Intramolecular five-membered ring compounds and their applications

Functional group-specific reduction of Cr(VI) by low molecular weight organic acids in frozen solution: Kinetics, mechanism and DFT calculation

Low molecular weight organic acids (LMWOA) are commonly present in natural water and play a pivotal role in the reduction of Cr(VI). In frozen solutions, the efficiency of Cr(VI) reduction is significantly enhanced due to the freezing concentration effect. However, this facilitation is found to be contingent upon the functional groups of LMWOA in this study. To be specific, LMWOA and Cr(VI) can form five-membered ring complexes, which greatly enhance electron transfer efficiency through Ligand-to-Metal Charge Transfer (LMCT). DFT calculations indicate that oxygen-containing groups located on carbon atoms at α positions play a crucial role in forming these complexes, ultimately determining the kinetics of Cr(VI) reduction. Moreover, freezing not only increases proton concentrations but also reduces free water molecule content in the liquid-like layer (LLL), thereby affecting LMWOA species through regulation of protonation and hydrolysis, and subsequently impacting reaction mechanisms. The stoichiometric ratios between LMWOA and Cr(VI) exceed theoretical values due to complexation with Cr(III). The reduction of Cr(VI) by LMWOA in frozen solutions is inhibited by soil solution, while the degree of inhibition varies among different types of LMWOA.

Reactivity of Schiff base-[C,N,S] pincer palladacycles: hydrolysis renders singular trinuclear, tetranuclear, and heteropentanuclear Pd3W2 coordinated complexes

Treatment of the Schiff base ligands a-f with Li2[PdCl4]/NaAcO in methanol under reflux gave the single nuclear palladacycles 1a-1f, with the metal atom bonded to a terdentate monoanionic [C,N,S] iminic ligand and to a chloride ligand that completes the palladium coordination sphere. Reaction of 1a-1c with silver perchlorate/triphenylphosphine in acetone at room temperature yielded the single nuclear complexes 2a-2c as the perchlorate salts, after substitution of the chloride ligand by a triphenylphosphine. However, reaction of a-c with Na2[PdCl4]/NaAcO in methanol at room temperature also gave compounds 1a-1c albeit contaminated with small amounts of the corresponding free aldehyde (mixture A). Reaction of mixture A with silver perchlorate/triphenylphosphine in acetone at room temperature gave analogously 2a-2c with some of the corresponding free aldehyde (mixture B). Attempts to purify mixtures A and B via recrystallization produced single crystals of 5 and 6 respectively: two serendipitously formed complexes, bearing thiomethyl aniline and/or acetate ligands, and void of aldehyde or iminic residue; the structures contain eight- and six-membered rings of alternating palladium and nitrogen atoms, respectively. To clarify this situation the aniline itself was reacted with palladium(II) acetate or with Na2[PdCl4]; in the latter case after recrystallization a unique behavior is revealed, giving rise to a tetranuclear complex containing a Pd4N4 ring with three differing coordination environments on the palladium atoms. Treatment of 1d with Ph2PCH2PPh2 (dppm)/AgClO4 or with Ph2PCH2(PPh2)W(CO)5/AgClO4 gave 3d, with a mono-coordinated dppm ligand, and 4d, respectively; complex 3d could not be converted into 4d by reaction with W(CO)5(THF). Recrystallization of 4d gave a still further noticeable species, complex 8: a pentanuclear trans-configured heterometallic mixed valent Pd(II)/W(0) linear complex with the palladium atoms supported by two acetate and two thiomethyl aniline bridging ligands. The complexes were fully characterized by microanalysis, IR, 1H, and 31P NMR spectroscopies, as appropriate. The X-ray single-crystal analyses for compounds 1b, 5, 6, 7 and 8 are described.

