A series of temperature-dependent Cd^II-complexes containing an important family of N-rich heterocycles from in situ conversion of pyridine-type Schiff base

Abstract

An efficient method for the synthesis of a wide variety of N-rich heterocycles has been systematically explored. The synthetic protocol involves a solvothermal in situ metal–ligand reaction of pyridine-type Schiff base N-(2-pyridylmethyl)-pyridine-2-carbaldimine (L) with Cd²⁺ ions, resulting in the efficient formation of nine temperature-dependent Cd^II-complexes 1–9 supported by six types of N-rich heterocycles L^1–6. To the best of our knowledge, both the synthetic strategy with solvothermal in situ Cd^II-mediated Schiff base-conversion and N-rich heterocycle rings L^1–2 as well as cis-L⁶ are reported for the first time. Meanwhile, plausible in situ formation mechanisms of L^1–6 are also proposed.

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Introduction

Polyazaheteroaromatic compounds such as imidazo[1,5-a]-pyridine, pyrazine, piperazine, imidazolidine and imidazole, as well as their derivatives are well known for their importance in industry and are widely used as ligands in neutral and anionic forms in organic and organometallic chemistry.¹ Usually, the preparation of these organic nitrogen-containing heterocycles involves the formation of C–C and C–N bonds mediated by transition metals. It has been well documented that most stoichiometric and catalytic C–C and C–N bond-forming reactions mediated by the middle and late transition metals typically follow oxidative addition, transmetalation and reductive elimination pathways,² which can assemble different kinds of aza-heterocycles in cumbersome steps. Obviously, these traditional synthetic methods are hard to avoid, especially for nitrogen-rich complicated heterocycles, with unfavorable factors such as multi-steps, difficult purification and separation as well as low yield, etc.

On the other hand, the recently developed solvothermal in situ metal–ligand reaction³ might be an overwhelming strategy for preparation of complicated aza-heterocycles as well as the expected metal-complexes simultaneously. For example, some substituted derivatives of inmidazo[1,5-a]pyridine, pyrazine and piperazine were efficiently synthesized from simple starting materials catalyzed by Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Ti⁴⁺ ions,⁴ although occasionally the resulting products may be mixtures which are difficult to purify. By contrast, only one example involving tetra-substituted imidazolidine/tri-substituted imidazole rings has been reported so far. Musie et al. recently found that Cu(II)-promoted imidazolidine ring was formed from Schiff base ligand of pyridine-2-imine benzoate in alkaline environment.⁵

Accordingly, it is desirable to explore systematically the formation conditions and coordination chemistry of asymmetric tetra-substituted imidazolidine/pyrizine and symmetric imidazo-[1,5-a]pyridine/tri-substituted imidazole by solvothermal in situ metal-catalyzed reaction of simple Schiff base. Of interest is the ecologically compatible synthesis of new heterocyclic compounds that are not easily accessible and that can be applied as pharmaceuticals or as ligands for the development of new catalysts and advanced luminescent/biological functional materials.⁶

Based on the above considerations, a series of complicated aza-heterocycles, such as 2,2′,2′′-(1-(pyridin-2-ylmethyl)imidazoli-dine-2,4,5-triyl)tripyridine (L¹), pyridin-2-yl(2,4,5-tri(pyridin-2-yl)-1,2-dihydroimidazol-3-yl)methanone (HL²), 2,3,5,6-tetra-(pyridin-2-yl)pyrazine (L³), 2,2′,2′′-(1H-imidazole-2,4,5-triyl)tripyrid-ine (HL⁴), 1-pyridineimidazo-[1,5-a]-pyridine (L⁵) and 1-(1,2-di(pyridin-2-yl)-2-(3-(pyridin-2-yl)H-imidazo[1,5-a]pyrid-in-1-yl)-ethyl)-3-(pyridin-2-yl)H-imid-azo[1,5-a]pyridine (cis/trans-L⁶) were prepared by cadmium-catalyzed solvothermal in situ formation from pyridine-type Schiff base of N-(2-pyridyl-methyl)pyridine-2-carbaldimine (L). Here, we describe the syntheses and coordination chemistry of these six N-rich heterobicycles L^1–6 by Cd^II-catalytic ring closure starting from the Schiff base L (Scheme 1). In addition, plausible in situ formation mechanisms of L^1–6 are also proposed. To the best of our knowledge, both the synthetic strategies with solvothermal in situ Cd^II-mediated Schiff base-conversion and ligands L^1–2 as well as cis-L⁶ are reported for the first time.

Scheme 1 Construction strategies of nine Cd^II complexes 1–9 containing six ligands L¹–L⁶ from in situ conversion of pyridine-type Schiff base N-(2-pyridylmethyl)pyridine-2-carbaldimine (L).

Results and discussion

Synthesis

The pyridine-type Schiff base L was prepared readily from the condensation of equivalent amounts of 2-pyridine formaldehyde and 2-pyridylethylamine. A series of reactions catalyzed by cadmium(II) from L were performed in situ as shown in Scheme 1, finally pale-yellow/reddish Cd^II-compounds 1–9 were obtained at different temperatures for three days. The temperature factor is essential for Cd-complexes and in situ formed aza-heterocycles, mainly deriving from these cycloaddition reactions in present cases being subject to tuning of the reaction temperature.

