Hydrogen and Water Splitting by Frustrated Lewis Pairs and Reactions thereof

The principle of combining a Lewis acid (LA) and a Lewis base (LB), prevented from the formation of classical Lewis adduct and thus possessing reactive potential, is known as frustrated Lewis pair (FLP). This powerful, yet simple concept allowed for a wide range of reactivities unprecedented for main group chemistry and is perceived as an alternative to transition metal catalysis. Among such reactivities, hydrogen activation and catalytic hydrogenations of unsaturated organic substrates, perhaps remain the most remarkable ones. In the literature review, our focus is limited to hydrogen activation and reactions thereof, and furthermore, FLPs comprising boron-centered LAs. First, mechanistic aspects of the process are discussed, including principles of assessment and modulation of Lewis acidity. Further, the review covers hydrogenation catalysis with FLPs, including hydrogenation of polar and non-polar substrates, functional group- and moisture- tolerant catalysts, and enantioselective hydrogenations. A separate section is devoted to intramolecular FLPs as a distinct subclass of FLPs. The results and discussion part summarizes the key findings reported in the attached original publications and can be divided into two subtopics. First is devoted to the development of linked FLPs aimed for generation of parahydrogen-induced hyperpolarization (PHIP). The phenomenon occurs upon parahydrogen (p-H2) pairwise splitting and causes substantial amplification of NMR signals of p-H2 originating fragments and neighboring nuclei. Therefore, the method can be promising for sensitivity enhancement in, e.g., NMR (MRI) signal imaging. In this work, the series of phenylene bridged ansa-aminoboranes (AABs) varied in the chemical environment around B were synthesised, and their ability to generate PHIP upon addition of p-H2 at ambient conditions was demonstrated. Kinetic, thermodynamic, and NMR parameters favorable for the efficient production of PHIP with such FLPs were formulated. Replacement of quadrupolar 14N nucleus of the amine site with 15N in the series led to spontaneous polarization transfer to nitrogen and strong signal enhancements. Further efforts were dedicated to overcoming the incompatibility of ansa-FLPs with water, bringing closer their realization as PHIP contrast agents in biologically relevant media. In this regard, thorough design of LA-LB active sites resulted in ansa-phosphinoborane capable of both H2 and H2O splitting in a reversible manner. Quite unexpectedly, this compound featured stoichiometric reduction of H2O to H2 via a proton “umpolung” mechanism. The second part is devoted to synthesizing (+)-camphor-based chiral boranes for asymmetric hydrogenation of imines. This study resulted in a highly enantioselective catalyst easily accessible from the synthetic point of view and shed light on the origin of the enantioselectivity.Lewis-hapon (LH) ja Lewis-emäksen (LE) yhdistelmä, jossa klassisen Lewis-adduktin muodostuminen on estynyt, tunnetaan turhautuneena Lewis -parina (Frustrated Lewis Pair, FLP). Tämä yksinkertainen konsepti mahdollistaa laajan kemiallisen reaktiivisuuden, joista monet ovat aikaisemmin tuntemattomia pääryhmän alkuaineiden kemiassa. Reaktiivisuuksista molekulaarisen vedyn aktivoiminen ja tyydyttymättömien orgaanisten substraattien katalyyttiset vedytykset ovat ehkä kaikkein merkittävimpiä, ja aikaisemmin tunnettu vain siirtymämetallikatalyyteille. Kirjallisuuskatsauksen painopiste on vedyn aktivoinnissa ja jatkoreaktioissa, ja turhautuneissa Lewis -pareissa, joiden LH:t sisältävät booria. Kirjallisuuskatsauksen aluksi käsitellään reaktiivisuuden perusteita, mukaan lukien Lewisin happamuuden arvioinnin ja moduloinnin periaatteet. Tämän lisäksi katsauksessa käsitellään vedytyskatalyysiä mukaan lukien polaaristen ja poolittomien substraattien vedytys, enantioselektiivinen vedytus ja katalyyttien kykyä sietää funktionaalisia ryhmiä ja kosteutta. Kirjallisuuskatsauksessa on myös erillinen osio omistettuna molekyylinsisäisille FLP:ille. Tulokset ja keskustelut –osio tiivistää alkuperäisissä julkaisuissa raportoidut keskeiset havainnot, jotka voidaan jakaa kahteen osaan. Ensimmäinen osa on omistettu molekyylinsisäisten FLP:iden kehittämiseen, joiden tarkoituksena on muodostaa hyperpolarisaatio paravedyn indusoimana (PHIP). Hyperpolarisaatio tapahtuu paravedyn (p-H2) aktivoinnilla aiheuttaen p-H2:sta peräisin olevien fragmenttien ja vierekkäisten ytimien NMR-signaalien huomattavan vahvistumisen. Siksi menetelmä voi olla lupaava herkkyyden parantamiseksi esim. NMR (MRI) -signaalikuvauksessa. Väitöskirjatyössä valmistettiin sarja fenyylisillallisia ansa-aminoboraaneja (AAB), joiden LH:n kemiallista rakennetta muokattiin. Näillä ABB:la tutkittiin PHIP:n muodostumista eri reaktio-olosuhteissa ja määritettiin optimaaliset kineettiset, termodynaamiset sekä NMR-parametrit. Huomionarvoista on myös, että amiinin (LE) 14N ytimen korvaaminen 15N:llä johti spontaaniin polarisaation siirtymiseen typelle ja siten voimakkaaseen signaalin kasvamiseen. Työssä tutkittiin myös mahdollisuutta valmistaa molekyylinsisäisiä FLP rakenteita, jotka kykenesivät H2:n aktivoimiseen veden läsnä ollessa. Tämä mahdollistaisi FLP:n hyödyntämisen PHIP-varjoaineina biologisesti merkityksellisissä liuoksissa. Tältä osin on huomionarvoista on, että LA-LB:n aktiivisten kohtien perusteellinen suunnittelu johti ansa-fosfinoboraaniin, joka pystyy sekä H2:n että H2O:n reversiibeliin aktivoimiseen. Yllättäen tämä yhdiste mahdollisti myös stoikiometrisen H2O:n pelkistyksen H2:ksi protonien "umpolung"-mekanismin kautta. Tulokset ja keskustelut –osion toisessa osassa käsitellään(+)-kamferipohjaisten kiraalisten boraanien valmistamista ja käyttöä imiinien asymmetrisessä vedytyksessä. Tämä tutkimus johti erittäin enantioselektiiviseen katalyyttiin, joka on synteettisestä näkökulmasta helposti saatavilla. Tutkimus myös valaisi enantioselektiivisyyden alkuperää FLP katalysoiduissa reaktioissa

