Bipyridine-type Ligands Research Articles

Tunably strained metallacycles enable modular differentiation of aza-arene C–H bonds

The precise activation of C–H bonds will eventually provide chemists with transformative methods to access complex molecular architectures. Current approaches to selective C–H activation relying on directing groups are effective for the generation of five-membered, six-membered and even larger ring metallacycles but show narrow applicability to generate three- and four-membered rings bearing high ring strain. Furthermore, the identification of distinct small intermediates remains unsolved. Here, we developed a strategy to control the size of strained metallacycles in the rhodium-catalysed C−H activation of aza-arenes and applied this discovery to tunably incorporate the alkynes into their azine and benzene skeletons. By merging the rhodium catalyst with a bipyridine-type ligand, a three-membered metallacycle was obtained in the catalytic cycle, while utilizing an NHC ligand favours the generation of the four-membered metallacycle. The generality of this method was demonstrated with a range of aza-arenes, such as quinoline, benzo[f]quinolone, phenanthridine, 4,7-phenanthroline, 1,7-phenanthroline and acridine. Mechanistic studies revealed the origin of the ligand-controlled regiodivergence in the strained metallacycles.

Rational Design and Assembly of Two-Dimensional Layered Metal–Organic Frameworks: Structure, Morphology, Fluorescence Regulation, and High Iodine Adsorption

Two-dimensional (2D) layered metal–organic frameworks (LMOFs), an emerging type of 2D materials, have aroused significant attention recently. Although a great number of LMOFs with diverse topologies have been reported, their predictable preparation remains challenging. Inspired by the transformation of three-dimensional pillared-layer MOFs (PLMOFs) to 2D layers via solvent-assisted link exchanging, we developed herein a novel de novo synthesis strategy, i.e., in situ capping, for the rational fabrication of 2D LMOFs. During PLMOF construction, the pillars are in situ substituted by terminal ligands to seal both sides of the instantaneously generated layers, forming the targeted 2D LMOFs. Employing such a strategy, 2D LMOFs {[Zn(hsb-2)(benzoate)2]·0.5H2O}n (HSB-W11), [Zn(hsb-2)(2-benzoylbenzoate)2]n (HSB-W12), [Zn(hsb-2)(2-acetylbenzoate)2]n (HSB-W13), and {[Zn(hsb-2)(2-naphthoate)2]·DMF·H2O}n (HSB-W14) were obtained predictably by exchanging the terephthalate pillar (bdc) of HSB-W1 (a 3D PLMOF composed of bdc and 1,2-bis(4′-pyridylmethylamino)-ethane) with the capping ligand benzoate or its analogues. The strategy is also valid for other PLMOF systems with bipyridine-type ligands as pillars, X-pcu-1-Zn, for example. Furthermore, the undersized materials of HSB-W11–W14 have been prepared by the instant in situ exfoliation method. The structure, morphology, and fluorescence were tuned by changing the anchoring terminal ligands. In addition, LMOFs HSB-W11–W14 were utilized as I2 adsorbents; remarkably, HSB-W13 displayed a record-high volatile iodine uptake at room temperature. The in situ capping strategy has many merits including high designability and efficiency, facile green synthesis, and atom economy, which may be widely adapted to fabricate diverse 2D LMOFs.

Dual Photoredox Ni/Benzophenone Catalysis: A Study of the NiII Precatalyst Photoreduction Step.

The combination of NiIIX2 salts with a bipyridine-type ligand and aromatic carbonyl-based chromophores has emerged as a benchmark precatalytic system to efficiently conduct cross-couplings mediated by light. Mechanistic studies have led to two scenarios in which Ni0 is proposed as the catalytic species. Nonetheless, in none of these studies has a NiII to Ni0 photoreduction been evidenced. By exploiting UV-visible, nuclear magnetic resonance, resonance Raman, electron paramagnetic resonance, and dynamic light scattering spectroscopies and also transmission electron microscopy, we report that, when photolyzed by UVA in alcohols, the structurally defined [NiII2(μ-OH2)(dtbbpy)2(BPCO2)4] complex 1 integrating a benzophenone chromophore is reduced into a diamagnetic NiI dimer of the general formula [NiI2(dtbbpy)2(BPCO2)2]. In marked contrast, in THF, photolysis led to the fast formation of Ni0, which accumulates in the form of metallic ultrathin Ni nanosheets characterized by a mean size of ∼100 nm and a surface plasmon resonance at 505 nm. Finally, it is shown that 1 combined with UVA irradiation catalyzes cross-couplings, that is, C(sp3)-H arylation of THF and O-arylation of methanol. These results are discussed in light of the mechanisms proposed for these cross-couplings with a focus on the oxidation state of the catalytic species.

