Isolation and crystal and molecular structures of [(C5H2Br3)2Fe], [(C5HBr4)2Fe] and [(C5Br5)(C5Br4HgBr)Fe]

Hexa- and octabromoferrocene, as well as a mercury derivative of decabromoferrocene, were isolated from mixtures of polybromoferrocenes and characterized by X-ray diffraction. The crystals show halogen and hydrogen bonding, which act co-operatively.


Introduction
'Noncovalent interactions' are found in nearly all disciplines of chemistry, biochemistry and biology, and have been studied, at least in part, for quite a while (Hobza & Ř ezá č, 2016). This term brings together such apparently different interactions as hydrogen, halogen, lone-pair-, anion-, cation-andbonding, and these interactions can either act independently or co-operatively (Mahadevi & Sastry, 2016;Portela & Ferná ndez, 2021). Among these, halogen bond(ing) has been studied continuously at a high level since about 1995. The last comprehensive review dates back to 2016 (Cavallo et al., 2016). A look at SciFinder shows since then nearly 2000 new entries for the years 2021 and 2022, and already 492 entries with the concept 'Halogen Bonding' (accessed on May 26th, 2022). The vast majority of these studies are centred on organic or biological systems, with a focus on crystal engineering (Mukherjee et al., 2014). Relatively rarely studied were metal-containing systems (Brammer et al., 2008), in particular, organometallic systems have so far been restricted to a few metal carbonyls, ruthenium-complexed aryl iodides (Kelly & Holman, 2022) and one study on 1,1 0 -dihaloferrocenes (Shimizu & Ferreira da Silva, 2018). Our group has been working on polyhalogenated metallocenes for quite a while (Sü nkel & Motz, 1988;Sü nkel & Hofmann, 1992;Sü nkel et al., 1994Sü nkel et al., , 2015Sü nkel & Bernhartzeder, 2011), and some very recent reports on the synthesis and crystal structure determinations of [(C 5 H n Br 5-n )(C 5 H 2 Br 3 )Fe] (n = 1 or 2; Butler et al., 2021;Butler, 2021) and [(C 5 H n Br 5-n )(C 5 Br 5 )Fe] (n = 0 or 1; Rupf et al., 2022) prompted us to report on our synthetic and crystallographic studies of polybromoferrocenes. A special focus is made on the occurrence of halogen and hydrogen bonding in these systems.

Experimental
2.1. Synthesis and crystallization 2.1.1. Reaction of 1,1 0 0 0 ,2,2 0 0 0 -tetrabromoferrocene (1) with LiTMP in a 1:10 molar ratio and C 2 H 2 Br 4 . A solution of 1 (243 mg, 0.48 mmol) in tetrahydrofuran (THF; 2 ml) was added to a freshly prepared solution of LiTMP (4.8 mmol) in THF (4 ml) at À30 C. After stirring for 5 h, the temperature was lowered to À78 C and C 2 H 2 Br 4 (0.6 ml, 5.0 mmol) was added. With continuous stirring, the temperature was raised to ambient temperature over a period of 16 h. After this, water (10 ml) was added and the mixture was extracted with several 10 ml portions of CH 2 Cl 2 . The combined extracts were washed with water, then dried with MgSO 4 and completely evaporated in vacuo. The residue was taken up in the minimum amount of petroleum ether and chromatographed on an alumina column (20 Â 2 cm), using petroleum ether as eluent. 21 fractions were collected and examined by mass spectroscopy and selected fractions were examined by 1 H NMR spectroscopy (Figs. S1 and S2 in the supporting information). All fractions contained mixtures of polybromoferrocenes. While the first fraction consisted of a mixture of penta-, hexa-and heptabromoferrocene, the intermediate fractions contained hexa-, hepta-and octabromoferrocene, and the last fraction was a mixture of hepta-and octabromoferrocene with traces of nonabromoferrocene. Crystals of 1,1 0 ,2,2 0 ,3,3 0 -hexabromoferrocene (3) were obtained by slow evaporation of the sixth fraction in a refrigerator, and crystals of 1,1 0 ,2,2 0 ,3,3 0 ,4,4 0 -octabromoferrocene (5) were obtained from the last fraction by the same method. All other fractions were also recrystallized from different solvents (petroleum ether, Et 2 O and CH 2 Cl 2 ), but yielded neither crystals nor 'pure' powders (according to 1 H NMR spectra taken after redissolution).

