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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 CO2. In the reverse reaction of HCOOH dehydrogenation, it is approximately twice less active but has no induction period., Inorganic Chemistry, 61 (27), ISSN:0020-1669, ISSN:1520-510X

The dinuclear aluminum salt {[iBu2(DMA)Al]2(μ-H)}+[B(C6F5)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 (DMAC16) 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.

Metal catalyzed polymerizations are among the most important chemical reactions, accounting for the production of about 400 million tons per year of polymeric materials, 50% of which are polyolefins. The CIRCC research units at the University of Salerno, founded by the late Professor Adolfo Zambelli, a coworker of Giulio Natta and a pioneer in the studies of stereospecific polymerization catalysts, has a consolidated expertise in this field. Although often considered a “mature” area of research, olefin polymerization catalysis continues to drive great interest of both industrial and academic scientists. On the other hand, strong political and economic pressure toward the development of “green” and possibly biodegradable alternatives to olefin-based polymers stimulated our group to direct increasing research efforts in the area of sustainable polymers. In this perspective, we focus on the most recent work from the CIRCC research units involved in homogeneous catalysis for polymerization of a variety of monomers, with the aim to address how the concepts and the expertise developed for olefin polymerization can be applied to the development of different metal-catalyzed polymerizations and copolymerizations. Of course, although the results are discussed in the frame of the most important literature contributions, a comprehensive review of such a wide and diversified topic is out of the scope of the paper. References to reviews covering the different types of metal catalyzed polymerizations are provided in each chapter.

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.

The solution structure and dynamics of three prototypical bis-indenyl ansa-zirconocenium methyl cations (Me2Si(2-methyl-4-phenyl-6-tert-butylindenyl)2ZrMe+, [1Me]+ Me2Si(2,4-dimethylindenyl)2ZrMe+, [2Me]+ Me2Si(indenyl)2ZrMe+, [3Me]+) paired with the weakly coordinating B(C6F5)4- anion have been investigated by NMR spectroscopy in aromatic hydrocarbons with different polarities (toluene, ϵr = 2.38; chlorobenzene, ϵr = 5.69; 1,2-difluorobenzene (ODFB), ϵr = 13.38). These highly electrophilic cations are stabilized by solvent coordination, as evidenced by the unequivocal identification of [1Me·C7H8]+B(C6F5)4-, a rare example of a toluene-stabilized ion pair that has been fully characterized in solution. Combining spectroscopic and DFT methods allowed us to evaluate how solvent coordination modulates the dynamic processes typical of this class of catalysts. An exchange between the two faces of coordinated toluene (face inversion, FI) occurs without solvent decomplexation (ΔH⧧FI = 14.6 kcal mol-1 ΔS⧧FI = 3 cal mol-1 K-1 ΔG⧧FI(298) = 13.7 kcal mol-1) and is about 20 times faster than the exchange between coordinated and free solvent (solvent decomplexation, SD, ΔH⧧SD = 17.9 kcal mol-1 ΔS⧧SD = 10 cal mol-1 K-1 ΔG⧧SD(298) = 14.9 kcal mol-1) and ion pair symmetrization (IPS, ΔH⧧IPS = 18.6 kcal mol-1 ΔS⧧IPS = 12 cal mol-1 K-1 ΔG⧧IPS(298) = 14.9 kcal mol-1). For the ion pairs [1-3Me·solvent]+B(C6F5)4-, IPS rate constants (kIPS) do not correlate with the solvent polarity (kIPS(ODFB) > kIPS(toluene) > kIPS(chlorobenzene)). For the more coordinating toluene and chlorobenzene solvents, ΔG⧧IPS nicely tracks with the amount of positive charge at the metal, increasing as the positive charge increases (qZrMe2,CM5; 1Me2 > 3Me2 > 2Me2). In contrast, in the less coordinating ODFB, differences in the IPS barriers for [1-3Me·ODFB]+B(C6F5)4- appear to correlate better with steric parameters, as measured by the percent buried volume on the open quadrants (%VBur,open) for the corresponding neutral precursors (%VBur,open = 34.8, 35.5, 34.6% for 1Me2, 2Me2, and 3Me2, respectively).

An integrated thermodynamic and kinetic study was carried out to clarify how the formation of ionic aggregates higher than ion pairs affects the rate of olefin insertion into the Zr–C bond. To this...

Measuring accurate translational self-diffusion coefficients (Dt ) 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 (VH ) 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 Dt values of three isosteric Cp2 MCl2 complexes (Cp=cyclopentadienyl, M=Ti, Zr, Hf), having significantly different molecular mass, provided an empirical demonstration that VH is the critical molecular property affecting Dt . This central concept served to clarify the assumptions behind the derivation of Dt =ƒ(M) power laws from the Stokes-Einstein equation. Some pitfalls in establishing log (Dt ) 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.

The aquifer of “mineral” water, historically known for its curative properties, is an identifying characteristic of the Anticolana valley. This area hosted a coppice chestnut forest for a long time. Under the forest, there is an important aquifer, historically renowned and widely recognized for preventing renal stone formation and or facilitating their expulsion. This mineral water pre- vents the formation of calcium oxalate and phosphate crystals in the kidney and promotes their dissolutions through soluble calcium complexes. The forest environment soil is particularly rich in humification products owing to the interaction of the rainwater–litter–soil system. The fulvic fraction is soluble in water under all pH conditions and enriches the water basin. We aimed to test these hypotheses and assess how the coppice chestnut forest is involved in fulvic acid production. Fulvic fractions isolated and purified from soil samples and mineral water (550 μg L−1) were analyzed by GC-MS, FTIR, and NMR. Data from different sources were compared, showing sufficient similarities to state that the fulvic acids isolated from the water come from the processes that take place between the stems and the chestnut litter. The chestnut forest provides enrichment to water quality, which is a distinctive piece of information in defining water enhancement strategies, establishing soil management and designating sustainable forest management.

