Syntheses and Crystal Structures of Two Metal–Organic Frameworks Formed from Cd²⁺ Ions Bridged by Long, Flexible 1,7-bis(4-Pyridyl)heptane Ligands with Different Counter-Ions

Abstract

The ethanol–water layered syntheses and crystal structures of the coordination polymers [Cd(C₁₇H₂₂N₂)₂(H₂O)₂]·2(ClO₄)·C₁₇H₂₂N₂·C₂H₅OH 2 and [Cd(C₁₇H₂₂N₂)₂(NO₃)₂] 3 are reported, where C₁₇H₂₂N₂ is a flexible spacer, 1,7-bis(4-pyridyl)heptane. In compound 2, trans-CdO₂N₄ octahedral nodes are linked by pairs of bridging ligands to result in [001] looped polymeric chains. The chains stack in the [100] direction to form (010) pseudo layers. Sandwiched between them are secondary sheets of free ligands, perchlorate ions and ethanol solvent molecules. Hydrogen bonds between these species help to consolidate the structure. Compound 3 contains trans-CdO₂N₄ octahedral nodes as parts of regular 4⁴ nets, which propagate in the (103) plane. Three independent nets are interpenetrated.

1. Introduction

Dipyridyl ligands, in which a pair of pyridine (py) rings are linked by a flexible alkyl chain [py–(CH₂)_n–py (n ≥ 2)], are effective species for constructing metal–organic frameworks (MOFs) and coordination networks [1,2,3,4,5,6,7,8,9,10] but appear to be understudied compared to dicarboxylate linkers. For example, the ligand 1,4-bis(4-pyridyl)butane (C₁₄H₁₆N₂) has been crystallised with Cu²⁺ cations and glutarate ions, forming a two-dimensional grid in which the copper ions are held in pairs by four bridging carboxylate groups, and the nitrogen ligands coordinate at the two apical sites of the dimers to form infinite sheets in [Cu₂(C₅H₆O₄)₂(C₁₄H₁₆N₂)]·2(CH₃CN) (Cambridge Structural Database ref. code ECINEM) [1]. The same ligand has been crystallised hydrothermally with Cd²⁺ ions and dicarboxylate anions, forming “helicates” in [Cd(C₁₄H₁₆N₂)(C₁₀H₈O₄)(H₂O)]·H₂O (MUHQAI), in which the metal ions adopt trigonal–bipyramidal coordination geometries [3]. When Zn²⁺ ions and benzotriazole-5-carboxylic acid are combined with 1,4-bis(4-pyridyl)butane, an unusual double-layer framework topology arises in [Zn₂(C₇H₃N₃O₂)₂(C₁₄H₁₆N₂)]·2(DMF)·5(H₂O) (DMF = dimethylformamide, C₃H₇NO) (TITBOP), which incorporates tetrahedral ZnN₃O nodes [4].

The next member of this ligand series, 1,5-bis(4-pyridyl)pentane (C₁₅H₁₈N₂), can be crystallised with several divalent metal ions, including cobalt, nickel, copper and cadmium. The Co²⁺ (DITYAH), Ni²⁺ (DITYEL) and Cu²⁺ (DITXUA) compounds are isostructural, with the formula [M(C₁₅H₁₈N₂)₂(NO₃)₂] [6], and consist of interpenetrated three-dimensional networks with the charge-balancing nitrate ions bonded to the metal ions, which adopt trans-MN₄O₂ coordination geometries. The cobalt and nickel octahedra are almost regular, while the copper octahedra show typical Jahn–Teller elongations of the bonds to the axial O atoms. Combining cadmium ions with 1,5-bis(4-pyridyl)pentane and different counter-ions lead to strikingly different structures: [Cd₂(C₁₅H₁₈N₂)₃(NO₃)₄]·H₂O (LOPNUZ) [2] contains distorted CdO₄N₃ pentagonal bipyramids arising from the coordination of three ligands and two chelating nitrate ions: the ligands link the metal ions into infinite sheets. In [Cd(C₁₅H₁₈N₂)₂(H₂O)₂]·2(ClO₄)·C₁₇H₂₂N₂·C₂H₅OH (TOKCON), where perchlorate ions provide charge balance, a completely different structure featuring a squashed two-dimensional 4⁴ net with linking trans-CdN₄O₂ nodes arises, where the O atoms are parts of water molecules. The ClO₄^– ions and “free” (uncoordinated) ligand molecules accept O–H⋯O and O–H⋯N hydrogen bonds, respectively, from the water molecules [7].

