Polydentate N,O-Ligands Possessing Unsymmetrical Urea Fragments Attached to a p-Cresol Scaffold

In this study, three series of polydentate N,O-ligands possessing unsymmetrical urea fragments attached to a p-cresol scaffold are obtained, namely mono- and bi-substituted open-chain aromatics, synthesised using a common experiment, as well as fused aryloxazinones. Separate protocols for the preparation of each series are developed. It is found that in the case of open-chain compounds, the reaction output is strongly dependent on both bis-amine and carbamoyl chloride substituents, while oxazinones can be effectively obtained via a common protocol. The products are characterized via 1D and 2D NMR spectra in solution and using single-crystal XRD. A preliminary study on the coordination abilities of the products performed via ITC shows that there are no substantial interactions in the pH range of 5.0–8.5 in general.


Introduction
Solvent extraction is among the most powerful techniques applied in separation science and technology [1][2][3][4][5][6][7][8] at both the laboratory and industrial scale.In particular, synergistic extraction is an active field of research and development in modern coordination chemistry, having a great impact on metal separation and treatment and the recycling of industrial wastes [9][10][11][12][13][14][15].The proper selection of a combination of extractants and synergists results in a multiple increase in the extraction efficiency and the better separation of metals.Therefore, the discovery of new extraction systems is a topical and rapidly evolving trend in modern chemistry.
Recently, we reported on the efficiency of two novel polydentate ligands, shown in Figure 1, as synergists in the isolation and separation of metal ions [41].These ligands possess unsymmetrical urea fragments attached to a p-cresol scaffold and can be generally divided into open-chain substituted aromatics (S1) and fused aryloxazinones (S2).The concept was to design polydentate ligands with variable coordination abilities, controlled according to differences in the substitution pattern and geometry of the molecules [42].The compounds were obtained in a common experiment and were isolated from the complex mixture with a low overall yield [43].
Molecules 2023, 28, x FOR PEER REVIEW 2 of 28 The compounds were obtained in a common experiment and were isolated from the complex mixture with a low overall yield [43]. Herein, we report on the optimization of separate synthetic protocols for each ligand series, solution and solid-state characterization, and report a preliminary study on the coordination properties of the products.

Results and Discussion
Our efforts were initially directed towards open-chained ligands due to the observed diverse extraction efficiency of S1 towards various metal ions [41].Starting bis-amines were obtained by grinding commercially available 2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde and primary amine in solventless conditions, followed by sodium borohydride reduction.As reported [43], the compounds S1 and S2 were isolated from a very complex mixture obtained via the direct reaction of the corresponding p-cresol-based secondary bis-amine (1b), phosgene as toluene solution and aniline in the presence of pyridine as a base.In an attempt to override the formation of oxazinone-ring-containing compounds, the reaction among bis-amine 1, phosgene and primary amine was performed as a two-step protocol; the initial formation of carbamic chloride and the subsequent reaction with bis-amine (Scheme 1).Three types of N-substituents were chosen, namely phenyl, benzyl and phenethyl, i.e., aryl, benzyl and alkyl, in order to tune the nitrogen basicity and steric flexibility.The conditions were varied and it was found that the optimal conditions for the predominant formation of product 2 or 3 are different for each example, and that the reaction out-put is strongly dependent on the base, reagents proportions and solvent; meanwhile, prolongation and dilution have no significant impact.Selected results are summarized in Table 1.
The transformation performed between aniline-derived bis-amine (1a) and phenylcarbamic chloride in the presence of pyridine as a base led to the formation of two main products, isolated in a moderate overall yield (up to 57%); mono-and bis-acylated ligands Herein, we report on the optimization of separate synthetic protocols for each ligand series, solution and solid-state characterization, and report a preliminary study on the coordination properties of the products.

Results and Discussion
Our efforts were initially directed towards open-chained ligands due to the observed diverse extraction efficiency of S1 towards various metal ions [41].Starting bis-amines were obtained by grinding commercially available 2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde and primary amine in solventless conditions, followed by sodium borohydride reduction.As reported [43], the compounds S1 and S2 were isolated from a very complex mixture obtained via the direct reaction of the corresponding p-cresol-based secondary bis-amine (1b), phosgene as toluene solution and aniline in the presence of pyridine as a base.In an attempt to override the formation of oxazinone-ring-containing compounds, the reaction among bis-amine 1, phosgene and primary amine was performed as a two-step protocol; the initial formation of carbamic chloride and the subsequent reaction with bis-amine (Scheme 1).
The compounds were obtained in a common experiment and were isolated from the complex mixture with a low overall yield [43]. Herein, we report on the optimization of separate synthetic protocols for each ligand series, solution and solid-state characterization, and report a preliminary study on the coordination properties of the products.

