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18F‐Labeled molecular tracers and subsequent positron emission tomography are indispensable molecular imaging tools in medical diagnosis and research. The preparation of 18F‐labeled molecular tracers involves critical steps such as the 18F‐labeling reaction, work‐up, and 18F‐product purification, which are governed by 18F‐labeling chemistry. Since direct incorporation of 18F in aqueous media exhibits many advantages in practice, this Review summarizes the existing 18F‐labeling methods in aqueous media, which are sorted by atoms forming chemical covalent bonds with F. The Review is focused on the respective reaction mechanism, the water effect and the applications of these methods for the development of 18F‐radiopharmaceuticals. The research progress on aqueous nucleophilic labeling methods using [18F]F− as the 18F source has been mainly discussed. [ABSTRACT FROM AUTHOR]

Thia‐Michael additions (1,4‐additions of a thiol to a Michael acceptor) are generally catalyzed by an external Brønsted or Lewis base. A spontaneous (uncatalyzed) Michael addition of thiols to α‐trifluoromethyl acrylates is described, as well as its application to the very efficient preparation of a thermoset. A thorough mechanistic investigation, based on an experimental kinetic study and on DFT calculations, is presented for the addition of arene‐ and alkanethiols to tert‐butyl trifluoromethyl acrylate in polar aprotic solvents, unveiling a probable solvent‐assisted proton transfer in the rate‐determining step and a considerable lowering of the energy barrier induced by the CF3 group. [ABSTRACT FROM AUTHOR]

The cationic complex [Ni(H)(OH)]+ was previously found to activate dioxygen and methane in gas phase under single collision conditions. These remarkable reactivities were thought to originate from a non‐classical electronic structure, where the Ni‐center adopts a Ni(II), instead of the classically expected Ni(III) oxidation state by formally accepting an electron from the hydroxo ligand, which formally becomes a hydroxyl radical in the process. Such radicaloid oxygen moieties are envisioned to easily react with otherwise inert substrates, mimicking familiar reactivities of free radicals. In this study, the reductive activation of dioxygen by [Ni(H)(OH)]+ to afford the hydroperoxo species was investigated using coupled cluster, multireference ab initio and density functional theory calculations. Orbital and wave function analyses indicate that O2 binding tranforms the aforementioned non‐classical electronic structure to a classical Ni(III)‐hydroxyl system, before O2 reduction takes place. Remarkably, we found no evidence for a direct involvement of the radicaloid hydroxyl in the reaction with O2, as is often assumed. The function of the redox non‐innocent character of the activator complex is to protect the reactive electronic structure until the complex engages O2, upon which a dramatic electronic reorganization releases internal energy and drives the chemical reaction to completion. [ABSTRACT FROM AUTHOR]

Molybdenum dinitrogen complexes have played a major role as catalytic model systems of nitrogenase. In comparison, analogous tungsten complexes have in most cases found to be catalytically inactive. Herein, a tungsten complex was shown to be supported by a pentadentate tetrapodal (pentaPod) phosphine ligand, under conditions of N2 fixation, primarily catalyzes the hydrogen evolution reaction (HER), in contrast to its Mo analogue, which catalytically mediates the nitrogen‐reduction reaction (N2RR). DFT calculations were employed to evaluate possible mechanisms and identify the most likely pathways of N2RR and HER activities exhibited by Mo‐ and W‐pentaPod complexes. Two mechanisms for N2RR by PCET are considered, starting from neutral (M(0) cycle) and cationic (M(I) cycle) dinitrogen complexes (M=Mo, W). The latter was found to be energetically more favorable. For HER three scenarios are treated; that is, through bimolecular reactions of early M‐NxHy intermediates, pure hydride intermediates or mixed M(H)(NxHy) species. [ABSTRACT FROM AUTHOR]

UV irradiation of solutions of a guanidinate coordinated dimagnesium(I) compound, [{(Priso)Mg}2] 3 (Priso=[(DipN)2CNPri2]−, Dip=2,6‐diisopropylphenyl), in either benzene, toluene, the three isomers of xylene, or mesitylene, leads to facile activation of an aromatic C−H bond of the solvent in all cases, and formation of aryl/hydride bridged magnesium(II) products, [{(Priso)Mg}2(μ‐H)(μ‐Ar)] 4–9. In contrast to similar reactions reported for β‐diketiminate coordinated counterparts of 3, these C−H activations proceed with little regioselectivity, though they are considerably faster. Reaction of 3 with an excess of the pyridine, p‐NC5H4But (pyBut), gave [(Priso)Mg(pyButH)(pyBut)2] 10, presumably via reduction of the pyridine to yield a radical intermediate, [(Priso)Mg(pyBut⋅)(pyBut)2] 11, which then abstracts a proton from the reaction solvent or a reactant. DFT calculations suggest two possible pathways to the observed arene C−H activations. One of these involves photochemical cleavage of the Mg−Mg bond of 3, generating magnesium(I) doublet radicals, (Priso)Mg⋅. These then doubly reduce the arene substrate to give "Birch‐like" products, which subsequently rearrange via C−H activation of the arene. Circumstantial evidence for the photochemical generation of transient magnesium radical species includes the fact that irradiation of a cyclohexane solution of 3 leads to an intramolecular aliphatic C−H activation process and formation of an alkyl‐bridged magnesium(II) species, [{Mg(μ‐Priso−H)}2] 12. Furthermore, irradiation of a 1 : 1 mixture of 3 and the β‐diketiminato dimagnesium(I) compound, [{(DipNacnac)Mg}2] (DipNacnac=[HC(MeCNDip)2]−), effects a "scrambling" reaction, and the near quantitative formation of an unsymmetrical dimagnesium(I) compound, [(Priso)Mg−Mg(DipNacnac)] 13. Finally, the EPR spectrum (77 K) of a glassed solution of UV irradiated 3 is dominated by a broad featureless signal, indicating the presence of a doublet radical species. [ABSTRACT FROM AUTHOR]

Vitrimers are a third class of polymers gathering the mechanical properties and solvent resistance of 3D thermosets and the reprocessability of thermoplastics. This unique behaviour is due to the triggering of certain covalent exchange reactions that allow the network to rearrange upon application of a stimulus. The constitutive feature of vitrimers is the adoption of a glass‐like viscosity during the rearrangement of the network, often due to an associative mechanism for the exchange reaction. Transesterification networks are one of the most studied type of vitrimers that usually require the incorporation of a catalyst, implying the associated drawbacks. Following up on a recent report on catalyst‐free transesterification vitrimers in which the ester functions are particularly reactive thanks to the presence of fluorine atoms in α‐ or β‐position, parallel DFT calculations and an experimental kinetic study on model molecules are presented in order to quantitatively assess the effect of neighbouring fluorinated groups on the transesterification reaction rate. [ABSTRACT FROM AUTHOR]

