Estudo DFT da influência de aminas cíclicas como ligantes em complexos do tipo Diclorobis (trifenilfosfina)(amina) rutênio (II) em reações de ROMP de norborneno

Abstract

Density Functional Theory was employed to evaluate the substituent effect of cyclic amines on the catalytic properties of the[RuCl2(PPh3)2(amine)] complexes in norbornene ROMP (Ring Opening Metathesis Polimerization). The possible configurations that amines may assume in these catalysts were analyzed by considering the coordination of the amine in the equatorial position or in the axial position in a square base pyramid; the most stable isomer formation mechanism of the complexes and their respective ROMP mechanisms. Six cyclic amines were analyzed differing from each other by the addition of one CH2 unit in the cycle, in increasing order of ring size: aziridine, azetidine, pyrrolidine, piperidine, perhydroazepine and azocane. All isomers exhibit preferential coordination of the amine at the equatorial position of the [RuCl2(PPh3)2(amine)] complex. Regarding the investigation of the possible mechanism of formation of the substituted complexes, we observed that through mechanism A, which occurs through an initial associative step, there is a lower energy cost expressed in negative enthalpy values and lower Gibbs free energy value, followed by the uncoordination of PPh3 (triphenylphosphine), which dissociates spontaneously, with the aziridine complex being the exception. In norbornene ROMP, the coordination of etyldiazoacetate occurs spontaneously for all substituted complexes. Subsequently, the substituted complexes show a differentiated behavior with the dissociation of a PPh3 molecule becoming more spontaneous when the number of CH2 units in the ring increases, possibly regulated by a rearrangement in the coordination sphere, due to the size of the ligands.The dissociation of the second molecule of PPh3 corresponds to the stage with the highest energy cost for the reaction, characterized by being an endothermic process with ∆H ranging from 33.56 to 37.95 kcal.mol-1, and not spontaneous, presenting positive values of ∆G ranging from 20.31 to 22.58 kcal.mol-1.The difference in the thermodynamic parameters of the amines in this step was very discreet, believing that there is not so much influence of the CH2 units in the cycle. In the other stages, there was greater variation in the values of ∆H and ∆G for the different complexes, but there were no trends, since, when moving from one stage of the mechanism to another, those that obtained greater reaction favorability did not obtain this favoritismo in subsequent stages. Polynorbornene formation occurs spontaneously for all amines as it corroborates with experimental studies and validates or model used in this work, as the synthesized complexes (pyrrolidine, piperidine, perhydroazepine and azocane) are active for norbornene ROMP, showing quantitative yields under most reaction conditions, differing between themselves as to the characteristics of the polymers obtained.