Supramolecular motifs in metal complexes of Schiff bases. Part 5. Zinc(II)-assisted self-assembly of some bis-N,N- and N,O-bidentate Schiff bases and chiral packing modes in solid state

Abstract

Zinc(II)-assisted self-assembly of a new bis-N,O-bidentate Schiff base ligand, (N-salicylidene-4,4′-diaminodiphenyl)methane (L¹⁶) with two chelating sites linked by a spacer group (-C₆H₄CH₂C₆H₄-), afforded in high yield the double-helical dinuclear complex (L¹⁶ : Zn^II = 2∶2). Single-crystal X-ray analyses demonstrated clearly that the two Zn^II centers have a distorted tetrahedral (T_d) coordination sphere with two-wrapped ligands. The analogous Schiff base, bis[4-(2-pyridylmethyleneaminophenyl]methane (L¹⁷) was also designed to self-assemble in the presence of metal ions, leading to a triple-helical dinuclear (L¹⁷∶Zn^II = 3∶2) supramolecular motif. Each zinc ion has six-coordinate octahedral geometry with six nitrogens from three ligands. These unprecedented helical motifs in the solid state and solution seem to be induced by the geometrical preference for octahedral or tetrahedral coordination mode of the Zn^II ion and the interligand π-stacking interactions between the spacer groups of L¹⁶ and L¹⁷. Electrospray mass spectrometry proved a very useful characterisational tool in detecting the distribution of supramolecular species in solution. Use of a N,N ′-bis(2-pyridylmethylene)-1,4-diaminobenzene (L³⁵) with a rigid phenylene spacer in its center resulted in the one-dimensional zigzag polymeric structure ((L³⁵∶Zn^II)_∞ = (1∶1)_∞) where the multiple π-stacking interactions operate between the aromatic rings of linking ligands. Each zinc ion is octahedrally coordinated by two N,N-bidentate arms of two different ligands and two cis oxygens from N,N-dimethylformamide as coordinating solvent.

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Introduction

Recent synthetic advances using Schiff base ligands ^1–5 with aromatic spacer groups and other flexible bis-bidentate ligands ^6–15 as building blocks for metal-assisted self-assembly (metallosupramolecular chemistry) have demonstrated the requirements for the close and tunable control of the coordination sphere of the metal ion and the weak aromatic interactions between the spacer groups in the ligand. The mechanism and structure for the formation of metal-assisted self-assembled complexes are often impossible to predict in advance and many subtle factors such as π–π interactions,^2,16 templated-anions,^17–19 trapped-cations,²⁰ deprotonation of the site remote from the metal center ^7b,21 and the preferred coordination geometry of metal ion ²² can influence the final product.

Achiral bridged Schiff bases, the previously designed series of bis-bidentate ligands shown in Scheme 1, have been shown to spontaneously wrap around copper(II) ion to create a self-assembled helical multinuclear complex in the solid state.¹ Formation of some helical multinuclear supramolecular motifs has been explained on the basis of self-assembly processes involving π stacking of the spacer group (X) with free rotational freedom and a flexible coordination mode for tetrahedral and square-planar geometries of the Cu^II ion.

Scheme 1

Zinc(II) ion has been widely found in several zinc-containing metalloenzymes such as zinc-peptidases,²³ human carbonic anhydrase,²⁴ and alkalinephosphatase.²⁵ The coordination mode for octahedral or tetrahedral geometries of zinc(II) ion may affect the formation and mechanism of the self-assembly processes of bis-bidentate Schiff bases. Furthermore, the CH ⋯ π interactions, which have recently received much attention in supramolecular architectures, have also been important in a variety of molecular phenomena at physical, chemical, and biological levels.²⁶ We report here a one-pot self-assembly synthesis of several zinc(II) complexes utilizing weak CH ⋯ π and π ⋯ π interactions of a novel type of Schiff base ligand, L¹⁶, L¹⁷, and L³⁵ as shown in Scheme 2. The variations in (a) metal-binding site from pyridylmethylene N,N moiety to salicylidene O,N one (L¹⁷→L¹⁶) and (b) spacer group from diphenylmethylene -C₆H₄CH₂C₆H₄- to phenylene -C₆H₄- (L¹⁷→ L³⁵) may produce some zinc(II) complexes with unprecedented supramolecular motifs.

Scheme 2 (a) Change in the coordination site from N,N bidentate to N,O bidentate. (b) Change into the shorter spacer group.

