Supramolecular Synthons: Will Giant Rigid Superspheres Do?

For the first time, the concept of supramolecular synthons was applied to giant rigid superspheres based on pentaphosphaferrocene [CpRFe(η5-P5)] (R = Me, Et) and Cu(I) halides, which reach 2.1-3.0 nm in diameter. Two supramolecular synthons, σ-π and π-π, are discovered based on halogen···CpR and Cp*···Cp* specific interactions, respectively. The geometry of the synthons is reproducible in a series of crystal structures of various supramolecules. The σ-π synthon alone is realized more frequently for Br-containing superspheres. A combination of the σ-π and π-π synthons is more typical for Cl-containing supramolecules. Each supramolecule can bear up to nine synthons to give mostly 2D and 3D architectures.

The notion of a supramolecular synthon in supramolecular chemistry as a repeating structural pattern based on intermolecular interactions was formulated by G. Desiraju in 1995.[1−3] The arrangement of molecules or ions can be regarded as a synthon if they predictably assemble via specific nonbonding interactions and realize a robust geometry in the crystal. This concept is mostly used for the design of organic solids and therefore is best developed for various D-H···A hydrogen as well as halogen bonds. For synthons constructed on CH···π, CH···O, and Cl···O interactions, the energies of interaction were estimated.[4] The examples of synthon application to transition metal complexes are rarer. In 1999, Bernhardt first used copper(II) complexes with a heterocycle melamine appended to a 14-membered polyazamacrocyclic ring to construct 1D and 2D arrays in the cocrystals with barbituric cyanuric acids.[5] Since 2003, Reger et al. have reported on synthons based on mononuclear complexes of transition metals. The rigid geometry of the complexes allowed control of the mutual arrangement of the interacting aromatic ligands.[6−11] Brammer et al. used the ability of transition metal halides to form C-X···X′-M halogen bonds to construct new metal-based supramolecular synthons.[12] Recently, the synthons were reported for polynuclear transition metal complexes[13,14] and even that was based on Ag–Ag bonds.[15−17] Among organometallic compounds, M-C=O···π(aryl) interactions were treated as supramolecular synthons.[18]

Despite its progress, the concept of supramolecular synthons might be restricted by the size of the molecule that bears the corresponding synthon. For small molecules, the formation of the directed nonbonding interactions can be predicted with high probability. However, for a large molecule or molecular ion, its flexibility, chemical complexity, and steric demand of substituents make the prediction less straightforward. The question arises as to whether a nanometer-scale rigid molecule is capable of forming a synthon.

Pentaphosphaferrocene-Based Supramolecules

Since 2003, we have systematically investigated the reaction of pentaphosphaferrocene CpRFeP5 (1), (1a: Cp* = η5-C5Me5; 1b: CpEt = η5-C5Me4Et; 1c: CpBn = η5-C5(CH2Ph)5 and 1d: CpBIG = η5-C5(4-nBuC6H4)5) with copper halides, which has proven to be the way to an excellent class of rigid spherical supramolecules (Table ).[19−27] These products are reproducibly formed in high yields and possess various molecular structures mostly of fullerene or fullerene-like topology. The resemblance to fullerene structure is predetermined by a pentagonal cyclo-P5 ligand of building block 1. These ligands connect single CuX (X = Cl, Br, I) units or more extended copper halide frameworks to give completely inorganic rigid scaffolds that enumerate from 80 up to 312 noncarbon atoms.[28,29] Thereby, the smallest supramolecule, [{CpRFe(η5-P5)}12(CuX)20] (2), is an inorganic analogue of C80 based on 12 units of 1a or 1c and 20 CuX (X = Cl, Br) units (Figure a).[23−27] This 80-vertex supramolecule (2) is a hollow sphere of 2.3 and 3.14 nm in diameter, depending on the size of CpR. The supramolecule with fullerene-like topology, [{CpRFe(η5-P5)}12{CuX}25(MeCN)10] (3), is known only for 1a and 1b moieties.[19−22] Its 90-vertex core is related to the 80-vertex sphere and differs only by an equatorial part of 5X and 5{Cu(MeCN)2} units (Figure b).

Table 1

Supramolecular Synthons in 1a- and 1b-Based Supramolecules

	compound	synthon	d1, Åa	φ1, degb	d2, Åc	φ2, degd	ref
2a	(o-C2B10H12)0.5@[(1a)12(CuBr)18.8]·7.33C7H8·0.67MeCN	4(σ–π)	3.21–3.39	162.5–175.8			(24)
2b	(o-C2B10H12)0.5@[(1a)12(CuBr)18.8]·3.82C7H8·2.23MeCN	4(σ–π)	3.24–3.50	162.4–173.3			(24)
2c	Cp2Fe@[(1a)12(CuCl)20]						(23)
2d	CpCrAs5@[(1a)12(CuCl)20]						(22)
3a	[1a]@[(1a)12(CuBr)25(MeCN)10]·2.1C6H4Cl2·MeCN	4(σ–π)	3.30–3.38	165.0–177.7			(19)
3b	[1a]@[(1a)12(CuBr)25(MeCN)10]·10.4C7H8·0.8MeCN	6(σ–π)	3.31–3.42	166.0–176.8			(19)
3c	[1a]@[(1a)12(CuBr)25(MeCN)10]·5C7H8·17.7MeCN	6(σ–π)	3.29–3.95	162.4–168.1			(19)
3d	[(CpCr)2As5]@[(1a)12(CuBr)25(MeCN)10]·10C7H8·3MeCN	6(σ–π)	3.32–3.44	166.1–176.2			(19)
3e	[1b]@[(1b)12(CuBr)25(MeCN)10]·2CH2Cl2·1.5MeCN	7(σ–π), 2(dbl σ–π)	3.51–3.70,	168.0–175.9,			(20)
			3.83–3.85	171.6–171.9
3f	[1a]@[(1a)12(CuBr)25(MeCN)10]·2.9C6H4Cl2·3.9MeCN	4(σ–π), 2(π–π)	3.31–3.56	163.8–172.7	3.72	154.2	(19)
3g	[1a]0.5@[(1a)12Cu25Cl24(MeCN)9][1a]0.5@ [(1a)12Cu25Cl26(MeCN)9]·12C7H8·1.5MeCN	4(σ–π), 3/2(π–π)e	3.25–3.40	160.5–173.7	3.30–3.60	129.0–153.8	(19)
3h	[1a]0.5@[(1a)12(CuCl)25(MeCN)10]·6CH2Cl2·1.5MeCN	2(σ–π), 2(π–π)	3.20	163.1	5.26	180.0	(19)
3i	[1a]0.6@[(1a)12(CuCl)25(MeCN)10]·9.5THF·2MeCN	2(σ–π), 6(π–π)	3.16–3.85	154.6–180	3.58	168.8	(19)
3j	[Cu(MeCN)4]+[(1a)0.5@(1a)12(CuCl)25(MeCN)10]3 {1a}0.5@[(1a)12Cu24Cl25(MeCN)8]-·34CH2Cl2	7/5(σ–π)e, π–π	3.18–3.77	160.0–177.9	3.44–3.70	140.0–173.4	(21)
4	[1a]@[(1a)9(CuCl)10]·2C7H8	8(σ–π), 4(π–π)	3.33–3.41	159.1–160.1	3.42–3.60	161.3–175.1	(26)
5	C60@[(1a)13(CuCl)26(H2O)2(MeCN)9]·6C6H4Cl2·MeCN	4(σ–π)	3.63f	154.1			(27)

The distance (X···Cp*) between the center of a C5 ring and a halide (X = Cl, Br) of the neighboring supramolecules.

The angle (∠Cu-X···Cp*) between the center of a C5 ring and the direction of a CuX bond (X = Cl, Br) of the neighboring supramolecules.

The distance (Cp*···Cp*) between the centers of the C5 ring (Cp*···Cp*) of the neighboring supramolecules, Å.

The angle (∠Fe-Cp*···Cp*) between the direction of an Fe-Cp* π-bond and the center of the neighboring C5 ring (see Figure ).

