The interplay between coordination and non-covalent interactions in three zinc coordination polymers

Abstract

The synthesis and characterization of three zinc(II) polymers, [(μ-OAc)₃Zn₂L¹]_n·0.5nH₂O (1), [(μ-OAc)₃Zn₂L²]_n (2), [(μ-OAc)₃Zn₂L³]_n (3), using three tridentate ligands, HL¹ {2-(((2-(diethylamino)ethyl)amino)methyl)phenol}, HL² {4-chloro-2-(((3-(dimethylamino)propyl)amino)methyl)phenol} and HL³ {4-bromo-2-(((3-(dimethylamino)propyl)amino)methyl)phenol}, have been reported in this manuscript. The structures were confirmed by SC-XRD analysis. The DFT study analyzes the cooperative roles of bridging acetate co-ligands and NH⋯O hydrogen bonds. A structural survey of the Cambridge Structural Database (CSD) reveals that polymeric complexes derived from reduced bicompartmental Schiff base ligands are rare, emphasizing the unique structural features and supramolecular assembly observed in the present work.

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Introduction

The use of Schiff bases ligands for the designed synthesis of metal complexes has been well established for many years.^1–8 Material scientists have explored the possibility of such complexes to be used as functional materials.^9–12 Many such complexes have been widely used in inorganic biochemistry, medicinal chemistry, catalysis, medical imaging, optical materials and thin films.^13–18 Using these Schiff base complexes as ferromagnets were also reported in the literature.^19–25 Among various ligands, salen type tetradentate N₂O₂ donor Schiff bases, produced by the condensation of several amines with salicylaldehyde derivatives, are widely used by synthetic inorganic chemists for the synthesis of many mono, di and polynuclear complexes with potential application in bio-mimicking catalysis, molecular magnetism, opto-electronic research, crystal engineering, etc.^26–32N-alkyl diamines were also used to produce varieties of tridentate N₂O donor Schiff bases, which produced transition and non-transition metal complexes of interesting structures and applications.^33–38 Use of the reduced analogues of these Schiff bases for the syntheses of various complexes was also reported in the literature.³⁹

On the other hand, synthetic inorganic chemists and material scientists were attracted towards the synthesis and characterization of zinc complexes with Schiff base and reduced Schiff base ligands for the potential application of many zinc complexes in bio-mimetic catalysis, in photocatalytic degradation of organic dyes, in the fabrication of electronic and opto-electronic devices, sensing of nitroaromatic explosives, etc.^40–52 The coordination number of zinc may also vary in complexes primarily because of the d¹⁰ electronic configuration of zinc with no CFSE in any geometry.^53–61 Supramolecular chemists and crystal engineers are also interested in estimating the energy of several non-covalent interactions in the solid state structures of different diamagnetic zinc complexes.⁶² Apart from H-bonding, other significant non-covalent interactions, such as C–H–π, π–π, cation–π, lone pair–π, and anion–π, have been recognized in different zinc complexes.^63–65 Suitable tuning of these interactions has been utilized in several branches of biology, catalysis and supramolecular chemistry.^66–69 Spodium bonding (i.e. σ-hole) interactions have also been studied in several zinc complexes with Schiff base or reduced Schiff base ligands.^70–72

In the present work, three zinc(II) complexes, [(μ-OAc)₃Zn₂L¹]_n·0.5nH₂O (1), [(μ-OAc)₃Zn₂L²]_n (2) and [(μ-OAc)₃Zn₂L³]_n (3) have been synthesized using three reduced Schiff base ligands {HL¹ {2-(((2-(diethylamino)ethyl)amino)methyl)phenol, HL² {4-chloro-2-(((3-(dimethylamino)propyl)amino)methyl)phenol} and HL³ {4-bromo-2-(((3-(dimethylamino)propyl)amino)methyl)phenol} (Scheme 1) and characterized. Non-covalent interactions in their solid state structures were studied by DFT, MEP and QTAIM/NCIPlot calculations. It is found that the complexes are predominantly stabilized by moderately strong Zn–O(acetate) bonds. At the same time, NH⋯O interactions are important to stabilize the assemblies. To further contextualize the structures reported herein, we have conducted a systematic analysis of the Cambridge Structural Database, highlighting the limited number of related coordination polymers derived from reduced Schiff base ligands (Scheme 1).

Scheme 1 Schematic representation of the reduced Schiff base ligands HL¹–HL³.

Experimental

Chemicals and solvents were procured from Sigma-Aldrich.

Synthesis of the ligands

The reduced Schiff base ligands HL¹–HL³ have been synthesized following the methods reported in the literature^63,73,74 and are briefly described in the ESI.†

Synthesis of complexes

[(μ-OAc)₃Zn₂L¹]_n·0.5nH₂O (1). With continuous stirring, a methanol solution (5 mL) of the ligand HL¹ (∼1 mmol) was added to a solution (5 mL) containing ∼2 mmol zinc(II) acetate dihydrate (0.439 g) in methanol. Colourless crystals of complex 1 appeared at the bottom of the reaction vessel after ten days on slow evaporation of the solution in an open atmosphere.

Yield: 0.398 g (∼74%) based on zinc(II) acetate dihydrate. Anal. Calc. for C₃₈H₆₂N₄O₁₅Zn₄ (FW: 1076.39): C, 42.40; H, 5.81; N, 5.20%. Found: C, 42.4; H, 5.6; N, 5.3%, FT-IR (KBr, cm⁻¹): 3283–3175 (ν_N–H), 2992–2859 (ν_C–H), 1615–1549 (ν_COO⁻), 1486–1382 (ν_COO⁻). UV-vis, λ_max (nm), [ε_max (L mol⁻¹ cm⁻¹)] (MeOH): 285 (1.705 × 10³), 234 (5.108 × 10³).

[(μ-OAc)₃Zn₂L²]_n (2) and [(μ-OAc)₃Zn₂L³]_n (3). Complexes 2 and 3 were synthesised in similar processes to that used for the synthesis of complex 1, except that HL² and HL³ respectively were used instead of HL¹.

Complex 2, yield: 0.434 g (∼79%) based on zinc(II) acetate dihydrate. Anal. Calc. for C₁₈H₂₇ClN₂O₇Zn₂ (FW: 549.65): C, 39.34; H, 4.95; N, 5.10%. Found: C, 39.3; H, 4.9; N, 5.1%, FT-IR (KBr, cm⁻¹): 3260–3190 (ν_N–H), 2986–2810 (ν_C–H), 1620–1526 (ν_COO⁻), 1484–1368 (ν_COO⁻). UV-vis, λ_max (nm), [ε_max (L mol⁻¹ cm⁻¹)] (MeOH): 297 (2.725 × 10³), 242 (11.54 × 10³).

Complex 3, yield: 0.457 g (∼77%) based on zinc(II) acetate dihydrate. Anal. Calc. for C₁₈H₂₇BrN₂O₇Zn₂ (FW: 594.10): C, 36.39; H, 4.58; N, 4.72%. Found: C, 36.3; H, 4.5; N, 4.8%, FT-IR (KBr, cm⁻¹): 3263–3201 (ν_N–H), 3037–2819 (ν_C–H), 1625–1506 (ν_COO⁻), 1482–1366 (ν_COO⁻). UV-vis, λ_max (nm), [ε_max (L mol⁻¹ cm⁻¹)] (MeOH): 296 (2.529 × 10³), 242 (11.712 × 10³).

Instrumental details

The details of instruments and software are given in the ESI.†

Theoretical methods

Theoretical calculations were performed to complement the experimental findings and gain insights into the electronic structure and non-covalent interactions of the synthesized Zn complexes. All density functional theory (DFT) calculations were carried out using the TURBOMOLE software package (version 7.7).⁷⁵ The PBE0 functional⁷⁶ and D4 dispersion correction⁷⁷ were employed to account for hybrid exchange–correlation effects and dispersion interactions, in conjunction with the def2-TZVP basis set.⁷⁸ The X-ray crystallographic coordinates of the studied complexes were used as the initial geometries without further optimization. The major component of the disorder in complexes 2 and 3 was used. Molecular electrostatic potential (MEP) surfaces were computed at the same level of theory, with an isodensity value of 0.001 atomic units (a.u.) defining the molecular surface.

