A water-soluble heterometallic iron(iv)–ruthenium(iv) μ-nitrido porphyrin complex: synthesis, structure and catalytic activity

Abstract

A heterometallic Fe(IV)–Ru(IV) nitrido complex bearing a water-soluble porphyrin ligand has been synthesized, and its redox and catalytic properties have been investigated. Treatment of Na₄[Fe(TPPS)(H₂O)₂] ([TPPS]⁴⁻ = meso-tetrakis(4-sulfonatophenyl)porphyrinate) with [Ru(N)(L_OEt)Cl₂] (L_OEt⁻ = [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]⁻) afforded the water-soluble μ-nitrido complex Na₄[(H₂O)(TPPS)Fe(μ-N)RuCl₂(L_OEt)] (Na₄[1]). Cation metathesis of Na₄[1] with Ph₄PCl gave (Ph₄P)₄[(H₂O)(TPPS)Fe(μ-N)RuCl₂(L_OEt)] ((Ph₄P)₄[1]), which has been characterized by single-crystal X-ray diffraction. The observed Ru–N(nitride) [1.701(5) Å] and Fe–N(nitride) [1.676(5) Å] distances and the Fe–N–Ru angle [171.4(3)°] of diamagnetic [1]⁴⁻ are indicative of the Fe(IV)NRu(IV) bonding behavior. The cyclic voltammogram of Na₄[1] in 0.1 M HCl displayed reversible redox couples at +0.66 and +0.94 V versus a saturated calomel electrode, which are assigned metal-centred and porphyrin-based oxidations, respectively. Treatment of Na₄[1] with Ag(CF₃SO₃) and PPh₃ resulted in the formation of Na₃([(H₂O)(TPPS)Fe(μ-N)Ru(L_OEt)Cl₂{Ag(PPh₃)}]) (2). Na₄[1] is capable of catalyzing sulfide oxidation with H₂O₂ in aqueous medium. For example, in MeCN/H₂O (1 : 1, v/v), in the presence of 2 mol% of Na₄[1], methyl p-tolyl sulfide was oxidized by H₂O₂ to afford methyl p-tolyl sulfoxide selectively in 95% yield. Electrospray ionization mass spectrometry indicated that the reaction of Na₄[1] with H₂O₂ generated new species with m/z = 430.2228 and 434.2198 corresponding to [1 + O]⁴⁻ and [1 + 2O]⁴⁻, respectively, which are possible active intermediates in the catalytic sulfide oxidation.