Recent Advances in the Nickel-Catalyzed Alkylation of C-H Bonds

Functionalization of C-H bonds has emerged as a powerful strategy for converting inert, nonfunctional C-H bonds into their reactive counterparts. A wide range of C-H bond functionalization reactions has become possible by the catalysis of metals, typically from the second row of transition metals. First-row transition metals can also catalyze C-H functionalization, and they have the merits of greater earth-abundance, lower cost and better environmental friendliness in comparison to their second-row counterparts. C-H bond alkylation is a particularly important C-H functionalization reaction due to its chemical significance and its applications in natural product synthesis. This review covers Ni-catalyzed C-H bond alkylation reactions using alkyl halides and olefins as alkyl sources.

Investigating the Mechanism of Diethylenetriamine Pentamethylene Phosphonic Acid as a Depressant for Calcite Flotation of Fluorite

Fluorite and calcite have been attracting research attention for a long time. This paper reports on an investigation of the use of diethylene triamine pentamethylphosphonic acid (DTPMPA) as a chelating inhibitor. DTPMPA was used as a chelating inhibitor to study the flotation, separation, and adsorption behaviors of fluorite and calcite minerals. The microflotation experiment showed that the maximum separation of fluorite and calcite can be achieved with a DTPMPA dosage of 1.5 × 10-4 mol/L under weakly alkaline conditions (pH = 8). Zeta potential measurement, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy were used to confirm that DTPMPA was adsorbed on the surface of calcite, inhibiting NaOl adsorption. Additionally, density functional theory calculations showed that oxygen in the DTPMPA phosphate group formed the most stable bidentate binuclear adsorption configuration by chelating with calcium on the calcite surface. Through detection analysis and simulation calculations, the results showed that DTPMPA exhibited significantly weaker adsorption on fluorite compared to that on calcite, highlighting its selective inhibition ability on calcite.

Four-membered C^N chelation in main-group organometallic chemistry

Main-group organometallic compounds containing four-membered C^N chelating rings are being studied because of the interest in harnessing the enhanced reactivity of such compounds which arises as a result of the release of steric strain. In this article, we have reviewed the literature on these systems. This review is organised in terms of the types of ligand systems that allow the assembly of such compounds, viz., compounds containing aliphatic amine motifs, pyridine motifs and aniline motifs. In addition to a discussion on the synthesis and structure, we also examine the reactivity and applications of the main-group element compounds involved. In particular, applications involving H₂ activation, carbonyl activation, olefin reduction, C-H activation, hydroalumination, cyanamide oligomerisation, borylation of olefins and heteroarenes, isocyanate activation and C-C bond activation are discussed.

Palladium-Catalyzed PIDA-Mediated δ-C(sp3 )-H Acetoxylation of Amino Acid Derivatives: Overriding Competitive Intramolecular Amination

The selective δ-C(sp3 )-H acetoxylation of N-(SO2 Py)-protected amino acid derivatives has been accomplished by using palladium-catalysis and PhI(OAc)2 (PIDA) as both terminal oxidant and acetoxy source. The distinct structural and electronic features of the SO2 Py compared to more traditional carbonyl-based directing groups is essential to override the otherwise more favourable competitive intramolecular C-H amination. The δ-site selectivity predominates over traditionally more favorable 5-membered cyclopalladation at competitive γ-CH2 . Experimental and DFT mechanistic studies provide important insights about the mechanism and the underlying factors controlling the chemo- and regioselectivity.

Classical vs. Non-Classical Cyclometalated Pt(II) Complexes

Rollover cyclometalated complexes constitute a family of derivatives which differ from classical cyclometalated species in certain aspects. Various potential application fields have been developed for both classes of compounds, which have both similarities and differences. In order to uncover the relationships and distinctions between these two families of compounds, four Pt(II) cyclometalated complexes derived from 2-phenylpyridine (ppy) and 2,2'-bipyridine (bpy), assumed as prototypical ligands, were compared. For this study, an electron rich isostructural and isoelectronic pair of compounds, [Pt(N^C)Me(PPh3)], and an electron-poorer compound, [Pt(N^C)Cl(PPh3)] were chosen (N^C = ppy or bpy). DFT calculations, cyclic voltammetry, and UV-Vis spectra also helped to shed light into these species. Due to the presence of the more electronegative nitrogen in place of a C-H group, the rollover bpy-H ligand becomes a slightly weaker donor than the classical ppy-H ligand, and hence, generates (slightly) more stable cyclometalated complexes, lower energy frontier molecular orbitals, and electron-poorer platinum centers. On the whole, it was revealed that classical and rollover complexes have overall structural similarity, which contrasts to their somewhat different chemical behavior.

