Directing 2D-Coordination Networks: Combined Effects of a Conformationally Flexible 3,2':6',3″-Terpyridine and Chain Length Variation in 4'-(4-n-Alkyloxyphenyl) Substituents.

The synthesis and characterization of 4′-(4-n-propoxyphenyl)-3,2′:6′,3″-terpyridine is described. Five 2D-coordination networks have been isolated by crystal growth at room temperature from reactions of Co(NCS)2 with 4′-(4-n-alkyloxyphenyl)-3,2′:6′,3″-terpyridines in which the n-alkyl group is ethyl, n-propyl, n-butyl, n-pentyl and n-hexyl in ligands 2–6, respectively. The single-crystal structures of [{Co(2)2(NCS)2}.0.6CHCl3]n, [{Co(3)2(NCS)2}.4CHCl3.0.25H2O]n, [{Co(4)2(NCS)2}.4CHCl3]n, [Co2(5)4(NCS)4]n and [Co(6)2(NCS)2]n have been determined, and powder X-ray diffraction has demonstrated that the single-crystal structures are representative of the bulk materials. Each compound possesses a (4,4) net with Co centres as 4-connecting nodes. For the assemblies containing 2, 3 and 4, the (4,4) net comprises two geometrically different rhombuses, and the nets pack in an ABAB... arrangement with cone-like arrangements of n-alkyloxyphenyl groups being accommodated in a similar unit in an adjacent net. An increase in the n-alkyloxy chain length has two consequences: there is a change in the conformation of the 3,2′:6′,3″-tpy metal-binding domain, and the (4,4) net comprises identical rhombuses. Similarities and differences between the assemblies with ligands 2–6 and the previously reported [{Co(1)2(NCS)2}.3MeOH]n and [{Co(1)2(NCS)2}.2.2CHCl3]n in which 1 is 4′-(4-methoxyphenyl)-3,2′:6′,3″-terpyridine are discussed. The results demonstrate the effects of combining a variable chain length in the 4′-(4-n-alkyloxyphenyl) substituents of 3,2′:6′,3″-tpy and a conformationally flexible 3,2′:6′,3″-tpy metal-binding domain.

We have previously shown that crystal growth by layering a methanol solution of Co(NCS) 2 over a chloroform solution of 1 yields either [Co (1) 3 ] n ((4,4)-net), depending on the crystallization conditions. However, powder X-ray diffraction (PXRD) revealed that in experiments where crystals selected for single crystal X-ray diffraction proved to be the (4,4)-nets, the 1D-polymer [Co(1)(NCS) 2 (MeOH) 2 ] n was always the dominant product [19]. The solution absorption spectrum of 3 ( Figure 2) is reminiscent of the spectra of other members of the homologous series of ligands shown in Scheme 2 [11,19,20], and the absorptions are assigned to a combination of spin-allowed π*←n and π*←π transitions.

Reactions of Compounds 2-6 with Co(NCS) 2 and Bulk Material Characterization
Methanol solutions of cobalt(II) thiocyanate were layered over chloroform solutions of ligands 2-6 under the same room temperature conditions. For each ligand, the reactions were carried out using metal:ligand molar ratios of 1:1, 2:1 and 1:2, and the conditions detailed in Sections 3.2-3.6 correspond to the setups in which crystals grew. Irrespective of the initial metal:ligand ratios, the crystalline products were coordination networks in which the Co:ligand ratio was 1:2. After selection of single crystals for X-ray diffraction, the bulk materials were analyzed by PXRD and solid-state IR spectroscopy, and where multiple experiments were carried out (see Sections 3.2-3.4), cell checks were carried out on single crystals from different batches. Powder patterns were fitted to those predicted from the single-crystal structures (Figure 3), and the data confirmed that crystals selected for single-crystal diffraction analysis were representative of the bulk samples for each of the coordination networks discussed in the next section. The fingerprint regions of the IR spectra of single crystals or microcrystalline samples (Figures S7-S11 in the supporting information) are all similar, consistent with analogous coordination behaviors of ligands 2-5 in the coordination assemblies. Each spectrum exhibits bands in the region between 2820 and 3100 cm −1 characteristic of the aromatic C-H stretches of the ligands. A strong absorption band in each IR spectrum at 2068 cm −1 (compounds with 2 and 3), 2062 cm −1 (with 4), 2074 cm −1 (with 5) and 2072 cm −1 (with 6) arises from the coordinated thiocyanate ligands. [Co(6) 2 (NCS) 2 ] n were selected. Each experimental powder pattern was fitted to the predicted pattern from the single crystal determination. The black dots correspond to the best fit from the profile matching refinement. Lower vertical green marks denote the Bragg peak positions. The bottom (blue) line in each plot represents the difference between the experimental and calculated points.

