Single-strand helical complexes constructed from 2-pyridinyl-3-pyridinylmethanone: tuning the helical pitch length by variation of metal cation and/or counter anion

Abstract

The new ligand 2-pyridinyl-3-pyridinylmethanone (L) proves to be an excellent building block for the construction of single-strand helical architectures. A series of helical complexes have been synthesized by the reaction of L with various metal salts, in which L exhibits three kinds of coordination modes involving two kinds of bridging conformations, resulting in four types of single-strand helical chains. The counter anions in the series of 2₁ helical silver(I) complexes {[Ag(L)]X}_∞ (X = NO₃, 1; PF₆, 2; BF₄, 3; ClO₄, 4; CF₃CO₂, 5; CF₃SO₃, 6) are fully or partially embedded inside the cylindrical helix, and the pitch length corresponds not only to the size of the anion but also to its manner of docking into the groove of the helix. Formation of the helical structure in {[Cu(L)(CH₃CN)(H₂O)(ClO₄)]ClO₄}_∞ (7) is driven by Ow–H⋯O (perchlorate) hydrogen bonding that leads to a stable triangular motif which rigidly fixes the configuration of the helix. In {[Co(L)(H₂O)₃](ClO₄)₂·2H₂O}_∞ (8) and {[Zn(L)(H₂O)₃](CF₃SO₃)₂·H₂O}_∞ (9), similar helical chains without anion embedment suggest that the pitch length can be tuned by the size of metal cations. Notably, complex {[Ag(L)]CF₃SO₃}_∞ (10), a conformational polymorph of 6, has a 4₁ helix induced by argentophilic interaction.

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Introduction

The helical organization of molecules is a commonly observed phenomenon throughout nature, and hence a comprehensive understanding of the factors governing the formation of helical structures is of great importance not only in mimicking nature but also in preprogramming molecules to give specific architecture and defined functionality.^1–5 The construction of metal–organic helical coordination polymeric structures has been achieved with strand-like ligands that undergo wrapping to generate helicity, and short bridging ligands with helical information encoded by conformational restriction mostly lead to single-strand helices.^4,6,7

The self-organization process of ligands with metal ions to form defined architectures and functionalities depends on the selection of optimal components such as the coordination geometry of the metal ions, the binding site of the donating atoms, and the length of the spacers that link the binding sites.^2–4,8,9 Since ligands generally have specific backbones and linking information and the coordination geometries of various metal ions are so different, it remains a challenge to develop useful ligands for the generation of helical moieties adaptive to a wide range of metal cations and counter anions.^10–12

In contrast to extensive studies on the coordination chemistry of di-2-pyridinylmethanone (di-2-pyridyl ketone),¹³ metal complexes of its N-positional isomer 2-pyridinyl-3-pyridinylmethanone (L) has remained unexplored so far (Scheme 1). With its two differentiable pyridyl rings bearing variable dihedral angles within a limited range, this flexible ligand is potentially capable of linking metal ions into an infinite helix. Here we report a series of helical complexes generated from L and various metal ions (Ag⁺, Cu²⁺, Co²⁺ and Zn²⁺) and counter anions (NO₃⁻, PF₆⁻, BF₄⁻, ClO₄⁻, CF₃SO₃⁻ and CF₃CO₂⁻).

Scheme 1

Experimental

General

Diethyl ether was purchased from LAB-SCAN and further refluxed over sodium and benzophenone. All other chemicals were obtained commercially from Aldrich and used without further purification.

Elemental analyses of C, H and N were performed by the MEDAC LTD Brunel Science Centre, United Kingdom. IR spectra were recorded with a Nicolet Impact 420 FT-IR spectrometer using KBr pellets. ¹H NMR spectra were taken at 300 Hz with a Bruker-300 spectrometer using CDCl₃ as solvent. Mass spectrometry was conducted on a ThermoFinnigan MAT 95 XL spectrometer.

