Synthesis and Reactivity of a Cyclooctatetraene-Like Polyphosphorus Ligand Complex [Cyclo-P8]

The thermolysis of Cp′′′Ta(CO)₄ with white phosphorus (P₄) gives access to [{Cp′′′Ta}₂(μ,η^{2 : 2 : 2 : 2 : 1 : 1}-P₈)] ( A ), representing the first complex containing a cyclooctatetraene-like (COT) cyclo-P₈ ligand. While ring sizes of n >6 have remained elusive for cyclo-P_n structural motifs, the choice of the transition metal, co-ligand and reaction conditions allowed the isolation of A . Reactivity investigations reveal its versatile coordination behaviour as well as its redox properties. Oxidation leads to dimerization to afford [{Cp′′′Ta}₄(μ₄,η^{2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1}-P₁₆)][TEF]₂ ( 4 , TEF=[Al(OC{CF₃}₃)₄]⁻). Reduction, however, leads to the fission of one P−P bond in A followed by rapid dimerization to form [K@[2.2.2]cryptand]₂[{Cp′′′Ta}₄(μ₄,η^{2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1}-P₁₆)] ( 5 ), which features an unprecedented chain-type P₁₆ ligand. Lastly, A serves as a P₂ synthon, via ring contraction to the triple-decker complex [{Cp′′′Ta}₂(μ,η^{6 : 6}-P₆)] ( B ).     

Design Strategies for Diradical Boron/Nitrogen Doped Carbon-Based Materials

New heterocyclic diradicaloids based on boron and nitrogen-doped polycyclic systems with open-shell ground-states are obtained via concomitant structural and quinoidal extensions, thus allowing to merge the best of both design strategies. A combination of experimental characterization and theoretical calculations have helped disclose their electronic structure, as well as rationalize their associated magnetic and photophysical properties, spanning the chemical space of available molecular templates for cutting-edge applications in organic electronics and spintronics.     

Catalytic Alkyne Semihydrogenation with Polyhydride Ni/Ga Clusters

The bimetallic, decanuclear Ni₃Ga₇-cluster of the formula [Ni₃(GaTMP)₃(μ²-GaTMP)₃(μ³-GaTMP)] ( 1 , TMP=2,2,6,6-tetramethylpiperidinyl) reacts reversibly with dihydrogen under the formation of a series of (poly-)hydride clusters 2 . Low-temperature 2D NMR experiments at −80 °C show that 2 consist of a mixture of a di- ( 2_Di ), tetra- ( 2_Tetra ) and hexahydride species ( 2_Hexa ). The structures of 2_Di and 2_Tetra are assessed by a combination of 2D NMR spectroscopy and DFT calculations. The cooperation of both metals is essential for the high hydrogen uptake of the cluster. Polyhydrides 2 are catalytically active in the semihydrogenation of 4-octyne to 4-octene with good selectivity. The example is the first of its kind and conceptually relates properties of molecular, atom-precise transition metal/main group metal clusters to the respective solid-state phase in catalysis.     

The Crucial Role of Sb2F10 in the Chemical Synthesis of F2

The purely chemical synthesis of fluorine is a spectacular reaction which for more than a century had been believed to be impossible. In 1986, it was finally experimentally achieved, but since then this important reaction has not been further studied and its detailed mechanism had been a mystery. The known thermal stability of MnF₄ casts serious doubts on the originally proposed hypothesis that MnF₄ is thermodynamically unstable and decomposes spontaneously to a lower manganese fluoride and F₂. This apparent discrepancy has now been resolved experimentally and by electronic structure calculations. It is shown that the reductive elimination of F₂ requires a large excess of SbF₅ and occurs in the last reaction step when in the intermediate [SbF₆][MnF₂][Sb₂F₁₁] the addition of one more SbF₅ molecule to the [SbF₆]⁻ anion generates a second tridentate [Sb₂F₁₁]⁻ anion. The two tridentate [Sb₂F₁₁]⁻ anions then provide six fluorine bridges to the Mn atom thereby facilitating the reductive elimination of the two fluorine ligands as F₂.     

Tuning C1/C2 Selectivity of CO2 Electrochemical Reduction over in-Situ Evolved CuO/SnO2 Heterostructure

Heterostructured oxides with versatile active sites, as a class of efficient catalysts for CO₂ electrochemical reduction (CO₂ER), are prone to undergo structure reconstruction under working conditions, thus bringing challenges to understanding the reaction mechanism and rationally designing catalysts. Herein, we for the first time elucidate the structural reconstruction of CuO/SnO₂ under electrochemical potentials and reveal the intrinsic relationship between CO₂ER product selectivity and the in situ evolved heterostructures. At −0.85 V_RHE, the CuO/SnO₂ evolves to Cu₂O/SnO₂ with high selectivity to HCOOH (Faradaic efficiency of 54.81 %). Mostly interestingly, it is reconstructed to Cu/SnO_2-x at −1.05 V_RHE with significantly improved Faradaic efficiency to ethanol of 39.8 %. In situ Raman spectra and density functional theory (DFT) calculations reveal that the synergetic absorption of *COOH and *CHOCO intermediates at the interface of Cu/SnO_2-x favors the formation of *CO and decreases the energy barrier of C−C coupling, leading to high selectivity to ethanol.     

