A conformational role for NifW in the maturation of molybdenum nitrogenase P-cluster

Reduction of dinitrogen by molybdenum nitrogenase relies on complex metalloclusters: the [8Fe:7S] P-cluster and the [7Fe:9S:Mo:C:homocitrate] FeMo-cofactor. Although both clusters bear topological similarities and require the reductive fusion of [4Fe:4S] sub-clusters to achieve their respective assemblies, P-clusters are assembled directly on the NifD₂K₂ polypeptide prior to the insertion of FeMo-co, which is fully assembled separately from NifD₂K₂. P-cluster maturation involves the iron protein NifH₂ as well as several accessory proteins, whose role has not been elucidated. In the present work, two NifD₂K₂ species bearing immature P-clusters were isolated from an Azotobacter vinelandii strain in which the genes encoding NifH and the accessory protein NifZ were deleted, and characterized by X-ray absorption spectroscopy and EPR. These analyses showed that both NifD₂K₂ complexes harbor clusters that are electronically and structurally similar, with each NifDK unit containing two [4Fe:4S]^2+/+ clusters. Binding of the accessory protein NifW parallels a decrease in the distance between these clusters, as well as a subtle change in their coordination. These results support a conformational role for NifW in P-cluster biosynthesis, bringing the two [4Fe:4S] precursors closer prior to their fusion, which may be crucial in challenging cellular contexts.

Upon binding of NifW, a subtle conformation change occurs in NifD₂K₂, decreasing the distance between the two [4Fe:4S] clusters precursors of the P-cluster in nitrogenase.     

Insights into the Two-Electron Reductive Process of [FeFe]H2ase Biomimetics: Cyclic Voltammetry and DFT Investigation on Chelate Control of Redox Properties of [Fe2(CO)4(κ2-Chelate)(μ-Dithiolate)]

The electrochemical reduction of complexes [Fe₂(CO)₄(κ²-phen)(μ-xdt)] (phen=1,10-phenanthroline; xdt=pdt ( 1 ), adt^iPr ( 2 )) in MeCN-[Bu₄N][PF₆] 0.2 m is described as a two-reduction process. DFT calculations show that 1 and its monoreduced form 1⁻ display metal- and phenanthroline-centered frontier orbitals (LUMO and SOMO) indicating the non-innocence of the phenanthroline ligand. Two energetically close geometries were found for the doubly reduced species suggesting an intriguing influence of the phenanthroline ligand leading to the cleavage of a Fe−S bond as proposed generally for this type of complex or retaining the electron density and avoiding Fe−S cleavage. Extension of calculations to other complexes with edt, adt^iPr bridge and even virtual species [Fe₂(CO)₄(κ²-phen)(μ-adt^R)] (R=CH(CF₃)₂, H) or [Fe₂(CO)₄(κ²-phen)(μ-pdt^R)] (R=CH(CF₃)₂, iPr) showed that the relative stability between both two-electron-reduced isomers depends on the nature of the bridge and the possibility to establish a remote anagostic interaction between the iron center {Fe(CO)₃} and the group carried by the bridged-head atom of the dithiolate group.     

The spin-coupling model of zero-field splitting for trimeric [3Fe–4S] and mixed-metal [3FeZn–4S] clusters of ferredoxins from Pyrococcus furiosus