Syntheses

2-Pyridinyl-3-pyridinylmethanone (L). Under the protection of N₂, 2-bromopyridine(6.32 g) in 20 ml anhydrous diethyl ether were added slowly into 25.2 ml n-butyllithium (15% solution in hexane) in 50 ml anhydrous diethyl ether at −50 °C. After addition was complete, the mixture was stirred at −50 °C for 0.5 hour. After cooling to −90 °C, methyl nicotinate (5.4 g) in 50 ml anhydrous diethyl ether was added to the mixture with vigorous stirring within 30 minutes. After further stirring at −90 °C for another 30 minutes, the solution was allowed to slowly warm to room temperature and stand overnight. The reaction solution was quenched with 40 ml HCl solution in water and methanol (water/methanol/conc. HCl = 5 : 5 : 1) at 0 °C and extracted with dichloromethane. The combined organic extract was then dried over anhydrous sodium sulfate, concentrated in vacuo, and purified by chromatography on silica-gel using ethyl acetate/hexane as the eluent. The colorless needle-like product of L (4.7 g) was obtained in 64% yield; mp 69.1–69.6 °C; ¹H NMR (300 M Hz, DMSO-d₆): δ 9.13 (d, J = 2.1 Hz, 1H), 8.80 (dd, J₁ = 1.5, J₂ = 4.8 Hz, 1H), 8.75 (d, J = 4.8 Hz, 1H), 8.34 (td, J₁ = 2.1, J₂ = 7.8 Hz, 1H), 8.11 (s, 1H), 8.10 (s, 1H), 7.72 (dd, J₁ = 4.8, J₂ = 7.5 Hz, 1H), 7.58 (dd, J₁ = 4.8, J₂ = 7.8 Hz, 1H); IR (KBr): 1668s, 1578s, 1471w, 741m, 704s cm⁻¹; MS m/z: 185.07 ([MH]⁺); Anal. for C₁₁H₈N₂O: Calcd. C 71.73, H 4.38, N 15.20; Found C 71.65, H 4.37, N 15.19%.

{[Ag(L)]NO₃}_∞ (1). A mixture of L (37 mg, 0.2 mmol) and AgNO₃ (34 mg, 0.2 mmol) were dissolved in 2 ml acetonitrile with stirring at room temperature. After filtration, the resulting yellow solution was layered by diethyl ether to give yellow crystals of 1 in 29% yield. Anal. for C₁₁H₈N₃O₄Ag: Calcd. C 37.31, H 2.28, N 11.86; Found C 37.28, H 2.28, N 11.79%. IR (KBr): 1666s, 1581m, 1471w, 1412m, 1383vs, 1311m, 1250w, 1142w, 1088w, 1034w, 993w, 941w, 829w, 800w, 742w, 702m, 667w, 619w cm⁻¹.

{[Ag(L)]PF₆}_∞ (2). L (37 mg, 0.2 mmol) and AgPF₆ (50 mg, 0.2 mmol) were dissolved in a mixed solvent of 2 ml methanol and 2 ml acetonitrile with stirring at room temperature. The resulting solution was filtered and allowed to stand for two weeks to give colorless crystals of 2 in 84% yield. Anal. for C₁₁H₈F₆N₂OPAg: Calcd. C 30.23, H 1.85, N 6.41; Found C 30.27, H 1.78, N 6.35%. IR (KBr): 1666s, 1581m, 1471w, 1417w, 1335w, 1311m, 1292w, 1250w, 1142w, 1088w, 1038w, 995w, 943w, 837brs, 742w, 702m, 665w, 619w, 561s cm⁻¹.

{[Ag(L)]BF₄}_∞ (3). Colorless crystals of 3 were obtained in the same manner as for 2 in 47% yield. Anal. for C₁₁H₈BF₄N₂OAg: Calcd. C 34.87, H 2.13, N 7.39; Found C 35.32, H 2.17, N 7.50%. IR (KBr): 1666s, 1581m, 1471w, 1417w, 1385w, 1311m, 1250w, 1082brs, 941w, 831w, 798w, 742w, 704m, 667w, 621w, 526w cm⁻¹.

