B-Methylated Amine-Boranes:Substituent Redistribution, Catalytic Dehydrogenation, and Facile Metal-Free Hydrogen Transfer Reactions

Although the dehydrogenation chemistry of amine-boranes substituted at nitrogen has attracted considerable attention, much less is known about the reactivity of their B-substituted analogues. When the B-methylated amine-borane adducts, RR′NH·BH₂Me (<b>1a</b>: R = R′ = H; <b>1b</b>: R = Me, R′ = H; <b>1c</b>: R = R′ = Me; <b>1d</b>: R = R′ = <i>i</i>Pr), were heated to 70 °C in solution (THF or toluene), redistribution reactions were observed involving the apparent scrambling of the methyl and hydrogen substituents on boron to afford a mixture of the species RR′NH·BH_3–<i>x</i>Me_<i>x</i> (<i>x</i> = 0–3). These reactions were postulated to arise via amine-borane dissociation followed by the reversible formation of diborane intermediates and adduct reformation. Dehydrocoupling of <b>1a</b>–<b>1d</b> with Rh­(I), Ir­(III), and Ni(0) precatalysts in THF at 20 °C resulted in an array of products, including aminoborane RR′NBHMe, cyclic diborazane [RR′N–BHMe]₂, and borazine [RN–BMe]₃ based on analysis by in situ ¹¹B NMR spectroscopy, with peak assignments further supported by density functional theory (DFT) calculations. Significantly, very rapid, metal-free hydrogen transfer between <b>1a</b> and the monomeric aminoborane, <i>i</i>Pr₂NBH₂, to yield <i>i</i>Pr₂NH·BH₃ (together with dehydrogenation products derived from <b>1a</b>) was complete within only 10 min at 20 °C in THF, substantially faster than for the N-substituted analogue MeNH₂·BH₃. DFT calculations revealed that the hydrogen transfer proceeded via a concerted mechanism through a cyclic six-membered transition state analogous to that previously reported for the reaction of the <i>N</i>-dimethyl species Me₂NH·BH₃ and <i>i</i>Pr₂NBH₂. However, as a result of the presence of an electron donating methyl substituent on boron rather than on nitrogen, the process was more thermodynamically favorable and the activation energy barrier was reduced

Information Systems in Earth Management From Science to Application Results from the First Funding Period

The Editors and the Publisher can not be held responsible for the opinions expressed and the statements made in the articles published, such responsibility resting with the author

Electron beam-induced deposition of platinum from Pt(CO)₂Cl₂ and Pt(CO)₂Br₂

Two platinum precursors, Pt(CO)2Cl2 and Pt(CO)2Br2, were designed for focused electron beam-induced deposition (FEBID) with the aim of producing platinum deposits of higher purity than those deposited from commercially available precursors. In this work, we present the first deposition experiments in a scanning electron microscope (SEM), wherein series of pillars were successfully grown from both precursors. The growth of the pillars was studied as a function of the electron dose and compared to deposits grown from the commercially available precursor MeCpPtMe3. The composition of the deposits was determined using energy-dispersive X-ray spectroscopy (EDX) and compared to the composition of deposits from MeCpPtMe3, as well as deposits made in an ultrahigh-vacuum (UHV) environment. A slight increase in metal content and a higher growth rate are achieved in the SEM for deposits from Pt(CO)2Cl2 compared to MeCpPtMe3. However, deposits made from Pt(CO)2Br2 show slightly less metal content and a lower growth rate compared to MeCpPtMe3. With both Pt(CO)2Cl2 and Pt(CO)2Br2, a marked difference in composition was found between deposits made in the SEM and deposits made in UHV. In addition to Pt, the UHV deposits contained halogen species and little or no carbon, while the SEM deposits contained only small amounts of halogen species but high carbon content. Results from this study highlight the effect that deposition conditions can have on the composition of deposits created by FEBID.</p

Mechanisms of the Thermal and Catalytic Redistributions, Oligomerizations, and Polymerizations of Linear Diborazanes