Refinement
Compound 3: SHELXT (Sheldrick, 2015a) provided the complete molecule of 3 on the first run. The following difference Fourier synthesis (see Fig. S6 of the supporting information) showed two electron-density maxima (Q15 and Q16 in Fig. S6; d = 2.66 and 2.48 e Å À3 ) at radial distances of 1.54 and 1.40 Å from ring atoms C24 and C14, respectively. Despite these short distances (more typical for C-C bonds), we assigned these peaks to Br atoms (first named X1 and X2) with very low site-occupancy factors, since from the preceeding synthesis no other elements could have been present. The following refinement, however, showed rather short intermolecular distances (2.581/3.328 and 2.955/3.106 Å ) from these positions to atoms Br21 i /Br22 i and Br11 i /Br12 i , respectively [symmetry code: (i) x, y À 1, z] (Fig. S7). It was concluded that X1/X2 could not be present in the same molecule as Br11/Br12/Br21 (and eventually Br22 also) and therefore it was assumed that compound 3 (with Br11-Br13 and Br21-Br23) cocrystallized with very small amounts (ca 3%) of compound 2 [with Br13-Br14 (= X2) and Br22-Br24 (= X1)], and this model was used for the subsequent refinements. The refinement procedure was as follows: first, it was assumed that all Br atoms would have the same isotropic U values and then the site-occupation factors for X1 = Br24/H24 and X2 = Br14/ H14, as well as Br11/H11, Br12/H12, Br21/H21 and Br23/H23, were allowed to refine. The site-occupancy factors were then fixed at these values and the U values were allowed to refine freely for the main components even anisotropically. Any attempts to produce longer C14-Br14 and C24-Br24 'bonds' via the use of restraints met with failure. It should be noted at this point that the crystal structure of 1,1 0 ,2,4-tetraiodoferrocene showed a similar disorder and an apparent 'bond shortening', which the authors were able to resolve (Evans et al., 2021).
Compound 5: the measured crystal was recognized as a twin (two domains, rotated by 180 around 010) and a HKLF5 data file was created. The scale factor BASF refined to a final value of 0.17818. The refinement proceeded without any problems, and no signs of disorder were found.
Crystal data, data collection and structure refinement details of all compounds are summarized in Table 1.
high yields of 1,1 0 ,2,2 0 ,3,3 0 -hexabromoferrocene (3), contaminated, however, with heptabromoferrocene (4) and octabromoferrocene (5). All attempts to repeat this procedure on compound 3 met with failure, due to the very low solubility of this compound. On the other hand, the preparation of 'permercurated ferrocenes' followed by the addition of KBr 3 , first reported in 1977, then later in 1994, 1997 and 2022, suffered from difficulties due to solubility problems (Boev & Dombrovskii, 1977;Han et al., 1994;Neto et al., 1997;Rupf et al., 2022). For example, Han and co-workers showed that using Hg(O 2 CCF 3 ) 2 as the mercuration agent gave a 'mixture of at least four partially brominated ferrocenes'. When they used Hg(O 2 CCH 3 ) 2 as the mercuration agent, decabromoferrocene (7) could be isolated in 60% yield, contaminated, however, with at least two partially brominated ferrocenes. Rupf and coworkers repeated this latter experiment and showed that besides 7 also nonabromoferrocene (6) and nonabromo-(bromomercurio)ferrocene (8) were formed (based on 13 C NMR spectroscopy; a closer look at Fig. S20 of their supporting information shows the additional formation of octabromoferrocene 5 and the bromomercurioferrocenes [C 10 H n Br 9-n HgBrFe] with n = 1 and 2). When they used Hg(O 2 CC 3 H 7 ) 2 , they apparently obtained a mixture of 7 and 8 with no other contaminants (based on NMR and IR). Neto and co-workers reported the use of Hg(O 2 CCCl 3 ) 2 as the mercuration agent, the transformation of the apparently formed [C 10 (HgO 2 CCl 3 ) 10 Fe] to the decachloromercurioferrocene, followed by reaction with KBr 3 to give pure 7 [characterization by NMR and IR spectroscopy, and elemental analysis (C and Fe)].
We decided to look at the lithiation reactions with LiTMP as the lithiating reagent, C 2 H 2 Br 4 as the brominating agent and THF as the reaction medium at low temperatures again. We started with a solution of 1,1 0 ,2,2 0 -tetrabromoferrocene (1; purity > 95%) in THF and treated it with ten molar equivalents of LiTMP, followed by the addition of tetrabromoethane. After standard work-up, a chromatographic separation was attempted. Since no band formation was recognizable, 21 fractions with equal volume were collected. Fractions 1, 10, 12 and 21 were examined by mass spectrometry (Fig. S1), while fractions 4, 6, 8 and 21 were studied by NMR spectroscopy (Fig. S2). All fractions were left standing in open vials for slow evaporation of the solvent. From these crystallization attempts, fractions 1, 6 and 21 gave crystals. The observation that some compounds were present in nearly all fractions is most likely due to the low solubility in the eluting solvent, which led to 'smearing' over the length of the chromatography column. The use of different solvent mixtures for elution (PE/ Et 2 O, PE/THF and PE/CH 2 Cl 2 ; PE is petroleum ether) increased the solubility, but did not improve the resolution of the compounds. This problem might have been overcome by the use of high-performance liquid chromatography (HPLC); however, this was not available to us.
The mass spectrum of fraction 1 showed the presence of 2 (m/z = 579.7), 3 (m/z = 659.6) and 4 (m/z = 737.5), with 2 as the main component. In both of fractions 10 and 12, 4 was the main component, contaminated by 2 (traces), 3 and 5 (m/z = 817.5). Finally, the mass spectrum of fraction 21 showed 5 as the main component, contaminated by 4 and traces of 6 (m/z = 895.2). The 1 H NMR spectra of fractions 4, 6 and 8 showed different mixtures of compounds 3 ( = 4.47) and 4 ( = 4.72 and 4.43) [assignments based on Butler (2021)]. The 1 H NMR spectrum of fraction 21 (in dimethyl sulfoxide) showed three very weak signals at = 5.33, 5.20 and 4.76, which might be assigned to 4 and 5 by comparison with the mass spectra (no other NMR data in this solvent were available). The crystals obtained from fraction 1 suffered from disorder or cocrystallization effects, which could not be properly resolved. The crystals from fraction 6 also showed disorder, which could, however, be successfully modelled as cocrystallization of compounds 3 (ca 97% contribution) and 2. Fraction 21 yielded pure crystals of compound 5. Fig. 1 shows the structural formulae of the compounds discussed in this study.
We also repeated the permercuration of ferrocene according to Winter and co-workers, using Hg(O 2 CCH 3 ) 2 as the mercurating agent and dichloroethane as the solvent, followed by bromination with KBr 3 . Chromatography of the crude reaction product yielded two fractions (Han et al., 1994). The first contained, according to its mass spectrum (Fig. S3), a mixture of bromoferrocenes 5-7 (with compound 6 dominating), while the second consisted of a mixture of bromomercurioferrocenes [C 10 H n Br 10-n HgFe] (n = 0-2; Fig. S4). Fig. S5 shows the 1 H NMR spectrum of the first fraction, which apparently consists of four proton-containing substances, of which one dominates. Although we did not perform these sigma-hole interactions Acta Cryst. (2022). C78, 578-590 Blockhaus and Sü nkel Crystal and molecular structures of polybromoferrocene 581 Table 2 Overview of the CSD structures of polyhaloferrocenes substituted on both rings.  Rupf et al. (2022) experiments, it can be assumed that compounds 5-7 are formed by further bromination of [C 10 H n Br 10-n HgFe]. Therefore, we conclude that neither the permercuration nor the bromination reactions are complete. Although all fractions were used for crystallization attempts, only crystals of compound 8 could be obtained.

