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Fourteen Ag( i ), Au( i ), Ni( ii ), Pd( ii ), and Pt( ii ) complexes with macrocyclic tetradentate N-heterocyclic carbene (NHC) ligands were prepared via reactions between three macrocyclic tetrabenzimidazolium salts and metal precursors. All except two Au complexes were characterized using single-crystal X-ray diffraction. Three different structures, including a trinuclear one containing a NHC–Ag–(H 2 O) moiety and a hexanuclear propeller-like supramolecular assembly, are found for Ag–NHC complexes. Nine complexes of group 10 metal ions adopt square-planar geometry, in which the different ring-sizes of the macrocyclic tetracarbene ligands lead to a variation of metal–carbene bond lengths. π–π stackings are observed between the rigid aromatic benzimidazole rings in the nickel group complexes.
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A Rigidified Macrocyclic Grubbs Complex: A Rare Example of In - and Out -Isomers that Show a Dramatic Difference in Catalytic Reactivity
The design of a rigidified macrocyclic N-heterocyclic carbene (NHC) ligand led to the formation and structural characterization of in- and out-Ru carbene complexes. In this study, introduction of a conformational lock was used to rigidify heteroaryl-aryl bonds and thereby enforce a more perpendicular dihedral angle. A forcing metalation step was needed to form the isomeric Ru carbene complexes (Grubbs complexes). The major isomer had the Ru carbene fragment located outside the macrocyclic ring whereas the minor isomer had the Ru carbene inside the macrocyclic ring. The two new Ru carbene complexes are the first examples of in- and out-isomers of a Grubbs-type complex. The solid state structures of each isomeric ruthenium carbene complex was determined by x-ray diffraction studies. The two Ru complexes showed significantly different catalytic reactivity in the ring-closing metathesis (RCM) of the benchmark substrate, diethyl diallylmalonate. We performed computational studies to determine rotational barriers; scalable energetic barriers were found in the unmetallated NHC ligand, favoring the in-isomer by 2.4 kcal/mol. These calculations, coupled with attempted interconversion of isomers, support a mechanism featuring rotational isomerization of the NHC nucleophile in a preequilibrium step before metalation.
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- Award ID(s):
- 2154997
- PAR ID:
- 10543482
- Editor(s):
- Chirik, Paul
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Organometallics
- Volume:
- 43
- Issue:
- 18
- ISSN:
- 0276-7333
- Page Range / eLocation ID:
- 2132 to 2146
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.organomet.4c00331
