Aggregation and Degradation of White Phosphorus Mediated by N‐Heterocyclic Carbene Nickel(0) Complexes

The reaction of zerovalent nickel compounds with white phosphorus (P₄) is a barely explored route to binary nickel phosphide clusters. Here, we show that coordinatively and electronically unsaturated N‐heterocyclic carbene (NHC) nickel(0) complexes afford unusual cluster compounds with P₁, P₃, P₅ and P₈ units. Using [Ni(IMes)₂] [IMes=1,3‐bis(2,4,6‐trimethylphenyl)imidazolin‐2‐ylidene], electron‐deficient Ni₃P₄ and Ni₃P₆ clusters have been isolated, which can be described as superhypercloso and hypercloso clusters according to the Wade–Mingos rules. Use of the bulkier NHC complexes [Ni(IPr)₂] or [(IPr)Ni(η⁶‐toluene)] [IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene] affords a closo‐Ni₃P₈ cluster. Inverse‐sandwich complexes [(NHC)₂Ni₂P₅] (NHC=IMes, IPr) with an aromatic cyclo‐P₅^? ligand were identified as additional products.     

NHC-assisted Ni(II)-catalyzed acceptorless dehydrogenation of amines and secondary alcohols

A novel catalytic system including NiCl₂-NHC ligand precursor has been developed for the preparation of imines from amines and ketones from alcohols. Owing to the acceptorless dehydrogenation pathway for this reaction, the hydrogen gas is liberated as a by-product. The active catalyst is generated in situ by the reaction of nickel (II) chloride, bis (imidazolium) chlorides and potassium tert-butoxide. The products were obtained in good to excellent yields and a wide variety of amines and ketones were applied successfully in this protocol.     

Synthesis and crystal structures of [K(H<Subscript>2</Subscript>O)(Piv)]<Subscript>∞</Subscript> and [K<Subscript>2</Subscript>(Phen)(H<Subscript>2</Subscript>O)<Subscript>2</Subscript>(Piv)<Subscript>2</Subscript>]<Subscript>∞</Subscript>

A method for the synthesis of potassium pivalates (trimethylacetates) from potassium tert-butoxide and pivalic acid was proposed. The complexes of the formulas [K(H₂O)(Piv)]_∞(I) and [K₂(Phen)(H₂O)₂(Piv)₂]_∞ (II) (Piv denotes the pivalate anion and Phen denotes 1,10-phenanthroline) were obtained and characterized by elemental analysis and IR and ¹H NMR spectroscopy. The crystal structures of complexes I and II were determined using X-ray diffraction. Crystal structure I has a layered motif with two nonequivalent K atoms (C.N.s 5 + 2 and 6). The coordination of phenanthroline in II gives rise to a ribbon motif, the structure containing three nonequivalent K atoms (C.N.s 6, 6 + 1, and 8).     

Electrocatalytic H2 Generation from Water Relying on Cooperative Ligand Electron Transfer in “PN3P” Pincer-Supported NiII Complexes

Water is the most sustainable source for H₂ production, and the efficient electrocatalytic production of H₂ from mixed water/acetonitrile solutions by using two new air-stable nickel(II) pincer complexes, [Ni(κ³-2,6-{Ph₂PNR}₂(NC₅H₃)Br₂] (R=H I , Me II ) is reported. Hydrogen generation from H₂O/CH₃CN solutions is initiated at −2 V against Fc^+/0, and bulk electrocatalysis studies showed that the catalyst functions with an excellent Faradaic efficiency and a turnover frequency of 160 s⁻¹. A DFT computational investigation of the reduction behavior of I and II revealed a correlation of H₂ formation with charge donation from electrons originating in a reduced ligand-localized orbital. As a result, these catalysts are proposed to proceed by a novel mechanism involving electron/proton transfer between a Ni^0I species bonded to an anionic PN³P ligand (“L⁻/Ni^0I”) and a Ni^I-hydride (“Ni−H”). Furthermore, these catalysts are able to reduce phenol and acetic acid, more active proton sources, at lower potentials that correlate with the substrate pK_a.     

