Constructing chiral MOFs by functionalizing 4 , 2 ′ : 6 ′ , 4 ′′-terpyridine with long-chain alkoxy domains : rare examples of neb nets †

The coordination chemistry of the divergent ligands 4,2′:6′,4′′and 3,2′:6′,3′′-terpyridine is dominated by the assembly of extended 1D chain or 2D sheet structures or discrete metallomacrocycles. 4,2′:6′,4′′-Terpyridines (4,2′:6′,4′′-tpy) coordinate only through the outer two pyridine donors, providing the V-shaped tecton shown in Scheme 1. This coordination motif is well-established and entry into higherdimensionality architectures is typically accomplished by introducing donor groups in the 4′-position. In addition to the widespread use of pyridinyl and carboxylate functionalities, sulfonate donors have also been used to access 3D frameworks. 3D-networks utilizing only the two pendant donors of the 4,2′:6′,4′′-tpy domain are relatively rare, although the reaction of 4′-(pyridin-2-yl)-4,2′:6′,4′′-tpy and CoIJNCS)2 gives a 3D structure in which the pyridinyl substituent is not coordinated. This is an unusual case where combination of a simple 4,2′:6′,4′′-tpy ligand and Co(NCS)2 leads to a 3D framework; more usually, 2D (4,4) or (6,3) nets result. A (4,4) net also forms when CoCl2·6H2O reacts with 4′-(3,4dimethoxyphenyl)-4,2′:6′,4′′-tpy. Although coordination polymers and networks containing a wide variety of 4,2′:6′,4′′-tpy or 3,2′:6′,3′′-tpy ligands are known, it remains a challenge to code for 3D frameworks rather than 1D-chains or 2D-nets in the absence of additional coordination domains or of co-ligands. To direct the assembly of 3D architectures predicated upon 4,2′:6′,4′′and 3,2′:6′,3′′-tpy motifs, we have adopted two strategies. In the first, we have used ditopic ligands containing ‘back-toback’ 4,2′:6′,4′′-tpy or 3,2′:6′,3′′-tpy domains. In the second, we have selected metal-nodes which prefer 6-(or higher) coordination numbers, e.g. reaction between the tpy ligands and CoIJNCS)2 (ref. 16–18) or Cd(NO3)2·4H2O. 24–26

