Synthesis of Complexes with Protic NH,NH-NHC Ligands via Oxidative Addition of 2-Halogenoazoles to Zero-Valent Transition Metals

Abstract

A versatile, one-pot synthesis for the preparation of transition metal complexes bearing protic NH,NH-NHC ligands is disclosed. The reaction of unsubstituted 2-halogenoazoles with zerovalent metal complexes of the type [M(PPh 3 ) 4 ] (M = Pd, Pt) in the presence of NH 4 BF 4 proceeds by oxidative addition of the C2−X (X = Cl, Br, I) bond to the transition metal followed by protonation at the ring nitrogen atom, leading to complexes of the type trans-[MX(PPh 3 ) 2 (NH,NH-NHC)] (M = Pd II , Pt II ; NHC = benzimidazolylidene or imidazolylidene). The trans-complexes have been obtained in all cases. The time needed to complete the reaction depends on the halogen present in the azole, and the rate of the oxidative addition follows the order C2−I > C2−Br > C2−Cl. The N−H groups of the coordinated NH,NH-NHC have been deprotonated followed by reaction with MeI to give the NMe,NMe-substituted classical NHC ligands. The N−H groups of the protic NHCs are hydrogen bond donors. The formation of N−H•••O hydrogen bonds has been observed via 1 H NMR spectroscopy upon titration of the complexes bearing protic NHCs with DMPU.

Preparation of Complexes Bearing N‐Alkylated, Anionic or Protic CAACs Through Oxidative Addition of 2‐Halogenoindole Derivatives

“…The heterocyclic ligand features all characteristics of the known CAAC ligands of type A except for the C4‐C5 annulation and the unique N1‐alkyl substitution. Typical CAAC ligand properties were also observed in the 13 C NMR spectrum of cis ‐[ 3 ]BF 4 featuring the downfield shifted resonance for the C CAAC carbon atom at δ (C2)=242.2 ( cf the C NHC resonance at δ (C2)≈165–175 found in related NHC‐Pd complexes [9a,d,e] …”

“…Apart from azolium cations, [8b,d] neutral halogenoazoles can also oxidatively add to low‐valent transition metals to yield complexes bearing anionic azolato ligands, [9a–e] which after N‐protonation yield complexes bearing neutral protic NHC ( p NHC) [9a,c–e] ligands [15] . In order to prepare the analogous but currently unknown complexes bearing N1‐protonated CAAC ligands, neutral 2‐chloroindole 1 was reacted with [Pt(PPh 3 ) 4 ] to give complex trans ‐[ 5 ] bearing an anionic CAAC ligand with an unsubstituted ring‐nitrogen atom (Scheme 3, top).…”

Photoorganocatalytic Hydroacylation of Dialkyl Azodicarboxylates by Utilising Activated Ketones as Photocatalysts

“…Both the classical NHC bearing (type A )21 and the protic NHC bearing (type E )16c,d analogues of trans ‐[ 4 ] have been described. A comparison of the Pt‐C2/4 bond lengths in these complexes reveals that the N , N ‐dimethylimidazolylidene complex (type A ) features a Pt–C2 bond length of 1.995(5) Å, which is identical within experimental error to the Pt–C4 bond length observed for trans ‐[ 4 ].…”

Iridium(III) Complexes Bearing Chelating Bis‐NHC Ligands and Their Application in the Catalytic Reduction of Imines

“…However, this synthetic method involves the cumbersome preparation of the isocyanides, and hence some novel synthetic methodologies to afford transition metal complexes with protic NHC ligands have subsequently been developed. We have described the reaction of 2‐halo‐ N ‐alkylazoles and 2‐haloazoles with transition metal complexes to yield, in the presence of a proton source, complexes with protic NH,NR and NH,NH NHC ligands, respectively.…”