Several structures containing 1,6-bis(4-pyridyl)hexane (C₁₆H₂₀N₂) as the linking ligand are known. Manganese(II) cations, pairs of O,O′-chelating hexafluoro-acetylacetonato anions and bridging C₁₆H₂₀N₂ ligands combine together to generate zigzag chains containing cis-MnO₄N₂ octahedra in [Mn(C₁₆H₂₀N₂)₂(C₅HO₂F₆)₂] (WAWDEG) [9], whereas in [Mn(C₁₆H₂₀N₂)₂(NCS)₂(C₂H₆O)₂] (XIBYIQ) [10], the Mn²⁺ ions are coordinated by two N-bonded thiocyanate anions, two ethanol solvent molecules and two ligands to generate similar zigzag chains but with cis-MnO₂N₄ octahedral nodes. Combining Cu²⁺ with 1,6-bis(4-pyridyl)hexane and nitrate counter-ions leads to the very complex structure of [Cu(C₁₆H₂₀N₂)₃(NO₃)₂]·[Cu(C₁₆H₂₀N₂)₂(H₂O)(NO₃)]₂·2(NO₃)·C₂H₅OH (ODAHEG) [8], which contains two types of infinite chains in which the Jahn–Teller distorted copper ions are bonded to four ligand N atoms, nitrate O atoms and water molecules. One chain is “looped” (two bridging ligands connect every adjacent pair of metal ions), but the other has one bridging ligand and one “dangling” ligand only bonded to the copper atom from one of its N atoms. The striking structure of [Cu₂(C₁₆H₂₀N₂)₄(H₂O)₂]·4(NO₃)·C₁₆N₂₀N₂· C₂H₅OH·2H₂O (ODAHIK) [8] with the same ligand and counter-ion is completely different and consists of CuN₄O square pyramids connected by the bridging ligands into a fourfold interpenetrated three-dimensional network. The difference between ODAHEG and ODAHIK might arise due to the incorporation of uncoordinated water molecules from crystallisation into the latter structure, although this could hardly be predicted from an inspection of the formulae. Finally, in [Cd(C₁₆H₂₀N₂)₂(NO₃)₂] (LOPPAH), the combination of cadmium ions, nitrate ions and C₁₆H₂₀N₂ ligands leads to a novel structure featuring inclined, interpenetrating 4⁴ grids with octahedral CdN₄O₂ nodes [2].

To sum up, an important consequence of using neutral dipyridyl ligands as the metal-ion linkers rather than anions such as dicarboxylates is the requirement for charge-balancing anions, which, as noted above, may exert their own significant influence on the structure and prevent porosity [2,6,7,8]. Linkers with a longer, flexible spacer unit might give rise to elusive interpenetrated networks because of the greater space associated with the metal-ion coordination sphere in an infinite network [6]. Conversely, the longer alkyl chain might also result in disorder, which can hinder crystallographic studies. Either four pyridine nitrogen atoms might coordinate equatorially to the metal ion (giving a metal-to-ligand ratio of 1:2 as in most of the structures described above), with two counter-ions (most commonly nitrate or perchlorate) or water molecules coordinating apically, or six nitrogen atoms might coordinate octahedrally accompanied by uncoordinated counter-ions as in [Cd(C₁₂H₁₂N₂)₃]·2(ClO₄) (IDAJUS) [C₁₂H₁₂N₂ = 2-bis(4-pyridyl)ethane], where the metal-to-ligand ratio is 1:3 and a striking high-symmetry, triply interpenetrated network of infinitely extending cube-like ReO₃ topology building units arises [11]. The literature on bis(4-pyridyl)alkanes and M(II) ions was reviewed by us in 2007 [6]. It showed that the most common topologies are one-dimensional ribbons and two-dimensional nets, and two-dimensional or three-dimensional coordination networks are rare from these ligands.

The following novel papers are all relevant to this work because they incorporate pyridyl donors into spacers which bridge between metal ions, rigid or flexible [12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32].

