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Article

Coordination Chemistry of Polynitriles, Part XIII: Influence of 4,4′-Bipyridine on the Crystal and Molecular Structures of Alkali Metal Pentacyanocyclopentadienides

by
Patrick R. Nimax
† and
Karlheinz Sünkel
*
Department Chemistry, Ludwig Maximilians University Munich, 81377 Munich, Germany
*
Author to whom correspondence should be addressed.
†
Present Adress: Department of Chemical and Environmental Engineering of the Bilbao School of Engineering, University of the Basque Country, 48013 Bilbao, Spain.
Chemistry 2025, 7(5), 157; https://doi.org/10.3390/chemistry7050157
Submission received: 9 September 2025 / Revised: 20 September 2025 / Accepted: 25 September 2025 / Published: 1 October 2025
(This article belongs to the Section Inorganic and Solid State Chemistry)
Browse Figures
Versions Notes

Abstract

The reaction of 4,4′-bipyridine (C10H8N2) with the alkali metal pentacyanocyclopentadienides [Na{C5(CN)5}(MeOH)] and [KC5(CN)5] gives the coordination polymers [Na{C5(CN)5}(EtOH)(H2O)(C10H8N2)] (1) and [K{C5(CN)5}(H2O)2] • 2 (C10H8N2) (2) after recrystallization from EtOH. Both compounds show octahedral coordination around the metal ion with a NaN4O2 and KN2O4 environment. The [C5(CN)5] acts as a 1,1-bridging ligand in 1 and a 1,2-bridging ligand in 2. The 4,4′-bipyridine acts as a N,N′-bridging ligand between dimeric [Na2(EtOH)2(H2O)2(µ-{C5(CN)5}2] units, while it acts only as a guest molecule in the voids between polymeric [K(µ-H2O)4/2{µ-C5(CN)5}2/2]∞ chains. Both compounds employ multiple hydrogen bonds and π stacking to stabilize the crystalline structures.

1. Introduction

Metal–organic frameworks (MOFs) are a group of polymeric networks, composed of metal ions or small metal clusters as nodes and multidentate organic ligands as linkers. They were discovered approximately thirty years ago, and the interest in them is still growing. A search on SciFinder® (accessed on 2 September 2025) yields over 100,000 entries for the concept “MOFs” with over 15,000 publications alone in the year 2024 (Figure 1). They are characterized by being mostly highly crystalline and porous substances, with sometimes exceptionally high surface areas and large pore diameters, and have found numerous applications in catalysis [1,2,3], sensing and detection [4], gas adsorption [5], or diagnostic and therapeutic tools for cancer [6,7], to name just a few. Due to their high crystallinity, there has also been a tremendous amount of reported crystal structure determinations. A search in the Cambridge Structural Database (accessed on 1 September 2025, [8]) yields nearly 130,000 entries for the “MOF-Subset”. These large numbers are due not only to the large variety of combinations of metal cations and organic linkers, but also to anions, solvents, and guest molecules, which are usually situated in the large pores. Regarding the contributing metal ions, nearly all metals have been used, with the vast majority being transition metals and lanthanoids. In contrast, although being cheap and easily available as well as being non-toxic, s-block metal ions have been ignored for the generation of MOFs for a long time [9], although the situation is gradually improving in recent years [10,11,12,13].
When looking at the bridging multifunctional organic ligands, they can be divided mainly into two classes: aromatic (or less common, non-aromatic) polycarboxylic acid (salts) [14,15] and poly-amino or -imino type ligands. Of the latter, 4,4′-bipyridine is the most common linker ligand [16,17,18,19]. A search on the Cambridge Structural Database for the M-NC5H4-C5H4N linkage yields 7985 entries, of which 6620 contain the bipyridyl as a N,N′-bridging ligand. However, very few contain an alkali metal: 24 out of the 7985 and 16 out of the 6620, with the majority being Na compounds. In this context, it should be mentioned that 4,4′-bipyridine can be reduced to both a mono- and a di-anion, and thus the possibility of coordination polymers with redox-active ligands arises [20]. Another much smaller group of coordination polymers contains anionic polynitrile ligands, of which the tricyanomethanide and other “small cyano anions” [21,22], as well as the pentacyanocyclopentadienide [23,24,25], are the most common. With these anions, monodentate (terminal or bridging) and polydentate coordination can be observed, but in some cases, the solvent or water completely fills the coordination sphere of the metal, and the anion acts only as a “filler” of the crystalline voids, mostly accepting hydrogen bonds from the coordinated solvents. The influence of the addition of a linker ligand like 4,4′-bipyridine on the crystal structures of metal-polynitrile anion ligand combinations has also been studied [26,27]. To the best of our knowledge, however, no structural studies of alkali metal complexes, containing both polycyano anions and 4,4′-bipyridine ligands, have been reported so far. Here, we report our findings on the crystal structures of the products obtained from the reactions of [M{C5(CN)5}] (M = Na, K) with 4,4′-bipyridine.

