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DOI: 10.1039/D4NJ02476A (Paper) New J. Chem., 2024, 48, 15577-15589

Synthesis of a tube-like tetra-bipyridyl, tetra-ethylacetato-calix[4]arene in 1,3-alternate conformation and the unexpected crystal structure of its [bpy(CoCl2)]3-[bpy(CoCl2)(H2O)] complex†

Maxime Mourer a, Younes Bouizi c, Sébastien Leclerc d, Bernard Malaman b and Jean-Bernard Regnouf de Vains *a
aL2CM, UMR 7053 CNRS, Université de Lorraine, France. E-mail: jean-bernard.regnouf@univ-lorraine.fr
bIJL, UMR 7498, Université de Lorraine, France
cCRM2, UMR 7036 CNRS, Université de Lorraine, France
dLEMTA, UMR 7563 CNRS, Université de Lorraine, France

Received 29th May 2024 , Accepted 24th July 2024

First published on 8th August 2024

Abstract

Treating conic tetra-para-ethylacetato-calix[4]arene (1) with 2.2 equivalents of K2CO3 and 4 equivalents of 6-bromomethyl-6′-methyl-2,2′-bipyridine affords the expected fully substituted podand, mainly in its 1,3-alternate conformation (2). This tube-like bipyridyl derivative 2 was subjected to complexation studies with CoCl2·6H2O to give a blue tetra cobalt(II) complex species (3). This complex displays a unique group of 6 aromatic bipyridyl 1H-NMR resonance signals demonstrating its high symmetry, while X-ray diffraction studies showed the presence of 3 expected tetrahedral bpy(CoCl2) units and 1 original trigonal bipyramidal unit involving the adjunction of a fifth ligand assumed to be H2O that could participate in the crystal scaffold.

Introduction

Calixarene platforms offer a wide range of structural and functional modulations1–6 and are the subject of intense investigations, particularly for the building of highly organized metal chelators, with objectives of separation, purification, waste depollution, enzyme mimicry or detection.7–19

Our previous contribution to the field focused on the building of calixarene-based chelators based mainly on 2,2′-bipyridine complexing units, whose number and arrangement along the calix[4]arene axis generate a coordinating environment that is prone to forming complexes with metal ions via their preferred coordination geometry with high selectivity.20–34

We reported a recent and representative example that showed the feasibility of L2Cu(I)4 coordination with two shared Cu(I)(bipyridine)2 subunits assembling two conic conformers of the tetra-O-2,2′-bipyridyl derivative of tetra-para-tert-butyl-calix[4]arene.35

In order to widen the scope of such metal ion-based spatial organizing behavior to complexation in water or at the air–water interface, we designed the analogous conic ligand bearing, in place of tert-butyl groups, para-ethylacetate ones which are prone to generate water-solubility and/or amphiphilicity after hydrolysis.

Surprisingly, the tetra-ethylacetato ligand (2) thus obtained does not satisfy the structural/conformational requirements, i.e. the conic conformation of the calixarene core, instead it gives a 1,3-alternate tetrasubstituted analogue. This particular structure was found highly interesting for the building of new types of polynuclear complexes and coordination polymers. Various metal ions have thus been tested. Among these, cobalt(II) chloride gave blue monocrystals that were suitable for X-ray diffraction analysis.

We report here the synthesis of the new ligand 2, its complexation study with CoCl2·6H2O, and the characterization of the resulting complex.

The studies were carried out through complex formation survey using UV-visible spectroscopy titration, complex structure analysis in solution using ESI-MS and, even if paramagnetic, 1H- and 13C-NMR spectroscopy. We also studied the complex in the solid state through FTIR spectrometry, elemental analysis and X-ray diffraction. The (6,6′-dimethyl-2,2′-bipyridyl)Co(Cl)2 complex (4) was prepared in parallel for comparative studies.36

Results and discussion

Synthesis

Tetra ethyl ester 137 and 6-bromomethyl-6′-methyl-2,2′-bipyridine38 were synthesized according to the literature.

In a one-pot process that should involve the participation of ester groups in the complexation/chelation of K+ ions, the reaction was performed in refluxing MeCN with 2.2 equiv. of K2CO3 on calixarene 1, i.e. 4.4 basic units vs. 4 OH groups, followed by the reaction of 4.1 equivalents of 6-bromomethyl-6′-methyl-2,2′-bipyridine, affording the tetra-substituted podand (2). Surprisingly, 2 is formed in the 1,3-alternate conformation and isolated with a global yield of 40% (formal 90% yield via alkylation) after careful purification (Scheme 1). Such preferential alternate conformation was also observed with tetra-[4-cyanomethyl]-calix[4]arene in similar conditions.39,40


image file: d4nj02476a-s1.tif
Scheme 1 Synthetic pathway to complex 3.

The tetra cobalt(II) complex 3 was prepared by reaction of a solution of ligand 2 in CH2Cl2 with a solution of 4 equivalents of CoCl2·6H2O in EtOH at ambient temperature and under argon. Ethanol was chosen in order to avoid deleterious trans-esterification reactions that could occur with a different alcohol. The resulting blue solution containing ca. 11% EtOH became problematic with a moiré-effect under stirring. Evaporation of the two solvents to dryness afforded a blue solid that was taken in CH2Cl2, giving an insoluble material. After trituration, filtration, and rinsing, this insoluble material afforded the complex 3 with a yield of 82%.

In parallel, the reference 6′6′-dimethyl-2,2′-bipyridyl)Co(Cl)2 complex 4 (dmbpCoCl2) was prepared as a deep-blue green microcrystalline solid by reaction of 1 equivalent of CoCl2·6H2O in MeOH with 6,6′-dimethyl-2,2′-bipyridine in CHCl3.36

The ligand 2 was fully characterized by UV-vis and infrared spectroscopy, 1H and 13C-NMR spectroscopy, electrospray mass spectrometry, and combustion analysis. The results obtained were fully consistent with the proposed formula. The tetra cobalt(II) complex 3 was analyzed by UV-visible (titration and final characterization) and infrared spectroscopy. The combustion analysis was consistent with the proposed formula involving the presence of 3 complementary molecules of H2O. Results from the positive mode ESI mass spectrometry were consistent with the mono-, di- and tri-charged species incorporating ligand 2 and cobalt, but without expression of the molecular peak.

As previously explored,27 1D (1H-, 13C-) and 2D (COSY, HSQC) NMR spectroscopies were employed to analyze the paramagnetic complex.

UV-Visible titration

The blue color of complex 3 indicates that cobalt(II) is in a tetrahedral environment, favored by the steric hindrance generated by the bipyridyl 6-methyl and 6′-methylene groups and the two coordinating chlorides. The complexation of the ligand 2 by cobalt(II) was followed by UV-visible-near infrared titration between 250 and 1000 nm, of 3 mL of a 1.006 × 10−5 M CH2Cl2 solution of 2, with ca. 50 μL of a 2.412 × 10−3 M ethanolic solution of CoCl2·6H2O.

The results given in Fig. 1 show that the complexation is finalized at the 10th aliquot (50 μL) of CoCl2 solution, i.e., 4 equivalents, which is consistent with the expected M4L stoichiometry. Three main domains of transitions are visible.


image file: d4nj02476a-f1.tif
Fig. 1 UV-vis titration of ligand 2 (3 mL at 1.006 × 10−5 M in CH2Cl2) by a solution of CoCl2·6H2O, 2.412 × 10−3 M in EtOH. Wavelength window: 250–850 nm. (a) Pure ligand 2, (b)–(k): pure 2 plus 5 μL aliquots of CoCl2 solution up to 50 μL; (l) and (m) 55 and 60 μL. Equivalence was achieved at 50 μL of Co(II) solution.

The first one, in the UV-visible region of 250–450 nm (Fig. 2), is representative of effects of cobalt complexation on the bipyridyl chelating moieties. Notably, disappearance of the π–π* transition of the ligand centered at 292 nm (Abs = 0.686, ε = 68[thin space (1/6-em)]224 mol−1 L cm−1) to the benefit of a bathochromically shifted envelope including a new band at 317 nm (Abs = 0.446, ε = 44[thin space (1/6-em)]356 mol−1 L cm−1) and a shoulder at ca. 343 nm (Abs = 0.214, ε = 21[thin space (1/6-em)]312 mol−1 L cm−1) confirms the successful coordination.


image file: d4nj02476a-f2.tif
Fig. 2 UV-Vis titration of ligand 2 (3 mL at 1.006 × 10−5 M in CH2Cl2) by a solution of CoCl2·6H2O, 2.412 × 10−3 M in EtOH. Wavelength window: 250–450 nm. (a) Pure ligand 2, (b)–(k) pure 2 plus 5 μL aliquots of CoCl2 solution up to 50 μL; (l) and (m) 55 and 60 μL. Equivalence was achieved at 50 μL of Co(II) solution.

Two isosbestic points are observed at 308 nm (0 to 25 μL–0 to 2 equivalents of metal ion) and 301 nm (30 to 50 μL–2.4 to 4 equivalents of metal ion) (Fig. 3). Despite the poor resolution, the isosbestic points suggest the formation of the LM and LM2 species at 308 nm and LM3 and LM4 at 301 nm.


image file: d4nj02476a-f3.tif
Fig. 3 UV-Vis titration of ligand 2 (3 mL at 1.006 × 10−5 M in CH2Cl2) by a solution of CoCl2·6H2O, 2.412 × 10−3 M in EtOH. Isosbestic points wavelength window: 295–310 nm. (a) Pure ligand 2, (b)–(m) pure 2 plus 5 μL aliquots of CoCl2 solution.

The second domain, between 500 and 750 nm, is presented in Fig. 4. As in previous reports,27,29,36 it shows two weak transitions with maxima at 569 nm (Abs = 0.198, ε = 985 mol−1 L cm−1) and 658 nm (Abs = 0.357, ε = 1775 mol−1 L cm−1), which are attributed to metal-to-ligand-charge-transfer (MLCT) and d–d metal-centered-transitions (MCT), respectively. The low absorptivity of those transitions required titration with a solution that was 20 times more concentrated (2.011 × 10−4 M for 2 and 4.825 × 10−2 M for the cobalt salt solution).


image file: d4nj02476a-f4.tif
Fig. 4 UV-Vis titration of ligand 2 (3 mL at 2.011 × 10−4 M in CH2Cl2) by a 4.825 × 10−2 M solution of CoCl2·6H2O in EtOH. Wavelength window: 500–750 nm. (a) Pure ligand 2, (b)–(k) pure 2 plus 5 μL aliquots of CoCl2 solution up to 50 μL; (l)–(n) 55, 60 and 65 μL. Equivalence was achieved at 50 μL of Co(II) solution (here between 50 and 55 μL). Black dashed line: UV-visible spectrum of Co(dmbpy)(Cl)2 complex 4 in CH2Cl2 at 2.011 × 10−4 M.

Finally, a transition of very low intensity is present in the near infrared region at 966 nm (not shown on figures). This transition increased regularly with the addition of cobalt solution, and was characterized by a molar extinction coefficient of 160 mol−1 L cm−1, i.e., 40 by final Co/bipyridyl subunit.

The bold dashed line present in Fig. 4 corresponds to the (6,6′-dimethyl-2,2′-bipyridyl)Co(Cl)2 complex 4 (dmbp(CoCl2) in the visible region. It reveals the existence of similar transitions, located at λmax 571 nm (Abs = 0.053, ε = 262 mol−1 L cm−1) and λmax 654 nm (Abs = 0.106, ε = 530 mol−1 L cm−1), corresponding to transitions centered on a tetrahedral cobalt ion complexed by the 6,6′-dimethyl-2,2′-bipyridine moiety and two chlorine anions. Its similarity with curve d (third line over a) is in accordance with the first global equivalence during the formation of complex 3 (1.2 equivalents of CoCl2).

These observations point toward a CoA2B2 symmetry tetrahedral geometry around the cobalt ion, according to Banci et al.41 Of note, the same authors assume that a pentacoordination around Co(II) high spin generates transitions in the same range as our results with a characteristic transition at 625 nm within the envelope of the visible spectrum of 3, and also of 4 between 600 and 700 nm. Thus, it is possible that a part of Co/bipyridyl subunits in 3 is pentacoordinated in solution. This will be demonstrated by X-ray diffraction study in the solid state. On the basis of empirical approaches from spectroscopic and structural concordance studies, Bertini42 and Sartorius et al.43 assumed that transitions in the 500–700 nm region with an ε value inferior to 50 mol−1 L cm−1 correspond to a pseudooctahedral hexacoordination around high-spin Co(II). Meanwhile, ε values between 50 and 200 mol−1 L cm−1 would rather indicate a pentacoordination with variable geometry. Finally, ε values greater than 300 mol−1 L cm−1 corresponded to a pseudotetrahedral tetracoordination. Between these domains, or at the borderlines, two geometries can be proposed.

Following these rules, we suggest herein that complex 4 adopts a tetrahedral geometry due to its transition at ε654 = 530 mol−1 L cm−1, as with complex 3, for which the value of ε658 = 443 mol−1 L cm−1 by the bpyCoCl2 unit is observed.

Infrared spectroscopy

Infrared spectroscopy was performed in ATR mode (diamond) between 4000 and 400 cm−1 (2500, 25[thin space (1/6-em)]000 nm) (Fig. S1, ESI†) on the reference ligand 6,6′-dimethyl-2,2′-bipyridine (dmbp), the reference cobalt complex 6,6′-dimethyl-2,2′-bipyridyl(Co(Cl)2) 4 (dmbp(CoCl2)), on crystals of complex 3 and on the free ligand 2. No relevant bands were observed in the region 4000–2000 cm−1. Thus, only the spectra in the 2000–400 cm−1 region are given in Fig. 5.
image file: d4nj02476a-f5.tif
Fig. 5 2000–400 cm−1 FTIR (ATR mode) spectra of (a) ligand 2, (b) crystals of tetranuclear complex 3, (c) microcrystalline dmbp(CoCl2) complex 4, and (d) 6,6′-dimethyl-2,2′-bipyridine (dmbp).

Comparison of the free ligand 2 (spectrum a) and free dimethylbipyridine (spectrum d) shows good homology around 780 cm−1 (C[double bond, length as m-dash]C–H with two C–H neighbors), 1438 cm−1 (C[double bond, length as m-dash]C–H deformations, CH3 elongations) and 1572 cm−1 (pyridyl C[double bond, length as m-dash]N, C[double bond, length as m-dash]C valence vibrations). In addition, ligand 2 displays two other major bands at around 1148 cm−1 (calixarenic C–O ester and ether) and 1730 cm−1 (C(O)OEt functions).

For the bipyridyl complex 4 (spectrum c), we notably observe the splitting of the bipyridyl band at 1571.98 cm−1 into a sharp band at ca. 1599 cm−1. This is attributed to the pyridyl C[double bond, length as m-dash]N being directly impacted by the complexation to cobalt ion. The two remaining ones at around 1560 and 1566 cm−1 are attributed to calixarene C[double bond, length as m-dash]C valence vibrations. Similar transformations are observed for complex 3 (spectrum b), for which we also observe an impact of cobalt complexation on the sharp and intense ester band at 1730 cm−1 in ligand 2. This evolves towards a more complex structure, suggesting a possible interaction between the cobalt/bipyridyl subunits and carboxylates due to their proximity caused by the 1,3-alternate conformation.

