New Cyanido-Bridged Complexes of Zn(II) and/or Ag(I) with TPymT and Tptz Ligands: Synthesis, Structural and Fluorescent Properties

: The use of two triazine-derived pincer ligands led to the obtaining of a heterometal-lic compound and of an unexpected 3D coordination polymer (CP). Therefore, by reacting 2,4,6-tri(2-pyridyl)-1,3,5-triazine (tptz) with Zn(NO 3 ) 2 and K[Ag(CN) 2 ], the cyanido-bridged [Ag(CN)( m - CN)] 2 [Zn(tptz)(H 2 O)] ( 1 ) trinuclear complex was formed. Compound 1 crystallizes in the orthorhombic polar space group Aea 2 and the crystal packing involves argentophilic interactions. When 2,4,6-Tris(2-pyrimidyl)-1,3,5-triazine (TPymT) was used as a ligand, along with the same precursors as for 1 , a 3D CP was assembled, [Ag 6 (CN) 6 (TPymT) 2 ] ( 2 ). The formation of 2 was favored, most likely, by the dicyanoargentate(I) ion dissociation and its interesting topology is due to the bridging cyanide ligands and argentophilic interactions. The ﬂuorescence of both compounds was studied and compared to the emission features of their ligands. For the two coordination compounds, ligand-centered ﬂuorescence data are discussed.


Materials and Methods
The chemicals and the solvents used were purchased from commercial sources being of reagent grade and used without further purification.The 2,4,6-Tris(2-pyrimidyl)-1,3,5triazine (TPymT) ligand was obtained following the method described in the literature [24].
Synthesis of 1 and 2: 0.062 g TPymT (0.2 mmol) for 1/ 0.062 g tptz (0.2 mmol) for 2 in 10 mL MeCN was mixed with an aqueous solution of 0.11 g Zn(NO3)2•6H2O (0.4 mmol) and formed a pale yellow solution which was layered upon an aqueous solution of 0.039 We report here two new coordination compounds employing tptz and TpymT ligands, the cyanido-bridged [Ag(CN)(µ-CN)] 2 [Zn(tptz)(H 2 O)] trinuclear complex, (1), and the 3D CP of Ag(I), [Ag 6 (CN) 6 (TpymT) 2 ] n , (2), obtained in similar reaction conditions.The compounds were characterized through FTIR and UV-Vis spectroscopy.The crystal structures for 1 and 2 were determined by X-ray diffraction on single-crystal, whereas the powder XRD analysis was performed only for 1 (for 2, the rapid loss of crystallinity when exposed to X-ray radiation did not provide suitable PXRD data).The luminescent properties were also assessed for both compounds and their corresponding free ligands.

Materials and Methods
The chemicals and the solvents used were purchased from commercial sources being of reagent grade and used without further purification.The 2,4,6-Tris(2-pyrimidyl)-1,3,5triazine (TPymT) ligand was obtained following the method described in the literature [24].
Synthesis of 1 and 2: 0.062 g TPymT (0.2 mmol) for 1/ 0.062 g tptz (0.2 mmol) for 2 in 10 mL MeCN was mixed with an aqueous solution of 0.11 g Zn(NO 3 ) 2 •6H 2 O (0.4 mmol) and formed a pale yellow solution which was layered upon an aqueous solution of 0.039 g K[Ag(CN) 2 ] (0.2 mmol).Yellow crystals of 1 and 2 were obtained after a week.Yield: ca.40% based on TPymT (1) Physical Measurements.Elemental analyses (C, H, N) were carried out on a Perkin Elmer 2400 analyzer (Waltham, MA, USA).Room temperature infrared spectra of 1 and 2 were recorded on a FTIR Bruker Tensor V-37 spectrophotometer (Billerica, MA, USA) as KBr pellets in the 4000-400 cm −1 range.Additionally, room temperature UV-Vis spectra (diffuse reflectance technique) were measured with a JASCO V-770 NIR-UV-Vis spectrophotometer (Tokyo, Japan), using MgO as standard, in the 200-800 nm domain, while photoluminescence measurements (PL), on a JASCO FP 8300 spectrophotometer (Tokyo, Japan), using 255-400 nm excitation lines of the Xe light.Powder X-ray diffraction data (PXRD) for 1 and 2 were recorded on a Proto AXRD benchtop (Wroclaw, Poland) using Cu-Kα radiation, with a wavelength of 1.54059 Å, in the range of 2θ = 5-35 • .The PXRD measurements did not provide suitable diffraction data for compound 2 due to the rapid loss of crystallinity when exposed to X-ray radiation.
X-ray Data Collection and Structure Refinement.Single-crystal X-ray diffraction data for 1 and 2 were collected on an Oxford-Diffraction XCALIBUR Eos CCD diffractometer (Oxford Diffraction Limited, Oxfordshire, UK) with graphite-monochromated Mo-Kα.The CrysAlisPro package (Rigaku Oxford diffraction, Oxfordshire, UK) from Oxford Diffraction was used to determine the unit cell and for the data integration [26], for which the multiscan correction for absorption was applied.The ShelXT program [27] including intrinsic phasing method was used to solve the two structures, while the refinement of these structures has been done by full-matrix least-squares method on F 2 with ShelXL program.The interface for the ShelX programs was Olex2 software [28] and the drawings of the molecules were executed with the Diamond 4 program [29].All non-hydrogen atoms were refined anisotropically, while the hydrogen atoms were placed geometrically and refined using a riding model.A disorder model involving a shared occupation of 50:50 for C and N atoms was used to assign the carbon and nitrogen atoms from the cyanide group.Most likely, a small impurity consisting of chloride ions in the TPymT solution led to a substitutional disorder, the position of the C17N11 cyanide group being partially substituted by the Cl -species.The main crystallographic data for 1 and 2 are gathered in Table 2, whereas the selected distances and angles for 1 and 2 are displayed in Table S1.Specific details of each refinement are given in the crystallographic information files (CIFfiles).CCDC numbers: 2213972 (1), 2213973 (2).
, where n is the number of reflections and p is the total number of refined parameters.

Results
The reaction between tptz, a triazine derivative, with Zn(II) ions and dicyanoargentate(I) afforded a cyanido-bridged heterometallic complex with the formula: In this case, the [Ag(CN) 2 ] − complex ions acted as metalloligands toward the [Zn(tptz)(H 2 O)] 2+ unit, which was formed in situ from Zn(NO 3 ) 2 and tptz ligand.Based on our previous results, the dicyanoargentate(I) ion could dissociate in certain conditions [20].We reported the reaction of the [Ag(CN) 2 ] − complex ion with Fe(BF 4 ) 2 and TPymT, leading to the serendipitous assembly of a 3D CP of Ag(I) with a peculiar topology [20].Here, instead of Fe(II), we employed Zn(II) ions, in the same reaction conditions as for 1, and a 3D CP of Ag(I), 2, was also formed.Again, the only metal ion found in the structure is Ag(I), due to the dicyanoargentate(I) ion dissociation and the lack of coordination geometry preference of Ag(I), as opposed to Fe(II) and Zn(II).Additionally, the higher denticity of TPymT ligand compared to tptz molecule could also contribute to the formation of silver(I)-only extended structure.
The FTIR spectra for compounds 1 and 2 show bands characteristic to the cyanide ligands, terminal and bridging, at 2183 and 2150 cm −1 (1) and 2132 and 2120 cm −1 (2), respectively, as shown in Figures S1 and S2.The FTIR spectra display intense absorptions at 1554 and 1532 cm −1 for 1, and a large, strong band at 1534 cm −1 for 2, which could be attributed to the stretching vibrations of the aromatic C=N and C=C bonds of the triazine, pyridine and pyrimidine aromatic rings from the TPymT and tptz ligands, respectively.The bands assigned to the aromatic ring vibrations are found at 1376 and 1011 cm −1 for 1, and 1371 and 1000 cm −1 for 2. The aromatic C-H deformation vibrations appear, most likely, as bands at 766 cm −1 for 1, and 774 cm −1 for 2. The FTIR data is in accordance with the crystal structures of the compounds determined from single crystal X-ray diffraction data and XRD on powder for 1 (Figure S3).
C20-Ag1-C21 = 171.1(4)°,as presented in Figure S4 and Table S2.The angles formed by the cyanido ligands and the Ag(I) ions are 175.8(10) and 176.2(11)° for Ag1-C20-N9 and Ag1-C21-N10, and 173.8 (10) and 177.2(13) o for Ag2-C18-N7 and Ag2-C19-N8, respectively.The Ag-Ccyanido bond distances have values ranging between 2.040( 11) Å (Ag1-C21) and 2.051( 12) Å (Ag1-C20).The Zn(II) ion is surrounded by five N donor atoms, two Ncyanido and three nitrogen atoms from the tptz ligand, and one oxygen atom from the aqua ligand, in a distorted octahedral geometry, according to the calculations performed with the SHAPE 2.1 program, results included in Figure S4 and Table S2 [30,31].The tptz ligand coordinates in a mono-terpyridine mode to the Zn(II) ion, and occupy three of the four coordination sites in the equatorial plane of the octahedron, the bond distances being of 2.273(7), 2.214(7) and 2.084(7) Å for Zn1-N1, Zn1-N2 and Zn1-N3, respectively.These values are close to those reported for similar examples of Zn(II) complexes with tptz ligand [18].The cyanide ligands arranged, one in the equatorial position (C20N9) and the other in the axial position (C18N7), coordinate to the Zn(II) ion at a distance of 2.161 (10) Strong hydrogen bonds between the aqua ligand and the cyanido nitrogen atom stabilize a 2-D network, displayed in Figure S5.Table S3 contains the H-bond parameters for compound 1, the distances and angles.In addition, the π-π stacking interactions between the aromatic rings are shown in Figure S6, with the shortest distance of ca.3.44 Å.
Compound 2 crystallizes in the monoclinic space group P21/c.Figure 2 displays the asymmetric unit which is formed from two [Ag2(TPymT)] 2+ and two [Ag(CN)3] 2-units.The structure contains six crystallographically independent Ag(I) atoms.Two of them, Ag1 and Ag4, are pentacoordinated by three N atoms belonging to the TPymT ligand and two Ncyanido atoms, in a distorted square pyramidal geometry as calculated with SHAPE 2.1 software [30,31], see Figure S7 and Table S4.The Ag2 and Ag5 ions are both tetracoordinated by three N atoms pertaining to the TPymT ligand and one Ncyanide atom, in a distorted square planar geometry.The μ3-bridging TPymT ligand coordinates in a bis-terpyridine fashion to the Ag(I) atoms.The Ag-NTPymT bond distances range between 2.391(5) for Ag2-N5 and 2.6985(67) Å for Ag1-N4, close to those reported for similar compounds The Zn(II) ion is surrounded by five N donor atoms, two N cyanido and three nitrogen atoms from the tptz ligand, and one oxygen atom from the aqua ligand, in a distorted octahedral geometry, according to the calculations performed with the SHAPE 2.1 program, results included in Figure S4 and Table S2 [30,31].The tptz ligand coordinates in a monoterpyridine mode to the Zn(II) ion, and occupy three of the four coordination sites in the equatorial plane of the octahedron, the bond distances being of 2.273(7), 2.214(7) and 2.084(7) Å for Zn1-N1, Zn1-N2 and Zn1-N3, respectively.These values are close to those reported for similar examples of Zn(II) complexes with tptz ligand [18].The cyanide ligands arranged, one in the equatorial position (C20N9) and the other in the axial position (C18N7), coordinate to the Zn(II) ion at a distance of 2.161 (10) Strong hydrogen bonds between the aqua ligand and the cyanido nitrogen atom stabilize a 2-D network, displayed in Figure S5.Table S3 contains the H-bond parameters for compound 1, the distances and angles.In addition, the π-π stacking interactions between the aromatic rings are shown in Figure S6, with the shortest distance of ca.3.44 Å.
Compound 2 crystallizes in the monoclinic space group P2 1 /c. Figure 2 displays the asymmetric unit which is formed from two [Ag 2 (TPymT)] 2+ and two [Ag(CN) 3 ] 2− units.The structure contains six crystallographically independent Ag(I) atoms.Two of them, Ag1 and Ag4, are pentacoordinated by three N atoms belonging to the TPymT ligand and two N cyanido atoms, in a distorted square pyramidal geometry as calculated with SHAPE 2.1 software [30,31], see Figure S7 and Table S4.The Ag2 and Ag5 ions are both tetracoordinated by three N atoms pertaining to the TPymT ligand and one N cyanide atom, in a distorted square planar geometry.The µ 3 -bridging TPymT ligand coordinates in a bis-terpyridine fashion to the Ag(I) atoms.The Ag-N TPymT bond distances range between 2.391(5) for Ag2-N5 and 2.6985(67) Å for Ag1-N4, close to those reported for similar compounds [13,20].The other two silver(I) atoms, Ag3 and Ag6, are tricoordinated by three cyanido ligands in a distorted trigonal planar geometry with the angles involving the silver(I) atoms between 95.4 for N10-Ag3-N11 and 135.4 o for C35-Ag6-N25.
The cyanido ligands allow the structure to expand into 3D space, due to the μ2 coordination mode of the cyanide, which connects two silver(I) ions, Figure 3b,c.Thus, cyano groups C17N11 and C36N25 coordinate as μ2-C,C,N bridges to two Ag(I) ions, Ag3, Ag1b, and Ag6, Ag4c, respectively, while the other four cyanide groups act as μ-C,N linkers.In addition, the Ag1•••Ag3b (3.3740(11) Å) and Ag4•••Ag6c (3.0924(10) Å) argentophilic interactions contribute to the assembly of hexanuclear rectangles ({Ag1-Ag2-Ag3-Ag1b-A2b-Ag3b} and {Ag4-Ag5-Ag6-Ag4c-Ag5c-Ag6c}) connected through cyanide ligands, thus growing into the 3D network, Figure 3a.Along crystallographic c axis cyanido-bridged pseudo-rectangles interpenetrating in a zig-zag fashion are illustrated in Figure 3b.Additionally, the pseudo-rectangle units are seen to be linked through cyanide groups after an axis paralleling the crystallographic b axis, each unit being connected to four others.The cyanido ligands allow the structure to expand into 3D space, due to the µ 2 coordination mode of the cyanide, which connects two silver(I) ions, Figure 3b,c.Thus, cyano groups C17N11 and C36N25 coordinate as µ 2 -C,C,N bridges to two Ag(I) ions, Ag3, Ag1b, and Ag6, Ag4c, respectively, while the other four cyanide groups act as µ-C,N linkers.In addition, the Ag1•••Ag3b (3.3740(11) Å) and Ag4•••Ag6c (3.0924(10) Å) argentophilic interactions contribute to the assembly of hexanuclear rectangles ({Ag1-Ag2-Ag3-Ag1b-A2b-Ag3b} and {Ag4-Ag5-Ag6-Ag4c-Ag5c-Ag6c}) connected through cyanide ligands, thus growing into the 3D network, Figure 3a.Along crystallographic c axis cyanidobridged pseudo-rectangles interpenetrating in a zig-zag fashion are illustrated in Figure 3b.Additionally, the pseudo-rectangle units are seen to be linked through cyanide groups after an axis paralleling the crystallographic b axis, each unit being connected to four others.[13,20].The other two silver(I) atoms, Ag3 and Ag6, are tricoordinated by three cyanido ligands in a distorted trigonal planar geometry with the angles involving the silver(I) atoms between 95.4 for N10-Ag3-N11 and 135.4 o for C35-Ag6-N25.The cyanido ligands allow the structure to expand into 3D space, due to the μ2 coordination mode of the cyanide, which connects two silver(I) ions, Figure 3b,c.Thus, cyano groups C17N11 and C36N25 coordinate as μ2-C,C,N bridges to two Ag(I) ions, Ag3, Ag1b, and Ag6, Ag4c, respectively, while the other four cyanide groups act as μ-C,N linkers.In addition, the Ag1•••Ag3b (3.3740(11) Å) and Ag4•••Ag6c (3.0924(10) Å) argentophilic interactions contribute to the assembly of hexanuclear rectangles ({Ag1-Ag2-Ag3-Ag1b-A2b-Ag3b} and {Ag4-Ag5-Ag6-Ag4c-Ag5c-Ag6c}) connected through cyanide ligands, thus growing into the 3D network, Figure 3a.Along crystallographic c axis cyanido-bridged pseudo-rectangles interpenetrating in a zig-zag fashion are illustrated in Figure 3b.Additionally, the pseudo-rectangle units are seen to be linked through cyanide groups after an axis paralleling the crystallographic b axis, each unit being connected to four others.Diffuse reflectance UV-Vis spectra recorded on polycrystalline powders of the ligands, tptz and TPymT, and of complexes 1 and 2, are shown in Figure S8.As expected, the UV-Vis spectra of compounds 1 and 2 relative to spectra of the corresponding ligands, tptz and TPymT, respectively, are similar, since Ag(I) complexes cannot exhibit d-d transitions.The split broad band with maxima at 270 and 320 nm, and a shoulder at 400 nm, for the tptz ligand, determined by n-π* and π-π* transitions within the triazine and pyridyl aromatic rings are shifted to 280, 360 and 420 nm, respectively, for 1.The same tendency is noticed for the TPymT free ligand and 2, namely, the bathochromic shift of the band at 325 nm and the shoulder at 440 nm for TPymT arising from n-π* and π-π* transitions within the triazine and pyrimidyl aromatic rings, to 385 and 480 nm for 2. The absorption shifts of the complexes can be assigned to the metal-metal to ligand charge transfer (MMLCT) most likely determined by the strong argentophilic interactions along with low-lying ligand π* orbitals [32,33].

Solid State Fluorescence
Silver(I) complexes can exhibit thermally activated delayed fluorescence (TADF), although they need ligands with a high electron donating character, such as phosphines, and even negatively charged ones [34].In addition, the ligand should contain electron donor and acceptor moieties [35].Moreover, phosphorescent Ag(I) coordination compounds have been previously reported, showing longer emission lifetimes and higher Stokes shifts compared to the corresponding ligands [36][37][38].In our case, the photoluminescent properties of the Ag(I) complexes are determined by intra-ligand π-π* fluorescence [22].Solid state emission spectra for complexes 1 and 2 and their ligands were performed at room temperature.In Figure 4, the emission spectra for 1 and tptz, excited at 370 nm, and for 2 and TPymT excited at λ exc = 255 nm are represented.A series of papers concluded that the complexation of Ag(I) determines the fluorescence quenching for Ag(I) coordination compounds [39].Compounds 1 and 2 exhibit ligand-centered fluorescence for which the numerous argentophilic interactions could play a beneficial role in both cases.Thus, when excited at 370 nm, a blue emission occurs, with maximum at ca. 470 nm with a shoulder at 540 nm, for tptz free ligand.For 1 the bands are slightly shifted compared to the tptz ligand, to ca. 480 nm and 550 nm.The luminescent properties are comparable to other complexes of Zn(II) with the tptz ligand [19].It is worth mentioning that the fluorescence spectra were measured under the same conditions (parameters and sample weight) for the tptz, TPymT free ligands, 1 and 2, respectively.Therefore, the emissive study and the parallel between ligands and complexes were accurate.Figure S9a shows the overlayed emission spectra of the free tptz ligand and compound 1 at λ exc = 350, 360 and 370 nm, whereas the emission bands for 1, excited at 350, 360, 380, 390 and 400 nm, are represented in Figure S9b.Compound 2 exhibits fluorescence in the blue region through two structured bands with peaks at ca. 440 and 470 nm, similar to the TPymT ligand in terms of wavelength and intensity and can be assigned to intra-ligand π-π* transitions.Conversely, our reported 3D compound of Ag(I) with TPymT ligand showed a weaker emission as a broad band [20].Figure S10 displays the emission bands for TPymT excited at 295, 300 nm, and 340, and 350 nm, the emission that was partially quenched for 2.

Figure 3 .
Figure 3.A fragment of the 3D structure of compound 2 showing (a) the argentophilic interactions and the cyanide μ2-C,C,N bridges; (b) packing motif in the ab crystallographic plane (the aromatic rings were omitted for clarity), dark blue lines correspond to the cyanido bridging groups; (c) the

Figure 3 .
Figure 3.A fragment of the 3D structure of compound 2 showing (a) the argentophilic interactions and the cyanide μ2-C,C,N bridges; (b) packing motif in the ab crystallographic plane (the aromatic rings were omitted for clarity), dark blue lines correspond to the cyanido bridging groups; (c) the

Figure 3 .
Figure 3.A fragment of the 3D structure of compound 2 showing (a) the argentophilic interactions and the cyanide µ 2 -C,C,N bridges; (b) packing motif in the ab crystallographic plane (the aromatic rings were omitted for clarity), dark blue lines correspond to the cyanido bridging groups; (c) the structure of 2 growing along crystallographic b axis; (Ag atoms are represented with magenta color; N atoms are represented with blue color; C atoms are represented with grey color and the disordered N/C atoms are represented with light blue color).Symmetry codes: a = 1 + x, 3/2 − y, 1 2 + z; b = 2 − x, −y, 1 − z; c = 1 − x, 2 − y, 1 − z; d = −1 + x, 3/2 − y, − 1 2 + z.

Table 1 .
Crystal data and structure refinement for 1 and 2.

Table 2 .
Crystal data and structure refinement for 1 and 2.