2. Materials and Methods

Solvents used for synthesis and recrystallization were of analytical grade and were obtained commercially without further purification. The reagents, 4,4′-bipyridine and KCl, were also obtained commercially and were used as obtained. [Na{C5(CN)5}(MeOH)] was prepared as described by us previously [28]. The corresponding potassium compound, [KC5(CN)5], was obtained analogously from KCl and [AgC5(CN)5] in MeOH, in 90% yield.

2.1. Preparation of Compounds

2.1.1. Reaction of [Li{C5(CN)5}(H2O)] with 4,4′-Bipyridine

A solution of [Li{C5(CN)5}(H2O)] (108 mg, 0.50 mmol) in MeOH (20 mL) was treated with a solution of 4,4′-bipyridine (78 mg, 0.50 mmol) in MeOH (20 mL), and stirred for two hours at r.t. After evaporation of the solvent in vacuo to ca. 1 mL, CH2Cl2 was added, and the obtained yellow powder was dried in vacuo. Yield 193 mg.
Mass spectrometry: FAB+: m/z = 157.0 (C10H8N2 + H+); FAB−: m/z = 190.3 (C5(CN)5−). Elemental Analysis: Found: C, 60.12; H, 3.10; N, 24.67%. Calculated for [Li{C5(CN)5}(C10H8N2)(H2O)4]: C, 59.98; H, 3.46; N, 24.07%.

2.1.2. Reaction of [Na{C5(CN)5}(MeOH)] with 4,4′-Bipyridine

A solution of [Na{C5(CN)5}(MeOH)] (103 mg, 0.42 mmol) in MeOH (20 mL) was treated with a solution of 4,4′-bipyridine (66 mg, 0.42 mmol) in MeOH (20 mL) and stirred for two hours at 40 °C. After evaporation of the solvent in vacuo, a colorless solid was obtained, which was washed with CH2Cl2 and dried in vacuo.
1H NMR (400 MHz, DMSO-d6): δ = 9.14 m, 8.56 m. Mass spectrometry: FAB+: m/z = 157.0 (C10H8N2 + H+); FAB−: m/z = 190.3 (C5(CN)5−). Elemental Analysis: Found: C, 57.80; H, 3.10; N, 22.81%. Calculated for [Na{C5(CN)5}(C10H8N2)(MeOH)0.65(H2O)2.15]: C, 57.83; H, 3.50; N, 22.85%.
Recrystallization from EtOH gave a small number of colorless blocks suitable for X-ray diffraction.

2.1.3. Reaction of [KC5(CN)5] with 4,4′-Bipyridine

(a) A solution of [KC5(CN)5] (138 mg, 0.60 mmol) in MeOH (20 mL) was treated with a solution of 4,4′-bipyridine (94 mg, 0.60 mmol) in MeOH (20 mL) and stirred for 2 h at 40 °C. After evaporation of the solvent, a colorless solid was obtained, which was washed with CH2Cl2 and dried in vacuo.
1H NMR (270 MHz, D2O): δ = 8.39 (d, J = 6.4 Hz), 7.93 (d, J = 6.4 Hz). Mass spectrometry: FAB+: m/z = 157.0 (C10H8N2+H+); FAB−: m/z = 190.3 (C5(CN)5−). Elemental Analysis: Found: C, 57.80; H, 2.56; N, 23.10%. Calculated for [K{C5(CN)5}(C10H8N2)1.11(H2O)2]: C, 58.17; H, 2.96; N, 23.07%.
Recrystallization from either MeOH or EtOH gave colorless blocks, suitable for X-ray diffraction. Both materials showed the same unit cell.
(b) A solution of [KC5(CN)5] (115 mg, 0.50 mmol) in MeOH (20 mL) was treated with solid 4,4′-bipyridine (78 mg, 0.50 mmol) and stirred for two hours at r.t. After evaporation of the solvent in vacuo to ca. 1 mL, CH2Cl2 was added, and the obtained yellow powder was dried in vacuo. Yield 141 mg.
Elemental Analysis: Found: C, 54.01; H, 4.10; N, 20.99%. Calculated for [K{C5(CN)5}(C10H8N2)1.25(H2O)4.16]: C, 54.00; H, 3.69; N, 21.00%.

