Phoenix

A novel chiral coordination polymer, [Cu­(C₆H₅CH­(OH)­COO)­(μ-C₆H₅CH­(OH)­COO)] (<b>1</b>-L and <b>1</b>-D), was synthesized through a reaction of copper acetate with l-mandelic acid at room temperature. Although previously reported copper mandelate prepared by hydrothermal reaction was a centrosymmetric coordination polymer because of the racemization of mandelic acid, the current coordination polymer shows noncentrosymmetry and a completely different structure from that previously reported. The X-ray crystallography for <b>1</b>-L revealed that the copper center of the compound showed a highly distorted octahedral structure bridged by a chiral mandelate ligand in the unusual coordination mode to construct a one-dimensional (1D) zigzag chain structure. These 1D chains interdigitated each other to give a layered structure as a result of the formation of multiple aromatic interactions and hydrogen bonds between hydroxyl and carboxylate moieties at mandelate ligands. The coordination polymer <b>1</b>-L belongs to the noncentrosymmetric space group of C2 to show piezoelectric properties and second harmonic generation (SHG) activity

Model Studies of Methyl CoM Reductase: Methane Formation via CH₃–S Bond Cleavage of Ni(I) Tetraazacyclic Complexes Having Intramolecular Methyl Sulfide Pendants

The Ni­(I) tetraazacycles [Ni­(dmmtc)]⁺ and [Ni­(mtc)]⁺, which have methylthioethyl pendants, were synthesized as models of the reduced state of the active site of methyl coenzyme M reductase (MCR), and their structures and redox properties were elucidated (dmmtc, 1,8-dimethyl-4,11-bis­{(2-methylthio)­ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane; mtc, 1,8-{bis­(2-methylthio)­ethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane). The intramolecular CH₃–S bond of the thioether pendant of [Ni^I(dmmtc)]­(OTf) was cleaved in THF at 75 °C in the presence of the bulky thiol DmpSH, which acts as a proton source, and methane was formed in 31% yield and a Ni­(II) thiolate complex was concomitantly obtained (Dmp = 2,6-dimesityphenyl). The CH₃–S bond cleavage of [Ni^I(mtc)]⁺ also proceeded similarly, but under milder conditions probably due to the lower potential of the [Ni^I(mtc)]⁺ complex. These results indicate that the robust CH₃–S bond can be homolytically cleaved by the Ni­(I) center when they are properly arranged, which highlights the significance of the F430 Ni environment in the active site of the MCR protein

Coordination of Methyl Coenzyme M and Coenzyme M at Divalent and Trivalent Nickel Cyclams: Model Studies of Methyl Coenzyme M Reductase Active Site

Divalent and trivalent nickel complexes of 1,4,8,11-tetraazacyclotetradecane, denoted as cyclam hereafter, coordinated by methyl coenzyme M (MeSCoM^–) and coenzyme M (HSCoM^–) have been synthesized in the course our model studies of methyl coenzyme M reductase (MCR). The divalent nickel complexes Ni­(cyclam)­(RSCoM)₂ (R = Me, H) have two trans-disposed RSCoM^– ligands at the nickel­(II) center as sulfonates, and thus, the nickels have an octahedral coordination. The SCoM^2– adduct Ni­(cyclam)­(SCoM) was also synthesized, in which the SCoM^2– ligand chelates the nickel via the thiolate sulfur and a sulfonate oxygen. The trivalent MeSCoM adduct [Ni­(cyclam)­(MeSCoM)₂]­(OTf) was synthesized by treatment of [Ni­(cyclam)­(NCCH₃)₂]­(OTf)₃ with (^<i>n</i>Bu₄N)­[MeSCoM]. A similar reaction with (^<i>n</i>Bu₄N)­[HSCoM] did not afford the corresponding trivalent HSCoM^– adduct, but rather the divalent nickel complex polymer [−Ni^II(cyclam)­(CoMSSCoM)−]_<i>n</i> was obtained, in which the terminal thiol of HSCoM^– was oxidized to the disulfide (CoMSSCoM)^2– by the Ni­(III) center

Heterolytic Activation of Dihydrogen Molecule by Hydroxo-/Sulfido-Bridged Ruthenium–Germanium Dinuclear Complex. Theoretical Insights

Heterolytic activation of dihydrogen molecule (H₂) by hydroxo-/sulfido-bridged ruthenium–germanium dinuclear complex [Dmp­(Dep)­Ge­(μ-S)­(μ-OH)­Ru­(PPh₃)]⁺ (<b>1</b>) (Dmp = 2,6-dimesitylphenyl, Dep = 2,6-diethylphenyl) is theoretically investigated with the ONIOM­(DFT:MM) method. H₂ approaches <b>1</b> to afford an intermediate [Dmp­(Dep)­(HO)­Ge­(μ-S)­Ru­(PPh₃)]⁺-(H₂) (<b>2</b>). In <b>2</b>, the Ru–OH coordinate bond is broken but H₂ does not yet coordinate with the Ru center. Then, the H₂ further approaches the Ru center through a transition state <b>TS</b>_{<b>2</b>–<b>3</b>} to afford a dihydrogen σ-complex [Dmp­(Dep)­(HO)­Ge­(μ-S)­Ru­(η²-H₂)­(PPh₃)]⁺ (<b>3</b>). Starting from <b>3</b>, the H–H σ-bond is cleaved by the Ru and Ge–OH moieties to form [Dmp­(Dep)­(H₂O)­Ge­(μ-S)­Ru­(H)­(PPh₃)]⁺ (<b>4</b>). In <b>4</b>, hydride and H₂O coordinate with the Ru and Ge centers, respectively. Electron population changes clearly indicate that this H–H σ-bond cleavage occurs in a heterolytic manner like H₂ activation by hydrogenase. Finally, the H₂O dissociates from the Ge center to afford [Dmp­(Dep)­Ge­(μ-S)­Ru­(H)­(PPh₃)]⁺ (<b>PRD</b>). This step is rate-determining. The activation energy of the backward reaction is moderately smaller than that of the forward reaction, which is consistent with the experimental result that <b>PRD</b> reacts with H₂O to form <b>1</b> and H₂. In the Si analogue [Dmp­(Dep)­Si­(μ-S)­(μ-OH)­Ru­(PPh₃)]⁺ (<b>1</b>_<b>Si</b>), the isomerization of <b>1</b>_<b>Si</b> to <b>2</b>_<b>Si</b> easily occurs with a small activation energy, while the dissociation of H₂O from the Si center needs a considerably large activation energy. Based on these computational findings, it is emphasized that the reaction of <b>1</b> resembles well that of hydrogenase and the use of Ge in <b>1</b> is crucial for this heterolytic H–H σ-bond activation

[3:1] Site-Differentiated [4Fe–4S] Clusters Having One Carboxylate and Three Thiolates

[4Fe–4S] clusters modeled after those in organisms having three cysteine thiolates and one carboxylate were synthesized by using the tridentate thiolato chelate. X-ray structural analysis reveals that the carboxylates coordinate to the unique irons in an η¹ manner rather than η². Redox potentials show a positive shift from that of the cluster having ethanethiolate and the tridentate thiolato chelate. These properties conform to the arrangement of the [4Fe–4S] clusters in the electron transfer systems included in <i>Rc</i> dark operative protochlorophyllide oxidoreductase (DPOR) and formaldehyde oxidoreductase (FOR) with <i>Pf</i> ferredoxin