Sterically tunable phosphenium cations: Synthesis and characterization of bis(arylamino)phosphenium ions, phosphinophosphenium adducts, and the firstwell-defined rhodium phosphenium complexes

A family of bis(arylamino)chlorophosphines of the general formula CIPN(Ar)C H2CH2N(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. Ster ic interactions involving the 2,6-aryl substituents in 3b,c result in hinde red rotation about the N-C-Ar bond, as evidenced by H-1 NMR spectroscopy. T reatment of halophosphines 3a-c with AgOSO2CF3 (AgOTf) or Tl{B[3,5-(CF3)(2) -C6H3](4)}) (TlBArF) affords the cationic bis(arylamino)phosphenium compoun ds [PN(Ar)CH2CH2N(Ar)] (A), (4a-e; A = OTf, BArF), in high yield. Phospheni um cations 4a-d reversibly form adducts with trimethylphosphine. The struct ure of the phosphinophosphenium adduct 5c (Ar = 2,6-(CHMe2)(2)-C6H3) has be en determined by single-crystal X-ray diffraction techniques, revealing bot h the steric influences of the 2,6-Ar substituents and the electronic natur e of the bonding in 5. Treatment of Wilkinson's catalyst, RhCl(PPh3)(3), wi th 1 equiv of 4a gives the first well-defined Rh phosphenium complex, {tran s-RhCl(PPh3)(2)[PN(4-MeO-C6H4)CH2CH2N(4-MeO-C6H4)]}(OTf) (6), which is isol ated 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 evidence d by the analogous reaction between RhCl(PPh3)(3) and mesityl-substituted 4 b, which affords products analogous to 4a,c as well as the isomeric Rh phos phenium complex 7a, having cis PPh3 ligands. P-31 NMR spectroscopic paramet ers for 6 and 7a,b are consistent with Rh-P multiple bonding.