Hydrogen Bonds, Halogen Bonds, and Other Non-Covalent Interactions in a Series of Iodocymantrenes [Mn(C₅I_nH_5−n)(CO)₂L], L = CO, PPh₃, and n = 1–5

Abstract

In this study, the molecular and crystal structures of iodocymantrenes [Mn(C₅I_nH_5−n)(CO)₂(PPh₃)] (1b n = 1; 2, n = 2; 3, n = 3) are reported and compared with the known structures of [Mn(C₅I_nH_5−n)(CO)₃] (1a, n = 1; 5, n = 5) and [Mn(C₅I₄H)(CO)₂(PPh₃)] (4). In the crystals, many weak interactions like H bonds (H…O, H…I, H…π), halogen bonds (I…I, I…O, I…C, I…π), and π-π contacts are found. Hirshfeld analyses show that H bonding is far more important when the PPh₃ ligand is present, and this is mainly based on dispersive interactions. However, without the PPh₃ ligand, H…I and other I…X contacts are the most frequently observed intermolecular interactions.

1. Introduction

“A halogen bond occurs, when there is evidence of a net attractive interaction between an electrophilic region associated with a halogen atom in a molecular entity and a nucleophilic region in another, or the same, molecular entity” [1]. While this IUPAC definition of the halogen bond is generally accepted, there is still some debate when it comes to details. Usually, one differentiates between “true” halogen bonds between two halogen atoms of the type C-X…X’-C and halogen-Lewis base adducts of the type C-X…A. Both types are usually “characterized by X…X’ or X…A distances significantly shorter than the sum of van der Waals radii” [2]. For the former, two types—Type I and Type II—can be identified, while for the latter, “strong linearity” is postulated. Type I interactions are characterized by both angles Θ₁ and Θ₂ being close to 180°, while in Type II, one angle is close to 180°, and the other close to 90° [3] (Scheme 1).

Another definition assigns Type I to situations where |Θ₁ − Θ₂| ≤ 15° and Type II for situations with |Θ₁ − Θ₂| > 30°, while cases for which 15° < |Θ₁ − Θ₂| ≤ 30° are assigned a “quasi-type I/type II”. At the same time, it was stated that “Type I … is not a halogen bond” [4]. The directionality of halogen bonds and hydrogen bonds is regarded as proof of the mainly electrostatic nature of both interactions [5]. Besides the directionality issue, another problem arises with the distance criterion, which is strongly related to the van der Waals radii. Something quite often ignored is the fact that “intermolecular interactions and their influences in supramolecular phenomena are determined by energies, but a different property, geometry, is commonly measured” and “… the notion that ‘stronger is shorter’ is generally fallacious” [6]. Therefore, it has been suggested that in order not to overlook the important features of supramolecular interactions, larger van der Waals radii than those tabulated by Pauling and Bondi should be used [6,7]. As most organic molecules also contain hydrogen, the question also arose that if both hydrogen bonds and halogen bonds are so similar [4], would they be “competitors”, or would they enhance each other [8]? Both situations were found [8,9]. Combinations of halogen bonding with hydrogen bonding are particularly important in coordination chemistry, especially in organometallic chemistry, and additional interactions like X…O, X…N, X…π are also possible [10]. Hydrogen bonding of the C-H…O type in metal carbonyls was described nearly 30 years ago [11], while the possible importance of halogen bonds in metal complexes was discovered only 12 years later [12]. Such organometallic complexes were mostly metal carbonyls [13,14,15], but halo-metallocenes [9,16,17] were also studied to determine their halogen bonding interactions. A special, rather rare kind of iodine … π(Cp) interaction was found in some ferrocene systems [18].

Our group has been working on polyhalogenated cymantrenes for a while, and recently, we reported the synthesis of a series of iodocymantrenes [Mn(C₅I_nH_5−n)(CO)₂(PPh₃)], n = 1–4, (1b, 2, 3, 4), including the molecular structure of a tetraiodo compound [19]. Here, we report the molecular and crystal structures of the first three members of this series and compare them with the known structures of 4 and [Mn(C₅I_nH_5−n)(CO)₃] (n = 1 or 5) [13,20], with a focus on the occurrence of hydrogen and halogen bonding and other non-covalent interactions. Scheme 2 shows the molecular structural formulae of all compounds discussed here.