NMR spectroscopy

As previously shown with some parent paramagnetic cobalt(II) complexes of bipyridyl calixarenes,27,29 NMR spectroscopy was useful to attempt elucidation of the structure of complex 3. The (6,6′-dimethyl-2,2′-bipyridyl)Co(Cl)2 complex 4 served as reference for the chemical shifts of the bipyridine units.

Thus, in CDCl3 (Fig. S2c, ESI†), the symmetric complex 4 displays 4 resonance signals spread between 80 and −40 ppm. These features are characteristic of the paramagnetic influence of Co(II): three singlets at 69.75, 47.23 and −15.03 ppm integrating to 2 H each and with half-value widths of ca. 19 Hz, which were attributed to the six bipyridyl aromatic protons, and one broad singlet strongly upfield shifted at ca. −21 ppm, integrating to 6 H with an important half-value width of 145 Hz, attributed to the methyl groups.

For the tetranuclear complex 3, 1D (1H-; 13C-) and 2D (COSY, HSQC) experiments were employed. 1D 1H-NMR was realized with a solution of 3 in a mixture of CD3CN and CD2Cl2 at RT between 80 and −50 ppm. The 1D spectrum reveals the high symmetry of 3 in solution (Fig. 6), exhibiting 13 relevant signals. Four signals occur in the very low fields at ca. 74.1, 67.2, 53.8 and 32.0 ppm, with half-value widths between 35 and 25 Hz and integration of 4 H each. In addition to the solvent resonance signals at 5.42 and 1.96 ppm in the normal fields, two resonance signals appear at ca. 7.4 and 4.3 ppm, with half-value widths of close to 20 Hz and integrations of 8 and 12 H, respectively. In the high to very high fields, seven resonance signals appear at ca. −1.3, −3.7, −8.4, −15.0, and −19.2 ppm, with half-value widths comprising between 14 and 50 Hz, integrating for 8, 8, 8, 4 and 4 H, respectively, and at ca. −22.9 and −44.9 ppm with larger half-value widths of ca. 250 and 400 Hz, integrated to 12 and 8 H, respectively. The COSY experiment (Fig. 6) exhibits two triads at 74.07, 53.81, −15.00 and 67.16, 31.95, −19.18 ppm that were attributed to pyridyl aromatic protons,29 although their exact distribution within the biheterocycle was unclear.


image file: d4nj02476a-f6.tif
Fig. 6 COSY NMR experiment on complex 3 in CD3CN + CD2Cl2 (a and b refer to pyridyl subunits description).

According to the upfield shifting effect of the paramagnetic cobalt(II) on the CH3 groups of complex 4, and on the basis of the integration values, the two large resonance signals at ca. −22.9 and −44.9 ppm correspond to the bipyridyl CH3- and CH2O-groups, respectively. Unfortunately, no long range COSY correlation with their close aromatic protons were found.

With respect to their integrations, a correlation between the two abovementioned signals at ca. 7.4 and 4.3 ppm confirmed their attribution to the ethyl ester groups. Of note, their unusual chemical shifts suggest they are under the paramagnetic influence of cobalt(II) (Fig. 6, inset).

With the help of the 1H,13C-HSQC experiment (Fig. S3, ESI†), the three residual resonance signals integrated to 8 H each at −8.43, −3.73 and −1.33 ppm were attributed to the bridging methylene groups (ArCH2Ar), the calixarenic aromatic protons (ArH) and the acetyl methylenes (OCH2C(O)OR), respectively.

Because of the lack of solubility of 3 and 4 in pure or mixed CDCl3, CD2Cl2, CD3OD or CD3CN, the 13C-NMR analysis carried out between −40 and 800 ppm44 showed a low signal-to-noise ratio, making it difficult to interpret.

The d6-DMSO was found to decomplex cobalt from bipyridine (loss of the characteristic blue colour and release of free bipyridine in solution). However, (H)DMF apparently did not, and afforded (as with pure CoCl2·6H2O) deep blue solutions of 3 and 4 with higher concentrations. The proton-coupled 13C-NMR analysis of 4 (Fig. S4, ESI†) exhibited 6 resonance signals of equal integrations, comprising 3 doublets at 295.13, 313.24 and 475.2 ppm, with JH–C values of 170 Hz, corresponding to pyridyl CH, and two broad signals at 348.00 and 362.00 ppm which were attributed to the four quaternary carbons. The absence of quartet multiplicity for the CH3 signal at 138.79 ppm may be due to the half-value width (145 Hz) of its 1H-resonance signal.

Unfortunately, no data related to a paramagnetic cobalt complex were obtained with complex 3, with weak and badly resolved resonance signals located between 0 and 180 ppm that may correspond to the released ligand.

To overcome the solubility and concentration constraints, we attempted a 1H,13C-HSQC experiment in a mixture of CD3CN and CD2Cl2 between −100/+300 ppm and −30/+80 ppm for the carbon and proton dimensions, respectively. The 2D spectrum thus obtained (Fig. S3, ESI†) exhibited well-defined correlations for the solvent and for some calixarene fragments in the 0 to 130 ppm 13C- and −5 to +10 ppm 1H-region. No other correlations, notably for the 1H resonance signals of bipyridines, were found.

The 13C-characterization of complex 3 was finally obtained in a CD3CN + CDCl3 mixture, with an accumulation period of 72 h, between 700 and −100 ppm. Resonance peaks were found in the window of 500 to −25 ppm, as represented in Fig. 7 and described in the Experimental section.


image file: d4nj02476a-f7.tif
Fig. 7 13C-NMR spectrum of complex 3 (RT, CD3CN, CDCl3, 72 h; selected window: 500 to −25 ppm).

X-Ray diffraction analysis

Complex 3 crystallizes as blue parallelepipedic single crystals by slow evaporation of an acetonitrile/CH2Cl2 solution.

The structure was solved at 90 K, and Table 1 summarizes the relevant information on the data collection and structure refinements.

Table 1 Summary of single-crystal data collection and structure refinement conditions of complex 3
Structural formula C92H88Cl8Co4N8O13
Space group P[1 with combining macron]
Temperature [K] 90(2)
Formula weight 2033.1
Wavelength (Å) 0.71073
Crystal system Triclinic
Crystal colour Blue
Unit cell dimensions
a [Å] 13.1367(19)
b [Å] 16.011(2)
c [Å] 23.770(4)
α [°] 75.598(4)
β [°] 83.619(5)
γ [°] 71.724(4)
V [Å3] 4595.2(12)
Z 2
Crystal description Parallelepiped
Calculated density (g cm−3) 1.469
Absorption coefficient [mm−1] 1.008
F(000) 2088
Crystal dim [mm] 0.2 × 0.2 × 0.4
Theta range [°] 3–24
h_min, h_max −12, 12
k_min, k_max −14, 14
l_min, l_max −21, 21
R(int) 0.085
Reflections collected 76[thin space (1/6-em)]342
Reflections unique 7572
Data/restraints/parameters 7572/0/1109
Goodness-of-fit 1.07
Final R value 0.062 for 7572 data
Final R1 [I > 4σ] 0.041 for 5930 data
Final wR2 [I > 4σ] 0.0904
Largest difference [e Å−3] 0.41 and −0.30

The last refinements give rise to the formula C92H88Cl8Co4N8O13 with Z = 2.

