Rhenium(I) complexes from 2-(2'-hydroxyphenyl)benzimidazolyl based bis-chelating ligand and 4-(amino)pyridine/4-(dimethylamino)pyridine
Graphical abstract
Introduction
The rhenium-carbonyl based complexes have been gaining intense research interest in the fields of bio-imaging, anticancer agents, photosensitizer as well as photocatalyst for carbon dioxide reduction to useful chemicals, molecular sensors due to their intrinsic properties such as kinetic inertness, photo-stability, photo-physical and electrochemical properties.[1], [2], [3], [4], [5], [6], [7], [8], [9] The ligand design plays a crucial role in tuning the excited state properties of these complexes. Few organic heteroaromatic motifs including chelating ligands, 2,2′-bipyridine, 1,10-phenanthroline and their structural analogous, and monodentate donors, pyridine, phosphine, and their derivatives, have been widely employed for making the complexes.[1], [2], [3], [4], [5], [6], [7], [8], [9] Due to their importance in various fields, designing new complexes have been increasing steadily in order to tune the photophysical properties, in particular, shifting the absorption and emission towards the red region. We have been using commercially known chelating ligands, 2-(2'-hydroxyphenyl)benzimidazole, 2-(2'-hydroxyphenyl)benzothiazole, 2-(2'-hydroxyphenyl)benzoxazole, 2-(imidazo[1,2-a]pyridin-2-yl)phenol and designing new bis-chelating ligands based on 2-(2'-hydroxyphenyl)benzimidazolyl core for making luminescent acyclic mononuclear to tetranuclear complexes, and supramolecular coordination complexes (SCCs) including helicates, mesocates and tetrahedrons in the presence of pyridine/benzimidazolyl based neutral nitrogen donor.[10] The complexes comprising of fac-Re(CO)3, phenoxybenzimidazolyl and pyridine/benzimidazolyl donor are stable and display phosphorescent emission. In continuation of the research in the field, 4-(amino)pyridine derivatives, rigid bis-chelating ligand containing two units of 2-(2'-hydroxyphenyl)benzimidazolyl and phenyl spacer are chosen instead of benzimidazolyl motif, and flexible bis-chelating ligand, respectively, in order to make a dinuclear fac-Re(CO)3 core–based complexes with enhanced photophysical properties.
Herein, we report two fac-Re(CO)3 core based complexes with general formula, fac-[{Re(CO)3(L′)}2(L)](where, L′ = 4-(amino)pyridine = 4-AP for 1, L′ = 4-(dimethylamino)pyridine = 4-DMAP for 2, and L = 1,4-phenylene-bis(2-(2′-hydroxyphenyl)benzimidazolyl = H2-L). The complexes were synthesized by treating Re2(CO)10, H2-L and L′ via a one-pot solvothermal approach. The complexes were characterized using various analytical and spectroscopic methods. The molecular structure of the complex 2 was confirmed from a single-crystal X-ray diffraction analysis. The photophysical properties of the complexes were studied in the solution state.
Section snippets
Results and discussion
The treatment of Re2(CO)10, H2-L, 4-AP or 4-DMAP, mesitylene:hexane or mesitylene via a one-pot solvothermal approach ( resulted in complexes 1−2 (Scheme 1). The yellow color complexes are air and moisture stable. The complexes are sparingly soluble in DMSO. The ATR−IR spectra of the complexes display three carbonyl stretching bands in the range 2008−1864 cm−1, corresponding to the fac-Re(CO)3 core. [10] The 1H NMR of complex 1 was recorded in a mixture of solvent, CDCl3:DMSO-d6 (5:1, v/v). The
General data
Re2(CO)10, 4-(amino)pyridine, 4-(dimethylamino)pyridine, o-phenylenediamine, 2-hydroxybenzaldehyde, cesium carbonate, copper iodide, 1,10-phenanthroline, mesitylene, hexane and spectroscopic grade solvents, DMSO and DMF, were obtained from commercial sources and used as received. The ligand H2-L was synthesized by the previously reported method.[10i] Elemental analyses were performed on the Flash EA 1112 series CHNS analyser. ATR-IR spectra were recorded on a Nicolet iS5 FT-IR spectrometer.
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 University of Hyderabad, DST (PURSE, FIST programs) and UGC-India (CAS, UPE programs) for financial support. IM thanks DST for INSPIRE fellowship. MP and MS thank Dr. R. V. Krishnakumar (Department of Physics, Thiagarajar College, Madurai) and Networking Resource Centre (SoC, UoH) for scientific support.
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