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Development of Au-based Multinuclear Complexes with N-Heterocyclic Carbene Ligands

張, 茜 東京大学 DOI:10.15083/0002001525

2021.09.08

概要

1. Introduction
Gold compounds have shown promise as catalysts for molecular transformation and luminescence materials. In particular, AuI compounds have unique characteristics such as carbophilic π-acidity, redox activity, and aurophilicity due to the d10 closed shell electron configuration of AuI ions in a linear two-coordinate geometry. Among them, multinuclear AuI compounds, in which two or more AuI ions are bonded or located close to each other, have been extensively investigated to gain knowledge about the influence of aurophilic interactions on the structure-specific electronic properties leading to multi-electron redox catalysts and photoluminescence. Over the last two decades, there has been a steep rise in research related to AuI complexes ligated by N-heterocyclic carbene (NHC) ligands. Compared to traditional phosphine ligands, NHC ligands have a stronger σ-donating property, and manipulation of the electronic and steric characters of NHC ligands can be more easily achieved by N-functionalization. In this study, I focused on the construction of multinuclear AuI complexes with NHC ligands. Specifically, a carbon-centered CAuI 6 cluster and a macrocyclic dinuclear AuI compound based on bis-NHC ligands have been developed for Au-based multinuclear complexes with structure-specific properties.

2. Synthesis and characterization of carbon-centered AuI complexes supported by NHC ligands
In my master course study, I constructed a carbon-centered AuI cluster, [C(AuI L)6] 2+, supported by 1,3-diisopropylimidazol-2-ylidene (L = IiPr) ligands by the reaction of [(IiPrAu)3O](BF4) with (trimethylsilyl)diazomethane in CH2Cl2 in the presence of triethylamine to produce chemically stable dark-brown solid. Based on this, detailed characterization of the product and comparison with those of phosphine ligand supporting counterparts were conducted based on 1 H and 13C NMR, ESI-MS, absorption/emission, and XRD analyses in this study.

A single crystal of [C(AuIiPr)6](BF4)2 suitable for XRD measurement was obtained by slow evaporation from a CH2Cl2/n-hexane solution as a yellow block crystal. The AuI cluster has an antiprism structure with D3h symmetry, and the bond lengths of Au(1)–Au(1)’ and Au(1)–Au(1)” are 3.0548(3) and 2.9282(3) Å, respectively. This result suggests strong Au-Au interactions in the cluster. The Au– Au distances are comparable to those of a triphenylphosphine supported counterparts (2.887(1)– 3.226(1) Å). Notably, the whole crystal structure is not highly symmetrical due to the packing force arising from the bulky IiPr ligands

Photoluminescence measurement of [C(AuIiPr)6](BF4)2 cluster in the solid state revealed that it is luminescent with green emission under irradiation of UV light (λex = 370 nm). The emission had a longer wavelength (λem = 547 nm) compared with that of a triphenylphosphine supported carbon-centered cluster (λem = 537 nm). This difference may come from the stronger σ-coordination ability of NHC ligands (Figure 1). On the other hand, no emission was observed for [C(AuIiPr)6](BF4)2 in solution at room temperature.

3. Au-mediated macrocycle with bis-NHC ligands
NHCs can be easily modified with many types of organic ligand donors at the nitrogen atoms directing toward metal array. In this study, aiming at linearly arranging heterometallic coinage metal ions with a close distance to study the influence of metallophilic interactions on metal array-specific properties, I developed a bis-NHC ligand L with a pyridine-centered heterocycle-coupled spacer. Since NHC ligands have a higher affinity to AuI ions than pyridine and furan, which prefer AgI or CuII , ligand L was expected to produce a linear heterometallic AuI – M–AuI (M = AgI or CuII) complex in a ladder or helical double-stranded structure.

