A Highly Efficient and Inexpensive Palladium-Salen Complex for Room Temperature Suzuki-Miyaura Reaction

Abstract

The palladium catalysed Suzuki-Miyaura cross-coupling reaction is one of the most important strategy for the synthesis of biaryls, which are building blocks of numerous vital organic compounds used in pharmaceuticals, fineand agro-chemical industries. The Suzuki-Miyaura reaction relies on the cross-coupling between aryl halides and easily accessible organoborons in presence of catalytic amount of palladium-based salts or complexes. Many ideal developments have been attained till now to improve the efficiency of this reaction and to minimize the issues related to environment pollution. Conventionally, the C-C bond formation reactions are performed using phosphine ligated palladium complexes, which show excellent activity in this transformation. However most of the phosphine ligands are toxic and unstable in air or moisture, and are also known to increase the possibility of side reactions, thereby lowering the yield of desired cross-coupled products. Recently, different nitrogen based ligands such as N-heterocyclic carbenes, amines, oximes, amides and their co-ordination with palladium have attracted considerable attention for SuzukiMiyaura Cross-Coupling reaction with potentiality to overcome some of drawbacks face by traditional phosphine ligands. So attention has been given to design catalyst containing chelating multi-dentate Schiff-base ligands. The water insoluble property of these ligands restricts their use upto undesirable organic solvents such as DMF, toluene, THF etc. Thus, to practice the principles of green chemistry, researchers have developed efficient catalytic protocols to conduct Suzuki-Miyaura cross-coupling reaction under easily available and non-toxic aqueous media. Moreover, there has been a tremendous rise in interest for employing aryl chloride as substrates for Suzuki-Miyaura cross-coupling, as they are easily accessible economical compared to their bromide and iodide counterparts. Herein we wish to report a facile and efficient room temperature Suzuki-Miyaura cross-coupling reaction strategy using a unique Palladium salen complex as catalyst in greener solvent. It is notable to mention that these multidanted ligands, due to the presence of multiple bonding sites is expected to increase the steric congestion around the metal centre which is considered to be the vital step facilitating the reductive elimination step in the cross-coupling mechanism. Initially, we have synthesised and characterized two novel palladium complexes with a previously reported tetradentate Schiff-base ligand N,N'-bis(salicylidene)-phenylmethanediamine (L1) and its reduced form (L2), ligand (Scheme 1). The new palladium complexes C1 and C2 were prepared by refluxing methanolic solution of the corresponding tetradentate ligands L1 and L2 with equimolar amount of palladium acetate. The newly synthesized complexes C1 and C2 were characterized by elemental analysis, IR, Hand CNMR and mass spectral data. The spectral analysis of prepared complexes corroborates with the reported one. The value of elemental analyses and the appearances of molecular ion peaks in ESI-mass spectra of complexes C1 and C2 support their proposed composition. In the FTIR spectra of complexes, the value for ν (C=N) stretching vibration of the free ligand at about 1617 cm gets considerably shifted to lower frequency at 1606 cm after complexation, implying the coordination of imine nitrogen with palladium owing to the donation of electrons from nitrogen atom to the empty d-orbitals of the metal. This signal was absent in ligand L2 and in its corresponding complex C2, indicating the successful reduction of C=N bond. In complex C2 the band at 3160 cm due to the ν (NH) stretching frequency of the L2 undergoes a considerable shift towards 3080 cm after complexation. The complex C2 has been shifted about 80 cm, indicating strong coordination of nitrogen to palladium metal atom. The v (O-H) signal pertaining to free ligands (3425 cm in FTIR and 13.5 ppm in H NMR) was found to be absent in both complex C1 and C2, supporting the deprotonation of phenolic moiety and