The odd number of oxygen atoms, i.e., 13, is due to the presence of one supposed molecule of water attached to one of the four cobalt centers. As no hydrogen atom can be positioned with certainty, the choice between the neutral water molecule or the anionic hydroxyl group as the ligand must take into account the presence of a cobalt(II) or a cobalt(III) species, respectively, which will be discussed.

This analysis also confirms the coordination sphere of the three complex subunits, named Co1bpyCl2, Co2bpyCl2, and Co3bpyCl2, which adopt a pseudo-tetrahedral geometry. Remarkably, the fourth unit Co4bpyCl2(O) is a trigonal bipyramidal penta-coordinated species involving an oxygen ligand (O41), in addition to the two bipyridyl nitrogen atoms and the two chlorine anions.

Fig. 8 shows the crystal packing of complex 3 at 90 K and the voluminous solvent-accessible (but empty) void volume estimated to be 86 Å3 by the unit cell (ca. 2% of the cell volume).


image file: d4nj02476a-f8.tif
Fig. 8 Crystal packing at 90 K of complex 3 (capped sticks, no hydrogens) along the a-axis (6.5 layers), b-axis (3 layers) at the left, and c-axis (2 layers) down.

Fig. 9 presents the two molecules of complex 3 within one unit cell, and confirms the 1,3-alternate conformation of the calix[4]arene platform foreseen from 1H- and 13C-NMR analyses of the ligand 2, with ArCH2Ar singlet resonance signals at 3.75 and 37.45 ppm, respectively. As expected, the calixarene platform bears four dimethylbpy(CoCl2) subunits, each attached to the platform via an oxa-methylene arm.


image file: d4nj02476a-f9.tif
Fig. 9 The two molecules of the complex 3 in the unit cell at 90 K. ADPs ellipsoids at 50% probability level.

Fig. 10 and Table S1 (ESI†) show the quasi-perfectly square-tubular structure of the calixarene platform, with the four phenyl rings found parallel to their alternate parent, and in the prolongation along the C2 axis, orthogonal to their spanning ones. The pairs Co1bpyCl2/Co3bpyCl2 and Co2bpyCl2/Co4bpyCl2(O) are found in alternate position on opposite sides of the calixarene core.


image file: d4nj02476a-f10.tif
Fig. 10 Focusing on the square-tubular section structure of the calixarene at 90 K.

From Fig. 10 and 11, it can be seen that the Co1bpyCl2 unit is aligned perpendicular to its tethered phenolic unit, with CoCl2 oriented to the phenyl side. Co2bpyCl2 and Co4bpyCl2(O) are aligned almost perpendicular to their tethered phenolic units, with their CoCl2 oriented also on the phenyl side. However, Co2bpyCl2 leans 15 degrees to the plane of its phenyl group, and is oriented towards the face of Co1bpyCl2. Meanwhile, Co4bpyCl2(O) leans 12 degrees to the plane of its phenyl group, and is oriented towards the face of Co3bpyCl2. Finally, the latter is oriented with an angle of ca. 124° with respect to its tethered phenyl ring.


image file: d4nj02476a-f11.tif
Fig. 11 C2 axis oriented lateral views of: (left) Molecule 1 and (right) molecule 2 of complex 3 in the unit cell at 90 K.
Geometries around cobalt ions. Geometric parameters related to these subunits are described in Tables 2–4 and Table S2 (ESI†).
Table 2 Selected geometric parameters in [bipyridine(CoCl2)] subunits
Bond lengths around cobalt(II) centres [Å]
Co1 Unit Co2 Unit
N11–Co1 2.034(4) N21–Co2 2.050(5)
N12–Co1 2.049(5) N22–Co2 2.018(5)
Cl11–Co1 2.211(2) Cl21–Co2 2.211(2)
Cl12–Co1 2.202(2) Cl22–Co2 2.219(2)
Co3 Unit Co4 Unit
N31–Co3 2.031(5) N41–Co4 2.066(5)
N32–Co3 2.006(5) N42–Co4 2.079(5)
Cl31–Co3 2.217(2) Cl41–Co4 2.366(2)
Cl32–Co3 2.220(2) Cl42–Co4 2.362(2)
O41–Co4 2.057(4)
Bond angles around cobalt(II) centres [°]
Co1 Unit Co2 Unit
N11–Co1–N12 79.7(2) N21–Co2–N22 81.0(2)
N11–Co1–Cl11 111.40(13) N21–Co2–Cl21 109.66(14)
N11–Co1–Cl12 119.01(14) N21–Co2–Cl22 112.74(14)
N12–Co1–Cl11 111.08(13) N22–Co2–Cl21 109.66(14)
N12–Co1–Cl12 116.53(13) N22–Co2–Cl22 122.22(15)
Cl11–Co1–Cl12 114.43(7) Cl21–Co1–Cl22 116.96(07)
Co3 Unit Co4 Unit
N31–Co3–N32 81.3(2) N41–Co4–N42 78.5(2)
N31–Co3–Cl31 116.21(14) N41–Co4–Cl41 93.14(13)
N31–Co3–Cl32 113.10(14) N41–Co4–Cl42 96.40(14)
N32–Co3–Cl31 115.10(14) N42–Co4–Cl41 92.94(13)
N32–Co3–Cl32 116.48(14) N42–Co4–Cl42 93.28(13)
Cl31–Co3–Cl32 111.63(7) Cl41–Co4–Cl42 169.49(7)
O41–Co4–Cl41 85.42(14)
O41–Co4–Cl42 84.23(14)
O41–Co4–N41 145.0(2)
O41–Co4–N42 136.6(2)

Table 3 Distortion of bipyridine subunits [°]
Co1 Unit Co2 Unit
C15–C16–C17–C18 −6.0(1) C25–C26–C27–C28 −9.0(1)
N11–C16–C17–N12 −4.0(8) N21–C26–C27–N22 −8.4(8)
Co3 Unit Co4 Unit
C35–C36–C37–C38 4.0(1) C45–C46–C47–C48 1.0(1)
N31–C36–C37–N32 2.2(8) N41–C46–C47–N42 1.0(8)

Table 4 Particular geometric parameters in subunit Co4 of complex 3
Distances around Co4 at the unit cell interface (along the c axis) [Å]
Co4–(Co3)′ 5.700(1) Cl41–(Cl31)′ 4.801(3) O41–(Cl31)′ 3.302(6)
Cl41–(Co3)′ 6.124(2) Cl41–(Cl32)′ 5.680(2) O41–(Cl32)′ 3.223(4)
Cl42–(Co3)′ 5.879(2) Cl42–(Cl31)′ 5.075(3) Co4–(Cl31)′ 4.467(2)
O41–(Co3)′ 3.906(5) Cl42–(Cl32)′ 5.491(2) Co4–(Cl32)′ 5.278(2)
Angles around Co4 at the unit cell interface (along the c axis) [°]
(Cl31)′–Co4–(Cl32)′ 43.27(3) (Co3)′–(Cl31)′–O41 87.8(1)
(Cl31)′–Co4–(Co3)′ 21.05(2) (Co3)′–(Cl32)′–O41 89.7(1)
(Cl32)′–Co4–(Co3)′ 22.92(2) (Cl31)′–O41–(Cl32)′ 68.5(1)

The tetrahedral geometry of Co1, Co2 and Co3 is comparable with the crystallographic data available for the free (6,6′-dimethyl-2,2′-bipyridyl)Co(Cl)2 complex 4,45,46 and with the parent complexes previously published by our group.27,29 The three bipyridine units are slightly distorted in terms of planarity, with the N–C–C–N dihedral angles comprising between 4 and 7 degrees, showing the adaptation of the biheterocycles to the strained tetrahedral geometry.

