Guest Editor Christian Müller presents an overview of the papers in this Cluster Issue commemorating the synthesis of the first stable low-coordinate organophosphorus compound in 1966.

This microreview describes the progress that has been made during the last few years in the synthesis, coordination chemistry, and application of donor-functionalized 3H-1,2,3,4-triazaphospholes, the phosphorus analogues of the well-known 1,2,3-triazoles. The latter class of compounds has attracted tremendous interest in various areas of chemistry due to their facile assembly through “click” chemistry. In contrast, the phosphorus-containing analogues have received much less attention, although their synthesis is fairly straightforward, starting from organic azides and phosphaalkynes, without the need for use of any catalyst. These fascinating low-coordinate phosphorus heterocycles have conjugated π systems with high degrees of aromaticity and possess rather high π density at their phosphorus atoms, due to significant N–C=P N⁺=C–P^– conjugation. It has been shown recently that [3+2] cycloaddition between functionalized organic azides and phosphaalkynes also allows the incorporation of additional donor substituents into specific positions of the phosphorus heterocycle, and both chelating and polydentate triazaphospholes are nowadays synthetically accessible. These compounds have been used to prepare the first coordination compounds based on triazaphospholes, making use of the chelate effect. The recently used strategies in this area facilitate, for the first time, synthetic access to a completely new set of triazaphospholes as well as their corresponding metal complexes, leading to a much broader scope for potential applications. As a very recent highlight, experimental and theoretical findings have established triazaphospholes as a new class of phosphorus-containing extended π systems for optoelectronic applications, because significant fluorescence emission of 2-pyridyl-functionalized triazaphosphole derivatives has been observed. New developments in the areas of homogeneous catalysis and materials sciences can consequently be foreseen in the near future.

The facile access to 3-bromo-2-pyrone allows the preparation of 6-bromo-2-trimethylsilyl-phosphinine by a [4+2] cycloaddition with Me₃Si-C≡P for the first time. The regioselectivity of this reaction could be verified by means of single crystal X-ray diffraction of the corresponding W⁰ complex. In the presence of ZnBr₂ and dppp (1,3-bis(diphenylphosphino)propane) as a bidentate ligand, the bromo-phosphinine quantitatively undergoes a Negishi cross-coupling reaction with PhLi that selectively leads to 6-phenyl-2-trimethylsilyl-phosphinine. This heterocycle could again be characterized by means of X-ray diffraction as a W⁰ complex. These results describe a new and convenient route to 2,6-disubstituted phosphinines that makes use of readily available starting materials.

Detailed studies on the reactivity of 2-(2′-pyridyl)-4,6-diphenylphosphinine (2) towards CF₃SO₃H and sulfur have been performed, and the results were compared with those for nonfunctionalized 2,4,6-triphenylphosphinine derivatives. Substantial differences between these heterocycles were observed, and the reaction products could be characterized crystallographically. The reactions of 2,4,6-triarylphosphinine sulfides with methanol led to different products, which could be characterized by NMR spectroscopy and X-ray crystal structure analysis. Interestingly, the outcomes of these transformations strongly depend on the presence of an additional donor functionality within the phosphorus heterocycle as well as the nature of the solvent and the reaction temperature. DFT calculations were performed to rationalize the different reaction pathways.

The development of a novel benzimidazole-derived bidentate P,N-ligand and its application in Ir-catalyzed hydrogenation is described. The ligand backbone was obtained through a one-pot tandem hydroformylation–cyclization sequence and the enantiomers of the generated alcohol were separated by chiral HPLC. By comparing the experimentally obtained CD spectra of the enantiomers with the simulated spectra generated from time-dependent DFT calculations, the absolute configuration could be obtained. The chiral alcohols could further be isolated on a larger scale after transesterification by using Candida Antarctica lipase B (Novozym 435) and could subsequently be converted into the corresponding chiral P,N-ligand by reaction with ClPPh₂. The coordination properties of the racemic P,N-ligand were investigated and the molecular structure of the Rh^I complex [(P,N)Rh(CO)Cl] was determined by X-ray crystal structure analysis. The corresponding chiral cationic Ir^I complex was used as catalyst for the enantioselective hydrogenation of prochiral N-phenyl-(1-phenylethylidene)amine and trans-α-methylstilbene. For the N-aryl-substituted imine, enantiomeric excesses of only 10 % were obtained, whereas the unfunctionalized olefin could be hydrogenated with enantiomeric excesses of up to 90 %. Interestingly, the modular synthetic access to the P,N-hybrid system described here allows facile modification of the ligand structure, which should extend the scope of such novel P,N-ligands for asymmetric catalytic conversions to a large extent in the future.

