Effects of N-oxidation on the molecular and crystal structures and properties of isocinchomeronic acid, its metal complexes and their supramolecular architectures: experimental, CSD survey, solution and theoretical approaches

Nine coordination complexes and polymer (M/L/X) based on Co, Ni, Zn, Cu (M), pyridine-N-oxide-2,5-dicarboxylic acid (H2pydco) (L) and either isonicotinamide (Ina), piperazine (pipz), 2,2′-bipyridine (bipy) and 1,10-phenanthroline (phen) (X) were synthesized and characterized by elemental analyses, infrared spectroscopy and single crystal X-ray diffraction. The resulting empirical formulae of the prepared complexes are [Co(H2O)6][Co(pydco)2(H2O)2]·2H2O (1), [M(pydco)(H2O)4]2 [M = Co (2), Ni (3), Zn (4)], [Co(pydco)(bipy)(H2O)2]·4H2O (5), [Co(pydco)(phen)(H2O)2]·5.135(H2O)·0.18(EtOH) (6), [Cu(Hpydco)(bipy)Cl]·2H2O (7), [Cu(Hpydco)(bipy)Cl]2·2H2O (8), and {[AgCu(H2O)2(phen)(pydco)]NO3}n (9). With the exception of 9, which forms an extended structure via multiple coordination modes, all the complexes contain (H)pydco as a bidentate ligand coordinated to the metal ion via the N-oxide and the adjacent carboxylate group oxygen atom, creating a chelate ring. The metal centers exhibit either distorted octahedral (1–6) or square pyramidal (7–9) geometry. Our results demonstrate that, when acting cooperatively, non-covalent interactions such as X–H⋯O hydrogen bonds (X = O, N, C), C–O⋯π and π⋯π stacking represent driving forces for the selection of different three-dimensional structures. Moreover, in compounds 2–4, 1D supramolecular chains are formed where O⋯π–hole interactions are established, which unexpectedly involve the non-coordinated carboxylate group. The non-covalent interaction (NCI) plot index analysis reveals the existence of the O⋯π–hole interactions that have been evaluated using DFT calculations. The Cremer and Pople ring puckering parameters are also investigated. The complexation reactions of these molecules with M were investigated by solution studies. The stoichiometry of the most abundant species in the solution was very close to the corresponding crystals. Finally, the effect of N-oxidation on the geometry of complexes has been also studied using the Cambridge Structural Database. It shows that complexes containing N-oxidized H2pydc are very rare.