2.2. Crystallography

Colorless blocks of compound 1 were mounted on an oxford Xcalibur2 diffractometer, (Oxford Diffraction Limited, 68, Milton Park, Abingdon, Oxfordshire. OX14 4RX. UK) while blocks of compound 2 were mounted on a bruker D8 Venture diffractometer (Bruker Corporation, 40 Manning Road, Billerica, MA 01821, USA). The structure of 1 was solved with sir2014 [29], while sir97 [30] was used for the solution of compound 2. Both structures were refined using shelxl 2019-2 [31]. Water and ethanol hydroxyl H atoms were localized in a Difference Fourier synthesis and freely refined. In the structure of compound 2, the water H atoms were localized in a difference Fourier synthesis and were refined with restraints for the O–H bond lengths and the K…H distances. All other H atoms were positioned geometrically and refined according to a standard “riding model”. Structure visualization was performed using the programs ortep3 for Windows [32] and mercury [33]. Further details of the data collections and refinements are collected in Table S1 of the Supplementary Materials. Both structures have been deposited at the CCDC, with deposition numbers 2486615-2486616.

3. Results

3.1. Synthesis

Treatment of the pentacyanocyclopentadienides [LM{C5(CN)5}] (LM = Li(H2O), Na(MeOH), or K) with one molar equivalent of 4,4′-bipyridine in MeOH yielded colorless products. While the elemental analyses of the crude products with Li or Na suggested the integration of one equivalent of bipyridine as well as water (and MeOH in the case of Na), the reactions in the K system apparently showed the presence of a molar excess of bipyridine, as well as water. Unfortunately, spectroscopic methods like NMR or MS did not provide any further information with respect to the stoichiometry and/or the concrete bonding situation. Therefore, analysis by X-ray diffraction was essential to clarify the situation. Recrystallization from EtOH yielded single-crystalline material for the Na and K compounds, which could be studied by X-ray crystallography. The crystals obtained from the Na reaction turned out to contain coordinated pentacyanocyclopentadienide, water, and EtOH, together with one coordinated 4,4′-bipyridine molecule. The crystals from the K reaction contained coordinated water and pentacyanocyclopentadienide, while two molecules of 4,4′-bipyridine acted as uncoordinated guest molecules. In light of the obtained elemental analyses, it cannot be excluded that the coordinated EtOH molecule of the Na compound is partially replaced by water, and that the two bipyridine molecule positions are not fully occupied.

3.2. Molecular Structures

3.2.1. [Na{C5(CN)5}(EtOH)(H2O)(4,4′-C10H8N2)] (1)