The oxidation of [(Cp*Mo)2(μ,η6:η6‐P6)] (1) with halogens or halogen sources was investigated. The iodination afforded the ionic complexes [(Cp*Mo)2(μ,η3:η3‐P3)(μ,η1:η1:η1:η1‐P3I3)][X] (X=I3−, I−) (2) and [(Cp*Mo)2(μ,η4:η4‐P4)(μ‐PI2)][I3] (3), while the reaction with PBr5 led to the complexes [(Cp*Mo)2(μ,η3:η3‐P3)(μ‐Br)2][Cp*MoBr4] (4) [(Cp*MoBr)2(μ,η3:η3‐P3)(μ,η1‐P2Br3)] (5) and [(Cp*Mo)2(μ‐PBr2)(μ‐PHBr)(μ‐Br)2] (6). The reaction of 1 with the far stronger oxidizing agent PCl5 was followed via time‐ and temperature‐dependent 31P{1H} NMR spectroscopy. One of the first intermediates detected at 193 K was [(Cp*Mo)2(μ,η3:η3‐P3)(μ‐PCl2)2][PCl6] (8) which rearranges upon warming to [(Cp*Mo)2(μ‐PCl2)2(μ‐Cl)2] (9), [(Cp*MoCl)2(μ,η3:η3‐P3)(μ‐PCl2)] (10) and [(Cp*Mo)2(μ,η4:η4‐P4)(μ‐PCl2)][Cp*MoCl4] (11), which could be isolated at room temperature. All complexes were characterized by single‐crystal X‐ray diffraction, NMR spectroscopy and their electronic structures were elucidated by DFT calculations. [ABSTRACT FROM AUTHOR]

To investigate how remotely induced changes in ligand folding might affect catalysis by organometallic complexes, dynamic α‐amino‐iso‐butyric acid (Aib) peptide foldamers bearing rhodium(I) N‐heterocyclic carbene (NHC) complexes have been synthesized and studied. X‐ray crystallography of a foldamer with an N‐terminal azide and a C‐terminal Rh(NHC)(Cl)(diene) complex showed a racemate with a chiral axis in the Rh(NHC) complex and a distorted 310 helical body. Replacing the azide with either one or two chiral L‐α‐methylvaline (L‐αMeVal) residues gave diastereoisomeric foldamers that each possessed point, helical and axial chirality. NMR spectroscopy revealed an unequal ratio of diastereoisomers for some foldamers, indicating that the chiral conformational preference of the N‐terminal residue(s) was relayed down the 1 nm helical body to the axially chiral Rh(NHC) complex. Although the remote chiral residue(s) did not affect the stereoselectivity of hydrosilylation reactions catalysed by these foldamers, these studies suggest a potential pathway towards remote conformational control of organometallic catalysts. [ABSTRACT FROM AUTHOR]

Oligomers of α‐aminoisobutyric acid (Aib) are achiral peptides that adopt 310 helical structures with equal population of left‐ and right‐handed conformers. The screw‐sense preference of the helical chain may be controlled by a single chiral residue located at one terminus. 1H and 19F NMR, X‐ray crystallography and circular dichroism studies on new Aib oligomers show that the incorporation of a chiral quaternary α‐trifluoromethylalanine at their N‐terminus induces a reversal of the screw‐sense preference of the 310‐helix compared to that of a non‐fluorinated analogue having an l‐α‐methyl valine residue. This work demonstrates that, among the many particular properties of introducing a trifluoromethyl group into foldamers, its stereo‐electronic properties are of major interest to control the helical screw sense. Its use as an easy‐to‐handle 19F NMR probe to reliably determine both the magnitude of the screw‐sense preference and its sign assignment is also of remarkable interest. [ABSTRACT FROM AUTHOR]

Fix me! Molybdenum–dinitrogen complexes containing ferrocenyldiphosphine and pentamethylcyclopentadienyl moieties as auxiliary ligands have been designed, prepared, and characterized (see scheme). The ferrocenyldiphosphine works as a unique ligand to inhibit dissociation of dinitrogen, as well as to make the coordinated molecular dinitrogen reactive toward electrophiles. [ABSTRACT FROM AUTHOR]

Indenyl (Ind) rhodium N‐heterocyclic carbene (NHC) complexes [Rh(η5‐Ind)(NHC)(L)] were synthesised for 1,3‐bis(2,6‐diisopropylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIPr) with L=C2H4 (1), CO (2 a) and cyclooctene (COE; 3), for 1,3‐bis(2,4,6‐trimethylphenyl)‐4,5‐dihydroimidazol‐2‐ylidene (SIMes) with L=CO (2 b) and COE (4), and 1,3‐bis(2,4,6‐trimethylphenyl)imidazol‐2‐ylidene (IMes) with L=CO (2 c) and COE (5). Reaction of SIPr with [Rh(Cp*)(C2H4)2] did not give the desired SIPr complex, thus demonstrating the "indenyl effect" in the synthesis of 1. Oxidative addition of HSi(OEt)3 to 3 proceeded under mild conditions to give the Rh silyl hydride complex [Rh(Ind){Si(OEt)3}(H)(SIPr)] (6) with loss of COE. Tethered‐fluorenyl NHC rhodium complexes [Rh{(η5‐C13H8)C2H4N(C)C2HxNR}(L)] (x=4, R=Dipp, L=C2H4: 11; L=COE: 12; L=CO: 13; R=Mes, L=COE: 14; L=CO: 15; x=2, R=Me, L=COE: 16; L=CO: 17) were synthesised in low yields (5–31 %) in comparison to good yields for the monodentate complexes (49–79 %). Compounds 3 and 1, which contain labile alkene ligands, were successful catalysts for the catalytic borylation of benzene with B2pin2 (Bpin=pinacolboronate, 97 and 93 % PhBpin respectively with 5 mol % catalyst, 24 h, 80 °C), with SIPr giving a more active catalyst than SIMes or IMes. Fluorenyl‐tethered NHC complexes were much less active as borylation catalysts, and the carbonyl complexes were inactive. The borylation of toluene, biphenyl, anisole and diphenyl ether proceeded to give meta substitutions as the major product, with smaller amounts of para substitution and almost no ortho product. The borylation of octane and decane with B2pin2 at 120 and 140 °C, respectively, was monitored by 11B NMR spectroscopy, which showed high conversions into octyl and decylBpin over 4–7 days, thus demonstrating catalysed sp3 C−H borylation with new piano stool rhodium indenyl complexes. Irradiation of the monodentate complexes with 400 or 420 nm light confirmed the ready dissociation of C2H4 and COE ligands, whereas CO complexes were inert. Evidence for C−H bond activation in the alkyl groups of the NHC ligands was obtained. [ABSTRACT FROM AUTHOR]

