Chloride- and Hydrosulfide-Bound 2Fe Complexes as Models of the Oxygen-Stable State of [FeFe] Hydrogenase.

[FeFe] hydrogenases demonstrate remarkable catalytic efficiency in hydrogen evolution and oxidation processes. However, susceptibility of these enzymes to oxygen-induced degradation impedes their practical deployment in hydrogen-production devices and fuel cells. Recent investigations into the oxygen-stable (Hinact) state of the H-cluster revealed its inherent capacity to resist oxygen degradation. Herein, we present findings on Cl- and SH-bound [2Fe-2S] complexes, bearing relevance to the oxygen-stable state within a biological context. A characteristic attribute of these complexes is the terminal Cl-/SH- ligation to the iron center bearing the CO bridge. Structural analysis of the t-Cl demonstrates a striking resemblance to the Hinact state of DdHydAB and CbA5H. The t-Cl/t-SH exhibit reversible oxidation, with both redox species, electronically, being the first biomimetic analogs to the Htrans and Hinact states. These complexes exhibit notable resistance against oxygen-induced decomposition, supporting the potential oxygen-resistant nature of the Htrans and Hinact states. The swift reductive release of the Cl-/SH-group demonstrates its labile and kinetically controlled binding. The findings garnered from these investigations offer valuable insights into properties of the enzymatic O2-stable state, and key factors governing deactivation and reactivation conversion. This work contributes to the advancement of bio-inspired molecular catalysts and the integration of enzymes and artificial catalysts into H2-evolution devices and fuel-cell applications.

Enantioselective Organocatalytic Three-Component Vinylogous Michael/Aldol Tandem Reaction among 3-Alkylidene oxindoles, Methyleneindolinones, and Aldehydes.

We reported a one-pot enantioselective three-component vinylogous Michael/aldol tandem reaction of prochiral 3-alkylidene oxindoles with methyleneindolinones and aldehydes using bifunctional organocatalysts. A variety of enantioenriched 3,3-disubstituted oxindoles 3 and spirolactones 4 were generated in moderate yields (up to 78%) with high stereoselectivities (up to >20:1 dr, >99% ee). Intriguingly, we observed that the aldol reaction with paraformaldehyde generates 3,3-disubstituted oxindoles 3 bearing a hydroxymethyl group, while the reaction with aliphatic aldehydes generates spirolactones 4.

Stable Bimetallic FeII/{Fe(NO)2}9 Moiety Derived from Reductive Transformations of a Diferrous-dinitrosyl Species.

A dimeric dithiolate-bridged species, [Fe(NO)(PS2)]2 (1) containing two {FeNO}7 units, can be isolated by treating [Fe(CO)2(NO)2] with PS2H2 (PS2H2 = bis(2-dimercaptophenyl)phenylphosphine). Crystallographic studies reveal the syn-configuration of NO units and the bridging thiolates in the butterfly shape of the 2Fe2S core. Addition of PPh3 to the solution of dinuclear 1 leads to the formation of mononuclear {FeNO}7 [Fe(NO)(PS2)(PPh3)] (2) that shows electrochemical responses similar to those of 1. One-electron reduction of 1 with Cp*2Co or KC8 results in the isolation of thiolate-bridged bimetallic DNIC, [(PS2)Fe(µ-PS2)Fe(NO)2]- ([3]-), confirmed by several spectroscopies including single-crystal X-ray diffraction studies. The bimetallic DNIC [3]- is a rare example obtained from the one-electron reduction of a dinuclear Fe-NO {FeNO}7 model complex. With the assistance of redox behaviors of 2, electrochemical studies imply that the reduction of 1 leads to the formation of a mononuclear {FeNO}8 [Fe(NO)(PS2)(THF)]- intermediate, which involves disproportionation or NO- transfer to yield [3]-. Based on IR data and magnetic properties, the electronic structure of [3]- can be described as a FeII/{Fe(NO)2}9 state. Isolation of the {Fe(NO)2}9 moiety coordinated by the Fe ancillary complex lends strong support to the NO scrambling behavior in the effectiveness of the activity of flavodiiron nitric oxide reductases (FNORs).

