# Impressive 1D (Ferrocenyl C6F5R )n Stacking Due to Cooperative Interactions in N-(Ferrocenylmethyl)Pentafluorobenzenecarboxamide: Four Crystal Structures and Contacts Analyses in N-(Ferrocenylalkyl)Benzenecarboxamides

John F Gallagher, Christian Jelsch, Peter T M Kenny, Alan J Lough

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John F Gallagher, Christian Jelsch, Peter T M Kenny, Alan J Lough. Impressive 1D (Ferrocenyl C6F5R )n Stacking Due to Cooperative Interactions in N-(Ferrocenylmethyl)Pentafluorobenzenecarboxamide: Four Crystal Structures and Contacts Analyses in N-(Ferrocenylalkyl)Benzenecarboxamides. Crystals, 2025, 15 (4), pp.299. 10.3390/cryst15040299. hal-05012425

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# Impressive 1D (Ferrocenyl···C₆F₅R···)ₙ Stacking Due to Cooperative Interactions in N-(Ferrocenylmethyl)Pentafluorobenzenecarboxamide: Four Crystal Structures and Contacts Analyses in N-(Ferrocenylalkyl)Benzenecarboxamides

John F. Gallagher ¹,* , Christian Jelsch ²,† , Peter T. M. Kenny ¹,³,† and Alan J. Lough ⁴

¹ School of Chemical Sciences, Dublin City University, D09 V209 Dublin, Ireland

² CRM2, CNRS UMR 7036, Faculté des Sciences et Technologies, Université de Lorraine, 54000 Nancy, France

³ National Institute for Cellular Biotechnology, Dublin City University, D09 V209 Dublin, Ireland

⁴ Department of Chemistry, University of Toronto, Toronto, ON M5S 3H6, Canada

* Correspondence: john.gallagher@dcu.ie

† This research paper is dedicated to our great friend and colleague, Professor Peter T. M. Kenny (1958–2022); a passionate and lifelong Glasgow Celtic and Chelsea football supporter.

**Abstract:** The crystal structures, interactions, and contacts analyses of four N-(ferrocenylalkyl)benzene-carboxamide derivatives are described as the N-(ferrocenylmethyl) benzenecarboxamide **4a**, N-(ferrocenylmethyl)-2,6-difluorobenzenecarboxamide **4e**, N-(ferrocenylmethyl)pentafluorobenzenecarboxamide **4f** and N-(ferrocenylethyl)-4-fluorobenzenecarboxamide **5**. Intermolecular amide···amide hydrogen-bonding interactions as 1D intermolecular chains are present in all four crystal structures, with N···O distances ranging from 2.819 (2) to 2.924 (3) Å. Three of the crystal structures have one molecule per asymmetric unit, except the phenyl **4a**, which has Z'=2. In the structure of **4a**, Fc(C-H)···(phenyl) and phenylC-H···π(C₅H₄) ring interactions dominate the interaction landscape, together with (1:1) face-to-face (phenyl)···(phenyl) and (C₅H₅)···(C₅H₅) ring stacked pairs (Fc = ferrocenyl moiety). In **4e**, interlocking ferrocenyls, short C-H···(C-F) and C-H···O hydrogen bonds are the only additional notable intermolecular interactions. In the pentafluorophenyl derivative **4f**, a remarkable selection of interactions is present with interwoven 1D ferrocenyl···(C₆F₅) stacking and C-H···F interactions; molecules aggregate forming impressive 1D columns comprising intertwined (Fc···C₆F₅···)ₙ ring stacking. In the ethyl bridged system **5**, C-H···F and C-H···π (arene) contacts with (4-fluorobenzene) ring···ring pairs combine and stack about inversion centres. The reported *para*-F substituted structure **REYWOU (4d)** is used for comparisons with the **4a**, **4e**, **4f**, and **5** crystal structures. In view of the rich interaction chemistry, contacts enrichment analyses of the Hirshfeld surface highlights several interesting features in all five ferrocenylalkylcarboxamide structures.

**Keywords:** aggregation; aromatic ring stacking; columnar; contact enrichment analysis; crystal structure; ferrocene; fluorine; hydrogen bonding

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Received: 20 February 2025

Revised: 14 March 2025

Accepted: 18 March 2025

Published: 25 March 2025

**Citation:** Gallagher, J.F.; Jelsch, C.; Kenny, P.T.M.; Lough, A.J. Impressive 1D (Ferrocenyl···C₆F₅R···)ₙ Stacking Due to Cooperative Interactions in N-(Ferrocenylmethyl) Pentafluorobenzenecarboxamide: Four Crystal Structures and Contacts Analyses in N-(Ferrocenylalkyl) Benzenecarboxamides. *Crystals* **2025**, *15*, 299. https://doi.org/10.3390/cryst15040299

**Copyright:** © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

After 70+ years of enormous developments, ferrocene chemistry continues to progress and grow unabated in a multitude of areas in chemistry and science [1–9]. There are many research fields that have overlapped with ferrocenes and aided in its development.
Significant branching into expanding multidisciplinary studies has arisen, and, overall, the growth and study of ferrocene crystal structures diversifies and shows no sign of abating [10]. This is noted by the number of structures incorporating ferrocene or as a derivative which is currently ~19,000 (18,797 structures without restrictions and including repeat structures 01/02), as listed in the Cambridge Structural Database (CSD) in January 2025 [10]. Of on-going interest in structural science, the ferrocene molecule [as Fe(cp)₂] and its derivatives are notable on account of their rich, robust chemistry and remarkable stability [1–3]. It is widely used due to the physicochemical changes and improvements imparted on the molecular systems under study. For example, areas such as bioorganometallic and related chemistry (incorporating the ferrocenyl moiety—Fc) have seen spectacular growth over the past few decades with applications in medicinal and anti-cancer studies [10–18].

Our research interests in ferrocene derivatives have focused on two specific areas as (i) hydrogen bonding and crystal engineering studies [9,19] and (ii) bioorganometallic ferrocene chemistry [13,14,20,21]. In the latter studies, ferrocenyl and ferrocenoyl groups were incorporated with conjugated linker groups such as benzene and naphthalene bonded specifically to amino acids and dipeptides [20,21]. An impetus for this research is their utilisation as new biomaterials and in medicinal chemistry/anti-cancer studies [20,21].

