Retracted Article: Synthesis of a chiral dinuclear Cu(II)-benzothiazolamine complex: evidence of cuprophilic interaction in its structure and exploration of its electrochemical properties and catalytic performance

Abstract

The synthesis of a chiral dinuclear [Cu(OAc)₂(L1)]₂ complex (A) and its analogues Cu(OAc)₂(L1)₂ (B), Cu(OAc)₂(L1)PPh₃ (C), CuBr(L1)PPh₃ (D), and Cu(OAc)₂(L2) (E) is described. The X-ray structure of A reveals a cuprophilic interaction (2.65 Å) and shows that L1 behaves as a monodentate ligand. The stereogenic centre in L1 aligns the NH group to form non-covalent interactions with the paddle-wheel acetate groups at variable distances (2.4–2.5 Å and 2.2–2.7 Å). Thermogravimetric analysis confirmed our hypothesis that two equivalents of L1 (B) or a combination of L1 and PPh₃ (C) would disrupt the cuprophilic interaction. All complexes, except D, showed irreversible redox waves by cyclic voltammetry. Complexes C and E have lower oxidative peaks (at 10 V s⁻¹) than complex A between +0.40 and +0.60 V. This highlights the influence of ligand(s) on the redox behaviour of Cu(II) complexes. The significance of this electrochemical behaviour was evident in the Chan–Lam (CL) coupling reaction, where 2.5 mol% of A successfully facilitated the formation of a C–N bond. This study showcased the structure, thermal stability, electrochemical properties and catalytic performance of a chiral dinuclear copper(II)-benzothiazolamine complex.

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Introduction

Benzothiazole is a privileged heterocyclic scaffold that is less explored as a ligand in transition-metal catalysis,^1a–c whereas similar motifs, such as benzimidazole and benzoxazole (Fig. 1A), have been studied extensively as ligands for the syntheses of metal complexes. For example, Pt(II) complexed with benzimidazole derivatives,^2a benzoxazole-based ligands complexed with 3d-metal ions,^2b and Cu(I) complexes constructed with different N-heterocyclic benzoxazole ligands^2c have been reported. Benzothiazolamines (nitrogen at the 2-position) are medically important. They are present as core motifs in pharmaceutical drugs such as anti-HIV 1 ^3a and antibacterial 2 ^3b agents (Fig. 1B). Fluorinated benzothiazole derivatives have been investigated as PET imaging agents for breast cancer^4a and as potential tracers for β-amyloid plaques in Alzheimer's disease.^4b Crucially, benzothiazole derivatives complexed with transition metals have been shown to possess biological activities such as antimicrobial activity,^5a antitumour activity,^5b inhibition of enzymes,^5c and in the treatment of Alzheimer's disease.^5d A previous study^1c has explored the synthesis and characterisation of palladium(II) complexed with achiral N-(benzothiazol-2-yl)benzamide 3 (Fig. 1D), which required the use of 1,2-bis(diphenylphosphino)ethane as a co-ligand. Herein, we report the synthesis and characterisation of a dinuclear paddle wheel copper(II) complex bearing (S)-L1 as a ligand (Fig. 1D). The research interest in paddle wheel dinuclear Cu(II) complexes has grown exponentially in recent years.^6a,b This is due to their tunable redox chemistry, biological relevance and their striking structural features, which make them attractive as building blocks for supramolecular metal–organic frameworks (SMOFs).^6c–g However, none of these examples possessed a chiral N-ancillary ligand. In addition to aurophility (Au^I–Au^I)^7a and argentophilicity (Ag^I–Ag^I),^7b cuprophilicity has been used to describe a closed-shell d¹⁰–d¹⁰ Cu^I–Cu^I interaction^7c–e but not a d⁹–d⁹ Cu^II–Cu^II interaction. Cuprophilic interaction is considered to be present if the Cu–Cu separation is shorter than the sum of the van der Waals radii of two Cu atoms (2.80 Å).^7f Ligands have been shown to support dinuclear copper complexes with Cu–Cu distances ranging from 2.5 to 4.0 Å.^8a,b Interestingly, ligands^9a–d complexed with copper(II) have rarely been used in CL reactions, and none of the reported examples are nitrogen-chelating monodentate chiral ligands.^9e

Fig. 1 (A) Heterocyclic motifs for ligand design; (B) medically relevant benzothiazole derivatives 1 and 2; (C) achiral 3 and (D) chiral (S)-L1 and (S)-L2.