2. Results

2.1. Molecular Structures of 1b, 2, and 3

Compound 1b crystallizes in the triclinic spacegroup P-1 with one molecule in the asymmetric unit. Figure 1 shows an ortep3 view of the molecular structure (“top view”, i.e., a view from above the molecule in a direction perpendicular to the plane of the cyclopentadienyl ring).

The iodo atom is in a transoid position with respect to the P atom. One CO ligand eclipses with one CH group of the Cp ligand, while the other bisects a C-C bond of the Cp ring. Important geometrical parameters of 1b, 2, and 3 are collected in

Table 1. Important bond parameters of the structures of 1b, 2, and 3.

Length [Å] or Angle [°]	1b	2/Mol. A	2/Mol. B	3
Mn–P	2.2352 (11)	2.248 (4)	2.240 (4)	2.260 (2)
C–I	2.076 (5)	2.073 (15) 2.079 (16)	2.05 (2) 2.08 (2)	2.081 (10) 2.074 (8) 2.069 (11)
Mn–CT 1	1.777 (2)	1.788 (7)	1.769 (9)	1.788 (4)
I-CT-Mn-P	162.0	163 91	166 94	162.7 91.3 18.9

¹ CT is the centroid of the Cp ring.

.

Compound 2 also crystallizes in the triclinic space group P-1 but with two molecules in the asymmetric unit. The crystal was twinned and contained 5.5% solvent-accessible voids, both leading to a relatively low precision of the bond parameters. Figure 2 shows an ortep3 view of its molecular structure (molecule A; the molecular structure of molecule B is depicted in Figure S1 of the Supplementary Information).

One of the iodo atoms is again in a nearly transoid position with respect to the P atom, while the other is at a right angle.

Compound 3 also crystallizes in the triclinic space group P-1, with one molecule in the asymmetric unit. Figure 3 presents an ortep3 view of its molecular structure.

One iodo atom is again in a transoid position, while the second is at a right angle, and one C-I bond nearly eclipses the Mn-P bond. One Mn-CO bond bisects a C-C bond of the Cp ring, while the other eclipses a C-H bond of the Cp ring.

When comparing the three structures, there is only a little influence of the substitutional degree on the bond parameters. The Mn–P bond length, as well as the Mn–Cp-ring-centroid distance, increase slightly with increasing numbers of I atoms, while the averaged C–I bond lengths are all the same in all compounds.

2.2. Intermolecular Contacts

In this section, the structures of 1b, 2, and 3 are compared with the known structures of 1a (CSD: DUKQUV), 4 (CSD: TRIZAN), and 5 (CSD: PIXWEY), which can be found in the Cambridge Structural Database [21]. The publication, including 1a, contains some discussion of intermolecular interactions, but not to the extent of the present paper. The other two compounds had only been discussed in the context of their molecular structures. In most cases, the suggestions of Dance [6] were followed, and intermolecular contacts up to a distance “sum of van der Waals radii plus 0.3 Å” were examined. All individual measurements are contained in Tables S2–S7 of the Supporting Information. For the figures, the program mercury was used, usually with its standard settings [22].

2.2.1. Hydrogen Bonds

Figure 4 displays the hydrogen bonding pattern around molecules of compounds 1b, 2, and 3. A corresponding representation of the hydrogen bonding observed in compounds 1a and 4 (compound 5 does not contain any H atoms) is shown in Figure S2 of the Supporting Information. The typical H bond parameters are collected in Table S2 of the Supporting Information.

In compound 1a (Figure S2, Table S2), all three carbonyl oxygen atoms accept H bonds, O3 accepts two, and O1 accepts three (when the distance criterion according to Dance is used; within the “classical” limits, indicated by orange highlighting in Table S2, there are still four CH…O contacts). The iodine atom does not accept any hydrogen bonds. The H…O distances are in the range of 2.55–2.97 Å, with C-H…O angles between 129 and 172°. The longest distance comes with the largest angle.

For compound 1b (Figure 4), both carbonyl O atoms, as well as the I atom, accept H bonds: O1 accepts three, I1 accepts two, and O2 accepts one. The H…O distances are in the range of 2.61–2.91 Å, with C-H…O angles between 143 and 175°. The H atoms belong to the Cp ring as well as the three phenyl rings.

