**Synthesis and Thermochromic Luminescence of Ag(I) Complexes Based on 4,6-Bis(diphenylphosphino)-Pyrimidine**

#### **Alexander V. Artem'ev \* , Maria P. Davydova, Alexey S. Berezin and Denis G. Samsonenko**

Nikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3, Acad. Lavrentiev Ave., 630090 Novosibirsk, Russia; m\_davydova@mail.ru (M.P.D.); berezin@niic.nsc.ru (A.S.B.); denis@niic.nsc.ru (D.G.S.)

**\*** Correspondence: chemisufarm@yandex.ru

Received: 6 August 2020; Accepted: 24 August 2020; Published: 26 August 2020

**Abstract:** Two Ag(I)-based metal-organic compounds have been synthesized exploiting 4,6-bis (diphenylphosphino)pyrimidine (L). The reaction of this ligand with AgNO<sup>3</sup> and AgBF<sup>4</sup> in acetonitrile produces dinuclear complex, [Ag2L2(MeCN)2(NO3)2] (**1**) and 1D coordination polymer, [Ag2L(MeCN)3]*n*(BF4)2*<sup>n</sup>* (**2**), respectively. In complex **<sup>1</sup>**, <sup>µ</sup>2-P,P′ -bridging coordination pattern of the ligand L is observed, whereas its µ4-P,N,N′ ,P′ -coordination mode appears in **2**. Both compounds exhibit pronounced thermochromic luminescence expressed by reversible changing of the emission chromaticity from a yellow at 300 K to an orange at 77 K. At room temperature, the emission lifetimes of **1** and **2** are 15.5 and 9.4 µs, the quantum efficiency being 18 and 56%, respectively. On account of temperature-dependent experimental data, the phenomenon was tentatively ascribed to alteration of the emission nature from thermally activated delayed fluorescence at 300 K to phosphoresce at 77 K.

**Keywords:** Ag(I) complexes; metal-organic coordination polymers; luminescence; thermally activated delayed fluorescence; phosphorescence; pyrimidylphosphines

#### **1. Introduction**

Recently, Ag(I)-based metal-organic compounds have attracted increased attention as promising antibacterial agents [1,2], luminescent sensorics [3,4], and potential emitters for lighting application [5–7]. The rich coordination abilities of Ag(I) ion coupled with easy accessibility of the organic ligands provide ample opportunities for the design of most diverse coordination architectures covering both simple molecules and sophisticated metal-organic frameworks [8–13]. The charge balancing anions (NO<sup>3</sup> <sup>−</sup>, OAc−, OTf−, ClO<sup>4</sup> <sup>−</sup>, BF<sup>4</sup> <sup>−</sup>, PF<sup>6</sup> <sup>−</sup>, halides, etc.,) can also determine the structure of products, which are self-assembled via the reactions of Ag(I) precursors with organic ligands [14–16]. Remarkably, such giant structural diversity of the Ag(I) compounds provides various tools for tuning their functional properties. For instance, they can be regulated by adjusting the electronic properties of organic ligands. Not the least, of the factors are supramolecular interactions of Ag···Ag, Ag···π, and Ag···X kind, which also can influence the properties of the Ag(I) compounds, e.g., luminescence [17–21].

Concerning luminescence of silver(I) metal-organic compounds, it was almost neglected for a long time, probably because of the preconception on photosensitivity of this class in principle. It was not until fairly recently the Ag(I) complexes were recognized as promising emitters, which can exhibit enhanced quantum efficiency coupled with short decay time [22–27]. Note that the luminescent properties of Ag(I) complexes primarily depend on the structure of the ligand environment. Compared to Cu(I) complexes showing a metal-to-ligand charge transfer (MLCT) luminescence [28,29], the Ag(I) analogues commonly emit metal-perturbed ligand-centered fluorescence [30–32], ligand-centered

phosphorescence [33], or dual emission [34]. This stems from the fact that the MLCT excitation is hampered for Ag(I) complexes because of the higher ionization potential of Ag<sup>+</sup> ion in comparison with the Cu<sup>+</sup> one [35]. This restriction, however, can be overcome using highly electron-donating ligands (e.g., phosphines) coupled with π-acceptors (chelating diimines, azines, etc.,). On account of highly electron-donating ligands, the silver d<sup>10</sup> orbitals begin to contribute to HOMO and near-HOMO, while the π-acceptors facilitate charge transfer from the metal. As a result, (M + L')LCT excited states can be generated, thereby inducing thermally activated delayed fluorescence (TADF) in Ag(I) compounds [22–27,36–42]. In the context of OLED application, it is relevant to note that the (M + L')LCT emission of Ag(I) benefits over that of Cu(I) analogues since the former (*i*) is shorter in the lifetimes [36,41,42], and (*ii*) generally appears in the higher energy domain [36,37,39,43–45]. On the whole, Ag(I) compounds that emit apparent luminescence at ambient temperature, especially, that of TADF nature, are still rare. π π

Herein, we report on the synthesis and investigation of two Ag(I) compounds derived from 4,6-bis(diphenylphosphino)pyrimidine and AgNO<sup>3</sup> or AgBF4. Both compounds manifest pronounced thermochromic luminescence, which appears as a reversible changing of the emission color from yellow at 300 K to orange/red at 77 K. It should be noted in this regard that the Ag(I) complexes endowed with such property are very limited in number [13,46–49].

#### **2. Results and Discussion**

#### *Synthesis and Characterization*

The starting ligand, 4,6-bis(diphenylphosphino)pyrimidine (L), has been synthesized by the treatment of 2,6-dichloropyrimidine with 2 equiv. of lithium diphenylphosphide [50]. The ligand has been tested in the reaction with AgNO<sup>3</sup> and AgBF<sup>4</sup> in acetonitrile using different reactant's molar ratios. It has been revealed that the interaction of AgNO<sup>3</sup> with ligand L in a 1:1 molar ratio affords dinuclear complex [Ag2L2(MeCN)2(NO3)2] (**1**) isolated as solvate **1**·MeCN (Scheme 1). Meanwhile, using AgBF<sup>4</sup> under similar conditions, the reaction provides cationic 1D coordination polymer (CP), [Ag2L(MeCN)3]*n*(BF4)2*<sup>n</sup>* (**2**) that also crystallizes as solvate **2**·MeCN (Scheme 1). The preparative yields of products **1** and **2** are 69 and 91%, respectively. ∙ ∙

**Scheme 1.** Synthesis of compounds **1** and **2**.

∙ ∙ The products obtained are off-white powders, which are well soluble in acetonitrile. Upon storage on air, both **1**·MeCN and **2**·MeCN easily lose acetonitrile molecules. Note that the desolvation is reversible: the recrystallization of the powders formed from acetonitrile leads again to the crystals of the above solvates. Both compounds have been characterized by single crystal X-ray diffractometry (sc-XRD), FT-IR, and UV-Vis abortion spectroscopy.

