Stabilization of Au NPs on symmetrical tridentate NNN-Pincer ligand grafted on magnetic support as water dispersible and recyclable catalyst for coupling reaction of terminal alkyne Nasrin Zohreh a,⇑ , Seyed Hassan Hosseini b , Mahboobeh Jahani a , Morena S. Xaba c , Reinout Meijboom c a Department of Chemistry, Faculty of Science, University of Qom, P. O. Box: 37185-359, Qom, Iran b Department of Chemical Engineering, University of Science and Technology of Mazandaran, Behshahr, Iran c Department of Chemistry, University of Johannesburg, P.O. Box 524, Auckland Park, 2006 Johannesburg, South Africa article info Article history: Received 30 August 2017 Accepted 24 October 2017 Keywords: Supported gold catalysis Pincer catalysts Propargylamine Heterogeneous catalyst abstract A symmetrical tridentate NNN-pincer ligand derived from 2-aminopyridine/cyanuric chloride was cova- lently grafted onto the surface of modified magnetic support to stabilize Au nanoparticles. Spectroscopic evidence (IR, XRD, EDX, TEM, and XRF) and other analyses (TGA, VSM, and H 2 -TPR) confirmed successful immobilization and desired catalyst structure. Catalyst loading of 0.07–0.1 mol% was shown to be very effective in the synthesis of propargylamines through addition of terminal alkynes to amine/aldehyde adduct (A 3 -coupling). The water-dispersible catalyst allows the use of water as an inexpensive solvent. The high accessibility and robustness of the catalyst enhanced reusability so that no deactivation of cat- alyst or aggregation of supported Au NPs is observed after seven repeated recycling. Control experiments also showed the true heterogeneous nature of the catalyst in the reaction without significant leaching. All experiments showed that the catalyst is comparable to or even more active than the corresponding spe- cies in solution. Ó 2017 Elsevier Inc. All rights reserved. 1. Introduction For centuries, gold had been considered a precious, purely dec- orative inert metal. However, the past two decades have witnessed a tremendous growth in the utilization of gold for numerous cat- alytic transformations [1–4]. Indeed, high Pauling electronegativity and electron affinity of gold, unique atomic structure, physical sta- bility [5,6], interesting interaction of gold with sulphur [7], quan- tum size effect of gold [8], the capability of fine-tuning of gold NPs by varying the size, shape, and crystal structure [9], and unique surface properties of gold have made it an attractive tran- sition metal for research rather than the other [10]. In the last 15 years, thousands of chemical transformations, importantly acti- vation of alkynes [11–16], have been effectively catalyzed by gold nanoparticles or homogeneous complexes of Au(I) [11], and Au(III) [11,17–20]. Despite the efficiency of homogeneous gold catalysts [21–24], a literature survey revealed that more than 0.5 mol% of catalyst loading is needed for the reaction [25–27] and only in a few cases, catalysts with high turnover numbers have been reported [21–24]. Notably, gold catalysts are expensive and unrecoverability of homogeneous gold catalysis is a common drawback. Thus, homogeneous gold catalysts would be too expen- sive to be useful for industrial processes. Accordingly, heteroge- neous recyclable gold catalysts have been designed through covalently immobilization of gold complexes or stabilization of gold nanoparticles onto the solid supports [28–33]. Resins [34], micro/mesoporous silica [33,35], polymers [36–38], and Fe 3 O 4 [39–41] are among the most preferred supports due to their large surface area. While there are many successes on the use of these heterogeneous gold catalysts, two major obstacles are still consid- erable: (1) gold leaching and (2) instability of gold NPs due to tend- ing to aggregate during the course of the reaction. Today, the ‘‘ligand design” [42,43] come into effect to solve these problems. In recent years there has been an increasing expansion in the development of pincer compounds, mainly explored by Gerhard van Koten and his co-workers [44–50], so that they have emerged as unique class of ligands in designing of effective metal-based cat- alysts which is along with the concept of ‘‘ligand design” [51–54]. An important characteristic of the pincer platforms is the fact that its three co-planar ligating sites are well organized by the back- bone of the ligand led to the formation of five- or six-membered chelate rings with a central bonded metal [55,56]. In most cases, the pincer platform itself acts as a non-innocent ligand or provides, https://doi.org/10.1016/j.jcat.2017.10.021 0021-9517/Ó 2017 Elsevier Inc. All rights reserved. ⇑ Corresponding author. E-mail addresses: n.zohreh@qom.ac.ir, nasrin.zohreh@gmail.com (N. Zohreh). Journal of Catalysis 356 (2017) 255–268 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat