The half-sandwich 18- and 16-electron arene ruthenium iminophosphonamide complexes

Novel half-sandwich 18ē and 16ē arene ruthenium iminophosphonamide complexes [(η⁶-C₆Me₆)RuCl{(R'N)₂PR₂}] (3a-c) and [(η⁶-C₆Me₆)Ru{(R'N)₂PR₂}]⁺(X^-) (4a-c) (a, R = Ph, R' = p-Tol; b, R = Et, R' = p-Tol; c, R = Ph, R' = Me. X = BF₄, PF₆ or BAr^F₄) were synthesized. The elongated Ru-Cl bond in the 18ē complexes is shown to dissociate even in apolar solvents to form the corresponding 16ē cations, which can be readily isolated as salts with non-coordinating anions. The coordinatively unsaturated 16ē complexes are stable species due to efficient π-electron donation from the nitrogen atoms of the zwitterionic NPN-ligand. The ruthenium iminophosphonamides are moderately active in the ROMP polymerization of norbornene; the 16ē complexes 4a,b yield high molecular weight polymers (M_n∼ 300 × 10³) with a narrow distribution M_w/M_n∼ 1.6, while the 18ē complexes 3a,b give polymers of lower molecular weight (M_n < 50 × 10³) with a wider polydispersity index M_w/M_n∼ 2.5.