As part of our ongoing studies in this area, we now describe the solvent-layering syntheses and crystal structures of two new coordination polymers prepared from 1,7-bis(4-pyridyl)heptane (C₁₇H₂₂N₂; bph) 1 (Figure 1) and cadmium(II) perchlorate or cadmium(II) nitrate, viz., [Cd(C₁₇H₂₂N₂)₂(H₂O)₂]·2(ClO₄)·C₁₇H₂₂N₂·C₂H₅OH 2 and [Cd(C₁₇H₂₂N₂)₂(NO₃)₂] 3, which shows the dramatic effect of changing the anion in this type of coordination network. The synthesis of the ligand was described in reference [6].

2. Experimental

2.1. Synthesis and Characterisation

Warning:

[Testimony from Mark R. St J. Foreman]

It is important to use KNH₂, not NaNH₂ for large scale reactions. The formation of NaNH₂ is much more exothermic than the formation of the KNH₂. This is thought to be based on lattice energies. I have had some truly scary moments in life making sodium amide but when I summoned up the courage to use potassium amide I found it was far safer to make. I did not have problems with liquid ammonia boiling violently. Also, the initial rate of reaction of solutions of potassium in liquid ammonia with the iron catalyst it much higher, I have never had a problem with unreacted potassium building up in the liquid ammonia but with sodium things can be a bit tricky.

1,7-bis(4-pyridyl)heptane [33]:

A flange flask equipped with a compressed air-driven mechanical stirrer and a cold finger condenser was assembled hot and allowed to cool while being purged with oxygen-free nitrogen gas. After cooling to room temperature, a mixture of solid carbon dioxide and acetone was added to the cold finger condenser. Into this flask, we decanted liquid ammonia (2 L). Potassium metal (23.4 g) was cut up and slowly added to liquid ammonia, which was gently refluxing in a flange flask. After the addition of the first sliver of potassium, a small amount of iron(III) chloride was added to form the catalyst for the reaction of ammonia with the solvated electron to form the potassium amide (KNH₂). After all the potassium had been reacted, the slurry of potassium amide was cooled to −78 °C using a cold bath made of solid carbon dioxide and acetone. After allowing the slurry to be stirred for one hour, the dropwise addition of a solution of 4-methylpyridine (60 mL) in diethyl ether (50 mL) was made. After allowing the mixture to stir at −78 °C for twenty minutes, the 1,5-dibromopentane (69 g. 41 mL) was added dropwise. The resulting mixture was stirred (3 h) at −78 °C before the addition of ammonium chloride (14 g). After further (1 h) stirring at −78 °C, the cold bath around the reaction flask was removed and the ammonia allowed to slowly evaporate. The next day, water (200 mL) was added to the brown residue in the flask, and the resulting mixture was extracted with diethyl ether (5 × 200 mL). The combined ethereal extracts were treated with potassium carbonate to remove water before filtration and evaporation to produce a heavy oil. The heavy oil was distilled in vacuum (0.1 mmHg) to provide 1,7-bis-(4-pyridyl) heptane.

Compound 2:

1,7-bis(4-Pyridyl)heptane 1 (0.100 g, 0.463 mmol) was dissolved in ethanol (5 mL) and layered onto a solution of Cd(ClO₄)₂·xH₂O (0.121 g, ~0.40 mmol) in water (5 mL). The solution was left to stand for two weeks, during which time colourless plates of compound 2 grew at the interface. These crystals were harvested via filtration and air dried (0.148 g, 67%). Found: C 55.3; H 6.2; N 7.4%. C₅₃H₇₆CdCl₂N₆O₁₁ requires C 55.0; H 6.6; N 7.3%; IR ν_max (KBr)/cm⁻¹ 3431s, 3052w, 2929s, 2853s, 1612s, 1558w, 1466w, 1425w, 1224w, 1193s, 1009s, 811w, 625s and 584w.

Compound 3:

1,7-bis(4-Pyridyl)heptane 1 (0.100 g, 0.463 mmol) was dissolved in ethanol (5 mL) and layered onto a solution of Cd(NO₃)₂·6H₂O (0.120 g, 0.389 mmol) in water (5 mL). The solution was left to stand for two weeks, during which time colourless blocks of compound 3 grew at the layer interface, which were collected and air dried (0.119 g, 54%). IR ν_max (KBr)/cm⁻¹ 2924s, 2856s, 1575s, 1557w, 1502w, 1485s, 1350s, 1226s, 1014s and 810s.

Caution! Cadmium salts are toxic, and perchlorates can be explosive. Work with small quantities, and take all appropriate safety precautions.

