CdII-Mediated Efficient Synthesis and Complexation of Asymmetric

Oct 9, 2014 - State Key Laboratory of Coordination Chemistry, Nanjing University, Hankou ... with corrections to some chemical formulas and the abstra...
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CdII-Mediated Efficient Synthesis and Complexation of Asymmetric Tetra-(2-pyridine)-Substituted Imidazolidine Yan-Jun Ou,† Zhi-Peng Zheng,†,‡ Xu-Jia Hong,†,‡ Lin-Tao Wan,† Lei-Ming Wei,† Xiao-Ming Lin,*,† and Yue-Peng Cai*,† †

School of Chemistry and Environment, Guangzhou Key Laboratory of Materials for Energy Conversion and Storage, and Theoretical Chemistry of Environment, Ministry of Education, South China Normal University, Guangzhou 510006, P.R. China ‡ State Key Laboratory of Coordination Chemistry, Nanjing University, Hankou Road 22, Nanjing, 210093 P.R. China S Supporting Information *

ABSTRACT: One convenient CdII-mediated C−C/C−N bond-forming strategy toward asymmetric tetra-(2-pyridine)substituted imidazolidine (L1), the basic framework of several natural products with bioactivity, has been found for the first time. In the reaction of tridentate N3-set neutral pyridine-type Schiff base ligand (L) possessing a [−HCNCH2−] linkage with CdCl2 at 70 °C for 3 days, one two-dimensional 44 topological layer [Cd3L1Cl6]n (1) could be obtained, in which ligand L1 resulted from [3 + 2] C−C/C−N asymmetric coupling dimerization of L. When equimolar amounts of NaSCN and NaNO3 were added to the reaction mixtures, one-dimensional chain [Cd2L1(SCN)Cl3]n (2) and zero-dimensional dinuclear Cd2L1(NO3)4(MeOH) (3) containing the same ligand L1 were also generated under the same reaction conditions, respectively. Obviously, the medium of the Cd2+ ion plays the key role in solvothermal in situ formation of ligand L1. Moreover, new ligand tetra-substituted imidazolidine (L1) could be obtained effectually from all three complexes 1−3 through the reactions of those compounds with Na2S.

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Meanwhile, the recent reports also show imines with the (−CHN−CH−)- moiety may be cycloadded into symmetric tetra-substituted piperazines with the central six-membered ring catalyzed by Fe2+ and Ti2+ ions.18−21 Accordingly, it is very meaningful to explore the formation conditions and coordination behaviors of asymmetric tetra-substituted imidazolidines by solvothermal in situ metal-catalyzed reaction of simple Schiff base ligands. Herein we report the synthesis and its CdIIcoordination chemistry of a new asymmetric imidazolidine containing 2-pyridine tetra-substitutents (L1) with about 90% yield, produced from cadmium(II)-mediated coupling cycloaddition reaction of the Schiff base N-(2-pyridylmethyl)pyridine-2-carbaldimine (L) (Scheme 1), in which three new resulting CdII coordination polymers, namely, [Cd3L1Cl6]n (1), [Cd2L1(SCN)Cl3]n (2) and Cd2L1(NO3)4(CH3OH) (3) (L1 =

he rapid and facile construction of structurally diverse imidazolidines via changing its substituted groups is very important in various fields, mainly deriving from the following considerations: (i) imidazolidines can act as intermediate in the nucleotide biosynthesis, and some metal complexes are confirmed to have good activity as cytotoxic metallopharmaceuticals.1−6 (ii) They are also significant building blocks, meanwhile, in bioactive compounds and carriers of the carbonyl compounds with pharmacological activity, and some of metal complexes are potential chemotherapeutic agents for DNA cleavage.7−10 (iii) Moreover, their transition-metal complexes were widely studied in the field of organic light-emitting diode (OLEDs),11,12 etc. Symmetric disubstituted imidazolidines were prepared early by Bischoff in 1898,13,14 and then the asymmetric disubstituted imidazolidines were also reported by Kliegel in 1977.15 In contrast, tetra-substituted imidazolidines are very rare. Grigg and co-workers16 in 1989 reported the first example of tetrasubstituted imidazolidine (asymmetric) obtained by thermal 1,2-prototropy and cycloaddition reaction of imine RCHNZH (R = 2-CH-pyridyl, Z = 3-CH-pyridyl) containing a 1.3:1 mixture of trans- and cis-isomers with 60% yield, in which two pyridine rings are in the 2-position and the remaining two ones are in the 3-position. Recently, Musie et al.17 found that Cu(II)promoted asymmetric tetra-substituted imidazolidine ring formation from the Schiff base ligand of pyridine-2-imine benzoate via an in situ metal−ligand reaction in an alkaline environment. © 2014 American Chemical Society

