Factors that Influence π- versus η2-Coordination of β-Diketiminato Ligands in Magnesium Complexes

Structural and Spectroscopic Studies of Steric and Electronic Substituent Effects in a Series of Magnesium Aminodiboranates Mg[(BH3)2NMeR]2

The magnesium N,N-dimethylaminodiboranate compound Mg[(BH₃)₂NMe₂]₂, the most volatile compound of magnesium known, serves as an excellent chemical vapor deposition (CVD) precursor for the growth of thin films such as the dielectric material MgO. To explore how the thermal stability and physical properties of magnesium aminodiboranates depend on the steric and electronic properties of the nitrogen-bound substituents, we have made a series of analogues of Mg[(BH₃)₂NMe₂]₂, in which one of the two methyl substituents on nitrogen is replaced with an ethyl, iso-propyl, or tert-butyl group. In the crystal structure of Mg[(BH₃)₂NMe(t-Bu)]₂, the magnesium center is coordinated to two chelating (BH₃)₂NMe(t-Bu) ligands, each of which binds in a κ²,κ² fashion so that the magnesium center forms eight Mg-H contacts. Unlike Mg[(BH₃)₂NMe₂]₂, however, which has a linear N···Mg···N angle and is an isolated molecule in the solid state, the N···Mg···N angle in Mg[(BH₃)₂NMe(t-Bu)]₂ is distinctly nonlinear (149.9°) because hydrogen atoms of BH₃ groups of nearby molecules form two additional Mg-H contacts with the magnesium center. When the complexes are heated in toluene solution, the (BH₃)₂NMeR^- groups reversibly undergo B-N bond cleavage (with concomitant migration of a hydrogen atom) to release the aminoborane BH₂═NMeR and form magnesium borohydride, Mg(BH₄)₂. For the methyl, ethyl, and iso-propyl derivatives, the equilibrium strongly favors Mg[(BH₃)₂NMeR]₂. In contrast, for the tert-butyl derivative, the equilibrium strongly favors BH₂═NMe(t-Bu) and Mg(BH₄)₂. The results suggest that more strongly electron-donating groups slightly strengthen the B-N bonds and disfavor B-N bond cleavage, provided that the groups are not too large. In contrast, sterically bulky ligands disfavor B-N bond reformation, thus promoting the dissociative equilibrium that involves B-N bond cleavage. Interestingly, the rates at which the complexes approach equilibrium depend only weakly on the nature of the R group, at least within the series studied. These findings are relevant to the potential use of magnesium aminodiboranates as CVD precursors to the superconducting phase MgB₂.

Cyclopentadienyl Complexes of the Alkaline Earths in Light of the Periodic Trends

The electron-rich cyclopentadienyl and the analogous indenyl and fluorenyl ligands (collectively denoted here as Cp') have been impactful in stabilizing electron-deficient metal centers including the highly electropositive alkaline earths. Being in the s-block, the group 2 metals follow a major periodic variation in their atomic and ionic properties which is reflected in those Cp' compounds. This article presents an overview of this class of compounds for all the five metals from beryllium to barium (radium is excluded for its radioactivity), highlighting their systematic variation.

Coordination of β-Ketoimine-Derived Ligands at Main Group and Transition Metals

The β-ketoimine HC[MeC(O)][MeC(NHt-Bu)] (1H) (Me = methyl) was used as a ligand in the synthesis of organo-aluminium and gallium compounds. With Al, the NH functionality was deprotonated to form the N,O-chelating β-ketoiminate ligand in Al{HC[MeC(O)][MeC(Nt-Bu)]}Me2 (3) (t-Bu = tertiary butyl), whereas the neutral form coordinated to trimethylgallium via the oxygen atom to form the adduct GaMe3·{HC[MeC(O)][MeC(NHt-Bu)]} (4). Reaction of 1H with Ar†NH2 (Ar† = 2-t-BuC6H4) afforded the new N-aryl β-ketoimine HC[MeC(O)][MeC(NHAr†)] (2H), which reacted with Pd(OAc)2 (OAc = acetate = CH3CO2–) to afford the heteroleptic dimer {Pd[HC(MeC(O))(MeC(NAr†))](μ-OAc)}2 ([5]2). The homoleptic bis(β-ketoiminate) Pd{HC[MeC(O)][MeC(NAr†)]}2 (6) was isolated as a minor product of this reaction. The crystal structures of compounds 3, 4, [5]2, and 6 are reported.

Remarkably Volatile Copper(II) Complexes of N,N‘-Unsymmetrically Substituted 1,3-Diketimines as Precursors for Cu Metal Deposition via CVD or ALD

To enhance the volatility of the copper precursor in the copper deposition process, it was envisioned that N,N'-unsymmetrically substituted 1,3-diketimines should be more volatile than their symmetrically substituted counterparts. A variety of Cu(II) (N,N'-unsymmetrically substituted 1,3-diketiminate) complexes have been synthesized and have proven to be much more volatile than their symmetrical counterparts. This makes the new materials particularly attractive to the ALD and CVD processes. Among the new compounds, 8-a and 8-b are sublimable even at room temperature.

Routes to N,N‘-Unsymmetrically Substituted 1,3-Diketimines

[reaction: see text] A series of novel N,N'-unsymmetrically substituted 1,3-diketimines (3, 12, and 27) have been synthesized from the reaction of exocyclic enaminoketones 8 with amines or metalloenamines (from 13 or 14) with imidoyl thioether 25 or 26.

Divergent coordination mode of magnesium and zinc alkyls supported by the bifunctional pyrrolylaldiminato ligand

The first magnesium and zinc alkyls derived from N-(iso-propyl)-pyrrolylaldimine have been synthesised and structurally characterised: both tBuM(N,N')-type compounds exist as three-coordinate monomers in benzene solution, but in the solid state the magnesium complex is a centrosymmetric dimer with Mg2(mu-N)2 bridges, whereas the zinc complex is a Zn...pi bonded dimer with a pi-coordinated pyrrole unit.