Multidentate Acyclic Neutral Ligands and Their Complexation

The properties of cyclic crown ethers are approximated by acyclic neutral ligands (popdands), which are compared and contrasted with open-chain bioionophores and acidic chelating agents in this article. Variations of the endo-polarophilicity/exo-lipophilicity balance, complex stability, ion slectivity can often be accomplished more easily, with greater versatility, and at less expense with acyclic polyethers than with their cyclic counterparts; complexation and decomplexation are generally faster in acyclic systems; and the pseudocavity usually has greater conformational flexibility. Acyclic crown ethers and open-chain cryptands stiffened by rigid “terminal groups” containing donor atoms readily form crystalline complexes of alkali and alkaline earth metals. Some oppen-chain neutral ligands form helical conformations in their crycstalin complexes. The observed coordination numbers and geometries are of theoretical interst. Attractive terminal group interactions lead to pseudocyclic species occupying a position intermediate between cyclic and acyclic ligands. It has recently proved possible to isolate crystalline complexes of alkali and alkaline earth metal ions with weakly donating oligo(ethylene glycol ethers) and with glycols; such complexes have also been obtained with sugars. Acyclic neutral ligands can serve as simple models of nigericin-type bioionophores and be used analytically in microelectrodes. The recently discoverd crystalline stoichiometric complexes formed by some acyclic neutral ligands with guest molecules such as urea, thiourea, and water provide a fresh insight into weak interactions between neutral molecules and for the development of urea receptors.     

含茂、非茂混合配体钛族络合物的合成与催化烯烃聚合的研究进展

目前金属有机钛族络合物的发展呈现出多元化的趋势:茂络合物、非茂络合物以及茂非茂混合配体络合物,而其中茂、非茂混合配体络合物是近年来发展的一个新亮点.关于茂钛族络合物、非茂钛族络合物近年已经有过许多的综述,而关于茂、非茂混合配体钛族络合物的综述却很少.综述了近年来茂、非茂混合配体钛族络合物合成与催化烯烃聚合的研究进展.     

Isocyanide‐Based Three‐Component Synthesis of Pyrano‐pyrido‐quinoxalines

Pyrano‐pyrido‐quinoxaline derivatives were synthesized in good yields by a three‐component reaction of isocyanides, dialkyl acetylenedicarboxylates, and pyrido[1,2‐a]quinoxaline‐triones in DMF at 100°.     

Charged Behaviour from Neutral Ligands: Synthesis and Properties of N‐Heterocyclic Pseudo‐amides

Deprotonation of the 1‐isopropyl‐3‐(phenylamino)pyridin‐1‐ium iodide gives the corresponding neutral betaine, which is formalised as a pyridinium‐amido ligand when coordinated to a metal. Spectroscopic, structural and theoretical methods have been used to investigate the metal–ligand bonding, ligand dynamics and electron distribution. Collectively, the data show that the ligand can be characterised as a pseudo‐amide and is a strong donor akin to alkyl phosphines and N‐heterocyclic carbenes. Furthermore, rotation about both N substituent C? N bonds occurs, which is in contrast to the two alternative pyridinium positional isomers that exhibit neutral resonance structures. For comparison, compounds and complexes derived from norharman were prepared, which contain an additional C? C bond supporting conjugation and the accessibility of a neutral resonance structure. Notwithstanding the formal neutral structure, norharman‐derived ligands are comparably strong donors, and have the additional advantage of exhibiting stability to dioxygen and water.     

Functionalized pentamethylferrocenes: Synthesis, structure, and electrochemistry

The advantageous properties of the Cp^* ligand — intensified electron donation, steric bulk, and enhanced solubility in comparison to the ubiquitous Cp ligand — are finding increasing use in organometallic chemistry. A systematic evaluation of synthetic routes to pentamethylferrocene compounds with a wide range of functionalities, including carboxyl, carbonyl, aminomethyl, vinyl, ethynyl, fulvenyl, cyclopentadienylmethyl, and others is reported. Spectroscopic, structural, and electrochemical properties of such functionalized pentamethylferrocenes Fc^*/2—R are compared to those of non-methylated ferrocenes Fc—R. The electronic influence of the Cp^* ligand in these unsymmetricalferrocenes Fc^*/2—R has been studied by cyclic voltammetry measurements, demonstrating a decrease in oxidation potential of −0.276 V in direct comparison to non-methylated ferrocenes Fc—R.     