The spin-coupling model of zero-field splitting (ZFS) is developed for trimeric [3Fe–4S] clusters. The correlations between the cluster ZFS parameters D_S and E_S of the states with total spin S and ZFS parameters D_i and E_i of individual ions were obtained for mixed-valent (MV) [3Fe–4S]⁰ clusters with high-spin ground state S_gr=2, for the MV iron core of the hetero-metal [3FeZn–4S]⁺ cluster with S_gr=5/2 and for the monovalent [3Fe–4S]⁺ cluster with S_gr=5/2 of Pyrococcus furiosus ferredoxin (Pf Fd). These correlations and the cluster ZFS parameters D_S and E_S depend on total spin S, intermediate spin S₁₂ and individual spins s_i. The spin-coupling model explains the experimentally observed negative cluster ZFS parameters of MV trimers in the [3Fe–4S]⁰ and [3FeZn–4S]⁺ clusters and the positive cluster ZFS parameter of the tetrameric MV [4Fe–4S]⁺ cluster and the monovalent [3Fe–4S]⁺ trimer of Pf Fd. Single-particle ZFS parameters D_i were obtained for the [3Fe–4S] trimers and [4Fe–4S]⁺ tetramer (S_gr=3/2) of Pf Fd. In distorted trimers, the cluster ZFS parameter D_S of anisotropy changes the value and sign under the variation of isotropic Heisenberg exchange or/and double exchange coupling due to the exchange admixture of the excited states. Experimentally observed peculiarities of effective hyperfine constants A_i for the MV trimer with S_gr=5/2 of the hetero-metal [3FeZn–4S]⁺ cluster were described in the spin-coupling exchange-resonance model with the exchange admixture of the excited states and non-equivalence of the states of different localization.     

Charged Si9 Clusters in Neat Solids and the Detection of [H2Si9]2− in Solution: A Combined NMR,Raman, Mass Spectrometric,and Quantum Chemical Investigation

Polyanionic silicon clusters are provided by the Zintl phases K₄Si₄, comprising [Si₄]⁴⁻ units, and K₁₂Si₁₇, consisting of [Si₄]⁴⁻ and [Si₉]⁴⁻ clusters. A combination of solid‐state MAS‐NMR, solution NMR, and Raman spectroscopy, electrospray ionization mass spectrometry, and quantum‐chemical investigations was used to investigate four‐ and nine‐atomic silicon Zintl clusters in neat solids and solution. The results were compared to ²⁹Si isotope‐enriched samples. ²⁹Si‐MAS NMR and Raman shifts of the phase‐pure solids K₄Si₄ and K₁₂Si₁₇ were interpreted by quantum‐chemical calculations. Extraction of [Si₉]⁴⁻ clusters from K₁₂Si₁₇ with liquid ammonia/222crypt and their transfer to pyridine yields in a red solid containing Si₉ clusters. This compound was characterized by elemental and EDX analyses and ²⁹Si‐MAS NMR and Raman spectroscopy. Charged Si₉ clusters were detected by ²⁹Si NMR in solution. ²⁹Si and ¹H NMR spectra reveal the presence of the [H₂Si₉]²⁻ cluster anion in solution.     

Protonation and Proton‐Coupled Electron Transfer at S‐Ligated [4Fe‐4S] Clusters

Biological [Fe‐S] clusters are increasingly recognized to undergo proton‐coupled electron transfer (PCET), but the site of protonation, mechanism, and role for PCET remains largely unknown. Here we explore this reactivity with synthetic model clusters. Protonation of the arylthiolate‐ligated [4Fe‐4S] cluster [Fe₄S₄(SAr)₄]^2? ( 1 , SAr=S‐2,4‐6‐(iPr)₃C₆H₂) leads to thiol dissociation, reversibly forming [Fe₄S₄(SAr)₃L]^1? ( 2 ) and ArSH (L=solvent, and/or conjugate base). Solutions of 2 +ArSH react with the nitroxyl radical TEMPO to give [Fe₄S₄(SAr)₄]^1? ( 1_ox ) and TEMPOH. This reaction involves PCET coupled to thiolate association and may proceed via the unobserved protonated cluster [Fe₄S₄(SAr)₃(HSAr)]^1? ( 1‐H ). Similar reactions with this and related clusters proceed comparably. An understanding of the PCET thermochemistry of this cluster system has been developed, encompassing three different redox levels and two protonation states.     

Nitrene Insertion into C?C and C?H Bonds of Diamide Diimine Ligands Ligated to Chromium and Iron

The impact of redox non‐innocence (RNI) on chemical reactivity is a forefront theme in coordination chemistry. A diamide diimine ligand, [{‐CHN(1,2‐C₆H₄)NH(2,6‐iPr₂C₆H₃)}₂]ⁿ (n=0 to −4), (dadi)ⁿ, chelates Cr and Fe to give [(dadi)M] ([ 1 Cr(thf)] and [ 1 Fe]). Calculations show [ 1 Cr(thf)] (and [ 1 Cr]) to have a d⁴ Cr configuration antiferromagnetically coupled to (dadi)²⁻*, and [ 1 Fe] to be S=2. Treatment with RN₃ provides products where RN is formally inserted into the C C bond of the diimine or into a C H bond of the diimine. Calculations on the process support a mechanism in which a transient imide (imidyl) aziridinates the diimine, which subsequently ring opens.     

Olefin Cycloadditions of the Electrophilic Phosphinidene Complex [iPr2N−P=Fe(CO)4]

Highly strained methylenephosphiranes are formed in the reaction of the new electrophilic phosphinidene complex [iPr₂N−P=Fe(CO)₄] with allenes. Remarkably, reaction with diallenes at 0°C also leads to a phosphirane, which rearranges upon warming to room temperature to a bis-isopropylidenephospholene (see scheme).     

Small Gold(I) and Gold(I)–Silver(I) Clusters by C−Si Auration

Auration of o-trimethylsilyl arylphosphines leads to the formation of gold and gold–silver clusters with ortho-metalated phosphines displaying 3c–2e Au−C−M bonds (M=Au/Ag). Hexagold clusters [Au₆L₄](X)₂ are obtained by reaction of (L−TMS)AuCl with AgX, whereas reaction with AgX and Ag₂O leads to gold–silver clusters [Au₄Ag₂L₄](X)₂. Oxo-trigold(I) species [Au₃O]⁺ were identified as the intermediates in the formation of the silver-doped clusters. Other [Au₅], [Au₄Ag], and [Au₁₂Ag₄] clusters were also obtained. Clusters containing PAu−Au−AuP structural motif display good catalytic activity in the activation of alkynes under homogeneous conditions.     

[(TeMe2)Mn(CO)4(μ5-Te)(μ4-Te)Mn4(CO)12]−: A Pentacoordinate Bridging Tellurido Ligand in a Square-Pyramidal Geometry

The first Te–Mn–CO clusters were obtained by the thermal reaction of K₂TeO₃ with [Mn₂(CO)₁₀] in MeOH. The basicity of the μ₄-Te ligand in the octahedral cluster anion [(μ₄-Te)₂Mn₄(CO)₁₂]²⁻ is demonstrated by its binding to the fragment [(TeMe₂)Mn(CO)₄]⁺ in an axial fashion to afford the novel cluster 1 .     

Construction of Co4 Atomic Clusters to Enable Fe−N4 Motifs with Highly Active and Durable Oxygen Reduction Performance

Fe−N−C catalysts with single-atom Fe−N₄ configurations are highly needed owing to the high activity for oxygen reduction reaction (ORR). However, the limited intrinsic activity and dissatisfactory durability have significantly restrained the practical application of proton-exchange membrane fuel cells (PEMFCs). Here, we demonstrate that constructing adjacent metal atomic clusters (ACs) is effective in boosting the ORR performance and stability of Fe−N₄ catalysts. The integration of Fe−N₄ configurations with highly uniform Co₄ ACs on the N-doped carbon substrate (Co₄@/Fe₁@NC) is realized through a “pre-constrained” strategy using Co₄ molecular clusters and Fe(acac)₃ implanted carbon precursors. The as-developed Co₄@/Fe₁@NC catalyst exhibits excellent ORR activity with a half-wave potential (E_1/2) of 0.835 V vs. RHE in acidic media and a high peak power density of 840 mW cm⁻² in a H₂−O₂ fuel cell test. First-principles calculations further clarify the ORR catalytic mechanism on the identified Fe−N₄ that modified with Co₄ ACs. This work provides a viable strategy for precisely establishing atomically dispersed polymetallic centers catalysts for efficient energy-related catalysis.     