The topological properties of electron density were analyzed using the *quantum theory of atoms in molecules (QTAIM) framework,^79,80 implemented in the Multiwfn program.⁸¹ The same software was employed for NCIplot analysis,^82,83 which identifies and visualizes non-covalent interactions through reduced density gradient (RDG) isosurfaces. Graphical representations of QTAIM and NCIplot results, including bond critical points (BCPs), bond paths, and RDG isosurfaces, were generated using the Visual Molecular Dynamics (VMD) program.⁸⁴

Results and discussion

Synthesis

A solution of N,N-diethyl-1,2-diaminoethane or N,N-dimethyl-1,3-diaminopropane and salicylaldehyde or 5-chlorosalicylaldehyde or 5-bromosalicylaldehyde in a 1 : 1 molar ratio in methanol was refluxed to produce N₂O donor Schiff base ligands following the literature method.^{27,33,85–90} Sodium borohydride was then used as a mild reducing agent to prepare the reduced analogues (HL¹–HL³) of the Schiff base ligands.^44,51,63,73 HL¹, HL² and HL³ produce the polymeric complexes 1–3 on reaction with zinc(II) acetate dihydrate in methanol. Synthetic routes to the complexes are briefly highlighted in Scheme 2. Suitable single crystals of the complexes were used to SC-XRD analysis. The crystallographic data and refinement details are shown in Table 1. Selected bond lengths and bond angles are given in Table 2 and Table 3 respectively.

Scheme 2 Synthetic routes to complexes 1–3.

Table 1 Crystal data and refinement details of complexes 1, 2 and 3

Complex	1	2	3
a R 1 = ∑||Fo| − |Fc||/∑|Fo|. b wR2 = ∑w(|Fo|^2 − |Fc|^2)^2/(∑w|Fo|^2)^1/2.
Formula	C38H62N4O15Zn4	C18H27ClN2O7Zn2	C18H27BrN2O7Zn2
Formula weight	1076.39	549.65	594.10
Temperature (K)	273(2)	150(2)	273(2)
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	Cc	P21/n	P21/n
a (Å)	12.6715(6)	9.4593(15)	9.483(3)
b (Å)	14.3504(7)	19.398(3)	19.780(6)
c (Å)	14.5469(9)	12.964(2)	13.087(4)
α	(90)	(90)	(90)
β	114.1020(10)	108.532(5)	108.890(10)
γ	(90)	(90)	(90)
Z	2	4	4
V (Å^3)	2414.6(2)	2255.5(6)	2322.6(13)
d calc (g cm^−3)	1.480	1.619	1.699
μ (mm^−1)	2.028	2.286	3.825
F(000)	1116	1128	1200.0
Total reflections	14 512	94 143	47 397
Unique reflections	4352	9592	4195
Observed data [I > 2σ(I)]	3547	6832	3371
No. of parameters	284	310	310
R(int)	0.0714	0.0700	0.0801
R 1,^a wR2^b (all data)	0.0526, 0.0963	0.0786, 0.1937	0.0501, 0.1040
R 1,^a wR2^b [I > 2σ(I)]	0.0429, 0.0929	0.0622, 0.1706	0.0378, 0.0956
CCDC No.	2383603	2383604	2383605

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Table 2 Selected bond lengths (Å) of the complexes 1–3

Complex	1	2	3
Zn(1)–O(1)	2.073(5)	2.106(2)	2.101(2)
Zn(1)–O(2)	2.159(6)	2.065(3)	2.058(3)
Zn(1)–O(4)	2.093(5)	2.187(2)	2.198(3)
Zn(1)–O(7)	2.182(5)	2.152(2)	2.171(2)
Zn(1)–N(1)	2.118(6)	2.149(3)	2.140(3)
Zn(1)–N(2)	2.235(6)	2.214(3)	2.214(3)
Zn(2)–O(1)	1.939(5)	1.957(2)	1.961(2)
Zn(2)–O(6)	1.918(5)	1.946(2)	1.941(3)
Zn(2)–O(5)	1.967(5)	1.962(2)	1.967(3)
Zn(2)–O(3)	1.972(6)	1.951(3)	1.951(3)