Introduction
Over the last few decades, considerable attention has been devoted to crystal engineering, which is the design and synthesis of solidstate molecular structures with desired properties, based on the understanding and exploitation of intermolecular interactions. A variety of non-covalent interactions including hydrogen bonding, p/p stacking, C-H/p and other interactions produce many interesting structures with 1D (pillar, chain or band), 2D (layer) and 3D (network) topologies. [1][2][3] Intermolecular forces are considerably weaker than covalent bonds, so that supramolecular species are thermodynamically less stable, kinetically more labile and dynamically more exible than molecules. Supramolecular chemistry is therefore concerned with weak bonds. In the production of supramolecular scaffolds several factors such as the type of metal, solvent, organic ligand and auxiliary ligand play crucial roles. 4 Auxiliary ligands such as 2,2 0 -bipyridine and 1,10phenanthroline are bidentate with two aromatic units each carrying an N-donor atom. The crystal structures formed by these ligands can display weak p/p interactions that are very important for the construction of multidimensional arrays. 5 In recent years, researchers have been attracted to the many special properties and fascinating applications of complexes containing dicarboxylic acid and pyridine rings. 6 These applications exist in many areas, such as catalysis, 7 antibacterial activity, 8 anticancer properties, 9 aqueous solution chemistry, 10 surface chemistry, 11 magnetism 12,13 and uorescence. 14,15 Our research group therefore set out to synthesize coordination complexes based on derivatives of pyridinedicarboxylic acids with a view to their signicant applications. [16][17][18][19][20] Among the structural isomers of 2,n-pyridinedicarboxylic acids (n ¼ 3-6), H 2 pydc (isocinchomeronic acid or pyridine-2,5dicarboxylic acid) is an appropriate candidate for constructing metallotectons due to its interesting structural features: it is a multidentate ligand containing N-and O-donor atoms; it has two carboxylates on opposite sides of the pyridine ring. These features confer the potential to construct higher-dimensional extended structures. 21,22 The carboxylate groups display various coordination modes including monodentate (terminal and bridging) and bidentate (chelating and bridging). 23 Meanwhile, 5-carboxylate has a stronger bridging capability than 2-carboxylate due to its strong electron-donating and electrostatic power. 24 N-oxidation of the pyridine ring adds a new coordination mode: whereas the nitrogen atom of the pyridine ring in H 2 pydc can donate one pair of electrons, the N-oxide group can donate two pairs and can therefore coordinate to a larger number of metal centers, resulting in a greater variety of bridging modes for H 2 pydco compared to H 2 pydc (see Scheme 1). [25][26][27] We were inspired to synthesize the Noxide form of the ligand by the work of Xiong et al. who demonstrated how N-oxide functionalization could greatly enhance the CO 2 separation properties of isoreticular MOFs. 28 Moreover, this family of pyridine N-oxides have been utilized as an anti-HIV agent, gas adsorbent, luminescent agent, etc., [28][29][30] which encouraged us to prepare new complexes with the pyridine-N-oxide-2,5-dicarboxylic acid. Our studies concur with previous investigations in showing that H 2 pydco tends to be an effective chelating and bridging ligand. 27 2 (phen)(pydco)]NO 3 } n (9), where H 2 pydco ¼ pyridine-N-oxide-2,5-dicarboxylic acid, Ina ¼ isonicotinamide, pipz ¼ piperazine, bipy ¼ 2,2 0 -bipyridine and phen ¼ 1,10-phenanthroline (see Scheme S1 †). By means of DFT calculations (M06-2X-D/ def2-TZVP), we have studied the importance of p-hole interactions involving the carboxylate group (as p-hole) donor in compounds 2-4 and the inuence of the metal center (M ¼ Co, Ni, Zn) on the interaction strength. Moreover, we have also performed the non-covalent interaction (NCI) index analysis in complexes 2-4 to conrm the existence of p-hole O/CO interactions and their interplay with H-bonding interactions. The complexation reactions of these supramolecular systems in aqueous solution were investigated by potentiometric pH titration method (solution studies) in order to characterize the stoichiometry of new species. The solution behavior of the investigated species, providing additional evidence of the interactions between adduct and metal ions, supporting the results obtained from the solid-state studies.     Index ranges

X-ray structure determination and structure renement
Single-crystal X-ray diffraction measurements were performed on a STOE IPDS-2T diffractometer with graphite monochromated Mo-Ka radiation. Single crystals were chosen using a polarizing microscope and were mounted on glass bers for data collection. Cell constants and orientation matrices were obtained by least-squares renement against setting angles for all reections. Diffraction data were collected as a series of 1 u scans and integrated using the Stoe X-AREA soware package. 31 Lorentz and polarization corrections were applied to the data and a numerical absorption correction was applied using X-RED 32 and X-SHAPE. 33 The structures were solved by direct methods 34 and subsequent difference Fourier maps and then rened on F 2 by full-matrix least-squares with anisotropic displacement parameters for all non-H atoms. 35 The atomic scattering factors were taken from International Tables for X-ray Crystallography. 36 All renements were performed within the Stoe X-STEP32 structure evaluation package. 37 Synthesis of pyridine-N-oxide-2,5-dicarboxylic acid    A solution of H 2 pydco (0.092 g, 0.5 mmol), bipy (0.078 g, 0.5 mmol) and CuCl 2 $2H 2 O (0.085 g, 0.5 mmol) in ethanol-water (1 : 1; 25 mL) was prepared and stirred at room temperature. Aer 15 min a suspension formed and the mixture was stirred for 4 h at room temperature. Two differently-colored acicular crystals [green (7) and blue (8)] were obtained by slow