Complex **1**·MeCN crystallizes in the monoclinic *P*21/*n* space group with one half molecule per asymmetric unit. As seen from Figure 1, the scaffold of **1** is formed by two Ag atoms bridged by two ligands L through phosphorus atoms so that the pyrimidine rings become coplanar. The latter are sandwiched in a "head-to-tail" manner with the distance between the average planes being 3.861 Å. The O atom of the NO<sup>3</sup> group and acetonitrile N atom complete the coordination sphere of Ag1 and Ag2 atoms to the distorted {Ag@P2ON} tetrahedron. The bond lengths around metal atoms are comparable with those of most related Ag(I) complexes [51–53]. In the crystal, molecules of **1** are associated together and with MeCN molecules via weak C–H···O and C–H···C contacts forming 3D supramolecular structure. ∙ ∙∙∙ ∙∙∙

∙ ′ ′ ′ ′ **Figure 1.** Molecular structure of **1**·MeCN. The aromatic H atoms and solvate molecules are omitted. Selected bond lengths (Å) and angles (◦ ): Ag1–P1 2.4224(13), Ag1–P2′ 2.4461(13), Ag1–O1N 2.273(9), Ag1–N1A 2.493(8); P1–Ag1–P2′ 118.48(4), P1–Ag1–O1N 123.4(4), P1–Ag1–N1A 107.0(3), P2′–Ag1–O1N 110.9(3), P2′–Ag1–N1A 96.8(3), O1N–Ag1–P1 134.1(3), O1N–Ag1–N1A 79.8(4). Symmetry code ('): 1 − *x*, 1 − *y*, 1 − *z*.

∙ ∙∙∙ ≈ ≈ ∙ ∙∙∙ ∙∙∙ ∙∙∙ ∙∙∙ CP **2**·MeCN crystallizes in the orthorhombic *P*21212<sup>1</sup> space group, and its crystals contain zig-zag chains propagating along *a* axis. The chains are built up from alternating ligand molecules and [Ag2(MeCN)3] units, which are linked via Ag–N and Ag–P bonds (Figure 2). The metal atoms of the [Ag2(MeCN)3] units are bridged by two P,N-faces of the adjacent ligands L in a "head-to-tail" fashion. The formed eight-membered cycles feature short Ag···Ag contact of 3.3352(4) Å that is consistent with twice van der Waals radius of Ag atom (3.44 Å [54]). Both Ag1 and Ag2 atoms are ligated by one MeCN molecule (dAg–N ≈ 2.33 Å); besides, a second MeCN molecule is weakly associated (dAg–N ≈ 2.65 Å) with Ag1 atom. As a result, Ag1 atom adopts a T-shaped {Ag@N2P} geometry, while Ag2 center has a distorted see-saw {Ag@N3P} environment. The Ag–N<sup>L</sup> and Ag–P distances are nearly the same as those in **1**·MeCN. The non-coordinated tetrafluoroborate anions and MeCN solvate molecules are associated with the [Ag2L(MeCN)3]*<sup>n</sup>* chains by means of van der Waals contacts such as C–H···F, C–H···N, Ag···F, and C···F.

− − λ ≈ FT-IR spectra of solid **1** and **2** are in agreement with sc-XRD data, showing characteristic vibrations of the ligand L along with stretching vibrations of the counter-ions (Figure S1). The N–O and B–F stretchings of the NO<sup>3</sup> <sup>−</sup> and BF<sup>4</sup> <sup>−</sup> groups appear as strong bands at 1380–1417 and 950–1200 cm–1 , correspondingly. The solid state UV-Vis spectra of **1** and **2** (plotted as Kubelka-Munk function, Figure S2) display broad bands expanding from the far-UV edge and falling close at about 400 and 440 nm, respectively (Figure S2). Each absorption band has two pronounced shoulders. The high-energy (HE) shoulders, with λmax ≈ 280 nm, are nearly the same for both compounds, while the low-energy (LE)

π π π

≈ ≈

ones maximize at ≈350 nm for **1** and at ≈390 nm for **2**. Considering the literature data [38], the HE absorption band can be attributed to intraligand π–π \* and n–π \* transitions. The LE band is likely associated with promotions of MLCT kind, which is typical for emissive Ag(I) complexes [22–27,36–40].

∙ − ′∙∙∙ ′ ′ ′ ′ ′ ′ ′ ′ ′ − − **Figure 2.** A fragment of the 1D chain of **2**·MeCN. The aromatic H atoms, [BF<sup>4</sup> ] <sup>−</sup> counterions and solvate molecules are omitted. Selected bond lengths (Å) and angles (◦ ): Ag1′ ···Ag2 3.3352(4), Ag1′–P2 2.3943(9), Ag1–N1 2.309(3), Ag1–N1A 2.320(4), Ag1–N4A 2.655(4), Ag2–P1′ 2.3622(9), Ag2–N2 2.242(3), Ag2–N2A 2.351(4); P2–Ag1′–Ag2 69.98(2), N1–Ag1–N1A 98.43(11), N1–Ag1–N4A 87.69(10), N1A–Ag1–N4A 81.28(13), P1′–Ag2–Ag1′ 72.95(2), N2–Ag2–Ag1′ 86.93(7), N2–Ag2–P1′ 152.18(7), N2–Ag2–N2A 92.88(12), N2A–Ag2–Ag1′ 165.92(12), N2A–Ag2–P1′ 111.31(9). Symmetry code ('): 0.5 + *x*, 1.5 − *y*, 1 − *z*.

τ Φ When UV-irradiated, solid compounds **1** and **2** exhibit yellow luminescence at ambient temperature. Upon cooling down to liquid nitrogen temperature, the luminescence strongly enhances, and its color changes to red-orange (for **1**) or red (for **2**) (Figures 3d and 4d). The thermochromic luminescence found appears to be reversible: warming the samples to 300 K recovers the initial emission chromaticity. Inspirited by these noticeable findings, we have studied the emission properties of the titled compounds at 77–300 K range. Temperature-dependent emission and excitation spectra of **1** and **2** are plotted in Figures 3 and 4, and the corresponding photophysical data are summarized in Table 1. As seen from the graphs, the emission spectra of **1** and **2** contain a broad band maximized at about 550 and 580 nm, accordingly. The corresponding emission colors on the CIE chromaticity diagram are consistent with those observed by the naked eye. The associated emission lifetimes (τobs) of **1** and **2** measured at 300 K are 15.5 and 9.4 µs, and the photoluminescence quantum yields (ΦPL) are 18 and 56%, respectively. The excitation profiles of **1** and **2** are presented by smooth bands that fall close at about 420 and 440 nm (Figures 3b and 4b). The excitation curves, therefore, resemble the absorption patterns (Figure S2). Note that the compounds studied do not possess excitation-dependent properties, which are quite common for Ag(I) complexes [55–57]. When the temperature is gradually lowered to 77 K, the emission bands of **1** and **2** are red-shifted by 30 and 22 nm (Figures 3a and 4a), thereby changing the emission color to red-orange and red, respectively (Figure 3c,d). Simultaneously, the lifetimes rise to 3970 µs (**1**) and 300 µs (**2**).

