fac-Re(CO)3 core-based complex featuring benzimidazole as pendant motif from hydroxyquinoline and pyridylbenzimidazole
Graphical abstract
Introduction
The design and synthesis of new acyclic- and cyclic- fac-[Re(CO)3]+ core-based complexes are continuously increasing due to their potential applications in pharmaceuticals and catalysis [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Recently, special attention has been focused on making acyclic rhenium complexes with chelating ligands due to their final complexes' stability and potentials in biology. Several ligands with chelating donors such as N^N, N^N−, N^C−, N^O−, O^N−, O^O−, N^S, N^S−, S^S, P^S, P^P, P^O, and P^O− are used. Among the various chelating ligands, 8-hydroxyquinoline and its derivatives (HOQN) have been used for making acyclic fac-[Re(CO)3]+ core-based neutral homoleptic dinuclear complexes (Type-I) and neutral heteroleptic mononuclear complexes (Type-II) along with neutral monodentate Lewis donors (Chart 1) [3]. The monodentate donors so far utilized are acetonitrile, pyridine, 4-hydroxypyridine, imidazole and its derivatives. Though the complexes display interesting properties and show potential applications in the field of electrocatalysts for CO2 reduction and luminescent materials, only countable number of the complexes are available. It is very important to mention that complexes-based on [ReO] core and 8-hydroxyquinolinate are well-known [4]. Similarly, complexes based on OQN with other metal ions are plentiful and have potential applications in medicinal field and materials [4]. Because of the importance of fac-[Re(CO)3(OQN)] core-based complexes in various research areas, efforts are being focused toward the design of new types of fac-[Re(CO)3(OQN)]-core based complexes by modulating 8-hydroxyquinoline core as well as ancillary ligands. We have been designing and synthesizing various types of fac-[Re(CO)3]+ core-based supramolecular coordination complexes (discrete cyclic systems) and acyclic complexes using various types of ligands [16]. To explore further, we envision that designing a small complex based on fac-[{Re(CO)3}{8-hydroxyquinolinate}]-motif decorated with biological motif units including benzimidazole as a non-coordinating motif at proper distance would result in a new type of complex system that can find potential applications in medicinal field [1]. The decorating benzimidazole motif in the rhenium complex may prove to be useful for further interaction with biological environment via intercalation and multiple hydrogen bonding interactions. However, remotely fixing benzimidazolyl motif in the complexes is challenging because of the high affinity of benzimidazole towards rhenium core.[3e, 4g-h, 16] Herein, we report a new type of hydroxyquinone-based fac-[Re(CO)3]+ complex, fac-[Re(CO)3(OQN)(pybimz)] (1) where pybimz = 2-(4′-pyridyl)benzimidazole, in which a free benzimidazole motif is remotely attached to the coordinated metal-complex. The results indicate that the combination of pyridine-benzimidazolyl core will be utilized for making fac-[Re(CO)3]+ complex with free benzimidazole core. The molecular structure of the complex was confirmed by single-crystal X-ray diffraction analysis. The molecules are arranged in the crystal structure as a cavity containing 3D polymeric network assisted by hydrogen bonding interactions. The absorption properties of the complex were studied both experimentally and theoretically.
Section snippets
Results and discussion
The treatment of Re2(CO)10, HOQN, pybimz, and toluene under solvothermal conditions resulted in pale yellow crystals of compound 1 (Scheme 1). The expected other side products in the reaction are dinuclear homoleptic complex [{Re(CO)3(OQN)}2] (I) and dinuclear heteroleptic complex [{Re(CO)3(OQN)}2(pybimz)] (II). The pybimz acts as bridging ligand in II. The other side product in all the cases is dihydrogen gas. Based on the obtained yield 69% (crystals), we assume that the major products
General data
Re2(CO)10 (98%), 4-pyridinecarboxaldehyde (97%), 8-hydroxyquinoline (98%), o-phenylene- diamine, and toluene were purchased and used as received. Spectroscopic grade solvent was used in the absorption measurements. Elemental analysis was performed on a Elemental Analyses system GmbH VarioEL elemental analyzer. Infrared spectrum was recorded on a Perkin-Elmer FT-IR spectrophotometer. 1H NMR spectra were recorded on Bruker Avance III 500 MHz instrument. Electronic absorption spectra were recorded
Appendix A. Supplementary material
CCDC no: 2071328 for 1. Copy of this information can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (Fax: +44-1223-336033; e-mail: [email protected] or www.ccdc.cam.ac.uk).
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.
Aknowledgements
We thank University of Hyderabad, DST (PURSE, FIST programs) and UGC-India (CAS, UPE programs) for financial support. IM thanks DST INSPIRE for fellowship.
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Both MP and IM contributed equally.