High-Valent Iridium Complexes Containing a Tripodal Bis- Cyclometalated C∧N∧C Ligand

Iridium complexes containing a bis-cyclometalated tripodal CANAC ligand have been synthesized, and their redox property and reactivity have been studied. The previously reported intermediate (1) prepared from IrCl3 center dot xH2O and bis(4-(tert-butyl)phenyl)methyl)pyridine (H2Bubnpy) has been used as a starting material for synthesis of Ir(CANAC) complexes. Treatment of 1 with CO, xylNC (xyl = 2,6-dimethylphenyl), PCy3 (Cy = cyclohexyl), and the Kla''ui tripodal ligand Na[Co(ri5-C5H5){P(O)(OEt)2}3] (NaLOEt) afforded [H3dtpny][Ir(Bubnpy)(CO)Cl2] (2), [Ir(Bubnpy)(CNxyl)2Cl] (3), [Ir(Bubnpy)(PCy3)Cl] (4), and [Ir(Bubnpy)(LOEt)] (6), respectively. Reaction of 5-coordinated 4 with CO gave the adduct [Ir(Bubnpy)(PCy3)Cl(CO)] (5). Treatment of 1 and 4 with [Ru(LOEt)(N)Cl2] afforded the respective heterometallic mu-nitrido complexes [(H2O)Cl(Bubnpy)Ir(mu-N)Ru(LOEt)Cl2] (7) and [(PCy3)Cl(Bubnpy)Ir(mu-N)Ru(LOEt)Cl2] (8). The Ir-N [1.887(5) angstrom] and Ru-N [1.661(5) angstrom] distances in 7 are indicative of the Ir(V)=N=Ru(IV) resonance structure. The cyclic voltammogram of 6 in CH2Cl2 displayed a reversible couple at ca. 0 V vs Fc+/0 (Fc = ferrocene) that is assigned as the Ir(IV/III) couple. Oxidation of 6 with Ag(OTf) (OTf- = triflate) afforded 6 center dot OTf that exhibited an EPR signal in CH2Cl2 at 4 K with gx = 2.06, gy = 1.91, and gz = 2.84 and Ir(IV) (I = 3/2) hyperfine coupling constants Axx, Ayy, and Azz of 60, 40, and 18 G, respectively, consistent with the Ir(IV) formulation. Treatment of 4 with PhICl2 yielded [Ir(Bubnpy)(PCy3)Cl2] (9), whereas the reaction of 7 with PhICl2 yielded mixed-valence [Cl2(Cl2Bubnpy)Ir(mu-N)Ru(LOEt)Cl2] (10) that contains a chlorinated CANAC ligand. DFT calculations indicated that the spin density in 10 is mostly found at the Ir atom, the chloride ligands, and a phenyl ring of the CANAC ligand. 1 and 4 can catalyze the oxidation of cyclooctene with PhIO, possibly via an Ir-oxo or Ir-PhIO active intermediate. The results of this work highlight the ability of the dianionic tripodal CANAC ligand to stabilize high-valent iridium.

Automated summary

In this study, iridium complexes containing a bis-cyclometalated tripodal CANAC ligand were synthesized and their redox property and reactivity were investigated. Various iridium complexes were successfully obtained through different reactions, and their structures and properties were further studied. The results highlighted the crucial role of the CANAC ligand in stabilizing high-valent iridium.