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.

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.

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.

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.

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.

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.

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.

The Front Cover shows the art gallery within the Beryllium Centre, which exhibits some of the latest samples of beryllium art. The current exhibition features the solid state structure of dinuclear [(PPh₃)BeCl₂]₂. In this piece, only one phosphine ligand can be accommodated due to the steric bulk of PPh₃. However, two smaller ligands, like PMePh₂, can find a place in the Beryllium Centre. Therefore, size restrictions apply with a threshold cone angle from 136° to 145°. This parameter dictates whether single or double admission is allowed. Furthermore, the electron donating abilities of the ligands determine whether they behave as spectator ligands or attack the solvent. Therefore, the walls are covered with advertising flyers by the stronger donors. More information can be found in the Research Article by M. R. Buchner and S. I. Ivlev.

A novel imidazolylidene backbone-based mesoionic carbene (MIC)−Mn(I) complex Mn-bim-MIC^imz is developed and explored in N-heteroarene hydrogenation process.

Abstract

Herein we report the first mesoionic carbene (MIC)-Mn(I) complex Mn-bim-MIC^imz derived from imidazolylidene motif. Structurally the octahedral Mn(I) complex Mn-bim-MIC^imz was assembled with an anionic benzimidazolato-anchored imidazolylidene MIC-based bidentate ligand (bim-MIC^imz ) and four CO ligands, as supported by detailed characterization using NMR and FTIR spectroscopy, mass spectrometry, and single crystal X-ray diffraction study. We reckoned that the bim-MIC^imz ligand would provide a robust and stable bonding with the Mn(I) centre, and also enhance electron density at the Mn(I) centre through its stronger σ-donating/weaker π-accepting property. These structural and electronic attributes triggered to exploit Mn-bim-MIC^imz in catalytic hydrogenation of N-heteroarenes, where efficient hydride (Mn−H) delivery is a key step.

The threshold cone angle of phosphines was determined, below which two ligands can be accommodated in the first ligand sphere of beryllium dihalide fragments. While [(PMe₂Ph)₂BeX ₂] (X=Cl, Br, I) and [(PMePh₂)₂BeX ₂] are mononuclear complexes, [(PPh₃)BeX ₂]₂ is dinuclear and exhibits dynamic behaviour in solution due to phosphine dissociation. This high dynamicity is the reason for halide exchange with dichloromethane.

Abstract

The phosphine complexes of beryllium chloride, bromide and iodide, [(PMe₂Ph)₂BeX ₂], [(PMePh₂)₂BeX ₂] and [(PPh₃)BeX ₂]₂ (X=Cl, Br, I) were prepared and characterised with multinuclear NMR spectroscopy. Additionally the molecular structure of dinuclear [(PPh₃)BeCl₂]₂ was determined with single crystal X-ray diffraction techniques. The threshold cone angle of the phosphines, below which two ligands can coordinate to the beryllium dihalide fragments, is between 136° and 145°. Halide-chloride exchange in dichloromethane is observed for [(PPh₃)BeBr₂]₂ and [(PPh₃)BeI₂]₂, which leads to the formation of [(PPh₃)BeCl₂]₂. Due to the relatively low Lewis basicity of PPh₃, it almost exclusively acts as a spectator ligand with only little formation of phosphonium cations.