Ligand L was synthesized from 2-acetylfuran in 6 steps (Scheme 1). Firstly, 2,6-difuryl pyridine 1 was prepared by a coupling reaction of two 2-acetyl furan derivatives, and then brominated by N-bromosuccinimide (NBS) to afford 2. Compound 2 was reacted with imidazole using a CuI catalyst to produce a bis-imidazole 3, followed by alkylation at the terminal nitrogen atoms by n-butyl chloride to obtain a bis-imidazolium salt, H2L·Cl2. The counter anion was then replaced using AgPF6 to obtain H2L·(PF6)2. Complexation of H2L·(PF6)2 with equimolar amount of thtAuCl (tht = SC4H8) in the presence of base formed a macrocyclic dinuclear AuI complex, [L2AuI 2](PF6)2 in 60% yield, which is air- and light-stable at room temperature. Its NMR spectra in CDCl3 well established a high symmetrical structure in solution. The 13C signals assignable to Ccarbene atoms were observed at 182.2 ppm, in the range for cationic two-coordinate NHC–Au–NHC compounds. The molecular structure of [L2AuI 2](PF6)2 was determined by single-crystal X-ray analysis (Figure 2). The intramolecular Au–Au distance is ca. 13.5 Å, which is too far for intramolecular aurophilic interactions. Notably, as the macrocyclic cavity is very narrow and there is steric hindrance between the proton atoms on the furan rings, the AuI -mediated macrocycle is distorted to adapt a helical structure while the single crystal is a racemic mixture.

H2L·(PF6)2 and [L2AuI 2](PF6)2 were investigated with respect to their photoluminescence properties (Figure 3). At room temperature, H2L·(PF6)2 produced intense blue-purple luminescence both in solution and solid state. [L2AuI 2](PF6)2 showed a photoluminescence behavior similarly to that of H2L·(PF6)2, except that the intensity both in solution and solid state were much decreased. This result indicates that the emission mainly comes from the linked heteroaromatic rings, while significant quenching takes place in [L2AuI 2](PF6)2 due to the steric and electronic changes caused by the complexation with AuI ions.

The central pyridine moiety of the ligand L in [L2AuI 2](PF6)2 can possibly coordinate to other metal ions to enable linear heterometallic arrangement. In expectation that a CuII ion would bridge two pyridine ligands in the macrocycle, 1.5 eq of CuII was added to a solution of [L2AuI 2](PF6)2 in CH3CN. However, instead of generation of [L2AuI 2CuIICl2](PF6)2, oxidation of AuI to AuIII was observed, as the signals for L2AuI 2, L2AuI AuIII and L2AuIII 2 were confirmed by NMR measurement (Figure 4). Further addition of CuII to the solution resulted in conversion from [L2AuI 2](PF6)2 to a dinuclear AuIII macrocyclic compound [L2AuIII 2Cl4](PF6)2. Unfortunately, the isolation of [L2AuIII 2Cl4](PF6)2 was not successful because of its decomposition, as L2AuI 2 and L2AuI AuIII species were observed again after purification probably due to their disproportionation. In order to fully study the redox behaviors of dinuclear macrocyclic compound [L2AuI 2]·(PF6)2 and to achieve redox-induced helicity switching in the solid state, later, freshly prepared dichloroiodobenzene as an oxidant was added to a solution of [L2AuI 2]·(PF6)2 in CH3CN to generate fully oxidized dinuclear AuIII macrocylic compound [L2AuIII 2Cl4]·(PF6)2, which was obtained as a stable white solid. Further study of the redox behavior and introduction of other heterometallic ions into macrocyclic [L2AuI 2](PF6)2 is on going.

4. Conclusion
In this study, I have examined Au-based multinuclear complexes with N-heterocyclic carbene ligands using a carbon atom center or a linearly linking bis-monodentate ligand as the template, with a focus on the influence of carbene coordination and aurophilic interactions on the structure and electronic properties of the resultant complexes. Consequently, a carbon-centered CAu6 cluster with antiprism arrangement as well as a macrocyclic dinuclear AuI compound based on bis-NHC ligands were obtained and their luminescent properties and redox behaviors have been clarified. Further studies on their photochemical and electronic properties and multi-electron-based catalytic activities are now underway.

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