Timothy P. Hanusa

Non-cyclopentadienyl organometallic compounds of calcium, strontium and barium

Abstract The use of sterically demanding ligands has allowed the organometallic compounds of the heavy alkaline-earth metals calcium, strontium and barium to emerge from the shadow cast by the far better studied organomagnesium Grignard reagents. Metallocenes and other cyclopentadienyl-based complexes have been the most intensively investigated, but in the past decade a wealth of new non-cyclopentadienyl compounds have been characterized. A broad range of structure types are known, encompassing σ- and π-bound anionic ligands, and Lewis base adducts with neutral donors. In this review, crystallographically characterized complexes are discussed, and current interpretations of the bonding in heavier Group 2 element compounds are examined. Recent applications of non-cyclopentadienyl compounds in organic synthesis are surveyed.

Synthesis and Crystal Structure of the Bis(allyl)calcium Complex [Ca{C3(SiMe3)2H3}2⋅(thf)2]

Two η3-allyl ligands in an anti configuration are present in the title compound (structure depicted). Even though the Ca−C bonds are about the length expected for cyclopentadienyl complexes, the bis(trimethylsilyl)allyl anion displays hydrogen-atom distortions similar to those found in other main group and transition metal π-allyl complexes.

Structural Lessons from Main-Group Metallocenes

Abstract The metallocenes of the Group 2 and Group 14 elements display a higher degree of structural and chemical similarity than would be expected based on the differences in their electronic configurations alone. A comparison of their structures suggests that the interplanar ring angles in both are primarily determined by the size and shape of the cyclopentadienyl ligands; the two additional valence electrons in the Group 14 compounds exert little stereochemical influence. Although the two metallocene families display differences in reactivity, the presence or absence of metal valence electrons is not always a useful criterion for distinguishing between them. In Group 14 metallocenes, the electrons are not efficient donors to electrophiles, and sufficiently bulky cyclopentadienyl rings can interfere with the extent of redox reactions. Examination of main-group metallocene melting points reveals a trend based on the symmetry and flexibility of the cyclopentadienyl ring. In rigorously monomeric species, c...

Cyclopentadienyl ring metathesis with bis(pentamethylcyclopentadienyl)calcium as a route to mixed ring organolanthanide complexes; the crystal structure of (C5Me5)2Nd(C5H5)

Abstract Bis(pentamethylcyclopentadienyl)calcium, (C 5 Me 5 ) 2 Ca, can be made by the reaction of Ca[N(SiMe 3 ) 2 ] 2 with C 5 Me 5 H in toluene. It undergoes cyclopentadienyl ring metathesis with tris(cyclopentadienyl)lanthanide complexes, Cp 3 Ln (Ln = La, Nd, Sm) in toluene to generate the mixed ring complexes (C 5 Me 5 ) 2 LnCp. The X-ray crystal structure of (C 5 Me 5 ) 2 NdCp shows that the complex is a sterically crowded monomer with η 5 -C 5 Me 5 and Cp rings. The average NdC distances for both the C 5 Me 5 and Cp rings are 2.76–2.79 A.

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.

Solution synthesis of calcium, strontium and barium metallocenes

Abstract Bis(cyclopentadienyl)metal complexes of calcium, strontium and barium can be prepared in synthetically useful yields (> 65%) from the reaction of lithium or potassium cyclopentadienides and the appropriate metal dihalides in THF. The THF-soluble complexes (C5H5)2Ca(THF)2 and (Me5C5)2M(THF)2 (M = Ca, Sr, Ba) are extracted from the reaction of the appropriate potassium cyclopentadienide with a metal halide; the THFinsoluble (C5H5)2M(THF)x(x ≈ 1 for Sr; x ≈ 0.25 for Ba) remain after the reaction of a lithium cyclopentadienide and the metal iodide.

Synthese und Kristallstruktur des Bis(allyl)‐Calciumkomplexes [Ca{C3(SiMe3)2H3}2(thf)2]

Zweiη3-Allylliganden in anti-Konfiguration weist die Titelverbindung auf (Struktur im Kristall siehe Bild). Zwar haben die Ca-C-Bindungen etwa die Lange, die man fur Cyclopentadienylkomplexe erwartet, doch ahneln die H-Atom-Positionen im Bis(trimethylsilyl)allyl-Anion eher denen anderer Hauptgruppen- und Ubergangsmetall-π-Allyl-Komplexe.

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.

Formation and solid state structure of a tetranuclear oxoaryloxide cluster of barium, [Ba4(µ4-O)(µ2-OC6H2(CH2NMe2)3-2,4,6)6]·3(toluene)

The tetranuclear oxo–aryloxide cluster, [Ba4(µ4-O)(µ2-OC6H2(CH2NMe2)3-2,4,6)6]·3(toluene), synthesized from the reaction of K[(OC6H2(CH2NMe2)3-2,4,6] and BaI2 in tetrahydrofuran, crystallizes from toluene as a tetrahedron of barium atoms encapsulating a single oxo ligand, the metal centres coordinated by six µ2-aryloxide groups and the nitrogen atoms of the ortho-dimethylaminomethyl groups.