H-Bonds, π-Stacking and (Water)O-H/π Interactions in (µ4-EDTA)Bis(Imidazole) Dicopper(II) Dihydrate

crystals Article H-Bonds, π-Stacking and (Water)O-H/πInteractions in (µ4-EDTA)Bis(Imidazole) Dicopper(II) Dihydrate Jeannette Carolina Belmont-Sánchez 1, María Eugenia García-Rubiño 2, Antonio Frontera 3, Josefa María González-Pérez 1, Alfonso Castiñeiras 4and Juan Niclós-Gutiérrez 1,*   Citation: Belmont-Sánchez, J.C.; García-Rubiño, M.E.; Frontera, A.; González-Pérez, J.M.; Castiñeiras, A.; Niclós-Gutiérrez, J. H-Bonds, π-Stacking and (Water)O-H/π Interactions in (µ4-EDTA)Bis (Imidazole) Dicopper(II) Dihydrate. Crystals 2021,11, 48. https:// doi.org/10.3390/cryst11010048 Received: 17 December 2020 Accepted: 6 January 2021 Published: 8 January 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Department of Inorganic Chemistry, Faculty of Pharmacy, University of Granada, 18071 Granada, Spain; [email protected] (J.C.B.-S.); [email protected] (J.M.G.-P.) 2Departamento Fisicoquímica, Facultad de Farmacia, Universidad de Granada, 18071 Granada, Spain; [email protected] 3Departament de Química, Universitat de les Illes Balears, Crta. de Valldemossa km 7.5, 07122 Palma de Mallorca (Baleares), Spain; toni.fr[email protected] 4Department of Inorganic Chemistry, Faculty of Pharmacy, University of Santiago de Compostela, 15782 Santiago de Compostela, Spain; [email protected] *Correspondence: [email protected] Abstract: We synthesized and studied the polymeric compound {[Cu 2 ( µ4 -EDTA)(Him) 2 ] 2H 2 O} n ( 1 ). The single-crystal structure is reported along with an in depth characterization of its thermal stability (TGA), spectral properties (FT-IR, Vis-UV and RSE), and magnetic behavior. The crystal consists of infinite 2D-networks built by centrosymmetric dinuclear motifs, constructed by means of a bridging anti,syn-carboxylate group from each asymmetric unit. Each layer guides Him ligands toward their external faces. They are connected by intermolecular (Him)N-H ··· O(carboxylate) bonds and antiparallel π – π stacking between symmetry related pairs of Him ligands, and then pillared in a 3D-network with parallel channels, where disordered water molecules are guested. About half of the labile water is lost from these channels over a wide temperature range (r.t. to 210 ◦ C) before the other one, most strongly retained by the cooperating action of (water)O1-H(1A) ··· O(carboxylate) and (water) O1-H(1B) ···π (Him) interactions. The latter is lost when organic ligands start to burn. ESR spectra and magnetic measurements indicated that symmetry related Cu(II) centers connected by the bridging carboxylate groups behave magnetically not equivalently, enabling an exchange interaction larger than their individual Zeeman energies. Keywords: crystal structure; copper(II); EDTA; imidazole; polymer; H-bonding; π – π stacking; water O-H/πinteraction 1. Introduction Ethylenediaminetetraacetic acid (H 4 EDTA) and its different anionic forms generate an enormous diversity of metal chelate complexes. The ability of EDTA to form up to five stable five-membered chelate rings around a metal center is only one among all these possibilities, frequently enriched in structurally well documented compounds where µ - EDTA bridging forms increases its denticity. Now we are interested in using the tetravalent EDTA anion as a bridging µ -chelator for two metallic centers (M), thus generating M 2 ( µ - EDTA) motifs which can form dinuclear complex molecules, as well as polymeric networks of different dimensionalities. In these compounds, at least a tridentate role for each half- EDTA should be expected, where each M center should be chelated by a N,N-methylene- aminodiacetate(2-) group, -CH 2 -N(CH 2 CO 2- ) 2 (here after a mida group). A search in the Cambridge Structural Database (CSD, version 5.41, update August 2020) for “any metal” (M) in M 2 ( µ -EDTA) motifs affords 58 different structures (number of examples for each M indicated in parenthesis): Mo (36), W (6), Cu(5), U (3), Tc (2), Sn (2), Re (1), Zr (1), Al (1), or Ba (1). Since some literature is certainly old and/or is not in English, a reference code in Crystals 2021,11, 48. https://doi.org/10.3390/cryst11010048 https://www.mdpi.com/journal/crystals Crystals 2021,11, 48 2 of 15 CSD database is provided for such compounds in the text. The highest µn -EDTA denticity in these kind of compounds has been reported for the Ba(II) derivative, [Ba 2 ( µ12 -EDTA)] (TAHFUF in CSD) [1]. As far as concerns M 2 ( µ -EDTA) compounds (M being now a first-row transition metal ion) the crystal structures of salts [Fe II (H 2 O) 6 ][Mo 2 ( µ -EDTA)( µ -O) 2 ] · 5H 2 O (UCETAX) [ 2 ] and [Ni(H 2 O) 6 ][Mo 2 ( µ -EDTA)( µ -S) 2 ] · 2H 2 O (LOVKUE) [ 3 ] have been reported along with those of five Cu(II) compounds with “Cu( µ -EDTA)Cu” motifs [ 4 – 7 ]. These copper(II) compounds comprise, among others (see below) two 2D-polymers also having Na(I) ions, Kegging poly-oxo-wolframate anions, as well as bounded to Na(I)-aqua ligands and crystallization water molecules [ 4 ]. The other three are {[Cu 2 ( µ4 -EDTA)(H 2 O) 2 ] · 2H 2 O} n (CUEDTA01, here after the so-called the binary dinuclear chelate) [ 5 ], [Cu 2 ( µ4 -EDTA)(py) 2 (H 2 O) 2 ] · 2H 2 O (py = pyridine, NAMJOB) [ 6 ], and {[Cu 2 ( µ4 -EDTA)(3hpy) 2 ]} n (3hpy = 3-hydroxypyridine, PEZRES ) [7]. See additional comments on the three compounds in Sections 3.1 and 3.2 As a part of a program on ternary copper(II) complexes with amino-polycarboxylate chelators and five (imidazole) or six-membered (pyrimidine’s) N-heterocyclic ligands related to structural formulas of purine nucleic basis, here we report the synthesis, crystal structure, (thermal, spectral and magnetic) physical properties, and density functional theory (DFT) calculations of {[Cu2(µ4-EDTA)(Him)2]·2H2O}n(1). 2. Materials and Methods 2.1. Reagents Malachite Cu 2 CO 3 (OH) 2 (Aldrich), H 4 EDTA acid (TCI) and imidazole (Alfa Aesar) were used as received. 2.2. Crystallography A blue needle crystal of {[Cu 2 ( µ4 -EDTA)(Him) 2 ] · 2H 2 O} n ( 1 ) was mounted on a glass fiber and used for data collection. Diffraction data were obtained using a Bruker D8 VENTURE PHOTON III-14 diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). The data were processed with APEX2 [ 8 ] and corrected for absorption using SADABS [ 9 ]. The structure was solved by direct methods using the program SHELXS-2013 [ 10 ], and refined by full-matrix least-squares techniques against F 2 using SHELXL-2013 [ 10 ]. The O1 water atom is disordered over two positions; the occupancy factor for each one was refined, resulting in a value of 0.440(8) for O1 and 0.560(8) for O2. Positional and anisotropic atomic displacement parameters were refined for all non-hydrogen atoms. Hydrogen atoms were located in difference maps and included as fixed contributions riding on attached atoms with isotropic thermal parameters constrained to 1.2/1.5 U eq of the carrier atoms. Molecular graphics were generated with DIAMOND [ 11 ]. Crystal data, experimental details, and refinement results are summarized in Table 1. Crystallographic data for 1 has been deposited in the Cambridge Crystallographic Data Center, with the CCDC number 2047832. 2.3. Other Physical Measurements Analytical data (CHN) was obtained in a Fisons–Carlo Erba EA 1108 elemental microanalyzer (Carlo Erba Reagents, Sabadell, Spain). The copper(II) content was cheeked as CuO by the weight of the final residue in the thermo-gravimetric analysis (TGA) at 950 ◦ C within <1% of assumed experimental error. The FT-IR spectrum was recorded (KBr pellet) on a Jasco FT-IR 6300 spectrophotometer (JASCO Deutschland GmbH, Pfungstadt, Germany). The electronic (diffuse reflectance) spectrum was obtained with a Varian-Cary5E spectrophotometer (Agilent Technologies, Inc., Santa Clara, CA, USA). TGA was carried out (r.t.–950 ◦ C) in airflow (100 mL/min) by a Shimadzu Thermobalance TGA–DTG–50H (SHIMADZU DEUTSCHLAND GmbH, Duisburg, Germany), while a series of 36 timespaced FT-IR spectra were recorded with a coupled FT-IR Nicolet Magna 550 spectrometer (Thermo Fisher Scientific Inc., Madrid, Spain) to identify the evolved gasses. X-band EPR measurements were carried out on a Bruker ELEXSYS 500 spectrometer (Karlsruhe, Crystals 2021,11, 48 3 of 15 Germany) equipped with a super-high-Q resonator ER-4123-SHQ and standard Oxford low temperature devices. For Q-band studies, EPR spectra were recorded on a Bruker EMX system equipped with an ER-510-QT resonator. An NMR probe calibrated the magnetic field, and the frequency inside the cavity was determined with a Hewlett-Packard 5352B microwave frequency counter (Agilent Technologies, Madrid, Spain). Computer simulation: WINEPR-SimFonia, version 1.5, Bruker Analytische Messtechnik GmbH (Karlsruhe, Germany). Temperature dependent magnetic measurements were performed between 2 and 300 K with an applied field of 0.1 T using a commercial MPMS3 SQUID magnetometer (Quantum Design GmbH, Darmstadt, Germany). The experimental susceptibilities were corrected for the diamagnetism of the constituent atoms using Pascal tables. Table 1. Crystal data and structure refinement for {[Cu2(µ4-EDTA)(Him)2]·2H2O}n(1). Empirical Formula C8H12CuN3O5 Empirical formula weight 293.75 Temperature 100(2) K Wavelength 0.71073 Å Crystal system, space group Monoclinic, C2/c Unit cell dimensions a = 21.1313(13) Å, α= 90◦ b = 5.9280(4) Å, β= 110.474(3)◦ c = 18.1842(13) Å, γ= 70.545(2)◦ Volume 2134.0(3) Å3 Z, Calculated density 8, 1.829 Mg/m3 Absorption coefficient 2.062 mm−1 F(000) 1200 Crystal size 0.250 ×0.070 ×0.040 mm2 Theta range for data collection 3.588 to 30.5050◦ Limiting indices –28 ≤h≤30, –8 ≤k≤8, –25 ≤l≤25 Reflections collected / unique 32813 / 3255 [R(int) = 0.0462] Completeness to θ= 25.242 99.7% Absorption correction Semi-empirical from equivalents Max. and min. transmission 1.000 and 0.857 Refinement method Full-matrix least-squares on F2 Data/restraints/parameters 6551/0/307 Goodness-of-fit on F21.055 Final R indices [I > 2σ(I)] R1= 0.0278, wR2= 0.0619 R indices (all data) R1= 0.0354, wR2= 0.0663 Largest diff. peak and hole 0.461 and −0.590 e.Å−3 CCSD ref. number 2047832 2.4. Synthesis with Relevant Vis-UV and FTIR Spectral Data Compound 1 was obtained in a two-step process. First, Cu 2 CO 3 (OH) 2 (green malachite, 1 mmol, 0.22 g) and H 4 EDTA (1 mmol, 0.29 g) (Alpha-Aesar, Kandel, Germany) were reacted in water (100 mL) inside an open Kitasato flask at 50 ◦ C. Magnetic stirring was aided by hand shaking from time to time to promote a full reaction of these products, to give a clear blue solution of Cu 2 ( µ -EDTA) chelate. Heat was then ceased, and the solution filtered (without vacuum) to remove any malachite residue. Him (2 mmol, 0.19 g) was added to the filtrate, which produced an intensification of the blue color in the resulting solution. Slow evaporation of this mother liquor (about one week at r.t.) produced the first crystals of 1 which were collected by filtration, washed with water, and air-dried to be checked by FT-IR spectroscopy. The remaining mother liquor was placed in a crystallizer, and put inside a desiccator to permit a slow diethyl ether diffusion. The formation of blue crystals of 1 (many of them suitable for crystallographic purposes) was achieved in a high yield (~0.50 g, ~85%). Elemental analysis (%): Calc. for C 16 H 24 Cu 2 N 6 O 10 : C 32.71, H 4.12, N 14.31, Cu (as CuO) 27.08; Found: C 32.67, H 4.10, N 14.29, Cu 27.09 (as CuO, final residue at 950 ◦C, in the TGA curve). UV–vis spectrum data: rather symmetrical d-d band with maximum absorption at 665 nm. FT–IR data (cm −1 ): 3650–3200vbr νas / νs (H 2 O), Crystals 2021,11, 48 4 of 15 peaks at 3161w, 3144w, 3124w νas (NH), 3045w ν (C-H) arom , 2988, 2934 νas (CH 2 ), 2866, 2814 νas (CH 2 ), ~1640sh δ (H 2 O), 1611 νas (COO), 1550vw or 1512vw δ (N-H), 1389m νs (COO), 910w or 902w π (C-H) arom for only one C-H (usually expected at 900-860), 840w π (C-H) arom for two adjacent C-H (expected at 860-810). These spectra are reported as Supporting Information Figures S1 and S2, respectively. 2.5. Theoretical Methods The energies of the assemblies analyzed in this manuscript were computed using the Gaussian-16 program [ 12 ], as follows. The interaction energies ∆ E, were computed as the energy difference between the dimeric assembly and the sum of the energies of the monomers. The basis set superposition error was corrected using the Boys and Bernardi approach [ 13 ]. The def2-TZVP [ 14 , 15 ] basis set combined with the PBE0 functional [ 16 , 17 ] and the D3 dispersion correction [ 18 , 19 ] were used for the calculations. This level of theory (functional and basis set) has been used before to study noncovalent interactions in the solid state [ 20 – 24 ], including those analyzed in this work [ 25 – 27 ]. Moreover, X-ray coordinates instead of optimized geometries were used to investigate the energetic features of the noncovalent interactions in the solid state, because we were interested to analyze them as they stand in the solid state, instead of investigating the optimal complexation geometry. The MEP (molecular electrostatic potential) surfaces were computed using the 0.001 isosurface and visualized using Gaussview software [ 12 ] at the same level of theory. The noncovalent interaction (NCI) index [ 28 , 29 ] and quantum theory of atoms-in- molecules (QTAIM) [ 30 , 31 ] analyses were calculated using the wavefunction generated by Gaussian-16 as input for the AIMAll program [32]. 3. Results and Discussion 3.1. A Comment About the Synthesis of Compound 1 The two step synthetic procedure here reported for 1 agrees with the desired stoichiometric ratio Cu:EDTA:Him 2:1:2. If Him is not added (second step), the reaction between malachite and H 4 EDTA yields the binary dinuclear chelate, {[Cu 2 ( µ4 -EDTA)(H 2 O) 2 ] · 2H 2 O} n , whose crystal structure has been previously reported [ 5 ]. On the other hand, Sergienko et al. [ 6 ], reacting this binary dinuclear chelate (as stating reactant) and a large excess of Him (Him:Cu mole ratio = ~7:1) synthesized and X-ray characterized the salt [Cu(Him) 6 ][(Him) ( µ2 -EDTA)Cu · Cu(Him) 4· Cu( µ2 -EDTA)(Him)] · 6H 2 O (GEMPOE in CSD, with an averaged Him/Cu mole ratio 3/1) [ 6 ]. In this compound the centrosymmetric trinuclear complex anion has EDTA as pentadentate chelator (having a non-coordinated acetate arm) for both the terminal Cu(II) atoms as well as the µ2 -bridging ligand for the central Cu(Him) 4 linker moiety. This enables the N-binding of Him in one of the four closest sites of the elongated coordination of the metal center at both terminal Cu(µ2-EDTA)(Him) moieties. 3.2. Thermal Stability Under air-dry flow, the weight loss versus temperature TGA behavior of 1 consists of six steps (Figure 1). Experimental results, calculated loss weights, and evolved gases or final residue are reported in Table 2. Detailed information of this TGA is supplied as Supporting Information Figure S3. First, compound 1 loses one of two uncoordinated water molecules (calculated for 2 H 2 O molecules 6.133%!) before starting the burning of organic ligands above 210 ◦ C. Thus, the remaining water at 210 ◦C will be most likely lost during the second step, overlapped with the beginning of burning of organic ligands. The evolved gases suggest that this phenomenon first affects EDTA, but weight loss in the second to sixth steps advise against additional speculative attributions. Even so, it seems evident that the aromatic Him ligands burn over 500 ◦ C. Most organics burn below 515 ◦ C, yielding a non-pure CuO residue. Interestingly, a small weight loss during the last step leads to a final residue in excellent agreement with the calculated value for a pure CuO residue. Crystals 2021,11, 48 5 of 15 Crystals 2020, 10, x FOR PEER REVIEW 5 of 15 Figure 1. Weight loss versus temperature (in the range r.t. to 950 °C) in the thermo-gravimetric analysis (TGA) analysis of 1 (sample: 11.03 mg). Table 2. Summary of results obtained from the thermogravimetric analysis of compound 1. Step or R Temp. (°C) Time (min) Weight (%) Exp. Cal. Evolved Gases or Residue (R) 1 40–210 2–19 3.366 3.066 * 1 H 2 O *, CO 2 (t) 2 210–345 19–33 33.474 - H 2 O, CO 2 , CO (t) 3 345–400 33–38 3.704 - H 2 O, CO 2 , CO, CH 4 (t) 4 400–425 38–42 14.307 - H 2 O, CO 2 , CO, CH 4 (t) 5 425–515 42–51 14.817 - H 2 O, CO 2 , CO, CH 4 (t) N 2 O, NO, NO 2 6 515–950 51–91 1.317 - H 2 O, CO 2 , CO (t), CH 4 (t), N 2 O, NO, NO 2 R 950 95 27.088 27.080 2 CuO * Calculated for the loss of only 1 H 2 O. t = trace amounts. First, compound 1 loses one of two uncoordinated water molecules (calculated for 2 H 2 O molecules 6.133 %!) before starting the burning of organic ligands above 210 °C. Thus, the remaining water at 210 °C will be most likely lost during the second step, overlapped with the beginning of burning of organic ligands. The evolved gases suggest that this phenomenon first affects EDTA, but weight loss in the second to sixth steps advise against additional speculative attributions. Even so, it seems evident that the aromatic Him ligands burn over 500 °C. Most organics burn below 515 °C, yielding a non-pure CuO residue. Interestingly, a small weight loss during the last step leads to a final residue in excellent agreement with the calculated value for a pure CuO residue. 3.3. Copper(II) Coordination and Crystal Structure Compound 1 is a polymer that agrees with the formula {[Cu 2 (µ 4 -EDTA)(Him) 2 ]·2H 2 O} n . It consists of centro-symmetric dinucelar complex motifs and non-coordinated water (Figure 2). In each half of the dinuclear complex motif, the Cu(II) center exhibits a distorted square-based pyramidal surrounding (Table 3), type 4+1. The square base is supplied by the N(1), O(11), and O(21) donors from a mer-mida group from EDTA and the N(1) imidazole donor, occupying the four shortest coordination sites of the Cu(II). One of the acetate arms of the mida group acts as O(21)-monodentate, whereas the other one displays an anti,syn-O(11),O(12)-bridging function. This µ 2 -carbox- ylate group represents the main driving promoter for the polymerization of 1, supplying Figure 1. Weight loss versus temperature (in the range r.t. to 950 ◦ C) in the thermo-gravimetric analysis (TGA) analysis of 1(sample: 11.03 mg). Table 2. Summary of results obtained from the thermogravimetric analysis of compound 1. Step or R Temp. (◦C) Time (min) Weight (%) Exp. Cal. Evolved Gases or Residue (R) 1 40–210 2–19 3.366 3.066 * 1 H2O *, CO2(t) 2 210–345 19–33 33.474 - H2O, CO2, CO (t) 3 345–400 33–38 3.704 - H2O, CO2, CO, CH4(t) 4 400–425 38–42 14.307 - H2O, CO2, CO, CH4(t) 5 425–515 42–51 14.817 - H2O, CO2, CO, CH4(t) N2O, NO, NO2 6 515–950 51–91 1.317 - H2O, CO2, CO (t), CH4(t), N2O, NO, NO2 R 950 95 27.088 27.080 2 CuO * Calculated for the loss of only 1 H2O. t = trace amounts. 