Molecular structures
An intensely debated topic since the very early days of ferrocene chemistry was the question of the relative stability of the eclipsed and staggered conformers of this molecule. While the very first crystal structure determination of ferrocene (Fischer & Pfab, 1952) hinted at a staggered geometry, the most recent low-temperature IR and XANES (X-ray absorption near edge structure) spectra, as well as DFT (density functional theory) calculations showed that the eclipsed conformation is the energy minimum (Bourke et al., 2016;Silva et al., 2014). For ferrocenes substituted on both rings, an additional conformational isomerism arises from the possibility of different relative positions of the substituents (Scheme 1).
While theoretical calculations on 1,1 0 -dibromoferrocene showed that the two C 2 isomers (II and III in Scheme 1) are minimum conformations (Silva et al., 2014), in the crystal structure, only the less favourable C 2v structure (I in Scheme 1) was obtained (Hnetinka et al., 2004). To obtain an overview of the 'realized' structures, a Cambridge Structural Database (CSD; Groom et al., 2016) search on ferrocenes with at least one halogen substituent on each ring was undertaken. This search delivered 40 hits, of which 14 contained only halogen substituents: all four FdX 2 , two FdX 3 , three FdX 4 , FdCl 6 , FdBr 9 and FdBr 10 ; FdCl 2 was determined twice and FdI 4 exists as two positional isomers; 'Fd' is a common abbreviation for ferrocenes with substituents on both rings, while 'Fc' symbolizes ferrocenes with substituents only on one ring; strictly speaking, 'Fc' stands only for the [C 10 H 9 Fe] residue, while 'Fd' symbolizes a [(C 5 H 4 ) 2 Fe] group. The four 1,1 0 -dihaloferrocenes have been discussed already with respect to supramolecular interactions in general and halogen bonding in particular (Shimizu & Ferreira da Silva, 2018  Top view of the molecular structure of compound 3 (major orientation), with displacement ellipsoids drawn at the 30% probability level.