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Table 3 Selected bond angles (°) of the complexes 1–3

Complex	1	2	3
a symmetry transformation i = −½ + x, 3/2 − y, −½ + z (for complex 1) and ii = ½ + x, 1.5 − y, ½ + z (complexes 2 and 3).
O(1)–Zn(1)–O(2)	88.5(2)	90.32(9)	89.73(10)
O(1)–Zn(1)–N(1)	89.0(2)	89.72(9)	89.72(11)
O(2)–Zn(1)–N(1)	94.4(2)	178.15(10)	177.81(12)
O(1)–Zn(1)–O(4)	91.15(19)	87.30(9)	87.62(10)
O(2)–Zn(1)–O(4)	90.5(2)	91.51(11)	91.71(11)
N(1)–Zn(1)–O(4)	175.1(2)	90.33(10)	90.38(11)
O(1)–Zn(1)–O(7)^a	167.6(2)	90.48(9)	90.23(9)
O(2)–Zn(1)–O(7)^a	80.0(2)	95.50(10)	95.61(10)
N(1)–Zn(1)–O(7)^a	87.0(2)	82.65(9)	82.28(11)
O(4)–Zn(1)–O(7)^a	93.81(19)	172.66(10)	172.36(10)
O(2)–Zn(1)–N(2)	167.3(2)	87.36(11)	87.79(12)
O(1)–Zn(1)–N(2)	103.5(2)	176.75(10)	176.26(11)
N(1)–Zn(1)–N(2)	82.1(2)	92.67(11)	92.86(12)
O(7)–Zn(1)–N(2)	87.6(2)	92.00(12)	92.80(11)
O(4)–Zn(1)–N(2)	93.1(2)	90.48(12)	89.65(11)
O(6)–Zn(2)–O(1)	116.4(2)	112.28(10)	111.74(11)
O(6)–Zn(2)–O(3)	100.9(3)	101.40(11)	102.15(12)
O(1)–Zn(2)–O(3)	107.9(2)	105.53(10)	106.09(11)
O(6)–Zn(2)–O(5)	121.3(2)	121.72(10)	121.56(11)
O(1)–Zn(2)–O(5)	105.3(2)	104.39(10)	104.55(11)
O(3)–Zn(2)–O(5)	103.6(2)	110.59(13)	109.92(14)

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Solubility test

Solubilities of complexes 1, 2 and 3 in various media are shown in Table 4. All three complexes are soluble in methanol, DMSO and chloroform. On the other hand, only complex 1 was soluble in acetonitrile and DCM (Table 4).

Table 4 Solubility of complexes 1, 2 and 3 in various solvent media (‘+’ = soluble and ‘−’ = insoluble)

Solvent	Complex 1	Complex 2	Complex 3
H2O	−	−	−
MeOH	+	+	+
MeCN	+	−	−
DMF	+	+	+
DMSO	+	+	+
DCM	+	−	−
CHCl3	+	+	+

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IR and electronic spectra

In the IR spectrum of each complex, a distinct band is observed in the region 3283–3169 cm⁻¹ (in 1), 3260–3190 cm⁻¹ (in 2) or 3263–3201 cm⁻¹ (in 3) due to the presence of N–H stretching vibration.^56,57,91 Bands in the region 2992–2859 cm⁻¹ (in 1), 2986–2710 cm⁻¹ (in 2) and 3037–2819 cm⁻¹ (in 3) may be assigned as C–H stretching vibrations.^39,92 Two distinct bands at ∼1408 and 1572 cm⁻¹ (in 1), at ∼1401 and ∼1575 cm⁻¹ (in 2), or at ∼1400 and ∼1576 (in 3) cm⁻¹ may be assigned as symmetric and antisymmetric stretching vibrations of respectively metal-bound carboxylates, as are usually in other complexes.⁹³ Many bands are observed at the fingerprint region for all three complexes. The IR spectra of the complexes are shown in Fig. S1–S3 (ESI†).

