21 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
N‑Heterocyclic Carbene Complexes of Three- and Four-Coordinate Fe(I)
N-heterocyclic carbene complexes
of three- and four-coordinate
FeÂ(I), [FeÂ(L<sup>R</sup>)<sub>4</sub>]Â[PF<sub>6</sub>] (L<sup>R</sup> = 1,3-R<sub>2</sub>-4,5-dimethylimidazol-2-ylidene, R = Me (<b>2</b>), Et (<b>3</b>), <sup>i</sup>Pr (<b>4</b>))
and [FeÂ(L<sup>Mes</sup>)<sub>2</sub>(THF)]Â[PF<sub>6</sub>] (<b>5</b>) (L<sup>Mes</sup> = 1,3-bisÂ(2,4,6-trimethylphenyl)Âimidazol-2-ylidene),
were synthesized from successive reactions of [FeÂ(toluene)<sub>2</sub>]Â[PF<sub>6</sub>]<sub>2</sub> with 1 equiv of KC<sub>8</sub> and
L<sup>R</sup> (4 equiv for R = Me, Et, <sup>i</sup>Pr; 2 equiv for
R = Mes). The coordination geometry of the iron atom in these complexes
varies depending on the nature of the R group in L<sup>R</sup>: a
tetrahedral geometry was observed for <b>2</b>, a square-planar
one for <b>3</b> and <b>4</b>, and a three-coordinate
T-shaped one for <b>5</b>. In solution, <b>4</b> releases
L<sup>iPr</sup> ligand(s), while the L<sup>R</sup> ligands of the
other FeÂ(I) complexes remain firmly bound. Tetrahedral <b>2</b> and T-shaped <b>5</b> contain a high-spin (<i>S</i> = <sup>3</sup>/<sub>2</sub>) FeÂ(I) center, whereas square-planar <b>3</b> and <b>4</b> contain FeÂ(I) in the low-spin state (<i>S</i> = <sup>1</sup>/<sub>2</sub>)
N‑Heterocyclic Carbene Complexes of Three- and Four-Coordinate Fe(I)
N-heterocyclic carbene complexes
of three- and four-coordinate
FeÂ(I), [FeÂ(L<sup>R</sup>)<sub>4</sub>]Â[PF<sub>6</sub>] (L<sup>R</sup> = 1,3-R<sub>2</sub>-4,5-dimethylimidazol-2-ylidene, R = Me (<b>2</b>), Et (<b>3</b>), <sup>i</sup>Pr (<b>4</b>))
and [FeÂ(L<sup>Mes</sup>)<sub>2</sub>(THF)]Â[PF<sub>6</sub>] (<b>5</b>) (L<sup>Mes</sup> = 1,3-bisÂ(2,4,6-trimethylphenyl)Âimidazol-2-ylidene),
were synthesized from successive reactions of [FeÂ(toluene)<sub>2</sub>]Â[PF<sub>6</sub>]<sub>2</sub> with 1 equiv of KC<sub>8</sub> and
L<sup>R</sup> (4 equiv for R = Me, Et, <sup>i</sup>Pr; 2 equiv for
R = Mes). The coordination geometry of the iron atom in these complexes
varies depending on the nature of the R group in L<sup>R</sup>: a
tetrahedral geometry was observed for <b>2</b>, a square-planar
one for <b>3</b> and <b>4</b>, and a three-coordinate
T-shaped one for <b>5</b>. In solution, <b>4</b> releases
L<sup>iPr</sup> ligand(s), while the L<sup>R</sup> ligands of the
other FeÂ(I) complexes remain firmly bound. Tetrahedral <b>2</b> and T-shaped <b>5</b> contain a high-spin (<i>S</i> = <sup>3</sup>/<sub>2</sub>) FeÂ(I) center, whereas square-planar <b>3</b> and <b>4</b> contain FeÂ(I) in the low-spin state (<i>S</i> = <sup>1</sup>/<sub>2</sub>)