The Preparation and X-Ray Structure of [AsPh4][Ph2P(S)NSiMe3] · 0.5 THF

The reaction of Ph₂P(S)N(SiMe₃)₂ with potassium tert-butoxide in a 1:1 molar ratio produces K[Ph₂P(S)NSiMe₃], which was converted to the AsPh₄⁺ salt by metathesis with [AsPh₄]Cl. The X-ray crystal structure of [AsPh₄][Ph₂P(S)NSiMe₃] · 0.5 THF consists of noninteracting AsPh₄⁺ and Ph₂P(S)NSiMe₃^? ions with d(P? S) = 1.980(4) Å and d(P? N) = 1.555(8) Å. The PNSi bite angle in the anion is 136.3(5)°. Electrophilic attack by Ph₂P(S)Cl occurs at the sulfur atom of Ph₂P(S)NSiMe₃^?. The oxidation of the anion with iodine produces a disulfide which regenerates K[Ph₂P(S)NSiMe₂] upon treatment with potassium tert-butoxide.     

Tetrakis(acetonitrile)‐dibromo‐nickel(II)‐di‐acetonitrile, [Ni(CH3CN)4Br2]·2CH3CN

[Tetrakis(acetonitrile)‐dibromo‐nickel(II)]‐di‐acetonitrile was obtained from a solution of nickel(II) dibromide in acetonitrile at 258 K. The crystal structure [monoclinic, P2₁/n (no.14), a = 1005.5(5), b = 831.3(5) , c = 1131.7(5) pm, β = 106.263(5)°, V = 908.1(8)·10⁶ pm³, Z = 2, R₁ for 1580 reflections with I₀>2σ(I₀): 0.0505] contains sixfold coordinated Ni^II atoms. Two trans coordinating bromide anions and four equatorial acetonitrile molecules form an elongated octahedron around the central Ni^II atom. [Ni(CH₃CN)₄Br₂] octahedra are connected via hydrogen bonds to neighboring octahedra as well as to solvate acetonitrile molecules.     

Investigation of the Catalytic Activity of a 2‐Phenylidenepyridine Palladium(II) Complex Bearing 4,5‐Dicyano‐1,3‐bis(mesityl)imidazol‐2‐ylidene in the Mizoroki‐Heck Reaction

The phenylidenepyridine (ppy) palladacycles [PdCl(ppy)(IMes)] ( 4 ) [IMes = 1,3‐bis(mesityl)imidazol‐2‐ylidene] and [PdCl(ppy){(CN)₂IMes}] ( 6 ) [(CN)₂IMes = 4,5‐dicyano‐1,3‐bis(mesityl)imidazol‐2‐ylidene] were prepared by facile two step syntheses, starting with the reaction of palladium(II) chloride with 2‐phenylpyridine followed by subsequent addition of the NHC ligand to the precatalyst precursor [PdCl(ppy)]₂. Suitable crystals for the X‐ray analysis of the complexes 4 and 6 were obtained. It was shown that 6 has a shorter NHC‐palladium bond than the IMes complex 4 . The difference of the palladium carbene bond lengths based on the higher π‐acceptor strength of (CN)₂IMes in comparison to IMes. Thus, (CN)₂IMes should stabilize the catalytically active central palladium atom better than IMes. As a measure for the π‐acceptor strength of (CN)₂IMes compared to IMes, the selone (CN)₂IMes · Se ( 7 ) was prepared and characterized by ⁷⁷Se‐NMR spectroscopy. The π‐acceptor strength of 7 was illuminated by the shift of its ⁷⁷Se‐NMR signal. The ⁷⁷Se‐NMR signal of 7 was shifted to much higher frequencies than the ⁷⁷Se‐NMR signal of IMes · Se. Catalytic experiments using the Mizoroki‐Heck reaction of aryl chlorides with n‐butyl acrylate showed that 6 is the superior performer in comparison to 4 . Using complex 6 , an extensive substrate screening of 26 different aryl bromides with n‐butyl acrylate was performed. Complex 6 is a suitable precatalyst for para‐substituted aryl bromides. The catalytically active species was identified by mercury poisoning experiments to be palladium nanoparticles.     

Impedance measurements for nickel deposition in sulfate and chloride electrolytes

From the analysis of the impedance of nickel deposition, the electrode kinetics is shown to be dependent on the type of anion. In chloride electrolytes a slow electrode activation with cathodic polarization is predominant. In sulfate solutions a low-frequency capacitive feature, favored by a pH decrease, appears to result from interactions between the nickel and hydrogen discharges. An interpretation is proposed where the ad-ion Ni_ads^I acts as both a reaction intermediate and a catalyst associated with a propagating kink site, and where the adsorbed species H_ads^*, generated by the presence of Ni_ads^I, inhibit the hydrogen evolution. It is concluded that the active area is closely connected to the coverages by adsorbates.     

