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

Abstract

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.