The fractional number corresponds to the number of synthons (when different) per crystallographically unique molecule. For more detail, see Table S2 in the Supporting Information.

The center of the Cu···Cu bond is taken instead of Cu to measure d1 and φ1 for the bridging X = Cl atom.

Figure 1

Supramolecules 2–5 based on 1a and 1b.

We have also characterized two 1a-based supramolecules possessing unique structure. One of them comprises two equal open shells [{1a}9{CuCl}10] (4) of fullerene-like topology (Figure c).[26] The shells assemble in a capsule-like arrangement via cooperative specific P···P interactions and π–stacking of two guest Cp*FeP5 molecules. The other one is a rare example of C60 inclusion compound C60@[{1a}13(CuCl)26(H2O)2(MeCN)9] (5).[27] Its 99-vertex scaffold is formed by 26 Cu atoms, 13 units of 1a, and eight Cl-bridges (Figure d).

Despite different molecular structures, the supramolecules possess similar fragments that follow the fullerene topology, namely consisting of the adjacent five- and six-membered rings provided by cyclo-P5 ligands and Cu2P4-rings (Figure ). For this reason, the position of functional groups of interest for the synthon formation is also similar.

Figure 2

Minimal angles τ between Cp* and X functional groups (rays 1–4) for a fragment with fullerene topology of 2–4 (see Table 1S).

Superspheres as Potential Synthons: Specific Interactions and Their Geometric Requirements

The ability of supramolecules to construct a supramolecular synthon requires suitable nonbonding interactions and favorable arrangements of the corresponding functional groups. The outer surface of the supramolecules is shaped by the aromatic cyclopentadienyl ligands and by terminal or bridging halide anions. These functional groups enable intermolecular interactions of two different kinds.

The CpR ligands are capable of π–π interactions if an aromatic system is not hindered by a bulky substituent R. For this reason, only Cp* derivatives are prospective for the synthon formation. Dissimilarly, CpEt and the bulkier CpBn and CpBIG do not participate in the π–π interactions. Instead, CpBn forms intramolecular bonds of H···π type between benzyl rings.[30] Still, specific interactions are possible for CpEt derivatives that do not involve an aromatic system but rather a halide. The X anions are capable for interactions with both aromatic systems and another halide.[31−39] We have not observed halogen bonding between the superspheres, whereas X···Cp* interactions were described.[19] Among terminal and bridging X atoms, the former should be more prospective for the X···Cp* interactions due to the higher negative charge and better availability. In line with this, the known synthons involving halogen atoms list only structural units with terminal X. To the best of our knowledge, the X···π interactions, in contrast to X···X halogen bonding, were not yet systematically used to construct supramolecular synthons.

The 1-based supramolecules are rigid, furnishing a fixed arrangement of functional groups expected for the synthon formation. Moreover, a large number of these functional groups increases the probability of synthon realization and provides multiple choices in supramolecular organization of the crystal.

To construct possible supramolecular synthons, we should consider a way of nonbonding interactions of two neighboring superspheres. Obviously, two supramolecules can interact via either one X···Cp* or one Cp*···Cp* π-stacking. The realization of more than one nonbonding specific contact between a pair of superspheres is limited by the geometry of the mutual spatial distribution of X atoms and Cp* groups. The relative position of two functional groups can be characterized by the angle between vectors (rays) originating from the center of the supramolecule toward the functional group, i.e., the terminal X atom or the center of the adjacent Cp* ligand (Figure ). Hereafter, we call them the angles between functional groups (τX···Cp, τX···X, or τCp···Cp).

