Generation and direct EPR spectroscopic observation of triplet arylphosphinidenes: stabilisation versus internal rearrangements.

Triplet phosphinidenes, which have been postulated as important intermediates in numerous organophosphorus reactions, have been previously directly observed only in isolated cases. Recently we have published the first recorded EPR spectrum of triplet phosphinidene-mesitylphosphinidene (A. V. Akimov et al., Angew. Chem., Int. Ed., 2017, 56, 7944). In the present study we considered a series of triplet arylphosphinidenes which have been stabilised and detected for the first time using EPR spectroscopy by photolysis of 1-arylphosphiranes ArPC2H4 (Ar = C6H5, 9-anthracenyl, and 2,4,6-iPr3C6H2) in solid methylcyclohexane. We paid special attention to their magnetic parameters and the conditions of their stabilization during the photolytic cleavage of arylphosphiranes. An unusual influence of o-substituents on the spin-orbit component of the ZFS parameters D is observed. Surprisingly, photolysis of bulky arylphosphirane Mes*PC2H4 (Mes* = 2,4,6-ButC6H2) results in no formation of the stabilized triplet phosphinidene under similar experimental conditions. The performed quantum chemical calculations showed that the highly unstable singlet phosphinidene Mes*P undergoes an almost barrier-free rearrangement affording a stable insertion product, thereby hindering the conversion of the singlet intermediate to a more stable triplet phosphinidene.

The EPR Spectrum of Triplet Mesitylphosphinidene: Reassignment and New Assignment.

Low-temperature UV-photolysis of mesitylphosphiranes under highly anaerobic conditions leads to the formation of the triplet mesitylphosphinidene (MesP). The recorded X-band EPR spectrum of triplet MesP and the derived zero-field splitting parameter D=4.116 cm-1 differ significantly from those reported previously for this intermediate. New magnetic parameters of mesitylphosphinidene are discussed along with the results of DFT calculations.

Mechanism of intramolecular transformations of nickel phosphanido hydride complexes.

In solution, nickel phosphanido hydride complexes ([NiH{P(Ar)(H)}(dtbpe)], Ar = Dmp, Mes*) undergo a degenerate intramolecular exchange, with the Ni-H and P-H hydrogens and both halves of the dtbpe moiety interchanging. This intramolecular rearrangement was shown to occur in three steps: first, the hydride proton migrates to phosphorus, then the P-Aryl moiety rotates around the P-Ni bond, and finally the back migration of one proton to Ni completes the process. Both migration and rotation were determined to be characterized by high barriers (on the NMR time scale) and to depend on the type of aryl group at the terminal phosphorus. Compared to that observed for the Ni complexes, the same isomeric preference, but with a slower rate of intramolecular rearrangement, is predicted for the corresponding Pt complexes. An opposite isomeric preference, however, is expected for the corresponding Pd complexes. Thus, it is likely that some of the catalytic reactions of Pd and Pt complexes are driven by the relative thermodynamic stabilities of their main forms.