Synthesis of Functionalized Heteroleptic Germylene Alkoxides

N-coordinated Ge(II) alkoxides L¹(tBuO)Ge ( 1 ), L²(tBuO)Ge ( 2 ) and [L²(OtBu)Ge ⋅ BH₃] ( 4 ) were prepared. Effect of either chelating ligands L¹ and L² or Ge→B interaction on strength of the Ge−OtBu bond was studied by insertion reaction of PhNCO. As a result, the Ge(II) carbamate L²{[(tBuO)OC](Ph)N}Ge ( 3 ) was isolated. Alcoholysis exchange reactions of 1 and 2 with substituted phenols were studied to find an easy synthetic protocol for a synthesis of functionalized Ge(II) alkoxides. Reactions yielded Ge(II) alkoxides L^1,2(2-Br−C₆H₄O)Ge ( 5 for L¹, 8 for L²), L^1,2(2-MeNH−C₆H₄O)Ge ( 6 for L¹, 9 for L²), L^1,2(2-Ph₂P−C₆H₄O)Ge ( 7 for L¹, 10 for L²), L²(2-Br-3-OH−C₆H₃O)Ge ( 11 ) and L²(2-NC₅H₄O)Ge ( 12 ) containing the additional polar groups Y (Y=Br, MeNH, PPh₂, OH or N). Finally, phosphane decorated Ge(II) alkoxides 7 and 10 were tested as suitable ligands in reactions with (COD)W(CO)₄ and BH₃. As a consequence, new complexes [(κ²- 7 )W(CO)₄] ( 13 ) and [L¹(2-Ph₂P ⋅ {BH₃}-C₆H₄O)Ge ⋅ {BH₃}] ( 14 ) were isolated. All compounds were characterized by NMR and IR spectroscopy, and compounds 3 , 4 , 9 and 11 were additionally characterized by X-ray diffraction analysis.     

New Synthesis of Ferrocene Monocarboxylic Acid and Systematic Studies on the Preparation of Related Key‐Intermediates

An improved technique for the preparation of ferrocenecarboxaldehyde and two new methods for the synthesis of ferrocenemonocarboxylic acid from the aldehyde or from acetyl ferrocene are described. Using the aldehyde or the monocarboxylic acid as starting materials, some important ferrocene key intermediates were prepared: monochlorocarbonyl ferrocene, alkali metal and ammonium ferrocenoates, ferrocene carboxyaldehyde diethylacetal, ferrocenecarboxamide, et …, as well as a new ferrocene derivative: piperazinium diferrocenoate.     

Donor‐Acceptor‐Functionalized Tetraethynylethenes with Nitrothienyl Substituents: Structure‐Property Relationships

Tetraethynylethenes (TEEs) functionalized with donor (4‐(dimethylamino)phenyl) and acceptor (5‐nitro‐2‐thienyl) groups were prepared by Pd⁰‐catalyzed Sonogashira cross‐coupling reactions (Schemes 1 – 6). The physical properties of these novel chromophores were examined and compared with those of analogous systems containing 4‐nitrophenyl instead of 5‐nitro‐2‐thienyl acceptor groups. X‐Ray crystal‐structure analyses showed the π‐conjugated frameworks of 2 , 11 , and 13 , including the TEE core and all aryl moieties, to be nearly perfectly planar (Figs. 1, 3, and 4). In contrast, one 4‐(dimethylamino)phenyl group in 10 is rotated almost 90° out of the molecular plane, presumably due to crystal‐packing effects (Fig. 2). The analysis of bond lengths and bond angles revealed little, if any, evidence of intramolecular ground‐state donor‐acceptor interactions. The electrochemical behavior of nitrothienyl‐substituted TEEs is similar to that of the corresponding nitrophenyl‐functionalized derivatives (Table 3). The nitrothienyl groups were reduced at −1.23 V (vs. the ferrocene/ferricinium couple, Fc/Fc⁺), regardless of the degree or pattern of other substitutions. For nonsymmetrical TEE 13 , the reduction of the nitrothienyl group at −1.23 V is followed by a reduction of the nitrophenyl group at −1.40 V, a potential typical for the reduction of other nitrophenyl‐substituted TEEs, such as 17 – 20 . UV/VIS Spectroscopy showed a consistently lower‐energy absorption cutoff for nitrothienyl derivatives compared with the analogous nitrophenyl‐substituted TEEs that confirms a lowering of the HOMO‐LUMO gap as a result of nitrothiophene substitution (Figs. 5 and 6). A comparison of the tetrakis‐arylated TEEs 11 , 13 , and 20 clearly showed a steady bathochromic shift of the longest‐wavelength absorption maximum and the end‐absorption upon sequential replacement of nitrophenyl by nitrothienyl groups. Quantum‐chemical computations were performed to explain a number of complex features of the electronic absorption spectra. All empirical features of relevance in the experimental UV/VIS spectra for 2 , 5 , 6 , and 17 – 19 were correctly reproduced by computation (Tables 4 and 5). The combination of theory and experiment was found to be very useful to explain the particular acceptor properties of the 5‐nitro‐2‐thienyl group.     

