## 2 Results and Discussion
### The Chemical Nature of Zirconium in the L-Zr-X Samples
The lack of a peak at 833 cm\({}^{-1}\) and the steric  
Figure 1: IR spectra of IDA-S, L-S and L-Zr-S samples. hindrance of the ortho positions due to the steric demand of iminodiacetate groups confirm the meta-substitutions on the STY rings [24]. Even when we used a significant excess of zirconium salt, we could not saturate the iminodiacetate groups--the ratio of uncomplexed/chelated iminodiacetate groups in the L-Zr-X samples was \(\sim\)2:1, \(\sim\)3:1 and \(\sim\)4:1 for X=Cl, N and S, respectively. Since the iminodiacetic acid groups forming in the acidic environment during Zr loading rearrange into an N-protonated form with the formation of a carboxylate group, a \"free\" iminodiacetic group can act as a monovalent acid. It can be considered as only a monovalent anion, [HN(CH\({}_{2}\)COO)\({}_{2}\)]. Thus, the counter-ions of zirconium (IV) used to neutralize the excess of two charges of zirconium (IV) may be two ionic unchelated iminodiacetate groups (Scheme 1b). Due to intramolecular self-protonation, a minimum of two free iminodiacetate groups need to neutralize one chelated iminodiacetate-coordinated zirconium. Thus, the Zr-loading of iminodiacetate groups in the L-Zr-X samples may not be higher than 1/3 due to the 2:1 ratio of monovalent anionic and chelated iminodiacetate groups, respectively, as was found in the case of the zirconyl chloride-loaded L-Zr-Cl sample (Scheme 1b). To confirm the presence of the protonated/Zr-coordinated nitrogen atoms in the L-Zr-X samples, we used solid-state \({}^{15}\)N and \({}^{13}\)C CP MAS NMR spectroscopy. Deconvolution of the broad \({}^{15}\)N CP-MAS NMR signal of the iminodiacetate nitrogen resulted in two components for both samples, L-N and L-Zr-N (315/330 and 317/332 ppm, Figure 2a,b, respectively). The signal at 315 ppm may be attributed to the protonated N-atom, whereas the signal at 317 ppm can be attributed to the protonated and Zr-coordinated N-atom. This latter signal is broadened by the quadrupolar magnetic nature of the Zr. A weak \({}^{15}\)N signal of the unprotonated tertiary N atom (due to the equilibrium character of the protonation reaction) also appears in both spectra. The chemical shifts in the nitrogen atoms in protonated and coordinated forms (Scheme 1b) are similar--the electron-shielding environment is similar. In other words, the strength of the coordination of Zr to the nitrogen atom in the iminodiacetate group of L-Zr-X samples is close to the strength of the proton binding to the protonated nitrogen atoms in -HN(CH\({}_{2}\)COOH)(CH\({}_{2}\)COO) or -HN(CH\({}_{2}\)COO)\({}_{2}\) groups (Scheme 1b). When X= sulfate and nitrate, the chelated/unchelated iminodiacetate group ratios were 1:4 and 1:3, respectively. The free iminodiacetate groups are in their acidic form according to the low pH used during Zr loading and, due to self-protonation, these are in the [-CH\({}_{2}\)-NH(CH\({}_{2}\)COO)(CH\({}_{2}\)COOH) (Scheme 1b) form [29]. The asymmetric nature of these iminodiacetate groups results in two C=O carbonyl signals [28] in the \({}^{13}\)C NMR spectra of the L-X-Zr samples (Supplementary Materials, Figure S7). Due to the lack of foreign neutralizing ions in the L-Zr-X samples, a question arises: why is Zr loading so different depending on the chemical form of the zirconium salt used in the loading experiments? The zirconium(IV) ion easily hydrolyzes/polymerizes in aqueous solutions, even under highly acidic conditions [12]. The exclusive form of zirconium(IV) in aq. solutions above pH > 0 are zirconyl ions (ZrO\({}^{2+}\)), but the counter-ions have an enormous influence on the chemical nature of the species containing Zr. For example, solid zirconyl nitrate, ZrO(NO\({}_{3}\))2:5H\({}_{2}\)O, consists of H\({}_{2}\)[ZrO(NO\({}_{3}\))4].H\({}_{2}\)O anionic species. It completely dissociates in its aqueous solutions with the formation of a neutral ZrO(OH)NO\({}_{3}\), which could not be loaded on a cationic (Amberlite IR 100) ion exchanger [13]. Zirconium sulfate strongly hydrolyzes in water.