DOI: 10.7274/29429864
title: Terminal Nitride Complexes of Bis(iminoxolene) Iridium

description: Metal nitride complexes are common in Groups 5-8, but only two group 9 metal nitride complexes are known. These two 4-coordinate iridium nitrides have planar geometries which prevent the population of IrºN p* orbitals. In contrast, square pyramidal terminal iridium nitride complex has never been reported because only two d electrons can be accommodated in this geometry without populating p* orbitals, which would require the unprecedentedly high Ir(VII) oxidation state. This thesis explores the possibility that the unfavorable bonding between Ir and the nitride group in square pyramidal geometry can be avoided using redox-active ligands where electrons can be shuttled away from the metal and onto the ligand, in order to support the strong p donation of the nitride group.
The 6-coordinate bis(iminoxolene) iridium azide complexes Bu4N[(Diso)2Ir(N3)2] (Diso = (4,6-di-tert-butyl-2-(2,6-diisopropylphenylimino)benzoquinone) and Bu4N[(Dicy)2Ir(N3)2] (Dicy = (2,4-di-tert-butyl-6-((2,6-dicyclohexyl-4-(trifluoromethyl)phenyl)imino)cyclohexa-2,4-dien-1-one) are synthesized through the addition of (Bu4N)N3 to iridium monochloride precursors. The 6-coordinate (Cp2Co)[(CF3Tio)2Ir(N3)2] (CF3Tio = N-2'-(3,5,3'',5''-tetrakis(trifluoromethyl)-m-terphenyl)-4,6-di-tert-butyl-2-iminobenzoquinone) is synthesized through the addition of NaN3 to the 6-coordinate anionic precursor, (Cp2Co)[(CF3Tio)2IrCl2], at high temperatures. The Diso and CF3Tio metal azide complexes all exhibit bent N3 groups bound to iridium with Ir-N-N bond angles ranging from 120.3-120.8° and ??N3 (as) frequencies ranging from 2022-2023 cm-1. Pyridine can displace one of the azide ligands to form neutral monoazide complexes. The resulting Ir(N3)(Py) compounds have azide stretching frequencies ranging from 2037-2040 cm-1. This increase in frequency is attributed to an electrostatic effect rather than a difference in bond lengths for the bound N3 group.
The rate of dissociation of pyridine from the 6-coordinate Ir(N3)(Py) compounds differ greatly. The rate of substitution of pyridine by pyridine-d5 for (Diso)2Ir(N3)(Py) is complete within 15 min at room temperature. The estimated half-life for pyridine exchange for (Dicy)2Ir(N3)(Py) is approximately 4 hours at room temperature, and for the (CF3Tio)2Ir(N3)(Py) complex it is approximately 6 days at room temperature. (Diso)2Ir(N3)(Py) is dissolved in the absence of pyridine, a reactive terminal nitride can be observed by 1H NMR or UV-visible spectroscopy. The UV-Visible spectrum of this intermediate has a ?max of 387 nm, in agreement with TD-DFT calculations that predict an intense peak at 402 nm.
To probe the reactivity of a terminal nitride intermediate, the reaction of (Diso)2Ir(N3)(Py) with the oxygen atom donor, N-methylmorpholine N-oxide (NMO), was studied. UV-Vis kinetics shows the formation of (Diso)2Ir(NO) in a reaction that is inverse first order in pyridine and zero order in NMO, giving an experimental rate law of Rate = k[Ir]1[NMO]0[Py]-1. The mechanism involves initial pre-equilibrium loss of pyridine from (Diso)2Ir(N3)(Py), followed by rate-determining loss of N2 from the monoazide to form the nitride and a final fast step where the nitride is trapped by NMO to form the nitrosyl. The product nitrosyl compound, (Diso)2Ir(NO), has been characterized as having a N–O bond length of 1.122(12) Å, an Ir–NO bond angle of 125.0(9)°, and a ??N-O of 1578 cm-1. Preliminary data indicates that the transient nitride also reacts with triphenylphosphine to form a phosphiniminato complex, and with 1-hexene to form an iridium aziridide.
Azide abstraction from the 6-coordinate anionic complexes Bu4N[(Diso)2Ir(N3)2] and Bu4N[(Dicy)2Ir(N3)2] using trimethylsilyl triflate produce 5-coordinate neutral iridium monoazide products that are observable at low temperature by UV-Vis and NMR. The loss of N2 from (Dicy)2Ir(N3) towards terminal nitride formation was observed by UV-Vis and found to be first order in (Dicy)2Ir(N3) with k = 0.00157(2) s-1.
The bis(iminoxolene) terminal iridium nitride (Dicy)2IrN is isolated in pure form by a two-step route.  The monoazide species can be intercepted by acetonitrile to give isolable trans-(Dicy)2Ir(N3)(NCCH3), which then precipitates the terminal nitride upon dissolution and standing in benzene. There is good agreement between ?max observed at 382 and 620 nm for the nitride product in comparison to the TD-DFT spectrum predicting intense ?max at 402 and 609 nm. The ??IrºN stretch is tentatively assigned at 930 cm-1.
The square pyramidal d2 terminal iridium(VII) nitride (Dicy)2IrN is synthesized using redox-active iminoxolene ligands. DFT predicts a LUMO and LUMO+1 with Ir dp* character where the highly oxidized iridium avoids the population of IrºN p* orbitals. The HOMO and HOMO–1 of (Dicy)2IrN are almost exclusively RAO in character indicating fully reduced amidophenoxide-type ligands with DFT predicting an MOS of –1.90(7) for (ap)2IrN. The electrochemistry of (Dicy)2IrN shows the poor outer-sphere oxidizing power which demonstrates the strong stabilization of the Ir(VII) state by the nitride ligand. Synthesis of the nitride from a monoazide precursor (Dicy)2Ir(N3)(CH3CN) allows the iridium oxidation state to jump from III to VII in one step, while the iminoxolene ligands accept electrons from the iridium so the d2 center exhibits strong Ir-N p bonding.