Michael Cowley

We demonstrate that polymer electron acceptors with excellent all-polymer solar-cell (all-PSC) device performance can be developed from polymer electron donors by using BN units. By alleviating the steric hindrance effect of the bulky pendant moieties on the conjugated polymers that contain BN units, the π–π stacking distance of polymer backbones is decreased and the electron mobility is consequently enhanced by nearly two orders of magnitude. As a result, the power conversion efficiency of all-PSCs with the polymer acting as the electron acceptor is greatly improved from 0.12 % to 5.04 %. This PCE value is comparable to that of the best all-PSCs with state-of-the-art polymer acceptors.

From giver to taker: Incorporation of BN units into polymer electron donors has resulted in a series of polymer electron acceptors. Extending the length of the repeating units of the conjugated polymers alleviates the effect of steric hindrance from the pendant groups and promotes the π–π stacking of the polymer backbones. The all-polymer solar-cell device shows a power conversion efficiency (PCE) exceeding 5.0 %.

The first frustrated Lewis pair-catalyzed cycloisomerization of a series of 1,5-enynes was developed. The reaction proceeds via the π-activation of the alkyne and subsequent 5-endo-dig cyclization with the adjacent alkene. The presence of PPh₃ was of utmost importance on the one hand to prevent side reactions (for example, 1,1-carboboration) and on the other hand for the efficient protodeborylation to achieve the catalytic turnover. The mechanism is explained on the basis of quantum-chemical calculations, which are in full agreement with the experimental observations.

Cycloisomerization without a metal: The first FLP-catalyzed C−C bond forming reaction is described as proceeding through a cycloisomerization/protodeborylation sequence. The reaction mechanism is supported by X-ray crystal structure analysis of intermediates, kinetic experiments, and by quantum-mechanical calculations.

At ultrahigh pressure (>110 GPa), H₂S is converted into a metallic phase that becomes superconducting with a record T_c of approximately 200 K. It has been proposed that the superconducting phase is body-centered cubic H₃S (Im m, a=3.089 Å) resulting from the decomposition reaction 3 H₂S2 H₃S+S. The analogy between H₂S and H₂O led us to a very different conclusion. The well-known dissociation of water into H₃O⁺ and OH⁻ increases by orders of magnitude under pressure. H₂S is anticipated to behave similarly under pressure, with the dissociation process 2 H₂SH₃S⁺+SH⁻ leading to the perovskite structure (SH⁻)(H₃S⁺). This phase consists of corner-sharing SH₆ octahedra with SH⁻ ions at each A site (the centers of the S₈ cubes). DFT calculations show that the perovskite (SH⁻)(H₃S⁺) is thermodynamically more stable than the Im m structure of H₃S, and suggest that the A site hydrogen atoms are most likely fluxional even at T_c .

Under ultrahigh pressure (>110 GPa), H₂S is converted into a metallic phase that becomes superconducting with a record T_c of approximately 200 K. It is proposed that in this phase a dissociation of 2 H₂S into H₃S⁺ and SH⁻ is present, leading to the perovskite structure (SH⁻)(H₃S⁺). This phase consists of corner-sharing SH₆ octahedra with SH⁻ ions at the center of each S₈ cube.

Nanostructured Group 14 semiconductors attract significant attention because of a broad range of potential applications. In their Communication on page 2441 ff., T. Fässler, D. Fattakhova-Rohlfing et al. describe a general and controllable fabrication method for Ge nanomorphologies with tunable composition using the controlled reaction of [Ge₉]⁴⁻ Zintl clusters to a solid germanium phase. The image was designed by Christoph Hohmann, Nanosystems Initiative Munich.

We report the spectroscopic identification of the [B₃(NN)₃]⁺ and [B₃(CO)₃]⁺ complexes, which feature the smallest π-aromatic system B₃⁺. A quantum chemical bonding analysis shows that the adducts are mainly stabilized by L[B₃L₂]⁺ σ-donation.

Minimal π systems: The [B₃(NN)₃]⁺ and [B₃(CO)₃]⁺ complexes are identified through infrared photodissociation spectroscopy. The complexes feature the smallest π-aromatic system B₃⁺. Quantum chemical bonding analysis reveals that the adducts are stabilized by L[B₃L₂]⁺ σ-donation.

