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"Braunstein, Pierre"
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Bifunctional and recyclable polyesters by chemoselective ring-opening polymerization of a δ-lactone derived from CO2 and butadiene
When aiming at the direct use of CO
2
for the preparation of advanced/value-added materials, the synthesis of CO
2
/olefin copolymers is very appealing but challenging. The δ-lactone 3-ethylidene-6-vinyltetrahydro-2
H
-pyran-2-one (EVP), synthesized by telomerization of CO
2
with 1,3-butadiene, is a promising monomer. However, its chemoselective ring-opening polymerization (ROP) is hampered by unfavorable thermodynamics and the competitive polymerization of highly reactive C=C double bonds under usual conditions. Herein, we report the chemoselective ROP of EVP using a phosphazene/urea binary catalyst, affording exclusively a linear unsaturated polyester poly(EVP)
ROP
, with a molar mass (
M
n
) up to 16.1 kg·mol
−1
and a narrow distribution (
Ð
< 1.6), which can be fully recycled back to the pristine monomer, thus establishing a monomer-polymer-monomer closed-loop life cycle. In these polyesters, the CO
2
content reaches 33 mol% (29 wt%). The reasons for the unexpected chemoselectivity were investigated by Density-functional theory (DFT) calculations. The poly(EVP)
ROP
features two pendent C=C double bonds per repeating unit, which show distinct reactivity and thus can be properly engaged in sequential functionalizations towards the synthesis of bifunctional polyesters. We disclose here a methodology providing a facile access to bifunctional and recyclable polyesters from readily available feedstocks.
The δ-lactone 3-ethylidene-6-vinyltetrahydro-2H-pyran-2-one (EVP), synthesized by telomerization of CO
2
with 1,3-butadiene, is a promising monomer for the direct use of CO
2
in the synthesis of polymers, but its ring-opening polymerization (ROP) remains challenging. Here the authors report its chemoselective ROP using a phosphazene/urea binary catalyst, affording exclusively a linear unsaturated polyester which can be recycled back to the pristine monomer.
Journal Article
Controllable multiple-step configuration transformations in a thermal/photoinduced reaction
Solid-state photochemical reactions of olefinic compounds have been demonstrated to represent powerful access to organic cyclic molecules with specific configurations. However, the precise control of the stereochemistry in these reactions remains challenging owing to complex and fleeting configuration transformations. Herein, we report a unique approach to control the regiospecific configurations of C = C groups and the intermediates by varying temperatures in multiple-step thermal/photoinduced reactions, thus successfully realizing reversible ring closing/opening changes using a single-crystal coordination polymer platform. All stereochemical transitions are observed by in situ single-crystal X-ray diffraction, powder X-ray diffraction and infrared spectroscopy. Density functional theory calculations allow us to rationalize the mechanism of the synergistic thermal/photoinduced transformations. This approach can be generalized to the analysis of the possible configuration transformations of functional groups and intermediates and unravel the detailed mechanism for any inorganic, organic and macromolecular reactions susceptible to incorporation into single-crystal coordination polymer platforms.
Solid-state photochemical reactions of olefinic compounds provide access to organic cyclic molecules with specific configurations but the precise control of the stereochemistry in these reactions remains challenging. Here, the authors demonstrate control of the regiospecific configurations of C=C groups and the intermediates by varying temperatures in multi-step thermal and photoinduced ring opening and closing reactions using a single-crystal coordination polymer platform.
Journal Article
Interfacial Electronic Interactions Between Ultrathin NiFe‐MOF Nanosheets and Ir Nanoparticles Heterojunctions Leading to Efficient Overall Water Splitting
Creating specific noble metal/metal‐organic framework (MOF) heterojunction nanostructures represents an effective strategy to promote water electrolysis but remains rather challenging. Herein, a heterojunction electrocatalyst is developed by growing Ir nanoparticles on ultrathin NiFe‐MOF nanosheets supported by nickel foam (NF) via a readily accessible solvothermal approach and subsequent redox strategy. Because of the electronic interactions between Ir nanoparticles and NiFe‐MOF nanosheets, the optimized Ir@NiFe‐MOF/NF catalyst exhibits exceptional bifunctional performance for the hydrogen evolution reaction (HER) (η10 = 15 mV, η denotes the overpotential) and oxygen evolution reaction (OER) (η10 = 213 mV) in 1.0 m KOH solution, superior to commercial and recently reported electrocatalysts. Density functional theory calculations are used to further investigate the electronic interactions between Ir nanoparticles and NiFe‐MOF nanosheets, shedding light on the mechanisms behind the enhanced HER and OER performance. This work details a promising approach for the design and development of efficient electrocatalysts for overall water splitting. A meticulously designed heterojunction bifunctional catalyst Ir@NiFe‐MOF/NF, derived from anchoring Ir nanoparticles in NiFe‐MOF/NF nanosheet arrays, exhibits a robust Ir─O─Ni/Fe interface interaction confirmed by XPS, Raman, and XAFS analyses. Such interaction augments H2O and intermediate adsorption, resulting in the superior HER and OER activities of the catalyst, which can serve as a promising bifunctional candidate for efficient electrocatalytic water splitting.