Tetraazacoronenes and Their Dimers,Trimers and Tetramers

Tetraazacoronenes were synthesized from bay-functionalized tetraazaperylenes by Zr-mediated cyclization and four-fold Suzuki–Miyaura cross coupling. In the Zr-mediated approach, an η⁴-cyclobutadiene-zirconium(IV) complex was isolated as an intermediate to cyclobutene-annulated derivatives. Using bis(pinacolatoboryl)vinyltrimethylsilane as a C₂ building block gave the tetraazacoronene target compound along with the condensed azacoronene dimer as well as higher oligomers. The series of extended azacoronenes show highly resolved UV/Vis absorption bands with increased extinction coefficients for the extended aromatic cores and fluorescence quantum yields of up to 80 % at 659 nm.     

MIDA- and TIDA-Boronates Stabilize α-Radicals Through B−N Hyperconjugation

Multifunctional organoboron compounds increasingly enable the simple generation of complex, Csp³-rich small molecules. The ability of boron-containing functional groups to modify the reactivity of α-radicals has also enabled a myriad of chemical reactions. Boronic esters with vacant p-orbitals have a significant stabilizing effect on α-radicals due to delocalization of spin density into the empty orbital. The effect of coordinatively saturated derivatives, such as N-methyliminodiacetic acid (MIDA) boronates and counterparts, remains less clear. Herein, we demonstrate that coordinatively saturated MIDA and TIDA boronates stabilize secondary alkyl α-radicals via σ_B-N hyperconjugation in a manner that allows site-selective C−H bromination. DFT calculated radical stabilization energies and spin density maps as well as LED NMR kinetic analysis of photochemical bromination rates of different boronic esters further these findings. This work clarifies that the α-radical stabilizing effect of boronic esters does not only proceed via delocalization of radical character into vacant boron p-orbitals, but that hyperconjugation of tetrahedral boron-containing functional groups and their ligand electron delocalizing ability also play a critical role. These findings establish boron ligands as a useful dial for tuning reactivity at the α-carbon.     

Ammonia Synthesis at Room Temperature and Atmospheric Pressure from N2: A Boron-Radical Approach

Ammonia, NH₃, is an essential molecule, being part of fertilizers. It is currently synthesized via the Haber–Bosch process, from the very stable dinitrogen molecule, N₂ and dihydrogen, H₂. This process requires high temperatures and pressures, thereby generating ca 1.6 % of the global CO₂ emissions. Alternative strategies are needed to realize the functionalization of N₂ to NH₃ under mild conditions. Here, we show that boron-centered radicals provide a means of activating N₂ at room temperature and atmospheric pressure whilst allowing a radical process to occur, leading to the production of borylamines. Subsequent hydrolysis released NH₄⁺, the acidic form of NH₃. EPR spectroscopy supported the intermediacy of radicals in the process, corroborated by DFT calculations, which rationalized the mechanism of the N₂ functionalization by R₂B radicals.     

Alkaline-Earth Metal Mediated Benzene-to-Biphenyl Coupling

Complex [(^DIPePBDI)Ca]₂(C₆H₆), with a C₆H₆²⁻ dianion bridging two Ca²⁺ ions, reacts with benzene to yield [(^DIPePBDI)Ca]₂(biphenyl) with a bridging biphenyl²⁻ dianion (^DIPePBDI=HC[C(Me)N-DIPeP]₂; DIPeP=2,6-CH(Et)₂-phenyl). The biphenyl complex was also prepared by reacting [(^DIPePBDI)Ca]₂(C₆H₆) with biphenyl or by reduction of [(^DIPePBDI)CaI]₂ with KC₈ in presence of biphenyl. Benzene-benzene coupling was also observed when the deep purple product of ball-milling [(^DIPPBDI)CaI(THF)]₂ with K/KI was extracted with benzene (DIPP=2,6-CH(Me)₂-phenyl) giving crystalline [(^DIPPBDI)Ca(THF)]₂(biphenyl) (52 % yield). Reduction of [(^DIPePBDI)SrI]₂ with KC₈ gave highly labile [(^DIPePBDI)Sr]₂(C₆H₆) as a black powder (61 % yield) which reacts rapidly and selectively with benzene to [(^DIPePBDI)Sr]₂(biphenyl). DFT calculations show that the most likely route for biphenyl formation is a pathway in which the C₆H₆²⁻ dianion attacks neutral benzene. This is facilitated by metal-benzene coordination.     

Efficient Reverse Intersystem Crossing in Carbene-Copper-Amide TADF Emitters via an Intermediate Triplet State

The mechanism behind reverse intersystem crossing (rISC) in metal-based TADF emitters is still under debate. Thermal rISC necessitates small singlet/triplet energy gaps as realized in donor-acceptor systems with charge-transfer excited states. However, their associated spin-orbit couplings are too small to account for effective rISC. Here, we report the first nonadiabatic dynamics simulation of the rISC process in a carbene-copper(I)-carbazolyl TADF emitter. Efficient rISC on a picosecond time scale is demonstrated for an initial triplet minimum geometry that exhibits a perpendicular orientation of the ligands. The dynamics involves an intermediate higher-lying triplet state of metal-to-ligand charge transfer character (³MLCT), which enables large spin-orbit couplings with the lowest singlet charge transfer state. The mechanism is completed in the S₁ state, where the complex can return to a co-planar coordination geometry that presents high fluorescence efficiency.