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

Abstract

In an effort to synthesize cyclometalated iridium complexes containing a tridentate C^N^C ligand, transmetallation of [Hg(HC^N^C)Cl] (1) (H₂C^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 Ir₄Hg₂ complex, [Cl(κ²C,N-HC^N^C)(cod)IrHgIr(cod)Cl₂]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)Cl₂] (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 PhICl₂ gave [Ir(C^N^C)(cod)Cl]·HgCl₂ (5·HgCl₂) 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)(H₂O)](OTf) (6) that can catalyze the cyclopropanation of styrene with ethyl diazoacetate. Reaction of 1 and [Ir(CO)₂Cl(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 (PPh₄)[Ir(CO)₂Cl₂] and sodium acetate in acetonitrile gave [Ir(C^N^C)(κ²C,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(κ²C,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(PPh₃)₃Cl] with 1 and sodium acetate in refluxing thf led to isolation of the previously reported compound [Ir(κ²P,C-C₆H₄PPh₂)₂(PPh₃)Cl] (10). The crystal structures of 2–5, 8, 8′, 9 and 10 have been determined.