Journal Article
In situ observation of a stepwise 2 + 2 photocycloaddition process using fluorescence spectroscopy
Using highly sensitive and selective in situ techniques to investigate the dynamics of intermediates formation is key to better understand reaction mechanisms. However, investigating the early stages of solid-state reactions/transformations is still challenging. Here we introduce in situ fluorescence spectroscopy to observe the evolution of intermediates during a two-step [2 + 2] photocycloaddition process in a coordination polymer platform. The structural changes and kinetics of each step under ultraviolet light irradiation versus time are accompanied by the gradual increase-decrease of intensity and blue-shift of the fluorescence spectra from the crystals. Monitoring the fluorescence behavior using a laser scanning confocal microscope can directly visualize the inhomogeneity of the photocycloaddition reaction in a single crystal. Theoretical calculations allow us to rationalize the fluorescence behavior of these compounds. We provide a convenient strategy for visualizing the solid-state photocycloaddition dynamics using fluorescence spectroscopy and open an avenue for kinetic studies of a variety of fast reactions.
Using highly sensitive and selective in situ techniques to investigate the dynamics of intermediates formation is key to better understand reaction mechanisms but investigating the early stages of solid-state reactions or transformations is still challenging. Here the authors use in situ fluorescence spectroscopy to observe the evolution of intermediates during a two-step [2 + 2] photocycloaddition process in a coordination polymer.
Journal Article
Tetraolefin stereospecific photodimerization and photopolymerization in coordination polymers
In the context of the highly desirable design and preparation of smart materials with adjustable properties based on photoreactive compounds, we report two unique photoreactive coordination polymers (CPs),{[Zn(4-Iba)
2
(4-tkpvb)]·H
2
O}
n
(1
, 4-HIba = 4-iodobenzoic acid, 4-tkpvb = 1,2,4,5-tetrakis(4-pyridylvinyl)benzene) and [Cd(1,4-bdc)(4-tkpvb)]
n
(
2
, 1,4-H
2
bdc = 1,4-benze-nedicarboxylic acid). Depending on the metal ions and auxiliary carboxylate ligands used, the 4-tkpvb ligands in
1
and
2
adopt different mutual arrangements, which facilitate the photodimerization and photopolymerization reactions in single crystal to single crystal (SCSC) transformations. Upon UV light irradiation which results in the dimerization of the 4-tkpvb ligands, the one-dimensional (1D) zigzag chainsof
1
are transformed into a two-dimensional (2D) network of {[Zn(4-Iba)
2
(bpbtpvcb)
0.5
]·H
2
O}
n
(
1a
, bpbtpvcb = 1,3-bis(4-pyridyl)-2,4-bis(2,4,5-tri(2-(4-pyridyl)vinyl) phenylcyclobutane). Similarly, in the crystals of
2
, all 4-tkpvb ligands experience a photopolymerization reaction to form an unprecedented 1D linear organic polymer, poly-1,3-bis(4-pyridyl)-2,5-bis(2-(4-pyridyl)-vinyl) phenyl)cyclobutane (poly-bpbpvpcb). Furthermore, the 2D network of
2
is converted into a three-dimensional (3D) framework of [Cd
n
(1,4-bdc)
n
(poly-bpbpvpcb)] (
2a
). This work not only offers a new protocol to selectively synthesize new supramolecular cyclic compounds, but also expands the application of photopolymerization to the synthesis of macromolecules.