In the electronic spectra of the complexes (Fig. S4–S6, ESI†), recorded in 10⁻⁴ M methanol solution, the bands at 234 and 285 nm (in 1), at 242 and 297 nm (in 2) and at 242 and 296 nm (in 3) may be assigned as intra-ligand π→π* and n→π* transitions respectively.^94–96

Description of structures

[(μ-OAc)₃Zn₂L¹]_n·0.5nH₂O (1). Complex 1 crystallizes in the monoclinic space group Cc, forming a 1D chain (Fig. 1). The asymmetric unit (Fig. 2) contains a hexa-coordinated Zn(1) and a tetra-coordinated Zn(2). The tridentate N,N,O-donor reduced Schiff base ligand, (L¹)⁻, exhibits a μ₂-η¹:η¹:η² coordination model in that Zn(1) is coordinated by two amine nitrogen atoms, N(1) and N(2), and a phenolate oxygen atom, O(1), in addition to two acetate oxygen atoms, O(2) and O(4), of two bridging acetate anions and an oxygen atom O(7)ⁱ from a symmetry related (symmetry transformation, i = −½ + x, 1.5 − y, −½ + z) acetate to complete the distorted octahedral geometry. On the other hand, Zn(2) is tetrahedrally coordinated by one phenolate oxygen atom, O(1), and three acetate oxygen atoms, O(3), O(5), and O(6), from bridging acetate groups. Tetra-coordinated zinc complexes generally adopt either tetrahedral or square planar geometries. The τ₄ index (τ₄ = {360 − (α + β)}/141, where α and β represent the two largest ligand–metal–ligand bond angles) is a useful tool in determining the actual geometry.⁹⁷ A perfect square planar geometry yields a τ₄ value of 0, while a regular tetrahedral geometry gives a value close to 1. For the Zn(2) center, the calculated τ₄ value is 0.867, indicating that the geometry is slightly distorted tetrahedral. The two largest ligand–metal–ligand angles are 121.3(2)° [O(6)–Zn(2)–O(3)] and 116.4(2)° [O(1)–Zn(2)–O(6)] around the Zn(2) center. A half-chair conformation is found for the saturated five member chelate ring [Zn(1)–N(1)–C(8)–C(9)–N(2)] (Fig. S7, ESI†) with puckering parameters q = 0.458(10) Å; ϕ = 120.5(8)°.^98–100

Fig. 1 Perspective view of the 1-D polymeric chain of complex 1. Only the relevant hydrogen atoms are shown for clarity.

Fig. 2 Perspective view of the asymmetric unit of complex 1. Only the relevant hydrogen atom is shown for clarity. i = symmetry transformation = −½ + x, 3/2 − y, −½ + z.

[(μ-OAc)₃Zn₂L²]_n (2) and [(μ-OAc)₃Zn₂L³]_n (3). X-ray crystallographic analysis reveals that complexes 2 and 3 are isostructural one-dimensional polymers, crystallizing in the monoclinic space group P2₁/n, as illustrated in Fig. 3. The asymmetric units of both complexes (Fig. 4) contain two zinc(II) centers, a single tridentate N,N,O-donor reduced Schiff base ligand {(L²)⁻ for complex 2 and (L³) for complex 3} and three bridging acetate groups. The reduced Schiff base ligands coordinate in a μ₂-η¹:η¹:η² mode. The phenolate oxygen atom, O(1) bridges the two zinc(II) centers, Zn(1) and Zn(2). The Zn(1) center is coordinated by two amine nitrogen atoms [N(1) and N(2)], on one phenolate oxygen atom, O(1) of the reduced Schiff base units, (L²)⁻ or (L³)⁻, and two acetate oxygen atoms, O(2) and O(4). An additional acetate oxygen atom, O(7)′, from a symmetry related (symmetry transformation, ii = ½ + x, 3/2 − y, ½ + z) neighbouring molecule coordinates Zn(1) center to complete its distorted octahedral geometry. This O(7)^a atom is trans to O(4) and it should be noted that the Zn(1) environment in complexes 2 and 3 is different from that in complex 1 where O(7)^a is trans to the O(1) atom that bridges the two zinc atoms. In complexes 2 and 3 the Zn(2) center is tetra-coordinated, bonded to one phenolate oxygen atom, O(1), from the reduced Schiff base units {(L²)⁻ or (L³)⁻} and three acetate oxygen atoms, O(3), O(5), and O(6), of three bridging acetate anions. The τ₄ index is calculated as 0.894 for complex 2 and 0.899 for complex 3, confirming distorted tetrahedral environments around Zn(2) in both structures. The two largest ligand–metal–ligand angles around Zn(2) are 121.72(10)° [O(6)–Zn(2)–O(5)] and 112.28(10)° [O(1)–Zn(2)–O(6)], in complex 2; and are 121.56(11)° [O(6)–Zn(2)–O(5)] and 111.74(11)° [O(1)–Zn(2)–O(6)] in complex 3.