Scheme 1. Asymmetric Imidazolidine L1 from Solvothermal in Situ Ligand Generation of Pyridine-Type Schiff Base L Mediated by Cd2+ Ion

Received: June 21, 2014 Revised: October 4, 2014 Published: October 9, 2014 5339

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2,2′,2″-(1-(pyridin-2-yl-methyl) imidazolidine-2,4,5-triyl-tripyridine), are involved. To the best of our knowledge, this is the first example of a Cd2+-mediated C−C/C−N bond-forming strategy toward tetra-substituted imidazolidine in situ from a pyridine-type Schiff base. Pale-yellow crystals of 1−3 (ca. 75−88% yield) were formed from the in situ solvothermal reaction of N-(2-pyridylmethyl)pyridine-2-carbaldimine (L) with CdCl2/CdCl2 + NaSCN/ CdCl2 + NaNO3 in the mixed solvents of methanol and pyridine (v:v = 3:1) at 70 °C or so for 3 days (see Scheme 1). The component diversification of the resulting CdII-L1 nets can be tuned simply via adding auxiliary anions such as SCN− and NO3−. Elemental analysis and PXRD (Figure S1 in Supporting Information) proved the phase purity of the bulk materials. Compounds 1−3 are air-stable and insoluble in water and most organic solvents. One of the most interesting features for the synthesis is the direct dehydrogenative [3 + 2] coupling cyclization involving C−C and C−N bond-forming from the Schiff base N-(2pyridylmethyl)-pyridine-2-carbaldimine mediated by Cd2+. It has been reported previously that dehydrogenative coupling of Schiff base N-(2-pyridylmeth-yl)-pyridine-2-carbaldimine under solvothermal conditions leads to the functional tetradentate ligand of tetra-substituted piperazine.20 In fact, in the process of optimizing synthetic conditions of complexes 1−3, we found that the same reactants at 70 °C for 3 days could produce three corresponding Cd-complexes containing tetra-substituted 2pyridyl imidazolidine, which is very difficultly obtained by common organic synthetic reactions. In order to obtain 1, a reaction containing equimolar amounts of L with CdCl2 was carried out in the mixed solvents of methanol and pyridine at 70 °C for 3 days. 1 crystallized as pale yellow crystals suitable for X-ray structure analysis. The Xray diffraction analysis reveals that 1 exhibits a two-dimensional (2-D) layer-like structure (Figure 1c). The asymmetric unit of 1 involves three Cd2+ ions, one in situ formed L1 ligand, and six Cl− ions. The central Cd2 ion was coordinated by four bridging Cl− ions (Cl2, Cl3, Cl4, Cl5) and one terminal Cl− ion (Cl6), displaying distorted trigonal bipyramidal geometry. Both adjacent Cd1 and Cd3 ions were hexa-coordinated by four μ2-bridging Cl− ions (Cl1, Cl2, Cl3, Cl1a) and two nitrogen atoms from one ligand L1 (N4, N6) for Cd1 ion, and two μ2bridging chloride ions were hexa-coordinated by four μ2bridging Cl− ions (Cl1, Cl4, Cl5) and four nitrogen atoms from another ligand L1 (N1b, N2b, N3b, N5b) for Cd3 ion, adopting a slightly distorted octahedral coordination geometry. The Cd− N (2.277(4)−2.489(4) Å) and Cd−Cl (2.421(2)−2.802(2) Å (Table S2) bond lengths are typical for cadmium(II) compounds with similar pyridine ligands.22,23 The neutral ligand L1 coordinated in a tetradentate N4-chelate and bidentate N2-chelate fashions to two CdII ions thus forming four five-membered chelate rings (Figure 1a). Via 10 μ2bridging Cl atoms, six CdII ions (Cd1−Cd3, Cd1a-Cd3a) are connected to form the smallest neutral repeating unit Cd6L1Cl12 as depicted in Figure 1a, in which each repeating unit links four adjacent same units to form a 2-D layer with 44 topology as shown in Figure 1c,d. In addition, noticeable interlayer C−H···Cl contacts between the CH groups of the pyridyl rings (donors) and the coordinated chloride ions (acceptors) from another layer are determined (Table S3). Hence, a three-dimensional (3-D) supramolecular network is formed in the crystal packing through those interlayer C−H··· Cl hydrogen bonds (Figure S2).