Co4 adopts a trigonal bipyramid geometry, likely preferred due to the coordination of a fifth, oxygen-based, ligand. This ligand completes the coordination sphere along with the two bipyridyl nitrogens (N41 and N42) and its oxygen atom (O41) found in a N41,N42-pinched triangular base, and the two chloride anions (Cl41 and Cl42) opposed at the apexes of each pyramid.

Thus, angles measured around Co4 in the triangular base are 78.5° for N41–Co4–N42, and 136.6° and 145.0° for O41–Co4–N41 and O41–Co4–N42, respectively. The chlorine atoms slightly lean towards the calixarene platform, with angles of ca. 85° with respect to O41, and form an angle of only 169.50°, which is far from the expected 180°.

Surprisingly, no similar geometry with bipyridine-based cobalt(II) complexes was found in the literature. The closest structure, proposed by Tran and Marsura,47 involves the 6-methyl-6′-[2-oxa-propylbipyrazinyl]-2,2′-bipyridine ligand. The complexation of cobalt(II) is achieved by the two bipyridyl nitrogen atoms, two chlorine anions and the oxygen of the oxapropyl arm, in a penta-coordinated mode, with adoption of a distorted tetrahedral geometry capped by the ether oxygen at ca. 2.4 Å from cobalt. In this case, the degrees of freedom along the sigma bonds allows the positioning of this ether oxygen near the cobalt center. In our case, the steric hindrance of the calixarene moiety prevents such an orientation of the corresponding analogous phenoxy oxygen in complex 3.

Sykes et al.48 described another example involving the rigid phenanthroline ligand. This mono-phenanthrolyl, dichloro, mono DMSO cobalt(II) complex displays a penta-coordinate structure intermediate between the trigonal bipyramidal and square pyramidal geometries, where the two Nphen atoms are adjacent, the two chlorines are perpendicular from each other, and the DMSO ligand occupies the fifth position via one of its oxygens. Gennari and Duboc described a bipyridine functionalized by two thiol arms, and its Co(II) (dimeric: 2 Nbpy plus 2 bridging S and 1 bridging S–S by Co) or Co(III) (monomeric, 2 Nbpy plus 2 bridging S, 1 Cl) centers, each penta-coordinated with a distorted square pyramidal geometry.49,50

Other pentacoordinations of cobalt(II) have been also described with terpyridine and analogous ligands, which we nevertheless considered too far from bipyridine.51–53

Status of the O41 atom. The presence of the fifth oxygen ligands on the Co4 entity in complex 3 can be associated with either a hydration process conserving the Co(II) status, or a hydroxylation reaction, passing thus from Co(II) to Co(III).

The crystallization is realized by slow evaporation of the solvent, without specific protection from air, i.e., oxygen and water. However, the presence of water molecules inherent to the hexa aquo cobalt(II) salt used for complexation led us to the conclusion that this fifth ligand is H2O.

Due to the oxygen orientation, at least one of its hydrogen atoms could be involved in hydrogen bonds with the chlorine atoms of the Co3bpyCl2 subunit located in the neighbor cell (see Fig. 13), thus participating in the crystal cohesion/edification.

Of note, the divalent or trivalent status of Co4 could be linked to the fact that Co(II) should generate longer bonds with coordinating atoms than Co(III), due to their ionic radii.54

Surprisingly, to the best of our knowledge, no data related to Co(II) complexes involving H2O as the ligand were found. Meanwhile, one structure involving the monodentate –OH group attached to an octahedral Co(III) complexed by a bispidine has been described by Comba et al.,55 with the Co(III)–OH distance measured at 1.958 Å, 2.3% smaller than that of Co4–O.

A focus on bipyridine ligands and on their cobalt complexes gave interesting results on nitrogen–cobalt distances.

For example, the two bisthiolato-bipyridyl pentacoordinated Co(II) or Co(III) complexes reported by Gennari and Duboc show two dCo(III)–Nbpy value of 1.9915 and 2.0267 Å for the Co(III) species. Meanwhile, the distances in its reduced form increase to d(Co(II)–Nbpy) of 2.072 and 2.11 Å, respectively.49,50 Reimann et al. gave two dCo(III)–Nbpy values of 1.922 and 1.936 Å for a bis-bipyridyl, nitrato Co(III) octahedral complex, close to those of Gennari's species.56 Sengottuvelan et al. showed dCo(III)–Nbpy values of 1.9294(15) and 1.9186(18) Å for the octahedral hexacoordinated [Co(acac)(bpy)(N3)2·H2O] complex.57 Similar Nbpy–Co(III) distances comprising between 1.920 and 1.940 Å were described by Talwar et al. for the distorted octahedral hexacoordinated [Co(bpy)2CO3]·IO4 complex.58

More interesting for our purpose, Toyama et al. described Nbpy–Co(III) distances of 1.929(2) and 1.919(2) Å for the octahedral hexacoordinated cis-[Co(bpy)2(OH2)2][OTf]3·H2O complex species involving two 2,2′-bipyridines in the cis-position, and two coordinating H2O molecules. The distances between the oxygen atoms of these water molecules and the Co(III) center are not discussed, but examination of the cif file shows that they measure at 1.913(14) Å each, 7.5% shorter than O41–Co4 (2.057(4) Å).59

Through these results, we see that the average distance between cobalt(III) and bipyridyl N chelating atoms is about 1.94 Å, compared to the cobalt(II) species of Gennari at 2.07 Å, which is close to the distance dCo4–Nbpy values measured at 2.066 and 2.075 Å in complex 3. This allows us to describe Co4 as a pentacoordinated Co(II) species, indicating that the oxygen-containing fifth ligand must be neutral, i.e., a coordinating molecule of H2O.

Presence of hydrogen bonds around Co4. Fig. 12 presents two molecules of complex 3 arranged in two adjacent cells along the a-axis. It shows that the Co4 bpyCl2(O) subunit of the main unit cell looks towards the (Co3bpyCl2)′ subunit of the adjacent cell, of which cobalt is represented as (Co3)′.
image file: d4nj02476a-f12.tif
Fig. 12 Two molecules of complex 3 at 90 K in two adjacent unit cells, with a focus on the orientation of coordinating O41 atom in the pentacoordinated Co4 subunit towards the adjacent tetrahedral Co3 one.

Assuming the aqueous nature of O41, this part of the crystal is possibly the place of crystal stabilizing interactions; for example, via hydrogen bonding between hydrogen(s) attached to O41 and neighbor chlorine ions. This hypothesis is consistent with the work of Steiner.60 With help of the CSD database and considering the O–H bond length at 0.983 Å, Steiner related that, for H2O bonded to a transition metal atom, the H-bond with the Cl anion has a mean length of 2.182(8) Å and the mean O⋯Cl anion distance is 3.190 Å, if the angle at H is >140° and the directionality O–H–Cl is in the range of over 140° to 180°.