λ λ λ **Figure 3.** (**a**) Temperature-dependent emission spectra of **1** (λex = 385 nm); (**b**) temperature-dependent excitation spectra of **1** (λem = 580 nm); (**c**) temperature dependence of the emission chromaticity of **1** (λex = 385 nm); (**d**) emission color of sample **1** at 300 and 77 K. λ λ λ

λ λ λ λ **Figure 4.** (**a**) Temperature-dependent emission spectra of **2** (λex = 410 nm); (**b**) temperature-dependent excitation spectra of **2** (λem = 580 nm); (**c**) temperature dependence of the emission chromaticity of **2** (λex = 385 nm); (**d**) emission color of sample **2** at 300 and 77 K.

λ

λ

**Table 1.** Photophysical data for solid **1** and **2**.


Taken together, these observations suggest that TADF is likely responsible for the room temperature emission of **1** and **2**. The temperature dependence of the lifetimes, τobs (T), measured in 77–300 K window, supports this suggestion, following the equation intended for the TADF model [58]: τ

$$\tau\_{\rm obs}(T) = \left( 3 + \exp\left(-\frac{\Delta E\_{\rm ST}}{k\_B T}\right) \right) / \left(\frac{3}{\tau\_T} + \frac{1}{\tau\_S} \exp\left(-\frac{\Delta E\_{\rm ST}}{k\_B T}\right) \right) \tag{1}$$

wherein τ*<sup>S</sup>* and τ*<sup>T</sup>* are the lifetimes of prompt fluorescence and phosphorescence, respectively, ∆*EST* is the energy gap between the respective excited states (S<sup>1</sup> and T1), and *k<sup>B</sup>* is the Boltzmann constant. Applying this equation for fitting the datasets of Figure 5, the following values have been roughly estimated for **1**: ∆E(S1−T1) = 750 cm−<sup>1</sup> , fluorescence lifetime τ (S1) = 400 ns, and phosphorescence lifetime τ (T1) = 4000 µs. Analogously, the following values have been estimated for **2**: ∆E(S1−T1) = 1000 cm−<sup>1</sup> , τ (S1) = 35 ns, and τ (T1) = 300 µs. It should be emphasized that the given ∆E(S1−T1) values are purely evaluative because the Eq. 1 can be applied correctly when the emission quantum yields do not change much than the investigated temperature range [58]. Nevertheless, the estimated ∆E(S1−T1) magnitudes agree well with the common values for TADF-emitting Ag(I) and Cu(I) complexes, thus allowing to assume manifestation of TADF by **1** and **2**. ߬ௌ ்߬ ݇ ்ௌܧ∆ Δ − <sup>−</sup> τ τ Δ − <sup>−</sup> τ τ Δ − Δ −

λ λ λ λ **Figure 5.** (**a**) Temperature dependence of the emission lifetimes for **1** (λex = 385 nm, λem = 550 nm), and (**b**) for **2** (λex = 410 nm, λem = 580 nm).

τ τ Δ − τ As seen from Figure 5, the luminescence of both compounds at ambient temperature represents TADF because the τobs (T) curves attain the high-temperature plateau. The pure phosphorescence begins to appear when the τobs (T) curve reaches the low-temperature plateau. In the case of **2**, it occurs below 120 K, while complex **1**, possessing a narrower ∆E(S1−T1) gap, emits pure phosphorescence at 77 K and below. On the drop-down range of the τobs (T) curves, the contribution of the TADF gradually increases up to ~100% at 300 K, owing to the thermal population of the higher-lying S<sup>1</sup> state from the T<sup>1</sup> state. As a result, the emission energy shifts in the blue region upon warming from 77 to 300 K, resulting in thermochromic luminescence of **1** and **2**. Considering the literature data on the related Ag(I) complexes exhibiting TADF [22–27,36–42], we believe that the S<sup>1</sup> and T<sup>1</sup> excited states of **1** and **2** are of MLCT or (M + L')LCT nature. The DFT computations performed on complex **1** support this suggestion revealing that HOMO and near-HOMOs are contributed by silver d-orbitals and p-orbitals of nitrate oxygen atoms, while LUMO and near-LUMOs are pure pyrimidine π-orbitals (Figure S10). Note the previously reported Ag(I) complexes feature a similar HOMO/LUMO distribution pattern [22–27,36–42]. Thus, it can be assumed that the emissive excited state of **1** has a 1,3(M + L')LCT character (L' = NO3, and L = pyrimidyldiphosphine).

#### **3. Materials and Methods**

#### *3.1. General*

Synthesis of the ligand L was performed under argon atmosphere, while the compounds **1** and **2** were prepared under ambient conditions. AgNO<sup>3</sup> (≥99.9%, Aldrich, St. Louis, Missouri, MO, USA), AgBF<sup>4</sup> (≥99.9%, Aldrich), *n*-BuLi (2.5 M in hexanes, Aldrich), 4,6-dichloropyrimidine (97%, Aldrich), and diphenylphosphine (98%, Aldrich) were used as purchased. Prior to use, commercial tetrahydrofuran (THF, anhydrous, ≥99.9%, Aldrich) was purified by distillation over sodium/benzophenone under argon flow. Acetonitrile and dichloromethane were distilled over phosphorus pentoxide.

FT-IR spectra were measured on a Bruker Vertex 80 spectrometer (Bruker, Billerica, Massachusetts, MA, USA) at ambient temperature. The microanalyses were performed on a MICRO cube analyzer.

<sup>1</sup>H, <sup>13</sup>C, and <sup>31</sup>P{1H} NMR spectra were recorded using a Bruker AV-500 spectrometer at 500.13, 125.8 MHz and 202.46 MHz, respectively, with solvent peaks as reference. The <sup>31</sup>P{1H} NMR shifts are expressed with respect to 85% H3PO4/D2O as an external standard.

The microanalyses were performed on a MICRO cube analyzer Photoluminescence spectra were recorded on a Fluorolog 3 spectrometer (Horiba Jobin Yvon, Kyoto, Japan) with a cooled PC177CE-010 photon detection module equipped with an R2658 photomultiplier. The luminescence decays (Figures S5 and S6) were measured on the same instrument. The absolute values of PLQYs were recorded using a Fluorolog 3 Quanta-phi device (Horiba Jobin Yvon). The luminescence quantum yield at 77 K was obtained relative to the quantum yield of the same sample at 300 K. Independently, these relative quantum yields were calibrated by using the absolute PLQY values measured at 77 K. Temperature dependences of luminescence were carried out using Optistat DN optical cryostats (Oxford Instruments, Abingdon, UK).

