Visible-Light Activation of Diorganyl Bis(pyridylimino) Isoindolide Aluminum(III) Complexes and Their Organometallic Radical Reactivity

We report on the synthesis and characterization of a series of (mostly) air-stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible-light excitation. The redox non-innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon-centered radicals. The photo-induced homolytic cleavage of the Al-C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin-free Giese-type reactions and carbonyl alkylations under ambient conditions, which both indicated radical-polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench-stable aluminum organometallics.

Spectroscopic Identification of Diphosphene HPPH and Isomeric Diphosphinyldene PPH2

The simplest diphosphene HPPH and isomeric diphosphinyldene PPH₂ features prototype phosphorus-phosphorus multiple bonding properties that have been of long-standing interest in main-group chemistry. Herein, we report the observation of cis-HPPH, trans-HPPH, and PPH₂ among the respective laser photolysis products of phosphine (PH₃ ) and diphosphine (P₂ H₄ ) in solid N₂ - and Ar-matrices at 10 K. The identification of these P₂ H₂ isomers with matrix-isolation IR and UV/Vis spectroscopy is supported by D-isotope labeling and the quantum chemical calculations at the CCSD(T)-F12a/cc-pVTZ-F12 level using configuration-selective vibrational configuration interaction theory (VCI). Bonding analyses suggest that the two conformers of HPPH contain standard PP double bonds, whereas, PPH₂ resembles P₂ in having partial PP triple bond due to the H₂ P←P π bonding interaction.

Reduction of Na+ within a {Mg2 Na2 } Assembly

Ionic compounds containing sodium cations are notable for their stability and resistance to redox reactivity unless highly reducing electrical potentials are applied. Here we report that treatment of a low oxidation state {Mg₂ Na₂ } species with non-reducible organic bases induces the spontaneous and completely selective extrusion of sodium metal and oxidation of the Mg^I centers to the more conventional Mg^II state. Although these processes are also characterized by a structural reorganisation of the initially chelated diamide spectator ligand, computational quantum chemical studies indicate that intramolecular electron transfer is abetted by the frontier molecular orbitals (HOMO/LUMO) of the {Mg₂ Na₂ } ensemble, which arise exclusively from the 3s valence atomic orbitals of the constituent sodium and magnesium atoms.

Strain-Release Pentafluorosulfanylation and Tetrafluoro(aryl)sulfanylation of [1.1.1]Propellane: Reactivity and Structural Insight

We leveraged the recent increase in synthetic accessibility of SF5 Cl and Ar-SF4 Cl compounds to combine chemistry of the SF5 and SF4 Ar groups with strain-release functionalization. By effectively adding SF5 and SF4 Ar radicals across [1.1.1]propellane, we accessed structurally unique bicyclopentanes, bearing two distinct elements of bioisosterism. Upon evaluating these "hybrid isostere" motifs in the solid state, we measured exceptionally short transannular distances; in one case, the distance rivals the shortest nonbonding C⋅⋅⋅C contact reported to date. This prompted SC-XRD and DFT analyses that support the notion that a donor-acceptor interaction involving the "wing" C-C bonds is playing an important role in stabilization. Thus, these heretofore unknown structures expand the palette for highly coveted three-dimensional fluorinated building blocks and provide insight to a more general effect observed in bicyclopentanes.

Taming Heavier Group 14 Imine Analogues: Accessing Tin Nitrogen [Sn=N] Double Bonds and their Cycloaddition/Metathesis Chemistry

A systematic study to access stable stannaimines is reported, by combining different heteroleptic stannylenes with a range of organic azides. The reactions of terphenyl-/hypersilyl-substituted stannylenes yield the putative tin nitrogen double bond, but is directly followed by 1,2-silyl migration to give Sn^II systems featuring bulky silylamido ligands. By contrast, the transition from a two σ donor ligand set to a mixed σ-donor/π-donor scaffold allows access to three new stannaimines which can be handled at room temperature. The reactivity profile of these Sn=N bonded species is crucially dependent on the substituent at the nitrogen atom. As such, the Sn=NMes (Mes=2,4,6-Me₃ C₆ H₂ ) system is capable of activating a broad range of substrates under ambient conditions via 1,2-addition reactions, [2+2] and [4+2] cycloaddition reactions. Most interestingly, very rare examples of main group multiple bond metathesis reactions are also found to be viable.

