Homoleptic allyl complexes of chromium with trimethylsilylated ligands. Formation and molecular structure of {[1-(SiMe3)C3H4]2Cr}2, [1,3-(SiMe3)2C3H3]2Cr, and [1,1′,3-(SiMe3)3C3H2]2Cr

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.

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.

Bis(allyl)zinc revisited: sigma versus pi bonding of allyl coordination

The reinvestigation of two allyl zinc compounds, parent bis(allyl)zinc [Zn(C(3)H(5))(2)] (1) and 2-methallyl chloro zinc [Zn(C(4)H(7))Cl] (2), revealed two new coordination modes in the solid state for the allyl ligand, viz cis- and trans-μ(2)-η(1):η(1). These results call for modification of the conventional interpretation of zinc-allyl interactions. Computational results indicate that the classical η(3)-bonding mode of the allyl ligand is not favored in zinc compounds. A rare case of a zinc-olefin interaction in the dimer of [Zn(η(1)-C(3)H(5))(OC(C(3)H(5))Ph(2))] was found in the monoinsertion product of 1 with benzophenone.

Observation of Internal Electron Transfer in Bulky Allyl Ytterbium Complexes with Substituted Terpyridine Ligands

A series of new bulky allyl terpyridyl-ytterbium complexes have been synthesized to determine the effect of allyl ligands on the internal charge-transfer process that exists in these materials. Compared to the pentamethylcyclopentadienyl-ytterbocene compound Cp*2Yb(tpyCN) (nu(C(triple bond)N) = 2172 cm(-1)), the symmetrically substituted allyl complex [1,3-(SiMe3)2C3H3]2Yb(tpyCN) possesses a markedly lowered C(triple bond)N frequency of 2130 cm(-1). Furthermore, the electronic nature of these bulky allyl complexes can be tuned, as demonstrated by the C(triple bond)N frequency of the asymmetric derivatives [1-(SiMe3)C3H4]2Yb(tpyCN) and [1-(SiPh3)-3-(SiMe3)C3H3]2Yb(tpyCN) (2171 and 2164 cm(-1), respectively). The differences in these frequencies can be attributed to differences in the ligands' steric and electronic character. Single-crystal X-ray characterization of [1,3-(SiMe3)2C3H3]2Yb(tpy) reveals that the allyl moiety possesses shorter Yb-C and Yb-N bond distances than the Cp* analogue. The magnetic susceptibility data for [1,3-(SiMe3)2C3H3]2Yb(tpy) departs dramatically from the Curie law, with a room-temperature magnetic moment of 2.95 mu(B).

Trimethylsilylated Allyl Complexes of Nickel. The Stabilized Bis(π-allyl)nickel Complex [η3-1,3-(SiMe3)2C3H3]2Ni and Its Mono(π-allyl)NiX (X = Br, I) Derivatives

Reaction of 2 equiv of K[1,3-(SiMe3)2C3H3] with NiBr2(dme) in THF at -78 degrees C produces the orange pi-allyl complex [1,3-(SiMe3)2C3H3]2Ni (1). Unlike the pyrophoric (C3H5)2Ni, the trimethylsilylated derivative only slowly decomposes in air (from hours to days). Both eclipsed (1a) and staggered (1b) conformations are found in solution; the eclipsed form irreversibly converts to the thermodynamically more stable staggered conformation when heated above 85 degrees C. Single-crystal X-ray structures obtained for both 1a and 1b confirm that the allyl ligands are bound in a trihapto manner to the metals and that trimethylsilyl substituents are in syn, anti arrangements. Density functional theory calculations performed on the bis(allyl)nickel complexes indicate that the substituents exert little effect on the basic metal-ligand geometries. Trimethylphosphine is converted to tetramethyltetraphosphane, (MeP)4, on reaction with 1. In toluene, 3-bromo-1,3-bis(trimethylsilyl)propene reacts with (COD)2Ni to produce the dimeric purple complex {[1,3-(SiMe3)2C3H3]NiBr}2 (2a). Both NMR and X-ray crystallographic data establish that the allyl ligands are staggered and that the trimethylsilyl substituents are in a syn, syn conformation. NMR data indicate that the reaction of one equivalent of 1 with Br2 in benzene produces an analogous complex (2b) with the allyl ligand substituents in a syn, anti configuration. When 1 equiv of 1 is treated with I2 in hexanes, the dark red dimeric complex {[1,3-(SiMe3)2C3H3]NiI}2 (3) is formed. Its X-ray crystal structure demonstrates that both eclipsed (3a) and staggered (3b) allyl conformation are present. The trimethylsilyl groups on the allyl ligands are in syn, anti arrangements in the two forms.

Metal Allyl Complexes with Bulky Ligands: Stabilization of Homoleptic Thorium Compounds, [(SiMe3)nC3H5-n]4Th (n = 1, 2)

Unlike the parent (C3H5)4Th that decomposes at 0 degrees C, homoleptic tetra(allyl)thorium complexes [(SiMe3)nC3H5-n]4Th (n = 1, 2) have been prepared from ThBr4(thf)4 and K[(SiMe3)nC3H5-n] that are stable up to 90 degrees C (n = 1) or 124 degrees C (n = 2). The molecules, which are fluxional on the NMR time scale, contain the first structurally authenticated Th-allyl bonds. The trimethylsilyl groups cause relatively little perturbation in the core metal-allyl geometry but markedly increase the kinetic stability of the compounds.