Synthesis and Complexation Study of New Aminoalkynyl−amidinate Ligands

The current library of amidinate ligands has been extended by the synthesis of two novel dimethylamino-substituted alkynylamidinate anions of the composition [Me₂N−CH₂−C≡C−C(NR)₂]⁻ (R = ⁱPr, cyclohexyl (Cy)). The unsolvated lithium derivatives Li[Me₂N−CH₂−C≡C−C(NR)₂] ( 1 : R = ⁱPr, 2 : R = Cy) were obtained in good yields by treatment of in situ-prepared Me₂N−CH₂−C≡C−Li with the respective carbodiimides, R−N=C=N−R. Recrystallization of 1 and 2 from THF afforded the crystalline THF adducts Li[Me₂N−CH₂−C≡C−C(NR)₂] ⋅ nTHF ( 1 a : R = ⁱPr, n=1; 2 a : R = Cy, n=1.5). Precursor 2 was subsequently used to study initial complexation reactions with selected di- and trivalent transition metals. The dark red homoleptic vanadium(III) tris(alkynylamidinate) complex V[Me₂N−CH₂−C≡C−C(NCy)₂]₃ ( 3 ) was prepared by reaction of VCl₃(THF)₃ with 3 equiv. of 2 (75 % yield). A salt-metathesis reaction of 2 with anhydrous FeCl₂ in a molar ratio of 2 : 1 afforded the dinuclear homoleptic iron(II) alkynylamidinate complex Fe₂[Me₂N−CH₂−C≡C−C(NCy)₂]₄ ( 4 ) in 69 % isolated yield. Similarly, treatment of Mo₂(OAc)₄ with 3 or 4 equiv. of 2 provided the dinuclear, heteroleptic molybdenum(II) amidinate complex Mo₂(OAc)[Me₂N−CH₂−C≡C−C(NCy)₂]₃ ( 5 ; yellow crystals, 50 % isolated yield). The cyclohexyl-substituted title compounds 2 a , 4 , and 5 were structurally characterized through single-crystal X-ray diffraction studies.     

[Ni(NHC)2] as a Scaffold for Structurally Characterized trans [H−Ni−PR2] and trans [R2P−Ni−PR2] Complexes

The addition of PPh₂H, PPhMeH, PPhH₂, P(para-Tol)H₂, PMesH₂ and PH₃ to the two-coordinate Ni⁰ N-heterocyclic carbene species [Ni(NHC)₂] (NHC=IiPr₂, IMe₄, IEt₂Me₂) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H−Ni−PR₂] or novel trans [R₂P−Ni−PR₂] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe₄>IEt₂Me₂>IiPr₂) and phosphines are employed. P−P activation of the diphosphines R₂P−PR₂ (R₂=Ph₂, PhMe) provides an alternative route to some of the [Ni(NHC)₂(PR₂)₂] complexes. DFT calculations capture these trends with P−H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni−P bond. P−P bond activation from [Ni(NHC)₂(Ph₂P−PPh₂)] adducts proceeds with computed barriers below 10 kcal mol⁻¹. The ability of the [Ni(NHC)₂] moiety to afford isolable terminal phosphido products reflects the stability of the Ni−NHC bond that prevents ligand dissociation and onward reaction.     

Benzyl Borane NHC Adducts: Beyond B−C Bond Scission

Benzyl-substituted boronates and borates are widely employed as mild sources in radical or anionic transfer reactions of benzyl entities. In this process the B−C bond to the benzyl moiety is essentially ruptured. In contrast, reactions with retention of the B−C bond are poorly investigated although several other reactive sites in benzyl–boron systems are clearly inherent. In this respect, the novel reactivity of the representative borane adduct IiPr−BH₂Bn [IiPr=:C{N(iPr)CH}₂, Bn=CH₂C₆H₅] is demonstrated. Dihalogenation of the BH₂ entity is observed with BCl₃ and BBr₃, whereas BI₃ either affords IiPr−BHI₂ or proceeds with borylation of the aromatic phenyl ring to give a hydride-bridged bisborylated species. The photochemical mono- and dihalogenation of the benzylic CH₂ group was demonstrated with elemental bromine Br₂. The brominated product IiPr−BBr₂−CHBr−C₆H₅ was borylated at the benzylic carbon atom in an umpolung event with BI₃ to afford the zwitterion IiPr−BI−CH(BI₃)−C₆H₅.     