Figure 3
Top view of the molecular structure of compound 5, showing a whole molecule, with displacement ellipsoids drawn at the 30% probability level.

Figure 4
Top view of the molecular structure of 8, with displacement ellipsoids drawn at the 30% probability level. compounds, except for FdI 2 , FdBr 9 and FdBr 10 , showed eclipsed conformations (or nearly eclipsed in the case of FdBr 3 and FdI 3 , with torsion angles of ca 14 ). Within this group of eclipsed structures, most showed the apparent 'higher energy' conformations I and IV, respectively. Only FdBr 3 , FdI 3 and FdI 4 crystallized in the most stable form VI. Table 2 gives an overview on these structures.
Compound 3 crystallizes in the triclinic space group P1, with one molecule in the asymmetric unit. Fig. 2 shows the major orientation of the disordered molecule. As in most polyhaloferrocene structures (see Table 2), the cyclopentadienyl (Cp) rings are nearly perfectly eclipsed, planar and parallel to each other. All Br atoms are shifted slightly to the distal side of the Cp rings with respect to the Fe atom.
Compound 5 also crystallizes in the triclinic space group P1, however, as a twin with half a molecule in the asymmetric unit and the Fe atom residing on an inversion centre (Fig. 3). As a consequence of this, the Cp rings are perfectly staggered, with the two C-H bonds in relative transoid positions. Both Cp rings are planar and parallel to each other and the Br atoms are all shifted to the distal sides of the Cp rings, however, to a smaller extent than in the eclipsed structures mentioned before. The iron-centroid distance (determined within PLATON) also seems to be more dependent on the relative orientation of the Cp rings than on the degree of bromination. Table 3 collects important geometrical parameters of several polybromoferrocenes from the literature, together with those of compounds 3, 5 and 8.
Compound 8 crystallizes in the monoclinic space group P2 1 /n, with one molecule in the asymmetric unit (Fig. 4). The C10-Hg1-Br10 bond deviates slightly from being linear [171.0 (2) ]. The Cp rings are planar and parallel to each other, while their relative orientation is staggered. The distances from Fe1 to both Cp ring centroids are identical within 1. Except for atoms Br5 and Hg1, which are within the Cp ring planes, all the ring substituents are shifted again to the distal sides of the Cp rings. In comparison with the structure of the ferricenium salt 8 + ÁAsF 6 À , the C-Br bonds are slightly longer, Hirshfeld surfaces of compounds 3 (left), 5 (middle) and 8 (right), together with the closest contact atoms. Red spots show very close contacts between atoms inside and outside the Hirshfeld surface. The Hg compound differs from the other two by the appearance of such a red spot over the plane of the Cp ring. while the iron-centroid distances are significantly shorter in 8, which is quite usual when comparing ferrocenes with their oxidized counterparts (Rupf et al., 2022). For all three compounds, an analysis with PLATON showed no residual solvent-accessible voids (Spek, 2020).