Of note and particular interest to the present manuscript is the recent publication by Cockcroft and co-workers, where they reported the structure and properties of (C₆F₆)·ferrocene [or (C₆F₆)·FeCp₂] in a temperature range from 100 K up to ~309 K [5]. In this C₆F₆·FeCp₂ solvate adduct (from 100→290 K), the crystal structure comprises closely packed columns of alternating hexafluorobenzene C₆F₆ and FeCp₂ molecules arranged so that the hexafluorobenzene is sandwiched between the ferrocenes. It is stated that intermolecular quadrupolar forces between the C₆F₆ and FeCp₂ components are the main driving forces in the observed columnar and structural aggregation [5].

In the present manuscript, four new ferrocenecarboxamide crystal structures are reported [13,14] and analysed for comparisons using contacts enrichment analyses (Scheme 1). The structural changes that arise on changing the substitution pattern from phenyl to pentafluorobenzene derivatives are analysed. Comparisons are made with the published 4-fluorobenzene structure 4d (REYWOU) [14] and the related literature [5,10]. Additionally, the ethylene-bridged group (-CH₂CH₂-) in 5 provides an extra degree of flexibility in molecular structure. Previously, we have published research on conjugated ferrocene derivatives in structural systematic studies and biomaterials [9,19] and have noted that the number of compounds incorporating the 'FcCH₂NHC=O (arene)' moiety is still relatively low, with only eight structures available in the CSD, ([Fc = (η⁵-C₅H₅)Fe(η⁵-C₅H₄)]) [10]. These structures include UYENUJ [15] and MIRYIZ [18]. A total of thirty 'FcCH₂NHCO-C structures are noted in a more general study [10]. In order to further develop and expand this area of ferrocene structural research, four new ferrocene derivatives are presented with their crystal structures. It is noted in particular the diversity of packing modes and the importance of thorough analysis of the 3D structures for comparisons with related molecules [5,10].
<figure><img src="image_3.png" /><figcaption>**Scheme 1.** Schematic diagram of the ferrocene derivatives **4a**, **4d** [14], **4e**, **4f**, and **5**.</figcaption></figure>

## 2. Experimental

### 2.1. *Materials and Characterisation*

The chemicals, materials, spectroscopic, X-ray diffraction methods, and analytical equipment are as reported previously [9,13,14]. All chemicals used in the synthetic reactions were used as purchased from Sigma-Aldrich, Dublin, Ireland and the synthetic reactions utilised standard organic synthetic procedures, as described in earlier work [14] (ESI). The original numbering system for the compounds and structures presented herein is retained. Compounds **4b** and **4c** are the related *ortho*-F and *meta*-F isomers of the *para*-F substituted structure **REYWOU** (4d) [14]. Single crystals of **4a**, **4e**, **4f**, **5** were grown from dichloromethane solutions [14]. Single crystal X-ray diffraction methods and data collection strategies were routine [22] and structure solution and refinement for **4a**, **4e**, **4f**, **5** undertaken using the SHELXS and SHELXL4 programs [23]. Molecular graphics using the Mercury program provided ORTEP diagrams (with displacement ellipsoids drawn at the 30% probability level) and the hydrogen-bonding diagrams [24]. Analyses of the geometric data for comparison purposes were performed using SHELXL4 output together with the 'CALC ALL' routine in PLATON [25]. Salient features of the four structures are provided in Tables 1 and 2. Comparisons and analyses of the CSD [10] were undertaken using the latest version available in 2024 [Conquest version 24.2.0 (5.45 + update 3); build 415171].
<table><caption>**Table 1.** Selected crystallographic data (full details are available; ESI, Table S1).</caption><thead><tr><th>Structures</th><th>Crystal System;<br>Space Group</th><th>Z'</th><th>Volume (Å³); KPI</th><th>R, wR<sub>2</sub>R-Factors,<br>GoF</th></tr></thead><tbody><tr><td>4a</td><td>Monoclinic; P2<sub>1</sub>/n</td><td>2</td><td>2851.36 (18); 71.3</td><td>0.044, 0.126, 0.98</td></tr><tr><td>4d [14]</td><td>Orthorhombic; P2<sub>1</sub>2<sub>1</sub>2<sub>1</sub></td><td>1</td><td>1435.97 (4); 71.8</td><td>0.031, 0.065, 1.05</td></tr><tr><td>4e</td><td>Monoclinic; P2<sub>1</sub>/a</td><td>1</td><td>1524.92 (8); 69.3</td><td>0.034, 0.085, 1.05</td></tr><tr><td>4f</td><td>Monoclinic; P2<sub>1</sub>/c</td><td>1</td><td>1577.44 (10); 71.2</td><td>0.034, 0.086, 1.06</td></tr><tr><td>5</td><td>Orthorhombic; Pbca</td><td>1</td><td>3145.27 (16); 69.8</td><td>0.037, 0.092, 1.03</td></tr></tbody></table>

R-factor definitions as R [F² > 2σ(F²)], wR (F²).

<table><caption>**Table 2.** Selected interplanar angles and intermolecular distances (°, Å).</caption><thead><tr><th>Structures</th><th>C<sub>5</sub>H<sub>4</sub>/C<sub>6</sub> (°)</th><th>C<sub>5</sub>H<sub>4</sub>/Amide (°)</th><th>Amide...Amide Interaction (Å)</th><th>O=C-N-C<sub>torsion</sub></th></tr></thead><tbody><tr><td>4a</td><td>73.16 (11)/79.12 (11)</td><td>64.77 (12)/68.16 (12)</td><td>2.924 (3)/2.869 (3)</td><td>-1.6 (4)/4.1 (4)</td></tr><tr><td>4d [14]</td><td>77.80 (8)</td><td>69.80 (10)</td><td>2.972 (3)</td><td>2.1 (3)</td></tr><tr><td>4e</td><td>81.24 (7)</td><td>68.55 (10)</td><td>2.819 (2)</td><td>-7.6 (3)</td></tr><tr><td>4f</td><td>11.34 (11)</td><td>57.31 (9)</td><td>2.892 (2)</td><td>3.0 (3)</td></tr><tr><td>5</td><td>61.66 (7)</td><td>34.08 (12)</td><td>2.853 (2)</td><td>-3.8 (3)</td></tr></tbody></table>

## 2.2. Methods: Hirshfeld Surface Analysis Details

Analyses of the Hirshfeld surface (ESI; Figure S10 and Table S3) [26] and contacts enrichment ratio were performed to analyse different types of intermolecular interactions using procedures and programs previously reported [26,27].