Instantaneous Free Radical Scavenging by CeO2 Nanoparticles Adjacent to the Fe−N4 Active Sites for Durable Fuel Cells

To achieve the Fe−N−C materials with both high activity and durability in proton exchange membrane fuel cells, the attack of free radicals on Fe−N₄ sites must be overcome. Herein, we report a strategy to effectively eliminate radicals at the source to mitigate the degradation by anchoring CeO₂ nanoparticles as radicals scavengers adjacent (Sca^ad-CeO2) to the Fe−N₄ sites. Radicals such as ⋅OH and HO₂⋅ that form at Fe−N₄ sites can be instantaneously eliminated by adjacent CeO₂, which shortens the survival time of radicals and the regional space of their damage. As a result, the CeO₂ scavengers in Fe−NC/Sca^ad-CeO2 achieved ∼80 % elimination of the radicals generated at the Fe−N₄ sites. A fuel cell prepared with the Fe−NC/Sca^ad-CeO2 showed a smaller peak power density decay after 30,000 cycles determined with US DOE PGM-relevant AST, increasing the decay of Fe−NC_Phen from 69 % to 28 % decay.     

K6[Fe8.27(HPO3)12]: a new mixed‐valence iron phosphite

The title compound, hexapotassium octairon(II,III) dodecaphosphonate, exhibiting a two‐dimensional structure, is a new mixed alkali/3d metal phosphite. It crystallizes in the space group Rm, with two crystallographically independent Fe atoms occupying sites of m (Fe1) and 3m (Fe2) symmetry. The Fe2 site is fully occupied, whereas the Fe1 site presents an occupancy factor of 0.757 (3). The three independent O atoms, one of which is disordered, are situated on a mirror and all other atoms are located on special positions with 3m symmetry. Layers of formula [Fe₃(HPO₃)₄]²⁻ are observed in the structure, formed by linear Fe₃O₁₂ trimer units, which contain face‐sharing FeO₆ octahedra interconnected by (HPO₃)²⁻ phosphite oxoanions. The partial occupancy of the Fe1 site might be described by the formation of two [Fe(HPO₃)₂]⁻ layers derived from the [Fe₃(HPO₃)₄]²⁻ layer when the Fe1 atom is absent. Fe²⁺ is localized at the Fe1 and Fe2 sites of the [Fe₃(HPO₃)₄]²⁻ sheets, whereas Fe³⁺ is found at the Fe2 sites of the [Fe(HPO₃)₂]⁻ sheets, according to bond‐valence calculations. The K⁺ cations are located in the interlayer spaces, between the [Fe₃(HPO₃)₄]²⁻ layers, and between the [Fe₃(HPO₃)₄]²⁻ and [Fe(HPO₃)₂]⁻ layers.     

Synthesis and Structural Characterization of Divalent Transition Metal Alkynylamidinate Complexes

A series of new alkynylamidinate complexes of selected first and second row transition metals has been synthesized and fully characterized. Treatment of MCl₂ precursors (M=Mn, Fe, Co) with 2 equiv. of the lithium alkynylamidinates Li[c-C₃H₅−C≡C−C(NR′)₂] ⋅ THF (R′=ⁱPr (2), Cy (cyclohexyl) ( 2 )) afforded a series of binuclear complexes of the type M₂[c-C₃H₅−C≡C−C(NR)₂-κN:κN′]₂[c-C₃H₅−C≡C−C(NR)₂-κ²N,N′]₂ ( 3 : M=Mn, R=Cy; 4 a : M=Fe, R=ⁱPr; 4 b : M=Fe, R=Cy; 5 : M=Co, R=ⁱPr) with no significant metal-metal bonding. In marked contrast, a similar reaction of CrCl₂ with 2 equiv. of 1 afforded the homoleptic dinuclear chromium(II) complex Cr₂[c-C₃H₅−C≡C−C(NⁱPr)₂-κN:κN′]₄ ( 6 ) which supposedly comprises a Cr−Cr quadruple bond. Complex 6 could also be prepared in a more rational way and in better yield (61 %) by using dichromium(II) tetraacetate, Cr₂(OAc)₄, as starting material. Related reactions employing dimolybdenum(II) tetraacetate, Mo₂(OAc)₄, and 2 or 3 equiv. of 1 afforded the mixed-ligand paddle wheel-type complexes trans-Mo₂(OAc-κO:κO′)₂([c-C₃H₅−C≡C−C(NⁱPr)₂-κN:κN′]₂ ( 7 ) and Mo₂(OAc-κO:κO′)([c-C₃H₅−C≡C−C(NⁱPr)₂-κN:κN′]₃ ( 8 ). All title compounds were structurally characterized through single-crystal X-ray diffraction and spectroscopic techniques (NMR, IR, Raman).     

