Synthesis and reactivity of an iridium complex based on a tridentate aminophosphano ligand

Reactivity of Ir(I)-aminophosphane platforms towards oxidants

The iridium(I)-aminophosphane complex [Ir{κ3C,P,P'-(SiNP-H)}(cod)] has been prepared by reaction of [IrCl(cod)(SiNP)] with KCH3COO. DFT calculations show that this reaction takes place through an unexpected outer sphere mechanism (SiNP = SiMe2{N(4-C6H4Me)PPh2}2; SiNP-H = CH2SiMe{N(4-C6H4Me)PPh2}2). The reaction of [IrCl(cod)(SiNP)] or [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with diverse oxidants has been explored, yielding a range of iridium(III) derivatives. On one hand, [IrCl(cod)(SiNP)] reacts with allyl chloride rendering the octahedral iridium(III) derivative [IrCl2(η3-C3H5)(SiNP)], which, in turn, reacts with tert-butyl isocyanide yielding the substitution product [IrCl(η3-C3H5)(CNtBu)(SiNP)]Cl via the observed intermediate [IrCl2(η1-C3H5)(CNtBu)(SiNP)]. On the other hand, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with [FeCp2]X (X = PF6, CF3SO3), I2 or CF3SO3CH3 results in the metal-centered two-electron oxidation rendering a varied assortment of iridium(III) compounds. [Ir{κ3C,P,P'-(SiNP-H)}(cod)] reacts with [FeCp2]+ (1 : 2) in acetonitrile affording [Ir{κ3C,P,P'-(SiNP-H)}(CH3CN)3]2+ isolated as both the triflato and the hexafluorophosphato derivatives. Also, the reaction of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] with I2 (1 : 1) yields a mixture of iridium(III) derivatives, namely the mononuclear compound [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I, containing the η2,η3-cycloocta-2,6-dien-1-yl ligand, and two isomers of the dinuclear derivative [Ir2{κ3C,P,P'-(SiNP-H)}2(μ-I)3]I, the first species being isolated in low yield. DFT calculations indicate that [IrI(κ2P,P'-SiNP)(η2,η3-C8H11)]I forms as the result of a bielectronic oxidation of iridium(I) followed by the deprotonation of the cod ligand by iodide and the protonation of the methylene moiety of the [Ir{κ3C,P,P'-(SiNP-H)}] platform by the newly formed HI. Finally, the oxidation of [Ir{κ3C,P,P'-(SiNP-H)}(cod)] by methyl triflate proceeds via a hydride abstraction from the cod ligand, with the elimination of methane and the formation of the η2,η3-cycloocta-2,6-dien-1-yl ligand with the concomitant two-electron oxidation of the iridium centre. The crystal structures of selected compounds have been determined.

Cyclometalated iridium complexes based on monodentate aminophosphanes

Monodentate aminophosphanes HNP [NH(4-tolyl)PPh₂] and SiMe₃NP [SiMe₃N(4-tolyl)PPh₂] react with [Ir(μ-Cl)(cod)]₂ affording tetra- or pentacoordinate complexes of formula [IrCl(L)_n(cod)] (L = HNP, n = 1, 2; L = SiMe₃NP, n = 1). The reaction of [IrCl(SiMe₃NP)(cod)] with carbon monoxide smoothly renders [Ir(CO)₃(SiMe₃NP)₂][IrCl₂(CO)₂]. The reaction of HNP or SiMe₃NP with [Ir(CH₃CN)₂(cod)][PF₆] yields the cyclometalated iridium(III)-hydride derivatives [IrH{κ²C,P-NR(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] (R = H, SiMe₃) as a result of the intramolecular oxidative addition of the tolyl C²-H bond to iridium. The straighforward formation of [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺ was observed when the reaction was monitored by NMR spectroscopy at 233 K, whereas a more complex reaction sequence was observed in the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)]⁺, including the formation of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ and [Ir(cod)(HNP)₂]⁺. The "mixed" complex [IrH{κ²C,P-SiMe₃N(4-C₆H₃CH₃)PPh₂}(HNP)(cod)]⁺ was obtained upon reaction of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(cod)(CH₃CN)][PF₆] with SiMe₃NP at 233 K. Finally, the reaction of [Ir(CH₃CN)₂(coe)₂][PF₆] with SiMe₃NP or HNP resulted in the formation of [Ir(CH₃CN)₂(SiMe₃NP)₂][PF₆] and [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)][PF₆], respectively. Both the OC-6-35 and the OC-6-52 isomers of [IrH{κ²C,P-NH(4-C₆H₃CH₃)PPh₂}(HNP)₂(CH₃CN)]⁺ - featuring facial and meridional dispositions of the phosphorus atoms, respectively - were isolated depending on the reaction solvent. Several compounds described herein catalyse the dehydrogenation of formic acid in DMF, [IrCl(HNP)₂(cod)] being the most active, with TOF_{1 min} of about 2300 h^-1 (5 mol% catalyst, 50 mol% sodium formate, DMF, 80 °C).