Structure and formation mechanism of complexes 1–4

The reaction of ligand L with an equimolar amount of CdCl₂ at less than 60 °C for three days, gave 1-D compound [(Cd₆(L¹)₂Cl₁₃)-(Cd(L¹)Cl)]_n·2H₂O (1). In the presence of H₂O₂, however, 0-D binuclear compound [Cd(L²)Cl]₂ (2) with a carbonyl group in ligand HL² was obtained. The crystal structure of the pale yellow block compound 1 is shown in Fig. 1, exhibiting a 1-D zigzag-chain structure. The asymmetric unit of 1 consists of one six nuclear anion [Cd₆Cl₁₃(L¹)₂]⁻ unit (Fig. 1a), one cation [CdL¹Cl]⁺ unit (Fig. 1c), and two water molecules. In the anionic part, six Cd^II ions present three types of coordination mode, namely, a tetrahedral geometry for Cd6 atom supported by four chloride ions (one μ₂-Cl10, and three terminal Cl11, Cl12, Cl13), a distorted square pyramid for Cd4 atom surrounded by three chloride ions (two μ₂-Cl7/Cl9, and one terminal Cl8) and two nitrogen atoms (N10, N11) from the same ligand L¹. The distorted octahedral geometries for four Cd1, Cd2, Cd3 and Cd5 atoms, in which Cd1 was surrounded by two μ₂-Cl1/Cl2 ions and four nitrogen atoms (N1, N2, N3 and N6) from the same ligand L¹, Cd2 by six μ₂-Cl ions (Cl1, Cl2, Cl3, Cl5, Cl6, Cl7), Cd3 by two chloride ions (one μ₂-Cl3, one terminal Cl4) and four nitrogen atoms (N7, N8, N9 and N12) from the same ligand L¹, and Cd5 by four μ₂-Cl ions (Cl5, Cl6, Cl9 and Cl10) and two nitrogen atoms (N4 and N5) from the same ligand L¹. Six adjacent Cd^II atoms were connected in a Cd₆Cl₁₃ unit through a single or double μ₂-bridging Cl atom (Fig. S1†), which was further linked into 1D wave-like anionic chain (1) (Fig. 1d and e) with the smallest repeating unit [Cd₆Cl₁₃(L¹)₂]⁻ moiety via μ₂-η¹:η¹:η¹:η¹:η¹:η¹ coordination mode of ligand L¹ (Fig. 1b and e). On the other hand, the Cd7 atom in each cationic unit is hexa-coordinated by one terminal chloride atom (Cl14) and five nitrogen atoms from the same ligand L¹ (N13, N14, N15, N17, N18) with μ₁-η¹:η¹:η¹:η¹:η¹ coordination mode to form a [CdL¹Cl]⁺ unit presenting distorted octahedral geometry (Fig. 1c). All Cd–Cl and Cd–N bond distances are from 2.839 (4) to 2.883 (4) and 2.439 (4) to 2.483 (4) Å, respectively (Table S2†), which are similar to those reported.⁷

Fig. 1 In 1, (a and b) the coordination environment of six Cd²⁺ ions and coordination mode of ligand L¹ in anionic part. (c) The structure of cationic moiety and another coordination mode of ligand L¹. (d and e) 1-D wave-like anionic chain with the smallest unit [Cd₆Cl₁₃(L¹)₂] along b axis. The symmetric codes: (a) 1 − x, 0.5 + y, 0.5 − z; (b) 1 − x, −0.5 − y, 0.5 − z.

Compound 2 is a 0-D dinuclear structure with an asymmetric unit consisting of one crystallographically unique Cd^II atom, one coordinated chloride ion, one (L²)⁻ ligand and one uncoordinated methanol molecule as shown in Fig. 2. Different from those in 1, the Cd^II atom in 2 is only penta-coordinated in a distorted square pyramid with N4Cl donor set, in which the coordination sphere around Cd1 consists of one terminal Cl atom and three pyridyl N atoms (N1, N3, N4a) from the same ligand (L²)⁻, one imidazolium N atom (N2) from another ligand (L²)⁻. Compared with ligand L¹ in 1, ligand HL² in 2 may derive from oxidation of the methylene on L¹ into a carbonyl group and the dehydrogenation of the imidazolidine core ring into a delocalized imidazolium ring, accompanied by the protonation of the pyridine nitrogen atom (Scheme 2). Therefore, ligand (L²)⁻ in 2 shows μ₂-η¹:η¹:η¹:η¹:η¹ coordination mode, further forming dinuclear compound 2 with similar bond distances of Cd–N (2.160 (5) to 2.572 (5) Å). Similar to complex 1, a 3-D supramolecular network of 2 constructed by hydrogen bonds C–H…Cl(O) is shown in Fig. S2.†

Fig. 2 Molecular structure of 2 with partial atomic labels, and two uncoordinated methanol molecules are omitted for clarity. The symmetric code: (a) 1 − x, 2 − y, 1 − z.