2.2. Crystal Structure Determinations

The intensity data for compounds 2 and 3 were collected using a Nonius Kappa CCD diffractometer (Mo Kα radiation, λ = 0.71073 Å, T = –153 °C). For compound 2, a colourless plate of dimensions 0.20 × 0.15 × 0.02 mm was chosen for data collection, and for compound 3, a colourless block 0.25 × 0.20 × 0.15 mm was used. Empirical (multi-scan) absorption corrections were applied at the data-reduction stage. The crystal of 2 was found to be non-merohedrally twinned, and some overlapped reflections from the two domains were excluded from the refinement. The structures were routinely solved via direct methods using SHELXS-97 [34] (space groups P2₁ for compound 2 and P2₁/n for compound 3), and the atomic models were developed and refined against |F|² with SHELXL-2019 [35]. The O-bound H atoms in compound 2 were found in difference maps and refined as riding atoms in their as-found relative positions with U_iso(H) = 1.2U_eq(O). The C-bound H atoms for both structures were geometrically placed and modelled as riding atoms with C–H = 0.95–0.99 Å depending on hybridisation and U_iso(H) = 1.2U_eq(C). The structures were verified and analysed with PLATON [36], and the molecular graphics were generated with ORTEP-3 [37] and Mercury [38]. Crystal data for compounds 2 and 3 are summarised in

Table 1. Key crystallographic data for compounds 2 and 3.

	2	3
Empirical formula	C53H76CdCl2N6O11	C34H34CdN6O6
Mr	1156.50	745.15
Crystal system	Monoclinic	Monoclinic
Space group	P21 (No. 4)	P21/n (No. 14)
a (Å)	11.1612 (3)	23.0560 (3)
b (Å)	16.2897 (5)	7.72740 (10)
c (Å)	15.3959 (5)	30.3995 (6)
β (°)	95.1973 (11)	102.8379 (7)
V (Å3)	2787.66 (15)	5280.68 (14)
Z	2	6
ρcalc (g cm–3)	1.378	1.406
μ (mm−1)	0.550	0.672
Data collected (2θ < xx°)	13,913	19,872
Unique data	13,913	10,201
RInt	– *	0.058
Flack parameter	0.05 (3)
R (F)	0.078	0.044
wR (F2)	0.217	0.112
CCDC deposition number	2,405,487	2,405,488

* Data not merged for a non-merohedral twin.

, and selected geometrical data are presented in

Table 2. Selected geometrical data (Å, °) for compound 2.

Cd1–O1	2.328 (6)		Cd1–N1	2.349 (5)
Cd1–N3	2.350 (6)		Cd1–O2	2.357 (7)
Cd1–N4 i	2.357 (6)		Cd1–N2 ii	2.359 (6)
C3–C6–C7–C8	61.9 (13)		C6–C7–C8–C9	173.8 (7)
C7–C8–C9–C10	–171.3 (7)		C8–C9–C10–C11	178.9 (7)
C9–C10–C11–C12	74.5 (10)		C10–C11–C12–C15	173.1 (7)
C20–C23–C24–C25	61.7 (10)		C23–C24–C25–C26	–177.8 (7)
C24–C25–C26–C27	178.2 (8)		C25–C26–C27–C28	179.3 (8)
C26–C27–C28–C29	–178.1 (7)		C27–C28–C29–C32	174.8 (7)
C37–C40–C41–C42	174.9 (7)		C40–C41–C42–C43	175.0 (7)
C41–C42–C43–C44	53.8 (11)		C42–C43–C44–C45	163.2 (9)
C43–C44–C45–C46	–69.9 (12)		C44–C45–C46–C49	–171.2 (8)
O1–H1o⋯O8 iii	0.85	2.03	2.877 (9)	179
O1–H2o⋯N6 iii	0.84	1.97	2.778 (10)	163
O2–H3o⋯O6 iv	0.86	2.09	2.787 (9)	138
O2–H4o⋯N5 v	0.84	2.04	2.760 (10)	144
O11–H11o⋯O4 vi	0.84	2.18	3.018 (10)	174
C17–H17⋯O3 v	0.95	2.55	3.253(12)	131
C18–H18⋯O2	0.95	2.51	3.185 (11)	128
C35–H35⋯O9 i	0.95	2.57	3.429 (12)	151
C47–H47⋯O5 vii	0.95	2.53	3.422 (13)	156
C51–H51⋯O9	0.95	2.58	3.376 (11)	141

The four values for the hydrogen bonds refer to the D–H, H⋯A and D⋯A separations (Å) and the D–H⋯A angle (°), respectively. Symmetry codes: (i) x, y, z + 1; (ii) x, y, z − 1; (iii) x + 1, y, z; (iv) 1 − x, ½+y, 1 − z; (v) 1 − x, ½ + y, 2 − z; (vi) x − 1, y, z − 1; (vii) x − 1, y, z.

and

Table 3. Selected geometrical data (Å, °) for compound 3.