Results and Discussion
Our efforts were initially directed towards open-chained ligands due to the observed diverse extraction efficiency of S1 towards various metal ions [41].Starting bis-amines were obtained by grinding commercially available 2-hydroxy-5-methyl-1,3-benzenedicarboxaldehyde and primary amine in solventless conditions, followed by sodium borohydride reduction.As reported [43], the compounds S1 and S2 were isolated from a very complex mixture obtained via the direct reaction of the corresponding p-cresol-based secondary bis-amine (1b), phosgene as toluene solution and aniline in the presence of pyridine as a base.In an attempt to override the formation of oxazinone-ring-containing compounds, the reaction among bis-amine 1, phosgene and primary amine was performed as a two-step protocol; the initial formation of carbamic chloride and the subsequent reaction with bis-amine (Scheme 1).Three types of N-substituents were chosen, namely phenyl, benzyl and phenethyl, i.e., aryl, benzyl and alkyl, in order to tune the nitrogen basicity and steric flexibility.The conditions were varied and it was found that the optimal conditions for the predominant formation of product 2 or 3 are different for each example, and that the reaction out-put is strongly dependent on the base, reagents proportions and solvent; meanwhile, prolongation and dilution have no significant impact.Selected results are summarized in Table 1.
The transformation performed between aniline-derived bis-amine (1a) and phenylcarbamic chloride in the presence of pyridine as a base led to the formation of two main products, isolated in a moderate overall yield (up to 57%); mono-and bis-acylated ligands Three types of N-substituents were chosen, namely phenyl, benzyl and phenethyl, i.e., aryl, benzyl and alkyl, in order to tune the nitrogen basicity and steric flexibility.The conditions were varied and it was found that the optimal conditions for the predominant formation of product 2 or 3 are different for each example, and that the reaction output is strongly dependent on the base, reagents' proportions and solvent; meanwhile, prolongation and dilution have no significant impact.Selected results are summarized in Table 1.
The transformation performed between aniline-derived bis-amine (1a) and phenylcarbamic chloride in the presence of pyridine as a base led to the formation of two main products, isolated in a moderate overall yield (up to 57%); mono-and bis-acylated ligands 2aa and 3aa (Scheme 1, Table 1), i.e., only N-acylated products.On the contrary, the oxygen was also attacked when using N,N-diisopropylethylamine (DIPEA), leading to the formation of compounds 4 (Figure 2 and Figure S1) and 5 (Figure 3), together with 2aa in a 51% overall yield (entry 1).For this reason, all further experiments were performed in the presence of pyridine as a base.ence of pyridine as a base.As seen in Table 1, increasing the carbamic chloride portion leads to an increase in the percentage of product 3 in general, while the solvent effect is dependent on the Nsubstituents.The best conversion is achieved in toluene, except in ligands 2ac/3ac, where the overall yield is higher in dichloroethane (entries 9 vs. 10), 2ba/3ba (entries 12 vs.13) and 2cc/3cc (entries 26 vs. 28), and where the results in toluene and benzene are commensurable.However, the best conditions for the preparation of a particular ligand, 2 or 3, are more sensitive to the solvent used.All carbamic chlorides operate the most effectively in toluene, except for a few examples.Phenylcarbamic chloride is the most effective in benzene only for the ligand 2ba (entry 12).Similar conversions with benzylcarbamic chloride are achieved for 2cb in toluene and dichloroethane (entries 22 vs. 25).When using phenylethylcarbamic chloride, benzene is the right solvent for 2bc (entry 16) and 2cc (entry 26), dichloroethane is the right solvent for 3ac (entry 10), while the yields of 3cc are identical in toluene and dichloroethane (entries 28 vs. 29).The best results for all bis-amines are also obtained in toluene, with few exceptions.Starting from 1a, dichloroethane is the preferable solvent only for ligand 3ac (entry 10).The best yields from 1b are obtained in benzene for ligands 2ba (entry 12) and 2bc (entry 16).When starting from 1c, benzene is the right solvent for 2cc (entry 26), whereas the results in toluene and dichloroethane are comparable for ligands 2cb (entries 22 vs. 25) and 3cc (entries 28 vs. 29).
The structures of the products are assigned using 1D and 2D NMR spectra and confirmed via single-crystal XRD of selected samples.The NMR spectra show characteristics for each series pattern (Table 2).Separate signals are observed for each group in compounds 2, while all groups of both p-cresol scaffold and substituents show common signals in symmetrical molecules 3, the effect being the most discernible for CH protons of pcresol scaffold (CH-3 and CH-5) and methylene groups, which are in an area free of other signals.The latter is demonstrated in the example of the 1b-derived ligands 2ba and 3ba, as shown in Figure 4.
At the same time, the chemical shifts in some signals are strongly dependent on the substitution pattern (Table 2).The proton resonances for CH-3 and CH-5 of the p-cresol scaffold are shifted upfield and downfield, respectively, in mono-substituted ligands obtained from 1a (R1 = Ph, 2aa, 2ab, 2ac); meanwhile, in bi-substituted analogues, the signals are in between.For the rest of the ligands (R1 = Bn, phenethyl), the chemical shifts in the two signals in the spectra of ligand 2 and the signal in the spectra of 3 possess very close values.The signals for both bridged methylene groups (CH2-Cq-2 and CH2-Cq-6) are shifted upfield in the spectra of all ligands of 2 with respect to the corresponding signal in 3. The carbon resonances for CH-3 and CH-5 are slightly shifted downfield in 3 with respect to 2 in all couples, except in ligands 2ab/3ab and 2ac/3ac, in which the signal of As seen in Table 1, increasing the carbamic chloride portion leads to an increase in the percentage of product 3 in general, while the solvent effect is dependent on the N-substituents.The best conversion is achieved in toluene, except in ligands 2ac/3ac, where the overall yield is higher in dichloroethane (entries 9 vs. 10), 2ba/3ba (entries 12 vs.13) and 2cc/3cc (entries 26 