Direct on-line method to monitor the dynamic structure of noncovalent titanium complexes in solution by using cold-spray ionization time-of-flight mass spectrometry.

In this paper, we report the feasibility of using the on-line cold-spray ionization time-of-flight mass spectrometry (CSI-TOFMS) method to monitor directly the labile noncovalent organometallic complexes formed in the substitution reaction between a chiral titanium asymmetric catalyst and organic acid in the toluene solvent. The comparison between the conventional ESI and CSI-TOFMS spectra of [(S)1]2Ti2[(S)2]2 in THF indicated that the CSI-TOFMS is an effective method to characterize the structures of labile organometallic complexes in solution. A new strategy was designed so that [(S)1]2Ti2[(S)2]2 and organic acid solution were delivered independently to the CSI ion source by two infusion syringe pumps, and they mixed at a triple valve, and then the substitution reaction occurred in the sprayer. The mix and reaction time was approximately 5 s and can be adjusted by the length of the PEEK tube and the flow rate. The reaction products were ionized and detected directly by the CSI-TOFMS. Organic acids, such as formic, acetic, phenylpropionic, anisic, and para-methoxylphenylactic acids, were investigated. From the on line CSI-TOFMS spectra, it is observed clearly that one or two (S)2 group(s) can be substituted by the added acid to form new species. The results obtained here demonstrated that the designed strategy with on line CSI-TOFMS is effective and reliable to elucidate the dynamic structure of noncovalent titanium complexes in reaction solution.