The λ³-phosphinine derivatives 2,6-diphenyl-4-(p-tolyl)phosphinine and 2-(2′-pyridyl)-4,6-diphenylphosphinine were quantitatively converted into the corresponding λ⁴-phosphinine anions by reaction with phenyllithium. Systematic hydrolysis experiments with H₂O and MeOH show that a subtle interplay between the pK_a values of the generated 1,2-dihydrophosphinine derivatives and the pK_b value of the formed bases, LiOH and LiOCH₃, respectively, leads either to the kinetic or thermodynamic product. The coordination chemistry of the λ⁴-phosphinine anions towards Rh^I was further investigated, and the anions based on 2,6-diphenyl-4-(p-tolyl)-phosphinine and 2-(2′-pyridyl)-4,6-diphenylphosphinine demonstrate different coordination modes. Whereas the former coordinates in an η⁵ fashion towards the Rh^I atom, the latter acts as a bidentate chelating ligand with an η¹ coordination to the metal center through the phosphorus lone pair.

This microreview describes progress in the development and coordination chemistry of donor-functionalized phosphinines – the phosphorus analogues of pyridines – that has been made during the last five years. The stepwise assembly of 2,4,6-triarylphosphinines starting from functionalized benzaldehyde and acetophenone derivatives allows the incorporation of additional substituents into specific positions of the aromatic phosphorus heterocycle. This strategy can be used to synthesize chelating phosphinines, which is an essential aspect especially for the preparation of phosphinine-based transition metal complexes containing metal centres in medium-to-high oxidation states. Access to such coordination compounds used to be very difficult because monodentate phosphinines cannot be used for this purpose, due to their weak σ-donor properties but strong π-accepting capacities. Moreover, metal complexes of less highly substituted phosphinines turned out to be extremely sensitive to nucleophilic attack, making their straightforward synthesis, characterization and application rather unattractive. The recent strategies in this area facilitate synthetic access to a completely new set of phosphinine-metal complexes for the first time, leading to a much broader scope for potential applications. New developments in the areas of homogeneous catalysis and materials sciences can consequently be foreseen in the near future.

Invited for this month's cover is the group of Christian Müller at the Freie Universität Berlin. The image shows the most recent developments in the coordination chemistry of functionalized, π-accepting phosphinines.

The novel atropisomeric pyridine derivative rac-10 has been synthesized and structurally characterized. In contrast to its phosphorus analogue 3, axially chiral 10 has a considerably lower rotational barrier as estimated by DFT calculations. However, the presence of the two enantiomers could be confirmed by means of chiral analytical HPLC analysis and by protonation experiments with a chiral acid. Compound rac-10 could be further dehydrogenated by treatment with DDQ to the benzo(h)quinoline derivative rac-12. This conversion failed for the phosphorus analogue rac-3. Interestingly, although 2,4,6-triarylphosphinines undergo facile CH activation with [Cp*IrCl₂]₂ in the presence of NaOAc, this reaction does not proceed with the corresponding pyridine derivatives. On the other hand, the latter ones can be selectively ortho-metalated with Pd(OAc)₂, leading to acetate-bridged dimeric species, which could be unambiguously confirmed by means of X-ray crystal structure analysis. The treatment of phosphinines with Pd(OAc)₂ led instead to the formation of the unusual cofacial oxidative coupling products 16 and 17, which consist of a phosphorus-containing cage structure.

