Rücktitelbild: The Mo?Mo Quintuple Bond as a Ligand to Stabilize Transition‐Metal Complexes (Angew. Chem. 31/2015)

Herein, we report the employment of the Mo Mo quintuple bonded amidinate complex to stabilize Group 10 metal fragments {(Et₃P)₂M} (M=Pd, Pt) and give rise to the isolation of the unprecedented δ complexes. X‐ray analysis unambiguously revealed short contacts between Pd or Pt and two Mo atoms and a slight elongation of the Mo Mo quintuple bond in these two compounds. Computational studies show donation of the Mo Mo quintuple‐bond δ electrons to an empty σ orbital on Pd or Pt, and back‐donation from a filled Pd or Pt d_π orbital into the Mo Mo δ* level (LUMO), consistent with the Dewar–Chatt–Duncanson model.     

Density functional study of the sigma and pi bond activation at the Pd=X (X = Sn, Si, C) bonds of the (H2PC2H4PH2)Pd=XH2 complexes. Is the bond cleavage homolytic or heterolytic?

The mechanism for the activation of the sigma bonds, the O-H of H2O, C-H of CH4, and the H-H of H2, and the pi bonds, the C[triple bond]C of C2H2, C=C of C2H4, and the C=O of HCHO, at the Pd=X (X = Sn, Si, C) bonds of the model complexes (H2PC2H4PH2)Pd=XH2 5 has been theoretically investigated using a density functional method (B3LYP). The reaction is significantly affected by the electronic nature of the Pd=X bond, and the mechanism is changed depending on the atom X. The activation of the O-H bond with the lone pair electron is heterolytic at the Pd=X (X = Sn, Si) bonds, while it is homolytic at the Pd=C bond. The C-H and H-H bonds without the lone pair electron are also heterolytically activated at the Pd=X bonds independent of the atom X, where the hydrogen is extracted as a proton by the Pd atom in the case of X = Sn, Si and by the C atom in the case of X=C because the nucleophile is switched between the Pd and X atoms depending on the atom X. In contrast, the pi bond activation of C[triple bond]C and C=C at the Pd=Sn bond proceeds homolytically, and is accompanied by the rotation of the (H2PC2H4PH2)Pd group around the Pd-Sn axis to successfully complete the reaction by both the electron donation from the pi orbital to Sn p orbital and the back-donation from the Pd dpi orbital to the pi orbital. On the other hand, the activation of the C=O pi bond with the lone pair electron at the Pd=Sn bond has two reaction pathways: one is homolytic with the rotation of the (H2PC2H4PH2)Pd group and the other is heterolytic without the rotation. The role of the ligands controlling the activation mechanism, which is heterolytic or homolytic, is discussed.     

A Two‐Coordinate Cobalt(II) Imido Complex with NHC Ligation: Synthesis,Structure, and Reactivity

The synthesis, structural characterization, and reactivity of the first two‐coordinate cobalt complex featuring a metal–element multiple bond [(IPr)Co(NDmp)] ( 4 ; IPr=1,3‐bis(2′,6′‐diisopropylphenyl)imidazole‐2‐ylidene; Dmp=2,6‐dimesitylphenyl) is reported. Complex 4 was prepared from the reaction of [(IPr)Co(η²‐vtms)₂] (vtms=vinyltrimethylsilane) with DmpN₃. An X‐ray diffraction study revealed its linear C Co N core and a short Co N distance (1.691(6) Å). Spectroscopic characterization and calculation studies indicated the high‐spin nature of 4 and the multiple‐bond character of the Co N bond. Complex 4 effected group‐transfer reactions to CO and ethylene to form isocyanide and imine, respectively. It also facilitated E H (E=C, Si) σ‐bond activation of terminal alkyne and hydrosilanes to produce the corresponding cobalt(II) alkynyl and cobalt(II) hydride complexes as 1,2‐addition products.     

Enhanced π‐back‐donation resulting in the trans labilization of a pyridine ligand in an N‐heterocyclic carbene (NHC) PdII precatalyst: a case study

