Single Crystal X-Ray Diffraction
X-ray crystallography reveals the title co-crystal to comprise 1,3,5,7-tetraazatricyclo[220.127.116.11,7
]decane (HMTA, 1) and 4-fluorophenol (4-FP) in the ratio 1:1, but with four crystallographically nonequivalent HMTA molecules and four crystallographically nonequivalent 4-FP molecules in the asymmetric unit (Figure 1
). Thus, the co-crystal crystallizes in the triclinic space group P-1
with four molecular aggregates in the asymmetric unit (Z′ = 4), viz. aggregates A, B, C and D; and eight independent molecules (Z′′ = 8). Contrary to other structures with Z′ > 1, no appreciable differences between the conformations of the four symmetrically independent co-crystals can be observed.
The molecular structure of the four crystallographically independent two-molecule aggregates forming the asymmetric unit of the crystal structure of the title compound is shown in Figure 1
. Molecular aggregates B−D all have a 1:1 ratio, and all have N···H−O−hydrogen bonds between the two species but differ in their structural parameters and crystal packing, as described below. Although the 4-FP:HMTA adduct is a 1:1 two-molecule aggregate of a phenol and HMTA, similar to the co-crystal of 2,4,6-trinitrophenol and 4-hydroxy-3-methoxybenzaldehyde with a hydrogen-bonded co-crystal involving monodonor molecules and using one N atom in the molecule of HMTA [15
], the title compound is the first case that crystallizes with more of one molecule in the asymmetric unit, which is an intriguing structural parameter indicated by the X-ray diffraction studies.
To a first approximation, the structure of A resembles those of B−D. However, although the HMTA molecules are structurally rigid due to the absence of conformational flexibility, we observed that the molecules are crystallographically nonequivalent due to differences in the C-N bond lengths. This is evidenced by the differences in lengths of the C−N bonds around the nitrogen atoms involved in the hydrogen bonds with the 4-FP molecules, hereafter referred to as α bonds, which range from 1.476(9) to 1.493(10) Å, 1.462(10) to 1.500(10) Å, 1.442(10) to 1.504(10) Å and 1.468(10) to 1.504(10) Å for the A–D HMTA molecules. The lengths of these bonds (except for the α3 bond in C, Table 2
), are relatively longer on average than those of HMTA itself (1.462(5) Å) [16
]. In addition, adjacent C−N bonds (referred to as β) are generally shorter relative to HMTA, ranging from 1.445(10) to 1.456(10) Å, 1.452(10) to 1.469(10) Å, 1.443(11) to 1.494(10) Å and 1.431(10) to 1.481(10) Å for the A−D HMTA molecules, which is consistent with an anomeric effect [17
]. However, comparing the bond lengths observed for A−D to those of previous studies reported in the literature for the O−H···N hydrogen-bonded aggregate of HMTA with hydroxybenzoic acid [17
], we found that not all the β bonds are shorter than the corresponding α bonds and the bond in HMTA, for example, α2 and β2 in B, β3 and β3 in C and β1 and β2 in D (Table 2
). However, the most significant differences in the N−C bond lengths within the four HMTA molecules appear at the other six N−C bonds involving the carbon atoms attached to two different nitrogen atoms (excluding N1), named γ bonds. The γ6 in D and γ3 and γ5 bonds in C are significantly shorter than the N−C bonds of the uncomplexed HMTA or those of the 1:1 hydroxybenzoic acid adduct (1.460(2) to 1.480(6)) [17
]. On the other hand, the γ4 and γ5 bonds in D are longer than those in the 1:1 hydroxybenzoic acid adduct. The average values of all N−C bond lengths in the HMTA moiety are 1.469, 1.469, 1.466 and 1.473 Å for A, B, C and D, respectively. Despite these variations, the intra-ring bond angles in the HMTA rings of the four aggregates are not very different, with average values of the C−N−C bond angles of 108.0°, 107.7°, 107.8° and 108.0° for the A, B, C and D units of HMTA, respectively, and N−C−N bond angles of 112.3°, 112.7°, 112.6° and 112.4°, respectively. In the same way, and similar to the 1:1 co-crystal of HMTA with 4-hydroxy-3-methoxybenzaldehyde [18
], the puckering parameters [19
] for the four six-membered N−C−N−C−N−C rings of each HMTA molecule span an experimentally equivalent range, with a mean deviation of all atoms from their mean-ring planes of 0.236 for A and D and 0.235 for B and C. The abovementioned ring puckering data indicate that the bond-length variations do not alter the intra-ring angles and the tetrahedral cage-like structure in the four HMTA molecules.