The fingerprint plots of contacts around the molecules in the four crystal packings of **4a**, **4e**, **4f**, **5** were computed with the CrystalExplorer17 program [26]. The contact statistics and enrichment ratios were obtained using the MoProViewer program (2024 version) [27]. In the case of compound **4a** (with Z' = 2), the Hirshfeld surface was computed around two independent molecules not in contact with each other in the crystal packing. In the contacts analysis, the charged HN hydrogen atoms bound to nitrogen were differentiated from the hydrophobic hydrogen HC atoms linked to a carbon.

In the asymmetric unit of the phenyl derivative **4a**, molecules A and B have similar conformations, and although PLATON [25] suggests space group C2/c, the results do not support this space group assignment (as noted in the following section and ESI). Geometric differences between molecules A and B can be demonstrated by the Fe1A-C11A-C2A-N1A dihedral angle = -179.48 (18)° and for Fe1B-C11B-C2B-N1B = 175.51 (19)°. This demonstrates a significant 4° angle difference (with a detailed explanation provided in the ESI). Additional differences can be noted between molecules A and B (e.g., amide···amide distances) in the following section on the crystal and molecular structure discussion of **4a**. The root mean square deviation of molecule A with the inverted B molecule is 0.10 Å and maximum atomic deviation is 0.16 Å.

## 2.3. Four Crystal and Molecular Structures

### N-(Ferrocenylmethyl)benzenecarboxamide structure (4a) with Z' = 2

The phenyl derivative **4a** crystallises with two molecules A and B (Z' = 2) in the asymmetric unit of space group P2₁/n (No. 14) (Scheme 1; Figure 1). Molecules A and B adopt similar conformations, and molecular similarities are demonstrated by their root mean square (rms) bond and angle deviations of 0.007 Å and 0.47° (Mercury = 0.10 Å) [25]. In the ferrocenyl moiety, the C₅ rings are almost eclipsed, as noted by the five HC···CH
torsion angles. Pairs of molecules aggregate in a hydrogen bonded cyclic arrangement by face-to-face $C_6 \cdots C_6$ stacked rings and C-H···π($C_5H_4$) interactions. The amide conformation is shown by $c_pC-C-N-C=O$ torsion angles of -80.9 (3)° in (A) and 79.0 (3)° in (B), as influenced by the $C_2A/B$ hinge, whereas the $C_5H_4/C_6$ interplanar angles are 73.16 (11)° and 79.12 (11)° (Table 2). Molecular differences are exemplified by the cross-molecule Fe1$_{A/B}$···C34$_{A/B}$ distances of 9.207 (3) Å and 9.147 (3) Å for A and B, respectively, differing by 0.06 Å. The packing index (KPI) = 71.3 [25].

The asymmetric unit in **4a** comprises A and B molecules in a cyclic arrangement, linked by two phenylC-H···π($C_5H_4$) interactions and oriented with their NHCOC$_6H_5$ groups parallel as a face-to-face $C_6 \cdots C_6$ pair (Figure 1b). Geometric data reveal the C-H···C$_5$ (centroid) as C34A-H34A···Cg4$^i$ [2.78 Å, 156°, 3.663 (3) Å] and C34B-H34B···Cg1$^i$ [2.84 Å, 158°, 3.744 (4) Å]; the $C_6 \cdots C_5H_4$ interplanar angles are 78.19 (11)° and 74.12 (11)°. For the NHCOC$_6H_5 \cdots C_6H_5$ CONH stacked pair, the shortest contacts are C31A···C33B = 3.401 (4) Å and C31B···C33A = 3.411 (4) Å, with an interplanar angle of 2.93 (17)°; the $C_6 \cdots C_6$ centroid distance = 3.8141 (18) Å [25]. The A/B molecular $C_5H_5$ interplanar angle within the asymmetric unit is 5.60 (10)°.

**4a** exhibits a wide range of intermolecular interactions comprising 1D intermolecular amide···amide chains, Fc(C-H)···C$_6$ ring, C-H···π($C_5H_4$) interactions, (1:1) face-to-face (phenyl) ring···ring and ($C_5H_5$) ring···ring pairs, where [Fc = (η$^5$-C$_5H_5$)Fe(η$^5$-C$_5H_4$)]). These interactions are noted including those in the A, B intricate molecular pair. The 1D amide···amide interactions form (A···A···)$_n$ and (B···B···)$_n$ chains along the *b*-axis direction with hydrogen-bonding distances N1A···O1A$^i$ = 2.924 (3) Å and N1B···O1B$^{ii}$ = 2.869 (3) Å (symmetry operations *i*, *ii*; ESI, Table S2). The amide···amide interaction between molecules of A is augmented by a $C_5H_4$ ring oriented side on and interacting with a symmetry related (C31A, ..., C36A)$^i$ phenyl ring as C12A-H12A···π(Cg3)$^i$; [2.73 Å, 163°, 3.652 (3) Å]. The asymmetric Fc-H···C$_6$ interaction in molecule A differs from that in B, where two FcC···C$_6$ manifest in side-on 2 × ($C_5H_4$)···C$_6$ contacts rather than as a single C-H···Cg (in A). The associated data are C15B-H15B···C31B$^{ii}$ [2.83 Å; 159°; 3.730 (4) Å] and C25B-H25B···C34B$^{ii}$ [2.84 Å; 156°; 3.730 (5) Å].

Both 1D undulating molecular chains (and interlinked by phenylC-H···π($C_5H_4$) interactions and (1:1) $C_6 \cdots C_6$ stacked pairs) generate a 1D column aligned along the *b*-axis and ½ × *c* cell length wide. Columns, comprising the (A···)$_n$ and (B···)$_n$ chains, are linked by face-to-face $C_5H_5 \cdots C_5H_5$ ring overlaps and paired into slightly ruffled sheets parallel to (2 0–5). The $c_pC \cdots C_{p2}$ contact separations are rather short, i.e., C23A···C24B$^{vx}$ = 3.303 (5) Å. The $C_5H_5 \cdots C_5H_5$ interplanar angle is 8.15 (19)°, and rings are moderately offset with a $C_5H_5$ ring centroid with Cg···Cg distance = 3.857 (2) Å. This is depicted by face-to-face $C_5H_5 \cdots C_5H_5$ and C-H···π weak H-bonds (Figure 1c). Sheets are linked by the remaining face-to-face $C_5H_4 \cdots C_5H_4$ contacts. In addition, ferrocene moieties interlock (using their C-Hs), and this is particularly notable for molecule A where the $c_p$ rings and Fc(C-H) form a dual ‘cog-like’ intermolecular aggregation.

