Suzuki−Miyaura coupling and O−arylation reactions catalysed by palladium(II) complexes of bulky ligands bearing naphthalene core, Schiff base functionality and biarylphosphine moiety.

Schiff bases L1 [i.e., 2-(diphenylphosphino)-N-(naphthalen-1-ylmethylene)ethanamine], L2 [i.e., 2- (diphenylphosphino)-N-(naphthalen-2-ylmethylene)ethanamine], L3 [i.e., 2-(1-(2-(diphenylphosphino)ethylim- ino)ethyl)naphthalen-1-ol] and L4 [i.e., 2-((2-(diphenylphosphino)ethylimino)methyl)naphthalen-1-ol] have been synthesized using a straightforward methodology which involves a condensation reaction between H 2 N−CH 2 −CH 2 −PPh 2 and appropriate carbonyl compound. Due to the presence of diphenylphosphine (−PPh 2) moiety and >C = N− functionality, these compounds behave as ligands and undergo complexation reaction with palladium on treatment with Na 2 PdCl 4 to yield the palladium(II) complexes (1–4). Ligands as well as complexes have been characterized using standard NMR spectroscopic techniques. ESI−MS and single crystal X−ray diffraction studies corroborate the structures of complexes. Crystal structures of complexes 1−3 reveal clearly that the geometry around Pd centre is distorted square planar. Ligands L1 and L2 are coordinated to Pd centre in bidentate (P, N type) mode, however, L3 and L4 act as a tridentate (P,N,O type) ligand and bind with metal in anionic mode. The Pd P and Pd N bond distances in complexes 1−3 are in the ranges 2.204−2.212 Å and 2.023−2.072 Å, respectively. Complex 3 [i.e., PdCl(L3 −H)] also has a Pd-O bond, the length of which is found to be 2.009(3) Å. All the complexes have potential for catalysing O-arylation (C-O coupling) of phenol and Suzuki-Miyaura coupling (SMC) reactions. Both bromoarenes and chloroarenes can be used as substrates in Suzuki coupling and converted into biaryl derivatives. For O-arylation reactions of phenol, bromoarenes are used as arylating agents. For catalysis of such reactions (i.e., C-O coupling), high (0.1 mol%) catalyst loading is required. However, Suzuki reactions require low (0.001 mol%) loading of catalysts to occur with bromoarenes and give the products. The high potential of the complexes is also evident from the fact that they also convert different aryl chlorides into the coupled products in Suzuki coupling. 31P{1H} NMR data reveal that the electronic environments of nuclei of phosphorous donors are closely similar in all the four ligands. Similar magnitude of deshielding of the 31P{1H} signals in all the complexes indicate that, while forming the dative bond, the P donor of all the ligands transfer the electron density to the palladium to a similar extent. Hence, the electronic effects created by the ligands through the phosphorous donor are similar in all the complexes. Therefore, it is inferred that variation in their catalytic performance is because of difference in the binding mode of the ligand and/or minor alteration in the architecture of organic ligand. Amongst them, complex 2 shows the highest catalytic activity, and the least active catalyst is complex 3 for C-C coupling reactions. For C-O coupling reactions, the efficiencies of complexes 1 and 2 are slightly higher than those of complexes 3 and 4. [ABSTRACT FROM AUTHOR]