Palladacycles of Thioethers Catalyzing Suzuki-Miyaura C-C Coupling: Generation and Catalytic Activity of Nanoparticles Gyandshwar Kumar Rao, Arun Kumar, Satyendra Kumar, Umesh B. Dupare, and Ajai K. Singh* Department of Chemistry, Indian Institute of Technology Delhi, New Delhi-110016, India. * S Supporting Information ABSTRACT: Tridentate thioether ligands, 2-HO-4-R-C 6 H 3 - (C 6 H 4 )CHNH(CH 2 ) 3 SPh [R = H (L1) or -OMe (L2)] react with Na 2 PdCl 4 , giving palladacycles [PdCl(C - ,N,S)] (1: (C - ,N,S) = L1-H; 2: (C - ,N,S) = L2-H). The 1 H and 13 C{ 1 H} NMR spectra of ligands and their palladacycles have been found to be characteristic. Complexes 1 and 2 have also been characterized with HR-MS. The crystal structure of 2 has been solved. The Pd-S bond length is 2.428(2) Å, and palladium has a nearly square planar geometry. During the course of catalysis of Suzuki-Miyaura C-C coupling using 1 and 2 as catalysts, unexpected formation of Pd 16 S 7 nanoparticles (NPs) has been observed with both complexes. This is the first time that such an observation has been made with palladacycles of thioethers used in this coupling reaction. The efficiency of 2 in carrying out the coupling is significantly lower than that of 1. Complex 2 has an additional -OMe group in the ligand structure, and the size of Pd 16 S 7 NPs formed from this complex are larger (6 nm) than those obtained from 1 (2 nm). ■ INTRODUCTION Transition metal-catalyzed C-C bond forming reactions are powerful synthetic tools in organic chemistry. 1,2 Palladium species constitute a class of versatile and useful catalysts for such organic transformations. The facile interchange between Pd(0) and Pd(II) or Pd(II) and Pd(IV) and the tolerance of palladium compounds to many functional groups present on substrate are mainly responsible for their versatility. 2 Not only Suzuki-Miyaura coupling but other C-C bond forming reactions such as ethylene oligo/polymerization, Heck and Negishi coupling, etc., also rely on this facile interconversion of oxidation states. The most important Suzuki-Miyaura catalysts include complexes of Pd(II) with bulky and electron-rich phosphines 3,4 and carbenes, 5 and palladacycles, 2,6,7 owing to their high efficiency and the ease with which they can be modified. In the last few decades interest in palladium complexes of ligands containing a combination of various donor groups, such as Suzuki-Miyaura catalysts, has grown significantly as the distinct features of each donor atom can confer unique properties to the complex. 2,6-8 Sulfur has been incorporated in framework of many ligands, and Pd(II) complexes of sulfated Schiff bases, 9 pincer type S ligands, 7 and some other S- containing ligands 10 have emerged as a family of air-stable, moisture-insensitive, and efficient catalysts. The sulfur-ligated palladacycles are important among this family of Suzuki catalysts, and in recent years an increase in research on them has been noticed. They have shown good promise for the coupling of aryl bromides and iodides, but most of them 7c-g,8,9a,c-e have not been reported to transform effectively aryl chlorides, which are the cheapest and most readily available among the aryl halides. The L1 and L2 synthesized as a part of our research program on chalcogenated Schiff bases 11 and their reduced forms have been found worth studying to design thioether palladacycles 1 and 2, which may show potential as Suzuki catalysts. The in situ formation of Pd NPs has been reported in Suzuki-Miyaura coupling reactions catalyzed with palladium complexes of organosulfur ligands. 7b These Pd NPs are reported to be stabilized in the presence of additive n- Bu 4 NBr. 7g,11d However, in some cases the absence of this additive gives a better yield. 7g There is enough evidence that suggests that Pd(0) species leached from the surface of such NPs are the true catalysts during the course of reaction, and the role of the ligand (including its architecture) is limited to affecting size, dispersion, and the chemical nature of the NPs. However, formation and involvement of NPs of a palladium sulfide phase in the catalytic Suzuki coupling reaction have never been reported. Suzuki-Miyaura coupling carried out in the presence of palladacycles 1 and 2 results in the formation of nanosized particles of composition Pd 16 S 7 , which appear to play a role in the catalysis of the coupling via generation of Pd(0) species. The formation of the Pd 16 S 7 phase in Suzuki coupling has been noticed for the first time. The unexpectedly high difference in activities of 1 and 2 has been observed. The complex 2, having an additional -OMe (with respect to 1), has been made a part of this study, as its single crystal structure Received: March 11, 2013 Published: April 4, 2013 Article pubs.acs.org/Organometallics © 2013 American Chemical Society 2452 dx.doi.org/10.1021/om4001956 | Organometallics 2013, 32, 2452-2458