Journal Article
Ligand Control of the Metal Coordination Sphere: Structures, Reactivity and Catalysis
Two major aspects of coordination/organometallic chemistry are discussed in this article: (i) the use of functional chelating ligands to stabilize metal complexes while allowing easy stereodifferentiation within the coordination sphere and (ii) the choice of suitable ligands to stabilize challenging ‘underligated’ metal complexes with electronically highly unsaturated metal centres, thus potentially displaying unusual reactivity. In both cases, the relevance to homogeneous catalysis will be discussed. Deux aspects majeurs de la chimie de coordination/organométallique sont discutés dans cet article : d’une part, l’utilisation de ligands chélatants fonctionnels pour stabiliser des complexes métalliques en permettant une différenciation stéréoélectronique aisée au sein de la sphère de coordination et, d’autre part, la sélection de ligands adaptés à la stabilisation de complexes métalliques sous-coordinés dont les centres métalliques sont hautement insaturés électroniquement et donc potentiellement le siège d’une réactivité inhabituelle. Dans les deux cas, nous évoquerons la pertinence de ces approches en catalyse homogène.
Journal Article
Flexible Vertex Engineers the Controlled Assembly of Distorted Supramolecular Tetrahedral and Octahedral Cages
Designing and building unique cage assemblies attract increasing interest from supramolecular chemists but remain synthetically challenging. Herein, we propose the use of a flexible vertex with adjustable angles to selectively form highly distorted tetrahedral and octahedral cages, for the first time, in which the flexible vertex forms from the synergistic effect of coordination and covalent interactions. The inherent interligand angle of the vertex can be modulated by guest anions present, which allows for the fine-tuning of different cage geometries. Furthermore, the reversible structural transformation between tetrahedral and octahedral cages was achieved by anion exchange monitored by mass spectrometric technique, the smaller anions favoring tetrahedral cages, while the larger anions supporting octahedral cages. Additionally, the KBr-based cage thin films exhibited prominent enhancement of their third-order NLO responses in two or three orders of magnitude compared to those obtained for their corresponding solutions. This work not only provides a new methodology to build irregular polyhedral structures in a controlled and tunable way but also provides access to new kinds of promising functional optical materials.
Journal Article
Characterization of cobalt phosphide nanoparticles derived from molecular clusters in mesoporous silica
The synthesis of well dispersed cobalt phosphide nanoparticles (NPs) in SBA-15 mesoporous silica by wet impregnation of the molecular cluster [Co
4
(CO)
10
(μ-dppa)] (
1
) (dppa = HN(PPh
2
)
2
) is described. The thermal activation of the silica impregnated precursor under a H
2
/N
2
(5/95 %) stream at different temperatures to form NPs was studied and it was found that the size of the latter is limited in the 5.5–6.5 nm range by the size of the pores. The obtained materials were characterized by various analytical methods. The porosity and the structure of the mesoporous silica supports were analyzed by N
2
adsorption/desorption and small-angle X-ray diffraction. The nanoparticles were characterized by wide-angle X-ray diffraction, transmission electron microscopy in conventional and scanning modes, electron tomography, energy-dispersive X-ray spectroscopy, and magnetic measurements. Cobalt phosphide NPs of few nanometers were observed in the pores of SBA-15.
Journal Article
Accelerating water dissociation at carbon supported nanoscale Ni/NiO heterojunction electrocatalysts for high-efficiency alkaline hydrogen evolution
The synergistic catalysis of heterojunction electrocatalysts for the multi-step process in hydrogen evolution reaction (HER) is a promising approach to enhance the kinetics of alkaline HER. Herein, we proposed a strategy to form nanoscale Ni/NiO heterojunction porous graphitic carbon composites (Ni/NiO-PGC) by reduction-pyrolysis of the preformed Ni-metal-organic framework (MOF) under H
2
/N
2
atmosphere. Benefiting from low electron transfer resistance, increased number of active sites, and unique hierarchical micro-mesoporous structure, the optimized Ni/NiO-PGC
10−1−400
exhibited excellent electrocatalytic performance and robust stability for alkaline HER (
η
10
= 30 mV, 65 h). Density functional theory (DFT) studies revealed that the redistribution of electrons at the Ni/NiO interface enables the NiO phase to easily initiate the dissociation of alkaline H
2
O, and shifts down the d-band center of Ni and optimizes the H* adsorption-desorption process of Ni, thereby leading to extremely high HER activity. This work contributes to a further understanding of the synergistic promotion of the multi-step HER processes by heterojunction electrocatalysts.
Journal Article