The rmsd of molecular overlap between **4a** (molecule B or inverted A) and the **REY-WOU (4d)** structure (*N*--(ferrocenylmethyl)-*para*-fluorobenzene carboxamide is 0.3 Å [14]. Differences are primarily due to the orientation of the unsubstituted η$^5$-C$_5H_5$ cyclopentadienyl ring. As such, **REYWOU** will therefore be discussed primarily in terms of molecular interactions [14].

**N-(Ferrocenylmethyl)-4-fluorobenzenecarboxamide [REYWOU; 10], 4d [14]**

The molecular structure of **4d** has been described [14] (Figure 2) and salient features of the crystal structure are only provided herein for comparisons with **4a**, **4e**, **4f**, and **5**. It is noted that **4d** has a similar conformation to **4a** with $c_pC-C-N-C=O$ torsion angle of 89.6 (3)° as influenced by the $C_2A/B$ hinge; the $C_5H_4/C_6$ interplanar angle = 77.80 (8)°.
Amide···amide interactions dominate with molecules aligning in the *a*-axis direction (N1···O1<sup>xvi</sup> = 2.972 (2) Å). This H-bond is bifurcated, being augmented by *ortho*-C32-H32···O1<sup>xvi</sup> contacts (d<sub>C···O</sub> = 3.485 (3) Å), generating 1D chains incorporating R<sub>1</sub><sup>2</sup> (7) hydrogen bonded rings (symmetry operations as noted in Table S2, **ESI**). This type of H-bond aggregation is frequently observed in benzamide structures and 1D chains aggregate through a combination of intermolecular contacts [9].

<figure><img src="image_4.png" /><figcaption>**Figure 1.** a–c ORTEP diagrams of molecules A and B (**top**), together with the asymmetric unit (**centre**) in **4a**. A view of the *arene*C–H···π(**cp**) and **cp**C–H···(**arene**) interactions with the principal interacting H atoms coloured in **green** (**bottom**). The arrow highlights both the Fc···Fc stacking and Fc(**C–H**)···C<sub>6</sub> ring interactions.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Crystallographic autostereogram of an array of interactions in the (100) plane in 4d [REYWOU] [14] showing a relay of [Fe(C₅-H)···π(C₅H₄)Fe(C₅-H)···π(C₅H₄)···]n contacts and including the unit cell. The C₅, as indicated, highlights the capped (stacked) nature of the (1:1) C₅:C₆ (face-to-face) ring overlap.</figcaption></figure>

Molecules pack along the *b*-axis direction via an extensive combination of face-to-face C₅H₅···C₆ stacked pairs [interplanar angle = 8.45 (13)°], relays of C₅-H···π(C₅H₄) interactions [C21-H21···Cg1ₓvii; 2.75 Å, 176°, 3.680 (3) Å] and HC-H···C₅H₄ contacts [H2B···C13ₓvii = 2.79 Å] (Figure 2). Weak intermolecular contacts as F34···C1-N1ₓviii = 3.185 (2) Å, with an angle for C34-F34···C1ₓviii = 134.46 (15)° augment the interactions. In summary, key interactions involve amide···amide/C32-H32···O1ₓvi pairing along the *a*-axis direction and the interaction rich combination of (1:1) stacking and contacts along [010]. The zig-zag relay of [Fc(C₅-H)···π(C₅H₄)Fe(C₅-H)···]n contacts, with the C₅H₅ capped by a (1:1) stack with the C₆ ring, is notable and can also be denoted simply by (Fc-H···Fc-H···)n (Figure 2).

### N-(Ferrocenylmethyl)-2,6-difluorobenzenecarboxamide or 4e

In the structure of **4e** (Figure 3), intermolecular amide···amide hydrogen bonding generates 1D chains along the *a*-axis direction with N1-H1···O1ᵢᵢᵢ data as 2.00 (3) Å, 156 (2)°, 2.819 (2) Å (*iii* = ½ + x, ½-y, z; ESI, Table S2). The interactions arise in tandem with alkylC2-H2A···(C36-F36)ᵢᵢᵢ interactions [for C36; data are 2.72 Å, 164°, 3.679 (2) Å; ESI, Figure S4]. The 1D chains are linked by intermolecular C35-H35···O1ᵢᵢᵢ interactions (*iᵢ* = ½ - x, -½ + y, 1 - z), with the formation of the (C31, ..., C36) sandwich about inversion centres, with Cg3···Cg3ₓᵢᵢ = 3.5765 (12) Å; Cg3···C₆ₓᵢᵢ = 3.3773 (9) Å, (xᵢᵢ = 1 - x, -y, 1 - z; ESI, Table S2).

The *ortho*-fluorine F32 and F36 atoms do not partake directly in significant intermolecular interactions. However, F36 is positioned at 2.81 Å from H2A (0.15 Å longer than the sum of the van der Waals radii 1.20 + 1.47 Å) [25]. The C2-H2A···F36ᵢᵢᵢ angle = 139° and assisting the C2-H2A···C36ᵢᵢᵢ contact. Of interest is that the unsubstituted C₅H₅ ring does not form any face-to-face stacks with either of the C₅ or C₆ rings. On the other hand, there are two phenyl C-H···π interactions on one side of the C₅H₅. It can be stated that heterocycles are more likely to be involved in parallel stacking than carbocyclic rings [27,28]. Not surprisingly, the C₆H₃F₂ rings form face-to-face pairs about inversion centres and the closest C···C contact is C34···C36ₓᵢᵢ = 3.410 (3) Å and the centroid Cg3···Cg3ₓᵢᵢ distance is 3.5765 (12) Å.
<figure><img src="image_6.png" /><figcaption>**Figure 3.** ORTEP diagrams of 4e with displacement ellipsoids at the 30% probability level. A view of the face-to-face ring···ring stacked pair with tandem amide···amide and C-H···C interactions (interacting H atoms coloured in **dark green**). A general CPK view of the amide···amide (N1···O1) chain and crystal packing and showing the unit cell.</figcaption></figure>