Figure 1. In 1, (a) the coordination environment of three Cd2+ ions and coordination modes of ligand L1 and six Cl− ions in the smallest repeating unit Cd6L1Cl12, (b) chair-five numbered imidazolidine ring of ligand L1, (c) 2-D layer in the bc plane, (d) 44 topological network, where the smallest repeating unit Cd6L1Cl12 as one 4-connected node. The symmetric codes: (a) 1 − x, 1 − y, −z; (b) 0.5 − x, 0.5 + y, 0.5 − z; (c) 0.5 + x, 0.5 − y, 0.5 − z; (d) 0.5 − x, −0.5 + y, 0.5 − z.

For studying the role of anions upon the formation of ligand L1 in situ, NaSCN and NaNO3 were added to the reaction mixture of 1, and pale yellow and colorless block single crystals 2 and 3 were generated at 70 °C for more 3 days under the same reaction conditions, respectively. It is interesting to find that tetra-substituted imidazolidine derivative L1 from Cdmediated C−C/C−N coupling could still be observed under these conditions. Elemental analyses gave the components of two complexes: [Cd2L1(SCN)Cl3]n (2) and Cd2L12(NO3)4(CH3OH) (3), as characterized by X-ray single-crystal diffraction. The structure of 2 reveals an interesting 1-D wave-like chain with the asymmetric unit consisting of two crystallographically unique Cd1 and Cd2 atoms, three chlorides (Cl1, Cl2, and Cl3), one thiocyanate ion, and one ligand L1 as provided in Figure 2a. Cd1 is hexa-coordinated by four nitrogen atoms 5340

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Figure 3. Molecular structure of compound 3.

Similar to those in 1 and 2, ligand L1 acts as a sexa-dentate bridge to connect two CdII atoms, forming binuclear compound 3 with similar bond distances of Cd−O (2.253(5)−2.525(5) Å) and Cd−N (2.301(6)−2.536(5) Å).10 Similar to complexes 1 and 2, the 3-D supramolecular network of 3 constructed by hydrogen bonds C(O)-H···O is shown in Figure S4. From detail structural analyses of above three complexes 1−3 in the solid state, the effect of anions clearly does not play the key role in the C−C/C−N bond coupling from Schiff base L. Considering the proposed mechanism of nucleophilic cycloadditions in some related systems from the literature,7 we think that the formation of ligand L1 resulted from [3 + 2] cycloaddition of Schiff base N-(2-pyridylmethyl)-pyridine-2carbaldi-mine (L) mediated by Cd2+. It is well-known that the activity of a C−H bond in the α-position to an imino group −CHN−CH- is significantly increased after the imino nitrogen atom is ligated to a metal center.24−26 Pyridine, as the basic acceptor, has the ability to deprotonate the imino carbon-bound hydrogen atom to form 1,3-dipole. According to the classification of Huisgen, the 1,3-dipole of CN+-C− can be represented as X=Y+-Z− of allylic type.27 Meanwhile, to the best of our knowledge, the 1,3-dipolar cycloaddition is also a very effective strategy in the synthesis of five-membered heterocyclic compounds.28 Such the carbanion formed at C6′ attached C7 atom of the electrophilic carbon at CN to form a C−C bond. Then N2 with a negative charge attacks at C7′ and form the five-membered ring followed by protonation of the imidazolidine ring, finally leading to the occurrence of [3 + 2] cycloaddition and formation of cadmium-imidazolidine complex L1-Cd (Scheme 2). In 1−3, the core imidazolidine ring of ligand L1 exhibits a half-chair configuration with a mean distance of 0.825 Å between C18 atom and the least-squares plane (N3, N4, C12, C19) (Figure 4a). And sexa-dentate ligand L1 presents the same coordination mode of μ2-η1:η1:η1:η1:η1:η1, namely, L1 may be regarded as the fusion from a tetra-dentate tripod ligand and a bidentate ligand via imidazolidine ring (Figure 4b). More importantly, the tetra-substituted imidazolidine (L1) could be easily obtained in high yield from the reaction of three corresponding complexes 1−3 with Na2S. The resulting pale yellow semisolid was confirmed by EA, 1H NMR, and MS as denoted in Figures S5 and 6. Thermal stability of three compounds 1−3 (Figure S7, SI) was examined in a N2 atmosphere from room temperature to 800 °C. From the TG curve of compound 3, we can find that one methanol molecule was lost in the approximate temperature from 93 to 117 °C (calcd 3.56%, found 3.62%). When the