High-Frequency Fe-H and Fe-H2 Modes in a trans-Fe(η2-H2)(H) Complex: A Speed Record for Nuclear Resonance Vibrational Spectroscopy.

Nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT) are complementary tools for studying the vibrational and geometric structures of specific isotopically labeled molecular systems. Here we apply NRVS and DFT to characterize the trans-[57Fe(η2-H2)(H)(dppe)2][BPh4] [dppe = 1,2-bis(diphenylphosphino)ethane] complex. Heretofore, most NRVS observations have centered on the spectral region below 1000 cm-1, where the 57Fe signal is strongest. In this work, we show that state-of-the-art synchrotron facilities can extend the observable region to 2000 cm-1 and likely beyond, in measurements that require less than 1 day. The 57Fe-H stretch was revealed at 1915 cm-1, along with the asymmetric 57Fe-H2 stretch at 1774 cm-1. For a small fraction of the H2-dissociated product, the 57Fe-H stretch was detected at 1956 cm-1. The unique sensitivity to 57Fe motion and the isolated nature of the Fe-H/H2 stretching modes enabled NRVS to quantitatively analyze the sample composition.

Synthesis and Luminescence Properties of Two-Electron Bimetallic Cu-Ag and Cu-Au Nanoclusters via Copper Hydride Precursors.

The synthesis, structural characteristics, and photophysical properties of luminescent Cu-rich bimetallic superatomic clusters [Au@Cu12(S2CNnPr2)6(C≡CPh)4]+ (1a+), [Au@Cu12{S2P(OR)2}6(C≡CPh)4]+ (2+), (2a+ = iPr; 2b+ = nPr), [Au@Cu12{S2P(C2H4Ph)2}6(C≡CPh)4]+ (2c+), and [Ag@Cu12{S2P(OnPr)2}6(C≡CPh)4]+ (3+) were studied. Compositionally uniform clusters 1+-3+ were isolated from the reaction of dithiolato-stabilized, polyhydrido copper clusters with phenylacetylene in the presence of heterometal salts. By using X-ray diffraction, the structures of 1a+, 2a+, 2b+, and 3+ were able to be determined. ESI-mass spectrometry and elemental analysis confirmed their compositions and purity. The structural characteristics of these clusters are similar with respect to displaying gold (or silver)-centered Cu12 cuboctahedra surrounded by six dithiocarbamate/dithiophosph(in)ate and four alkynyl ligands. The doping of Au and Ag atoms into the polyhydrido copper nanoclusters significantly enhances their PL quantum yields from Ag@Cu12 (0.58%) to Au@Cu12 (55%) at ambient temperature in solution. In addition, the electrochemical properties of the new alloys were investigated by cyclic voltammetry.

Organocatalytic asymmetric allylic alkylation of 2-methyl-3-nitroindoles: a route to direct enantioselective functionalization of indole C(sp3)-H bonds.

We here described a direct catalytic asymmetric functionalization of 2-methylindoles using organocatalysis. An efficient asymmetric allylic alkylation reaction with respect to 2-methyl-3-nitroindoles and racemic Morita-Baylis-Hillman carbonate has been achieved by using a chiral biscinchona alkaloid catalyst, which provided the functionalized indole derivatives in good yields and enantioselectivities.

Photoinduced NO and HNO Production from Mononuclear {FeNO}6 Complex Bearing a Pendant Thiol.

