Synthesis, structures and electrochemical properties of nitro- and amino-functionalized diiron azadithiolates as active site models of Fe-only hydrogenases

Complex [[(mu-SCH2)2N(4-NO2C6H4)]Fe2(CO)6] (4) was prepared by the reaction of the dianionic intermediate [(mu-S)2Fe2(CO)6](2-) and N,N-bis(chloromethyl)-4-nitroaniline as a biomimetic model of the active site of Fe-only hydrogenase. The reduction of 4 by Pd-C/H2 under a neutral condition afforded complex [[(mu-SCH2)2N(4-NH2C6H4)]Fe2(CO)6] (5) in 67 % yield. Both complexes were characterized by IR, 1H and 13C NMR spectroscopy and MS spectrometry. The molecular structure of 4, as determined by X-ray analysis, has a butterfly 2Fe2S core and the aryl group on the bridged-N atom slants to the Fe(2) site. Cyclic voltammograms of 4 and 5 were studied to evaluate their redox properties. It was found that complex 4 catalyzed electrochemical proton reduction in the presence of acetic acid. A plausible mechanism of the electrocatalytic proton reduction is discussed.     

Synthesis, structure and electrochemical property of diphenylacetypene-substituted diiron azadithiolates as active site of Fe-only hydrogenases

The model complexes 1-3 of functionalized azadithiolate (ADT)-bridged Fe-only hydrogenases, [Fe₂(Co)₆(μ-ADT)C₆H₄CCR] [R = C₆H₄NO₂-4 (1), C₆H₅ (2), C₆H₄OCH₃-4 (3)] have been synthesized in high yields under mild conditions by using Sonogashira reaction. Spectroscopic study and X-ray crystal structural analysis of 1 demonstrate that the model complexes retain the butterfly structure of 2Fe2S model analogues. The intermolecular C-H?O, C-H?π hydrogen bonding and π-π interactions play important roles in molecular packing of 1. In the presence of HOAc, complex 1 features the catalytic electrochemical proton reduction.     

Synthesis and structural characterization of diiron azadithiolate complexes relevant to the active site of [FeFe] hydrogenases

Two N-functionally substituted diiron azadithiolate complexes, [(µ-SCH₂)₂NCH₂CH₂OC(O)C₆H₄I-p]Fe₂(CO)₆ (1) and {[(µ-SCH₂)₂NCH₂CH₂OC(O)C₆H₄I-p]Fe₂(CO)₅Ph₂PCH}₂ (2) as models for the active site of [FeFe] hydrogenases, have been prepared and fully characterized. Complex 1 was prepared by the reaction of [(µ-SCH₂)₂NCH₂CH₂OH]Fe₂(CO)₆ with p-iodobenzoic acid in the presence of 4-dimethylaminopyridine (DMAP) and N,N′-dicyclohexylcarbodiimide (DCC) in 78% yield. Further treatment of 1 with 1 equiv. of Me₃NO?·?2H₂O followed by 0.5 equiv. of trans-1,2-bis(diphenylphosphino)ethylene (dppe) affords 2 in 60% yield. The new complexes 1 and 2 were characterized by IR and ¹H (¹³C, ³¹P) NMR spectroscopic techniques and their molecular structures were confirmed by X-ray diffraction analysis. The molecular structure of 1 has two conformational isomers, in one isomer its N-functional substituent is axial to its bridged nitrogen and in the other isomer its N-functional substituent is equatorial. The crystal structure of 2 revealed that its N-functional substituents are equatorial to its nitrogens and dppe occupies the two apical positions of the square-pyramidal irons.     

Synthesis,structures and electrochemical properties of hydroxyl- and pyridyl-functionalized diiron azadithiolate complexes

The hydroxyl- and pyridyl-functionalized diiron azadithiolate complexes [{(μ-SCH₂)₂N(CH₂CH₂OH)}Fe₂(CO)₆] (1) and [{(μ-SCH₂)₂N(CH₂CH₂OOCPy)}Fe₂(CO)₆] (Py = pyridyl) (2) were prepared as biomimetic models of the active site of Fe-only hydrogenases. Both complexes were characterized by MS, IR, ¹H NMR spectra and elemental analysis. The molecular structures of 1 and 2 were determined by single crystal X-ray analysis. A network is constructed by intermolecular H-bonds in the crystals of 1. An S?O intermolecular contact was found in the crystals of 2, which is scarcely found for organometallic complexes. Cyclic voltammograms of 1 and 2 were studied to evaluate their redox properties.     