Catalyst Engineering through Heterobidentate (N–X-Type) Ligand Design for Iridium-Catalyzed Borylation

AbstractIridium-catalyzed C–H activation and borylation reactions operate under mild conditions that enable easy and atom-economical installation of the versatile boronate ester group in (het)arenes and alkanes. The standard catalytic system for iridium-catalyzed borylation uses [Ir(cod)(OMe)]2 as a precatalyst, a bipyridine type ligand, and B2pin2 or HBpin as the borylating agent. Initially, a bipyridine-ligated trisboryl–iridium complex is generated that enables the borylation reaction and the regioselectivity is mainly governed by the sterics of substituents present on the ring. As a result, monosubstituted and 1,2-disubstituted arenes give mixtures of isomers. Significant efforts by several research groups have overcome the selectivity issue for directed proximal C–H borylation by introducing a directing group and newly developed ligands. This short review aims to summarize recent elegant discoveries in directed C(sp2)–H and C(sp3)–H borylation by using heterobidentate ligand (P/N–Si, N–B, and N–C) coordinated iridium catalysts.1 Introduction2 Iridium-Catalyzed Directed C–H Borylation of C(sp2)–H Bonds3 Iridium-Catalyzed Directed C–H Borylation of C(sp3)–H Bonds4 Conclusions

Complexation Behavior of Pinene–Bipyridine Ligands towards Lanthanides: The Influence of the Carboxylic Arm

The complexation behavior of two novel, chiral pinene–bipyridine-type ligands ((–)-HL1 and (–)-HL2) containing a carboxylic arm towards lanthanide Ln(III) (Ln = La, Eu, Lu) ions was investigated through spectroscopic methods. The association constants of the mononuclear complexes determined from the UV-Vis titrations indicated that the ligand (–)-HL1 possessing a shorter carboxylic arm formed more stable complexes compared with (–)-HL2, whose carboxylic arm had one more methylene unit. This is due to the formation of more stable seven-member metal chelate rings in the first case as compared with the eight-member metal chelate rings in the second. IR and fluorescence spectroscopy provided additional information about the structure of these complexes.

Hetero-Bis-Conjugation of Bioactive Molecules to Half-Sandwich Ruthenium(II) and Iridium(III) Complexes Provides Synergic Effects in Cancer Cell Cytotoxicity.

Four bipyridine-type ligands variably derivatized with two bioactive groups (taken from ethacrynic acid, flurbiprofen, biotin, and benzylpenicillin) were prepared via sequential esterification steps from commercial 2,2'-bipyridine-4,4'-dicarboxylic acid and subsequently coordinated to ruthenium(II) p-cymene and iridium(III) pentamethylcyclopentadienyl scaffolds. The resulting complexes were isolated as nitrate salts in high yields and fully characterized by analytical and spectroscopic methods. NMR and MS studies in aqueous solution and in cell culture medium highlighted a substantial stability of ligand coordination and a slow release of the bioactive fragments in the latter case. The complexes were assessed for their antiproliferative activity on four cancer cell lines, showing cytotoxicity to the low micromolar level (equipotent with cisplatin). Additional biological experiments revealed a multimodal mechanism of action of the investigated compounds, involving DNA metalation and enzyme inhibition. Synergic effects provided by specific combinations of metal and bioactive fragments were identified, pointing toward an optimal ethacrynic acid/flurbiprofen combination for both Ru(II) and Ir(III) complexes.