Potassium Ion Conductivity in the Cubic Labyrinth of a Piezoelectric,Antiferromagnetic Oxoferrate(III) Tellurate(VI)

Orange-colored crystals of the oxoferrate tellurate K_12+6xFe₆Te_4−xO₂₇ [x=0.222(4)] were synthesized in a potassium hydroxide hydroflux with a molar water–base ratio n(H₂O)/n(KOH) of 1.5 starting from Fe(NO₃)₃ ⋅ 9H₂O, TeO₂ and H₂O₂ at about 200 °C. By using (NH₄)₂TeO₄ instead of TeO₂, a fine powder consisting of microcrystalline spheres of K_12+6xFe₆Te_4−xO₂₇ was obtained. K_12+6xFe₆Te_4−xO₂₇ crystallizes in the acentric cubic space group I 3d. [Fe^IIIO₅] pyramids share their apical atoms in [Fe₂O₉] groups and two of their edges with [Te^VIO₆] octahedra to form an open framework that consists of two loosely connected, but not interpenetrating, chiral networks. The flexibility of the hinged oxometalate network manifests in a piezoelectric response similar to that of LiNbO₃.The potassium cations are mobile in channels that run along the <111> directions and cross in cavities acting as nodes. The ion conductivity of cold-pressed pellets of ball-milled K_12+6xFe₆Te_4−xO₂₇ is 2.3×10⁻⁴ S ⋅ cm⁻¹ at room temperature. Magnetization measurements and neutron diffraction indicate antiferromagnetic coupling in the [Fe₂O₉] groups.     

(Hydroxyalkyl)cob(III)alamins as Competitive Inhibitors in Coenzyme B12-Dependent Enzymic Reactions: 1H-NMR Structure Analysis and Kinetic Studies with Glycerol Dehydratase and Diol Dehydratase

A series of (hydroxyalkyl)cobalamins, i.e., 1a – d , (HO−(CH₂)_n−Cbl, n=2 – 5), two diastereoisomeric (2,3-dihydroxypropyl)cobalamins, i.e., 2a , b ([(R)- and (S)-[(HO)₂pr]-Cbl) and their diastereoisomeric `base-off' analogues, the (Coβ-2,3-dihydroxypropyl-[1′-O-(p-tolyl)cobamides]) 3a , b ([(R)- and (S)-(HO)<sub>2</sub>pr]-PTC) were prepared and characterized by their 500-MHz <sup>1</sup>H-NMR spectra. The inhibitory activities of these compounds and of hydroxocobalamin (HO−Cbl) and (Coα-cyano)(Coβ-hydroxo)[1′-O-(p-tolyl)cobamide] (HO−PTC) were tested with two coenzyme-B<sub>12</sub>-dependent enzymes: glycerol dehydratase (GDH) and propane-1,2-diol dehydratase (DDH) (Table 4). The hydroxyalkyl and dihydroxypropyl derivatives of cobalamin acted as strong competitive inhibitors of coenzyme B<sub>12</sub> (5′-deoxy-5′-adenosylcobalamin, Ado−Cbl) for both enzymes, with K<sub>i</sub> values falling within the range defined by HO−Cbl (best inhibitor) and CN−Cbl (K<sub>i</sub> /K<sub>m</sub> ratio of ca. 2). The short-chain HO−(CH<sub>2</sub>)<sub>n</sub>−Cbl ( 1a , b ; n=2 or 3) exhibited K<sub>i</sub> equal to the K<sub>m</sub> for Ado−Cbl. The [(R)- and (S)-(HO)<sub>2</sub>pr]−Cbl ( 2a , b ) and the long-chain HO−(CH<sub>2</sub>)<sub>n</sub>−Cbl ( 1c , d ; n=4, 5) were less efficient inhibitors, with [(S)-(HO)<sub>2</sub>pr]−Cbl ( 2a ) performing slightly better than the (R)-diastereoisomer 2b for both enzymes. The `base-off' analogues, Ado−PTC and [(R)- and (S)-(HO)₂pr]−PTC ( 3a , b ), were moderate inhibitors with K_i/K_m ratios of 4.5 – 28 for GDH or 7 – 13 for DDH. [(S)-(HO)₂pr]−PTC ( 3a ) was the best inhibitor in this group. The non-alkylated analogue (HO,CN)−PTC proved to be a very poor inhibitor. These results confirm that the `base-on' binding mode of coenzyme B₁₂ is preferred for GDH and DDH. The increase in K_i for PTC- vs. Cbl-type inhibitors may result from the entropic penalty required for folding of the PTC nucleotide chain into a Cbl-like loop conformation. Hydrophilic interactions between the β-ligand of the inhibitor and ribosyl- or substrate-binding sites may make an important contribution to the formation or stabilization of the apoenzyme-inhibitor complex, especially for the PTC derivative.     

