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[Cp*Ir(R-pica)Cl] (Cp*=pentamethylcyclopentadienyl anion, pica=2-picolineamidate) complexes bearing carbohydrate substituents on the amide nitrogen atom (R=methyl-β-D-gluco-pyranosid-2-yl, 1; methyl-3,4,6-tri-O-acetyl-β-D-glucopyranosid-2-yl, 2) were tested as catalysts for formic acid dehydrogenation in water. TOF MAX values over 12000 h -1 and 50000 h -1 were achieved at 333 K for 1 and 2, respectively, with TON values over 35000 for both catalysts. Comparison with the simpler cyclohexyl-substituted analogue (3) indicated that glucosyl-based complexes are much better performing under the same experimental conditions (TOF MAX =5144 h -1 , TON=5000 at pH 2.5 for 3) owing to a lower tendency to isomerize to the less active k 2 -N,O isomer upon protonation. The 5-fold increase in TOF MAX observed for 2 with respect to 1 is reasonably due to an optimal steric protection by the acetyl substituent, which may prevent unproductive inner-sphere reactivity. These results showcase a powerful strategy for the inhibition of the common deactivation pathways of [Cp*Ir(R-pica)X] catalysts for FA dehydrogenation, paving the way for the development of better performing hydrogen storage systems., (© 2024 Wiley-VCH GmbH.)

A yet-outstanding supramolecular chemistry challenge is isolation of novel varieties of stacked complexes with finely-tuned donor-acceptor bonding and optoelectronic properties, as herein reported for binary adducts comprising two different cyclic trinuclear complexes (CTC@CTC'). Most previous attempts focused only on 1-2 factors among metal/ligand/substituent combinations, resulting in heterobimetallic complexes. Instead, here we show that, when all 3 factors are carefully considered, a broadened variety of CTC@CTC' stacked pairs with intuitively-enhanced intertrimer coordinate-covalent bonding strength and ligand-ligand/metal-ligand dispersion are attained (d M-M' 2.868(2) Å; ΔE>50 kcal/mol, an order of magnitude higher than aurophilic/metallophilic interactions). Significantly, CTC@CTC' pairs remain intact/strongly-bound even in solution (K eq 4.67×10 5  L/mol via NMR/UV-vis titrations), and the gas phase (mass spectrometry revealing molecular peaks for the entire CTC@CTC' units in sublimed samples), rather than simple co-crystal formation. Photo-/electro-luminescence studies unravel metal-centered phosphorescence useful for novel all metal-organic light-emitting diodes (MOLEDs) optoelectronic device concepts. This work manifests systematic design of supramolecular bonding and multi-faceted spectral properties of pure metal-organic macrometallacyclic donor/acceptor (inorganic/inorganic) stacks with remarkably-rich optoelectronic properties akin to well-established organic/organic and organic/inorganic analogues., (© 2024 Wiley-VCH GmbH.)

Cationic cyclometalated hafnocenes [Cp Pr Cp CH2CH2CH2 Hf][B(C 6 F 5 ) 4 ] ( 4 Pr ) and [Cp i Bu Hf][B(C CH2CH(Me)CH2 Hf][B(C 6 F 5 ) 4 ] ( 4a iBu and 4b HfMe][B(C iBu ) were synthesized from the corresponding [(Cp Pr ) 2 HfMe][B(C 6 F 5 ) 4 HfMe][B(C 1 Pr ) and [(Cp i Bu ) 2 ) complexes via C-H activation. 6 F 5 ) 4 ] ( 1 iBu ) complexes via C-H activation. 4a iBu , 4b iBu , and 4 Pr , mimicking a propagating M-polymeryl species (M = transition metal) with or without a β-methyl branch on the metalated chains, serve to investigate whether and how the nature of the last inserted olefin molecules changes the structure, stability, and reactivity of the corresponding heterobimetallic complexes, formed in the presence of aluminum- or zinc-alkyl chain transfer agents (CTAs), which are considered relevant intermediates in coordinative chain transfer polymerization (CCTP) and chain shuttling polymerization (CSP) technologies. NMR and DFT data indicate no major structural difference between the resulting heterobridged complexes, all characterized by the presence of multiple α-agostic interactions. On the contrary, thermodynamic and kinetic investigations, concerning the reversible formation and breaking of heterobimetallic adducts, demonstrate that isomer 4a iBu , in which the β-Me is oriented away from the reactive coordination site on Hf, but not 4b iBu , having the β-Me pointing in the opposite direction, is capable of reacting with CTAs. Quantification of kinetic rate constants highlights that the formation process is rate limiting and that the nature of the last inserted α-olefin unit modulates transalkylation kinetics. The reaction of 4a iBu , 4b iBu , and 4 Pr with diisobutylaluminum hydride (DiBAlH) allows the interception and characterization of new heterobinuclear and heterotrinuclear species, featuring both hydride and alkyl bridging moieties, which represent structural models of elusive intermediates in CCTP and CSP processes, capturing the instant when an alkyl chain has just transferred from a transition metal to a main group metal, while the two metals remain engaged in a single heterobimetallic intermediate.