Experimental

Preparation of Schiff base ligands, L¹⁷, L¹⁶, and L³⁵, and their Zn^II complexes

Ligand L¹⁷ was synthesized by usual Schiff-base condensation of bis(4-aminophenyl)methane and pyridine-2-aldehyde.² To a stirred solution of bis(4-aminophenyl)methane (6.09 g, 0.0307 mol) in ethanol (300 ml) at room temperature was added dropwise an ethanolic solution of pyridine-2-aldehyde (6.58 g, 0.0615 mol). After the addition was complete, the reaction mixture was heated to 70–80 °C. The resultant precipitate was filtered off, washed with ethanol, and dried in air to afford 9.0 g (78%) of pale-yellow crystalline product. Calcd. for C₂₅H₂₀N₄: C, 79.76; H, 5.35; N, 14.88; found: C, 79.59; H, 5.43; N, 14.93%. ¹H-NMR (400 MHz, DMF-d₇ (99.5%), 25 °C, internal ref. TMS): δ 8.74 (2H, ddd, J = 4.9, 2.0, 0.9, pyridyl H⁶), 8.65 (2H, s, -CH[double bond, length half m-dash]N-), 8.21 (2H, ddd, J = 7.8, 2.0, 1.0, pyridyl H³), 7.98 (2H, td, J = 7.8, 1.5, pyridyl H⁴), 7.54 (2H, ddd, J = 7.5, 5.0, 1.4, pyridyl H⁵), 7.36 and 7.40 (8H, dd, J = 7.8, 1.5, aminophenyl), 4.09 (2H, s, -CH₂-).

Ligand L¹⁶ was synthesized in 90% yield by heating ethanolic solutions containing bis(4-aminophenyl)methane (1 equiv.) and salicylaldehyde (2 equiv.). Data for ligand L¹⁶: fluorescent yellow powder, 90% yield (Calcd. for C₂₇H₂₂N₂O₂; C, 79.78; H, 5.45; N, 6.89. Found: C, 79.78; H, 5.65; N, 6.89%). ¹H-NMR (400 MHz, CDCl₃, 25 °C, internal ref. TMS): δ 13.29 (2H, ddd, OH ⋯ H), 8.62 (2H, s, -CH[double bond, length half m-dash]N-), 7.38 (2H, d, J = 7.3, sal-ad H⁶), 7.37 (2H, td, sal-ad H⁴), 7.27 and 7.23 (8H, ddd, bis(amino)phenyl), 7.02 (2H, d, J = 7.8, sal-ad H³), 6.94 (2H, td, J = 7.8, 1.0, sal-ad H⁵), 4.04 (2H, s, -CH₂-).

Ligand L³⁵ was synthesized by the same condensation of 1,4-phenylenediamine and pyridine-2-aldehyde and obtained as dark orange crystals, yield 70% (Calcd. for C₁₈H₁₄N₄: C, 75.5; H, 4.92; N, 19.56. Found: C, 75.36; H, 5.06; N, 19.25%). ¹H-NMR (400 MHz, CDCl₃, 30 °C, internal ref. TMS): δ 8.73 (2H, ddd, J = 4.9, 1.5, 1.0, pyridyl H⁶), 8.67 (2H, s, -CH[double bond, length half m-dash]N-), 8.22 (2H, ddd, J = 7.8, 1.3, 1.0, pyridyl H³), 7.83 (2H, td, J = 7.3, 1.5, pyridyl H⁴), 7.38 (2H, ddd (overlapped with diaminobenzene), pyridyl H⁵), 7.38 (4H, s, diaminobenzene).

Synthesis and characterization of the zinc(II) complexes of L¹⁷ and L¹⁶ used in this study were carried out by the usual synthetic method.^2,3 For example, a mixture of L¹⁷ (0.64 g, 1.7 mmol) and Zn(ClO₄)₂·6H₂O (0.33 g, 1.7 mmol) in CH₃OH (50 ml) was stirred at room temperature for several hours. The resultant pale-yellow precipitate was filtered off and air-dried. The yields of this material were ca. 50–60%, but were not optimised. Pale-yellow crystals were obtained by slow diffusion of diethyl ether in the DMF–MeCN solution (Found: C, 53.62; H, 3.97; N, 10.50; Cl, 7.84%. Calcd. for C₇₅H₆₀N₁₂O₁₆Cl₄Zn₂·DMF·2MeCN: C, 54.32; H, 4.05; N, 11.58; Cl, 7.82%).

Reaction of L¹⁶ (3 equiv.) with Zn(ClO₄)₂·6H₂O (2 equiv.) and K₂CO₃ (2 equiv.) in methanol at room temperature affords a fluorescent yellow solid (Found: C, 53.69; H, 4.05; N, 4.62%. Calcd. for C₅₄H₅₀N₄O₁₆Cl₂Zn₂: C, 53.48; H, 4.15; N, 4.62%) which is soluble in CHCl₃. Fluorescent-yellow crystals were obtained by slow diffusion of diethyl ether into the CHCl₃ solution of the metal complex.

Reaction of a methanol solution of 1 equiv. of the ligand L³⁵ with Zn(ClO₄)₂·6H₂O (1 equiv.) at room temperature leads to the deep orange solids with L³⁵∶Zn^II = 2∶2 and 3∶2 ratio as judged by elemental analysis. The yield was in the range 60–70%. (Calcd. forC₅₄H₄₂N₁₂O₁₆Cl₄Zn₂ as 3∶2 complex: C, 46.74; H, 3.05; N, 12.11; Cl, 10.22 and C₃₆H₂₈N₈O₁₆Cl₄Zn₂ as 2∶2 complex: C, 39.26; H, 2.56; N, 10.17; Cl, 12.87. Found: C, 42.48; H, 3.40; N, 11.04; Cl, 10.07%). X-Ray quality crystals were grown by slow diffusion of diethyl ether into a DMF solution of the crude material.