The C–N bond formation via cross-coupling (transition metal catalysed) can be achieved using Buchwald–Hartwig (palladium),^10a,b Ullmann-type (copper)^10c or CL (copper)^10d,e reaction conditions. Although these methods are complementary, the key difference is that aryl boronic acids are employed in CL reactions (Scheme 1) as one of the coupling partners and most reported examples required high loadings of copper (20–100%) to promote the reaction. Although a firm mechanistic understanding of CL reactions has been established, the roles of oxidant and Cu^I intermediate are poorly understood.^11a,b

Scheme 1 Chan–Lam coupling of amines with boronic acids.

Results and discussion

Synthesis and structure

The synthesis of (S)-L1 was achieved using commercially available precursors (S)-1-phenylethylamine and 2-chlorobenzothiazole (Scheme 2).^11c This method is column chromatography free, and it avoids the use of carbon disulfide (CS₂) or thiols.^11d,e The treatment of (S)-L1 with N-bromosuccinimide in dichloromethane (DCM) at room temperature (RT) for 12 h provided (S)-L2.^11f

Scheme 2 Syntheses of (S)-L1, (S)-L2 and the Moiras complex (S)-A. For the synthesis of complexes B–E, see Scheme S1.†

Moiras complex A was generated by mixing (S)-L1 with Cu(OAc)₂, in a ratio of 1 : 1, in tetrahydrofuran (THF) and methanol (MeOH) at 65 °C for 30 minutes. Its dinuclear and paddle wheel structure was confirmed by X-ray crystallography (Fig. 2). Also, (S)-L1 was shown to act as a monodentate ligand, chelated to copper via the endocyclic nitrogen (N1 or N3). The initial hypothesis expected N2 (or N4) also to coordinate with copper, but this postulated interaction did not occur. This is probably due to the bond angles of N1–C7–N2 or N3–C22–N4, which are 124.5° and 124.1°, respectively.

Fig. 2 X-ray structure of the Moiras complex (S)-A. Atomic displacement ellipsoids for non-H atoms drawn at the 50% probability level.

The cuprophilic interaction between d⁹–d⁹ Cu^II–Cu^II in A is 2.65 Å, which is shorter than the sum of the van der Waals radii of two copper atoms (2.80 Å). The van der Waals radius for copper is 1.40 Å (Fig. 3 and S4†). Although complex A is C₂-symmetrical, the N1–Cu2 and N3–Cu1 distances are relatively the same, being 2.17 and 2.18 Å, respectively. However, the distance of the intramolecular non-covalent interactions (O⋯H) between the paddle wheel acetate oxygen atoms and NH (next to the stereogenic centre) of L1 varied. The respective distances between O1 and O3 with H2 (in L1 on the right-hand side of the complex) are 2.4 Å and 2.5 Å, whereas the respective distances between O6 and O7 with H4 (in L1 on the left-hand side of the complex) are 2.2 Å and 2.7 Å (Fig. 3).

Fig. 3 X-ray structure of the Moiras complex (S)-A showing the van der Waals (vdw) radius of each atom (vdw radius for Cu = 1.40 Å) using PyMOL.

With the knowledge of the behaviour of (S)-L1 as a ligand and the nature (paddle wheel, cuprophilic interaction and intramolecular hydrogen bonding) of the structure of complex A, we investigated the possibility of chelating two monodentate ligands to one copper atom, i.e., using two equivalents of L1 or a combination of one equivalent of L1 and triphenylphosphine (PPh₃) each with one equivalent of Cu(OAc)₂ (Scheme S1†). This endeavour generated complexes B and C. We also explored copper in the +1 oxidation state by mixing copper bromide dimethyl sulfoxide (CuBr·DMS) with one equivalent of L1 and one equivalent of PPh₃ to generate complex D, which should provide a comparison with complex C (i.e., Cu^Ivs. Cu^II). To understand the influence of halogen on the ligand and/or paddle wheel complexes, we generated complex E using one equivalent of (S)-L2 and one equivalent of Cu(OAc)₂.

Thermal properties of complexes A–C and E

The influence of intra- or intermolecular non-covalent interactions on the boiling point of compounds has been well studied and established. The stronger the interaction the higher the boiling point. Herein, we conducted thermogravimetric analysis (TGA) on complex A to elucidate the influence of non-covalent interactions (cuprophilic and H-bond interactions) on its thermal stability. At 200 °C, A lost just 25% of its weight.

