Fabian Dielmann

A Crystalline Singlet Phosphinonitrene: A Nitrogen Atom–Transfer Agent

N on P Nitrogen atoms form strong, relatively unreactive triple bonds with themselves (in N2) and with carbon (in cyanide and nitriles). In contrast, binding to transition metals often leaves an otherwise naked nitrogen center more prone to reactivity. Dielmann et al. (p. 1526) prepared a compound with nitrogen bound to divalent phosphorus, which acted more like a metal than a light element. Although the compound, formally a nitrene, was sufficiently stable to isolate at room temperature and characterize by x-ray diffraction, it transferred the nitrogen efficiently to unsaturated carbon compounds. A phosphorus fragment supports an otherwise uncoordinated nitrogen atom in a stable motif previously seen only with metals. A variety of transition metal–nitrido complexes (metallonitrenes) have been isolated and studied in the context of modeling intermediates in biological nitrogen fixation by the nitrogenase enzymes and the industrial Haber-Bosch hydrogenation of nitrogen gas into ammonia. In contrast, nonmetallic nitrenes have so far only been spectroscopically observed at low temperatures, despite their intermediacy in a range of organic reactions. Here, we report the synthesis of a bis(imidazolidin-2-iminato)phosphinonitrene, which is stable at room temperature in solution and can even be isolated in the solid state. The bonding between phosphorus and nitrogen is analogous to that observed for metallonitrenes. We also show that this nitrido phosphorus derivative can be used to transfer a nitrogen atom to organic fragments, a difficult task for transition metal–nitrido complexes.

A Nano‐sized Supramolecule Beyond the Fullerene Topology

The reaction of [CpBnFe(η5-P5)] (1) (CpBn=η5-C5(CH2Ph)5) with CuI selectively yields a novel spherical supramolecule (CH2Cl2)3.4@[(CpBnFeP5)12{CuI}54(MeCN)1.46] (2) showing a linkage of the scaffold atoms which is beyond the Fullerene topology. Its extended CuI framework reveals an outer diameter of 3.7 nm—a size that has not been reached before using five-fold symmetric building blocks. Furthermore, 2 shows a remarkable solubility in CH2Cl2, and NMR spectroscopy reveals that the scaffold of the supramolecule remains intact in solution. In addition, a novel 2D polymer [{CpBnFe(η5-P5)}2{Cu6(μ-I)2(μ3-I)4}]n (3) with an uncommon structural motif was isolated. Its formation can be avoided by using a large excess of CuI in the reaction with 1.

Reversible Carbon Dioxide Binding by Simple Lewis Base Adducts with Electron-Rich Phosphines

For the efficient utilization of carbon dioxide as feedstock in chemical synthesis, low-energy-barrier CO2 activation is a valuable tool. We report a metal-free approach to reversible CO2 binding under mild conditions based on simple Lewis base adducts with electron-rich phosphines. Variable-temperature NMR studies and DFT calculations reveal almost thermoneutral CO2 binding with low-energy barriers or stable CO2 adduct formation depending on the phosphines donor ability. The most basic phosphine forms an air-stable CO2 adduct that was used as phosphine transfer agent, providing a convenient access to transition-metal complexes with highly electron-rich phosphine ligands relevant to catalysis.

Tunable Porosities and Shapes of Fullerene‐Like Spheres

The formation of reversible switchable nanostructures monitored by solution and solid-state methods is still a challenge in supramolecular chemistry. By a comprehensive solid state and solution study we demonstrate the potential of the fivefold symmetrical building block of pentaphosphaferrocene in combination with CuI halides to switch between spheres of different porosity and shape. With increasing amount of CuX, the structures of the formed supramolecules change from incomplete to complete spherically shaped fullerene-like assemblies possessing an Ih-C80 topology at one side and to a tetrahedral-structured aggregate at the other. In the solid state, the formed nano-sized aggregates reach an outer diameter of 3.14 and 3.56 nm, respectively. This feature is used to reversibly encapsulate and release guest molecules in solution.

Crystalline, Lewis base-free, cationic phosphoranimines (iminophosphonium salts).