vs. 28), and where the results in toluene and benzene are commensurable.However, the best conditions for the preparation of a particular ligand, 2 or 3, are more sensitive to the solvent used.All carbamic chlorides operate the most effectively in toluene, except for a few examples.Phenylcarbamic chloride is the most effective in benzene only for the ligand 2ba (entry 12).Similar conversions with benzylcarbamic chloride are achieved for 2cb in toluene and dichloroethane (entries 22 vs. 25).When using phenylethylcarbamic chloride, benzene is the right solvent for 2bc (entry 16) and 2cc (entry 26), dichloroethane is the right solvent for 3ac (entry 10), while the yields of 3cc are identical in toluene and dichloroethane (entries 28 vs. 29).The best results for all bis-amines are also obtained in toluene, with few exceptions.Starting from 1a, dichloroethane is the preferable solvent only for ligand 3ac (entry 10).The best yields from 1b are obtained in benzene for ligands 2ba (entry 12) and 2bc (entry 16).When starting from 1c, benzene is the right solvent for 2cc (entry 26), whereas the results in toluene and dichloroethane are comparable for ligands 2cb (entries 22 vs. 25) and 3cc (entries 28 vs. 29).
The structures of the products are assigned using 1D and 2D NMR spectra and confirmed via single-crystal XRD of selected samples.The NMR spectra show characteristics for each series pattern (Table 2).Separate signals are observed for each group in compounds 2, while all groups of both p-cresol scaffold and substituents show common signals in symmetrical molecules 3, the effect being the most discernible for CH protons of p-cresol scaffold (CH-3 and CH-5) and methylene groups, which are in an area free of other signals.The latter is demonstrated in the example of the 1b-derived ligands 2ba and 3ba, as shown in Figure 4.  Several phases appropriate for single-crystal XRD were grown and analysed.The ORTEP views are shown in Figure 5.A comparison between the two types of ligand shows that the fragments bearing unsymmetrical urea moiety possess conserved molecular geometry, while the remaining part(s) of the molecules seems to be more flexible and lead to variations in the molecular geometry and distance between the coordination centres.At the same time, the chemical shifts in some signals are strongly dependent on the substitution pattern (Table 2).The proton resonances for CH-3 and CH-5 of the pcresol scaffold are shifted upfield and downfield, respectively, in mono-substituted ligands obtained from 1a (R 1 = Ph, 2aa, 2ab, 2ac); meanwhile, in bi-substituted analogues, the signals are in between.For the rest of the ligands (R 1 = Bn, phenethyl), the chemical shifts in the two signals in the spectra of ligand 2 and the signal in the spectra of 3 possess very close values.The signals for both bridged methylene groups (CH 2 -C q -2 and CH 2 -C q -6) are shifted upfield in the spectra of all ligands of 2 with respect to the corresponding signal in 3. The carbon resonances for CH-3 and CH-5 are slightly shifted downfield in 3 with respect to 2 in all couples, except in ligands 2ab/3ab and 2ac/3ac, in which the signal of compound 3 is in between those of the corresponding ligand 2; meanwhile, that for the methylene group of 3 is in between both signals in the spectra of 2 in all examples.
Several phases appropriate for single-crystal XRD were grown and analysed.The ORTEP views are shown in Figure 5.A comparison between the two types of ligand shows that the fragments bearing unsymmetrical urea moiety possess conserved molecular geometry, while the remaining part(s) of the molecules seems to be more flexible and lead to variations in the molecular geometry and distance between the coordination centres.The latter is illustrated in the example of ligands 2ba and 3ba in Figure 5f.Compound 2ab crystallizes in the monoclinic Cc space group with two molecules in the ASU.The overlay of the two molecules provides an RMSD of 1.0635 Å (Figure S2a), and shows that the molecular geometry differs due to the flexibility of the side chains and the observed rotation of the terminal benzyl moieties around the N-C bond, e.g., rotamers or conformational isomerism is plausible.For both molecules, an intramolecular O-H…O hydrogen bond stabilizes the molecular geometry.A classical N-H…O interaction between adjacent molecules produces C 2 2(8) chains, propagating along the c axis.Although Compound 2ab crystallizes in the monoclinic Cc space group with two molecules in the ASU.The overlay of the two molecules provides an RMSD of 1.0635 Å (Figure S2a), and shows that the molecular geometry differs due to the flexibility of the side chains and the observed rotation of the terminal benzyl moieties around the N-C bond, e.g., rotamers or conformational isomerism is plausible.For both molecules, an intramolecular O-H. ..O hydrogen bond stabilizes the molecular geometry.A classical N-H. ..O interaction between adjacent molecules produces C 2 2 (8) chains, propagating along the c axis.Although several weak interactions, namely C-H. ..O, N-H. ..π (Table S6, Figure 6a,b), are detected, the threedimensional packing molecules of the molecules of 2ab produce pseudo-layers stacked along a (Figure S2b).Compound 2ac crystallizes in the orthorhombic Pca21 space group with one molecule in the ASU.The hydrogen bonding interactions are similar to those observed in compound 2ab: one intramolecular O5-H5…O9 and intermolecular N2-H2…O9 (Figure 6c and Table S7).The intermolecular hydrogen bond again produces a chain motif C 1 1(4) propagating along a. Again, the presence of an aromatic ring in the molecule of 2ac is responsible for several weak interactions (Figure 6).The combination of hydrogen bonds and a weak interaction produces pseudo-layers stacked along c (Figure S3).The hydrogen bonding patter in 2ba, 2bc and 3ba is analogous to 2ab and 2ac.The basic difference is the presence of two intramolecular hydrogen bonds instead of one.The intermolecular interaction again produces chains with graphsets C 1 1(10) for 2ba, 2bc and C 1 1(12) 3ba (Figure 7 and Tables S8-S10).The three-dimensional arrangement of the molecules again produces pseudo-layers, as shown in Figure S4.Compound 2ac crystallizes in the orthorhombic Pca2 1 space group with one molecule in the ASU.The hydrogen bonding interactions are similar to those observed in