RESUMO
The synthesis and properties of photoswitchable electron-rich phosphines containing N-heterocyclic imines equipped with a photochromic dithienylethene unit are reported. Heteronuclear NMR spectroscopy and UV/vis studies reveal that the imine substituents undergo reversible electrocyclic ring-closing and ring-opening reactions upon exposure to UV and visible light, respectively. The photoisomerization alters the electron-donating ability of the phosphines by up to ΔTEP = 8 cm-1.
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Isolable singlet carbenes are among the most important tools in chemistry, but generally require the interaction of two substituents with the electron deficient carbon atom. We herein report a synthetic approach to monosubstituted phosphinocarbenes via deprotonation of hitherto unknown diprotic terminal methylene phosphonium ions. Two methylene phosphonium salts bearing bulky N-heterocyclic imine substituents at the phosphorus atom were isolated and fully characterized. Deprotonation studies indicate the formation of transient monosubstituted carbenes that undergo intermolecular cycloadditions or intramolecular Buchner ring expansion to afford a cycloheptatriene derivative. The reaction mechanism of the latter transformation was elucidated using DFT calculations, which reveal the ambiphilic nature of the phosphinocarbene enabling the insertion into the aromatic C-C bond. Additional computational studies on the role of substituent effects are presented.
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1,4-Diphosphabarrelenes are bicyclic diphosphines relevant, for example, for the generation of polymetallic coordination compounds. However, current synthetic protocols either suffer from low yields or require multiple reaction steps. Herein, we report the one-step synthesis of pyrrole-based 1,4-diphosphabarrelenes that are obtained in very good yields from the reaction of 1,2,5-trimethylpyrrole with 1,2-bis(dichlorophosphino)ethane or 1,2-bis(dichlorophosphino)benzene. The new caged diphosphines are strong donor ligands and act as bridging ligand in nickel(0), rhodium(I), iridium(I) and copper(I) coordination compounds.
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The thermolysis of Cp'''Ta(CO)4 with white phosphorus (P4 ) gives access to [{Cp'''Ta}2 (µ,η2 : 2 : 2 : 2 : 1 : 1 -P8 )] (A), representing the first complex containing a cyclooctatetraene-like (COT) cyclo-P8 ligand. While ring sizes of n >6 have remained elusive for cyclo-Pn structural motifs, the choice of the transition metal, co-ligand and reaction conditions allowed the isolation of A. Reactivity investigations reveal its versatile coordination behaviour as well as its redox properties. Oxidation leads to dimerization to afford [{Cp'''Ta}4 (µ4 ,η2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1 -P16 )][TEF]2 (4, TEF=[Al(OC{CF3 }3 )4 ]- ). Reduction, however, leads to the fission of one P-P bond in A followed by rapid dimerization to form [K@[2.2.2]cryptand]2 [{Cp'''Ta}4 (µ4 ,η2 : 2 : 2 : 2 : 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1 -P16 )] (5), which features an unprecedented chain-type P16 ligand. Lastly, A serves as a P2 synthon, via ring contraction to the triple-decker complex [{Cp'''Ta}2 (µ,η6 : 6 -P6 )] (B).
RESUMO
Phosphino(silyl)carbenes are known for their ineptitude to form transition metal complexes. We describe the synthesis of phosphino(silyl)carbenes bearing N-heterocyclic imine groups and show that these isolable, crystalline carbenes readily form stable copper(I) and gold(I) complexes. The solid-state structures of the free carbenes and their transition metal complexes are reported.