Cd1–O1	2.311 (2)		Cd1–N1	2.326 (3)
Cd1–O4	2.350 (2)		Cd1–N5	2.351 (3)
Cd1–N3	2.377 (3)		Cd1–N4 i	2.385 (3)
Cd2–O7	2.310 (2)		Cd2–O7 ii	2.310 (2)
Cd2–N6 iii	2.331 (3)		Cd2–N6 iv	2.331 (3)
Cd2–N2	2.370 (3)		Cd2–N2 ii	2.370 (3)
C3–C6–C7–C8	65.2 (4)		C6–C7–C8–C9	170.7 (3)
C7–C8–C9–C10	–55.9 (4)		C8–C9–C10–C11	–53.9 (4)
C9–C10–C11–C12	176.2 (3)		C10–C11–C12–C15	70.0 (4)
C20–C23–C24–C25	–65.4 (4)		C23–C24–C25–C26	–169.1 (3)
C24–C25–C26–C27	66.8 (4)		C25–C26–C27–C28	175.5 (3)
C26–C27–C28–C29	174.1 (3)		C27–C28–C29–C32	176.5 (3)
C37–C40–C41–C42	–173.5 (3)		C40–C41–C42–C43	–172.6 (3)
C41–C42–C43–C44	–64.7 (5)		C42–C43–C44–C45	–170.6 (3)
C43–C44–C45–C46	63.5 (5)		C44–C45–C46–C49	162.3 (3)
C17–H17⋯N6 iv	0.95	2.60	3.281 (4)	129
C51–H51⋯O9 v	0.95	2.51	3.297 (4)	140
C2–H2⋯O4 vi	0.95	2.50	3.208 (4)	131
C5–H5⋯O4	0.95	2.57	3.225 (4)	126
C11–H11B⋯O5 vii	0.99	2.55	3.434 (4)	148
C12–H12A⋯O6 vii	0.99	2.54	3.411 (4)	147
C14–H14⋯O8 viii	0.95	2.56	3.203 (4)	125
C47–H47⋯O7 iii	0.95	2.51	3.191 (4)	129
C47–H47⋯O8 iii	0.95	2.52	3.438 (4)	163
C18–H18⋯N4 i	0.95	2.57	3.232 (4)	127
C24–H24B⋯O9 ix	0.99	2.47	3.261 (4)	136
C31–H31⋯O9 x	0.95	2.46	3.351 (5)	157
C30–H30⋯O1 xi	0.95	2.39	3.057 (4)	127
C34–H34⋯O4 xi	0.95	2.39	3.069 (4)	128
C46–H46A⋯O3 xii	0.99	2.41	3.287 (4)	148

The four values for the hydrogen bonds refer to the D–H, H⋯A and D⋯A separations (Å) and the D–H⋯A angle (°), respectively. Symmetry codes: (i) ½ − x, y + 3/2, ½ − z; (ii) 2 − x, 2 − y, −z; (iii) 2 − x, 1 − y, −z; (iv) x, y − 3, z; (v) x, y + 3, z; (vi) x, y − 1, z; (vii) 1 − x, −y, −z; (viii) 2 − x, 1 − y, −z; (ix) x − 1, y + 1, z; (x) x − 1, y, z; (xi) ½ − x, y − 3/2, ½ − z; (xii) 3/2 − x, y + 3/2, ½ − z.

. Full details can be found in the deposited cif files. Structural data for other MOFs and their ref. codes were accessed using the Cambridge Structural Database [39]. The mode of interpenetration was worked out from the X-Ray single-crystal data [38].