All oxygen atoms and three of the iodine atoms of the two molecules of compound 2 in the crystal accept H bonds: O11 accepts three, O12 accepts two, O21 accepts three, O22 accepts two, I11 accepts one, I12 accepts two, and I21 accepts two (Figure 4, Table S2). The H…O distances are in the range of 2.60–3.01 Å, with C-H…O angles between 123 and 171°. The H…I distances lie between 3.02 and 3.38 Å, with C-H…I angles between 134 and 169°. Only two H atoms are attached to the Cp rings (and only of molecule A); all others are bound to the six phenyl rings on both molecules A and B.

Both O atoms and all three I atoms of compound 3 accept hydrogen bonds from the three phenyl rings—the Cp H atoms are not involved. The H…O distances are in the range of 2.65–2.85 Å, with C-H…O angles between 120 and 175°, while the H…I distances are in the range of 3.08–3.41 Å, with C-H…I angles between 124 and 164°. The largest angle is associated with the shortest distance.

In compound 4, all oxygen atoms except O21 on molecule B accept H bonds from the three phenyl rings of molecule A and two phenyl rings of molecule B (Figure S2, Table S2). The H…O distances range from 2.63 to 2.91 Å, with C-H…O angles between 121 and 155°. Two of the iodine atoms of molecule A and three of molecule B accept H bonds (I12 accepts three, I13 accepts three, I22 accepts three, I23 accepts one, and I24 accepts three), both from the Cp rings and all six phenyl rings. The H…I distances range from 3.11 to 3.48 Å, with C-H…I angles between 121 and 146°. The largest angle is associated with the shortest distance.

2.2.2. C-H…C and C…C Contacts

These contacts are closely related to C-H…π and π-π interactions. Figure 5 shows these interactions in compounds 1b, 2, and 3, while the corresponding interactions of compounds 1a and 4 (there are no such contacts in compound 5) are shown in Figures S3 and S4. Numerical parameters are collected in Tables S3 and S4.

In compound 1a (Figure S3), there is one C-H…C contact between a Cp H atom and a carbonyl C atom. As the same C-H bond also makes a H bond to the attached O atom, it is probably better to describe these two interactions as C-H…π(CO) interaction. In addition, there is also a π-π interaction between one carbonyl group and one C-C bond of a Cp ring.

In compound 1b, there are C-H…C interactions, both of the “localized” (with one C atom) and the “semi-localized” (with two C-atoms) type between phenyl rings, which are consequently of the C-H…π type [23]. At the same time, there are several C-C interactions between pairs of Cp rings, with distances ranging from 3.21 to 3.68 Å, which qualify them as π-π interactions (Figure 5).

Table S3 shows that there are many more C-H…C interactions with donors from molecule A than from molecule B of compound 2 (15 vs. 7). All donors and most acceptors are C-H bonds from phenyl rings. Only Cp carbon C13 and carbonyl carbons C16 and C26 act as acceptors. All types of C-H…π interactions, localized, semi-localized, and delocalized, can be found. C-C contacts are only found between pairs of Cp ligands in molecule B, which qualifies them as π-π contacts.

In compound 3, carbonyl C atom C6 accepts one C-H…C bond. Since the same C-H bond donates a H bond to the attached oxygen atom O1, this interaction is better described as a C-H…π(CO) bond. One interaction between phenyl rings 1 and 2 can be regarded as “localized” π contact, one between phenyl rings 2 and 3 can be regarded as “semi-localized”, and one between rings 3 and 1 can be regarded as “delocalized”, with the H…C distances spread between 2.96 and 3.18 Å. There are also several close contacts between pairs of Cp rings that can therefore be characterized as π-π interactions.

There are many C-H…C contacts in compound 4. Most of them occur between phenyl rings (usually of the semi-localized” C-H…π type). There is, however, a rather unusual interaction between two phenyl C-H bonds and three Cp-carbon atoms that occurs pairwise (Figure S4). At the same time, there are also close C…C contacts between two carbon atoms of a phenyl ring with four carbons of a Cp ring; thus, an unusual π-π contact between one phenyl double bond and the π system of one Cp ring occurs. The C…C distances of this particular interaction are in the range of 3.36–3.56 Å.

2.2.3. I…O and I…I Contacts

The “halogen-bonding” I…O and I…I contacts of compounds 1b, 2, and 3 are depicted in Figure 6, those of compounds 1a and 4 are depicted in Figure S5, and those of compound 5 are depicted in Figure S6. The corresponding numerical data are contained in Tables S5 and S6. The occurrence of an I…O contact in compound 1a has already been reported [13].

