Pd(II) and Pt(II) complexes of α-keto stabilized sulfur ylide: Synthesis, structural, theoretical and catalytic activity studies
Graphical abstract
Introduction
Sulfur ylides are a group of reactive compounds which have attracted considerable interest in recent years [1], [2], [3]. This is not least because of their significant role in organic synthesis, including Johnson-Corey-Chaykovsky reaction [4], which is due to the high stability and ambidentate character as ligand can engage as an appropriate ligand for organometallic synthesis [5], [6], [7]. The coordination chemistry of these compounds has been investigated extensively and it was found that they can bond to a metal center through the carbanion or the enolate oxygen (Scheme 1) [8], [9], [10]. From consideration of the Hard-Soft-Acid-Base principle (HSAB) [11], it can be hypothesized that the title ligands tend to coordinate to soft metal ions via the soft ylidic carbon as opposed to the hard carbonyl oxygen atom. The more observed Cα-coordination mode corresponds to the transition metal complexes of Hg(II) [9], Cd(II) [10], Ag(I) [3] and Pd(II) [12], while the O-coordination mode of these compounds is limited to a small number of complexes with hard oxophilic metals such as tungsten [13].
The palladium catalyzed Suzuki-Miyaura reaction is one of the most reliable methods for the construction of Csp2Csp2 bond [14], [15], [16]. Although this reaction originally catalyzed by palladium complexes containing bulky phosphine ligands [17], [18], but most of them are moisture/air sensitive and have environmental considerations. Application of more stable and cheaper complexes containing sulfur or nitrogen ligands was developed to eliminate or reduce costs, operational hazards and environmental pollution [19], [20], [21]. Very recently, our group reported the synthesis of sulfur-containing palladium complexes and successful application as catalyst precursors for the Mizoroki-Heck reaction [22]. Furthermore, attempts to use more economical aryl chlorides which are less reactive than aryl bromides and iodides in CC coupling reactions showing a noteworthy growth [23], [24], [25]. Mostly electron-poor aryl chlorides, which are easier to activate, have been used successfully in these reactions and non-activated aryl chlorides or even deactivated aryl chlorides such as chloroanisoles were coupled by applying palladium complexes in combination with phosphonium salts [26].
Notwithstanding this background, we choose the Pd(II) and Pt(II) complexes of sulfur ylides as attractive candidates for the synthesis, X-ray characterization and theoretical studies. Although, the complexation of sulfur ylides to various transition metals has been extensively studied, however, application of the Pd(II) complexes of sulfur ylides in the Suzuki-Miyaura reaction of less reactive aryl chlorides is the key advantage of this study.
Section snippets
Physical measurements and materials
All of the reactions were carried out under air. All the solvents and starting materials were purchased from commercial sources and used without further purification. The starting material [PdCl2(cod)] and [PtCl2(cod)] were prepared according to the published procedures [27]. Melting points were measured on a SMP3 apparatus. IR spectra were recorded in the range of 4000–400 cm−1 and on a Shimadzu 435-U-04 spectrophotometer from KBr pellets. NMR spectra (1H and 13C) were recorded on 500 MHz,
Synthesis
As shown in Scheme 2, dimethyl sulfide reacts with the 2, 3′dibromoacetophenone in equimolar ratios, forming the sulfonium salt 1 in 95% yields. The resulted salt was treated with NaOH 10% to obtain the sulfur ylides 2 in 85% yields. Reaction of this ylide with [MCl2(cod)] (M = Pd and Pt) in 2:1 ratio gives the new square-planar complexes cis- and trans-[MCl2(Me2SCHC(O)C6H4-m-Br)2] (M = Pd, cis- and trans-3; M = Pt, cis- and trans-4). Recrystallization of these complexes from DMSO solution
Conclusion
In summary, the present study describes the synthesis and characterization of sulfonium salt 1, sulfur ylide 2, and its new Pd(II) and Pt(II) complexes cis- and trans-3 and 4. On the basis of physico-chemical and spectroscopic data, we propose C-coordination of ylide 2 to the metal ions, which is further confirmed by the X-ray crystal structures. Density functional theory study has been done on structures and nature of MC bonding in [YMCl2DMSO] (M = Pd, Pt) complexes via NBO and EDA analysis.
Acknowledgements
The authors are grateful to the Bu Ali Sina University for funding of our research and partial support of this work.
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