Catalytic activity in olefin polymerization depends not only on the catalyst but also, crucially, on activator/alkylator/scavenger “packages.” Along with binary mixtures containing Lewis or Bronste...

There is considerable interest in dynamic materials featuring modular components with nano-scale dimensions and controlled responsiveness to external stimuli. Supramolecular polymers are a class of materials that fulfil all these conditions well. Here, we present a family of host–guest supramolecular polymers that combine the outstanding complexing properties of tetraphosphonate cavitands toward N-methylpyridinium guests with molecular switching. The designed monomer is a cavitand featuring four inward facing PO groups at the upper rim and a single N-methylpyridinium unit at the lower rim, forming instantaneously a polymeric species in solution, thanks to the high complexation constants measured for these host–guest interactions. This system has been analyzed by NMR spectroscopy and electrochemical techniques. In order to interpret the results of diffusion-sensitive experiments, we took advantage of the X-ray crystal structure obtained for the polymeric species and developed an original treatment for the PGSE data by non-linear fitting. The analysis of the experimental data identified an isodesmic polymerization model at a monomer concentration below 20 mM, driven by intrachain host–guest interactions, and an additional level of tetrameric bundle aggregation above 20 mM, due to interchain dipolar and quadrupolar interactions. Two orthogonal disassembly procedures have been implemented: electrochemical reduction for the linear chains and solvent-driven dissolution for the bundles.

Two hybrid materials (1_TiO2 and 2_TiO2) were designed and prepared by anchoring novel [Cp*Ir(κ3-O,N,O-glyphosate)] (1; glyphosate=N-(phosphonomethyl)glycine) and [Cp*Ir(κ3-O,N,O-glyphosine)] (2; glyphosine=N,N-bis(phosphonomethyl)glycine) complexes onto rutile TiO2. Characterization in solution (NMR spectroscopy) and solid state (X-Ray diffractometry) indicates that 1 and 2 are stabilized by the two arms of the glycine fragment, whereas they have (2) or can easily generate (1 and 2) a dandling phosphonate arm suitable for their anchoring on TiO2. As a matter of fact, they exhibit rather elongated Ir–OP (1: 2.1405 Å, 2: 2.142 Å) and short Ir–OC (1: 2.0784 Å, 2: 2.086 Å) and Ir–N (1: 2.155 Å, 2: 2.207 Å) bonds. NMR spectra of 2 show a dynamic process that exchanges the two phosphonate moieties, consistently with an easy –OP detachment from Ir. Both molecular and heterogenized species were tested as catalysts in water oxidation (WO) driven by NaIO4. 1 (TOF up to 96 min−1) and 2 (26 min−1) proved to be effective molecular catalysts; more importantly, 1_TiO2 and 2_TiO2 showed very high activity (TOF ∼200 min−1), which remained rather constant over seven successive runs, even if substantial leaching of the noble metal in solution occurred during the first catalytic tests. TON was very high and limited only by the amount of NaIO4 used. These results show the potentialities of mixed carboxylate/phosphonate ligands for the development of highly active water oxidation catalysts.

Identifying structure-property correlations for molecular water oxidation catalysts (WOCs) is strongly hampered by the complexity of the water oxidation (WO) reaction mechanism and catalyst in situ modifications. Here, the substitution effects for [Cp*Ir(X-pic)NO3] WOCs (pic = κ2-pyridine-2-carboxylate complexes) have been systematically explored by synthesizing several complexes differing for the nature of the X substituent on the pyridine-carboxylate ligand and testing them in WO driven by NaIO4. The activity data were correlated with the σ-Hammett parameter of the various substituents, which was proven to effectively reproduce the trends in electron donor properties of the ligands. The measured TOFmax values were found to roughly increase with the σ-Hammett parameter (i.e., with decreasing electron density at the metal center), reaching plateaux at activity values comparable to that of the simpler [Cp*Ir(H2O)3](NO3)2 catalyst. This trend has been typically observed for WOCs following a water nucleophilic attack mechanism. However, nuclear magnetic resonance studies on the reaction between [Cp*Ir(X-pic)NO3] and NaIO4 suggest that the activity is actually determined by the ease of pyridine-carboxylate ligand loss, likely being a necessary step toward precatalyst activation. This indicates that the same active species is generated from all [Cp*Ir(X-pic)NO3] starting complexes, albeit at different rates and/or in different amounts. A somewhat clear picture therefore appears to emerge, partially in contradiction with some of the hypotheses previously proposed: the active species should contain some degradation fragments of the Cp∗ (based on previous literature studies) and no X-pic derived ligands (based on the results reported in this work).

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 eq. 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 modelling 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 ; Δ G # ≈ 11 kcal/mol, Δ G ≈ -5 kcal/mol). Intramolecular hydrogen liberation from I NH2 is predicted to be rather prohibitive (Δ G # ≈ 26 kcal/mol, Δ G ≈ 23 kcal/mol), 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%) was 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 be also 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 1c . DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.

The cationic complexes [Cp*(tBu3P=N)Ti(η3-CH2C(C5H11)CH2)]+ (Ti-Alla+) and [Cp*(tBu3P=N)Ti(η3-CH2C(CH3)CH(C4H9))]+ (Ti-Allb+), bearing η3-allyl-type fragments coordinated to a Ti(IV) center with Cp* and phosphinimide ancillary ligands, were easily synthesized from the corresponding cationic Ti-benzyl complex by reaction with 2-methyl-1-heptene. The unusual stability of these allyl species at room temperature allowed detailed NMR spectroscopic investigation of their structure and dynamic behavior in solution. The spectroscopic results provided supporting experimental evidence for η3-coordination mode of the allyls and allowed to determine their preferential configuration. Furthermore, quantitative analysis of the exchange kinetics between syn and anti allylic protons was carried out by 1H EXSY NMR spectroscopy. The activation parameters for the latter process indicate that the faster exchange observed with Ti-Alla+ (ΔH‡ ≈ 16 kcal mol−1; ΔS‡ ≈ 0 cal mol−1 K−1) compared to Ti-Allb+ (ΔH‡ ≈ 11 kcal mol−1; ΔS‡ ≈ −19 cal mol−1 K−1) is primarily of entropic origin. A tentative interpretation for the higher stability of Ti-Alla+ and Ti-Allb+, compared to their bis-Cp titanocene analogues reported in the literature, is proposed based on steric considerations. Finally, an exploratory reaction, using benzaldehyde as representative substrate, was carried out to demonstrate the capability of Ti-Alla+ and Ti-Allb+ to serve as allylating agents for carbonyl compounds.

