Co‐Catalyzed Metal‐Ligand Cooperative Approach for N‐alkylation of Amines and Synthesis of Quinolines via Dehydrogenative Alcohol Functionalization

Posted on 12 October 2023 by

A well-defined Co-complex (1 a), bearing 2-(phenyldiazenyl)-1,10-phenanthroline ligand, catalyzed sustainable synthesis of various N-alkylated amines, indole, and substituted quinolines are reported using the readily available alcohols as the alkylating agents.

Abstract

Herein we report a cobalt-catalyzed sustainable approach for C−N cross-coupling reaction between amines and alcohols. Using a well-defined Co-catalyst 1 a bearing 2-(phenyldiazenyl)-1,10-phenanthroline ligand, various N-alkylated amines were synthesized in good yields. 1 a efficiently alkylates diamines producing N, N′-dialkylated amines in good yields and showed excellent chemoselectivity when oleyl alcohol and β-citronellol, containing internal carbon-carbon double bond were used as alkylating agents. 1 a is equally compatible with synthesizing N-heterocycles via dehydrogenative coupling of amines and alcohols. 1H-Indole was synthesized via an intramolecular dehydrogenative N-alkylation reaction, and various substituted quinolines were synthesized by coupling of 2-aminobenzyl alcohol and secondary alcohols. A few control reactions and spectroscopic experiments were conducted to illuminate the plausible reaction mechanism, indicating that the 1 a-catalyzed N-alkylation proceeds through the borrowing hydrogen pathway. The coordinated arylazo ligand participates actively throughout the reaction; the hydrogen eliminated during dehydrogenation of alcohols was set aside in the ligand backbone and subsequently gets transferred in the reductive amination step to imine intermediates yielding N-alkylated amines. On the other hand, 1 a-catalyzed quinoline synthesis proceeds through dehydrogenation followed by successive C−C and C−N coupling steps forming H₂O₂ as a by-product under air.

Regioselective Rhodium‐Catalyzed 1,2‐Hydroboration of Pyridines and Quinolines Enabled by the Tris(8‐quinolinyl)phosphite Ligand

Posted on 12 October 2023 by

A rhodium(I) complex of the tris(8-quinolinyl)phosphite ligand, P(OQuin)₃, enables the 1,2-regioselective hydroboration of pyridines and quinolines. P(OQuin)₃ coordinates to rhodium as a bidentate P,N chelate ligand. Reactivity studies of 1 showed that the PPh₃ ligand can be substituted intermolecularly by PCy₃ and intramolecularly by a second unit 8-quinolyl of P(OQuin)₃.

Abstract

A Rh(I) complex [κ²(P,N)-{P(Oquin)₃}RhCl(PPh₃)] (1) bearing the P,N ligand tris(8-quinolinyl)phosphite, P(Oquin)₃, has been synthesized and structurally characterized. The molecular structure of complex 1 shows that P(Oquin)₃ acts as a bidentate P,N chelate ligand. Reactivity studies of 1 reveal that the triphenylphosphine ligand can be replaced by Pcy₃ or removed upon oxidation with concomitant coordination of a second 8-quinolyl unit of P(Oquin)₃. In addition, the Rh(III) complex [RhCl₂{OP(Oquin)₂}] (3), resulting from treating 1 with either wet CDCl₃ or, sequentially, with HCl and water, was identified by X-ray diffraction analysis. Complex 1 catalyzes the 1,2-regioselective hydroboration of pyridines and quinolines, affording N-boryl-1,2-dihydropyridines (1,2-BDHP) and N-boryl-1,2-hydroquinolines (1,2-BDHQ) in high yield (up to >95 %) with turnover numbers (TONs) of up to 130. The system tolerates a variety of substrates of different electronic and steric nature. In comparison with other transition-metal-based hydroboration catalysts, this system is efficient at a low catalyst loading without the requirement of base or other additives.

Selective Zinc‐Catalyzed 1,2‐hydroboration of N‐heteroaromatics via a Non‐Hydride Mechanism

Posted on 12 October 2023 by

New zinc hydride catalyst allows for selective 1,2-hydroboration of nitrogen heteroaromatics with decreased catalyst load and under mild conditions. The 1,2-regioselectivy is provided by a non-hydride mechanism relying on the ambiphilic properties of the zinc complex.