Impressive, intricate stacking in N-(Ferrocenylmethyl)pentafluorobenzenecarboxamide

The pentafluorobenzene derivative (4f) (Figure 4) exhibits an array of intermolecular interactions that incorporate N-H···O=C hydrogen bonding as 1D chains in the c-axis direction in tandem with local (1:1) C₅H₄···C₆F₅ ring overlay between the substituted C₅H₄ and C₆F₅ rings. Chains crosslink, forming sheets via C-H···O/F contacts and tight C₅H₅···C₆F₅ ring stacking on the opposite side of the C₆F₅ ring. Sheets aggregate by C-H···F contacts and sheet formation involves all three carbocyclic rings (as 2 × C₅; C₆). The ferrocenyl (Fc) moiety is effectively sandwiched by C₆F₅ rings forming a 1D stack. Four C-H···F interactions per 4f molecule involve three of the C₆F₅ fluorine atoms as F32, F35, and F36, (symmetry codes *vi* to *ix*) with shortest C-H···F-C contact being C13-H13···F35*vi* = 2.55 Å for H···F (angle = 133°).

<figure><img src="image_7.png" /><figcaption>**Figure 4.** *Cont.*</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Figure 4.** (a) ORTEP diagram of 4f with displacement ellipsoids at the 30% probability level. Three CPK packing diagrams of the (Fc···C₆F₅···)ₙ 1D stacking along the a + c direction, (b) as a chain with amides (ball and stick), and the space where Fc moieties occupy, (c) an autostereogram of the stacking and including the unit cell, and (d) a view of the C₆F₅ groups in green and five C₆F₅ groups depicted as ball and stick.</figcaption></figure>

Overall, the impressive feature of the **4f** crystal packing is the intricate (ferrocene···C₆F₅···)ₙ alternating (Fc group)···ring overlap, where the Fc group and C₆F₅ ring alternate as (1:1)ₙ pairing in 1D columns that lie parallel to the (−1 0 1) plane (ESI, Figures S5–S8). Each **4f** molecule contributes the ferrocenyl and C₆F₅ to two distinct 1D columns that are reinforced by crosslinking amide···amide interactions (alternate explanation as above), (ESI, Figure S6). At a local level the C₆F₅CONH- moiety can be analysed further. Each amide···amide interaction facilitates formation of a compact, localised (1:1) face-to-face C₅H₄···C₆F₅ ring stacking (per amide···amide link). Therefore, the C₅H₄···C₆F₅ ring overlap as a (1:1) stacked pair arises in tandem with each amide···amide interaction as it propagates in the *c*-axis direction. The crystal structure
aggregation is enhanced as the remaining C₅H₅ ring (in each **4f** molecule) is critical in the formation of sheets with 1D columns of (Fc···C₆F₅···)ₙ ring stacking. Overall the (Fc···C₆F₅···)ₙ ring stacking is both elegant and efficient, where the Fc group is sandwiched between two C₆F₅ rings of **4f** molecules from different 1D chains. Diagrams are used to highlight and show how it manifests in the **4f** crystal structure (ESI, Figures S4–S7).

Table 3 highlights the geometric details of **4f** and stacking distances Cg1···Cg3<sup>v</sup> = 3.5720 (12) Å between the C₅H₄ (Cg1 centroid) and C₆F₅ rings (Cg3 centroid). The tighter Cg2···Cg3<sup>xx</sup> is 3.5399 (13) Å (Cg2 = C₅H₅), where Cg represents the ring centroids. The perpendicular centroid···ring distances are 3.2530 (10) Å and 3.3133 (8) Å, with three C···C distances < 3.40 Å [and the shortest is C25···C32<sup>xx</sup> = 3.319 (3) Å]. The C₅ and C₆ ring interplanar angle is 10.05 (12)°. For comparison with **4f** and of particular note is the **SUFLIR** crystal structure of a (1:1) co-crystal of ferrocene·C₆F₆ [5,10] and with thirteen datasets collected over a temperature range (from 100 K to 304 K). In Table 3, comparisons are made between **SUFLIR** at 100 K (KPI = 72.4 [25]) and the present study of **4f** (at 150 K).

<table><caption>**Table 3.** Comparative analysis of the Fc···C₆ stacking in **SUFLIR** [5] and **4f** (present work) [25].</caption><thead><tr><th>Geometric Data</th><th>SUFLIR (100 K) [5] (Å,°)</th><th>4f (Å,°)</th></tr></thead><tbody><tr><td>Fe···C<sub>5</sub> (centroid); (Å,°) <sup>a</sup></td><td>1.6467 (8), 1.6479 (8); 179.53 (4)</td><td>1.6482 (9), 1.6530 (10); 178.75 (6)</td></tr><tr><td>C<sub>5</sub>···C<sub>6</sub>F<sub>6</sub> [5] or C<sub>5</sub>···C<sub>6</sub>F<sub>5</sub> <sup>b</sup></td><td>3.5825 (10); 3.6052 (11)</td><td>3.5399 (13); 3.5720 (12)</td></tr><tr><td>Dihedral angle (planes)° <sup>c</sup></td><td>7.67(9); 8.86 (9) <sup>d</sup></td><td>2.76 (12); 2.88 (10)</td></tr><tr><td>C-F···C<sub>5</sub> ring centroid (Å)</td><td>3.5949 (13); 3.6210 (14)</td><td>3.5179 (16); 3.6620 (15)</td></tr><tr><td>C-F···C<sub>5</sub> ring centroid (Å)</td><td>3.3342 (18); 3.3370 (18)</td><td>3.257 (2); 3.377 (2)</td></tr><tr><td>C-F···C<sub>5</sub> (°)</td><td>68.05 (8); 67.13 (8)</td><td>67.79 (10); 67.22 (9)</td></tr><tr><td>Shortest stack cpC···C<sub>arene</sub></td><td>3.310 (2), 3.337 (2), 3.340 (2)</td><td>3.319 (3), 3.327 (3), 3.387 (3)</td></tr><tr><td>Shortest stack cpH···F-C</td><td>3.089, 3.167, 3.176, 3.226</td><td>3.21, 3.29, 3.29, 3.35</td></tr></tbody></table>

