New half-sandwich (η6-p-cymene)ruthenium(II) complexes with benzothiazole hydrazone Schiff base ligand: Synthesis, structural characterization and catalysis in transamidation of carboxamide with primary amines

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Highlights

Abstract

Few half-sandwich (η6-p-cymene) ruthenium(II) complexes supported by benzothiazole hydrazone Schiff bases were synthesized. The new complexes possess the general formulae [Ru(η6-p-cymene)(L)Cl] (1-3) (L = salicyl((2-(benzothiazol-2-yl)hydrazono)methylphenol) (SAL-HBT), 2-((2-(benzothiazol-2-yl)hydrazono)methyl)-6 methoxyphenol) (VAN-HBT) or naphtyl-2-((2-(benzothiazol-2-yl)hydrazono)methyl phenol) (NAP-HBT). All compounds were fully studied by analytical, spectroscopic techniques (IR, NMR) and also by mass spectrometry. The solid state structure of the complex 3 reveals the coordination of p-cymene moieties with ruthenium(II) in a three-legged piano-stool geometry along with benzothiazole hydrazone Schiff base ligand in a monobasic bidentate fashion. The catalytic properties of the complexes were screened in transamidation of primary amide with amines after optimization with respect to solvent, substituents, time and catalyst loading. The results show that the complex 3 is the most efficient catalyst for the transamidation of carboxamides with amines.

Introduction

Amide functional groups present in numerous compounds ranging from biologically active natural products to pharmaceuticals [1]. Some examples of these include lipitor (atorvastatin), fentanyl anaesthetic, metolachlor, capsaicin and nylon. They are also found within biological systems as the peptide bonds in proteins rendering them essential for life. As a result, several methods have been established for the synthesis of amides [2], [3], [4], [5], [6] including reactions of carboxylic acid derivatives (other than amides) with amines or ammonia [7], hydration of nitriles [5] and reaction of amines with aldehydes or alcohols [8] in addition to some recognized name reactions [9]. All of these methods possess their own merits along with certain shortcomings including utilization of stoichiometric amount of activating reagents, long reaction times, harsh reaction conditions, limited substrate scopes and formation of significant amount of chemical waste. In the search of cost-effective and more atom economical protocol, one of the methods employed to synthesis amide was transamidation, catalyzed by homogeneous transition metal catalysts. Stahl [10], Williams [11], Beller [12] and other groups [13] reported elegant methods for transamidation by employing homogeneous transition metal catalytic systems. Among the homogeneous metal catalysts, Ru compounds emerged out to be the most promising candidate. In the past decade, many researchers utilized ruthenium metal catalysts for transmidation reaction including our own group [14]. Despite good results and advances were achieved in the previous reports, still it is interesting to synthesis new homogeneous catalysts with better catalytic activity.

In recent years, cyclometalated organometallic ruthenacycles flourish as an active class of catalysts. The aromatic π-ligand stabilizes and protects the metal centre from rapid oxidation [15]. These Ru(II) arene complexes display a three-legged piano stool structure creates the possibility of introducing different types of ligands into the octahedral metal centre [16] and also successfully utilized as better catalysts for several catalytic reactions such as C−H bond activation, hydrogenation reactions of unsaturated bonds of carbonyl, alkene and imines bonds, oxidative Heck reactions, oxidative C–C coupling and C−N coupling (amidation and hydroamination) [17], [18], [19], [20], [21], [22]. The results of the above reactions show that varying the complex framework through modification of the arene (η6R-arene) and the other auxiliary ligands is crucial for tuning chemical reactivity. In this context, many different supporting auxiliary ligands have been used in combination with the Ru(II) arene scaffold to provide different reactivity profiles. The recent reports show that benzothiazole hydrazones attract considerable attention as versatile ligands since they possess both hard and soft donor atoms (e.g. NS or XNS, X = N, O, and S) in their skeleton [23].

Based on the above facts and continuation of our research on the synthesis, characterization and catalytic applications of transition metal based catalyst, we here in describe synthesis of three half-sandwich (η6-p-cymene)ruthenium(II) complexes bearing benzothiazole hydrazone Schiff base ligand with different wingtip substituent in the benzene ring. The new complexes were characterized by analytical, spectroscopic techniques (IR, NMR), mass spectrometry and single crystal XRD. The synthesized complexes were used as catalysts in the transamidation of primary amide with amines. The effects of solvent, substituents, time and catalyst loading on the catalytic activity were also investigated.

Section snippets

General strategy

Commonly available RuCl3.3H2O was used as supplied from Sigma Aldrich. All chemicals and solvents were acquired from Merck or Aldrich. Thin-layer chromatography (Merck 1.05554 aluminum sheets precoated with silica gel 60 F254) was used for reaction observance and the spots were visualized with UV light at 254 nm or under iodine. Column chromatography refinement was done for the complexes using silica gel (200–400 mesh). Melting points were checked in open capillary tubes on a Technico micro

Synthesis and spectral characterization

The reaction of 2-hydrazino benzothiazole with salicylaldehyde, o-vanillin or 2-hydroxy-1-naphthaldehyde in ethanol gives the desired Schiff base ligands SAL-HBT, VAN-HBT or NAP-HBT in good yields. The reaction of ligands with [Ru(η6-p-cymene)Cl2]2 in ethanol under reflux affords the new complexes (1-3) (Scheme 1). All the complexes are non-hygroscopic solids, air-stable in solid state and solution at room temperature; soluble in common organic solvents such as acetone, benzene, chloroform,

Conclusions

Synthesis, structural characterization and catalytic activity of half-sandwich (η6-p-cymene) Ruthenium(II) complexes were presented in this work. The benzothiazole hydrazone Schiff base ligands coordinated with Ru(II) ion as monobasic bidentate N^N fashion. The pseudo-octahedral “piano-stool” geometry compensated by p-cymene ring (η6), one Cl and monobasic bidentate (N^N) benzothiazole hydrazone Schiff base ligands have been revealed by single crystal X-ray diffraction study (Complex 3).

CRediT authorship contribution statement

Subbarayan Vijayapritha: Conceptualization, Investigation, Writing - original draft. Periasamy Viswanathamurthi: Validation, Methodology, Formal analysis, Writing - review & editing, Visualization, Supervision, Project administration.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We thank Department of Chemistry, Gandhigram Rural Institute-Deemed University, Gandhigram (NMR spectra) and SAIF, Gauhati University, Guwahati (single crystal X-Ray) for their help in characterization studies.

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