Synthesis, spectroscopic and structural characterization of Co(II)-pullulan complexes by UV-Vis, ATR-FTIR, MALDI-TOF/TOF MS and XRD

[Display omitted] •Stable Co(II)-RLMP complexes were synthesized and isolated in the solid state.•Co(II)-RLMP complex formation is confirmed by UV-Vis and ATR-FTIR spectroscopy.•MALDI-TOF/TOF MS results confirmed d-Glcp as the RLMP repeating unit.•XRD results show a low crystallinity form of Co(II)-RLMP.•Different model structures of synthesized Co(II)-RLMP complexes are proposed. Complexes of Co(II) ion with reduced low-molar pullulan (RLMP) (Mw 6000 g/mol) were synthesized in aqueous solutions at boiling temperature in the pH range from 7.5 to 13.5. Obtained Co(II)-RLMP complexes, with cobalt content ∼2–8% (AAS), were characterized by UV-Vis spectrophotometry, FTIR spectroscopy (ATR-FTIR, FT-IRIS), MALDI-TOF/TOF MS, and XRD. Tetragonally distorted Oh coordination of Co(II) ions with O ligand atoms in synthesized complexes is suggested based on the spectrophotometric data. No influence of complexation process on the 4C1 chair conformation of the d-glucopyranose units of pullulan was detected by ATR-FTIR measurements and FT-IRIS showed high homogeneity of synthesized complexes. Some additional depolymerization of pullulan during complex synthesis was indicated by MALDI-TOF/TOF MS but it also revealed good stability of complexes with much weaker binding of Co(II) ion in low molar mass fragments. Even in complexes with highest Co(II) ion content a low degree of crystallinity was detected by XRD analysis.