A comparative study of palladium-based coordination compounds with bidentate (N,N, P,P and P,O) ligands; Design, synthesis, X-ray structural, catalytic activity and DFT studies

Abstract

This account describes our recent studies on four new asymmetric cyclometallated Pd(II) complexes with a formula [L1 → Pd ← L2](ClO4) (L1 = benzylamine and L2 = bis (diphenylphosphino) methane oxide (dppmo) (3), 1,2-bis (diphenylphosphino) ethane (dppe) (4), bispyridine (bipy) (5) and 1, 10-phenanthroline (phen) (6)). These complexes have been prepared by the reaction of palladium(II) acetate with benzylamine ligand in equimolar ratios in toluene produced the [Pd(C6H4CH2NH2)(µ-OAc)]2 complex 1. A metathesis reaction between complex 1 and NaCl in water/acetone (1:10) exchanged the acetato ligands of 1 with chloro ligands. This produced the dinuclear cyclopalladated compound [Pd(C6H4CH2NH2)(µ-Cl)]2 complex 2. The following addition of dppmo, dppe, bipyridine and phenanthroline ligands, respectively, to the solution of 2 in CH2Cl2 generated the corresponding complexes. These complexes have been fully characterized by FT-IR, 1H, 13C and 31P NMR spectroscopic methods and other conventional techniques such as elemental analysis (CHN). Further, the solid-state structure of the complex 3 was determined by the single-crystal X-ray diffraction methods, showing that this complex consist of two five-membered rings formed by coordination of the dppmo ligand through the one phosphorus atom and one oxygen atom and coordination of the benzylamine ligand through the one carbon atom and one nitrogen atom to the metal center. Therefore, the catalytic activity of complex 3, using Suzuki and Heck coupling reactions, has been evaluated and compared. Using theoretical methods the structures of the asymmetric palladacycle complexes [L1 → Pd ← L2](ClO4) with different coordination mode, (where L1 = benzylamine (N,C coordination) and L2 = dppmo (P,O coordination) (3), dppe (P,P coordination) (4) and phen (N,N coordination) (6)), were compared with each other. In this regard density function theory (DFT) calculations at the BP86/def2-SVP level of theory were applied. The strength and nature of donor-acceptor bonds between the L1 and L2 and Pd fragments in the [L1 → Pd ← L2](ClO4) were carried out by natural bond orbital analysis (NBO) and energy-decomposition analysis (EDA) as well as their natural orbitals for chemical valence variation (EDA-NOCV).