Besides the “short” I…O contact in compound 1a (shorter than the sum of van der Waals radii) reported already, there are also two rather long ones (I…O ≈ 3.8 Å), both carbonyl oxygens of another molecule, translated in the x direction. The shortest contact is associated with the largest angle at iodine (nearly linear) and at oxygen O2 (slightly larger than a right angle), making this a clearly Type II interaction. The same O atom also has a long contact time with a symmetry-related I atom, with nearly identical contact angles at I and O atoms, making this a Type I interaction. There are no I…I contacts in this compound.

Compound 1b has only one I…O contact, which is also very long (>3.7 Å), and, according to the |θ₁-θ₂| criterion, has to be regarded as a quasi-Type I/Type II interaction. There is no I…I interaction.

In compound 2, two I…O contacts can be found; one I of molecule A interacts with an O atom of molecule B, and vice versa. The former is shorter than the sum of van der Waals radii and is, therefore, a Type II interaction. The longer interaction is of quasi-Type I/Type II. There are two I…I interactions, one shorter and one longer than the sum of van der Waals radii. The angles at the I atoms are between 116 and 142° and are thus neither linear nor perpendicular. However, they can be assigned to Type I (the “shorter”) and quasi-Type I/Type II.

For compound 3, only one I…O contact can be found, which is rather short. While the angle at I2 is close to linear, the angle at the O atom is “intermediate”, and therefore the contact is quasi-Type I/Type II. All I atoms are involved in I…I contacts: I1 in three, I2 is involved in one, and I3 is involved in two. Only the “symmetrical” contact between atoms I3…I3 is shorter than the sum of van der Waals radii and with θ₁ = θ₂ =154°, a typical Type I contact.

In compound 4, one I atom of molecule A and two I atoms of molecule B show I…O contacts, two of which are below the sum of van der Waals radii and are Type II interactions. The “long” contact is of Type I. All iodine atoms of molecule A are involved in I…I contact, while only two iodines of molecule B interact with other iodines. Atoms I21 and I14 are even involved in two I…I contacts each. Two of these contacts are relatively short, while the other two are rather long. Three contacts are Type II, and one is quasi-Type I/Type 2.

Compound 5 shows a large number (14) of I…O contacts. There are three I atoms of molecule A, two of molecule B, two of molecule C, and three of molecule D. Some of them take part even in two (I16 and I20), and one takes part in three such contacts (I10). All O atoms act as acceptors, with two from two I atoms (O1 and O12). Most of these contacts (11) are longer than the sum of van der Waals radii. The angles at iodine span the range from 62 to 165°, while the ones at oxygen show less spread, being between 87 and 159°. In five cases, the angle at iodine is larger than the one at oxygen. Seven interactions are of Type I, two of quasi-Type I/Type II, and seven are of Type II. There are seven I…I contacts, in which 12 I atoms are involved. Two of them (I1 and I7) take even part in two I…I contacts each, while eight do not take part in any.

2.2.4. I…C Contacts

These interactions are I…π-ring interactions [24,25], which can further be differentiated into “localized”, “semi-localized”, and “delocalized” contacts. Figure 7 shows the I…C contacts for compounds 1b, 2, and 3, while Figure S7 shows the corresponding interactions for compounds 1a, 4, and 5.

In compound 1a, atom I1 has four contacts with four carbonyl O atoms, one each with C6 and C8 and two with C7 of two different molecules. As mentioned above, there are also contacts to carbonyl oxygen atoms O1 and O2, and therefore, they are better described as I…π (CO) contacts with carbonyls C6-O1 and C7-O2. As the I1…C8 distance is much larger than the other two, this contact is less significant.

In compound 1b, there are three contacts of the I atom to three C atoms of the same phenyl ring, and therefore, it seems appropriate to describe this interaction as an I…π (phenyl) interaction. Alternatively, considering that there is a short distance to the middle C-atom and two approximately equally longer to the two “outer” C atoms, one might consider this interaction similar to a σ-π-allyl interaction.

In compound 2, all iodine atoms take part in I…C interactions, two of them taking part in two. I11 interacts with two C atoms of a Cp ligand, which makes this interaction a semi-localized I…π(Cp) contact. I12 interacts with two carbon atoms of a phenyl ring, which makes this interaction an I…π(phenyl) contact. The remaining two iodine atoms make only one I…C contact each, one with a Cp carbon and one with a phenyl ring. As the I…C distance of the former is the longest of all (3.89 Å) combined with the smallest angle at I (71.3°), it is doubtful that this interaction is of any importance.

