 24 °C. Electrochemical investigation of the resulting arylcopper complexes uncovered reversible redox events that led to the synthesis and isolation of a rare mixed-valence organocopper complex [Cu2(μ-η(1):η(1)-Ph)DPFN](NTf2)2 in high yield. The solid-state structure of the mixed-valence μ-phenyl complex exhibits inequivalent copper centers, despite a short Cu···Cu distance. Electronic and variable-temperature electron paramagnetic resonance spectroscopy of the mixed-valence μ-phenyl complex suggest that the degree of spin localization is temperature-dependent, with a high degree of spin localization observed at lower temperatures. Electronic structure calculations agree with the experimental results and suggest that the spin is localized almost entirely on one metal center.

Z-type interactions between bis(perfluorophenyl)zinc and platinum(II) diaryl complexes supported by 1,10-phenanthroline (phen), 2,2′-bipyridine (bpy), and bis(dimethylphosphino)ethane (dmpe) ligands are reported. In the solid state, the nature of the Pt–Zn interaction depends on the bidentate ligand; the phen-supported complex exhibits an unsupported Pt–Zn bond, while the dmpe derivative features additional bridging aryl interactions. A strongly accelerated rate of reductive elimination is observed for phen- and bpy-supported complexes, while aryl exchange between Pt and Zn is observed for the dmpe complex.

E-cadherin (E-cad) is a transmembrane adhesion glycoprotein, the expression of which is often reduced in invasive or metastatic tumors. To assess E-cad's distribution among different types of cancer cells, we used bisulfite-sequencing for detailed, base-by-base measurement of CpG methylation in E-cad's promoter region in the NCI-60 cell lines. The mean methylation levels of the cell lines were distributed bimodally, with values pushed toward either the high or low end of the methylation scale. The 38 epithelial cell lines showed substantially lower (28%) mean methylation levels compared with the nonepithelial cell lines (58%). The CpG site at -143 with respect to the transcriptional start was commonly methylated at intermediate levels, even in cell lines with low overall DNA methylation. We also profiled the NCI-60 cell lines using Affymetrix U133 microarrays and found E-cad expression to be correlated with E-cad methylation at highly statistically significant levels. Above a threshold of ∼20% to 30% mean methylation, the expression of E-cad was effectively silenced. Overall, this study provides a type of detailed analysis of methylation that can also be applied to other cancer-related genes. As has been shown in recent years, DNA methylation status can serve as a biomarker for use in choosing therapy. [Mol Cancer Ther 2007;6(2):391–403]

The compound [Co2(μ-OH)2(OH2)2(DPFN)][NO3]4 is a molecular structural analog of proposed active sites of cobalt phosphate water oxidation catalysts. Computational studies on this system indicate feasible catalytic pathways to oxygen formation, despite the low electrocatalytic activity observed for [Co2(μ-OH)2(OH2)2(DPFN)][NO3]4. Electrochemical and reactivity studies implicate the binding of phosphate to the dicobalt core, which may inhibit water oxidation catalysis.