5 research outputs found
Phoenix
A novel chiral coordination polymer, [CuÂ(C<sub>6</sub>H<sub>5</sub>CHÂ(OH)ÂCOO)Â(μ-C<sub>6</sub>H<sub>5</sub>CHÂ(OH)ÂCOO)]
(<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 CH<sub>3</sub>–S Bond Cleavage of Ni(I) Tetraazacyclic Complexes Having Intramolecular Methyl Sulfide Pendants
The NiÂ(I) tetraazacycles [NiÂ(dmmtc)]<sup>+</sup> and
[NiÂ(mtc)]<sup>+</sup>, 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-bisÂ{(2-methylthio)Âethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane;
mtc, 1,8-{bisÂ(2-methylthio)Âethyl}-1,4,8,11-tetraaza-1,4,8,11-cyclotetradecane).
The intramolecular CH<sub>3</sub>–S bond of the thioether pendant
of [Ni<sup>I</sup>(dmmtc)]Â(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 NiÂ(II) thiolate complex
was concomitantly obtained (Dmp = 2,6-dimesityphenyl). The CH<sub>3</sub>–S bond cleavage of [Ni<sup>I</sup>(mtc)]<sup>+</sup> also proceeded similarly, but under milder conditions probably due
to the lower potential of the [Ni<sup>I</sup>(mtc)]<sup>+</sup> complex.
These results indicate that the robust CH<sub>3</sub>–S bond
can be homolytically cleaved by the NiÂ(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<sup>–</sup>) and coenzyme M (HSCoM<sup>–</sup>) have been
synthesized in the course our model studies of methyl coenzyme M reductase
(MCR). The divalent nickel complexes NiÂ(cyclam)Â(RSCoM)<sub>2</sub> (R = Me, H) have two trans-disposed RSCoM<sup>–</sup> ligands
at the nickelÂ(II) center as sulfonates, and thus, the nickels have
an octahedral coordination. The SCoM<sup>2–</sup> adduct NiÂ(cyclam)Â(SCoM)
was also synthesized, in which the SCoM<sup>2–</sup> ligand
chelates the nickel via the thiolate sulfur and a sulfonate oxygen.
The trivalent MeSCoM adduct [NiÂ(cyclam)Â(MeSCoM)<sub>2</sub>]Â(OTf)
was synthesized by treatment of [NiÂ(cyclam)Â(NCCH<sub>3</sub>)<sub>2</sub>]Â(OTf)<sub>3</sub> with (<sup><i>n</i></sup>Bu<sub>4</sub>N)Â[MeSCoM]. A similar reaction with (<sup><i>n</i></sup>Bu<sub>4</sub>N)Â[HSCoM] did not afford the corresponding trivalent
HSCoM<sup>–</sup> adduct, but rather the divalent nickel complex
polymer [−Ni<sup>II</sup>(cyclam)Â(CoMSSCoM)−]<sub><i>n</i></sub> was obtained, in which the terminal thiol of HSCoM<sup>–</sup> was oxidized to the disulfide (CoMSSCoM)<sup>2–</sup> by the NiÂ(III) center
Heterolytic Activation of Dihydrogen Molecule by Hydroxo-/Sulfido-Bridged Ruthenium–Germanium Dinuclear Complex. Theoretical Insights
Heterolytic activation of dihydrogen
molecule (H<sub>2</sub>) by
hydroxo-/sulfido-bridged ruthenium–germanium dinuclear complex
[DmpÂ(Dep)ÂGeÂ(μ-S)Â(μ-OH)ÂRuÂ(PPh<sub>3</sub>)]<sup>+</sup> (<b>1</b>) (Dmp = 2,6-dimesitylphenyl, Dep = 2,6-diethylphenyl)
is theoretically investigated with the ONIOMÂ(DFT:MM) method. H<sub>2</sub> approaches <b>1</b> to afford an intermediate [DmpÂ(Dep)Â(HO)ÂGeÂ(μ-S)ÂRuÂ(PPh<sub>3</sub>)]<sup>+</sup>-(H<sub>2</sub>) (<b>2</b>). In <b>2</b>, the Ru–OH coordinate bond is broken but H<sub>2</sub> does not yet coordinate with the Ru center. Then, the H<sub>2</sub> further approaches the Ru center through a transition state <b>TS</b><sub><b>2</b>–<b>3</b></sub> to afford
a dihydrogen σ-complex [DmpÂ(Dep)Â(HO)ÂGeÂ(μ-S)ÂRuÂ(η<sup>2</sup>-H<sub>2</sub>)Â(PPh<sub>3</sub>)]<sup>+</sup> (<b>3</b>). Starting from <b>3</b>, the H–H σ-bond is cleaved
by the Ru and Ge–OH moieties to form [DmpÂ(Dep)Â(H<sub>2</sub>O)ÂGeÂ(μ-S)ÂRuÂ(H)Â(PPh<sub>3</sub>)]<sup>+</sup> (<b>4</b>). In <b>4</b>, hydride and H<sub>2</sub>O 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 H<sub>2</sub> activation by hydrogenase. Finally, the
H<sub>2</sub>O dissociates from the Ge center to afford [DmpÂ(Dep)ÂGeÂ(μ-S)ÂRuÂ(H)Â(PPh<sub>3</sub>)]<sup>+</sup> (<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 H<sub>2</sub>O to form <b>1</b> and H<sub>2</sub>. In the Si analogue [DmpÂ(Dep)ÂSiÂ(μ-S)Â(μ-OH)ÂRuÂ(PPh<sub>3</sub>)]<sup>+</sup> (<b>1</b><sub><b>Si</b></sub>),
the isomerization of <b>1</b><sub><b>Si</b></sub> to <b>2</b><sub><b>Si</b></sub> easily occurs with a small activation
energy, while the dissociation of H<sub>2</sub>O 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 η<sup>1</sup> manner rather than η<sup>2</sup>. 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