A series of 2,4,6-triarylphosphinines were prepared and investigated in the base-assisted cyclometalation reaction using [Cp*IrCl₂]₂ (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) as the metal precursor. Insight in the mechanism of the CH bond activation of phosphinines as well as in the regioselectivity of the reaction was obtained by time-dependent ³¹P{¹H} NMR spectroscopy. At room temperature, 2,4,6-triarylphosphinines instantaneously open the Ir-dimer and coordinate in an η¹-fashion to the metal center. Upon heating, a dissociation step towards free ligand and an Ir-acetate species is observed and proven to be a first-order reaction with an activation energy of ΔE_A=56.6 kJ mol⁻¹ found for 2,4,6-triphenylphosphinine. Electron-donating substituents on the ortho-phenyl groups of the phosphorus heterocycle facilitate the subsequent cyclometalation reaction, indicating an electrophilic CH activation mechanism. The cyclometalation reaction turned out to be very sensitive to steric effects as even small substituents can have a large effect on the regioselectivity of the reaction. The cyclometalated products were characterized by means of NMR spectroscopy and in several cases by single-crystal X-ray diffraction. Based on the observed trends during the mechanistic investigation, a concerted base-assisted metalation–deprotonation (CMD) mechanism, which is electrophilic in nature, is proposed.

The design and preparation of an asymmetrically substituted and bulky phosphinine was achieved by introducing sterically demanding substituents into specific positions of a rigid phosphorus-heterocyclic framework. Compound 5 shows, at the same time, axial chirality and a sufficiently high energy barrier for internal rotation to prevent enantiomerization. Both enantiomers of 5 were isolated by means of chiral analytical HPLC, and their absolute configurations could be assigned by combining experimental data and DFT calculations. Despite its substitution pattern, 5 can still coordinate to transition-metal centers through the lone pair of electrons on the phosphorus atom. Rapid CH activation on the adjacent aryl substituent at the 2-position of the phosphorus heterocycle was achieved by using [{Cp*IrCl₂}₂] (Cp*=1,2,3,4,5-pentamethylcyclopentadienyl) as a metal precursor. A racemic mixture of 5 was applied as a π-accepting low-coordinate phosphorus ligand in the Rh-catalyzed hydroformylation of trans-2-octene, which showed a clear preference for the formation of 2-methyloctanal.

Rh^III and Ir^III complexes based on the λ³-P,N hybrid ligand 2-(2′-pyridyl)-4,6-diphenylphosphinine (1) react selectively at the PC double bond to chiral coordination compounds of the type [(1H⋅OH)Cp*MCl]Cl (2,3), which can be deprotonated with triethylamine to eliminate HCl. By using different bases, the pK_a value of the POH group could be estimated. Whereas [(1H⋅O)Cp*IrCl] (4) is formed quantitatively upon treatment with NEt₃, the corresponding rhodium compound [(1H⋅O)Cp*RhCl] (5) undergoes tautomerization upon formation of the λ⁵σ⁴-phosphinine rhodium(III) complex [(1⋅OH)Cp*RhCl] (6) as confirmed by single-crystal X-ray diffraction. Blocking the acidic POH functionality in 3 by introducing a POCH₃ substituent leads directly to the λ⁵σ⁴-phosphinine iridium(III) complex (8) upon elimination of HCl. These new transformations in the coordination environment of Rh^III and Ir^III provide an easy and general access to new transition-metal complexes containing λ⁵σ⁴-phosphinine ligands.

The bidentate P,N hybrid ligand 1 allows access for the first time to novel cationic phosphinine-based Rh^III and Ir^III complexes, broadening significantly the scope of low-coordinate aromatic phosphorus heterocycles for potential applications. The coordination chemistry of 1 towards Rh^III and Ir^III was investigated and compared with the analogous 2,2′-bipyridine derivative, 2-(2′-pyridyl)-4,6-diphenylpyridine (2), which showed significant differences. The molecular structures of [RhCl(Cp*)(1)]Cl and [IrCl(Cp*)(1)]Cl (Cp*=pentamethylcyclopentadienyl) were determined by means of X-ray diffraction and confirm the mononuclear nature of the λ³-phosphinine–Rh^III and Ir^III complexes. In contrast, a different reactivity and coordination behavior was found for the nitrogen analogue 2, especially towards Rh^III as a bimetallic ion pair [RhCl(Cp*)(2)]⁺[RhCl₃(Cp*)]⁻ is formed rather than a mononuclear coordination compound. [RhCl(Cp*)(1)]Cl and [IrCl(Cp*)(1)]Cl react with water regio- and diastereoselectively at the external PC double bond, leading exclusively to the anti-addition products [MCl(Cp*)(1H⋅OH)]Cl as confirmed by X-ray crystal-structure determination.