Single crystal X-ray structure analysis

Single crystals suitable for X-ray analysis of 1 ([Zn₂(L¹⁷)₃](ClO₄)₂), 2 ([Zn₂(L¹⁶ − 2H)₂]) and 3 ([Zn^IIL³⁵]·(ClO₄)₂) were obtained from DMF–MeCN/ether, CHCl₃/ether, and DMF/ether, respectively. Precise data collection and crystal parameters for 1, 2 and 3 are summarized in Table 1. CCDC reference number 188/193. The structure was solved by direct method ²⁷ and expanded using Fourier techniques.²⁸ The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included but not refined. All calculations were performed using the teXsan ²⁹ crystallographic software package of Molecular Structure Coorporation.

Table 1 Crystallographic data, data collection and refinement parameters for 1·DMF·2CH₃CN, 2·2CHCl₃, and 3·3DMF

	1·DMF·2CH3CN	2·2CHCl3	3·3DMF
a R = Σ ¶Fo| − |Fc||/Σ/Fo|.b Rw = [Σω(|Fo| − |Fc|)^2/Σω|Fo|^2]^1/2, ω = 1/σ^2(Fo) = [σ^2(Fo) + p^2(Fo)^2/4]^−1.
Formula	C82H73N15O17Cl4Zn2	C56H42N4O4Cl6Zn2	C27H35N7O11Cl2Zn
M	1813.14	1178.45	769.90
Crystal system	Monoclinic	Monoclinic	Monoclinic
Space group	C2/c (no. 15)	P21/c (no. 14)	P22/a (no. 14)
Unit cell dimensions
a/Å	54.89(2)	18.263(4)	13.472(2)
b/Å	13.833(2)	16.089(3)	18.812(3)
c/Å	22.01(1)	19.160(4)	14.538(4)
β/ °	95.40(3)	109.22(3)	106.17(1)
U/Å^3	16639.9	5316(2)	3539(2)
Z	8	4	4
T/K	298(2)	298(1)	173(1)
μ(Mo-Kα)/cm^−1	7.82	12.54	9.09
No. of reflections measured	10268	7883	5297
Residuals: R,^aRw^b	0.061, 0.084	0.077, 0.095	0.052, 0.075

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Results and discussion

UV/VIS titrations in the formation of Zn^II complexes with L¹⁶, L¹⁷, and L³⁵

The UV/VIS spectrum for L¹⁶ is quite sensitive to the acidity of the solution and the presence of Cu^II ion. Fig. 1(a) represents the change in the UV/VIS spectrum of L¹⁶ upon addition of Zn^II ion in ethanol. Four isosbestic points are observed at λ = 289, 314.5, 370, and 454 nm. Upon increasing Zn^II concentration, there is a decrease in absorbance of the ligand π–π* (CH[double bond, length half m-dash]N) band at 345 nm and emergence of a new peak at 394 nm due to deprotonation of the phenol group and the Zn^II coordination. A molar ratio plot at 345 and 394 nm demonstrates the formation of a Zn^II∶L¹⁶ = 1∶1 ratio complex, as judged by the clear inflection at [Zn^II]/[L¹⁶] = 1.

Fig. 1 UV/VIS spectral changes of ligands L¹⁶ (a), L¹⁷ (b) and L³⁵ (c) in ethanol upon addition of Zn(CH₃COO)₂·2H₂O. [L¹⁶] = [L¹⁷] = [L³⁵] = 6.50 × 10⁻⁵ mol dm⁻³.

A smaller UV/VIS change (Fig. 1(b)) is observed in the case of the L¹⁷–Zn²⁺ system and an isosbestic point is not observed under similar conditions. An inflection at [Zn^II]/[L¹⁷] = ca. 0.66 (λ_obs = 330.5 nm) may suggest the formation of Zn^II∶L¹⁷ = 2∶3 complex. UV/VIS titration (Fig. 1(c)) was performed also in the L³⁵/Zn²⁺ system. Since L³⁵ has a considerably delocalized π-conjugate system, the π–π* band due to the coordination of the Zn^II ion shows a large red shift up to 450 nm. The magnitude of this red-shift on coordination is approximately 100 nm, suggesting that the induced π-conjugation in the five-membered chelate rings occurs effectively through the spacer in the L³⁵–Zn²⁺ system. The result is a deepening in colour from colourless to orange–yellow. Some isosbestic points at 281.5 and 395 nm and a molar ratio plot suggest the formation of Zn^II∶L³⁵ = 2∶3 complex. It is interesting to note that the UV/VIS absorption change at higher Zn^II concentration of [Zn^II]/[L³⁵] >  ca. 0.6 reverses both at λ = 366 and 441 nm.

