Binding of Propylene Oxide to Porphyrin− and Salen−M(III) Cations, Where M = Al, Ga, Cr, and Co

The binding of propylene oxide (PO) to a series of metal cations LM(III)+, where for L = tetraphenylporphyrin
(TPP) M = Al, Ga, Cr, and Co, and for L = (R,R)-N,N‘-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediamine
(salen) M = Al and Cr, was studied in the gas phase by electrospray tandem mass spectroscopy, and the relative
stabilities of LM(PO)2+ and LM(PO)+ cations were determined. The chromium(III) and aluminum(III) cations most
tenaciously bind PO, and for M = Al, coordination to the TPP ligated metal center was favored relative to salen.
For (TPP)M(PO)2+, the dissociation of PO followed the order M = Al > Cr, but for (TPP)M(PO)+ the dissociation
was M = Cr > Al. The single-crystal structural determinations on (R,R-salen)AlOCHMe(S)CH2Cl·0.5PO and
(R,R-salen)AlO2CMe·1.5py grown in neat PO and pyridine, respectively, reveal five-coordinate aluminum(III) centers
with the alkoxide/acetate ligands in the axial position of a square-based pyramid. These results are discussed in
terms of the reactivity of these metal complexes in ring-opening polymerizations and copolymerizations with PO
and CO2, respectively.

本研究采用电喷雾串联质谱法（electrospray tandem mass spectroscopy）在气相条件下，探究了环氧丙烷（propylene oxide, PO）与一系列金属阳离子配合物LM(III)+的配位结合行为：当配体L为四苯基卟啉（tetraphenylporphyrin, TPP）时，中心金属M分别为Al、Ga、Cr及Co；当配体L为(R,R)-N,N’-双(3,5-二叔丁基水杨醛亚胺)-1,2-环己二胺（(R,R)-N,N‘-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexenediamine, salen）时，中心金属M为Al和Cr。本研究同时测定了LM(PO)₂⁺与LM(PO)⁺阳离子的相对稳定性。实验结果表明，铬(III)与铝(III)阳离子对PO的结合最为牢固；对于中心金属为Al的体系，相较于salen配体体系，TPP配位的金属中心更易发生配位作用。对于(TPP)M(PO)₂⁺配合物，PO的解离活性顺序为M=Al > Cr；而对于(TPP)M(PO)⁺配合物，其PO解离活性顺序则为M=Cr > Al。本研究还对分别在纯PO与吡啶（pyridine）溶剂中培养得到的(R,R-salen)AlOCHMe(S)CH₂Cl·0.5PO与(R,R-salen)AlO₂CMe·1.5py单晶进行结构解析，结果显示二者的铝(III)中心均为五配位构型，烷氧基/乙酸根基配体处于四方锥结构的轴向位点。最后，本文结合这类金属配合物分别与PO进行开环聚合、与CO₂进行共聚反应的反应活性，对上述实验结果展开了讨论。