compound 2ab: one intramolecular O5-H5. ..O9 and intermolecular N2-H2. ..O9 (Figure 6c and Table S7).The intermolecular hydrogen bond again produces a chain motif C 1  1 (4) propagating along a. Again, the presence of an aromatic ring in the molecule of 2ac is responsible for several weak interactions (Figure 6).The combination of hydrogen bonds and a weak interaction produces pseudo-layers stacked along c (Figure S3).The hydrogen bonding patter in 2ba, 2bc and 3ba is analogous to 2ab and 2ac.The basic difference is the presence of two intramolecular hydrogen bonds instead of one.The intermolecular interaction again produces chains with graphsets C 1  1 (10) for 2ba, 2bc and C 1 1 ( 12) 3ba (Figure 7 and Tables S8-S10).The three-dimensional arrangement of the molecules again produces pseudo-layers, as shown in Figure S4.A two-step sequence was chosen for the preparation of fused aryloxazinone ligands with an unsymmetrical urea fragment (7).Bis-amines 1 were submitted to a reaction with phosgene solution (Scheme 2).The proportions, base, and reaction duration were varied and it was found that two equivalents of phosgene, pyridine as a base, and a 2 h reaction time at room temperature are the optimal conditions.The intermediate products 6 were easily isolated via chromatography in 40-50% yields.Further prolongation did not result in better conversion, while the use of equimolar amounts of the reagents significantly decreased the yields.It has to be noted that a side-product was isolated in a low yield (2%) from the reaction of 1a in parallel with chloride 6a, whose structure was determined to be, via NMR and XRD, amino oxazinone 8a (Figure 8), the precursor of 6a.Based on this observation, it can be suggested that phenol oxygen is initially attacked by phosgene, followed by cyclization and N-acylation.The second step was performed in different proportions and with different bases, A two-step sequence was chosen for the preparation of fused aryloxazinone ligands with an unsymmetrical urea fragment (7).Bis-amines 1 were submitted to a reaction with phosgene solution (Scheme 2).The proportions, base, and reaction duration were varied and it was found that two equivalents of phosgene, pyridine as a base, and a 2 h reaction time at room temperature are the optimal conditions.The intermediate products 6 were easily isolated via chromatography in 40-50% yields.Further prolongation did not result in better conversion, while the use of equimolar amounts of the reagents significantly decreased the yields.A two-step sequence was chosen for the preparation of fused aryloxazinone ligands with an unsymmetrical urea fragment (7).Bis-amines 1 were submitted to a reaction with phosgene solution (Scheme 2).The proportions, base, and reaction duration were varied and it was found that two equivalents of phosgene, pyridine as a base, and a 2 h reaction time at room temperature are the optimal conditions.The intermediate products 6 were easily isolated via chromatography in 40-50% yields.Further prolongation did not result in better conversion, while the use of equimolar amounts of the reagents significantly decreased the yields.It has to be noted that a side-product was isolated in a low yield (2%) from the reaction of 1a in parallel with chloride 6a, whose structure was determined to be, via NMR and XRD, amino oxazinone 8a (Figure 8), the precursor of 6a.Based on this observation, it can be suggested that phenol oxygen is initially attacked by phosgene, followed by cyclization and N-acylation.The second step was performed in different proportions and with different bases, solvents, temperatures, and reaction durations.Finally, the conditions were optimized and the products were isolated via chromatography in a 87% yield after 19 h of stirring at It has to be noted that a side-product was isolated in a low yield (2%) from the reaction of 1a in parallel with chloride 6a, whose structure was determined to be, via NMR and XRD, amino oxazinone 8a (Figure 8), the precursor of 6a.Based on this observation, it can be suggested that phenol oxygen is initially attacked by phosgene, followed by cyclization and N-acylation.A two-step sequence was chosen for the preparation of fused aryloxazinone ligands with an unsymmetrical urea fragment (7).Bis-amines 1 were submitted to a reaction with phosgene solution (Scheme 2).The proportions, base, and reaction duration were varied and it was found that two equivalents of phosgene, pyridine as a base, and a 2 h reaction time at room temperature are the optimal conditions.The intermediate products 6 were easily isolated via chromatography in 40-50% yields.Further prolongation did not result in better conversion, while the use of equimolar amounts of the reagents significantly decreased the yields.It has to be noted that a side-product was isolated in a low yield (2%) from the reaction of 1a in parallel with chloride 6a, whose structure was determined to be, via NMR and XRD, amino oxazinone 8a (Figure 8), the precursor of 6a.Based on this observation, it can be suggested that phenol oxygen is initially attacked by phosgene, followed by cyclization and N-acylation.The second step was performed in different proportions and with different bases, solvents, temperatures, and reaction durations.Finally, the conditions were optimized and the products were isolated via chromatography in a 87% yield after 19 h of stirring at The second step was performed in different proportions and with different bases, solvents, temperatures, and reaction durations.Finally, the conditions were optimized and the products were isolated via chromatography in a 87% yield after 19 h of stirring at 50 • C in dichloroethane with 2.5 equivalents of primary amine, in the absence of another base.The results are summarized in Table 3.The structures of the products were assigned using 1D and 2D NMR spectra.The chemical shifts in the bridge methylene (CH 2 -C q -2 and CH 2 -C q -6) and skeleton methyne (CH-3 and CH-5) groups are dependent on the substitution pattern within the series, the effect being more significant on methylene group resonances (Table 4).The influence of the chloride 6 substituent (R 1 ) is inessential in general, while that of primary amine (R 2 ) causes substantial shifts in particular signals.The latter is illustrated for the ligands obtained with 6a-c and aniline in Figure 9.

