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($\mu$2-$\eta$4-N-(2-Butynyl)phthalimide)(hexacarbonyl)dicobalt

Isabelle Jourdain, Michael Knorr, Tom Charenton, Carsten Strohmann,
Jan-Lukas Kirchhoff, Mohamed Othman

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Isabelle Jourdain, Michael Knorr, Tom Charenton, Carsten Strohmann, Jan-Lukas Kirchhoff, et al.. (μ2-η4-N-(2-Butynyl)phthalimide)(hexacarbonyl)dicobalt. Molbank, 2023, 2023 (1), pp.M1545. 10.3390/M1545.10.3390/M1545 hal-03943146

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Short Note

(μ₂-η⁴-N-(2-Butynyl)phthalimide)(hexacarbonyl)dicobalt

Isabelle Jourdain ¹,* , Michael Knorr ¹,* , Tom Charenton ¹, Carsten Strohmann ², Jan-Lukas Kirchhoff ²
and Mohamed Othman ³

¹ Institut UTINAM UMR 6213 CNRS, Université de Franche-Comté, 16 Route de Gray, 25030 Besançon, France

² Anorganische Chemie, Technische Universität Dortmund, Otto-Hahn Straße 6, 44227 Dortmund, Germany

³ URCOM, UNILEHAVRE, Normandie University, FR3021, EA 3221, 25 Rue P. Lebon, 76058 Le Havre, France

* Correspondence: isabelle.jourdain@univ-fcomte.fr (I.J.); michael.knorr@univ-fcomte.fr (M.K.);
Tel.: +33-3-8166-6270 (M.K.)

**Abstract:** The reaction of [Co₂(CO)₈] with an equimolar amount of the internal alkyne N-(2-butynyl)phthalimide (1-Phthalimido-2-butyne) 1 in heptane solution yields the title compound [Co₂(CO)₆(μ-phthalimidoCH₂C≡CMe)] 2. Compound 2 has been characterized using IR, ¹H and ¹³C NMR spectroscopy; the tetrahedrane-type cluster framework has been ascertained using a single-crystal X-ray diffraction study performed at 100 K.

**Keywords:** cobalt; carbonyl; triphenylphosphite; crystal structure; molecular cluster

# 1. Introduction

Since the first reports dating from the 1950s on the reaction of dicobalt octacarbonyl with alkynes yielding dicobaltatetrahedranes [Co₂(CO)₆(μ-RC≡CR')][1,2], there is nowadays a plethora of articles dealing with this organometallic reaction [3]. The interest in this research is not only driven by merely synthetic aspects, i.e., spectroscopic and structural characterization of these cage-like species, but also by the use of these compounds as precursors for further modifications in material sciences [4] and organic syntheses [5,6]. An important example for the latter domain is the Pauson-Khand reaction, in which [Co₂(CO)₈] reacts with an alkyne, an alkene, and carbon monoxide to form an α,β-cyclolopentenone [7–11]. In addition to the applications using dicobaltatetrahedranes as synthetic tools, this class of dinuclear organometallic compounds is emerging in bio-organometallic chemistry [12–21]. Some examples of dicobaltatetrahedranes, ligated using terminal functionalized alkynes and displaying biological activity, are depicted in Scheme 1.

Among the organic heterocyclic compounds showing a promising biological and medicinal activity, there are also some representatives based on the phthalimido scaffold [22–24]. In 2005, Gust et al. also reported the reaction of propargyl phthalimide with [Co₂(CO)₈] and determined the cytotoxicity and DNA binding efficiency of the resulting complex N-(2-propynyl)phthalimide]hexacarbonyldicobalt (see compound V of Scheme 1) [21]. In the context of our research on Co-Co carbonyl complexes towards various alkynes producing dicobaltatetrahedranes [25,26], we attempted to synthetize a Co-Co complex ligated using an internal alkyne, namely 1-Phthalimido-2-butyne 1, for upcoming biological studies. Apart from the purely synthetic aspect, one of the objectives was to compare the impact of the replacement of a terminal alkyne by an internal one (Phtal-C≡CH vs. Phtal-C≡CMe).

