Synthesis, Characterization and Catalytic Application of Highly Cationic Ruthenium Complexes Supported by alpha,alpha'-Diimine

Abstract

Abstract Development of novel organometallic complexes has been an area of significant interest over several decades owing to their numerous applications, particular in the catalytic transformation of organic molecules. Several strategies have been implemented to develop newer organometallic complexes with greater functionality and selectivity. Ligand design has remained a key in changing the structure and geometry of the active metal complex. Advances in ligand design have resulted in increased enantioselectivity, high conversion rates and ability to react at near ambient temperature which assists in reducing energy demands and ultimately CO2 emissions. In this work, the α,α′-diimine ligand (N,N′-dibenzylidene-ethylene diamine) was employed and the substituent pattern of the flanking N-aryl was modified by using different anilines with o-CH3, o,p-CF3, o-OCH3, F etc.). A total of 13 different structurally different α,α′-diimine ligands were prepared and characterized through NMR. It was observed that when α,α′-diimine ligand was prepared using 3,5-bis-trifluoromethyl aniline, 2,4- bisfluoromethyl aniline or 2-tert-butylaniline, the yields of 56%, 55% and 38% were obtained respectively. This series of structurally varying ,-diimine ligands were reacted with two types of 6-arene Ru(II) chloro dimers. In order to facilitate rapid anion exchange, different reagents were employed including ammonium hexafluorophosphate (NH4PF6), silver hexafluorophosphate (AgPF6), etc. In total, 27 complexes were generated in this study. The structures of all 3 complexes were confirmed using X-ray diffraction. Since the Ru(II) complexes featured two neutral ligands, there is a possibility that highly Lewis acidic Ru-based dications could be generated in-situ through the use of a Cl-abstraction reagent, in this case sodium tetrakis-3,5-bis-(trifluoromethyl)-phenyl borate, a reagent commonly used in the field of cationic polymerization. Three different complexes were tested to catalytically promote the Diels Alder reaction and to determine the effectiveness in terms of conversion rate, but also in the selectivity of exo/endo isomers. The main benchmarking reaction employed acrolein or methacrolein as the dienophile, and cyclopentadiene as the diene. All complexes tested showed excellent conversion rates ranging between 84-100% with catalysts featuring the strong electron withdrawing group CF3 on the flanking N-aryls showing the highest level of conversion. In all cases, acrolein demonstrated more rapid conversion than methacrolein, based on the steric difference of both dienophiles. Additionally, when the diene was 1,3- cyclohexanedione, the conversion rate again increased, however, the highest conversion rates were obtained when bromo-acrolein was the dienophile. The results of this study clearly indicated that these dicationic ruthenium(II) ,‘-diimine based complexes have excellent efficiency in Diels- Alder reactions in terms of conversion rates. The enantioselectivity was not as good as reported by other Ru(II) complexes in the literature and the observed results depended upon the substituent pattern of the flanking N-aryl on the ,‘-diimine ligand. When the substituent was ortho- methyl, the catalysts enabled the formation of endo isomer in higher ratios than exo in the case of acrolein and higher exo than endo for methacrolein. For catalysts bearing electron withdrawing groups on the flanking N-aryl groups of the ,‘-diimine ligand, the differential ratio between the exo/endo isomers was lower and was in accordance with the reduced steric bulk around the site where the dienophile coordinates. Keywords: α,α′- Diimine ligand; Ruthenium, Lewis Acid, Catalysts, Diels-Alder; Organometallic Complexes.