Compound 1 crystallizes in the triclinic space group P-1 with two formula units in the unit cell. Figure 2 shows the asymmetric unit. While the stoichiometric formula suggests a coordination number of 4, this is misleading. In reality, both the pentacyanocyclopentadienide and the bipyridine act as bridging ligands and thus lead to an octahedral coordination (NaN4O2) around the Na ion (Figure 3). The [C5(CN)5] anion bridges two Na cations unsymmetrically via only one nitrile nitrogen atom (N1), while the bipyridine ligand uses both its N atoms symmetrically for coordination. The two symmetry-equivalent nitrile N atoms are approximately orthogonal, while the two different bipyridines are linearly arranged. The distance between the doubly nitrile N1-bridged Na ions is 3.832(1) Å. The two halves of the bipyridine molecule are nearly coplanar with an interplanar angle of 1.8(3)° and a distance (C13–C18) of 1.491(3) Å.
For comparison, the molecular structure of the precursor compound [Na{C5(CN)5}(MeOH)] contains also an octahedral NaN4O2 environment; however, the four N atoms are derived from four different cyano groups of four different anions, and the two MeOH molecules act as doubly bridging ligands, giving a dimeric molecule in the crystal with a distance of 3.685(1) Å between the Na centers [28]. Furthermore, the structure of 1 can be compared with the structure of [Na(dp-bian)(4,4′-bipy)2] (dp-bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) [20]. This compound contains bridging and terminal bipyridine ligands, with Na–Nbipy distances of 2.495(2) to 2.534(2) Å for the bridging and 2.470(2) Å for the terminal ligand. The bridging bipyridine ligands are completely planar with a central C–C bond of 1.491(3) Å, while the terminal bipyridine contains two twisted halves of the molecule with an interplanar angle of 49.73(6)° and a central C–C bond of 1.480(2) Å. Further comparable structures containing Na(4,4′-Bipyridine) units are [FeNa(N3)4(C10H8N2)]n [34], [Na2Mn2(4,4′-bipy)7(2,2′-bipy)4] [35], and [Na(4,4′-bipy)(en)] [36]. In these compounds, Na–N distances of 2.464(3)/2.419(3) Å, 2.546(4)–2.826(4) Å and 2.426(1) Å, respectively. The bipyridine ligands are twisted by 28.4(2)° in the Fe–Na compound with a central C–C bond of 1.480(3) Å, in the Mn–Na compound twisted by 16.7(5)/8.4(6)° with central C–C bonds of 1.486(5)/1.463(5) Å and in the [Na(en)] compound the two halves of the bipyridine are almost coplanar (interplanar angle 0.6(2)°) with a central C–C bond of 1.424(2) Å.

3.2.2. [K{C5(CN)5}(H2O)2] • 2 (4,4′-C10H8N2) (2)

Compound 2 crystallizes in the monoclinic space group C2/c with half a molecule (half of the coordination entity and two half-bipyridines) in the asymmetric unit (Figure 4). A C2 symmetry axis runs along the C3-C8-N3 group and thus generates the second half of the cyclopentadienyl ligand. Similarly, a C2 symmetry axis runs through the center of the C13-C13′ bond (at 0, y, 0.75). However, an inversion center is situated at the middle of the C23-C23′ bond (at −0.75, 0.25, 0). Again, the stoichiometric formula is misleading. Each K ion is coordinated by four µ2-bridging water molecules and two µ2-bridging pentacyanocyclopentadienides, thus producing a KN2O4 octahedron (Figure 5).
For comparison, the precursor compound [KC5(CN)5] also crystallizes in space group C2/c with a unique K environment and a C2 symmetry axis across the [C5(CN)5] ligand. However, in this compound, the anion uses all its nitrile groups for coordination, with one of them bridging even two K ions, and K–N distances range between 2.78 and 2.95 Å [24]. Comparable 4,4′-bipyridine-containing K compounds are difficult to find. In [K(dp-bian)(4,4′-bipy)2], two bridging bipyridine ligands are present, with K–N distances of 2.802(2) and 2.879(2) Å [20].
The different symmetry operators (C2 axis vs. inversion center) lead to different relative orientations of the two halves of the bipyridine molecules: molecule 1 (N10-C11-C12-C13-C14-C15)2 contains two “twisted” halves connected by C13–C13′ (1.481(3) Å) with an interplanar angle of 34.3(2)°, while molecule 2 (N20-C21-C22-C23-C24-C25)2 contains two nearly coplanar halves connected by C23–C23′ (1.489(2)Å) with an interplanar angle of 0.2(2)°. In the above-mentioned [K(dp-bian)] compound, the two halves of the bipyridine molecules are twisted with an interplanar angle of 26.87(4)° and a central C–C bond of 1.484(2) Å.

3.3. Crystal Structures and Intermolecular Interactions

Coordination polymers containing 4,4′-bipyridine are characterized by an “interplay of coordinative, hydrogen bonding and π–π stacking interactions” [37]. Depending on the metal, metal–metal, metal–anion, and metal-π interactions can contribute to the overall structure [38]. Thus, we could show in our recently published study on the crystal structures of mixed alkaline-earth pentacyanocyclopentadienide-4,4′-bipyridine compounds that a complicated interplay of hydrogen bridges and π–π stacking interactions occurs. Nitrile nitrogens and/or uncoordinated bipyridine N atoms acted as hydrogen bond acceptors towards O–H (from coordinated water or alcohol molecules) and/or C–H bonds of coordinated or uncoordinated 4,4′-bipyridines. Pentacyanocyclopentadienide anions showed π stacking either exclusively with other anions or with the aromatic bipyridine rings [27].