We report the synthesis and characterization of novel triphosphine calix[6]arene ligands. These supramolecular wheels, with recognition features governed by the hydrogen‐bonding domain, were employed to synthesize multitasking trinuclear gold(I) complexes as a new platform for the synthesis of interwoven (pseudo)rotaxane species. In parallel, the multivalent, metal‐bonded upper rim displayed catalytic features promoting highly selective gold‐catalyzed cycloisomerization reactions of 1,6‐enynes. [ABSTRACT FROM AUTHOR]

Two series of the ferrocenyl and ruthenocenyl analogues of etoposide bearing 1,2,3‐triazolyl or aminoalkyl linker were synthesized and evaluated for their cytotoxic properties, influence on the cell cycle, ability to induce tubulin polymerization, and inhibition of topoisomerase II activity. We found that the replacement of the etoposide carbohydrate moiety with a metallocenyl group led to organometallic conjugates exhibiting differentiated antiproliferative activity. Biological studies demonstrated that two ferrocenylalkylamino conjugates were notably more active than etoposide, with submicromolar or low‐micromolar IC50 values towards SW620, etoposide‐resistant SW620E, and methotrexate‐resistant SW620M cancer cell lines. Moreover, the simplest ferrocenylmethylamino conjugate exerted dual inhibitory action against tubulin polymerization and topoisomerase II activity while other studied compounds affected only topoisomerase II activity. [ABSTRACT FROM AUTHOR]

cis and trans‐copper(II) porphyrin dimers have been synthesized, in which two CuII porphyrin macrocycles are bridged through a rigid ethene linker for possible through‐space and through‐bond spin‐couplings between the paramagnetic CuII centers. It has been found that the two macrocycles come closer after 1 e− oxidation, however, they move far apart upon further 1 e− oxidation leading to transformation of the cis to the trans isomer. Detailed investigations are performed here on the interactions between the two porphyrin macrocycles, between two unpaired spins of closely spaced CuII centers, and also between the unpaired spins of metal and porphyrin π–cation radicals. Spectroscopic investigations along with the X‐ray structure of the 2 e−‐oxidized complex displayed strong electronic communications through the bridge between two porphyrin π–cation radicals. The counterion I9− is stabilized in an unusual trigonal‐pyramidal structure in the 2 e−‐oxidized complex in which the central iodide ion is bound with four iodine (I2) molecules. Variable‐temperature magnetic study revealed strong antiferromagnetic coupling between the two porphyrin π–cation radical spins (Jr–r) in the 2 e−‐oxidized complex. DFT calculations suggest stabilization of the triplet state, which is also in good agreement with the experiment. Ab initio molecular dynamics allowed the variation of the structural details to be followed upon stepwise oxidations and also the final isomerization process including its associated energy barrier. [ABSTRACT FROM AUTHOR]

The palladium‐catalyzed reaction of 4‐iodo‐2‐quinolones with activated alkynes was investigated. Cyclopenta[de]quinoline‐2(1 H)‐ones and/or phenanthridine‐6(5 H)‐ones were obtained through [3+2] annulation involving aromatic C−H activation or [2+2+2] annulation involving vinylic C−H activation, respectively. Reasonable mechanisms for the formation of these annulation products have been proposed based on density functional theory calculations. [ABSTRACT FROM AUTHOR]

Spin‐forbidden reactions are frequently encountered when transition metal oxo species are involved, particularly in oxygen transfer reactivity. The computational study of such reactions is challenging, because reactants and products are located on different spin potential energy surfaces (PESs). One possible approach to describe these reactions is the so‐called minimum energy crossing point (MECP) between the diabatic reactants and products PESs. Alternatively, inclusion of spin‐orbit coupling (SOC) effects allows to locate a saddle point on a single adiabatic PES (TS SOC). The TS SOC approach is rarely applied because of its high computational cost. Recently evidence for a TS SOC impact on significantly lowering the activation barrier in dioxygen addition to a carbene‐gold(I)‐hydride complex reaction (Chem. Sci. 2016, 7, 7034–7039) or even on predicting a qualitatively different reaction mechanism in mercury methylation by cobalt corrinoid (Angew. Chem. Int. Ed. 2016, 55, 11503–11506) has been put forward. Using MECP and TS SOC approaches a systematic analysis is provided here of three prototypical transition metal oxo spin‐forbidden processes to investigate their implications on reactivity. Cycloaddition of ethylene to chromyl chloride (CrO2Cl2+C2H4), iron oxide cation insertion into the hydrogen molecule (FeO++H2) and H‐abstraction from toluene by a MnV‐oxo‐porphyrin cation (MnOP(H2O)++C6H5CH3) are case studies. For all these processes the MECP and TS SOC results are compared, which show that the spin‐forbidden reactivity of transition metal oxo species can be safely described by a MECP approach, at least for the first‐row transition metals investigated here, where the spin‐orbit coupling is relatively weak. However, for the Mn‐oxo reactivity, the MECP and TS SOC have been found to be crucial for a correct description of the reaction mechanism. In particular, the TS SOC approach allows to straightforwardly explore detailed features of the adiabatic potential energy surface which in principle could affect the overall reaction rate in cases where the involved diabatic PESs are tricky. [ABSTRACT FROM AUTHOR]

The aerial oxidation of PdII to PdIV has emerged as an integral component of sustainable catalytic C−H functionalization processes. However, a proper understanding of the factors that control the viability of this oxidative process remains elusive. An investigation of the intricate mechanism of the transmetalation reaction of the aerial oxidative transformation of [(Me3tacn)PdIIMe2] (Me3tacn=N,N′,N′′‐trimethyl‐1,4,7‐triazacyclononane) to [(Me3tacn)PdIVMe3]+ has been conducted by using DFT, along with multireference methods, such as second‐order n‐electron valence‐state perturbation theory (NEVPT2) with complete active space self‐consistent field theory (CASSCF). The present endeavor predicts that the thermodynamics and kinetics of the oxygen activation step are primarily dictated by the polarity of the solvents, which determine the amount of charge transfer to the oxygen molecule from the PdII center. Additionally, it is observed that the presence of a protic solvent has a significant effect on the spin–orbit coupling term at the minimum energy crossing point of the triplet and singlet surfaces. Moreover, it is shown that the intermetal ligand‐transfer phenomenon is an important instance of an oxygen‐assisted SN2 reaction. [ABSTRACT FROM AUTHOR]