Insertion of CO2 Mediated by a (Xantphos)NiI–Alkyl Species

The incorporation of CO₂ into organometallic and organic molecules represents a sustainable way to prepare carboxylates. The mechanism of reductive carboxylation of alkyl halides has been proposed to proceed through the reduction of Ni^II to Ni^I by either Zn or Mn, followed by CO₂ insertion into Ni^I‐alkyl species. No experimental evidence has been previously established to support the two proposed steps. Demonstrated herein is that the direct reduction of (tBu‐Xantphos)Ni^IIBr₂ by Zn affords Ni^I species. (tBu‐Xantphos)Ni^I‐Me and (tBu‐Xantphos)Ni^I‐Et complexes undergo fast insertion of CO₂ at 22 °C. The substantially faster rate, relative to that of Ni^II complexes, serves as the long‐sought‐after experimental support for the proposed mechanisms of Ni‐catalyzed carboxylation reactions.     

Synthesis and Characterization of NiIIIN3S2 Complexes as Active Site Models for the Oxidized Form of Nickel Superoxide Dismutase

Nickel complexes, [Ni(H₂BA^RTPP)](ClO₄)₂ (R=Ph for 1 or iPr for 2 ), supported by a pentadentate ligand H₂BA^RTPP were synthesized and oxidized to form Ni^III species having a N3S2 coordination environment to mimic the active site of the oxidized form of nickel superoxide dismutase (NiSODox). The Ni^III species 2 ⁺ exhibited a rhombic signal with g values at 2.15, 2.12 and 2.02 similar to that of NiSODox. DFT calculations revealed that 2 ⁺ has an unpaired electron primarily located in the d orbital of the Ni^III center, which strongly overlaps with the p_z orbital of the axial pyridine nitrogen of H₂BA^PrTPP.     

Studies on the Chemistry of Thienoannelated O , N - and S , N -Containing Heterocycles, 28 [1]. Synthesis of Imidazo[1,5- d ]thieno[2,3- b ][1,4]thiazine Derivatives as GABA -Receptor Ligands

 GABA-receptor-ligands are still very interesting in drug-development. Usually benzodiazepines are used in the treatment but they have serious side-effects. Thus, a recently synthesized quioxaline derivative which showed reduced side-effects in an animal model was used as a model-substance. The cyclus was modified to optimize the pharmacological profile. Accordingly, a series of imidazo-thieno-thiazines was synthesized starting from 5-acetyl-2-chloro-3-nitrothiophene to yield 6-ethyl-2,3-dihydro-1H-thieno[2,3-b][1,4]thiazine-2-one. Reaction with potassium tert-butoxide and diethylchlorophosphate gave an intermediate, which resulted in the desired ring system after adding the corresponding isocyanides and potassium tert-butoxide.     

Control of Hydrolysis and Condensation Reactions of Titanium tert-butoxide by Chemical Modification with Catechol

Titanium tert-butoxide (Ti(OC(CH₃)₃)₄; Ti(O ^t Bu)₄) was chemically modified with catechol (C₆H₄(OH)₂) and hydrolysis and condensation behavior of a resultant modified alkoxide was studied. Spectroscopic results revealed that the reaction between titanium tert-butoxide and catechol resulted in the formation of catecholate groups (C₆H₄O₂ ^2–) bound to titanium and corresponding release of tert-butanol. The mass spectrometry and cryoscopy indicated that main species was a dimer [(C₆H₄O₂)₂Ti₂(O ^t Bu)₄]. The hydrolysis of the modified alkoxide in the system with Ti:tetrahydrofuran (THF):H₂O = 1:10:x (x = 0.5–10) resulted in the partial hydrolysis, and all the hydrolyzed products after the drying under reduced pressure were soluble in THF and chloroform.     

