Synthesis and Characterization of Calcium <i>N</i>,<i>N</i>‑Dimethylaminodiboranates as Possible Chemical Vapor Deposition Precursors

The reaction of CaBr₂ with 2 equiv of sodium <i>N,N</i>-dimethylaminodiboranate, Na­(H₃BNMe₂BH₃) in Et₂O at 0 °C followed by crystallization and drying in vacuum yields the unsolvated calcium compound Ca­(H₃BNMe₂BH₃)₂, <b>1</b>. Before the vacuum drying step, the colorless crystals obtained by crystallization consist of the diethyl ether adduct Ca­(H₃BNMe₂BH₃)₂(Et₂O)₂, <b>2</b>. If the reaction of CaBr₂ with 2 equiv of Na­(H₃BNMe₂BH₃) is carried out in the more strongly coordinating solvent tetrahydrofuran (thf), the solvate Ca­(H₃BNMe₂BH₃)₂(thf)₂, <b>3</b>, is obtained. This compound does not desolvate as easily in vacuum as the diethyl ether compound <b>2</b>. Treating the thf adduct <b>3</b> with 1,2-dimethoxyethane (dme), bis­(2-methoxyethyl) ether (diglyme), or <i>N,N</i>,<i>N</i>′<i>,N</i>′-tetramethylethylenediamine (tmeda) in thf affords the new compounds Ca­(H₃BNMe₂BH₃)₂(dme), <b>4</b>, Ca­(H₃BNMe₂BH₃)₂(diglyme), <b>5</b>, and Ca­(H₃BNMe₂BH₃)₂(tmeda), <b>6</b>, respectively, in greater than 60% yields. Treatment of <b>3</b> with 2 equiv of the crown ether 12-crown-4 in thf affords the charge-separated salt [Ca­(12-crown-4)₂]­[H₃BNMe₂BH₃]₂, <b>7</b>. Crystal structures of all the Lewis base adducts are described. Compounds <b>2</b>–<b>6</b> all possess chelating κ²-BH₃NMe₂BH₃-κ² groups, in which two hydrogen atoms on each boron center are bound to calcium. Compound <b>7</b> is the only ionic compound in the series; the Ca atom is completely encapsulated by two 12-crown-4 rings, and the anions are charge-separated counterions within the unit cell. When heated, the dme, diglyme, and tmeda compounds <b>4</b>, <b>5</b>, and <b>6</b> melt without decomposition, and can be sublimed readily under reduced pressure (1 Torr) at 90 °C (<b>4</b>) and 120 °C (<b>5, 6</b>). The dme adduct is one of the most volatile calcium compounds known, and is a promising CVD precursor for the growth of calcium-containing thin films

Synthesis and Characterization of Calcium <i>N</i>,<i>N</i>‑Dimethylaminodiboranates as Possible Chemical Vapor Deposition Precursors