Mass spectra of Zn^II complexes with L¹⁶, L¹⁷, and L³⁵ complexes

Positive electrospray ionization mass spectrometry (ESI-MS) of the L¹⁶–Zn^II complex in MeOH (trace CHCl₃) shows the presence of several aggregated species (Fig. 2). The fairly weak peak at m/z = 1879.1 could be assignable to the tetranuclear mono-protonated species, [Zn₄(L¹⁶ − 2H)₄]H⁺, (4∶4)⁺. Two peaks corresponding to the binuclear mono-protonated [Zn₂(L¹⁶ − 2H)₂]H⁺, (2∶2)⁺ species and trinuclear mono-protonated [Zn₃(L¹⁶ − 2H)₃]H⁺, (3∶3)⁺ species are strongly observed at m/z = 941.0 and 1408.9, respectively. The experimental isotope profiles were in excellent agreement with the theoretical isotope distributions expected for the [Zn₃(L¹⁶ − 2H)₃]H⁺.

Fig. 2 ESI mass spectrum of a methanol solution of 2.

The solution behaviour of the L¹⁷–Zn^II complex was analyzed by two types of mass spectrometry, ESI-MS and fast atom bombardment mass spectrometry (FAB-MS). FAB-MS of the L¹⁷–Zn^II complex is very complicated but informative on the successive loss of ClO₄⁻ or HClO₄ from neutral species, 1 ([Zn₂(L¹⁷)₃](ClO₄)₂) (Fig. 3). The FAB-MS (3-nitrobenzyl alcohol as a matrix in DMF as solvent) shown in Fig. 3 displays several characteristic peaks for (Zn^II∶L¹⁷)ⁿ⁺ = (1∶1)⁺, (1∶2)⁺, (2∶2)⁺, and (2∶3)⁺ species (minus ClO₄⁻ or HClO₄), where M_1∶1, M_1∶2, M_2∶2, and M_2∶3 denote the species [ZnL¹⁷](ClO₄)₂, [Zn(L¹⁷)₂](ClO₄)₂, [Zn₂(L¹⁷)₂](ClO₄)₄, and [Zn₂(L¹⁷)₃](ClO₄)₄, respectively. The peaks for higher aggregates [M_2∶3 − ClO₄ − (HClO₄)₂]⁺, [M_2∶3 − ClO₄ − HClO₄]⁺, and [M_2∶3 − ClO₄]⁺ can be detected at m/z = 1358.0, 1458.0, and 1559.0, respectively.

Fig. 3 FAB mass spectrum for 1.

ESI-MS of the L¹⁷–Zn^II complex in MeOH is significantly different from the FAB-MS observed in the same complex (Fig. 4). The most important peak corresponding to the dinuclear-triple motif of crystal structure 1 (vide infra) is [M_2∶3 − ClO₄]⁺ at m/z = 1558.6. It is noted that minor 1∶1 complexes that are solvated by several MeOH molecules ([M_1∶1(MeOH)₂ − ClO₄]⁺ and [M_1∶1(MeOH)₃ − ClO₄]⁺) are clearly detected at m/z = 606.0 and 636.1.

Fig. 4 ESI mass spectrum of a methanol solution of 1.

Fig. 5 shows the ESI-MS of the L³⁵–Zn^II complex in methanol. The peaks at m/z = 349.2, 451.2, and 637.5 could be assigned to lower aggregates [M_1∶1 − ClO₄ − HClO₄]⁺, [M_1∶1 − ClO₄]⁺, and [M_1∶2 − ClO₄ − HClO₄]⁺. Higher aggregates such as Zn^II∶L³⁵ = 2∶2 and 2∶3 complexes are not observed in this system.

Fig. 5 ESI mass spectrum of a methanol solution of 3.

X-Ray crystal structure of L¹⁷–Zn^II complex, 1

Table 1 lists relevant X-ray data for the crystal parameters of 1. As expected, the formation of a dinuclear structure (Zn^II∶L¹⁷ = 2∶3) with a triple-helical motif is confirmed as shown in Fig. 6. The complex cation [(Zn^II)₂(L¹⁷)₃]⁴⁺ contains two Zn^II ions and three wrapped ligands L¹⁷. Selected bond lengths and angles for 1 are presented in Table 2.

Table 2 Selected bond lengths (Å) and angles (°) for [(Zn^II)₂(L¹⁷)₃](ClO₄)₄ (1) with esds in parentheses