Synthesis, characterization and electrochemical behavior of some N-heterocyclic carbene-containing active site models of [FeFe]-hydrogenases

Treatment of parent compounds [(μ-SCH₂)₂X]Fe₂(CO)₆ (A, X = O; B, X = NBu-t; C, X = NC₆H₄OMe-p) with N-heterocyclic carbene I_Mes (I_Mes = 1,3-bis(mesityl)imidazol-2-ylidene) generated in situ through reaction of imidazolium salt I_Mes ·HCl with n-BuLi or t-BuOK afforded the monocarbene-substituted complexes [(μ-SCH₂)₂X]Fe₂(CO)₅(I_Mes) (1, X = O; 2, X = NBu-t; 3, X = NC₆H₄OMe-p). Similarly, the monocarbene and dicarbene-substituted complexes [(μ-SCH₂)₂NBu-t]Fe₂(CO)₅[I^∗_Mes(CH₂)₃I^∗_Mes]·HBr (4) and [(μ-SCH₂)₂CH₂Fe₂(CO)₅]₂[μ-I^∗_Mes(CH₂)₃I^∗_Mes] (5, I^∗_Mes = 1-(mesityl)imidazol-2-ylidene) could be prepared by reactions of parent compound B with the mono-NHC ligand-containing imidazolium salt [I^∗_Mes(CH₂)₃I^∗_Mes] · HBr and parent compound [(μ-SCH₂)₂CH₂]Fe₂(CO)₆ (D) with di-NHC ligand I^∗_Mes(CH₂)₃I^∗_Mes (both NHC ligands were generated in situ from reaction of n-BuLi with imidazolium salt [I^∗_MesI^∗_Mes(CH₂)₃I^∗_Mes] · 2HBr), respectively. The imidazolium salt [I^∗_Mes(CH₂)₃I^∗_Mes] · 2HBr was prepared by reaction of 1-(mesityl)imidazole with Br(CH₂)₃Br. All the new model compounds 1-5 and imidazolium salt [I^∗_Mes(CH₂)₃I^∗_Mes] · 2HBr were fully characterized by elemental analysis, spectroscopy, and X-ray crystallography. On the basis of electrochemical studies of 1 and 2, compound 2 was found to be a catalyst for proton reduction to hydrogen. In addition, an EECC mechanism for this electrocatalytic reaction is preliminarily suggested.     

Synthesis,characterization, and crystal structures of N-functionalized diiron azadithiolate complexes related to the active site of [FeFe]-hydrogenases

A series of N-functionalized diiron azadithiolate complexes, [(µ-SCH₂)₂NCH₂CO₂Me]Fe₂(CO)_5?L [L?=?CO (1); PPh₃ (2); Ph₂PCH₂PPh₂ (3)], as active site models of [FeFe]-hydrogenases has been prepared and characterized. While 1 was prepared by a sequential reaction of (µ-HS)₂Fe₂(CO)₆ with two equiv. of aqueous HCHO, followed by treatment of (µ-HOCH₂S)₂Fe₂(CO)₆ with one equiv. of H₂NCH₂CO₂Me in 46% yield; 2 and 3 were prepared by a carbonyl substitution reaction of 1 with PPh₃ or Ph₂PCH₂PPh₂ in the presence of Me₃NO?·?2H₂O in 90% and 85% yields, respectively. The crystal structures of 1 and 2 revealed that the substituent attached to the bridgehead nitrogen occupies an equatorial position and the PPh₃ ligand resides in an axial position of the square pyramid of Fe2.     

An approach to water-soluble hydrogenase active site models: Synthesis and electrochemistry of diiron dithiolate complexes with 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane ligand(s)

In order to improve the hydro- and protophilicity of the active site models of the Fe-only hydrogenases, three diiron dithiolate complexes with DAPTA ligand(s) (DAPTA = 3,7-diacetyl-1,3,7-triaza-5-phosphabicyclo[3.3.1]nonane), (μ-pdt)[Fe(CO)₃][Fe(CO)₂(DAPTA)] (1, pdt = 1,3-propanedithiolato), (μ-pdt)[Fe(CO)₂(DAPTA)]₂ (2) and (μ-pdt)[Fe(CO)₂(PTA)][Fe(CO)₂(DAPTA)] (3), were prepared and spectroscopically characterized. The water solubility of DAPTA-coordinate complexes 1-3 is better than that of the PTA-coordinate analogues. With complexes 1-3 as electrocatalysts, the overvoltage is reduced by 460-770 mV for proton reduction from acetic acid at low concentration in CH₃CN. Significant decrease, up to 420 mV, in reduction potential for the Fe(I)Fe(I) to Fe(I)Fe(0) process and the curve-crossing phenomenon are observed in cyclic voltammograms of 2 and 3 in CH₃CN/H₂O mixtures. The introduction of the DAPTA ligand to the diiron dithiolate model complexes indeed makes the water solubility of 2 and 3 sufficient for electrochemical studies in pure water, which show that the proton reduction from acetic acid in pure water is electrochemically catalyzed by 2 and 3 at ca. −1.3 V vs. NHE.     