There are only two rather long (>3.90 Å) I…C contacts in compound 3, one to a Cp ring atom and one to a phenyl ring carbon. As the former one has a very small angle at I3 (63°), it can be assumed that this interaction is not very important. With an angle of ca. 120° at I1, this interaction is of the “localized” I…π(phenyl) type.

C…I interactions are very important in the crystal structure of compound 4. There are 14 such contacts: all iodine atoms take part except for one (I12). Two take part in only one I…C interaction, three take part in two, and two take part in three. All contacts except for two involve only phenyl carbons, while Cp carbon atom C25 interacts with two iodine atoms, I11 and I24. The shortest interaction (I11…C224, 3.32 Å) shows a close to linear angle at the iodine atom (175°). The interactions of I22 with C203/C204 and I23 with C114/C115/C116 are semi-localized I…π(phenyl) contacts.

Compound 5 employs 7 I…C contacts. All are longer than the sum of van der Waals radii, with distances between 3.80 and 3.93 Å. Two of these are of the I…π(CO) type, and one is of the I…π(Cp) type.

2.2.5. Packing Plots

The superposition of all these interactions (and a few others that have not been discussed here in detail) leads to the actual arrangement of molecules in the crystal, visible in the packing plots. These are displayed for compounds 1b, 2, and 3 in Figure 8, while the packing plots of the other compounds are displayed in Figure S8 of the Supporting Information.

2.3. Hirshfeld Analysis

Hirshfeld analyses allow the quantitative analysis of intermolecular interactions. For this reason, we applied the program CrystalExplorer, ver. 21.5. [26]. While this program has many features, we used only the Fingerprint plots for the present purpose [27], as well as the calculation of intermolecular energies [28,29,30]. A nice overview of the usefulness of this program can be found in [31]. Due to the twinning and occurrence of voids, Fingerprint plots were not produced for compound 2, and due to computational problems with handling 40 iodine atoms in the unit cell of 5, we refrained from calculations of interaction energies for this compound.

2.3.1. Fingerprint Plots

All points on a Hirshfeld surface are characterized by a combination of the closest distances to atoms outside the surface (d_e) and inside the surface (di). A “Fingerprint plot” is a two-dimensional representation of all the d_i/d_e pairs occurring in a crystal (d_i values on the “x” and d_e values on the “y” axis) using color coding; with increasing numbers of occurrences, the color changes from gray to dark blue to light blue to green to red. For more details, see the original publication: [27]. The “complete” Fingerprint plots (superposition of all kinds of contacts X…Y) of compounds 1b and 3 are displayed in Figure 9, while the corresponding plots of compounds 1a, 4, and 5 are shown in Figure S9.

The quantitative evaluation of these Fingerprint plots allows for a comparison of the distributions of individual types of contacts across the Hirshfeld surface (

Table 2. Fingerprint plots: contributions (%) of the individual contacts.

Comp	H-H	C-C	H-C	H-O	H-I	I-O	I…I	I…C
1a	3.2	1.0	12.3	44.5	9.4	12.0	0	8.6
1b	43.8	0.5	19.6	17.7	13.2	1.4	0	3.0
3	31.9	1.9	18.8	13.9	21.1	4.1	7.0	0.9
4	23.1	2.6	16.5	9.7	31.5	4.4	5.3	4.8
5	–	0.5	–	–	–	23.4	42.0	15.8

).

The H…H contacts are the most frequently observed for compounds 1b and 3, while H…O contacts dominate for 1a, H…I contacts for compound 4 and I…I contacts for 5. The large importance of contacts H…X in the crystals of 1b, 3, and 4 is, in part, due to the statistical weight of 15 C-H bonds alone in the PPh₃ ligand. A bit astonishing is the non-linearity of the percentages of I…I (and I…C) contacts with increasing iodine content.

2.3.2. Interaction Energies

For the calculation of interaction energies, one molecule is selected. All contact atoms in a sphere with a radius of 3.8 Å (a larger value has to be chosen when there is more than one molecule in the asymmetric unit) around the center of this molecule are determined, and all incomplete fragments are completed. Then, the program tonto (integrated into the CrystalExplorer program suite [32]) is used to determine all interaction energies between all pairs of molecules. The program allows us to choose between a faster HF/3-21G and a more accurate B3LYP/DGDZVP energy model. The complete listing of all interacting molecule pairs with |E_tot| > 10 kJ/mol is displayed obtained with the accurate basis set in

Table 3. Complete listing of the B3LYP/DGDZVP interaction energies [kJ/mol] of 1a,b, 3, and 4.

