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

N‑Heterocyclic Carbene Complexes of Three- and Four-Coordinate Fe(I)

N-heterocyclic carbene complexes of three- and four-coordinate Fe­(I), [Fe­(L^R)₄]­[PF₆] (L^R = 1,3-R₂-4,5-dimethylimidazol-2-ylidene, R = Me (<b>2</b>), Et (<b>3</b>), ⁱPr (<b>4</b>)) and [Fe­(L^Mes)₂(THF)]­[PF₆] (<b>5</b>) (L^Mes = 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene), were synthesized from successive reactions of [Fe­(toluene)₂]­[PF₆]₂ with 1 equiv of KC₈ and L^R (4 equiv for R = Me, Et, ⁱPr; 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^R: 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^iPr ligand(s), while the L^R ligands of the other Fe­(I) complexes remain firmly bound. Tetrahedral <b>2</b> and T-shaped <b>5</b> contain a high-spin (<i>S</i> = ³/₂) Fe­(I) center, whereas square-planar <b>3</b> and <b>4</b> contain Fe­(I) in the low-spin state (<i>S</i> = ¹/₂)

N‑Heterocyclic Carbene Complexes of Three- and Four-Coordinate Fe(I)

N-heterocyclic carbene complexes of three- and four-coordinate Fe­(I), [Fe­(L^R)₄]­[PF₆] (L^R = 1,3-R₂-4,5-dimethylimidazol-2-ylidene, R = Me (<b>2</b>), Et (<b>3</b>), ⁱPr (<b>4</b>)) and [Fe­(L^Mes)₂(THF)]­[PF₆] (<b>5</b>) (L^Mes = 1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene), were synthesized from successive reactions of [Fe­(toluene)₂]­[PF₆]₂ with 1 equiv of KC₈ and L^R (4 equiv for R = Me, Et, ⁱPr; 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^R: 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^iPr ligand(s), while the L^R ligands of the other Fe­(I) complexes remain firmly bound. Tetrahedral <b>2</b> and T-shaped <b>5</b> contain a high-spin (<i>S</i> = ³/₂) Fe­(I) center, whereas square-planar <b>3</b> and <b>4</b> contain Fe­(I) in the low-spin state (<i>S</i> = ¹/₂)

Synthesis of V/Fe/S Clusters Using Vanadium(III) Thiolate Complexes Bearing a Phenoxide-Based Tridentate Ligand

Vanadium­(III) thiolate complexes carrying a phenoxide-based tridentate ligand were prepared from the reactions of V­(NMe₂)₄ with the protonated forms of tridentate ligands (H₂­(O,P,O) = bis­(3,5-di-<i>tert</i>-butyl-2-hydroxy­phenyl)­phenylphosphine or H₂(O,O,O) = bis­(3,5-di-<i>tert</i>-butyl-2-hydroxy­phenyl)­phenyl­phosphine­oxide) and thiols (HSR; R = mesityl (Mes), 2,4,6-<i>ⁱ</i>Pr₃C₆H₂ (Tip)). The vanadium–thiolate complexes were subjected to the V/Fe/S cluster synthesis via treatment with an Fe­(II) thiolate complex [(TipS)­Fe]₂­(μ-SDmp)₂ (<b>4</b>, Dmp = 2,6-(mesityl)₂­C₆H₃) and elemental sulfur in toluene, leading to the formation of two new V/Fe/S clusters. One is an edge-bridged double-cubane-type [VFe₃S₄]-[VFe₃S₄] cluster [(O,P,O)­VFe₃S₄­(SDmp)­(HNMe₂)]₂ (<b>5</b>) having face-capping tridentate (O,P,O) ligands on vanadium atoms. The other is a [VFe₃S₄-Fe] cluster [(μ-O,O,O)­VFe₃S₄­(SDmp)­(STip)­Fe­(μ-SDmp)] (<b>6</b>), the core of which consists of a cubane-type [VFe₃S₄] 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^–) 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

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

Interconversion between [Fe₄S₄] and [Fe₂S₂] Clusters Bearing Amide Ligands

Structural conversion of [Fe₄S₄] clusters into [Fe₂S₂] clusters has been suggested to be a fundamental process for some O₂-sensing proteins. While the formation of [Fe₂S₂] clusters from synthetic [Fe₄S₄] clusters has been unprecedented, an all-ferric [Fe₄S₄]⁴⁺ cluster Fe₄S₄{N­(SiMe₃)₂}₄ (<b>1</b>) was found to split in the presence of pyridines, giving [Fe₂S₂] clusters Fe₂S₂­{N­(SiMe₃)₂}₂(L)₂ (<b>2</b>, L = pyridines). The structural conversion between [Fe₄S₄] and [Fe₂S₂] 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 [Fe₂S₂] clusters was also induced by chemical reductions of Fe₂S₂­{N­(SiMe₃)₂}₂(Py)₂ (Py = pyridine), and the resultant [Fe₄S₄] clusters [<b>1</b>]⁻ and [<b>1</b>]^2– were found to split into two [Fe₂S₂] clusters by oxidation with [Cp₂Fe]⁺ 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 bis­(trimethylsilyl)­amide and thiolate ligands were synthesized from the reactions of Fe­{N­(SiMe₃)₂}₂ (<b>1</b>) with 1 equiv of thiol HSR (R = C₆H₅ (Ph), 4-^tBuC₆H₄, 2,6-Ph₂C₆H₃ (Dpp), 2,4,6-ⁱPr₃C₆H₂ (Tip)) and subsequent treatment with O₂. The trinuclear clusters [{(Me₃Si)₂N}­Fe]₃(μ₃-O)­{μ-S­(4-RC₆H₄)}₃ (R = H (<b>3a</b>), ^tBu (<b>3b</b>)) were obtained from the reactions of <b>1</b> with HSPh or HS­(4-^tBuC₆H₄) and O₂, while we isolated a tetranuclear cluster [{(Me₃Si)₂N}₂Fe₂(μ-SDpp)]₂(μ₃-O)₂ (<b>4</b>) as crystals from an analogous reaction with HSDpp. Treatment of a tertrahydrofuran (THF) solution of <b>1</b> with HSTip and O₂ resulted in the formation of a dinuclear complex [{(Me₃Si)₂N}­(TipS)­(THF)­Fe]₂(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 ruthenium­(II) thiolate complex, affording a ruthenium­(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]­nonane dimer [(9-BBN)₂], 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 [Fe₈S₆O] with an Oxygen Unsymmetrically Bridging Two Proto-Fe₄S₃ Cubes: Relevancy to the Substrate Binding Mode of the FeMo Cofactor

An oxygen-encapsulated iron sulfido cluster, [(DmpS)­Fe₄S₃O]­[(DmpS)­Fe₄S₃]­(μ-SDmp)₂(μ-OCPh₃) (<b>2</b>; Dmp = 2,6-(mesityl)₂C₆H₃), has been synthesized by the reaction of the preformed dinuclear iron thiolate/alkoxide [(Ph₃CO)­Fe]₂(μ-SDmp)₂ (<b>1</b>) with ¹/₈S₈ and ¹/₄H₂O in toluene. In the [Fe₈S₆O] core, the oxygen atom bridges unsymmetrically two incomplete Fe₄S₃ 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