C2-symmetric Ligands Research Articles

Synthesis of Nanometer-Sized Cylinder-Like Structures from a 1,3,5-Triphenylbenzene-Bridged Tris-NHC Ligand and AgI, AuI, and CuI

The metal-controlled self-assembly of nanometer-sized trinuclear cylinder-like assemblies is described. The 1,3,5-triphenylbenzene-derived trisimidazolium salt [1](PF6)3 reacts with 1.5 equiv of Ag2O to yield the trinuclear AgI hexacarbene complex [2](PF6)3, where three AgI ions are sandwiched in between two C3-symmetric tris-NHC ligands. Transmetalation of [2](PF6)3 with 3 equiv of [AuCl(SMe2)] or CuI yields the trinuclear AuI hexacarbene complex [3](PF6)3 or the trinuclear CuI hexacarbene complex [4](PF6)3, respectively, with retention of the metallosupramolecular structure. The 1,3,5-triphenylbenzene ligand in the gold complex adopts a nonplanar geometry to allow π interactions between the central phenyl rings.

Formation of an Oxidant-Sensible Pd(II) Coordination Compound and Its 1H NMR Specific Characterization: A Preparative and Analytical Challenge in Current Coordination Chemistry

A three-part experiment that leads to the synthesis of palladium(II) complex starting from a C3-symmetric triaminoguanidinium-based ligand is presented. In the first part, the preparation of tris-benzylidenetriaminoguanidinium chloride ([H6Br3L]Cl) by an acidic catalyzed 3-fold imine formation reaction of 5-bromo-2-hydroxybenzaldehyde and triaminoguanidinium chloride is described. Starting from the second part, the reaction procedures are performed under inert gas atmosphere. The conversion of PdCl2 with acetonitrile to give [Pd(MeCN)4]Cl2 as a precursor is performed at 80 °C. The third part of the experiment is a three-step procedure that begins with deprotonation of [H6Br3L]Cl, followed by transfer of [Pd(MeCN)4]Cl2 to the reaction flask, where the chelation of Pd(II)-precursor leads to the in situ species [Pd(MeCN)H3Br3L]. Addition of PPh3 as the final reaction step yields [Pd(PPh3)H3Br3L]. This sequence of experiments provides an excellent example of ligand and transition metal preparation and its subsequent complexation to form an oxidant-sensitive organometallic species. Undergraduate students get the opportunity to learn Schlenk techniques and to investigate 1H NMR spectra of organic and organometallic compounds starting from relatively simple to more advanced spectra. The distinct result of the chemical shift of the 4JP–H-affected doublet mirrors the preparative success of the student and demonstrates the utility of 1H NMR experiments.

Construction of a Highly Distorted Benzene Ring in a Double Helicene

A P-fused double helicene consisting of a highly distorted benzene ring, with a bending angle of 23°, has been synthesized by a tandem intramolecular phospha-Friedel-Crafts reaction. Despite the distortion and reduced aromaticity, the double helicene shows thermal and chemical stability. These are important features that make these compounds attractive for applications as a new C2-symmetric bisphosphine ligand. The simple strategy proposed in this work can be used to prepare a diverse range of distorted molecules.

Synthesis of two triarylboron-functionalized metal-organic frameworks: in situ decarboxylic reaction, structure, photoluminescence, and gas adsorption properties.

Two 3D noninterpenetrating porous metal-organic frameworks (PMOFs) [Cd3(L(1))2(DMA)2]·DMA [1, H3L(1) = tris(p-carboxylic acid)tridurylborane] and [Zn3(L(2))3(H2O)2]·5H2O·2EtOH [2, H2L(2) = 4,4'-((2,3,5,6-tetramethylphenyl)boranediyl)bis(2,3,5,6-tetramethylbenzoic acid)] were synthesized by employment of a C3-symmetric ligand (H3L(1)) to assembly with Cd(NO3)2 or Zn(NO3)2. Complex 1 exhibits a (3, 6)-connected topological network based on a Cd3 cluster and Y-shaped trinodal organic linker. Complex 2 shows a 6-connected topology, since in situ decarboxylic reaction of the initial H3L(1) occurred to generate a new ligand, H2L(2), which can be considered as a linear linker. Both 1 and 2 exhibit blue fluorescence. Significantly, complex 1 with larger channels is unstable upon the removal of guest molecules. In contrast, activated 2 exhibits higher stability and permanent porosity.