Hafnium catalysts for olefin polymerization are often very sensitive to the nature of cocatalysts, especially if they contain "free" aluminium trialkyls. Herein, cocatalyst effects in Hf-catalysed propene polymerization are examined for four Hf catalysts belonging to the family of C S -symmetric (Hf-CS-Met) and C 2 -symmetric (Hf-C2-Met) metallocenes, as well as of octahedral (Hf-OOOO) and pentacoordinated (Hf-PyAm) "post-metallocenes". The performance of the recently developed {[iBu 2 (PhNMe 2 [B(C 2 (μ-H)} + [B(C 6 F 5 ) 4 ] - (AlHAl) cocatalyst is compared with that of established systems like methylalumoxane, phenol-modified methylalumoxane and trityl borate/tri-iso-butylaluminium. The worst catalytic performance is observed with MAO. Conversely, the best cocatalyst varies depending on the Hf catalyst used and the performance indicator of interest, highlighting the complexity and importance of selecting the right precatalyst/cocatalyst combination. AlHAl proved to be a suitable system for all catalysts tested and, in some cases, it provides the best performance in terms of productivity ( e.g. with hafnocenes). Furthermore, it generally leads to high molecular weight polymers, also with catalysts enabling easy chain transfer to Al like Hf-PyAm. This suggests that AlHAl has a low tendency to form heterodinuclear adducts with the cationic active species, therefore preventing the formation of dormant sites and/or termination events by chain transfer to Al.

Group 4 metal-Salan olefin polymerization catalysts typically have relatively low activity, being slowed down by a pre-equilibrium favoring a non-polymerization active resting state identified as a mer-mer isomer (MM); formation of the polymerization active fac-fac species (FF) requires isomerization. We now show that the chemistry is more subtle than previously realized. Salan variations bearing large, flat substituents can achieve very high activity, and we ascribe this to the stabilization of the FF isomer, which becomes lower in energy than MM. Detailed in situ NMR studies of a fast ( o -anthracenyl) and a slow ( o - t Bu) Salan precursors, suitably activated, indicate that preferred isomers in solution are different: the fast catalyst prefers FF while the slow catalyst prefers a highly distorted MM geometry. Crystal structures of the activated o -anthracenyl substituted complex with a moderately (chlorobenzene) and, more importantly, a weakly coordinating solvent (toluene) in the first coordination sphere emphasize that the active FF isomer is preferred, at least for the benzyl species. Site epimerization (SE) barriers for the fast catalyst (Δ S > 0, dissociative) and the slow catalyst (Δ S < 0, associative) in toluene corroborate the solvent role. Diagnostic NMe 13 C chemical shift differences allow unambiguous detection of FF or MM geometries for seven activated catalysts in different solvents, highlighting the role of solvent coordination strength and bulkiness of the ortho -substituent on the isomer equilibrium. For the first time, active polymeryl species of Zr-Salan catalysts were speciated. The slow catalyst is effectively trapped in the inactive MM state, as previously suggested. Direct observation of fast catalysts is hampered by their high reactivity, but the product of the first 1-hexene insertion maintains its FF geometry.

The dinuclear aluminum salt {[ i Bu 2 (DMA)Al] 2 ( μ -H)} + [B(C 6 F 5 ) 4 ] - ( AlHAl ; DMA = N , N -dimethylaniline) is the prototype of a new class of molecular cocatalysts for catalytic olefin polymerization, its modular nature offering easy avenues for tailoring the activator to specific needs. We report here, as proof of concept, a first variant ( s-AlHAl ) bearing p -hexadecyl- N , N -dimethylaniline (DMA C16 ) units, which enhances solubility in aliphatic hydrocarbons. The novel s-AlHAl was used successfully as an activator/scavenger in ethylene/1-hexene copolymerization in a high-temperature solution process., Competing Interests: The authors declare no conflict of interest.