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 μβvec = −26,000 × 10–48 esu (at 1907 nm) in dilute nonpolar CH2Cl2 solution, which is maintained in polar DMF (μβvec= −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 reve...

The solution structure of AlMe2 F and its reactivity with a prototypical ansa-metallocene have been investigated by advanced NMR techniques, in an attempt to indirectly shed some light on the structure and working principles of methylalumoxane (MAO) mixtures in olefin polymerization. In solution, AlMe2 F gives rise to a complex equilibrium of oligomeric species, including a heterocubane [(Me2 Al)4 F4 ] tetramer, resembling the behavior of MAO. This complex mixture reacts with (ETH)ZrMe2 (ETH=rac-[ethylenebis(4,5,6,7-tetrahydro-1-indenyl)]) to afford [(ETH)ZrMeδ+ (μ-F)(AlMe2 F)n AlMe3δ- ] inner-sphere ion pairs through successive insertions/deinsertions of AlMe2 F units into the Zr⋅⋅⋅(μ-F) bond.

We recently reported evidence that a cyclometalated intermediate can facilitate the polymerization of 1-hexene to append polymer chains to the termini of propyl groups of the Me2Hf(Cpn-Propyl)2 catalyst precursor. Herein we provide further mechanistic details on the activation of Me2Hf(Cpn-Propyl)2 by B(C6F5)3 and the polymerization of 1-hexene mainly by applying a battery of mass spectrometry-based techniques. First, a combination of MALDI and CID fragmentation is used to characterize the high molecular mass region (up to 6 kDa) of the isolated poly(1-hexene) material with attached metallocene. The CID fragmentation patterns are explained by relatively low-energy ligand losses and higher energy hydrocarbon chain degradation via C–C bond cleavage and 1,3-hydrogen shift reactions. Further mechanistic insights are gained by investigating 1-hexene polymerization reaction employing a properly 13C-labeled catalyst activated by B(C6F5)3. Mass spectrometry analyses, along with supporting NMR experiments, indicat...

“Structural” AlMe2(bht) was found to be an integral building block of the “TMA-free” olefin polymerization cocatalyst MAO/BHT (TMA = trimethylaluminum, MAO = methylaluminoxane, BHT = 2,6-di-tert-butyl-4-methylphenol). NMR studies show that treatment of MAO/BHT with the neutral N-donor pyridine (py) leads to selective generation of neutral AlMe2(bht)(py). The reaction of MAO/BHT with 2,2'-bipyridine (bipy) generates cationic [AlMe(bht)(bipy)]+ fragments (bht = BHT phenolate), analogously to what happens when unmodified MAO is treated with bipy, causing [AlMe2(bipy)]+ formation. This suggests that the activation of an olefin polymerization precatalyst LnMX2 (X = Me or Cl) can occur via an indirect pathway involving [AlMeR]+ (R = Me or bht) transient species (rather than direct Cl/Me abstraction by the Al-cages) not only when unmodified MAO is exploited but also in the case of “TMA-free” BHT-modified MAO. The high chemoselectivity of py for neutral Al adducts however presents a distinct difference to unmodified MAO. These observations indicate that a) “structural” TMA is converted into “structural” AlMe2(bht) upon reaction of MAO with BHT and that b) MAO/BHT is harder to ionize than unmodified MAO. A refined formula and cluster size estimation [(AlOMe)0.87(AlMe2bht)0.13]n (n = 57–84) is proposed for MAO/BHT based on this new experimental evidence, accounting for the presence of “structural” AlMe2(bht) units.

The self-aggregation of neutral [RuCl2(eta6-p-cymene)(CAP)] (2; p-cymene = 1-methyl-4-(propan-2-yl)benzene) and ionic [RuCl(eta6-p-cymene)(L)(CAP)](PF6) (3, L = CAP; 4, L = MeCN) and [RuCl2(eta6-p-cymene)(CAP-H)](BPh4) (5; CAP-H = monoprotonated CAP) ruthenium(II) arene complexes, bearing the water-soluble phosphine CAP (1,4,7-triaza-9-phosphatricyclo[5.3.2.1]tridecane), in aprotic polar solvents, was investigated by means of PGSE (pulsed field gradient spin-echo) diffusion NMR. In addition, the analogous neutral [RuCl2(eta6-p-cymene)(PTA)] (1) and monoprotonated [RuCl2(eta6-p-cymene)(PTA-H)](BPh4) (6) PTA (1,3,5-triaza-7-phosphaadamantane) complexes were investigated in acetonitrile-d3. Complexes 1-4 do not exhibit any tendency to self-aggregate in acetonitrile-d3 and acetone-d6. In contrast, 5 and 6 monoprotonated complexes undergo a peculiar self-aggregation process, involving almost exclusively the cation, leading to the main formation of dicationic species held together by intermolecular hydrogen bonds. This has been clearly demonstrated by 1H diffusion NMR experiments, which allowed measurement of an average hydrodynamic volume for the cation that was more than double that expected, whereas that of the counterion is only slightly higher than that of the free anion. The trend of the aggregation number (N+) as a function of the concentration of 5 and 6 was fitted using the equation of the equal K indefinite model of association, leading to DeltaG°298 K = -3.1 and -3.0 kcal mol-1, respectively, for the formation of H-bonded dications. The critical role played by the protonated -NH unit in establishing hydrogen bonding was further supported by the detection of 6+2 dinuclear species in acetonitrile-d3 solutions containing equimolar mixtures of 2 and 6. The self-aggregation free energy of 2 (DeltaG°298 K = -3.1 kcal mol-1) and 6+ (DeltaG°298 K = -2.9 kcal mol-1) in such mixed solutions is practically identical with those of 5 and 6 in acetonitrile-d3.