Chiral Palladium(II) and Nickel(II) Complexes with C2-Symmetrical Tridentate Bis(oxazoline) Ligands: Synthesis, Characterization, and Catalytic Norbornene Polymerization

A series of chiral palladium(II) and nickel(II) complexes bearing a C2-symmetric monoanionic tridentate bis(oxazoline) ligand, (R2-(S,S)-BOZ)M(X) (1–6: R = CH(CH3)2, M = Pd, X = OAc (1); R = CH(CH3)2, M = Pd, X = Cl (2); R = Ph, M = Pd, X = Cl (3); R = Ph, M = Ni, X = Cl (4); R = CH(CH3)2, M = Ni, X = Cl (5); R = CH(CH3)2, M = Pd, X = OTf (6)), have been synthesized and structurally characterized. The experimental results demonstrate that such chiral palladium(II) and nickel(II) complexes bearing C2-symmetric tridentate ligands in which the monoanionic group is located inside are effective for norbornene polymerization. In the presence of various cocatalysts such as MAO, MMAO, and activator/AlR3, these chiral palladium(II) complexes exhibit much higher activities of up to 4.8 × 108 g of PNB (mol of Pd)−1 h–1 for the vinylic polymerization of norbornene, affording insoluble polynorbornenes with high packing density. In contrast, the chiral nickel(II) complexes show relatively low activities of ca. 4.5 × 107 g of PNB (mol of Ni)−1 h–1 and produce both insoluble polynorbornenes and soluble high-molecular-weight polynorbornenes with moderate molecular weight distributions.

ChemInform Abstract: The Synthesis of Chiral Amino Diol Tridentate Ligands and Their Enantioselective Induction During the Addition of Diethylzinc to Aldehydes.

Abstract A series of C2-symmetric chiral amino diol tridentate ligands 3a–g were prepared from achiral bulky organolithiums, achiral bulky primary amines, and optically active epichlorohydrin (ECH). The prepared C2-symmetric chiral amino diol tridentate ligands were capable of inducing enantioselectivity in the model reaction of aromatic and aliphatic aldehydes with diethylzinc with an ee of up to 96%. The enantioselectivity can be modulated by adjusting the steric hindrance of the achiral reagents employed in the synthesis of the chiral ligand. The configuration of the addition product depended on the configuration of the amino diol ligands, which can be simply controlled as desired by using the ECH with the desired configuration during the preparation of the ligand.

Facile and efficient synthesis of C2-symmetrical 1,3,5-triazine polycarboxylate ligands under microwave irradiation

A rapid and efficient method for preparation of C2-symmetrical 1,3,5-triazine polycarboxylate ligands was developed. The reactions included either selective mono- or di-substitution of 2,4,6-trichloro-1,3,5-triazine with various nucleophiles containing carboxyl group(s), followed by nucleophilic displacement of the remained chloride(s) in aqueous media under microwave irradiation. Novel C2-symmetrical tripodal ligands were afforded in good yields and purities under short reaction time with simple work-up, which are potentially useful as structural directing units in metal-organic frameworks.

Design of C2-symmetric salen ligands and their Co(II)- or Yb(III)-complexes, and their role in the reversal of enantioselectivity in the asymmetric Henry reaction

Reversal of enantioselectivity in asymmetric Henry reactions was achieved with novel chiral C2-symmetric salen ligands bearing morpholine moieties by changing the Lewis acid center from Co(II) to Yb(III). The possible transition state models were supported by mass spectrometry experiments.