Synthesis of {[AgCu(H 2 O) 2 (phen)(pydco)]NO 3 } n (9)
H 2 pydco (0.092 g, 0.5 mmol) and NaOH (0.040 g, 1 mmol) were dissolved in deionized water (7.5 mL) and stirred for 30 min at room temperature. In a separate beaker CuCl 2 $2H 2 O (0.085 g, 0.5 mmol) was dissolved in deionized water (5 mL) and AgNO 3 powder (0.255 g, 1.5 mmol) was added. The AgCl precipitate which formed was ltered off and phen (0.099 g, 0.5 mmol) was added to the clear ltrate. This mixture was added into the solution of H 2 pydco and NaOH and stirred for 1 h at room temperature. The mixture was sealed in a 25 mL Teon-lined reactor. The reactor was heated at 130 C for 8 h and then cooled to room temperature at a rate of 10 C h À1 . Blue acicular crystals of 9 (m.p. 245 C) were obtained and the nal yield of 9 was 0.25 g (40% based on Cu). Anal. calcd for C 19

Theoretical methods
The calculations of the noncovalent interactions were computed using the Gaussian-09 program package. 42 We have used the M06-2X DFT functional in combination with Grimme's dispersion correction 43 because it is convenient for describing the weak noncovalent interactions properly. Moreover, it is  recommended for system with transition metals. 44 In order to describe correctly the p-hole interactions we have used the crystallographic coordinates and the def2-TZVP basis set for all atoms. This procedure and level of theory have been successfully used to evaluate similar interactions. 45 The interaction energies were computed by calculating the difference between the energies of isolated monomers and their assembly. The interaction energies were calculated with correction for the basis set superposition error (BSSE) by using the Boys-Bernardi counterpoise technique. 46 The NCI plot 47 iso-surfaces have been used to characterize noncovalent interactions. They correspond to both favorable and unfavorable interactions, as differentiated by the sign of the second density Hessian eigenvalue and dened by the isosurface color. The color scheme is a redyellow-green-blue scale with red for r cut + (repulsive) and blue for r cut À (attractive). 48 The Gaussian-09 M06-2X/def2TZVP level of theory wave function has been used to generate the NCI plot and the MEP surfaces.

Potentiometric pH titrations
Potentiometric measurements were performed for solutions in a 50 mL double-walled glass vessel using a Model 686 Metrohm Tiroprocessor equipped with a combined glass-calomel electrode. The temperature was xed at 25.0 AE 0.1 C. The ionic strength was adjusted to 0.1 M by use of KNO 3 . A CO 2 -free atmosphere for the base (carbonate-free 0.095 M sodium hydroxide) was ensured throughout. The concentrations of H 2 pydco (L) and phen (Q) were 3.0 Â 10 À3 M and potentiometric pH titrations carried out in the absence and presence of 1.5 Â 10 À3 M of the metal ions. Metal complexes protonation and stability constants and ligands protonation constants were calculated using the program BEST introduced by Martell

Infrared spectroscopy
The broad and strong bands at 3000-3500 cm À1 can be attributed to the stretching vibrations n(OH) of lattice and coordinated water molecules and n(]C-H) in the aromatic rings. They are also indicative of the presence of hydrogen bonding. 13,55 The strong n as (COO À ) and the n s (COO À ) bands in free H 2 pydco 1726 and 1419 cm À1 are shied in the complexes to the lower frequencies in the range 1674-1628 cm À1 and 1398-1372 cm À1 , respectively. These results indicate that in these complexes the carboxylate group of pydco coordinates to the transition metal ions through deprotonation. 56 Furthermore, the strong absorption bands in the range of 1550-1600 cm À1 can be attributed to the n(C]C) and n(C]N) vibration of aromatic pyridyl ring for all these complexes. The IR spectrum of H 2 pydco (see Fig. S4 †) shows a strong band at 1230 cm À1 due to the presence of N-O group conrming the synthesis of the ligand. 27 Bands in the 1228-1205 cm À1 region for the complexes (see

CSD search
A search of the Cambridge Structural Database (CSD) for complexes of pyridine-2,5-dicarboxylic acid ligands ( Fig. 1) returned 419 entries of complexes with 2,5-pydc, but only four complexes containing 2,5-pydco, the latter being lanthanoid complexes. 27 This dearth of structurally-characterized complexes of all isomers of pydco spurred our efforts to prepare and characterize new supramolecular coordination complexes of 2,5-pydco with various transition metals.

