Manipulating connecting nodes through remote alkoxy chain variation in coordination networks with 4′-alkoxy-4,2′:6′,4′′-terpyridine linkers

The effects of increasing the length of the alkoxy substituent in 4′-alkoxy-4,2′:6′,4′′-terpyridines when they are combined with cadmium(II) nitrate under conditions of room temperature crystallization and in the same cadmium:ligand (1:3) ratio have been investigated. The divergent ligand 4′-n-propoxy-4,2′:6′,4′′-terpyridine (2) reacts with Cd(NO3)2·4H2O to give [{Cd2(NO3)4(2)3}·3CHCl3]n in which the Cd atoms act as 3-connecting nodes and assemble into a (6,3) net with each ligand 2 linking adjacent Cd atoms. One of the three independent n-propoxy groups nestles into a cleft in the next 2-dimensional sheet; this ‘tail-in-pocket’ interaction restricts the length of the alkyl chain that can be accommodated. Replacing the n-propoxy by an n-pentoxy, n-hexoxy or n-heptoxy substituent results in a switch from a (6,3) to (4,4) net; in [{Cd2(NO3)4(3)4}·3CHCl3]n (3 = 4′-n-pentoxy-4,2′:6′,4′′-terpyridine) and [{Cd2(NO3)4(4)4}·CHCl3·MeOH]n (4 = 4′-n-hexoxy-4,2′:6′,4′′-terpyridine), each Cd atom is a 4-connecting node with trans-nitrato ligands, while in [{Cd(NO3)2(5)2}·2MeOH]n (5 = 4′-n-heptoxy-4,2′:6′,4′′-terpyridine) a cis-arrangement of nitrato ligands is observed. The reaction between Cd(NO3)2·4H2O and 4 was also investigated using a 1:1 ratio of reagents; this leads to the assembly of the 1-dimensional ladder [Cd2(NO3)4(MeOH)(4)3]n in which each Cd atom is a 3-connecting node. In each structure, face-to-face π-stacking of the central pyridine rings or of pyridine/phenyl rings of ligands in adjacent sheets or chains is a primary packing interaction; the role of van der Waals interactions as the chain length increases is discussed. Powder diffraction confirmed that each coordination polymer or network characterized by single crystal X-ray crystallography was representative of the bulk sample. The solid-state emission properties of ligands 2, 3 and 4 and their coordination polymers are reported; the blue emission of the free ligands is red-shifted by up to 59 nm upon formation of the coordination networks, and quantum yields are in the range 11–22%.


Crystallography
Crystallographic data were collected on a Bruker-Nonius Kappa APEX diffractometer; data reduction, solution and refinement used APEX2 25 and CRYSTALS. 26 diffraction data were collected on a Stoe Stadi P powder diffractometer.Structural diagrams and structural analysis were carried out using Mercury v. 3.5.1, 27,28and TOPOS. 29The solvent region of three of the structures treated with the program SQUEEZE, 30 and the electron density removed was equated to appropriate solvent molecules and added to relevant formulae (see below).High thermal motion in some of the alkyl chains meant that some carbon atoms had to be refined isotropically; some bond parameters in these chains were restrained to chemically reasonable values.all data = 0.0586), wR 2 = 0.1212 (wR 2 all data = 0.1228), gof = 0.9870.CCDC 1402482.
Photoluminescence.Solid-state quantum yields were measured using a Hamamatsu absolute PL quantum yield spectrometer C11347 Quantaurus_QY.Lifetimes and emission spectra of solid samples were measured using a Hamamatsu Compact Fluorescence lifetime Spectrometer C11367 Quantaurus-Tau; an LED light source with excitation wavelength of 280 nm was used.