Structural differences between **SUFLIR** and **4f** (as noted in Table 3) are relatively small [25]. Distances between stacked rings (from the centroid···centroid for C₅, C₆F₆) are 0.03–0.04 Å shorter in **4f**, while the interplanar (dihedral) angles are closer to co-planarity and differing by 5–6° from **SUFLIR** [5]. The reason for the differences may be more steric than electronic and influenced by the carboxamide···carboxamide hydrogen bonding in **4f** and short contacts involving the methylene CH₂ group H2B hydrogen atom. The C-F···C₅ geometric data are more symmetrical in **SUFLIR** [5], but with shorter contact distances in **4f**. However, the cpC-H···F-C distances are shorter in **SUFLIR** (by 0.05–0.10 Å) for their three shortest H···F contacts and this may be influenced to an extent by differences in data collection temperatures (100 vs. 150 K).

In the stacking behaviour of **SUFLIR** [5] and **4f**, it is important to consider factors that influence their packing. Benzene (−33.3 ± 2.1) [29] and Ferrocene (−30 ± 7) [30] have large negative molecular quadrupole moments (QM), expressed in 10<sup>−40</sup> C m². This contrasts with hexafluorobenzene (C₆F₆) which has a moment of similar magnitude but with a positive value as determined experimentally as +31.7 (±1.7) × 10<sup>−40</sup> C m² [29]. Therefore, it can be evaluated that the quadrupole moments for the Fc-C and C-C₆F₅ moieties are relatively similar in magnitude but opposite in sign. A guideline estimate of quadrupole moments are ca. −20 and +20 × 10<sup>−40</sup> C m<sup>−2</sup> for the Fc-C and C-C₆F₅ moieties, respectively [29–31]. Hence, the observation for **4f** that favourable ring stacking occurs with conformational change and in tandem with amide···amide directed hydrogen bonding in

<footnote><sup>a</sup> Ferrocene C₅ ring geometric data (distances in Å; angle °). <sup>b</sup> Defined as centroid···centroid (Cg) distances (Å) for the C₅ or C₆ rings. <sup>c</sup> Dihedral angle between the Fc C₅ ring and C₅···C₆F₆ [5] or C₅···C₆F₅ present work ring planes [25]. <sup>d</sup> As 7.8° and 9.1° using Mercury [5,24].</footnote>
the crystal state. In **4f**, the methylene -CH₂- spacer group facilitates conformational change and the ability to form interweaved 1D stacked columns using the ferrocenyl moiety (Fc) and the pentafluorobenzene C₆F₅ rings.

**N-(Ferrocenylethyl)-4-fluorobenzene-carboxamide or 5.**

The molecular conformation of **5** (Figure 5) (with an ethylene -CH₂CH₂- bridge as compared to the methylene -CH₂- bridge for the **4x** series) is broadly similar to the pentafluorobenzene derivative **4f**. Having the additional -CH₂- spacer group provides extra molecular flexibility. In terms of molecular conformation, the C11-C3-C2-N1 torsion angle is 176.19 (19)° and C3-C2-N1-C1 is 81.0 (3)°. The ferrocenyl C₅ rings are effectively eclipsed and with all five H-C···(C)H < 0.3°, whereas in **4f** the C₅ rings deviate from being fully eclipsed by ~18°.

Amide···amide interactions dominate as N1-H1···O1x forming 1D chains along the *b*-axis direction. These chains are linked about inversion centres by C21-H21···F34<sup>xii</sup> contacts (**ESI**; Table S2). The F34 atom also engages in a weak contact as C25-H25···F34<sup>xi</sup> = 3.554 (3) Å (Table S2); with two C-H···F34 forming a bifurcated hydrogen-bonding system. In addition, compact ring···ring pairing about inversion centres involves the aromatic C₆ groups and with shortest C31···C32<sup>xii</sup> contact distance of 3.451 (3) Å. In addition, weak C-H···π(arene) contacts form as C36-H36···C32<sup>x</sup> (H36···C32<sup>x</sup> = 2.82 Å; C36-H36···C32<sup>x</sup> = 173°) with C33-H33···O1<sup>xiii</sup> interactions. Of further structural interest is that ferrocene carboxamide structures containing an ethylene (-CH₂-CH₂-) group or even just a two-carbon atom chain as a linker group are relatively rare in the CSD [10].

<figure><img src="image_9.png" /><figcaption>**Figure 5.** *Cont.*</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Figure 5.** (a) ORTEP diagram of the molecular structure of 5 with displacement ellipsoids at the 30% probability level. Structure 5 differs from 4d [REYWOU] by an extra -CH₂- group. (b) An ORTEP diagram of 5 showing the amide···amide 1D chains along the *b*-axis direction. (c) Two identical views (ORTEP, CPK) of a molecular pair of 5 highlighting the arene ring···ring stacking and C-H···F hydrogen-bonding interactions.</figcaption></figure>
## 2.4. Hirshfeld Surface Analysis: Contacts Enrichment Analyses of Five Structures (4a, 4d-f, and 5)

MoProViewer software (version 2024) [27] was used to investigate the intermolecular interactions and the contacts enrichment on the Hirshfeld surfaces of the five molecules (4a, 4d-f, and 5) in their crystal packings [26]. The enrichment values ($E_{XY}$) were obtained as the ratio between the proportions of actual contacts ($C_{XY}$) and equiprobable (random) contacts ($R_{XY}$), the latter being obtained via probability products ($R_{XY} = S_X \times S_Y$), where $S_X$ is the surface content in chemical species X. Contacts X...Y, which are over-represented with respect to the share of X and Y chemical species on the Hirshfeld surface, have enrichments larger than unity. The chemical nature of the contacts and their enrichment data values in the crystal structures are listed in the **ESI** (Table S3).