Light triggers the formation of HNO from a metal-nitrosyl species, facilitated by an intramolecular pendant thiol proton. Two {FeNO}6 complexes (the Enemark-Felthan notation), [Fe(NO)(TMSPS2)(TMSPS2H)] (1, TMSPS2H2 = 2,2'-dimercapto-3,3'-bis(trimethylsilyl)diphenyl)phenylphosphine; H is a dissociable proton) with a pendant thiol and [Fe(NO)(TMSPS2)(TMSPS2CH3)] (2) bearing a pendant thioether, are spectroscopically and structurally characterized. Both complexes are highly sensitive to visible light. Upon photolysis, complex 2 undergoes NO dissociation to yield a mononuclear Fe(III) complex, [Fe(TMSPS2)(TMSPS2CH3)] (3). In contrast, the pendant SH of 1 can act as a trap for the departing NO radical upon irradiation, resulting in the formation of an intermediate A with an intramolecular [SH···ON-Fe] interaction. As suggested by computational results (density functional theory), the NO stretching frequency (νNO) is sensitive to the intramolecular interaction between the pendant ligand and the iron-bound NO, and a shift of νNO from 1833 (1) to 1823 cm-1 (A) is observed experimentally. Subsequent photolysis of the intermediate A results in HNO production and a thiyl group that then coordinates to the Fe center for the formation of [Fe(TMSPS2)2] (4). In contrast with the common acid-base coupling pathway, the HNO is not voluntarily yielded from 1 but rather is generated by the photopromoted pathway. The photogenerated HNO can further react with [MnIII(TMSPS3)(DABCO)] (TMSPS3H3 = (2,2'2''-trimercapto-3,3',3''-tris(trimethylsilyl)triphenylphosphine; DABCO = 1,4-diazabicyclo[2.2.2]octane) in organic media to yield anionic [Mn(NO)(TMSPS3)]- (5-) with a {MnNO}6 electronic configuration, whereas [MnIII(TMSPS3)(DABCO)] reacts with NO gas for the formation of a {MnNO}5 species, [Mn(NO)(TMSPS3)] (6). Effective differentiation of the formation of HNO from complex 1 with the pendant SH versus NO from 2 with the pendant SMe is achieved by the employment of [MnIII(TMSPS3)(DABCO)].

Diindeno-Fused Dibenzo[a,h]anthracene and Dibenzo[c,l]chrysene: Syntheses, Structural Analyses, and Properties.

Diindeno-fused dibenzo[a,h]anthracene 6 and diindeno-fused dibenzo[c,l]chrysene 9 contain the key moieties 1,4-quinodipropene (1,4-QDP) and 2,6-naphthoquinodipropene (2,6-NQDP), respectively, and they both have an open-shell singlet ground state. The latter compound exhibits a strong biradical character and interesting properties, including a low ΔET-S (2.44 kcal mol-1 ), a small HOMO-LUMO gap (1.06 eV), a wide photoabsorption range (250-1172 nm), and a large two-photon absorption cross-section (σ=1342±56 GM). This work verifies that 6 has a slightly larger HOMO-LUMO gap and ΔET-S than its helical isomer diindeno[2,1-f:1',2'-j]picene (DIP), but is a much stronger two-photon absorber, verifying the important effect of geometry on the photophysical properties.

NO-to-[N2O2]2--to-N2O Conversion Triggered by {Fe(NO)2}10-{Fe(NO)2}9 Dinuclear Dinitrosyl Iron Complex.

Flavodiiron nitric oxide reductases (FNORs) evolved in some pathogens are known to detoxify NO via two-electron reduction to N2O to mitigate nitrosative stress. In this study, we describe how the electronically localized {Fe(NO)2}10-{Fe(NO)2}9 dinuclear dinitrosyl iron complex (dinuclear DNIC) [(NO)2Fe(µ-bdmap)Fe(NO)2(THF)] (2) (bdmap = 1,3-bis(dimethylamino)-2-propanolate) can induce a reductive coupling of NO to form hyponitrite-coordinated tetranuclear DNIC, which then converts to N2O. Upon the addition of 1 equiv of NO into the dinuclear {Fe(NO)2}10-{Fe(NO)2}9 DNIC 2, the proposed side-on-bound [NO]--bridged [(NO)2Fe(µ-bdmap)(κ2-NO) Fe(NO)2] intermediate may facilitate intermolecular (O)N-N(O) bond coupling to yield the paramagnetic tetranuclear quadridentate trans-hyponitrite-bound {[(NO)2Fe(µ-bdmap)Fe(NO)2]2(κ4-N2O2)} that transforms to [Fe(NO)2(µ-bdmap)]2, along with the release of N2O upon Hbdmap (1,3-bis(dimethylamino)-2-propanol) added.

