Synthesis and characterization of cyclometallated complexes of imines: 4-ClC6H4CH=NCH2(4’-FC6H4) and 2,3-F2C6H3CH=NCH2(4’-FC6H4). Expanding the scope of cyclopalladations

Abstract

The treatment of 4-fluorobenzylamine with 4-chlorobenzaldehyde or 2,3-difluorobenzaldehyde in acetone at 80ºC afforded the corresponding imines (1), 4-ClC6H4CH=NCH2(4’-FC6H4) and 2,3-F2C6H3CH=NCH2(4’-FC6H4) respectively, which were isolated in a yield >90%. The same reaction was performed in milder conditions obtaining similar results. The imines (1) reacted with a stoichiometric amount of Pd(OAc)2 in acetic acid affording the corresponding acetato-bridged five-membered ortho-cyclopalladated dimers (2) in 44-69% yield. The reaction was reproduced in different conditions to improve the yield, obtaining the best results at 85ºC for 1h. Compounds 2 were converted by a metathesis reaction with an excess of LiCl and LiBr in acetone into the corresponding chloro- or bromo-bridged dinuclear cyclopalladated compounds. The stoichiometric amount of PPh3 and dppe was added to the halogen-bridge compounds to obtain the mononuclear compounds trans-N,L-[Pd{RHC=NCH2(4’-FC6H4)}(X)(L)] [3-A (R= 4-ClC6H4, X= Br, L= PPh3); 3-B (R= 2,3-F2C6H3, X = Br, L = PPh3); 4-A (R= 4-ClC6H4, X= Cl, L= PPh3); and the dinuclear compounds 5-A (R= 4-ClC6H4, X= Br, L= dppe); 5-B (R= 2,3-F2C6H3, X= Br, L= dppe)] in 50-90% yield. The compounds obtained were characterized by 1H NMR, 31P NMR spectroscopy −in CDCl3 solution−, elemental analyses, mass spectrometry, and IR spectroscopy. Some tendencies have been found in this work: i) the E isomer of the imine was isolated in all reactions; ii) the ortho-fluoro atoms show an interaction with the imine proton; iii) the endo-palladacycle is the thermodynamic control isomer; iv) the aromatic protons of the metallated ring are high field shifted in compounds containing phosphines, showing a trans arrangement between the phosphines and the nitrogen atom in agreement with the transphobia; v) the methinic proton signal in proton NMR spectra is displaced toward high fields confirming the nitrogen-metal bond.