The solid-state reflectance spectra were recorded on a Shimadzu UV-3101 spectrophotometer (Shimadzu Corporation, Kyoto, Japan). Samples were prepared by a thorough grinding of a mixture of a complex (*ca.* 2 mol %) with BaSO4. The reflectance data were converted into a spectrum applying a Kubelka-Munk function using BaSO<sup>4</sup> as a standard.

DFT computations of **1** were performed using the hybrid B3LYP functional [59] combined with the def2TZVP basis sets [60]. The calculations were performed for single point geometry taken from X-ray coordinates for non-hydrogen atoms. The computations were proceeded using Gaussian 09 suite [61].

#### *3.2. Synthesis of 4,6-Bis(diphenylphosphino)pyrimidine (L)*

*n*-BuLi in hexanes (2.5 M, 15 mL) was added dropwise to a solution of diphenylphosphine (6.139 g, 0.033 mol) in absolute THF (40 mL) at −20 ◦C. The mixture was kept at −20 ◦C and stirred for 1 h. Then, at the same temperature, suspension of 4,6-dichloropyrimidine (2.384 g, 0.016 mol) in THF (10 mL) was added dropwise. The resulting mixture was warmed to 40 ◦C and stirred for 4 h. After that H2O (50 mL) was added and the quenched mixture was extracted with CH2Cl<sup>2</sup> (3 × 30 mL). The organic extracts were washed with H2O (3 × 10 mL), dried with Na2SO4, and evaporated in vacuum. The crude product obtained was recrystallized from MeOH/CH2Cl<sup>2</sup> (10:1, *v*/*v*) to give colorless crystals of L. Yield: 4.087 g (57%). <sup>1</sup>H NMR (500.13 MHz, CDCl3) δ 9.20 (s, 1H, C2–H in Pym), 7.35–7.31 (m, 20H in Ph), 6.69 (s, 1H, C5–H in Pym). <sup>13</sup>C{1H} NMR (126 MHz, CDCl3) δ 174.4 (d, *J* = 7.0 Hz, C<sup>4</sup> and C<sup>6</sup> in Pym), 156.8 (t, *J* = 9.3 Hz, C<sup>2</sup> in Pym), 134.4 (s, *o*-Ph), 134.3 (s, *o*-Ph), 133.6 (d, *J* = 8.8 Hz, *i*-Ph), 132.1

(d, *J* = 9.6 Hz, C<sup>5</sup> in Pym), 129.7 (s, *p*-Ph), 128.9 (d, *J* = 7.8 Hz, *m*-Ph). <sup>31</sup>P{1H} NMR δ (202.47 MHz, CDCl3) −2.62. FT-IR (KBr, cm−<sup>1</sup> ): 424 (w), 434 (w), 444 (m), 463 (w), 484 (m), 500 (vs), 544 (w), 608 (w), 619 (w), 696 (vs), 743 (vs), 766 (m), 783 (w), 889 (w), 978 (w), 999 (m), 1026 (m), 1070 (w), 1097 (m), 1157 (w), 1184 (w), 1260 (s), 1310 (w), 1331 (w), 1435 (s), 1479 (s), 1535 (s), 1584 (w), 2984 (vw), 3049 (w), 3071 (vw).

#### *3.3. [Ag2L2(NO3)2(MeCN)2]*·*MeCN (1*·*MeCN)*

A solution of L (50 mg, 0.11 mmol) and AgNO<sup>3</sup> (20 mg, 0.11 mmol) in CH3CN (1 mL) was stirred at room temperature for 30 min. The precipitated white powder of the **1**·CH3CN was centrifuged and dried in air. White powder. Yield: 50 mg (69%). Single crystals of **1**·CH3CN were grown by vapor diffusion of diethyl ether into the CH3CN solution for overnight. FT-IR (thin film, cm−<sup>1</sup> ): 474 (m), 505 (s), 692 (vs), 746 (s), 997 (w), 1028 (w), 1099 (m), 1265 (m), 1288 (s), 1385 (s), 1420 (s), 1437 (vs), 1481 (m), 1495 (m), 1539 (vs), 2251 (w), 2294 (vw), 2921 (w), 2936 (vw), 2994 (vw), 3056 (w). Anal. Calcd: C56H44Ag2N6P4O<sup>6</sup> (1236.62) C, 54.4; H, 3.6; N, 6.8. Found: C, 54.4; H, 3.4; N, 6.7. Since the solvate **1**·CH3CN quickly loses the molecules of acetonitrile upon storage, the microanalysis was calculated on [Ag2(L)2(NO3)2].

#### *3.4. [Ag2L(MeCN)3]n(BF4)2n*·*MeCN (2*·*MeCN)*

A solution of L (50 mg, 0.11 mmol) and AgBF<sup>4</sup> (43 mg, 0.22 mmol) in CH3CN (1 mL) was stirred at room temperature for 30 min. To the resulting solution, diethyl ether (5 mL) was then added and the precipitate formed was centrifuged and dried in air. White powder. Yield: 100 mg (91%). Single crystals of **2**·CH3CN were grown by vapor diffusion of diethyl ether into the CH3CN solution for overnight. FT-IR (cm−<sup>1</sup> ): 478 (w), 507 (m), 519 (m), 692 (S), 746 (m), 997 (m), 1063 (vs), 1084 (vs), 1097 (vs), 1163 (w), 1184 (w), 1287 (w), 1308 (w), 1437 (s), 1454 (w), 1481 (w), 1497 (w), 1560 (m), 1634 (w), 2253 (w), 2298 (w), 2388 (w), 2971 (w), 3009 (vw), 3063 (w). Anal. Calcd: C28H22Ag2B2F8N2P<sup>2</sup> (837.78) C, 40.1; H, 2.6; N, 3.3. Found: C, 40.0; H, 2.6; N, 3.5. Since the solvate **2**·CH3CN quickly loses the molecules of acetonitrile upon storage, the microanalysis was calculated on [Ag2L](BF4)2.

#### *3.5. X-ray Crystallography*

Single crystals of **1**·MeCN and **2**·MeCN were grown by diffusion of diethyl ether vapors into a MeCN solutions at ambient temperature for overnight. The X-ray data and the details of the refinement are summarized in Table S1. Diffraction data were collected on an automated Agilent Xcalibur diffractometer equipped with an area AtlasS2 detector (graphite monochromator, λ (Mo Kα) = 0.71073 Å, ω-scans, Agilent, Santa Clara, California, CA, USA). Integration, absorption correction, and determination of unit cell parameters were performed using the CrysAlisPro program package [62]. The structures were solved by dual space algorithm (SHELXT [63]) and refined by the full-matrix least squares technique (SHELXL [64]) in the anisotropic approximation (except hydrogen atoms). Positions of hydrogen atoms of organic ligands were calculated geometrically and refined in the riding model.