In comparison, complex B has a lower thermal stability than A (Fig. 4). This confirmed our hypothesis that two equivalents of L1 would disrupt the non-covalent interactions observed in A. This result also highlights the influence of these interactions on the thermal properties of an organometallic complex. Overall complex C has higher thermal stability than A, possibly due to the presence of PPh₃. However, A lost just 5% of its weight at 173 °C, whereas C lost 5% of its weight at a lower temperature (161 °C).

Fig. 4 TGA analysis. Comparison of the thermal properties of complexes A (green), B (red), and C (blue). For DSC analysis, see the ESI.†

Electrochemical properties of complexes A–E

The electrochemical behaviour of complexes A–E was investigated using cyclic voltammetry (CV). The measurements were conducted within a potential range of +0.75 to −0.25 (V vs. SCE), employing different scan rates starting from 0.1 V s⁻¹ to 10 V s⁻¹. As expected, the anodic and cathodic currents generally increased with an increase in scan rate from 0.1 V s⁻¹ to 10 V s⁻¹ and there was a cathodic shift in the reduction peak positions and a slight anodic shift in the oxidation peak positions with an increase in scan rates. Complexes A, B, C and E displayed an irreversible process at approximately +0.5 V vs. SCE, which did not appear to become reversible as the scan rate increased.

Previous electrochemical study has shown that the redox reaction of the Cu^II–Cu^III couple was an irreversible process.¹² Within the context of a copper-catalysed organic reaction catalytic cycle, this electrochemical behaviour displayed by complex A (B, C or E) would suggest a facile and irreversible oxidative addition. The oxidation of Cu^II to Cu^III has been invoked and accepted as critical in the early step of the CL reaction mechanism,^11a,b but the fate of Cu^III afterwards (i.e., Cu^III to Cu^I) is still debatable.^9e Therefore, we explored the electrochemical behaviour of complex D (Cu^I) and compared it to that of complex C (Cu^II). Complex D possesses a fully reversible redox wave at +0.4 V vs. SCE and has a higher oxidative peak potential than complex C (Fig. 5). At 10 V s⁻¹ scan rate, complex E has a lower oxidative peak potential than complex A from the range of +0.40 to +0.60 V (vs. SCE) (L1vs.L2), whereas the oxidative potentials of A and B are more similar. Interestingly, complex B has a slightly higher anodic potential than complex C at +0.45 V vs. SCE (L1vs. PPh₃). These results demonstrate the influence of ligands on the electrochemical behaviour of Cu^II complexes.

Fig. 5 The overlay of cyclic voltammograms of complexes A–E at 10 V s⁻¹. For the cyclic voltammogram of each complex at different scan rates, see Fig. S5–9.†

Catalytic performance of complex A in the Chan–Lam reaction

The knowledge of the structure, thermal properties and electrochemical behaviour of complex A justified the exploration of its catalytic activity in the CL reaction. Critically, this study was carried out in a manner that should explain the roles of an oxidant, Cu^I (using complex D), and a base. Contentiously, a role for Cu^IV as a putative intermediate in the proposed CL reaction mechanism was considered.^13a–c Notably, Cu^III complexes were once deemed highly reactive, non-existent, and unstable. Now, there are numerous reports of isolable, crystallographically characterised, and even chemically inert complexes formally bearing Cu^III centers.^14a–e Initial studies tested the use of 2.5 mol% Cu(OAc)₂ and conducted the reaction without complex A (Table 1, entries 1 and 2). The latter stage of the previously reported CL reaction mechanism^11a suggested that oxygen (in air) acts as an oxidant that converts Cu^I into Cu^II.

Table 1 Exploration of complex A for C–N bond formation^a

Entry	R	Deviation from the reaction conditions	Product	Yield^c (%)
a Reaction conditions: 4 (1.57 mmol, 1.0 eq.), 5a–e (2.35 mmol, 1.5 eq.), pyridine (1.57 mmol, 1.0 eq.), PhMe (5 mL), the Moiras complex A (0.039 mmol, 0.025 eq.), with or without air, and room temperature (RT). b The reaction mixture was evacuated under vacuum and backfilled with N2 twice. c Isolated yields.
1	3-Me	Using Cu(OAc)2	6a	10
2	3-Me	Without complex A	6a	—
3	3-Me	Using complex D	6a	—
4	3-Me	Without pyridine	6a	—
5^b	3-Me	Under N2 atmosphere	6a	—
6	3-Me	1-Methylpyrrolidine as solvent	6a	12
7	3-Me	Pyridine as solvent	6a	8
8	3-Me	THF as solvent	6a	50
9	3-Me	DCM as solvent	6a	94
10	3-Me	None	6b	92
11	3-F	None	6b	90
12	4-F	None	6c	87
13	4-OMe	None	6d	85
14	Napthyl	None	6e	90