ChemInform Abstract: The Design and Synthesis of Planar Chiral Ligands and Their Application to Asymmetric Catalysis

AbstractReview: [49 refs.

Synthesis and Characterization of Iron Trisphenolate Complexes with Hydrogen-Bonding Cavities

A new family of C3-symmetric ligands, featuring phenolate donors and a secondary coordination sphere, have been synthesized. We report the synthesis and subsequent coordination chemistry of these new tripodal N-anchored tris(phenolate) chelates, [tris(5-tert-butyl-3-N-carboxamide-2-hydroxybenzyl)amines] (H3(R)SalAmi), to iron(II), iron(III), and zinc(II). These electron-rich complexes have intramolecular hydrogen bonds, and therefore the potential to stabilize biologically relevant substrates in small-molecule activation chemistry.

Secondary Building Unit (SBU) Controlled Formation of a Catalytically Active Metal–Organic Polyhedron (MOP) Derived from a Flexible Tripodal Ligand

A nanosized truncated octahedron-shaped metal–organic-polyhedron (MOP) namely [{Cu12(TMBTA)8(DMA)4(H2O)8}·8H2O·X] MOP-TO (X = 48 H2O molecules as per SQUEEZE calculation and TGA data) was successfully derived from a flexible C3-symmetric ligand (2,4,6-trimethylbenzene)-1,3,5-triacetic acid (TMBTA) and Cu(NO3)2 in dimethyl acetamide (DMA) and EtOH under solvothermal conditions. The nanocage was well-characterized by single crystal X-ray diffraction (SXRD) and electron spray ionization mass spectrometry (ESI-MS). Remarkably, the nanocage molecules could be seen under high-resolution transmission electron microscope (HR-TEM). It was evident that the Cu(II) paddle wheel secondary building unit (CPWSBU) was responsible for the formation of the nanocage, as the corresponding reactions of TMBTA with other metal ions (e.g., Co(II) and Zn(II) resulted in the formation of two coordination polymers, namely [{Co(μ-TMBTA)(H2O)4}]∝ TMBTA-Co(II) and [{(H2O)2Zn(μ-TMBTA)Zn·K}·2H2O]∝ TMBTA-Zn(II). Interestingly, the nanocage MOP-TO was exploited in catalyzing (2,2,6,6-tetramethylpiperidin-1-yl)oxy (TEMPO)-assisted aerobic oxidation of benzyl alcohol to benzaldehyde.

The synthesis of chiral amino diol tridentate ligands and their enantioselective induction during the addition of diethylzinc to aldehydes

A series of C2-symmetric chiral amino diol tridentate ligands 3a–g were prepared from achiral bulky organolithiums, achiral bulky primary amines, and optically active epichlorohydrin (ECH). The prepared C2-symmetric chiral amino diol tridentate ligands were capable of inducing enantioselectivity in the model reaction of aromatic and aliphatic aldehydes with diethylzinc with an ee of up to 96%. The enantioselectivity can be modulated by adjusting the steric hindrance of the achiral reagents employed in the synthesis of the chiral ligand. The configuration of the addition product depended on the configuration of the amino diol ligands, which can be simply controlled as desired by using the ECH with the desired configuration during the preparation of the ligand.

The Design and Synthesis of Planar Chiral Ligands and Their Application to Asymmetric Catalysis

Ferrocene- and ruthenocene-based planar chiral ligands have been developed and used in transition-metal-catalyzed asymmetric allylic substitutions, asymmetric hydrogenations, and asymmetric conjugate addition reactions. The most common ferrocene ligands, which have bis(oxazoline) substituents on the cyclopentadiene rings, have been modified to create new planar chiral C 2-symmetric ligands, some of which have shown excellent potential in the aforementioned reactions. A series of planar chiral ruthenocene ligands have also been developed and their activities differ from those of their ferrocene counterparts. 1 Introduction 2 The Design and Synthesis of Planar Chiral Ligands 2.1 Ferrocene-Based Ligands 2.2 Ruthenocene-Based Ligands 3 Applications of Planar Chiral Ligands in Asymmetric Reactions 3.1 Asymmetric Allylic Alkylation 3.2 Asymmetric Allylic Amination 3.3 Asymmetric Allylic Alkylation with Enamines as Nucleophiles 3.4 Asymmetric Alkylation of Aryl Aldehydes 3.5 Asymmetric Hydrogenation 3.6 1,4-Asymmetric Conjugation Reactions 4 Summary