Zn(1)–N(1)	2.158(6)	Zn(1)–N(2)	2.195(5)
Zn(1)–N(5)	2.186(7)	Zn(1)–N(6)	2.193(6)
Zn(1)–N(9)	2.154(6)	Zn(1)–N(10)	2.176(6)
Zn(2)–N(3)	2.174(6)	Zn(2)–N(4)	2.132(7)
Zn(2)–N(7)	2.247(6)	Zn(2)–N(8)	2.167(6)
Zn(2)–N(11)	2.200(6)	Zn(2)–N(12)	2.187(6)
C(6)–N(2)	1.290(9)	C(20)–N(3)	1.264(9)
C(31)–N(6)	1.265(9)	C(45)–N(7)	1.270(9)
C(56)–N(10)	1.268(8)	C(70)–N(11)	1.277(8)
N(1)–Zn(1)–N(2)	76.7(2)	N(1)–Zn(1)–N(5)	91.3(3)
N(1)–Zn(1)–N(6)	164.7(2)	N(1)–Zn(1)–N(9)	98.4(2)
N(1)–Zn(1)–N(10)	94.5(2)	N(2)–Zn(1)–N(5)	87.6(2)
N(2)–Zn(1)–N(6)	93.5(2)	N(2)–Zn(1)–N(9)	173.6(2)
N(2)–Zn(1)–N(10)	99.9(2)	N(5)–Zn(1)–N(6)	76.5(2)
N(5)–Zn(1)–N(9)	96.7(2)	N(5)–Zn(1)–N(10)	171.4(2)
N(6)–Zn(1)–N(9)	92.1(2)	N(6)–Zn(1)–N(10)	98.7(2)
N(9)–Zn(1)–N(10)	76.2(2)
N(3)–Zn(2)–N(4)	77.3(3)	N(3)–Zn(2)–N(7)	96.0(2)
N(3)–Zn(2)–N(8)	170.1(2)	N(3)–Zn(2)–N(11)	100.6(2)
N(3)–Zn(2)–N(12)	97.9(2)	N(4)–Zn(2)–N(7)	93.2(2)
N(4)–Zn(2)–N(8)	97.1(3)	N(4)–Zn(2)–N(11)	173.3(2)
N(4)–Zn(2)–N(12)	98.4(2)	N(7)–Zn(2)–N(8)	76.1(2)
N(7)–Zn(2)–N(11)	93.4(2)	N(7)–Zn(2)–N(12)	163.6(2)
N(8)–Zn(2)–N(11)	85.8(2)	N(8)–Zn(2)–N(12)	90.9(2)
N(11)–Zn(2)–N(12)	75.4(2)
Zn(1)–N(1)–C(1)	128.6(5)	Zn(1)–N(1)–C(5)	114.5(5)
Zn(1)–N(2)–C(6)	111.3(5)	Zn(1)–N(2)–C(7)	127.6(5)
Zn(1)–N(5)–C(26)	130.0(6)	Zn(1)–N(5)–C(30)	112.7(5)
Zn(1)–N(6)–C(31)	113.2(5)	Zn(1)–N(6)–C(32)	129.3(5)
Zn(1)–N(9)–C(51)	126.2(6)	Zn(1)–N(9)–C(55)	114.3(4)
Zn(1)–N(10)–C(56)	114.2(4)	Zn(1)–N(10)–C(57)	127.4(4)
Zn(2)–N(3)–C(17)	127.5(5)	Zn(2)–N(3)–C(20)	112.6(6)
Zn(2)–N(4)–C(21)	114.1(6)	Zn(2)–N(4)–C(25)	128.4(6)
Zn(2)–N(7)–C(42)	130.3(5)	Zn(2)–N(7)–C(45)	111.9(5)
Zn(2)–N(8)–C(46)	114.4(4)	Zn(2)–N(8)–C(50)	128.2(6)
Zn(2)–N(11)–C(67)	125.0(4)	Zn(2)–N(11)–C(70)	114.0(5)
Zn(2)–N(12)–C(71)	114.3(5)	Zn(2)–N(12)–C(75)	127.5(5)
C(10)–C(13)–C(14)	114.7(7)	C(35)–C(38)–C(39)	114.5(7)
C(60)–C(63)–C(64)	114.7(6)

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Fig. 6 X-ray crystal structure for 1. Perchlorate ions are omitted for clarity.

The Zn^II ions (Zn(1) and Zn(2)) are six-coordinated, being bound by three pyridine nitrogens (py) and three azomethine nitrogens (CH[double bond, length half m-dash]N) from three L¹⁷ ligands (L¹⁷(1)/C(1)–C(25), N(1)–N(4), L¹⁷(2)/C(26)–C(50), N(5)–N(8) and L¹⁷(3)/C(51)–C(75), N(9)–N(12)). The lengths of the Zn–N(py) bonds [2.154(6)–2.186(7) Å] are somewhat shorter than those of Zn–N(CH[double bond, length half m-dash]N) [2.176(6)–2.195 Å]. A slight trans influence is observed in the longer Zn(1)–N(5) (2.186(7) Å) and shorter Zn(1)–N(10) (2.176(6) Å). A pseudo-octahedral geometry around Zn(1) is given as judged by the N–Zn–N bond angles N(py)–Zn(1)–N(py), N(py)–Zn(1)–N(CH[double bond, length half m-dash]N) and N(CH[double bond, length half m-dash]N)–Zn(1)–N(CH[double bond, length half m-dash]N). The coordination geometry around Zn(2) is almost the same as that of Zn(1). The interatomic Zn(1) ⋯ Zn(2) separation is found to be 11.431 Å. Interestingly, a chiral cavity, which could accommodate small ions such as halide ion, is observed between the three spacer groups. The ClO₄⁻ ions are too large to be included at the center of the cavity.