Scheme 2 Proposed mechanism for the formation of tetra-substituted 2-pyridyl imidazolidine L¹/imidazolium (L²)⁻ in the cadmium(II) complexes 1–2. Reaction conditions: (1) α-H abstraction/pyridine, 60 °C. (2) [3 + 2] cycloaddition/H⁺, 60 °C. (3) Rearrangement and dehydrogenated in presence of with H₂O₂, 100 °C.

In the reaction of ligand L with an equimolar amount of CdCl₂ at a temperature of more than 70 °C for three days, the following were obtained: two 0-D compounds [Cd(L³)Cl₂]₂ (3) (at 80 °C) and Cd₂(L³)Cl₄·H₂O (4) (at 100 °C) involving pyrizine-core L³ ligand, respectively. The crystal structure of the pale yellow block compound 3 is shown in Fig. 3, exhibiting a 0-D binuclear structure. The asymmetric unit of 3 involves one Cd²⁺ ion, one in situ formed L³ ligand, and two Cl⁻ ions. The coordination geometry around Cd1 is octahedral with three nitrogen atoms (N1, N2, N3) from one L³ ligand and three chlorine atoms (Cl1, Cl2, Cl2a), in which Cl2, N1, N2, N3 atoms occupy the corners of a square, and Cl1 and Cl2a atoms at the apical positions. The ligand L³ in 3 has the same coordination mode I: μ₁-η¹:η¹:η¹, namely, which as a terminal tridentate ligand using the three nitrogen atoms (N1, N2, N3) on the same side of L³ coordinates to one Cd²⁺ ion to form a 0-D dinuclear structure via two μ₂-bridging Cl2 atoms. The Cd–N and Cd–Cl bond distances are 2.345 (8)–2.370 (7) Å and 2.470 (3)–2.863 (3) Å, respectively. And intermolecular hydrogen bonding C–H…Cl interaction resulting in 1-D supramolecular chain further stabilizes crystal structure (Fig. S3 and Table S3†).

Fig. 3 Molecular structure of 3 with partial atomic labels, and the symmetric code: (a) 1 − x, 1 − y, 2 − z.

The pale yellow prism complex 4 crystallized in a monoclinic space group C2/c with two Cd^II atoms, four chlorine atoms, one ligand L³, and one crystallization water molecule in one molecule as depicted in Fig. 4. The cadmium center is penta-coordinated to three nitrogen atoms (N1, N2, N3) from the same L³ and two terminal chlorine atoms (Cl1, Cl2) in a slightly distorted trigonal bipyramidal geometry with normal Cd–N(Cl) bond distances and comparable to previously reported metric data for similar complexes. The ligand L³ in 4 adopts another coordination mode II: μ₂- η¹:η¹:η¹:η¹:η¹:η¹ to bridge two CdCl₂ units and generates a dinuclear compound 4. Considering the weak interaction of the Cl2 atom to the Cd1 center with Cl2…Cd1 separation of 3.15 Å, the 1-D chain displayed in Fig. S4† is formed in ac plane. Meanwhile, some interchain hydrogen bonding C–H…Cl interactions contribute to the stability of the packing structure (Fig. S5 and Table S3†).

Fig. 4 Molecular structure of 4 with partial atomic labels, and lattice water molecule is omitted for clarity. The symmetric code: (a) 0.5 − x, 0.5 − y, 1 − z.

From the above analyses and Scheme 1, ligand L¹ in compound 1 has an imidazolidine-core ring with three 2-pyridyl substituents in the 2,4,5-positions and one pyridin-2-yl-methyl group in the 3-position. Different from L³, ligand (L²)⁻ in 2 contains a planar imidazolium-core ring and its 3-position connects one 2-pyridyl group via a carbonyl group. Both compounds 3 and 4 contain ligand L³, which is pyrazine-core ring with four 2-pyridyl substituents in 2,3,5,6-positions (Scheme 1). Three ligands L^1–3 can be successfully isolated from the reactions of those compounds containing corresponding ligands with Na₂S. Obviously, Schiff base L in the assembly process of four compounds 1–4 occurred in situ cycloaddition reactions to form three different ligands L^1–3.

On the basis of the literature,⁸ two possible mechanisms should be addressed, shown in Schemes 2 and 3. At a low temperature (60 °C), we think that the formation of ligand L¹ resulted from [3 + 2] cycloaddition of Schiff base N-(2-pyridylmethyl)-pyridine-2-carbaldimine (L) mediated by Cd²⁺. It is known that the acidity of a C–H bond in the α-position to an imino group –CH=N–CH– is markedly increased if the imino nitrogen atom is coordinated to a metal center.⁸ Basic acceptors, such as pyridines, have the ability to deprotonate the imino carbon-bound hydrogen atom to form a 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. To the best of our knowledge, the 1,3-dipolar cycloaddition is also a general and powerful method for the synthesis of five-membered heterocyclic compounds.⁹ The carbanion formed at C6′ is attached to the C7 atom from another ligand L to form a C–C bond. Meanwhile, due to steric hindrance from the coordination of N1′ to the Cd^II center, N2 with a negative charge attacks the C7′ atom from another ligand L to form a C–N bond, completing the [3 + 2] cycloaddition reaction to give ligand L¹ with a half-chair configuration imidazolidine-core (Scheme 2).¹⁰ However, accompanied by a temperature rise and addition of oxidant H₂O₂, the center imidazoldine ring was oxidized and dehydrogenated into a planar imidazole-core ring with tetra-2-pyridyl substituents,¹¹ accompanied by carbonylation of the methylene linked in the 3-position and protonation of the pyridine nitrogen atom, further forming ligand (L²)⁻ (Scheme 2).