One‐electron reduction of corannulene, C20H10, with Li metal in diglyme resulted in crystallization of [{Li+(diglyme)2}4(C20H10.−)2(C20H10‐C20H10)2−] (1), as revealed by single‐crystal X‐ray diffraction. This hybrid product contains two corannulene monoanion‐radicals along with a dianionic dimer, crystallized with four Li+ ions wrapped by diglyme molecules. The dimeric (C20H10‐C20H10)2− anion provides the first crystallographically confirmed example of spontaneous radical dimerization for C20H10.−. The C−C bond length between the two C20H10.− bowls of 1.588(5) Å is consistent with the single σ‐bond character of the linker. The trans‐disposition of two bowls in the centrosymmetric (C20H10‐C20H10)2− dimer is observed with the torsion angle around the central C−C bond of 180°. Comprehensive theoretical analysis of formation/decomposition processes of the dimeric dianion has been carried out in order to evaluate the nature of bonding and energetics of the C20H10.− coupling. It is found that such σ‐bonded dimers are thermodynamically unstable due to large preparation energy and repulsive Pauli component of the bonding, but kinetically persistent due to a high energy barrier provided by the existing spin‐crossing point. [ABSTRACT FROM AUTHOR]

The practical application of Shilov‐type Pt catalysis to the selective hydroxylation of terminal aliphatic C−H bonds remains a formidable challenge, due to difficulties in replacing PtIV with a more economically viable oxidant, particularly O2. We report the potential of employing FeCl2 as a suitable redox mediator to overcome the kinetic hurdles related to the direct use of O2 in the Pt reoxidation. For the selective conversion of butyric acid to γ‐hydroxybutyric acid (GHB), a significantly enhanced catalyst activity and stability (turnover numbers (TON)>30) were achieved under 20 bar O2 in comparison to current state‐of‐the‐art systems (TON<10). In this regard, essential reaction parameters affecting the overall activity were identified, along with specific additives to attain catalyst stability at longer reaction times. Notably, deactivation by reduction to Pt0 was prevented by the addition of monodentate pyridine derivatives, such as 2‐fluoropyridine, but also by introducing varying partial pressures of N2 in the gaseous atmosphere. Finally, stability tests revealed the involvement of PtII and FeCl2 in catalyzing the non‐selective overoxidation of GHB. Accordingly, in situ esterification with boric acid proved to be a suitable strategy to maintain enhanced selectivities at much higher conversions (TON>60). Altogether, a useful catalytic system for the selective hydroxylation of primary aliphatic C−H bonds with O2 is presented. [ABSTRACT FROM AUTHOR]

The hydrogenolysis of mono‐ and dinuclear PdII hydroxides was investigated both experimentally and computationally. It was found that the dinuclear μ‐hydroxide complexes {[(PCNR)Pd]2(μ‐OH)}(OTf) (PCNH=1‐[3‐[(di‐tert‐butylphosphino)methyl]phenyl]‐1H‐pyrazole; PCNMe=1‐[3‐[(di‐tert‐butylphosphino)methyl]phenyl]‐5‐methyl‐1H‐pyrazole) react with H2 to form the analogous dinuclear hydride species {[(PCNR)Pd]2(μ‐H)}(OTf). The dinuclear μ‐hydride complexes were fully characterized, and are rare examples of structurally characterized unsupported singly bridged μ‐H PdII dimers. The {[(PCNMe)Pd]2(μ‐OH)}(OTf) hydrogenolysis mechanism was investigated through experiments and computations. The hydrogenolysis of the mononuclear complex (PCNH)Pd‐OH resulted in a mixed ligand dinuclear species [(PCNH)Pd](μ‐H)[(PCC)Pd] (PCC=a dianionic version of PCNH bound through phosphorus P, aryl C, and pyrazole C atoms) generated from initial ligand "rollover" C−H activation. Further exposure to H2 yields the bisphosphine Pd0 complex Pd[(H)PCNH]2. When the ligand was protected at the pyrazole 5‐position in the (PCNMe)Pd−OH complex, no hydride formed under the same conditions; the reaction proceeded directly to the bisphosphine Pd0 complex Pd[(H)PCNMe]2. Reaction mechanisms for the hydrogenolysis of the monomeric and dimeric hydroxides are proposed. [ABSTRACT FROM AUTHOR]

Despite being weak attractive forces, closed‐shell metallophilic interactions play important roles in the Group 11 metal complexes on their diverse structural and physical features. A plethora of experimental and computational studies has thus been dedicated to such weak attractive d10–d10 interactions, particularly aurophilic and argentophilic interactions. Although d10–d10 CuI–CuI forces had been recognized for four decades, cuprophilic interactions are less explored and they are best evidenced by single‐crystal X‐ray crystallographic analysis on CuI complexes and aggregates thereof, by which precise information about the Cu⋅⋅⋅Cu contacts, shorter than the sum of two van der Waals radii (3.92 Å) between the copper centers concerned can be obtained. Based on recently compelling experimental and spectroscopic evidence for intra‐ and intermolecular cuprophilic interactions in copper chemistry, the present Minireview summarizes recent progress in the past three decades in the synthesis and structures of multinuclear homometallic copper complexes, whereby supported and unsupported d10–d10 CuI–CuI interactions are at work.(Non)‐debatable interactions? The characterizations of cuprophilic interactions by X‐ray crystallography and Raman spectroscopy in complexes containing multinuclear copper(I) centers is presented. Furthermore, factors affecting the cuprophilic interactions (e.g., ligand scaffold, donor nature, coordination number, charge) are discussed. [ABSTRACT FROM AUTHOR]

Despite being weak attractive forces, closed‐shell metallophilic interactions play important roles in the Group 11 metal complexes on their diverse structural and physical features. A plethora of experimental and computational studies has thus been dedicated to such weak attractive d10–d10 interactions, particularly aurophilic and argentophilic interactions. Although d10–d10 CuI–CuI forces had been recognized for four decades, cuprophilic interactions are less explored and they are best evidenced by single‐crystal X‐ray crystallographic analysis on CuI complexes and aggregates thereof, by which precise information about the Cu⋅⋅⋅Cu contacts, shorter than the sum of two van der Waals radii (3.92 Å) between the copper centers concerned can be obtained. Based on recently compelling experimental and spectroscopic evidence for intra‐ and intermolecular cuprophilic interactions in copper chemistry, the present Minireview summarizes recent progress in the past three decades in the synthesis and structures of multinuclear homometallic copper complexes, whereby supported and unsupported d10–d10 CuI–CuI interactions are at work. [ABSTRACT FROM AUTHOR]