Electronic and steric factors for enhanced selective synthesis of 2-ethyl-1-hexanol in the Ir-complex-catalyzed Guerbet reaction of 1-butanol

1-Butanol is a potential bio-based fermentation product obtained from cellulosic biomass. As a value-added chemical, 2-ethyl-1-hexanol (2-EH) can be produced by Guerbet conversion from 1-butanol. This work reports the enhanced catalytic Guerbet reaction of 1-butanol to 2-EH by a series of Cp*Ir complexes (Cp*: 1,2,3,4,5-pentamethylcyclopenta-1,3-diene) coordinated to bipyridine-type ligands bearing an ortho-hydroxypyridine group with an electron-donating group and a Cl− anion. The catalytic activity of the Cp*Ir complex increased by increasing the electron density of the bipyridine ligand when functionalized with the para-NMe2 and ortho-hydroxypyridine groups. A record turnover number of 14047 was attained. A mechanistic study indicated that the steric effect of the ethyl group on the α-C of 2-ethylhexanal (2-EHA) and the conjugation effect of C=C–C=O in 2-ethylhex-2-enal (2-EEA) benefits the high selectivity of 2-EH from 1-butanol by inhibiting the cross-aldol reaction of 2-EHA and 2-EEA with butyraldehyde. Nuclear magnetic resonance study revealed the formation of a carbonyl group in the bipyridine-type ligand via the reaction of the Cp*Ir complex with KOH.

A 1D palladium coordination polymer and its catalytic activity in microwave-assisted Sonogashira reactions

A palladium coordination polymer, [Pd(dppp)(L)](OTf)2·(H2O)] (1) (dppp = 1,3-bis(diphenylphosphino)propane), was prepared from a bipyridine-type ligand (L = (4-py)–CH = N–C10H6–N = CH–(4-py)) and [Pd(dppp)](OTf)2. An X-ray diffraction study showed that it has a one-dimensional zigzag chain structure and the triflate ions act as counterions. Compound 1 exhibited Sonogashira coupling reactivity in air under microwave conditions.

Structure-dependent regioselectivity of a roll-over cyclopalladation occuring at 2,2′-bipyridine-type ligands

In this work, different bipyridine-analogue ligands bearing a dimethylamino group in the meta-position of one of the heterocyclic rings were synthesized and reacted with palladium(II) acetate under identical conditions. Cyclometallated palladium(II) complexes with C,N- or C,N,N’-coordinating chelate ligands are formed which were characterized by elemental analysis, 1H and 13C NMR spectroscopy, and single crystal X-ray diffraction analysis. In the case of the mononuclear, C,N,N’-coordinated complex, which is formed by an attack of the palladium(II) site at of the N-methyl groups, the primarily coordinating acetato ligand is exchanged against a chlorido ligand, which is liberated from the solvent dichloromethane by a nucleophilic substitution reaction. In contrast, cyclometallation occurring at one of the six-membered heterocycles leads to dinuclear acetato-bridged palladium(II) complexes.

Urea-accelerated Iridium-catalyzed 2-Position-selective C–H Borylation of Indole Derivatives

Abstract We developed iridium-catalyzed 2-position-selective C–H borylation of indole derivatives by modifying the structure of a bipyridine-type ligand. The yields ranged from good to excellent, even on a gram scale, and were dramatically improved by the addition of a urea derivative catalyst. The introduced boryl group was converted to chloro, bromo, and phenyl groups without isolation of the borylated intermediate. Preliminary results indicated the importance of hydrogen bonding between the substrate and the urea additive.

Bis-conjugation of Bioactive Molecules to Cisplatin-like Complexes through (2,2'-Bipyridine)-4,4'-Dicarboxylic Acid with Optimal Cytotoxicity Profile Provided by the Combination Ethacrynic Acid/Flurbiprofen.

