Sterically Tunable Phosphenium Cations:  Synthesis and Characterization of Bis(arylamino)phosphenium Ions, Phosphinophosphenium Adducts, and the First Well-Defined Rhodium Phosphenium Complexes

A family of bis(arylamino)chlorophosphines of the general formula (Ar = 4-MeO-C 6 H 4, 3a; Ar = 2,4,6-Me 3 -C 6 H 2, 3b; Ar = 2,6-(CHMe 2 ) 2 -C 6 H 3, 3c ) has been prepared from PCl 3 and the appropriate diamine. Steric interactions involving the 2,6-aryl substituents in 3b, c result in hindered rotation about the N−C Ar bond, as evidenced by 1 H NMR spectroscopy. Treatment of halophosphines 3a − c with AgOSO 2 CF 3 (AgOTf) or Tl{B[3,5-(CF 3 ) 2 -C 6 H 3 ] 4 } (TlBAr F ) affords the cationic bis(arylamino)phosphenium compounds ( 4a − e; A = OTf, BAr F ), in high yield. Phosphenium cations 4a − d reversibly form adducts with trimethylphosphine. The structure of the phosphinophosphenium adduct 5c (Ar = 2,6-(CHMe 2 ) 2 -C 6 H 3 ) has been determined by single-crystal X-ray diffraction techniques, revealing both the steric influences of the 2,6-Ar substituents and the electronic nature of the bonding in 5 . Treatment of Wilkinson's catalyst, RhCl(PPh 3 ) 3, with 1 equiv of 4a gives the first well-defined Rh phosphenium complex, ( 6 ), which is isolated in 80% yield. In contrast, treatment of Wilkinson's catalyst with 4c results in quantitative formation of [Rh(PPh 3 ) 3 ](OTf) and chlorophosphine 3c . The influence of the phosphenium N−Ar substituents is further evidenced by the analogous reaction between RhCl(PPh 3 ) 3 and mesityl-substituted 4b, which affords products analogous to 4a, c as well as the isomeric Rh phosphenium complex 7a, having cis PPh 3 ligands. 31 P NMR spectroscopic parameters for 6 and 7a, b are consistent with Rh−P multiple bonding.