Studies of the Carbon Dioxide and Epoxide Coupling Reaction in the Presence of Fluorinated Manganese(III) Acacen Complexes: Kinetics of Epoxide Ring-Opening

Abstract

Five-coordinate manganese(III) complexes of N, N'-bis(trifluoroacetylacetone)-1,2-ethylenediimine (tfacacen) have been synthesized and structurally characterized by X-ray crystallography. The presence of the electron-withdrawing -CF3 substituents enhances the electrophilicity of the metal center in these (tfacacen)MnX (X=Cl, N3, NCO, NCS) derivatives when compared with their (acacen)MnX (acacen=N, N'-bis(acetylacetone)-1,2-ethylenediimine) analogs. This is demonstrated by the increased propensity of the Mn(III) center in the tfacacen complexes to bind a sixth ligand. Binding studies were performed utilizing the upsilonN3 stretching frequency in (tfacacen)MnN3, which is sensitive to the coordination of a ligand at the vacant axial site. Of importance, cyclohexene oxide was shown to readily bind to (tfacacen)MnN3, thereby providing an opportunity for directly monitoring the dependence of the epoxide ring-opening process on the metal complex concentration. In this instance, as has been amply demonstrated in the (salen)CrX case, the ring opening of cyclohexene oxide was found to be second-order in [(tfacacen)MnN3], with an activation energy of 71.0+/-6.0 kJ/mol. In the presence of strongly coordinating anions or amine bases, the rate of epoxide ring opening by (tfacacen)MnN3 was greatly retarded. The manganese cyanate and thiocyanate complexes were examined in an effort to develop other initiators for epoxide ring opening which provide readily accessible infrared spectroscopic probes. Indeed, the thiocyanate ligand was found to be well-suited for monitoring the epoxide ring-opening reaction by infrared spectroscopy.

Mechanistic Insight into the Initiation Step of the Coupling Reaction of Oxetane or Epoxides and CO2 Catalyzed by (salen)CrX Complexes

“…7 In the presence of various cocatalysts, for example, heterocyclic amines and anionic nucleophiles, however, (salen)MX (M ) Cr and Co) are very reactive catalysts for the coupling of epoxides and CO 2 . 4 The chromium catalytic system has been similarly shown to be active for the copolymerization of oxetane and CO 2 via a closely related mechanistic pathway.…”

Green Synthesis of Novel Polyesterurethane Materials from Epoxides and Carbon Dioxide by New Set of One-Dimensional Coordination Polymer Catalyst

“…The activation of the CO 2 is supposed to take place via the free pyridine base present in the precyclophane ligand ( L ) in the complex 1a followed by the insertion of the CO 2 into the metal–oxygen bond. However, in case of zinc-based catalyst 1b , the presence of the nitrate anion and a different coordination environment may possibly slow down the epoxide ring opening and initiation of the catalysis reaction, 53 which might be the reason behind the poor efficiency of 1b for the cycloaddition of epoxide and CO 2 . This is supported by the fact that the coordination polymer 1b catalyzes the cycloaddition reaction of the epoxides and CO 2 with a high turnover number in the presence of the cocatalyst TBAB.…”

X-Ray crystal structures of five-coordinate (salen)MnN3 derivatives and their binding abilities towards epoxides: chemistry relevant to the epoxide–CO2 copolymerization process

“…These m N 3 vibrational modes correspond to trans-(salen)Mn(N 3 ) 2 − and free azide, respectively. 8 It is evident that azide binding is not quantitative. That is, under these conditions an equilibrium is established in solution (eqn (2)), where the equilibrium position is favored to the left.…”

“…For comparison, the m N 3 vibrational mode in (tfacacen)Mn(N 3 ) 2 − appears at 2027 cm −1 , about 23 cm −1 lower in frequency from the mono azide derivative. 8 (2) Fig. 5 FTIR spectra of 0.012 M dichloromethane solutions of (1) complex 1, (2) complex 2 and (3) complex 3 with the addition of 1 equiv.…”

“…Relevant to this point, we have recently published studies on five-coordinate d 4 high-spin Mn(III) complexes containing a N 2 O 2 −2 ligand framework closely related to the salen system, where the ability of the manganese center to bind a sixth ligand can be controlled by altering the substituents on the tetradentate ligand. 8 That is, it was dramatically demonstrated that whereas (acacen)MnX does not bind a weakly coordinating ligand like an epoxide, the addition of electron-withdrawing trifluoromethyl substituents in (tfacacen)MnX complexes readily leads to the coordination of epoxides (Fig. 1).…”