The molecular structure of the benzimidazol‐2‐ylidene–PdCl₂–pyridine‐type PEPPSI (pyridine‐enhanced precatalyst, preparation, stabilization and initiation) complex {1,3‐bis[2‐(diisopropylamino)ethyl]benzimidazol‐2‐ylidene‐κC²}dichlorido(pyridine‐κN)palladium(II), [PdCl₂(C₅H₅N)(C₂₃H₄₀N₄)], has been characterized by elemental analysis, IR and NMR spectroscopy, and natural bond orbital (NBO) and charge decomposition analysis (CDA). Cambridge Structural Database (CSD) searches were used to understand the structural characteristics of the PEPPSI complexes in comparison with the usual N‐heterocyclic carbene (NHC) complexes. The presence of weak C—H…Cl‐type hydrogen‐bond and π–π stacking interactions between benzene rings were verified using NCI plots and Hirshfeld surface analysis. The preferred method in the CDA of PEPPSI complexes is to separate their geometries into only two fragments, i.e. the bulky NHC ligand and the remaining fragment. In this study, the geometry of the PEPPSI complex is separated into five fragments, namely benzimidazol‐2‐ylidene (Bimy), two chlorides, pyridine (Py) and the Pd^II ion. Thus, the individual roles of the Pd atom and the Py ligand in the donation and back‐donation mechanisms have been clearly revealed. The NHC ligand in the PEPPSI complex in this study acts as a strong σ‐donor with a considerable amount of π‐back‐donation from Pd to C_carbene. The electron‐poor character of Pd^II is supported by π‐back‐donation from the Pd centre and the weakness of the Pd—N(Py) bond. According to CSD searches, Bimy ligands in PEPPSI complexes have a stronger σ‐donating ability than imidazol‐2‐ylidene ligands in PEPPSI complexes.     

Synthesis and Reactivity of a Neutral,Three‐Coordinate Platinum(II) Complex Featuring Terminal Amido Ligation

A crystallographically characterized three‐coordinate, formally 14 electron Pt^II complex 1 featuring terminal amido ligation is reported. Computational analysis revealed relatively weak π donation from the amide lone pair to platinum and supports a 14‐electron assignment for 1 . Stoichiometric reactivity studies confirmed the viability of net O H and C H addition across, as well as isonitrile insertion into, the terminal platinum–amido linkage of 1 .     

Propane σ‐Complexes on PdO(101): Spectroscopic Evidence of the Selective Coordination and Activation of Primary C?H Bonds

Achieving selective C H bond cleavage is critical for developing catalytic processes that transform small alkanes to value‐added products. The present study clarifies the molecular‐level origin for an exceptionally strong preference for propane to dissociate on the crystalline PdO(101) surface via primary C H bond cleavage. Using reflection absorption infrared spectroscopy (RAIRS) and density functional theory (DFT) calculations, we show that adsorbed propane σ‐complexes preferentially adopt geometries on PdO(101) in which only primary C H bonds datively interact with the surface Pd atoms at low propane coverages and are thus activated under typical catalytic reaction conditions. We show that a propane molecule achieves maximum stability on PdO(101) by adopting a bidentate geometry in which a H Pd dative bond forms at each CH₃ group. These results demonstrate that structural registry between the molecule and surface can strongly influence the selectivity of a metal oxide surface in activating alkane C H bonds.     

Palladium‐Catalyzed C?S Activation/Aryne Insertion/Coupling Sequence: Synthesis of Functionalized 2‐Quinolinones

The insertion of an aryne into a C S bond can suppress the addition of an S nucleophile to the aryne in the presence of palladium. Catalyzed by Pd(OAc)₂, a wide range of α‐carbamoyl ketene dithioacetals readily react with arynes to selectively afford functionalized 2‐quinolinones in high yields under neutral reaction conditions by a C S activation/aryne insertion/intramolecular coupling sequence. The attractive feature of the new strategy also lies in the versatile transformations of the alkythio‐substituted quinolinone products.     

Theoretical studies on the reaction mechanism of palladium(0)-catalyzed addition of thiocyanates to alkynes

The reaction mechanism of the Pd(0)-catalyzed alkyne cyanothiolation reaction is investigated by MP2, CCSD(T) and the density functional method B3LYP. The overall reaction mechanism is examined. The B3LYP results are consistent with the results of CCSD(T) and MP2 methods for the isomerization, acetylene insertion and reductive elimination steps, but not for the oxidative addition step. For the oxidative addition, the bisphosphine and monophosphine pathways are competitive in B3LYP, while the bisphosphine one is preferred for CCSD(T) and MP2 methods. The electronic mechanisms for the oxidative addition of thiocyanate HS-CN to Pd(PH(3))(2) and Pd(PH(3)) and for the acetylene insertion into Pd-S and Pd-CN are discussed in terms of the electron-donation and back-donation. The chemo-selectivity that acetylene inserts into the Pd-S bond rather than into the Pd-CN bond is due to the involvement of the S p orbital. It is the doubly occupied S p unhybridized orbital that donates an electron to the alkylene pi* anti-bonding orbital, which makes insertion into Pd-S bond more favorable than into the Pd-CN bond. During the insertion into the Pd-S bond, the S sp(2) hybrid orbital and unhybridized p orbital transform into each other, while the C sp hybrid orbital shifts its direction for insertion into Pd-CN bond. By using the monosubstituted acetylenes (CN, Me and NH(2)), the influence of substituents at acetylene on the chemo- and regio-selectivities is analyzed.     

