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. 2025 Aug 13:10.1039/d5dt01576f.
doi: 10.1039/d5dt01576f. Online ahead of print.

IPaulHet - spatially-defined, wingtip-flexible, N,C-chelating oxazole and thiazole donor N-heterocyclic carbene ligands

Pamela Podchorodecka  1 , Błażej Dziuk  2 , Roman Szostak  3 , Michal Szostak  4 , Elwira Bisz  1
Affiliations

IPaulHet - spatially-defined, wingtip-flexible, N,C-chelating oxazole and thiazole donor N-heterocyclic carbene ligands

Pamela Podchorodecka et al. Dalton Trans. .

Abstract

N-Heterocyclic carbenes (NHCs) are among the most versatile ligands in transition metal catalysis with their steric and electronic properties playing a critical role in governing reactivity and selectivity. In this area, the wingtip unsymmetrical IPaul ligand introduced by Nelson and co-workers offers a unique balance of steric bulk and flexibility characterized by spatially-defined steric features. Herein, we report a new class of IPaul-based ligands bearing benzoxazole and benzothiazole donor wingtips. The synthesis, application, and structural and electronic characterization are described. These ligands retain the defining 'bulky yet flexible' profile of IPaul, while enabling precise control over the catalytic pocket geometry through N-heteroaryl wingtip substitution. We present their coordination chemistry with Ag(I), Pd(II), Rh(I), and Se as well as catalytic studies in cross-coupling and hydrosilylation catalyzed by Ag, Pd, and Rh complexes. By combining the steric asymmetry of IPaul with the chelating flexibility of N-azole donors, this ligand class provides stabilization of reactive metal centers and is well-suited for diverse catalytic applications. We anticipate that the combination of steric flexibility with N,C-chelation in versatile N-heterocyclic carbenes will be of broad interest across organometallic, inorganic and catalytic chemistry.

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Conflict of interest statement

Conflicts of interest

There are no conflicts to declare.

Figures

Fig. 1.
Fig. 1.
State-of-the-art of sterically-demanding N-heterocyclic carbenes in inorganic and organometallic chemistry.
Fig. 2.
Fig. 2.
X-ray crystal structures of complexes 4a (I), 4b (II), 5b (III) and Ag(IPaul)Cl (IV). Two views for 4a and 4b: front (A); side (B). Hydrogen atoms have been omitted for clarity. Selected bond lengths [Å] and angles [°], see the SI.† (C) Topographical steric map of complexes 4a, 4b, 5b and Ag(IPaul)Cl showing %Vbur per quadrant. 4a: CCDC 2452824 4b: 2452825 and 5b: CCDC 2452826.
Fig. 3.
Fig. 3.
(A) HOMO and LUMO energy levels (eV). (B) HOMO-1, LUMO+6 and LUMO (eV) of IPaulOxa calculated at B3LYP 6–311++g(d,p). See SI.
Scheme 1
Scheme 1
Synthesis of Imidazolium Precursors. (A) Synthesis of 1-Arylimidazole 2, (B) Synthesis of Imidazolium Salts 3a–3b
Scheme 2
Scheme 2
Synthesis of Ag(I)–NHC Complexes 4a–4b
Scheme 3
Scheme 3
Synthesis of Pd(II)–NHC Complexes 5a–5b
Scheme 4
Scheme 4
Synthesis of Se–NHC Complexes 6a–6b
Scheme 5
Scheme 5
Synthesis of Rh(I)–NHC Complexes 7a–7b
Scheme 6
Scheme 6
Catalytic Activity of [IPaulHet–M] Complexes
See this image and copyright information in PMC

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