Synthesis of Bimetallic Copper-Rich Nanoclusters Encapsulating a Linear Palladium Dihydride Unit.

The structurally precise Cu-rich hydride nanoclusters [PdCu14 H2 (dtc/dtp)6 (C≡CPh)6 ] (dtc: di-butyldithiocarbamate (1); dtp: di-isopropyl dithiophosphate (2)) were synthesized from the reaction of polyhydrido copper clusters [Cu28 H15 (S2 CNn Bu2 )12 ]+ or [Cu20 H11 {S2 P(Oi Pr)2 }9 ] with phenyl acetylene in the presence of Pd(PPh3 )2 Cl2 . Their structures and compositions were determined by single-crystal X-ray diffraction and the results supported by ESI-mass spectrometry. Hydride positions in 1 were confirmed by single-crystal neutron diffraction. Each hydride is connected to one Pd0 and four CuI atoms in slightly distorted trigonalbipyramidal geometry. The anatomies of clusters 1 and 2 are very similar and DFT calculations allow rationalizing the interactions between the encapsulated [PdH2 ]2- unit and its Cu14 bicapped icosahedral cage. As a result, Pd has the highest coordination number (14) so far recorded.

Heteroatom-Doping Increases Cluster Nuclearity: From an [Ag20 ] to an [Au3 Ag18 ] Core.

A templated galvanic exchange performed on [Ag20 {Se2 P(OiPr)2 }12 ] of C3 symmetry with three equiv AuI yields a mixture of [Au1+x Ag20-x {Se2 P(OiPr)2 }12 ]+ (x=0-2) from which [Au@Ag20 {Se2 P(OiPr)2 }12 ]+ and [Au@Au2 Ag18 {Se2 P(OiPr)2 }12 ]+ are successfully characterized to have T and C1 symmetry, respectively. Crystal structural analyses combined with DFT calculations on the model compounds explicitly demonstrate that the central Ag0 of Ag20 being oxidized by AuI migrates to the protecting atomic shell as a new capping AgI , and both second and third Au dopants prefer occupying non-adjacent icosahedron vertices. The differences in symmetry, T and C1 , are manifested in the spatial orientation of their protecting atomic shell composed of eight capping Ag atoms as well as re-construction upon the replacement of Ag atoms on the vertices of AuAg12 icosahedral core with second and third Au dopants. As a result, a unique pathway for substitutional-doped clusters with increased nuclearity is proposed.

Electron Delocalization of Mixed-Valence Diiron Sites Mediated by Group†10 Metal Ions in Heterotrimetallic Fe-M-Fe (M=Ni, Pd, and Pt) Chain Complexes.

The heterotrimetallic complexes [FeMFe(dpa)4 Cl2 ] (M=Ni (1), Pd (2), and Pt (3); dpa- =dipyridylamido) featuring two high-spin iron centers linked by Group 10 metals were synthesized and their physical properties were investigated. Oxidation of 1-3 with suitable oxidants in CH2 Cl2 solution yielded the mixed-valent species [1]+/2+ -[3]+/2+ . The solution properties of [1]0/+/2+ -[3]0/+/2+ were characterized by 1 H NMR and UV/Vis/NIR spectroscopy as well as spectroelectrochemisty. The mixed-valent states of [1]+ -[3]+ obtained by electrochemical or chemical oxidation are classified as class II valence delocalization. The solid-state structures of 1-3, [1]+ , [3]+ , and [1]2+ were determined by single-crystal X-ray diffraction analysis, exhibiting a linear metal framework with an approximate D4 symmetry. The spin states and magnetic properties were studied by using SQUID magnetometry, EPR and Mössbauer spectroscopy, and DFT calculations. Antiferromagnetic interactions between terminal high-spin iron centers are present within [1]0/+/2+ -[3]0/+/2+ and the |J| values increase with the central metal ion changing from Ni to Pt. The DFT calculations reproduce the antiferromagnetic coupling and ascribe it to a σ-type exchange pathway. The substitution of the central metal not only influences the spin-spin interactions but also the degree of electronic delocalization between the terminal iron sites along the Fe-M-Fe chains.