Comp	Type	Symm.op.	R [Å]	Eele	Epol	Edis	Erep	Etot	Contribution
1a (1)	A	1.5 − x, −y, z + ½	6.97	−11.8	−1.9	−18.4	15.5	−20.4	H…π(CO), I…O
	B	x ± ½, ½ − y, 1 − z	7.60	−7.9	−1.6	−21.1	15.9	−18.2	H…π(CO), H…π(Cp), I…π(CO)
	C	x ± 1, y, z	7.27	−13.1	−0.5	−20.5	24.5	−16.9	I…π(CO)
	D	2 − x, -y − ½, 1.5 − z	6.60	−14.4	−1.8	−15.6	22.6	−16.2	H…I, H…π(CO). I…π(CO)
	E	x − ½, −y – ½, 1 − z	6.18	−6.0	−0.8	−17.2	14.9	−12.7	H…π(CO), I…O
1b (2)	A	1 − x, 1 − y, 1 − z	10.18	−22.0	−1.7	−65.1	47.0	−52.1	C-H…π(Ph), π(Ph)…π(Ph)
	B	1 − x, −y, 1 − z	8.00	−11.0	−5.6	−51.6	31.8	−41.1	H…H, H…C
	C	2 − x, 1 − y, 2 − z	7.31	−37.9	−4.8	−49.3	82.3	−35.8	I…π(CC). I…O, H…π(CO), H…I
	D	1 − x, −y, 2 − z	8.36	−15.3	−1.4	−35.3	31.9	−28.3	CH…π(Cp), H…I
	E	2 − x, 1 − y, 1 − z	9.49	−10.8	−2.0	−27.9	15.8	−27.5	H…H, H…π(Ph), H…O
	F	1 − x, 1 − y, 2 − z	8.43	−1.4	−0.8	−22.3	4.8	−18.6	CH…π, H…H
	G	x, y − 1, z	10.60	−9.80	−1.3	−20.4	19.4	−17.1	H…H, H…O, H…I, H…π(CO), CH…π
	H	x − 1, y, z	10.34	−7.9	−1.6	−15.3	11.1	−16.0	H…H, H…π(CO)
3 (3)	A	1 − x, 1 − y, 1 − z	11.05	−23.3	−2.0	−66.3	51.7	−51.9	C-H…π(Ph), H…H
	B	1 − x, −y, −z	7.28	−25.4	−1.6	−63.4	58.6	−47.1	π(Cp)…π(Cp), H…I
	C	1 − x, −y, 1 − z	9.84	−12.3	−5.6	−57.1	36.7	−44.2	C-H…π(Ph), H…H, H…O
	D	−x, 1 − y, 1 − z	10.85	−16.2	−2.0	−40.6	30.6	−35.0	CH…π, H…H, O…C
	E	1 − x, 1 − y, −z	7.85	−19.3	−1.3	−46.1	50.5	−30.3	H…I
	F	−x, 1 − y, −z	7.92	−19.7	−3.1	−38.7	49.8	−26.0	H…I, I…O
	G	x − 1, y, z	10.09	−8.0	−1.6	−24.4	16.2	−20.8	CH…π(CO), H…H, CH…π(Cp)
	H	x, y − 1, z	10.57	−8.9	−1.1	−19.5	16.4	−17.1	H…H, H…I, H…π(CO), H…O, O…π(CC)
	I	x, y, z + 1	13.14	−1.4	−0.4	−9.7	0.0	−10.2	H…I
4 (4)	A	1 − x, −y, 1 − z	8.57	−22.4	−4.2	−70.0	71.5	−43.7	H…H, H…π(CO), H…π(Cp), H…I
	B	1 − x, 1 − y, 1−z	11.08	−11−6	−2.6	−75.7	64.6	−40.2	H…H, π(Ph)…π(Ph)
	C	Mol.B: x, y, z	11.59	−46.6	−2.9	−48.0	92.2	−36.2	H…H, H…π(Ph), H…I, I…π(Ph)
	D	½ − x, y ± ½, ½ − z	8.32	−46.6	−2.9	−48.0	92.2	−36.2	H…I, I…I
	E	Mol. B: x, y − 1, z	11.16	−46.6	−2.9	−48.0	92.2	−36.2	H…π(CO)
	F	x ± ½, ½ − y, z ± ½	11.62	−2.9	−0.8	−20.3	7.7	−16.5	H…H, H…π(Cp), H…I
	G	Mol.B: 1 − x, 1 − y, 1 − z	12.51	−2.9	−0.8	−20.3	7.7	−16.5	H…H, H…π(Ph)

