Synthesis and Redox Properties of (3-Phenothiazinomesityl)- and (4-Phenothiazinoduryl)dimesitylphosphines and the Corresponding Arsines

To construct the novel redox systems possessing the trimesitylphosphine- or trimesitylarsine-type
substructure as a reversible redox site, dimesityl(3-phenothiazinomesityl)phosphine, dimesityl(4-phenothiazinoduryl)phosphine, and the corresponding arsines were synthesized (mesityl = 2,4,6-trimethylphenyl, duryl = 2,3,5,6-tetramethylphenyl). The key synthetic intermediates, (3-bromomesityl)dimesitylphosphine, (4-bromoduryl)dimesitylphosphine, and the analogous arsines were prepared by
successive addition of the corresponding Grignard reagents to phosphorus or arsenic trichloride. The
(bromoaryl)phosphines and -arsines were converted to the corresponding (iodoaryl)phosphines and -arsines
and coupled with phenothiazine in the presence of copper to afford the phenothiazinophosphino- and
phenothiazinoarsinobenzenes. The cyclic voltammograms of the phenothiazinopnictogenobenzenes thus
obtained exhibit two-step redox waves corresponding to oxidation on the pnictogen as well as phenothiazine
redox centers. The phenothiazino group contributes to stability of the redox systems, and the
phenothiazinophosphinobenzenes display two-step nearly reversible redox waves at −78 °C. On the other
hand, the cyclic voltammograms of the phenothiazinoarsinobenzenes consist of the first reversible wave
followed by the second irreversible wave, suggesting decomposition at the unstable arsenic redox center.
The pnictogen redox centers of the phenothiazinopnictogenobenzenes are unstable as compared with
those of the corresponding trimesityl derivatives. Chemical oxidation of the phenothiazinophosphinobenzenes and phenothiazinoarsinobenzenes by tris(4-bromophenyl)aminium perchlorate, which can oxidize
trimesitylphosphine and trimesitylarsine to the corresponding cation radicals, was studied by EPR.
However, only the nitrogen-centered cation radical was observed probably because of the instability of
the phosphorus as well as arsenic radical centers.