Cationic phosphoranimines have been postulated as intermediates in phosphazene polymerization chemistry. However, the high electrophilicity of the phosphorus center has so far prevented their characterization. Here, we report the synthesis of two Lewis base-free iminophosphonium salts, obtained by reaction of a stable phosphinonitrene with methyl trifluoromethanesulfonate and trifluoromethanesulfonic acid. These cationic species were characterized by NMR spectroscopy and single-crystal X-ray diffraction analysis. Using 4-(dimethylamino)pyridine, a corresponding Lewis-base adduct has also been isolated.

Imidazolin‐2‐ylidenaminophosphines as Highly Electron‐Rich Ligands for Transition‐Metal Catalysts

A variety of chemical transformations benefit from the use of strong electron-donating ancillary ligands, such as alkylphosphines or N-heterocyclic carbenes when electron-rich metal centers are required. Herein, we describe a facile and highly modular access to monodentate and bidentate imidazolin-2-ylidenamino-substituted phosphines. Evaluation of the phosphines electronic properties substantiate that the formal replacement of alkyl or aryl groups by imidazolin-2-ylidenamino groups dramatically enhance their donor ability beyond that of alkylphosphines and even N-heterocyclic carbenes. The new phosphines have been coordinated onto palladium(II) centers, and the beneficial effect of the novel substitution patterns has been explored by using the corresponding complexes in the palladium-catalyzed Suzuki-Miyaura cross-coupling reaction of non-activated aryl chloride substrates.

Isolation of Bridging and Terminal Coinage Metal–Nitrene Complexes

Transition metal complexes featuring a metal-nitrogen multiple bond have been widely studied due to their implication in dinitrogen fixation and catalytic nitrogen-carbon bond formation. Terminal copper- and silver-nitrene complexes have long been proposed to be the key intermediates in aziridination and amination reactions using azides as the nitrogen source. However, due to their high reactivity, these species have eluded isolation and spectroscopic characterization even at low temperatures. In this paper we report that a stable phosphinonitrene reacts with coinage metal trifluoromethanesulfonates, affording bridging and terminal copper- and silver-nitrene complexes, which are characterized by NMR spectroscopy and single crystal X-ray diffraction analysis.

Intramolecular phosphorus-phosphorus bond formation within a Co2P4 core.

The reduction of [(Cp‴Co)2(μ,η(2:2)-P2)2] (Cp‴ = 1,2,4-tBu3C5H2) with the samarocenes, [(C5Me4R)2Sm(THF)n] (R = Me or n-propyl), gives [(Cp‴Co)2P4Sm(C5Me4R)2]. This is the first example of an intramolecular P-P coupling in a polyphosphide complex upon reduction of the transition metal. The formation of the P-P bond is not a result of the direct reduction of the phosphorus atoms but is induced by a rearrangement of the positive charges between the metal atoms.

Tris(imidazolin‐2‐ylidenamino)phosphine: A crystalline phosphorus(III) superbase that splits carbon dioxide

We report the synthesis and remarkable properties of the phosphine P(NIiPr)3 (NIiPr=1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidenamino), a crystalline solid accessible through a short and scalable route. Evaluation of the electron-donor properties reveals a Tolman electronic parameter (TEP) value of 2029.7 cm-1 for the new phosphine that is significantly lower than those of all known phosphines or even N-heterocyclic carbenes. Moreover, P(NIiPr)3 is more basic [pKBH+ (MeCN)=38.8] than Verkades proazaphosphatranes, thus being the strongest reported nonionic phosphorus(III) superbase. The coordination chemistry of the new phosphine towards different metal centers has been explored, and due to its unique electron-releasing character, the phosphine is capable of capturing and cleaving the CO2 molecule.

Reactivity of a Stable Phosphinonitrene towards Small Molecules

The room-temperature stable phosphinonitrene 1 undergoes a thermal rearrangement into heterocycle 2 through a process involving a nitrene insertion into a CH bond. In the presence of acetonitrile, a nitrene-acetonitrile adduct has been isolated; then it first rearranges into a ketenimine and subsequently into a rare example of diazaphosphete. Compound 1 also splits water, carbon dioxide, carbon disulfide, and elemental sulfur, although it reacts with white phosphorus, leading to a P5 N cluster formally resulting from the insertion of the PN moiety into a P-P edge of the P4 tetrahedron.