Tris(N-pyrrolidinyl)phosphine substituted diiron dithiolate related to iron-only hydrogenase active site: Synthesis, characterization and electrochemical properties

A tris(N-pyrrolidinyl)phosphine (P(NC₄H₈)₃) monosubstituted complex, [(μ-pdt)Fe₂(CO)₅P(NC₄H₈)₃] (2) was synthesized as a functional model of the hydrogen-producing capability of the iron hydrogenase active site. The structure was fully characterized by X-ray crystallography. IR and electrochemical studies have indicated that the P(NC₄H₈)₃ ligand has better electron-donating ability than that of those phosphine ligands, such as PMe₃, PTA (1,3,5-triaza-7-phosphaadamantane), PMe₂Ph PPh₃, and P(OEt)₃. The electrocatalytic activity of 2 was recorded in CH₃CN in the absence and presence of weak acid, HOAc. The cathodic shift of potential at −1.98 V and the dependence of current on acid concentration have indicated that complex 2 can catalyze the reduction of protons to hydrogen at its Fe⁰Fe^I level in the presence of HOAc. IR spectroelectrochemical experiments are conducted during the reduction of 2 under nitrogen and carbon monoxide, respectively. The formation of a bridging CO group during the reduction of 2 at −1.98 V has been identified using IR spectroelectrochemical techniques, and an electrocatalytic mechanism of 2 consistent with the spectroscopic and electrochemical results is proposed.     

Preparation,characterization and electrochemistry of an iron-only hydrogenase active site model covalently linked to a ruthenium tris(bipyridine) photosensitizer

An NH₂-functionlized [Fe₂S₂] model complex of the iron-only hydrogenase active site was covalently linked to the tris(bipyridine)ruthenium photosensitizer. The [RuFeFe] trinuclear complex 1 was characterized by MS, IR, UV-vis, ¹H & ¹³C NMR spectra. A quasi-reversible reduction peak at ?1.41 V versus Ag/Ag⁺ for the Fe^IFe^I/Fe^IFe⁰ process is observed in the cyclic voltammogram of 1.     

Synthesis,crystal structures and electrocatalytic properties of bridgehead-C-functionalized diiron dithiolate complexes

To investigate the influence of bridgehead-C functionality in diiron dithiolate complexes on the molecular structure and electrocatalytic properties of [FeFe]-hydrogenase models, three new bridgehead-C-functionalized model complexes 1–3 have been synthesized and structurally characterized. Treatments of parent complex [(μ-SCH₂)₂CHCO₂H][Fe₂(CO)₆] (A) with the esterification agents o-MeC₆H₄OH, p-ClC₆H₄OH, or p-HOC₆H₄CHO in the presence of 4-dimethylaminopyridine and dicyclohexylcarbodiimide in CH₂Cl₂ at room temperature resulted in formation of [(μ-SCH₂)₂CHCO₂R][Fe₂(CO)₆] (R = o-MeC₆H₄–, 1; p-ClC₆H₄–, 2; p-OHCC₆H₄–, 3) in 53–55% yields. The new complexes 1–3 were characterized by elemental analysis, IR and NMR spectroscopy, and especially determined by X-ray crystallography. The electrochemical properties of 1–3 and the electrocatalytic H₂ evolution catalyzed by 1 have been investigated by cyclic voltammetry, where 1 is a catalyst for HOAc proton reduction to H₂ under electrochemical conditions.     

Synthesis and characterization of carboxy-functionalized diiron model complexes of [FeFe]-hydrogenases: Decarboxylation of Ph2PCH2COOH promoted by a diiron azadithiolate complex