{[Ag(L)]ClO₄}_∞ (4). Reaction of L (37 mg, 0.2 mmol) and AgClO₄ (40 mg, 0.2 mmol) in a mixed solvent of 4 ml methanol and 4 ml acetonitrile at room temperature gave a light-yellow solution, which after filtration and standing for two weeks deposited light-yellow crystals of 4 in 90% yield. Anal. for C₁₁H₈ClN₂O₅Ag: Calcd. C 33.75, H 2.06, N 7.15; Found C 33.70, H 2.00, N 7.02%. IR (KBr): 1666s, 1581m, 1471w, 1435w, 1417w, 1334w, 1313m, 1294w, 1250w, 1088brs, 993w, 939w, 829w, 798w, 742w, 704m, 667w, 629m cm⁻¹.

{[Ag(L)]CF₃CO₂}_∞ (5). L (37 mg, 0.2 mmol) and AgCF₃CO₂ (44 mg, 0.2 mmol) were dissolved in a mixed solvent of 1 ml methanol and 1 ml acetonitrile at room temperature. After filtration, the resulting solution was diffused by diethyl ether to give yellow crystals of 5 in 37% yield. Anal. for C₁₃H₈F₃N₂O₃Ag: Calcd. C 38.55, H 1.99, N 6.91; Found C 38.45, H 2.00, N 6.92%. IR (KBr): 1670s, 1581m, 1471w, 1429w, 1311m, 1250w, 1207s, 1134m, 1086w, 1036w, 993w, 941w, 837w, 803w, 744w, 704m, 667w, 621w cm⁻¹.

{[Cu(L)(CH₃CN)(H₂O)(ClO₄)]ClO₄}_∞ (7). L (37 mg, 0.2 mmol) and Cu(ClO₄)₂·6H₂O (74 mg, 0.2 mmol) were dissolved in 5 ml CH₃CN with stirring. After 30 minutes, the reaction solution was filtered and allowed to stand for several weeks to give green crystals of 7 in 37% yield. Anal. for C₁₃H₁₃Cl₂N₃O₁₀Cu: Calcd. C 30.88, H 2.59, N 8.31; Found C 30.66, H 2.65, N 8.14%. IR (KBr): 3465m, 1662m, 1630m, 1599m, 1575w, 1425w, 1325m, 1265w, 1152vs, 1119vs, 1085vs, 1025m, 962w, 829w, 802w, 756w, 698m, 677w, 629s cm⁻¹.

{[Co(L)(H₂O)₃](ClO₄)₂·2H₂O}_∞ (8). L (37 mg, 0.2 mmol) and Co(ClO₄)₂·6H₂O (37 mg, 0.2 mmol) were dissolved in a mixed solvent of 2 ml CH₃CN and 2 ml CH₃OH with stirring. After 30 minutes, the reaction solution was filtered and allowed to stand to give orange crystals of 8 after several weeks. Yield: 57%. Anal. for C₁₁H₁₈Cl₂N₂O₁₄Co: Calcd. C 24.83, H 3.41, N 5.26; Found C 25.14, H 2.88, N 5.19%. IR (KBr): 3361brs, 1672m, 1630s, 1589s, 1479w, 1425m, 1335s, 1263m, 1203w, 1143vs, 1113vs, 1088vs, 1034w, 1018m, 956m, 941w, 833w, 798w, 756w, 704m, 675m, 627s cm⁻¹.

{[Zn(L)(H₂O)₃](CF₃SO₃)₂·H₂O}_∞ (9). Colorless crystals of 9 were obtained using the same method as for 8 in 37% yield. Anal. for C₁₃H₁₆F₆N₂O₁₁S₂Zn: Calcd. C 25.19, H 2.60, N 4.52; Found C 25.08, H 2.30, N 4.55%. IR (KBr): 3512brs, 1672s, 1632s, 1603s, 1477w, 1429m, 1281vs, 1254vs, 1171vs, 1038vs, 953m, 853w, 800w, 754m, 704m, 646s, 579m, 521m cm⁻¹.