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Therefore, entry 3 investigated the catalytic ability of Cu^I in the CL reaction in air, but it was ineffective. Entry 4 showcased the important role of a base (pyridine) towards the feasibility of the reaction – presumably, it helped generate anionic intermediate I (Scheme 3). Entry 5 obviates the relevance of, or the need for, an oxidant in this reaction. Entries 6 and 7 suggest that nitrogen-containing solvents can displace L1 from copper; hence, the yields of both reactions were like that of entry 1. Furthermore, entry 1 vs. entry 10 showcased the influence and importance of the ligand. A moderate yield was obtained if THF was used as solvent (entry 8), but DCM (entry 9) performed well like PhMe (entry 10; see Fig. S1–3† for the behaviour of complex A in different solvents). Notably, the reaction proceeded well with aromatic amine 4 and aryl boronic acids 5a–e (entries 10–14). However, we obtained homo-coupling products with aliphatic amines. Literature reports have shown that aryl boronic acids can be coupled with aliphatic amines as well as N-containing heterocycles.¹⁵ Presumably, the non-covalent interactions (π–π stacking) in the highly ordered ternary putative complex J (Scheme 3) are necessary for the C–N bond formation. Based on the electrochemical data (and using alkyl amines), G to H is irreversible, but H to J will likely be reversible due to the entropy penalty. In comparison, a prior convoluted mechanism^11a proposed that the copper acetate dimer was deprotonated to the mononuclear square planar Cu^II catalyst, followed by Lewis-pairing, and then transmetallation and subsequent disproportionation to generate Cu^III. This Cu^III would then undergo reductive elimination to generate the product, as well as Cu^I species. The intermediate Cu^II species may then be regenerated under oxidative conditions from Cu^I. Overall, our work demonstrated the catalytic performance of a paddle wheel Cu(II) bearing a chiral benzothiazolamine ligand (complex A) in the Chan–Lam (CL) coupling reaction, which was driven by non-covalent interactions via a putative Cu^IV intermediate.

Scheme 3 Proposed mechanism for complex A catalytic activity in the Chan–Lam reaction.

Conclusions

We have synthesised a novel paddle wheel Cu(II) complex A bearing a chiral benzothiazolamine monodentate ligand (L1). The single-crystal X-ray crystallography revealed a cuprophilic interaction between the two copper atoms at a distance (2.65 Å) lower than the sum of the van der Waals radii of two Cu atoms (2.80 Å). Within this complex four hydrogen bonds between NH (of L1) and oxygen atoms of the acetate group also exist, with varying distances. We investigated the influence of different ligands (L1vs. L2vs. PPh₃) and the number of ligands (combination of L1 and PPh₃) on the thermal properties and electrochemical behaviour of Cu^II complexes. The hydrogen-bond interaction within complex A and the π–π interaction between the aryl moieties of the two coupling partners on Cu^IV facilitated the formation of a C–N bond.

Data availability

All experimental procedures, characterisation data, and NMR spectra are available in the ESI.†

Author contributions

O. Stephen Ojo: conceptualisation, methodology, writing – original and final draft, data curation for all sections, and synthesis and characterisation of L1, L2, and complexes A–E. NMR and infra-red spectroscopy results and melting points comply with the ESI.† Halilu Sale: studied the electrochemical properties of complexes A–E and compiled their data. Mark D. Symes: reviewed, edited, and supervised the electrochemical study. Claire Wilson: X-ray crystallography analysis and data refinement.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support from the School of Chemistry, University of Glasgow is gratefully acknowledged. Thanks to Andrew Monaghan for TGA and DSC analysis of complexes A–C and E. Special thanks to Guilia Pellegrino for laboratory assistance and Professor Haralampos N. Moiras for helpful discussion. We have applied for a Creative Commons Attribution (CC BY) licence.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2292157. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3dt02994h

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