Metallorganische Pinzetten-Komplexe von Elementen der Platingruppe: Sensoren, Schalter und Katalysatoren

Sterically and Electronically Flexible Pyridylidene Amine Dinitrogen Ligands at Palladium: Hemilabile cis/trans Coordination and Application in Dehydrogenation Catalysis

Ligand design is crucial for the development of new catalysts and materials with new properties. Herein, the synthesis and unique hemilabile coordination properties of new bis-pyridylidene amine (bis-PYE) ligands to palladium, and preliminary catalytic activity of these complexes in formic acid dehydrogenation are described. The synthetic pathway to form cationic complexes [Pd(bis-PYE)Cl(L)]X with a cis-coordinated N,N-bidentate bis-PYE ligand is flexible and provides access to a diversity of PdII complexes with different ancillary ligands (L=pyridine, DMAP, PPh3 , Cl, P(OMe)3 ). The 1 H NMR chemical shift of the trans-positioned PYE N-CH3 unit is identified as a convenient and diagnostic handle to probe the donor properties of these ancillary ligands and demonstrates the electronic flexibility of the PYE ligand sites. In the presence of a base, the originally cis-coordinated bis-PYE ligand adopts a N,N,N-tridentate coordination mode with the two PYE units in mutual trans position. This cis-trans isomerization is reverted in presence of an acid, demonstrating a unique structural and steric flexibility of the bis-PYE ligand at palladium in addition to its electronic adaptability. The palladium complexes are active in formic acid dehydrogenation to H2 and CO2 . The catalytic performance is directly dependent on the ligand bonding mode, the nature of the ancillary ligand, the counteranion, and additives. The most active system features a bidentate bis-PYE ligand, PPh3 as ancillary ligand and accomplishes turnover frequencies up to 525 h-1 in the first hour and turnover numbers of nearly 1000, which is the highest activity reported for palladium-based catalysts to date.

Diphosphino-Functionalized 1,8-Naphthyridines: a Multifaceted Ligand Platform for Boranes and Diboranes

A 1,8-naphthyridine diphosphine (NDP) reacts with boron-containing Lewis acids to generate complexes featuring a number of different naphthyridine bonding modes. When exposed to diborane B2 Br4 , NDP underwent self-deprotonation to afford [NDP-B2 Br3 ]Br, an unsymmetrical diborane comprised of four fused rings. The reaction of two equivalents of monoborane BBr3 and NDP in a non-polar solvent provided the simple phosphine-borane adduct [NDP(BBr3 )2 ], which then underwent intramolecular halide abstraction to furnish the salt [NDP-BBr2 ][BBr4 ], featuring a different coordination mode from that of [NDP-B2 Br3 ]Br. Direct deprotonation of NDP by KHMDS or PhCH2 K generates mono- and dipotassium reagents, respectively. The monopotassium reagent reacts with one or half an equivalent of B2 (NMe2 )2 Cl2 to afford NDP-based diboranes with three or four amino substituents.

Synthesis and Characterization of Cobalt NCN Pincer Complexes

A series of cobalt complexes, stabilized by a monoanionic tridentate NCN pincer ligand, was synthetized and characterized. Preparation of the paramagnetic 15 VE complex [Co(NCNCH2-Et)Br] (1) was accomplished by transmetalation of Li[2,6-(Et2NCH2)2C6H3] with CoBr2 in THF. Treatment of this air-sensitive compound with NO gas resulted in the formation of the diamagnetic Co(III) species [Co(NCNCH2-Et)(NO)Br] (2) as confirmed by X-ray diffraction. This complex features a strongly bent NO ligand (Co-N-O∠135.0°). The νNO is observed at 1609 cm-1 which is typical for a bent metal-N-O arrangement. Coordinatively unsaturated 1 could further be treated with pyridine, isocyanides, phosphines and CO to form five-coordinate 17 VE complexes. Oxidation of 1 with CuBr2 led to the formation of the Co(III) complex [Co(NCNCH2-Et)Br2]. Treatment of [Co(NCNCH2-Et)Br2] with TlBF4 as halide scavenger in acetonitrile led to the formation of the cationic octahedral complex [Co(NCNCH2-Et)(MeCN)3](BF4)2. A combination of X-ray crystallography, IR-, NMR- and EPR-spectroscopy as well as DFT/CAS-SCF calculations were used to characterize all compounds.