3. Results and Discussion

3.1. Structure of Compound 2

The asymmetric unit of compound 2 consists of a Cd²⁺ ion, three C₁₇H₂₂N₂ ligands, two perchlorate ions, two water molecules and an ethanol solvent molecule (Figure 2). The cadmium ion is coordinated by four ligand nitrogen atoms (from the N1/N2 and N3/N4 ligands) and two water O atoms to generate a regular trans-CdO₂N₄ octahedron (Table 2), with the minimum/maximum cis and trans bond angles being 81.7 (3)/95.5 (3)° and 171.79 (17)/175.7 (3)°, respectively. The bond valence sum (BVS) [40], in valence units, for the metal ion is 1.99, being in very good agreement with the expected value of 2.00.

In the N1/N2 ligand, the dihedral angle between the aromatic rings is 69.5 (4)°. The alkyl chain linking the pyridine rings is characterised by six torsion angles (Table 2). Working from C3 in the N1 ring to C15 in the N2 ring (see Figure 2), the chain has a gaaaga conformation, where g = gauche (modulus of torsion angle lies between 50° and 70°) and a = anti (modulus of torsion angle > 170°). For the N3/N4 ligand, the dihedral angle between the pyridine rings is 80.7 (4)°, and the alkyl-chain conformation is gaaaaa (N3 ring → N4 ring). For the N5/N6 ligand, which is not bonded to the metal ion, the inter-ring dihedral angle is 33.4 (4)°, and the chain conformation is aagaga (N5 ring → N6 ring). The deviations of the metal ions from their attached rings with nitrogen atoms N1–N4 are as follows: 0.840 (1) Å, −0.069 (1) Å, −0.586 (1) Å and 0.452 (1) Å. The mean Cl–O bond length in the perchlorate ions in compound 2 is 1.439 Å, which is quite typical [41].

In the extended structure of compound 2, the N1/N2 and N3/N4 ligands bridge the cadmium nodes into [001] infinite cationic looped chains of formula [Cd(C₁₇H₂₂N₂)₂(H₂O)]²⁺_n (Figure 3), with a metal–metal separation of 15.3959 (10) Å, which is much longer than the shortest inter-chain Cd⋯Cd separation of 9.9070 (9) Å. There are 32 atoms (two bph ligands and two metal atoms) in each loop of the polymeric chain. These chains stack in the [100] direction to generate (010) pseudo layers in the crystal. Sandwiched between the pseudo layers are (010) sheets of free ligands (i.e., the N5/N6 molecule), perchlorate anions and ethanol solvent molecules of crystallisation (Figure 4). Hydrogen bonding (Table 2) appears to play an important role in the cohesion of the layers: each water molecule bonded to the cadmium ion partakes in one O–H⋯O bond to a perchlorate ion and one O–H⋯N hydrogen bond to a nitrogen atom of the free ligand. Finally, the OH group of the ethanol molecule forms an O–H⋯O link to a perchlorate ion. There are also several weak C–H⋯O interactions present.

3.2. Structure of Compound 3

The asymmetric unit of compound 3 consists of two Cd²⁺ ions (one of which lies on a crystallographic inversion centre), three C₁₇H₂₂N₂ ligands and three nitrate anions (Figure 5). This leads to the uncommon situation of Z = 6 for a monoclinic crystal based on the simplest empirical formula for 3 of Cd(C₁₇H₂₂N₂)₂(NO₃)₂. Both the cadmium ions are coordinated by four ligand N atoms and two monodentate nitrate O atoms to generate a fairly regular trans-CdO₂N₄ octahedron in each case (Table 3). The bond valence sum for Cd is 2.00 and for Cd2 (site symmetry $\overline{1}$) is 2.06.

In the N1/N2 ligand, the dihedral angle between its aromatic rings is 51.32 (12)°, and the linking –(CH₂)₇– alkyl chain has a gaggag conformation (in the sense of progressing from the N1 ring to the N2 ring). The pyridine rings in the N3/N4 ligand subtend a dihedral angle of 60.05 (13)°, and its alkyl-chain conformation is gagaaa (N3 ring → N4 ring). For the N5/N6 ligand, the inter-ring dihedral angle is 79.45 (11)°, and the chain conformation is aagaga (N5 ring → N6 ring). The deviations of the metal ions from their attached rings with nitrogen atoms N1–N6, respectively, are as follows: Cd1 = 0.068 (5) Å; Cd2 = 0.378 (5) Å; Cd1 = 0.177 (5) Å; Cd1 = 0.137 (5) Å; Cd1 = 0.074 (5) Å; Cd2 = 0.234 (5) Å. The mean N–O bond length in the nitrate ions for the oxygen atoms (O1, O4 and O7) also bonded to the metal ions of 1.281 Å is significantly longer than that of the free oxygen atoms (1.233 Å).