The 12 available Cp* and 20 terminal halides for the 80- and 90-vertex superspheres are uniformly distributed over the surface of the superspheres (Figure S1). Their spatial arrangement is different however. The Cp* ligands form an icosahedron in 2 and bicapped pentagonal prism in 3. The terminal halide ligands enclose pentagonal dodecahedron and pentagonal prism with pentagonal trapezoidal prismatic caps. Thus, supramolecules 2 and 3 can form 12 synthons based on Cp*···Cp* interactions. The spatial arrangement of 12 equal spheres is maximally possible as known for close sphere packings. The minimal angle τCp···Cp between adjacent Cp* ligands in 2 and 3 is close to 60°, which is required for a hexagonal layer to form (Table S1). On the other hand, the 20 X···Cp* intermolecular contacts cannot appear all at once for steric reasons, and the maximal possible number is also restricted to 12. In contrast to the Cp* groups, the X atoms are not prearranged for 12 contacts with neighboring superspheres. The minimal angle τX···X being close to 40° in 2 and 3 is too acute as well as the angle τX···Cp (Table S1). Hence, the possible number of such synthons per one supramolecule is further restricted. These angles vary because of deviations of supramolecules 3–5 from the perfect sphere and can therefore influence the ability of a supramolecule to form synthons. For example, in total in 4, 18 Cp* and 20 terminal X (X = Cl) are distributed over the ellipsoidal surface. The τ angles in nonfullerene fragments are also listed in Table S1.

Despite some steric restrictions, the number of combinations of superspheres via synthons is nevertheless so numerous that an abundance of the supramolecular architectures is possible.

Synthon Types and Their Geometry: the σ–π Synthons

The most frequent σ–π synthon is based on halogen···π interactions (Table ).[31−39] The terminal X ligand (X = Cl, Br) of one supersphere interacts with the aromatic Cp* ligand of the other one (Figure a,b).

Figure 3

Inorganic scaffolds of 2–5 and corresponding supramolecular synthons: σ–π in superspheres (a) 2 and (d) 5, (b) double σ–π synthon in 3e, and (c) π–π synthon in 4.

The geometry of the X···π interactions requires that the X atom face a π system at a distance shorter than the sum of the van der Waals radii of sp2-hybridized carbon and halide. The values beyond 3.45 and 3.53 Å exceed the sum of the van der Waals radii for X = Cl and Br.[40,41] In addition, the Cu–X bond should most preferably be perpendicular to the π-system with the X atom pointing to its center. In superspheres, the Cl···π and Br···π distances are in the ranges 3.16–3.85 and 3.21–3.95 Å, respectively (Table ). The bridging X atoms are not expected to form the synthon. However, a peculiar interaction based on a μ-Cl anion is found for the 99-vertex supramolecule 5 (Figure d). Expectedly, the Cl···π distance (3.63 Å) is in the upper range for a synthon based on a terminal Cl anion. This example seems to be unique because most of the bridging X atoms of the supramolecules are not available for the intermolecular interactions. In supersphere 3, the bridging X atoms are shielded by the [Cu(MeCN)2]+ units of the equatorial part.

The isostructural compounds E4@[{1a}10(CuI)(MeCN)6], where n = 30.1 and E = P4 or n = 29.6 and E = As4, are based on the extended CuI framework providing only bridging iodides.[42] Consequently, no specific interactions between the supramolecules are found except for the elongated (4.42–4.43 Å) van der Waals I···I contacts. The iodide-based synthons are therefore not yet found for supramolecules.

Another unique example of a “double” σ–π synthon is found in 3e, which is the only example of derivative 1b (Figure b).

The robust geometry of the underlying interaction is a crucial requirement for a synthon. The X···π contacts amount to d1 of 3.5[2] Å and φ1 angle of 156[4]° for Cl···π and 3.4[2] Å and φ1 of 169[5]° for Br···π (Table ). The values seem to be reproducible with sufficient accuracy for the non-valent contacts (see Figure S2). Interestingly, for heavier halide, the average lengths are comparable or even shorter, and the angles are more obtuse. Similar findings were reported by Irving for the C–Cl···Cp synthons: The shorter the perpendicular distance Cl···Cp, the closer are the angles to 180°.[43] The total range of the angles is reported to vary in the range of 90–180°, which is much narrower for the supramolecules (Table ), and the tendency to form 180° is not observed. For the double synthon, the d1 value is reasonably elongated to 3.8 Å for both Br···π contacts, but the geometry of the interaction is angularly distorted. Although the φ1 angles are 171–172°, the Cu–Br bonds are inclined with respect to the Cp* plane by a rather obtuse angle of 127°. To the best of our knowledge, no synthons based on M-X···aryl are described, and for the reported synthons based on Cp groups or Br anions, the geometric characteristics are not discussed.