The reaction of CaBr₂ with 2 equiv of sodium <i>N,N</i>-dimethylaminodiboranate, Na­(H₃BNMe₂BH₃) in Et₂O at 0 °C followed by crystallization and drying in vacuum yields the unsolvated calcium compound Ca­(H₃BNMe₂BH₃)₂, <b>1</b>. Before the vacuum drying step, the colorless crystals obtained by crystallization consist of the diethyl ether adduct Ca­(H₃BNMe₂BH₃)₂(Et₂O)₂, <b>2</b>. If the reaction of CaBr₂ with 2 equiv of Na­(H₃BNMe₂BH₃) is carried out in the more strongly coordinating solvent tetrahydrofuran (thf), the solvate Ca­(H₃BNMe₂BH₃)₂(thf)₂, <b>3</b>, is obtained. This compound does not desolvate as easily in vacuum as the diethyl ether compound <b>2</b>. Treating the thf adduct <b>3</b> with 1,2-dimethoxyethane (dme), bis­(2-methoxyethyl) ether (diglyme), or <i>N,N</i>,<i>N</i>′<i>,N</i>′-tetramethylethylenediamine (tmeda) in thf affords the new compounds Ca­(H₃BNMe₂BH₃)₂(dme), <b>4</b>, Ca­(H₃BNMe₂BH₃)₂(diglyme), <b>5</b>, and Ca­(H₃BNMe₂BH₃)₂(tmeda), <b>6</b>, respectively, in greater than 60% yields. Treatment of <b>3</b> with 2 equiv of the crown ether 12-crown-4 in thf affords the charge-separated salt [Ca­(12-crown-4)₂]­[H₃BNMe₂BH₃]₂, <b>7</b>. Crystal structures of all the Lewis base adducts are described. Compounds <b>2</b>–<b>6</b> all possess chelating κ²-BH₃NMe₂BH₃-κ² groups, in which two hydrogen atoms on each boron center are bound to calcium. Compound <b>7</b> is the only ionic compound in the series; the Ca atom is completely encapsulated by two 12-crown-4 rings, and the anions are charge-separated counterions within the unit cell. When heated, the dme, diglyme, and tmeda compounds <b>4</b>, <b>5</b>, and <b>6</b> melt without decomposition, and can be sublimed readily under reduced pressure (1 Torr) at 90 °C (<b>4</b>) and 120 °C (<b>5, 6</b>). The dme adduct is one of the most volatile calcium compounds known, and is a promising CVD precursor for the growth of calcium-containing thin films

Lanthanide <i>N</i>,<i>N</i>-Dimethylaminodiboranates as a New Class of Highly Volatile Chemical Vapor Deposition Precursors

New lanthanide <i>N</i>,<i>N</i>-dimethylaminodiboranate (DMADB) complexes of stoichiometry Ln­(H₃BNMe₂BH₃)₃ and Ln­(H₃BNMe₂BH₃)₃(thf) have been prepared, where Ln = yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, and lutetium, except that isolation of the desolvated complexes proved difficult for Eu and Yb. The tetrahydrofuran (thf) complexes are all monomeric, and most of them adopt 13-coordinate structures in which each DMADB group chelates to the metal center by means of four B–H···Ln bridges (each BH₃ group is κ²<i>H</i>; i.e., forms two B–H···Ln interactions). For the smallest three lanthanides, Tm, Yb, and Lu, the metal center is 12 coordinate because one of the DMADB groups chelates to the metal center by means of only three B–H···Ln bridges. The structures of the base-free Ln­(H₃BNMe₂BH₃)₃ complexes are highly dependent on the size of the lanthanide ions: as the ionic radius decreases, the coordination number decreases from 14 (Pr) to 13 (Sm) to 12 (Dy, Y, Er). The 14-coordinate complexes are polymeric: each metal center is bound to two chelating DMADB ligands and to two “ends” of two ligands that bridge in a Ln­(κ³<i>H</i>-H₃BNMe₂BH₃-κ³<i>H</i>)­Ln fashion. In the 13-coordinate complexes, all three DMADB ligands are chelating, but the metal atom is also coordinated to one hydrogen atom from an adjacent molecule. The 12-coordinate complexes adopt a dinuclear structure in which each metal center is bound to two chelating DMADB ligands and to two ends of two ligands that bridge in a Ln­(κ²<i>H</i>-H₃BNMe₂BH₃-κ²<i>H</i>)­Ln fashion. The complexes react with water, and the partial hydrolysis product [La­(H₃BNMe₂BH₃)₂(OH)]₄ adopts a structure in which the lanthanum and oxygen atoms form a distorted cube; each lanthanum atom is connected to three bridging hydroxyl groups and to two chelating DMADB ligands. One B–H bond of each chelating DMADB ligand forms a bridge to an adjacent metal center. Field ionization MS data, melting and decomposition points, thermogravimetric data, and NMR data, including an analysis of the paramagnetic lanthanide induced shifts (LIS), are reported for all of the complexes. The Ln­(H₃BNMe₂BH₃)₃ compounds, which are highly volatile and sublime at temperatures as low as 65 °C in vacuum, are suitable for use as chemical vapor deposition (CVD) and atomic layer deposition (ALD) precursors to thin films

