Mechanochemical Formation, Solution Rearrangements, and Catalytic Behavior of a Polymorphic Ca/K Allyl Complex.

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 )2 C3 H3 ]- ) with CaI2 yields 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 of 1 to 2 occurs within minutes. The structures of 1 and 2 have been modeled with DFT calculations, and 2 initiates 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).

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.

Disappearing Polymorphs in Metal-Organic Framework Chemistry: Unexpected Stabilization of a Layered Polymorph over an Interpenetrated Three-Dimensional Structure in Mercury Imidazolate.

The "disappearing polymorph" phenomenon is well established in organic solids, and has had a profound effect in pharmaceutical materials science. The first example of this effect in metal-containing systems in general, and in coordination-network solids in particular, is here reported. Specifically, attempts to mechanochemically synthesize a known interpenetrated diamondoid (dia) mercury(II) imidazolate metal-organic framework (MOF) yielded a novel, more stable polymorph based on square-grid (sql) layers. Simultaneously, the dia-form was found to be highly elusive, observed only as a short-lived intermediate in monitoring solvent-free synthesis and not at all from solution. The destabilization of a dense dia-framework relative to a lower dimensionality one is in contrast to the behavior of other imidazolate MOFs, with periodic density functional theory (DFT) calculations showing that it arises from weak interactions, including structure-stabilizing agostic C-H⋅⋅⋅Hg contacts. While providing a new link between MOFs and crystal engineering of organic solids, these findings highlight a possible role for agostic interactions in directing topology and stability of MOF polymorphs.

Exploration of Mechanochemical Activation in Solid-State Fluoro-Grignard Reactions.

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.

An η3 -Bound Allyl Ligand on Magnesium in a Mechanochemically Generated Mg/K Allyl Complex.

Milling two equivalents of K[1,3-(SiMe3 )2 C3 H3 ] (=K[A']) with MgX2 (X=Cl, Br) produces the allyl complex [K2 MgA'4 ] (1). Crystals grown from toluene are of the solvated species [((η6 -tol)K)2 MgA'4 ] ([1⋅2(tol)]), a trimetallic monomer with both bridging and terminal (η1 ) allyl ligands. When recrystallized from hexanes, the unsolvated 1 forms 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 by 1, with activities that are comparable to K[A'] and greater than the homometallic magnesium complex [{MgA'2 }2 ].

Halide metathesis in overdrive: mechanochemical synthesis of a heterometallic group 1 allyl complex.

As a synthesis technique, halide metathesis (n RM + M'X n → R n M' + n MX) normally relies for its effectiveness on the favorable formation of a metal halide byproduct (MX), often aided by solubility equilibria in solution. Owing to the lack of significant thermodynamic driving forces, intra-alkali metal exchange is one of the most challenging metathetical exchanges to attempt, especially when conducted without solvent. Nevertheless, grinding together the bulky potassium allyl [KA']∞ (A' = [1,3-(SiMe3)2C3H3]-) and CsI produces the heterometallic complex [CsKA'2]∞ in low yield, which was crystallographically characterized as a coordination polymer that displays site disorder of the K+ and Cs+ ions. The entropic benefits of mixed Cs/K metal centers, but more importantly, the generation of multiple intermolecular K…CH3 and Cs…CH3 interactions in [CsKA'2]∞, enable an otherwise unfavorable halide metathesis to proceed with mechanochemical assistance. From this result, we demonstrate that ball milling and unexpected solid-state effects can permit seemingly unfavored reactions to occur.

Mechanochemically Driven Transformations in Organotin Chemistry: Stereochemical Rearrangement, Redox Behavior, and Dispersion-Stabilized Complexes.

