Synthesis and crystal structure of a 3D copper(II)–silver(I) coordination polymer assembled through hydrogen bonding, π–π stacking and metal–π interactions, {[Cu(phen)₂(CN)][Ag(CN)₂] · 3H₂O}_n (phen = 1,10-phenanthroline)

1 Introduction

Transition metal cyanides such as [Fe(CN)₆]³⁻ [1–6], [Ni(CN)₄]²⁻ [7–11], [Au(CN)₂]⁻ [12–17], and [Ag(CN)₂]⁻ [18–29] are widely used as building blocks for the preparation of supramolecular coordination systems. The rigidity of such frameworks may allow shape and size selective inclusion of organic solvents, water molecules, and aromatic compounds for potential applications in the area of catalysis [1, 12, 29, 30]. Among the cyanidometalates, [Ag(CN)₂]⁻ is particularly interesting because the resulting coordination polymers are usually supported by Ag–Ag (argentophilic) interactions [18, 27–29]. The [Ag(CN)₂]⁻ anion is labile and may be transformed into other secondary products such as [Ag₂(CN)₃]⁻ according to the following equilibria [26]:

K[Ag(CN)2] ⇄ KCN  + AgCNAgCN + K[Ag(CN)2] ⇄ K[Ag2(CN)3]AgCN + 2K[Ag(CN)2] ⇄ K2[Ag3(CN)5]

As a result, a variety of products consisting of [Ag(CN)₂]⁻ [18–25], [Ag₂(CN)₃]⁻ [26], and [Ag₃(CN)₅]²⁻ anions [27] have been detected in the reactions of [Ag(CN)₂]⁻. In addition, polymers incorporating pentameric units, [{Ag(CN)₂}⁻]₅, have also been observed [28]. The Ag–CN system is also of considerable interest because cyanide solutions form the basis for an important method of extracting silver metal from silver ore [31].

We have been investigating the structural properties of M(II)–Ag(I) coordination polymers that contain the [Ag(CN)₂]⁻ anion as a bridging unit [18–20]. In the present study, we attempted to prepare a coordination polymer consisting of [Cu(phen)₂]²⁺ and [Ag(CN)₂]⁻ ions, but the product isolated was found to be {[Cu(phen)₂(CN)][Ag(CN)₂] · 3H₂O}_n, in which the [Ag(CN)₂]⁻ anions are bound to the copper complex cations through metal–π and nitrilo–π interactions. The crystal structure is characterized further by significant π–π stacking interactions between aromatic rings and hydrogen bonding to form a 3D network.

2 Experimental section

3 Results and discussion

The title complex was prepared by the reaction of CuCl₂ · 2H₂O, phen, and K[Ag(CN)₂] mixed in a molar ratio of 1:2:2. In the IR spectrum of 1, two ν(CN) bands are observed, one at 2140 cm⁻¹ and the other one at 2095 cm⁻¹, while for K[Ag(CN)₂] only one ν(C≡N) appears at 2140 cm⁻¹. The presence of two bands indicates that the complex contains two kinds of cyanide environments. The IR spectrum of phen has the characteristic bands for ν(C=N) at 1618 and 1589 cm⁻¹ and ν(C=C) at 1505 and 1423 cm⁻¹ [18]. For 1 the ν(C=N) vibrations of phen were observed at 1626 and 1583 cm⁻¹, while the ν(C=C) bands appeared at 1519 and 1428 cm⁻¹. The presence of these bands indicates the coordination of phen to the metal. A broad peak at 3321 cm⁻¹ due to the O–H stretch suggests the presence of hydrogen bonds in the complex.