Tuning the Stability and Reactivity of Metal‐bound Alkylperoxide by Remote Site Substitution of the Ligand

An alkylperoxonickel(II) complex with hydrotris(3,5‐diisopropyl‐4‐bromo‐1‐pyrazolyl)borate, [Ni^II(OOtBu)(Tp^iPr2,Br)] ( 3a ), is synthesized, and its chemical properties are compared with those of the prototype non‐brominated ligand derivative [Ni^II(OOtBu)(Tp^iPr2)] ( 3b ; Tp^iPr2=hydrotris(3,5‐diisopropyl‐1‐pyrazolyl)borate). Same synthetic procedures for the prototype 3b and its precursors can be employed to the synthesis of the Tp^iPr2,Br analogues. The dimeric nickel(II)‐hydroxo complex, [(Ni^IITp^iPr2,Br)₂(μ‐OH)₂] ( 2a ), can be synthesized by the base hydrolysis of the labile complexes [Ni^II(Y)(Tp^iPr2,Br)] (Y=NO₃ ( 1a ), OAc ( 1a′ )), which are obtained by the metathesis of NaTp^iPr2,Br with the corresponding nickel(II) salts, and the following dehydrative condensation of 2a with the stoichiometric amount of tert‐butylhydroperoxide yields 3a . The unique structural characteristics of the prototype 3b , that is, highly distorted geometry of the nickel center and intermediate coordination mode of the O O moiety between η¹ and η², are kept in the brominated ligand analogue 3a . The introduction of the electron‐withdrawing substitutents on the distal site of Tp^R affects the thermal stability and reactivity of the nickel(II)‐alkylperoxo species.     

Hydride Reactivity of NiII?X?NiII Entities: Mixed‐Valent Hydrido Complexes and Reversible Metal Reduction

After the lithiation of PYR‐H₂ (PYR^2?=[{NC(Me)C(H)C(Me)NC₆H₃(iPr)₂}₂(C₅H₃N)]^2?), which is the precursor of an expanded β‐diketiminato ligand system with two binding pockets, its reaction with [NiBr₂(dme)] led to a dinuclear nickel(II)–bromide complex, [(PYR)Ni(μ‐Br)NiBr] ( 1 ). The bridging bromide ligand could be selectively exchanged for a thiolate ligand to yield [(PYR)Ni(μ‐SEt)NiBr] ( 3 ). In an attempt to introduce hydride ligands, both compounds were treated with KHBEt₃. This treatment afforded [(PYR)Ni(μ‐H)Ni] ( 2 ), which is a mixed valent Ni^I? μ‐H? Ni^II complex, and [(PYR‐H)Ni(μ‐SEt)Ni] ( 4 ), in which two tricoordinated Ni^I moieties are strongly antiferromagnetically coupled. Compound 4 is the product of an initial salt metathesis, followed by an intramolecular redox process that separates the original hydride ligand into two electrons, which reduce the metal centres, and a proton, which is trapped by one of the binding pockets, thereby converting it into an olefin ligand on one of the Ni^I centres. The addition of a mild acid to complex 4 leads to the elimination of H₂ and the formation of a Ni^IINi^II compound, [(PYR)Ni(μ‐SEt)NiOTf] ( 5 ), so that the original Ni^II(μ‐SEt)Ni^IIX core of compound 3 is restored. All of these compounds were fully characterized, including by X‐ray diffraction, and their molecular structures, as well as their formation processes, are discussed.     

Zincocene and Dizincocene N‐Heterocyclic Carbene Complexes and Catalytic Hydrogenation of Imines and Ketones

The N‐heterocyclic carbene (NHC) adducts Zn(Cp^R)₂(NHC)] (Cp^R=C₅HMe₄, C₅H₄SiMe₃; NHC=ItBu, IDipp (Dipp=2,6‐diisopropylphenyl), IMes (Mes=mesityl), SIMes) were prepared and shown to be active catalysts for the hydrogenation of imines, whereas decamethylzincocene [ZnCp*₂] is highly active for the hydrogenation of ketones in the presence of noncoordinating NHCs. The abnormal carbene complex [Zn(OCHPh₂)₂(aItBu)]₂ was formed from spontaneous rearrangement of the ItBu ligand during incomplete hydrogenation of benzophenone. Two isolated Zn^I adducts [Zn₂Cp*₂(NHC)] (NHC=ItBu, SIMes) are presented and characterized as weak adducts on the basis of ¹³C NMR spectroscopic and X‐ray diffraction experiments. A mechanistic proposal for the reduction of [ZnCp*₂] with H₂ to give [Zn₂Cp*₂] is discussed.     

