N‑Heterocyclic Carbene Complexes of Nickel, Palladium, and Iridium Derived from Nitron: Synthesis, Structures, and Catalytic Properties

The mesoionic compound (1,4-diphenyl-1,2,4-triazol-4-ium-3-yl)­phenylazanide, commonly referred to as Nitron, has been employed as a “crypto-NHC” to afford 1,2,4-triazolylidene compounds of nickel, palladium, and iridium. Specifically, Nitron reacts with NiBr2, PdCl2, and [Ir­(COD)­Cl]2 to afford the N-heterocyclic carbene complexes (NitronNHC)2NiBr2, (NitronNHC)2PdCl2, and (NitronNHC)­Ir­(COD)­Cl, respectively. The lattermost compound reacts with (i) CO to afford the dicarbonyl compound (NitronNHC)­Ir­(CO)2Cl and (ii) CO, in the presence of PPh3, to afford the monocarbonyl compound (NitronNHC)­Ir­(PPh3)­(CO)­Cl. Structural studies on (NitronNHC)­Ir­(COD)­Cl and (NitronNHC)­Ir­(CO)2Cl indicate that NitronNHC has a stronger trans influence than does Cl; furthermore, IR spectroscopic studies on (NitronNHC)­Ir­(CO)2Cl indicate that NitronNHC is electronically similar to the structurally related Enders carbene but is less electron donating than imidazol-2-ylidenes with aryl substituents. Significantly, the NitronNHC ligand affords catalytic systems, as illustrated by the ability of (NitronNHC)­Ir­(CO)2Cl to effect (i) the dehydrogenation of formic acid, (ii) aldehyde hydrosilylation, (iii) dehydrocoupling of hydrosilanes and alcohols, and (iv) ketone reduction via transfer hydrogenation.