Evidence for C-F Bond Formation through Formal Reductive Elimination from Tellurium(VI)

The fundamental challenge of C-F bond formation by reductive elimination has been met by compounds of select transition metals and fewer main group elements. The work detailed herein expands the list of main group elements known to be capable of reductively eliminating a C-F bond to include tellurium. Surprising and novel modes of both sp² and sp³ C-F bond formation were observed alongside formation of Te^IV cations during two separate attempts to synthesize/characterize fluorinated organotellurium(VI) cations in superacidic media (SbF₅ /SO₂ ClF). Following detailed low-temperature NMR experiments, the mechanisms of the two unique reductive elimination reactions were probed and investigated using density functional theory (DFT) calculations. Ultimately, we found that an "indirect" reductive elimination pathway is likely operative whereby Sb plays a key role in fluoride abstraction and C-F bond formation, as opposed to unimolecular reductive elimination from a discrete Te^VI cation.

Synthesis of Molecular Phenylcalcium Derivatives: Application to the Formation of Biaryls

Hydrocarbon‐soluble β‐diketiminato phenylcalcium derivatives, which display various modes of Ca−μ₂‐Ph−Ca bridging, are accessible from reactions of Ph₂Hg and [(BDI)CaH]₂. Although the resultant compounds are inert toward the C−H bonds of benzene, they yield selective and uncatalyzed biaryl formation when reacted with readily available aryl bromides.

Ylide-stabilized phosphenium cations: Impact of the substitution pattern on the coordination chemistry

Although N‐heterocyclic phosphenium (NHP) cations have received considerable research interest due to their application in organocatalysis, including asymmetric synthesis, phosphenium cations with other substitution patterns have hardly been explored. Herein, the preparation of a series of ylide‐substituted cations of type [YPR]⁺ (with Y=Ph₃PC(Ph), R=Ph, Cy or Y) and their structural and coordination properties are reported. Although the diylide‐substituted cation forms spontaneous from the chlorophosphine precursor, the monoylidylphosphenium ions required the addition of a halide‐abstraction reagent. The molecular structures of the cations reflected the different degrees of electron donation from the ylide to the phosphorus center depending on the second substituent. Molecular orbital analysis confirmed the stronger donor properties of the ylide systems compared to NHPs with the mono‐ylide substituted cations featuring a more pronounced electrophilicity. This was mirrored by the reaction of the cations towards gold chloride, in which only the diylide‐substituted cation [Y₂P]⁺ formed the expected LAuCl]⁺ complex, while the monoylide‐substituted compounds reacted to the chlorophosphine ligands by transfer of the chloride from gold to the phosphorus center. These results demonstrate the tunability of ylide‐functionalized phosphorus cations, which should allow for further applications in coordination chemistry in the future.

Taming the Lewis Superacidity of Non-Planar Boranes: C-H Bond Activation and Non-Classical Binding Modes at Boron

The rational design of a geometrically constrained boron Lewis superacid featuring exceptional structure and reactivity is disclosed. It allowed the formation of non-classical electron deficient B-H-B type of bonding which was supported by spectroscopic and X-ray diffraction parameters as well as computational studies. Taming the pyramidal Lewis acid electrophilicity through weak coordinating anion dissociation enabled a series of highly challenging chemical transformations such as Csp 2 -H and Csp 3 -H activation under frustrated Lewis pair regime and the cleavage of Csp 3 -Si bonds. The demonstration of such type of rich chemical behavior and flexibility on a single molecular compound make it a unique mediator of chemical transformations generally restricted to transition metals.

Antimony(III) Halide-Assisted Stereospecific Coordination of Thione

The antimony halide-aided stereospecific coordination of a cyclic thiourea-type of ligand is observed for the first time. The antimony(III) imidazole thione complexes syn-[(L¹ )SbCl₃ ] (syn-1) and anti-[(L¹ )SbBr₃ ] (anti-2) have been synthesized in very good yield by the reaction between the spatially defined steric impact ligand [(IPaul)S] (L¹ ) ([(IPaul)S]=1,3-bis(2,4-methyl-6-diphenyl phenyl)imidazole thione) and corresponding antimony halide. The stereoselective formation of complexes syn-1 and anti-2 has been confirmed by both NMR and single-crystal X-ray diffraction studies. Interestingly the stereospecific nature of syn-1 and anti-2 remains intact in solution. Furthermore, the thermal stability of antimony(III) imidazole thione complexes were examined by TGA analysis.