Computational Study of Bridge Splitting, Aryl Halide Oxidative Addition to PtII , and Reductive Elimination from PtIV : Route to Pincer-PtII Reagents with Chemical and Biological Applications

Density functional theory computation indicates that bridge splitting of [Pt^II R₂ (μ-SEt₂ )]₂ proceeds by partial dissociation to form R₂ Pt^a (μ-SEt₂ )Pt^b R₂ (SEt₂ ), followed by coordination of N-donor bromoarenes (L-Br) at Pt^a leading to release of Pt^b R₂ (SEt₂ ), which reacts with a second molecule of L-Br, providing two molecules of PtR₂ (SEt₂ )(L-Br-N). For R=4-tolyl (Tol), L-Br=2,6-(pzCH₂ )₂ C₆ H₃ Br (pz=pyrazol-1-yl) and 2,6-(Me₂ NCH₂ )₂ C₆ H₃ Br, subsequent oxidative addition assisted by intramolecular N-donor coordination via Pt^II Tol₂ (L-N,Br) and reductive elimination from Pt^IV intermediates gives mer-Pt^II (L-N,C,N)Br and Tol₂ . The strong σ-donor influence of Tol groups results in subtle differences in oxidative addition mechanisms when compared with related aryl halide oxidative addition to palladium(II) centres. For R=Me and L-Br=2,6-(pzCH₂ )₂ C₆ H₃ Br, a stable Pt^IV product, fac-Pt^IV Me₂ {2,6-(pzCH₂ )₂ C₆ H₃ -N,C,N)Br is predicted, as reported experimentally, acting as a model for undetected and unstable Pt^IV Tol₂ {L-N,C,N}Br undergoing facile Tol₂ reductive elimination. The mechanisms reported herein enable the synthesis of Pt^II pincer reagents with applications in materials and bio-organometallic chemistry.

Terminal Alkyne Coupling Reactions Through a Ring: Effect of Ring Size on Rate and Regioselectivity

Terminal alkyne coupling reactions promoted by rhodium(I) complexes of macrocyclic NHC-based pincer ligands-which feature dodecamethylene, tetradecamethylene or hexadecamethylene wingtip linkers viz. [Rh(CNC-n)(C2 H4 )][BArF 4 ] (n=12, 14, 16; ArF =3,5-(CF3 )2 C6 H3 )-have been investigated, using the bulky alkynes HC≡CtBu and HC≡CAr' (Ar'=3,5-tBu2 C6 H3 ) as substrates. These stoichiometric reactions proceed with formation of rhodium(III) alkynyl alkenyl derivatives and produce rhodium(I) complexes of conjugated 1,3-enynes by C-C bond reductive elimination through the annulus of the ancillary ligand. The intermediates are formed with orthogonal regioselectivity, with E-alkenyl complexes derived from HC≡CtBu and gem-alkenyl complexes derived from HC≡CAr', and the reductive elimination step is appreciably affected by the ring size of the macrocycle. For the homocoupling of HC≡CtBu, E-tBuC≡CCH=CHtBu is produced via direct reductive elimination from the corresponding rhodium(III) alkynyl E-alkenyl derivatives with increasing efficacy as the ring is expanded. In contrast, direct reductive elimination of Ar'C≡CC(=CH2 )Ar' is encumbered relative to head-to-head coupling of HC≡CAr' and it is only with the largest macrocyclic ligand studied that the two processes are competitive. These results showcase how macrocyclic ligands can be used to interrogate the mechanism and tune the outcome of terminal alkyne coupling reactions, and are discussed with reference to catalytic reactions mediated by the acyclic homologue [Rh(CNC-Me)(C2 H4 )][BArF 4 ] and solvent effects.

Computational Analysis of Mesomerism in para-Substituted mer-NCN Pincer Platinum(II) Complexes, Including its Relationships with Hammett σp Substituent Parameters

Density Functional Theory studies of square-planar Pt^II pincer structures, (4-Z-NCN)PtCl ([4-Z-NCN]^- =[4-Z-2,6-(Me₂ NCH₂ )₂ C₆ H₂ -N,C,N]^- , Z=H, NO₂ , CF₃ , CO₂ H, CHO, Cl, Br, I, F, SMe, SiMe₃ , tBu, OH, NH₂ , NMe₂ ), enable characterisation of mesomerism for the pincer-Pt interaction. Relationships between Hammett σ_p substituent parameters of Z and DFT data obtained from NBO6 and AOMix computation are used to probe the interaction of the 5d_yz orbital of platinum with π-orbitals of the arene ring. Analogous computation for 2,6-(Me₂ CH₂ )₂ C₆ H₃ Z (Z=H, CF₃ , CHO, Cl, Br, I, F, SMe, SiMe₃ , tBu, OH, NH₂ ) and (4-H-NCN)PtZ allows an estimation of the relative substituent effects of "(CH₂ NMe₂ )₂ PtZ" on π-delocalisation in the pincer system.

