Both new compounds
1 and
2 were obtained in crystalline form. All samples were characterized by IR and UV–Vis spectra, TGA, and CHN elemental analysis (
Figures S1–S3, ESI†) followed by a single-crystal X-ray diffraction analysis (
Figure 1,
Table 1, and
Figures S4–S8,
Tables S1 and S2, ESI†). The symmetrically independent parts are presented in
Figures S4 and S5 (ESI†). The results of data collection and structure refinement have been summarized in
Table 1. Compounds 1 and 2 crystallizes in the trigonal R
and triclinic P
crystallographic space group, respectively. The crystal structures of both compounds consist of pentadecanuclear cyanide-bridged clusters {Co
9W
6L
x(MeOH)
24-x} (
Figure 1) due to the cluster surface decoration with monodentate pyridine
N-oxide (pyNO, L
1, x = 12) for
1 or 4-phenylpyridine
N-oxide (4-phpyNO, L
2, x = 7) for
2. The cluster core in both cases is analogous to the six capped body-centered cubes of broad M
9M’
6 family. In both cases, the central [Co1(μ-NC)
6]
2+ moiety with almost perfect octahedron geometry (
Tables S3 and S4, ESI†) is surrounded by six cyanide-bridges directed toward six [W(µ-CN)
5(CN)
3]
3− blocks located in the vertices of super-octahedron. The eight remaining Co
2+ ions are located in the corners of the super-cube, each accepting three cyanide-bridges from the neighboring W centers, reaching the general composition fac-[Co
II(µ-NC)
3L
y(MeOH)
3-y]
−. In compound
1, two crystallographically independent external fac-[Co2/3(μ-NC)
3(pyNO)
1.5(MeOH)
1.5]
2+ moieties are distinguished, with occupancy 0.25, 0.5, 0.75, or 1.0 of the organic and solvent ligands (see
Figure S4, ESI†). In compound
2, four crystallographically independent external cobalt moieties were found: one fac-[Co3(μ-NC)
3(MeOH)
3]
2+, two fac-[Co2/5(μ-NC)
3(4-phpyNO)(MeOH)
2]
2+, and one fac-[Co4(μ-NC)
3(4-phpyNO)
1.5(MeOH)
1.5]
2+ (with occupancy 0.5 or 1.0 of the organic and solvent ligands, see
Figure S5, ESI†). All Co
2+ ions exhibit practically O
h geometry (
Tables S3 and S4, ESI†). The Co-N/O distances in
1 and
2 are in the range of 2.061
–2.123 Å, which is typical for Co
9W
6-based clusters with monodentate ligands. All [W(µ-(CN)
5(CN)
3]
3− ions connect five cobalt ions, while three cyanide ligands are terminal. Chosen distances and angles are collected in
Tables S1 and S2 (ESI†). All values are in line with the related literature data [
13,
14,
15,
16,
17].
The application of a new type of ligands to fifteen-centered clusters allowed obtaining a completely new character of the ligand shell. In
1, pyNO ligands connected to the Co3 moieties interacts with each other with π–π interactions, which has not been seen in M
9M’
6L
x clusters with monodentate coordination of ambidentate linkers (L = pyrazine mono-
N-oxide (pzmo)) and 4,4-bipyridine mono-
N-oxide (4,4’-bpdo,
Figure S6, ESI†) [
16]. Supramolecular arrangement is also dominated by this type of interaction, but there are also weak O3
MeOH∙∙∙N8
CN hydrogen bonds in the direction [001]. Each cluster is surrounded symmetrically by six other clusters, creating a densely packed 3D supramolecular architecture. The shortest intercluster distance is 5.22 Å, which conforms to the absence of additional solvent molecules in the molecular architecture. No interactions between 4-phpyNO ligands coordinated to the same metallic center are observed in the case of
2, which is in line with a relatively large freedom of rotation of the phenyl ring. The crystal structure of
2 is completed with additional crystallization MeOH molecules, creating a network of hydrogen bond synthons, as well as uncoordinated 4-phpyNO molecules (with occupancy 0.5 or 1.0;
Figure S6, ESI†). The face-to-face or edge-to-face π–π interactions between coordinated and uncoordinated ligands form supramolecular layers perpendicular to the [010] direction, which are additionally stabilized by O5M
MeOH–N38
CN hydrogen bonds. Longer ligand and also the presence of uncoordinated 4-phpyNO molecules in
2 compared to
1 result in the larger separation of clusters. The shortest distance between them is 7.23 Å, and the clusters are not distributed in such a symmetrical manner as in the case of
1. The structural uniformity has been confirmed using powder X-ray diffraction data (
Figure S9, ESI†).