A facile route to PtII complexes doubly functionalized with bioactive molecules through a bipyridine-type ligand is described. Initially, ligands LEE (containing two ethacrynic acid units), LEF (ethacrynic acid+flurbiprofen) and LEB (ethacrynic acid+biotin) were obtained in moderate to good yields from 2,2'-bipyridine-4,4'-dicarboxylic acid. Subsequent reaction of the ligands with [PtCl2 (DMSO)2 ] afforded complexes [PtCl2 (LEE )] (2), [PtCl2 (LEF )] (3) and [PtCl2 (LEB )] (4) in high yields. All compounds were fully characterized by analytical and spectroscopic methods. Complexes 2-4 are highly stable in water/DMSO solution at 37 °C after 72 h, whereas progressive release of the bioactive fragments was detected in a cell culture medium. The compounds were assessed for their in vitro antiproliferative activity towards tumorigenic A2780, A2780cisR and Y79 cells and non-tumourigenic HEK293 cells. In particular, the combination of ethacrynic acid and flurbiprofen in 3 overcomes cisplatin-based resistance and provides strong cancer cell selectivity. Enzyme inhibition assays on human GST P1 and human COX-2 and cross-experiments with complex 1, analogous to 2-4 but lacking bio-groups, revealed a clear synergy between the PtII frame and the bioactive organic components.

Copper(II) and cobalt(II) complexes based on methyl trifluorosalicylate and bipyridine-type ligands: Synthesis and their antimicrobial activity

The synthesis of bis(salicylato)Cu(II) and Co(II) chelates based on methyl trifluorosalicylate and the corresponding M(OAc)2 in the presence of alkali is reported. The following reaction with 2,2′-bipyridine and 1,10-phenanthroline in refluxing methanol affords heteroligand complexes containing methyl trifluorosalicylate and azaheterocyclic ligands in the ratio 2:1. However, the refluxing of Co(II) chelates in ethanol resulted in transesterification of methyl ester group to form heteroligand complexes of ethyl trifluorosalicylate while Cu(II) chelates underwent hydrolysis of ester group to obtain heteroligand complexes of trifluorosalicylic acid. The structures of obtained metal complexes were confirmed by X-Ray diffraction experiment, IR spectroscopy and elemental analysis. The antimycotic and antibacterial activities of synthesized metal complexes were investigated. The heteroligand metal complexes were found to exhibit moderate anti-Candida albicans activity. Among new series of complexes, the most activity showed the mixed metal complexes of trifluorosalicylates and 1,10-phenanthroline revealing a wide range of action in relation to pathogenic fungi and bacteria.

Copper(II) and Cobalt(II) Complexes Based on Methyl Trifluorosalicylate and Bipyridine-Type Ligands: Synthesis and Their Antimicrobial Activity

The synthesis of bis(salicylato)Cu(II) and Co(II) chelates based on methyl trifluorosalicylate and the corresponding M(OAc) 2 in the presence of alkali is reported. The following reaction with 2,2’-bipyridine and 1,10 phenanthroline in refluxing methanol affords heteroligand complexes containing methyl trifluorosalicylate and azaheterocyclic ligands in the ratio 2:1. However, the refluxing of Co(II) chelates in ethanol resulted in transesterification of methyl ester group to form heteroligand complexes of ethyl trifluorosalicylate while Cu(II) chelates underwent hydrolysis of ester group to obtain heteroligand complexes of trifluorosalicylic acid. The structures of obtained metal complexes were confirmed by X-Ray diffraction experiment, IR spectroscopy and elemental analysis. The antimycotic and antibacterial activities of synthesized metal complexes were investigated. The heteroligand metal complexes were found to exhibit moderate anti-Candida albicans activity. Among new series of complexes, the most activity showed the mixed metal complexes of trifluorosalicylates and 1,10-phenanthroline revealing a wide range of action in relation to pathogenic fungi and bacteria.

Strongly visible light-absorbing metal-organic frameworks functionalized by cyclometalated ruthenium(ii) complexes.

Four different ruthenium(ii) complexes were incorporated into the metal–organic framework (MOF) UiO-67 using three different synthetic strategies: premade linker synthesis, postsynthetic functionalization, and postsynthetic linker exchange. One of these complexes was of the type (N–N)3Ru2+, and three of the complexes were of the type (N–N)2(N–C)Ru+, where N–N is a bipyridine-type ligand and N–C is a cyclometalated phenylpyridine-type ligand. The resulting materials were characterized by PXRD, SC-XRD (the postsynthetic functionalization MOFs), N2 sorption, TGA-DSC, SEM, EDS, and UV-Vis spectroscopy, and were digested in base for subsequent 1H NMR analysis. The absorption profiles of the MOFs that were functionalized with cyclometalated Ru(ii) complexes extend significantly further into the visible region of the spectrum compared to the absorption profiles of the MOFs that were functionalized with the non-cyclometalated reference, (N–N)3Ru2+.