Synthesis of V/Fe/S Clusters Using Vanadium(III) Thiolate Complexes Bearing a Phenoxide-Based Tridentate Ligand
VanadiumÂ(III) thiolate complexes
carrying a phenoxide-based tridentate ligand were prepared from the
reactions of VÂ(NMe<sub>2</sub>)<sub>4</sub> with the protonated forms
of tridentate ligands (H<sub>2</sub>Â(O,P,O) = bisÂ(3,5-di-<i>tert</i>-butyl-2-hydroxyÂphenyl)Âphenylphosphine or
H<sub>2</sub>(O,O,O) = bisÂ(3,5-di-<i>tert</i>-butyl-2-hydroxyÂphenyl)ÂphenylÂphosphineÂoxide)
and thiols (HSR; R = mesityl (Mes), 2,4,6-<i><sup>i</sup></i>Pr<sub>3</sub>C<sub>6</sub>H<sub>2</sub> (Tip)). The vanadium–thiolate
complexes were subjected to the V/Fe/S cluster synthesis via treatment
with an FeÂ(II) thiolate complex [(TipS)ÂFe]<sub>2</sub>Â(μ-SDmp)<sub>2</sub> (<b>4</b>, Dmp = 2,6-(mesityl)<sub>2</sub>ÂC<sub>6</sub>H<sub>3</sub>) and elemental sulfur in toluene, leading to
the formation of two new V/Fe/S clusters. One is an edge-bridged double-cubane-type
[VFe<sub>3</sub>S<sub>4</sub>]-[VFe<sub>3</sub>S<sub>4</sub>] cluster
[(O,P,O)ÂVFe<sub>3</sub>S<sub>4</sub>Â(SDmp)Â(HNMe<sub>2</sub>)]<sub>2</sub> (<b>5</b>) having face-capping tridentate
(O,P,O) ligands on vanadium atoms. The other is a [VFe<sub>3</sub>S<sub>4</sub>-Fe] cluster [(μ-O,O,O)ÂVFe<sub>3</sub>S<sub>4</sub>Â(SDmp)Â(STip)ÂFeÂ(μ-SDmp)] (<b>6</b>), the core of which consists of a cubane-type [VFe<sub>3</sub>S<sub>4</sub>] unit and an external iron atom. The external iron
is bound to an SDmp ligand and two oxygen atoms of the tridentate
(O,O,O) ligand. Cluster <b>6</b> is structurally relevant to
the active site of nickel-dependent CO dehydrogenase, and their common
structural features include a cubane-type unit with a heterometal,
one more iron atom besides the cubane unit, and a bridging ligand
between the external iron and the heterometal of the cubane unit
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
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
Interconversion between [Fe<sub>4</sub>S<sub>4</sub>] and [Fe<sub>2</sub>S<sub>2</sub>] Clusters Bearing Amide Ligands
Structural conversion
of [Fe<sub>4</sub>S<sub>4</sub>] clusters into [Fe<sub>2</sub>S<sub>2</sub>] clusters has been suggested to be a fundamental process
for some O<sub>2</sub>-sensing proteins. While the formation of [Fe<sub>2</sub>S<sub>2</sub>] clusters from synthetic [Fe<sub>4</sub>S<sub>4</sub>] clusters has been unprecedented, an all-ferric [Fe<sub>4</sub>S<sub>4</sub>]<sup>4+</sup> cluster Fe<sub>4</sub>S<sub>4</sub>{NÂ(SiMe<sub>3</sub>)<sub>2</sub>}<sub>4</sub> (<b>1</b>) was found to split
in the presence of pyridines, giving [Fe<sub>2</sub>S<sub>2</sub>]
clusters Fe<sub>2</sub>S<sub>2</sub>Â{NÂ(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>(L)<sub>2</sub> (<b>2</b>, L = pyridines).
