The First Synthesis of the Sterically Encumbered Adamantoid Phosphazane P 4 (N t Bu) 6 : Enabled by Mechanochemistry

Rigid PN cages as 3-dimensional building blocks for crystalline or amorphous networked materials

Three-dimensional covalent connectors are valuable synthons for accessing crystalline or amorphous networks. Currently, fused polycyclic alkanes are employed as connectors in this context. We debut phosphorus-nitrogen (PN) cages as new 3-dimensional (3-D) inorganic connectors that yield crystalline and amorphous networks, including examples with gas porosity. We show that the high tunability of PN cages accelerates network diversification and the presence of a responsive 31P NMR spectroscopic handle provides structural insight. Collectively, this work unlocks a new and convenient 3-D synthon for reticular chemistry.

Disentangling the Effect of Pressure and Mixing on a Mechanochemical Bromination Reaction by Solid-State NMR Spectroscopy

Mechanical forces, including compressive stresses, have a significant impact on chemical reactions. Besides the preparative opportunities, mechanochemical conditions benefit from the absence of any organic solvent, the possibility of a significant synthetic acceleration and unique reaction pathways. Together with an accurate characterization of ball-milling products, the development of a deeper mechanistic understanding of the occurring transformations at a molecular level is critical for fully grasping the potential of organic mechanosynthesis. We herein studied a bromination of a cyclic sulfoximine in a mixer mill and used solid-state nuclear magnetic resonance (NMR) spectroscopy for structural characterization of the reaction products. Magic-angle spinning (MAS) was applied for elucidating the product mixtures taken from the milling jar without introducing any further post-processing on the sample. Ex situ ¹³ C-detected NMR spectra of ball-milling products showed the formation of a crystalline solid phase with the regioselective bromination of the S-aryl group of the heterocycle in position 4. Completion is reached in less than 30 minutes as deduced from the NMR spectra. The bromination can also be achieved by magnetic stirring, but then, a longer reaction time is required. Mixing the solid educts in the NMR rotor allows to get in situ insights into the reaction and enables the detection of a reaction intermediate. The pressure alone induced in the rotor by MAS is not sufficient to lead to full conversion and the reaction occurs on slower time scales than in the ball mill, which is crucial for analysing mixtures taken from the milling jar by solid-state NMR. Our data suggest that on top of centrifugal forces, an efficient mixing of the starting materials is required for reaching a complete reaction.

(PNSiMe3 )4 (NMe)6 : A Robust Tetravalent Phosphaza-adamantane Scaffold for Molecular and Macromolecular Construction

Tetraarylmethanes and adamantanes are important rigid covalent connectors that play a four-way scaffolding role in molecular and materials chemistry. We report the synthesis of a new tetravalent phosphaza-adamantane cage, (PNSiMe₃ )₄ (NMe)₆ (2), that shows high thermal, air, and redox stability due to its geometry. It nevertheless participates in covalent four-fold functionalization reactions along its periphery. The combination of a robust core and reactive corona makes 2 a convenient inorganic scaffold upon which tetrahedral molecular and macromolecular chemistry can be constructed. This potential is demonstrated by the synthesis of a tetrakis(bis(phosphine)iminium) ion (in compound 3) and the first all P/N poly(phosphazene) network (5).

Mechanochemical Access to a Short-Lived Cyclic Dimer Pd2 L2 : An Elusive Kinetic Species En Route to Molecular Triangle Pd3 L3 and Molecular Square Pd4 L4

Pd-based molecular square Pd₄ L₄ and triangle Pd₃ L₃ represent the molecular ancestors of metal-coordination polyhedra that have been an integral part of the field for the last 30 years. Conventional solution-based reactions between cis-protected Pd ions and 2,2'-bipyridine exclusively give Pd₄ L₄ and/or Pd₃ L₃ as the sole products. We herein show that, under solvent-free mechanochemical conditions, the self-assembly energy landscape can be thermodynamically manipulated to form an elusive cyclic dimer Pd₂ L₂ for the first time. In the absence of solvent, Pd₂ L₂ is indefinitely stable in the solid-state, but converts rapidly to its thermodynamic products Pd₃ L₃ and Pd₄ L₄ in solution, confirming Pd₂ L₂ as a short-lived kinetic species in the solution-based self-assembly process. Our results highlight how mechanochemistry grants access to a vastly different chemical space than available under conventional solution conditions. This provides a unique opportunity to isolate elusive species in self-assembly processes that are too reactive to both "see" and "capture".