The determination of organic substances impurities in sodium nitrate by the methods of gas chromatography and gas chromatography - mass-spectrometry

В настоящее время в нитрате натрия нормируются примеси металлов и неорганических солей. Сведения о содержании примесей органических веществ, являющихся источником углерода в получаемых на основе нитрата натрия оптических материалах, отсутствуют. В данной работе проведены идентификация и количественное определение примесей углеводородов в нитрате натрия с использованием высокочувствительных и экспрессных методов газовой хроматографии и хромато-масс-спектрометрии. Для концентрирования примесей применены методы парофазной макроэкстракции и жидкофазной микроэкстракции. Применение метода парофазной макроэкстракции показало присутствие примесей легких углеводородов С₁-С₄, метод жидкофазной микроэкстракции позволил выявить примеси труднолетучих веществ - высокомолекулярных углеводородов С₁₅-С₁₇. Установлено, что в процессе парофазной макроэкстракции происходит разложение примесей углеводородов С₁₅-С₁₇. Поэтому надежное определение углеводородного примесного состава нитрата натрия возможно с применением жидкофазной микроэкстракции. Показано, что концентрация углеводородов С₁₅-С₁₇ в нитрате натрия составляет 2×10⁻⁶ - 2×10⁻⁵ % мас. Источники примесей высокомолекулярных углеводородов С₁₅-С₁₇ в нитрате натрия связаны скорее всего с причинами технологического характера, т.к. эти углеводороды являются основными составляющими масел, применяемых для герметизации аппаратуры, используемой при синтезе NaNO₃.At the present time the impurities of metals and inorganic salts are normalized in sodium nitrate. The data on the content of organic substances impurities, which are the source of carbon in the prepared based on sodium nitrate optical materials, are not available. In this work, the identification and determination of hydrocarbon impurities in sodium nitrate was carried out with the use of highly sensitive and express methods of gas chromatography and gas chromatography-mass-spectrometry. For the pre-concentration of the impurities the methods of vapor-phase macro-extraction and of liquid phase micro-extraction were investigated. The use of the method of vapor-phase macro-extraction indicated the presence of the impurities of light С1-С4 hydrocarbons; the method of liquid phase micro-extraction made it possible to identify the impurities of semi-volatile С15-С17 hydrocarbons. It was found that in the process of vapor-phase macro-extraction, the decomposition of С15-С17 hydrocarbons takes place. That is why the reliable determination of hydrocarbon impurity composition is possible with the use of liquid phase micro-extraction. It was shown that the concentration of С15-С17 hydrocarbons in sodium nitrate is 2·10⁻⁶ - 2·10⁻⁵ mas. %. Accounting for the possible sources of hydrocarbon impurities in sodium nitrate, it is possible to state that the presence of the impurities of high molecular С15-С17 hydrocarbons is of the technological nature since these hydrocarbons are the main constituents of oils applied for pressurization of apparatus used for synthesis of NaNO3

