The Fascinating Flexibility and Coordination Modes of a Pentamethylene Connected Macrocyclic CNC Pincer Ligand.

Ronja Jordan,Doris Kunz

doi:10.3390/molecules26061669

Abstract

The coordination chemistry of an electron-rich macrocyclic CNC pincer-ligand consisting of two pentamethylene tethered N-heterocyclic carbene moieties on a carbazole backbone (bimcaC5) is investigated by mainly NMR spectroscopy and X-ray crystal structure analysis. A bridging coordination mode is found for the lithium complex. With the larger and softer potassium ion, the ligand adopts a facial coordination mode and a polymeric structure by intermolecular potassium nitrogen interactions. The facial coordination is also confirmed at a Cp*Ru fragment, while C-H activation under dehydrogenation at the alkyl chain is observed upon reaction with [Ru(PPh3)3Cl2]. In contrast, Pd(OAc)2 reacts under C-H activation at the central carbon atom of the pentamethylene tether to an alkyl-pincer macrocycle.

Highlights

Pincer-ligands, whose wingtips are tethered with simple or unfunctionalized hydrocarbon chains, are still very rare and an exception among phosphine pincer complexes [1,2].For geometric reasons, bis(N-heterocyclic carbene) pincer ligands are more suitable for tethering
When we reported on the improved synthesis of the so-called bimca ligand [15] (1,8bis(imidazolin-2-ylidene)-3,8-di-tert-butylcarbazolide), a monoanionic CNC ligand, we showed that a still relatively concentrated reaction mixture of 1,8-bis(imidazol-1-yl)3,8-di-tert-butylcarbazol and 1,5-dibromopentane leads selectively to the bisimidazolium macrocycle 1a (HbimcaC5 )·2HBr [13]
The first step of the synthesis of metal complexes is the generation of the monoanionic carbene ligand bimcaC5 with 3 equiv. of a strong base, preferentially an alkali metal base

Summary

Introduction

Bis(N-heterocyclic carbene) pincer ligands are more suitable for tethering. CNC ligands that contain alkyl tethers between 8 and 16 C-atoms [3,4,5]. Their transition metal complexes showed particular properties depending on the ring size [6,7,8], but no reactivity at the tether itself. Tethered CNC pincer complexes with smaller ring sizes are scarce [9], potential ligand precursors are well known [10,11,12,13,14]. The interaction with the metal center should become stronger and influence its reactivity by steric restrictions

Methods

Results

Conclusion