(N),C,N-Coordinated Heavier Group 13-15 Compounds: Synthesis, Structure and Applications

The aim of this review is to summarize recent achievements in the field of (N),C,N-coordinated group 13-15 compounds not only regarding their synthesis and structure, but mainly focusing on their potential applications. Relevant compounds contain various types of N-coordinating ligands built up on an ortho-(di)substituted phenyl platform. Thus, group 13 and 14 derivatives were used as single-source precursors for the deposition of semiconducting thin films, as building blocks for the preparation of high-molecular polymers with remarkable optical and chemical properties or as compounds with interesting reactivity in hydrometallation processes. Group 15 derivatives function as catalysts in the Mannich reaction, in the allylation of aldehydes or activation of CO₂ . They were used as transmetallation reagents in transition metal catalysed coupling reactions. The univalent species serve as ligands for transition metals, activate alkynes or alkenes and are utilized as catalysts in the transfer hydrogenation of azo-compounds.

C-As Bond Formation Reactions for the Preparation of Organoarsenic(III) Compounds

Potential widespread applications of organoarsenic chemistry have been limited by the inherent lack of safe and effective As-C bond formation reactions. Several alternative reagents and methods have been developed in the last few decades to address the hazards and drawbacks associated with traditional arsenic synthetic strategies. Herein, this minireview summarizes the advances made in nucleophilic, electrophilic, radical and metal-mediated As(III)-C bond formations while specifically highlighting the behavior of arsenic synthons with various well-established reagents (eg. Grignard reagents, organolithium compounds, organometallic reagents, radical initiators and Lewis/Brønsted bases). Avenues for asymmetric synthesis are also discussed, as are recent advances in organoarsenic chemistry suggesting that arsines exhibit novel reactivities independent from that of other relatively more well explored Group V cogeners.

Biological pincer complexes

At least two types of pincer complexes are known to exist in biology. A metal-pyrroloquinolone quinone (PQQ) cofactor was first identified in bacterial methanol dehydrogenase, and later also found in selected short-chain alcohol dehydrogenases of other microorganisms. The PQQ-associated metal can be calcium, magnesium, or a rare earth element depending on the enzyme sequence. Synthesis of this organic ligand requires a series of accessory proteins acting on a small peptide, PqqA. Binding of metal to PQQ yields an ONO-type pincer complex. More recently, a nickel-pincer nucleotide (NPN) cofactor was discovered in lactate racemase, LarA. This cofactor derives from nicotinic acid adenine dinucleotide via action of a carboxylase/hydrolase, sulfur transferase, and nickel insertase, resulting in an SCS-type pincer complex. The NPN cofactor likely occurs in selected other racemases and epimerases of bacteria, archaea, and a few eukaryotes.

Macrocyclic Metal Complexes Bearing Rigid Polyaromatic Ligands: Synthesis and Catalytic Activity

We synthesised palladium and platinum complexes possessing cyclic and acyclic pincer-type polyaromatic ligands and investigated their structural effect on the catalysis. The pincer-type bis(6-arylpyridin-2-yl)benzene skeleton was constructed via Kröhnke pyridine synthesis under transition metal-free conditions on gram-scale quantity. Ligand structure significantly influenced catalytic activity toward the platinum-catalysed hydrosilylation of diphenyl acetylenes, despite the ligand-independence of the conformations and electronic properties of these complexes.

Employing Aryl-Linked Bis-mesoionic Carbenes as a Pincer-Type Platform to Access Ambient-Stable Palladium(IV) Complexes

The study of palladium(IV) species has great implications for Pd^II /Pd^IV -mediated catalysis. However, most of the Pd^IV complexes rapidly decompose under ambient conditions, which makes the isolation, characterization and further reactivity study very challenging. The reported ancillary ligand platforms to stabilize Pd^IV species are dominated by chelating N-donors such as bipyridines. In this work, we present two Pd^IV complexes with scarcely used C-donors as the supporting platform. The anionic aryl donor and MIC (MIC=mesoionic carbene) are combined in a [CC'C]-type pincer framework to access a series of ambient-stable Pd^IV tris(halido) complexes. Their synthesis, solid-state structures, stability, and reactivity are presented. To the best of our knowledge, the work presented herein reports the first isolated Pd^IV -MIC as well as the first Pd^IV carbene-based aryl pincer.

Csp3 -H Activation without Chelation Assistance in an Iridium Pincer Complex Forming Cyclometallated Products

Cyclometallation of 8-methylquinoline and 2-(dimethylamino)-pyridine in an iridium-based pincer complex is described. The C-H activation of 2-(dimethylamino)pyridine is not chelation assisted, which has not been described before for Csp³ -H bonds in cyclometallation reactions. The mechanism of the cyclometallation of 2-(dimethylamino)pyridine was studied by DFT calculations and kinetic measurements.