Scheme 3 Proposed mechanism for the formation of tetra-substituted 2-pyridyl pyrazine L³ in the cadmium(II) complexes 3 and 4.

At a high temperature (80 °C above), first the Schiff base L coordinates to the cadmium center, followed by deprotonation of the methylene proton (α-H) by the base (pyridine) to produce a strong nucleophilic carbanion at C6. Subsequently, nucleophilic attack of the carbanion at the electrophilic carbon C7′ from an uncoordinated/coordinated ligand L forms a C–C bond, which increases the acidity of the C–H bond at the heterocyclic carbon C6′ atom (α-position), and easily produces a new carbanion formed at C6′. This allows the next nucleophilic attack at the imino carbon C7, simultaneously resulting in another C–C bond coupling and completing the [3 + 3] cycloaddition reaction.^1,2,12 Finally, the dehydrogenation reaction in the presence of the base pyridine provides compound 4 with an imidazolidine-core tetra-substituents L³, or compound 3 with a bridging effect of the coordinated chlorine atoms (Scheme 3).

In order to further support the above proposed reaction mechanism, a control experiment was carried out in the temperature range 50 to 120 °C, in which one similar ligand with a substituted methyl at C7 atom (L′ in Scheme S1†) was synthesized and the same reaction mediated by Cd^II was attempted (see ESI†). The single-crystal analytical result indicates that the reaction of L′ with CdCl₂ in methanol containing pyridine always gives only single C–C coupling⁷ product 1′ (Fig. S6, 7 and Table S1†), and fully agrees with the proposed C–C coupling reaction mechanism shown in Scheme 2 and 3.

Structure and formation mechanism of complex 5

Interestingly, in the presence of one equivalent of 2-pyridylethylamine, reaction of ligand L with CdCl₂ generated pale-yellow dinuclear compound 5 in three days. X-ray single-crystal diffraction confirms its component of [Cd₂(L⁴)Cl₂]·CH₃OH·0.5H₂O (5) with in situ formed ligand L⁴ bearing an imidazole-core, and its molecular structure is shown in Fig. 5. Crystal structural analysis shows that a molecule of 5 contains two crystallographically independent penta-coordinated cadmium(II) atoms with the distorted trigonal bipyramidal geometries for the Cd1 and Cd2 atoms (see Table S2† for bond lengths and angles), two in situ formed ligands L⁴, two terminal coordinated chlorine atoms, one uncoordinated methanol molecule and half a lattice water molecule. Each ligand L⁴ having an imidazole-core ring is different from the ligand L¹ with an imidazolidine-core ring in 1, presenting a tetra-dentate μ₂-η¹:η¹:η¹:η¹ coordination mode. In addition, π…π packing interaction between the two parallel pyridyl rings with two centroid separation of 3.536 Å from two ligands L⁴ further stabilizes the dinuclear molecule. And intermolecular hydrogen bonding C–H…Cl interactions result in the assembly of a 3-D surpramolecular network (Table S3 and Fig. S8†).

Fig. 5 Molecular structure of the cadmium(II) complex 5 with atomic labels; the uncoordinated methanol and lattice water molecules are omitted for clarity. The yellow dashed line represents intramolecular π…π packing interaction between the two almost parallel pyridine rings.

Although the synthesis and structure of ligand HL⁴ have been reported in the literature,¹³ no example involves the solvothermal in situ metal–ligand reaction and its coordination chemistry. Accordingly, the in situ formation mechanism of HL⁴ is proposed in Scheme 4. First the Schiff base ligand L and 2-pyridyl-ethylamine co-coordinate to cadmium(II) ions to generate intermediate A, and then due to the coordination effect, α-H atoms of Schiff base ligand L and 2-pyridylethylamine are activated to readily generate the carbanion B intermediate in the presence of the base (pyridine), which further facilitates intramolecular nucleophilic attacks at the imino carbons from Schiff base L and the amino nitrogen from 2-pyridylethylamine to form C–C and C–N bonds, finally completing [3 + 2] cycloaddition reaction and offering compound 5 containing a tri-substituted 2-pyridyl imidazole HL⁴ after dehydrogenation. Both ligands L³ and HL⁴ can be successfully isolated from the reactions of those compounds containing corresponding ligands with Na₂S.¹⁴

Scheme 4 Proposed mechanism for the formation of 2,2′,2′′-(1H-imidazole-2,4,5-triyl)-tripyrid-ine (HL⁴) in the cadmium(II) complex 6.