Hirshfeld analysis and intermolecular contacts
To gain some insight into the intermolecular interactions at work in these compounds, a Hirshfeld analysis was undertaken, using the program CrystalExplorer (Spackman et al., 2021). Fig. 5 shows the Hirshfeld surfaces of the three compounds, together with the closest contact atoms (within 3.8 Å ).
The so-called 'fingerprint plots', which summarize all contacts between atoms inside and outside the Hirshfeld surface (Spackman & McKinnon, 2002;Spackman & Jayatilaka, 2009), are shown in Fig. 6. A common feature of all three plots is the occurrence of a red stripe around the main diagonal, reaching from ca d e /d i = 1.8/1.8 to 2.2/2.2, which corresponds to a large number of BrÁ Á ÁBr contacts (d e + d i = 3.6 to 4.4 Å ; the sum of the van der Waals radii of two Br atoms is 3.70 Å ).
The very different appearance of these plots is mainly due to the decreasing number of H atoms present. Table 4 and Due to purely statistical effects (there are eight H atoms in FdBr 2 , four in 3, two in 5 and none in 8), the absolute numbers cannot be compared directly. However, it is quite obvious that the importance of CÁ Á ÁH and especially HÁ Á ÁH contacts decreases drastically with increasing bromine content, while the importance of BrÁ Á ÁBr contacts increases in the same direction. At the same time, it appears quite interesting that HÁ Á ÁBr contacts are very important in all compounds where H atoms are present.
To obtain a more detailed picture of the individual interactions, a Mercury analysis was undertaken (Macrae et al., 2020) 3.3.1. Hydrogen bonds. The structures of the known polyhaloferrocenes collected in Table 2 Table 3 Important geometrical parameters of compounds 3, 5 and 8 in comparison with literature data for closely related compounds.
FdBr 2 is 1,1 0 -dibromoferrocene; 'Ct' is the abbreviation for the 'centroid' of the Cp rings, as calculated by the corresponding feature in PLATON (Spek, 2020); (Br-Cp) is the distance of the Br atoms from the Cp plane.
When using the standard settings of Mercury, no hydrogen bonds are indicated for compound 3. However, when increasing the limit by 0.2 Å , four (obviously very weak) hydrogen bonds appear [ Fig. 7 (left) and Table 5].
As can be seen from Fig. 7 (left), the atom pairs H14/Br11 and H24/Br21 connect the individual molecules in the y direction, while the pairs H15/Br13 and H25/Br23 join them in the x direction. Atoms Br12 and Br22 are not involved in hydrogen bonding.
In compound 5, the standard settings of Mercury suffice to show that only atom H5 (and, of course, its inversionsymmetry-generated counterpart H5 0 ) engages in a symmetrical bifurcated hydrogen bond (Type A in Scheme 2) with atoms Br2 and Br3 [ Fig. 7 (right) and Table 5]. As the figure shows, these interactions join individual molecules in the y direction.

Halogen bonding and other BrÁ Á ÁBr interactions.
In the discussion of halogen bonding, a distinction is usually made between XB Type I and XB Type II. According to the IUPAC and IUCr classifications, Type I contacts are geometry based, arising from close-packing requirements, while Type II arise from interactions between an electron-rich region on one halogen atom and an electron-deficient region on the other. The distinction can be made on the basis of the angles Â1 and Â2, which occur at halogen atoms X and X 0 of R-XÁ Á ÁX 0 -R 0 , and their difference (Scheme 3).
Usually it is assumed that for 0 < |Â1 -Â2| < 15 , a Type I contact is formed, while for Type II contacts, 30 < |Â1 -Â2| < 105 is found, and only the latter are regarded as real halogen bonds (Cavallo et al., 2016). In a more recent article, a distinction into Types I-IV was suggested, but was still based on these angles (Ibrahim et al., 2022): Type I: 90 < Â1 ' Â2 < 180 ; Type II: Â1 = 180 and Â2 = 90 ; Type III: Â1 ' Â2 = 180 ; Type IV: Â1 ' Â2 = 90 (obviously, Types III and IV are only extrema of the more general Type I). The forces behind these attractions are either van der Waals (Type I), electrostatic (Type II) or dispersion (Types III and IV) forces. It was further found that 'the Type I interactions were more frequent at the shortest distances' (Cavallo et al., 2016). Table 6 Table 4 Individual contributions (%) of the different interactions present in the crystal structures of FdBr 2 , 3, 5 and 8.