The synthesis and characterization of conformational polymorphs 6 (space group P2₁/c) and 10 (space group P4₁2₁2 and P4₃2₁2) with the same structural formula {[Ag(L)]CF₃SO₃}_∞ have been reported in a recent communication.¹⁴

X-Ray diffraction studies

Single-crystal X-ray diffraction measurements were carried out on a Bruker SMART 1000 CCD diffractometer operating at 50 KV and 30 mA using Mo-Kα radiation (λ = 0.71073 Å). Each selected crystal was mounted inside a Lindemann glass capillary for data collection at 293 K. Data collection and reduction were performed using the SMART and SAINT software.¹⁵ An empirical absorption correction was applied using the SADABS program.¹⁶ All the structures were solved by direct methods and refined by full-matrix least squares on F² using the SHELXTL program package.¹⁷ All non-hydrogen atoms were subjected to aniostropic refinement, and all hydrogen atoms except those of water molecules were added in idealized positions and refined isotropically. Hydrogen atoms of aqua ligands in complexes 7–9 and hydrogen atoms of the lattice water molecules in complex 9 were located from difference electronic Fourier maps. Hydrogen atoms of the lattice water molecules in complex 9 could not be located. The corresponding counter anions in complexes 1–5 and 7–8 were treated using a disordered model. Crystal data and details of the refinement for 1–10 are summarized in Table 1.

CCDC reference numbers 278821–278828.

See http://dx.doi.org/10.1039/b510569b for crystallographic data in CIF or other electronic format.

Table 1 Crystallographic data for complexes 1–10

Complex	1	2	3	4	5	6	7	8	9	10
a R 1 = Σ‖Fo| − |Fc‖/Σ|Fo|. b wR 2 = {Σ[w(Fo^2 − Fc^2)^2]/Σ[w(Fo^2)^2]}^1/2.
Empirical formula	C11H8N3O4Ag	C11H8F6N2OPAg	C11H8BF4N2OAg	C11H8ClN2O5Ag	C13H8F3N2O3Ag	C12H8F3N2O4SAg	C13H13Cl2N3O10Cu	C11H18Cl2N2O14Co	C13H16F6N2O11S2Zn	C12H8F3N2O4SAg
M	354.07	437.03	378.87	391.51	405.08	441.13	505.70	532.10	619.77	441.13
Crystal size/mm	0.46 × 0.36 × 0.25	0.20 × 0.18 × 0.09	0.45 × 0.17 × 0.16	0.43 × 0.24 × 0.17	0.45 × 0.32 × 0.30	0.45 × 0.23 × 0.15	0.43 × 0.43 × 0.38	0.34 × 0.25 × 0.14	0.50 × 0.42 × 0.24	0.34 × 0.30 × 0.17
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Orthorhombic	Monoclinic	Monoclinic	Tetragonal
Space group	P21/c	P21/c	P21/c	P21/c	P21/c	P21/c	Pbca	P21/n	P21/c	P41212
a/Å	7.9514(6)	8.6733(7)	8.0172(5)	8.1126(9)	7.7407(5)	9.5431(6)	18.919(1)	13.0805(8)	10.1623(5)	10.2355(4)
b/Å	12.1163(9)	12.193(1)	12.4067(8)	12.513(1)	13.6362(8)	12.4438(8)	8.1263(5)	11.1580(7)	11.5785(5)	10.2355(4)
c/Å	12.4823(9)	13.270(1)	13.2083(9)	13.418(2)	13.5921(9)	13.3198(9)	24.802(1)	13.8114(8)	19.8500(9)	28.105(2)
α/°	90	90	90	90	90	90	90	90	90	90
β/°	107.077(1)	101.673(2)	110.355(1)	111.047(2)	109.424(1)	105.165(1)	90	95.398(1)	97.017(1)	90
γ/°	90	90	90	90	90	90	90	90	90	90
V/Å^3	1149.5(2)	1374.4(2)	1231.8(1)	1271.2(3)	1353.0(2)	1526.7(2)	3813.0(4)	2006.9(2)	2318.1(2)	2944.4(3)
Z	4	4	4	4	4	4	8	4	4	8
D calc/g cm^−3	2.046	2.112	2.043	2.046	1.989	1.919	1.762	1.761	1.776	1.990
μ(Mo-Kα)/mm^−1	1.768	1.654	1.681	1.816	1.539	1.509	1.485	1.195	1.344	1.565
F(000)	696	848	736	768	792	864	2040	1084	1248	1728
Reflections collected	4920	7267	6505	6706	7177	10599	19358	10620	12107	15946
Independent reflections (Rint)	2035 (0.0243)	2422 (0.0510)	2165 (0.0327)	2241 (0.0367)	2388 (0.0284)	3927 (0.0338)	3354 (0.0393)	3535 (0.0372)	4087 (0.0350)	2596 (0.0193)
Observed reflections [I > 2σ(I)]	1791	1895	1909	1834	1974	2687	2742	2846	3625	2490
Parameters	199	226	217	217	226	208	318	340	324	208
Goodness-of-fit	1.136	1.119	1.066	1.206	1.066	1.049	1.169	1.131	1.120	1.080
R 1 [I > 2σ(I)]^a	0.0368	0.0525	0.0365	0.0573	0.0350	0.0536	0.0466	0.0502	0.0460	0.0229
wR 2 (all data)^b	0.0966	0.1399	0.0988	0.1526	0.0934	0.1669	0.1235	0.1438	0.1322	0.0582

