Heteroleptic Triazole-Bisoxazoline Iron Complexes Reveal Lability of the Iron-Carbene Bond Even Within a Chelating...

N-Heterocyclic carbenes have proven to be excellent ligands for transition metals, with numerous applications in catalysis and beyond. However, they have also displayed lability with first row transition metals, largely due to the hard-soft mismatch of the metal-carbon bond. Chelation is often considered a suitable methodology for supporting the labile M-C bond through the introduction of a strongly coordinating donor site such as hard phenolates. Herein, we demonstrate that chelating phenolate-carbene ligands are kinetically labile in iron(II) complexes. Specifically, heteroleptic iron complexes [Fe(C^O)(N^N)] were synthesized composed of a phenolate-functionalized triazolylidene (CO) ligand and N,N-bidentate coordinating bisoxazoline ligand (N^N). Stability studies by ¹H NMR spectroscopy showed that the heteroleptic complexes preferentially convert to their corresponding homoleptic complexes [Fe(C^O)₂] and [Fe(N^N)₂], indicating reversible decoordination of the carbene phenolate chelate from the iron center. The rate of this rearrangement is dependent on the substituents on the ligands and increases for triazolylidene wingtip groups mesityl (Mes) < di(isopropyl)aryl (DIPP) < adamantyl (Ad), with significant ligand redistribution for DIPP and Ad systems observed even at room temperature. The most stable heteroleptic complex featured mesityl wingtips on the triazole and phenyl groups as oxazoline substituents and displayed signs of ligand exchange only after 16 h at room temperature. This substitutional lability of carbene ligands even when supported by a phenolate chelating group has direct consequences when designing iron complexes for catalytic applications.