In all five crystal packings, the strong N-H···O=C hydrogen bond between amide groups has extremely high enrichment ratios $E_{O,H_n}$ greater than 14.8, as it constitutes by far the most attractive electrostatic interaction (**ESI**, Table S3). The fingerprint plots (**ESI**, Figure S10) display all two spikes at short distance corresponding to this N-H···O=C hydrogen bond. The hydrophilic content (O and $H_N$) on the surface is always below 7%. As a consequence of the high hydrophobic content in $H_C$ and C atoms, the most abundant contacts in all cases (except in the fluorine rich **4f**) are C···$H_C$ followed by $H_C···H_C$ hydrophobic contacts. All of the fingerprint plots show abundant C···H and H···H contacts (**ESI**, Figure S10). The next most abundant contacts in the crystal structures of **4a**, **4d**, **4e**, and **5** are the C···C and the F···$H_C$ weak hydrogen bonds in **4d**, **4e**, and **5**. The H···F contacts show spikes within the fingerprint clouds at the shortest distances of $d_i = 1.2/d_e = 1.4$ Å. In all structures, with the exception of the penta-fluorobenzene derivative **4f**, the enrichments of the hydrophobic contacts C···C, $H_C···H_C$, and C···$H_C$ do not deviate much from the unitary ratios (in the range $0.83 < E < 1.23$).

The penta-fluorinated compound **4f** is peculiar in that the C···C contacts are significantly enriched (with $E_{CC} = 2.2$) due to the (Fc···C$_6$F$_5$···)$_n$ 1D ring···ring stacking. The C$^\delta$-H$^{\delta+}$ and C$^{\delta+}$F$^{\delta-}$ groups in the two carbocyclic rings form attractive electrostatic interactions. The fluorine content on the surface is 23.9% and with an enrichment value of 1.54. Therefore, the F···$H_C$ weak hydrogen bonds turn out to be the most abundant contact type in the penta-fluorinated **4f** crystal structure. The F···F contact is 3.9% in **4f**, whereas the other four structures **4a**, **4d**, **4e**, and **5** (which contain less fluorine atoms) have this value at 0. On the other hand, the C···$H_C$ weak hydrogen bonds are quite under-represented in **4f** (with $E = 0.49$), presumably due to competition of the slightly stronger F···$H_C$ interactions. The fingerprint plots for **4f** are depicted in Figure 6 and can be compared with the other four crystal structures of **4a**, **4d**, **4e**, and **5** (**ESI**; Figure S10).

<figure><img src="image_11.png" /><figcaption>**Figure 6.** *Cont.*</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Figure 6.** Fingerprint plots of the main contacts on the Hirshfeld surface of 4f [26].</figcaption></figure>

## 2.5. Comparisons with Ferrocene Derivatives That Exhibit Distinct Aggregation and Packing

Analysis of the Cambridge Structural Database was performed (with ca. 400 structures retrieved in a search as Fc\_C₆F₁) for structures similar to 4f and C₆F₆·FcCp₂ (SUFLIR) [5]. The search demonstrates that the vast majority of ferrocene derivatives incorporating at least one aromatic fluorine (as C₆F) **do not** stack in extended columns but rather are 'capped' in a variety of ways by either C-H···π(arene), C-F···(cp) contacts or (1:1) face to face pairing of aromatic rings [10]. There are several notable examples of a diverse range of packing types involving ferrocene groups either on their own or in combination with other rings. These structures include GEHNAJ (ferrocene·decafluorobiphenyl) [6], LUZJIB (ferrocene·decafluorophenanthrene) [32], SUFLIR01-09 (ferrocene·hexafluorobenzene) [5,10].

The crystal structure of UNEWES reveals slight undulating 1D chains incorporating alternating ferrocenyl Fc···C₆FH₄ pairs sandwiched and linked between C-H···π(cp) interactions (ESI, Figure S9) [33]. At the other extreme, in the crystal structure of UY-IHER or 3-ferrocenyl-1-(4-fluorophenyl)-5-methyl-1H-pyrazole, zigzag chains of ferrocenyl moieties are linked solely by C-H···π(cp) interactions [34]. In ADIQAF or rac-1,1'-bis(pentafluorophenyl)-3,3'-di-t-butylferrocene the pentafluorobenzene ring forms a 1D column [35], whereas in LUZJIB, the decafluorophenanthrene (X) forms stacks as (Fc···X···X···)n [32]. Another structural example is that of FANNUE (01) (ESI, Figure S9) where the ferrocene stacks and interacts with a hexakis(μ₂-3,4,5,6-tetrafluorobenzene-1,2-diyl)Hg₆ (L) complex such that the structure consists of (L···ferrocene···L···)n stacking [36]. In QAPYAI [37] the Fc moiety and a pyrrotriazole ring form a stack along the b-axis direction and this aggregation arrangement is reminiscent of the 4f crystal structure.

A related search of the CSD [10] was undertaken specifically for ferrocene centroid...C₆ ring centroid distances that are <3.60 Å, with C₅/C₆ interplanar angles < 15° (arbitrary values) and with optional fluorine atoms. This CSD search reveals several structures including MANRIC, SAMTEH, APUJIH, QIGRUW, and VIBSEF [10,38]. The vast majority of these structures exhibit a short range stack (depending on the structures and symmetry) and are usually capped at either end by C-H···π(arene) contacts, thereby preventing 1D stack formation. In the 1D stack of MANRIC, the unsubstituted C₅ and C₆ ring [C14, ..., C19] centroids are separated by 3.457 (2) Å with an interplanar angle of 4.22 (18)°, arising in a centrosymmetric stacked dimer that stacks with symmetry related dimers [38]. Dimers link into 1D chains by overlapping rings with interplanar distances of 3.4154 (8) Å. Much research is on-going and as this is not a review, a more extensive analysis will be published at a future date. Of additional note is that Horie and co-workers have studied ferrocene ring rotation in interlocked molecules in single crystals [39].

Competition between hydrogen and halogen bonding in intermolecular interactions continues to attract much attention, aiding in our understanding of intermolecular interactions and aggregation in molecular crystals [40–45]. In this present structural study, we
show the roles that ferrocenyl moieties and fluorinated benzene rings play in aggregation and stacking [5,10]. Several recent publications have compared series of related structures in terms of the effects of the different types of interactions that can arise with substitution pattern [41–45]. Understanding the competition between intermolecular processes in structures and the profound effects that they can have in aggregation and packing where compounds differ as isomers, or by one atom substitution has been explored in halogenated benzamides [42], carboxamides [43], isophthalamides [44], and Schiff bases [45]. The future of this structural research will expand with even more expansive studies (and resulting structural data) on how substitution patterns influence structure together with the range and types of competition between intermolecular interactions [10].

