This journal is © The Royal Society of Chemistry 2019 Chem. Commun., 2019, 55, 5025--5028 | 5025 Cite this: Chem. Commun., 2019, 55, 5025 A covalently linked dimer of [Ag 25 (DMBT) 18 ] À † Mohammad Bodiuzzaman, a Abhijit Nag, a Raghu Pradeep Narayanan, a Ankush Chakraborty, b Ranjit Bag, b Ganesan Paramasivam, a Ganapati Natarajan, a Govindasamy Sekar, b Sundargopal Ghosh b and Thalappil Pradeep * a We report the first example of a covalently bound dimer of monolayer protected atomically precise silver nanocluster [Ag 25 (DMBT) 18 ] À (DMBT stands for 2,4-dimethylbenzenethiol). Covalently linked dimers could be important to design new cluster assembled materials with composite properties. Atomically precise noble metal nanoclusters protected with ligands have emerged as a new research frontier in nanoscience, due to their unique optical and chemical properties as well as promising applications. 1–4 Out of these, the molecular systems studied in detail include [Au 102 ( p-MBA) 44 ], [Au 25 (SCH 2 CH 2 ) 18 ], [Au 38 (SCH 2 CH 2 ) 24 ], [Ag 44 ( p-MBA) 30 ], [Ag 25 (SPhMe 2 ) 18 ], [Ag 29 (BDT) 12 (PPh 3 ) 4 ], [Ag 40 (SPh- Me 2 ) 24 (PPh 3 ) 8 ], [Ag 46 (SPhMe 2 ) 24 (PPh 3 ) 8 ] etc. , 5–12 which exist in various charge states (the abbreviations used are presented in the ESI †). Single crystal X-ray diffraction has been utilized for their structural analysis while mass spectrometry is an important tool to understand their formulae and supramolecular interactions. 13,14 Dimers of Au 25 (SR) 18 (SR: phenyl ethanethiolate and butane thiolate) were first reported by our group using studies in the gas phase. 15 This kind of dimer was not observed for structurally similar [Ag 25 (DMBT) 18 ] À clusters due to the lack of metallophilic interactions. Recently, synthesis of new superstructures by the assembly of molecular pieces of metals has attracted great attention. 16–18 The creation of superstructures using clusters without changing the original structure of the building block is a challenging task. The ligand exchange reaction is suitable in this regard and various studies have been done on gold clusters to create super- structures. 16,18 Ligands containing two reactive rigid thiol groups can be efficient to create the assembly of building blocks. This aspect of creating superstructures has not been explored in the case of silver nanoclusters. In this communication, we demonstrate the synthesis of a covalently linked dimer of a monolayer protected cluster (MPC), [Ag 25 (DMBT) 18 ] À via a ligand-exchange reaction, wherein a complex of ruthenium bipyridyl-4-4 0 -dithiol [Ru(bpy) 2 bpy(CH 2 SH) 2 ] 2+ replaces two dimethylbenzenethiol ligands and creates a dimer. The products were characterized by optical absorption spectroscopy, infrared spectroscopy and detailed high resolution electrospray ionization mass spectrometry (HR ESI MS). [Ag 25 (DMBT) 18 ] À and [Ru(bpy) 2 bpy(CH 2 SH) 2 ](PF 6 ) 2 were synthesized by following reported procedures 6,19 and characterized by optical absorption spectroscopy and HR ESI MS (Fig. S1 and S2, ESI†). The ligand exchange reaction which leads to dimerization of [Ag 25 (DMBT) 18 ] À was carried out by mixing [Ru(bpy) 2 bpy(CH 2 SH) 2 ] 2+ and [Ag 25 (DMBT) 18 ] À in acteonitrile (see the Experimental section of the ESI† for details). The cleaned product obtained from the ligand exchange reaction was analysed via UV-vis spectroscopy. The optical absorption spectra of [Ag 25 (DMBT) 18 ] À , its dimer and [Ru(bpy) 2 bpy(CH 2 SH) 2 ] 2+ are compared as shown in Fig. 1. The spectrum of the dimer changed slightly compared to the parent cluster from which we infer that the cluster core remains intact. There is a slight red shift of 600 nm and a change was observed close to absorption at 400 nm which indicates optical coupling between [Ru(bpy) 2 bpy(CH 2 SH) 2 ] 2+ and the cluster. This type of minor change in the optical absorption spectrum ensures that the building blocks are intact which is essential for creating superstructures. HR ESI MS was used to show the formation of the dimer of [Ag 25 (DMBT)] À . Sample preparation and instrumental details are given in the ESI.† Under optimized conditions for collecting the data, full range HR ESI MS shows two peaks, at an m/z value of 5167 and an m/z value of 5504. Expansion of the peak at an m/z value of 5167 exhibits a characteristic peak to peak separation of 1 which confirms its unit charge. This species is [Ag 25 (DMBT)] À , confirmed by its perfect match of experimental and theoretical mass spectra. Similarly, the expanded peak at m/z 5504 shows the characteristic peak to peak separation of 0.5 which confirms its doubly charged state. This peak is assigned as the dimer of [Ag 25 (DMBT) 18 ] À linked by the [Ru(bpy) 2 bpy(CH 2 SH) 2 ] 2+ linker. The assignment of the peak was supported by the well matching of simulated and experimental isotopic distributions (Fig. 1). a Department of Chemistry, DST Unit of Nanoscience (DST UNS) and Thematic Unit of Excellence (TUE), Indian Institute of Technology Madras, Chennai 600036, India. E-mail: pradeep@iitm.ac.in b Department of Chemistry, Indian Institute of Technology Madras, Chennai 600036, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/c9cc01289c Received 14th February 2019, Accepted 2nd April 2019 DOI: 10.1039/c9cc01289c rsc.li/chemcomm ChemComm COMMUNICATION Published on 02 April 2019. Downloaded by Indian Institute of Technology Chennai on 8/28/2019 6:53:07 AM. View Article Online View Journal | View Issue