Synthesis and Metal Complex Formation of Nitrogen-Oxygen Donor Macrocyclic Ligands

Reduction of 2,3:9,10-dibenzo-5,12-diaza-1,8-dioxacylotetradecane-5,11-diene( 1 ) with sodium borohydride in ethanol generates 2,3:9,10-dibenzo-5,12-diaza-1,8-dioxacyclotetradecane( 2 ) in high yield. The macrocyclic ligand 2,3:9,10-dibenzo-5,12-diaza-1,8-dioxacyclotetradecane-N,N′-diacetic acid ( 3 ) has been synthesized and characterized. In addition, reaction of ( 2 ) with an excess of acrylonitrile gives 2,3:9,10-dibenzo-5,12-diaza-1,8-dioxacyclotetradecane-N,N′-di(2-cyanoethyl) ( 4 ). Several new complexes of these ligands with nickel(II), copper(II) and cobalt(II) have been prepared and identified. The structures of these complexes are proposed on the basis of elemental analysis, ¹H, ¹³C-NMR, IR, UV-VIS spectra, magnetic susceptibility and conductivity data.     

Coordinative Behaviour of Tridentate NS2‐Donor Ligands with Cadmium: Synthesis and Crystal Structures of New Complexes

The reaction of cadmium salts with various amounts of the tridentate NS₂‐chelating ligands 1‐(2‐mercapto‐acetophenone)‐4‐triphenylmethylthiosemicarbazone (H₂L¹) and 1‐(5‐mercapto‐3‐methyl‐1‐phenylpyrazole‐4‐carboxaldehyde)‐4‐triphenyl‐methylthiosemicarbazone (H₂L²) in the presence of bases like N‐methylimidazole (N–MeIm), pyridine (py) or triethylamine (Et₃N) provided a series of novel mono‐, di‐, tri‐ and heptanuclear cadmium complexes. They are of the general formulas [CdL¹(N–MeIm)]₂ ( 1 ), [CdL¹(py)]₂ ( 2 ), [CdL²(N–MeIm)]₂ ( 3 ), [CdL²(py)₃] · 0.25 C₆H₁₄ · 0.5 py ( 4 ), [Et₃NH]₂[Cd₃L ] · 7 MeOH ( 5 ), [Et₃NH]₂[Cd₃L ] ( 6 ) and [Et₃NH]₂[Cd₇L ] · 14 MeOH ( 7 ). The compounds were characterized by elemental analysis, IR‐ and ¹H‐NMR‐spectroscopy. Single‐crystal X‐ray structure analyses are reported for the complexes 2 , 4 , 5 and 7 . While 2 has a dimeric structure where each cadmium ion is pentacoordinated in a N₂S₃‐environment, 4 consists of a monomeric cadmium center with distorted octahedral N₄S₂‐coordination. The complexes 5 and 7 exhibit new structural types for tri‐ and heptanuclear cadmium compounds. It is shown that sulfur bridging might proceed via arylthiolates, iminothiolates or even both functions of the ligand. Aggregation is influenced by various factors like solvents, counterions and ligand properties.     

Topotactic Synthesis of Phosphabenzene‐Functionalized Porous Organic Polymers: Efficient Ligands in CO2 Conversion

Progress toward the preparation of porous organic polymers (POPs) with task‐specific functionalities has been exceedingly slow—especially where polymers containing low‐oxidation phosphorus in the structure are concerned. A two‐step topotactic pathway for the preparation of phosphabenzene‐based POPs (Phos‐POPs) under metal‐free conditions is reported, without the use of unstable phosphorus‐based monomers. The synthetic route allows additional functionalities to be introduced into the porous polymer framework with ease. As an example, partially fluorinated Phos‐POPs (F‐Phos‐POPs) were obtained with a surface area of up to 591 m² g^?1. After coordination with Ru species, a Ru/F‐Phos‐POPs catalyst exhibited high catalytic efficiency in the formylation of amines (turnover frequency up to 204 h^?1) using a CO₂/H₂ mixture, in comparison with the non‐fluorinated analogue (43 h^?1) and a Au/TiO₂ heterogeneous catalysts reported previously (<44 h^?1). This work describes a practical method for synthesis of porous organic phosphorus‐based polymers with applications in transition‐metal‐based heterogeneous catalysis.     

A New Class of Tunable Dendritic Diphosphine Ligands: Synthesis and Applications in the Ru‐Catalyzed Asymmetric Hydrogenation of Functionalized Ketones

A series of tunable G₀–G₃ dendritic 2,2′‐bis(diphenylphosphino)‐1,1′‐binaphthyl (BINAP) ligands was prepared by attaching polyaryl ether dendrons onto the four phenyl rings on the P atoms. Their ruthenium complexes were employed in the asymmetric hydrogenation of β‐ketoesters, α‐ketoesters, and α‐ketoamides to reveal the effects of dendron size on the catalytic properties. The second‐ and third‐generation catalysts exhibited excellent enantioselectivities, which are remarkably higher than those obtained from the small molecular catalysts and the first‐generation catalyst. Molecular modeling indicates that the incorporation of bulky dendritic wedges can influence the steric environments around the metal center. In addition, the ruthenium catalyst bearing a second‐generation dendritic ligand could be recycled and reused seven times without any obvious decrease in enantioselectivity.     

Competition Between Thiol and Phosphine Ligands During the Synthesis of Au Nanoclusters

Reduction of a mixture of Ph₃PAuCl and CH₃(CH₂)₅SH with NaBH₄ yields predominately phosphine encapsulated nanoclusters with Au cores <1 nm, similar to the product isolated when the alkane thiol is not present in the reaction. When Et₃N is added to a solution of Ph₃PAuCl and CH₃(CH₂)₅SH, a Au–S bond is formed, and the subsequent reduction of this thiolate results in the formation of >2 nm core thiol encapsulated Au nanoclusters as the majority product. This latter reduction has been examined in more detail through in situ ³¹P NMR experiments, and a solution exchange reaction is observed wherein the PPh₃ generated by the reduction displaces thiol from the surface of the nanocluster product. This thiol displacement occurs with loss of a Au atom from the nanocluster core, as observed by NMR. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.

钐－二氯二茂钛体系促进的碳－硫键的还原断裂－二烷基二硫醚的合成

A mild and rapid method was found for the reductive cleavage of the carbon-sulfur bond in the alkylthiocyanates by samarium/Cp2TiC12 system to give the corresponding dialkyl disulfides in moderate to good yields.     

Donor‐Flexible Nitrogen Ligands for Efficient Iridium‐Catalyzed Water Oxidation Catalysis

A pyridylideneamide ligand with variable donor properties owing to a pronounced zwitterionic and a neutral diene‐type resonance structure was used as a dynamic ligand at a Cp* iridium center to facilitate water oxidation catalysis, a reaction that requires the stabilization of a variety of different iridium oxidation states and that is key for developing an efficient solar fuel device. The ligand imparts high activity (nearly three‐fold increase of turnover frequency compared to benchmark systems), and exceptionally high turnover numbers, which indicate a robust catalytic cycle and little catalyst degradation.     

Synthesis and Complexation Properties of Pyrimidine-Derived Crown Ether Ligands

Five new proton-ionizable macrocyclic ligands containing a pyrimidone-subcyclic unit, 6–10 , were prepared from the previously prepared pyrimidinocrown ethers 1–5 (see Figure 1 and Scheme 1). One of the new proton-ionizable crown ethers is chiral. The proton-ionizable pyrimidonocrown ethers were prepared in high yields by treating the appropriate methoxy-substituted pyrimidinocrown with 5 M sodium hydroxide in a 50% alcohol-water mixture. Complexation properties of four of the pyrimidine-derived macrocycles were studied by various nmr techniques. Pyrimidono-18-crown-6 (9) forms a strong complex with benzylammonium perchlorate and also forms a complex with benzylamine. (S, S)-Dimethyl-substituted pyrimidino- and pyrimidono-18-crown-6 ligands 4 and 9 form stronger complexes with the (R)-form of α-(1-naphthyl)ethylammonium perchlorate than with the (S)-form. (S, S)-Dimethyl-substituted pyrimidono-18-crown-6 ( 9 ) also forms a stronger complex with (R)-α-(1-naphthyl)ethylamine than with the (S)-form. The crystal structure for compound 7 is reported.     