Energy-Efficient Hydrogen Evolution by Fe-S Electrocatalysts: Mechanistic Investigations.

The intrinsic catalytic property of a Fe-S complex toward H2 evolution was investigated in a wide range of acids. The title complex exhibited catalytic events at -1.16 and -1.57 V (vs Fc+/Fc) in the presence of trifluoromethanesulfonic acid (HOTf) and trifluoroacetic acid (TFA), respectively. The processes corresponded to the single reduction of the Fe-hydride-S-proton and Fe-hydride species, respectively. When anilinium acid was used, the catalysis occurred at -1.16 V, identical with the working potential of the HOTf catalysis, although the employment of anilinium acid was only capable of achieving the Fe-hydride state on the basis of the spectral and calculated results. The thermodynamics and kinetics of individual steps of the catalysis were analyzed by density functional theory (DFT) calculations and electroanalytical simulations. The stepwise CCE or CE (C, chemical; E, electrochemical) mechanism was operative from the HOTf or TFA source, respectively. In contrast, the involvement of anilinium acid most likely initiated a proton-coupled electron transfer (PCET) pathway that avoided the disfavored intermediate after the initial protonation. Via the PCET pathway, the heterogeneous electron transfer rate was increased and the overpotential was decreased by 0.4 V in comparison with the stepwise pathways. The results showed that the PCET-involved catalysis exhibited substantial kinetic and thermodynamic advantages in comparison to the stepwise pathway; thus, an efficient catalytic system for proton reduction was established.

Anion-Directed Metallocages: A Study on the Tendency of Anion Templation.

Self-assembly of Cu(NO3 )2 ⋅3 H2 O and di(3-pyridylmethyl)amine (dpma) with addition of different acids (HNO3 , HOAc, HCl, HClO4 , HOTf, HPF6 , HBF4 , and H2 SO4 ) afforded a family of anion-templated tetragonal metallocages with a cationic prismatic structure of [(Gn- )⊂{Cu2 (Hdpma)4 }](8-n)+ (Gn- =NO3- , PF6- , SiF62- ) with different ligating anions/solvents (NO3- , Cl- , ClO4- , OTf- , H2 O) outside the cage. Systematic competitive experiments have rationalized the tendency of anion templation towards the formation of metallocages [(Gn- )⊂{Cu2 (Hdpma)4 }](8-n)+ as occurring in the order SiF62- ≈PF6- >NO3- >SO42- ≈ClO4- ≈BF4- . This sequence is mostly elucidated by shape control over size selectivity and electrostatic attraction between the cationic {Cu2 (Hdpma)4 }8+ host and the anionic guests. In addition, these results have also roughly ranked the anion coordination ability in the order Cl- , ClO4- , OTf- >NO3- >BF4- , CH3 SO4- . Magnetic studies of metallocages 1 t and 2-4 suggest that the fitted magnetic interaction, being weakly magnetically coupled overall, is interpreted as a result of the combination of intracage ferromagnetic coupling integrals and intercage antiferromagnetic exchange; both contributions are very weak and comparable in strength.

Generation of a Mn(IV)-Peroxo or Mn(III)-Oxo-Mn(III) Species upon Oxygenation of Mono- and Binuclear Thiolate-Ligated Mn(II) Complexes.

