Trinodal Self-Penetrating Nets from Reactions of 1,4-Bis(alkoxy)-2,5-bis(3,2’:6’,3’’-terpyridin-4’-yl)benzene Ligands with Cobalt(II) Thiocyanate

The tetratopic ligands 1,4-bis(2-ethylbutoxy)-2,5-bis(3,2’:6’,3”-terpyridin-4’-yl)benzene (1) and 1,4-bis(3-methylbutoxy)-2,5-bis(3,2’:6’,3”-terpyridin-4’-yl)benzene (2) have been prepared and characterized by 1H and 13C{1H} NMR, IR, and absorption spectroscopies and mass spectrometry. Reactions of 1 and 2 with cobalt(II) thiocyanate under conditions of crystal growth at room temperature result in the formation of [{Co(1)(NCS)2}·MeOH·3CHCl3]n and [{Co(2)(NCS)2}·0.8MeOH·1.8CHCl3]n. Single-crystal X-ray diffraction reveals that each crystal lattice consists of a trinodal self-penetrating (6.8)(6.8)(6.8)2 net. The nodes are defined by two independent cobalt centres and the centroids of two crystallographically independent ligands which are topologically equivalent.


Synthesis of [{Co(1)(NCS) 2 }·MeOH·3CHCl 3 ] n
A MeOH (8 mL) solution of Co(NCS) 2 (1.75 mg, 0.01 mmol) was layered over a CHCl 3 (5 mL) solution of ligand 1 (7.41 mg; 0.01 mmol) in a crystallization tube (inner dimeter = 13.6 mm, volume = 24 mL). This was left to stand at room temperature. Orange block-like crystals visible to the eye were first obtained after 15 days, and a single crystal was selected for X-ray diffraction after another four days. A portion of the remaining crystals was mounted wet in a sample holder (to avoid loss of solvent from the crystals) and analyzed by powder X-ray diffraction (PXRD). 2 (1.75 mg, 0.01 mmol) was layered over a CHCl 3 (5 mL) solution of ligand 2 (6.33 mg, 0.01 mmol) in a crystallization tube (inner dimeter = 13.6 mm, volume = 24 mL) which was left to stand at room temperature. Orange block-like crystals visible to the eye were first obtained after seven days, and an X-ray quality crystal was selected after two months. A portion of the remaining crystals was mounted wet in a sample holder (to avoid loss of solvent from the crystals) and analyzed by PXRD.
In [{Co(1)(NCS) 2 }·MeOH·3CHCl 3 ] n , one CHCl 3 molecule was located and refined, and the Olex2 implementation of SQUEEZE [33] was used to treat the rest of the solvent region. A solvent mask was calculated, and 1044.0 electrons were found in a volume of 3742.0 Å −3 in one void. This is consistent with the presence of 2CHCl 3 and 1CH 3 OH per formula unit which account for 1072.0 electrons. The formulae and dependent numbers were adapted to account for this. Due to their thermal motion, the aliphatic chains had to be heavily restrained with SIMU, SADI, and DFIX restraints and were refined isotropically. H atoms were positioned geometrically, then allowed to ride on their parent atom.
In [{Co(2)(NCS) 2 }·0.8MeOH·1.8CHCl 3 ] n , the aliphatic chains display orientational and vibrational disorder and were modelled with the use of SADI and DFIX restraints, one chain over two sites of partial occupancies 0.60/0.40 and the other chain with equal occupancy sites. Some atoms were refined isotropically. Disorder in one thiocyanate ligand was modelled with two sites for the S atom of partial occupancies 0.75/0.25. All the solvent molecules present in this structure were modelled with partial occupancies. H atoms were positioned geometrically, then allowed to ride on their parent atoms.
Phase purity of bulk samples of the two compounds was confirmed at room temperature by PXRD in transmission mode using a Stoe Stadi P diffractometer equipped with a Cu Kα1 radiation (Ge(111) monochromator) and a DECTRIS MYTHEN 1K detector. The single crystal of [{Co(1)(NCS) 2 }·MeOH·3CHCl 3 ] n is representative of the main phase of the bulk sample. Minor phases are also present in the bulk, which can be attributed to solvent loss during sample preparation and measurement. The bulk sample of [{Co(2)(NCS) 2 }·0.8MeOH·1.8CHCl 3 ] n corresponds to the material characterized in the single crystal structure. The reflections of the main phases were indexed with a monoclinic cell in the space group C2/c (No. 15). The whole-pattern decomposition (profile matching) analysis of the diffraction patterns was done by the package FULLPROF SUITE [34][35][36][37] (version July-2019) using a previously determined instrumental resolution function (based on the silicon NIST standard 640d). The crystal model for the two compounds was taken from the single crystal diffraction data. Refined parameters were zero shift, sample displacement, transparency, lattice parameters, and peaks shapes (Y) as a Thompson-Cox-Hastings pseudo-Voigt function.