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**Citation:** Jourdain, I.; Knorr, M.; Charenton, T.; Strohmann, C.; Kirchhoff, J.-L.; Othman, M. (μ₂-η⁴-N-(2-Butynyl)phthalimide) (hexacarbonyl)dicobalt. *Molbank* **2023**, *2023*, M1545. https://doi.org/10.3390/M1545

Academic Editor: Rodrigo Abonia

Received: 8 December 2022

Revised: 26 December 2022

Accepted: 6 January 2023

Published: 11 January 2023

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**Copyright:** © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
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4.0/).
<figure><img src="image_5.png" /><figcaption>**Scheme 1.** Examples of some dicobaltatetrahedranes displaying biological activity (I, III–V: antipro-liferative activity against cancer cells; II: hormonally active compound).</figcaption></figure>

**2. Results and Discussion**

The title compound **2** was obtained via treatment of [Co₂(CO)₈] with an equimolar amount of *N*--(2-butynyl)phthalimide **1** in heptane solution at 60 °C as shown in Scheme 2. Upon cooling, small orange-red needle-shaped crystals of air-stable **2** were isolated, and elemental analysis confirmed their composition as [Co₂(CO)₆(µ-phthalimidoCH₂C≡CH)].

<figure><img src="image_6.png" /><figcaption>**Scheme 2.** Synthesis of the title compound 2.</figcaption></figure>

The IR spectrum of this product in cyclohexane, shown in Figure 1, reveals the signals of terminal carbonyl with intense ν(CO) vibrations at 2093, 2055, and 2028 cm⁻¹. In addition, two further absorptions at 1775 and 1727 cm⁻¹ are attributed to the carbonyls' stretching modes (symmetric and antisymmetric) of the imide function [27]. These values fit well with those measured in cyclohexane for [Co₂(CO)₆(µ-phthalimidoCH₂C≡CH)] at 2094, 2057, and 2030 as well as 1775 and 1727 cm⁻¹ (Figure S1). The latter complex has already been described and characterized using IR spectroscopy in solid state as KBr pellets (2095, 2054, 2036, 2016 (Co-CO); 1712 (C=O) cm⁻¹) [21]. The infrared spectrum of **2** in the solid state was also recorded in attenuated total reflectance (ATR) mode and is presented in the Supplementary Materials (Figure S2). Despite being a monocrystalline solid, the carbonyl absorptions were not fully resolved (2093, 2052, 2030, and 2008 as well as 1766 and 1708 cm⁻¹).
<figure><img src="image_7.png" /><figcaption>**Figure 1.** IR spectrum of compound 2 recorded in cyclohexane.</figcaption></figure>

The ¹H-NMR recorded in CDCl₃ reveals a singlet at δ 2.65 due to the acetylenic methyl group; a further singlet at δ 5.03 is attributed to the enantiotopic methylene hydrogens. The remaining resonances at δ7.74 and 7.88 are assigned to the aromatic cycle (Figure 2). In the proton-decoupled ¹³C-NMR spectrum depicted in Figure 3, only a broadened resonance centered at δ 199.3 could be observed for the six Co-bound carbonyl groups, suggesting a fluxional behavior of the COs. In contrast, the resonance of the two chemically equivalent imido C=O carbonyl groups at δ 167.9, the two acetylenic carbons at δ 93. 2 and 91.8, as well as those of the CH₂ and CH₃ carbons gave rise to well-resolved signals.

<figure><img src="image_8.png" /><figcaption>**Figure 2.** ¹H-NMR spectrum (400 MHz, CDCl₃) of compound 2 at 25 °C. The asterisk * denotes the chloroform resonance.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Figure 3.** $^{13}$C({^1}H)-NMR spectrum (100.62 MHz, CDCl$_3$) of compound 2 at 25 °C. The asterisk * denotes the CDCl$_3$ resonance.</figcaption></figure>

In addition to the spectroscopic characterization of 2 in solution, the complex was also unambiguously analyzed using X-ray diffraction performed at 100 K. Complex 2 crystallizes in the triclinic space group *P*-1; the asymmetric unit contains two independent molecules with slightly different bond lengths and angles. As shown in Figure 4, in this organometallic species the former alkyne is almost orthogonally µ<sub>2</sub>-η<sup>4</sup>-coordinated with respect to the two crystallographically non-equivalent Co1 and Co2 centers, which are linked through a metal–metal bond of 2.4688(14) Å.