Abstract

A novel bidentate amine-imine ligand precursor LH has been synthesized. This compound was reacted with ZnMe₂ to generate the zinc methyl complex, LZnMe (4). The latter compound was fully characterized by NMR spectroscopy and single crystal X-ray diffraction. Compound 4 is a catalyst for the hydroboration and hydrosilylation of N-heterocycles, but with moderate catalytic activity. A more active catalyst, the zinc hydride complex LZnH (5) was synthesized by reacting the lithium salt LLi with ZnCl₂ followed by sequential reaction with tBuOK and PhMeSiH₂. Compound 5 catalyzes the selective 1,2-hydroboration of nitrogen heteroaromatics with decreased catalyst load and under mild conditions. Deuterium-labeling experiments and kinetic studies provided insight into the possible reaction mechanism. It is proposed that hydride transfer to the substrate proceeds directly from the reductant (borane) via a six-membered transition state facilitated by the catalyst, in which it plays an ambiphilic role, activating the substrate via coordination to the Lewis acidic zinc and enhancing the hydricity of the borane through coordination to the zinc hydride.

Silyl‐ and Germyl‐Substituted Diorganophosphonites

Posted on 12 October 2023 by

The syntheses of triorganosilyl- and triorganostannyl-substituted diorganophosphonites as well as an evaluation of their Lewis basicity are reported. Reactions of triphenylsilyl-dialkylphosphonites with selenium proceed via initial formation of spectroscopically observable P-oxidation products that are unprecedented for phosphine derivatives with P−Si bonds.

Abstract

Reactions of metalated diorganophosphonite boranes with triorganosilyl and -germyl halides provided borane adducts of diorgano(tetryl)phosphonites. Further treatment with excess Et₃N or DABCO yielded the borane-free species (RO)₂P-ER′₃ (E=Si, Ge; R, R′=alkyl, aryl). The products of all reactions were characterized by elemental analyses and NMR data, and in selected cases by MS and single-crystal XRD studies. Reactions of selected ligands with Ni(CO)₄ and selenium were shown to produce Ni(CO)₃-complexes or diorgano(tetryl) phosphonoselenoates (RO)₂(R′₃E)P=Se, respectively, which were identified spectroscopically but could not be isolated. Evaluation of the TEP and ¹ J _PSe coupling constants were used for a first assessment of the electron donor properties of the new molecules.

Immobilization of Iridium Triazolylidene Complexes into Polymer Scaffolds and Their Application in Water Oxidation

Posted on 12 October 2023 by nPascal Knörr, nNicolas Lentz, nMartin Albrechtn

Iridium complexes containing a robustly bound phenyl-triazolylidene ligand were post-functionalized to be embedded within polyethyleneterephthalate (PET)-type polymers and investigated in catalytic water oxidation.

Abstract

A triazolylidene irdium complex was postmodified with simple methods to introduce two alcohol groups in the triazolylidene backbone. The reaction of this difunctionalized iridium triazolylidene unit with terephthalic acid chloride allowed for integrating these iridium complexes into a polymeric assembly. Both the monomeric complexes as well as the polymerized systems showed activity in water oxidation driven by cerium ammonium nitrate as a chemical oxidant with comparable catalytic performance. Post-reaction analysis of the aqueous reaction solution by ICP MS showed a partial loss of iridium from the polymer into the aqueous phase under catalytic conditions, indicating a need for more robust polymer supports for this type of application.

NHC‐Adducts of Cyclopentadienyl‐Substituted Alanes

Posted on 12 October 2023 by nLuis Werner, nSophie Mann, nUdo Radiusn

Reaction of the NHC-stabilized mono- and bis-iodo-alanes with NaCp afforded the corresponding Cp-substituted compounds that show dismutation in solution. Using magnesocene MgCp₂ as Cp source led to isolation of tetranuclear Al₂Mg₂ compounds with interesting structural motifs including bridging hydrides between Al and Mg centers.

Abstract

The mono- and bis-iodo-substituted NHC-stabilized alanes (NHC) ⋅ AlH₂I and (NHC) ⋅ AlHI₂ offer a convenient entry for further substitution reactions at aluminum. Reactions of (NHC) ⋅ AlH₂I 1–4 with one equivalent of NaCp afforded the adducts (NHC) ⋅ AlH₂Cp 9–12 (NHC=Me₂Im^Me (9), iPr₂Im^Me (10), iPr₂Im (11), Dipp₂Im (12)). Alane adducts with two Cp substituents (NHC) ⋅ AlHCp₂ 13–16 (NHC=Me₂Im^Me (13), iPr₂Im^Me (14), iPr₂Im (15), Dipp₂Im (16)) were prepared by the analogous reaction of (NHC) ⋅ AlHI₂ 5–8 using two equivalents of NaCp. The unusual dimeric adducts ((NHC) ⋅ AlH₂Cp ⋅ CpMgI)₂ 17–19 (NHC=Me₂Im^Me (17), iPr₂Im^Me (18), iPr₂Im (19)) were obtained from the reaction of 1–3 with MgCp₂.