RESUMO
Fluorinated group 16 moieties are attractive building blocks in synthetic chemistry but only few synthetic methods are available to prepare them. Herein, we report a new oxidative fluorination reagent capable of stabilizing reactive fluorinated anions. It consists of an SF5 - anion and a chemically inert phosphonium cation and is exceptionally thermally stable. Accordingly, it was used to generate the SeF5 - and TeF5 - anions from the elemental chalcogens and to prepare the unknown tetrafluoro(phenyl)-λ5 -selenate PhSeF4 - and -tellurate PhTeF4 - from the corresponding diphenyl dichalcogenides. In addition, we show that further derivatization of [PhTeF4 ]- by oxidation to trans-PhTeF4 O- and subsequent alkylation gives access to a new class of trans-(alkoxy)(phenyl)tetrafluoro-λ6 -tellanes (trans-PhTeF4 OR), thus providing an approach to introduce the functional group into organic molecules.
RESUMO
N-Heterocyclic carbenes (NHCs) are commonly prepared by deprotonation of azolium salts using strong anionic bases. This reaction is often unselective, yielding alkali metal NHC complexes or dimerized NHCs. Alternatively, free NHCs are obtained by the dechlorination of 2-chloroazolium salts using electron-rich phosphines. PPh3 , PCy3 , and PtBu3 are unsuitable for Cl+ abstraction, while the sterically encumbered tris(1,3-tert-butylimidazolidin-2-ylidenamino)phosphine 1 selectively removes Cl+ from 2-chloroazolium salts. Since bulky 1 does not bind to metal complexes, it was used for the preparation of NHC complexes via inâ situ Cl+ abstraction from 2-chloroazolium salts. The dechlorination was employed for the site-selective monometallation with IrI , IrIII , RhI , RhIII , and RuII of a bis-NHC precursor composed of a 2-chlorobenzimidazolium and a 2-chlorobenzimidazole group, followed by the preparation of the heterobimetallic IrIII /PdII complex [18](BF4 )2 by a dechlorination/oxidative addition reaction sequence.
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We report the synthesis and properties of the much sought-after tris(1,1,3,3-tetramethylguanidinyl) phosphine P(tmg)3 , a crystalline, superbasic phosphine accessible through a short and scalable procedure from the cheap and commercially available bulk chemicals 1,1,3,3-tetramethylguanidine, tris(dimethylamino)-phosphine and phosphorus trichloride. The new phosphine exhibits exceptional electron donor properties and readily forms transition metal complexes with gold(I), palladium(II) and rhodium(I) precursors. The formation of zwitterionic Lewis base adducts with carbon dioxide and sulfur dioxide was explored. In addition, the complete series of phosphine chalcogenides was prepared from the reaction of P(tmg)3 with N2 O and the elemental chalcogens.
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Using light as an external stimulus to alter the reactivity of Lewis bases is an intriguing tool for controlling chemical reactions. Reversible photoreactions associated with pronounced reactivity changes are particularly valuable in this regard. We herein report the first photoswitchable nitrogen superbases based on guanidines equipped with a photochromic dithienylethene unit. The resulting N-heterocyclic imines (NHIs) undergo reversible, near quantitative electrocyclic isomerization upon successive exposure to UV and visible irradiation, as demonstrated over multiple cycles. Switching between the ring-opened and ring-closed states is accompanied by substantial pKa shifts of the NHIs by up to 8.7 units. Since only the ring-closed isomers are sufficiently basic to activate CO2 via the formation of zwitterionic Lewis base adducts, cycling between the two isomeric states enables the light-controlled capture and release of CO2 .
RESUMO
While the metathesis reaction between alkynes and thiocarbonyl compounds has been thoroughly studied, the reactivity of alkynes with isoelectronic main group R2E=S compounds is rarely reported and unknown for [R2P=S]+ analogues. We show that thiophosphonium ions, which are the isoelectronic phosphorus congeners to thiocarbonyl compounds, undergo [2 + 2]-cycloaddition reactions with different alkynes to generate 1,2-thiaphosphete ions. The four-membered ring species are in an equilibrium state with the corresponding P=C-C=S heterodiene structure and thus undergo hetero-Diels-Alder reactions with acetonitrile. Heteroatom and substituent effects on the energy profile of the 1,2-thiaphosphete formation were elucidated by means of quantum chemical methods.
