Coordination Chemistry Of Metal Complexes And Organic Bases Studied By Electrospray Ionization Mass Spectrometry

Electrospray ionization (ESI) is a soft ionization method that has proven to be useful for the analysis of thermally sensitive and nonvolatile substances and, for this reason, has been employed widely for the characterization of biomolecules, synthetic oligomers and polymers, and inorganic and organometallic complexes. ESI can keep any weakly covalent or noncovalent intact in a complex. In addition, its ability to detect ionic species insolution , provides a convenient technique for direct observation of reaction intermediates. In the process of monitoring organic reactions, electrospray ionization mass spectrometry (ESI-MS) can provide m/z and the isotope distribution patterns of the important intermediates, which maybe offer crucial information for the mechanism study. Based on the points above, ESI in combination with tandem mass spectrometry (MS/MS), is employed to study the coordination chemistry of metal complexes and organic bases, and directly observes the copolymerization of propylene and carbon dioxide catalyzed by chromium complexes and DMAP, in order to study the reaction pathway.The positive ESI mass spectra of SalenCrX and SalanCrX (where the tetradentate N,N'-disubstituted bis(aminophenoxide) is designated as Salan, a saturated version of the Schiff-base Salen ligand) complexes in the presence of DMAP have demonstrated striking diffierence in the coordination behavior. With the increasing amounts of DMAP, [SalenCr]⁺ cations are prone to bind two DMAP molecules to form six-coordinated complex ions, while [SalanCr]⁺ cations are inclined to pick up one molecule of DMAP to form five-coordinated complex ions which are found to be relatively unstable. The remarkable difference in the coordination of these two complexes with DMAP, results in significant difference in catalytic activity for the copolymerization of carbon dioxide and propylene oxide (PO). In the presence of 1 equiv DMAP, the activity of SalanCrX was about 30 times that of SalenCrX at ambient temperature. ESI-MS studies suggest that the coordination modes of the two chromium complexes with DMAP and the induction periods of CO₂/PO copolymerization and the rate of polymer-chain growth with the two catalyst systems are very different. A straightforward approach to insight into the details of copolymerization of CO₂ and epoxides by SalanCr(III)X/DMAP catalyst system is provided.Furthermore, ESI-MS/MS has been employed to study the stability and selectivity of SalenMX (M = Al, Mn, Cr, Co) and organic bases. The experimental results indicate that SalenMX (M = Al, Mn) are prone to bind one organic base molecule, while SalenMX (M = Cr, Co) are inclined to pick up two molecules. The stability of the metal complexes binding with organic bases is correlated to the Lewis acidity of central metals and the Lewis base of organic bases. The stronger the Lewis acidity of the central metal and the Lewis base of organic bases are, the tighter the affinity of organic bases towards cental metal. The ligands of metal complexes have little influence on the stability. To a certain extent, the stability of the coordination of organic bases and SalenMX reflects the Lewis acidity of the central metal.