Fig. 1 shows the asymmetric unit of [Co 2 (NCS) 4 (4) 4 ] n with symmetry-generated atoms.Each of the three independent Co atoms is octahedrally sited with a trans arrangement of N-bonded thiocyanato ligands.The thiocyanates containing S2, S4 and S6 are disordered and each has been modelled over two sites of 60.3/39.7,62.1/37.9 and 56.4/43.6%occupancies, respectively.Each of the four independent ligands 4 coordinates through the outer two pyridine rings, consistent with previously reported coordination modes. 1,2The Co-N bond distances (see caption to Fig. 1) and bond angles within the coordination spheres are unexceptional.No significant deviations from planarity are noted for the tpy units; twist angles between pairs of bonded pyridine rings range from 6.5 to 16.3°.Each phenyl ring is, as expected on steric grounds, twisted with respect to the pyridine ring to which it is attached (range of angles = 36.1 to 37.6°).
The structure shown in Fig. 1 propagates into a 3D framework.The differences in coordination environments about Co1, Co2 and Co3 in [Co 2 (NCS) 4 (4) 4 ] n are structurally insignificant and the framework is uninodal.Fig. 2 shows views down the crystallographic c and b-axes, generated using a combination of TOPOS 29 and Mercury (v.3.7). 30,31The 4-fold screw axes that define the chirality (i.e.opposite handednesses in each of space groups P4 1 2 1 2 and P4 3 2 1 2, Fig. S2 †) of the lattice run along the c-axis.Views down either the a or b-axes (Fig. 2, right) reveal the 6-membered rings present in the 4-connected net.The combination of 4-coordinate nodes and 6-membered rings is reminiscent of diamond.However, whereas a diamond net consists of interconnected 6 4 cage units, Fig. 3 illustrates that the 4-connected framework in [Co 2 (NCS) 4 (4) 4 ] n is constructed from 6 6 cage units. 28Each Co⋯Co edge of the 3D framework is spanned by one ligand 4 and the hexyloxy-chains are in close to extended conformations, directed along the a and b-axes (Fig. 4).
The reaction of 5 with CoĲNCS) 2 led to single crystals of [{Co 2 (NCS) 4 (5) 4 }•2CHCl 3 •MeOH] n .The compound crystallizes in the tetragonal space group P4 1 2 1 2, once again signifying a chiral network.The asymmetric unit contains three independent Co atoms and four independent ligands 5, and structural details within the coordination sphere of each Co atom are analogous to those shown in Fig. 1    Replacing ligand 4 ( n hexyloxy chains) by 5 ( n nonyloxy chains) has little effect on the overall structure, although the unit cell expands slightly along the a and b axes in response to the accommodation of the longer alkyl chains which are oriented along these axes (Fig. 4).In the two enantiomorphs In addition to the scarcity of the neb net among coordination polymers and hydrogen-bonded networks, 32 we note that the space groups P4 1 2 1 2 and P4 3 2 1 2 are relatively rare.A search of the Cambridge Structural Database 33,34 (CSD which contains ∼800 000 entries) using Conquest 35 v. 5.37 (with November 2015 update) revealed 1563 and 1346 hits for P4 1 2 1 2 and P4 3 2 1 2, respectively.
Fig.1shows the asymmetric unit of [Co 2 (NCS) 4 (4) 4 ] n with symmetry-generated atoms.Each of the three independent Co atoms is octahedrally sited with a trans arrangement of N-bonded thiocyanato ligands.The thiocyanates containing S2, S4 and S6 are disordered and each has been modelled over two sites of 60.3/39.7,62.1/37.9 and 56.4/43.6%occupancies, respectively.Each of the four independent ligands 4 coordinates through the outer two pyridine rings, consistent with previously reported coordination modes.1,2The Co-N bond distances (see caption to Fig.1) and bond angles within the coordination spheres are unexceptional.No significant deviations from planarity are noted for the tpy units; twist angles between pairs of bonded pyridine rings range from 6.5 to 16.3°.Each phenyl ring is, as expected on steric grounds, twisted with respect to the pyridine ring to which it is attached (range of angles = 36.1 to 37.6°).The structure shown in Fig.1propagates into a 3D framework.The differences in coordination environments about Co1, Co2 and Co3 in [Co 2 (NCS) 4 (4) 4 ] n are structurally insignificant and the framework is uninodal.Fig.2shows views down the crystallographic c and b-axes, generated using a combination of TOPOS29 and Mercury (v.3.7).30,31The 4-fold screw axes that define the chirality (i.e.opposite handednesses in each of space groups P4 1 2 1 2 and P4 3 2 1 2, Fig.S2 †) of the lattice run along the c-axis.Views down either the a or b-axes (Fig.2, right) reveal the 6-membered rings present in the 4-connected net.The combination of 4-coordinate nodes and 6-membered rings is reminiscent of diamond.However, whereas a diamond net consists of interconnected 6 4 cage units, Fig.3illustrates that the 4-connected framework in [Co 2 (NCS) 4 (4) 4 ] n is constructed from 6 6 cage units.28Each Co⋯Co edge of the 3D framework is spanned by one ligand 4 and the hexyloxy-chains are in close to extended conformations, directed along the a and b-axes (Fig.4).The reaction of 5 with CoĲNCS) 2 led to single crystals of [{Co 2 (NCS) 4 (5) 4 }•2CHCl 3 •MeOH] n .The compound crystallizes in the tetragonal space group P4 1 2 1 2, once again signifying a chiral network.The asymmetric unit contains three independent Co atoms and four independent ligands 5, and structural details within the coordination sphere of each Co atom are analogous to those shown in Fig.1for [Co 2 (NCS) 4 (4) 4 ] n .As in the latter, the Co-N(NCS) bond distances (range 2.065(5) to 2.107(5) Å) in [{Co 2 (NCS) 4 (5) 4 }•2CHCl 3 •MeOH] n are shorter than the C-N(tpy) bond lengths (range 2.153(5) to

Fig. 3
Fig. 3 (a) TOPOS/Mercury representation of the structure of [Co 2 (NCS) 4 (4) 4 ] n showing Co nodes.(b) The fundamental 6 6 unit of the neb topology excised from the framework shown in (a).