Crystal Structures
Single crystal X-ray diffraction confirmed that the pink crystals grown by layering comprised [{Co (2) 4 ] n and [Co(6) 2 (NCS) 2 ] n . The compounds incorporating ligands 2, 3 and 4 (ethoxy, n-propoxy and n-butoxy, respectively) crystallize in the tetragonal space groups P4/ncc or P-42 1 c (see Sections 3.8-3.10), while those containing ligands 5 and 6 (n-pentyloxy and n-hexyloxy) crystallize in the monoclinic space groups P2 1 /n and P2 1 /c, respectively. ORTEP diagrams of the asymmetric units with symmetry-generated atoms are displayed in Figures S12-S16. We discuss the structures together and focus on differences that correlate to the lengthening of the n-alkyloxy chain. In all the compounds, each cobalt atom is in an octahedral environment with a trans-arrangement of thiocyanato ligands and is bonded to one pyridine ring of four different 3,2 :6 ,3"-tpy units. In  Table 1 and are unexceptional. In keeping with expectations, the central pyridine ring of each ligand is non-coordinated.  (7), 178.59 (7) 179.24 (7) 1 Only the trans angles are given. 2 Two independent ligands.
[{Co(4) 2 (NCS) 2 } . 4CHCl 3 ] n , the 3,2 :6 ,3"-tpy domains all adopt the conformation depicted in Scheme 5a. In contrast, on going to the ligands with the n-pentyloxy and n-hexyloxy chains, the conformation switches to that shown in Scheme 5b. The angles between the least squares planes of the pyridine rings in the 3,2 :6 ,3"-tpy unit also change ( Table 2). These changes are associated with a modification of the rhombuses that make up the (4,4) net, as discussed below.  We consider first the (4,4) nets assembled with Co(NCS) 2 Figure S16 in the supporting information). This leads to a cone-like arrangement, and adjacent sheets pack with the cones nesting into one another ( Figure 5a). The sheets pack in an ABAB... manner with adjacent sheets twisted through 90 • with respect to the next (Figure 5b). The transition from an ethoxy to n-propoxy to n-butoxy substituent has little impact on the structure, with the distances between the mean planes that are constructed through the cobalt atoms in a sheet increasing only from 9.07 Å in [{Co (2) We note that π-stacking interactions play no role in the assembly.  Hydrogen atoms and solvent molecules are omitted.

Compound 3
4-Hydroxybenzaldehyde (3.05 g, 25.0 mmol) was dissolved in EtOH (50 mL), then 3-acetylpyridine (6.06 g, 5.51 mL, 50.0 mmol) and crushed KOH (2.81 g, 50.0 mmol) were added to the solution. Aqueous NH 3 (32%, 80 mL) was slowly added to the reaction mixture. This was stirred at room temperature overnight. The excess of NH 3 was removed under vacuum, then water (100 mL) was added to the reaction mixture and CH 2 Cl 2 was used to extract impurities. Aqueous HCl (ca. 4%) was added to the aqueous phase until the pH = 8 and the formation of a solid was observed. The yellow solid was collected by filtration, washed with water (3 × 10 mL) and dried in vacuo. The 1 H-NMR spectrum of the product was in agreement with that previously reported for 4 -(4-hydroxyphenyl)-3,2 :6 ,3"-terpyridine (3.10 g, 9.53 mmol, 38.1%) [22]. All the intermediate (3.10 g, 9.53 mmol) was dissolved in DMF (40 mL), and the solution was heated to 70 • C. Then, anhydrous K 2 CO 3 (3.95 g, 28.6 mmol) was added, and the color changed from yellow to red-brown. After 5 min, 1-bromopropane (1.29 g, 0.96 mL, 10.5 mmol) was added to the reaction mixture. This was stirred at 70 • C overnight, then it was cooled to room temperature and poured into ice water and stirred for 20 min. The brown solid that formed was dissolved in CHCl 3 and was washed with aqueous K 2 CO 3 solution. The organic layers were dried over MgSO 4 , and the solvent was then removed. The solid product was purified by column chromatography (SiO 2 , ethylacetate:cyclohexane 1:1, R f = 0.16). Compound 3 was obtained as a pale yellow solid (1.  Figure S6 in the supporting information)/cm −1 : 3041w, 2962w, 2934w, 1603vs, 1515vs, 1243vs, 1182s, 1024s, 804vs, 700vs.
In experiment II, a solution of Co(NCS) 2 (10.6 mg, 0.060 mmol) in MeOH (6 mL) was layered over a CHCl 3 solution (4 mL) of 2 (10.6 mg, 0.030 mmol) in a crystallization tube (i.d. = 13.6 mm, 24 mL). Pink block-like crystals grew after four days (total mass of product was ca. 10 mg), and a single crystal was selected. A cell check confirmed a match with the crystal structurally characterized from experiment I. The remaining crystals in the tube were washed with MeOH and CHCl 3 , dried under vacuum and analyzed by PXRD and FT-IR spectroscopy. IR (selected bands, see Figure S7 in the supporting information)/cm −1 : 2975w, 2068vs, 1601vs, 1514vs, 1242vs, 1179s, 1050s, 815s, 747vs, 703vs.
In experiment III, a solution of Co(NCS) 2 (2.7 mg, 0.015 mmol) in MeOH (6 mL) was layered over a CHCl 3 solution (4 mL) of 2 (10.6 mg, 0.030 mmol) in a crystallization tube (i.d. = 13.6 mm, 24 mL). Pink block-like crystals grew after five days (total mass of product was ca. 10 mg), and a single crystal was selected for X-ray diffraction. A cell check confirmed a match with the crystal structurally characterized from experiment I. (3)