In the extended structure of compound 3, the three ligand molecules are all bridging to generate a square grid propagating in the (103) plane with a regular 4⁴ topology [2,7,8] (Figure 6, Figure 7 and Figure 8). Each Cd1 atom is linked to two other Cd1 atoms and two Cd2 atoms in a pseudo “cis” square planar arrangement; each Cd2 atom is linked to four Cd1 atoms: the Cd⋯Cd distances within this network are 17.1075 (4) Å, 18.2357 (5) Å and 18.2592 (4) Å. The shortest out-and-back circuit for both Cd1 and Cd2 encompasses no fewer than 64 atoms (four bph ligands and four metal atoms). The shortest metal⋯metal separation in the structure of compound 3 of 7.7274 (4) Å arises due to interpenetration of the 4⁴ nets (see below).

The 4⁴ nets in compound 2 do not occur in isolation, but a triply interpenetrated layer structure arises, with each infinite 4⁴ grid interpenetrated by two others. It may be seen in Figure 6 that the individual grids (coloured red, purple and blue) alternate with respect to the [010] direction. Finally, the (103) interpenetrated sheets stack together to yield the crystal structure of compound 3 (Figure 7).

4. Conclusions

The syntheses and structures of [Cd(C₁₇H₂₂N₂)₂(H₂O)₂]·2(ClO₄)·C₁₇H₂₂N₂·C₂H₅OH 2 and [Cd(C₁₇H₂₂N₂)₂(NO₃)₂] 3 have been described. The same metal ion and bridging ligand in combination with different counter-ions has resulted in very different structures. The different geometries of the flexible dph ligands (as quantified by their alkyl chain torsion angles) are notable, with no fewer than five different conformations occurring, as noted above.

[Cd(C₁₅H₁₈N₂)₂(H₂O)₂]·2(ClO₄)·C₁₅H₁₈N₂·C₂H₅OH (TOKCON) [7], in which the ligand has two fewer methylene groups, has a very similar formula to that of compound 2, but its structure is different: TOKCON consists of a squashed two-dimensional 4⁴ grid rather than the one-dimensional looped chains seen in 2, but both contain trans-CdO₂N₄ nodes and layers containing free ligands, perchlorate ions and ethanol solvent molecules. The hydrogen-bonding pattern of O–H⋯O and O–H⋯N links from the coordinated water molecules is almost the same in both structures.

Compound 3 consists of a novel array of triply interpenetrated 4⁴ two-dimensional grids, which is facilitated by the long spacer chain of seven carbon atoms in the bph ligand, which are not disordered. It may be noted that [Cd₂(C₁₃H₁₄N₂)₄(NO₃)₄(H₂O)] (LOPNEJ) [2] where C₁₃H₁₄N₂ is the shorter ligand 1,3-bis(4-pyridyl)propane containing three methylene groups, contains doubly interpenetrated 4⁴ grids with two different types of trans-CdN₄O₂ nodes (one with two nitrate anions and one with one nitrate ion and one water molecule). The fascinating area of interpenetrated MOF structures has recently been reviewed by Gupta and Vittal [41].

Other MOFs containing 1,7-bis(4-pyridyl)heptane characterised by single-crystal diffraction include [Zn₂(OH)(C₁₇H₂₂N₂)₂]·3(ClO₄)·½(H₂O) (WETWOH) [5] (a search for WETWOH in the CSD also matches with a completely different zirconium organometallic compound) and [Cu₂(C₁₇H₂₂N₂)₄(NO₃)₃]·NO₃·H₂O (DITYIP) [6]. WETWOH shows unusual hydroxyl-bridged Zn(OH)Zn dimeric units, with one of the distinct ligands bridging the dimers into chains and the other “dangling”, being bonded to the zinc ion from just one of its N atoms: the interpretation of the structure of WEETWOH was hindered by severe disorder of the ligands. In DITYIP, the two distinct copper nodes form CuN₄O square-based pyramids and trans-CuN₄O₂ Jahn–Teller distorted octahedra. These are linked by the ligands into a triply interpenetrated three-dimensional network based on (6⁵.8²) nets [6].

These materials may find applications in gas storage, separation, catalysis and slow-release cadmium materials. The interpenetration networks may become either two or just one network as they crystallise with guest molecules, which stabilise the large framework.