Ti2Nb6Cl14O4: A Unique 2D–1D Network Combination in Niobium Cluster Chemistry

Layers of niobium clusters that are linked to each other through zigzag chains of edge-sharing [TiCl₄O₂] ocatahedra are the central structural features of the title compound. Bridging of the chains by [(Nb₆Cl₈ⁱO₄ⁱ)Cl₆^a]⁶⁻ clusters results in the formation of empty tunnels (a section of the structure is shown on the right).     

On the Reactivity of Silylated Ge9 Clusters: Synthesis and Characterization of [ZnCp*(Ge9{Si(SiMe3)3}3)], [CuPiPr3(Ge9{Si(SiMe3)3}3)], and [(CuPiPr3)4{Ge9(SiPh3)2}2]

We report on the synthesis of new derivatives of silylated clusters of the type [Ge₉(SiR₃)₃]^? (R = SiMe₃, Me = CH₃; R = Ph, Ph = C₆H₅) as well as on their reactivity towards copper and zinc compounds. The silylated cluster compounds were synthesized by heterogeneous reactions starting from the Zintl phase K₄Ge₉. Reaction of K[Ge₉{Si(SiMe₃)₃}₃] with ZnCl₂ leads to the already known dimeric compound [Zn(Ge₉{Si(SiMe₃)₃}₃)₂] ( 1 ), whereas upon the reaction with [ZnCp*₂] the coordination of [ZnCp*]⁺ to the cluster takes place (Cp*=1,2,3,4,5‐pentamethylcyclopentadienyl) under the formation of [ZnCp*(Ge₉{Si(SiMe₃)₃}₃)] ( 2 ). A similar reaction leads to [CuPiPr₃(Ge₉{Si(SiMe₃)₃}₃)] ( 3 ) from [CuPiPr₃Cl] (iPr=isopropyl). Further we investigated the novel silylated cluster units [Ge₉(SiPh₃)₃]^? ( 4 ) and [Ge₉(SiPh₃)₂]^? ( 5 ), which could be identified by mass spectroscopy. Bis‐ and tris‐silylated species can be synthesized by the respective stoichiometric reactions, and the products were characterized by ESI‐MS and NMR experiments. These clusters show rather different reactivity. The reaction of the tris‐silylated anion 4 with [CuPiPr₃Cl] leads to [(CuPiPr₃)₃Ge₉(SiPh₃)₂]⁺ as shown from NMR experiments and to [(CuPiPr₃)₄{Ge₉(SiPh₃)₂}₂] ( 6 ), which was characterized by single‐crystal X‐ray diffraction. Compound 6 shows a new type of coordination of the Cu atoms to the silylated Zintl clusters.