Nonagostic M···H–C Interactions. Synthesis, Characterization, and DFT Study of the Titanium Amide Ti₂Cl₆[N(<i>t</i>-Bu)₂]₂

The compound Ti₂Cl₆[N­(<i>t</i>-Bu)₂]₂ (<b>1</b>) has been synthesized by treating TiCl₄ with di­(<i>tert</i>-butyl)­amine, HN­(<i>t</i>-Bu)₂. Compound <b>1</b> crystallizes in two different polymorphs from pentane, both conforming to the space group <i>P</i>2₁/<i>n</i>. In both polymorphs, <b>1</b> exhibits a close Ti···C contact of 2.634(3) Å between titanium and a γ-methyl group in one of the two <i>tert</i>-butyl groups of the bound amido ligand. Interestingly, the γ-methyl group adopts a rotational conformation that maximizes the Ti···H distances, the shortest of which are 2.36(2) and 2.62(2) Å. Even though the former distance is within the range characteristic of agostic interactions, the rotational orientation of the methyl group suggests that the Ti···H interactions are repulsive rather than attractive. DFT and NBO analysis confirms this supposition: there is no evidence of weakening of the C–H bond closest to the titanium and no evidence of significant overlap of titanium orbitals with the C–H bonding orbitals of the γ-methyl group involved in the close contact. Further evidence that the close contact is repulsive was obtained from a DFT study of a series of related complexes in which the N­(<i>t</i>-Bu)₂ ligand is replaced with a NR­(<i>t</i>-Bu) ligand, where the substituent R not involved in the close contact is Et, Me, or SiMe₃. All of these latter substituents, which are sterically smaller than a <i>t</i>-Bu group, enable the amide group to pivot in such a way as to move the <i>tert</i>-butyl group farther from the metal center. The results suggest that the short Ti···C and Ti···H distances seen crystallographically for <b>1</b> are actually the result of intraligand and interligand steric repulsions involving the amide substituent not involved in the close contact. The lack of an agostic interaction despite the close contact (and the low electron count of the Ti center) is ascribed to the strong σ- and π-donor properties of the amide and chloride ligands, which raise the energies of the empty orbitals on Ti

Lanthanide <i>N</i>,<i>N</i>-Dimethylaminodiboranates as a New Class of Highly Volatile Chemical Vapor Deposition Precursors

New lanthanide <i>N</i>,<i>N</i>-dimethylaminodiboranate (DMADB) complexes of stoichiometry Ln­(H₃BNMe₂BH₃)₃ and Ln­(H₃BNMe₂BH₃)₃(thf) have been prepared, where Ln = yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, and lutetium, except that isolation of the desolvated complexes proved difficult for Eu and Yb. The tetrahydrofuran (thf) complexes are all monomeric, and most of them adopt 13-coordinate structures in which each DMADB group chelates to the metal center by means of four B–H···Ln bridges (each BH₃ group is κ²<i>H</i>; i.e., forms two B–H···Ln interactions). For the smallest three lanthanides, Tm, Yb, and Lu, the metal center is 12 coordinate because one of the DMADB groups chelates to the metal center by means of only three B–H···Ln bridges. The structures of the base-free Ln­(H₃BNMe₂BH₃)₃ complexes are highly dependent on the size of the lanthanide ions: as the ionic radius decreases, the coordination number decreases from 14 (Pr) to 13 (Sm) to 12 (Dy, Y, Er). The 14-coordinate complexes are polymeric: each metal center is bound to two chelating DMADB ligands and to two “ends” of two ligands that bridge in a Ln­(κ³<i>H</i>-H₃BNMe₂BH₃-κ³<i>H</i>)­Ln fashion. In the 13-coordinate complexes, all three DMADB ligands are chelating, but the metal atom is also coordinated to one hydrogen atom from an adjacent molecule. The 12-coordinate complexes adopt a dinuclear structure in which each metal center is bound to two chelating DMADB ligands and to two ends of two ligands that bridge in a Ln­(κ²<i>H</i>-H₃BNMe₂BH₃-κ²<i>H</i>)­Ln fashion. The complexes react with water, and the partial hydrolysis product [La­(H₃BNMe₂BH₃)₂(OH)]₄ adopts a structure in which the lanthanum and oxygen atoms form a distorted cube; each lanthanum atom is connected to three bridging hydroxyl groups and to two chelating DMADB ligands. One B–H bond of each chelating DMADB ligand forms a bridge to an adjacent metal center. Field ionization MS data, melting and decomposition points, thermogravimetric data, and NMR data, including an analysis of the paramagnetic lanthanide induced shifts (LIS), are reported for all of the complexes. The Ln­(H₃BNMe₂BH₃)₃ compounds, which are highly volatile and sublime at temperatures as low as 65 °C in vacuum, are suitable for use as chemical vapor deposition (CVD) and atomic layer deposition (ALD) precursors to thin films