To discuss the above structural description, we consider below the observations for the entire M
9M’
6L
x family of compounds, focusing on (i) the molar ratios in the parent solution and in the final product, and (ii) on the structural disorder in the ligand shell. All previous reports on functionalized fifteen-centered cluster cores were associated with
O,
O-,
N,
O-,
N,
N-, or
N,
N,
N-donor ligands providing their monodentate (
m), bidentate (
b), or tridentate (
t)
local coordination: Mn
9W
6L
x (L = 4,4′-bipyridine, x = 4,
m;
trans-1,2-di(4-pyridyl)ethylene, x = 5,
m; 4,4′-dipyridyl disulfide, x = 4,
m; 4,4′-di-tert-butyl-2,2′-bipyridine, x = 8,
b; 4,7-di-phenyl-1,10-phenantroline, x = 8, (
b), Fe
9M′
6(Me
3tacn)
8 (Me
3tacn = 1,4,7-trimethyl-1,4,7-triazacyclononane; M′ = W, Re,
t), Co
9W
6L
x (L = 4,4′-bipyridine di-
N-oxide, x = 12,
m; (R/S)methylpyridinemethanol, x = 8,
b; 2,2′-bipyridine di-
N,
N-oxide, x = 6/7,
b; 2,2′-bipyridine mono-
N-oxide, x = 6 or 8
b; 4,4′-bipyridine mono-
N-oxide, x = 4 or 6,
m; pyrazine mono-
N-oxide, x = 5,
m), Ni
9W
6L
x (L = 2,2′-bipyridine, x = 8,
b; 4,4′-dimetyl-2,2′-bipyridine, x = 8,
b; 5,5′-dimetyl-2,2′-bipyridine, x = 8,
b; 4,4′-di-tert-butyl-2,2′-bipyridine, x = 8,
b; 3,4,7,8-tetrametyl-1,10-phenantroline, x = 6,
b; 2,2′-bi(4,5-dihydrothiiazine), x = 8,
b; (
R/S)-2-(1-hydroxyethyl)pyridine, x = 8,
b), Ni
9Mo
6L
x (L = 2,2′-bipyridine, x = 8,
b; 3,4,7,8-tetrametyl-1,10-phenantroline, x = 6,
b), and Co
1Cu
8W
6(Me
3tacn)
8,
t [
12,
13,
14,
15,
16,
17,
18,
19,
20]. The formation of stable crystalline product depended strongly on L: M
2+ molar ratio in solution. For the chelating ligands, the L:M
2+ of 1:1 was sufficient to observe a complete or an almost complete capping of the peripheric M
2+ sites, with the resulting L:M
2+ ratio of 8:8 (prevalently), 7:8 or 6:8 in the cluster coverage (in the case of bis-chelating ligands), and 8:8 (in the case of tridentate Me
3tacn), and 16:8, 14:8, or 12:8, and 24:8, respectively, counting separately each coordinated donor atom. For the linker-type ligands, the effective growth of the crystals was observed only by some excess of L, 1.5:1, 5:1, 7:1, or 20:1, with respect to the M
2+, to give the cluster coverage ratio L:M
2+ between 12:8 and 4:8. In this work, for the first time, we have the opportunity to present compounds based on 15-centered clusters containing exclusively monodentate (without the bridging function) ligands, pyNO, and 4-phpyNO. To achieve effective crystallization, the use of a minimum of 30-fold excess of the ligand was required, with the resulting coverage L: M
2+ ratio 12:8 (
1) and 7:8 (10:7 ratio involving all 4-phpyNO in the crystal structure) (
2) observed. Although no direct evidence was shown for the presence of complete 15-nuclear species in solution, the thermodynamic equilibria can be inferred to operate in solution, 15-nuclear cores acting as super-complexes with eight triple coordination sites located at the corners of super-cube sublattice. The above diversification in the ease of the crystal formation is in agreement with the thermodynamic prediction of complex stability considering the chelate effect, or “anchoring” due to the bridging, against the “simple” coordination of monodentate ligand. The ligand shell disorder in
1 and
2 is unprecedented along the entire family, and may be understood in terms of the degree of M–L bond rigidity and intercluster interactions inscribed in the ligand structure and M–L bonding mode. The longer linker ligands (e.g., 4,4′-bipyridine mono-
N-oxide, 4,4′-bpmo; 4,4′-bipyridine di-
N-oxide, 4,4′-bpdo) [
16,
17] form quite easily the bridging connections between clusters, and/or participate in the hydrogen bond supramolecular interaction network. On the other hand, the convergent bidentate ligand (frequently equipped with the remote substituents, e.g., 4,4′-dimetyl-2,2′-bipyridine, 4,4′-di-tert-butyl-2,2′-bipyridine, or 4,7-di-phenyl-1,10-phenantroline) [
20] fill the intercluster space with the π–π synthons or van der Waals contacts. In both cases, the degree of freedom is strongly limited, which prevents the disorder. Interestingly, the dataset for pyrazine mono-
N-oxide (pzmo) ligand [
16], the shorter analog of 4,4′-bpmo, fairly resembles that of pyNO in
1: (i) one of the pzmo ligands reveals severe positional disorder, and (ii) preparation of the product requires the ratio L:Co
2+ ratio of 20:1, a little less than 30:1 in
1. The occurrence of disorder in both cases may be correlated with the small volumes of both ligands, whereas the differences are definitely related to the natural preferences for intermolecular interactions. Coming to the compound
2, the monodentate or crystallization form of 4-phpyNO moiety can be confronted with the diversity of coordination modes, bridging and monodentate, noted also both for Co
9W
6(4,4′-bpmo)
4;6 and Co
9W
6(4,4′-bpdo)
12.