High-molar-mass poly(alkylphosphinoboranes) are currently not accessible by conventional metal-catalyzed dehydropolymerization. However, the mild thermolysis of a Lewis base stabilized phosphinoborane is an alternative metal-free approach. In their Communication on page 13782 ff., M. Scheer, I. Manners, and co-workers describe their successful synthesis of poly(tert-butylphosphinoborane) by this promising strategy.

Cation–π interactions are one of the most important classes of noncovalent bonding, and are seen throughout biology, chemistry, and materials science. However, in almost every documented case, these interactions play only a supporting role to much stronger covalent or dative bonds, thus making examples of exclusive cation–π bonding exceedingly rare. In this study, a neutral diboryne molecule is found to encapsulate the light alkali metal cations Li⁺ and Na⁺ in the absence of a net charge, covalent bonds, or lone-pair donor groups. The resulting encapsulation complexes are, to our knowledge, the first structurally authenticated species in which a neutral molecule binds the light alkali metals exclusively through cation–π interactions.

No help required: Cation–π interactions are one of the most important classes of noncovalent bonding; however, examples of exclusive cation–π bonding are exceedingly rare. A neutral diboryne molecule has been found to encapsulate Li⁺ and Na⁺ in the absence of a net charge, covalent bonds, or lone-pair donor groups. In the resulting encapsulation complexes, a neutral molecule binds the light alkali metals exclusively through cation–π interactions.

Photolysis of the cyclic phosphine oligomer [PPh]₅ in the presence of pentaarylboroles leads to the formation of 1,2-phosphaborines by the formal insertion of a phenylphosphinidene fragment into the endocyclic CB bond. The solid-state structure features a virtually planar central ring with bond lengths indicating significant delocalization. Appreciable ring current in the 1,2-phosphaborine core, detected in nuclear independent chemical shift (NICS) calculations, are consistent with aromatic character. These products are the first reported 1,2-BPC₄ conjugated heterocycles and open a new avenue for BP as a valence isoelectronic substitute for CC in arene systems.

Jamming PB into benzene: 1,2-Phosphaborines were synthesized by the ring expansion reaction of boroles with the cyclic phosphine [PPh]₅ under UV irradiation. The products were structurally characterized revealing a planar central ring. The nature of the bonding was analyzed computationally and indicated that the heterocycle had appreciable aromatic character.

We report the evaporation of a stable cyclic silylene and its oxidation (with ozone or N₂O) through oxygen atom transfer to form the corresponding silanone under matrix isolation conditions. As uncomplexed silanones are rare owing to their very high reactivity, this method provides an alternative route to these sought-after molecules. The silanone, as well as a novel bicyclic silane with a bridgehead silicon atom derived from an intramolecular silylene CH bond insertion, were characterized by comparison of high-resolution infrared spectra with density functional theory (DFT) computations at the M06-2X/cc-pVDZ level of theory.

One atom at a time: Oxygen atom transfer from ozone to a stable silylene provides access to a new cyclic silanone. This bimolecular atom transfer reaction was achieved under matrix isolation conditions through co-deposition of the silylene and ozone. Conclusive evidence for the silanone is provided by comparison of experimental and computed IR spectra, including isotopological ¹⁶O/¹⁸O replacements. Atom colors: Si=purple, C=black, O=red, H=blue.

We report the first direct spectroscopic observation by electron paramagnetic resonance (EPR) spectroscopy of a triplet diradical that is formed in a thermally induced rotation around a main-group π bond, that is, the SiSi double bond of tetrakis(di-tert-butylmethylsilyl)disilene (1). The highly twisted ground-state geometry of singlet 1 allows access to the perpendicular triplet diradical 2 at moderate temperatures of 350–410 K. DFT-calculated zero-field splitting (ZFS) parameters of 2 accurately reproduce the experimentally observed half-field transition. Experiment and theory suggest a thermal equilibrium between 1 and 2 with a very low singlet–triplet energy gap of only 7.3 kcal mol⁻¹.