Abstract: A practical homogeneous and gas‐liquid palladium‐catalyzed Catellani‐type reaction using a continuous‐flow platform is described. The implementation of continuous‐flow technology allowed the acceleration of the transformation and, for the first time, expansion of the chemical space to gaseous olefins (i.e., ethylene, propylene and 3,3,3‐trifluoropropene), thus providing a safe and practical approach to sterically hindered ortho‐disubstituted styrenes and vinyl arenes. The complete control over the stoichiometry of gaseous reagents through flow technology proved essential for directing the selectivity of the Catellani reaction to the desired products. [ABSTRACT FROM AUTHOR]

Abstract: Helical α‐aminoisobutyric acid (Aib) foldamers show great potential as devices for the communication of conformational information across phospholipid bilayers, but determining their conformation in bilayers remains a challenge. In the present study, Raman, Raman optical activity (ROA), infrared (IR) and vibrational circular dichroism (VCD) spectroscopies have been used to analyze the conformational preferences of Aib foldamers in solution and when interacting with bilayers. A 310‐helix marker band at 1665–1668 cm−1 in Raman spectra was used to show that net helical content increased strongly with oligomer length. ROA and VCD spectra of chiral Aib foldamers provided the chiroptical signature for both left‐ and right‐handed 310‐helices in organic solvents, with VCD establishing that foldamer screw‐sense was preserved when the foldamers became embedded within bilayers. However, the population distribution between different secondary structures was perturbed by the chiral phospholipid. These studies indicate that ROA and VCD spectroscopies are valuable tools for the study of biomimetic structures, such as artificial signal transduction molecules, in phospholipid bilayers. [ABSTRACT FROM AUTHOR]

Abstract: A spin‐forbidden chemical reaction involves a change in the total electronic spin state from reactants to products. The mechanistic study is challenging because such a reaction does not occur on a single diabatic potential energy surface (PES), but rather on two (or multiple) spin diabatic PESs. One possible approach is to calculate the so‐called “minimum energy crossing point” (MECP) between the diabatic PESs, which however is not a stationary point. Inclusion of spin–orbit coupling between spin states (SOC approach) allows the reaction to occur on a single adiabatic PES, in which a transition state (TS SOC) as well as activation free energy can be calculated. This Concept article summarizes a previously published application in which, for the first time, the SOC effects, using spin–orbit ZORA Hamiltonian within density functional theory (DFT) framework, are included and account for the mechanism of a spin‐forbidden reaction in gold chemistry. The merits of the MECP and TS SOC approaches and the accuracy of the results are compared, considering both our recent calculations on molecular oxygen addition to gold(I)‐hydride complexes and new calculations for the prototype spin‐forbidden N2O and N2Se dissociation reactions. [ABSTRACT FROM AUTHOR]

Abstract: Oxidation of nonmetallic singlet molecules by oxygen has its own share of intricacies. Herein, by means of DFT and ab initio techniques, mechanistic details of the aerial oxidation of an N‐heterocyclic carbene (NHC) stabilized diphosphorus complex are revealed. This particular oxidation process is known to produce an unexpected P−P bond containing diphosphorus tetroxide complex, instead of the more thermodynamically stable oxo‐bridged (P−O−P) compound. These findings suggest that the P−P bond containing less stabilized species is a kinetically controlled product (KCP) and obtained due to the presence of lower lying transition states (TSs) in the pathway leading to its formation, relative to the higher lying corresponding minimum‐energy crossing points (MECPs) present in the pathway involved in the formation of the oxo‐bridged species, which is the thermodynamically controlled product (TCP). Thus, an intriguing variant of the well‐known KCP/TCP phenomenon is presented here, in which the KCP is formed not by merely traditionally known lower barrier heights of TSs involved in the formation of KCP, but by faster transmission of a system through a low barrier TS relative to a higher lying MECP. Additionally, the faster kinetics of an irreversible unimolecular O−O dissociation step, which avoids the formation of the TCP is a contributing factor in dictating the fate of the reaction. The insights provided herein may help to understand the oxidation of other P−P‐containing species, such as black phosphorene. [ABSTRACT FROM AUTHOR]

Abstract: Though dinitrosyl‐iron complexes (DNICs) are largely believed to act as NO carriers, several experiments on model DNICs have suggested that they can also act as nitrating agents in presence of dioxygen. Oxygen activation by DNICs has been implicated as a possible route for protein tyrosine nitration (PTN), which leads to neurodegenerative disorders. Herein using static and dynamic theoretical techniques we unravel a previously unknown dual state mechanistic paradigm for dioxygen activation of a biomimetic nitrating DNIC complex leading to phenolic nitration. Our computations reveal that the model DNIC, the ground electronic state of which is singlet, has a low‐lying triplet state and an inherent singlet–triplet spin‐crossover of DNICs can be triggered by fluxional changes in the bite angle of the two NO ligands. The presence of a low‐lying triplet state in the DNIC affords an avenue for O2 activation other than a direct O2 activation by O2‐induced spin‐crossover of the singlet ground state. These two low‐lying channels facilitate the formation of a peroxynitrite species. Nitration of phenolic substrates is facilitated by the release of NO2. The corresponding minimum energy crossing points (MECP) have been located. Along the reaction path, the changes in the electronic structure scenarios have been studied and interpreted. Our report also sheds light on the plausible mechanistic pathway of PTN by reactive species formed once O2 activation by DNICs have been achieved. [ABSTRACT FROM AUTHOR]

Abstract: The ability of neutral transition‐metal hydrides to serve as a source of hydride ion H− or proton H+ is well appreciated. The hydride ligands possessing a partly negative charge are proton accepting sites, forming a dihydrogen bond, M−Hδ−⋅⋅⋅δ+HX (M=transition metal or metalloid). On the other hand, some metal hydrides are able to serve as a proton source and give hydrogen bond of M−Hδ+⋅⋅⋅X type (X=organic base). In this paper we analyse recent works on transition‐metal and boron hydrides showing i) how formation of an intermolecular complex between the reactants changes the Z−H (M−H and X−H) bond polarity and ii) what is the implication of such activation in the mechanisms of hydrides reactions. [ABSTRACT FROM AUTHOR]