Manganese-catalyzed hydrogenation reactions have aroused widespread interest in recent years. Among the catalytic systems described, especially PNP- and NNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, ketones, nitriles, aldimines and esters. Furthermore, NNP-Mn pincer compounds are efficient catalysts for the hydrogenolysis of less reactive amides, ureas, carbonates, and carbamates. Herein, the synthesis and application of specific imidazolylaminophosphine ligands and the corresponding Mn pincer complexes are described. These new catalysts have been characterized and studied by a combination of experimental and theoretical investigations, and their catalytic activities have been tested in several hydrogenation reactions with good to excellent performance. Especially, the reduction of N-heterocycles can be performed under very mild conditions., (© 2022 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The development of an efficient heterogeneous catalyst for storing H 2 into CO 2 and releasing it from the produced formic acid, when needed, is a crucial target for overcoming some intrinsic criticalities of green hydrogen exploitation, such as high flammability, low density, and handling. Herein, we report an efficient heterogeneous catalyst for both reactions prepared by immobilizing a molecular iridium organometallic catalyst onto a high-surface mesoporous silica, through a sol-gel methodology. The presence of tailored single-metal catalytic sites, derived by a suitable choice of ligands with desired steric and electronic characteristics, in combination with optimized support features, makes the immobilized catalyst highly active. Furthermore, the information derived from multinuclear DNP-enhanced NMR spectroscopy, elemental analysis, and Ir L 3 -edge XAS indicates the formation of cationic iridium sites. It is quite remarkable to note that the immobilized catalyst shows essentially the same catalytic activity as its molecular analogue in the hydrogenation of CO 2 . In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period.

The degradation pathways of highly active [Cp*Ir(κ 2 -N,N-R-pica)Cl] catalysts (pica=picolinamidate; 1 R=H, 2 R=Me) for formic acid (FA) dehydrogenation were investigated by NMR spectroscopy and DFT calculations. Under acidic conditions (1 equiv. of HNO 3 ), 2 undergoes partial protonation of the amide moiety, inducing rapid κ 2 -N,N to κ 2 -N,O ligand isomerization. Consistently, DFT modeling on the simpler complex 1 showed that the κ 2 -N,N key intermediate of FA dehydrogenation (I NH ), bearing a N-protonated pica, can easily transform into the κ 2 -N,O analogue (I NH2 ; ΔG ≠ ≈11 kcal mol -1 , ΔG ≈-5 kcal mol -1 ). Intramolecular hydrogen liberation from I NH2 is predicted to be rather prohibitive (ΔG ≠ ≈26 kcal mol -1 , ΔG≈23 kcal mol -1 ), indicating that FA dehydrogenation should involve mostly κ 2 -N,N intermediates, at least at relatively high pH. Under FA dehydrogenation conditions, 2 was progressively consumed, and the vast majority of the Ir centers (58 %) were eventually found in the form of Cp*-complexes with a pyridine-amine ligand. This likely derived from hydrogenation of the pyridine-carboxiamide via a hemiaminal intermediate, which could also be detected. Clear evidence for ligand hydrogenation being the main degradation pathway also for 1 was obtained, as further confirmed by spectroscopic and catalytic tests on the independently synthesized degradation product 1 c. DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible., (© 2020 Wiley-VCH GmbH.)

The development of efficient water oxidation catalysts (WOCs) is of key importance in order to drive sustainable reductive processes aimed at producing renewable fuels. Herein, two novel dinuclear complexes, [(Cp*Ir) 2 (μ-κ 3 -O,N,O-H 4 -EDTMP)] (Ir-H 4 -EDTMP, H 4 -EDTMP 4- = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir) 2 (μ-κ 3 -O,N,O-EDTA)] (Ir-EDTA, EDTA 4- = ethylenediaminetetraacetate), were synthesized and completely characterized in solution, by multinuclear and multidimensional NMR spectroscopy, and in the solid state, by single crystal X-Ray diffraction. They were supported onto rutile TiO 2 nanocrystals obtaining Ir-H 4 -EDTMP@TiO 2 and Ir-EDTA@TiO 2 hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO 4 , providing almost quantitative yields, and TON values only limited by the amount of NaIO 4 used. As for the molecular catalysts, Ir-H 4 -EDTMP (TOF up to 184 min -1 ) exhibited much higher activity than Ir-EDTA (TOF up to 19 min -1 ), likely owing to the higher propensity of the former to generate a coordination vacancy through the dissociation of a Ir-OP bond (2.123 Å, significantly longer than Ir-OC, 2.0913 Å), which is a necessary step to activate these saturated complexes. Ir-H 4 -EDTMP@TiO 2 (up to 33 min -1 ) and Ir-EDTA@TiO 2 (up to 41 min -1 ) hybrid materials showed similar activity that was only marginally reduced in the second and third catalytic runs carried out after having separated the supernatant, which did not show any sign of activity, instead. The observed TOF values for hybrid materials are higher than those reported for analogous systems deriving from heterogenized mononuclear complexes. This suggests that supporting dinuclear molecular precursors could be a successful strategy to obtain efficient heterogenized water oxidation catalysts., (Copyright © 2020 Science China Press. Published by Elsevier B.V. All rights reserved.)