Sc3+ Chloro and Alkyl Complexes Coordinated by Pincer NHC‐Tethered Bis(phenolate) Ligands

Posted on 12 October 2023 by

A series of chloro and alkyl complexes of scandium bearing pincer NHC ligands of different σ-donor abilities were prepared in good to high yields. For both types of compounds, two synthetic routes were employed.

Abstract

Bis(phenolate) ligands with benzimidazole-2-ylidene (L¹) and tetrahydropyrimidine-2-ylidene (L² ) linkers proved to be suitable coordination environments for the synthesis of isolable Sc³⁺ chloro and alkyl complexes. The treatment of Sc(CH₂SiMe₃)₃(THF)₂ with equimolar amounts of [L^1,2H₃ ]Cl afforded chloro complexes L^1,2ScCl(solv) ₂ (solv=THF, Py) in 76–85 % yields. L^1,2ScCl(THF) ₂ were also prepared by the salt metathesis reactions of ScCl₃ with [L^1,2 ]Na₂ generated from [L^1,2H₃ ]Cl and 3 equiv. of NaN(SiMe₃)₂ (−40 °C, THF) and isolated in somewhat lower yields (68–73 %). L²ScCl(THF) ₂ was subjected to the alkylation reaction with LiCH₂SiMe₃ affording alkyl derivative [L²Sc(CH₂SiMe₃ )] ₂ . This compound can be alternatively prepared by the subsequent reactions of [L²H₃ ]Cl with equimolar amount of NaN(SiMe₃)₂ and Sc(CH₂SiMe₃)₃(THF)₂. In the dimeric alkyl compound [L²Sc(CH₂SiMe₃ )] ₂ , one of the phenoxide groups of the dianionic ligand is coordinated to one scandium center, while the second one features μ-bridging coordination with two metal centers.

Crystalline Rhodium‐Tin Complexes with Radical Trianion and Tetraanion Phthalocyanine Ligands: Observation of Nimine(Pc)−Rh Coordination Bond

Posted on 12 October 2023 by

The crystalline complexes were obtained by the interaction of [Sn^II(Pc⁽ⁿ⁺²⁾⁻)]ⁿ⁻ (n=1, 2) anions with {(COD)RhCl}₂. Heterometallic anionic assemblies [(COD)Rh(Cl)⋅Sn^II(Pc⁽ⁿ⁺²⁾⁻)]⁻ with tin-rhodium bonds based on paramagnetic Pc⋅³⁻ or diamagnetic Pc⁴⁻ species were formed.

Abstract

Crystalline {Cryptand(Na⁺)}[(COD)Rh^ICl⋅Sn^II(Pc³⁻)]⁻⋅2C₆H₄Cl₂ (1) and {Cryptand(Cs⁺)}[(COD)Rh^I⋅Sn^II(Pc⁴⁻)]⁻⋅C₆H₅CH₃ (2) complexes were obtained via the interaction of [Sn^II(Pc³⁻)]⁻ and [Sn^II(Pc⁴⁻)]²⁻, respectively, with organometallic {(COD)RhCl}₂ dimer (COD is 1,5-cyclooctadiene). Dissociation of {(COD)RhCl}₂ followed by the Rh−Sn binding is observed at the formation of 1. Elimination of the chlorine atom at the rhodium atom is observed in 2, and rhodium is additionally coordinated to the imine nitrogen atom of Pc⁴⁻. The complexes contain mono- Pc⋅³⁻ and doubly reduced Pc⁴⁻ species, respectively, that is supported by the data of XRD analysis as well as optical and magnetic properties of 1 and 2. There is an alternation of C-N_imine bonds in the macrocycles, which gradually increases with increasing negative charge on the macrocycle. The difference between shorter and longer bonds increases from 0.051 Å in Pc³⁻ to 0.075 Å in Pc⁴⁻. The formation of 1 is accompanied by an essential blue shift of the Q-band of starting SnPc and the appearance of a new intense band at 1031 nm. The even stronger shift of the Q-band is observed in the spectrum of 2, but the band in the near-IR range becomes weaker. The value of effective magnetic moment of 1 is 1.76 μ _B at 300 K corresponding the contribution of the Pc³⁻ radical trianions (S=1/2). Only weak magnetic coupling with the Weise temperature of −3 K is observed in 1 due to weak π–π interaction between the macrocycles in the chains. Paramagnetic Pc³⁻ species additionally monitored by EPR spectroscopy show a strong temperature dependence of g-factor and linewidth of the EPR signal. Complex 2 is diamagnetic and EPR silent.