Water Reduction and Dihydrogen Addition in Aqueous Conditions With ansa-Phosphinoborane

Ortho -phenylene-bridged phosphinoborane (2,6-Cl 2 Ph) 2 B-C 6 H 4 -PCy 2 1 was synthesized in three steps from commercially available starting materials. 1 reacts with H 2 or H 2 O under mild conditions to form corresponding zwitterionic phosphonium borates 1-H 2 or 1-H 2 O . NMR studies revealed both reactions to be remarkably reversible. Thus, when exposed to H 2 , 1-H 2 O partially converts to 1-H 2 even in the presence of multiple equivalents of water in the solution. The addition of parahydrogen to 1 leads to nuclear spin hyperpolarization both in dry and hydrous solvents, confirming the dissociation of 1-H 2 O to free 1 . These observations were supported by computational studies indicating that the formation of 1-H 2 and 1-H 2 O from 1 are thermodynamically favored. Unexpectedly, 1-H 2 O can release molecular hydrogen to form phosphine oxide 1-O . Kinetic, mechanistic, and computational (DFT) studies were used to elucidate the unique ?umpolung? water reduction mechanism.Peer reviewe

Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst

Parahydrogen-induced polarization is a nuclear spin hyperpolarization technique that can provide strongly enhanced NMR signals for catalytic hydrogenation reaction products and intermediates. Among other matters, this can be employed to study the mechanisms of the corresponding chemical transformations. Commonly, noble metal complexes are used for reactions with parahydrogen. Herein, we present a PHIP study of metal-free imine hydrogenations catalyzed by the ansa-aminoborane catalyst QCAT. We discuss the reaction mechanism by showing the pairwise nature of the initial hydrogen activation step that leads to the formation of the negative net nuclear spin polarization of N-H hydrogen in the QCAT-H-2 intermediate, enabling the further transfer of parahydrogen-originating protons to the imine substrate with the accumulation of hyperpolarized amine products. Parahydrogen-induced polarization also demonstrates the reversibility of the catalytic cycle.Peer reviewe

Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst

Parahydrogen-induced polarization is a nuclear spin hyperpolarization technique that can provide strongly enhanced NMR signals for catalytic hydrogenation reaction products and intermediates. Among other matters, this can be employed to study the mechanisms of the corresponding chemical transformations. Commonly, noble metal complexes are used for reactions with parahydrogen. Herein, we present a PHIP study of metal-free imine hydrogenations catalyzed by the ansa-aminoborane catalyst QCAT. We discuss the reaction mechanism by showing the pairwise nature of the initial hydrogen activation step that leads to the formation of the negative net nuclear spin polarization of N-H hydrogen in the QCAT-H-2 intermediate, enabling the further transfer of parahydrogen-originating protons to the imine substrate with the accumulation of hyperpolarized amine products. Parahydrogen-induced polarization also demonstrates the reversibility of the catalytic cycle.Peer reviewe