Description of the crystal structures
The crystallographic data for compounds 1-9 is shown in Table  1. In addition, selected bond lengths, valence angles and hydrogen bond geometries are given in Tables S1 and S2 in the ESI. †

Crystal structure of 1
Single crystal X-ray diffraction analysis reveals that 1 crystallizes in the monoclinic space group C2/c. The asymmetric unit of 1 contains two Co(II) ion, one pydco 2À anion, four coordinated water molecules (one and three for Co1 and Co2, respectively) and one uncoordinated water molecule (see Fig. 2). Co1 is coordinated by two mutually trans carboxylate O donors and two mutually trans Noxide O donors from two pydco 2À ligands to form the equatorial  plane, and by two apical oxygen atoms from two water molecules to complete the distorted octahedral geometry. Co2 is coordinated by six water molecules in a regular octahedral geometry. The O-H/O hydrogen bonds create motifs of graph-set notation R 2 1 (4), R 2 2 (11), R 2 2 (13) and C 2 2 (8) which extend complex 1 along the crystallographic b direction. Interestingly, the structure exhibits bifurcated O-H/O interactions with the oxygen atom of the 5carboxylate group acting as a hydrogen bond acceptor from two different O-H donors, while two of the water molecules coordinated to Co2 act as bifurcated donors (see Fig. 3). Each unit comprising one cationic and one anionic complex is linked together through O-H/O hydrogen bonds between uncoordinated water molecules and 2-carboxylate oxygen atoms or water molecules coordinated to Co2, resulting in a zigzag chain structure along the c direction, as shown in Fig  Crystal structure of 5 Complex 5 crystallizes in the triclinic space group P 1. The Co(II) ion (see Fig. 2) occupies a general position and exhibits distorted octahedral coordination from one bidentate pydco 2À ligand, one bipy ligand and two water molecules. The equatorial plane is occupied by one water molecule, two bipy nitrogen atoms and O1 from an N-oxide, while axial positions are Fig. 13 Hpydco À and pydco 2À coordination modes observed in 1-9.  (Fig. 6).
In addition, hydrogen bonding (C11H11/O4; 2.440Å) and two types of p/p stacking between pyridine rings (with two different types of centroid/centroid separations of 3.585, and 3.606Å) leads to a third chain along the a axis (see Fig. S10 †). 59 In general, the chains are packed via O-H/O, C-O/p, p/p stacking interactions in three directions to cooperate threedimensional supramolecular structure.

Crystal structure of 6
A view of complex 6 with labeling of selected atoms is shown in Fig. 2. The asymmetric unit contains a Co(II) ion occupying a general position and exhibiting distorted octahedral coordination geometry, in which the apical positions are occupied by O6 from a water molecule and N2 from a phen ligand; the equatorial plane is dened by O7 from water molecule, N3 from a phen ligand, O1 from an N-oxide group and O4 from the 2carboxylate group of a pydco 2À ligand.
Due to the presence of disorder, it was a challenge to identify the solvent molecules, but we have been able to conclude that the asymmetric unit contains ve molecules of water and 0.2 molecules of ethanol. Molecules of 6 are linked into dimers by O-H/O hydrogen bonds between two water molecules from one complex and two oxygen atoms of the 5-carboxylate group from an adjacent complex, resulting in homosynthons with graph-set notation R 2 2 (8). Dimers are linked into chains along the b axis by two lattice water molecules which form O-H/O hydrogen bonds between oxygen atoms from an N-oxide group and from an O atom from the 2-carboxylate group (see Fig. 7).
The dimers repeat along a axis via O-H/O hydrogen bonds between a coordinated water molecule and oxygen atoms of 2carboxylate group from pydco 2À ligand forming onedimensional ladders (see Fig. 8). These ladders are attached to each other via hydrogen bonding (C17H17/O4; 2.510Å), p/ p (with two different types of centroid/centroid separations of 3.627 and 3.671Å), CH/p (with a centroid/H18C18 separation of 3.5325Å), and C-O/p (with a centroid/O5C7 separation of 3.317Å) interactions between pyridine and phenyl rings of phen and thereby generate two-dimensional sheets ( Fig. S11 and S12 †).