A thiolate-bridged binuclear complex [PPN]2[(MnII(TMSPS3))2] (1, PPN = bis(triphenylphosphine)iminium and TMSPS3H3 = (2,2',2″-trimercapto-3,3',3″-tris(trimethylsilyl)triphenylphosphine)), prepared from the reaction of MnCl2/[PPN]Cl and Li3[TMSPS3], converts into a mononuclear complex [PPN][MnII(TMSPS3)(DABCO)] (2) in the presence of excess amounts of DABCO (DABCO = 1,4-diazabicyclo[2.2.2]octane). Variable temperature studies of solution containing 1 and DABCO by UV-vis spectroscopy indicate that 1 and 2 exist in significant amounts in equilibrium and mononuclear 2 is favored at low temperature. Treatment of 1 or 2 with the monomeric O2-side-on-bound [PPN][MnIV(O2)(TMSPS3)] (3) produces the mono-oxo-bridged dimer [PPN]2[(MnIII(TMSPS3))2(µ-O)] (4). The electrochemistry of 1 and 2 reveals anodic peak(s) for a MnIII/MnII redox couple at shifted potentials against Fc/Fc+, indicating that both complexes can be oxidized by dioxygen. The O2 activation mediated by 1 and 2 is investigated in both solution and the solid state. Microcrystals of 2 rapidly react with air or dry O2 to generate the Mn(IV)-peroxo 3 in high yield, revealing a solid-to-solid transformation and two-electron reduction of O2. Oxygenation of 1 or 2 in solution, however, is affected by diffusion and transient concentration of dioxygen in the two different substrates, leading to generation of 3 and 4 in variable ratios.

Eight-Electron Silver and Mixed Gold/Silver Nanoclusters Stabilized by Selenium Donor Ligands.

The first atomically and structurally precise silver-nanoclusters stabilized by Se-donor ligands, [Ag20 {Se2 P(Oi Pr)2 }12 ] (3) and [Ag21 {Se2 P(OEt)2 }12 ]+ (4), were isolated by ligand replacement reaction of [Ag20 {S2 P(Oi Pr)2 }12 ] (1) and [Ag21 {S2 P(Oi Pr)2 }12 ]+ (2), respectively. Furthermore, doping reactions of 4 with Au(PPh3 )Cl resulted in the formation of [AuAg20 {Se2 P(OEt)2 }12 ]+ (5). Structures of 3, 4, and 5 were determined by single-crystal X-ray diffraction. The anatomy of cluster 3 with an Ag20 core having C3 symmetry is very similar to that of its dithiophosphate analogue 1. Clusters 4 and 5 exhibit an Ag21 and Au@Ag20 core of Oh symmetry composed of eight silver capping atoms in a cubic arrangement and encapsulating an Ag13 and Au@Ag12 centered icosahedron, respectively. Both ligand exchange and heteroatom doping result in significant changes in optical and emissive properties for chalcogen-passivated silver nanoparticles, which have been theoretically confirmed as 8-electron superatoms.

Anion-Directed Copper(II) Metallocages, Coordination Chain, and Complex Double Salt: Structures, Magnetic Properties, EPR Spectra, and Density Functional Study.

A series of Cu(II) metallo-assemblies showing anion-directed structural variations, including five metallocages [(G(n-) )⊂{Cu2 (Hdpma)4 }]((8-n)+) (A(-) )8-n (G(n-) =NO3 (-) , ClO4 (-) , SiF6 (2-) , BF4 (-) , SO4 (2-) ; A(-) =NO3 (-) , ClO4 (-) , BF4 (-) , CH3 SO4 (-) ; Hdpma=bis(3-pyridylmethyl)ammonium cation), a complex double salt, namely, (H3 dpma)4 (CuCl4 )5 Cl2 , and a coordination chain, namely, [Cu2 (dpma)(OAc)4 ], are reported. The influence of the anion can be explained by its coordinating ability, the affinity of which for the Cu(II) center interferes significantly with metallocage formation, and its shape, which offers host-guest recognition ability to engage in weak metal-anion coordination and hydrogen bonding to the organic ligand, which are responsible for metallocage templation. EPR studies of these metallocages in the powder phase at room temperature and 77 K showed a trend of the g values (g|| >2.10>g⊥ >2.00) indicating a dx2-y2 -based ground state with square-pyramidal geometry for the Cu(II) centers. The magnetism of these metallocages can be interpreted as the result of a combination of relatively small magnetic coupling integrals and a substantial contribution of temperature-independent paramagnetism (TIP). The weak magnetic interaction is corroborated by the results of DFT calculations and the EPR spectra. Availability of the low-lying state for spin population was confirmed by a magnetization study, which revealed a magnetic moment approaching 2Nß, which would explain the presence of the larger TIP term.