Table 4.
Selected 1 H and 13 C NMR resonances (ppm) for the ligands 7.

Lig.
CH-3 CH-3 The structures of the products were confirmed via the single-crystal XRD of selected samples, as shown in Figure 10.The overlay of the molecules present in the ASU of the crystal structures revealed that the geometry of the benzoxazinone unit is highly conserved (Figure 11).In 6a, only an acceptor (C=O) is available and the molecular geometry minimizes its surficial area/interactions by closing on itself in order to promote halogen bonding and π. ..π/CH 3 interactions (Figure 12).In 7aa, 7ab, and 7bb, a donor and acceptor are present and the crystal structure stabilization is "dominated" by a hydrogenbonding NH. ..O interaction.In 7aa and 7ab, neighbouring molecules produce a zig-zag chain with a C 1 1 (10) graphset (Figure 13a,b, Tables S11 and S12), while in 7bb, the formation of a dimmer is preferred (R 2 2 (20), Figure 13c, Table S13).A comparison between the geometry of the three types of ligands, namely 2, 3 and 7, shows that the open-chain substituted compounds are oriented towards the optimal intramolecular H-bonding of the urea's heteroatoms, while the preferred geometry of oxazinones is driven by intermolecular bonding, as illustrated in Figure 14.At the same time, two types of H-bonding are observed in the open-chain compounds.In some products, like 2ab and 2ac, H-bonding involves hydroxyl proton and carbonyl oxygen (Figure 14b), while hydroxyl proton and nitrogen are bonded in others (Figure 14c).The structures of the products were confirmed via the single-crystal XRD of selected samples, as shown in Figure 10.The overlay of the molecules present in the ASU of the crystal structures revealed that the geometry of the benzoxazinone unit is highly con-        A comparison between the geometry of the three types of ligands, namely 2, 3 and shows that the open-chain substituted compounds are oriented towards the optimal i tramolecular H-bonding of the urea s heteroatoms, while the preferred geometry of ox zinones is driven by intermolecular bonding, as illustrated in Figure 14.At the same tim two types of H-bonding are observed in the open-chain compounds.In some produc like 2ab and 2ac, H-bonding involves hydroxyl proton and carbonyl oxygen (Figure 14 while hydroxyl proton and nitrogen are bonded in others (Figure 14c).A comparison between the geometry of the three types of ligands, namely 2, 3 and 7, shows that the open-chain substituted compounds are oriented towards the optimal intramolecular H-bonding of the urea s heteroatoms, while the preferred geometry of oxazinones is driven by intermolecular bonding, as illustrated in Figure 14.At the same time, two types of H-bonding are observed in the open-chain compounds.In some products, like 2ab and 2ac, H-bonding involves hydroxyl proton and carbonyl oxygen (Figure 14b), while hydroxyl proton and nitrogen are bonded in others (Figure 14c).Finally, a preliminary study on the coordination abilities of the products was performed by using isothermal titration calorimetry (ITC).This test is a sensitive and effective method that can provide information about complexation reactions and hence the strength of metal-ligand coordination.In this work, ITC is employed to detect the interactions of calcium (II), lead (II) and potassium (I) ions with the synthesized polydentate N,O-ligands (Table S6) at an approximately neutral pH range (5.0-8.5).The latter is chosen in an attempt to keep the conditions as green as possible.Typical representations of the ITC data showing interaction and a lack of interaction are shown in Figure 15a,b, respectively.Finally, a preliminary study on the coordination abilities of the products was performed by using isothermal titration calorimetry (ITC).This test is a sensitive and effective method that can provide information about complexation reactions and hence the strength of metal-ligand coordination.In this work, ITC is employed to detect the interactions of calcium (II), lead (II) and potassium (I) ions with the synthesized polydentate N,O-ligands (Table S6) at an approximately neutral pH range (5.0-8.5).The latter is chosen in an attempt to keep the conditions as green as possible.Typical representations of the ITC data showing interaction and a lack of interaction are shown in Figure 15a,b, respectively.
Finally, a preliminary study on the coordination abilities of the products was performed by using isothermal titration calorimetry (ITC).This test is a sensitive and effective method that can provide information about complexation reactions and hence the strength of metal-ligand coordination.In this work, ITC is employed to detect the interactions of calcium (II), lead (II) and potassium (I) ions with the synthesized polydentate N,O-ligands (Table S6) at an approximately neutral pH range (5.0-8.5).The latter is chosen in an attempt to keep the conditions as green as possible.Typical representations of the ITC data showing interaction and a lack of interaction are shown in Figure 15a,b, respectively.The remaining data are given in Table S14 (Supplementary Material).The fitted association constants of the interaction of compounds 2ba, 2bc, 2cb, 3bb, and 3cc with metals are provided in Table 5 and illustrated in Figures S5-S10.The remaining data are given in Table S14 (Supplementary Material).The fitted association constants of the interaction of compounds 2ba, 2bc, 2cb, 3bb, and 3cc with metals are provided in Table 5 and illustrated in Figures S5-S10.
Table 5. Summary of the ITC results for the titration of either Ca(II) chloride, Pb(II) chloride or KCl with 2ba, 2bc, 2cb, 3bb, 3cc derivatives; all interactions are for pH 5.0 and are conducted at 25 • C with 125 rpm continuous stirring; Ka is the association constant; n represents the stoichiometry, e.g., the number of molecules that associate to a metal ion.The ITC data reveal that most of the compounds do not interact with the particular metal ions tested.Only a few unveil interactions with a sensible Ka strength, mostly in slightly acidic conditions.The latter shows that the ligands likely need more acidic or more basic media to bind metal ions.This suggestion outlines a possible direction for further study regarding the interaction of compounds in a more broad pH range with an enlarged panel of metal ions.