Nuclear Magnetic Resonance (NMR) is an analytical technique extensively used in almost every chemical laboratory for structural identification. This technique provides statistically equivalent signals in spite of using spectrometer with different hardware features and is successfully used for the traceability and quantification of analytes in food samples. Nevertheless, to date only a few internationally agreed guidelines have been reported on the use of NMR for quantitative analysis. The main goal of the present study is to provide a methodological pipeline to assess the reproducibility of NMR data produced for a given matrix by spectrometers from different manufacturers, with different magnetic field strengths, age and hardware configurations. The results have been analyzed through a sequence of chemometric tests to generate a community-built calibration system which was used to verify the performance of the spectrometers and the reproducibility of the predicted sample concentrations., (Copyright © 2020 Elsevier B.V. All rights reserved.)

The established model cluster (AlOMe) 16 (AlMe 3 ) 6 for methylaluminoxane (MAO) cocatalyst has been studied by density functional theory, aiming to rationalize the different behaviors of unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di- tert -butyl-4-methylphenol), highlighted in previous experimental studies. The tendency of the three model Lewis acidic sites A - C to release neutral Al fragments (i.e., AlMe 2 R; R = Me or bht) or transient aluminum cations (i.e., [AlMeR] + ) has been investigated both in the absence and in the presence of neutral N-donors. Sites C are most likely responsible for the activation capabilities of TMA-rich MAO, but TMA depletion destabilizes them, possibly inducing structural rearrangements. The remaining sites A and B , albeit of lower Lewis acidity, should be still able to release cationic Al fragments when TMA-depleted modified MAOs are treated with N-donors (e.g. [AlMe(bht)] + from MAO/BHT). These findings provide tentative interpretations for earlier observations of donor-dependent ionization tendencies of MAO and MAO/BHT and how TMA depleted MAOs can still be potent activators.

Invited for the cover of this issue is the group of Cristiano Zuccaccia at the Università degli Studi di Perugia. The image depicts a relation of the nuances of chemical bonding to the diverse ways that animals "bind" to their natural surroundings. Read the full text of the article at 10.1002/chem.201905699., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Reactions between the π-acidic cyclic trimetallic coinage metal(I) complexes {[Cu(μ-3,5-(CF 3 ) 2 pz)] 3 , {[Ag(μ-3,5-(CF 3 ) 2 pz)] 3 , and {[Au(μ-3,5-(CF 3 ) 2 pz)] 3 with TTF, DBTTF and BEDT-TTF give rise to a series of coinage metal(I)-based new binary donor-acceptor adducts {[Cu(μ-3,5-(CF 3 ) 2 pz)] 3 DBTTF} ( 1 ), {[Ag(μ-3,5-(CF 3 ) 2 pz)] 3 DBTTF} ( 2 ), {[Au(μ-3,5-(CF 3 ) 2 pz)] 3 DBTTF} ( 3 ), {[Cu(μ-3,5-(CF 3 ) 2 pz)] 3 TTF} ( 4 ), {[Ag(μ-3,5-(CF 3 ) 2 pz)] 3 TTF} ( 5 ), {[Au(μ-3,5-(CF 3 ) 2 pz)] 3 TTF} ( 6 ), {[Cu(μ-3,5-(CF 3 ) 2 pz)] 3 BEDT-TTF} ( 7 ), {[Ag(μ-3,5-(CF 3 ) 2 pz)] 3 BEDT-TTF} ( 8 ), and {[Au(μ-3,5-(CF 3 ) 2 pz)] 3 BEDT-TTF} ( 9 ), where pz = pyrazolate, TTF = tetrathiafulvalene, DBTTF = dibenzotetrathiafulvalene, and BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene. This series of binary donor-acceptor adducts has been found to exhibit remarkable supramolecular structures in both the solid state and solution, whereby they exhibit supramolecular stacked chains and oligomers, respectively. The supramolecular solid-state and solution binary donor-acceptor adducts also exhibit superior shelf stability under ambient laboratory storage conditions. Structural and other electronic properties of solids and solutions of these adducts have been characterized by single-crystal X-ray diffraction (XRD) structural analysis, 1 H and 19 F NMR, UV-vis-near-IR spectroscopy, Fourier transform infrared, and computational investigations. The combined results of XRD structural data analysis, spectroscopic measurements, and theoretical studies suggest sustenance of the donor-acceptor stacked structure and electronic communication in both the solid state and solution. These properties are discussed in terms of potential applications for this new class of supramolecular binary donor-acceptor adducts in molecular electronic devices, including solar cells, magnetic switching devices, and field-effect transistors.

