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

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.

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 [7,8,9,10,11]. In addition to the applications using dicobaltatetrahedranes as synthetic tools, this class of dinuclear organometallic compounds is emerging in bio-organometallic chemistry [12,13,14,15,16,17,18,19,20,21]. 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 [22,23,24]. In 2005, Gust et al. also reported the reaction of propargyl phtalimide 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).

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≡CMe)].

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).

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.

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 µ₂-η⁴-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.

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 [{Co₄(μ₂-CO)₃(CO)₉] 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 [phthalimidoCH₂C≡CCH₂phthalimido] (CSD refcode ECUFOB) (1.323(9) vs. 1.191(4) Å) [28].

Although there are numerous examples of crystallographically characterized hexacarbonyldicobalt compounds which are µ₂-η²,η² capped by an internal alkyne, a survey of the CSD data base revealed only two other examples containing a -CH₂-C≡C-Me motif as is present in 2, namely (trans-5,8,8-trimethyl-trans-1,4,5,7-tetrahydro-cis-4-(2-butynyl)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. Comparison of relevant bond lengths (Å) and angles (°) in 2 and the crystallographically characterized [Co₂(CO)₆(µ-RCH₂C≡CCH₃)] tetrahedranes A and B.

	2	A	B
Co–Co	2.4679(14)	2.4688(9)	2.4690(9)
C–Calkyne	1.330(9)	1.325(6)	1.326(6)
Calkyne–CMe	1.481(9)	1.486(6)	1.492(6)
Calkyne–CH2	1.499(9)	1.500(7)	1.493(6)
Co–Calkyne	1.972(6)	1.974(5)	1.972(4)
Co–Cpseudo-equatorial	1.815(7)	1.823(5)	1.820(5)
Co–Cpseudo-axial	1.797(7)	1.785(6)	1.787(5)
CSD reference	This work	DOTKEC [29]	HUKCIA [30]

and compared with those of compounds A and B (averaged bond lengths).

In the molecular cell of 2 with Z = 4, there are no intermolecular contacts deserving any discussion (Figure 6).

3. Materials and Methods

N-(2-butynyl)phthalimide 1 was commercially purchased from Aldrich. ¹H- and ¹³C-NMR spectra were recorded on a Brucker AC 400 (Bruker, Wissembourg, France) at 400 and 100.62 MHz, respectively, using CDCl₃ as solvent. Infrared spectra were recorded on a Vertex 70 spectrometer (Bruker, Wissembourg, France) in ATR mode or in solution.

(µ₂-η⁴-N-(2-butynyl)phthalimide)(hexacarbonyl)dicobalt: Alkyne 1 (199.2 mg, 1 mmol) was added to a stirred solution of Co₂(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 C₁₈H₉Co₂NO₈ (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 %. ¹H-NMR (CDCl₃) at 298 K: δ 2.65 (s, CH₃), 5.03 (s, NCH₂), 7.74 (br, CH Ar), 7.88 (br, CH Ar) ppm. ¹³C{¹H}-NMR (CDCl₃) at 298 K: δ 20.3 (CH₃), 39.7 (NCH₂), 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_α radiation was employed instead of MoK_α 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 C₁₈H₉Co₂NO₈, M = 485.12 g.mol⁻¹, orange-red needles, crystal size 0.377 × 0.194 × 0.15 mm³, triclinic, space group P-1: a = 7.4317(4)Å, b = 13.4351(7) Å, c = 19.2794(11) Å, α = 78.462(3°, β = 84.011(3)°, γ = 89.156(3)°, V = 1875.73(18) ų, Z = 4, D_calc = 1.718 g/cm³, T = 100 K, h = −8 ≤ h ≤ 8, k = −15 ≤ k ≤ 16, I = 0 ≤ l ≤ 16, GOF = 1.077, R₁ = 0.0855, wR2 = 0.2428 for 6962 reflections with I > = 2σ (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 F² (SHELXL, 2015 [31,32,33]).

Data were collected using graphite-monochromated CuK_α 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-phthalimidoCH₂C≡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.