General
All reagents were purchased from Aldrich, Merck and Fluka, and were used without any further purification.The deuterated solvents were purchased from Deutero GmbH.Fluka silica gel (TLC-cards 60778 with fluorescent indicator 254 nm) was used for TLC chromatography and R f -value determination.Merck Silica gel 60 (0.040-0.063 mm) was used for the flash chromatography purification of the products.The melting points were determined in capillary tubes using the SRS MPA100 OptiMelt (Sunnyvale, CA, USA) automated melting point system with a heating rate of 1 • C per min.The NMR spectra were recorded on the Bruker Avance II+ 600 spectrometer (Rheinstetten, Germany) in CDCl 3 ; the chemical shifts were quoted in ppm in δ-values against tetramethylsilane (TMS) as an internal standard, and the coupling constants were calculated in Hz.The assignment of the signals was confirmed by applying two-dimensional COSY, NOESY, HSQC and HMBC techniques.The spectra were processed using the Topspin 2.1 program.The turbo spray mass spectra of compounds 2aa, 3ba, 6a, 4, 7aa, 7ab, and 7bb were obtained using API 150EX (AB/MAS Sciex), and the ESI spectra of compounds 3aa, 2ab, 3ab, 2ac, 3ac, 2bb, 3bb, 2bc, 3bc, 2ca, 3ca, 2cb, 3cb, 2cc, 3cc, 5, 6c, 7ac, 7bc, 7ca, 7cb, and 7cc were obtained using the Single Quadrupole Liquid Chromatograph Mass Spectrometer Shimadzu LCMS-2020.The spectra were processed using Xcalibur Free Style program version 4.5 (Thermo Fisher Scientific Inc., Waltham, MA, USA).
The characterization of compounds 2ba and 7ba is given in ref. [43].Compound 6b was an object of individual further study and its characterization will be published in due course.