A triplet diradical that is formed in a thermally induced rotation around a main-group π bond, that is the SiSi double bond of 1, was directly observed by EPR spectroscopy. Both experiment and theory support a thermal equilibrium between singlet 1 and the perpendicular triplet diradical 2.

The regioregular synthesis of the first azaborine oligomers and a corresponding conjugated polymer was accomplished by Suzuki–Miyaura coupling methods. An almost perfectly coplanar syn arrangement of the heterocycles was deduced from an X-ray crystal structure of the dimer, which also suggested that NH⋅⋅⋅π interactions play an important role. Computational studies further supported these experimental observations and indicated that the electronic structure of the longer azaborine oligomers and polymer resembles that of poly(cyclohexadiene) more than poly(p-phenylene). A comparison of the absorption and emission properties of the polymer with those of the oligomers revealed dramatic bathochromic shifts upon chain elongation, thus suggesting highly effective extension of conjugation.

A BN serial: A regioregular conjugated polymer and short model oligomers constructed solely from 1,2-azaborine units by Suzuki–Miyaura cross-coupling have been synthesized and found to adopt an unusual syn conformation stabilized by NH⋅⋅⋅π interactions. The optoelectronic properties of the polymer more closely resemble the computationally predicted properties of poly(cyclohexadiene) rather than those of poly(p-phenylene). pin=pinacol.

The BN analogue of ortho-benzyne, 1,2-azaborine, is generated by flash vacuum pyrolysis, trapped under cryogenic conditions, and studied by direct spectroscopic techniques. The parent BN aryne spontaneously binds N₂ and CO₂, thus demonstrating its highly reactive nature. The interaction with N₂ is photochemically reversible. The CO₂ adduct of 1,2-azaborine is a cyclic carbamate which undergoes photocleavage, thus resulting in overall CO₂ splitting.

Azaborine in a flash: The boron–nitrogen derivative of ortho-benzyne, 1,2-azaborine, can be synthesized by flash vacuum pyrolysis (FVP) and trapped under cryogenic conditions to form a Lewis acid/base complex with nitrogen. Irradiation generates the free 1,2-azaborine which readily reacts with dinitrogen at slightly elevated temperatures.

The radical anion of octa-tert-butyloctasilacubane was generated and isolated. The EPR spectrum showed the satellites due to the tertiary ¹³C nuclei of the eight tert-butyl groups. The X-ray crystallographic analysis showed that the SiSi bonds are shortened and the SiC bonds are elongated compared with those of octa-tert-butyloctasilacubane. These results are well explained by the distribution of an unpaired electron in the singly occupied molecular orbital (SOMO).

The radical anion of octa-tert-butyloctasilacubane was generated and isolated (see picture). The X-ray crystallographic analysis showed that the SiSi bonds are shortened and the SiC bonds are elongated compared with those of octa-tert-butyloctasilacubane. These results are well explained by the distribution of an unpaired electron in the singly occupied molecular orbital (SOMO).

The first example of the homonuclear pyramidanes, pentagermapyramidane, was synthesized, fully characterized, and computationally studied to reveal its peculiar structural features and the nature of its apex-to-base bonding interactions. Both solid-state and solution structures of pentagermapyramidane are discussed based on the computed stabilities of its square-pyramidal and distorted forms.

Germanium pyramids: The homonuclear pentagermapyramidane Ge[Ge₄(SiMetBu₂)₄] (1) was synthesized and characterized. Crystal structures of two structural variations of 1 are reported: the distorted pyramidal structure 1 a, corresponding to the energy minima on the Ge₅R₄ potential energy surface (PES), and the square-planar pyramidal 1 b, representing a transition state on the PES.

Silicon analogues of the most prominent carbon nanostructures, namely, hollow spheroidals such as C₆₀ and the fullerene family, have been unknown to date. Herein we show that discrete Si₂₀ dodecahedra, stabilized by an endohedral guest and valence saturation, are accessible in preparative yields through a chloride-induced disproportionation reaction of hexachlorodisilane in the presence of tri(n-butyl)amine. X-ray crystallography revealed that each silicon dodecahedron contains an endohedral chloride ion that imparts a net negative charge. Eight chloro substituents and twelve trichlorosilyl groups are attached to the surface of each cluster in a strictly regioregular arrangement, a thermodynamically preferred substitution pattern according to quantum-chemical assessment. Our results demonstrate that the wet-chemical self-assembly of a complex, monodisperse Si nanostructure is possible under mild conditions starting from simple Si₂ building blocks.