Ion pairing is known to play a key role in stoichiometric and catalytic reactions mediated by charged transition metal complexes, especially when they are conducted in low-polar solvents. Molecular olefin polymerization catalysis by group IV metal complexes represents a textbook case in this respect, due to the high electrophilicity of the involved cationic active species and the very low relative permittivity (~ 2) of the commonly used hydrocarbon solvents. As a matter of fact, ion pairing has a huge impact on activity and selectivity of olefin polymerization catalysts. Ion pairing is strictly related to the activation process, since the cationic active species is usually generated by the reaction of a neutral precursor with a suitable cocatalyst (activator). The counterion generated from the latter reaction has to be weakly coordinating in order to disfavor its competition with the olefin for the occupancy of the coordination vacancy of the active metal center. Herein, after having recalled some generalities of olefin polymerization and nature of activators, using ion pairing as a reading key, the main findings concerning with the structure and self-aggregation tendency of ion pairs relevant to olefin polymerization, as deduced by means of advanced NMR techniques, are described. Representative examples of structure/activity relationships are then discussed, suggesting some rationale to interpret the different catalytic behavior of various ion pairs. Finally, cases in which the activation process, besides providing the expected active ion pair, leads to further layers of complexity (in situ ligand modification, dimer formation, etc.) are illustrated.

International audience; 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-κ3-O,N,O-H4-EDTMP)] (Ir-H4-EDTMP, H4-EDTMP4− = ethylenediamine tetra(methylene phosphonate)) and [(Cp*Ir)2(μ-κ3-O,N,O-EDTA)] (Ir-EDTA, EDTA4− = 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 TiO2 nanocrystals obtaining Ir-H4-EDTMP@TiO2 and Ir-EDTA@TiO2 hybrid materials. Both molecular complexes and hybrid materials were found to be efficient catalysts for WO driven by NaIO4, providing almost quantitative yields, and TON values only limited by the amount of NaIO4 used. As for the molecular catalysts, Ir-H4-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-H4-EDTMP@TiO2 (up to 33 min−1) and Ir-EDTA@TiO2 (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.

The established model cluster (AlOMe)16(AlMe3)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., AlMe2R; 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., A model cluster for methylaluminoxane (MAO) was used to study the tendency of model Lewis acidic sites to release neutral Al fragments or transient aluminum cations both for unmodified MAO and TMA-depleted MAO/BHT (TMA = trimethylaluminum; BHT = 2,6-di-tert-butyl-4-methylphenol). Albeit only sites of lower Lewis acidity are found in TMA depleted modified MAOs, they can release cationic Al fragments when treated with certain N-donors. This shows how TMA depleted MAOs can still be potent activators.

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.

Reactions between the π-acidic cyclic trimetallic coinage metal(I) complexes {[Cu(μ-3,5-(CF3)2pz)]3, {[Ag(μ-3,5-(CF3)2pz)]3, and {[Au(μ-3,5-(CF3)2pz)]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-(CF3)2pz)]3DBTTF} (1), {[Ag(μ-3,5-(CF3)2pz)]3DBTTF} (2), {[Au(μ-3,5-(CF3)2pz)]3DBTTF} (3), {[Cu(μ-3,5-(CF3)2pz)]3TTF} (4), {[Ag(μ-3,5-(CF3)2pz)]3TTF} (5), {[Au(μ-3,5-(CF3)2pz)]3TTF} (6), {[Cu(μ-3,5-(CF3)2pz)]3BEDT-TTF} (7), {[Ag(μ-3,5-(CF3)2pz)]3BEDT-TTF} (8), and {[Au(μ-3,5-(CF3)2pz)]3BEDT-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, 1H and 19F 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.

Intramolecular activation of hydrocarbyls to form metallacyclic complexes is a relatively fast process in cationic hafnocene catalysts bearing propyl-substituted Cp ligands. The resulting metallacycles are effective 1-hexene polymerization catalysts with activities comparable to that of the nonmetalated precursor. Ad hoc polymerizations of 1-hexene, using (CpPr)2HfMe2 as catalyst precursor, allow the isolation and characterization, via nuclear magnetic resonance (NMR) and matrix-assisted laser desorption ionization (MALDI) techniques, of polymers containing (CpCH2–CH2–CR3)2HfCl2 (R = H or polymeryl) units. The polymeryl substitutions arise from irreversible incorporation of polymer chains onto the cyclopentadienyl ligand substituent(s) via metallacycle intermediates. As a consequence of such “self-modification”, multiple active sites are generated by a nominally single-site catalyst; this may explain the broadening of the molecular weight distribution (MWD) and chemical composition distribution (CCD) obse...

The generation of solar fuels by means of a photosynthetic apparatus strongly relies on the development of an efficient water oxidation catalyst (WOC). Cerium ammonium nitrate (CAN) is the most commonly used sacrificial oxidant to explore the potentiality of WOCs. It is usually assumed that CAN has the unique role to oxidatively energize WOCs, making them capable to offer a low-energy reaction pathway to transform H2O to O2. Herein, we show that CAN might have a much more relevant and direct role in WO, mainly related to the capture and liberation of O–O-containing molecular moieties.

