Versatile Reactivity of Half-Sandwich Rhodium(III) Iminophosphonamide Complexes

Novel 18e and 16e pentamethylcyclopentadienyl rhodium(III) complexes [(eta 5-C5Me5)RhX(NPN)] (1a,b, X = Cl; 2a-c, X = PF6, BAr4F) with chelating zwitterionic iminophosphonamide (NPN) ligands (Ph2P(NR)(NR '); a, R = R ' = p-Tol; b, R = p-Tol, R ' = Me; c, R = R ' = Me) were synthesized and characterized by single-crystal X-ray diffraction. In the 16e complexes 2, the rhodium (Rh) atom is efficiently stabilized by pi-donation of unshared N electrons, thus hampering coordination of the external ligands and rendering the 18e complexes labile. Due to low coordination enthalpy, the cationic 18e monocarbonyl and pyridine adducts 2amiddotL are stable only at low temperatures. At room temperature, 2middotCO adducts readily give stable carbonyl-carbamoyl complexes [(eta 5-C5Me5)Rh(CO){(CO(NR ')Ph2P(NR)}]+ (4) formed as a result of CO insertion into the Rh-N bond, thus showing high nucleophilicity of the N atoms in 18e complexes. High basicity of the Na+NPN- precursors caused side deprotonation of the eta 5-C5Me5 ligand during the synthesis of 1 that yields unstable fulvene Rh(I) complexes [(eta 4-C5Me4CH2)Rh{Ph2P(NR)(NR ')2}] (3a,b). Complex 3a undergoes a facile reaction with isoprene to yield an unusual [(eta 5:eta 1-C5Me4(CH2)C(Me)= CHCH2)Rh(NPN)] complex-the first example of intermolecular 1,4-metallacycloaddition of diene to the Rh-fulvene complex.