RESUMO
Novel multistimuli-responsive phosphine ligands comprising a redox-active [3]dioxaphosphaferrocenophane backbone and a P-bound imidazolin-2-ylidenamino entity that allows switching by protonation are reported. Investigation of the corresponding metal complexes and their redox behaviour are reported and show the sensitivity of the system towards protonation and metal coordination. The experimental findings are supported by DFT calculations. Protonation and oxidation events are applied in Rh-catalysed hydrosilylations and demonstrate a remarkable influence on reactivity and/or selectivity.
RESUMO
Phosphorus(v) sulfide and Lawesson's reagent are commonly used thionating reagents which are considered to operate after dissociation into highly reactive dithiophosphorane fragments. We report the synthesis and properties of a monomeric thiophosphonium ion [R2P[double bond, length as m-dash]S]+. The highly electrophilic species reacts with carbonyls in oxo-for-sulfido exchange reactions at room temperature and undergoes phosphorus-chalcogen bond metathesis reactions with phosphine chalcogenides.
RESUMO
While the metathesis reaction between alkynes and carbonyl compounds is an important tool in organic synthesis, the reactivity of alkynes with isoelectronic main-group R2EâO compounds is unexplored. Herein, we show that oxophosphonium ions, which are the isoelectronic phosphorus congeners to carbonyl compounds, undergo [2 + 2] cycloaddition reactions with different alkynes to generate 1,2-oxaphosphete ions, which were isolated and structurally characterized. The strained phosphorus-oxygen heterocycles open to the corresponding heterodiene structure at elevated temperature, which was used to generate six-membered phosphorus heterocycles via hetero Diels-Alder reactions. Insights into the influence of the substituents at the phosphorus center on the energy profile of the oxygen atom transfer reaction were obtained by quantum-chemical calculations.
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A systematic study on the reactivity of the triple-decker complex [(Cp'''Co)2 (µ,η4 :η4 -C7 H8 )] (A) (Cp'''=1,2,4-tritertbutyl-cyclopentadienyl) towards sandwich complexes containing cyclo-P3 , cyclo-P4 , and cyclo-P5 ligands under mild conditions is presented. The heterobimetallic triple-decker sandwich complexes [(Cp*Fe)(Cp'''Co)(µ,η5 :η4 -P5 )] (1) and [(Cp'''Co)(Cp'''Ni)(µ,η3 :η3 -P3 )] (3) (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) were synthesized and fully characterized. In solution, these complexes exhibit a unique fluxional behavior, which was investigated by variable temperature NMR spectroscopy. The dynamic processes can be blocked by coordination to {W(CO)5 } fragments, leading to the complexes [(Cp*Fe)(Cp'''Co)(µ3 ,η5 :η4 :η1 -P5 ){W(CO)5 }] (2 a), [(Cp*Fe)(Cp'''Co)(µ4 ,η5 :η4 :η1 :η1 -P5 ){(W(CO)5 )2 }] (2 b), and [(Cp'''Co)(Cp'''Ni)(µ3 ,η3 :η2 :η1 -P3 ){W(CO)5 }] (4), respectively. The thermolysis of 3 leads to the tetrahedrane complex [(Cp'''Ni)2 (µ,η2 :η2 -P2 )] (5). All compounds were fully characterized using single-crystal X-ray structure analysis, NMR spectroscopy, mass spectrometry, and elemental analysis.