MAO​/BHT (MAO = methylalumoxane; BHT = 2,​6-​di-​tert-​butyl-​4-​methylphenol) cocatalyst for olefin polymn. has been investigated by NMR spectroscopy. It has been found that it consists of oligomeric [AlOMe0.9(bht)​0.1]​n cages and monomeric MeAl(bht)​2. Diffusion NMR indicates an av. n for Al-​clusters of 62-​96, i.e. 2-​3 times higher than that estd. for unmodified MAO under analogous conditions (n ≈ 26-​41)​. The reactivity of MAO​/BHT has been explored by monitoring the activation of the Cp*-​phosphinimide titanium dichloride precatalyst Cp*(tBu3P=N)​TiCl2. Comparison with independently synthesized model species and DFT modeling allowed characterization of the reaction mixts. obtained at varying aluminum to titanium ratios. Homodinuclear adducts [Cp*(tBu3P=N)​TiX]​2(μ-​Y)​+ (X, Y = Me or Cl) forming Outer Sphere Ion Pairs (OSIPs) with MAO​/BHT-​derived anions are dominant at low Al​/Ti ratios, whereas mononuclear Inner Sphere Ion Pairs (ISIPs) [Cp*(tBu3P=N)​Ti-​X]​+[MAO​/BHT]​-​ are formed at high Al​/Ti ratios; both types of species are found to be viable precursors for the cationic active species. Activation of dibenzyl analog Cp*(tBu3P=N)​TiBn2 results in the clean formation of [Cp*(tBu3P=N)​Ti-​Bn]​+[MAO​/BHT]​-​ OSIP, giving sharp 1H and 31P NMR signals; this reaction was exploited to quantify the amt. of strongly acidic sites on Al-​clusters, shedding further light on the structure and properties of MAO​/BHT.

Iridium complexes bearing pyridine triazolylidene ligands with variable steric hindrance, derived by the presence of an R group (R=H, Me, Et, nPr, iPr, Bu, and Oct) onto the N1-nitrogen, have been synthesized, fully characterized and tested as water oxidation catalysts (WOCs), using chemical sacrificial oxidants (CAN and NaIO4) or in photocatalytic experiments ([Ru(bpy)3]Cl2 as phosensitizer and Na2S2O8 as an electron acceptor). The catalytic activity is barely affected by the nature of R when WO is driven by NaIO4 (1 min−1

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.

The solution structure of AlMe

The novel iridium complex [IrCp*(NCMe)2(PPh2Me)][PF6]2 (I) efficiently catalyzed the Meyer–Schuster rearrangement of selected arylpropargylic alcohols into α,β-unsaturated aldehydes under mild conditions and without the need of a co-catalyst. A mechanism involving a (hydroxy)alkenylcarbene intermediate is proposed.

Complexes [Cp*Ir(ppy)NO3] (ppy = 2-phenylpyridine, 1), [Cp*Ir(2′,4′-F2-ppy)NO3] (2b), [Cp*Ir(κ2-C2,C2′-NHC)Cl], (κ2-C2,C2′-NHC = κ2-C2,C2′-1,3-bis(4-(N,N-dimethylamino)phenyl)-imidazol-2-ylidene, 3), [Cp*Ir(bpyr)(SOMe2)] (bpyr = bis-diethyl-pyrrole, 4), [Cp*Ir(bimH2)Cl]Cl (bimH2 = 2,2′-bisimidazole, 5) and [Cp*IrCl(μ2-κ2-κ1-bimH)IrCl2Cp*] (6) have been synthesized and characterized in solution by multinuclear and multidimensional NMR spectroscopy. The structures in the solid state of 3 and 6 have been determined by means of single-crystal diffraction X-Ray studies. Complexes 1–6 were found to be effective catalysts for water oxidation to molecular oxygen driven by cerium ammonium nitrate. The highest activity was observed for 4 (TOFmax = 12 min−1, TON > 500), 5 (TOFmax = 23 min−1, TON > 500) and 6 (TOFmax = 51 min−1, TON > 500) bearing more electron-donating ligands.

The reaction of [Cp*Ir(bzpy)NO3 ] (1; bzpy=2-benzoylpyridine, Cp*=pentamethylcyclopentadienyl anion), a competent water-oxidation catalyst, with several oxidants (H2 O2 , NaIO4 , cerium ammonium nitrate (CAN)) was studied to intercept and characterize possible intermediates of the oxidative transformation. NMR spectroscopy and ESI-MS techniques provided evidence for the formation of many species that all had the intact Ir-bzpy moiety and a gradually more oxidized Cp* ligand. Initially, an oxygen atom is trapped in between two carbon atoms of Cp* and iridium, which gives an oxygen-Ir coordinated epoxide, whereas the remaining three carbon atoms of Cp* are involved in a η(3) interaction with iridium (2 a). Formal addition of H2 O to 2 a or H2 O2 to 1 leads to 2 b, in which a double MeCOH functionalization of Cp* is present with one MeCOH engaged in an interaction with iridium. The structure of 2 b was unambiguously determined in the solid state and in solution by X-ray single-crystal diffractometry and advanced NMR spectroscopic techniques, respectively. Further oxidation led to the opening of Cp* and transformation of the diol into a diketone with one carbonyl coordinated at the metal (2 c). A η(3) interaction between the three non-oxygenated carbons of "ex-Cp*" and iridium is also present in both 2 b and 2 c. Isolated 2 b and mixtures of 2 a-c species were tested in water-oxidation catalysis by using CAN as sacrificial oxidant. They showed substantially the same activity than 1 (turnover frequency values ranged from 9 to 14 min(-1) ).