The σ–π synthon occurs in almost all supramolecules containing terminal halide. Among them, solely the Br···Cp* synthon is formed in all but one of the supramolecules (Table ). It gives shorter contacts to the aromatic fragment than Cl···Cp*, the larger Br radius notwithstanding. The Br···Cp* synthon can be regarded as the only directed intermolecular interaction that influences the crystal packing. In contrast, the Cl···Cp* interaction always appears along with the π–π synthon. The only exception is supramolecule 5, where Cl has a bridging function.

Supramolecular Architectures Based on the σ–π Synthons

One supramolecule in 2–5 bears from two up to nine σ–π supramolecular synthons with a predomination of even numbers (Table , Figure ). Two synthons per supramolecule give obviously either linear (3i) or zigzag (3h) chains depending on the mutual position of the functional groups involved (Table , Figure S3). In the linear chain, “head-to-tale” orientation of the synthons takes place, where each supramolecule provides a Cp* group and a bromide on the opposite sides. These groups are not strictly opposed to each other with the angle τBr···Cp* of 165.3°, and the linear chain can form only due to a counter bent on a bromide afforded by angle φ1 (Table ). The zigzag chain is based on “head-to-head” position of the synthons, which are placed at an acute angle when looking from the center of the supramolecule.

Figure 4

Supramolecular architectures based on the σ–π supramolecular synthon.

Table 2

Synthon-Based Motifs in the Structures of Supramolecules 2–5

		topology based on synthonb
Da	N	σ–π	π–π	σ–π + π–π
1D	3h	zigzag chain	linear chain	2,4C5
2D	2a, 2b	kgm (2D)		kgm
	3a	sql (2D)		sql
	5	sql (2D)		sql
3D	3b, 3d	pcu (3D)		pcu
	3c	pcu (3D)		pcu
	3e	ncj (3D)		ncj
	3f	sql (2D)	zigzag chain	pcu
	3g	sql (2D)	band	6,7T2
	3i	linear chain	hxl	hex
	3j	5,7T8 (3D)	linear chain	7,7-cc
	4	bcu (3D)	sql	fcu

Dimensionality of architecture based on both σ–π and π–π synthons.

Topological symbols determined according to ToposPro based on notations,[44,45] as resulting from σ–π synthons, π–π synthons, and from both types of synthons.

The topological motif is not listed in the ToposPro database.[45]

Four synthons always form a square layer for supramolecules 3 (in 3a, 3f, and 3g) and 5. As the molecular structure does not allow four right τ angles between the functional groups, the square net must always be distorted. The typical layer in 3 is almost flat, and each mesh is a rhomb provided by two obtuse (91–121°) and two acute (67–73°) τCp*···Cp* and τX···Cp* angles. For 3a and 3f, containing one crystallograpically unique supramolecule, the same groups are always involved in the formation of the synthon-based layer. For 3g, with its two independent supramolecules, another configuration of the layer is realized (Figure S4). In 5, the square layer is puckered, and all of the τCl···Cp* angles between the synthon-forming groups are acute at 58–76° (Figure S5).

In contrast, an almost ideal Kagome layer appears for superspheres 2 (in triclinic 2a and 2b, Figure a). The layer is constructed by three types of crystallographically unique supramolecules, providing 2Cp* + 2Br, 4Cp*, or 4Br groups for the synthon formation (Figure S9, Table S2). Interestingly, the mutual arrangement of the synthon-forming groups is similar for each of the three unique supramolecules. The resulting arrangement is flat, and the τ angles vary in the range 64.0–77.3° for triangular meshes and in 102.3–116.0° for hexagonal ones (relative to 60 and 120° in the ideal Kagome pattern).