Crystal structure of 7
Complex 7 crystallizes in the triclinic space group P 1. The asymmetric unit contains a ve-coordinated complex of Cu(II) which adopts a distorted square pyramidal coordination geometry ‡ in which the basal plane is occupied by two bipy nitrogen atoms and two oxygen atoms from one Hpydco À ligand, while the axial position is occupied by a chloride ion. The asymmetric unit is completed by two uncoordinated water molecules. A displacement ellipsoid plot of complex 7 with selected atoms labeling scheme is shown in Fig. 2. The chloride anions and uncoordinated water molecules participate in O-H/O and O-H/Cl hydrogen bonding and thereby form chains along the b axis with motifs described by the graph-set notation C 2 2 (8) (see Fig. 9). Another chain is generated through weak C-H/O, C-H/Cl, C-H/p (with a centroid/H5C5 separation of 3.535Å), and p/ p (with a centroid/centroid separations of 3.631Å) interactions (see Fig. S13 in the ESI †). Moreover, the third dimension is generated via strong O-H/O hydrogen bonds between water molecules and 5-carboxylate groups of pydco 2À which lead to the formation of a motif described by the graph-set notation R 4 4 (12) (see Fig. S14 in the ESI †). According to Fig. S15 in the ESI, † the three-dimensional structure of compound 7 has  (1) 69 ‡ The distorted square pyramidal coordination geometry is indicated by the value of s 5 (0.230). The parameter s 5 is dened as (b À a)/60, where b and a are the largest angles subtended at the metal center). For ideal square pyramidal s 5 is 0 and for ideal trigonal bipyramidal geometry is 1. 60 expanded via different interactions by connection of all above chains.

Crystal structure of 8
Single crystal X-ray diffraction analysis shows that compound 8 crystallizes in the triclinic space group P 1. A displacement ellipsoid drawing of 8 with selected atoms labelled is shown in Fig. 2. The asymmetric unit of 8 contains one Cu(II) ion, one bipy ligand, one Hpydco À anion, one chloride anion and one uncoordinated water molecule wherein hydroxyl proton H2 is shared between two O2 atoms with 50% occupancy. 61 A distorted square pyramidal geometry, is dened by two nitrogen atoms from a bipy ligand and two oxygen atoms from a Hpydco À ligand forming the equatorial plane with a chloride anion in the axial position.
(H6B/O6; 2.59(3)Å) and (C1H1/O6; 2.554Å) hydrogen bonds between water molecules and C1 of pyridine ring from pydco 2À ligand cross-ink the zigzag chains into twodimensional sheets. These O-H/O hydrogen bonds lead to the formation of a motif with the graph-set notation R 2 2 (4). Weak p/p interactions with a centroid/centroid separations of 3.775Å occur between the pyridine rings of two bipy ligands in adjacent chains (see Fig. S16 †). The structure is extended parallel to a axis via weak C-H/O interactions between uncoordinated water molecules and the pyridine rings of pydco 2À and bipy ligands (see Fig. S17 †). Finally, covalent bonds and non-covalent interactions such as hydrogen bonds and p/p stacking in these three chains expand the structure to give a three-dimensional network (Fig. S18 †).
Thus, a one-dimensional coordination polymer is linked by further interactions into a three-dimensional network (see Fig. S20 †). The aim was to explore how altering the coordination mode of the ligand changes the molecular structures, non-covalent interactions and therefore the supramolecular structures of these complexes. The rst-row transition metals are borderline acids and they have a greater affinity for nitrogen than for oxygen atoms. However, it could be anticipated that the presence of two donor oxygen atoms in the H 2 pydco ligand would lead to their chelating the metal ion and the formation of a six-membered ring. Indeed, if the ligand can assume a suitable conformation, the formation of sixmembered ring causes little or no strain on the bond angles of the ligand or at the metal. 66 We observed two coordination modes for the pydco ligand in our complexes (see Fig. 13).
Based on the CSD search, there are three complexes with the same structures as complexes 1, 3, and 6 in this work. In Table  2 Table 3 indicate that the six-membered chelating rings created by coordination of ligand to the metal ions adopt the following conformations: (i) 1 has two rings with half-chair and skew-boat conformations; (ii) Each of 2, 3 and 4 complexes have two rings in which they have a skew-boat conformation in one ring. In another ring, they have state between skew-boat and boat conformations.
(iii) 5 and 6 each have one ring with a skew-boat conformation; (iv) 7, 8, and 9 each have one ring with a half-chair conformation; In Fig. 14 we compare the structures of compounds 1, 3, and 6 with previously synthesized complexes of H 2 pydc. [67][68][69] The formation of 5-membered chelate rings by the H 2 pydc ligand frequently resulted in complexes with planar conformations, while the H 2 pydco ligand formed complexes with 6-membered chelate rings displaying twisted conformations. N-oxide functionalization of the pyridine ring of the ligand led to the formation of complex and interesting supramolecular frameworks.