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Results and discussion

X-Ray crystallographic characterization showed that complexes 1–9 exhibit a single-strand helical chain structure winding around a 2₁ axis whereas 10 has a 4₁ axis.¹⁴ The key tectonic element that leads to the wide adaptation of L to a wide variety of metal ions and counter anions in forming helical architectures can be ascribed to its flexible conformation. Rotation of the 2- and 3-pyridyl rings about the respective C–C(carbonyl) single bonds result in a wide range of bridging conformations (angular parameters θ and ϕ, calculated from the torsion angle M–N1⋯N2–M′) corresponding to the size variation of both the metal ions and the counter anions (Fig. 1). Ligand L exhibits two kinds of conformation in this series of helical complexes: in conformation A, the two pyridyl nitrogen atoms are anti-related to generate an obtuse bridging angle θ, while conformation B has a syn-related pair to form an acute bridging angle ϕ. Conformation A is more frequently observed in the metal complexes of L, in which variation of the bridging angle θ yields spring-like helices of different pitch lengths. Compared to the linear coordination geometry of the silver(I) ion,¹⁸ the metal(II) ions Cu²⁺, Co²⁺ and Zn²⁺ are each trans-coordinated by the nitrogen atoms of two independent ligands L, its octahedral coordination sphere being completed by the carbonyl oxygen and supplementary ligands such as aqua ligand and acetonitrile molecule. Selected bond lengths and angles, together with selected shape parameters of complexes 1–10 are listed in Table 2.

Table 2 Atom labeling of the 2-pyridinyl-3-pyridinylmethanone ligand (L) and selected bond lengths and bond angles, together with selected shape parameters for complexes 1–10

Complex	1	2	3	4	5	6	7	8	9	10
M–N bond lengths/Å	2.177(3)	2.160(5)	2.151(4)	2.162(6)	2.212(3)	2.163(4)	1.997(4)	2.121(4)	2.139(3)	2.170(3)
	2.189(3)	2.170(5)	2.175(4)	2.179(6)	2.242(3)	2.176(4)	1.997(4)	2.123(4)	2.141(3)	2.194(3)
N1–M–N2′ angle/°	172.5(1)	177.7(2)	175.9(1)	174.8(2)	167.4(1)	176.4(2)	167.5(2)	166.4(1)	162.7(1)	157.9(1)
M⋯O (carbonylic) distance/Å	2.838(4)	2.697(5)	2.692(3)	2.720(6)	2.698(3)	2.757(4)	2.008(3)	2.127(3)	2.244(2)	2.744(3)
C5–C6–C8 angle/°	118.5(3)	119.0(5)	119.1(4)	119.9(6)	121.0(3)	118.1(4)	124.4(4)	123.8(4)	122.1(3)	119.5(3)
N1–C5–C6–O torsion angle/°	31.1	26.9	27.6	26.9	−26.2	−24.1	2.2	−3.2	−9.7	23.2
O–C6–C8–C7 torsion angle/°	−140.2	−134.2	−140.0	−140.5	142.9	136.0	38.6	140.6	141.9	−133.1
Dihedral angle of the two pyridyl rings/°	65.2	68.2	64.2	63.8	57.3	65.8	44.5	40.9	44.1	69.3
M–N1⋯N2–M′ torsion angle (θ or ϕ)/°	−109.2	−111.5	113.3	113.0	119.4	−111.1	43.9	146.1	145.9	−95.7
Pitch length of helix/Å	12.116	12.193	12.407	12.513	13.636	12.444	8.126	11.158	11.578	28.105

