Visible Light Assisted Photocatalytic [3 + 2] Azide−Alkyne “Click” Reaction for the Synthesis of 1,4-Substituted 1,2,3-Triazoles Using a Novel Bimetallic Ru−Mn Complex Pawan Kumar, † Chetan Joshi, † Ambrish K. Srivastava, ‡ Piyush Gupta, § Rabah Boukherroub, ∥ and Suman L. Jain* ,† † Chemical Sciences Division, CSIR-Indian Institute of Petroleum, Mohkampur, Dehradun 248005 India ‡ Department of Physics, University of Lucknow, University Road, Lucknow, Uttar Pradesh 226007, India § Anaytical Sciences Division, CSIR-Indian Institute of Petroleum, Mohkampur, Dehradun 248005 India ∥ Institute of Electronics, Microelectronics and Nanotechnology (IEMN), UMR CNRS8520, Lille1 University, Avenue Poincare ́ -BP60069, 59652 Villeneuve d’Ascq, France * S Supporting Information ABSTRACT: A photoactive bimetallic complex comprising a photo- sensitizer ruthenium unit and a catalytic Mn(I) unit connected via a bipyrimidine (bpm) bridging ligand is prepared and used for the first time for developing a light induced copper catalyzed [3 + 2] azide− alkyne “click” (CuAAC) reaction for the formation of 1,2,3-triazoles under visible light irradiation. The developed bimetallic complex exhibited enhanced activity as both the photosensitizer ruthenium unit as well as manganese catalyst unit are attached in a single molecule, providing efficient electron transfer for the photochemical reduction of Cu(II) to Cu(I) in situ which subsequently was used for the cycloaddition of azides with terminal alkynes to give 1,4-disubstituted 1,2,3-triazoles in the presence of triethylamine as a sacrificial donor. KEYWORDS: Photocatalyst, Click reaction, Ruthenium, Manganese, Visible light, Redox catalyst, Triazoles ■ INTRODUCTION Development of sustainable chemical synthesis in order to diminish the detrimental environmental impact associated with chemical industries is a prime objective in present day chemistry. Sunlight, being an abundant, safe, and easily available energy resource, holds great potential in driving environ- mentally benign organic transformations. 1 Importantly, light induced reactions provide room temperature chemical syn- thesis, and also avoid thermally induced side reactions. However, simple organic molecules mainly absorb only ultraviolet (UV) light, which is only 5% of the solar spectrum and requires special vessels for reactions. Owing to these limitations, development of visible light assisted photocatalytic reactions is receiving particular interest in current decades. 2,3 In this regard, a plethora of selective organic transformations on a semiconductor photocatalyst have been developed, which can be performed in common glass reactors. 4,5 However, lower efficiency and poor product yields are the common drawbacks of such catalytic systems. Transition metal complexes such as ruthenium or iridium metal complexes and metal free organic dyes have also been acknowledged as excellent homogeneous photocatalysts for a series of organic transformations under visible light irradiation. 6,7 The copper catalyzed azide−alkyne cycloaddition (CuAAC) also known as a “click reaction” is a well-accepted, widely utilized, reliable, and straightforward approach to transform organic azides and terminal alkynes into the corresponding 1,4-disubstituted 1,2,3-triazoles. 8 Owing to the unique features of the CuAAC reaction, such as high efficiency, high yields, and mild reaction conditions, this has been established to be a powerful tool in organic synthesis, medicinal chemistry, polymer chemistry, and surface mod- ifications. 9,10 Furthermore, the products of CuAAC reactions, such as 1,4-disubstituted 1,2,3-triazoles, have been employed as ligands for catalysts and as building blocks for luminescent metal complexes. In this context, Bai et al. have recently reported the use of CuAAC reactions in the syntheses of nitrogen containing ligands such as pyridine, pyrazole, and benzyltriazole hybridized 1,2,3-triazole ligands and their application to support luminescent Cu(I) and Zn(II) clusters and polymeric complexes. 11,12 The click reaction is generally carried out by using a catalytic mixture containing Cu(II) with a reducing agent (usually sodium ascorbate). The direct use of Cu(I)/metallic copper or its clusters is also possible; however, the formation of undesirable alkyne−alkyne homocoupling Received: July 15, 2015 Revised: September 16, 2015 Published: December 2, 2015 Research Article pubs.acs.org/journal/ascecg © 2015 American Chemical Society 69 DOI: 10.1021/acssuschemeng.5b00653 ACS Sustainable Chem. Eng. 2016, 4, 69−75