New polynuclear compounds based on N-benzyliminodipropionic acid: solution studies, synthesis, and X-ray crystal structures

Abstract Three new polynuclear compounds based on a dicarboxylic acid ligand are reported. In particular, two Cu(II) coordination compounds, [Cu2(H2O)6(Hbzlidp)2](CF3SO3)2·2H2O (1) and [Cu(NO3)(Hbzlidp)]∞ (2) (bzlidp2− = N-benzyliminodipropionate anion), and a Ni(II) dinuclear compound, [Ni2(H2O)4(bzlidp)2] (3), were synthesized and characterized by IR spectroscopy, elemental analysis and single crystal X-ray diffraction. Different structures were obtained depending on the reaction conditions. The structural analyses reveal that 1 was formed by dinuclear [Cu2(H2O)6(Hbzlidp)2]2+ units built by two copper(II) ions joined through two Hbzlidp− ligands, while 2 was formed by pairs of Cu(II) centers bridged by four syn,syn carboxylate groups to generate “paddle wheel” units. The dinuclear copper units are arranged in a rhombus type grid, in a 2-D layer structure. In both cases, the N was protonated and not coordinated to the metal center. Compound 3 was formed by [Ni2(H2O)4(bzlidp)2] neutral dinuclear units, with octahedral Ni(II) centers. Solution studies of the ligand–M(II) systems (M(II) = Mn, Co, Ni, Cu, Zn, Cd, and Pb) were also carried out.


Introduction
Coordination polymers are infinite crystalline lattices composed of inorganic nodes (metal ions or clusters) connected by organic polytopic ligands that act as linkers [1]. Solid-state materials with different ligand. H 2 bzlidp was prepared based on a slight modification of a previously reported protocol for the synthesis of the N-methyliminodipropionic acid (H 2 midp) [31]. The authors reported the preparation of a Cu(II) compound ([Cu(H 2 O)midp]·H 2 O) where the coordination geometry of each Cu ion can be described as a slightly distorted square pyramid and the midp 2− is a tetradentate bridging ligand resulting in infinite chains, which extend along [010]. Other related iminodipropionic ligands have been employed in the preparation of Cu(II) mononuclear coordination compounds. For instance, with the o-anisidine-N,Ndi-3-propionate anion (o-andp 2− ) a mononuclear compound, [Cu(o-andp)(H 2 O) 2 ]·2H 2 O, was obtained where the Cu coordination polyhedron is a distorted tetragonal bipyramid (4 + 2) and the ligand is a tetradentate chelator, forming three chelate rings [32]. With p-toluidine-N,N-di-3-propionate (p-Tdp 2− ) and aniline-N,N-di-3-propionate (adp 2− ), mononuclear complexes [Cul(H 2 O) 2 ] (l = p-Tdp 2− or adp 2− ) were prepared and show a square pyramid Cu coordination polyhedron with the ligand as tridentate chelator. However, no polynuclear species were reported with these ligands [33,34].

General information
all common laboratory chemicals were reagent grade, purchased from commercial sources and used without purification. Infrared spectra, as KBr pellets, were obtained from a FTIR Shimadzu IR-Prestige-21 spectrophotometer from 4000 to 400 cm −1 . Elemental analyses (C, H, N, and S) were performed on a Flash 2000 instrument. NMR spectra were obtained on a Bruker avance DPX-400 instrument. Proton chemical shifts (δ) are reported in ppm downfield from TMS as an internal reference. Thermogravimetric analyses (TGa) were performed in a Shimadzu TGa-50 instrument with a Ta 50 I interface using platinum cells. TGa results were recorded under a nitrogen stream (50 ml min −1 ) and with a heating rate of 1.0 °C min −