More recently, Pethes et al. showed that a H-bond between H2O and the Cl anion is acceptable if the H⋯Cl anion is less than 2.8 Å, and the angle Cl–O–H is smaller than 30 degrees.61

According to Fig. 13 and Table 5, a model drawing was prepared (Fig. 14) in which two hydrogen atoms are added to O41, respecting the distances H⋯O of 0.983 Å and the H–O–H angle of 104.5°. This modelling approach, even if poor scientifically, allows the building of a precession cone around O41 at the summit and with the two H (Ha and Hb) on the large rim. Their positioning is arbitrarily symmetrical with regards to the O41–Co3′ axis, and within the Cl32′–O41–Cl31′ plane. This give the Ha⋯Cl32′ and Hb⋯Cl31′ distances of 2.30 Å and 2.36 Å, respectively, which are inferior to the maximum limit of 2.80 Å described by Pethes, but slightly greater than the 2.182 Å given by Steiner.


image file: d4nj02476a-f13.tif
Fig. 13 Probable hydrogen bonding between two neighboring complex 3 molecules at the common border of two unit cells, involving the pentacoordinated Co4 entity and the tetrahedral one (Co3)′, at 90 K.
Table 5 Particular geometric parameters involving two hypothetical hydrogens Ha and Hb around O41 between subunits Co4 and Co3′ of complex 3. Evaluation of possible hydrogen bonds
Distances [Å] Angles [°]
a According to Steiner.60 b Arbitrary positioning of Ha and Hb in the plane O41–(Cl31)′–(Co3)′–(Cl32)′. c Distances deduced from drawing (Fig. 14). d esd not available.
O41–(Co3)′ 3.906(5) (Cl31)′–O41–(Cl32)′ 68.5(1)
O41–(Cl31)′ 3.302(6) (Cl31)′–O41–(Co3)′ 34.56(1)
O41–(Cl32)′ 3.223(4) (Cl32)′–O41–(Co3)′ 34.64(6)
(Co3)′–(Cl31)′ 2.217(2) (Cl31)′–(Co3)′–(Cl32)′ 111.63(7)
(Co3)′–(Cl32)′ 2.220(2)
O41–Ha 0.983ad (Cl32)′–O41–Hab 17.00d
O41–Hb 0.983ad (Cl31)′–O41–Hbb 18.00d
(Cl32)′–Ha 2.30cd (Cl32)′–Ha–O41b 157.00d
(Cl31)′–Hb 2.36cd (Cl31)′–Hb–O41b 155.00d
Torsion angles [°]
(Cl31)′–(Co3)′–(Cl32)′–O41 −14.3(1)
O41–(Cl31)′–(Co3)′–(Cl32)′ 14.0(1)
(Cl32)′–O41–(Cl31)′–(Co3)′ −9.57(8)
(Co3)′–(Cl31)′–O41–(Cl32)′ −9.57(8)
(Co3)′–(Cl32)′–O41–(Cl31)′ 9.55(8)


image file: d4nj02476a-f14.tif
Fig. 14 Schematic representation of Co4/Co3′ subunit environment at the cell interface. Evaluation of possible hydrogen bonds involving the H2O molecule bound to Co4 and chlorine atoms bond to neighbor Co3′.

The Cl32′–O41–Ha and Cl31′–O41–Hb angles of 17° and 18°, respectively, are inferior to the maximum limit of 30° given by Pethes. Meanwhile, the O41–Ha–Cl32′ and O41–Hb–Cl31′ angles of 157 and 155°, respectively, are within the 140° and 180° limits given by Steiner. The O41⋯Cl32′ and O41⋯Cl31′ distances of 3.223 Å and 3.302 Å, respectively, are slightly greater than the mean distance value of 3.190 Å given by Steiner.

These results support the hypothesis that at least one hydrogen bond between H–O41–H and one chlorine ligand of the neighbor molecule of 3 exists in the crystal.

Experimental part

General

Chemistry. IRTF spectroscopy was performed in ATR mode on diamond crystal on a Shimadzu IRAffinity-1S spectrophotometer, at ambient temperature in the 400 to 4000 cm−1 wavenumber range. UV-Visible spectra were obtained between 2500 and 1000 nm using a UV MC2 apparatus (SAFAS, Monaco) with 110-QS Hellma quartz cells (10 mm light path, 3.5 mL capacity), λmax in nm and ε in mol−1 dm3 cm−1. 1H- and 13C-NMR spectra were recorded on a Bruker AVANCE III 400 MHz (chemical shifts in ppm, J in Hz). Mass spectra (electrospray ionization – ESI) were recorded on a Bruker micrOTOFQ or a Xevo G2 XS Waters apparatus, at the Service Commun de Spectrométrie de Masse Organique, Nancy (MASSLOR).

Elemental analysis was performed on a Flash Smart (Thermofisher) apparatus at the Service de Microanalyse, Nancy (SYNBION). Merck TLC plates were used for chromatographic analysis (SiO2, ref. 1.05554; Al2O3, ref. 1.05581). Separations by column chromatography were done on aluminium oxide 90 standardized Merck 1.01097 and silica gel 60 M Macherey-Nagel (0.04–0.063 mm). All commercially available products were used without further purification, unless otherwise specified.

Single-crystal studies. A single crystal of 0.2 × 0.2 × 0.4 mm (parallelepipedic shape) was glued onto the tip of a quartz fiber, and mounted on a goniometric head. Data acquisition was performed at 90 K using a Bruker APEX-II diffractometer equipped with a QAZAR CCD detector using Mo Kα radiation (λ = 0.71073 Å) at the Institut Jean Lamour of Nancy. Cell refinement and data reduction were carried out using the APEX2 Software.62 The structure solution was obtained using direct methods, refined using full-matrix least-squares techniques with olex2-1.363 and refined with SHELXL-2018/3 software, implemented in the WinGX suite.64
NMR studies. NMR experiments were performed on a Bruker Avance III spectrometer operating at 9.4 Tesla (400 MHz and 100.6 MHz for 1H and 13C, respectively), using a Bruker 5 mm BBFO probe. Pulse widths were 15.6 ms for 1H and 10.5 ms for 13C. All experiments were performed at room temperature. Experiments were processed using the Bruker Topspin software package. For the paramagnetic cobalt complexes, the 1H, 13C decoupled from proton, 1H–1H DQF-COSY, 1H–13C HSQC NMR experiments were run with increased spectral width, given in the following section.
Acquisition parameters. 1D 1H spectral width 60 kHz, 1D 13C spectral width 40 kHz, and repetition time of 2 s.
Double quantum filtered COSY experiment. 4k complex points × 512 increments; 8 scans per increment; spectral width 60 kHz; the cardinal sine window function was used in both domains prior to Fourier transformations; final matrix size: 1k × 1k.
1H–13C HSQC experiment. 1k × 256 data set, 128 scans per increment; 1H spectral width 80 kHz; 13C spectral width 40 kHz; the cardinal sine window function was used in both domains prior to Fourier transformations; final matrix size: 1k × 1k.