Synthesis and Characterization of Ligands 1 and 2
The strategy for the preparation of compounds 1 and 2 was similar to that used to synthesize related tetratopic ligands [11] and is adapted from a procedure reported in the literature [38]. The route is summarized in Scheme 5. MALDI-TOF mass spectra of 1 and 2 are shown in Figures

Synthesis and Characterization of Ligands 1 and 2
The strategy for the preparation of compounds 1 and 2 was similar to that used to synthesize related tetratopic ligands [11] and is adapted from a procedure reported in the literature [38]. The route is summarized in Scheme 5. MALDI-TOF mass spectra of 1 and 2 are shown in Figures  The 1 H and 31 C{ 1 H} NMR spectra of 1 and 2 were recorded in CDCl3 and signals were assigned using the COSY, NOESY, HMQC and HMBC spectra. The spectroscopic signatures of the tpy domains are not affected by the change in the alkoxy substituent, and the aliphatic regions of the spectra are consistent with the 2-ethylbutoxy and 3-methylbutoxy groups in 1 and 2, respectively ( Figure 1). HMQC and HMBC spectra of the two compounds are shown in Figures S5-S8 (see Supplementary material). The solution absorption spectra of compounds 1 and 2 are displayed in Figure  2 and are similar to that observed for 1,4-bis(n-octyloxy)-2,5-bis(3,2':6',3''-terpyridin-4'-yl)benzene [13]. Absorptions arise from spin-allowed π*←π and π*←n transitions. The 1 H and 31 C{ 1 H} NMR spectra of 1 and 2 were recorded in CDCl 3 and signals were assigned using the COSY, NOESY, HMQC and HMBC spectra. The spectroscopic signatures of the tpy domains are not affected by the change in the alkoxy substituent, and the aliphatic regions of the spectra are consistent with the 2-ethylbutoxy and 3-methylbutoxy groups in 1 and 2, respectively (Figure 1). HMQC and HMBC spectra of the two compounds are shown in Figures S5-S8 (see Supplementary material). The solution absorption spectra of compounds 1 and 2 are displayed in Figure 2 and are similar to that observed for 1,4-bis(n-octyloxy)-2,5-bis(3,2':6',3"-terpyridin-4'-yl)benzene [13]. Absorptions arise from spin-allowed π*←π and π*←n transitions.

Crystal Growth and Powder X-ay Diffraction
Single crystals of [{Co(1)(NCS)2} . MeOH . 3CHCl3]n and [{Co(2)(NCS)2} . 0.8MeOH . 1.8CHCl3]n were grown under ambient conditions by layering a methanol solution of Co(NCS)2 over a chloroform solution of ligand 1 or 2. After selection of crystals for single-crystal X-ray diffraction studies (see Section 3.2), the bulk materials were analyzed by powder X-ray diffraction (PXRD). Despite careful handling of the crystals, loss of solvent could not be prevented completely, and as a result, the PXRD spectra were of poor quality. Figure 3 shows a refinement of the PXRD pattern for each bulk material compared with that determined from the single crystal structure of each of

Crystal Growth and Powder X-ay Diffraction
Single crystals of [{Co(1)(NCS)2} . MeOH . 3CHCl3]n and [{Co(2)(NCS)2} . 0.8MeOH . 1.8CHCl3]n were grown under ambient conditions by layering a methanol solution of Co(NCS)2 over a chloroform solution of ligand 1 or 2. After selection of crystals for single-crystal X-ray diffraction studies (see Section 3.2), the bulk materials were analyzed by powder X-ray diffraction (PXRD). Despite careful handling of the crystals, loss of solvent could not be prevented completely, and as a result, the PXRD spectra were of poor quality. Figure 3 shows a refinement of the PXRD pattern for each bulk material compared with that determined from the single crystal structure of each of

Crystal Growth and Powder X-ray Diffraction
Single crystals of [{Co(1)(NCS) 2 }·MeOH·3CHCl 3 ] n and [{Co(2)(NCS) 2 }·0.8MeOH·1.8CHCl 3 ] n were grown under ambient conditions by layering a methanol solution of Co(NCS) 2 over a chloroform solution of ligand 1 or 2. After selection of crystals for single-crystal X-ray diffraction studies (see Section 3.2), the bulk materials were analyzed by powder X-ray diffraction (PXRD). Despite careful handling of the crystals, loss of solvent could not be prevented completely, and as a result, the PXRD spectra were of poor quality. Figure 3 shows a refinement of the PXRD pattern for each bulk material compared with that determined from the single crystal structure of each of