The crystallographic data and details of the structure refinements are summarized in Table S1. CCDC 2020455 and 2020456 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Center at http://www.ccdc. cam.ac.uk/data\_request/cif.

#### **4. Conclusions**

Thus, two Ag(I) metal-organic compounds have been synthesized by the treatment of 4,6-bis(diphenylphosphino)pyrimidine (L) with AgNO<sup>3</sup> and AgBF<sup>4</sup> in acetonitrile. It has been revealed that the interaction with AgNO<sup>3</sup> results in neutral dinuclear complex, [Ag2L2(MeCN)2(NO3)2], while the reaction with AgBF<sup>4</sup> produces cationic 1D zig-zag polymer, [Ag2L(MeCN)3]*n*(BF4)2*n*. The structure of the complex is built up from two Ag(I) ions bridged by the two ligands in a

µ2-P,P′ -manner. The 1D chains of the polymer are assembled by alternating ligand (µ4-N,P,N′ ,P′ ) and [Ag2(MeCN)3] units, interconnected through Ag–N and Ag–P bonds. Both title compounds feature pronounced thermochromic luminescence, which appears as reversible yellow-to-orange changing of the emission color during the cooling-warming cycling (300–77 K). The detailed temperature-dependent photophysical study has shown that the ambient temperature photoluminescence of the above compounds may be tentatively ascribed to TADF. At 77 K, they certainly emit pure phosphorescence. The distinct thermochromic behavior of the complexes designed makes them promising materials for luminescent thermometry. From the fundamental viewpoint, the findings reported contribute to coordination chemistry and photophysics of Ag(I)-based metal-organic compounds.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2304-6740/8/9/46/s1, Figure S1: FT-IR spectra of **1** and **2** displayed in the fingerprint range; Figure S2: Solid state absorption spectra of **1** and **2** plotted as a Kubelka–Munk function; Figure S3: Temperature dependence of the integral intensity of the emission of **1** (λex = 365 nm); Figure S4: Temperature dependences of the integral intensity of the emission of **2** recorded at λex = 410 nm (*left*) and λex = 365 nm (*right*); Figure S5: Emission decay profiles of **1** recorded at different temperatures (λex = 385 nm, λem = 550 nm); Figure S6: Emission decay profiles of **2** recorded at different temperatures (λex = 410 nm, λem = 580 nm); Figure S7: <sup>1</sup>H NMR spectrum of the ligand **L** (CDCl<sup>3</sup> ); Figure S8: J-modulated <sup>13</sup>C NMR spectrum of the ligand **L** (CDCl<sup>3</sup> ); Figure S9: <sup>31</sup>P{1H} NMR spectrum of the ligand **L** (CDCl<sup>3</sup> ); Figure S10: Four lowest unoccupied and 4 highest occupied MOs (iso-value = 0.045) for the S<sup>0</sup> state of the complex **1** calculated at the B3LYP/def2TZVP level; Table S1: X-Ray crystallographic data for **1**·CH3CN and **2**·CH3CN; the CIF and the checkCIF output files are included in the Supplementary Materials.

**Author Contributions:** Project conceptualization, administration, supervision, writing—review and editing, and funding acquisition, A.V.A.; investigation, writing, data curation, visualization, M.P.D. and A.S.B.; crystallography, D.G.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by Russian Science Foundation (Project 18-73-10086).

**Acknowledgments:** We thank Evgeniya Doronina (A.E. Favorsky Irkutsk Institute of Chemistry, Irkutsk, Russian Federation) for help with the DFT calculations.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

#### *Article* **E**ff**ect of Coordinating Solvents on the Structure of Cu(II)-4,4**′ **-bipyridine Coordination Polymers**

#### **Marzio Rancan 1,\* , Alice Carlotto <sup>2</sup> , Gregorio Bottaro <sup>1</sup> and Lidia Armelao 1,2,\***


Received: 23 July 2019; Accepted: 13 August 2019; Published: 19 August 2019

**Abstract:** Solvent can play a crucial role in the synthesis of coordination polymers (CPs). Here, this study reports how the coordinating solvent approach (CSA) can be used as an effective tool to control the nature of the final CP. This study exploited the system Cu(II)-4,4′ -bipyridine coupled to different coordinating solvents, such as DMA, DMF and DMSO. This allowed the isolation and structurally characterization of four new CPs: three 2D layered networks and one 1D chain. Moreover, it was evidenced that even adventitious water can play the role of the coordinating solvent in the final CP.

**Keywords:** coordination polymer; MOF; CP; dimensionality control; Cu(II)-4,4′ -bipyridine; dipyridil ligand; copper

#### **1. Introduction**

Coordination polymers (CPs) and metal organic frameworks (MOFs) have attracted increasing interest over the last two decades since the features of these systems are potentially useful in several cutting-edge research areas [1–3]. Countless studies have been devoted by researchers to these compounds, however there are still controversial opinions as to whether a real design of these systems can be applied [4–6]. In fact, the predictability of the final network can be a challenge since it is a consequence of the self-assembly process that involves competing, reversible and simultaneous interactions among the metal, ligand, counterion and solvent, just to mention the main chemical actors. In this context, the energy of the metal-ligand bond plays a crucial role. Metal ions and charged ligands (for instance carboxylates) can give quite strong bonds (200–400 kJ/mol ca.) paving the route to the so-called reticular chemistry that allows a good design of the final MOF [7]. This strategy has been used to develop large families of structures where the network can be designed combining the starting building blocks and at the same time, the pores size can be controlled by simply varying the length of the ligand maintaining the same network topology and obtaining isoreticular MOFs [8,9]. On the other hand, when considering weaker interactions, as for instance metal ions and neutral ligands (60–180 kJ/mol ca.), the final coordination outcome is not easy to control, both in terms of topology and network dimensionality. In fact, as the interaction energy between metal and ligand decreases, the system is more prone to be affected by other parameters, such as the counterion and the solvent. In particular, when considering Cu(II) and the 4,4′ -bipyridine ligand (bpy), many different CPs and MOFs can be obtained. The isolated structure strongly depends on the counterion leading, for instance, to systems with different dimensionalities (1D, 2D and 3D) and topologies [10–14]. In addition, the solvent can play an important role. For instance, the 2D [Cu(bpy)2(CF3SO3)2]*<sup>n</sup>* framework can be transformed into a hydrogen bond assisted 3D framework through a solvent (H2O) mediated process [15] and solvent dependent routes were developed to obtain 2D or 3D [Cu(bpy)2(CF3SO3)2]*<sup>n</sup>*

