**4. Conclusions**

The Schotten–Baumann (SB) reaction has been successfully adapted for the derivatization of MS hardly ionizable Re(VII) chlorocomplexes. We systematically studied the reaction of the Re(VII) bis(catechol) chlorocomplex with the set of halogen and alkyl anilines as derivatization agents. The SB reaction products are easily ionizable under common ESI conditions providing structurally characteristic molecular and fragment anions. Based on DFT computation, the effect of Re-N bond shortening in the course of complex deprotonation was simulated and also correlated with the basicity of aniline derivative used as a derivatization agent. Our conclusions follow the known relation between the basicity of the reaction environment and the yield of the SB reaction. However, an attempt to increase the yield of the derivatization reaction by adding triethylamine (TEA) to the reaction mixture was unsuccessful. Such a conclusion probably refers to the fast reaction providing the dioxorhenium complex as a competition to the SB reaction itself.

The shortening of the Re-N bond throughout neutral molecule deprotonation and concurrent prolongation of Re-O(20) makes the cleavage of one or both ligands the most probable initial fragmentation pathway of studied complexes, leading to the formation of abundant M-L and M-2L anions.

Although the fragmentation of all studied complexes was analogous, we observed a notable difference concerning the formation of ReO<sup>3</sup> − (*m*/*z* 235) ion in a fragmentation scheme of [Re(O)(Cat)2PIPA]−. Unlike the other complexes, this ion is not formed directly from an M-2L fragment but through an aryl-vinyl stabilized unique *m*/*z* 325, unseen in other studied complexes' fragmentation schemes.

**Supplementary Materials:** The following are available online, Figures S1–S13 and Tables S1–S7. Figure S1. HR ESI–MS/MS spectra of complex 1; collision energy, CE, was 40 eV, Figure S2. HR ESI– MS/MS spectra of complex 2; collision energy, CE, was 40 eV, Figure S3. HR ESI–MS/MS spectra of complex 3; collision energy, CE, was 40 eV, Figure S4. HR ESI–MS/MS spectra of complex 4; collision energy, CE, was 40 eV, Figure S5. HR ESI–MS/MS spectra of complex 5; collision energy, CE, was 40 eV, Figure S6. Pro-posed fragmentation scheme of complex 1, Figure S7. Proposed fragmentation scheme of complex 3, Figure S8. Proposed fragmentation scheme of complex 4, Figure S9. Proposed fragmentation scheme of complex 5, Figure S10. CID diagram of complex 1, Figure S11. CID diagram of complex 3, Figure S12. CID diagram of complex 4. Figure S13. Graph of calculated bond elongation of prepared complexes. Table S1. Cartesian coordinates calculated for the optimized neutral structure of complex 1, Table S2. Cartesian coordinates calculated for the optimized neutral structure of complex 2, Table S3. Cartesian coordinates calculated for the optimized neutral structure of complex 3, Table S4. Cartesian coordinates calculated for the optimized neutral structure of complex 4, Table S5. Cartesian coordinates calculated for the optimized neutral structure of complex 5, Table S6. Chemical names, labels, and formulas of prepared rhenium complexes, Table S7. Calculated bond elongation of prepared complexes.

**Author Contributions:** M.Š. conceived, designed and performed the experiments; M.Š. and I.J. wrote the paper; M.V. was responsible for data analysis and visualization. All authors have read and agreed to the published version of the manuscript.

**Funding:** Financial support from Charles University Centre of Advanced Materials (CUCAM) (OP VVV Excellent Research Teams, project number CZ.02.1.01/0.0/0.0/15\_003/ 0000417) is greatly acknowledged.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Data is contained within the article or Supplementary Materials.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds mentioned in the materials section are available from the authors.
