Diazadiene Complexes of the Heavy Alkaline-Earth Metals Strontium and Barium: Structures and Reactivity

1,4-Diaza-1,3-diene (=DAD) complexes of the heavy alkaline-earth metals strontium and barium have been synthesized by direct metalation of <i>N</i>,<i>N</i>′-bis­(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (<b>1</b>, =DAD^Dipp). The reaction with Sr metal afforded a mixture of the red enediamide-type derivative (DAD^Dipp)­Sr­(DME)₂ (<b>2</b>, DME = 1,2-dimethoxyethane) and black (DAD^Dipp)₂Sr­(DME) (<b>3</b>), which contains two coordinated DAD radical anions. With barium, only the radical anion derivative (DAD^Dipp)₂Ba­(DME) (<b>4</b>) was formed in 82% yield. For the first time, transfer of a DAD radical anion ligand from an alkaline-earth metal to a rare-earth metal has been achieved. Reaction of <b>4</b> with [{(Ph₂SiO)₂O}₂{Li­(THF)₂}₂]­HoCl (<b>5</b>) afforded the novel (DAD)holmium bis­(disiloxanediolate) complex [{(Ph₂SiO)₂O}₂{Li­(THF)₂}₂]­Ho­(DAD^Dipp) (<b>6</b>). All new complexes (<b>2</b>–<b>4</b> and <b>6</b>) have been structurally characterized by X-ray diffraction. In addition, the radical anion complexes <b>3</b>, <b>4</b>, and <b>6</b> were characterized by their EPR spectra

Diazadiene Complexes of the Heavy Alkaline-Earth Metals Strontium and Barium: Structures and Reactivity

1,4-Diaza-1,3-diene (=DAD) complexes of the heavy alkaline-earth metals strontium and barium have been synthesized by direct metalation of <i>N</i>,<i>N</i>′-bis­(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (<b>1</b>, =DAD^Dipp). The reaction with Sr metal afforded a mixture of the red enediamide-type derivative (DAD^Dipp)­Sr­(DME)₂ (<b>2</b>, DME = 1,2-dimethoxyethane) and black (DAD^Dipp)₂Sr­(DME) (<b>3</b>), which contains two coordinated DAD radical anions. With barium, only the radical anion derivative (DAD^Dipp)₂Ba­(DME) (<b>4</b>) was formed in 82% yield. For the first time, transfer of a DAD radical anion ligand from an alkaline-earth metal to a rare-earth metal has been achieved. Reaction of <b>4</b> with [{(Ph₂SiO)₂O}₂{Li­(THF)₂}₂]­HoCl (<b>5</b>) afforded the novel (DAD)holmium bis­(disiloxanediolate) complex [{(Ph₂SiO)₂O}₂{Li­(THF)₂}₂]­Ho­(DAD^Dipp) (<b>6</b>). All new complexes (<b>2</b>–<b>4</b> and <b>6</b>) have been structurally characterized by X-ray diffraction. In addition, the radical anion complexes <b>3</b>, <b>4</b>, and <b>6</b> were characterized by their EPR spectra

Integrating AlN with GdN Thin Films in an in Situ CVD Process: Influence on the Oxidation and Crystallinity of GdN

The application potential of rare earth nitride (REN) materials has been limited due to their high sensitivity to air and moisture leading to facile oxidation upon exposure to ambient conditions. For the growth of device quality films, physical vapor deposition methods, such as molecular beam epitaxy, have been established in the past. In this regard, aluminum nitride (AlN) has been employed as a capping layer to protect the functional gadolinium nitride (GdN) from interaction with the atmosphere. In addition, an AlN buffer was employed between a silicon substrate and GdN serving as a seeding layer for epitaxial growth. In pursuit to grow high-quality GdN thin films by chemical vapor deposition (CVD), this successful concept is transferred to an in situ CVD process. Thereby, AlN thin films are included step-wise in the stack starting with Si/GdN/AlN structures to realize long-term stability of the oxophilic GdN layer. As a second strategy, a Si/AlN/GdN/AlN stacked structure was grown, where the additional buffer layer serves as the seeding layer to promote crystalline GdN growth. In addition, chemical interaction between GdN and the Si substrate can be prevented by spatial segregation. The stacked structures grown for the first time with a continuous CVD process were subjected to a detailed investigation in terms of structure, morphology, and composition, revealing an improved GdN purity with respect to earlier grown CVD thin films. Employing thin AlN buffer layers, the crystallinity of the GdN films on Si(100) could additionally be significantly enhanced. Finally, the magnetic properties of the fabricated stacks were evaluated by performing superconducting quantum interference device measurements, both of the as-deposited films and after exposure to ambient conditions, suggesting superparamagnetism of ferromagnetic GdN grains. The consistency of the magnetic properties precludes oxidation of the REN material due to the amorphous AlN capping layer