Complexes possessing rare "tertiary" sulfonamide nitrogen-to-metal bonds of normal length: fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes with hydrophilic sulfonamide ligands.

Tertiary sulfonamide nitrogen-to-metal bonds of normal length are very rare. We recently discovered such a bond in one class of fac-[Re(CO)3(N(SO2R)(CH2Z)2)](n) complexes (Z = 2-pyridyl) with N(SO2R)dpa ligands derived from di-(2-picolyl)amine (N(H)dpa). fac-[M(CO)3(N(SO2R)(CH2Z)2)](n) agents (M = (186/188)Re, (99m)Tc) could find use as radiopharmaceutical bioconjugates when R is a targeting moiety. However, the planar, electron-withdrawing 2-pyridyl groups of N(SO2R)dpa destabilize the ligand to base and create relatively rigid chelate rings, raising the possibility that the rare M-N(sulfonamide) bond is an artifact of a restricted geometry. Also, the hydrophobic 2-pyridyl groups could cause undesirable accumulation in the liver, limiting future use in radiopharmaceuticals. Our goal is to identify a robust, hydrophilic, and flexible N(CH2Z)2 chelate framework. New C2-symmetric ligands, N(SO2R)(CH2Z)2 with (Z = CH2NH2; R = Me, dmb, or tol), were prepared by treating N(H)dien(Boc)2, a protected diethylenetriamine (N(H)dien) derivative, with methanesulfonyl chloride (MeSO2Cl), 3,5-dimethylbenzenesulfonyl chloride (dmbSO2Cl), and 4-methylbenzenesulfonyl chloride (tolSO2Cl). Treatment of fac-[Re(CO)3(H2O)3](+) with these ligands, designated as N(SO2R)dien, afforded new fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes. Comparing the fac-[Re(CO)3(N(SO2Me)dien)]PF6 and fac-[Re(CO)3(N(SO2Me)dpa)]PF6 complexes, we find that the Re(I)-N(sulfonamide) bonds are normal in length and statistically identical and that the methyl (13)C NMR signal has an unusually upfield shift compared to that in the free ligand. We attribute this unusual upfield shift to the fact that the sulfonamide N undergoes an sp(2)-to-sp(3) rehybridization upon coordination to Re(I) in both complexes. Thus, the sulfonamide N of N(SO2R)dien ligands is a good donor, even though the chelate rings are conformationally flexible. Addition of the strongly basic and potentially monodentate ligand, 4-dimethylaminopyridine, did not affect the fac-[Re(CO)3(N(SO2tol)dien)]PF6 complex, even after several weeks. This complex is also stable to heat in aqueous solution. These results indicate that N(SO2R)dien ligands form fac-[Re(CO)3(N(SO2R)dien)]PF6 complexes sufficiently robust to be utilized for radiopharmaceutical development.

Coordinating ligand functionalized AgNPs for colorimetric sensing: effect of subtle structural and conformational change of ligand on the selectivity

Metal coordinating C2-symmetric phenolic chelating ligands (1–3) were used to prepare surface functionalized AgNPs for selective colorimetric sensing of metal cations and anions.

A streamlined synthesis of extended thiophloroglucinol ligands and their trinuclear NiII3 complexes

A protocol for the synthesis of trinucleating C3-symmetric ligands based on a central meta-phenylene bridging 1,3,5-trimercaptobenzene (thiophloroglucinol) backbone has been established. The key compound turned out to be the trialdehyde obtained from the triple nucleophilic attack of dimethyldithiocarbamate at 1,3,5-tribromo-2,4,6-triformylbenzene. Reacting this trialdehyde with six equivalents of a primary amine results in the simultaneous dithiocarbamate cleavage and Schiff-base formation providing the extended thiophloroglucinol ligands H3bertdien, H6bert(Me), H6bert(t-Bu2), and H6habbi. Reaction with Ni(II) leads to the formation of the trinuclear Ni(II)3 complexes [(bertdien)Ni(II)3](X)3 (X = BPh4(-), BF4(-)), [(bert(Me))Ni(II)3], [(bert(t-Bu2))Ni(II)3], and [(habbi)Ni(II)3], which are characterized spectroscopically, electrochemically, and crystallographically.

Ligand-Based Carbon–Nitrogen Bond Forming Reactions of Metal Dinitrosyl Complexes with Alkenes and Their Application to C–H Bond Functionalization

Over the past few decades, researchers have made substantial progress in the development of transition metal complexes that activate and functionalize C-H bonds. For the most part, chemists have focused on aliphatic and aromatic C-H bonds and have put less effort into complexes that activate and functionalize vinylic C-H bonds. Our groups have recently developed a novel method to functionalize vinylic C-H bonds that takes advantage of the unique ligand-based reactivity of a rare class of metal dinitrosyl complexes. In this Account, we compare and discuss the chemistry of cobalt and ruthenium dinitrosyl complexes, emphasizing alkene binding, C-H functionalization, and catalysis. Initially discovered in the early 1970s by Brunner and studied more extensively in the 1980s by the Bergman group, the cyclopentadienylcobalt dinitrosyl complex CpCo(NO)2 reacts reversibly with alkenes to give, in many cases, stable and isolable cobalt dinitrosoalkane complexes. More recently, we found that treatment with strong bases, such as lithium hexamethyldisilazide, Verkade's base, and phosphazene bases, deprotonates these complexes and renders them nucleophilic at the carbon α to the nitroso group. This conjugate anion of metal dinitrosoalkanes can participate in conjugate addition to Michael acceptors to form new carbon-carbon bonds. These functionalized cobalt complexes can further react through alkene exchange to furnish the overall vinylic C-H functionalized organic product. This stepwise sequence of alkene binding, functionalization, and retrocycloaddition represents an overall vinylic C-H functionalization reaction of simple alkenes and does not require directing groups. We have also developed an asymmetric variant of this reaction sequence and have used this method to synthesize C1- and C2-symmetric diene ligands with high enantioinduction. Building upon these stepwise reactions, we eventually developed a simple one-pot procedure that uses stoichiometric amounts of a cobalt dinitrosoalkane complex for both inter- and intramolecular C-H functionalization. We can achieve catalysis in one-pot intramolecular reactions with a limited range of substrates. Our groups have also reported an analogous ruthenium dinitrosyl complex. In analogy to the cobalt complex, this ruthenium complex reacts with alkenes in the presence of neutral bidentate ligands, such as TMEDA, to give octahedral dinitrosoalkane complexes. Intramolecular functionalization or cyclization of numerous ruthenium dinitrosoalkane complexes proceeds under mild reaction conditions to give the functionalized organic products in excellent yields. However, despite extensive efforts, so far we have not been able to carry out intermolecular reactions of these complexes with a variety of electrophiles or C-H functionalization reactions. Although additional work is necessary to further boost the catalytic capabilities of both cobalt and ruthenium dinitrosyl complexes for vinylic C-H functionalization of simple alkenes, we believe this ligand-based vinylic C-H functionalization reaction has provided chemists with a useful set of tools for organic synthesis.