networks [16]. In this context, the authors previously showed that a coordinating solvent, such as dimethyl sulfoxide (DMSO), can be used to tune and control the dimensionality of the final CP [17]. This coordinating solvent approach (CSA) promotes dimensional variability, driving the formation of Cu–bpy architectures, such as a 3D nanoporous network ({[Cu2(bpy)4(DMSO)3(ClO4)](ClO4)3·2DMSO}*n*, **1**) or a 1D chiral chain ({[Cu(bpy)2(DMSO)4](ClO4)2}*n*, **2**) just changing the crystallization conditions (i.e., the presence of a co-solvent or evaporation rate). In solution, DMSO and bpy molecules establish a series of dynamic equilibria to coordinate the metal center during the self-assembly process. The coordinating solvent can block a different number of coordination sites leading to different CP architectures. Recently, the authors have also demonstrated that Cu–bpy bonds and coordinating solvents can be used to reversibly self-assemble mechanically interlocked CPs, such as the coordination-driven polyrotaxane-like architectures [18], when employing a Cu–metallocycle as a platform for the self-assembly of the Cu–bpy extend architectures.

Herein, this study extended CSA to other coordinating solvents (*N*,*N*-dimethylformamide, DMF; *N*,*N*-dimethylacetamide, DMA; and mixtures of them) and their effect on the final Cu–bpy based CPs were studied. Moreover, it was also demonstrated that even water can participate as a coordinating solvent during the dynamic equilibria that lead to the final CP. All the new compounds have been isolated as single crystals and structurally characterized resulting in three new 2D and one 1D CPs.

#### **2. Results**

All the CPs (**3**–**6**) were synthesized by dissolving Cu(ClO4)2·6(H2O) in a coordinating solvent and adding bpy in a 1:2 ratio. The solvent evaporation or diffusion of a co-solvent allowed isolating single crystals of the compounds. All the structures were solved by the single-crystal X-ray crystallographic method and their phase purity confirmed by powder X-ray diffraction (PXRD, Figure S1). The important refinement and geometric parameters are shown in Table 1.


**Table 1.** Crystal data and structures refinement.

By using DMA, light blue single crystals of {[Cu(bpy)2(H2O)2](ClO4)2·2DMA}*<sup>n</sup>* (**3**) were obtained in good yield (70% ca.). An X-ray analysis evidenced that four bpy molecules in the equatorial plane and two water molecules in apical positions coordinated the copper center, Figure 1a. The copper atom has a Jahn–Teller distorted octahedral coordination with Cu–N distances of 2.027(4), 2.036(6) Å and 2.047(6) (for Cu1–N1, Cu1–N2 and Cu1–N3, respectively) and a Cu1–O1 bond length of 2.482(2) Å. The water molecule forms H-bonds with a ClO<sup>4</sup> <sup>−</sup> anion (2.275(9) Å) and with a DMA molecule (1.972(5) Å). The network develops as a 2D CP, leading to layers of grids composed of repeating squares with

Cu–bpy–Cu sides (Figure 1b). Two slightly different Cu–bpy–Cu distances can be found, one of 11.1385(7) Å coinciding with the *b* axis and the other of 11.1165(7) Å along the *c* axis equal to half its length. The 2D layers are at a distance of *a*/2 (7.3462(5) Å) and mismatched along the *b* axis with a value of *b*/2 (Figure 1c). This packing of the grids layers leads to the formation of two different alternating channels (Figure 1d) that occupy the 66% of unit cell volume. In one case, the plane containing the bpy molecules of a grid is parallel to the grow direction of the channel. In the other one, that plane is perpendicular to the channel direction. Perchlorate anions and DMA molecules that lay between the grid layers fill the channels. −

− − − − − **Figure 1.** (**a**) Coordination environment of the copper center in compound **3** (thermal ellipsoids drawn at the 50% probability level). (**b**) 2D network (single layer); (**c**) view along the *ab* plane of alternating and mismatched grids; (**d**) channels formed by the 2D layers (green points). Color code: Cu, purple; O, red; N, blue; C, grey; Cl, green; H, white. Anions, DMA molecules and H atoms omitted for clarity in the packing figures. Symmetry operations: *i* = 1 − *x*, −1 + *y*, 1/2 − *z*; *ii* = 1 − *x*, +*y*, 1/2 − *z*.

When DMF was used in place of DMA, the solvent evaporation at open air led to a major crop (yield 40% ca.) of deep blue single crystals of {[Cu(bpy)2(DMF)2](ClO4)2·2(bpy)}*<sup>n</sup>* (**4**) along with a minor fraction (yield 3% ca.) of light blue single crystals of {[Cu(bpy)2(H2O)2](ClO4)2·2(bpy)·2(H2O) }*<sup>n</sup>* (**5**).

Considering compound **4**, four bpy molecules in the equatorial plane coordinate the copper center and the apical positions are occupied by DMF molecules (Figure 2a). Copper has a Jahn–Teller distorted octahedral coordination with Cu–N distances of 2.0250(12), 2.0564(12) Å (for Cu1–N1 and Cu1–N2, respectively) and a Cu1–O1 bond length of 2.5180(12) Å. Even **4** is a 2D CP with layers of grids (Figure 2b) composed of repeating squares with Cu–bpy–Cu sides of 11.1640(3) Å and one Cu···Cu diagonal coinciding with *b* axis (15.7544(4) Å). The 2D layers in **4** are at a distance of 10.3 Å and mismatched of 5.6 Å (Figure 2c). This layer packing leads to the formation of only one kind of channels (Figure 2d) that occupies 57% of the unit cell volume. These channels are filled by the ClO<sup>4</sup> − anions that lay between the grid layers and by uncoordinated bpy molecules hosted in the Cu–bpy squares. In particular, each square hosts two bpy molecules that interacts with the CP network with a series of CH···π interactions and among them by π···π stacking (Figure 2e,f). −

π π π

π π π − − − − − − **Figure 2.** (**a**) Coordination environment of the copper center in compound **4** (thermal ellipsoids drawn at the 50% probability level). (**b**) 2D network (single layer); (**c**) view along the *ac* plane of alternating and mismatched grids; (**d**) channels formed by the 2D layers (green points). (**e**) Two uncoordinated bpy molecules hosted by a Cu–bpy square. (**f**) CH···π interactions and π···π stacking in the host-guest ensemble. Color code: Cu, purple; O, red; N, blue; C, grey. Anions and H atoms omitted for clarity. Symmetry operations: *i* = 2 − *x*, 1 − *y*, 1 − *z*; *ii* = 1/2 + *x*, 3/2 − *y*, 1/2 + *z*; *iii* = 3/2 − *x*, 1/2 + *y*, 1/2 − *z*.