Structure and formation mechanism of complexes 6–8

Changing the reactants again, in the presence of one equivalent of 2-pyridine formaldehyde, ligand L reacted with one equivalent of CdCl₂ to offer three 0-D compounds 6–8 containing ligands L^5–6 in the temperature range 80–135 °C and one 2-D layer-like compound [Cd₂L³Cl₃(pic)]_n (9) containing ligand L³, accompanied by H₂O₂ oxidation of pyridine formaldehyde into picolinate (pic) at a temperature of more than 80 °C, respectively. Compound 6 crystallized as pale yellow crystals suitable for X-ray structure analysis (Fig. 6). The mononuclear complex 6 has an essentially undistorted square pyramidal coordination geometry around the cadmium(II) ion satisfied by three nitrogen atoms (N1, N2, N4) from in situ formed ligand L⁵ and two terminal coordinated chlorine atoms (Cl1, Cl2). The Cd–N bond of (2.240 (5)–2.477 (5) Å), and Cd–Cl (2.416 (2)–2.468 (2) Å) bond distances are in the range expected for this coordination geometry with cadmium(II). Due to intermolecular hydrogen bonding C–H…Cl and π…π packing interactions, a 2-D layer-like structure is assembled (Fig. S9 and Table S3†).

Fig. 6 Molecular structure of the cadmium(II) complex 6 with atomic labels.

Single-crystal X-ray diffraction reveals that compound 7 crystallizes in the monoclinic crystal system of the space group C2/c.

As shown in Fig. 7, a molecule of 7 contains two crystallographically independent Cd(II) ions, one in situ formed ligand L⁶, two terminal Cl⁻ ions and one lattice methanol molecule. Each Cd(II) ion is penta-coordinated by three nitrogen atoms (N1, N2, N3/N5, N6, N7) from the same ligand L⁶ and two terminal chloride atoms (Cl1, Cl2/Cl3, Cl4). One ligand L⁶, which adopts cis-conformation of four coordinated terminal groups referred to C6–C24 single bond, coordinates to 2 equiv. of Cd²⁺ ions, showing μ₂-η¹:η¹:η¹:η¹:η¹:η¹ coordination mode. Meanwhile, the existence of intermolecular hydrogen bonding C(O)–H…Cl interactions makes dinuclear molecular unit [Cd₂(cis-L⁶)Cl₄] to be assembled into 3-D supramolecular network as shown in Fig. S10.†

Fig. 7 Molecular structure of the cadmium(II) complex 7 containing cis-L⁶ with partial atom labels. The uncoordinated methanol is omitted for clarity.

However, compound 8 crystallizes in the triclinic crystal system of the space group P1̄. As shown in Fig. 8, a molecule of 8 possesses a crystallographically imposed inversion center located at the mid-point of the C13–C13a single bond. Cd1 ion is penta-coordinated by three nitrogen atoms (N1, N2, N3) from the same ligand L⁶ and two terminal chloride atoms (Cl1, Cl2) and structurally similar to that of dinuclear 7. One ligand L⁶, which is trans-conformation of four coordinated terminal groups referred to C13–C13a bond, coordinates to 2 equiv. of Cd²⁺ ions showing μ₂-η¹:η¹:η¹:η¹:η¹:η¹ coordination mode. Meanwhile, the existence of intermolecular hydrogen bonding C–H…Cl and π…π packing interactions makes dinuclear molecular [Cd₂L⁶Cl₄] unit to be assembled into a 2-D supramolecular layer with the thickness of 0.681 Å as shown in Fig. S11.†

Fig. 8 Molecular structure of the cadmium(II) complex 8 with partial atom labels. The symmetric code: (a) −x, 2 − y, 2 − z.

For ligands L^5–6, the possible formation mechanism is outlined in Scheme 5.^7,15 Initially, the carbanion formed at the C6 atom from intermediate B increases the nucleophilicity at the heterocyclic nitrogen atom N3, which facilitates a nucleophilic attack at the imino carbon C7 to form C–N bond,¹⁶ resulting in the formation of intermediate C. And then coordination of the nitrogen atom of picolinaldehyde to Cd²⁺ enhances the electrophilicity of the carbon atom of the aldehyde, triggering the formation of an alcoholic intermediate D via the nucleophilic attack of C to the carbonyl electrophile. Finally, cleavage of the C–O bond in D gives a radical intermediate E. Next, two competitive reactions may occur for E, which subsequently either obtains proton from intermediate A to furnish the formation of ligand L⁵, or undergoes dimerization to give L⁶. Because of the free rotation of C–C (C6–C24 for 7 and C13–C13a for 8) single bond and the steric hindrance from its four connecting groups, ligand L⁶ shows two conformations of cis-/trans-form in the formed compounds referring to C–C single bond such as complex 7 (cis-form, dynamic stability) and 8 (trans-form, thermodynamic stability). Moreover, by increasing the reaction temperature or prolonging the reaction time, complex 7 may be irreversibly transformed into 8.

Scheme 5 Proposed mechanism for the in situ formation of ligands L^5–6 in the cadmium(II) complexes 6–8.

Similar to the above control experiment, reaction of ligand L′ with CdCl₂ and KSCN in the presence of one equivalent 2-pyridine formaldehyde always provides single 1-D product 2′ for 50–120 °C (Fig. S12 and Table S1†), also further displaying that the proposed C–N formation mechanism in Scheme 5 is reasonable.