Results and discussion
n-Propoxy-tailed 4,2′:6′,4″-terpyridine We recently reported that under crystallization conditions in MeOH/CHCl 3 , ligand 23 The ladder possesses a Cd : ligand ratio of 2 : 3, and both the rungs and rails of the ladder are defined by bridging ligands (Fig. 1).In order to probe the effects of lengthening the alkoxy chain in the linker, crystallization experiments combining CdĲNO 3 ) 2 •4H 2 O with ligands 2, 3, 4 or 5 with ratios of Cd : ligand of 1 : 3 in MeOH/CHCl 3 were conducted.These resulted in the growth of X-ray quality crystals.
Structural analysis of a crystal selected from the bulk sample of crystals grown from the reaction of 2 Ĳn-propoxy-tailed ligand) and CdĲNO 3 ) 2 •4H 2 O confirmed the formation of ĳ{Cd 2 ĲNO 3 ) 4 Ĳ2) 3 } •3CHCl 3 ] n .Cell checks on other crystals from the batch revealed consistent cell parameters.A comparison of the powder diffraction pattern for a batch of ground crystals from the bulk sample, with that of predicted from the single crystal structure is shown in Fig. S1.† The data are consistent with the single crystal being representative of the bulk sample.
In (4,4) nets lie parallel to the ab-plane.The alkoxy chains are in extended or close to extended conformations and are directed approximately along the c-axis (Fig. 6b).This has the effect of locking the nets with the 4-membered macrocycles almost directly over one another as shown in  8a).Additional voids in the lattice are occupied by solvent molecules.SQUEEZE 30 was used to treat the solvent region; the rationalized solvent content in the structurally similar coordination networks in ĳ{Cd 2 ĲNO 3 ) 4 Ĳ3) 4 }•3CHCl 3 ] n and ĳ{Cd 2 ĲNO 3 ) 4 Ĳ4) 4 } •CHCl 3 •MeOH] n is consistent with the greater spatial demands of ligand 4 versus 3.As in the (4,4) networks with ligands 3 and 4, the Cd atom in ĳ{CdĲNO 3 ) 2 Ĳ5) 2 }•2MeOH] n is octahedrally sited and is coordinated by four 4,2′:6′,4″-tpy and two nitrato ligands.
However, the nitrato groups are mutually cis (Fig. 9a), in contrast to trans-arrangements in ĳ{Cd 2 ĲNO 3 ) 4 Ĳ3) 4 }•3CHCl 3 ] n and ĳ{Cd 2 ĲNO 3 ) 4 Ĳ4) 4 }•CHCl 3 •MeOH] n .Bond parameters for the coordination sphere are given in the caption to Fig. 9a.The alkoxy chain with O1 is in a fully extended conformation while that with O2 is folded out of the plane of the phenyl ring to which it is bonded.The Cd atoms act as 4-connecting nodes and assemble into a (4,4) net.Compared to the up/up/ up/down arrangement of bridging ligands around each 4-membered metallomacrocycle in •2MeOH] n adopt an up/down/up/down arrangement (Fig. 9b).The two independent Cd⋯Cd distances within the (4,4) net are 12.938(2) and 12.630(2) Å.The Cd nodes in each net lie in a plane (deviation is ±0.1 Å); the sheets stack over one another (Fig. 10a) and the shortest Cd⋯Cd separations are 8.566(2) and 8.950(2) Å.
Fig. 10b shows a superimposition of the structure and TOPOS representation of part of one (4,4) net in ĳ{CdĲNO 3 ) 2 -Ĳ5) 2 }•2MeOH] n .A comparison of this with Fig. 6a and 7 illustrates that both cis and trans-arrangements of nitrato ligands lead to cavities in the network through which long alkoxy chains from the next sheet can penetrate.This is shown for 4,2′:6′,4″-tpy domains in adjacent sheets interact through π-stacking of the central pyridine rings (Fig. 11b).The angle between the least squares planes of the rings containing N2 ad N5 i (symmetry code i = 1 − x, 1 − y, 1 − z) is 5.4°and the ring-plane to centroid separation is 3.57 Å.This type of interaction is found in a range of coordination polymers involving 4,2′:6′,4″-tpy ligands. 10     A change in the ratio of cadmium : ligand For the coordination networks described above, the ratio of cadmium : ligand in the crystallization experiments was 1 : 3 leading to networks with a 2 : 3 ratio of Cd : ligand 2, or 1 : 2 for ligands 3, 4 or 5.For the hexoxy-tailed ligand 4, a crystallization experiment was also run with a 1 : 1 ratio of Cd : ligand.Structural analysis (see Fig. S5 † for powder diffraction data) revealed the formation of the 1-dimensional coordination polymer ĳCd 2 ĲNO 3 ) 4 ĲMeOH)Ĳ4) 3 ] n which crystallizes in the triclinic space group P1 ¯, and possesses a ladder structure (Fig. 12 and S6 †).The structure is similar to that found for ĳ{Cd 2 ĲNO 3 ) 4 Ĳ1) 3 }•CHCl 3 •MeOH] n . 23The asymmetric unit contains two independent Cd atoms and three independent ligands 4. Atom Cd1 is 8-coordinate (Fig. 12