General Procedure for the Preparation of Ligands 2 and 3
Step 1: To a commercial 15% solution of phosgene in toluene (2 mmol), an amine (1 mmol) was added in an argon atmosphere and the mixture was stirred at room temperature for 40 min.The mixture was bubbled with argon to eliminate the excess phosgene and the crude carbamic chloride (CC) was further used without purification.
Step 2: To a solution of bis-amine (1) and pyridine in toluene, carbamic chloride was added portion-wise in an argon atmosphere and the mixture was stirred at room temperature for 24 h.The products were partitioned between toluene and brine.The organic layer was washed with 10 % aq.HCl and then with brine, dried over MgSO 4 , and evaporated to dryness.The product was purified via flash chromatography on silica gel by using a mobile phase with a gradient of polarity from DCM to 2% MeOH/DCM.The reagents' proportions and the corresponding yields are summarized in Table 1.

General Procedure for the Preparation of Ligands 7
General procedure for the preparation of chlorides 6: To a solution of bis-amine 1 (1 mmol) and pyridine (2 mmol), toluene phosgene (2 mmol, as 15% commercial solution in toluene) was added and the mixture was stirred at room temperature in an argon atmosphere for 2 h.The products were partitioned between toluene and brine.The organic layer was washed with 10 % aq.HCl and then with brine, dried over MgSO4, and evaporated to dryness.The product was purified via flash chromatography on silica gel by using a mobile phase with a gradient of polarity from DCM to 0.4% MeOH/DCM.
Structure and numeration scheme of intermediates 6, ligands 7, and their precursors 8.This numeration scheme was chosen in an attempt to facilitate a comparison with ligands 2 and 3.