Theoretical study
The theoretical study is devoted to analyze some unconventional non-covalent interactions observed in the solid-state In particular, we have estimated the energy associated to p-hole interactions that are established between the coordinated carboxylate group and the uncoordinated one, which is the p-hole donor. It should be mentioned that p-hole interactions in X-ray structures have been studied by Bürgi and Dunitz in 1975, 72 thus revealing the trajectory along which a nucleophile attacks the p-hole of carbonyl group. More recently, the importance of n / p* interactions in proteins from a lone pair of electrons (n) to the antibonding orbital (p*) of carbonyl group has been demonstrated. 73 Moreover, operative p-holes have been described nitroderivatives, 74 group 13 molecules 75 and acyl carbon containing molecules. 76 First of all, we have computed the molecular electrostatic potential (MEP) plotted onto the approximate van der Waals surface (isosurface 0.001 au) in order to investigate the electron rich and electron poor region of the complex. Since compounds 2-4 are isostructural, we have used compound 4 as a model because the closed-shell electronic conguration of this complex (d 10 metal center) facilitates the computational Table 4 Overall stability and stepwise protonation constants of 2,5-pydco, 1,10-phenanthroline and 2,2 0 -bipyridine and recognition constants for their interaction in aqueous solution at 25 C m ¼ 0.  analysis. The MEP surface of compound 4 is given in Fig. 14a and it can be observed the most positive region is located at the H-atoms of the coordinated water molecules (+99 kcal mol À1 ).
This is due to the coordination of the water molecules to the metal center that increases the acidity of the H atoms. The most negative region is located at the O-atoms of the uncoordinated 2] (f), 2,5-pydco/Co 2+ [1 : 1] (g), 2,5-pydco/Ni 2+ (h), 2,5-pydco/Zn 2+ (i), 2,2 0bipyridine/Co 2+ (j), 2,5-pydco/2,2 0 -bipyridine/Co 2+ (k), 1,10-phenantroline/Co 2+ (l), 2,5-pydco/1,10-phenantroline/Co 2+ (m), 2,5-pydco/Cu 2+ (n), 2,2 0 -bipyridine/Cu 2+ (o), 2,5-pydco/2,2 0 -bipyridine/Cu 2+ (p), 1,10-phenantroline/Cu 2+ (q), 2,5-pydco/Ag + (r), 1,10-phenantroline/Ag + (s), 2,5pydco/1,10-phenantroline/Ag + /Cu 2+ (t). Table 5 Overall stability constants of 2,5-pydco/2,2-bipyridine or 1,10-phenanthroline/M n+ /N + (l/q/m/n) binary, ternary and quaternary systems in aqueous solution at 25 C m ¼ 0. This journal is © The Royal Society of Chemistry 2019 carboxylate group (À95 kcal mol À1 ), as expected. Therefore, the most favored interaction from an electrostatic point of view is an H-bond between the M À OH 2 and the carboxylate group. As a matter of fact, this molecule forms self-assembled dimer in the solid state due to the formation of four strong H-bonds, as shown in Fig. 15b. The formation energy of this dimer is very large DE 1 ¼ À112.8 kcal mol À1 , in good agreement with the MEP analysis and solid-state structure. In order to further characterize this assembly, we have used the NCI plot index computational tool. Non-covalent interactions are efficiently visualized and identied by using the NCI plot tool. It allows an easy assessment of host-guest complementarity and the extent to which weak interactions stabilize a complex. For the theoretical model of the assembly used in Fig. 15b we have computed the NCI plot that is represented in Fig. 15c. It can be observed several small and dark blue isosurfaces that characterize the intramolecular H-bonds that conrm the strong nature of these bonds. It is important to highlight that the MEP surface of the monomer of compound 4 does not exhibits a positive p-hole over the carboxylate group, which is expected taking into consideration that the negative charge is located in this group. However, if the MEP surface is computed for the H-bonded dimer shown Fig. 15b, a small p-hole appears over the C atoms (see Fig. 16). The MEP surface shown in Fig. 16a also reveals a negative MEP at the O-atom of the coordinated carboxylate group (À55 kcal mol À1 ). The MEP over the C-atom of the carboxylate that establishes the double H-bond with the coordinated water molecules is very small +2.5 kcal mol À1 (see Fig. 16b). Nevertheless, it explains the formation of the innite supramolecular chain in the solid state (see Fig. 16b) where the H-bonded self-assembled dimer interacts with the neighbor molecules in the X-ray structure by means of double p-hole interactions. Both O/C distances are shorter than the sum of their van der Waals radii (3.22Å) and quite directional.
We have evaluated energetically the p-hole complexes in complexes 2-4, see Fig. 17. It can be observed that the geometric features of the three complexes are almost identical and also the interaction energies, thus suggesting that the type of metal center has a little inuence on the interaction energy. The interaction energies are weak (around 2.5 kcal mol À1 each O/ p-hole) in good agreement with the small MEP value at the phole.
We have also computed the NCI plots of the p-hole dimers represented in Fig. 18. The existence and weak attractive nature of the O/p-hole interactions is conrmed by the presence of green isosurfaces located between the O and C-atoms of carboxylate. Moreover, the NCI plot also reveals the existence of p-p stacking interactions since a more extended isosurface located between the aromatic ligands also appears upon complexation.