Equilibrium studies
The protonation constants of bzlidp 2− were determined through four potentiometric titrations (ca. 100-150 experimental points each) in the concentration range 1-4 mmol l −1 . Then, the behavior of the ligand in the presence of metal cations M(II) (M = Mn, Co, Ni, Cu, Zn, Cd, and Pb) was analyzed through at least six potentiometric titrations (ca. 100-150 experimental points each) for each system, at metal cation concentrations ranging from 1 to 3 mmol l −1 , and ligand to metal cation total molar ratios varying from 5 : 1 to 1 : 2. The pH interval from 2 to the pH where solid formation was evidenced was covered (pH value for the solid formation was always higher than six). In those cases, the formation of solid M(II) hydroxide was consistent with previous reports on metal hydroxide solubilities [35]. The formation constants of soluble hydroxo species of M(II) were also determined analogously under the same experimental conditions. Four titrations of M(II) solutions (ca. 80 experimental points each) were carried out in the concentration interval 1-3 mmol l −1 . The obtained values for these hydrolysis constants were taken into account in the Hyperquad input for the subsequent formation constants calculation.
In each potentiometric experiment, the solutions were poured into a 50 ml titration cell. after thermal equilibrium was reached, hydrogen ion concentrations were determined by successive readings, each performed after a small incremental addition of standard 0.1 mol l −1 NaOH solution. The titrant addition and e.m.f. measurements were carried out using an automatic titrator, Mettler-Toledo Dl50-Graphix. The ionic strength was kept almost constant throughout the titrations by using solutions containing 0.50 mol l −1 Me 4 NCl and relatively low initial concentrations of metal ions and bzlidp (the sum of these reactants initial concentrations did not contribute more than 2% to the total ionic strength). Pre-saturated argon (free of CO 2 ) was bubbled through the solutions during titrations to eliminate the effect of atmospheric carbon dioxide, and the temperature was kept at 20.0 (±0.1) °C. Equilibrium attainment after each titrant addition was verified by controlling the deviation of successive e.m.f. readings. Independent stock solutions were used in some titrations to check reproducibility. The cell electrode potential E° and the acidic junction potential were determined according to reported procedure [36] from independent titrations of the strong acid with the titrant solution. In this way, the pH scale was the free hydrogen concentration scale. The calibration in the alkaline range was checked by recalculating K w values for each system. The obtained values (average log 10 K w = −13.90) were always checked to assure they were in line with previously reported data under similar experimental conditions [35].
Data were analyzed using the HyPERQuaD program [37], and species distribution diagrams were produced using the HySS program [38]. The fit of the values predicted by the model to the experimental data was estimated on the basis of the σ parameter, corresponding to the scaled sum of squared differences between predicted and experimental values. Many other possible stoichiometries were tried for each system, and final models were selected on the basis of the σ parameter, the model confidence level estimator, Chi squared, and the internal consistency of data reflected in standard deviations of the formation constants [37].

Synthesis of N-benzyliminodipropionic acid (H 2 bzlidp)
N-benzyliminodipropionic acid (H 2 bzlidp) was prepared based on a slight modification of a previously reported protocol [31]. acrylic acid (31.7 g, 0.44 mol) was added to 20 ml of an 8 mol l −1 solution of benzylamine in dry ethanol (17.1 g, 0.16 mol). The mixture was heated overnight at boiling. The product precipitated as a white solid which was filtered and washed with cold ethanol (32.97 g, 0.13 mol, 82%. yield). 1
Green-blue crystals adequate for X-ray analysis were obtained from a water solution of the product by slow diffusion of acetonitrile. anal. Calcd for C 13

X-ray crystallography
The X-ray diffraction data for all compounds were collected with an agilent SuperNOVa diffractometer with microfocus X-ray using Cu Kα radiation (λ = 1.54184 Å). CrysalisPro [39] software was used to collect, index, scale, and apply analytical absorption correction based on the faces of the crystal. The structure was solved by a dual-space algorithm using the SHElXT program [40]. Fourier recycling and leastsquares refinement were used for the model completion with SHElXl-2014 [41][42][43]. all non-hydrogen atoms have been refined anisotropically and all hydrogens have been placed in geometrically suitable positions and refined as riding with isotropic thermal parameters related to the equivalent isotropic thermal parameter of the parent. Hydrogens were geometrically positioned with C-H = 0.93 Å and uiso(H) = 1.2 ueq(C), except for hydrogens on N in 1 and 2 that were determined by Fourier differences Table 1. Crystallographic data for 1-3. and refined freely. also hydrogens from water molecules of 3 and one water molecule of compound 1 could be localized in this form and refined. Crystal data, collection procedures, and refinement results are summarized in table 1.

Solution studies
Values for the protonation equilibrium constants of bzlidp 2− were required to study the metal complexation reactions. The obtained results at 20.0 °C in 0.50 mol l −1 Me 4 NCl are: log K H 1 = 8.856 (3); log H 2 = 12.789(5); log H 3 = 15.624 (7); σ = 0.2. The titration curves of H 2 bzlidp with base have three equivalent points, the third one corresponding to the presence of the N-connecting atom. No previous reports are available for comparison [35]. The first protonation constant shows lower basic character than that reported for iminodipropionato ligand, due to the presence of the benzyl substituent on nitrogen, an electron-withdrawing group [44]. N-benzyliminodiacetate also shows the same behavior. In fact, its first protonation constant has a very similar value to that of bzlidp: log K H 1 = 8.87 at 25 °C and 0.1 M ionic strength [35]. also in line with previous reports, the protonation constants of the propionic carboxylate groups in bzlidp are higher than those of the iminodiacetato ligand or N-benzyliminodiacetato, due to the presence of longer chains. [45] These facts mean that, both for bzlidp 2− and benzyliminodiacetate, the amino group is expected to be predominantly protonated in aqueous solution below pH 8.8.
The following step was the study of the M(II)-bzlidp interaction. The divalent cations used for this study were Mn, Co, Ni, Cu, Zn, Cd, and Pb, and the results are shown in table 2. Stability of complexes containing 3d metal ions follows the Irving-Williams trend. The species detected were in all cases [Ml] and [M(Hl)] + (l = bzlidp 2− ). Complex species [Ml] (Co, Ni, Cu) had been reported for various dipropionic acid ligands [31,44,46]. In this work, we have additionally detected the protonated species, with a stability constant value that is fairly independent of the identity of the metal ion. Figure 2 shows the speciation diagrams for the copper and nickel containing systems. It is important to note that protonated species are not predominant, but they are formed for pH values around 4. The speciation diagrams for the rest of the metal ions that were analyzed can be found in the Supporting Information, figures S1-S5.