The relationship between chiral centers and the helical-screw control of their peptides has already been reported, but it has yet to be elucidated in detail. A chiral four-membered ring α,α-disubstituted α-amino acid with a (R,R)-butane-2,3-diol acetal moiety at the γ-position, but no α-chiral carbon, was synthesized. X-ray crystallographic analysis unambiguously revealed that its homo-chiral heptapeptide formed right-handed (P) and left-handed (M) 310-helical structures at a ratio of 1:1. They appeared to be enantiomeric at the peptide backbone, but diastereomeric with fourteen (R)-configuration chiral centers. Conformational analyses of homopeptides in solution also indicated that diastereomeric (P) and (M) helices existed at approximately equal amounts, with a slight preference toward right-handedness, and they quickly interchanged at room temperature. The circumstances of chiral centers are important for the control of their helical-screw direction. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations have been used to study the oxidative addition of aryl halides to complexes of the type [Ni(PMenPh(3− n))4], revealing the crucial role of an open-shell singlet transition state for halide abstraction. The formation of NiI versus NiII has been rationalised through the study of three different pathways: (i) halide abstraction by [Ni(PMenPh(3− n))3], via an open-shell singlet transition state; (ii) SN2-type oxidative addition to [Ni(PMenPh(3− n))3], followed by phosphine dissociation; and (iii) oxidative addition to [Ni(PMenPh(3− n))2]. For the overall reaction between [Ni(PMe3)4], PhCl, and PhI, a microkinetic model was used to show that our results are consistent with the experimentally observed ratios of NiI and NiII when the PEt3 complex is used. Importantly, [Ni(PMenPh(3− n))2] complexes often have little, if any, role in oxidative addition reactions because they are relatively high in energy. The behaviour of [Ni(PR3)4] complexes in catalysis is therefore likely to differ considerably from those based on diphosphine ligands in which two coordinate Ni0 complexes are the key species undergoing oxidative addition. [ABSTRACT FROM AUTHOR]

Peptide foldamers have been studied for over two decades and numerous sequence patterns have been shown to form well-defined three-dimensional arrangements in solution. In particular, helices of various geometries have been described. In this article, different concepts concerning the construction of helical foldameric peptides, for which the possibility of governing the sense of the formed helix was evidenced, are presented and discussed. [ABSTRACT FROM AUTHOR]

The 2-aminophenylaluminum dihydride (2-TMP-C6H4)AlH2 ( 2) has been prepared and characterized for the first time. Compound 2 features an intramolecular N⋅⋅⋅Al donor-acceptor bond. 2 reacted with N-methylpyrrole and N-methylindole (both at 50 °C) by means of the elusive AlH C(sp2)−H dehydroalumination to aluminum heteroaryls ( 3 and 4). Moreover, 2 reacted with PhCCSiMe3 (at room temperature) and Ph2CCNR (R= iPr or 2,6- iPr2C6H3, at −30 to 20 °C ) to yield aluminaindene heterocycle ( 8) and alumina-aza-naphthalene heterocycle ( 9 and 10), respectively. These reactions underwent hydroalumination followed by AlH C(sp2)−H dehydroalumination. The reaction mechanism has been studied by combining experiment and quantum chemical calculations, with the result that the key heteroarene or arene C(sp2)−H bond activation is involved under cooperative interaction by the inherent N/Al donor/acceptor pair. The reported reactions open a straightforward route to heteroaryl and unique heterocyclic aluminum compounds. [ABSTRACT FROM AUTHOR]

Metadynamics simulations were used to describe the conformational energy landscapes of several helically folded aromatic quinoline carboxamide oligomers bearing a single chiral group at either the C or N terminus. The calculations allowed the prediction of whether a helix handedness bias occurs under the influence of the chiral group and gave insight into the interactions (sterics, electrostatics, hydrogen bonds) responsible for a particular helix sense preference. In the case of camphanyl-based and morpholine-based chiral groups, experimental data confirming the validity of the calculations were already available. New chiral groups with a proline residue were also investigated and were predicted to induce handedness. This prediction was verified experimentally through the synthesis of proline-containing monomers, their incorporation into an oligoamide sequence by solid phase synthesis and the investigation of handedness induction by NMR spectroscopy and circular dichroism. [ABSTRACT FROM AUTHOR]

The dinuclear nickel complexes [Ni2L(μ-O2CR)](ClO4) [R=Me ( 4), R=OMe ( 6)], where L2− is a 24-membered macrocyclic N6S2 ligand, react readily with excess I2 in MeCN solution at 4 °C to form stable mono-(I2) and bis-(I2) charge-transfer (CT) adducts of the type [Ni2L(μ-O2CR)(I2) n]+ ( n=1 or 2) containing linear RS-I-I linkages. Three new CT compounds, namely, [Ni2L(OAc)(I2)](I2)(I3) ( 5), [Ni2L(O2COMe)(I2)](I5) ⋅MeCN ( 7⋅MeCN), and [Ni2L(O2COMe)(I2)2](I5) ⋅MeCN ( 8⋅MeCN) as well as the triiodide salt [Ni2L(OAc)](I3) ( 9) were synthesized and fully characterized. A common feature of the CT adducts is a polyiodide matrix, which surrounds the individual complex molecules, stabilized by secondary I⋅⋅⋅I interactions with the CT linkages. The scatter in both the RS−I (2.6 to 3.0 Å) and the I−I bond lengths (2.7 to 3.0 Å) is indicative of both a variable strength of the RS−→I2 bond and a varying degree of charge transfer. An analysis of the structural parameters was undertaken accompanied by DFT calculations to quantify the donating ability of the bridging thiolate functions and to shed more light on the bonding in this rare sort of charge-transfer complexes. The stability of the CT complexes and the results of preliminary transport measurements are also reported. [ABSTRACT FROM AUTHOR]

The introduction of fluorine groups to pharmaceutical compounds can have a dramatic effect on the lipophilicity and metabolic stability of the molecule in vivo. Around 20 % of drugs contain at least one fluorine atom. The trifluoromethyl group is known to have beneficial effects and can dramatically affect the biological activity when substituted for a methyl group, for example. In any case, the direct and late-stage introduction of a trifluoromethyl group is a powerful transformation in the tool box of the medicinal chemist. The use of methyl fluorosulfonyldifluoroacetate (MFSDA) as a relatively inexpensive reagent for trifluoromethylation was first reported in 1989; however, in our opinion it has been somewhat underutilised. Herein, a comprehensive review of trifluoromethylation using MFSDA is reported, which we hope will further expose readers to this useful reagent. [ABSTRACT FROM AUTHOR]

The development of regioselective anti-Markovnikov alkene's hydroamination is a long-standing goal in catalysis. The [Rh(COD)(DPEphos)]+ complex is the most general and regioselective group 9 catalyst for such a process. The reaction mechanism for intermolecular hydroamination of alkenes catalyzed by [Rh(DPEphos)]+ complex is analyzed by means of DFT calculations. Hydroamination (alkene vs. amine activation routes) as well as oxidative amination pathways are analyzed. According to the computational results the operating mechanism can be generally described by alkene coordination, amine nucleophilic addition, proton transfer through the metal center and reductive elimination steps. The mechanism for the formation of the oxidative amination side product goes via a β-elimination after the nucleophilic addition and metal center protonation steps. The origin of the regioselectivity for the addition process (Markovnikov vs. anti-Markovnikov additions) is shown to be not charge but orbitally driven. Remarkably, η2 to η1 slippage degree on the alkene coordination mode is directly related to the regioselective outcome. [ABSTRACT FROM AUTHOR]