Figure 8
Packing plot of compound 3, viewed along c. Colour codes as defined by Mercury: carbon dark grey, hydrogen light grey, iron orange and bromine brown; the red lines are unexpanded contacts and the cyan lines are expanded contacts. Table 5 Hydrogen-bond parameters (Å , ) in compounds 3 and 5. the structural parameters of compounds 3, 5 and 8, while Figs. S9-S11 show Mercury representations of these interactions. All the listed BrÁ Á ÁBr contacts in Table 6 are well below the sum of the van der Waals radii (3.70 Å ; Bondi, 1964). When using the |Â1 -Â2| criterion, most interactions classified as Type II are also 'real' halogen bonds. To look at the structural consequences of this halogen bonding, a visualization of the packing plots should be helpful. Fig. 8 shows how the BrÁ Á ÁBr interactions join the individual molecules of compound 3 in the direction of the xy diagonal (b-a vector). It can also be seen that there are two intramolmolecular BrÁ Á ÁBr contacts of Type IV, emphasized in italic in Table 5. Two Br atoms (Br12 and Br22) are not involved in BrÁ Á ÁBr interactions. Fig. 9 shows that in compound 5 the BrÁ Á ÁBr contacts join the individual molecules in the z direction. All eight Br atoms are involved in BrÁ Á ÁBr interactions. In addition, there is also somestacking in the x direction; the centroids of two adjacent Cp rings are only 3.773 Å apart, while the ring planes have an interplanar distance of 3.507 Å (corresponding to an angle of 21.6 between the Ct-Ct 0 vector and the plane normal).
In bromomercurio compound 8, matters are a bit more complicated. Fig. 10 shows that BrÁ Á ÁBr contacts join the individual molecules in all directions. All Br atoms, except for Br2, Br5 and Br8, are involved in BrÁ Á ÁBr contacts.

Figure 9
Packing plot of compound 5, viewed along b. Colour codes as defined by Mercury: carbon dark grey, hydrogen light grey, iron orange and bromine brown; the red lines are unexpanded contacts and the cyan lines are expanded contacts.

Figure 10
Packing plots of compound 8, viewed along a (left) and along b (right). Colour codes as defined by Mercury: carbon dark grey, hydrogen light grey, iron orange and bromine brown; the red lines are unexpanded contacts and the cyan lines are expanded contacts.
interactions (Schmidbaur & Schier, 2015) can be excluded. In the crystal of the ferricenium complex 8 + ÁAsF 6 À , there is also a Hg 2 Br 2 ring with significantly shortened intermolecular HgÁ Á ÁBr contacts of 3.061 Å and a HgÁ Á ÁHg distance of 3.993 Å (Rupf et al., 2022). Furthermore, there are BrÁ Á Á contacts of 3.543 Å to a close Cp ring, in addition to a weak interaction between two Cp rings (Fig. 12);stacking occurs between two inversion-related C 5 Br 5 rings. Since the difference between the Ct-Ct 0 distance of 3.756 Å and the perpendicular distance between the Cp ring planes (3.690 Å ) is rather small (corresponding to an angle of 10.8 between the Ct-Ct 0 vector and the plane normal), it can be regarded as a 'true'interaction (though rather weak).

Co-operativity between HÁ Á ÁBr and BrÁ Á ÁBr contacts.
The importance of co-operativity in noncovalent interactions in general (Mahadevi & Sastry, 2016) and for the interplay of halogen and hydrogen bonds (Decato et al., 2021;Portela & Ferná ndez, 2021) in particular has been recognized in recent years and has been modelled by DFT calculations. This interplay has also been discussed for the 1,1 0 -dihaloferrocenes (Shimizu & Ferreira da Silva, 2018). In the preceding sections, we have discussed the individual contributions in compounds 3 and 5, and a look at Fig. 13 (and Tables 4 and 5) shows that also in these compounds HB and XB work together on the same halogen atoms.

Energetics of the intermolecular interactions found
in compounds 3, 5 and 8. The program CrystalExplorer allows for the calculation of interaction energies using the DFT program TONTO at the HF/3-21G level (Mackenzie et al., 2017). Fig. 14 shows the results of calculations for compounds 3 and 5 (apparently, due to the presence of Hg, the program cannot calculate wavefunctions for compound 8).
Inspection of the numerical values shows that the total interaction energies are stronger for compound 3. This is apparently due to the larger repulsion terms for 5, because both the largest dispersion and the largest electrostatic terms are found in compound 5. Another graphical representation ('energy frameworks') of the individual contributions can be seen in Fig. 15.

sigma-hole interactions
Acta Cryst. (2022). C78, 578-590 Blockhaus and Sü nkel Crystal and molecular structures of polybromoferrocene 587 Figure 11 The Hg 2 Br 2 ring in compound 8. Colour codes as defined by Mercury: carbon dark grey, hydrogen light grey, iron orange and bromine brown; the red lines are unexpanded contacts and the cyan lines are expanded contacts. Generic atom labels without symmetry codes ahve been used.

Figure 12
Partial packing diagram of compound 8, showing the BrÁ Á Á andcontacts (Å ). Colour codes as defined by Mercury: carbon dark grey, hydrogen light grey, iron orange, mercury blue and bromine brown; the red lines are unexpanded contacts and the cyan lines are expanded contacts. Ct1 0 /Ct1 00 and Ct2 0 / Ct2 00 are the centroids of inversion-related cyclopentadienyl rings, with one Fe atom between Ct1 0 and Ct2 0 , and another between Ct1 00 and Ct2 00 .

Comparison with halogen bonding in other halofer-
rocenes FdX n with X 6 ¼ Br and n > 2. At this point, it seems worthwhile to look at the occurrence of halogen bonding in the other polyhaloferrocenes mentioned in Table 2. As mentioned already, this study has been performed for the 1,1 0dihaloferrocenes before, and therefore these structures will sigma-hole interactions 588 Blockhaus and Sü nkel Crystal and molecular structures of polybromoferrocene Acta Cryst. (2022). C78, 578-590

Figure 13
Co-operativity of hydrogen and halogen bonding in compounds 3 and 5. Colour codes as defined by Mercury: carbon dark grey, hydrogen light grey, iron orange and bromine brown; the red lines are unexpanded contacts and the cyan lines are expanded contacts.

Figure 14
Interaction energies (HF/3-21G) for compounds 3 (left) and 5 (right) (standard program settings). The colour codes in the images refer to the tables below them. not be considered here again. Instead, the structure of the homoannularly substituted pentabromoferrocene (FcBr 5 ; Sü nkel & Bernhartzeder, 2011) is included (Table 7). All the listed XÁ Á ÁX contacts are below the sum of the van der Waals radii and of Type II except for the chloro compound (0.004 Å longer than this sum and Type I). This result (the increasing importance of XÁ Á ÁX contacts when going from X = Cl to X = I) parallels the observations in the FdX 2 sytems. In addition to the XÁ Á ÁX interactions, C-HÁ Á ÁX hydrogen bonds are important for all compounds, especially the chloro compound.
interactions are very strong for FdI 4 (virtually no displacement of the Cp rings of different molecules), while in FdCl 4 , the shift between the perpendicular projection of one centroid to the centroid of a neighbouring molecule is quite substantial. In FdI 3 , C-HÁ Á Á interactions seem to be of some importance, while in FcBr 5 , a weak C-BrÁ Á Á interaction can be observed.

Table 6
Characteristics of the BrÁ Á ÁBr interactions found in compounds 1, 3, 5, 8 and FdBr 2 . io)ferrocene. Hexabromoferrocene shows an eclipsed conformation of the Cp rings, as was also found for the already known structures of 1,1 0 -dibromo-and 1,1 0 ,2,2 0 -tetrabromoferrocene. Ferrocenes with a higher bromine content apparently prefer a staggered conformation, as was observed before for nona-and decabromoferrocene. All three title compounds show a combination of halogen bonding with either hydrogen bonding or interactions. Dispersion interactions appear to be stronger than electrostatic interactions. Table 7 Characteristics of the XÁ Á ÁX interactions in FdCl 4 , FdI 3 , FdI 4 and FcBr 5 .  where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 1.63 e Å −3 Δρ min = −1.24 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.