Synthesis of the complexes
The copper complexes were prepared by direct reaction of copper salts with H 2 bzlidp in aqueous solution at pH 3.0-4.0 and in different ligand to metal molar ratios. according to the solution studies, this pH, where the predominant form of the ligand is Hl − , should be adequate to favor formation of complexes bearing the monoprotonated form of the ligand. [Cu(Hbzlidp)] + was detected in this pH interval. The nickel complex was prepared by reaction of nickel acetate with H 2 bzlidp in a 1 : 1 metal to ligand molar ratio, in aqueous solution at pH 4.2, followed by diffusion of acetone. Even though at this acidic pH the species [Ni(Hl)] + is expected to be present, 3 was the only product obtained by this synthetic procedure. The crystalline solids were characterized by elemental analysis and infrared (IR) spectroscopy. IR spectra of 1 and 3 show broad bands at 2800-3600 cm −1 , which suggests the presence of water in the structures. This signal is absent in 2. The three complexes exhibit bands at 2800-2970 cm −1 that correspond to the v C-H vibrations of the -CH 2 -groups in the carbon chains on bzlidp ligands. The absence of the characteristic absorption of v as (COOH) near 1700 cm −1 indicates that carboxylate groups of the ligand are not fully protonated in the complexes. The v as (COO − ) vibrations occurred at ca. 1600 cm −1 , while those of v s (COO − ) appeared at ca. 1420 cm −1 . The large difference in the v as -v s values (around 180 cm −1 ) excludes the presence of chelating bidentate coordination modes of the ligand [47]. The bands at 1261, 1031, and 638 cm −1 account for the presence of the CF 3 SO − 3 in 1 [48]. The coordinated NO − 3 in 2 has bands at 1416 and 1321 cm −1 which correspond to the asymmetric and symmetric stretching vibrations in a monodentate coordination mode [49]. The TGa curves for 1 and 2 show a decomposition process that begins about 200 °C. For 1, a dehydration process (with a weight loss of 10.9%) is shown below 100 °C.

Crystal structures
It was possible to obtain single crystals for 1 by slow evaporation of the solvent at room temperature, while crystals of 2 and 3 were obtained by slow diffusion of acetonitrile or acetone, respectively.

Crystal structure of [Ni 2 (H 2 O) 4 (bzlidp) 2 ] (3)
Selected bond lengths and angles for 3 are presented in table 3. The structure is formed by [Ni 2 (H 2 O) 4 (bzlidp) 2 ] neutral dinuclear units, built by two nickel ions joined through two bzlidp 2− ligands as shown in figure 8. The bzlidp 2− are tridentate ligands, coordinating through two oxygens of the carboxylate groups, one of which is bridging and coordinated to the other Ni(II) of the molecule. Each Ni(II) is octahedral, and the coordination sphere is completed by nitrogen of bzlidp 2− and two water molecules (average Ni-OW distance 2.0719(12) Å) in a cis position. The intramolecular Ni-Ni average distance is 3.1938(5) Å. The unit cell of 3 is shown in figure 9. The units present intramolecular hydrogen bonds, between the free carboxylate oxygen O2 of one half of the molecule, and one of the water molecules in the other half, and between carboxylic O3 and the O1W of a coordinated water (figure 10 and table S3, Supplementary Material).

Conclusion
The dicarboxylic acid compound H 2 bzlidp showed interesting versatility as a ligand. It can assemble metal ions into polynuclear systems with different patterns of connection. In this way, it was possible to synthesize and determine the structure of three different polynuclear complexes, in which the ligand was a bridge, although with different coordination modes. Control of the coordination abilities of bzlidp was achieved by appropriate selection of the reaction conditions. In particular, acidic pH values led this tridentate ligand to be ditopic due to the N protonation, and a dinuclear, cyclic structure was obtained. When an excess of copper was employed, a more extended system was obtained, with each carboxylate group connecting two copper centers. Finally, in the case of nickel, which favors octahedral geometries, a dinuclear, more compact structure was formed, with the deprotonated N also coordinated to the metal center.

Supplementary material
CCDC 1454769-1454771 contains the supplementary crystallographic data for 1-3. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/cif.