Ylide‐Functionalized Phosphines: Strong Donor Ligands for Homogeneous Catalysis

Phosphines are important ligands in homogenous catalysis and have been crucial for many advances, such as in cross‐coupling, hydrofunctionalization, or hydrogenation reactions. Herein we report the synthesis and application of a novel class of phosphines bearing ylide substituents. These phosphines are easily accessible via different synthetic routes from commercially available starting materials. Owing to the extra donation from the ylide group to the phosphorus center the ligands are unusually electron‐rich and can thus function as strong electron donors. The donor capacity surpasses that of commonly used phosphines and carbenes and can easily be tuned by changing the substitution pattern at the ylidic carbon atom. The huge potential of ylide‐functionalized phosphines in catalysis is demonstrated by their use in gold catalysis. Excellent performance at low catalyst loadings under mild reaction conditions is thus seen in different types of transformations.     

Synthesis of Ethylene‐bridged Heterocyclic Bidentate P(III)N‐Ligands — Oxidation and Complexation Studies

A series of phosphor(III)inanone ligands 4‐7 , linked by ethylene bridges between the nitrogen atoms of the heterocyclic rings, were synthesized by the reaction of the bis‐PCl derivative 3 with the appropriate trimethylsilylamines. The bis‐phosphor(V)inanone compounds 8‐11 were obtained by the oxidation of 4‐7 with hexafluoroacetone (HFA). Oxidation of 4 and 6 with tetrachloro‐orthobenzoquinone (TOB) gave the bis‐phosphor(V)inanones 12 and 13 . The reaction of 4‐6 with [Pt(COD)Cl₂] led to the platinum complexes 14‐16 . All the σ³‐phosphorinanone compounds 4‐7 and the σ⁵‐phosphorinanone compounds 8‐10 , 12 and 13 exist as a mixture of two conformers, as indicated by two signals in the ³¹P‐NMR spectra. However, compounds 9 and 11 exist as single conformers, both display only one sharp singlet in the ³¹P‐NMR spectra. The Pt‐complexes 15 and 16 contain two conformers; one conformer of 16 could be isolated by crystallization. X‐ray crystal structure determinations for compounds 8 , 14 and 16 were conducted, revealing inversion symmetry for 8 and cis arrangement for 14 and 16 .     

Synthesis,Characterization and Crystal Structures of New Difurylphosphido-bridged Dinuclear Ruthenium Carbonyl Complexes Derived from Ferrocenylacetylene Ligands

The difurylphosphido-bridged dinuclear complex [Ru₂(CO)₆(μ-PFu₂)(μ-η¹,η²-Fu)] (Fu = 2-furyl) 1 readily reacts with two equivalents of each of the terminal alkynes HC≡CR (R = Fc, p-C₆H₄Fc, p-C₆H₄NO₂, Fc = Fe(η⁵-C₅H₅)(η⁵-C₅H₄)) by an interesting head-to-tail ynyl coupling with a furan group to form a series of phosphido-bridged diruthenium compounds containing a novel furyl-substituted C₄ hydrocarbyl chain of stoichiometry [Ru₂(CO)₄(μ-PFu₂){μ-η¹,η¹,η²,η³-RCC(H)C(R)C(H)Fu}] (R = Fc 2, p-C₆H₄Fc 3, p-C₆H₄NO₂ 4) in moderate to good yields. Reaction of 1 with an equimolar amount of HC≡CFc and HC≡C(p-C₆H₄NO₂) afforded a pair of isomers of [Ru₂(CO)₄(μ-PFu₂){μ-η¹,η¹,η²,η³-R¹CC(H)C(R²)C(H)Fu}] (R¹ = Fc, R² = p-C₆H₄NO₂ 5a; R¹ = p-C₆H₄NO₂, R² = Fc 5b) together with a small mixture of 4. X-ray crystal structures of 2, 3, 5a and 5b are reported. All of these new alkyne-derived dinuclear complexes are electron precise with 34 cluster valence electrons in which the μ-η¹,η²-furyl ligand acts as a three-electron donor and the μ-phosphido Ru₂ framework is retained in the products upon alkyne coupling reactions. The resulting organic fragment of each complex is coordinated to the Ru atoms via a π, a π-allyl and two σ bonds, and donates seven electrons to the metal core. Dedicated to the memory of Professor F. Albert Cotton.