3.3. Copper(II) Coordination and Crystal Structure Compound 1 is a polymer that agrees with the formula {[Cu 2 ( µ4 -EDTA)(Him) 2 ] · 2H 2 O} n . It consists of centro-symmetric dinucelar complex motifs and non-coordinated water ( Figure 2 ). In each half of the dinuclear complex motif, the Cu(II) center exhibits a distorted square-based pyramidal surrounding (Table 3), type 4+1. The square base is supplied by the N(1), O(11), and O(21) donors from a mer-mida group from EDTA and the N(1) imidazole donor, occupying the four shortest coordination sites of the Cu(II). One of the acetate arms of the mida group acts as O(21)-monodentate, whereas the other one displays an anti,syn-O(11),O(12)-bridging function. This µ2 -carboxylate group represents the main driving promoter for the polymerization of 1 , supplying to each Cu(II) center of its distal O12 6= 1 donor atom, at the largest bond distance of 2.488(1) Å. The square base of the coordination polyhedron is slightly distorted, and it reveals the Addison–Reedijk parameter (also the so-called trigonal index) τ , estimated for 1 from values of their trans-basal coordination angles ( θ or ϕ ) as τ = ( θ−ϕ )/60 = (172.70 − 168.16)/60 = 0.076 (Table 3). All structural coordination parameters in 1 are in agreement with an expectable Jahn–Teller distortion for copper(II) complexes, always related to its stable electronic configurations ([Ar]3d 9 ) and the useful “hole formalism” (versus a 3d 1 ). As reported for the closely related compounds with pyridine coligands, [Cu 2 ( µ4 -EDTA)(py) 2 (H 2 O) 2 ] · 2H 2 O [ 6 ] and {[Cu 2 ( µ4 -EDTA)(3hpy) 2 ]} n [ 7 ], the efficient coordination of imidazole is due to the mutual affinity between the Pearson’s borderline acid Cu(II) and N-heterocyclic borderline basis, Crystals 2021,11, 48 6 of 15 as well as to the mer-mida tridentate group. This enables the binding of the N-Him donor at one among the four closest donor sites around the copper(II) center. This is seen in many other related five-coordinated Cu(II) compounds (of the type 4+1) that also promote a displacement of the metal center (from the mean basal coordination plane) toward the distal donor atom (0.03 Å in 1). Crystals 2020, 10, x FOR PEER REVIEW 6 of 15 to each Cu(II) center of its distal O12≠1 donor atom, at the largest bond distance of 2.488(1) Å. The square base of the coordination polyhedron is slightly distorted, and it reveals the Addison–Reedijk parameter (also the so-called trigonal index) τ, estimated for 1 from values of their trans-basal coordination angles (θ or φ) as τ = (θ−φ)/60 = (172.70−168.16)/60 = 0.076 (Table 3). All structural coordination parameters in 1 are in agreement with an expectable Jahn–Teller distortion for copper(II) complexes, always related to its stable electronic configurations ([Ar]3d9) and the useful “hole formalism” (versus a 3d1). As reported for the closely related compounds with pyridine coligands, [Cu2(µ4- EDTA)(py)2(H2O)2]·2H2O [6] and {[Cu2(µ4-EDTA)(3hpy)2]}n [7], the efficient coordination of imidazole is due to the mutual affinity between the Pearson’s borderline acid Cu(II) and N-heterocyclic borderline basis, as well as to the mer-mida tridentate group. This enables the binding of the N-Him donor at one among the four closest donor sites around the copper(II) center. This is seen in many other related five-coordinated Cu(II) compounds (of the type 4+1) that also promote a displacement of the metal center (from the mean basal coordination plane) toward the distal donor atom (0.03 Å in 1). Figure 2. Structure and numbering scheme for non-hydrogen atoms in the asymmetric unit of 1. The water molecule has partial occupancies in the two disordered represented positions. Symmetry codes: a = #1 = −x + 1/2, y − 1/2, −z + 1/2, b = #2 = −x + 1/2, −y + 1/2, −z, e = #5 = −x + 1/2, −y − 1/2, −z. Table 3. Coordination bond lengths (Å) and angles (°) in the crystal of compound 1, {[Cu2(µ4- EDTA)(Him)2]·2H2O}n. See Figure 2 for numbering scheme of relevant atoms. Atoms Distance or Angle Cu(1)-O(11) 1.9389(12) Cu(1)-N(1) 1.9451(14) Cu(1)-O(21) 1.9608(12) Cu(1)-N(10) 2.0207(14) Cu(1)-O(12)#1 2.4878(12) O(11)-Cu(1)-N(1) 97.23(5) O(11)-Cu(1)-O(21) 168.16(5) (φ) N(1)-Cu(1)-O(21) 94.22(5) O(11)-Cu(1)-N(10) 84.79(5) N(1)-Cu(1)-N(10) 172.70(5) (θ) O(21)-Cu(1)-N(10) 84.23(5) O(11)-Cu(1)-O(12)≠1 88.90(5) N(1)-Cu(1)-O(12)# 1 89.50(5) O(21)-Cu(1)-O(12)#1 88.19(5) N(10)-Cu(1)-O(12)#1 97.57(5) Symmetry code: #1 − x + 1/2, y − 1/2, −z + 1/2. Figure 2. Structure and numbering scheme for non-hydrogen atoms in the asymmetric unit of 1 . The water molecule has partial occupancies in the two disordered represented positions. Symmetry codes: a = #1 = − x + 1/2, y − 1/2, − z + 1/2, b = #2 = − x + 1/2, − y + 1/2, − z, e = #5 = − x + 1/2, − y −1/2, −z. Table 3. Coordination bond lengths (Å) and angles ( ◦ ) in the crystal of compound 1 , {[Cu 2 ( µ4 - EDTA)(Him)2]·2H2O}n. See Figure 2for numbering scheme of relevant atoms. Atoms Distance or Angle Cu(1)-O(11) 