Linear diborazanes R₃N–BH₂–NR₂–BH₃ (R = alkyl or H) are often implicated as key intermediates in the dehydrocoupling/dehydrogenation of amine-boranes to form oligo- and polyaminoboranes. Here we report detailed studies of the reactivity of three related examples: Me₃N–BH₂–NMe₂–BH₃ (<b>1</b>), Me₃N–BH₂–NHMe–BH₃ (<b>2</b>), and MeNH₂–BH₂–NHMe–BH₃ (<b>3</b>). The mechanisms of the thermal and catalytic redistributions of <b>1</b> were investigated in depth using temporal-concentration studies, deuterium labeling, and DFT calculations. The results indicated that, although the products formed under both thermal and catalytic regimes are identical (Me₃N·BH₃ (<b>8</b>) and [Me₂N–BH₂]₂ (<b>9a</b>)), the mechanisms of their formation differ significantly. The thermal pathway was found to involve the dissociation of the terminal amine to form [H₂B­(μ-H)­(μ-NMe₂)­BH₂] (<b>5</b>) and NMe₃ as intermediates, with the former operating as a catalyst and accelerating the redistribution of <b>1</b>. Intermediate <b>5</b> was then transformed to amine-borane <b>8</b> and the cyclic diborazane <b>9a</b> by two different mechanisms. In contrast, under catalytic conditions (0.3–2 mol % IrH₂POCOP (POCOP = κ³-1,3-(OP<i>t</i>Bu₂)₂C₆H₃)), <b>8</b> was found to inhibit the redistribution of <b>1</b> by coordination to the Ir-center. Furthermore, the catalytic pathway involved direct formation of <b>8</b> and Me₂NBH₂ (<b>9b</b>), which spontaneously dimerizes to give <b>9a</b>, with the absence of <b>5</b> and BH₃ as intermediates. The mechanisms elucidated for <b>1</b> are also likely to be applicable to other diborazanes, for example, <b>2</b> and <b>3</b>, for which detailed mechanistic studies are impaired by complex post-redistribution chemistry. This includes both metal-free and metal-mediated oligomerization of MeNHBH₂ (<b>10</b>) to form oligoaminoborane [MeNH–BH₂]_<i>x</i> (<b>11</b>) or polyaminoborane [MeNH–BH₂]_<i>n</i> (<b>16</b>) following the initial redistribution reaction

Electron beam-induced deposition of platinum from Pt(CO)₂Cl₂ and Pt(CO)₂Br₂

Two platinum precursors, Pt(CO)2Cl2 and Pt(CO)2Br2, were designed for focused electron beam-induced deposition (FEBID) with the aim of producing platinum deposits of higher purity than those deposited from commercially available precursors. In this work, we present the first deposition experiments in a scanning electron microscope (SEM), wherein series of pillars were successfully grown from both precursors. The growth of the pillars was studied as a function of the electron dose and compared to deposits grown from the commercially available precursor MeCpPtMe3. The composition of the deposits was determined using energy-dispersive X-ray spectroscopy (EDX) and compared to the composition of deposits from MeCpPtMe3, as well as deposits made in an ultrahigh-vacuum (UHV) environment. A slight increase in metal content and a higher growth rate are achieved in the SEM for deposits from Pt(CO)2Cl2 compared to MeCpPtMe3. However, deposits made from Pt(CO)2Br2 show slightly less metal content and a lower growth rate compared to MeCpPtMe3. With both Pt(CO)2Cl2 and Pt(CO)2Br2, a marked difference in composition was found between deposits made in the SEM and deposits made in UHV. In addition to Pt, the UHV deposits contained halogen species and little or no carbon, while the SEM deposits contained only small amounts of halogen species but high carbon content. Results from this study highlight the effect that deposition conditions can have on the composition of deposits created by FEBID.ImPhys/Microscopy Instrumentation & Technique

Mechanisms of the Thermal and Catalytic Redistributions, Oligomerizations, and Polymerizations of Linear Diborazanes

Linear diborazanes R₃N–BH₂–NR₂–BH₃ (R = alkyl or H) are often implicated as key intermediates in the dehydrocoupling/dehydrogenation of amine-boranes to form oligo- and polyaminoboranes. Here we report detailed studies of the reactivity of three related examples: Me₃N–BH₂–NMe₂–BH₃ (<b>1</b>), Me₃N–BH₂–NHMe–BH₃ (<b>2</b>), and MeNH₂–BH₂–NHMe–BH₃ (<b>3</b>). The mechanisms of the thermal and catalytic redistributions of <b>1</b> were investigated in depth using temporal-concentration studies, deuterium labeling, and DFT calculations. The results indicated that, although the products formed under both thermal and catalytic regimes are identical (Me₃N·BH₃ (<b>8</b>) and [Me₂N–BH₂]₂ (<b>9a</b>)), the mechanisms of their formation differ significantly. The thermal pathway was found to involve the dissociation of the terminal amine to form [H₂B­(μ-H)­(μ-NMe₂)­BH₂] (<b>5</b>) and NMe₃ as intermediates, with the former operating as a catalyst and accelerating the redistribution of <b>1</b>. Intermediate <b>5</b> was then transformed to amine-borane <b>8</b> and the cyclic diborazane <b>9a</b> by two different mechanisms. In contrast, under catalytic conditions (0.3–2 mol % IrH₂POCOP (POCOP = κ³-1,3-(OP<i>t</i>Bu₂)₂C₆H₃)), <b>8</b> was found to inhibit the redistribution of <b>1</b> by coordination to the Ir-center. Furthermore, the catalytic pathway involved direct formation of <b>8</b> and Me₂NBH₂ (<b>9b</b>), which spontaneously dimerizes to give <b>9a</b>, with the absence of <b>5</b> and BH₃ as intermediates. The mechanisms elucidated for <b>1</b> are also likely to be applicable to other diborazanes, for example, <b>2</b> and <b>3</b>, for which detailed mechanistic studies are impaired by complex post-redistribution chemistry. This includes both metal-free and metal-mediated oligomerization of MeNHBH₂ (<b>10</b>) to form oligoaminoborane [MeNH–BH₂]_<i>x</i> (<b>11</b>) or polyaminoborane [MeNH–BH₂]_<i>n</i> (<b>16</b>) following the initial redistribution reaction