Nonsimple relationships between the P∗-chiral diamidophosphite and the arylphosphine moieties in Pd-catalyzed asymmetric reactions: combinatorial approach and P,P∗-bidentate phosphine-diamidophosphites

A small family of P,P∗-bidentate C1-symmetric ligands containing 1,3,2-diazaphospholidine rings with stereogenic phosphorus atoms has been prepared. Palladium catalytic systems with these phosphine-diamidophosphites afforded 95% and 63% ees in asymmetric allylic substitution and desymmetrization processes, respectively. The influence of the nature of both the phosphine and diamidophosphite moieties of these compounds on the enantioselectivity is discussed. The ‘mixed-ligand approach’ in Pd-catalyzed asymmetric allylation with participation of some new P∗-monodentate diamidophosphites and PPh3 is also considered.

Palladium-Catalyzed Efficient Enantioselective Synthesis of Chiral Allenes: Steric and Electronic Effects of Ligands

Asymmetric synthesis of chiral allenes starting from prochiral substrates under mild reaction conditions promoted by Pd-SYNPHOS catalyst is reported. This protocol provides an efficient access to various enantioenriched aryl- and alkyl- substituted allenes, which are versatile building blocks of high utility to both organic and medicinal chemists, in excellent isolated yields (up to 96%) and high enatiomeric ratio values (up to 95:5). In addition, a comparative study using several C2-symmetric atropisomeric diphosphine ligands revealed the overwhelming impact of the steric and electronic properties of the ligands for the catalytic efficiency of this process.

Studies on Metal–Organic Frameworks of Cu(II) with Isophthalate Linkers for Hydrogen Storage

Hydrogen (H2) is a promising alternative energy carrier because of its environmental benefits, high energy density, and abundance. However, development of a practical storage system to enable the "Hydrogen Economy" remains a huge challenge. Metal-organic frameworks (MOFs) are an important class of crystalline coordination polymers constructed by bridging metal centers with organic linkers. MOFs show promise for H2 storage owing to their high surface area and tuneable properties. In this Account, we summarize our research on novel porous materials with enhanced H2 storage properties and describe frameworks derived from 3,5-substituted dicarboxylates (isophthalates) that serve as versatile molecular building blocks for the construction of a range of interesting coordination polymers with Cu(II) ions. We synthesized a series of materials by connecting linear tetracarboxylate linkers to {Cu(II)2} paddlewheel moieties. These materials exhibit high structural stability and permanent porosity. Varying the organic linker modulates the pore size, geometry, and functionality to control the overall H2 adsorption. Our top-performing material in this series has a H2 storage capacity of 77.8 mg g(-1) at 77 K, 60 bar. H2 adsorption at low, medium, and high pressures correlates with the isosteric heat of adsorption, surface area, and pore volume, respectively. Another series, using tribranched C3-symmetric hexacarboxylate ligands with Cu(II), gives highly porous (3,24)-connected frameworks incorporating {Cu(II)2} paddlewheels. Increasing the length of the hexacarboxylate struts directly tunes the porosity of the resultant material from micro- to mesoporosity. These materials show exceptionally high H2 uptakes owing to their high surface area and pore volume. The first member of this family reported adsorbs 111 mg g(-1) of H2, or 55.9 g L(-1), at 77 K, 77 bar, while at 77 K, 1 bar, the material adsorbs 2.3 wt % H2. We and others have since achieved enhanced H2 adsorption in these frameworks using combinations of polyphenyl groups linked by alkynes. The maximum storage achieved for one of the enhanced materials is 164 mg g(-1) at 77 K, 70 bar, but because of its low density, its volumetric capacity is only 45.7 g L(-1). We attribute the significant adsorption of H2 at low pressures to the arrangement of the {Cu24(isophthalate)24} cuboctahedral cages within the polyhedral structure. Free metal coordination positions are the first binding sites for D2, and these frameworks have two types of Cu(II) centers, one with its vacant site pointing into the cuboctahedral cage and another pointing externally. D2 molecules bind first at the former position and then at the external open metal sites. Design of ligands and complexes is key for enhancing and maximizing H2 storage, and although current materials operate at 77 K, research continues to explore routes to high capacity H2 storage materials that can function at higher temperatures.