ОПРЕДЕЛЕНИЕ ПРИМЕСЕЙ ОРГАНИЧЕСКИХ ВЕЩЕСТВ В НИТРАТЕ НАТРИЯ МЕТОДАМИ ГАЗОВОЙ ХРОМАТОГРАФИИ И ХРОМАТО-МАСС-СПЕКТРОМЕТРИИ

At the present time the impurities of metals and inorganic salts are normalized in sodium nitrate. The data on the content of organic substances impurities, which are the source of carbon in the prepared based on sodium nitrate optical materials, are not available. In this work, the identification and determination of hydrocarbon impurities in sodium nitrate was carried out with the use of highly sensitive and express methods of gas chromatography and gas chromatography–mass-spectrometry. For the pre-concentration of the impurities the methods of vapor-phase macro-extraction and of liquid phase micro-extraction were investigated. The use of the method of vapor-phase macro-extraction indicated the presence of the impurities of light С1-С4 hydrocarbons; the method of liquid phase micro-extraction made it possible to identify the impurities of semi-volatile С15-С17 hydrocarbons. It was found that in the process of vapor-phase macro-extraction, the decomposition of С15-С17 hydrocarbons takes place. That is why the reliable determination of hydrocarbon impurity composition is possible with the use of liquid phase micro-extraction. It was shown that the concentration of С15-С17 hydrocarbons in sodium nitrate is 2·10-6 - 2·10-5 mas. %. Accounting for the possible sources of hydrocarbon impurities in sodium nitrate, it is possible to state that the presence of the impurities of high molecular С15-С17 hydrocarbons is of the technological nature since these hydrocarbons are the main constituents of oils applied for pressurization of apparatus used for synthesis of NaNO3.Key words: sodium nitrate, impurity composition, hydrocarbons, gas chromatography, chromato-mass-spectrometryDOI: http://dx.doi.org/10.15826/analitika.2015.19.2.012В настоящее время в нитрате натрия нормируются примеси металлов и неорганических солей. Сведения о содержании примесей органических веществ, являющихся источником углерода в получаемых на основе нитрата натрия оптических материалах, отсутствуют. В данной работе проведены идентификация и количественное определение примесей углеводородов в нитрате натрия с использованием высокочувствительных и экспрессных методов газовой хроматографии и хромато-масс-спектрометрии. Для концентрирования примесей применены методы парофазной макроэкстракции и жидкофазной микроэкстракции. Применение метода парофазной макроэкстракции показало присутствие примесей легких углеводородов С1–С4, метод  жидкофазной микроэкстракции позволил выявить примеси труднолетучих веществ – высокомолекулярных углеводородов С15–С17. Установлено, что в процессе парофазной макроэкстракции происходит разложение примесей углеводородов С15 – С17. Поэтому надежное определение углеводородного примесного состава нитрата натрия возможно с применением жидкофазной микроэкстракции. Показано, что концентрация углеводородов С15 –С17  в нитрате натрия составляет 2×10-6 – 2×10-5 % мас. Источники примесей высокомолекулярных углеводородов С15–С17 в нитрате натрия связаны скорее всего с причинами   технологического характера, т.к. эти углеводороды являются основными составляющими масел, применяемых для герметизации аппаратуры, используемой при синтезе NaNO3.Ключевые слова: нитрат натрия, примесный состав, углеводороды, газовая хроматография, хромато-масс-спектрометрия(Russian)DOI: http://dx.doi.org/10.15826/analitika.2015.19.2.012 V.А. Krylov1, 2, Т.G. Sorochkina11G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, Nizhny Novgorod, Russian Federation2N.I. Lobachevski Nizhny Novgorod State University, Nizhny Novgorodрод, Russian Federatio

Parahydrogen-Induced Polarization in Hydrogenation Reactions Mediated by a Metal-Free Catalyst

We report nuclear spin hyperpolarization of various alkenes achieved in alkyne hydrogenations with parahydrogen over a metal-free hydroborane catalyst (HCAT). Being an intramolecular frustrated Lewis pair aminoborane, HCAT utilizes a non-pairwise mechanism of H-2 transfer to alkynes that normally prevents parahydrogen-induced polarization (PHIP) from being observed. Nevertheless, the specific spin dynamics in catalytic intermediates leads to the hyperpolarization of predominantly one hydrogen in alkene. PHIP enabled the detection of important HCAT-alkyne-H-2 intermediates through substantial H-1, B-11 and N-15 signal enhancement and allowed advanced characterization of the catalytic process.Peer reviewe

Origin of Stereoselectivity in FLP-Catalyzed Asymmetric Hydrogenation of Imines

Development of metal-free strategies for stereoselective hydrogenation of unsaturated substrates is of particular interest in asymmetric synthesis. The emerging chemistry of frustrated Lewis pairs offers a promising approach along this line as demonstrated by recent achievements. However, the stereocontrol elements in these reactions are not clearly recognized thus far. Herein, we analyze the origin of stereoinduction in direct hydrogenation of imines catalyzed by a set of chiral boranes. We use the tools of computational chemistry to describe the elementary steps of the catalytic cycle, and we pay special attention to the stereoselectivity-determining hydride transfer process. The enantioselectivities predicted by the applied computational approach are in very good agreement with previous experimental observations. We find that the stereoselectivity is governed by a thermodynamically less favored conformer of the borohydride intermediate and not by the experimentally observed form. The most favored hydride transfer transition states are stabilized by specific aryl-aryl and alkyl-aryl noncovalent interactions, which play an important role in stereoinduction. This computational insight is exploited in proposing additional borane variants to improve the enantioselectivity, which could be demonstrated experimentallyPeer reviewe