<figure><img src="image_10.png" /><figcaption>**Figure 4.** Molecular structure of one of two independent molecules of 2. All H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Co1–Co2 2.4688(14), Co1–C2 1.980(6), Co1–C3 1.952(6), Co2–C2 1.984(6), Co2–C3 1.954(7), Co1–C16 1.812(7), Co1–C17 1.784(8), Co1–C18 1.831(7), Co2–C13 1.794(7), Co2–C14 1.812(7), Co2–C15 1.805(7), C2–C3 1.323(9); Co1–Co2–C3 50.75(15), Co2–Co1–C2 50.83(19), Co1–C2–Co2 77.0(2), Co1–C3–Co2 78.4(2).</figcaption></figure>
The resulting tetrahedral skeleton, reminiscent of that of organic tetrahedranes (https://en.wikipedia.org/wiki/Tetrahedrane, accessed on 26 December 2022), allows for considering this 40-electron species an organometallic cluster of dimetallatetrahedranes, in which two $Co(CO)_3$ units of the parent cluster $[Co_4(\mu_2-CO)_3(CO)_6]$ are replaced by isolobal C-R fragments, forming the edges of the cluster core. The former alkyne unit lost its linearity; the torsion angle C1-C2-C3-C4 changed to -3.1(18)°. The coordination sphere around each Co atom is completed by three terminal carbonyls. The mean Co-C bond length of the four pseudo-equatorial carbonyls is somewhat longer than that of the two pseudo-axial C=O ligands (1.815(7) vs. 1.789(7) Å). The C2-C3 bond distance of **2** is much shorter than that of a C-C single bond, but considerably elongated with respect to the C≡C bond reported for [phthalimido$CH_2C≡CCH_2phthalimido] (CSD refcode ECUFOB) (1.323(9) vs. 1.191(4) Å) [28].

Although there are numerous examples of crystallographically characterized hexacarbonyldicobalt compounds which are $\mu_2-\eta^2,\eta^2$ capped by an internal alkyne, a survey of the CSD data base revealed only two other examples containing a $-CH_2-C≡C-Me$ motif as is present in **2**, namely (trans-5,8,8-trimethyl-trans-1,4,5,7-tetrahydro-cis-4-(2-butyныл)bicyclo(5.1.0)octan-3-one)(hexacarbonyl)dicobalt (A) and (pent-3-yn-1-ol)(hexacarbonyl)dicobalt (B) (Figure 5) [29,30]. The most relevant averaged bond lengths (comprising molecules 1 and 2) of **2** are listed in Table 1 and compared with those of compounds A and B (averaged bond lengths).

<figure><img src="image_11.png" /><figcaption>**Figure 5.** Presentation of the two structurally characterized dicobaltatetrahedranes (A) and (B) incorporating a $-CH_2-C≡C-Me$ motif.</figcaption></figure>

<table><caption>**Table 1.** Comparison of relevant bond lengths (Å) and angles (°) in **2** and the crystallographically characterized $[Co_2(CO)_6(\mu-RCH_2C≡CCH_3)]$ tetrahedranes A and B.</caption><thead><tr><th></th><th>2</th><th>A</th><th>B</th></tr></thead><tbody><tr><td>Co-Co</td><td>2.4679(14)</td><td>2.4688(9)</td><td>2.4690(9)</td></tr><tr><td>C-C<sub>alkyne</sub></td><td>1.330(9)</td><td>1.325(6)</td><td>1.326(6)</td></tr><tr><td>C<sub>alkyne</sub>-C<sub>Me</sub></td><td>1.481(9)</td><td>1.486(6)</td><td>1.492(6)</td></tr><tr><td>C<sub>alkyne</sub>-C<sub>H2</sub></td><td>1.499(9)</td><td>1.500(7)</td><td>1.493(6)</td></tr><tr><td>Co-C<sub>alkyne</sub></td><td>1.972(6)</td><td>1.974(5)</td><td>1.972(4)</td></tr><tr><td>Co-C<sub>pseudo-equatorial</sub></td><td>1.815(7)</td><td>1.823(5)</td><td>1.820(5)</td></tr><tr><td>Co-C<sub>pseudo-axial</sub></td><td>1.797(7)</td><td>1.785(6)</td><td>1.787(5)</td></tr><tr><td>CSD reference</td><td>This work</td><td>DOTKEC [29]</td><td>HUKCIA [30]</td></tr></tbody></table>

In the molecular cell of **2** with Z = 4, there are no intermolecular contacts deserving any discussion (Figure 6).
<figure><img src="image_12.png" /><figcaption>**Figure 6.** View of the molecular cell of **2** showing the two pairwise arranged independent molecules.</figcaption></figure>

**3. Materials and Methods**

$N$-(2-butynyl)phthalimide 1 was commercially purchased from Aldrich. $^1$H- and $^{13}$C-NMR spectra were recorded on a Bruker AC 400 (Bruker, Wissembourg, France) at 400 and 100.62 MHz, respectively, using $CDCl_3$ as solvent. Infrared spectra were recorded on a Vertex 70 spectrometer (Bruker, Wissembourg, France) in ATR mode or in solution.