In CP **5**, the coordination environment of the Cu2<sup>+</sup> ions is similar, as in CP **3**. The four bpy molecules in the equatorial plane and the two water molecules in apical positions coordinate the copper center (Figure 3a). The metal atom has a Jahn–Teller distorted octahedral coordination with Cu–N distances of 2.0670(19), 2.041(3) Å and 2.045(3) (for Cu1–N1, Cu1–N2 and Cu1–N3, respectively) and a Cu1–O1 bond length of 2.4965(19) Å. Similar to **3** and **4**, the network develops as a 2D CP, leading to layers of grids composed of repeating squares with Cu–bpy–Cu sides (Figure 3b). Further in CP **5**, the apical water molecules form H-bonds with a ClO<sup>4</sup> <sup>−</sup> anion (2.128(3) Å) and with an uncoordinated bpy molecule (2.005(3) Å). The side of the bpy molecule not involved in the H-bond is diagonally pointing towards the center of a Cu–bpy square of a second grid layer. In this case, the starting DMF coordinating solvent is not found in the structure. Two slightly different Cu–bpy–Cu distances can be found: one of 11.1702(4) Å coinciding with the *b* axis and the other of 11.2841(6) Å. The 2D layers in **5** are at a closer distance (8.3 Å) compared to **3** and only slightly mismatched (Figure 3c). This layer packing leads to the formation of only one kind of channels (Figure 3d) that occupies 70% of the unit cell volume. Perchlorate anions and bpy molecules, that lay between and inside (bpy) the grid layers, fill those channels. −

− − − − **Figure 3.** (**a**) Coordination environment of the copper center in compound **5** (thermal ellipsoids drawn at the 50% probability level). (**b**) 2D network (single layer); (**c**) view along the *ac* plane of alternating and slightly mismatched grids; (**d**) channels formed by the 2D layers (green points). Color code: Cu, purple; O, red; N, blue; C, grey; Cl, green; H, white. Anions, uncoordinated bpy molecules and H atoms omitted for clarity in the packing figures. Symmetry operations: *i* = 1/2 − *x*, +*y*, 1/2 − *z*; *ii* = +*x*, −1 − *y*, +*z*.

It is worthy to note that both CPs **3** and **5** are obtained with coordinated water molecules instead of the solvents, DMA or DMF. This is an important difference compared to what happens when using DMSO as a coordinating solvent [17]. In that case, slow evaporation led to a Cu–bpy based 3D CP (**1**) while faster evaporation gave a 1D chiral chain (**2**). In both cases, the only solvent coordinating the

Cu center was DMSO. However, it was postulated that in the CSA, the role of water as competitive coordinating solvent may not be excluded. As a matter of fact, synthesis with DMA and DMF confirmed our hypothesis. The sources of water can be found in the hydration water molecules of the starting Cu(ClO4)2, in the non-anhydrous solvents, or even in the evaporation performed at open air.

Finally, the employment of mixtures of coordinating solvents was explored. DMA/DMF, DMF/DMSO and DMA/DMSO 1:1 solutions evaporation did not give any crystalline product suitable for single crystal X-ray diffraction. Very slow vapor diffusion of diethylether in a DMF/DMSO mixture led to light blue single crystals of {[Cu(bpy)2(DMF)(DMSO)](ClO4)2}*<sup>n</sup>* (**6**, Figure 4) in low yields (10% ca.). The X-ray structure shows that copper has a Jahn–Teller distorted octahedral coordination with the equatorial positions taken by two bpy molecules (Cu1–N1 and Cu1–N2 2.004(2) Å) and two DMSO molecules (Cu1–O1 2.0114(15) Å). The axial positions are occupied by two DMF molecules (Cu1–O2 2.3662(17) Å). The CP develops as a 1D chain with a Cu–bpy–Cu distance of 11.0913(7) Å coinciding with *b* axis. The chain grow linearly since the two bpy molecules lay in equatorial *trans* positions. On the contrary, when using only DMSO, the bpy ligands take two equatorial *cis* positions leading to a helicoidal 1D CP isolated as enantiopure single crystals (**2**) [17].

− − − − − **Figure 4.** (**a**) Coordination environment of the copper center in compound **6** (thermal ellipsoids drawn at the 50% probability level). (**b**) 1D chains along the plane *cb*. Color code: Cu, purple; O, red; N, blue; C, grey; S, yellow. Anions and H atoms omitted for clarity. Symmetry operations: *i* = 3/2 − *x*, +*y*, 1 − *z*; *ii* = 3/2 − *x*, −1 + *y*, 1 − *z*. − − − − −

#### **3. Discussion**

By employing strong coordinating solvents coupled to a neutral ligand, such as bpy, competitive coordination equilibria are introduced in the self-assembly process. The core concept of the CSA is depicted in Figure 5. The results obtained in the authors current and former [17] studies on the effect of coordinating solvents towards the final Cu–bpy based CPs are summarized in Table 2.

**Figure 5.** Scheme of the competing equilibria between the coordinating solvent and bridging ligand leading to different coordination polymers (CPs) through the coordinating solvent approach (CSA).


**Table 2.** The effect of coordinating solvents towards the final Cu–bpy based CPs.

1 slow evaporation; <sup>2</sup> in the presence of a non-coordinating solvent; <sup>3</sup> major product; <sup>4</sup> minor product.

The competitive equilibria are driven towards the CP by solvent evaporation or by introducing a co-solvent to change the medium features and to induce crystallization. Hence, when dissolving the metal center in a coordinating solvent, the process leading to the formation of the final CP can be described as the subsequent substitution of the coordinated solvent molecules by the bridging ligands. Thus, the solvent becomes itself a ligand that can compete with the bpy. This allows obtaining CPs with different networks and dimensionalities according to the solvent nature, the crystallization technique and to the remaining number of the coordinated solvent molecules versus the divergent ligands. Competitive species are present and this can lead to obtaining byproducts in low yields as in the case of CP **5**. This byproduct of CP **4** appears in the last stages of solvent evaporation and can be easily avoided by stopping the evaporation before completeness. Some general trends for the CSA applied to Cu–bpy systems can be found. The bridging bpy molecules always occupy the equatorial positions of the Cu octahedral coordination sphere. When four solvent molecules coordinate the copper (II) ion, 1D chains can be obtained, with the bpy molecules either in *trans* (CP **6**) or *cis* (CP **2**) positions. The presence of two apical solvent molecules in the coordination sphere always leads to the same structural motif, i.e., Cu–bpy squares forming extended grid-like architectures that can develop towards 2D (CPs **3**, **4**, **5**) or 3D (CP **1**) networks. Finally, it is worth noting that, when using DMA or DMF, it is likely to obtain CPs with coordinated water molecules instead of DMA or DMF ones, either as a single isolated product (**3**) or as an impurity (**5**).

#### **4. Materials and Methods**

#### *4.1. Synthesis*

The reagents were purchased from Sigma-Aldrich (St. Louis, MO, USA) and used as received. The elemental analyses were carried out with a Flash 2000 Thermo Scientific analyzer (Thermo Fisher Scientific, Cambridge, UK) at the Department of Chemical Sciences of the University of Padova.