⁽¹⁾ 13 molecules in calculation. There is one more contributor, at +0.2 kJ/mol; ⁽²⁾ 21 molecules in calculation. There are 7 more contributors between −8.5 and +0.2; ⁽³⁾ 16 molecules in calculation. There are 2 more contributors, −7.9 and −5.7; ⁽⁴⁾ 25 molecules in calculation: 11 mol. A, 14 mol. B. There are 12 more contributors, between −2.5 and +0.1.

, while the results for the “faster” HF-3-21G model can be found in the Supporting Information. The labels “A, B, C…” in the “Type” column are just given as an ordering scheme with respect to the total energies. The total interaction energy is composed of an electrostatic term E_ele, a polarization term E_pol, a dispersion term E_dis, and a repulsion term E_rep. Using an independent training set, a calculation of the total energy is possible using scale factors E_tot = 1.019 E_ele + 0.651 E_pol + 0.901 E_disp + 0.811 E_rep for the HF calculations and E_tot = 1.057 E_ele + 0.740 E_pol + 0.871 E_disp + 0.618 E_rep for the B3LYP calculations. The program produces as output a listing of all found interaction energies, listed together with the distance of molecular centroids of interacting molecular pairs. R.

As can be seen, the total interaction energies of the three PPh₃-containing compounds are much larger than for the pure carbonyl complex 1a. The major reason for this difference comes from the much larger importance of dispersion terms for the PPh₃ complexes. For all compounds and in all contributing molecular pairs, the dispersion term is the most important, although, in a few cases (Type “D” in 1a, Type “C” in 1b, and Types “C,D,E” in 4, the electronic term also reaches high values. A closer look at the individual interactions is possible when looking at the corresponding molecular pairs (see the last column in Table 3). While in compound 1a, H…π(CO) and I…π(CO) are involved in nearly all interactions, in the PPh₃-containing compounds, H…H and C-H…π interactions contribute the most.

3. Discussion

There were no unusual features of the molecular structures. They just proved that in the di- and tri-substituted complexes 2 and 3, the iodine atoms were in relative 1,2 and 1,2,3 positions, respectively. However, the examination of the intermolecular interactions produced some unusual features. First, it seemed worthwhile that the different iodine atoms participated in the iodine-related H…I, I…I, I…O, and I…C.

Table 4. Participation of the individual iodine atoms in all compounds in H…I, I…I, I…O, I…C interactions.

Comp	I Atom	H…I	I…I	I…O	I…C
1a	I1			+	+
1b	I1	+		+	+
2	I11	+	+	+	+
	I12	+	+		+
	I21	+	+		+
	I22		+	+	+
3	I1	+	+		+
	I2	+	+	+
	I3	+	+		+
4	I11		+		+
	I12	+	+
	I13	+	+		+
	I14		+	+	+
	I21		+		+
	I22	+		+	+
	I23	+			+
	I24	+	+	+	+
5	I1		+		+
	I2		+	+
	I3
	I4		+	+
	I5			+
	I6
	I7		+
	I8		+
	I9			+	+
	I10		+	+	+
	I11		+
	I12		+
	I13		+	+
	I14
	I15		+	+	+
	I16			+
	I17
	I18		+
	I19			+
	I20		+	+	+

shows all the results obtained in this study.

In crystals of compounds 2 and 4, there were two symmetry-independent molecules, and for compound 5, there were four, with a total of 37 different iodine atoms in these structures. There were only two that took part in all four different interactions, but four did not take part in any. It should be stressed once again that we used Dance’s distance criterion (ΣvdW + 0.3 Å) in our studies. The tables in the Supporting Information list a total of 225 contacts, of which only 50 (22%) would be below the sum of van der Waals radii. A total of 101 contacts contain iodine, of which 30 (30%) fulfill the “classical” distance criterion.