As simple as this: A stable, crystalline [20]silafullerane forms in preparatively useful yields through wet-chemical self-assembly from Si₂Cl₆ and chloride ions in the presence of an amine. Each silicon dodecahedron contains an endohedral chloride ion that imparts a net negative charge. Eight chloro substituents and twelve trichlorosilyl groups are attached to the surface of each cluster in a strictly regioregular arrangement.

NHC adducts of the stannylene Trip₂Sn (Trip=2,4,6-triisopropylphenyl) were reacted with zero-valent Ni, Pd, and Pt precursor complexes to cleanly yield the respective metal complexes featuring a three-membered ring moiety Sn-Sn-M along with carbene transfer onto the metal and complete substitution of the starting ligands. Thus the easily accessible NHC adducts to stannylenes are shown to be valuable precursors for transition-metal complexes with an unexpected SnSn bond. The complexes have been studied by X-ray diffraction and NMR spectroscopy as well as DFT calculations. The compounds featuring the structural motif of a distannametallacycle comprised of a [(NHC)₂M⁰] fragment and Sn₂Trip₄ represent rare higher congeners of the well-known olefin complexes. DFT calculations indicate the presence of a π-type Sn–Sn interaction in these first examples for acyclic distannenes symmetrically coordinating to a zero-valent transition metal.

Differing analogues: On reacting the carbene adduct of the stannylene [Trip₂Sn] (Trip=2,4,6-triisopropylphenyl) with zero-valent Group 10 complexes, symmetrically coordinating complexes of the distannene [Sn₂Trip₄] to Ni, Pd, and Pt have been obtained. Their structural and spectroscopic properties are presented and discussed.

Reaction of the tin cluster Sn₈(Ar)₄ (Ar=C₆H₂-2,6-(C₆H₃-2,4,6-Me₃)₂) with excess ethylene or dihydrogen at 25 °C/1 atmosphere yielded two new clusters that incorporated ethylene or hydrogen. The reaction with ethylene yielded Sn₄(Ar)₄(C₂H₂)₅ that contained five ethylene moieties bridging four aryl substituted tin atoms and one tin–tin bond. Reaction with H₂ produced a cyclic tin species of formula (Sn(H)Ar)₄, which could also be synthesized by the reaction of {(Ar)Sn(μ-Cl)}₂ with DIBAL-H. These reactions represent the first instances of direct reactions of isolable main-group clusters with ethylene or hydrogen under mild conditions. The products were characterized in the solid state by X-ray diffraction and IR spectroscopy and in solution by multinuclear NMR and UV/Vis spectroscopies. Density functional theory calculations were performed to explain the reactivity of the cluster.

Tinned small molecules: Reactions of the tin cluster Sn₈(Ar)₄ under mild conditions yields two new insertion compounds from incorporation of ethylene (see picture; left) or H₂ (right; Sn blue, C gray). To our knowledge, these are the first reactions between a stable main-group cluster and small molecules.

Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) are supremely important techniques with numerous applications in almost all branches of science. However, until recently, NMR methodology was limited by the time constant T₁ for the decay of nuclear spin magnetization through contact with the thermal molecular environment. Long-lived states, which are correlated quantum states of multiple nuclei, have decay time constants that may exceed T₁ by large factors. Here we demonstrate a nuclear long-lived state comprising two ¹³C nuclei with a lifetime exceeding one hour in room-temperature solution, which is around 50 times longer than T₁. This behavior is well-predicted by a combination of quantum theory, molecular dynamics, and quantum chemistry. Such ultra-long-lived states are expected to be useful for the transport and application of nuclear hyperpolarization, which leads to NMR and MRI signals enhanced by up to five orders of magnitude.

A long-lived nuclear singlet: A molecular system based on a ¹³C₂-labelled naphthalene core has been designed to support long-lived nuclear singlet order in solution. A nuclear singlet lifetime exceeding one hour has been achieved in room-temperature solution.