Fig. 3 Perspective views of 1D polymeric chains of complexes 2 (top) and 3 (bottom). Only the major component of the disordered aliphatic ring is shown. Hydrogen atoms bonded to carbon atoms are not shown.

Fig. 4 Perspective views of the asymmetric units of complexes 2 (left) and 3 (right). Only the major component of the disordered aliphatic ring is shown. Only relevant hydrogen atoms are shown for clarity. Symmetry transformation, ii = ½ + x, 3/2 − y, ½ + z.

Hirshfeld surface analysis

Fig. S8 (ESI†) displays the Hirshfeld surfaces of complexes 1–3, which are mapped over d_norm, shape index, and curvedness within the range of −0.1 Å to 1.5 Å. To enhance the visibility of the molecular moiety around which these surfaces are calculated, the surfaces are rendered transparent. The dominant intermolecular interactions in complexes 2 and 3, specifically O⋯H/H⋯O contacts, are clearly visible as red spots on the d_norm surfaces of all three complexes as shown in Fig. S8 (ESI†). In addition, other visible spots on the Hirshfeld surfaces correspond to H⋯H contacts. These interactions, indicated by smaller surface areas and lighter color tones, represent weaker and longer contacts compared to hydrogen bonds. The C⋯H/H⋯C interactions appear as distinct spikes in the two-dimensional fingerprint plots (Fig. 5). These plots reveal complementary regions, where one molecule acts as a donor (d_e > d_i) and the other as an acceptor (d_e < d_i). Furthermore, the fingerprint plots can be decomposed to highlight the contributions from different types of interactions, which may otherwise overlap in the full fingerprint profile.¹⁰¹

Fig. 5 Fingerprint plot: different contacts contributed to the total Hirshfeld surface area of complexes 2 and 3.

In complexes 2 and 3, the O⋯H/H⋯O interactions contribute 20.5% and 20.1%, respectively, to the Hirshfeld surface area. These interactions are clearly observed in the 2D fingerprint plots as two well-defined spikes: for complex 2, the O⋯H interaction corresponds to the lower spike with values d_i = 1.25 Å and d_e = 0.9 Å. The H⋯O interaction appears as the upper spike with d_i = 0.9 Å and d_e = 1.25 Å. Similarly, for complex 3, the O⋯H contact is represented by the lower spike at d_i = 1.30 Å and d_e = 0.95 Å. The H⋯O interaction is shown as the upper spike with d_i = 0.95 Å and d_e = 1.30 Å. These features appear as bright red spots on the d_norm surfaces, indicating significant hydrogen bonding. In addition to O⋯H interactions, halogen-involved contacts are also prominent in complexes 2 and 3: in complex 2, the Cl⋯H/H⋯Cl interactions account for 11.6% of the Hirshfeld surface. The Cl⋯H interaction appears as a lower spike at d_i = 1.7 Å and d_e = 1.1 Å, while the H⋯Cl interaction forms the upper spike at d_i = 1.1 Å and d_e = 1.7 Å. In complex 3, the Br⋯H/H⋯Br interactions contribute 12.5% to the surface. The Br⋯H interaction corresponds to a lower spike at d_i = 1.8 Å and d_e = 1.1 Å, whereas the H⋯Br contact appears as the upper spike at d_i = 1.1 Å and d_e = 1.8 Å. All of these halogen-based contacts are also visible as bright red regions on the d_norm surface, suggesting relatively strong non-covalent interactions. Additionally, C⋯H/H⋯C interactions are evident in both complexes: for complexes 2 and 3, the C⋯H interaction shows a lower spike at d_i = 1.75 Å and d_e = 1.1 Å and the H⋯C interaction forms an upper spike at d_i = 1.1 Å and d_e = 1.75 Å.