Solution studies
In these experiments, the completely protonated forms of 2,5pydco (L), 2,2 0 -bipyridine (Q) and 1,10-phenanthroline (Q 0 ) were titrated with a standard NaOH solution in order to investigate their stoichiometry and protonation constants. The resulting values for the overall stability and stepwise protonation constants of L, Q and Q 0 as well as the recognition constants for the L-Q and L-Q 0 proton transfer systems are listed in Table 4. The resulting protonation constants are in satisfactory agreement with those reported in the literature; the observed difference is due to different conditions. The corresponding experimental pH titration proles are shown in Fig. S21(a-c). † As can be seen, in all cases the potentiometric titration curves are depressed in the presence of the metal ions, indicating their strong interactions with metal ions. As it is obvious from Table  4, the most abundant proton transfer species for 2,5-pydco/2,2 0bipy system present at pH 2.3-2.6 (46.2%), and 3.7 (33.2%) are 2,5-pydcoH3-2,2 0 -bipy (log K ¼ 7.72) and 2,5-pydcoH2-2,2 0bipy (log K ¼ 3.93). In the 2,5-pydco/1,10-phen system the most abundant proton transfer species are 2,5-pydcoH2-1,10-phen (52.3%, log K ¼ 10.47) and 2,5-pydcoH3-1,10-phen (34.0%, log K ¼ 13.23).

Conclusions
In summary, we have successfully synthesized and modied the H 2 pydco ligand from the N/O-donor ligand H 2 pydc in high yield by a new procedure to create a new O-only donor set than subsequent synthesis of the chlorinated derivative of this ligand. The results indicate that altering the donor set of the ligand profoundly affects the molecular and supramolecular structures of the corresponding transition metal complexes. The H 2 pydco ligand exhibits exceptional abilities to link metallic centers via its versatile chelating and bridging coordination modes. In this regard, we synthesized nine complexes with different architectures. In these complexes, H 2 pydco acts as a bidentate ligand through one oxygen atom of the carboxylate group and one oxygen atom of the N-oxide group. The formation of 5-membered chelate rings by the H 2 pydc ligand frequently resulted in complexes with planar conformations, while the H 2 pydco ligand formed complexes with 6-membered chelate rings displaying twisted conformations. N-oxide functionalization of the pyridine ring of the ligand led to the formation of complex and interesting supramolecular frameworks. The present study demonstrates that in these complexes cooperativity between various strong hydrogen bonds and a range of relatively weak p/p, C-H/p and C-O/p interactions is necessary for the construction of the supramolecular frameworks. DFT studies combined with MEP and NCI plot computational tools have been used to characterize and rationalize the O/p-hole interaction that is energetically very weak due to the small p-hole over the carboxylate group. Such p-hole appears due to the participation of the COO À group in strong Hbonding interactions. The formation of binary, ternary and quaternary complexes in solution with stoichiometries very close to those of the solid state is strongly supported by the results of the potentiometric pH titration studies in aqueous solutions and the stoichiometry of the most abundant species in the solution was very similar to the corresponding crystalline complexes.

Conflicts of interest
There are no conicts to declare.