Two carboxy-functionalized diiron complexes [{(μ-SCH₂)₂X}{Fe(CO)₃}{Fe(CO)₂L}] (X = NC₃H₇, L = Ph₂PCH₂CH₂COOH, 4; X = CH₂, L = Ph₂PCH₂COOH, 5) were prepared, as biomimetic models of the [FeFe] hydrogenase active site, from the CO-replacement of [{(μ-SCH₂)₂NC₃H₇}Fe₂(CO)₆] (1) and (μ-pdt)Fe₂(CO)₆ (2) by phosphine ligands in CH₃CN at 40 °C, respectively. In contrast, the reaction of 1 with Ph₂PCH₂COOH under the same condition afforded complex [{(μ-SCH₂)₂NC₃H₇}{Fe(CO)₃}{Fe(CO)₂(Ph₂PCH₃)}] (3) with a decarboxylated phosphine ligand. The molecular structures of complexes 3-5 were determined by X-ray crystallographic analyses, which show that they have similar frameworks with the phosphine ligand on the apical position. The interesting C-H···S contacts between the methylene hydrogen atoms of the PhCH₂COOH ligand and the μ-S atoms of the pdt-bridge are found in the crystal of 5. According to the experimental evidence, a plausible mechanism, via sequential phosphine coordination, N-protonation, and decarboxylation steps, is proposed for the formation of 3 and for explanation of the contrastive reactivities of the adt- (2-aza-1,3-propanedithiolato) and the pdt- (1,3-propanedithiolato) bridged diiron complexes toward decarboxylation of the Ph₂PCH₂COOH ligand.     

Synthesis, structure and magnetic properties of new vanadate PbCo2V2O8

A new vanadate PbCo₂V₂O₈ was obtained through the study of PbO-CoO-V₂O₅ ternary system. The crystal structure was determined by Rietveld method, indicating that PbCo₂V₂O₈ has a tetragonal structure of space group I41cd with a spiral chain along the c-axis. Magnetic properties of the titled compound were investigated by means of susceptibility, magnetization, and heat capacity measurements. The results show that PbCo₂V₂O₈ is a quasi-one-dimensional canted antiferromagnet with Neel temperature of ∼4 K, being consistent with its crystal structure.     

Synthesis, structure, and properties of four ternary compounds: CaSrTt, Tt=Si, Ge, Sn, Pb

The title compounds were synthesized and characterized by structural measurements and electronic structure calculations. Single-crystal X-ray diffraction analyses established that they all have the orthorhombic inverse-PbCl₂-type structure (Pnma, Z=4, a=8.108(2), 8.124(2), 8.421(2), 8.509(2) Å; b=4.944(1), 4.949(1), 5.168(1), 5.189(1) Å; c=9.170(2), 9.184(2), 9.685(2), 9.740(2) Å, respectively). The tetrel (Tt) atoms are situated in tricapped trigonal prisms of ordered Sr and Ca atoms in which the smaller Ca atoms play a distinctive role. The structure is distinguishable from the Co₂Si type by its more nearly ideal 6+3 (TCTP) environment about Tt rather than a higher coordination by cations. Other representations of the two structural types are also considered. Electronic band structure calculations suggest that the compounds are semiconductors, in agreement with literature data on their Ae₂Tt analogues.     

Synthesis of N-substituted 1,2,3,4-tetrahydrobenz[g]isoquinoline-5,10-diones

The synthesis of 1,2,3,4-tetrahydrobenz[g]isoquinoline-5,10-dione hydrochlorides is reported starting from 1,4-dihydrobenz[g]isoquinoline-3(2H)-ones or ethyl (3-aminomethyl-1,4-dimethoxynaphth-2-yl)acetates. A third strategy relies on the synthesis of the title compounds via an N-protected 2-(3-bromomethyl-1,4-dimethoxynaphth-2-yl)ethylamine. The synthesized 1,2,3,4-tetrahydro-benz[g]isoquinoline-5,10-diones are a new class of quinones, which have not been reported yet.     

Synthesis, structure and properties of a new Zintl phase: SrLiSb

A new ternary antimonide SrLiSb has been synthesized and characterized using single-crystal X-ray studies. It is found to crystallize in the anti-PbCl₂ structure type with orthorhombic cell (centrosymmetric S.G., Pnma; , , ) and is isostructural to its calcium analogue (CaLiSb). However, BaLiSb has been reported to crystallize in the hexagonal space group P6₃/mmc. As in the Ca and Ba analogues, antimony is present as isolated Sb³⁻ ions making SrLiSb electron precise and hence is expected to behave as a classical Zintl compound. The magnetic susceptibility measurements show the diamagnetic nature and the conductivity is temperature independent, both verifying the classical Zintl nature of SrLiSb.     