Measuring accurate translational self-diffusion coefficients (D t ) by NMR techniques with modern spectrometers has become rather routine. In contrast, the derivation of reliable molecular information therefrom still remains a nontrivial task. In this paper, two established approaches to estimating molecular size in terms of hydrodynamic volume (V H ) or molecular weight (M) are compared. Ad hoc designed experiments allowed the critical aspects of their application to be explored by translating relatively complex theoretical principles into practical take-home messages. For instance, comparing the D t values of three isosteric Cp 2 MCl 2 complexes (Cp=cyclopentadienyl, M=Ti, Zr, Hf), having significantly different molecular mass, provided an empirical demonstration that V H is the critical molecular property affecting D t . This central concept served to clarify the assumptions behind the derivation of D t =ƒ(M) power laws from the Stokes-Einstein equation. Some pitfalls in establishing log (D t ) versus log (M) linear correlations for a set of species have been highlighted by further investigations of selected examples. The effectiveness of the Stokes-Einstein equation itself in describing the aggregation or polymerization of differently shaped species has been explored by comparing, for example, a ball-shaped silsesquioxane cage with its cigar-like dimeric form, or styrene with polystyrene macromolecules., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Twisted intramolecular charge transfer (TICT) chromophores exhibit uniquely large second-order optical nonlinearities ( μβ). However, their promise as electro-optic (E-O) materials is yet untapped, reflecting a strong tendency to aggregate in low-polarity media, leading to a dramatic fall in μβ. Until now, TICT chromophores in deaggregating polar solvents suffered decreased response due to polarity-driven changes in electronic structure. Here we report a new series of benzimidazolium-based TICT chromophores with interaryl torsional angles in the range of 64-77°. The most twisted, B2TMC-2, exhibits a large μβ vec = -26,000 × 10 -48 esu (at 1907 nm) in dilute nonpolar CH 2 Cl 2 solution, which is maintained in polar DMF ( μβ vec = -20,370 × 10 -48 esu) as measured by DC electric field-induced second harmonic generation (EFISH). Sterically enforced interaryl torsional angles are confirmed by single-crystal X-ray diffraction and solution phase Nuclear Overhauser Effect (NOE) NMR, and spectroscopic characterization reveals a zwitterionic/aromatic ground state electronic structure associated with the high μβ. We show that increasingly disrupted conjugation is correlated with increased μβ even at intermediate twist angles. The excellent performance and reduced aggregation in polar solvents opens new avenues for bridging microscopic and macroscopic chromophore performance.

Water oxidation (WO) is a central reaction in the photo- and electro-synthesis of fuels. Iridium complexes have been successfully exploited as water oxidation catalysts (WOCs) with remarkable performances. Herein, we report a systematic study aimed at benchmarking well-known Ir WOCs, when NaIO 4 is used to drive the reaction. In particular, the following complexes were studied: cis-[Ir(ppy) 2 (H 2 O) 2 ]OTf (ppy=2-phenylpyridine) (1), [Cp*Ir(H 2 O) 3 ]NO 3 (Cp*=1,2,3,4,5-pentamethyl-cyclopentadienyl anion) (2), [Cp*Ir(bzpy)Cl] (bzpy=2-benzoylpyridine) (3), [Cp*IrCl 2 (Me 2 -NHC)] (NHC=N-heterocyclic carbene) (4), [Cp*Ir(pyalk)Cl] (pyalk=2-pyridine-isopropanoate) (5), [Cp*Ir(pic)NO 3 ] (pic=2-pyridine-carboxylate) (6), [Cp*Ir{(P(O)(OH) 2 } 3 ]Na (7), and mer-[IrCl 3 (pic)(HOMe)]K (8). Their reactivity was compared with that of IrCl 3 ⋅n H 2 O (9) and [Ir(OH) 6 ] 2- (10). Most measurements were performed in phosphate buffer (0.2 m), in which 2, 4, 5, 6, 7, and 10 showed very high activity (yield close to 100 %, turnover frequency up to 554 min -1 with 10, the highest ever observed for a WO-driven by NaIO 4 ). The found order of activity is: 10>2≈4>6>5>7>1>9>3>8. Clues concerning the molecular nature of the active species were obtained, whereas its exact nature remains doubtfully., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)