Arylimido complexes of ruthenium(IV) porphyrins

Abstract

Treatment of [Ru(tbpp)O₂ ] [H₂tbpp = 5,10,15,20-tetrakis(p-tert -butylphenyl)porphyrin] with SiMe₃Cl gave [Ru(tbpp)Cl₂ ] in good yield. Reaction of [Ru(tbpp)Cl₂ ] with para-substituted anilines NRH₂ (R = p-XC₆H₄ 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 ¹H 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 RN=PR₃′. The reduction of [Ru(tbpp)(NR)] by PMe₂Ph 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 (k₁ ) and phosphine binding constant (K) and for the reduction of [Ru(tbpp)(NC₆H₄Me-p)] by PMe₂Ph at 25.0 °C in toluene solution were determined to be (6.86 ± 0.19) × 10^-4 s^-1 and (23.6 ± 6.5) × 10³ 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)(NC₆H₄X-p)] by tertiary PMe₂Ph the rate decreases in the order X = I > H ≈ Cl > Me. The imido transfer from [Ru^V(tbpp)(NC₆H₄Me-p)] to PMe₂Ph is about 60 times faster than that from [Ru^IV(tbpp)(NC₆H₄Me-p)].

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