1.9389(12) Cu(1)-N(1) 1.9451(14) Cu(1)-O(21) 1.9608(12) Cu(1)-N(10) 2.0207(14) Cu(1)-O(12)#1 2.4878(12) O(11)-Cu(1)-N(1) 97.23(5) O(11)-Cu(1)-O(21) 168.16(5) (ϕ) N(1)-Cu(1)-O(21) 94.22(5) O(11)-Cu(1)-N(10) 84.79(5) N(1)-Cu(1)-N(10) 172.70(5) (θ) O(21)-Cu(1)-N(10) 84.23(5) O(11)-Cu(1)-O(12)6=1 88.90(5) N(1)-Cu(1)-O(12)# 1 89.50(5) O(21)-Cu(1)-O(12)#1 88.19(5) N(10)-Cu(1)-O(12)#1 97.57(5) Symmetry code: #1 −x + 1/2, y −1/2, −z + 1/2. The crystal of 1 consists of an infinite network, having non-coordinated water molecules and polymeric 2D-layers of the metal complex. Such layers run parallel to the bc plane, in such a manner that Him ligands are oriented toward their external faces. They are then pillared along the a axis, connected by an extensive network of N(3)-H(3) ··· O(22) 6= 3 interactions (Figure 3a and Table 4). Such interactions are efficiently reinforced by π -staking between appropriate pairs of symmetry related anti-parallel Him ligands from adjacent layers: inter-centroids distance d c-c = 3.45 Å (depicted in Figure 3b), inter-planar distance Crystals 2021,11, 48 7 of 15 d π-π = 3.24 Å, dihedral angle α = 0 ◦ , slipping angles β = γ = 20.2 ◦ ). Consequently, both intermolecular interactions infer a strong 3D cohesion, at the same time generating funnels parallel to the aaxis (Figure 3a) where water molecules are hosted with certain disorder, in two non-equivalent positions (see Section 2.2), identified by the O-number of water in Figure 2. Crystals 2020, 10, x FOR PEER REVIEW 7 of 15 The crystal of 1 consists of an infinite network, having non-coordinated water molecules and polymeric 2D-layers of the metal complex. Such layers run parallel to the bc plane, in such a manner that Him ligands are oriented toward their external faces. They are then pillared along the a axis, connected by an extensive network of N(3)-H(3)···O(22) ≠ 3 interactions (Figure 3a and Table 4). Such interactions are efficiently reinforced by πstaking between appropriate pairs of symmetry related anti-parallel Him ligands from adjacent layers: inter-centroids distance dc-c = 3.45 Å (depicted in Figure 3b), inter-planar distance dπ-π = 3.24 Å, dihedral angle α = 0°, slipping angles β = γ = 20.2°). Consequently, both intermolecular interactions infer a strong 3D cohesion, at the same time generating funnels parallel to the a axis (Figure 3a) where water molecules are hosted with certain disorder, in two non-equivalent positions (see Section 2.2), identified by the O-number of water in Figure 2. (a) (b) Figure 3. (a) Layers of the polymeric metal complex of 1, pillared along the a axis of the crystal connected by (Him)N···O(carboxylate). Non-coordinated water and other intermolecular interactions omitted for clarity, to depict void channels where water is hosted. (b) Crystal structure fragment showing the π-stacking interaction between a pair of antiparallel Him ligands, belonging to the external faces of two pillared layers of the polymeric metal complex of 1. Table 4. Hydrogen bonds in the crystal of {[Cu2(µ4-EDTA)(Him)2]·2H2O}n (1) [Å, °]. D = H-donor atom, A = H-acceptor atom. D-H···A D(D···A) <(D-H···A) N(3)-H(3)···O(22) ≠ 3 2.774(2) 169.8 O(1)-H1A)···O(12) ≠ 7 2.869(4) 154.7 Symmetry codes: c = #3 = −x, −y, −z, d = ≠4 = −x + 1/2, y + 1/2, −z + 1/2, e = #5 = −x + 1/2, −y − 1/2, −z, g = #7 = x − 1/2, y − 1/2, z. The observed “host” role for the 3D-crystal network here reported as “guest disordered water” requires some comments. First of all, the labile O(2)H2 falls inside channels without being involved in identified intermolecular interactions. In clear contrast, the water numbered as O(1)H2 is retained inside the channels by the cooperating action of two weak intermolecular forces: the O1-H1A···O12)≠7 H-bond (#7 = x − 1/2, y − 1/2, z, see Table 4) and the quite unusual O1-H1B/π(Him) interaction (Figure 4). Now, the TGA behavior Figure 3. ( a ) Layers of the polymeric metal complex of 1 , pillared along the a axis of the crystal connected by (Him)N ··· O(carboxylate). Non-coordinated water and other intermolecular interactions omitted for clarity, to depict void channels where water is hosted. ( b ) Crystal structure fragment showing the π -stacking interaction between a pair of anti-parallel Him ligands, belonging to the external faces of two pillared layers of the polymeric metal complex of 1. Table 4. Hydrogen bonds in the crystal of {[Cu 2 ( µ4 -EDTA)(Him) 2 ] · 2H 2 O} n ( 1 ) [Å, ◦ ]. D = H-donor atom, A = H-acceptor atom. D-H···A D(D···A) <(D-H···A) N(3)-H(3)···O(22) 6=3 2.774(2) 169.8 O(1)-H1A)···O(12) 6=7 2.869(4) 154.7 Symmetry codes: c = #3 = − x, − y, − z, d = 6= 4 = − x + 1/2, y + 1/2, − z + 1/2, e = #5 = − x + 1/2, − y − 1/2, − z, g=#7=x−1/2, y −1/2, z. The observed “host” role for the 3D-crystal network here reported as “guest disordered water” requires some comments. First of all, the labile O(2)H 2 falls inside channels without being involved in identified intermolecular interactions. In clear contrast, the water numbered as O(1)H 2 is retained inside the channels by the cooperating action of two weak intermolecular forces: the O1-H1A ··· O12) 6= 7 H-bond (#7 = x − 1/2, y − 1/2, z, see Table 4) and the quite unusual O1-H1B/ π (Him) interaction (Figure 4). Now, the TGA behavior of 1 can be rationalized, emphasizing that the “more labile water” is retained in a surprisingly broad range of temperatures (r.t.