Aminodiboranates, diaminodiboranates, and dialkyltriazenides: Syntheses, structures, and applications for chemical vapor deposition

Caroff, Christopher M

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https://hdl.handle.net/2142/120200 Copy

Description

Title

Aminodiboranates, diaminodiboranates, and dialkyltriazenides: Syntheses, structures, and applications for chemical vapor deposition

Author(s)

Caroff, Christopher M

Issue Date

2023-03-27

Director of Research (if dissertation) or Advisor (if thesis)

Girolami, Gregory

Doctoral Committee Chair(s)

Girolami, Gregory

Committee Member(s)

• Fout, Alison
• Abelson, John
• Olshansky, Lisa

Department of Study

Chemistry

Discipline

Chemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• aminodiboranates
• diaminodiboranates
• dialkyltriazenides
• chemical vapor deposition
• precursor

Abstract

The magnesium N,N-dimethylaminodiboranate compound Mg[(BH3)2NMe2]2 or Mg(DMADB)2, the most volatile compound of magnesium known, serves as an excellent 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 analogs of Mg(DMADB)2 in which one of the two methyl substituents on nitrogen is replaced with an ethyl, iso-propyl, or tert-butyl group. The crystal structure of one of these complexes, Mg[(BH3)2NMe(t-Bu)]2, has been determined; the magnesium center in the methyl/tert-butyl compound 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. Interestingly, the N•••Mg•••N angle is distinctly nonlinear (149.9°) because hydrogen atoms of BH3 groups of nearby molecules form two additional Mg-H contacts with the magnesium center. The structure differs significantly from that of Mg(DMADB)2, which has a linear N•••Mg•••N angle and no intermolecular contacts; we ascribe these structural differences to solid state packing effects. 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, which has been predicted to occur by cleavage of the B-N bonds to form the aminoborane BH2=NR2 and H2 as byproducts. Two solvent-free magnesium complexes bearing substituted diaminodiboranate ligands of the form (NR2BH2NR2BH3)- have been synthesized, where the NR2 groups are NMe2 or a pyrrolidinyl ring N(C4H8). The mono-thf adduct Mg[N(C4H8)BH2N(C4H8)BH3]2(thf) is also reported, along with the aminodiboranate salt [Mg(NH3)4(thf)2]¬[(BH3)2NH2]2, which was obtained unintentionally in efforts to prepare the unsubstituted magnesium diaminodiboranate complex. Crystal structures and 1H and 11B NMR data are reported for all four complexes. The two solvent-free magnesium diaminodiboranate complexes have very similar structures but very different physical properties: the N,N-dimethyl compound is a highly volatile, low melting solid whereas the pyrrolidinyl analog does not melt or sublime at low temperatures. Thermal decomposition studies in toluene suggest that these magnesium diaminodiboranates decompose to form the cyclic aminoborane dimer (NMe2BH2)2 and magnesium hydrides. Our findings suggest that Mg(NMe2BH2NMe2BH3)2 is a potentially useful precursor for the deposition of MgH2 by CVD. Two new cobalt complexes bearing substituted diaminodiboranate ligands of the form (NR2BH2NR2BH3)- have been synthesized, where the NR2 groups are NMe2 or a pyrrolidinyl ring N(C4H8). Crystal structures, NMR, IR, EPR, and magnetic data are reported for both complexes. Despite the similarity of the ligands, the two cobalt complexes have very different molecular structures and magnetic properties: the N,N-dimethyl compound is a low-spin pseudo-square-planar complex whereas the pyrrolidinyl analog is a high-spin pseudo-tetrahedral complex. These cobalt diaminodiboranate complexes are rare examples of homoleptic cobalt compounds bearing borohydride type ligands. Five new homoleptic transition metal complexes bearing substituted diaminodiboranate ligands of the form M(NR2BH2NR2BH3)2 have been synthesized where the NR2 groups are NMe2 [M = V, Cr, Fe] or a pyrrolidinyl ring N(C4H8) [M = Cr, Fe]. A new heteroleptic TiIII chloride complex, Ti(NMe2BH2NMe2BH3)2(Cl) has also been synthesized. Crystal structures, NMR, and IR data are reported for the new complexes and magnetic data are reported for the Fe complexes. The homoleptic transition metal complexes adopt either pseudo-square-planar or pseudo-tetrahedral geometries depending on the identity of the metal center and display a wide range of thermal properties and volatilities. These transition metal diaminodiboranate complexes are rare examples of homoleptic transition metal compounds bearing borohydride type ligands. Several dialkyltriazenide complexes of the lanthanide elements neodymium, europium, and erbium have been prepared; these include the homoleptic complex Er(ButN3But)3, the tetrahydrofuran monoadducts Ln(ButN3But)3(THF) where Ln = Nd or Eu, and the lithium salts [Li(THF)][Ln(MeN3But)4] where Ln = Eu or Er. Crystal structures, NMR, and IR data are reported for all complexes. The di-tert-butyltriazenide complexes are thermally stable, sublime at reasonably low temperatures, and show smooth volatilization without decomposition by thermogravimetric analysis, which make them potentially useful in lanthanide separation processes and as chemical vapor deposition precursors for lanthanide nitrides and other phases.

Graduation Semester

2023-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2023 Christopher Caroff

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