RESUMO
Electron-rich tertiary phosphines are valuable species in chemical synthesis. However, their broad application as ligands in catalysis and reagents in stoichiometric reactions is often limited by their costly synthesis. Herein, we report the synthesis and properties of a series of phosphines with 1-alkylpyridin-4-ylidenamino and 1-alkylpyridin-2-ylidenamino substituents that are accessible in a very short and scalable route starting from commercially available aminopyridines and chlorophosphines. The determination of the Tolman electronic parameter (TEP) value reveals that the electron donor ability can be tuned by the substituent pattern at the aminopyridine backbone and it can exceed that of common alkylphosphines and N-heterocyclic carbenes. The potential of the new phosphines as strong nucleophiles in phosphine-mediated transformations is demonstrated by the formation of Lewis base adducts with CO2 and CS2 . In addition, the coordination chemistry of the new phosphines towards CuI , AuI , and PdII metal centers has been explored, and a convenient procedure to introduce the most basic phosphine into metal complexes starting from air-stable phosphonium salt is described.
RESUMO
Three-coordinate main group Lewis acids are exceedingly important reagents in chemical synthesis. In contrast to the well-established chemistries of neutral group 13 and cationic group 14 species, isoelectronic group 15 element dications are unknown. In this work, we use stabilizing N-heterocyclic imine substituents to isolate and characterize phosphorandiylium dications ([R3P]2+) and show that the electrophilicity at the phosphorus atoms is controlled by the π-electron-donating ability of these subtituents. Structural, spectroscopic and theoretical results reveal that the phosphorus dications adopt a perfectly trigonal-planar geometry with the electron-deficient phosphorus centres being well separated from the borate anions. The reactivity of the dications reveal their exceptional Lewis acidity at phosphorus; the adjacent nitrogen atoms, however, are weakly basic, resulting in transformations such as chloride ion abstraction from Me3SiCl and the selective monodefluorination of trifluoromethyl groups.
RESUMO
Stimuli-responsive ancillary ligands are valuable tools to control the activity and selectivity of transition-metal catalysts. The synthesis and characterization of a series of metal complexes containing phosphines with proton-responsive imidazolin-2-ylidenamino substituents are reported. Determination of the ligand-donor properties revealed that protonation of each substituent increases the Tolman electronic parameter (TEP) of the phosphine by 22â cm-1 , hence allowing for switching of the electron-donor power of phosphine 2 within an unprecedented range (ΔTEP=43.4â cm-1 ).
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The particularly basic phosphines 1a-c readily form isolable, zwitterionic Lewis base adducts with SO2 that were fully characterized including by X-ray diffraction studies. Computational and reactivity studies show that these adducts readily release SO at room temperature driven by the formation of the corresponding phosphine oxides.
RESUMO
Oxophosphonium ions (R2 P=O)+ are fascinating chemical intermediates related to the well-known acylium cations (RC=O)+ , and comprise a tricoordinate phosphorus(V) center with a phosphorus-oxygen double bond. Here, we report the synthesis of two oxophosphonium ions stabilized by bulky imidazolin-2-imine and imidazolin-2-olefin substituents attached to phosphorus. The novel species were characterized by NMR spectroscopy and single-crystal X-ray diffraction analysis, and the bonding situation was probed by DFT calculations. Determination of the acceptor number and the fluoride ion affinity revealed that the choice of the substituents has a strong influence on the electrophilicity of the phosphorus center. Additionally, the formation of Lewis base adducts with pyridine derivatives and the reactivity with isopropyl alcohol was explored.
RESUMO
The development of new methods for the chemical activation of the extremely inert greenhouse gas sulfur hexafluoride (SF6 ) not only is of current environmental interest, but also offers new opportunities for applications of SF6 as a reagent in organic synthesis. We herein report the first nucleophilic activation of SF6 by Lewis bases, namely by phosphines, which results either in its complete degradation to phosphine sulfides and difluorophosphoranes or in the selective conversion of SF6 into a bench-stable, crystalline salt containing the SF5- anion. Quantum chemical calculations reveal a nucleophilic substitution mechanism (SN 2) for the initial fluorine abstraction from SF6 by the phosphine. Furthermore, a scalable one-pot procedure for the complete decomposition of SF6 into solid, nonvolatile products is presented based on cheap and commercially available starting materials.