Heterocyclic-2-thione derivatives of metals incorporating cyclopalladated azobenzenes of variable nuclearity and N,S-bridged 1D polymer of [bis(pyridine-2-thiolato)mercury(ii)]

In this investigation, the reactivity of azobenzene-based cyclopalladated precursors with a series of heterocyclic-2-thiones is described.

Benzodiazepines: Drugs with Chemical Skeletons Suitable for the Preparation of Metallacycles with Potential Pharmacological Activity

The synthesis of organometallic compounds with potential pharmacological activity has attracted the attention of many research groups, aiming to take advantage of aspects that the presence of the metal-carbon bond can bring to the design of new pharmaceutical drugs. In this context, we have gathered studies reported in the literature in which psychoactive benzodiazepine drugs were used as ligands in the preparation of organometallic and metal complexes and provide details on some of their biological effects. We also highlight that most commonly known benzodiazepine-based drugs display molecular features that allow the preparation of metallacycles via C-H activation. These organometallic compounds merit further attention regarding their potential biological effects, not only in terms of psychoactive drugs but also in the search for drug replacements, for example, for cancer treatments.

N-(9,10-Dioxo-9,10-dihydroanthracen-1-yl)-2-methylbenzamide

In the present Short Note, we report a synthesis for the title compound, N-(9,10-dioxo-9,10-dihydroanthracen-1-yl)-2-methylbenzamide by reacting 2-methylbenzoyl chloride (or 2-methylbenzoic acid) with 1-aminoanthraquinone. The synthesized target compound was fully characterized by various spectroscopic methods (1H-NMR, 13C-NMR, IR, GC-MS). The importance of this compound lies its possession of an N,O-bidentate directing group, potentially suitable for metal-catalyzed C-H bond functionalization reactions.

N-(2-Hydroxy-1,1-dimethylethyl)-3-methylbenzamide

The title compound, N-(2-hydroxy-1,1-dimethylethyl)-3-methylbenzamide was synthesized by reacting 3-methylbenzoyl chloride or 3-methylbenzoic acid with 2-amino-2-methyl-1-propanol. In the present report, the synthesized target compound was fully characterized by various spectroscopic methods (1H NMR, 13C NMR, IR, GC-MS), its composition confirmed by elemental analysis, and its structure determined and confirmed by X-ray analysis. The importance of this compound lies in its possession of an N,O-bidentate directing group. Such a structural motif is potentially suitable for metal-catalyzed C–H bond functionalization reactions.

Switching the site-selectivity of C-H activation in aryl sulfonamides containing strongly coordinating N-heterocycles

The limitations of arene C-H functionalization of aryl sulfonamides containing strongly coordinating N-heterocycles were overcome using a Rh(iii) catalyst. The site-selectivity of C-H carbenoid functionalization at the ortho position relative to either the sulfonamide or N-heterocycle directing groups was elegantly switched using solvents of different polarities and different additive concentrations. Importantly, sulfonamide-group-directed ortho-C-H carbenoid functionalization tolerated strongly coordinating N-heterocycles, including pyridine, pyrrole, thiazole, pyrimidine, and pyrazine. Density functional theory (DFT) calculations were performed to rationalize the reaction mechanisms and the influence of reaction polarity.

N-Benzo[c][1,2,5]thiazol-4-yl-3-trifluoromethylbenzamide

The title compound, N-benzo[c][1,2,5]thiazol-4-yl-3-trifluoromethylbenzamide (1) was synthesized by reacting 3-trifluoromethylbenzoyl chloride (4) and 4-aminobenzo[c][1,2,5]thiadiazole (5). The compound was characterized by various spectroscopic methods (1H NMR, 13C NMR, IR, GC-MS) and its composition confirmed by elemental analysis. The importance of this compound lies in its possession of an N,N-bidentate directing group. Such a structural motif is potentially suitable for metal-catalyzed C-H bond functionalization reactions.