3.3.1. Hydrogen Bonding: O–H…X and C–H…X

Figure 6 shows the H-bonding interactions in compound 1, and Table 1 collects the relevant H-bond parameters.
Water oxygen O1 acts as a H-bond donor towards nitrile N atoms N2 and N5, and as a H-bond acceptor towards bipyridine H atoms H14 and H19. Nitrile N atom N4 accepts a H bond from ethanol OH group O2-H1E, while N3 accepts a H bond from bipyridine H atom H12. In addition, ethanol O atom O2 also accepts a weak intramolecular H bond from bipyridine H atom H16.
For the K compound 2, the situation is a bit more complicated. The water molecules bridge pairwise two inversion-related K ions along the c direction to give polymeric [K(H2O)4/2]∞ chains, with distances of 3.819 Å between the K ions. The pentacyanocyclopentadienide ion uses two neighboring (however, symmetry-related by a C2 axis) cyano nitrogen atoms to bridge two translation-related (in c direction) K ions in relative 1,3 position in the aforementioned chain (Figure 7).
Both bipyridine guests act as O–H…N H bond acceptors (Figure 8) and C–H…N H bond donors (Figure 9). The two bipyridine molecules have different bridging “tasks”: while molecule 1 (the “twisted” bipyridine) connects the octahedra chains in a direction accepting H bonds exclusively via water hydrogen H2, the planar molecule 2 connects the chains along the ab diagonals exclusively via water hydrogen H1.
In addition to the bridging of the octahedra chains via accepting H bonds from the coordinated water molecules, the bipyridine guest molecules donate H bonds both to other bipyridine molecules and to pentacyanocyclopentadienide nitrile N atoms.

3.3.2. π Stacking Interactions

In compound 1, π–π stacking occurs between bipyridine moieties and both pentacyanocyclopentadienide anions and other bipyridine ligands (Figure 10). Each bipyridine moiety is “sandwiched” by one other (inversion-related) bipyridine and one [C5(CN)5] anion. The closest C…C contacts are between 3.45 and 3.60 Å (which also corresponds approximately to the distance between neighboring planes). The distances between the centroids of the cyclopentadienyl ring (“Cp”) and the centroids of the two pyridine rings are 3.952 and 4.348 Å, with angles between the CTcp–CTpy vectors and the CTpy–CTpy vector of 54 and 63°, respectively. The distances between the closest centroids of the inversion-related bipyridines are 3.751 Å, and the angle between inversion-related bipyridine CT–CT vectors and the long axis” of the 4,4′-bipyridine molecule amounts to 79°. Figure S1 of the Supplementary Materials shows an ortep3 view of a [C5(CN)5], (bipy)2-“triple decker”.
Compound 2 shows a different kind of π-stacking (Figure 11). The planar bipyridine forms an alternating stack with the [C5(CN)5] anions. The closest contacts occur between atoms C1 and C6 on the anion and C21 and C25 of the bipyridine, ranging from 3.38 to 3.45 Å. The molecules of the twisted bipyridine show weak C…C contacts between the “inner” carbon atoms C14-C13-C13′-C14′, with C–C distances ranging from 3.43 to 3.49 Å.

4. Discussion

The reaction of 4,4′-bipyridine yields two different kinds of products with the pentacyanocyclopentadienides of Na and K. While the bipyridine molecule acts as a bridging ligand between Na ions, it acts only as a guest molecule in the crystal structure of the K compound. At first sight, this observation seems a bit unexpected, but it is consistent with the general findings about reported crystal structures of bipyridine compounds with the alkali metals. As was mentioned already in the Introduction, there are far more Na-containing structures with bridging bipyridines than ones containing K. In addition, this finding also parallels the observations made by us in our study on the crystal structures of bipyridine-containing alkaline-earth pentacyanocyclopentadienides; in these compounds, the heavier elements apparently also prefer integration of the bipyridine molecules as guest molecules over their coordination [27]. Apparently, the “hardness” of the metal ion is of minor importance in these compounds, with respect to preferences for ligand donor atoms. The [C5(CN)5] anion, which used four of its cyano groups for coordination to Na and all five cyano groups for coordination to K, employs only one CN group for coordination to Na and two for coordination to K. Again, it seems unexpected that the electron-poorer cyano nitrogen is preferred over the electron-rich bipyridine N donor for K. Apparently the electrostatic interaction of metal cation and pentacyanocyclopentadienide anion is more important. Both metals show octahedral coordination, with a NaN4O2 and a KO4N2 donor set. All O donors in the K compound and one in the Na compound are water molecules, while the remaining O donor is an ethanol molecule. This latter difference appears a bit unusual, as both crystals were obtained from EtOH solution, and one should expect that the harder Na cation should show a higher preference for the harder O donor than the softer K ion. It can therefore not be excluded that the crystals picked for measurement are unrepresentative of the bulk product. This view is additionally supported by the fact that the crystal of compound 2 contains apparently a 2:1 bipyridine/K ratio, although equimolar amounts of [KC5(CN)5] and 4,4′-bipyridine were used. On the one hand, such an observation has been made before in the synthesis of many bipyridine-containing MOFs. On the other hand, due to the correlation between thermal parameters and site-occupation factors, an independent refinement of these parameters does not appear possible. What might have helped to resolve this issue would have been a density determination of the crystals, which was unfortunately not performed, in part due to difficulties finding an appropriate solvent mixture.
Both compounds show extensive hydrogen bonding, both between OH or CH groups and N atoms (either bipyridine N atoms or nitrile N atoms) or O atoms. There are, however, slight differences between the compounds. In the Na compound, both water and ethanol ligands act as O–H hydrogen donors towards nitrile N atoms, while the water ligands in 2 donate H bonds only to the bipyridine N atoms. The water ligand in 1 also accepts hydrogen bonds from bipyridine C–H groups, while no such interaction is found in the K compound. One nitrile N atom in 1 and three in 2 accept hydrogen bonds from bipyridine C–H groups. All bipyridine N atoms accept C–H…N hydrogen bonds in compound 2, while no such interaction is observed in 1, which is not unexpected, since in the Na compound the bipyridine coordinates to two Na ions via both its N atoms.
π-stacking is observed in both compounds. In compound 1, a stack sequence [-Cp-Cp-bipy-bipy-]∞ is observed, while in compound 2, two different stacks can be found: a [-Cp-bipy-]∞ and a [-bipy-bipy-]∞ stack. While the closest C…C contacts in the range 3.40–3.50 Å are observed in both structures, they do not correspond to the “classical” π stacks, where the vectors if centroids of neighboring aromatic rings are close to orthogonal to the plane of these rings [39].

5. Conclusions

In summary, the main influence of introducing 4,4′-bipyridine into alkali metal pentacyanocyclopentadienides is the reduction in the number of coordinating nitrile N atoms, without eliminating them completely from the coordination sphere. The bipyridine molecule acts either as an N,N-bridging ligand or as a guest molecule in the voids of the crystal structure, interacting with the rest of the molecule via hydrogen bonds and π-stacking. Considering the increased interest in MOFs of the alkali metals, we could show that it is worthwhile looking at the possibilities that arise when replacing the “standard” polycarboxylate ligands with a combination of polynitrile anions and multifunctional aromatic N donor ligands. Already, apparently “small” changes as going from Na to K, induce significant structural changes, and the question arises as to what influences a subtle change within the substitutional patterns of the polynitrile anion and/or the bipyridine might create. Furthermore, both the crystal structures reported here show no void volume and cannot be considered as “porous”. Considering the numerous non-covalent interactions seen in both structures, it seems unlikely that the bipyridine molecules in these compounds could be easily removed and/or replaced. Again, the situation might change completely, by substituting one cyano substituent of the anion with H or instead using 4,4′-bipyridine, pyrazine, or a derivative with a “bridge” between the two pyridyl halves. The present study also does not yet answer questions on the influence of reaction stoichiometry as well as the used solvent (including the absence or presence of moisture), and therefore, there remains a lot of work to do.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/chemistry7050157/s1. Figure S1: the π stacking of [C5(CN)5] anions and bipyridine in compound 1; Figure S2: the π stacking of [C5(CN)5] anions and bipyridine in compound 2; Table S1: experimental parameters of the crystal structure determinations.