Conclusions
The divergent N,N-binding mode of 4,2′:6′,4″-terpyridines is readily exploited for the formation of coordination polymers and networks, but determining factors that can encourage the formation of 2-and 3-dimensional networks remains under-developed.In this work, we have demonstrated the structural consequences of increasing the length of the alkoxy substituent in 4′-alkoxy-4,2′:6′,4″-terpyridines when these ligands combine with CdĲNO 3 ) 2 •4H 2 O with a Cd : ligand ratio of 1 : 3. Ligand 2 contains a 4′-n-propoxy substituent and forms ĳ{Cd 2 ĲNO 3 ) 4 Ĳ2) 3 }•3CHCl 3 ] n consisting of a (6,3) net.n-Propoxy chains protrude from the sheet and are involved in 'tail-in-pocket' interactions which interlock the sheets.Although preliminary data indicate that an n-butoxy chain causes no major structural perturbation, introduction of longer chains cause a switch from a (6,3) to (4,4) net.The change is consistent with the pockets in which the smaller chains are accommodated when (6,3) sheets pack together are too small to accommodate longer chains.In ĳ{Cd 2 ĲNO 3 ) 4 Ĳ3) 4 }•3CHCl 3 ] n and ĳ{Cd 2 ĲNO 3 ) 4 Ĳ4) 4 }•CHCl 3 •MeOH] n each 4-connecting Cd atom has a trans-arrangement of nitrato ligands, while in ĳ{CdĲNO 3 ) 2 Ĳ5) 2 }•2MeOH] n , they are cis.The reaction between CdĲNO 3 ) 2 •4H 2 O and 4 using a 1 : 1 ratio of Cd : ligand switches the assembly to a 1-dimensional ladder; in ĳCd 2 ĲNO 3 ) 4 ĲMeOH)Ĳ4) 3 ] n each Cd atom is a 3-connecting node.Face-to-face π-interactions between arene rings (either pyridine/pyridine or pyridine/phenyl) in adjacent sheets or chains are a common feature of the coordination networks, and van der Waals interactions between n-hexoxy chains play a dominant role in the packing of ladders in ĳCd 2 ĲNO 3 ) 4 ĲMeOH)Ĳ4) 3 ] n .In the solid state, the coordination polymers are blue emitters; values of λ max em are red-shifted up to 59 nm with respect to the free ligand, and quantum yields are in the range 11-22%.

From
n-propoxy to n-pentoxy and n-hexoxy tails Reaction of CdĲNO 3 ) 2 •4H 2 O with 3 or 4 (n-pentoxy or n-hexoxy derivatives, Scheme 3) leads to ĳ{Cd 2 ĲNO 3 ) 4 Ĳ3) 4 }•3CHCl 3 ] n or ĳ{Cd 2 ĲNO 3 ) 4 Ĳ4) 4 }•CHCl 3 •MeOH] n , respectively.The compounds are structurally analogous, crystallizing in the space group P2 1 /c with similar cell dimensions.Cell parameters consistent with those in the experimental section for ĳ{Cd 2 ĲNO 3 ) 4 Ĳ3) 4 } •3CHCl 3 ] n and ĳ{Cd 2 ĲNO 3 ) 4 Ĳ4) 4 }•CHCl 3 •MeOH] n were obtained for represenative crystals chosen from the bulk sample.Fig. S2 and S3 † compare the powder diffraction patterns predicted from the single crystal structures of the two coordination polymers with those of ground crystals from the bulk material, and with experimental powder patterns for the precursors.The data confirm that the single crystals chosen were representative of the bulk samples.

Fig. 4
Fig. 4 Accommodation of the 4-propoxyphenyl unit (red) containing atom O3 in a pocket comprising three ligands 2; short H⋯H contacts (2.14 and 2.49 Å) are shown in green.See text for details of the π-stacking interaction.

Fig. 6
Fig. 6 (a) Superimposition of the structure and TOPOS representation of part of the (4,4) net in ĳ{Cd 2 ĲNO 3 ) 4 Ĳ3) 4 }•3CHCl 3 ] n viewed down the c-axis.(b) View down the a-axis showing projection of the ligands above and below the sheet.Nitrato ligands, H atoms and solvent molecules omitted.
Fig. 11b also shows that the depth of penetration of one sheet into the next is such that the 4-phenyl substituent in ligand 5 lies within the (4,4) net of Cd atoms.Again this mimics the situation in the networks involving ligands 3

and 4 ,
and leads to a similar inter-sheet separation; shortest Cd⋯Cd distances between nets are 8.566(2) and 8.950(2) Å with ligand 5 versus 8.940(2) and 8.894(2) Å with 3, and 8.468(1) and 8.433(1) Å with 4. Thus, despite the change in the local environment at the Cd centre on going from a transto cis-ligand arrangement, the packing interactions between the sheets remain similar.