Synthesis of substituted pyrido[3´,2´:4,5]thieno[3,2-c]isoquinolin-5(6H)-ones and their sulfinyl and sulfonyl derivatives

A method for the synthesis of previously unknown pyrido[3´,2´:4,5]thieno[3,2-c]isoquinolin-5(6H)-ones was suggested, which includes a condensation reaction of substituted 3-cyanopyridine-2(1H)-thiones with methyl 2-(chloromethyl)benzoate and subsequent treatment of the condensation products with potassium tert-butoxide. The oxidation of the condensation products to sulfoxides or sulfones and subsequent treatment of these compounds with potassium tert-butoxide led to substituted pyrido[3´,2´:4,5]thieno[3,2-c]isoquinolin-5(6H)-one 11-oxides or substituted pyrido[3´,2´:4,5]thieno[3,2-c]isoquinolin-5(6H)-one 11,11-dioxides.

     

Theoretical Study on the Mechanism of Ni‐Catalyzed Alkyl–Alkyl Suzuki Cross‐Coupling

Ni‐catalyzed cross‐coupling of unactivated secondary alkyl halides with alkylboranes provides an efficient way to construct alkyl–alkyl bonds. The mechanism of this reaction with the Ni/ L1 ( L1 =trans‐N,N′‐dimethyl‐1,2‐cyclohexanediamine) system was examined for the first time by using theoretical calculations. The feasible mechanism was found to involve a Ni^I–Ni^III catalytic cycle with three main steps: transmetalation of [Ni^I( L1 )X] (X=Cl, Br) with 9‐borabicyclo[3.3.1]nonane (9‐BBN)R₁ to produce [Ni^I( L1 )(R₁)], oxidative addition of R₂X with [Ni^I( L1 )(R₁)] to produce [Ni^III( L1 )(R₁)(R₂)X] through a radical pathway, and C? C reductive elimination to generate the product and [Ni^I( L1 )X]. The transmetalation step is rate‐determining for both primary and secondary alkyl bromides. KOiBu decreases the activation barrier of the transmetalation step by forming a potassium alkyl boronate salt with alkyl borane. Tertiary alkyl halides are not reactive because the activation barrier of reductive elimination is too high (+34.7 kcal mol^?1). On the other hand, the cross‐coupling of alkyl chlorides can be catalyzed by Ni/ L2 ( L2 =trans‐N,N′‐dimethyl‐1,2‐diphenylethane‐1,2‐diamine) because the activation barrier of transmetalation with L2 is lower than that with L1 . Importantly, the Ni⁰–Ni^II catalytic cycle is not favored in the present systems because reductive elimination from both singlet and triplet [Ni^II( L1 )(R₁)(R₂)] is very difficult.     

Intramolecular Cyclization of Furfuryl 2-Propynyl Ether

Intramolecular cyclization of furfuryl 2-propynyl ether in the presence of a catalytic amount ofpotassium tert-butoxide in tert-butyl alcohol gives 3a'6-epoxy-1'3-dihydro-6H-isobenzofuran. The cyclizationis presumed to be favored by preliminary isomerization of the 2-propynyl group into allenyl. Heating of thecyclization product with excess potassium tert-butoxide in tert-butyl alcohol results in cleavage of the epoxybridge with formation of 6-hydroxy-1'3-dihydroisobenzofuran.     

Studies on the Chemistry of Thienoannelated O , N - and S , N -Containing Heterocycles, 28 [1]. Synthesis of Imidazo[1,5- d ]thieno[2,3- b ][1,4]thiazine Derivatives as GABA -Receptor Ligands

Summary.  GABA-receptor-ligands are still very interesting in drug-development. Usually benzodiazepines are used in the treatment but they have serious side-effects. Thus, a recently synthesized quioxaline derivative which showed reduced side-effects in an animal model was used as a model-substance. The cyclus was modified to optimize the pharmacological profile. Accordingly, a series of imidazo-thieno-thiazines was synthesized starting from 5-acetyl-2-chloro-3-nitrothiophene to yield 6-ethyl-2,3-dihydro-1H-thieno[2,3-b][1,4]thiazine-2-one. Reaction with potassium tert-butoxide and diethylchlorophosphate gave an intermediate, which resulted in the desired ring system after adding the corresponding isocyanides and potassium tert-butoxide. Corresponding author. E-mail: thomas.erker@univie.ac.at Received August 6, 2002; accepted August 13, 2002