The molecular structure of the asymmetric unit of 1 along with the crystallographic numbering scheme is illustrated in Fig. 1. Selected bond lengths and bond angles are listed in Table 2. The crystal structure of the complex consists of [Cu(phen)₂(CN)]⁺ cations, [Ag(CN)₂]⁻ anions, and three water molecules. The Cu(II) ions are coordinated to four nitrogen atoms of phen ligands and one cyanide carbon atom assuming a distorted square pyramidal geometry. The phen ligands are in the square basal plane, and the CN group is present at the apical position of the square pyramid. The [Ag(CN)₂]⁻ anions are connected to the complex cation through metal–π and nitrilo–π interactions as illustrated in Fig. 2. The cis N–Cu–N angles vary from 80.10(8)° to 93.01(8)°, while the trans N–Cu–N angles are 100.70(7)° and 169.55(8)°. The cis N–Cu–C angles are greater than the cis N–Cu–N angles due to the strain of the phen ring. The Cu–C≡N unit is nearly linear with a bond angle of 178.7(2)°. The coordination environment of non-coordinated [Ag(CN)₂]⁻ ions is known to be linear. The coordination environment of the [Ag(CN)₂]⁻ units is also close to linear with Ag–C≡N angles at 177.0(2)° and 179.3(3)°. The Cu–N bond lengths are with 2.016(3)–2.137(2) Å comparable to those in known structures of copper(II) complexes of phen [33–37].

Fig. 1:

Ortep diagram (50 % probability ellipsoids) of the molecular structure of 1. All carbon-bonded hydrogen atoms are omitted for clarity. Dotted lines indicate hydrogen bonds. The sign refers to the calculated interplanar angle between mean planes built up by atoms of differently colored areas.

Fig. 2:

Illustration of interactions of the [Ag(CN)₂]⁻ fragment of 1 with [Cu(phen)₂(CN)]⁺ ions by means of metal–π and nitrilo–π interactions. All carbon-bonded hydrogen atoms are omitted for clarity. Bond distances between interacting atoms: Ag1A–C10B = 3.28 Å, Ag1A–C19 = 3.27 Å, C24–C27A = 3.31 Å, C23–C27A = 3.30 Å, C27A–C24B = 3.51 Å, C19B–C27A = 3.50 Å, N3–N7A = 3.27 Å, C23–N7A = 3.28 Å, N7A–C19B = 3.32 Å. Labels A and B refer to fragments of asymmetric units with the following symmetry codes: A = x, 1.5 − y, z + 1.5; B = 1 − x, y+ ½, 2.5 − z.

In the solid state, the compound 1 forms a 3D network which comprises both intermolecular hydrogen bonds and different types of π interactions. Thereby, all hydrogen atoms of the three crystallographically independent H₂O molecules, cf. Figure 1, are involved in hydrogen bonds to cyano ligands coordinated to the Cu1 or Ag1 atoms. Selected data of the hydrogen bonds are given in Table 3. A view on a selected cut-off of the 3D network of 1 is shown in Fig. 2, displaying the metal–π and nitrilo–π interactions. As this illustration cannot show individual interactions, Figs. 2–4 illustrate the π–π interactions in more detail, whereby the same labeling code has been applied.

Fig. 3:

Selected part of the 3D network formed by 1 in the solid state. All carbon bonded hydrogen atoms are omitted for clarity. Dotted lines indicate intermolecular hydrogen bonds and open bonds indicate principal π interactions. Labels A to F refer to selected fragments with the following symmetry operations: A = 1 − x, 1 − y, 3 − z; B = 1 − x, y+ ½, 2.5 − z; C = x, ½ − y, z − ½; D = 2 − x, y+ ½, 2.5 − z; E = 2 − x, 1 − y, 3 − z; F = x, y, z + 1.

Fig. 4:

Illustration of the interactions of [Cu(phen)₂(CN)]⁺ of 1 with each other by means of π–π stacking of two C₆H₂ aromatic rings in two different perspective views. All carbon-bonded hydrogen atoms are omitted for clarity. Label A refers to a symmetry-generated molecule of [Cu(phen)₂(CN)]⁺, symmetry code: A = 2 − x, 1 − y, 2 − z. The sign refers to the interplanar angle between interacting C₆H₂ aromatic rings and D refers to the distance of the geometrical centroids of the atoms C4–C7, C11, C12 and C4A–C7A, C11A, C12A, respectively.

The present study describes the crystal structure of a unique example of polymeric copper(II) complexes, namely {[Cu(phen)₂(CN)][Ag(CN)₂] · 3H₂O}_n (1). The complex forms in the solid state a 3D network through hydrogen bonding and different types of π interactions.