Crystal Growth of [{Co
In experiment I, a solution of Co(NCS) 2 (5.3 mg, 0.030 mmol) in MeOH (5 mL) was layered over a CHCl 3 solution (4 mL) of 3 (11.0 mg, 0.030 mmol) in a crystallization tube (i.d. = 13.6 mm, 24 mL). Pink block-like crystals grew after one day (total mass of product was ca. 10 mg), and a single crystal was selected for X-ray diffraction. The structural data confirmed a formulation of In experiment II, a solution of Co(NCS) 2 (10.6 mg, 0.060 mmol) in MeOH (5 mL) was layered over a CHCl 3 solution (4 mL) of 3 (11.0 mg, 0.030 mmol) in a crystallization tube (i.d. = 13.6 mm, 24 mL). Pink block-like crystals grew after one day, and a single crystal was selected for X-ray diffraction. A cell check confirmed a match with the crystal structurally characterized from experiment I. The remaining crystals in the tube were washed with MeOH and CHCl 3 , dried under vacuum and analyzed by PXRD and FT-IR spectroscopy. IR (selected bands, see Figure S8 in the supporting information)/cm −1 : 2968w, 2068vs, 1602vs, 1514vs, 1241vs, 1179s, 977m, 816s, 748vs, 704vs.
In experiment II, a solution of Co(NCS) 2 (10.6 mg, 0.060 mmol) in MeOH (6 mL) was layered over a CHCl 3 solution (4 mL) of 4 (11.4 mg, 0.030 mmol) in a crystallization tube (i.d. = 13.6 mm, vol. = 24 mL). Pink plate-like crystals grew after four days, and a single crystal was selected for X-ray diffraction. A cell check confirmed a match with the crystal structurally characterized from experiment I. The remaining crystals in the tube were washed with MeOH and CHCl 3 , dried under vacuum and analyzed by PXRD and FT-IR spectroscopy. IR (selected bands, see Figure S9 in the supporting information)/cm −1 : 2958w, 2062vs, 1600vs, 1514vs, 1240vs, 1180s, 1032m, 815s, 748vs, 703vs.
Powder X-Ray Diffraction (PXRD) patterns were collected at room temperature in transmission mode using a Stoe Stadi P diffractometer equipped with a Cu Kα1 radiation (Ge  4 ] n and [Co(6) 2 (NCS) 2 ] n were indexed with the monoclinic cells P2 1 /n and P2 1 /c, respectively. The profile matching analysis [33][34][35] of the diffraction patterns was performed with the package FULLPROF SUITE [35,36] (version July-2019) using a previously determined instrument resolution function based on a NIST640d standard. The structural models were taken from the single crystal X-Ray diffraction refinements. The refined parameters in Profile matching were: zero shift, lattice parameters, peak asymmetry, sample transparency and peak shapes as a Thompson-Cox-Hastings pseudo-Voigt function. The refinements confirmed that the bulk samples were consistent with the single crystal structures for all the compounds.