Oxidation by dioxygen has a rich repertoire of mechanistic intricacies. Herein, we report a hitherto unknown paradigm of dioxygen activation reaction which propagates through a four center two electron (4c-2e) bound species. Using static DFT and ab initio quantum chemical techniques we have unraveled the oxidation pathway for hydrazine and its methylated analogues by dioxygen which involves formation of this unconventional 4c-2e bonded species en route to the oxidation products. Inconvertible evidence in favor of such an unprecedented dioxygen activation route is provided by capturing the events of formation of the 4c-2e species in aqueous phase for hydrazine and its congeners and the experimentally observed products from the respective 4c-2e species, like H2O2 and N2H2, diazene in the case of hydrazine using Car-Parrinello molecular dynamics simulations. [ABSTRACT FROM AUTHOR]

The bioconjugation of polyoxometalates (POMs), which are inorganic metal oxido clusters, to DNA strands to obtain functional labeled DNA primers and their potential use in electrochemical detection have been investigated. Activated monooxoacylated polyoxotungstates [SiW11O39{Sn(CH2)2CO}]8− and [P2W17O61{Sn(CH2)2CO}]6− have been used to link to a 5′-NH2 terminated 21-mer DNA forward primer through amide coupling. The functionalized primer was characterized by using a battery of techniques, including electrophoresis, mass spectrometry, as well as IR and Raman spectroscopy. The functionality of the POM-labeled primers was demonstrated through hybridization with a surface-immobilized probe. Finally, the labeled primers were successfully used in the polymerase chain reaction (PCR) and the PCR products were characterized by using electrophoresis. [ABSTRACT FROM AUTHOR]

Constrained aminols from oxazanorbornene derivatives have the geometrical features to be used as β-turn inducers. Four different stereoisomers were prepared and spectroscopically characterized (MD calculations, NMR-titration and VT-NMR experiments). Temperature coefficients in DMSO are indicative for the existence of an intramolecular hydrogen bond. Chirooptical properties revealed a β-turn arrangement of all the synthesized compounds, where, depending on the absolute configuration of the cyclopentane spacer, they can be labeled as left- or right-handed turns. [ABSTRACT FROM AUTHOR]

A simple synthetic method allows the one-pot assembly of C3-symmetric, 44-core-valence-electron, triangular Pd or Pt clusters and their heterobimetallic mixed Pd/Pt analogues. These mixed metal complexes are the first examples of stable triangular all-metal heteroaromatics. In contrast to traditional heteroaromatic molecules formed combining main-group elements, they actually retain structural and electronic features of their homonuclear analogues. [ABSTRACT FROM AUTHOR]

The impact of reversible bond formation between a growing radical chain and a metal complex (organometallic-mediated radical polymerization (OMRP) equilibrium) to generate an organometallic intermediate/dormant species is analyzed with emphasis on the interplay between this and other one-electron processes involving the metal complex, which include halogen transfer in atom transfer radical polymerization (ATRP), hydrogen-atom transfer in catalytic chain transfer (CCT), and catalytic radical termination (CRT). The challenges facing the controlled polymerization of 'less active monomers' (LAMs) are outlined and, after reviewing the recent achievements of OMRP in this area, the perspectives of this technique are analyzed. [ABSTRACT FROM AUTHOR]

Low-temperature irradiation of linear [3]- and [4]phenylene cyclopentadienylcobalt complexes generates labile, fluxional η4-arene complexes, in which the metal resides on the terminal ring. Warming induces a haptotropic shift to the neighboring cyclobutadiene rings, followed by the previously reported intercyclobutadiene migration. NMR scrutiny of the primary photoproduct reveals a thermally accessible 16-electron cobalt η2-triplet species, which, according to DFT computations, is responsible for the rapid symmetrization of the molecules along their long axes. Calculations indicate that the entire haptotropic manifold along the phenylene frame is governed by dual-state reactivity of alternating 18-electron singlets and 16-electron triplets. [ABSTRACT FROM AUTHOR]

An unprecedented diversity of high-order bromine catenates (anionic polybromides) was generated in a tetraalkylphosphonium-based room temperature ionic liquid system. Raman spectroscopy was used to identify polybromide monoanions ranging from [Br5]− to [Br11]− in the bulk solution, while single-crystal X-ray diffraction identified extended networks of linked [Br11]− units, forming a previously unknown polymeric [Br24]2− dianion. This represents the largest polybromide species identified to date. In combination with recent work, this suggests that other, higher order molecular polybromide ions might be isolated. [ABSTRACT FROM AUTHOR]

Nearly isosteric oxo to thioxo substitution was employed to interrogate the structure of foldamers with a urea backbone and explore the relationship between helical folding and hydrogen-bonding interactions. A series of oligomers with urea bonds substituted by thiourea bonds at discrete or all positions in the sequence have been prepared and their folding propensity was studied by using a combination of spectroscopic methods and X-ray diffraction. The outcome of oxo to thioxo replacements on the helical folding was found to depend on whether central or terminal ureas were modified. The canonical helix geometry was not affected upon insertion of thioureas close to the negative end of the helix dipole, whereas thioureas close to the positive pole were found to increase the terminal flexibility and cause helix fraying. Perturbation was amplified when a selenourea was incorporated instead, leading to a structure that is only partly folded. [ABSTRACT FROM AUTHOR]

The Soai reaction and the Viedma deracemization of racemic conglomerate crystal mixtures are experimental pieces of evidence of the ability of enantioselective autocatalytic coupled networks to yield absolute asymmetric synthesis. Thermodynamically open systems or systems with non-uniform energy distributions may lead to chiral final states and, in systems able to come into thermodynamic equilibrium with their surroundings, to kinetically controlled absolute asymmetric synthesis. The understanding of network parameters and of the thermodynamic scenarios that may lead to spontaneous mirror symmetry breaking (SMSB) could assist in the development of new methods for asymmetric synthesis and enantioselective polymerizations (e.g., replicators), and to frame reasonable speculations on the origin of biological homochirality. [ABSTRACT FROM AUTHOR]