Saturated Heavier Group 14 Compounds as ‐Electron‐Acceptor (Z‐Type) Ligands

This review article describes the chemistry of transition‐metal complexes containing heavier group 14 elements (Si, Ge, and Sn) as the σ‐electron‐acceptor (Z‐type) ligands and discusses the characteristics of bonds between the transition metal and Z‐type ligand. Moreover, we review the iridium hydride mediated cleavage of E–X bonds (E=Si, Ge; X=F, Cl), where the key intermediates are pentacoordinate silicon or germanium compounds bearing a dative M→E bond.     

Alkyne Insertion into the M?P and M?H Bonds (M=Pd,Ni, Pt,and Rh): A Theoretical Mechanistic Study of the C?P and C?H Bond‐Formation Steps

In hydrogen‐metal‐phosphorus (H M P) transition metal complexes (proposed as intermediates of H P bond addition to alkynes in the catalytic hydrophosphorylation, hydrophosphinylation, and hydrophospination reactions), alkyne insertion into the metal‐hydrogen bond was found much more facile compared to alkyne insertion into the metal‐phosphorus bond. The conclusion was verified for different metals (Pd, Ni, Pt, and Rh), ligands, and phosphorus groups at various theory levels (B3LYP, B3PW91, BLYP, MP2, and ONIOM). The relative reactivity of the metal complexes in the reaction with alkynes was estimated and decreased in the order of Ni>Pd>Rh>Pt. A trend in relative reactivity was established for various types of phosphorus groups: PR₂>P(O)R₂>P(O)(OR)₂, which showed a decrease in rate upon increasing the number of the oxygen atoms attached to the phosphorus center.     

Mechanism of methylacetylene bisselenation catalyzed by palladium complex from density functional study

The reaction mechanism of Pd(0)-catalyzed methylacetylene bisselenation reaction is investigated by using the density functional method. The overall reaction mechanism involves the oxidative addition, insertion, and reductive elimination steps. The regioselectivity has been investigated for the methylacetylene insertion into Pd-Se bond of both cis and trans palladium complexes. It is found that the methylacetylene insertion into Pd-Se bond of the trans palladium complex using the substituted carbon atom attached to selenyl group is preferred among the four pathways of methylacetylene insertion processes. The electronic mechanisms on the methylacetylene insertion into Pd-Se bond are discussed in terms of the Frontier molecular orbital interactions. In addition, the influence of carbon monoxide on methylacetylene bisselenation was studied and found that the methylacetylene coordination and insertion into Pd-Se bond take place first generating the Pd-C bond, followed by CO insertion into the Pd-C bond.     

Palladium‐Catalyzed Dearomative Cyclocarbonylation by C?N Bond Activation

A fundamentally novel approach to bioactive quinolizinones is based on the palladium‐catalyzed intramolecular cyclocarbonylation of allylamines. [Pd(Xantphos)I₂], which features a very large bite angle, has been found to facilitate the rapid carbonylation of azaarene‐substituted allylamines into bioactive quinolizinones in good to excellent yields. This transformation represents the first dearomative carbonylation and is proposed to proceed by palladium‐catalyzed C N bond activation, dearomatization, CO insertion, and a Heck reaction.     

Mechanisms in the Reaction of Palladium(II)–π‐Allyl Complexes with Aryl Halides: Evidence for NHC Exchange Between Two Palladium Complexes

A detailed experimental and DFT study (PBE level) of the reaction of [Pd(η³‐C₃H₅)(tmiy)(PR₃)]BF₄ (tmiy=tetramethylimidazolin‐2‐ylidene, PR₃=phosphane), precursors to monoligated Pd⁰ species, with aryl electrophiles yielding 2‐arylimidazolium salt is reported. Experiments establish that an autocatalytic ligand transfer mechanism is preferred over Pd^IV and σ‐bond metathesis pathways, and that transmetalation is the rate‐determining step. Calculations indicate that the key step involves the concerted exchange of NHC and iodo ligands between two different Pd^II complexes. This is corroborated by experimental results showing the slower reaction of complexes containing the bulkier dipdmiy (dipdmiy = diisopropyldimethylimidazolin‐2‐ylidene).     

Enantioselective Palladium‐Catalyzed Insertion of α‐Aryl‐α‐diazoacetates into the O?H Bonds of Phenols

A palladium‐catalyzed asymmetric O H insertion reaction was developed. Palladium complexes with chiral spiro bisoxazoline ligands promoted the insertion of α‐aryl‐α‐diazoacetates into the O H bond of phenols with high yield and excellent enantioselectivity under mild reaction conditions. This palladium‐catalyzed asymmetric O H insertion reaction provided an efficient and highly enantioselective method for the preparation of synthetically useful optically active α‐aryl‐α‐aryloxyacetates.     