We examined how many iodine atoms take part in either H…I or I…I contacts, but not both. This is the case for seven iodine atoms. The number of those that exclusively take part in H…I or I…O contacts is only three, while nine take part in both H…I and I…I contacts, and five participate in both H…I and I…O contacts. So, it can be concluded that these interactions are sometimes, but not always, cooperative.

When looking at the occurrence of π interactions, it can be stated that the majority of them are of the C-H…π type, mostly with phenyl rings, but sometimes with Cp rings, too. There are also several π contacts of iodine atoms, mostly with phenyl rings, but sometimes also with metal carbonyl groups.

Finally, when comparing the interactions for which the largest energies were calculated, iodine atoms were only involved in compounds 1a and 4. In agreement with the results obtained from the Fingerprint plots, the large number of C-H groups in the PPh₃ ligand makes interactions involving the phenyl groups the most important.

4. Materials and Methods

The compounds used for the newly described structure determinations were synthesized and characterized as previously described by us [19]. The syntheses never produced pure compounds (purity usually between 80 and 90%): in all cases, spectra showed contamination by starting materials, stereoisomers, and/or decomposition products. Due to the instability of all iodocymantrenes (always light-sensitive; thermal instability increases with increasing iodine content), purification attempts via chromatography might remove the starting materials and stereoisomers but lead to higher contamination with decomposition products. The recrystallization steps used for the preparation of crystals led to very few crystals, the amounts of which were never sufficient for other methods of characterization.

NMR spectra were measured on jeol ecp-270 or ex-400 instrument, (both jeol Germany, Freising, Germany) using CD₂Cl₂ or CDCl₃ as solvent. The chemical shifts were obtained relative to the residual solvent signals, as defined by the MestReNova software (Version 14.1.1-24751, Mestrelab Research S.L., Santiago de Compostela, Spain) (δ_CDCl3 = 7.260 and 77.16 ppm, respectively; δ_CHDCl2 = 5.320 and 53.84 ppm, respectively). Mass spectra were obtained on Finnigan MAT 90 and JEOL Mstation 700 (jeol Germany, Freising, Germany) instruments, in DEI or FAB mode. Chromatographic separations were performed in Schlenk glass frits of ca. 20 mm diameter with a filling hight of ca. 15 cm of silica gel, which had been suspended in petroleum ether. A slight overpressure of nitrogen was used to promote the migration of the bands through the column.

All crystals were measured on a BRUKER D8 VENTURE system (Bruker Corp., Billerica, MA 01821, USA). Crystals of compound 2 were obtained as twins. Refinement was possible using a HKLF 5 file with a “BASF” scale factor of ca. 0.22. In addition, there were relatively large solvent-accessible voids, which were modeled via the squeeze option within platon. Still, it was necessary to apply many restraints on the anisotropic thermal parameters (DELU in shelxl). The experimental details of the structure determinations are collected in Table S1 of the Supporting Information. The software package WINGX v.2023.1 [33] was used for structure solution (SHELXT [34], refinement (SHELXL 2018/3, [35]), evaluation (PLATON, [36]), and graphical representation (ORTEP3 and MERCURY [22]). Carbon-bound hydrogen atoms were treated with a riding model using the AFIX command of SHELXL.

Regarding the computational methods used, the reader is referred to the original publications.

5. Conclusions

Iodocymantrenes show a broad spectrum of intermolecular interactions, from C-H…X over I…X to π-π contacts. An analysis of the Fingerprint plots shows that for the tricarbonyl complexes H…O (1a) and I…I (5), contacts dominate the intermolecular interactions. In the presence of a triphenylphosphine ligand, the importance of H…H and H…O contacts decreases with increasing iodine content, while the number of H…I contacts increases in the same direction. As a consequence, while the H…H contacts are the most dominant for compound 1b, the H…I contacts are the most important for compound 4. The calculation of interaction energies shows that for all compounds, the dispersion term is the largest contributor to the total interaction energy. It might be worthwhile to prepare the remaining three iodotricarbonyl complexes (with two, three, and four iodo substituents) and examine their crystal structures as well. This might allow for separating the effects of the iodine atoms from the effect of the PPh₃ ligand. Furthermore, in the case of the ferrocene system, not all iodoferrocenes are known. After their successful synthesis and a comprehensive study of their crystal structures, a further comparison would be possible to differentiate between influences of carbonyl and cyclopentadienyl ligands and perhaps of the metal. But first of all, these compounds would have to be isolated!