[Ag20 {S2 P(OR)2 }12 ]: A Superatom Complex with a Chiral Metallic Core and High Potential for Isomerism.

The synthesis and structural determination of a silver nanocluster [Ag20 {S2 P(OiPr)2 }12 ] (2), which contains an intrinsic chiral metallic core, is produced by reduction of one silver ion from the eight-electron superatom complex [Ag21 {S2 P(OiPr)2 }12 ](PF6 ) (1) by borohydrides. Single-crystal X-ray analysis displays an Ag20 core of pseudo C3 symmetry comprising a silver-centered Ag13 icosahedron capped by seven silver atoms. Its n-propyl derivative, [Ag20 {S2 P(OnPr)2 }12 ] (3), can also be prepared by the treatment of silver(I) salts and dithiophosphates in a stoichiometric ratio in the presence of excess amount of [BH4 ](-) . Crystal structure analyses reveal that the capping silver-atom positions relative to their icosahedral core are distinctly different in 2 and 3 and generate isomeric, chiral Ag20 cores. Both Ag20 clusters display an emission maximum in the near IR region. DFT calculations are consistent with a description within the superatom model of an 8-electron [Ag13 ](5+) core protected by a [Ag7 {S2 P(OR)2 }12 ](5-) external shell. Two additional structural variations are predicted by DFT, showing the potential for isomerism in such [Ag20 {S2 P(OR)2 }12 ] species.

[Cu13 {S2 CNn Bu2 }6 (acetylide)4 ]+ : A Two-Electron Superatom.

The first structurally characterized copper cluster with a Cu13 centered cuboctahedral arrangement, a model of the bulk copper fcc structure, was observed in [Cu13 (S2 CNn Bu2 )6 (C≡CR)4 ](PF6 ) (R=C(O)OMe, C6 H4 F) nanoclusters. Four of the eight triangular faces of the cuboctahedron are capped by acetylide groups in µ3  fashion, and each of the six square faces is bridged by a dithiolate ligand in µ2 ,µ2 fashion, which leads to a truncated tetrahedron of twelve sulfur atoms. DFT calculations are fully consistent with the description of these Cu13 clusters as two-electron superatoms, that is, a [Cu13 ]11+ core passivated by ten monoanionic ligands, with an a1 HOMO containing two 1S jellium electrons.

A reversible proton relay process mediated by hydrogen-bonding interactions in [FeFe]hydrogenase modeling.

A reversible and temperature-dependent proton-relay process is demonstrated for a Fe2 complex possessing a terminal thiolate in the presence of nitrogen-based acids. The terminal sulfur site (S(t) ) of the complex forms a hydrogen-bond interaction with N,N-dimethylanilinium acid at 183 K. The Fe2 core, instead, is protonated to generate a bridging hydride at 298 K. Reversibility is observed for the tautomerization between the hydrogen-bonded pair and the Fe-hydride species. X-ray structural analysis of the hydrogen-bonded species at 193 K reveals a short N(H)⋅⋅⋅S(t) contact. Employment of pyridinium acid also results in similar behavior, with reversible proton-hydride interconversion. DFT investigation of the proton-transfer pathways indicates that the pKa value of the hydrogen-bonded species is enhanced by 3.2 pKa units when the temperature is decreased from 298 K to 183 K.