Arylimido complexes of ruthenium(IV) porphyrins

Abstract

Treatment of [Ru(tbpp)O 2 ] [H 2 tbpp = 5,10,15,20-tetrakis(p-tert -butylphenyl)porphyrin] with SiMe 3 Cl gave [Ru(tbpp)Cl 2 ] in good yield. Reaction of [Ru(tbpp)Cl 2 ] with para-substituted anilines NRH 2 (R = p-XC 6 H 4 where X = Me, H, Cl or I) afforded the first arylimidoruthenium(IV) complexes [Ru(tbpp)(NR)]. These are paramagnetic with µ eff ca. 2.8 µ B and display 1 H NMR spectra that are typical for paramagnetic ruthenium(IV) porphyrins. The cyclic voltammograms of [Ru(tbpp)(NR)] exhibit reversible Ru V –Ru IV and Ru IV –Ru III couples. Treatment of [Ru(tbpp)(NR)] with Ag I or Ce IV afforded the imidoruthenium(V) complex [Ru(tbpp)(NR)] + . The complexes [Ru(tbpp)(NR)] underwent imido-group transfer reactions with tertiary phosphines to give [Ru(tbpp)(PR′ 3 ) 2 ] and RNPR 3 ′. The reduction of [Ru(tbpp)(NR)] by PMe 2 Ph shows saturation kinetics, in which the rate is first order in [Ru IV ]. The mechanism proposed for the Ru-mediated imido transfer involves reversible binding of phosphine to Ru IV and rate-limiting intramolecular imido-group transfer. The first-order rate constant (k 1 ) and phosphine binding constant (K) and for the reduction of [Ru(tbpp)(NC 6 H 4 Me-p)] by PMe 2 Ph at 25.0 °C in toluene solution were determined to be (6.86 ± 0.19) × 10 -4 s -1 and (23.6 ± 6.5) × 10 3 mol dm -3 , respectively. The activation enthalpy (ΔH ‡ ) and entropy (ΔS ‡ ) for the above reaction are 125 ± 1 kJ mol -1 and 113 ± 21 J K -1 mol -1 , respectively. For the reduction of para-X-substituted arylimido complexes [Ru(tbpp)(NC 6 H 4 X-p)] by tertiary PMe 2 Ph the rate decreases in the order X = I > H ≈ Cl > Me. The imido transfer from [Ru V (tbpp)(NC 6 H 4 Me-p)] to PMe 2 Ph is about 60 times faster than that from [Ru IV (tbpp)(NC 6 H 4 Me-p)].