These interactions, like others, manifest as red zones on the d_norm surfaces, indicating their presence and relevance in the supramolecular structure.

PXRD patterns

The phase purity and uniformity of the bulk materials were confirmed by the close match between the experimental PXRD patterns and the simulated patterns derived from SC-XRD data. The experimental and simulated XRD patterns for complex 2 are shown in Fig. 6, while similar patterns for complex 3 are presented in Fig. S9 (ESI†).

Fig. 6 The experimental and simulated PXRD patterns of complex 2.

Theoretical study

Three new Zn complexes have been synthesized and characterized by X-ray crystallography, utilizing different reduced Schiff base ligands. First, the DFT study focuses on the coordination polymers (1–3), analyzing the cooperative roles of bridging acetate co-ligands and NH⋯O hydrogen bonds. As shown in Fig. 7, the bridging acetate ligands are highlighted with a pink background; and the NH⋯O interactions between the coordinated amino groups and the oxygen atoms of the acetate ligands are indicated by dashed lines. Notably, the hydrogen-bonding interactions are shorter in complex 1 compared to complexes 2 and 3. Further analysis (vide infra) explores the relative contributions of coordination bonds and NH⋯O contacts in propagating the dinuclear Zn monomers into coordination polymers.

Fig. 7 Partial views of the X-ray structures of coordination polymers 1 (a), 2 (b) and 3 (c). H⋯O distances in Å are shown. N⋯O distances are 3.041(7), 3.168(4), and 3.201(4) Å and N–H⋯O angles are 164(4), 169(4), and 172(4)° respectively. H-atoms apart from the NH groups are omitted for clarity.

To evaluate the coordination bond connecting the dinuclear Zn complexes [Zn₂L(OAc)₂] units, we employed a theoretical reaction in which the bridging acetate ligand was rotated to disrupt the coordination bond while ensuring no additional interactions were established by the acetate ligand (see Scheme 3). The energy difference between the two theoretical models represents the Zn–O(acetate) coordination bond energy. The dissociation bond energy values range from 28.6 kcal mol⁻¹ in complex 1 to 35.0 kcal mol⁻¹ in complex 2, with a value of 32.6 kcal mol⁻¹ for complex 3. These results indicate that the Zn–O coordination bonds are moderately strong.

Scheme 3 (a) Reaction used to estimate the Zn–O(acetate) dissociation energy in coordination polymer 1; (b) reaction used to estimate the Zn–O(acetate) dissociation energy in coordination polymers 2 and 3.

To estimate the energies of the NH⋯O interactions, we used QTAIM analysis and the value of the potential energy density (V) at the bond critical point characterizing the NH⋯O hydrogen bond, applying the equation E = ½ × V. The results, shown in Fig. 8, indicate modest interaction energies: −3.8 kcal mol⁻¹ for complex 1, −2.1 kcal mol⁻¹ for complex 2, and −1.6 kcal mol⁻¹ for complex 3. This trend correlates well with the experimental distances observed in Fig. 8. These modest energy values suggest that the formation of the coordination polymers is primarily governed by the coordination bonds, while the NH⋯O interactions play a secondary role, likely fine-tuning the final geometry of the polymer.