Figure 5

Supramolecular assemblies based on the (a, b) σ–π or (c, d) π–π synthons: (a) Kagome pattern in 2a, (b) square layer in 3a, (c) band in 3g, and (d) trigonal layer in 3i.

Six σ–π synthons always yield a distorted 3D primitive cubic framework for compounds with one crystallographically unique supramolecule (3b–d). Analogously to the case of a square layer, nondistorted arrangement of the framework cannot be realized due to inappropriate arrangement of the synthon-forming groups (Figure S4a). Six synthons on average are also formed in 3j, but two unique supramolecules bear different numbers of synthons, 5 and 7, resulting in peculiar 5,7T8 net (Table ).

Eight σ–π synthons in ellipsoidal supramolecule 4 give 8-connected framework bcu.[46] It can be derived from body-centered cubic packing when only the closest eight neighbors are taken into account (Figure f, Figure S10). In the structure of supramolecule 3e, the maximal number of synthons is realized. In addition to seven σ–π “single” synthons, two “double” σ–π ones are formed per one supramolecule (Figure S7). The resulting 9-connected net belongs to topological type ncj (Table , Figure e).

π−π Synthons and Their Supportive Function

The π–π synthons found for supramolecules 3 and 4 demonstrate a classical face-to-face and a slipped (or offset) π-stacking geometry (Figure c,d).[47] The interplanar Cp*···Cp* distances with average values of 3.7[5] Å are typical for π-stacking. The slipped geometry predominates, which is signified by φ2 values with an average of 159[17]° (Table ). The large deviation displays that the formation of the π–π synthon is not so specific and must play a supportive role in addition to the σ–π synthon.

The formation of the π–π synthons might be favored by only the weakening of the σ–π interactions by using, for example, Cl instead of Br. On the other hand, these synthons can be hampered by using aromatic solvents. As a typical example, the supramolecules in 2a and 2b are surrounded by numerous toluene molecules, which block the Cp* ligands. For this reason, only σ–π synthons are found in 2a and 2b, whereas in the ultimate case of 2c and 2d, no synthons are formed. Solvent molecules like toluene, 1,2-dichlorobenzene, or acetonitrile can also mediate π–π interactions of the superspheres (3a, 3f, 3g, Figure S6). However, the interaction with a solvent cannot be regarded as a synthon because of its unpredictability. Conversely, such solvents have often been used along with copper bromide, which could cause the lack of a π–π synthon. For this reason, the conclusion on its robustness could be doubted. Indeed, most crystal structures of Br-containing 2 and 3 are solvates with aromatic solvents. However, in Br-containing 3f, the formation of the π–π synthon is allowed despite the presence of 1,2-dichlorobenzene. In Cl-containing 3g and 4, the presence of toluene did not block the π–π interactions. In addition, another typical solvent, dichloromethane, can also interact with the Cp* ligand in σ–π mode. This factor also diminishes the occurrence of the π–π synthon.

Another factor that impedes the formation of the π–π synthon is the number of terminal X anions per supramolecule, which either predominates or is nearly equal to the number of Cp* ligands. The ratio of Cu-Xterm:1a/1b is 1.67 for 2 and 3, 0.9 for 4, and 1.31 for 5 (as follows from 20:12, 18:20, and 17:13, respectively). Indeed, in 4, where Cp* groups predominate over X, four π–π synthons per molecule are formed.

Along with the already mentioned steric reasons and the competition with the solvent molecules, the σ–π synthon can be regarded as the most important factor in the aggregation of 1a- and 1b-containing supramolecules. Hence, it is not surprising that no structure is based solely on the π–π synthons. As mentioned above, the σ–π synthons usually predominate in every structure. The only contrary example is 3i, where the π–π synthon gives rise to a trigonal layer of the supramolecules (Figure d). The layer is flat and only slightly distorted as follows from τCp*···Cp* angles of 56.7–66.5° compared to the ideal 60°. Most of the supramolecular agglomerates based on the π–π synthon are 1D or, rarely, 2D (Table ).

