1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 z Catalysis Tetranuclear 3 d/4 f Coordination Complexes as Homogeneous Catalysts for Bis(indolyl)methane Syntheses N. V. T. Sai Manoj Gorantla, [a] Pulikanti Guruprasad Reddy, [b] Sayed Mohmmed Abdul Shakoor, [b] Rajkumar Mandal, [a] Sudipta Roy,* [b] and Kartik Chandra Mondal* [a] Two novel 3d (Ni)/4f (Dy and Y) metal ions and polydentate organic ligands (H 2 L1 and H 2 L2) based tetranuclear coordina- tion complexes, [(L1) 4 Ni II 2 Dy III 2 (DMF) 2 (NO 3 ) 2 ]⋅DMF (1) and [(L2) 4 Ni II 2 Y III 2 (DMF) 2 (NO 3 ) 2 ]⋅DMF⋅H 2 O(2), have been synthesized and characterized using IR, elemental analysis and single crystal X-ray diffraction studies. The application of these complexes as C À C bond coupling homogeneous catalysts has been logically shown. The complex 1 acts as an excellent catalyst (with a very low catalyst loading of 0.17 mol%) for the syntheses of a wide range of bis(indolyl)methane derivatives in good yields using various indoles and aromatic/hetero aromatic aldehydes under optimized reaction conditions. A plausible reaction mechanism has been proposed showing the coordinative binding of the aromatic aldehyde at Ni II centre along with the associative π- stacking interaction between the ligand (H 2 L1) and the phenyl ring of the aromatic aldehyde and/or indole leading to the formation of the desired bis(indolyl)methanes under the metal assisted C À C bond forming reaction with subsequent elimina- tion of water molecule in polar protic solvent. Introduction Polynuclear coordination complexes (PCC) based on 3d/4f metals and polydentate organic ligands have attracted increas- ing interest because of their fascinating electronic and optical properties leading to the applications in various fields including conventional homogeneous catalysis, photocatalysis, optical devices, imaging, molecular magnetism and luminescence. [1] Till date, a large number of 3d/4f complexes have been synthesized and reported having bi-, [2] tri- [3] and tetra-nuclear cores [4] with diverse molecular structures and interesting top- ologies. However, the use of such complexes as homogeneous catalysts for organic transformations are so far less explored. [5] It is reported that the spatial distance (typically in the range of 3.5-6 Å) between the metal atoms in 3d/4 f complexes can play a crucial role for converting the substrates coordinated to the metal centre into the corresponding products in high yields. [5a,1f] The structural tuning of the organic ligands and the nature of the 3d/4f metals are the two crucial points to keep in mind, while designing a new catalytically active coordination complex. [1f] In this context, Shibasaki et al. had reported a variety of asymmetric organic transformations; such as syn- selective asymmetric nitro-Mannish reactions [5c] and anti-selec- tive asymmetric Henry reactions [5e] using an insitu generated 3d/4f Schiff base complex. Following this initial approach, several well characterized 3d/4f complexes having the formula [(3d) 2 II (4f 2 ) III L 4 ] 2 + (3d = Ni, Co, Cu, Zn; 4f = Y, Sm, Dy, Gd, Sm) had been reported [6] by Kostakis et al. for SuzukiÀMiyaura coupling, [6d,e] domino electro-cyclization [6a,f] and FriedelÀCrafts alkylation [5a] reactions. However, the catalyst loading for some of these reactions are very high. [5a] In addition, similar type of 3d/4f complexes/helicates had also been reported to show catalytic activities for water oxidation [5b] and cyclic carbonates syntheses. [7] However, the applications of novel tetranuclear 3d/4f complexes as homogeneous catalysts for the syntheses of biologically attractive organic hetero-cycles have not been well explored so far. Organic heterocycles and their derivatives are well known to exhibit a wide range of biological activities. [8] Particularly, indole derivatives received significant importance in the field of synthetic chemistry, since most of these compounds exist in natural products and bioactive compounds. [9] Among those, C3 substituted indole alkaloids such as bis(indolyl)methanes (BIMs) have attracted immense interest because of their enormous biological relevance. [9b,10] For example, many of these com- pounds are used as drug molecules having antibacterial, anti- inflammatory, analgesic and anticancer properties. [10] In addi- tion, BIMs are highly beneficial to promote estrogen metabo- lism in humans. [11] Synthetically, BIMs received significant importance as one of the key intermediate to synthesize various natural products. [12] Some of the naturally existing bioactive BIMs are 1,1,3-tris(3-indolyl)butane, trisindoline, arun- dine and vibrindole. [12b] So far, various research groups have reported different methods for BIMs syntheses in a one pot fashion. [13] Among those, one of the important facile route is the nucleophilic addition of aldehydes/ketone by indole derivatives using Lewis acid catalyst. [5a,14] Subsequently, a [a] N.V.T.SaiManojGorantla,R.Mandal,Dr.K.C.Mondal Department of Chemistry, Indian Institute of Technology-Madras, Chennai-600036, India E-mail: csdkartik@iitm.ac.in [b] Dr.P.GuruprasadReddy,Dr.S.M.AbdulShakoor,Dr.S.Roy Department of Chemistry, Indian Institute of Science Education and Research (IISER) Tirupati, Karakambadi Road, Tirupati-517507, Andhra Pradesh, India E-mail: roy.sudipta@iisertirupati.ac.in Supporting information for this article is available on the WWW under https://doi.org/10.1002/slct.201901215 Communications DOI: 10.1002/slct.201901215 7722 ChemistrySelect 2019, 4, 7722–7727 © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim