Synthesis, Structure and Physical Properties of Cd-Coordination Polymer with (4, 5, 10)-Connected Topology Based on Trinuclear Clusters

Experimental Section

Materials and Physical Measurements. The 1,3-bis(1,2,4-triazol-1-yl)propane (btp) ligand was synthesized according to the literature method.8 All other reagents and solvents were commercial available and used without further purifi-cation. Infrared spectrum was obtained within the 4000−400 cm−1 as KBr disks on a VECTOR 22 spectrometer. Elemental analysis was performed on a Perkin Elmer 240C elemental analyzer. Fluorescent spectrum was recorded on a Fluoro Max−P spectrophotometer. Thermal gravimetric analysis (TGA) was collected on a Perkin-Elmer Pyris 1 TGA analy-zer from room temperature to 650 °C with a heating rate of 20 °C min−1 under nitrogen.

Synthesis of [Cd4(tbip)4(btp)·H2O]n (1). A mixture of Cd(NO3)2·6H2O (34.6 mg, 0.100 mmol), H2tbip (44.4 mg, 0.200 mmol), btp (17.8 mg, 0.100 mmol), KOH (22.4 mg, 0.400 mmol) in H2O (10 mL) was sealed in a 16 mL Teflon-lined stainless steel container and heated at 180 °C for 72 h. After cooling to room temperature, colorless block crystals were collected by filtration and washed by water and ethanol several times. (yield 13.6%, based on btp). Elemental analysis for C55H60Cd4N6O17 (Mr = 1526.73): C 43.27, H 3.96, N 5.50%; found: C 43.38, H 3.97, N 5.52%. Selected IR (KBr) spectra for 1: m (cm−1) 3612 (w), 3066 (w), 1634 (s), 1570 (s), 1521 (s), 1423 (m), 1442 (w), 1331 (w), 1042 (m), 831 (w), 813 (m), 731 (w), 702 (w).

Structural Determination and Refinement. Structural data for 1 were collected on a Bruker Smart Apex CCD with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at 291 K. Absorption correction was applied using multi-scan technique. The structure was solved by direct method using the program SHELXL−97 and refined by full−matrix least−squares technique on F2 with SHELXL−97.9 Six C atoms within two tert-butyl groups (C10, C11, C12, C22, C23, C24) were modelled as disordered over two sites. In complex 1, two sets of positions are defined by atoms C10A/C11A/C12A and C10B/C11B/C12B, with refined site occupancies of 0.347 (5) and 0.653 (5), respectively. The C22, C23 and C24 atoms are refined as disordered over two positions, with site occupancies of 0.82 (3) and 0.18 (3). All non-hydrogen atoms were located in difference Fourier maps and refined with anisotropic temperature parameters. All H atoms were refined isotropically, with the isotropic vibration parameters related to the non−H atom to which they are bonded. A summary of the structural determination and refinement for the title complex is summarized in Table S1 (Supporting Information) and the selected bond distances and angles are shown in Table S2

Figure 1.View of the asymmetric unit in complex 1. Six C atoms within two tert-butyl groups (C10, C11, C12, C22, C23, C24) show orientational disorder. All hydrogen atoms are omitted for clarity.

 

Results and Discussions

The single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the centrosymmetric orthorhombic space group Fddd with an asymmetric unit consisting of three crystallographically distinct divalent cadium atoms (two of which, Cd1 and Cd3, rest on an inversion center), two types of separate fully deprotonated tbip2- (tbip–A, O1–O4; tbip–B, O5–O8) ligands, one half of a btp ligand as well as one-half water molecules of crystallization (Figure 1). The Cd1 and Cd3 centers both are six-coordinated by six oxygen donor atoms from chelating carboxylate groups belonging to six different tbip2- ligands, resulting in a octa-hedral coordination environment. On the other hand, Cd2 displays a distorted {CdO5N} octahedral geometry, with the axial positions taken up by one triazole nitrogen donor atom from one btp ligand and one oxygen donor atom from chelating carboxylate group belonging to one tbip2- ligand as shown in Scheme 1. The equatorial plane filled by four oxygen atom donors from three chelating carboxylate groups from another three different tbip2- ligands. The Cd–N bond length is 2.306(3) Å and the Cd–O bond lengths are in the range 2.191(2)–2.600(2) Å, which are well comparable to those reported for other cadmium complexes,10 and the O–Cd–O bond angles fall in the range of 52.42(7)–175.84(9)°.

Scheme 1.View of the coordination mode of Cd2 metal in complex 1.

Scheme 2.View of the coordination mode of tbip2- ligands in complex 1.

Notably, two types of tbip2- ligands show different coordi-nation modes in complex 1. One tbip2- ligand links four Cd(II) cations in μ2-η1: η1, μ2-η1: η1 coordination fashions through its two carboxylate groups, whereas the remaining one links six Cd(II) cations in μ3-η2: η1, μ3-η2: η2 coordi-nation fashions through its two carboxylate groups as shown in Scheme 2. In these manners, three crystallographically independent Cd(II) centers are linked by tbip2- ligands with these coordination modes into a triangular trinuclear Cd(II) clusters (Figure 2) with Cd…Cd distances in the range of 3.2921(6)–5.6861(7) Å. As shown in Figure 3, these trinu-clear Cd(II) clusters are further connected by oxygen atoms of four carboxylate groups from two different tbip2- ligands to form an infinite rod-shaped secondary building units (SBUs) along a-axis. In such SBUs, the trinuclear Cd(II) clusters are arranged in nonlinear fashion with intercluster center distance of 5.224 Å. Moreover, the tbip2- ligands link rod-shaped SBUs to give rise to a complicated three-dimen-sional framework. On the other hand, the btp exhibits the gauche–gauche conformation with the dihedral angle bet-ween two triazole planes of 37.8(2)°, bridging two Cd2 atoms in complex 1 as shown in Scheme 3. The Cd2…Cd2 contact distances through btp ligand is 9.5813(13) Å. The 3D networks only constructed by tbip2- ligands are further connected by btp ligands to result in a new three-dimensional framework of complex 1 (Figure 4). Moreover, the lattice water molecule (O9) participates in two H-bonds [O(9)–H(9A)…O(1) and O(9)–H(9B)…O(2)#1 ; symmetry codes: #1 5/4-x, y, 1/4-z], which further stabilize the 3D networks architecture.

Figure 2.View of triangular trinuclear Cd(II) cluster.

Figure 3.The infinite rod-shaped SBU along a-axis.

Scheme 3.View of the coordination mode of btp ligand in complex 1.

A further insight into the nature of this intricate framework can be acquired by using topological analysis. A topological analysis of this net was performed with TOPOS software.11 In complex 1, each tbip–A ligand links four trinuclear Cd(II) clusters and serves as a 4-connected node, and each tbip–B ligand bridges five trinuclear Cd(II) clusters and acts as a 5-connected node; while each trinuclear Cd(II) cluster separate-ly joins four tbip–A ligands, five tbip–B ligands and another one trinuclear Cd(II) clusters, thus, it can be simplified as a 10-connected node. As a whole, the topology of the resulting 3D framework can be described as a unique trinodal (4, 5, 10)-connected net with the Schlali symbol of (3.48.5) (32.428.510.65) (43.62.7) (Figure 5). To the best of our knowledge, this trinodal topology has neither been reported. The un-ligated water molecules occupy a solvent-accessible incipi-ent space comprising 2.7% of the unit cell volume, accord-ing to PLATON.12

Figure 4.A view of the 3D framework of complex 1.

Figure 5.A schematic representation of the (4, 5, 10)-connected topology of complex 1.

Figure 6.Thermogravimetric curve of complex 1.

Figure 7.The solid-state emission spectra of complex 1.

The thermostability of 1 was studied by thermogravimetric analyses (TGA) (Figure 6). Thermogravimetric analysis of 1 demonstrates that the first weight loss before 108 °C corre-sponds to the release of the water molecules of crystalli-zation (obsd. 1.98%, calcd. 1.18%). The organic ligands began to decompose at 225 °C, with the 33.99% mass remnant at 620 °C consistent with production of CdO (33.64% calcd).

The fluorescent property of complex 1 was studied in the solid state at room temperature. The free H2tbip shows emission peaks at 320 nm.13 Irradiation of crystalline samples of complex 1 with ultraviolet light (λex = 295 nm) in the solid state resulted in intense emission violet visible light emission with a λmax of 343 nm (Figure 7). According to a recent review of d10 metal coordination polymer lumine-scence, the emissive behavior of 1 can be ascribed to ligand-centered electronic transitions.14

In summary, a new Cd(II) coordination polymer, [Cd4-(tbip)4(btp)·H2O]n (1) was successfully synthesized through a hydrothermal reaction of Cd(II) ions, 5-tert-butyl iso-phthalic acid (H2tbip) and 1,3-bis(1,2,4-triazol-1-yl)propane (btp). The complex features 3-D framework with (4, 5, 10)-connected (3.48.5) (32.428.510.65) (43.62.7) topology based on trinuclear Cd(II) cluster motifs. In addition, complex 1 exhibits strong fluorescent emissions in the solid state at room temperature.