Overall Supramolecular Architectures Based on the σ–π and π–π Synthons

As was shown above, the predominating σ–π synthon usually constructs 2D or even 3D supramolecular architectures. Compared to this, the impact of the π–π synthon may appear to be not so important. However, the role of the π–π synthon cannot be underestimated, because every time σ–π synthon provides a 1D or 2D architecture, the π–π synthon extends it to a 3D one (Table ). For example, the π–π-based trigonal layers in 3i connect the Cl···π-based linear chains in a primitive hexagonal framework (Figure a). The distorted square layers are joined by the zigzag chains in 3f or bands, consisting of edge-sharing hexagons, in 3g to give 3D primitive cubic or 6,7-connected architectures, respectively (Figure b,c). In 3j, linear chains of π–π synthons span a peculiar 5,7-connected framework based on σ–π synthons resulting in 7-connected framework (Figure d). The most interesting case is the structure of supramolecule 4. This is the only example when all possible intermolecular contacts between the superspheres in their face-centered cubic packing are granted by σ–π or π–π synthons (Figure e).

Figure 6

3D supramolecular architectures built on σ–π and π–π supramolecular synthons. Dark and light edges correspond to the σ–π and the π–π synthons, respectively.

Synthons in 1-Based Polymeric Structures

A number of polymeric structures based on 1 are known to date (Table S3).[48,49] These polymers can give insight into the appearance of the σ–π and π–π synthons in the supramolecules. Among the polymers, 1D and 2D are the most interesting, because in 3D frameworks, the mutual arrangement of synthon-forming Cp* and X groups is dictated by covalent bonding. Surprisingly, the Cp*···Cp* interactions are not found in the structure of 1a-based polymers (Table S3). It has already been mentioned that the coordination mode of the X atom is an important factor for the formation of the synthons in supramolecules; for polymers, the bridging role of X predominates, which is not favorable for the halide-based synthons. In the supramolecules, only once μ-Cl···Cp* synthon was found for 5. Indeed, in the structures with only bridging halides, no σ–π synthons occur.

The terminal atoms X are present only in five structures of 2D polymers belonging to three structural types, [(1a)(CuX)] (X = Cl, Br and I) and [(1a)(CuX)]·0.5C6H6 (X = Br, I).[26,48,49] The connectivity of the layers in all these structures is the same (Figure a). Similarly to the supramolecules, they are constructed from the same single CuX and 1a building units. Moreover, each Cu atom is coordinated to three 1a building blocks and a terminal X atom. However, in [(1a)(CuX)]·0.5C6H6, the Cp* groups are blocked by the H···π interactions with benzene molecules. Only in the solvent-free polymer do the terminal halides form σ–π synthons involving each X and Cp* group and join the layers into the 3D framework (Figure b).

Figure 7

Connectivity of (a) 2D [(Cp*FeP5) (CuI)] polymer and (b) interlayer σ–π synthons based on I···Cp* interactions.

Thereby, the first example of directed I···Cp* interaction is found in the 1a-based compound. With d1 = 3.75 Å and φ1 = 153.6°, this interaction resembles the synthon-forming Cl···Cp* and Br···Cp* ones. For this reason, it is expected for the supramolecules with terminal iodides. Hopefully, further investigations on CuI-based superspheres will reveal this type of supramolecular synthon.

Conclusions

The concept of supramolecular synthons is first extended to the giant pentaphosphaferrocene-based organometallic supramolecules. Their molecular structure is rigid, and the outer surface is mostly furnished by the CpR ligands (CpR = Cp* or CpEt) and halides with prearranged mutual orientation. These functional groups are capable of the π–π and halogen−π specific intermolecular interactions, which steadily occur in the series of the supramolecules irrespective of the molecular structure and the nature of the halide (Cl, Br). The outstanding reproducibility of these intermolecular patterns allows for the introduction of σ–π supramolecular synthons, which provide an additional agglomeration of the superspheres in the crystal via X···CpR interactions with reproducible geometrical characteristics. The assembly via a σ–π synthon yields in 1D and mostly 2D or 3D supramolecular architectures. In addition, a π–π synthon often plays a supporting role and increases the dimensionality of the supramolecular aggregation.