s-Block Metal Dibenzoazepinate Complexes: Evidence for Mg–Alkene Encapsulation

Stereochemical Stability of Planar-Chiral Benzazepine Tricyclics: Inversion Energies of P- and S-Alkene Ligands

Inversion barriers ΔG‡ for planar chiral phosphine-alkene and sulfonamide-alkene hybrid ligands based on phenyl-dibenz[b,f]azepine have been determined by density-functional theory calculations. Analysis of the structural and electronic characteristics of the minima and transition states explains the magnitudes of ΔG‡ and the geometrical changes during the inversion process. The steric repulsion caused by bulky substituents attached to the azepine nitrogen atom has a pronounced effect on the ΔG‡ value, explaining, inter alia, the stereochemical stability of the P- and S-alkene ligands when compared to the fluxional parent compound where X = H.

s-Block chemistry in weakly coordinating solvents

Alkaline earth metal catalysis has been a growing field in recent years. To enhance reactivity and to understand the metal-substrate interactions in more detail, reactions are increasingly carried out in weakly coordinating solvents. This article gives an overview over the two main approaches to facilitate this, which are either through the employment of highly dipolar haloaryls as solvents, or by increasing the solubility of the ligand systems. The resulting coordination modes and reactivities are presented together with the synthetic strategies. Additionally, the latest results of group 1 complex chemistry in aliphatic solvents are illustrated and future challenges are highlighted.

Retro-Diels-Alder decomposition of norbornadiene mediated by a cationic magnesium complex

First evidence for the coordination of norbornadiene (nbd) and dicyclopentadiene (dcpd) with the main group metal Mg is provided by the crystal structures of adducts with cationic β-diketiminate (BDI) Mg complexes. While the dcpd complex is thermally stable, [(BDI)Mg+·nbd][B(C6F5)4-] shows slow room temperature retro-Diels-Alder decomposition to give a complex with the cation (BDI)Mg(C5H5)Mg(BDI)+.

Unsupported Mg-Alkene Bonding

The first intermolecular early main group metal-alkene complexes were isolated. This was enabled by using highly Lewis acidic Mg centers in the Lewis base-free cations (Me BDI)Mg+ and (tBu BDI)Mg+ with B(C6 F5 )4 - counterions (Me BDI=CH[C(CH3 )N(DIPP)]2 , tBu BDI=CH[C(tBu)N(DIPP)]2 , DIPP=2,6-diisopropylphenyl). Coordination complexes with various mono- and bis-alkene ligands, typically used in transition metal chemistry, were structurally characterized for 1,3-divinyltetramethyldisiloxane, 1,5-cyclooctadiene, cyclooctene, 1,3,5-cycloheptatriene, 2,3-dimethylbuta-1,3-diene, and 2-ethyl-1-butene. In all cases, asymmetric Mg-alkene bonding with a short and a long Mg-C bond is observed. This asymmetry is most extreme for Mg-(H2 C=CEt2 ) bonding. In bromobenzene solution, the Mg-alkene complexes are either dissociated or in a dissociation equilibrium. A DFT study and AIM analysis showed that the C=C bonds hardly change on coordination and there is very little alkene→Mg electron transfer. The Mg-alkene bonds are mainly electrostatic and should be described as Mg2+ ion-induced dipole interactions.

Large decanuclear calcium and strontium hydride clusters

The largest, most hydride-rich, Ca₁₀H₁₆ cluster is formed by condensation of two smaller Ca₆H₉ octahedrons (the isostructural Sr hydride cluster is also reported).

Low Valent Magnesium Chemistry with a Super Bulky β-Diketiminate Ligand

The steric bulk of the well-known ^DIPP BDI ligand (CH[C(CH₃ )N-DIPP]₂ , DIPP=2,6-diisopropylphenyl) was increased by replacing isopropyl for isopentyl groups. This very bulky ^DIPeP BDI ligand could not stabilize the radical species (^DIPeP BDI)Mg^. : reduction of (^DIPeP BDI)MgI with Na gave (^DIPeP BDI)₂ Mg₂ with a rather long Mg-Mg bond of 3.0513(8) Å. Addition of TMEDA prior to reduction gave complex (^DIPeP BDI)₂ Mg₂ (C₆ H₆ ), which could also be obtained as its THF adduct. It is speculated that combination of a bulky spectator ligand and TMEDA prevents dimerization of the intermediate Mg^I radical, which then reacts with the benzene solvent. Complex (^DIPeP BDI)₂ Mg₂ (C₆ H₆ ), which formally contains the anti-aromatic anion C₆ H₆ ^2- , reacted with tBuOH as a Brønsted base to 1,3- and 1,4-cyclohexadiene and with H₂ as a two electron donor to (^DIPeP BDI)₂ Mg₂ H₂ and C₆ H₆ . It also reductively cleaved the C-F bond in fluorobenzene and gave (^DIPeP BDI)MgPh, (^DIPeP BDI)MgF, and C₆ H₆ .

Complexation of asymmetric diborenes with magnesium bromide

The synthesis of neutral Mg complexes of asymmetric diborenes is reported. X-ray diffraction analysis reveals that the Mg center is unsymmetrically coordinated to the B2 units of diborenes. Theoretical calculations manifest the donation of electrons from the π-type orbitals of diborenes to an empty orbital of Mg.

Highly Lewis acidic cationic alkaline earth metal complexes

Cationic Lewis base-free β-diketiminate (BDI) complexes of Mg and Ca have been isolated as their B(C₆F₅)₄^- salts. The cation (BDI)Mg⁺ shows an extraordinarily strong Lewis acidity that can compete with strong Lewis acids like B(C₆F₅)₃ and (BDI)AlMe⁺. Its highly electrophilic nature is exemplified by isolation of an 3-hexyne adduct.

Coordination of arenes and phosphines by charge separated alkaline earth cations

Generation of ‘naked’ β-diketiminato magnesium and calcium cations allows the isolation of benzene, toluene, 1,4-difluorobenzene and terminal phosphine adducts.

Tethered cationic alkaline earth – olefin complexes

The utilisation of a specifically tailored aminofluoroalcohol possessing both a methoxy and an olefin dangling side arms enables the preparation of the first examples of discrete calcium- and strontium-olefin cationic complexes.