Syntheses

5,11,17,23-Tetra(acetic acid)-25,26,27,28-tetrakis[(6′-methyl-2.2′-bipyridyl-6-yl)methoxy]calix[4]arene, 5,11,17,23-tetraethyl ester.
2 . K2CO3 (0.20 g, 1.43 × 10−3 mol) was added to a solution of 5,11,17,23-tetra(acetic acid)-calix[4]arene-25,26,27,28-tetrol, 5,11,17,23-tetraethyl ester 137 (0.50 g, 0.65 × 10−3 mol) in dry MeCN (50 mL) under argon. The resulting suspension was stirred under reflux for 30 min. 6-Bromomethyl-6′-methyl-2,2′-bipyridine38 (0.70 g, 2.66 × 10−3 mol) was then added, and reflux was continued under argon over 20 h (TLC monitoring: SiO2, CH2Cl2, EtOH 3%). After cooling, the solvent was evaporated to dryness and the residue was dissolved in CH2Cl2. The inorganic insoluble material was filtered off and the filtrate was concentrated prior purification by column chromatography (Al2O3, CH2Cl2/hexane, 90[thin space (1/6-em)]:[thin space (1/6-em)]10) to give the ligand 2. White powder. (0.37 g; 40%). IR (pure; ATR): 785.03 (strong, C[double bond, length as m-dash]C–H with two C–H neighbors); 1033.84 (medium, C–O–C); 1147.64 (large, strong, C–O ester and ether); 1438.89 (strong, sharp, C[double bond, length as m-dash]C–H deformations, CH3 elongations); 1571.89 (strong, pyridyl C[double bond, length as m-dash]N, C[double bond, length as m-dash]C valence vibrations); 1730.15 (strong, COOEt); 2790–3145 (broad, C[double bond, length as m-dash]C–H). UV-Vis (CH2Cl2): 292 (68224). 1H-NMR (400 MHz, CDCl3 (7.26 ppm), Fig. S5, ESI†): 1.10 (t, J = 7.1 Hz, 12 H, OCH2CH3), 2.70 (s, 12 H, CH3 bpy), 2.74 (s, 8 H, CH2COOEt), 3.75 (s, 8 H, ArCH2Ar), 3.93 (q, J = 7.15 Hz, 8 H, OCH2CH3), 4.96 (s, 8 H, OCH2bpy), 6.67 (s, 8 H, ArH), 6.75 (d, J = 7.5 Hz, 4 H, ArH bpy), 7.21 (d, J = 7.5 Hz, 4 H, ArH bpy), 7.75 (t, J = 7.7 Hz, 8 H, ArH bpy), 8.28 (d, J = 7.8 Hz, 4 H, ArH bpy), 8.48 (d, J = 7.6 Hz, 4 H, ArH bpy). 13C-NMR (100 MHz, CDCl3 (77.0 ppm)): 14.08 (OCH2CH3), 24.62 (CH3 bpy), 37.45 (ArCH2Ar), 40.20 (CH2COOEt), 60.53 (OCH2CH3), 72.70 (OCH2bpy), 118.35, 119.59, 121.53, 123.50 (C3, C3′, C5, C5′), 128.02 (Cp), 131.26 (Cm), 134.12 (Co), 136.91, 137.39, 153.96, 154.72, 155.20, 157.29, 157.91 (C2, C2′, C4, C4′, C6, C6′, Cipso), 171.69 (CO). Anal. calc. for C92H88O12N8 (1497.73): C 73.78, H 5.92, N 7.48; found: C 74.21, H 5.93, N 7.50. Positive mode ESI-MS: 1498.66 [M + H+]+, 1314.57 [M − C12H11N2 + H+]+, 1131.48 [M − 2 C12H11N2 + H+]+, 948.38 [M − 3 C12H11N2 + H+]+, 749.33 [M + 2 H+]2+/2, 500.22 [M + 3 H+]3+/3.
5,11,17,23-Tetra(acetic acid)-25,26,27,28-tetrakis[(6′-methyl-2.2'-bipyridyl-6-yl)methoxy]calix[4]arene, 5,11,17,23-tetraethyl ester, tetra (dichlorocobalt(II)) complex.
3 . An ultramarine blue solution of CoCl2⋅6H2O (105.5 × 10−3 g; 4.433 × 10−4 mol; 4.0 equiv.) in EtOH (2.0 mL) was added to a stirred solution of ligand 2 (166.0 × 10−3 g, 1.108 × 10−4 mol) in CH2Cl2 (12 mL) under Ar, to give a blue solution at ca. 11% of EtOH after rinsing with CH2Cl2 (4 mL). After 2 minutes of stirring, the solution became cloudy with a nice blue moiré-effect. Stirring was continued under Ar for 16 h, and the solvents were then evaporated to dryness. The resulting solid was treated with CH2Cl2 and the insoluble blue material was recovered by filtration on a crucible (0.188 g after drying under vacuum, 82%). The blue filtrate was then evaporated to dryness (0.035 g), in accordance with a quantitative yield. IR (pure, ATR): 785.03 (medium, C[double bond, length as m-dash]C–H), 1143.79 (m, C–O ester and ether), 1598.98 (C[double bond, length as m-dash]N), 1716.64 (m, COOEt). UV-Vis (CH2Cl2): 317 (44[thin space (1/6-em)]356), 343 (sh, 21[thin space (1/6-em)]312), 569 (985), 658 (1775). 1H-NMR (400 MHz, CD3CN (1.96 ppm) + CD2Cl2 (5.42 ppm), −90 to +120 ppm)): −44.92 (br s, Δν½ = 407 Hz, 8 H, OCH2bpy); −22.89 (br s, Δν½ = 246 Hz, 12 H, CH3bpy); −19.18 (br s, Δν½ = 31 Hz, 4 H, Hpy b); −15.01 (br s, Δν½ = 25 Hz, 4 H, Hpy a); −8.43 (br s, Δν½ = 50 Hz, 8 H), −3.73 (br s, Δν½ = 38 Hz, 8 H), −1.33 (br s, Δν½ = 14 Hz, 8 H, CH2COOEt; 4.27 (br s, Δν½ = 17 Hz, 12 H, OCH2CH3); 7.36 (br s, Δν½ = 22 Hz, 8 H, OCH2CH3); 31.96 (br s, Δν½ = 35 Hz, 4 H, Hpy b); 53.81 (br s, Δν½ = 25 Hz, 4 H, Hpy a); 67.16 (br s, Δν½ = 28 Hz, 4 H, Hpy b); 74.07 (br s, 4 H, Δν½ = 25 Hz, 4 H, Hpy a). 13C-NMR (partial resolution, HSQC; −40 to +800 ppm; CD3CN (1.972/1.32 + 118.26 ppm) + CD2Cl2 (5.412/55.11 ppm)): 19.03 (coupled to 1H 4.27; OCH2CH3); 38.84 (coupled to 1H −1.32; CH2COOEt); 64.51 (coupled to 1H 7.36, OCH2CH3); 120.37 (coupled to 1H −3.73, ArH); 13C-NMR (CD3CN (1.32 + 117.45 ppm) + CDCl3 (78.57 ppm): 19.08 (q, JH–C = 126 Hz, Δν½ = 12 Hz, OCH2CH3 from HSQC); 27.96 (t, JH–C = 125 Hz, Δν½ = 19 Hz, Ar–CH2–Ar); 38.63 (t, JH–C = 129 Hz, Δν½ = 12 Hz, CH2COOCH2CH3); 63.80 (t, JH–C = 145 Hz, Δν½ = 12 Hz, OCH2CH3 from HSQC); 81.88 (s, Δν½ = 30 Hz, Cquat); 119.26 (s, Δν½ = 12 Hz, Cipso,ortho,para of Ar); 119.63 (d, J = 157 Hz, Δν½ = 24 Hz, CH of Ar); 134.49 (s, Δν½ = 14 Hz, Cipso,ortho,para of Ar); 171.17 (s, Δν½ = 14 Hz, Cipso,ortho,para of Ar); 183.20 (s, Δν½ = 38 Hz, C(O)); 263.16 (d, JH–C = 165 Hz, Δν½ = 22 Hz, CH of bpy); 297.52 (d, JH–C = 165 Hz, Δν½ = 22 Hz, CH of bpy); 319.22 (d, JH–C = 165 Hz, Δν½ = 22 Hz, CH of bpy); 321.60 (d, JH–C = 165 Hz, Δν½ = 17 Hz, CH of bpy); 331.31 (s, Δν½ = 40 Hz, C2,2′,6,6′ of bpy); 375.33 (s, Δν½ = 40 Hz, C2,2′,6,6′ of bpy); 404.90 (br s, C2,2′,6,6′ of bpy); 428.00 (br s, C2,2′,6,6′ of bpy); 457.16 (d, JH–C = 173 Hz, Δν½ = 28 Hz, CH of bpy); 480.63 (d, JH–C = 173 Hz, Δν½ = 28 Hz, CH of bpy). Anal. calc. for C92H88Cl8Co4N8O12, 3 H2O (2071.14; powder before crystallization): C 53.35, H 4.57, N 5.41, O 11.59; found: C 53.03, H 4.37, N 5.37, O 11.31. Positive mode ESI-MS (Fig. S6 and S7, ESI†): 1535.92, 1536.92, 1537.92, 1538.92 [M − 4 (CoCl2) + K+]+; 1519.95, 1520.95 [M − 4 (CoCl2) + Na+]+; 1496.98, 1497.98 [M − 4 (CoCl2) + H+]+; 1131.48, 1132.49, 1133.48, 1134.47 [M − 4 (CoCl2) − 2(CH3bpyCH2) + H+]+; 1017.75, 1018.42, 1018.75, 1019.09 [2 L + Co2+ + H+]3+/3; 880.70, 881.20, 881.70, 882.20, 882.70, 883.20, 883.70, 884.21, 884.71, 885.19, 885.71 [M − 2 (CoCl2) + Li + H+]2+/2; 813.77, 814.28, 814.77, 815.28, 815.79, 816.27, 816.78, 817.27, 817.77, 818.28, 818.77, 819.27, 819.77 [M − 3 (CoCl2) − Cl− + K+]2+/2; 798.79, 799.28, 799.79, 800.29, 800.79, 801.30, 801.80, 802.30 [M − 3 (CoCl2) − Cl− + Li+]2+/2; 795.79, 796.28, 796.78 [M − 3 (CoCl2) − Cl− + H+]2+/2.
(6′6′-Dimethyl-2,2′-bipyridyl)Co(Cl)2; dmbp(CoCl2).
4 . An ultramarine blue solution of CoCl2·6H2O (0.272 g, 1.05 equiv.) in CH3OH (5 mL) was added dropwise to a solution of 6′,6′-dimethyl-2,2′-bipyridine (0.2 g, 1.085 × 10−3 mole) in CHCl3 (10 mL). This resulted in a quasi-immediate precipitation of a blue microcrystalline solid. The mixture was stirred under argon for two days and the precipitate was filtered off, rinsed with CHCl3 and then Et2O to afford the deep-blue green microcrystalline complex 4 (0.32 g; 94%) after drying under high vacuum. IR (pure, ATR): 792.74 (strong, C[double bond, length as m-dash]C–H), 1560.20 (C[double bond, length as m-dash]C), 1598.99 (C[double bond, length as m-dash]N). UV-Vis (CH2Cl2): 316 (10[thin space (1/6-em)]168), 339 (5780), 571 (263), 654 (530). 1H-NMR (CDCl3 (7.26 ppm)): −21.64 (br s, 6 H, Δν½ = 145 Hz, CH3bpy), −15.03 (s, 2 H, Δν½ = 18 Hz, H(3) or H(4) or H(5) bpy), 47.23 (s, 2 H, Δν½ = 19 Hz, H(3) or H(4) or H(5) bpy), 69.75 (2 H, Δν½ = 19 Hz, H(3) or H(4) or H(5) bpy). 1H-NMR (CD3OD 3.34 ppm + CDCl3 7.26 ppm; −80 to +120 ppm): −35.20 (br s, 6 H, Δν½ = 770 Hz, CH3bpy), −9.95 (br s, 2 H, Δν½ = 130 Hz, H(3) or H(4) or H(5) bpy), 48.89 (br s, 2 H, Δν½ = 70 Hz, H(3) or H(4) or H(5) bpy), 73.17 (br s, 2 H, Δν½ = 80 Hz, H(3) or H(4) or H(5) bpy). 13C-NMR ((H)DMF): 33.74 (q, JH–C = 138 Hz, CH3 of DMF); 39.11 (q, JH–C = 138 Hz, CH3 of DMF); 165.85 (d, JH–C = 192 Hz, HC(O) of DMF); 139.78 (br s, Δν½ = 40 Hz, CH3 of bpy); 295.13 (d, JH–C = 170 Hz, Δν½ = 50 Hz, HC of pyr); 313.24 (d, JH–C = 170 Hz, Δν½ = 16 Hz, HC of pyr); 348.00 (br s, Δν½ = 400 Hz, Cquat of pyr); 362.00 (br s, Δν½ = 240 Hz, Cquat of pyr); 475.20 (d, JH–C = 170 Hz, Δν½ = 77 Hz, HC of pyr). Anal. calc. for C12H12Cl2CoN2 (314.075): C 45.89, H 3.85, N 8.92; found: C 46.00, H 3.90, N 8.92.
Crystallisation conditions.
Complex 3. Deep-blue crystals suitable for X-ray diffraction analysis were obtained after some days in a vial equipped with a hood in polypropylene crossed by an injection needle, by slow evaporation of CH2Cl2 from a solution of complex 3 in MeCN (ca. 10 mg for 12 mL) and CH2Cl2. Crystals were maintained in the mother liquor to avoid irreversible and deleterious desiccation.