Figure 2. In 2, (a) the coordination environment of two Cd2+ ions and coordination modes of ligand L1 in the smallest repeating unit Cd2L1(SCN)Cl3, (b) 1-D chain along the b axis, (c) 1-D wave-like topological chain, where the smallest repeating unit Cd2(SCN)L1Cl3 as one 2-connected node. The symmetric codes: (a) 0.5 − x, −0.5 + y, 0.5 − z; (b) 0.5 − x, 0.5 + y, 0.5 − z.

from three pyridyl rings (N1, N2, N5) and one central imidazolidine ring (N3) of the same lignad L1 and two chlorine atoms (Cl2, Cl3) with slightly distorted octahedral coordination geometry. However, Cd2 is penta-coodinated in a square pyramidal coordination geometry by two pyridyl N atoms, one thiocyanate nitrogen atom, and two chlorides. Considering weak coordination of Cl3 atom to Cd2 center, Cd2 also has octahedral coordination geometry. Each L1 ligand links to two CdII centers together with one SCN− ion and three Cl− ions as the basic building unit (namely, the smallest repeating unit) for constructing one-dimensional (1-D) chain as depicted in Figure 2b,c. The Cd−N bond distances range from 2.266(5) to 2.463(3) Å, and the Cd−Cl bond lengths are in the range of 2.472(1)−2.623(1) Å, which are all within the normal range found in other reported experiments.22,23 Interchain hydrogen bonding C−H···Cl(S) interactions result in the assembly of a 3D supramolecular network in the ac plane (Figure S3). Compound 3 is a zero-dimensional (0-D) dinuclear structure with the asymmetric unit consisting of two crystallographically unique CdII atoms, four coordinated nitrate ions, one ligand L1, and one coordinated methanol molecule as shown in Figure 3. Different from those in 1 and 2, each Cd atom in 3 is heptacoordinated in monocapped octahedrons with N4O3 donor set for Cd1 and N2O5 donor set for Cd2, in which the coordination sphere around Cd1 consists of three pyridyl N atoms (N2, N3, N4), one imidazolidine N atom (N5), and three O atoms of two nitrate ions (O8, O10, O11). However, the heptacoordination of Cd2 atom is satisfied by one pyridyl N1 atom, another imidazolidine N6 atom, four nitrate O atoms (O1, O2, O4, O6), and one coordinated methanol O13 atom. 5341

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Scheme 2. Formation of tetra-Substituted 2-pyridyl Imidazolidine L1 in the Cadmium(II) Complexes

Obviously, this synthesis strategy may expand a wide interest in the construction of novel nitrogen-rich multidentate asymmetric imidazolidines relating to metal ion induced imine bond activation chemistry and relevant metal−organic frameworks or inorganic−organic hybrid materials with advanced luminescent and biological functions.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

Additional structural figures for the related compounds 1−3; the TG curves, solid-state emission, IR spectra, XRD patterns, and UV-absorption of compounds 1−3; and 1H NMR, 13C NMR, MS for ligand L1, tables of selected bond lengths, bond angles, crystal data, and structure refinement, as well as X-ray crystallographic files in CIF format for three compounds 1, 2, and 3. The CCDC reference numbers are CCDC 987346, 987347, and 987348 for 1, 2, and 3, respectively. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Authors

*(Y.-P.C.) E-mail: [email protected]. Fax: +86-020-39310. Tel: +86-020-39310383. *(X.-M.L.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial aid from the National Natural Science Foundation of P. R. China (Grant No. 91122008, 21471061, and 21071056), the Doctoral Program of Higher Education of China (Grant No. 20124407110007), Science and Technology Planning Project of Guangdong Province, Guangzhou, China (Grant No. 2013B010403024).

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Figure 4. In compounds 1−3, (a) the central five-membered imidazolidine rings with half-chair configuration, (b) the coordination mode of ligand L1.

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on October 13, 2014, and was reposted on October 22, 2014, with corrections to some chemical formulas and the abstract graphic on the first page of the Communication.

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