Homoleptic Gadolinium Amidinates as Precursors for MOCVD of Oriented Gadolinium Nitride (GdN) Thin Films

Five new homoleptic gadolinium tris-amidinate complexes are reported, which were synthesized via the salt-elimination reaction of GdCl₃ with 3 equiv of lithiated symmetric and asymmetric amidinates at ambient temperature. The Gd-tris-amidinates [Gd­{(N^<i>i</i>Pr)₂­CR}₃] [R = Me (<b>1</b>), Et (<b>2</b>), ^<i>t</i>Bu (<b>3</b>), ^<i>n</i>Bu (<b>4</b>)] and [Gd­{(NEt)­(N^<i>t</i>Bu)­CMe}₃] (<b>5</b>) are solids at room temperature and sublime at temperatures of about 125 °C (6 × 10^–2 mbar) with the exception of compound <b>4</b>, which is a viscous liquid at room temperature. According to X-ray diffraction analysis of <b>3</b> and <b>5</b> as representative examples of the series, the complexes adopt a distorted octahedral structure in the solid state. Mass spectrometric (MS) data confirmed the monomeric structure in the gas phase, and high-resolution MS allowed the identification of characteristic fragments, such as [{(N^<i>i</i>Pr)₂CR}GdCH₃]⁺ and [{(N^<i>i</i>Pr)₂CR}GdNH]⁺. The alkyl substitution patterns of the amidinate ligands clearly show an influence on the thermal properties, and specifically, the introduction of the asymmetric carbodiimide leads to a lowering of the onset of volatilization and decomposition. Compound <b>5</b>, which is the first Gd complex with an asymmetric amidinate ligand system to be reported, was, therefore, tested for the MOCVD of GdN thin films. The as-deposited GdN films were capped with Cu in a subsequent MOCVD process to prevent postdeposition oxidation of the films. Cubic GdN on Si(100) substrates with a preferred orientation in the (200) direction were grown at 750 °C under an ammonia atmosphere and exhibited a columnar morphology and low levels of C or O impurities according to scanning electron microscopy, Rutherford backscattering, and nuclear reaction analysis

Homoleptic Gadolinium Amidinates as Precursors for MOCVD of Oriented Gadolinium Nitride (GdN) Thin Films

Five new homoleptic gadolinium tris-amidinate complexes are reported, which were synthesized via the salt-elimination reaction of GdCl₃ with 3 equiv of lithiated symmetric and asymmetric amidinates at ambient temperature. The Gd-tris-amidinates [Gd­{(N^<i>i</i>Pr)₂­CR}₃] [R = Me (<b>1</b>), Et (<b>2</b>), ^<i>t</i>Bu (<b>3</b>), ^<i>n</i>Bu (<b>4</b>)] and [Gd­{(NEt)­(N^<i>t</i>Bu)­CMe}₃] (<b>5</b>) are solids at room temperature and sublime at temperatures of about 125 °C (6 × 10^–2 mbar) with the exception of compound <b>4</b>, which is a viscous liquid at room temperature. According to X-ray diffraction analysis of <b>3</b> and <b>5</b> as representative examples of the series, the complexes adopt a distorted octahedral structure in the solid state. Mass spectrometric (MS) data confirmed the monomeric structure in the gas phase, and high-resolution MS allowed the identification of characteristic fragments, such as [{(N^<i>i</i>Pr)₂CR}GdCH₃]⁺ and [{(N^<i>i</i>Pr)₂CR}GdNH]⁺. The alkyl substitution patterns of the amidinate ligands clearly show an influence on the thermal properties, and specifically, the introduction of the asymmetric carbodiimide leads to a lowering of the onset of volatilization and decomposition. Compound <b>5</b>, which is the first Gd complex with an asymmetric amidinate ligand system to be reported, was, therefore, tested for the MOCVD of GdN thin films. The as-deposited GdN films were capped with Cu in a subsequent MOCVD process to prevent postdeposition oxidation of the films. Cubic GdN on Si(100) substrates with a preferred orientation in the (200) direction were grown at 750 °C under an ammonia atmosphere and exhibited a columnar morphology and low levels of C or O impurities according to scanning electron microscopy, Rutherford backscattering, and nuclear reaction analysis