The Corey–Pauling–Koltun (CPK) model for [(Zn^II)₂(L¹⁷)₃]⁴⁺ shown in Fig. 7 illustrates the multiple π–π and CH–π aromatic interactions, which operate particularly in some interactive moieties between pyridine–spacer groups (2.8–3.4 Å) and spacer–spacer groups (2.9–3.1 Å), and which create the dinuclear triple-helical structure. The flexibility around the methine group of the spacer (-C₆H₄CH₂C₆H₄-) may allow suitable rotational motion of the aromatic rings and CH–π interactions, thereby tuning the triple-helical structure of 1.

Fig. 7 Space-filling representation of 1. Perchlorate ions are omitted for clarity.

X-Ray crystal structure of L¹⁶–Zn^II complex, 2

The X-ray crystal structure of [(Zn^II)₂(L¹⁶)₂]·2CHCl₃ (2·2CHCl₃) shows clearly the dinuclear double-helical structure for the neutral complex 2 (Fig. 8). Two Zn^II ions (Zn(1) and Zn(2)) and two L¹⁶ ligand molecules (L¹⁶(1)/C(1)–C(27), N(1), N(3), O(1) and O(3) and L¹⁶(2)/C(28)–C(54), N(2), N(4), O(2) and O(4)) which are wrapped around the Zn–Zn axis are involved in this supramolecular motif. Table 3 shows the selected bond lengths and angles for 2.

Table 3 Selected bond lengths (Å) and angles (°) for [(Zn^II)₂(L¹⁶)₂] (2) with esds in parentheses

Zn(1)–O(1)	1.895(6)	Zn(1)–O(2)	1.907(6)
Zn(1)–N(1)	2.019(6)	Zn(1)–N(2)	2.030(6)
Zn(2)–O(3)	1.895(5)	Zn(2)–O(4)	1.894(6)
Zn(2)–N(3)	2.020(6)	Zn(2)–N(4)	2.019(7)
O(1)–C(1)	1.328(10)	O(2)–C(28)	1.32(1)
O(3)–C(27)	1.322(9)	O(4)–C(54)	1.340(10)
N(1)–C(7)	1.310(10)	N(2)–C(34)	1.297(10)
N(3)–C(21)	1.30(1)	N(4)–C(48)	1.298(9)
O(1)–Zn(1)–O(2)	111.0(3)	O(1)–Zn(1)–N(1)	95.5(2)
O(1)–Zn(1)–N(2)	129.1(3)	O(2)–Zn(1)–N(1)	126.8(3)
O(2)–Zn(1)–N(2)	95.0(2)	N(1)–Zn(1)–N(2)	102.9(2)
O(3)–Zn(2)–O(4)	112.6(3)	O(3)–Zn(2)–N(3)	96.0(2)
O(3)–Zn(2)–N(4)	119.3(3)	O(4)–Zn(2)–N(3)	129.9(3)
O(4)–Zn(2)–N(4)	96.3(2)	N(3)–Zn(2)–N(4)	104.5(2)
Zn(1)–O(1)–C(1)	126.1(6)	Zn(1)–O(2)–C(28)	127.6(5)
Zn(2)–O(3)–C(27)	126.6(5)	Zn(2)–O(4)–C(54)	122.5(6)
Zn(1)–N(1)–C(7)	119.3(5)	Zn(1)–N(1)–C(3)	120.8(5)
C(7)–N(1)–C(8)	119.7(6)	Zn(1)–N(2)–C(34)	121.3(5)
Zn(1)–N(2)–C(35)	117.9(5)	C(34)–N(2)–C(35)	119.1(6)
Zn(2)–N(3)–C(18)	118.2(5)	Zn(2)–N(3)–C(21)	120.4(5)
C(18)–N(3)–C(21)	120.5(7)	Zn(2)–N(4)–C(45)	120.3(5)
Zn(2)–N(4)–C(48)	119.2(6)
C(11)–C(14)–C(15)	116.2(7)	C(38)–C(41)–C(42)	115.2(6)

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Fig. 8 X-ray crystal structure for 2 (a) and space-filling representation of 2 (b).

The Zn^II ions are four-coordinated with two phenolate oxygens and two azomethine nitrogens. The two Zn–O bonds are similar (1.895(6) and 1.907(6) Å) and the Zn–N (azomethine) bond 2.019(6)–2.030(6) Å appreciably shorter than the corresponding one in the L¹⁷–Zn^II complex (2.174(6)–2.247(6) Å). The coordination environment is pseudo-tetrahedral as judged by the N–Zn–N, O–Zn–O and N–Zn–O bond angles. The Zn(1) ⋯ Zn(2) separation (11.725 Å) is longer than that of the L¹⁷–Zn^II complex (11.431 Å).

The aromatic rings in the spacer groups are almost parallel to each other (dihedral angle 19.6–21.9°). Effective π–π stacking is observed between these aromatic rings (3.4–3.9 Å). The flexible methine group of the spacer group (-C₆H₄CH₂C₆H₄-) and the phenolate coordinating group would lead to the double-helical motif. It is noted that two CHCl₃ molecules per complex are trapped within the right-hand cleft. These solvent molecules are held by weak C(H) ⋯ Cl hydrogen bonds (3.3–3.9 Å). In the crystal lattice, the left-hand side of aromatic rings of the complex are efficiently stacked with those of another complex molecule.