Synthesis, crystal structure and optical properties of BiMgVO5

The new vanadate BiMgVO₅ has been prepared and its structure has been determined by single crystal X-ray diffraction: space group P2₁/n, , , , β=107.38(5)°, wR₂=0.0447, R=0.0255. The structure consists of [Mg₂O₁₀] and [Bi₂O₁₀] dimers sharing their corners with [VO₄] tetrahedra. The ranges of bond lengths are 2.129-2.814 Å for Bi-O; 2.035-2.167 Å for Mg-O and 1.684-1.745 Å for V-O. V-O bond lengths determined from Raman band wavenumbers are between 1.679 and 1.747 Å. An emission band overlapping the entire visible region with a maximum around 650 nm is observed.     

Azadithiolates cofactor of the iron-only hydrogenase and its PR3-monosubstituted derivatives: Synthesis, structure, electrochemistry and protonation

The core structure (μ-SCH₂)₂NH[Fe₂(CO)₆] (5) of Fe-only hydrogenases active site model has been synthesized by the condensation of iron carbonyl sulfides, formaldehyde and silyl protected amine. Its monosubstituted complexes (μ-SCH₂)₂NH[Fe₂(CO)₅PR₃] (R = Ph (6), Me (7)) were accordingly prepared. The coordination configurations of 5 and 6 were characterized by X-ray crystallography. Protonation of complex 7 to form the N-protonated product occurs in an acetonitrile solution upon addition of triflic acid. The redox properties of these model complexes were studied by cyclic voltammetry.     

Synthesis, crystal structure and optical properties of the new lead fluoride borate—Pb2BO3F

A new compound, Pb₂BO₃F, has been grown by high temperature solution method from the PbO–PbF₂–B₂O₃ system for the first time. The crystal structure of this compound has been identified by single crystal X-ray diffraction analysis. It crystallizes in the hexagonal system, space group P6₃/m (No. 176) with unit-cell parameters a=7.2460(3) Å, c=14.5521(17) Å, Z=6, V=661.69(9) ų. Its structure was solved by the direct methods and refined to R₁=0.0163 and wR₂=0.0367. The structure of Pb₂BO₃F consists of the distorted PbO₃F₂ groups and BO₃ triangles, which are all symmetric with each other in the gestalt structure to the extent that the Pb₂BO₃F compound crystallizes in the symmetric space group. The powder X-ray diffraction pattern of the Pb₂BO₃F has been measured. The BO₃ functional groups presented in the sample were identified by FTIR spectrum. The DTA curve of Pb₂BO₃F suggests that Pb₂BO₃F melts congruently at 448 °C.     

Synthesis, crystal structure and optical properties of a novel sodium lead pentaborate, NaPbB5O9

A novel sodium lead pentaborate, NaPbB₅O₉, has been successfully synthesized by standard solid-state reaction. The single-crystal X-ray structural analysis showed that NaPbB₅O₉ crystallizes in the monoclinic space group P2₁/c with a=6.5324(10) Å, b=13.0234(2) Å, c=8.5838(10) Å, β=104.971(10)°, and Z=4. The crystal structure is composed of double ring [B₅O₉]³⁻ units, [PbO₇] and [NaO₇] polyhedra. [B₅O₉]³⁻ groups connect with each other forming two-dimensional infinite _∞[B₅O₉]³⁻ layers, while [PbO₇] and [NaO₇] polyhedra are located between the layers. [PbO₇] polyhedra linked together via corner-sharing O atom forming novel infinite _∞[PbO₆] chains along the c axis. The thermal behavior, IR spectrum and the optical diffuse reflectance spectrum of NaPbB₅O₉ were reported.     

Synthesis, structure and electronic structure of a new polymorph of CaGe2

Reported are the flux synthesis, the crystal structure determination, the properties and the band structure calculations of a new polymorph of CaGe₂, which crystallizes with the hexagonal space group P6₃mc (no. 186) with cell parameters of a=3.9966(9) and c=10.211(4) Å (Z=2; Pearson's code hP6). The structure can be viewed as puckered layers of three-bonded germanium atoms, , which are stacked along the direction of the c-axis in an ABAB-fashion. The germanium polyanionic layers are separated by the Ca cations. As such, this structure is closely related to the structure of the other CaGe₂ polymorph, which crystallizes with the rhombohedral CaSi₂ type in the R3¯m space group (No. 166), where the layers are arranged in an AA′BB′CC′-fashion, and are also interspaced by Ca²⁺ cations. LMTO calculations suggest that in spite of the formal closed-shell configuration for all atoms and the apparent adherence to the Zintl rules for electron counting, i.e., Ca²⁺[3b-Ge¹⁻]₂), the phase will be a poor metal due to a small Ca-3d-Ge-4p band overlap. Magnetic susceptibility measurements as a function of the temperature indicate that the new CaGe₂ polymorph exhibits weak, temperature independent, Pauli-paramagnetism.