Fig. 8 Fragment of the coordination polymers of complexes 1 (a), 2 (b) and 3 (c) with indication of the bond critical point (BCP, red sphere) and bond path (orange line) that characterize the NH⋯O(acetate) H-bonds. The estimated energy using the V values are indicated in red next to BCP.

CSD analysis

To further investigate the tendency of reduced Schiff base ligands, similar to the N,N,O-donor type used in this work, to form coordination polymers via bridging anionic coligands, we conducted a search of the Cambridge Structural Database (CSD, version 5.45. November 2023). This search yielded 407 structures featuring reduced bicompartmental Schiff base ligands coordinated to transition metals. These reduced ligands are considerably less common than their conventional (non-reduced) counterparts, for which 3753 structures were identified using the same criteria. A manual inspection of the reduced-ligand entries revealed only 10 structures forming coordination polymers, indicating that such assemblies are relatively rare. The corresponding CSD reference codes, transition metals involved, and bridging anionic coligands are summarized in Table 5.^71,102–106 Notably, only one previous example involves Zn, highlighting the uniqueness of the Zn complexes reported in this study. Among the other transition metals, Cu is the most frequently observed, followed by Cd. However, in the case of Cu, only azido and chloride ligands are found as bridges; the acetate ligand employed in the present work appears in just one known structure.

Table 5 Reference codes, metals and bridging coligands of coordination polymers extracted from the CSD involving O,N,N-donor reduced Schiff-base ligands

CSD reference code	Metal	Bridging co-ligand	Ref.
AMAKOP	Ni	1,4-Dicarboxybenzene-2,5-dicarboxylato	102
RAFXIH	Cd	Isophthalato	103
RAFXON	Cd	5-Aminoisophthalato	103
UKETUA	Cu	Azido-N^1,N^1	104
UKEVAI	Cu	Chloro	104
WEPSAO	Cu	Azido-N^1,N^1 & azido-N^1,N^3	105
WEPSES	Cu	Azido-N^1,N^3	105
YIKRET	Cu	Azido-N^1,N^1 & azido-N^1,N^3	106
YIKRIX	Cu	Azido-N^1,N^1 & azido-N^1,N^3	106
ZIKQIX	Zn	Acetate	71

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Two representative examples, AMAKOP and ZIKQIX, are shown in Fig. 9. Both structures display NH⋯O contacts similar to those observed in the X-ray structures reported in this study, highlighting the tendency of coordinated NH groups to engage in intramolecular hydrogen bonding interactions with the anionic coligands.

Fig. 9 Partial view of the X-ray structures ANAKOP (a) and ZIKQIX (b). H-atoms omitted apart from those participating in the NH⋯O H-bonds. Distances in Å.

Conclusion

In this work we successfully synthesized and characterized three new Zn coordination polymers. DFT and QTAIM analyses revealed that the coordination polymers are predominantly stabilized by moderately strong Zn–O(acetate) bonds (28.6–35.0 kcal mol⁻¹), with ancillary NH⋯O hydrogen bonds (−3.8 to −1.6 kcal mol⁻¹) playing an important secondary role in fine-tuning polymer geometry. MEP and QTAIM/NCIPlot analyses also highlight the roles of NH⋯O interactions in stabilizing these assemblies, emphasizing the balance of forces driving structural organization. These findings enhance our understanding of the interplay between coordination and non-covalent interactions in complex formation and provide a foundation for designing new materials with tailored structural and functional properties. CSD analysis supports the conclusion that coordination polymers derived from reduced Schiff base ligands, particularly those incorporating acetate bridges and group 12 metals, are uncommon, thus reinforcing the novelty and structural interest of the reported systems.

Data availability

All data underlying the results are available as part of the article and no additional source data are required.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

P. Middya thanks the UGC, India, for awarding a Senior Research Fellowship. AF is grateful to Projects PID2020-115637GB-I00 and PID2023-148453NB-I00 funded by the Ministerio de Ciencia, Innovación y Universidades of Spain MCIU/AEI/10.13039/501100011033 and FEDER, UE.

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1–S9. CCDC 2383603–2383605 for complexes 1–3 respectively. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d5ce00505a

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