Ball milling a mixture of the bulky allyl K[A'] {[A'] = [1,3-(SiMe3)2C3H3]-} and SnCl2 in a 2:1 ratio for 5 min leads to the tris(allyl) stannate [SnA'3K]∞, which forms a coordination polymer in the solid state. Longer grinding of the 2:1 mixture (15 min), or the use of a 3:1 ratio of reagents, initiates a disproportionation reaction and the chiral tetra(allyl)tin species [SnA'4] is produced. A small amount of a diastereomeric [SnA'4] complex with meso symmetry can also be isolated with extended grinding. These products have been structurally authenticated with single-crystal X-ray crystallography. The tetra(allyl) species [SnA'4] are sterically crowded and decompose relatively quickly (<1 h) in hydrocarbon solvents. In the solid state, they are much more persistent (several months) and evidently owe their stability to internal London dispersion interactions, as evidenced by multiple close H···H' interligand contacts. Dispersion-corrected DFT calculations have been used to confirm the critical contribution of dispersion interactions to their stability. None of these products are available in their isolated forms from solution-based reactions, demonstrating the ability of mechanochemical activation to access otherwise unobtainable transformations in organotin chemistry.

Structural distortions in M[E(SiMe3)2]3 complexes (M = group 15, f-element; E = N, CH): is three a crowd?

The tris(bistrimethylsilylamido) species P[N(SiMe3)2]3 (1) and As[N(SiMe3)2]3 (2) have been prepared through halide metathesis in high yield. Their single crystal X-ray structures, along with that of Sb[N(SiMe3)2]3 (3), complete the series of structurally authenticated group 15 M[N(SiMe3)2]3 complexes (the bismuth analogue (4) has been previously reported). All four complexes possess the expected pyramidal geometries, with progressively longer M-N bond distances from P to Bi but closely similar N-M-N angles (107-104°). The structures of 1-4 also display distortions that are similar to those in f-element M[N(SiMe3)2]3 and M[CH(SiMe3)2]3 complexes, in which M···(ß-Si-C) interactions have been identified. Such structural features include distorted M-(N,CH)-Si and (N,CH)-Si-C angles and close M···C and M···Si contacts. DFT calculations confirm that there are no M···(ß-Si-C) interactions in 1-4; the bond distortions appear to result from the particular steric crowding that arises in pyramidal M[(N,CH)(SiMe3)2]3 complexes. This is likely the source of the most of the distortions in the structures of the f-element analogues as well, even though the latter possess attractive M···Si-C interactions.

Cationic, neutral, and anionic allyl magnesium compounds: unprecedented ligand conformations and reactivity toward unsaturated hydrocarbons.

Starting from bis(allyl)magnesium [Mg(C(3)H(5))(2)], a set of cationic, neutral, anionic, and dianionic allyl magnesium compounds has been isolated and characterized, including [Mg(C(3)H(5))(THF)(5)][B(C(6)F(5))(4)] (3), [Mg(C(3)H(5))(2)(1,4-dioxane)(THF)] (2), [KMg(C(3)H(5))(3)(THF)] (6), and [MMg(C(3)H(5))(4)] (8: M = K(2); 9: M = Ca). In solution, the allyl ligands of the compounds display fluxional behavior, even at low temperatures. Single crystal X-ray analysis reveals unusual µ(2)-η(1):η(3)- and unprecedented µ(3)-η(1):η(3):η(3)-coordination modes in the heterobimetallic compounds 6 and [8·(THF)(2)]. Density functional theory calculations confirm that these metal-allyl conformations are energetically stable. The magnesium compounds have been investigated as initiators for butadiene polymerization and ethylene oligomerization. The heterobimetallic compounds display initiation properties, including higher reaction rates, that are distinctively different from those of the monometallic species. Reactivity trends depend on the formal charge of the magnesium compounds (dianionic, higher-order magnesiate > monoanionic, lower-order magnesiate) and on the nature of the counterion (K(+) > Ca(2+)).

Synthesis without solvent: consequences for mechanochemical reactivity.