However, the bond lengths in the four 4-fluorophenol rings are different. For example, the F−C bond lengths for the A and D 4-fluorophenol molecules, 1.395(8) and 1.378(8)Å, respectively, are relatively longer than the equivalent bonds in B and C and those of 4-fluorophenol (1.361 Å) [20
]. This is also seen in the greater disparity in the associated C13−C14 and C14−C15 bond lengths (Table 2
). We also observed that these bonds are relatively longer than those of 4-FP (1.365 and 1.368 Å) [20
]. Additionally, an inspection of the other C−C bond lengths, as seen in Table 2
for 4-FP molecules, shows that there is a slight asymmetry in the electronic distribution around the aromatic ring of the 4-FP molecules. For example, C12−C13 and C15−C16 are slightly elongated in comparison to 4-FP [20
]. Furthermore, the marked discrepancy between C13−C14 and C14−C15 with respect to the C12−C13 and C15−C16 distances, with the later being significantly longer in all cases, reflects a decrease in the contribution of the quinoid-type structure in the 4-FP rings [21
]. This is consistent with an increase in the σ-electron-withdrawing (inductive) effect of the fluorine substituent on the four independent molecules of 4-FP [21
Although 4-fluorophenol is supposed to form intermolecular C−H∙∙∙F hydrogen bonds, it is well known that the formation of these intermolecular hydrogen bonds with fluorine as an acceptor is possible in environments shielded from water and with no competing hydrogen-bond acceptors [22
]. However, similar to C−H···O hydrogen bonds [24
], the stabilization energy of the C−H···F interactions (< 5 kcal/mol) [26
] in the crystal lattice can compete with conformational forces and are responsible for the other structural changes in small molecules. In these cases, the close C−H···F contacts could play an important role in crystal packing [12
As indicated in PLATON [31
], the crystal structure features four O−H···N hydrogen bonds (Table 3
). These bonds occur such that each molecule of HMTA links one 4-FP molecule to form four two-molecule aggregates, as illustrated in Figure 1
. These aggregates are connected into a three-dimensional architecture by a large number of C−H···X interactions (X = F, O and N).
In the crystal, the two-molecule aggregates A and D are linked via a pair of C−H···F and C−H···O hydrogen bonds forming an inversion dimers with enlarged R88
(34) ring motifs (named A–D). Similar interactions between B and C aggregates forming a second R88
(34) ring motifs (named B–C). For example, for A–D supramolecular synthons, pairs of C(1D)−H(1D1)···F(1A) and C(5A)−H(5A1)···O(1D) interactions operate in conjunction with O(1A)−H(1A)···N(1A) and O(1D)−H(1D)∙∙∙N(1D) hydrogen bonds to close a thirty-four-membered synthon (Figure 2
). These inversion dimers are connected into chains propagating along the a
axis by two additional C−H···F interactions, (C(3A)−H(3A2)···F(1D) and C(12D)−H(12D)···F(1A)) and one C(3D)−H(3D2)···O(1A) hydrogen bond (Figure 3
). The chains are reinforced by aromatic π···π stacking [Cg
29 = 4.029(4) Å and Cg
32 = 4.037(4) Å; Cg
29 and Cg
32 are the centroids of the C11A−C16A and C11D–C16D rings, respectively] (Figure 4
Although in principle, symmetry-independent molecules must have different supramolecular networking, the hydrogen-bonding pattern in the B−C inversion dimer is similar to that observed in the A−D inversion dimer, but with different geometric parameters (Table 3
). Unlike in the A−D inversion dimer, each fluorine atom has only one contact with a hydrogen atom, while in the A−D dimer, F1 is in contact with two hydrogen atoms (Table 3
). There are two C−H···F and two C−H···O hydrogen bonds that are slightly longer than those in the crystal structure of the A−D inversion dimer and form columns in the R88
(34) inversion dimer with different orientations along the b
axis (Figure 5
), which is also controlled by weak aromatic π···π stacking interactions [centroid–centroid separations = 4.025(4) and 4.037(4) Å]. Furthermore, both columns are linked by a weak hydrogen bond (C(4A)-H(4A1)∙∙∙N(2C), Table 3
) that contributes to the crystal packing, forming a three-dimensional structure.
The most prominent feature of the molecular packing in the crystal of the title 1:1 adduct is the formation of the four two-molecule aggregates with different structural parameters (F−C, N−C, O−C and C−C bond lengths, Table 2
) and different hydrogen bonds with geometric parameters (Table 3
). In fact, although the hydrogen-bonding patterns are almost identical, there are differences in the donor–acceptor distances, which range from 3.567(11) to 3.505(10) Å for C−H···O hydrogen bonds and 3.190(8) to 3.224(9) Å for C−H···F hydrogen bonds. Thus, one might expect any C−H···O and C−H···F contacts to be different for the four two-molecule aggregates. However, the C···F distances are only 0.02 and 0.03 Å longer than the sum of the van der Waals radii (F = 1.47 Å, C = 1.70 Å) [32
] for the A and D two-molecule aggregate, while the interactions in B and C are 0.05 Å longer. On the other hand, the four C−H···O hydrogen bonds are longer than the sum of the van der Waals radii (O = 1.52 Å, C = 1.70 Å) [32
] by 0.31, 0.35, 0.28 and 0.25 Å (Table 3
). Based on their lengths, these are presumably significantly weaker than the C−H···F hydrogen bonds.
Compared to the F−C bond length of 1.361 Å in 4-fluorophenol, as stated before, we believe that the intermolecular C−H···F−C hydrogen bonding interaction is the key to the observed structural changes, especially the observation of these elongated F−C bonds, which include the F1A−C14A, F1C−C14C, F1A−C14A, and F1D−C14D bonds. The plot of F−C bond length versus C···F distance is shown in Figure 6
. The F−C bond length almost linearly increases with decreasing donor–acceptor distance (C···F distance). The presence of two C−H···F hydrogen bond interactions in F1 made the F−C distance notably longer. Thus, the F1A atom is distinct from the F1(B–D) atoms based on the number of interactions it forms.
In conclusion, we have shown that the addition of one fluorine atom at the para position on the phenolic ring has influence in the generation of supramolecular assemblies in HMTA-phenol adducts. The altered packing modes due to the nature of the interactions, such as the different weak intermolecular C−H···F−C hydrogen bonds, influence the crystal structure of this co-crystal. Unlike what is typical for structures with Z′ > 1, no appreciable differences between the conformations of the four symmetrically independent co-crystals could be observed.