ИДЕНТИФИКАЦИЯ ПРИМЕСЕЙ В ТЕТРАКИС(ТРИФТОРФОСФИНЕ) НИКЕЛЯ С ИСПОЛЬЗОВАНИЕМ МЕТОДА ХРОМАТО-МАСС-СПЕКТРОМЕТРИИ

The impurity composition of the vapor and liquid phase of a commercial sample of tetrakis(trifluorophosphine) nickel (TTFN) obtained by the reaction of nickel and phosphorus (III) fluoride was carried out for the first time by the method of gas chromatography-mass spectrometry. The analysis was performed using Agilent 6890/5973N chromatograph-mass spectrometer with a quadrupole mass analyzer. A sampling system has been developed that allowed the analysis of the vapor and liquid phase of the substance. To separate the components of the mixture, a quartz capillary column DB-5MS 30 m × 0.32 mm × 0.25 μm with a fixed phase methylsiloxane, containing 5% phenyl groups, was used. It was shown that its use made it possible to separate the impurities in both the low and the high boiling point components with respect to the main component. The identification of impurities was carried out by comparing their experimental mass spectra, obtained in the mode of recording the total ion current in the 12-450 amu mass scanning range, with the NIST database. In the sample of TTFN, the impurities of phosphorus (III) fluoride, dichloromethane, benzene, and C6–C8 hydrocarbons were identified. The coefficients of similarity of the mass spectra of these substances with respect to the library materials were in the range of 0.85 – 0.98. The impurities of Ni(PF3)3(PF2C2H5) and Ni(PF3)3(PF2ОC2H5), whose mass spectra were absent from the NIST library, were also identified. The identification of these substances assumed that they were the products of the interaction of the substance - the base of the TTFN with the impurity components of the mixture. In this case, the mass spectra of these substances and TTFN were compared with the further reconstruction of their structure by the fragment ions. The mass spectra of Ni(PF3)4, Ni(PF3)3(PF2C2H5) and Ni(PF3)3(PF2ОC2H5) were first obtained and described.Keywords: tetrakis(trifluorophosphine) nikel, impurities, identification, gas chromatography-mass spectrometry, capillary column DOI: http://dx.doi.org/10.15826/analitika.2018.22.3.010(Russian)А.Iu. Sozin, О.Iu. Chernova, T.G. Sorochkina, О. Iu. Troshin, A.D. Bulanov G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, Russian Federation, 603950, Nizhny Novgorod, Tropinina Str.,49 Методом хромато-масс-спектрометрии впервые проведено исследование примесного состава паровой и жидкой фазы коммерческого образца тетракис(трифторфосфина) никеля (ТТФН), полученного по реакции никеля и фторида фосфора (III). Анализ проводили с использованием хромато-масс-спектрометра Agilent 6890/5973N с квадрупольным масс-анализатором. Разработана система пробоотбора, позволяющая проводить анализ паровой и жидкой фазы вещества. Для разделения компонентов смеси использовали кварцевую капиллярную колонку DB-5MS 30 м × 0.32 мм × 0.25 мкм с неподвижной фазой метилсилоксаном, содержащем 5 % фенильных групп. Показано, что ее применение позволяет разделять примеси в ТТФН как с низкими, так и с достаточно высокими относительно основного компонента температуры кипения. Идентификацию примесей проводили сравнением их экспериментальных масс-спектров, полученных в режиме регистрации полного ионного тока в диапазоне сканирования масс 12–450 а.е.м. с данными базы NIST. В образце ТТФН идентифицированы примеси фторида фосфора (III), дихлорметана, бензола, углеводородов С6–С8. Коэффициенты подобия масс-спектров этих веществ относительно библиотечных составили 0.85–0.98. Идентифицированы примеси Ni(PF3)3(PF2C2H5) и Ni(PF3)3(PF2ОC2H5), масс-спектры которых отсутствует в библиотеке NIST. Идентификация этих веществ проведена, исходя из предположения, что они являются продуктами взаимодействия  вещества – основы ТТФН с примесными компонентами смеси. Для этого сравнивали масс-спектры этих веществ и ТТФН с дальнейшим восстановлением их структуры по осколочным ионам. Впервые получены и описаны масс-спектры Ni(PF3)4, Ni(PF3)3(PF2C2H5) и Ni(PF3)3(PF2ОC2H5).Ключевые слова: тетракис(трифторфосфин) никеля, примеси, идентификация, хромато-масс-спектрометрия, капиллярная колонкаDOI: http://dx.doi.org/10.15826/analitika.2018.22.3.01