General Procedure for the Preparation of Ligands 7
General procedure for the preparation of chlorides 6: To a solution of bis-amine 1 (1 mmol) and pyridine (2 mmol), toluene phosgene (2 mmol, as 15% commercial solution in toluene) was added and the mixture was stirred at room temperature in an argon atmosphere for 2 h.The products were partitioned between toluene and brine.The organic layer was washed with 10 % aq.HCl and then with brine, dried over MgSO 4 , and evaporated to dryness.The product was purified via flash chromatography on silica gel by using a mobile phase with a gradient of polarity from DCM to 0.4% MeOH/DCM.

General Procedure for the Preparation of Ligands 7
General procedure for the preparation of chlorides 6: To a solution of bis-amine 1 (1 mmol) and pyridine (2 mmol), toluene phosgene (2 mmol, as 15% commercial solution in toluene) was added and the mixture was stirred at room temperature in an argon atmosphere for 2 h.The products were partitioned between toluene and brine.The organic layer was washed with 10 % aq.HCl and then with brine, dried over MgSO4, and evaporated to dryness.The product was purified via flash chromatography on silica gel by using a mobile phase with a gradient of polarity from DCM to 0.4% MeOH/DCM.Structure and numeration scheme of intermediates 6, ligands 7, and their precursors 8.This numeration scheme was chosen in an attempt to facilitate a comparison with ligands 2 and 3.
Structure and numeration scheme of intermediates 6, ligands 7, and their precursors 8.This numeration scheme was chosen in an attempt to facilitate a comparison with ligands 2 and 3.
General procedure for the preparation of ligands 7: A mixture of chloride 6 (1 mmol) and primary amine (2.5 mmol) in dichloroethane was stirred at 50 • C for 19 h.The products were partitioned between dichloroethane and brine.The organic layer was washed with 10 % aq.HCl and then with brine, dried over MgSO 4 , and evaporated to dryness.The product was purified via flash chromatography on silica gel by using a mobile phase with a gradient of polarity from DCM to 1% MeOH/DCM.The yields are given in Table 3.

Figure 2 .
Figure 2. Structure and ORTEP view of the O-acylated product 4.

Figure 2 .
Figure 2. Structure and ORTEP view of the O-acylated product 4.

Figure 3 .
Figure 3. Structure and ORTEP view of the O,N-diacylated product 5.

Figure 3 .
Figure 3. Structure and ORTEP view of the O,N-diacylated product 5.

Figure 6 .
Figure 6.Visualization of the hydrogen bonds (a,c) and weak interactions (b,d) in 2ab and 2ac, respectively.

Figure 9 .
Figure 9. Methylene groups area of the 1 H (a) and 13 C (b) NMR spectra of products 7aa, 7ba and 7ca.

Figure 9 .
Figure 9. Methylene groups' area of the 1 H (a) and 13 C (b) NMR spectra of products 7aa, 7ba and 7ca.

Figure 11 .
Figure 11.Overlay of the two independent molecules in the asymmetric unit of 6a (grey) and 7aa (blue).

Figure 11 .
Figure 11.Overlay of the two independent molecules in the asymmetric unit of 6a (grey) and 7a (blue).

Figure 11 .
Figure 11.Overlay of the two independent molecules in the asymmetric unit of 6a (grey) and 7aa (blue).

Figure 12 .
Figure 12.Visualization of the weak interactions in the structure of 6a.

Figure 12 . 28 Figure 12 .
Figure 12.Visualization of the weak interactions in the structure of 6a.

Figure 15 .
Figure 15.Typical calorimetric titration isotherms of (a) the binding interaction between 2ba and Pb 2+ , along with the fitted association constant; (b) titration isotherm showing no interaction between 7ab and Ca 2+ ; (c) successful model fit for 2ba and; (d) unsuccessful attempt to model the interaction for 7ab.

Figure 15 .
Figure 15.Typical calorimetric titration isotherms of (a) the binding interaction between 2ba and Pb 2+ , along with the fitted association constant; (b) titration isotherm showing no interaction between 7ab and Ca 2+ ; (c) successful model fit for 2ba and; (d) unsuccessful attempt to model the interaction for 7ab.