Solvents are so nearly omnipresent in synthetic chemistry that a classic question for their use has been: "What is the best solvent for this reaction?" The increasing use of mechanochemical approaches to synthesis-by grinding, milling, extrusion, or other means-and usually with no, or only limited, amounts of solvent, has raised an alternative question for the synthetic chemist: "What happens if there is no solvent?" This review focuses on a three-part answer to that question: when there is little change ("solvent-optional" reactions); when solvent needs to be present in some form, even if only in the amounts provided by liquid-assisted (LAG) or solvate-assisted grinding; and those cases in which mechanochemistry allows access to compounds that cannot be obtained from solution-based routes. The emphasis here is on inorganic and organometallic systems, including selected examples of mechanosynthesis and mechanocatalysis. Issues of mechanochemical depictions and the adequacy of LAG descriptions are also reviewed.

Geometric effects in olefinic cation-π interactions with alkali metals: a computational study.

Although cation-π interactions commonly involve aromatic or heteroaromatic rings as the source of π-electrons, isolated and nonconjugated olefins are equally effective donors of π-electron density. Previous comparisons of these π-electron sources have indicated that the net energy of the binding interactions is not a simple additive function of the number of π-bonds involved. For instance, the enthalpy of binding (ΔH°) of Li(+), Na(+), or K(+) cations to two ethylene molecules or to one benzene molecule is approximately the same, despite the 4:6 ratio of π-electrons involved. This present density functional theory study indicates that geometric factors can partially account for the proportionally greater interaction energies of olefins, but whether they are symmetrically placed around the cation or grouped on one hemisphere has little effect on the binding energy. Instead, flexible ligands that permit olefinic π-electrons to be oriented more favorably toward the metal than those in rigid aromatic rings can be correlated with greater bonding. For Li(+) complexes, this appears to be an appreciable factor, although it is less significant with Na(+) and K(+) complexes. For all three cations, stronger polarization interactions with olefins compared to arenes contribute to the strength of cation-π interactions involving olefinic π-bonds.

2,4-Pentanediolate as an alkoxide/diketonate "hybrid" ligand and the formation of aluminum and zirconium derivatives.

When 2,4-pentanediol (2,4-H(2)pd) is deprotonated, the resulting dianion (2,4-pd) serves as a type of "hybrid" ligand, i.e., an alkoxide that possesses structural features of a ß-diketonate. 2,4-Pentanediol reacts with Al(O-s-Bu)(3) and Zr(O-i-Pr)(4) to form chelated multinuclear complexes. The aluminum-containing product is first isolated as the insoluble [Al(2,4-pd)(2,4-Hpd)](n); on sublimation, a hydrocarbon-soluble mixture of polymetallic species is generated. Mass spectral evidence suggests that both Al(4)(2,4-pd)(6) and Al(5)(2,4-pd)(7)(2,4-Hpd) are present. The zirconium complex is isolated as an adduct, [Zr(2,4-pd)(2)](2)·(2,4-H(2)pd). The pentanediolates decompose on heating to form Al(2)O(3) and ZrO(2). Unlike the mononuclear Al(acac)(3) and Zr(acac)(4) derivatives (acac = acetylacetonate), the formation of aggregates with the 2,4-pd ligand suggests that the latter has more coordinative flexibility. The geometries of several model aluminum complexes with oxygen donor ligands were studied with density functional theory methods. The optimized structures were used with the gauge, including atomic orbital (GIAO) method to calculate their (27)Al NMR magnetic shielding values for comparison with experiment.

Classical versus bridged allyl ligands in magnesium complexes: the role of solvent.

Magnesium allyl complexes are regularly isolated with classical, sigma-bonded ligands, and this has been thought to be their preferred mode of bonding. Density functional theory calculations confirm that such bonding is the most stable mode when coordinated bases are present, but in their absence, pi-bonded forms are expected to be lower in energy. The isolation of the unsolvated [Mg{C(3)(SiMe(3))(2)H(3)}(2)](2) complex supports this prediction, as it is a dinuclear species in which two allyl ligands bridge the metals and display cation-pi interactions with them.

Solution interaction of potassium and calcium bis(trimethylsilyl)amides; preparation of Ca[N(SiMe3)2]2 from dibenzylcalcium.