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Fig. 1 Two kinds of bridging conformations of 2-pyridinyl-3-pyridinylmethanone (L) in its coordination complexes, leading to infinite helices with different helical pitch lengths.

Reaction of L with various silver(I) salts gives rise to complexes 1–6 formulated as {[Ag(L)]X}_∞ (X = NO₃, 1; PF₆, 2; BF₄, 3; ClO₄, 4; CF₃CO₂, 5; CF₃SO₃, 6). All six complexes comprise an infinite single-strand helix constituted from the alternate linkage of silver(I) cations and ligands L, and each helical pitch contains two silver(I)–L units (Fig. 2). The silver(I) atom adopts an almost linear coordination geometry while L acts as a bidentate bridging ligand with its carbonyl oxygen atom weakly interacting with the silver(I) atom at a distance ranging from 2.69 Å to 2.84 Å. The cationic helix is charge-balanced by two parallel columns of the corresponding counter anions that are pinched fully or partially into the grooves. The right-handed and left-handed helixes co-exist in each silver(I) complex, and they are arrayed in an alternate fashion to generate a racemic crystal.

Fig. 2 Part (a) to (f) show the infinite helical chains with imbedded counter anions in complexes 1–6, respectively. The disordered atoms in the anions are omitted for clarity. Color scheme: C, light gray; H, white; O, red; N, deep blue; Ag, purple; Cl, light green; F, green; S, yellow; P, deep green; B, orange.

Comparison of the crystal structures of complexes 1–6 indicates that the helical pitch lengths can be correlated with the size of the embedded counter anions (Fig. 2).¹⁹ In another words, the pitch length of the spring-like helix can be tuned by the incorporation of counter anions with different sizes. With NO₃⁻ as the smallest anion within this series, complex 1 exhibits a helical pitch length of 12.116 Å, being significantly shorter than that of complex 5 (13.636 Å) with the larger CF₃CO₂⁻ anion.¹⁹ The PF₆⁻, BF₄⁻ and ClO₄⁻ anions of intermediate sizes lead to similar medium pitch lengths at 12.193, 12.407 and 12.513 Å for 2, 3 and 4, respectively. It is notable that, with CF₃SO₃⁻ as the largest within this series of anions, complex 6 only exhibits a helical pitch length of 12.444 Å. This apparent anomaly can be attributed to the fact that the CF₃SO₃⁻ anion is only partially mounted in the helix with the CF₃ group exposed (Fig. 2f). Hence the “effective volume” of the CF₃SO₃⁻ anion incorporated in the helix is similar to those of the PF₆⁻, BF₄⁻ and ClO₄⁻ anions. Therefore, the pitch length of a spring-like {[Ag(L)]X}_∞ 2₁ helical chain is dominated not only by the size of the inlaid counter anion but also by its mode of fitting to the grooves inside the helix.