X-Ray crystal structure of L³⁵–Zn^II complex, 3

Recrystallization from DMF–diethyl ether afforded orange blocks of 3·3DMF. The X-ray crystal structure (Fig. 9 and Table 4) of 3 shows a one-dimensional polynuclear structure as [Zn^IIL³⁵(DMF)₂]_n·(DMF)_n·(ClO₄)_2n. The short spacer group (-C₆H₄-) of L³⁵ prevents the structure from wrapping around the Zn^II ions as observed in L¹⁷–Zn^II and L¹⁶–Zn^II. Each zinc ion is six-coordinated with four nitrogens from two L³⁵ molecules. Two remaining coordination sites are occupied by two O-bound DMF molecules. Their Zn–O(DMF ) bonds are 2.082 and 2.045 Å, respectively. It is noted that few DMF complexes with first-row transition metals have been crystallographically characterized, as pointed out by Ward et al.³⁰ The Zn–N(CH[double bond, length half m-dash]N) [Zn(1)–N(2) 2.267 and Zn(1)–N(4) 2.238 Å] are appreciably longer than the Zn–N(py) bonds [Zn(1)–N(1) 2.143 and Zn(1)–N(3) 2.141 Å]. The coordination geometry at each Zn^II center is pseudo-octahedral with bond angle in the range of O–Zn–O 92.5°, O–Zn–N 87.5–166.9° and N–Zn–N 76.5–161.5°.

Table 4 Selected bond lengths (Å) and angles (°) for [Zn^IIL³⁵]·(ClO₄)₂· 3DMF (3·3DMF) with esds in parentheses

Zn(1)–O(1)	2.082(3)	Zn(1)–O(2)	2.045(4)
Zn(1)–N(1)	2.143(5)	Zn(1)–N(2)	2.267(4)
Zn(1)–N(3)	2.141(5)	Zn(1)–N(4)	2.238(4)
N(2)–C(6)	1.276(7)	N(4)–C(18)	1.274(7)
O(1)–Zn(1)–O(2)	92.5(1)	O(1)–Zn(1)–N(1)	92.2(2)
O(1)–Zn(1)–N(2)	164.7(2)	O(1)–Zn(1)–N(3)	100.7(1)
O(1)–Zn(1)–N(4)	87.5(1)	O(1)–Zn(1)–N(1)	102.2(2)
O(2)–Zn(1)–N(2)	98.6(1)	O(2)–Zn(1)–N(3)	90.6(2)
O(2)–Zn(1)–N(4)	166.9(2)	N(1)–Zn(1)–N(2)	75.2(2)
N(1)–Zn(1)–N(3)	161.5(1)	N(1)–Zn(1)–N(4)	90.9(2)
N(2)–Zn(1)–N(3)	89.8(2)	N(2)–Zn(1)–N(4)	84.1(1)
N(3)–Zn(1)–N(4)	76.5(2)
Zn(1)–O(1)–C(19)	124.7(3)	Zn(1)–O(2)–C(22)	128.6(4)
Zn(1)–N(1)–C(1)	125.9(3)	Zn(1)–N(1)–C(5)	115.3(4)
Zn(1)–N(1)–C(5)	115.3(4)	Zn(1)–N(2)–C(6)	110.6(3)
Zn(1)–N(2)–C(7)	125.9(3)	C(6)–N(2)–C(13)	120.9(4)
Zn(1)–N(3)–C(13)	126.9(4)	Zn(1)–N(3)–C(17)	114.9(4)
Zn(1)–N(4)–C(10*)	128.2(3)	Zn(1)–N(4)–C(18)	111.5(3)

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Fig. 9 X-ray crystal structure for 3·3DMF.

Chirality in packing mode

In the solid state of the L¹⁷–Zn^II complex, it is noted that the ClO₄⁻ distribution around the complex cation is unsymmetrical. Three counter ClO₄⁻ ions are located at the periphery of [(Zn^II)₂(L¹⁷)₃]⁴⁺ cation [Zn(1) ⋯ ClO₄⁻ (6.307 and 6.800 Å) and Zn(2) ⋯ ClO₄⁻ (7.579 Å)] while one ClO₄⁻ ion is situated far from the complex cation to fill the lattice space [Zn(1) ⋯ ClO₄⁻ and Zn(2) ⋯ ClO₄⁻ (12.814 and 12.365 Å)]. In addition, two acetonitrile molecules and one DMF molecule are placed in the pocket formed by the wrapped conformation of three L¹⁷ ligands.

The [(Zn^II)₂(L¹⁷)₃]⁴⁺ cations have Δ–Δ or Λ–Λ configuration with the Zn^II ions. A view from the a–c plane in the crystal packing of 1 is presented in Fig. 10(a). The Δ–Δ and Λ–Λ optical isomers of 1 are alternately arrayed in the a–c plane. Four Zn^II ions from two [(Zn^II)₂(L¹⁷)₃]⁴⁺ cations along the a-axis are situated in the same plane and form a rhombic arrangement with the sum of interior angles 360° (11.431 × 14.459 Å, 47.3 and 132.7°). The other interesting projection of the crystal packing diagram in the a–b plane is two intersected Δ–Δ or Λ–Λ isomers with the bite angle 56.6°.