Ca[N(SiMe(3))(2)](2) (1) is isolated in nearly quantitative yield from the room temperature reaction of Ca(CH(2)Ph)(2)(THF) and HN(SiMe(3))(2) in toluene. A commonly used preparation of 1 involving the reaction of potassium bis(trimethylsilyl)amide, K[N(SiMe(3))(2)] (2), with CaI(2) can produce material that contains substantial amounts of potassium, probably in the form of a calciate such as K[Ca{N(SiMe(3))(2)}(3)]. The favorable formation of K[Ca{N(SiMe(3))(2)}(3)] from 1 and 2 was confirmed with density functional theory calculations. Deliberate doping of solutions of 1 with 2 initially causes only an upfield shift in the single (1)H NMR resonance observed for 1; not until K/Ca ratios exceed 1:1 is the presence of the added potassium obvious by the appearance of an additional peak in the spectrum.

Mechanochemically directed metathesis in group 2 chemistry: calcium amide formation without solvent.

Ball milling CaI2 and [KN(SiMe3)2] in a 1 : 1 ratio without solvent, and then extracting the ground material with toluene, yields the synthetically valuable neutral amide [Ca(N(SiMe3)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.

Multicomponent Mechanochemical Synthesis of Cyclopentadienyl Titanium tert-Butoxy Halides, Cp x TiX y (O t Bu)4-(x+y) (x, y = 1, 2; X = Cl, Br).

Titanium tert-butoxy halides of the formula Cp x TiX y (O t Bu)4-(x+y) (x, y = 1, 2; X = Cl, Br) have been prepared thorough milling the reagents without solvent. In the case of the chloride derivatives, Cp2TiCl2 is used as a starting material; in the case of the bromides, a mixture of LiCp, TiBr4, and Li[O t Bu] is used. The stoichiometric ratios of the starting materials are reflected in the major products of the reactions. Single-crystal X-ray structures are reported for Cp2TiCl(O t Bu), Cp2TiBr(O t Bu), and CpTiBr2(O t Bu), as well as for Cp2TiCl(O i Pr) and a redetermination of Cp2TiCl(OMe). The tert-butoxy derivatives are notable for their nearly linear Ti-O-C angles (>170°) that reflect Ti-O π-bonding, an interpretation supported with density functional theory calculations.

Advances in organometallic synthesis with mechanochemical methods.

Solvent-based syntheses have long been normative in all areas of chemistry, although mechanochemical methods (specifically grinding and milling) have been used to good effect for decades in organic, and to a lesser but growing extent, inorganic coordination chemistry. Organometallic synthesis, in contrast, represents a relatively underdeveloped area for mechanochemical research, and the potential benefits are considerable. From access to new classes of unsolvated complexes, to control over stoichiometries that have not been observed in solution routes, mechanochemical (or 'M-chem') approaches have much to offer the synthetic chemist. It has already become clear that removing the solvent from an organometallic reaction can change reaction pathways considerably, so that prediction of the outcome is not always straightforward. This Perspective reviews recent developments in the field, and describes equipment that can be used in organometallic synthesis. Synthetic chemists are encouraged to add mechanochemical methods to their repertoire in the search for new and highly reactive metal complexes and novel types of organometallic transformations.

Reaction environment and ligand lability in group 4 Cp2MXY (X, Y = Cl, OtBu) complexes.

Despite their usefulness in catalytic and materials chemistry, the mixed cyclopentadienyl/alkoxide complexes of Ti, Zr, and Hf (Cp2M(OR)2) have few reliable synthetic routes available to them. We describe the use of mechanical ball milling to promote halide metathesis from Cp2MCl2, and compare these results to those obtained in hexanes and THF. Even without solvent, ring lability is extensive with titanium complexes, and alkoxide compounds with 0-3 Cp rings are isolated. The ball milling reactions are much faster than those in solution, but the distributions of products are similar to those obtained in hexanes, although different from those in THF. The range of compounds obtained from Zr and Hf starting materials is more limited, as Cp ring exchange does not occur.