Synthesis and Single Crystal Structure of Sodium Octahydrotriborate, NaB₃H₈

This paper describes a modified synthesis of NaB₃H₈ by the reduction of BH₃·THF with sodium dispersed on silica gel. Single crystals obtained from CH₂Cl₂ show conclusively that the space group is <i>Pmn</i>2₁, in contrast to the <i>Pmmn</i> space group previously deduced from powder diffraction data

Steric and Electronic Analyses of Ligand Effects on the Stability of σ‑Methane Coordination Complexes: A DFT Study

Developing efficient catalysts for methane functionalization is a longstanding goal in inorganic chemistry. Here, we present theoretical calculations to support efforts to synthesize σ-methane complexes that can be studied by NMR spectroscopy. The systems studied are osmium complexes of stoichiometry (C5R5)Os(diphosphine)(CH3)(H)+: when both cyclopentadienyl and diphosphine are relatively strong electron donors, the methyl/hydride structure is in rapid equilibrium with its σ-methane tautomer at low temperatures, as shown experimentally some years ago. Here, using density functional theory, we examine how changing the steric and electronic properties of the ancillary cyclopentadienyl and diphosphine ligands affects the relative energies of the two tautomers, with the goal of identifying a ligand set for which the σ-methane structure, rather than the methyl/hydride form, is the predominant species in equilibrium. We also examine how varying the ancillary ligands affects the barrier for methane dissociation. The calculations suggest that osmium complexes bearing weakly donating and sterically undemanding ligands stabilize the σ-methane structure both relative to its methyl/hydride tautomer and toward dissociation of the methane ligand. More specifically, osmium σ-methane complexes of fluorinated diphosphines (CF3)2PCH2P(CF3)2 and (CF3)2PCF2P(CF3)2 are predicted to be stable enough to be observed by variable-temperature NMR spectroscopy

Steric and Electronic Analyses of Ligand Effects on the Stability of σ‑Methane Coordination Complexes: A DFT Study

Developing efficient catalysts for methane functionalization is a longstanding goal in inorganic chemistry. Here, we present theoretical calculations to support efforts to synthesize σ-methane complexes that can be studied by NMR spectroscopy. The systems studied are osmium complexes of stoichiometry (C5R5)Os(diphosphine)(CH3)(H)+: when both cyclopentadienyl and diphosphine are relatively strong electron donors, the methyl/hydride structure is in rapid equilibrium with its σ-methane tautomer at low temperatures, as shown experimentally some years ago. Here, using density functional theory, we examine how changing the steric and electronic properties of the ancillary cyclopentadienyl and diphosphine ligands affects the relative energies of the two tautomers, with the goal of identifying a ligand set for which the σ-methane structure, rather than the methyl/hydride form, is the predominant species in equilibrium. We also examine how varying the ancillary ligands affects the barrier for methane dissociation. The calculations suggest that osmium complexes bearing weakly donating and sterically undemanding ligands stabilize the σ-methane structure both relative to its methyl/hydride tautomer and toward dissociation of the methane ligand. More specifically, osmium σ-methane complexes of fluorinated diphosphines (CF3)2PCH2P(CF3)2 and (CF3)2PCF2P(CF3)2 are predicted to be stable enough to be observed by variable-temperature NMR spectroscopy