The reactions of three iridium water‐oxidation catalysts {[Cp*IrL1L2L3]Xn; 1: L1, L2 = 2,2‐bipyridine (bpy), L3 = Cl, n = 1, X = Cl; 2: L1, L2 = 2‐benzoylpyridine (bzpy), L3 = NO3; 3: L1 = L2 = L3 = H2O, n = 2, X = NO3; Cp* = pentamethylcyclopentadienyl} with cerium ammonium nitrate (CAN) and NaIO4 (sacrificial oxidants, SOs) have been studied by Clark electrode measurements (both in solution and in the gas phase), on‐line mass spectrometry, manometry and UV/Vis spectroscopy. Furthermore, cyclic voltammetry has been applied to evaluate the relative tendency of 1 and 2 to be oxidized. The turnover frequency (TOF) increases as the ratio (R) between the concentration of the SO and that of the catalyst increases. O2 production with CAN is observed in experiments with R = 20 for 1 and 3, whereas O2 becomes detectable with 2 only when R = 40. Catalyst 2 has the highest tendency to be oxidized to IrIV and forms a blue intermediate I characterized by a UV/Vis band at 574 nm. The formation of I occurs with the same velocity as that of the production of O2, which indicates that I is a species directly involved in the catalytic cycle. The disappearance of I, when O2 evolution is finished, is a second‐order process more than one order of magnitude slower than O2 production and is strongly accelerated by the presence of benzyl alcohol. This suggests that I is a molecular species that slowly undergoes disproportion when catalysis is over. Experiments in which multiple aliquots of SO (CAN) were added (R = 20 and 40) indicate that catalysts 1–3 can reinitiate the catalytic cycle once they have been kept in a dormant state for 0–9 min; the TOFs of the second and third additions are approximately equal and higher than that of the first addition. By combining manometry and on‐line mass spectrometry measurements, it was found that O2 evolution is parallel to the production of a small amount of CO2 owing to catalyst degradation. The TOFs of the experiments performed with NaIO4 as the SO are about 2–3 times lower than those with CAN, but the same reactivity order is found 3 > 2 > 1. The activation parameters were evaluated with NaIO4 for all catalysts and with CAN for 2 at 10–45 °C. ΔG# is practically the same in all situations (25–26 kcal mol–1), whereas ΔH# is appreciably lower for 2 (13.1 kcal mol–1 with CAN and 13.3 kcal mol–1 with NaIO4) than for 1 (16 kcal mol–1) and 3 (16.9 kcal mol–1). The lowest enthalpic cost with 2 is balanced by the highest entropic cost (–41 cal mol–1 K–1) that approaches that typical for an associative bimolecular process.

The complex [Cp*Ir(pica)Cl] (1; pica = picolinamidate = κ2-pyridine-2-carboxamide) was found to be an effective catalyst for both water oxidation to molecular oxygen and NAD+/NADH transformations, which are the key reactions of light-dependent natural photosynthesis. In particular, 1 exhibits high activity in water oxidation driven by CAN and NaIO4. With the former, the initial TOF exceeds that of [Cp*Ir(pic)Cl] (2; pic = picolinate = κ2-pyridine-2-carboxylate), which is the fastest iridium catalyst reported to date, whereas with NaIO4 it compares well with the best catalysts. 1 exhibits top performances also in the hydrogenation of NAD+ with HCOOK, leading to the regiospecific formation of 1,4-NADH (pH 7) with TOF = 143 h–1, which is about 3 times higher than the previous highest value (54 h–1) reported for [Cp*Ir(4-(1H-pyrazol-1-yl-κN2)benzoic acid)(H2O)]SO4 (3). The activity seems to be critically affected by the presence of the NH functionality, as indicated by its drop of about 1 order of magnitude w...

The results of a systematic investigation on the photocatalytic water oxidation activity of eight Ir complexes are reported. In particular, [Cp*IrLn]Xm [1, Ln = (H2O)3, X−m = (NO3−)2; 2, Ln = N-dimethylimidazolin-2-ylidene and (Cl−)2; 4, Ln = 1,1′-dimethyl-3,3′-ethylene-diimidazol-2,2′- diylidene and Cl− X− = PF6−; 5, Ln = 2-phenylpyridine (ppy) and NO3−; 6, Ln = 2-benzoylpyridine (bzpy) and NO3−; 8, Ln = 2,2′-bipyridine (bpy) and Cl−], [Ir(HEDTA)Cl]Na (3) and [Ir(ppy)2(OH2)2]OTf (7), whose water oxidation activity driven by chemical oxidants was already proved, were tested as mediators (10 μM–100 μM) in photocatalytic experiments exploiting [Ru(bpy)3]Cl2 (1 mM) as photosensitizer and Na2S2O8 (2.5 mM–30 mM) as electron acceptor, at pH 5.2 and pH 8 by Na2SiF6/NaHCO3 and Na2B4O7 buffers, respectively. All complexes showed to be competent catalysts for the photolytic water oxidation. Best performances were obtained with 2 at pH 5.2, where TOF > 2.5 min−1, TON > 80 and quantum yield > 0.1. H/D kinetic isotopic effect was evaluated for 1, 2 and 8 and related to their tendency to undergo oxidative transformation as deduced by 1H NMR studies, using CH3COOH as molecular probe.

The development of an efficient water oxidation catalyst is crucial in the framework of constructing an artificial photo(electro)synthetic apparatus for the production of solar fuels. Herein, new hydroxy–pyridine–carboxylate iridium complexes are reported exhibiting high activity in water oxidation with both cerium ammonium nitrate and NaIO4 as sacrificial oxidants. With the latter, the catalytic activity strongly depends on the pH and position of the OH-substituent in the pyridine ring, reaching a record turnover frequency of 458 min–1 and turnover number (>14 500) limited only by the amount of NaIO4. Kinetic experiments measuring O2 evolution paralleled by NMR studies on oxidative transformation with NaIO4 suggest that Cp* of the catalyst is readily degraded, whereas the hydroxy–pyridine–carboxylate ligands remain coordinated at iridium, tuning its activity.

We demonstrate here that the Pulsed field Gradient Spin Echo (PGSE) NMR diffusion technique can be effectively used as a complementary tool for the characterization of mono- and multi-site intermolecular halogen bonding (XB) in solution. The main advantage of this technique is that it provides the possibility of unambiguously determining the stoichiometry of the supramolecular adduct, information that is particularly important when multi-site molecular systems are studied. As an example, PGSE NMR measurements in chloroform indicate that hexamethylenetetramine (HMTA), a potentially four-site XB acceptor, actually exploits only two sites for the interaction with the XB donor N-bromosuccinimide (NBS), leaving the other two nitrogen sites unoccupied. Charge displacement calculations suggest that this is due also to the anti-cooperativity of the XB interaction between HMTA and NBS.