–210 ◦ C), because it is hosted within channels. Starting from 210 ◦ C, the hardly retained water should be lost, overlapped to the beginning of the combustion of organic ligands. Crystals 2021,11, 48 8 of 15 Crystals 2020, 10, x FOR PEER REVIEW 8 of 15 of 1 can be rationalized, emphasizing that the “more labile water” is retained in a surprisingly broad range of temperatures (r.t.–210 °C), because it is hosted within channels. Starting from 210 °C, the hardly retained water should be lost, overlapped to the beginning of the combustion of organic ligands. Figure 4. Crystal structure fragment representing the intermolecular (water)O1-H1B···π(Him) interaction. In brackets, assumable geometric parameters (d, Θ, Φ) typically used to recognize this kind of interaction [33–38]: d(O)···Him(Cg, centroid) 3.31 (4.3) Å, Θ(O1-Cg-H1B) 15.57° (<25°), Φ(O1-H1B···Cg) 120.1 (120–180°). A graphical summary of all coordination bonds and intermolecular interactions contributing to the stability of 1 is illustrated in Figure 5. Figure 5. Intermolecular interactions contributing to the stability of the crystal of 1, uniting the association of the 2D layered coordination polymer (N-H···O, π–staking) and reinforcing the retention of half the water (cooperation of O-H···O, O-H/π). 3.4. Magnetic Porperties The X-band (9.395 GHz) EPR spectrum shows the characteristic features of a rhombic g tensor from 5 K to room temperature (Figure 6). The main components of the g tensor, Figure 4. Crystal structure fragment representing the intermolecular (water)O1-H1B ···π (Him) interaction. In brackets, assumable geometric parameters (d, Θ , Φ ) typically used to recognize this kind of interaction [ 33 – 38 ]: d(O) ··· Him(Cg, centroid) 3.31 (4.3) Å, θ (O1-Cg-H1B) 15.57 ◦ (<25 ◦ ), ϕ(O1-H1B···Cg) 120.1 (120–180◦). A graphical summary of all coordination bonds and intermolecular interactions contributing to the stability of 1is illustrated in Figure 5. Crystals 2020, 10, x FOR PEER REVIEW 8 of 15 of 1 can be rationalized, emphasizing that the “more labile water” is retained in a surprisingly broad range of temperatures (r.t.–210 °C), because it is hosted within channels. Starting from 210 °C, the hardly retained water should be lost, overlapped to the beginning of the combustion of organic ligands. Figure 4. Crystal structure fragment representing the intermolecular (water)O1-H1B···π(Him) interaction. In brackets, assumable geometric parameters (d, Θ, Φ) typically used to recognize this kind of interaction [33–38]: d(O)···Him(Cg, centroid) 3.31 (4.3) Å, Θ(O1-Cg-H1B) 15.57° (<25°), Φ(O1-H1B···Cg) 120.1 (120–180°). A graphical summary of all coordination bonds and intermolecular interactions contributing to the stability of 1 is illustrated in Figure 5. Figure 5. Intermolecular interactions contributing to the stability of the crystal of 1, uniting the association of the 2D layered coordination polymer (N-H···O, π–staking) and reinforcing the retention of half the water (cooperation of O-H···O, O-H/π). 3.4. Magnetic Porperties The X-band (9.395 GHz) EPR spectrum shows the characteristic features of a rhombic g tensor from 5 K to room temperature (Figure 6). The main components of the g tensor, Figure 5. Intermolecular interactions contributing to the stability of the crystal of 1 , uniting the association of the 2D layered coordination polymer (N-H ··· O, π –staking) and reinforcing the retention of half the water (cooperation of O-H···O, O-H/π). 