This self-adjustment of pitch length of the helix is realized mainly by adaptive variation of the bridging angle (θ) in ligand L. For complex 1, the value of θ is −109.2° for the smallest NO₃⁻ anion in this series, while in complex 5 the corresponding value is 119.4° for the CF₃CO₂⁻ anion with the largest incorporated volume. Taking into account the partial insertion of CF₃SO₃⁻ into the groove of the helix, the value of θ is similar for complexes 2, 3, 4 and 6, being −111.5, 113.3, 113.0 and −111.1°, respectively. The full incorporation of the larger CF₃CO₂⁻ anion into the helix leads to significant structural differences of complex 5 compared with complexes 1–4 and 6 (Table 2). The angle between the two single bonds about the carbonyl group is 121.0° for 5, bending in the opposite manner as compared to the value of 118.1–119.9° for the other five complexes. Furthermore, the dihedral angle between the pyridyl rings of ligand L in complex 5 (57.3°) is much smaller than those of the others (in the range 63.8–68.2°), being consistent with its longer pitch length. Concerning the coordination environment of the silver(I) atom, the Ag–N bond length is significantly longer for complex 5 (2.212 and 2.242 Å) as compared with those in other complexes (ranging from 2.151 to 2.189 Å), and the N–Ag–N angle deviates from linearity more for 5 (167.4°) than those in other complexes (172.5 to 177.7°).

Though complexes 1–6 all crystallized in space group P2₁/c, they exhibit two kinds of packing modes in their crystal structures: mode I for 1, 2 and 6; and mode II for 3–5, as shown in Fig. 3. Both modes comprise layers that are assembled from infinite single-strand helices by weak interaction between the silver(I) atom and the counter anion (ranging from 2.7 to 3.6 Å), and such layers are further linked into a three-dimensional network by C–H⋯anion interactions. The nature of the counter anion obviously plays an important role in dictating the packing mode that is preferentially adopted in a silver(I) complex, but the interplay of various weak intermolecular interactions makes it difficult to draw reliable predictions for specific anions.

Fig. 3 Two kinds of packing modes in complexes 1–6: mode I for 1, 2 and 6; and mode II for 3–5. Each parallelogram represents the projection of a single helix. The small circles in dark gray and large circles in light gray represent silver atoms and anions, respectively. The dotted lines indicate weak interactions between the silver cations and anions.

In complex 7, ligand L bridges consecutive copper(II) atoms in syn mode B, as opposed to the anti mode A observed in complexes 1–6 (Fig. 1), yielding a different infinite single-strand helical chain. In this chain, the slightly-distorted pyramidal coordination requirement of the copper(II) atom is satisfied by two consecutive L ligands acting in chelating and monodentate modes, an acetonitrile molecule and an aqua ligand (Fig. 4). At the opposite side of the pyramidal apex, an oxygen atom from a perchlorate group has a weak interaction with the copper(II) center at a distance of 2.599(4) Å. Hence the coordination geometry of the copper(II) center can also be seen as distorted octahedral. A similar helical structure has been reported by Feringa et al.,^6a in which each copper(I) atom is coordinated by two pyridine N atoms and two imine N atoms in a distorted tetrahedral geometry. It is noteworthy that intramolecular aqua–perchlorate hydrogen bonding (O8A⋯O1w 2.849 Å, O8A⋯H1wA 2.048 Å and O1w–H1wA⋯O8A 165.3°) rigidly fixes the helical structure (calculated for the disordered oxygen atom of the weakly ligated perchlorate anion, which is farthest away from the aqua ligand), as illustrated in Fig. 4(b).²⁰Via these hydrogen bonds, the crystallographically equivalent perchlorate anions are pinched inside the cylindrical helix in two columns. The second type of perchlorate anion is arranged in external columns that charge-balance the cationic helix. The pitch length of the helix in complex 7 is significantly shorter at 8.126 Å due to the specific ligation conformation adopted by L. The P- and M-helixes are packed alternately and cross-linked by C–H⋯O(perchlorate) hydrogen bonds, so that 7 exists as a racemate in the bulk material.

Fig. 4 (a) Infinite cationic helical chain in complex 7. (b) Helical structure with triangular motif consolidated by O_w–H⋯O(perchlorate) hydrogen bonding in 7. Color scheme: C, light gray; H, white; O, red; N, deep blue; Cu, turquoise; Cl, green.