The structural conversion between [Fe<sub>4</sub>S<sub>4</sub>] and
[Fe<sub>2</sub>S<sub>2</sub>] clusters appeared to be reversible,
and the thermodynamic parameters for the equilibrium reactions between <b>1</b> + L and <b>2</b> were determined. Assembly of two
[Fe<sub>2</sub>S<sub>2</sub>] clusters was also induced by chemical
reductions of Fe<sub>2</sub>S<sub>2</sub>Â{NÂ(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub>(Py)<sub>2</sub> (Py = pyridine), and the
resultant [Fe<sub>4</sub>S<sub>4</sub>] clusters [<b>1</b>]<sup>−</sup> and [<b>1</b>]<sup>2–</sup> were found
to split into two [Fe<sub>2</sub>S<sub>2</sub>] clusters by oxidation
with [Cp<sub>2</sub>Fe]<sup>+</sup> in the presence of pyridine
Oxido-Bridged Di-, Tri-, and Tetra-Nuclear Iron Complexes Bearing Bis(trimethylsilyl)amide and Thiolate Ligands
A series of di-, tri-, and tetra-nuclear iron-oxido clusters
with
bisÂ(trimethylsilyl)Âamide and thiolate ligands were synthesized from
the reactions of FeÂ{NÂ(SiMe<sub>3</sub>)<sub>2</sub>}<sub>2</sub> (<b>1</b>) with 1 equiv of thiol HSR (R = C<sub>6</sub>H<sub>5</sub> (Ph), 4-<sup>t</sup>BuC<sub>6</sub>H<sub>4</sub>, 2,6-Ph<sub>2</sub>C<sub>6</sub>H<sub>3</sub> (Dpp), 2,4,6-<sup>i</sup>Pr<sub>3</sub>C<sub>6</sub>H<sub>2</sub> (Tip)) and subsequent treatment with O<sub>2</sub>. The trinuclear clusters [{(Me<sub>3</sub>Si)<sub>2</sub>N}ÂFe]<sub>3</sub>(μ<sub>3</sub>-O)Â{μ-SÂ(4-RC<sub>6</sub>H<sub>4</sub>)}<sub>3</sub> (R = H (<b>3a</b>), <sup>t</sup>Bu (<b>3b</b>)) were obtained from the reactions of <b>1</b> with HSPh or HSÂ(4-<sup>t</sup>BuC<sub>6</sub>H<sub>4</sub>) and
O<sub>2</sub>, while we isolated a tetranuclear cluster [{(Me<sub>3</sub>Si)<sub>2</sub>N}<sub>2</sub>Fe<sub>2</sub>(μ-SDpp)]<sub>2</sub>(μ<sub>3</sub>-O)<sub>2</sub> (<b>4</b>) as crystals
from an analogous reaction with HSDpp. Treatment of a tertrahydrofuran
(THF) solution of <b>1</b> with HSTip and O<sub>2</sub> resulted
in the formation of a dinuclear complex [{(Me<sub>3</sub>Si)<sub>2</sub>N}Â(TipS)Â(THF)ÂFe]<sub>2</sub>(μ-O) (<b>5</b>). The molecular
structures of these complexes have been determined by X-ray crystallographic
analysis
Catalytic Generation of Borenium Ions by Cooperative B–H Bond Activation: The Elusive Direct Electrophilic Borylation of Nitrogen Heterocycles with Pinacolborane
The
B–H bond of typical boranes is heterolytically split
by the polar Ru–S bond of a tethered rutheniumÂ(II) thiolate
complex, affording a rutheniumÂ(II) hydride and borenium ions with
a dative interaction with the sulfur atom. These stable adducts were
spectroscopically characterized, and in one case, the B–H bond
activation step was crystallographically verified, a snapshot of the
σ-bond metathesis. The borenium ions derived from 9-borabicyclo[3.3.1]Ânonane
dimer [(9-BBN)<sub>2</sub>], pinacolborane (pinBH), and catecholborane
(catBH) allowed for electrophilic aromatic substitution of indoles.
The unprecedented electrophilic borylation with the pinB cation was
further elaborated for various nitrogen heterocycles
A Nitrogenase Cluster Model [Fe<sub>8</sub>S<sub>6</sub>O] with an Oxygen Unsymmetrically Bridging Two Proto-Fe<sub>4</sub>S<sub>3</sub> Cubes: Relevancy to the Substrate Binding Mode of the FeMo Cofactor
An oxygen-encapsulated iron sulfido cluster, [(DmpS)ÂFe<sub>4</sub>S<sub>3</sub>O]Â[(DmpS)ÂFe<sub>4</sub>S<sub>3</sub>]Â(μ-SDmp)<sub>2</sub>(μ-OCPh<sub>3</sub>) (<b>2</b>; Dmp = 2,6-(mesityl)<sub>2</sub>C<sub>6</sub>H<sub>3</sub>), has been synthesized by the reaction
of the preformed dinuclear iron thiolate/alkoxide [(Ph<sub>3</sub>CO)ÂFe]<sub>2</sub>(μ-SDmp)<sub>2</sub> (<b>1</b>) with <sup>1</sup>/<sub>8</sub>S<sub>8</sub> and <sup>1</sup>/<sub>4</sub>H<sub>2</sub>O in toluene. In the [Fe<sub>8</sub>S<sub>6</sub>O] core,
the oxygen atom bridges unsymmetrically two incomplete Fe<sub>4</sub>S<sub>3</sub> cubes, and two coordinatively unsaturated iron atoms
are weakly bound to mesityl rings. Relevance of the cluster structure
of <b>2</b> to the nitrogenase FeMo cofactor and its substrate
binding mode is discussed