Synthesis and Structural Diversity of Barium (<i>N</i>,<i>N</i>-Dimethylamino)diboranates

The reaction of a slurry of BaBr₂ in a minimal amount of tetrahydrofuran (THF) with 2 equiv of Na­(H₃BNMe₂BH₃) in diethyl ether followed by crystallization from diethyl ether at −20 °C yields crystals of Ba­(H₃BNMe₂BH₃)₂(Et₂O)₂ (<b>1</b>). Drying <b>1</b> at room temperature under vacuum gives the partially desolvated analogue Ba­(H₃BNMe₂BH₃)₂(Et₂O)_<i>x</i> (<b>1′</b>) as a free-flowing white solid, where the value of <i>x</i> varies from <0.1 to about 0.4 depending on whether desolvation is carried out with or without heating. The reaction of <b>1</b> or <b>1′</b> with Lewis bases that bind more strongly to barium than diethyl ether results in the formation of new complexes Ba­(H₃BNMe₂BH₃)₂(L), where L = 1,2-dimethoxyethane (<b>2</b>), <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetramethylethylenediamine (<b>3</b>), 12-crown-4 (<b>4</b>), 18-crown-6 (<b>5</b>), <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>′-tetraethylethylenediamine (<b>6</b>), and <i>N</i>,<i>N</i>,<i>N</i>′,<i>N</i>″,<i>N</i>″-pentamethylethylenetriamine (<b>7</b>). Recrystallization of <b>4</b> and <b>5</b> from THF affords the related compounds Ba­(H₃BNMe₂BH₃)₂(12-crown-4)­(THF)·THF (<b>4′</b>) and Ba­(H₃BNMe₂BH₃)₂(18-crown-6)·2THF (<b>5′</b>). In addition, the reaction of BaBr₂ with 2 equiv of Na­(H₃BNMe₂BH₃) in the presence of diglyme yields Ba­(H₃BNMe₂BH₃)₂(diglyme)₂ (<b>8</b>), and the reaction of <b>1</b> with 15-crown-5 affords the diadduct [Ba­(15-crown-5)₂]­[H₃BNMe₂BH₃]₂ (<b>9</b>). Finally, the reaction of BaBr₂ with Na­(H₃BNMe₂BH₃) in THF, followed by the addition of 12-crown-4, affords the unusual salt [Na­(12-crown-4)₂]­[Ba­(H₃BNMe₂BH₃)₃(THF)₂] (<b>10</b>). All of these complexes have been characterized by IR and ¹H and ¹¹B NMR spectroscopy, and the structures of compounds <b>1</b>–<b>3</b>, <b>4′</b>, <b>5′</b>, and <b>6</b>–<b>10</b> have been determined by single-crystal X-ray diffraction. As the steric demand of the Lewis bases increases, the structure changes from polymers to dimers to monomers and then to charge-separated species. Despite the fact that several of the barium complexes are monomeric in the solid state, none is appreciably volatile up to 200 °C at 10^–2 Torr

Solution-Mediated Selective Nanosoldering of Carbon Nanotube Junctions for Improved Device Performance

As-grown randomly aligned networks of carbon nanotubes (CNTs) invariably suffer from limited transport properties due to high resistance at the crossed junctions between CNTs. In this work, Joule heating of the highly resistive CNT junctions is carried out in the presence of a spin-coated layer of a suitable chemical precursor. The heating triggers thermal decomposition of the chemical precursor, tris(dibenzylideneacetone)dipalladium (Pd₂(dba)₃), and causes local deposition of Pd nanoparticles at the CNT junctions, thereby improving the on/off current ratio and mobility of CNT network devices by an average factor of ∼6. This process can be conducted either in air or under vacuum depending on the characteristics of the precursor species. The solution-mediated nanosoldering process is simple, fast, scalable with manufacturing techniques, and extendable to the nanodeposition of a wide variety of materials