In complexes 8 and 9, L exhibits a similar anti linking mode A as that in complexes 1–6, in contrast to the syn mode in 7 (Fig. 1), bridging the cobalt(II) or zinc(II) atoms into an infinite cationic single-strand helical chain (Fig. 5). The helices in 8 and 9 are very similar: besides the three binding sites from a pair of L ligands, both complexes have their octahedral metal coordination sphere completed by a facial group of the aqua ligands. The P- and M-helical chains are cross-linked alternately to form a racemic crystal via C–H⋯O(perchlorate) hydrogen bonds and inter-chain hydrogen bonds involving these aqua ligands. The counter anions and lattice water molecules lie outside the helical chains in 8 and 9, leading to the pitch lengths of 11.158 Å and 11.578 Å, respectively. With no pinched anion inside the helix, the pitch length is mainly dominated by the size of the metal cation, as evidenced by the shorter length for the cobalt(II) helix in 8 compared to the zinc(II) helix in 9 (Table 2). This is also supported by the significantly longer M(II)–N bond lengths for 8 (2.121(4) and 2.123(4) Å) than those for 9 (2.139(3) and 2.141(3) Å), as well as a slightly larger value of θ in 8 (146.1°) than that in 9 (145.9°).

Fig. 5 Part (a) and (b) show the cationic infinite helical chains in complexes 8 and 9, respectively. Color scheme: C, light gray; H, white; O, red; N, deep blue; Co, turquoise; Zn, green.

Three kinds of coordination mode of L are observed in a series of helical complexes, namely bidentate anti-bridging (in 1–6, and 10), tridentate syn-bridging (in 7) and tridentate anti-bridging (in 8 and 9) (Fig. 6). These coordination modes involve two kinds of bridging conformation A and B that lead to four types of infinite helical chains which can be easily recognized from views along the helical axis (Fig. 7). The cross-section of the infinite helix in complexes 1–6 and 8–9 are rectangular and that in complex 7 is rhombic, while that in complex 10 is square. Different bridging modes of L, as described by the obtuse bridging angle θ in conformation A as compared with the acute bridging angle ϕ in conformation B, account for the fact that the helical pitch lengths in complexes 1–6 and 8, 9 (11.158–13.636 Å) are correspondingly much longer than that in complex 7 (8.126 Å). The longer pitch lengths of complexes 1–6 (12.116–13.636 Å) compared to those of complexes 8 and 9 (11.158 Å and 11.578 Å) can be rationalized on the basis of both the larger cation size and the anion embedment in complexes 1–6.

Fig. 6 Coordination modes of 2-pyridinyl-3-pyridinylmethanone (L).

Fig. 7 Perspective view of the helix along the helical axis: (a) for complexes 1–6; (b) for complex 7; (c) for complexes 8 and 9; and (d) for complex 10.

Studies on the IR spectra of this series of helical complexes revealed that the vibrations of ligand L were affected only to a minor extent in its coordination complexes. As to the C=O stretching mode, L exhibits a sharp and strong peak at 1668 cm⁻¹ while all its silver(I) complexes show the same peak at 1666 cm⁻¹ except for the value of 1670 cm⁻¹ in complex 5, and its metal(II) complexes 7–9 show this peak at 1662, 1672 and 1672 cm⁻¹, respectively.

Conclusion

The present study has demonstrated that 2-pyridinyl-3-pyridinylmethanone (L) is a versatile building block for the generation of single-strand helical structures with metal cations and counter anions of different sizes under mild conditions. In the series of silver(I) helical complexes 1–6, with the counter anions pinched inside the 2₁ helix, the pitch lengths correlate with the “effective” anion volume embedded into the grooves. In the cases of complexes 7 and 10, ligand L takes a particular bridging mode in forming specific helical architectures. In complexes 8 and 9 that comprise helical chains with no embedded anion, the pitch lengths are dictated by the size of the metal cations. The present strategy of tuning the helical pitch length by variation of counter anions and even metal cations may be of value in the development of synthetic methodologies in crystal engineering and materials science.

Acknowledgements

This work is financially supported by the Hong Kong Research Grants Council (Ref. No. CUHK 402003).

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