Synthesis, Characterization, and Catalytic Activity of a Cubic [Mo3S4Pd] Cluster Bearing Bulky Cyclopentadienyl Ligands

Posted on 12 October 2023 by

A cubic metal-sulfur cluster, [Cp^SiEt3 ₃Mo₃S₄Pd]Cl (Mo₃Pd, Cp^SiEt3=C₅Me₄SiEt₃), was synthesized by the incorporation of the Pd ion into a Mo₃S₄ cluster [Cp^SiEt3 ₃Mo₃S₄] (Mo₃ ). Mo₃Pd promoted a two-electron reduction process and was utilized for the hydrogen evolution reaction (HER). The mechanism of the HER was determined by density functional theory (DFT) calculations.

Abstract

A cubic metal-sulfur cluster containing three Mo ions and a Pd ion, [Cp^SiEt3 ₃Mo₃S₄Pd]Cl (Mo₃Pd, Cp^SiEt3=C₅Me₄SiEt₃), was synthesized by the incorporation of the Pd ion into a Mo₃S₄ cluster [Cp^SiEt3 ₃Mo₃S₄] (Mo₃ ). Mo₃Pd was characterized by ¹H NMR, UV-vis, X-ray crystallography, and cyclic voltammetry measurements. The electrochemical measurements demonstrated reversible one- and two-electron reduction processes for Mo₃Pd, which suggested potential catalytic activity for two-electron substrate reductions such as hydrogen evolution reaction. Controlled potential electrolysis in the presence of Mo₃Pd and trifluoroethanol in THF solvent displayed H₂ formation with a constant current over 60 min. The amount of generated H₂ by Mo₃Pd was two times higher than Mo₃ , indicating the catalytic activity facilitated by the Pd center. The mechanism of the catalytic cycle was determined by density functional theory.

Single‐Source Precursors for the Chemical Vapor Deposition of Iron Germanides

Posted on 12 October 2023 by

The reaction of GeCl₂ ⋅ 1,4-dioxane with Fe₂(CO)₉ gives [Cl₂GeFe(CO)₄]₂, Cl₂Ge[Fe₂(CO)₈]Ge[Fe₂(CO)₈] or Ge[Fe₂(CO)₈]₂, depending on the educt ratio. [Cl₂GeFe(CO)₄]_2, Ge[Fe₂(CO)₈]₂ and ^Me₂iPr₂NHC ⋅ GeCl₂ ⋅ Fe(CO)₄ were characterized in their thermal decomposition behavior and applied as single source precursors in chemical vapor deposition, resulting in Fe_xGe_1-x thin films.

Abstract

Binary iron-germanium phases are promising materials in magnetoelectric, spintronic or data storage applications due to their unique magnetic properties. Previous protocols for preparation of Fe_xGe_y thin films and nanostructures typically involve harsh conditions and are challenging in terms of phase composition and homogeneity. Herein, we report the first example of single source chemical vapor deposition (CVD) of Fe_xGe_y films. The appreciable volatility of [Ge[Fe₂(CO)₈]₂], [Cl₂GeFe(CO)₄]₂ and ^Me₂iPr₂NHC ⋅ GeCl₂ ⋅ Fe(CO)₄ allowed for their application as precursors under standard CVD conditions (^Me₂iPr₂NHC=1,3-diisopropoyl-4,5-dimethylimidazol-2-ylidene). The thermal decomposition products of the precursors were characterized by TGA and powder XRD. Deposition experiments in a cold-wall CVD reactor resulted in dense films of Fe_xGe_y. During the optimization of synthetic conditions for precursor preparation the new iron-germanium cluster Cl₂Ge[Fe₂(CO)₈]Ge[Fe₂(CO)₈] was obtained in experiments with a higher stoichiometric ratio of GeCl₂ ⋅ 1,4-dioxane vs. Fe₂(CO)₉.