3.4. Magnetic Porperties The X-band (9.395 GHz) EPR spectrum shows the characteristic features of a rhombic g tensor from 5 K to room temperature (Figure 6). The main components of the g tensor, estimated by the comparison of the experimental spectra with those obtained by a computer simulation program working at the second order of the perturbation theory, are g 1 = 2.194, Crystals 2021,11, 48 9 of 15 g 2 = 2.125, and g 3 = 2.066 (<g> = 2.128). It is notable that these parameters do not agree with the axially elongated square-base pyramidal coordination of copper in 1 . This fact suggests the presence of exchange interactions between magnetically non-equivalent Cu 2+ ions, as confirmed by the 2.0 value deduced for the Hathaway G parameter [ 36 ]. In this way, Cu1 and Cu1 d (d = 6= 4 = − x + 1/2, y + 1/2, − z + 1/2) related by a two-fold screw axis and connected by a anti,syn-carboxylate bridge, with Cu ··· Cu d separation of 5.766(1) Å) are crystallographically equivalents, but not from a magnetic point of view. If the exchange interaction (J) between them is larger than the difference between their Zeeman energies, the individual resonances must be averaged. This would explain why the observed spectrum does not adequately reflect the coordination geometry of the environment of the Cu(II) ions. Crystals 2020, 10, x FOR PEER REVIEW 9 of 15 estimated by the comparison of the experimental spectra with those obtained by a computer simulation program working at the second order of the perturbation theory, are g 1 = 2.194, g 2 = 2.125, and g 3 = 2.066 (<g> = 2.128). It is notable that these parameters do not agree with the axially elongated square-base pyramidal coordination of copper in 1. This fact suggests the presence of exchange interactions between magnetically non-equivalent Cu 2+ ions, as confirmed by the 2.0 value deduced for the Hathaway G parameter [36]. In this way, Cu1 and Cu1 d (d = ≠4 = −x + 1/2, y + 1/2, −z + 1/2) related by a two-fold screw axis and connected by a anti,syn-carboxylate bridge, with Cu···Cu d separation of 5.766(1) Å) are crystallographically equivalents, but not from a magnetic point of view. If the exchange interaction (J) between them is larger than the difference between their Zeeman energies, the individual resonances must be averaged. This would explain why the observed spectrum does not adequately reflect the coordination geometry of the environment of the Cu(II) ions. Figure 6. X-band ESR spectra of compound 1. See text for the fitting parameters. The Q-band (34.1 GHz) room temperature spectrum (Figure 7) supports this hypothesis. It shows the overlapping of two signals, a rhombic one with similar g values to those deduced from the X-band spectrum (g 1 = 2.192, g 2 = 2.136 and g 3 = 2.063; <g> = 2.130, G = 2.0) and an axial one with g // = 2.260 and g⊥ = 2.063 (<g> = 2.130, G = 4.1). The g values of the axial signal are in good agreement with the structural characteristics of the CuN 2 O 3 chromophore. Thus, it appears that the powder Q band spectrum shows the simultaneous presence of averaged and individual resonances for Cu1 and Cu1 d ions. This behavior has already been observed previously, and occurs when the condition J>|g(Cu1)-g(Cu1 d )|βH is not fulfilled for all orientations of the magnetic field when the spectrum is recorded at Q-band [37]. Assuming near axial symmetry for the molecular g tensor, the following relations between the exchange and molecular components of the g tensors can be established [38]: 󰇛𝑔 󰇜𝑔 ∥ 𝑐𝑜𝑠𝛼𝑔  𝑠𝑖𝑛𝛼 󰇛𝑔 󰇜𝑔 ∥ 𝑠𝑖𝑛𝛼𝑔  𝑐𝑜𝑠𝛼 𝑔  𝑔  𝑐𝑜𝑠2𝛼  𝑔  𝑔   𝑔  𝑔   2𝑔   where 2α is the canting angle between the parallel axis of the two tensors, i.e., the canting angle between the normals to the equatorial planes of the two interacting chromophores. The value of 2α that can be deduced from the last equation (73.9°) is practically the same as can be obtained from the crystallographic data (73.8°). Therefore, the molecular g values in this compound are: g // = 2.261 and g⊥ = 2.063, in good agreement with a d x2−y2 ground state, as expected for Cu(II) ions in elongated square-pyramidal geometry. Figure 6. X-band ESR spectra of compound 1. See text for the fitting parameters. The Q-band (34.1 GHz) room temperature spectrum (Figure 7) supports this hypothesis. It shows the overlapping of two signals, a rhombic one with similar g values to those deduced from the X-band spectrum (g 1 = 2.192, g 2 = 2.136 and g 3 = 2.063; <g> = 2.130, G = 2.0) and an axial one with g // = 2.260 and g ⊥ = 2.063 (<g> = 2.130, G = 4.1). The g values of the axial signal are in good agreement with the structural characteristics of the CuN 2 O 3 chromophore. Thus, it appears that the powder Q band spectrum shows the simultaneous presence of averaged and individual resonances for Cu1 and Cu1 d ions. This behavior has already been observed previously, and occurs when the condition J>|g(Cu1)- g(Cu1 d )| β H is not fulfilled for all orientations of the magnetic field when the spectrum is recorded at Q-band [ 37 ]. Assuming near axial symmetry for the molecular g tensor, the following relations between the exchange and molecular components of the g tensors can be established [38]: (gex 1)2=g2 kcos2α+g2 ⊥sin2α (gex 2)2=g2 ksin2α+g2 ⊥cos2α gex 3=g⊥ cos2α=gex 1−gex 2 gex 1+gex 2−2gex 3 where 2 α is the canting angle between the parallel axis of the two tensors, i.e., the canting angle between the normals to the equatorial planes of the two interacting chromophores.