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 CIPN(Ar)CH2CH2N(Ar) (Ar = 4-MeO-C6H4, 3a; Ar = 2,4,6-Me-3-C6H2, 3b; Ar = 2,6-(CHMe2)(2)-C6H3, 3c) has been prepared from PCl3 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 H-1 NMR spectroscopy. Treatment of halophosphines 3a-c with AgOSO2CF3 (AgOTf) or Tl{B[3,5-(CF3)(2)-C6H3](4)}) (TlBArF) affords the cationic bis(arylamino)phosphenium compounds [PN(Ar)CH2CH2N(Ar)] (A), (4a-e; A = OTf, BArF), in high yield. Phosphenium cations 4a-d reversibly form adducts with trimethylphosphine. The structure of the phosphinophosphenium adduct 5c (Ar = 2,6-(CHMe2)(2)-C6H3) 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(PPh3)(3), with 1 equiv of 4a gives the first well-defined Rh phosphenium complex, {trans-RhCl(PPh3)(2)[PN(4-MeO-C6H4)CH2CH2N(4-MeO-C6H4)]}(OTf) (6), which is isolated in 80% yield. In contrast, treatment, of Wilkinson's catalyst with 4c results in quantitative formation of [Rh(PPh3)(3)](OTf) and chlorophosphine 3c. The influence of the phosphenium N-Ar substituents is further evidenced by the analogous reaction between RhCl(PPh3)(3) and mesityl-substituted 4b, which affords products analogous to 4a,c as well as the isomeric Rh phosphenium complex 7a, having cis PPh3 ligands. P-31 NMR spectroscopic parameters for 6 and 7a,b are consistent with Rh-P multiple bonding.