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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.

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...

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.

“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.

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.

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

Reactions of [{N−,N,Cnaphthyl−}HfMe][MeB(C6F5)3] (2) precatalyst with a series of α-olefins have been investigated in order to intercept the active polymerization species generated by an in situ modification of the precursor by insertion of a single monomer unit into the Hf−CAryl bond. In all cases the first migratory insertion of monomer occurs into the Hf−CAryl bond rather than the Hf−CAlkyl bond. A low-temperature polymerization with 170 equiv of 1-hexene activated with tris(pentafluorophenyl)borane (FAB) allows for the complete NMR characterization of a Hf−CAlkylaryl methyl cation. This structure agrees with DFT studies of the kinetically favored diastereomer, although a number of other structures are more stable thermodynamically. Attempts to trap an inserted catalyst through the stoichiometric addition of 2-vinylpyridine or 3-ethoxy-1-propylene led to complicated reactions, but in both cases, experimental and computational data suggest that both processes initiate through insertion of the substrate ...

Pyridyl-amido catalysts have emerged recently with great promise for olefin polymerization. Insights into the activation chemistry are presented in an initial attempt to understand the polymerization mechanisms of these important catalysts. The activation of C1-symmetric arylcyclometallated hafnium pyridyl-amido precatalysts, denoted Me2Hf{N(-),N,C(-)} (1, aryl = naphthyl; 2, aryl = phenyl), with both Lewis (B(C6F5)3 and [CPh3][B(C6F5)4]) and Bronsted ([HNR3][B(C6F5)4]) acids is investigated. Reactions of 1 with B(C6F5)3 lead to abstraction of a methyl group and formation of a single inner-sphere diastereoisomeric ion pair [MeHf{N(-),N,C(-)}][MeB(C6F5)3] (3). A 1:1 mixture of the two possible outer-sphere diastereoisomeric ion pairs [MeHf{N(-),N,C(-)}][B(C6F5)4] (4) is obtained when [CPh3][B(C6F5)4] is used. [HNR3][B(C6F5)4] selectively protonates the aryl arm of the tridentate ligand in both precatalysts 1 and 2. A remarkably stable [Me2Hf{N(-),N,C2}][B(C6F5)4] (5) outer-sphere ion pair is formed when the naphthyl substituent is present. The stability is attributed to a hafnium/eta(2)-naphthyl interaction and the release of an eclipsing H-H interaction between naphthyl and pyridine moieties, as evidenced through extensive NMR studies, X-ray single crystal investigation and DFT calculations. When the aryl substituent is phenyl, [Me2Hf{N(-),N,C2}][B(C6F5)4] (10) is originally obtained from protonation of 2, but this species rapidly undergoes remetalation, methane evolution, and amine coordination, giving a diastereomeric mixture of [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (11). This species transforms over time into the trianionic-ligated [Hf{N(-),C(-),N,C(-)}NR3][B(C6F5)4] (12) through activation of a C-H bond of an amido-isopropyl group. In contrast, ion pair 5 does not spontaneously undergo remetalation of the naphthyl moiety; it reacts with NMe2Ph leading to [MeHf{N(-),N}NMe2C6H4][B(C6F5)4] (7) through ortho-metalation of the aniline. Ion pair 7 successively undergoes a complex transformation ultimately leading to [Hf{N(-),C(-),N,C(-)}NMe2Ph][B(C6F5)4] (8), strictly analogous to 12. The reaction of 5 with aliphatic amines leads to the formation of a single diastereomeric ion pair [MeHf{N(-),N,C(-)}NR3][B(C6F5)4] (9). These differences in activation chemistry are manifested in the polymerization characteristics of these different precatalyst/cocatalyst combinations. Relatively long induction times are observed for propene polymerizations with the naphthyl precatalyst 1 activated with [HNMe3Ph][B(C6F5)4]. However, no induction time is present when 1 is activated with Lewis acids. Similarly, precatalyst 2 shows no induction period with either Lewis or Bronsted acids. Correlation of the solution behavior of these ion pairs and the polymerization characteristics of these various species provides a basis for an initial picture of the polymerization mechanism of these important catalyst systems.

A new class of platinum(II) compounds of the formula [PtMe(olefin)(N,N‘-imino−amide chelate)] is described (olefin = ethylene, propylene, styrene, allyl alcohol, methyl vinyl ketone, methyl acrylate; N,N‘-imino−amide chelate = N,N‘-(1,2-dimethyl-1,2-ethanediylidene)bis(2,6-diisopropylaniline)). The neutral complexes exhibit square-planar coordination with a cis arrangement of the olefin and the alkyl group. The compounds have been characterized through one- and two-dimensional NMR spectroscopy, which has disclosed that only one isomer is present in solution. This isomer exhibits the alkene and the N(amido) atom in trans positions, while the alkene substituent is oriented toward the Pt−Me vector. The nature and the arrangement of the ligands seem suitable to favor a π-back-donation contribution to the Pt−olefin bond to an extent unprecedented for square-planar compounds, as discussed on the basis of NMR parameters and theoretical calculations.

Complexes [Pd(η1,η2-C8H12OMe)bipy]+X- (2a−f) (where X = BPh4- (a), CF3SO3- (b), BF4- (c), PF6- (d), SbF6- (e), and B(3,5-(CF3)2C6H3)4- (f); bipy = 2,2‘-bipyridine; C8H12OMe = cyclooctenylmethoxy group) were synthesized by the reaction of the dimer [Pd(η1,η2-C8H12OMe)Cl]2 (1) with the bipy ligand in methanol containing Y+X- salts. They were characterized in solution by multinuclear and multidimensional NMR spectroscopy. The solid-state structure of complex 2d was obtained by X-ray single-crystal investigation. The catalytic activity of complexes 2 toward CO/styrene copolymerization in methylene chloride was tested and related to the type of counterion. The order of the catalytic activity of complexes 2a−f is the following: BPh4- ≪ CF3SO3- < BF4- < PF6- < SbF6- < B(3,5-(CF3)2C6H3)4-. If the copolymerization reactions are carried out in the presence of an excess of the bipy ligand, the anion effect is less important and the order is the following: BPh4- ≪ CF3SO3- < BF4- ≈ B(3,5-(CF3)2C6H3)4- ≈ PF6- ≈ SbF6-...