Zoltán Benkő

Sodium phosphaethynolate, Na(OCP), as a “P” transfer reagent for the synthesis of N-heterocyclic carbene supported P3 and PAsP radicals

Sodium phosphaethynolate, Na(OCP), reacts as a P− transfer reagent with the imidazolium salt [DippNHC–H][Cl] [DippNHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] to give the parent phosphinidene–carbene adduct, DippNHCPH, with the loss of CO. In a less atom economic reaction, the cage compound, P7(TMS)3 (TMS = SiMe3) reacts likewise with the imidazolium salt to yield DippNHCPH thereby giving two entry points into parent phosphinidene-based chemistry. From the building block DippNHCPH, the carbene-supported P3 cation [(DippNHC)2(μ-P3)][Cl] was rationally synthesized using half an equivalent of PCl3 in the presence of DABCO (1,4-diazabicyclo[2.2.2]octane). The corresponding arsenic analogue, [(DippNHC)2(μ-PAsP)][Cl], was synthesized in the same manner using AsCl3. The reduction of both [(DippNHC)2(μ-P3)][Cl] and [(DippNHC)2(μ-PAsP)][Cl] into their corresponding neutral radical species was achieved simply by reducing the compounds with an excess of magnesium. This allowed the electronic structures of the compounds to be investigated using a combination of NMR and EPR spectroscopy, X-ray crystallography, and computational studies. The findings of the investigation into (DippNHC)2(μ-P3) and (DippNHC)2(μ-PAsP) reveal the central pnictogen atom in both cases as the main carrier of the spin density (∼60%), and that they are best described as the P3 or PAsP analogues of the elusive allyl radical dianion. The phosphorus radical was also able to undergo a cycloaddition with an activated acetylene, followed by an electron transfer to give the ion pair [(DippNHC)2(μ-P3)][P3(C(COOMe))2].

Sodium Phosphaethynolate as a Building Block for Heterocycles

Phosphorus-containing heterocycles have evolved from laboratory curiosities to functional components, such as ligands in catalytically active metal complexes or molecular constituents in electronic devices. The straightforward synthesis of functionalized heterocycles on a larger scale remains a challenge. Herein, we report the use of the phosphaethynolate (OCP)(-) anion as a building block for various sterically unprotected and functionalized hydroxy substituted phosphorus heterocycles. Because the resulting heterocycles are themselves anions, they are building blocks in their own right and allow further facile functionalization. This property may be of interest in coordination chemistry and material science.

Synthesis and Characterization of Terminal [Re(XCO)(CO)2(triphos)] (X=N, P): Isocyanate versus Phosphaethynolate Complexes

The terminal rhenium(I) phosphaethynolate complex [Re(PCO)(CO)(2)(triphos)] has been prepared in a salt metathesis reaction from Na(OCP) and [Re(OTf)(CO)(2)(triphos)]. The analogous isocyanato complex [Re(NCO)(CO)(2)(triphos)] has been likewise prepared for comparison. The structure of both complexes was elucidated by X-ray diffraction studies. While the isocyanato complex is linear, the phosphaethynolate complex is strongly bent around the pnictogen center. Computations including natural bond orbital (NBO) theory, natural resonance theory (NRT), and natural population analysis (NPA) indicate that the isocyanato complex can be viewed as a classic Werner-type complex, that is, with an electrostatic interaction between the Re(I) and the NCO group. The phosphaethynolate complex [Re(P=C=O)(CO)(2)(triphos)] is best described as a metallaphosphaketene with a Re(I)-phosphorus bond of highly covalent character.

Redox‐Triggered Reversible Interconversion of a Monocyclic and a Bicyclic Phosphorus Heterocycle

Molecules which change their structures significantly and reversibly upon an oxidation or reduction process have potential as future components of smart materials. A prerequisite for such an application is that the molecules should undergo the redox-coupled transformation within a reasonable electrochemical window and lock into stable redox states. Sodium phosphaethynolate reacts with two equivalents of dicyclohexylcarbodiimide (DCC) to yield an anionic, imino-functionalized 1,3,5-diazaphosphinane [3 a](-). The oxidation of this anion with elemental iodine causes an intramolecular rearrangement reaction to give a bicyclic 1,3,2-diazaphospholenium cation [6](+). This umpolung of electronic properties from non-aromatic to highly aromatic is reversible, and the cation [6](+) is reduced with elemental magnesium to reform the 1,3,5-diazaphosphinanide anion [3 a](-). Theoretical calculations suggest that phosphinidene species are involved in the rearrangement processes.

N-Heterocyclic Carbenes as Promotors for the Rearrangement of Phosphaketenes to Phosphaheteroallenes: A Case Study for OCP to OPC Constitutional Isomerism.

The concept of isomerism is essential to chemistry and allows defining molecules with an identical composition but different connectivity (bonds) between their atoms (constitutional isomers) and/or a different arrangement in space (stereoisomers). The reaction of phosphanyl ketenes, (NHP)-P=C=O (NHP=N-heterocyclic phosphenium) with N-heterocyclic carbenes (NHCs) leads to phosphaheteroallenes (NHP)-O-P=C=NHC in which the PCO unit has been isomerized to OPC. Based on the isolation of several intermediates and DFT calculations, a mechanism for this fundamental isomerisation process is proposed.

Donor-free phosphenium-metal(0)-halides with unsymmetrically bridging phosphenium ligands.

