Iridium porphyrin complexes with μ-nitrido, hydroxo, hydrosulfido and alkynyl ligands

Abstract

Iridium porphyrin complexes containing μ-nitrido, hydroxo, hydrosulfido, and alkynyl ligands have been synthesized and structurally characterized, and their oxidation has been studied. The alkyl-Ir^III porphyrin complex [Ir(tpp)R] (tpp²⁻ = 5,10,15,20-tetraphenylporphyrin dianion; R = C₈H₁₃; 1) was synthesized by reaction of [Ir(cod)Cl]₂ (cod = 1,5-cyclooctadiene) with H₂tpp in refluxing monoethylene glycol. Treatment of 1 with PPh₃ and [(L_OEt)Ru(N)Cl₂] (L_OEt⁻ = [(η⁵-C₅H₅)Co{P(O)(OEt)₂}₃]⁻) gave [Ir(tpp)(R)(PPh₃)] (2) and the μ-nitrido complex [R(tpp)Ir(μ-N)RuCl₂(L_OEt)] (3), respectively. The cyclic voltammogram of 3 exhibited a reversible oxidation couple at 0.44 V versus Fc^+/0 (Fc = ferrocene). The oxidation of 3 with [(4-BrC₆H₄)₃N](SbCl₆) resulted in Ir–C bond homolysis and formation of the chloride complex [Cl(tpp)Ir(μ-N)RuCl₂(L_OEt)] (4). The short Ir–N(nitrido) bond distances in 3 [1.944(3) Å] and 4 [1.831(4) Å] are indicative of multiple bond character and thus these two μ-nitrido complexes can be described by the two resonance forms: Ir^III–NRu^VI and Ir^VNRu^IV. Similarly, the oxidation of 2 with [(4-BrC₆H₄)₃N](SbCl₆) yielded [Ir(tpp)Cl(PPh₃)] (5). Chloride abstraction of 5 with TlPF₆ in tetrahydrofuran (thf) afforded [Ir(tpp)(PPh₃)(thf)](PF₆) (6) that reacted with CsOH·H₂O and Li₂S to give the hydroxo [Ir(tpp)(OH)(PPh₃)] (7) and hydrosulfido [Ir(tpp)(PPh₃)(SH)] (8) complexes, respectively. Treatment of 6 with phenylacetylene in the presence of CuI and Et₃N yielded the bimetallic complex [Ir(tpp)(PPh₃)(μ-η¹:η²-CCPh)(CuI)] (9), whereas the transmetallation of 6 with LiCCPh afforded the mononuclear alkynyl complex [Ir(tpp)(PPh₃)(CCPh)] (10). The electrochemistry of the Ir porphyrin complexes has been studied using cyclic voltammetry. On the basis of the measured redox potentials of [Ir(tpp)(PPh₃)X], the ability of X⁻ to stabilize the Ir^IV state is ranked in the order: R⁻ > PhCC⁻ > Cl⁻ ∼ OH⁻. Oxidation of 8 and 9 with [(4-BrC₆H₄)₃N](SbCl₆) led to isolation of 5 and [Ir(tpp)(PPh₃)(H₂O)]⁺, respectively. The crystal structures of complexes 3, 4, and 7–10 have been determined.