Ab Initio energetics of Si?O bond cleavage

A multilevel approach that combines high‐level ab initio quantum chemical methods applied to a molecular model of a single, strain‐free Si O Si bridge has been used to derive accurate energetics for Si O bond cleavage. The calculated Si O bond dissociation energy and the activation energy for water‐assisted Si O bond cleavage of 624 and 163 kJ mol⁻¹, respectively, are in excellent agreement with values derived recently from experimental data. In addition, the activation energy for H₂O‐assisted Si O bond cleavage is found virtually independent of the amount of water molecules in the vicinity of the reaction site. The estimated reaction energy for this process including zero‐point vibrational contribution is in the range of −5 to 19 kJ mol⁻¹. © 2017 Wiley Periodicals, Inc.     

Cuprous Oxide Catalyzed Oxidative C?C Bond Cleavage for C?N Bond Formation: Synthesis of Cyclic Imides from Ketones and Amines

Selective oxidative cleavage of a C C bond offers a straightforward method to functionalize organic skeletons. Reported herein is the oxidative C C bond cleavage of ketone for C N bond formation over a cuprous oxide catalyst with molecular oxygen as the oxidant. A wide range of ketones and amines are converted into cyclic imides with moderate to excellent yields. In‐depth studies show that both α‐C H and β‐C H bonds adjacent to the carbonyl groups are indispensable for the C C bond cleavage. DFT calculations indicate the reaction is initiated with the oxidation of the α‐C H bond. Amines lower the activation energy of the C C bond cleavage, and thus promote the reaction. New insight into the C C bond cleavage mechanism is presented.     

Nickel‐Catalyzed Asymmetric Reductive Heck Cyclization of Aryl Halides to Afford Indolines

A nickel‐catalyzed asymmetric reductive Heck reaction of aryl chlorides has been developed that affords substituted indolines with high enantioselectivity. Manganese powder is used as the terminal reductant with water as a proton source. Mechanistically, it is distinct from the palladium‐catalyzed process in that the nickel–carbon bond is converted into a C−H bond to release the product through protonation instead of hydride donation followed by C−H reductive elimination on Pd.     

General Olefin Synthesis by the Palladium‐Catalyzed Heck Reaction of Amides: Sterically Controlled Chemoselective N?C Activation

Metal‐catalyzed reactions of amides proceeding via metal insertion into the N CO bond are severely underdeveloped due to resonance stabilization of the amide bond. Herein we report the first Heck reaction of amides proceeding via highly chemoselective N CO cleavage catalyzed by Pd⁰ utilizing amide bond ground‐state destabilization. Conceptually, this transformation provides access to a myriad of metal‐catalyzed transformations of amides via metal insertion/decarbonylation.     

Observation of Main‐Group Tricarbonyls [B(CO)3] and [C(CO)3]+ Featuring a Tilted One‐Electron Donor Carbonyl Ligand

A combined experimental and theoretical study on the main‐group tricarbonyls [B(CO)₃] in solid noble‐gas matrices and [C(CO)₃]⁺ in the gas phase is presented. The molecules are identified by comparing the experimental and theoretical IR spectra and the vibrational shifts of nuclear isotopes. Quantum chemical ab initio studies suggest that the two isoelectronic species possess a tilted η¹(μ₁‐CO)‐bonded carbonyl ligand, which serves as an unprecedented one‐electron donor ligand. Thus, the central atoms in both complexes still retain an 8‐electron configuration. A thorough analysis of the bonding situation gives quantitative information about the donor and acceptor properties of the different carbonyl ligands. The linearly bonded CO ligands are classical two‐electron donors that display classical σ‐donation and π‐back‐donation following the Dewar–Chatt–Duncanson model. The tilted CO ligand is a formal one‐electron donor that is bonded by σ‐donation and π‐back‐donation that involves the singly occupied orbital of the radical fragments [B(CO)₂] and [C(CO)₂]⁺.     

Development, mechanism, and scope of the palladium-catalyzed enantioselective allene diboration

In the presence of a chiral phosphoramidite ligand, the palladium-catalyzed diboration of allenes can be executed with high enantioselectivity. This reaction provides high levels of selectivity with a range of aromatic and aliphatic allene substrates. Isotopic-labeling experiments, stereodifferentiating reactions, kinetic analysis, and computational experiments suggest that the catalytic cycle proceeds by a mechanism involving rate-determining oxidative addition of the diboron to Pd followed by transfer of both boron groups to the unsaturated substrate. This transfer reaction most likely occurs by coordination and insertion of the more accessible terminal alkene of the allene substrate, by a mechanism that directly provides the eta3 pi-allyl complex in a stereospecific, concerted fashion.