Novel Iron(II) and Copper(II) Polymeric Coordination Compounds with N,N′-bipyridine-type Ligands: Synthesis and Characterization

Novel Iron(II) and Copper(II) Polymeric Coordination Compounds with N,N′-bipyridine-type Ligands: Synthesis and Characterization

Hydrogen Bond-Accelerated meta-Selective C–H Borylation of Aromatic Compounds and Expression of Functional Group and Substrate Specificities

We found that meta-selective C–H borylation of aromatic compounds was accelerated when using urea moiety-containing bipyridine-type ligands unlike in cases involving a bipyridine-type ligand without the urea moiety. The acceleration was due to the recognition and capture of the aromatic substrates by the urea moiety of the ligand by hydrogen bonding. The acceleration was further enhanced by modifying the electronic and steric properties of the ligand. The functional group and substrate specificities were also observed using the urea moiety-containing ligands.

Novel mixed-metal cubane-type {Mo3NiS4} and {Mo3PdS4} clusters coordinated with 2,2′-bipyridine type ligands

New cuboidal cluster complexes of general formula [Mo3S4(M′X)Cl3L3]0/+ (M′ = Ni0, Pd0; L = dnbpy – 4,4′-dinonyl-2,2′-bipyridine or dbbpy – 4,4′-di-tert-butyl-2,2′-bipyridine; X = tu, Cl) were synthesized by reactions of trinuclear precursors [Mo3S4Cl3L3]+ with Ni(COD)2 (COD = 1,5-cyclooctadiene) or Pd2(dba)3·CHCl3 (dba = dibenzylideneacetone) in the presence of thiourea (tu) in good yields. The complexes were characterized by elemental analysis, IR, 1H NMR and ESI-MS techniques. Crystal structures of [Mo3S4(Nitu)Cl3(dbbpy)3]Cl·tu (1a), [Mo3S4(Nitu)Cl3(dbbpy)3]Cl (1b) and [Mo3S4(Pdtu)Cl3(dbbpy)3]Cl (2) were determined by X-ray analysis. The DFT calculations of the electronic structure of [Mo3S4(Pd′X)Cl3(bpy)3]0/+ (X = Cl or tu) were carried out.

Iridium/Bipyridine-Catalyzed ortho-Selective C-H Borylation of Phenol and Aniline Derivatives.

An iridium-catalyzed ortho-selective C-H borylation of phenol and aniline derivatives has been successfully developed. Iridium/bipyridine-catalyzed C-H borylation generally occurred at the meta- and para-positions of aromatic substrates. Introduction of an electron-withdrawing substituent on the bipyridine-type ligand and a methylthiomethyl group on the hydroxy and amino groups of the phenol and aniline substrates, however, dramatically altered the regioselectivity, affording exclusively ortho-borylated products. The reaction proceeded in good to excellent yields with good functional group tolerance. C-H borylation was applied to the synthesis of a calcium receptor modulator.

Photoinduced solid-state coloring behavior of boronium complexes

Boronium complexes bearing a 9-borabicyclononane framework with a bipyridine-type ligand display photoinduced solid-state coloring behavior. While the identity of the substituents on the boron atom is critical to gain photoresponsive capability, modifying the nitrogen-containing ligand structure and its substituents provides a wide variation in the photoinduced solid color.

Synthesis of metalloporphyrin-based conjugated microporous polymer spheres directed by bipyridine-type ligands.

Zinc porphyrin (TP-Zn)-based conjugated microporous polymer (Zn-CMP) spheres were obtained via Sonagashira-Hagihara cross coupling reactions between 5,10,15,20-tetrakis(4-ethynylphenyl)porphyrin-Zn(II) and brominated monomers directed by bidentate bipyridine (BP)-type ligands for the first time, and the sphere diameters could be adjusted from 320 to 740 nm. The coordination between BP and TP-Zn was proved to be the key to forming spheres.