Reactivity Studies of Phosphinines: The Selenation of Diphenyl-Phosphine Substituents and Formation of a Chelating Bis(Phosphinine) Palladium(II) Complex

Peter A Cleaves,Stephen M Mansell,Ben Gourlay,Robert J Newland,Robert Westgate

doi:10.3390/inorganics10020017

Abstract

Phosphinines and donor-substituted phosphinines are of recent interest due to their use in homogeneous catalysis. In this article, a Pd(II) bis(phosphinine) complex was characterised and phosphorus–selenium coupling constants were used to assess the donor properties of the diphenylphosphine substituents of phosphinine ligands to promote their further use in catalysis. The selenation of 2,5-bis(diphenylphosphino)-3,6-dimethylphosphinine (5) and 2-diphenylphosphino-3-methyl-6-trimethylsilylphosphinine (6) gave the corresponding phosphine selenides 8 and 9, respectively, leaving the phosphinine ring intact. Multinuclear NMR spectroscopy, mass spectrometry and single crystal X-ray diffraction confirmed the oxidation of all the diphenylphosphine substituents with 1JP-Se coupling constants determined to be similar to SePPh3, indicating that the phosphinine rings were electronically similar to phenyl substituents. Solutions of 6 were found to react with oxygen slowly to produce the phosphine oxide 10 along with other by-products. The reaction of [bis{3-methyl-6-(trimethylsilyl)phosphinine-2-yl}dimethylsilane] (4) with [PdCl2(COD)] gave the chelating dichloropalladium(II) complex, as determined by multinuclear NMR spectroscopy, mass spectrometry and an elemental analysis. The molecular structure of the intermediate 2 in the formation of 4,6-di(tert-butyl)-1,3,2-diazaphosphinine (3) was also determined, which confirmed the structure of the diazaphosphacycle P(Cl){N=C(tBu)CH=C(tBu)-N(H)}.

Highlights

Phosphinines are the phosphorus analogue of pyridine
With respect to phosphinine chemistry, oxidation with sulfur and selenium is usually difficult with phosphinine sulfides initially only tentatively characterised as intermediates in the reaction of S with 2-Ph-4,5-Me2PC5H2 and S with 2,3-(PPh2)2-6-PhPC5H, principally identified by 31P NMR spectroscopy [31,32]
We describe the synthesis of a bis(phosphinine) palladium complex and crystallographic characterisation of the chlorodiazaphosphacyclic intermediate formed in the synthesis of the key starting material, 1,3,2-diazaphosphinine

Summary

Introduction

Phosphinines are the phosphorus analogue of pyridine. The initial discovery of 2,4,6-triphenylphosphinine by Märkl [1] and the parent, unsubstituted phosphinine by Ashe [2] are landmarks in low-coordinate main group chemistry and continue to inspire new research in the field [3,4]. The first well-characterised phosphinine sulfides were formed from the reactions of 2,6R2-3,5-Ph2PC5H (R = Ph, SiMe3) with S in toluene at 90 ◦C for 5–7 days and revealed 31P chemical shifts (159 and 194 ppm, respectively) greatly reduced from the starting materials (206 and 269 ppm, respectively) [33]. Their molecular structures were determined by X-ray crystallography; P=S bond lengths (1.916(1) and 1.929(1) Å, respectively) shorter than those in S=PPh3 (1.952 Å) were observed [33]. The first well-characterised and quantitatively formed phosphinine selenide was only synthesised recently using red selenium and 2,6-(SiMe3)2PC5H3; a 31P chemical shift of 170 ppm was observed [35]

Methods

Results

Conclusion