Synthesis and structure of complex 9

X-ray single-crystal diffraction reveals that compound 9 exhibits a 2-D layer-like structure with 4⁴ topology (Fig. 9(iii) and (iv)). The asymmetric unit of 9 involves two Cd²⁺ ions, one in situ formed L³ ligand, and three Cl⁻ ions (Fig. 9(i)). The central Cd1 atom was hepta-coordinated by three bridging chlorine atoms (Cl1, Cl2 Cl2a), three nitrogen atoms from one L³ ligand (N1, N2, N3), and one oxygen atom from picolinate, displaying a distorted decahedral geometry (Fig. S11a in ESI†). However, the Cd2 atom hexa-coordinated via four μ₂-bridging chlorine atoms (Cl1, Cl2 Cl3, Cl3b), one nitrogen and oxygen atoms from the same picolinate (N4, O1), shows a slightly distorted octahedral coordination geometry (Fig. S11b in ESI†). Through the bridges of two μ₂-O1, two μ₂-Cl1 and two μ₃-Cl2 atoms, four Cd^II atoms (Cd1, Cd1a, Cd2, Cd2a) were linked to form the tetra-nuclear Cd-cluster unit [Cd₄(μ₂-O)₂(μ₂-Cl)₂(μ₃-Cl)₂] with the distances between two adjacent Cd atoms from 3.862 to 3.888 Å (Fig. S11c†), which further assembled into a 1-D infinite cluster chain structure [Cd₄(μ₂-O)₂(μ₂-Cl)₂(μ₃-Cl)₂]_n running along the c-axis via double bridging μ₂-Cl3 atoms (Fig. 9(ii)). The in situ formed ligand L³ in 9 has a coordination mode II similar to that in 2, μ₂-η¹:η¹:η¹:η¹:η¹:η¹, namely, using its four pyridyl nitrogen (N1, N3, N1c, N3c) and two pyrazinyl nitrogen atoms (N2, N2c) to bridge two Cd^II atoms from two adjacent 1-D Cd^II-cluster chains, further resulting in the formation of a 2-D layer with 4⁴ topology as depicted in Fig. 9(iii) and (iv).

Fig. 9 In 9, (i) different coordination environment of two Cd²⁺ ions and coordination mode μ₂-η¹:η¹:η¹:η¹:η¹:η¹ of hexa-dentate ligand L³, (ii) 1-D [Cd₄Cl₆-(pic)₂]_n along c axis, (iii and iv) 2-D layer with 4⁴ topology. Symmetric codes: (a) 2 − x, −y, 1 − z; (b) 2 − x, −y, −z; (c) 1 − x, −y, 1 − z; (d) −1 + x, y, z.

In the assembly process of complex 9, due to the high reaction temperature (more than 80 °C), the added pyridine formaldehyde did not get involved in the solvothermal in situ Cd^II-L conversion reaction into L³, and was in situ oxidized to picolinate, participating in the assembly of 9.

Photophysical properties

The preliminary photoluminescent properties of complexes 1–9 and ligands L^1–6 in the solid state at room temperature have been investigated and are shown in Fig. S13.† The emission spectra of the complexes 1–9, respectively, resemble that of the ligands L^1–6 excluding the emission intensity, indicating the fluorescence of the complexes 1–9 are L-based emission. Clearly, 1 exhibits green emission with peak wavelengths at 443 nm, while compound 2 and compound 5 present blue emission with peak wavelength at 448 nm and 412 nm, respectively. Similar to the center of the five-membered ring, the different performance between 1, 2 and 5 could be caused by better rigidity for 2 and 5. The emission spectra of 3, 4 and 9 with the same ligand 3, excited at about 390 nm, exhibit different luminescence characteristics: 3 and 4 have a similar emission peak at 436 and 431 nm, but complex 9 has a strong emission peak at 451 nm that can tentatively be rationalized by the robust 2-D valence-bonded framework (in 9). Interestingly, the emission wavelength of complexes 6–8 decreased gradually. Accordingly, the spectral differences among the three complexes are intrinsic, and could plausibly be due to the aforementioned structural variation.

Conclusions

In summary, a convenient C–C/C–N bond-forming methodology towards six N-rich heterocycles from Cd^II-mediated in situ conversion of pyridine-type Schiff base and the coordination chemistry of nine corresponding Cd^II-complexes were systematically explored for the first time. The results show that the in situ formation of L^1–6 not only involved Cd^II-catalyzed [3 + 3]/[3 + 2] cycloaddition reactions, but also was closely related to the reaction temperature. Meanwhile, to the best of our knowledge, in situ formation of heterocyclic rings L^1–2 as well as cis-L⁶ are also reported for the first time. Obviously, the coordination-induced effect presented here offers the intriguing possibility of preparing a series of new asymmetric nitrogen heterocyclic rings displaced from the metals. Moreover, extension of these functional two-, three-, four-connected in situ formed aza-heterocyclic ligands to other metal salts and a further systematic investigation of their function are in progress.