A DFT study of various model systems has addressed the interference of catalytic chain transfer (CCT) as a function of the R2 substituent in the atom-transfer radical polymerization (ATRP) of styrene catalyzed by [FeCl2(R1NC(R2)C(R2)NR1)] complexes. All model systems used R1=CH3 in place of the experimental Cy and tBu substituents and 1-phenylethyl in place of the polystyrene (PS) chain. A mechanistic investigation of 1) ATRP activation, 2) radical trapping in organometallic-mediated radical polymerization (OMRP), and 3) pathways to the hydride CCT intermediate was conducted with a simplified system with R2=H. This study suggests that CCT could occur by direct hydrogen-atom transfer without any activation barrier. Further analysis of more realistic models with R2= p-C6H4F or p-C6H4NMe2 suggests that the electronic effect of the aryl para substituents significantly alters the ATRP activation barrier. Conversely, the hydrogen-atom-transfer barrier is essentially unaffected. Thus, the greater ATRP catalytic activity of the p-NMe2 system makes the background CCT process less significant. The DFT study also compares the [FeCl2(R1NC(R2)C(R2)NR1)] systems with a diaminobis(phenolato) derivative for which the CCT process shows even greater accessibility but has less incidence because of faster ATRP chain growth and interplay with a more efficient OMRP trapping. The difference between the two systems is attributed to destabilization of the FeII catalyst by the geometric constraints of the tetradentate diaminobis(phenolato) ligand. [ABSTRACT FROM AUTHOR]

The biological activity of antibiotic peptaibols has been linked to their ability to aggregate, but the structure-activity relationship for aggregation is not well understood. Herein, we report a systematic study of a class of synthetic helical oligomer (foldamer) composed of aminoisobutyric acid (Aib) residues, which mimic the folding behavior of peptaibols. NMR spectroscopic analysis was used to quantify the dimerization constants in solution, which showed hydrogen-bond donors at the N terminus promoted aggregation more effectively than similar modifications at the C terminus. Elongation of the peptide chain also favored aggregation. The geometry of aggregation in solution was investigated by means of titrations with [D6]DMSO and 2D NOE NMR spectroscopy, which allowed the NH protons most involved in intermolecular hydrogen bonds in solution to be identified. X-ray crystallography studies of two oligomers allowed a comparison of the inter- and intramolecular hydrogen-bonding interactions in the solid state and in solution and gave further insight into the geometry of foldamer-foldamer interactions. These solution-based and solid-state studies indicated that the preferred geometry for aggregation is through head-to-tail interactions between the N and C termini of adjacent Aib oligomers. [ABSTRACT FROM AUTHOR]

Novel κ3- N, N, O ligands tend to form 1D coordination polymer strands. Deposition of 1D structures on highly oriented pyrolytic graphite (HOPG) was achieved from diluted solutions and polymer strands have been studied on HOPG by AFM/STM. Single strands were mapped by STM and their electronic properties were subsequently characterized by current imaging tunneling spectroscopy (CITS). Periodic density functional calculations simulating a polymer strand deposited on a HOPG surface are in agreement with the zig-zag structure indicated by experimental findings. Both the observed periodicity and the Zn-Zn distances can be reproduced in the simulations. Van der Waals interactions were found to play a major role for the geometry of the isolated polymer strand, for the adsorption geometry on HOPG, as well as for the adsorption energy. [ABSTRACT FROM AUTHOR]

A thorough computational exploration of the mechanistic intricacies of the intermolecular hydroamination (HA) of vinylarenes by a recently reported class of kinetically stabilised iminoanilide [{N^N}Ae{N(SiMe3)2} ⋅(THF) n] alkaline-earth amido compounds (Ae=Ca, Sr, Ba) is presented. Two distinct mechanistic pathways for catalytic HA mediated by alkaline-earth and rare-earth compounds have emerged over the years that account equally well for the specific features of the process. On one hand, a concerted proton-assisted pathway to deliver the amine product in a single step can be invoked and, on the other, a stepwise σ-insertive pathway that comprises a rapid, reversible migratory olefin insertion step linked to a less facile, irreversible AeC alkyl bond aminolysis. The results of the study presented herein, which employed a heavily benchmarked and reliable DFT methodology, supports a stepwise σ-insertive pathway that involves fast and reversible migratory CC bond insertion into the polar AeN pyrrolido σ bond. This proceeds with strict 2,1 regioselectivity via a highly polarised four-centre transition state (TS) structure, linked to irreversible intramolecular AeC bond aminolysis of the alkaline-earth alkyl intermediate as the energetically favourable mechanism. Turnover-limiting aminolysis is consistent with the significant KIE measured; the DFT-derived effective barrier matches the Eyring parameter empirically determined for the best-performing {N^N}Ba(NR2) catalyst gratifyingly well. It also predicts the observed trend in reactivity (Ca2) catalysts revealed that the variation in the Aepyrrolido bond strength together with the degree of protection of the alkaline earth by a sterically encumbering iminoanilide ligand scaffold not only profoundly influences the performance in HA catalysis, but also the likelihood of traversing rival mechanistic pathways. [ABSTRACT FROM AUTHOR]

Thirteen different symmetric and asymmetric phosphinesulfonato palladium complexes ([{( X 1-Cl)-μ-M} n], M=Na, Li, 1=X(P^O)PdMe) were prepared (see Figure 1). The solid-state structures of the corresponding pyridine or lutidine complexes were determined for (MeO)2 1-py, ( iPrO)2 1-lut, (MeO,Me2) 1-lut, (MeO)3 1-lut, CF3 1-lut, and Ph 1-lut. The reactivities of the catalysts X 1, obtained after chloride abstraction with AgBF4, toward methyl acrylate (MA) were quantified through determination of the rate constants for the first and the consecutive MA insertion and the analysis of β-H and other decomposition products through NMR spectroscopy. Differences in the homo- and copolymerization of ethylene and MA regarding catalyst activity and stability over time, polymer molecular weight, and polar co-monomer incorporation were investigated. DFT calculations were performed on the main insertion steps for both monomers to rationalize the effect of the ligand substitution patterns on the polymerization behaviors of the complexes. Full analysis of the data revealed that: 1) electron-deficient catalysts polymerize with higher activity, but fast deactivation is also observed; 2) the double ortho-substituted catalysts (MeO)2 1 and (MeO)3 1 allow very high degrees of MA incorporation at low MA concentrations in the copolymerization; and 3) steric shielding leads to a pronounced increase in polymer molecular weight in the copolymerization. The catalyst properties induced by a given P-aryl (alkyl) moiety were combined effectively in catalysts with two different non-chelating aryl moieties, such as c HexO/(MeO)2 1, which led to copolymers with significantly increased molecular weights compared to the prototypical MeO 1. [ABSTRACT FROM AUTHOR]