The reduction of Cr(VI) to Cr(III) mediated by environmentally relevant carboxylic acids: State-of-the-art and perspectives

The detoxification process mediated by carboxylic acids (CAs) has received considerable spotlights since CAs are clean reagent and ubiquitous in the natural environments and effluents. Here, we present an exhaustive review on surface-bound/dissolved metals-catalyzed Cr(VI) reduction by CAs and CAs-mediated Cr(VI) reduction by many highly/poorly reductive reagents. The overall mechanisms of Cr(VI) reduction are mainly associated with the coordination of CAs with surface-bound/dissolved metals or Cr(VI,V,IV) species and the electron donating abilities of CAs. Additionally, the general decays of intermediate Cr(V,IV) complexes are clearly emerged in the Cr(VI) reduction processes. The performance of various reaction systems for Cr(VI) reduction that is greatly dependent on the operation parameters, including solution pH, reagent concentration, temperature, coexisting ions and gas atmosphere, are also critically commented. From the study survey presented herein, CAs-mediated Cr(VI) reduction processes exhibit good potential for remediation of various Cr(VI)-contaminated waters/sites. However, there is still a need to address the remained bottle-necks and challenges for the remediation of Cr(VI) mediated by CAs in the related natural attenuation cases and the treatment of industrial effluents. Overall, the present review offers the comprehensive understanding of the Cr(VI) reduction mediated by CAs and provide the engineering community with the guidelines for Cr(VI) remediation in the real-world applications.

Acylsilane directed aromatic C-H alkenylations by ruthenium catalysis

A ruthenium-catalyzed C-H alkenylation of aroylsilanes with electron-deficient alkenes was developed, using acylsilane as the directing group. The mild reaction conditions enable the tolerance of a wide scope of functionalities such as OMe, F, Cl, Br and CF3, providing a convenient and highly effective method for the synthesis of styrene derivatives bearing acylsilane. Steroid and heterocycles such as furan and thiophene were also well tolerated. Furthermore, acylsilane was efficiently converted to the corresponding aldehyde or carboxylic acid.

Mesityl(amidinato)tetrylenes as ligands in iridium(i) and iridium(iii) complexes: silicon versus germanium and simple κ1-coordination versus cyclometallation

The reactivity of the germylene Ge(^tBu₂bzam)Mes with [Ir₂(μ-Cl)₂(η⁴-cod)₂] and [Ir₂Cl₂(μ-Cl)₂(η⁵-Cp*)₂] differs considerably from that previously communicated for the isostructural silylene Si(^tBu₂bzam)Mes.

From Chemical Serendipity to Translational Chemistry: New Findings in the Reactivity of Palladacycles

In the world of science, in particular the section concerning the field of chemistry, when the results encountered during the experiment do not meet our expectations, our shrewdness may play an important role to open up new unexplored fields that could be much more interesting than what we were seeking. In those cases, our research undergoes an unforeseen shift, delivering novel and challenging results that may altogether alter our point of view and our future work. We have then struck serendipity. Specifically, in our investigation linked to palladacycles we have found that the new trends in their reactivity, as well as in their structure, have been, in many cases, related to this experience, broadening our research scope within this field. Herein, we describe our most relevant findings, which have shed new light upon the reactivity of palladacycles, thus opening new routes that lead to novel unexpected structures.

