Synthesis, structure and reactivity of iridium complexes containing a bis-cyclometalated tridentate C^N^C ligand

In an effort to synthesize cyclometalated iridium complexes containing a tridentate C^N^C ligand, transmetallation of [Hg(HC^N^C)Cl] ( 1 ) (H 2 C^N^C = 2,6-bis(4- tert -butylphenyl)pyridine) with various organoiridium starting materials has been studied. The treatment of 1 with [Ir(cod)Cl] 2 (cod = 1,5-cyclooctadiene) in acetonitrile at room temperature afforded a hexanuclear Ir 4 Hg 2 complex, [Cl(κ 2 C , N -HC^N^C)(cod)IrHgIr(cod)Cl 2 ] 2 ( 2 ), which features Ir-Hg-Ir and Ir-Cl-Ir bridges. Refluxing 2 with sodium acetate in tetrahydrofuran (thf) resulted in cyclometalation of the bidentate HC^N^C ligand and formation of trinuclear [(C^N^C)(cod)IrHgIr(cod)Cl 2 ] ( 3 ). On the other hand, refluxing [Ir(cod)Cl] 2 with 1 and sodium acetate in thf yielded [Ir(C^N^C)(cod)(HgCl)] ( 4 ). Chlorination of 4 with PhICl 2 gave [Ir(C^N^C)(cod)Cl]·HgCl 2 ( 5 ·HgCl 2 ) that reacted with tricyclohexylphosphine to yield Hg-free [Ir(C^N^C)(cod)Cl] ( 5 ). Chloride abstraction of 5 with silver( i ) triflate (AgOTf) gave [Ir(C^N^C)(cod)(H 2 O)](OTf) ( 6 ) that can catalyze the cyclopropanation of styrene with ethyl diazoacetate. Reaction of 1 and [Ir(CO) 2 Cl(py)] (py = pyridine) with sodium acetate in refluxing thf afforded [Ir(C^N^C)(HgCl)(py)(CO)] ( 7 ), in which the carbonyl ligand is coplanar with the C^N^C ligand. On the other hand, refluxing 1 with (PPh 4 )[Ir(CO) 2 Cl 2 ] and sodium acetate in acetonitrile gave [Ir(C^N^C)(κ 2 C , N -HC^N^C)(CO)] ( 8 ), the carbonyl ligand of which is trans to the pyridyl ring of the bidentate HC^N^C ligand. Upon irradiation with UV light 8 in thf was isomerized to 8′ , in which the carbonyl is trans to a phenyl group of the bidentate HC^N^C ligand. The isomer pair 8 and 8′ exhibited emission at 548 and 514 nm in EtOH/MeOH at 77 K with lifetime of 84.0 and 64.6 μs, respectively. Protonation of 8 with p -toluenesulfonic acid (TsOH) afforded the bis(bidentate) tosylate complex [Ir(κ 2 C , N -HC^N^C) 2 (CO)(OTs)] ( 9 ) that could be reconverted to 8 upon treatment with sodium acetate. The electrochemistry of the Ir(C^N^C) complexes has been studied using cyclic voltammetry. Reaction of [Ir(PPh 3 ) 3 Cl] with 1 and sodium acetate in refluxing thf led to isolation of the previously reported compound [Ir(κ 2 P , C -C 6 H 4 PPh 2 ) 2 (PPh 3 )Cl] ( 10 ). The crystal structures of 2-5 , 8 , 8′ , 9 and 10 have been determined. In an effort to synthesize cyclometalated iridium complexes containing a tridentate C^N^C ligand, transmetallation of