The performance of perovskite solar cells with inverted polarity (
p-i-n
) is still limited by recombination at their electron extraction interface, which also lowers the power conversion efficiency (PCE) of
p-i-n
perovskite-silicon tandem solar cells. A ~1 nm thick MgF
x
interlayer at the perovskite/C
60
interface through thermal evaporation favorably adjusts the surface energy of the perovskite layer, facilitating efficient electron extraction, and displaces C
60
from the perovskite surface to mitigate nonradiative recombination. These effects enable a champion
V
oc
of 1.92 volts, an improved fill factor of 80.7%, and an independently certified stabilized PCE of 29.3% for a ~1 cm
2
monolithic perovskite-silicon tandem solar cell. The tandem retained ~95% of its initial performance following damp-heat testing (85 Celsius at 85% relative humidity) for > 1000 hours.
Mechanistic studies involving synergistic experiment and theory were performed on the perfectly alternating copolymerization of 1-butene oxide and carbic anhydride using a (salph)AlCl/[PPN]Cl catalytic pair. These studies showed a first-order dependence of the polymerization rate on the epoxide, a zero-order dependence on the cyclic anhydride, and a first-order dependence on the catalyst only if the two members of the catalytic pair are treated as a single unit. Studies of model complexes showed that a mixed alkoxide/carboxylate aluminum intermediate preferentially opens cyclic anhydride over epoxide. In addition, ring-opening of epoxide by an intermediate comprising multiple carboxylates was found to be rate-determining. On the basis of the experimental results and analysis by DFT calculations, a mechanism involving two catalytic cycles is proposed wherein the alternating copolymerization proceeds via intermediates that have carboxylate ligation in common, and a secondary cycle involving a bis-alkoxide species is avoided, thus explaining the lack of side reactions until the polymerization is complete.
Site selectivity represents a key challenge for non-directed C−H functionalization, even when the C−H bond is intrinsically reactive. Here, we report a copper-catalyzed method for benzylic C−H azidation of diverse molecules. Experimental and density functional theory studies suggest the benzyl radical reacts with a Cu II -azide species via a radical-polar crossover pathway. Comparison of this method with other C−H azidation methods highlights its unique site selectivity, and conversions of the benzyl azide products into amine, triazole, tetrazole, and pyrrole functional groups highlight the broad utility of this method for target molecule synthesis and medicinal chemistry.
In a possibly biomimetic
fashion, formally copper(III)–oxygen
complexes LCu(III)–OH (1) and LCu(III)–OOCm
(2) (L2– = N,N′-bis(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamide,
Cm = α,α-dimethylbenzyl) have been shown to activate X–H
bonds (X = C, O). Herein, we demonstrate similar X–H bond activation
by a formally Cu(III) complex supported by the same dicarboxamido
ligand, LCu(III)–O2CAr1 (3, Ar1 = meta-chlorophenyl), and we compare
its reactivity to that of 1 and 2. Kinetic
measurements revealed a second order reaction with distinct differences
in the rates: 1 reacts the fastest in the presence of
O–H or C–H based substrates, followed by 3, which is followed by (unreactive) 2. The difference
in reactivity is attributed to both a varying oxidizing ability of
the studied complexes and to a variation in X–H bond functionalization
mechanisms, which in these cases are characterized as either a hydrogen-atom
transfer (HAT) or a concerted proton-coupled electron transfer (cPCET).
Select theoretical tools have been employed to distinguish these two
cases, both of which generally focus on whether the electron (e–) and proton (H+) travel “together”
as a true H atom, (HAT), or whether the H+ and e– are transferred in concert, but travel between different donor/acceptor
centers (cPCET). In this work, we reveal that both mechanisms are
active for X–H bond activation by 1–3, with interesting variations as a function of substrate
and copper functionality.
Single-site
heterogeneous catalysts (SSHCs) play important roles
in fundamental science and technology, owing to the molecular level
control of structure–support interactions that is possible
in these systems. Recently, SSHCs supported by acidic oxides have
attracted particular interest because catalytically active metal centers
can be formed at the surface sites. Here, we incorporated a palladium
SSHC in phosphated and sulfated metal–organic frameworks (MOFs),
hafnium-based MOF-808 (Hf-MOF-808-PO4 and Hf-MOF-808-SO4). The structural and electronic properties of the Pd(II)
sites coordinated to the acidic sites in these MOFs were investigated
through X-ray photoelectron spectroscopy, vibrational spectroscopy,
X-ray crystallographic techniques, catalytic studies, and quantum
mechanical electronic structure calculations employing density functional
theory. We demonstrated that the presence of node-bound acidic functional
groups stabilizes the Pd(II) site in these MOFs, resulting in enhanced
catalytic activities (compared to in the nonacid functionalized Hf-MOF-808)
in the oxidative Heck reaction where Pd(II) is the active species.
The density functional calculations support the interpretation that
the acid functionalization of the MOF node can stabilize the Pd(0)
intermediate state during the catalytic reactions, thereby suppressing
Pd(0) aggregation leading to catalyst deactivation. These findings
offer insights and methodology for the catalytic investigation of
SSHCs in MOFs.
Isosorbide
is a rigid, sugar-derived building block that has shown
promise in high-performance materials, albeit with a lack of available
controlled polymerization methods. To this end, we provide mechanistic
insights into the cationic and quasi-zwitterionic ring-opening polymerization
(ROP) of an annulated isosorbide derivative (1,4:2,5:3,6-trianhydro-d-mannitol, 5). Ring-opening selectivity of this
tricyclic ether was achieved, and the polymerization is selectively
directed toward different macromolecular architectures, allowing for
formation of either linear or cyclic polymers. Notably, straightforward
recycling of unreacted monomer can be accomplished via sublimation.
This work provides the first platform for tailored polymer architectures
from isosorbide via ROP.
A series
of complexes {[NBu4][LCuII(O2CR)]
(R = −C6F5, −C6H4(NO2), −C6H5, −C6H4(OMe), −CH3, and −C6H2(
i
Pr)3)} were characterized (with the complex R = −C6H4(m-Cl) having been published elsewhere (J. Am. Chem. Soc.201914117236)). All feature N,N′,N″-coordination of the supporting L2– ligand, except for the complex with R = −C6H2(
i
Pr)3, which exhibits N,N′,O-coordination. For
the N,N′,N″-bound complexes, redox
properties, UV–vis ligand-to-metal charge transfer (LMCT) features,
and rates of hydrogen atom abstraction from 2,4,6,-tri-t-butylphenol using the oxidized, formally Cu(III) compounds LCuIII(O2CR) correlated well with the electron donating
nature of R as measured both experimentally and computationally. Specifically,
the greater the electron donation, the lower is the energy for LMCT
and the slower is the reaction rate. The results are interpreted to
support an oxidatively asynchronous proton-coupled electron transfer
mechanism that is sensitive to the oxidative power of the [CuIII(O2CR)]2+ core.
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