#### 4.1.1. Synthesis of {[Cu(bpy)2(H2O)2](ClO4)2·2DMA}*<sup>n</sup>* (**3**)

Cu(ClO4)2·6H2O (37 mg, 0.1 mmol) was dissolved in 10 mL of DMA in a large beaker and 4,4′ -bipyridine (bpy, 30 mg, 0.2 mmol) was added. The slow evaporation of the solvent led to light blue single crystals suitable for an X-ray analysis. Yield 55 mg, 70% (based on copper). The elemental analysis for C28H38Cl2CuN6O12, exp (%): C 42.53, N 10.58, H 4.92; calc (%): C 42.84, N 10.70, H 4.88.

#### 4.1.2. Synthesis of {[Cu(bpy)2(DMF)2](ClO4)2·2(bpy)}*<sup>n</sup>* (**4**) and {[Cu(bpy)2(H2O)2](ClO4)2·2(bpy)·2(H2O)2}*<sup>n</sup>* (**5**)

Cu(ClO4)2·6H2O (37 mg, 0.1 mmol) was dissolved in 10 mL of DMF in a large beaker and 4,4′ -bipyridine (bpy, 30 mg, 0.2 mmol) was added. The slow evaporation of the solvent led to deep blue single crystals of **4**. If the solvent was left to completely evaporate, a second kind of light blue single crystals appeared in the last evaporation stages as an impurity (**5**, yield 3 mg, 3% ca.). Due to their different color, the two species can be easily manually separated. To avoid crystallization of CP **5**, single crystals of compound **4** was removed from the solution before complete evaporation with a yield of 43 mg, 40% (based on copper). The elemental analysis for **4**, C46H46Cl2CuN10O10, exp (%): C 53.31, N 13.44, H 4.56; calc (%): C 53.47, N 13.55, H 4.49. The elemental analysis for **5**, C40H40Cl2CuN8O11, exp (%): C 51.72, N 11.98, H 4.21; calc (%): C 51.93, N 11.88, H 4.27.

#### 4.1.3. Synthesis of {[Cu(bpy)2(DMF)(DMSO)](ClO4)2}*<sup>n</sup>* (**6**)

Cu(ClO4)2·6H2O (37 mg, 0.1 mmol)) was dissolved in 1 mL of a DMSO/DMF (1:1) solution and 4,4′ -bipyridine (bpy, 30 mg, 0.2 mmol) was added. The very slow diffusion of diethylether vapor gave light blue single crystals after 6 weeks. Yield 8 mg, 10% (based on copper). The elemental analysis for C20H34Cl2CuN4O12S2, exp (%): C 33.37, N 8.01, S 9.05, H 4.83; calc (%): C 33.31, N 7.77, S 8.89, H 4.75.

#### *4.2. Crystal Structure Determination*

The data were collected using an Oxford Diffraction Gemini E diffractometer (Oxford Diffraction, Oxfordshire, England), equipped with a 2K × 2K EOS CCD area detector and sealed-tube Enhance (Mo) and (Cu) X-ray sources. The single crystals of compounds were fastened on the top of a Lindemann glass capillary. The data were collected by means of the ω-scans technique using graphite-monochromated radiation. The detector distance was set at 45 mm. The diffraction intensities were corrected for Lorentz/polarization effects as well as with respect to the absorption. The empirical multi-scan absorption corrections using equivalent reflections were performed with the scaling algorithm SCALE3 ABSPACK. The data reduction, finalization and cell refinement were carried out through the CrysAlisPro software (1.171.38.46, Rigaku Oxford Diffraction, Rigaku Corporation, Oxford, UK). The accurate unit cell parameters were obtained by least squares refinement of the angular settings of the strongest reflections, chosen from the whole experiment. The structures were solved with *Olex2* [19] by using *ShelXT* [20] structure solution program by Intrinsic Phasing and refined with the *ShelXL* [21] refinement package using least-squares minimization. In the last cycles of refinement, non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included in the calculated positions, and a riding model was used for their refinement. For CP **3**, the indexing of the collected data and frame inspections clearly showed twining signals. The data were processed with the twin/multicristal routine of the CrysAlisPro software. The logout of the twin data reduction is given as Supplementary Materials. The twin data reduction with two components allowed solving the structure. The two components were rotated at 180◦ around the [001] vector. The final refined BASF parameter was 0.389(2). The specific refinement details for each compounds are embedded in their CIF files given as Supplementary Materials and that have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication (CCDC 1942113–1942116). Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44) 1223 336033; e-mail, deposit@ccdc.cam.ac.uk).

#### *4.3. Powder X-ray Di*ff*raction (PXRD)*

The PXRD patterns of CP **3** and **4** were collected with a Bruker D8 Advance diffractometer (Bruker AXS, Karlsruhe, Germany), in Bragg–Brentano geometry, using Cu Kα. The patterns were acquired in the 5◦−50◦ 2θ range (0.03◦ /step and 10 s/step). The PXRD patterns of CP **5** and **6** were collected with the powder diffraction tool of an Oxford Diffraction Gemini E diffractometer using Cu Kα. The powder diffraction images (20 frames) were collected over 100 s exposition time with a 90 degrees φ rotation and a detector distance of 120 mm.

#### **5. Conclusions**

This study analyzed and studied the effect of coordinating solvents on the final outcome of the self-assembly process towards Cu(II)-4,4′ -bipyridine based coordination polymers. Solvents such as DMA, DMF and DMSO can compete with the bridging ligand in occupying two or four coordination sites of the metal center. In this competition, also adventitious water can participate. This allowed the access to different coordination polymers. In particular, it was found that the presence of four coordinating solvent molecules lead to 1D polymers, while two solvent molecules, in the apical sites, always gave the same fundamental unit (Cu–bpy square) that developed in grid-like structures to give 2D or 3D networks. Our results show that the coordinating solvent approach (CSA) can be used as an effective tool to modulate and control the dimensionality, composition and network of coordination polymers.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2304-6740/7/8/103/s1, Figure S1: PXRD patterns; CIF and checkCIF files of the crystal structures, and twin logout for CP **3**.

**Author Contributions:** Conceptualization, M.R.; validation M.R. and L.A.; investigation M.R., A.C. and G.B.; writing—original draft preparation, M.R. and L.A.; writing—review and editing, all authors; funding acquisition, M.R. and L.A.

**Funding:** This research was funded by the University of Padova (grant: P-DISC #CARL-SID17 BIRD2017-UNIPD), project CHIRoN and by Ministero Istruzione Università e Ricerca, MIUR (PRIN 2015, 20154X9ATP, Progetti di Ricerca di Interesse Nazionale. APC was sponsored by MDPI.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).
