The case of a μ2-P aromatic phosphinine as a 4-electron donor forming σ- and π-three-center-two-electron bonds

Profound insight into the electronic structures of occasionally observed μ2-P bridging phosphinines remains limited. In this work, we present the isolation and X-ray crystallographic characterization of a dimeric Rh(I) phosphinine complex exhibiting both η1-P and μ2-P phosphinine coordination modes. Variable temperature NMR analyses and DOSY spectrum measurement confirmed the presence of two types of fluxional phenomena in solution: η1-P phosphinine bonding and dissociation, and η1-P and μ2-P equilibrium. DFT calculations in conjunction with single crystal X-ray diffraction studies suggest that the μ2-P phosphinines donate four electrons via a σ-lone pair and a high-lying π-type electron pair, instead of two σ-lone pairs, forming σ- and π-three-center-two-electron bonds. The stronger π-type interactions lead to longer P-C bonds and larger negative coordination chemical shifts for μ2-P phosphinines. However, the binding interactions of μ2-P are thermodynamically weaker than those of η1-P. Reactivity studies further confirm the labile nature of the μ2-P phosphinine bonds, which could be easily converted to an η1-P phosphinine.

New frontiers: 1,4-diphosphinines and P-bridged bis(NHCs)

This review describes synthetic concepts and breakthroughs in 1,4-diphosphinine and related P-bridged bis(NHCs) chemistry, covering the last four decades, starting from monocyclic 1,4-dihydro-1,4-diphosphinines in the early 80s to the most recent and promising achievements of tricyclic 1,4-dihydro-1,4-diphosphinines and tricyclic 1,4-diphosphinines. Theoretical aspects are presented for 1,4-dihydro- and 1,4-diphosphinines considering HOMO LUMO situations as well as the degree of aromaticity. Moreover, fundamental characteristics of analytical data of these compounds are highlighted with special focus on substituent effects, structural aspects and trends of electrophilicity. The two P-centers and the heterocyclic rings of 1,4-dihydro- and 1,4-diphosphinines constitute a broad platform for substitution, reduction/oxidation, alkylation, complexation and cycloaddition reactions, i.e., a comprehensive compilation of reactivity aspects is presented. Furthermore, very recent developments in the synthesis and reactivity of tricyclic PV/V- and PIII/III-bridged bis(imidazole-2-ylidenes) will be discussed together with new perspectives derived from an antiaromatic middle ring. In total, our intention is to show new frontiers, i.e., new synthetic paths, thus creating novel opportunities for potential applications in molecular and materials chemistry.

Substituent effects in pyridyl-functionalized pyrylium salts, pyridines and λ3,σ2-phosphinines: a fundamental and systematic study

A series of pyridyl-functionalized O⁺–, N–, and P– containing heterocycles have been prepared and investigated for substituent effects.

[Re(CO)3Cl(C5H4ClP)2] and [Re(CO)2Cl(C5H4ClP)3]: synthesis and characterization of two novel rhenium(i) phosphinine complexes

Two novel rhenium(i) phosphinine complexes [Re(CO)₃Cl(η¹-C₅H₄ClP)₂] (1) and [Re(CO)₂Cl(η¹-C₅H₄ClP)₃] (2) were synthesized and the molecular structure of both was determined by single crystal X-ray diffraction.

2-(2′-Pyridyl)-4,6-diphenylphosphinine versus 2-(2′-pyridyl)-4,6-diphenylpyridine: synthesis and characterization of novel Cr0, Mo0 and W0 carbonyl complexes containing chelating P,N and N,N ligands

Replacing nitrogen by phosphorus in otherwise similar structures changes the properties of the resulting compounds significantly due to the electronic differences that exist between these heteroatoms.

2,4,6-Triarylphosphinines versus 2,4,6-triarylpyridines: an investigation of the differences in reactivity between structurally related aromatic phosphorus and nitrogen heterocycles

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 CH 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.

Pd(II) and Pt(II) complexes of 2-(2'-pyridyl)-4,6-diphenylphosphinine: synthesis, structure, and reactivity

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.