Organometallic compounds [Cp*Ir(κ2-N,O)X] (κ2-N,O = 2-pyridinecarboxylic acid, ion(−1) (1), 2,4-pyridinedicarboxylic acid, ion(−1) (2), 2,6-pyridinedicarboxylic acid, ion(−1) (3); X– = Cl– (a), NO3– (b)) and [Ir(κ3-N,O,O)(1-κ-4,5-η2-C8H13)(MeOH)] (κ3-N,O,O = 2,6-pyridinedicarboxylic acid, ion(−2) (4)) are effective catalysts for the oxidative splitting of water to O2 driven by Ce4+. They show similar TOFLT values (long-term TOF, 2.6–7.4 min–1) while TOFIN values (initial TOF) strongly depend on the catalyst (1 ≫ 2 > 3 > 4), reaching a maximum value of 287 min–1 (4.8 s–1) for 1a, which is the highest TOF value ever reported for an iridium catalyst. Voltammetric measurements indicate that the oxidative processes of compounds 1–4 are located at values substantially less positive than that of [Cp*Ir(bzpy)NO3] (bzpy = 2-benzoylpyridine; ΔE ≈ 0.2–0.3 V), taken as reference catalyst for water oxidation. In particular, compound 3, having a pendant −COOH moiety in close proximity to an iridium coordination site, a...

The reaction of a series of dimethyl zirconocenes [Me2Si(Cp)2ZrMe2, 1; Cpt−bu2ZrMe2, 2; Cpn−bu2ZrMe2, 3; Ind2ZrMe2, 4; CpMe42ZrMe2, 5; Cp∗2ZrMe2, 6] with [HNMe2Ph][B(C6F5)4] was investigated by means of NMR spectroscopy. It was found that protonolysis of a Zr–Me group occurred generating a coordinative vacancy at the metal center and methane. Cations coming from 1–4 dimethyl precursors bound NMe2Ph, liberated from the protonation process, and formed zirconaaziridinium ion pairs {[Me2Si(Cp)2Zr(η2–CH2NMePh)][B(C6F5)4], 7; [Cpt−bu2Zr(η2–CH2NMePh)][B(C6F5)4], 8; [Cpn−bu2Zr(η2–CH2NMePh)][B(C6F5)4], 9; [Ind2Zr(η2–CH2NMePh)][B(C6F5)4], 10}, reasonably as a consequence of CH activation of one Me group of coordinated NMe2Ph and methane elimination. The intramolecular/interionic structures and dynamics of 7–10 ion pairs were investigated by 1H, 13C and 19F 1D-and 2D-NMR techniques. The reactions of 7 and 10 ion pairs with 2-methyl-1-heptene afforded stable diastereoisomeric ion pairs bearing a five-member azametallacycle.

The fluxional behavior of zirconaaziridinium ion pairs [Cp2Zr(η2-CH2NMePh][X] (X– = MeB(C6F5)3– (1a), B(C6F5)4– (1b)) in solution has been investigated by means of 2D EXSY and line shape analysis NMR methods at different temperatures. It is found that two dynamic processes are operative: (i) the inversion of the absolute configuration of the coordinated nitrogen atom (PI process) and (ii) the dynamic symmetrization of the ion pair occurring through a 180° rotation around Zr–CH2 bond and anion relocation from one side of the cation to the other (BS process). Analysis of the kinetic data indicates that both processes necessitate nitrogen decoordination from the metal that, by the way, is not the rate-determining step; the nature of the borate anion has a negligible effect on the rates of both PI and BS processes.

Sticky counterions: Low‐temperature kinetic NMR studies were performed to determine ΔH≠ and ΔS≠ values for the insertion of a single 2‐methyl‐1‐heptene molecule into a Zr-C bond of [Cp2Zr(η2‐CH2NMePh)][X] (1a: X−=MeB(C6F5)3−, 1b: B(C6F5)4−) in [D8]toluene and a 1:1 mixture of [D8]toluene and [D5]chlorobenzene. Both activation parameters critically depend on the interplay of the counterion and the solvent.

The relative cation−anion position in [Pt(Me)(η2-olefin)(N,N)]BF4 complexes (where N,N = 2,6-(R‘)2C6H3NC(R‘ ‘)C(R‘ ‘)N-2,6-(R‘)2C6H3, R‘ = H, Me, Et, and i-Pr, R‘ ‘ = H, Me; olefin = CH2CHR, R = H, Me, COOMe) in methylene chloride has been investigated by detecting specific interionic dipolar interactions in the 19F{1H}-HOESY NMR spectra. The counterion shows strong interionic contacts with R‘ ‘ and R‘ protons and weak contacts with R and olefinic protons only when R‘ < Et and R‘ ‘ = Me. For R‘ ≥ Et the accessibilty to the metal center is completely inhibited and the counterion is located above or below the backside of the diimine ligand. The same position is also observed when R‘ ‘ = H and R‘ = R = Me despite the limited steric hindrance of the substituents due to specific interactions between H‘ ‘ and the fluorine of BF4-. In the other cases (R‘ < Et), the counterion also interacts with Me, R, and olefinic protons, indicating that the accessibility to the metal center is not forbidden.

Zirconaaziridinium inner sphere ion pairs (ISIPs) bearing alkyl chains of different lengths, [Cp2Zr(η2-CH2NR1R2)+···X−] (R1 = Me, R2 = C2H5, X− = MeB(C6F5)3−, ZrNC2BN; R1 = Me, R2 = C16H33, X− = MeB(C6F5)3−, ZrNC16BN; R1 = R2 = C18H37, X− = MeB(C6F5)3−, ZrNC18BN; R1 = Me, R2 = Ph, X− = MeB(C6F5)3−, ZrNPhBN; R1 = R2 = C18H37, X− = B(C6F5)4−, ZrNC18BT; R1 = Me, R2 = Ph, X− = B(C6F5)4−, ZrNPhBT), have been synthesized by the reaction of [Cp2ZrMe+···MeB(C6F5)3−] or [Cp2ZrMe+···B(C6F5)4−] with the suitable tertiary amine followed by selective C−H activation of one Me group of the coordinated amine and methane elimination. Subsequent reaction of ISIPs with THF afforded the corresponding outer-sphere ion pairs (OSIPs) ([ZrNC2THF][BN], [ZrNC16THF][BN], [ZrNC18THF][BN], [ZrNPhTHF][BN], [ZrNC18THF][BT], [ZrNPhTHF][BT]) in which the anion has been displaced into the second coordination sphere. The interionic structure in solution (i.e., the relative cation−anion position) and the self-aggregation level of both ISIPs...