($\mu_2$-$\eta^4$-$N$-$(2$-butynyl)phthalimide)(hexacarbonyl)dicobalt: Alkyne 1 (199.2 mg, 1 mmol) was added to a stirred solution of $Co_2(CO)_8$ (342.0 mg, 1 mmol) in heptane (10 mL). An immediate gas evolution was observed. The reaction mixture was then heated to 60 °C for 5 h. The solution was cooled to room temperature prior to lowering its temperature to 4 °C. Product 2 crystallized as orange-red needles which were collected via filtration. Yield: 78%. Anal. Calc. for $C_{18}H_{9}Co_{2}NO_{8}$ (M.W = 485.14 g.mol$^{-1}$): C, 44.56; H, 1.87; N, 2.89%. Detected: C, 44.64; H, 1.92; N, 2.98 %. $^1$H-NMR ($CDCl_3$) at 298 K: $\delta$ 2.65 (s, $CH_3$), 5.03 (s, $NCH_2$), 7.74 (br, CH Ar), 7.88 (br, CH Ar) ppm. $^{13}$C{$^1$H}-NMR ($CDCl_3$) at 298 K: $\delta$ 20.3 ($CH_3$), 39.7 ($NCH_2$), 91.8 ($Cq$), 93.2 ($Cq$), 123.6 (CH), 132.0 ($Cq$), 134.4 (CH), 167.9 (C=O), 199.3 (br, CO) ppm.

Since the grown single-crystals of 2 used for the determination of the crystal structure were quite small, $CuK_\alpha$ radiation was employed instead of $MoK_\alpha$ radiation. Moreover, the crystals were twinned. The B-alert found in the checkcif is due to the high electronic residual around the Co–Co bonds, which could not be further refined due to the low resolution at 138.0°.

Crystal data for $C_{18}H_{9}Co_2NO_8$, $M = 485.12$ g.mol$^{-1}$, orange-red needles, crystal size 0.377 x 0.194 x 0.15 mm$^3$, triclinic, space group $P\bar{1}$: $a = 7.4317(4)$ Å, $b = 13.4351(7)$ Å, $c = 19.2794(11)$ Å, $\alpha = 78.462(3°)$, $\beta = 84.011(3°)$, $\gamma = 89.156(3°)$, $V = 1875.73(18)$ Å$^3$, $Z = 4$, $D_{calc} = 1.718$ g/cm$^3$, $T = 100$ K, $h = -8 \le h \le 8$, $k = -15 \le k \le 16$, $l = 0 \le l \le 16$, $GOF = 1.077$, $R_1 = 0.0855$, $wR^2 = 0.2428$ for 6962 reflections with $I > = 2\sigma$ (I) and 11958 independent reflections. Largest diff. peak/hole/e Å$^{-3}$ 1.35/-1.02. The structure was solved using direct methods and refined using full-matrix least-squares against $F^2$ (SHELXL, 2015 [31–33]).

Data were collected using graphite-monochromated $CuK_\alpha$ radiation $l = 1.54178$ Å and were deposited at the Cambridge Crystallographic Data Centre as CCDC 2219351.
(Supplementary Materials). The data can be obtained free of charge from the Cambridge
Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/getstructures.

**4. Conclusions**

We have demonstrated that addition of the internal alkyne *N*-(2-butynyl)phthalimide **1** to [Co₂(CO)₈] straightforwardly yields the stable complex [Co₂(CO)₆(μ-phthalimi-doCH₂C≡CMe)] **2** featuring a dimetallatetrahedrane framework. The investigation of the biological activity of **2** and other related compounds will be the topic of a forthcoming study.

**Supplementary Materials:** The following are available online, CIF file, Check-CIF report, and ATR-IR spectrum.

**Author Contributions:** I.J. and T.C. prepared the compound; C.S. and J.-L.K. collected the X-ray data and solved the structure; I.J., M.K. and M.O. designed the study; I.J. and M.K. analyzed the data and wrote the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Région Bourgogne-Franche-Comté, DeCOmAB project.

**Data Availability Statement:** The X-ray data are at CCDC as stated in the paper.

**Acknowledgments:** We thank Stéphanie Beffy for recording some IR and NMR spectra. J.-L.K. acknowledges the receipt of a Kekulé scholarship from the Fonds der Chemischen Industrie (VCI).

**Conflicts of Interest:** The authors declare no conflict of interest.

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