Fig. 10 Chiral packing mode in the solid state of 1 (a), 2 (b) and 3 (c).

A projection of the crystal packing of 2 in the b–c plane is presented in Fig. 10(b). Two layers, right-hand (D) and left-hand (L) helicates of the neutral complex 2 are observed in this plane. Sections of the aromatic rings depicted at a, b and c (dihedral angle 2°) have a face-to-face arrangement resulting in infinite π-stacking interactions (roughly the c axis in Fig. 10(b)) between adjacent D and L optical isomers of 2.

A polynuclear zigzag arrangement is observed in the crystal packing of 3 (Fig. 10(c)). In this arrangement, 1D chain with strong-conjugated π-system is formed in the alternating –Δ–Λ–Δ–Λ–Δ– chains. The Zn ⋯ Zn separation is 9.033 Å. Furthermore, duplicated π-stacking from overlap of aromatic rings in interligand py(A)-spacer(B)-py(C) sites are observed.

Solution structure of L¹⁷–Zn^II and L¹⁶–Zn^II complexes

The crystal structure of complex 1 shows the formation of dinuclear triple-helical [(Zn^II)₂(L¹⁷)₃]⁴⁺ species, while ESI mass spectrum indicates the mixture of some aggregated species such as (Zn^II∶L¹⁷)ⁿ⁺ = (1∶1)⁺, (1∶2)⁺, (2∶2)⁺, and (2∶3)⁺ species. If the L¹⁷–Zn^II complex is to retain a (2∶3)⁺ structure in solution, the ¹H NMR spectrum should exhibit one set of sharp signals. The ¹H NMR spectra in DMF-d₇ of complex 1 and a comparison with its ligand L¹⁷ are shown in Fig. 11.

Fig. 11 ¹H NMR spectra of L¹⁷ and its Zn^II complex [(Zn^II)₂(L¹⁷)₃](ClO₄)₄, 1.

The room temperature spectrum of 1 indicates peak broadening in all protons. This broadening may arise from instability of the complex or exchange on the NMR time scale between different species including ligand molecule. Sharper signals are obtained at − 20 °C. Since the exchange is fast on the ¹H NMR time scale, we cannot determine precisely from these spectra alone the extent of ligand dissociation by integrating peaks associated with the zinc-bound and dissociated L¹⁷ ligand. In general, the protons of H(1) and H(2) of the py moiety tend to shift downfield (H(1)* and H(2)*) upon complexation. In particular, the azomethine proton H(5) shows much a larger downfield shift due to the -CH(5)[double bond, length half m-dash]N- coordination to Zn^II ion. However, two sets of spacer group resonances at H(6)* and H(7)* show large upfield shifts. These pronounced upfield shifts may be attributed to the CH ⋯ π interaction between the spacer aromatic protons and py moiety, which is mentioned for the [(Zn^II)₂(L¹⁷)₃]⁴⁺ complex in the solid-state.

In contrast to complex 1, the ¹H NMR spectrum of 2 in CDCl₃ at 25 °C is simple to interprete and sharply resolved into the ligand peak and complex peak (Fig. 12). The integrating ratio of peak is about 1∶2 (ligand∶complex). Upon complexation of L¹⁶ with Zn^II ion, large upfield shift, due to the deprotonation of the phenolic group, is observed particularly in the protons of H(3) and H(5). The large upfield shift observed in the protons of spacer group (H(6)→H(6)* and H(7)→H(7)*) of 2 would be ascribed to the existence of CH ⋯ π interactions in solution.

Fig. 12 ¹H NMR spectra of L¹⁶ and its Zn^II complex [(Zn^II)₂(L¹⁶)₂], 2.

Conclusion

The series of Zn^II complexes with L¹⁶, L¹⁷ and L³⁵ have been found to show interesting variations of structure in the solid-state and solution. Three Zn^II-assisted supramolecular motifs were presented using the bis-bidentate Schiff bases L¹⁶, L¹⁷ and L³⁵, having flexible aromatic spacer groups. X-Ray crystal diffraction characterization revealed that the L¹⁶–Zn^II, L¹⁷–Zn^II and L³⁵–Zn^II complexes display dinuclear double-helical, dinuclear triple-helical and 1-D polynuclear architectures in the solid-state, depending on subtle differences of coordination site and spacer group in the ligand. UV/VIS, ESI-MS and ¹H NMR data were also consistent with the formation of these species in solution. The coordinatively flexible Zn^II ion and the aromatic–aromatic interactions between the flexible spacer group in the wrapped ligands would be a crucial factor for the formation of the supramolecular structure. Three types of chiral-linking mode have been generated for the crystal packing in the L¹⁶–Zn^II, L¹⁷–Zn^II and L³⁵–Zn^II complexes.

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