Synthesis, Structures, and Reactivity of Single and Double Cyclometalated Complexes Formed by Reactions of [Cp*MCl2]2 (M = Ir and Rh) with Dinaphthyl Phosphines

Reactions of two dinaphthyl phosphines with [Cp*IrCl₂]₂ have been carried out. In the case of di(α-naphthyl)phenylphosphine (1a), a simple P-coordinated neutral adduct 2a is obtained. However, tert-butyldi(α-naphthyl)phenylphosphine (1b) is cyclometalated to form [Cp*IrCl(P^C)] (3b). Complexes 2a and 3a undergo further cyclometalation to give the corresponding double cyclometalated complexes [Cp*Ir(C^P^C)] (4a,b) upon heating. In the presence of sodium acetate, reactions of 1a,b with [Cp*IrCl₂]₂ directly afford the final double cyclometalated complexes (4a,b). In the absence of acetate, [Cp*RhCl₂]₂ shows no reaction with 1a,b, whereas with acetate the reactions form the corresponding single cyclometalated complexes [Cp*RhCl(P^C)] (5a,b), which react with ^t BuOK to form the corresponding rhodium hydride complexes (6a,b). Treatment of 4a with CuCl₂ or I₂ leads to opening of two Ir-C σ bonds to yield the corresponding P-coordinated iridium dihalide (7 or 8) by means of an intramolecular C-C coupling reaction. A new chiral phosphine (11) is formed by the ligand-exchange reaction of 8 with PMe₃. Reactions of the single cycloiridated complex 3b with terminal aromatic alkynes result in the corresponding five- and six-membered doubly cycloiridated complex 12 and/or η²-alkene coordinated complexes 13-15; the latter discloses that the electronic effect of terminal alkynes affects the regioselectivity. While the single cyclorhodated complex 5b reacts with terminal aromatic alkynes to form the corresponding six-membered cyclometalated complexes 16a-c by vinylidene rearrangement/1,1-insertion. Plausible pathways for formation of insertion products 13-16 were proposed. Molecular structures of twelve new complexes were determined by X-ray diffraction.

Synthesis, structures, and reactivity studies of cyclometalated N-heterocyclic carbene complexes of ruthenium

Dehydrochlorination of Cp*Ru(IPr)Cl leads to an unusual C–C bond activation, yielding a cyclometalated Ru complex bearing an NHC-C(sp²) ligand. Reactivity studies of cyclometalated Ru complexes were explored.

Preparation and characterization of terdentate [C,N,N] acetophenone and acetylpyridine hydrazone platinacycles: a DFT insight into the reaction mechanism

The reaction of N-ortho-chlorophenyl-substituted acetylpyridine hydrazones (a and d) with K₂[PtCl₄] (n-butanol/water, 100 °C) gave mononuclear complexes 1a and 1d with the ligands as [N,N] bidentate. In contrast, the reaction of N-phenyl or N-meta-chlorophenyl hydrazones (b and c, respectively) under analogous reaction conditions gave the cycloplatinated species 2b and 2c with the ligand as [C,N,N] terdentate. The treatment of the mononuclear complexes 1a and 1d with NaOAc (n-butanol, 100 °C) gave the corresponding cycloplatinated complexes 2a and 2d. Acetophenone hydrazone platinacycle 2e was prepared in a similar fashion and its reaction with tertiary mono- and triphosphines gave mono- or trinuclear species depending on the reaction conditions. The X-ray crystal structures of some of these complexes showed interesting π-π slipped stacking interactions between metallacyclic rings which, according to NCI analyses, showed an aromatic character. With an aim to rationalize the different reactivities shown by acetylpyridine hydrazones and the precise role of the acetate anion, the energy profiles for the three main steps of cycloplatination (iminoplatinum complex formation, chelation and cyclometallation) have been determined by using the DFT (M06) methods. Calculations indicate that the cycloplatination of 1b proceeds via electrophilic substitution, involving the direct replacement of the chloride anion at the Pt(ii) centre with the N-phenyl moiety as the rate-determining step, to give an agostic intermediate 5b+ that, subsequently, leads to the elimination of a proton as hydrogen chloride. When present as an "external" base, acetate enters the coordination sphere around the Pt(ii) centre and facilitates hydrazone N-H deprotonation and electrophilic C-H activation through a dissociative route, leading to a Wheland-type σ-complex intermediate 9ac.