Experimental section

Materials and characterizations

All materials were reagent grade obtained from commercial sources and used without further purification; solvents were dried using standard procedures. Elemental analyses for C, H, N were performed on a Perkin-Elmer 240C analytical instrument. IR spectra were recorded on a Nicolet FT-IR-170SX spectrophotometer in KBr pellets. Thermogravimetric analyses were performed on Perkin-Elmer TGA7 analyzer with a heating rate of 10 °C min⁻¹ in flowing air atmosphere. The luminescent spectra for the solid state were recorded at room temperature on Hitachi F-2500 and Edinburgh-FLS-920 with a xenon arc lamp as the light source. In the measurements of emission and excitation spectra the pass width is 5.0 nm. UV-vis absorption spectra were recorded on a Shimadzu UV1800 UV-vis spectrophotometer. X-ray powder diffraction patterns were measured on a Bruker D8 Advance diffractometer at 40 kV and 40 mA with a Cu target tube and a graphite monochromator. Nitrogen and hydrogen adsorption isotherms were taken on a Beckman Coulter SA 3100 surface area and pore size analyzer.

X-ray data collection and structure refinement

Complexes 1–9 were characterized by single crystal X-ray diffraction. Suitable single crystals were mounted on a glass fiber and the intensity data were collected on a Bruker APEX II diffractometer at 298 K using graphite monochromatic Mo-Kα radiation (λ = 0.71073 Å). Absorption corrections were applied using the multi-scan program SADABS.¹⁷ Structural solutions and full-matrix least-squares refinements based on F² were performed with the SHELXS-97 (ref. 18) and SHELXL-97 (ref. 19) program packages, respectively. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms on organic ligands were generated by the riding mode (C–H 0.96 Å). A summary of the parameters for the data collection and refinements for nine complexes are given in Table S1.† Selected bond lengths and angles for complexes 1–9 are listed in Table S2.†

Syntheses of complexes 1–9

Synthesis of 1. A mixture of N-(2-pyridylmethyl)pyridine-2-carbaldimine (L) (0.0394 g, 0.2 mmol), CdCl₂ (0.0402 g, 0.2 mmol), MeOH (7 mL) and pyridine (3 mL) was sealed in a 15 mL Pyrex tube. The tube was heated for 3 days at 60 °C under autogenous pressure, and then slow cooling of the resultant solution to room temperature over 24 h gave pale yellow block single crystals of 1. The crystals were collected by filtration, washed with Et₂O (2 × 3 mL), and dried in air. Yield: 76% (based on Cd). Elemental analysis calcd (%) for C₇₂H₇₀Cd₇Cl₁₄N₁₈O₂: C, 34.52; H, 2.80; N, 10.07. Found: C, 34.47; H, 2.89; N, 10.05. IR frequencies (KBr, cm⁻¹): 3447 (s), 2957 (w), 2367 (w), 1638 (s), 1566 (s), 1420 (s), 1329 (s), 1260 (w), 1155 (s), 1053 (w), 1016 (w), 924 (w), 880 (w), 779 (s), 714 (w), 650 (w), 517 (s), 463 (w).

Synthesis of 2. A mixture of N-(2-pyridylmethyl)pyridine-2-carbaldimine (L) (0.0788 g, 0.4 mmol), CdCl₂ (0.0402 g, 0.2 mmol), H₂O₂ (0.1 mL, 30%), MeOH (7 mL) and pyridine (3 mL) was sealed in a 15 mL Pyrex tube. The tube was heated for 3 days at 100 °C under autogenous pressure, and then slow cooling of the resultant solution to room temperature over 24 h gave pale yellow block single crystals of 2. The crystals were collected by filtration, washed with Et₂O (2 × 3 mL), and dried in air. Yield: 58% (based on Cd). Elemental analysis calcd (%) for C₅₀H₄₂Cd₂Cl₂N₁₂O₄: C, 51.25; H, 3.59; N, 14.35. Found: C, 51.18; H, 3.64; N, 14.36. IR frequencies (KBr, cm⁻¹): 3447 (s, br), 2957 (w), 2363 (m), 2041 (w), 1773 (w), 1638 (s), 1560 (s), 1477 (m), 1419 (s), 1327 (s), 1157 (m), 916 (m), 877 (w), 779 (s), 704 (w), 669 (w), 517 (s), 463 (w).

Synthesis of 3. The preparation procedure is similar to that of compound 2 except that the temperature is 80 °C without oxidant. Finally, 3 is obtained as pale yellow crystals with yield 33% (based on Cd). Elemental analysis calcd (%) for C₄₈H₃₂Cd₂Cl₄N₁₂: C, 50.37; H, 2.80; N, 14.69. Found: C, 50.28; H, 2.83; N, 14.65. IR frequencies (KBr, cm⁻¹): 3431 (s, br), 2957 (m), 2371 (s), 2060 (s), 1638 (s), 1570 (s), 1420 (s), 1339 (s), 1258 (w), 1194 (w), 1153 (m), 1007 (m), 924 (w), 878 (w), 779 (s), 648 (w), 621 (m), 563 (w), 516 (s).

