A series of donor‐functionalized pyrylium salts have been prepared by classical condensation reactions which were further converted into the corresponding thienyl‐ and pyridyl‐substituted polydentate λ3‐phosphinines by reaction with P(SiMe3)3. Further chemical modification of these phosphorus heterocycles with Hg(OAc)2 in the presence of methanol resulted in the formation of λ5‐phosphinines. The photophysical properties of a selected series of thienyl‐ and pyridyl‐functionalized pyrylium salts, λ3‐ and λ5‐phosphinines, were investigated and the results compared and supported by theoretical calculations on the DFT level. Significant fluorescence was observed for the pyrylium salts and λ5‐phosphinines. In contrast, the heteroaromatic substituted λ3‐phosphinines show very little emission which is consistent with the low oscillator strength predicted by DFT calculations for this π→π* transition. Furthermore, all three classes of compounds show readily observable phosphorescence in solution, which was determined by time‐gated detection at low temperature.

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.

The coordination chemistry of the bidentate P,N hybrid ligand 2-(2'-pyridyl)-4,6-diphenylphosphinine (1) towards Pd(II) and Pt(II) has been investigated. The molecular structures of the complexes [PdCl(2)(1)] and [PtCl(2)(1)] were determined by X-ray diffraction, representing the first crystallographically characterized λ(3)-phosphinine-Pd(II) and -Pt(II) complexes. Both complexes reacted with methanol at the P=C double bond at an elevated temperature, leading to the corresponding products [MCl(2)(1H·OCH(3))]. The molecular structure of [PdCl(2)(1H·OCH(3))] was determined crystallographically and revealed that the reaction with methanol proceeds selectively by syn addition and exclusively to one of the P=C double bonds. Strikingly, the reaction of [PdCl(2)(1H·OCH(3))] with the chelating diphosphine DPEphos at room temperature in CH(2)Cl(2) led quantitatively to [PdCl(2)(DPEphos)] and phosphinine 1 by elimination of CH(3)OH and rearomatization of the phosphorus heterocycle.

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 C-H 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.

8-Hydroxy-6-methyl-5,6,7,8-tetrahydroimidazo[1,2-a]pyridine was formed selectively in high yields from N-(beta-methallyl)imidazole by a tandem hydroformylation-cyclization sequence, representing a novel one-pot catalytic synthesis of bicyclic imidazole derivatives.