Author Contributions

Conceptualization, P.R.N. and K.S.; methodology, P.R.N.; validation, P.R.N. and K.S.; formal analysis, P.R.N.; investigation, P.R.N.; resources, K.S.; data curation, K.S.; writing—original draft preparation, P.R.N.; writing—review and editing, K.S.; visualization, K.S.; supervision, K.S.; project administration, K.S. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

CCDC 2486615-2486616 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44-1223336033.

Acknowledgments

We thank our former Undergraduate student Andreas Drechsel for helpful assistance in performing the experiments.

Conflicts of Interest

The authors declare no conflicts of interest.

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Chemistry 07 00157 g001
Figure 1. Development of annual publications on MOFs according to scifinder.
Figure 1. Development of annual publications on MOFs according to scifinder.
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Figure 2. Molecular structure of compound 1: asymmetric unit (displacement ellipsoids shown at the 50% probability level; dotted lines refer to intramolecular H bonding).
Figure 2. Molecular structure of compound 1: asymmetric unit (displacement ellipsoids shown at the 50% probability level; dotted lines refer to intramolecular H bonding).
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Chemistry 07 00157 g003
Figure 3. The NaO2N4 coordination environment of compound 1. Bond lengths are as follows (in Å): Na1–N1 2.574(2); Na1–N1ii 2.746(2); Na1–N10 2.488(2); Na1–N11i 2.500(2); Na1–O1 2.364(2); Na1-O2 2.355(2).
Figure 3. The NaO2N4 coordination environment of compound 1. Bond lengths are as follows (in Å): Na1–N1 2.574(2); Na1–N1ii 2.746(2); Na1–N10 2.488(2); Na1–N11i 2.500(2); Na1–O1 2.364(2); Na1-O2 2.355(2).
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Figure 4. Molecular structure of compound 2: asymmetric unit (dotted line corresponds to H bonding).
Figure 4. Molecular structure of compound 2: asymmetric unit (dotted line corresponds to H bonding).
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Chemistry 07 00157 g005
Figure 5. The KO4N2 coordination environment in compound 2. Bond lengths are as follows (in Å): K1–N1 = K1–N1i 2.901(2); K1–O1 = K1–O1i 2.6830(18); K1–O1ii = K1–O1iii 2.6946(17).
Figure 5. The KO4N2 coordination environment in compound 2. Bond lengths are as follows (in Å): K1–N1 = K1–N1i 2.901(2); K1–O1 = K1–O1i 2.6830(18); K1–O1ii = K1–O1iii 2.6946(17).
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Chemistry 07 00157 g006
Figure 6. Packing diagram (mercury) of compound 1, viewed along a, showing the OH…X and CH…X interactions. Color coding corresponds to the standard settings of mercury; dotted red lines refer to “hanging” H bonds.
Figure 6. Packing diagram (mercury) of compound 1, viewed along a, showing the OH…X and CH…X interactions. Color coding corresponds to the standard settings of mercury; dotted red lines refer to “hanging” H bonds.
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Figure 7. The polyhedra chains [K(µ-H2O)4/2{µ-C5(CN)5}2/2]∞ in compound 2, viewed along b. a and c cell axes shown. Standard color settings of mercury. Bipyridine guest molecules omitted for clarity. The [C5(CN)5] anions are color-coded in dark green.
Figure 7. The polyhedra chains [K(µ-H2O)4/2{µ-C5(CN)5}2/2]∞ in compound 2, viewed along b. a and c cell axes shown. Standard color settings of mercury. Bipyridine guest molecules omitted for clarity. The [C5(CN)5] anions are color-coded in dark green.
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Figure 8. Packing plot of compound 2, viewed along c, showing the bridging of the octahedra chains by the bipyridine guest molecules. Color coding: dark green—[C5(CN)5] anions; dark blue—the “twisted” bipyridine molecule 1; magenta—the planar bipyridine molecule 2. Light blue dotted lines correspond to “expanded” H bonds, as defined by mercury.