Synthesis of 4. The preparation procedure is similar to that of compound 3 except the ratio of ligand and CdCl₂ being adjusted to 1 : 1. Finally 4 is obtained as pale yellow crystals with yield 45% (based on Cd). Elemental analysis calcd (%) for C₂₄H₁₈Cd₂Cl₄N₆O: C, 39.25; H, 2.45; N, 11.45. Found: C, 39.28; H, 2.43; N, 11.55. IR frequencies (KBr, cm⁻¹): 3431 (s), 2957 (m), 2332 (w), 2112 (s), 1854 (w), 1653 (s), 1638 (s), 1568 (s), 1429 (s), 1404 (s), 1339 (m), 1254 (w), 1196 (w), 1150 (m), 1011 (m), 924 (w), 878 (w), 779 (s), 750 (w), 718 (w), 635 (m), 559 (m), 517 (m), 465 (w).

Synthesis of 5. The preparation procedure is similar to that of compound 4 except more addition with 2-pyridylethyl-amine (0.0216 g, 0.2 mmol). Finally, 5 is obtained as pale yellow crystals with yield 35% (based on Cd). Elemental analysis calcd (%) for C₇₄H₅₈Cd₄Cl₄N₂₀O₃: C, 47.49; H, 3.10; N, 14.97. Found: C, 47.38; H, 3.03; N, 15.05. IR frequencies (KBr, cm⁻¹): 3447 (s, br), 2957 (w), 2367 (w), 2041 (w), 1647 (s), 1560 (s), 1522 (w), 1483 (w), 1431 (s), 1327 (s), 1258 (w), 1152 (m), 1013 (m), 976 (w), 878 (w), 843 (w), 779 (s), 719 (m), 650 (w), 517 (m), 459 (w).

Synthesis of 6. The preparation procedure is similar to that of compound 4 except more addition with picolinaldehyde (0.0214 g, 0.2 mmol) and at N₂ atmosphere. Finally, 6 is obtained as pale yellow crystals with yield 68% (based on Cd). Elemental analysis calcd (%) for C₁₈H₁₄CdCl₂N₄: C, 45.99; H, 2.98; N, 11.92. Found: C, 45.93; H, 2.99; N, 11.95. IR frequencies (KBr, cm⁻¹): 3422 (s, br), 2957 (w), 2372 (w), 1638 (s), 1593 (m), 1560 (s), 1487 (m), 1418 (s), 1375 (w), 1339 (m), 1254 (w), 1169 (m), 1074 (w), 999 (w), 843 (w), 781 (s), 760 (w), 681 (w), 633 (m), 517 (m), 463 (w).

Synthesis of 7. The preparation procedure is similar to that of compound 6 except the temperature up to 100 °C. Finally, 7 is obtained as pale yellow crystals with yield 53% (based on Cd). Elemental analysis calcd (%) for C₃₇H₃₁Cd₂Cl₄N₈O: C, 45.76; H, 3.19; N, 11.54. Found: C, 45.69; H, 3.28; N, 11.56. IR frequencies (KBr, cm⁻¹): 3431 (s), 2957 (w), 2363 (m), 2064 (m), 1639 (s), 1595 (s), 1560 (s), 1489 (m), 1420 (m), 1325 (m), 1256 (w), 1155 (m), 1007 (w), 878 (w), 777 (m), 623 (w), 517 (s).

Synthesis of 8. The preparation procedure is similar to that of compound 6 except the temperature 120 °C. Finally, 8 is obtained as pale yellow crystals with yield 58% (based on Cd). Elemental analysis calcd (%) for C₃₆H₂₆Cd₂Cl₄N₈: C, 46.09; H, 2.77; N, 11.95. Found: C, 46.05; H, 2.73; N, 11.91. IR frequencies (KBr, cm⁻¹): 3447 (s, br), 2957 (w), 2363 (w), 1639 (s), 1595 (s), 1560 (s), 1522 (w), 1487 (m), 1420 (m), 1339 (m), 1258 (w), 1190 (w), 1153 (w), 1080 (m), 1003 (m), 878 (w), 799 (s), 743 (w), 692 (m), 652 (w), 517 (m).

Synthesis of 9.

The preparation procedure is similar to that of compound 4 except more addition of H₂O₂. Finally, 9 is obtained as pale yellow crystals with yield 72% (based on Cd). Elemental analysis calcd (%) for C₁₈H₁₂Cd₂Cl₃N₄O₂: C, 33.36; H, 1.85; N, 8.65. Found: C, 33.28; H, 1.88; N, 8.58. IR frequencies (KBr, cm⁻¹): 3414 (s, br), 2926 (w), 2851 (w), 2369 (w), 2137 (w), 2066 (m), 1639 (s), 1560 (s), 1420 (s), 1325 (m), 1260 (w), 1151 (m), 1018 (w), 777 (m), 712 (w), 638 (w), 519 (s).

Acknowledgements

The authors are grateful for financial aid from the National Natural Science Foundation of P. R. China (Grant nos 21071056, 21471061 and 91122008), the Doctoral Program of Higher Education of China (Grant no. 20124407110007), Science and Technology Program of Guangzhou, China (Grant no. 2014J4100051).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1022100, 1022101, 987353, 987352, 987351, 987347, 1022102, 1022103 and 987349 for 1–9. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra16198j

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