Search for: All records

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⇄ η³shifts), have remained unknown. Standard metathetical approaches to their synthesis (e.g., with K[Ind′] + BeX₂in 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 BeBr₂and potassium indenides, leads to the production of discrete (indenyl)beryllium complexes, including [Be(C₉H₇)₂] (1) and [Be{1,3‐(SiMe₃)₂C₉H₅}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⁻¹higher in energy.

 

Without solvents present, the often far‐from‐equilibrium environment in a mechanochemically driven synthesis can generate high‐energy, non‐stoichiometric products not observed from the same ratio of reagents used in solution. Ball milling 2 equiv. K[A’] (A’=[1,3‐(SiMe₃)₂C₃H₃]⁻) with CaI₂yields a non‐stoichiometric calciate, K[CaA’₃], which initially forms a structure (1) likely containing a mixture of pi‐ and sigma‐bound allyl ligands. Dissolved in arenes, the compound rearranges over the course of several days to a structure (2) with only η³‐bound allyl ligands, and that can be crystallized as a coordination polymer. If dissolved in alkanes, however, the rearrangement of1to2occurs within minutes. The structures of1and2have been modeled with DFT calculations, and2initiates the anionic polymerization of methyl methacrylate and isoprene; for the latter, under the mildest conditions yet reported for a heavy Group 2 species (one‐atm pressure and room temperature).

 

Milling two equivalents of K[1,3‐(SiMe₃)₂C₃H₃] (=K[A′]) with MgX₂(X=Cl, Br) produces the allyl complex [K₂MgA′₄] (1). Crystals grown from toluene are of the solvated species [((η⁶‐tol)K)₂MgA′₄] ([1⋅2(tol)]), a trimetallic monomer with both bridging and terminal (η¹) allyl ligands. When recrystallized from hexanes, the unsolvated1forms a 2D coordination polymer, in which the Mg is surrounded by three allyl ligands. The C−C bond lengths differ by only 0.028 Å, indicating virtually complete electron delocalization. This is an unprecedented coordination mode for an allyl ligand bound to Mg. DFT calculations indicate that in isolation, an η³‐allyl configuration on Mg is energetically preferred over the η¹‐ (σ‐bonded) arrangement, but the Mg must be in a low coordination environment for it to be experimentally realized. Methyl methacrylate is effectively polymerized by1, with activities that are comparable to K[A′] and greater than the homometallic magnesium complex [{MgA′₂}₂].

 

Owing to the strength of the C–F bond, the ‘direct’ preparation of Grignard reagents, i.e., the interaction of elemental magnesium with an organic halide, typically in an ethereal solvent, fails for bulk magnesium and organofluorine compounds. Previously described mechanochemical methods for preparing Grignard reagents have involved ball milling powdered magnesium with organochlorines or bromines. Activation of the C–F bond through a similar route is also possible, however. For example, milling 1- and 2-fluoronaphthalene with an excess of magnesium metal for 2 h, followed by treatment with FeCl3 and additional milling, produces the corresponding binaphthalenes, albeit in low yields (ca. 20%). The yields are independent of the particular isomer involved and are also comparable to the yields from corresponding the bromonaphthalenes. These results may reflect similar charges that reside on the α-carbon in the naphthalenes, as indicated by density functional theory calculations. 

Mechanochemistry reveals a novel polymorph of the mercury(II) imidazolate framework, based on square-grid (sql) topology layers. Reaction monitoring and periodic density functional theory calculations show that the sql-structure is of higher stability than the previously reported three-dimensional framework, with the unexpected stabilization of the lower dimensionality structure arising from weak interactions that include short interlayer C-H···Hg contacts, highlighting a potential role for agostic interactions in directing metal-organic framework topology. 

Ball milling CaI 2 and [KN(SiMe 3 ) 2 ] in a 1 : 1 ratio without solvent, and then extracting the ground material with toluene, yields the synthetically valuable neutral amide [Ca(N(SiMe 3 ) 2 ) 2 ] in good yield, without the contamination by calciate species that complicates solution metathesis routes. The effects on yield of grinding time, milling frequency, and calcium halide identity are also examined.