Figure 8. Packing plot of compound 2, viewed along c, showing the bridging of the octahedra chains by the bipyridine guest molecules. Color coding: dark green—[C5(CN)5] anions; dark blue—the “twisted” bipyridine molecule 1; magenta—the planar bipyridine molecule 2. Light blue dotted lines correspond to “expanded” H bonds, as defined by mercury.
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Chemistry 07 00157 g009
Figure 9. The 4,4′-bipyridine molecules acting as C–H…N hydrogen bond donors. Same color coding as in Figure 8. The thin green lines correspond to the octahedra chains in Figure 8.
Figure 9. The 4,4′-bipyridine molecules acting as C–H…N hydrogen bond donors. Same color coding as in Figure 8. The thin green lines correspond to the octahedra chains in Figure 8.
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Figure 10. The π stacking between 4,4′-bipyridine and pentacyanocyclopentadienide ligands in compound 1. Color coding: dark green—[C5(CN)5] anions; light blue—4,4′-bipyridine ligands; light blue dotted lines refer to close C…C contacts; lines of the type –..– indicate N–Na bonds.
Figure 10. The π stacking between 4,4′-bipyridine and pentacyanocyclopentadienide ligands in compound 1. Color coding: dark green—[C5(CN)5] anions; light blue—4,4′-bipyridine ligands; light blue dotted lines refer to close C…C contacts; lines of the type –..– indicate N–Na bonds.
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Chemistry 07 00157 g011
Figure 11. The two different π stacks in the crystal structure of compound 2. Color coding: light green—the [K(H2O)4/2{C5(CN)5}]∞ chain; red—the “planar” bipyridine molecule 2; dark blue—the “twisted” bipyridine molecule 1. Dotted light blue lines indicate weak C…C contacts.
Figure 11. The two different π stacks in the crystal structure of compound 2. Color coding: light green—the [K(H2O)4/2{C5(CN)5}]∞ chain; red—the “planar” bipyridine molecule 2; dark blue—the “twisted” bipyridine molecule 1. Dotted light blue lines indicate weak C…C contacts.
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Table 1. Hydrogen bond parameters in compounds 1 and 2.
Table 1. Hydrogen bond parameters in compounds 1 and 2.
Comp.D-H…AD-H [Å] H…A [Å]D...A [Å]<(DHA) [°]Symm.op.
1O1-H1W…N20.85(3)2.06(3)2.894(3)166(3)−x,1−y, 1−z
O1-H2W…N50.94(4)2.01(5)2.932(3)167(4)1−x, 2−y, 1−z
O2-H1E…N40.91(3)2.02(3)2.927(3)177(3)x, y−1, z−1
C12-H12…N30.952.623.542(3)163x, y−1, z−1
C14-H14…O10.952.533.474(3)172−x, 1−y, 1−z
C19-H19…O10.952.573.518(3)176−x, 1−y, 1−z
2C12-H12…N20.952.733.539(2)144x+1/2, y+1/2, z
C14-H14…N30.952.573.437(3)152−x−1/2, −y+1/2, −z+1
C15-H15…N200.952.813.647(2)148−x−1, y, −z+1/2
C21-H21…N100.952.653.546(2)157−x−1, −y+1, −z+1
C24-H24…N20.952.503.323(2)145x, y, z−1
O1-H1…N200.88(2)2.00(2)2.875(2)177(2)−x−1, y, −z+1/2
O1-H2…N100.85(2)2.10(2)2.914(2)160(2)x, y, z
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Nimax, P.R.; Sünkel, K. Coordination Chemistry of Polynitriles, Part XIII: Influence of 4,4′-Bipyridine on the Crystal and Molecular Structures of Alkali Metal Pentacyanocyclopentadienides. Chemistry 2025, 7, 157. https://doi.org/10.3390/chemistry7050157

AMA Style

Nimax PR, Sünkel K. Coordination Chemistry of Polynitriles, Part XIII: Influence of 4,4′-Bipyridine on the Crystal and Molecular Structures of Alkali Metal Pentacyanocyclopentadienides. Chemistry. 2025; 7(5):157. https://doi.org/10.3390/chemistry7050157

Chicago/Turabian Style

Nimax, Patrick R., and Karlheinz Sünkel. 2025. "Coordination Chemistry of Polynitriles, Part XIII: Influence of 4,4′-Bipyridine on the Crystal and Molecular Structures of Alkali Metal Pentacyanocyclopentadienides" Chemistry 7, no. 5: 157. https://doi.org/10.3390/chemistry7050157

APA Style

Nimax, P. R., & Sünkel, K. (2025). Coordination Chemistry of Polynitriles, Part XIII: Influence of 4,4′-Bipyridine on the Crystal and Molecular Structures of Alkali Metal Pentacyanocyclopentadienides. Chemistry, 7(5), 157. https://doi.org/10.3390/chemistry7050157

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