A series of 4-X-1-methylpyridinium cationic nonlinear optical (NLO) chromophores (X = (E)-CH=CHC(6)H(5); (E)-CH=CHC(6)H(4)-4'-C(CH(3))(3); (E)-CH=CHC(6)H(4)-4'-N(CH(3))(2); (E)-CH=CHC(6)H(4)-4'-N(C(4)H(9))(2); (E,E)-(CH=CH)(2)C(6)H(4)-4'-N(CH(3))(2)) with various organic (CF(3)SO(3)(-), p-CH(3)C(6)H(4)SO(3)(-)), inorganic (I(-), ClO(4)(-), SCN(-), [Hg(2)I(6)](2-)) and organometallic (cis-[Ir(CO)(2)I(2)](-)) counter anions are studied with the aim of investigating the role of ion pairing and of ionic dissociation or aggregation of ion pairs in controlling their second-order NLO response in anhydrous chloroform solution. The combined use of electronic absorption spectra, conductimetric measurements and pulsed field gradient spin echo (PGSE) NMR experiments show that the second-order NLO response, investigated by the electric-field-induced second harmonic generation (EFISH) technique, of the salts of the cationic NLO chromophores strongly depends upon the nature of the counter anion and concentration. The ion pairs are the major species at concentration around 10(-3) M, and their dipole moments were determined. Generally, below 5x10(-4) M, ion pairs start to dissociate into ions with parallel increase of the second-order NLO response, due to the increased concentration of purely cationic NLO chromophores with improved NLO response. At concentration higher than 10(-3) M, some multipolar aggregates, probably of H type, are formed, with parallel slight decrease of the second-order NLO response. Ion pairing is dependent upon the nature of the counter anion and on the electronic structure of the cationic NLO chromophore. It is very strong for the thiocyanate anion in particular and, albeit to a lesser extent, for the sulfonated anions. The latter show increased tendency to self-aggregate.

A study was conducted to demonstrate in situ diversification of (pyridylamide)hafnium(IV) catalysts. An extensive density functional theory (DFT) study was conducted for several C 1-symmetric and C s-symmetric precatalysts to investigate the diversification of the (pyridylamide)hafnium(IV) catalysts. The study also investigated propylene polymerizations were investigated with two catalysts to find evidence of multisite behavior. It was observed that catalyst systems 1/Ph 3C][B-(C 6F 5) 4] and 2/Ph 3C][B(C 6F 5) 4] produced polypropylene samples with broad molecular weight distributions.

The mixed-alkyl metallocene complexes (IPCF)M(Me)(CH2SiMe3) (M = Zr, Hf; IPCF = Me2C(C5H4)(C13H8)) were synthesized by the reaction of (IPCF)M(Me)Cl (M = Zr, Hf) with Me3SiCH2MgCl. The crystal structures of (IPCF)Zr(CH2SiMe3)2, (IPCF)HfMe2, and (IPCF)Zr(Me)Cl were determined by X-ray diffraction. The kinetics of site epimerization of the ion pairs (IPCF)M(CH2SiMe3)(μ-Me)B(C6F5)3 and [(IPCF)MCH2SiMe3+···B(C6F5)4−] (M = Zr, Hf) were studied by variable-temperature NMR spectroscopy, while the solution ground-state structures of the ion pairs [LZrCH2SiMe3+···B(C6F5)4−] (L = SBI, IPCF; SBI = rac-Me2Si(Ind)2) were probed experimentally by 19F,1H HOESY NMR spectroscopy and theoretically by DFT and molecular dynamics calculations. They reveal differences in the strength of anion interactions between the SBI and IPCF systems which may be significant for their catalytic activity. The tetraarylborate salts are stabilized by agostic interactions to ligand Si−Me moieties, with Hf > Zr. The exchange rates of both the M...

The synthesis, structural and spectroscopic characterization, and nonlinear optical response properties of a “slightly” twisted zwitterionic 4-quinopyran electrooptic chromophore FMC, 2-{4-[1-(2-propylheptyl)-1Hpyridine-4-ylidene]cyclohexa-2,5-dienylidene}malononitrile, are reported. X-ray diffraction data and density functional theory (DFT) minimized geometries confirm that deletion of the four o-, o′-, o′′ -, and o′′′ -methyl groups in the parent chromophore TMC-2, 2-{4-[3,5-dimethyl-1-(2-propylheptyl)-1H-pyridin-4-ylidene]-3,5dimethylcyclohexa-2,5-dienylidene}malononitrile, relaxes the arene-arene twist angle from 89.6 to 9.0°. These geometrical changes result in a significantly increased contribution of the quinoidal structure to the molecular ground state of FMC (versus TMC-2), reduced solvatochromic shifts in the optical spectra, and a diminished electric-field-induced second-harmonic (EFISH) generation derived molecular hyperpolarizability (µ� )- 2340 × 10 -48 esu of DFMC, the dendrimer derivative of FMC ,v s-24000 × 10 -48 esu of TMC-2 )i n CH 2Cl2 at 1907 nm. Pulsed field gradient spin–echo (PGSE) NMR spectroscopy and EFISH indicate that the levels of FMC aggregation in solution are comparable to those of TMC-2 (monomers and dimers) in CH2Cl2 solution. B3LYP and INDO/S computation of chromophore molecular structure, aggregation, and hyperpolarizability trends are in good agreement with experiment.