Syntheses of Heteroleptic Amidinate Strontium Complexes Using a Superbulky Ligand

The borderless world of chemical bonding across the van der Waals crust and the valence region

The definition of the van der Waals crust as the spherical section between the atomic radius and the van der Waals radius of an element is discussed and a survey of the application of the penetration index between two interacting atoms in a wide variety of covalent, polar, coordinative or noncovalent bonding situations is presented. It is shown that this newly defined parameter permits the comparison of bonding between pairs of atoms in structural and computational studies independently of the atom sizes.

Synthesis, structure, and coordination chemistry of a neutral pyrrolyl-functionalized amidinate ligand

A new o-phenylene-bridged ligand incorporating neutral pyrrolyl and amidino moieties was designed and synthesized. The reactivity of the ligand was investigated and showed versatile coordination modes towards alkali- and rare-earth metals.

Calcium catalyzed enantioselective intramolecular alkene hydroamination with chiral C2-symmetric bis-amide ligands

The chiral building block (R)-(+)-2,2'-diamino-1,1'-binaphthyl, (R)-BINAM, which is often used as backbone in privileged enantioselective catalysts, was converted to a series of N-substituted proligands R1-H2 (R = CH2tBu, C(H)Ph2, PPh2, dibenzosuberane, 8-quinoline). After double deprotonation with strong Mg or Ca bases, a series of alkaline earth (Ae) metal catalysts R1-Ae·(THF)n was obtained. Crystal structures of these C2-symmetric catalysts have been analyzed by quadrant models which show that the ligands with C(H)Ph2, dibenzosuberane and 8-quinoline substituents should give the best steric discrimination for the enantioselective intramolecular alkene hydroamination (IAH) of the aminoalkenes H2C[double bond, length as m-dash]CHCH2CR'2CH2NH2 (CR'2 = CPh2, CCy or CMe2). The dianionic R12- ligand in R1-Ae·(THF)n functions as reagent that deprotonates the aminoalkene substrate, while the monoanionic (R1-H)- ligand formed in this reaction functions as a chiral spectator ligand that controls the enantioselectivity of the ring closure reaction. Depending on the substituent R in the BINAM ligand, full cyclization of aminoalkenes to chiral pyrrolidine products as fast as 5 minutes was observed. Product analysis furnished enantioselectivities up to 57% ee, which marks the highest enantioselectivity reported for Ca catalyzed IAH. Higher selectivities are impeded by double protonation of the R12- ligand leading to complete loss of chiral information in the catalytically active species.

Magnesium hydrides bearing sterically demanding amidinate ligands: synthesis, reactivity and catalytic application

The synthesis and characterisation of a series of magnesium complexes bearing sterically demanding amidinate ligands is reported; this includes magneisum amides (1a and 1b), hydrides (3a and 3b) and alkyl complexes (2b). The solid and solution state behaviour of the complexes has been investigated using single crystal X-ray diffraction and NMR spectroscopy, revealing the magnesium hydrides to exist as dimers in the solid state, dispite the sterically demanding ligand systems and showing a degree of monomeric character in solution. The stoichiometric and catalytic activity of the amidinate complexes were investigated, with the complexes found to efficiently mediate both the hydroamination of N,N'-diisopropylcarbodiimide and the Tishchenko reaction. The metal hydrides are highly reactive towards coordinating substrates, showing a significant increase in catalytic rate compared with more ubiquitous β-diketiminate magnesium hydrides.

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).

Nucleophilic Aromatic Substitution at Benzene with Powerful Strontium Hydride and Alkyl Complexes

Key to the isolation of the first alkyl strontium complex was the synthesis of a strontium hydride complex that is stable towards ligand exchange reactions. This goal was achieved by using the super bulky β-diketiminate ligand ^DIPeP BDI (CH[C(Me)N-DIPeP]₂ , DIPeP=2,6-diisopentylphenyl). Reaction of ^DIPeP BDI-H with Sr[N(SiMe₃ )₂ ]₂ gave (^DIPeP BDI)SrN(SiMe₃ )₂ , which was converted with PhSiH₃ into [(^DIPeP BDI)SrH]₂ . Dissolved in C₆ D₆ , the strontium hydride complex is stable up to 70 °C. At 60 °C, H-D isotope exchange gave full conversion into [(^DIPeP BDI)SrD]₂ and C₆ D₅ H. Since H-D exchange with D₂ is facile, the strontium hydride complex served as a catalyst for the deuteration of C₆ H₆ by D₂ . Reaction of [(^DIPeP BDI)SrH]₂ with ethylene gave [(^DIPeP BDI)SrEt]₂ . The high reactivity of this alkyl strontium complex is demonstrated by facile ethylene polymerization and nucleophilic aromatic substitution with C₆ D₆ , giving alkylated aromatic products and [(^DIPeP BDI)SrD]₂ .

Kinetic stabilization of low-oxidation state and terminal hydrido main group metal complexes by a sterically demanding N,N′-bis(2,6-terphenyl)triazenide

Thermally robust main group metal complexes featuring terminal hydride ligands are achieved by deploying a sterically demanding N,N′-bis(2,6-terphenyl)triazenide ligand.

The addition of Grignard reagents to carbodiimides. The synthesis, structure and potential utilization of magnesium amidinates

Direct synthesis of magnesium amidinates of the general formula [RNC(R1)NR]MgR2 has been performed from appropriate carbodimide and Grignard reagents (R = iPr, Cy, pTol; R1 = Me, nBu; R2 = nBu, Cl, I). Magnesium bisamidinates of the composition [RNC(R1)NR]2Mg(solvent)2 are accessible from [RNC(R1)NR]MgR2 and via the Schlenk-like equilibrium in coordinating solvents. The only isolated amidinatomagnesium halide, preserved in the dinuclear form of {[pTolNC(Me)N-pTol]Mg(THF)}2-μ-(THF)-μ-(Cl)2, has been obtained from the reaction of pTol-N[double bond, length as m-dash]C[double bond, length as m-dash]N-pTol with MeMgCl(THF)n in hexane. The reaction of pTol-N[double bond, length as m-dash]C[double bond, length as m-dash]N-pTol with two equivalents of MeMgI gives an unprecedented dinuclear cyclic adduct {μ-[pTolNC(Me)N-pTol][MgI(OEt2)]2-μ-Me}. The structures of these complexes have been investigated by NMR spectroscopy, sc-XRD and theoretical methods. Selected complexes have been tested as initiators of ring-opening polymerization reactions with various substrates, the copolymerization of oxiranes and CO2 as well as the esterification of lactides.

Structural diversity in a homologous series of donor free alkali metal complexes bearing a sterically demanding triazenide

The isolation and structural characterization of donor free alkali metal complexes of a bis(terphenyl) substituted triazenide are presented.

Barium complexes with crown-ether-functionalised amidinate and iminoanilide ligands for the hydrophosphination of vinylarenes

Two barium-amide complexes bearing crown-ether-functionalised amidinate and iminoanilide ligands that act as competent precatalysts for the intermolecular hydrophosphination of vinylarenes are presented.

Synthesis and catalytic activity of tridentate N-(2-pyridylethyl)-substituted bulky amidinates of calcium and strontium

Sterically protected tridentate amidinates of calcium have been prepared to study the intramolecular hydroamination of an aminoalkene.

Heterobimetallic Ba/Li and Ca/Li amides and diphenylmethanide

Rare examples of stable heterobimetallic Ca/Li and Ba/Li amido polymeric species, as well as an ate Ca/Li diphenylmethanide compound stable in ethers, have been structurally characterised. Intra- and intermolecular metal⋯H–Si interactions are shown to play a key role.

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 .