Single Crystal Structures
Ligands 1 and 2 react with Co(NCS)2 to form structurally similar three-dimensional networks [{Co(1)(NCS)2} . MeOH . 3CHCl3]n and [{Co(2)(NCS)2} . 0.8MeOH . 1.8CHCl3]n. The change from the 2-ethylbutoxy to 3-methylbutoxy substituents on the central phenylene spacer in the ligand has little impact on the structural assembly. Both compounds crystallize in the C2/c space group with similar cell dimensions (see Sections 2.10 and 2.11). We therefore discuss only one structure in detail. In [{Co(2)(NCS)2} . 0.8MeOH . 1.8CHCl3]n, the 3-methylbutoxy chains are disordered and were modelled using restraints. The structure chosen for the detailed discussion is therefore [{Co(1)(NCS)2} . MeOH . 3CHCl3]n. The asymmetric unit contains two independent cobalt atoms and two independent half-ligands 1 (Figure S9), and the second half of each ligand is generated by inversion. Atom Co1 resides on an inversion centre (Wyckoff notation n), while Co2 lies on a 2-fold axis. Figure 4 shows the repeat unit of the coordination network with symmetry-generated atoms, and bond distances for the coordination spheres of Co1 and Co2 are given in Table 1. All bond lengths are typical. Both atoms Co1 and Co2 are octahedrally coordinated with trans-arrangements of thiocyanato ligands, and N-Co-N bond angles lie in the 87.0(2)° to 93.1(3)° range. Each metal
The tpy unit of each independent ligand 1 is twisted to give a conformation close to conformation III in Scheme 1. Coordination occurs only through the two outer nitrogen atoms (see the discussion in the introduction). Angles between the least squares planes through pairs of pyridine rings containing N1/N2, N2/N3, N4/N5 and N5/N6 are 10.6(4), 18.6(4), 29 The structure propagates into a three-dimensional network which possesses four chemically distinct nodes, all of which are 4-connecting. These nodes are defined by atoms Co1 and Co2 and the centroids of the two independent phenylene rings. The two ligand nodes are topologically equivalent (this was confirmed using the program Systre [39]) and thus the underlying topological net is trinodal. The full vertex symbol is (6 2 .8 4 )(6 4 .8 2 )(6 5 .8) 2 , with the individual symbols referring to Co1, Co2 and the ligands, respectively. Figure 5 displays part of the net viewed down the aand c axes. A view down the b-axis is shown in Figure S12. In these representations, Co1 atoms are shown in blue, Co2 in orange, and the ligand nodes in green. Combination of Co2 and ligand nodes generates sheets that are coplanar with the bc-plane, while sheets composed of Co1 and ligand nodes slice obliquely through the unit cell. The net is self-penetrating, with the shortest circuits of the net penetrated by rods of the same net [40], and is highly unusual, with the net not appearing on the Reticular Chemistry Structure Resource (RCSR) database of 3D nets [41]. The self-penetration can be appreciated by inspection of Figure 6. The catenated shortest circuits comprise four Co1 atoms linked by ligand nodes, and three such circuits are depicted in red and magenta in Figure 6. This representation highlights the interlocking of the shortest circuits which produces the self-penetrating (6 2 .8 4 )(6 4 .8 2 )(6 5 .8) 2 net. The structure propagates into a three-dimensional network which possesses four chemically distinct nodes, all of which are 4-connecting. These nodes are defined by atoms Co1 and Co2 and the centroids of the two independent phenylene rings. The two ligand nodes are topologically equivalent (this was confirmed using the program Systre [39]) and thus the underlying topological net is trinodal. The full vertex symbol is (6 2 .8 4 )(6 4 .8 2 )(6 5 .8)2, with the individual symbols referring to Co1, Co2 and the ligands, respectively. Figure 5 displays part of the net viewed down the a-and c axes. A view down the b-axis is shown in Figure S12. In these representations, Co1 atoms are shown in blue, Co2 in orange, and the ligand nodes in green. Combination of Co2 and ligand nodes generates sheets that are coplanar with the bc-plane, while sheets composed of Co1 and ligand nodes slice obliquely through the unit cell. The net is self-penetrating, with the shortest circuits of the net penetrated by rods of the same net [40], and is highly unusual, with the net not appearing on the Reticular Chemistry Structure Resource (RCSR) database of 3D nets [41]. The self-penetration can be appreciated by inspection of Figure 6. The catenated shortest circuits comprise four Co1 atoms linked by ligand nodes, and three such circuits are depicted in red and magenta in Figure 6. This representation highlights the interlocking of the shortest circuits which produces the self-penetrating (6 2 .8 4 )(6 4 .8 2 )(6 5 .8)2 net.   The structural similarity between the nets in [{Co(1)(NCS) 2 }·MeOH·3CHCl 3 ] n and [{Co(2)(NCS) 2 }·0.8MeOH·1.8CHCl 3 ] n indicates that both the 2-ethylbutoxy and 3-methylbutoxy groups are accommodated within the cavities in the network and that there is little difference between the steric requirements of these substituents. Significantly, if the substituents are replaced by n-octyloxy chains in [{Co(3)(NCS) 2 }·4CHCl 3 ] n where 3 is 1,4-bis(n-octyloxy)-2,5-bis(3,2':6',3"-terpyridin-4'-yl)benzene, a switch in assembly is observed to an {4 2 .8 4 } lvt net [13]. In this structure, the long n-octyloxy chains are in non-extended conformations and each lies over a 3,2':6',3"-tpy unit with close C-H...π contacts.