Di(indenyl)beryllium

The bonding in beryllocene, [BeCp₂ ], took decades to establish, owing to its unexpected mixed hapticity structure (i.e., [Be(η⁵ -Cp)(η¹ -Cp)]). Beryllium complexes containing the indenyl ligand, which is a close relative of the cyclopentadienyl anion, but which is also known to exhibit its own bonding peculiarities (e.g., facile η⁵ ⇄ η³ shifts), have remained unknown. Standard metathetical approaches to their synthesis (e.g., with K[Ind'] + BeX₂ in an ether solvent) give rise to intractable oils from which nothing identifiable can be isolated. In contrast, mechanochemical preparation, involving the solvent-free grinding of BeBr₂ and potassium indenides, leads to the production of discrete (indenyl)beryllium complexes, including [Be(C₉ H₇ )₂ ] (1) and [Be{1,3-(SiMe₃ )₂ C₉ H₅ }Br] (2). The former displays η⁵ /η¹ -coordinated ligands in the solid state, but DFT calculations indicate that an η⁵ /η⁵ -conformation is less than 5 kcal mol^-1 higher in energy.

Sustainability Assessment of Mechanochemistry by Using the Twelve Principles of Green Chemistry

In recent years, mechanochemistry has been growing into a widely accepted alternative for chemical synthesis. In addition to their efficiency and practicality, mechanochemical reactions are also recognized for their sustainability. The association between mechanochemistry and Green Chemistry often originates from the solvent-free nature of most mechanochemical protocols, which can reduce waste production. However, mechanochemistry satisfies more than one of the Principles of Green Chemistry. In this Review we will present a series of examples that will clearly illustrate how mechanochemistry can significantly contribute to the fulfillment of Green Chemistry in a more holistic manner.

Mechanochemical Synthesis of Short DNA Fragments

We demonstrate the first mechanochemical synthesis of DNA fragments by ball milling, enabling the synthesis of oligomers of controllable sequence and length using multi-step, one-pot reactions, without bulk solvent or the need to isolate intermediates. Mechanochemistry allowed for coupling of phosphoramidite monomers to the 5'-hydroxyl group of nucleosides, iodine/water oxidation of the resulting phosphite triester linkage, and removal of the 5'-dimethoxytrityl (DMTr) protecting group in situ in good yields (up to 60 % over three steps) to produce DNA dimers in a one-pot manner. H-Phosphonate chemistry under milling conditions enabled coupling and protection of the H-phosphonate linkage, as well as removal of the 5'-DMTr protecting group in situ, enabling a one-pot process with good yields (up to 65 % over three steps, or ca. 87 % per step). Sulfurization of the internucleotide linkage was possible using elemental sulfur (S8) or sulfur transfer reagents, yielding the target DNA phosphorothioate dimers in good yield (up to 80 % over two steps). This work opens the door to creation of solvent-free synthesis methodologies for DNA and RNA therapeutics.

Porous Molecular Capsules as Non-Polymeric Transducers of Mechanical Forces to Mechanophores

Mechanical grinding/milling can be regarded as historically the first technology for changing the properties of matter. Mechanically activated molecular units (mechanophores) can be present in various structures: polymers, macromolecules, or small molecules. However, only polymers have been reported to effectively transduce energy to mechanophores, which induces breakage of covalent bonds. In this paper, a second possibility is presented-molecular capsules as stress-sensitive units. Mechanochemical encapsulation of fullerenes in cystine-based covalent capsules indicates that complexation takes place in the solid state, despite the fact that the capsules do not possess large enough entrance portals. By using a set of solvent-free MALDI (sf-MALDI) and solid-state NMR (ss-NMR) experiments, it has been proven that encapsulation proceeds during milling and in this process hydrazones and disulfides get activated for breakage, exchange, and re-forming. The capsules are porous and therefore prone to collapse under solvent-free conditions and their conformational rigidity promotes the collapse by the breaking of covalent bonds.

Switching Chemoselectivity: Using Mechanochemistry to Alter Reaction Kinetics

A reaction manifold has been discovered in which the chemoselectivity can be altered by switching between neat milling and liquid assisted grinding (LAG) with polar additives. After investigation of the reaction mechanism, it has been established that this switching in reaction pathway is due to the neat mechanochemical conditions exhibiting different kinetics for a key step in the transformation. This proof of concept study demonstrates that mechanochemistry can be used to trap the kinetic product of a reaction. It is envisaged that, if this concept can be successfully applied to other transformations, novel synthetic processes could be discovered and known reaction pathways perturbed or diverted.

Orthogonality in main group compounds: a direct one-step synthesis of air- and moisture-stable cyclophosphazanes by mechanochemistry

Mechanochemistry has been established to be an environmentally-friendly way of conducting reactions in a solvent-free manner.