Conclusions

A new calixarene ligand incorporating four ethylacetate groups at the “upper” rim and four dimethyl bipyridines at the “lower” rim was obtained surprisingly in the 1,3-alternate conformation, hence generating an interesting tube-like ligand that presents multi-chelating behavior for transition metal ions adopting the tetrahedral coordination geometry. Its neutral, blue, paramagnetic complex with CoCl2 has been prepared and characterized with UV-visible spectroscopy and NMR in solution, and by X-ray diffraction study in the solid state.

In solution, it showed the expected capacity of complexation of 4 cobalt units per calixarene by UV-vis titration. 1D- and COSY-1H-NMR operated between −50 and +80 ppm confirmed its paramagnetism, and showed its high symmetry with a unique bipyridyl resonance signal system. Its 13C-NMR spectrum was recorded over a window from 700 to −100 ppm, affording interesting results in terms of chemical shifts and structure elucidation for such paramagnetic complexes. The resulting predicted model structure involving four bipyridyl(CoCl2) subunits, in which Co(II) is coordinated in tetrahedral geometry by two bipyridyl N atoms and two chlorine ions, organized in 1,3-alternate conformation was partially confirmed in the solid state by X-ray diffraction analysis. In fact, one of the four bipyridyl(CoCl2) subunits, at the unit cell border and close to a nearby cell bipyridyl(CoCl2) unit incorporates an oxygen atom as the fifth ligand, generating a pentacoordinated cobalt complex in distorted trigonal bipyramid geometry with the two Cl at the apex, and the two N and O in the base triangle. The cobaltous status was postulated based on the bipyridyl N–Co distances of ca. 2.07 Å, indicating the neutral status of the oxygen and, by this way, its aqueous nature. This H2O molecule was finally described as a H-bond linker with one or two chlorine ligands of the tetrahedral bipyridyl(CoCl2) subunit of the neighbor cell, participating by its presence in the crystallization or stabilization. Further magnetism and electrochemistry studies of this paramagnetic species are underway.

Data availability

The data supporting this article NJ-ART-05-2024-002476 “Synthesis of tube-like tetra-bipyridyl, tetra-ethylacetato-calix[4]arene in 1,3-alternate conformation and the unexpected crystal structure of its [bpy(CoCl2)]3-[bpy(CoCl2)(H2O)] complex” proposed by Regnouf de Vains Jean-Bernard have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Ministère de l’Enseignement Supérieur et de la Recherche, the CNRS and the Région Lorraine for financial support. S. Adach from Service Central d’Analyse Elémentaire, Université de Lorraine and F. Dupire from Plateforme de Spectrométrie de Masse de l’Université de Lorraine are acknowledged for elemental analysis and mass spectrometry respectively, as well as M. Zimmermann for his technical assistance during initial complexation studies. We thank Prof. S. Bouguet-Bonnet, from CRM2 laboratory for fruitful discussions on NMR, and Dr M. Bouché from L2CM for her assistance in manuscript revision and corrections.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2357976. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj02476a
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