**3. Conclusions and Future Work**

In the five ferrocenyl carboxamide crystal structures (including **REYWOU 4d** [14]), the structural effects on molecular geometry and conformations are assessed in structural packing for the phenyl **4a**, *para*-fluorobenzene **4d**, 2,6-difluorobenzene **4e**, and the penta-fluorobenzene derivatives **4f**. Competition between amide···amide hydrogen bonding and various types of ring···ring stacking, as well as weaker interactions such as C-H···O and C-H···π(arene), is analysed and comparisons are made [46–50].

Stacking of C₆ rings of the mono- and di-fluorinated rings with ferrocenyl groups does not dominate in the crystal packing due to competition with stronger hydrogen-bonding interactions. However, additional fluorine atoms on the aromatic ring promote 1D ring stacking. The additional F atoms on a benzene ring system push the molecular quadrupole from benzene as negative (−) towards that of the positive (+) hexafluorobenzene molecule [30,31]. This can induce a favourable stacking arrangement with ferrocenes or ferrocenyl moieties (having a negative molecular quadrupole). With the possibility of ferrocenes stacking relatively easily with penta-fluorobenzene-based systems and with a favourable stacking conformation, it may be that there is a strong possibility that trifluoro- or tetra-fluorobenzenes (of whichever F atom substitution pattern) may be enough to influence and participate in stacking patterns such as the tight 1D stacking observed in **4f**.

We recognise that a rich, diverse range of fluoro-derived ferrocenes is available and are examining incorporation of fluoroaromatics into various ferrocenyl moieties or through the formation of co-crystals with ferrocene [5,10,51]. The ability of trifluoro- and tetrafluorinated aromatic rings is under examination to study the structural diversity types and tipping points towards capped or 1D stacked structures. This requires a large number of closely related crystal structures so that in-depth comparisons can be made [10]. The present series represents a basis for the development of a general class of fluorinated ferrocenyl carboxamides [14]. There is a vast research space for exploring combinations with halogenated aromatics with chlorine, bromine, and iodine [10,52–56]. Recent structures with interesting interactions and packing include **ZOHFIP** [52], **IDUGAT** [53], **WUMWEJ** [54], **APUJON** [55], and **UYUPEM** [56].

**Supplementary Materials:** The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst15040299/s1, Scheme S1: The four ferrocene crystal structures **4a**, **4e**, **4f**, and **5**, together with the previously reported **4d** structure [REYWOU] [10,14]; Table S1: Crystallographic Experimental details; Table S2: Selected hydrogen-bond parameters in **4a**, **4e**, **4f** and **5**; Figure S1: A view of the asymmetric unit (Z' = 2) highlighting differences between the A and B molecules in the crystal structure of **4a**. These applies to distances, torsion angles and amide···amide hydrogen bonding where differences of *ca.* 0.05–0.06 Å and 4–5° are noted between A and B; Figure S2: A view of the interactions in **4a**. The central cyclic hydrogen bonded A and B molecules with key H atoms in black with the (1:1) (C₅H₅) ring···ring* overlap depicted by the CPK atoms as noted in the diagram (left) and with all atoms as a CPK model (right); Figure S3: A
cross-section of the multitude of interactions that are present in **4d** or **REYWOU** [14]. The ferrocenyl moieties are depicted in dark green to highlight the relay of interactions; Figure S4: A view of the amide···amide interaction in **4e** together with the H2A···(C36-F36)³³ contact (ball and stick model: left) and with atoms as their van der Waals spheres (right); Figure S5: Views of the molecular chain generated by the **4f** molecules. 1D molecular zigzag chains as generated by the [C₅H₅···C₆F₅] ring pairs are shown and parallel to (-101) (in tandem with the amide···amide interaction); Figure S6: Three **4f** molecules in green have a C₆F₅ group from symmetry related molecules inserted between their ferrocenyl groups and the three **4f** have their C₆F₅ groups effectively forming a 'sandwich' with the ferrocenyl moieties of the molecules in red. This stacking propagates through the **4f** structure; Figure S7: Four views of a 1D chain involving seven molecules of **4f**. The fluorine atoms are depicted in light green colour on the C₆F₅ ring with a ferrocenyl moiety sandwiched between two C₆F₅ rings; Figure S8: A view of the stacking with two 1D stacks indicated by arrows in **4f**. One stack is effectively contained within the (-101) and (-202) planes. A similar CPK view (atoms as their van der Waals spheres) with molecules involved in one column highlighted in red and blue and how they contribute to the other neighbouring columns and molecules in grey; Figure S9: Notable structures as **SUFLIR** [5], **FANNUE**, **LOTCUS** and **UNEWES** [10]; Figure S10: Fingerprint plots of the main contacts on the Hirshfeld surface of the five compounds (CrystalExplorer21 program); Table S3: Contacts Enrichments Data.

**Author Contributions:** Conceptualization, J.F.G.; Methodology, C.J. and P.T.M.K.; Software, J.F.G., C.J. and A.J.L.; Validation, J.F.G., C.J. and A.J.L.; Formal analysis, P.T.M.K. and A.J.L.; Investigation, C.J. and P.T.M.K.; Data curation, J.F.G. and A.J.L.; Writing—review & editing, J.F.G.; funding acquisition, J.F.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was funded by PRTLI-III programme (Ireland). John F. Gallagher thanks the Faculty of Science and Health in Dublin City University for research funding.

**Informed Consent Statement:** All ethical guidelines in the experiments have been adhered to.

**Data Availability Statement:** CCDC (Deposition Number 2409147–2409150) contains the Supplementary Crystallographic Data for this paper. The CIF data can be obtained free of charge via https://www.ccdc.cam.ac.uk/ (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: deposit@ccdc.cam.ac.uk). The data are available as CIF files from the corresponding author Professor John F. Gallagher (Dublin City University).

**Acknowledgments:** We thank Paula Kelly for initial technical support. We especially pay tribute to Peter Kenny, a colleague with whom we shared many hours of witty conversation and laughter in The 'Slipper', Mulligans of Poolbeg street, discussing a variety of topics such as the Borg collective, the Baby's arm, Glasgow Celtic, Chelsea, Guinness, Pipes, Tobacco flavours, sign removal, and the infamous Cork '2000' incident, to name but a few....

**Conflicts of Interest:** The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the research work as reported in this scientific paper.

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