Synthesis And Catalytic Activity Of Metalloporphyrins

In this work a series of ionic metalloporphyrins, including manganese porphyrins, palladium porphyrins and gold porphyrins, were designed and synthesized. Empathically, the ionic manganese porphyrins (A3, B2, and C2) were selected as the model catalysts for alkane/alkene oxidations. The influences, like electron-withdrawing nature of the peripheral groups in the porphyrino ring, the incorporated counter anions, and the use of ionic liquid solvents, on the catalytic performance of the ionic manganese porphyrins were investigated in terms of activity, selectivity, and stability. Compared with the conventional neutral manganese porphyrins for alkane/alkene oxidations in organic solvents, the developed ionic manganese porphyrins dissolved in the similar structured IL solvents were endowed with advantageous activity and stability which depended on many factors including that:1) the IL solvent dually acted as the axial ligands, avoiding the involvement of the imidazole or pyridine (derivatives), which is susceptible to oxidation and then sacrificed by excess oxidant; 2) the ILs based on pyridinium or imidazolium with conjugated features could interact with the super-conjugated porphyrins through p-p interactions, which could inhibit the self-aggregation of the manganese porphyrins itselves, avoid the formation of theμ-oxo MnⅣ-porphyrin dimers, and improve the catalytic activity and stability; 3) the strong electron-withdrawing effect of the cations in the ionic manganese porphyrin could decrease the electron density of porphyrino rings and enhance the ability of manganese porphyrins against the oxidative degradation; 4) The involvement of [PW12O40]3- could further increase the stability of porphyrin cations and thus improve catalytic activity of the catalyst. 1. Synthesis and catalytic performance of A3 for ethylbenzene oxidationFor comparison, the corresponding neutral manganese porphyrin (A1), and the ionic manganese porphyrin (A3) with hexafluorophosphates as counteranions, were synthesized and investigated parallelingly in this study. The ionic manganese porphyrin of A2 and A3 featured with imidazolium cations at four meso-carbons proved to be the more efficient and stable catalysts for the oxidation of the less reactive ethylbenzene in contrast to the neutral A1. The UV-visible spectral analysis further proved that the use of [Bmim]PF6 as the solvent could completely inhibit the formation of the inactiveμ-oxo MnⅣ-porphyrin dimer, and the incorporation of imidazolium units with strong electron-withdrawing effect in ionic A2 and A3 gave rise to the stability against oxidation degradation. This work suggested that the incorporation of quaternary ammonium cations (like imidazolium units) with strong electron-withdrawing nature into porphyrino ring was a more effective strategy to suppress the oxidative degradation and then improve the activity of the manganese porphyrin in contrast of the roles of halides, and that the use of the IL with conjugated nature as the solvent was a very effective strategy to inhibit the formation of the inactiveμ-oxo MnⅣ-porphyrin dimer, in contrast to the means like introducing sterically bulky substituents in the porphyrino ring, or using excess N-containing axial ligands. Hoverer, the role of introducing tartrates as the corresponding counter anions was found to be negligible to the catalytic performance of the ionic manganese porphyrin2. Synthesis and catalytic performance of B2 for ethylbenzene (derivatives) oxidationsWithout involvement of the auxiliary axial ligands, the ionic compound B2 combined with the cationic Mn-porphyrin ([MnTMPyP]5+) and the a-Keggin type phosphotungstate ([PW12O40]3-), prove to be an efficient catalyst in room temperature IL of [BPy]BF4 for ethylbenzene oxidation in terms of activity and stability, in comparison to B1 only possessing the catalytic site of [MnTMPyP]5+ alone. The contribution of [PW12O40]3-anions to the activation of ethylbenzene proved to be negligible, but to the stability of [MnTMPyP]5+ against the oxidative degradation was of significance.3. Synthesis and catalytic performance of C2 for cyclooctene epoxidationThe incorporation of Br atoms with bulky size and strong electron-withdrawing nature onβ-positions of the porphine ring was aimed to further decrease the electron density on the porphine ring, which supposedly could give rise to the catalyst stability against oxidative degradation. The distortion of the porphine ring and the increased steric hindrance with introduction of Br atoms could inhibit the manganese porphyrin aggregation through p-p interactions, and avoid the formation ofμ-oxo MnⅣ-porphyrin dimers. However, catalytic experiments and the UV-visible spectral analysis proved that C2was a less efficient and stable catalyst for the epoxidation of cyclooctene in contrast to C1, which did not possess any Br atom. Probably the introduction of eight bulky Br atoms onβ-positions increased the porphine ring's tension which could made the porphyrin ring structure collapse more easily under oxidative environment.