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

Abstract

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 NiBr 2 , PdCl 2 , and [Ir(COD)Cl] 2 to afford the Nheterocyclic carbene complexes (Nitron NHC ) 2 NiBr 2 , (Nitron NHC ) 2 PdCl 2 , and (Nitron NHC )Ir(COD)Cl, respectively. The lattermost compound reacts with (i) CO to afford the dicarbonyl compound (Nitron NHC )Ir(CO) 2 Cl and (ii) CO, in the presence of PPh 3 , to afford the monocarbonyl compound (Nitron NHC )Ir(PPh 3 )-(CO)Cl. Structural studies on (Nitron NHC )Ir(COD)Cl and (Nitron NHC )Ir(CO) 2 Cl indicate that Nitron NHC has a stronger trans influence than does Cl; furthermore, IR spectroscopic studies on (Nitron NHC )Ir(CO) 2 Cl indicate that Nitron NHC is electronically similar to the structurally related Enders carbene but is less electron donating than imidazol-2-ylidenes with aryl substituents. Significantly, the Nitron NHC ligand affords catalytic systems, as illustrated by the ability of (Nitron NHC )Ir(CO) 2 Cl to effect (i) the dehydrogenation of formic acid, (ii) aldehyde hydrosilylation, (iii) dehydrocoupling of hydrosilanes and alcohols, and (iv) ketone reduction via transfer hydrogenation.

Direct arylation reaction catalyzed by a PEPPSI-type palladium complex with an amido-functionalized triazole-based mesoionic carbene ligand

“…The Pd–C (NHC) distance of 1.957(9) Å is much shorter than that of 2.0138(15) Å in the reported trans -Pd(NHC nitron ) 2 Cl 2 . 32…”

“…The Pd-C (NHC) distance of 1.957 (9) Å is much shorter than that of 2.0138(15) Å in the reported trans-Pd(NHC nitron ) 2 Cl 2 . 32 An inspection of Fig. 6 clearly shows that seemingly there is an intramolecular π-π stacking of the pyridyl and phenyl ring in the structure of 4b.…”

“…Nitron is a mesoionic compound consisting of a triazolium moiety and had been shown to be a stable NHC ligand precursor. 32,50,51 A representative palladium complex of NHC derived from nitron is shown in Fig. 1.…”

“…1 Representative palladium complexes with normal NHC and MIC ligands in the literature. 23,31,32 Fig. 2 Reported and new palladium(II) carbene complexes.…”

Nitron-derivative-based palladium carbene complexes: structural characterization, theoretical calculations, and catalytic applications in the Mizoroki–Heck coupling reaction

“…[41][42][43][44] Its tautomer form in solution assumes the form of a free NHC ligand which readily reacts with different transition metal precursors. [45][46][47][48] Previous studies have documented several palladium(II) NHC complexes derived from nitron (Scheme 2). 48,49 However, to our knowledge, there is no literature on their corresponding palladium(0) complexes incorporating nitron-based NHC ligands.…”