Reactions of (cod)MCl2 (cod = 1,5 cyclooctadiene, M = Pd, Pt) with N-heterocyclic secondary phosphines or diphosphines produced complexes [(NHP)MCl]2 (NHP = N-heterocyclic phosphenium). The Pd complex was also accessible from a chlorophosphine precursor and Pd2(dba)3. Single-crystal X-ray diffraction studies established the presence of dinuclear complexes that contain μ-bridging NHP ligands in an unsymmetrical binding mode and display a surprising change in metal coordination geometry from distorted trigonal (M = Pd) to T-shaped (M = Pt). DFT calculations on model compounds reproduced these structural features for the Pt complex but predicted an unusual C2v-symmetric molecular structure with two different metal coordination environments for the Pd species. The deviation between this structure and the actual centrosymmetric geometry is accounted for by the prediction of a flat energy hypersurface, which permits large distortions in the orientation of the NHP ligands at very low energetic cost. The DFT results and spectroscopic studies suggest that the title compounds should be described as phosphenium-metal(0)-halides rather than conventional phosphido complexes of divalent metal cations and indicate that the NHP ligands receive net charge donation from the metals but retain a distinct cationic character. The unsymmetric NHP binding mode is associated with an unequal distribution of σ-donor/π-acceptor contributions in the two M-P bonds. Preliminary studies indicate that reactions of the Pd complex with phosphine donors provide a viable source of ligand-stabilized, zerovalent metal atoms and metal(0)-halide fragments.

A Convenient Synthesis of 1,2,4‐ and 1,3,4‐Azadiphospholes

A new synthetic route to functionalized neutral and anionic azadiphospholes from easily accessible starting materials is described. Equimolar reaction of Na(OCP) and N-(2,6-dimethylphenyl)pivalimidoyl chloride 2 a cleanly affords the imidoxy-functionalized 1,2,4-azadiphosphole 3 a. Using Na(OCP) and imidoyl chloride in a 2:1 ratio leads to an anionic four-membered ring Na[4 a], which has been structurally characterized. During 16 h at room temperature, Na[4 a] rearranges to the anionic 1,3,4-azadiphospholide Na[5 a] with release of carbon monoxide. Applying the more sterically demanding N-(2,6-diisopropylphenyl)pivalimidoyl chloride allows isolation of the 1,3,4-azadiphospholide Na[5 b] in good yield (>70 %). Possible mechanisms leading to the new isomeric azadiphospholides have been investigated with the aid of high-level composite calculations.

Assembly and Disassembly of a Metastable Bis-phosphine-Based Copper(I) Helicate

The synthesis of chelate complexes from bidentate ligands that are formed by self-assembly of simpler monodentate fragments has recently been developed as a new concept with considerable potential for application in catalysis studies. Ligand assembly can be accomplished through direct pairing of fragments with complementary binding motifs, or by fixing two ligand fragments to a suitable template. The individual components normally join through noncovalent interactions such as hydrogen bonding, formation of coordinative bonds, or electrostatic attraction (e.g. anion sequestering), which allow rapid assembly and disassembly of the aggregates and the correction of improper connections. On the other hand, the same type of self-assembly processes was also successfully employed to generate supramolecular architectures such as molecular polygons and three-dimensional cages or polyhedra, and even chiral helicates. Controlled formation of these supramolecular architectures is normally accomplished either by mixing of preformed ligand strands with suitable metal ions or by hierarchical assembly of simple coordination compounds with additional binding sites or reactive units, which is triggered by, for example, displacement of a labile ligand, a redox reaction, or addition of suitable spacers. In both methodologies, constituents and final product are in equilibrium, and the supramolecular architecture is considered to form the most stable aggregate under the chosen reaction conditions. We showed previously that catechol phosphine 1 assembles with various Lewis acids to give template-based bidentate phosphine ligands that readily formed metal complexes. In particular, reaction with borates gave an anionic bidentate phosphine [HNEt3]2, which was easily converted into a chelate complex 3 a upon treatment with silver triflate (Scheme 1). In extending this chemistry to the other uni-

Annulated 1,3,4-azadiphospholides: Heterocycles with widely tunable optical properties

Phosphorus heterocycles find applications in the synthesis of π-conjugated compounds and as precursors for optoelectronic materials such as organic light-emitting diodes (OLEDs), electronic switches, and transistors. A high-yield, one-pot synthesis of anionic annulated 1,3,4-azadiphospholides from Na(OCP) and 2-chloropyridines is presented. The synthesis proceeds without the use of transition metals and tolerates a wide range of substrates. Cyano-substituted compounds are especially deeply colored and have absorption maxima which range from λmax =525 to 596 nm. The optical properties are dominated by the spatial separation of an electron acceptor and donor unit within one molecule (push-pull chromophore). The anionic 1,3,4-azadiphospholides are silylated to neutral siloxy compounds with a strong blue-shifted absorption. This reaction can be reversed by addition of fluoride ions, which allows fluoride ions to be detected in optically low concentrations.

Substituent effect on the aromaticity of the silolide anion

The effect of the substituents on the planarity and aromaticity of the silolide anion was studied computationally. It was revealed that π-electron acceptor groups (e.g., silyl or trimethylsilyl) at the α position of the ring reduce substantially the inversion barrier about the central silicon increasing the aromaticity according to isomeric stabilization energy (ISE) and NICS values. In the planar and highly aromatic silolide anions, the mesomeric structures with the largest weight exhibit a Si=C double bond and a negative charge which is located at the α or β carbons based on NRT calculations. 2,5-disilylsilacyclopentadienides coordinated by naked Li+ have planar minima, however, further coordination by THF molecules (as in a solution) reduces somewhat the flattening of the silicon pyramid. NMR calculations were carried out to understand the connection between the 29Si chemical shift and the aromaticity of the ring. It was revealed that the commonly accepted charge transfer–chemical shift relationship is strongly influenced by the substituents and the counter cation. THF complexation of the Li counter cation has a small influence on the NMR shift.