Chalcone and its derivatives, known as chalconoids, represent a well-known and biologically important group of natural compounds. Chalcones can be synthesized in plants as secondary metabolites with the help of an enzyme named chalcone synthase and they also serve as biosynthetic intermediates of flavonoids. Chalconoids are of great interest in medicinal chemistry because they express a broad spectrum of biological activities, such as antimicrobial, anticancer, anti-inflammatory, anti-HIV, and antimalarial. These key properties of chalconoids have been thoroughly reviewed in the literature [[1], [2], [3], [4], [5], [6], [7], [8]]. The structural arrangement of chalcones and chalcone-based compounds containing suitable substituents of R1 and/or R2 (see Scheme 1) provides a flexible ligand platform suitable for coordination to transition metals with the aim to prepare metal complexes with improved biological features as compared to parent-free chalcones.

To date, many complexes of such types have been synthesized and studied, showing remarkable results in the field of bioinorganic and medicinal chemistry in some cases [[9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21]]. For example, the biological activities of Cu(II) and Zn(II) complexes [9,10], Ru(II) complexes [11], Cu(II), Co(II), Zn(II) and Fe(II) complexes [12], Zn(II) complexes [13], Cu(II)-complexes [14,15], Co(II) and Cu(II) complexes [16], Co(II), Ni(II), Cu(II) and Zn(II) complexes [17], Pt(IV) complexes [18], Ru, Rh and Ir complexes [19], Zn(II), Pb(II), Cd(II) and Ni(II) complexes [20], and Pd(II)-complexes [21]. Moreover, the biological potential of transition metal complexes was also reviewed [22,23].

In recent years, the group of copper(II) heteroleptic complexes involving the combination of various diimine ligands and other suitable O(N),N(O)-donor ligands called “Casiopeínas” caught the attention of bioinorganic community as many of them showed promising antiproliferative properties against various types of cancer cells in vitro [24] and in vivo [25], and after careful deliberation one of them (Casiopeína III ia (CasIIIia)) was able to enter the Phase I of clinical trials (for a comprehensive review, see [26]). Recently, the group of Casiopeínas-family was enlarged by the fourth-generation compounds based on Cu(II)-diimine complexes involving hydrogenated tridentate Schiff bases of salen type [27] showing good cytotoxicity against HeLa cells at micromolar IC50 levels, associated with DNA binding ability of the complex and its ability to cleave DNA by oxidative and hydrolytic mechanisms. At the same time, other directions of research involving the Cu(II)–diimine complexes were also developed, leading to the research of the structures and biological activities of the Cu(II)–diimine complexes involving, for example, thiosemicarbazone derivatives [28] showing exceptional antiproliferative effects against HeLa cervical cancer cells at nanomolar IC50 levels, associated with DNA binding, ROS generation and resulting induction of apoptosis, or involving picolinato-ligands [29,30] showing exceptional activity towards ovarian carcinoma cells resistant to cisplatin (A2780cis) at submicromolar IC50 levels. A plethora of other structural types and mechanisms of actions of Cu(II)–diimine complexes was previously thoroughly reviewed by Santini et al. [31].

Bearing in mind the above-mentioned literary data and taking into account an increasing abundance of clinical trials involving the copper complexes dedicated to the diagnostics [32] and treatment of different types of cancers [33], we strived to continue our efforts to prepare the copper(II) complexes containing the derivatives of naturally-occurring compounds as ligands and study their anticancer potential. In the previous studies, our team focused on the study of copper(II) complexes with various chalcone derivatives based on 2′-hydroxychalcone [14,15]. These complexes, with general composition [Cu(Ln)(NN)]NO3, where N–N = 2,2′-bipyridine or 1,10-phenanthroline and their derivatives, and Ln = 2′-hydroxychalcone and its derivatives, expressed overall significant in vitro cytotoxicity on a panel human cancer cell lines together with relatively low toxicity against normal cells. The best-performing complexes showed IC50 values ≈ 1 μM together with good selectivity of normal cells against cancer ones. These findings motivated us to explore the potential use of a different chalcone skeleton. Thus, we assembled a dimeric form of the 2′-hydroxychalcone moiety that offers two possible coordination sites for transition metals. We prepared (E)-1-(2-hydroxyphenyl)-3-[4-[(E)-3-(2-hydroxyphenyl)-3-oxo-prop-1-enyl]phenyl]prop-2-en-1-one, herein labelled as the H2L ligand, see Scheme 1. This compound was synthesized for the first time by Pinto et al. [34] and to the best of our knowledge, as for transition metal complexes, only one dinuclear Ru(II)-complex with H2L of the composition [Ru2(μ-L)(DMSO)6Cl2] was prepared, characterized and studied for its DNA cleavage and Topoisomerase I inhibition up to now [35], which point out the novelty of the presented coordination compounds, especially as for copper(II) complexes.

No copper complexes based on the H2L backbone have been reported so far. Copper is a vital element that forms the active site of biomolecules that are key in several metabolic processes, such as those involved in electron transfer and oxygen activation and plays pivotal roles in the nervous system of higher eukaryotes, appearing in proteins that direct biogenesis of both catecholamine and peptidic hormones/neurotransmitters [36].

Herein, we present the synthesis and thorough characterization of a series of dinuclear copper(II) complexes of the composition [Cu(NN)(μ–L)Cu(NN)](NO3)2⋅nH2O (n = 0–2) (1–5), containing the bis(chalcone) bridging ligand (H2L) in combination with 1,10-phenanthroline and its derivatives (labelled as NN). Structurally, the copper complexes herein described and based on the H2L framework contain two copper centres that are spatially separated from each other, hence acting as magnetically independent metal/spin centres. Such organization of the copper complexes, commonly referred to as non-coupled binuclear copper complexes, is difficult to synthetically engineer and can be found in natural biosystems such as peptidylglycine α-amidating monooxygenase (PAM) and dopamine β-monooxygenase (DβM) [37]. We show that these binuclear copper systems can significantly influence the proliferation of cancer and normal cells and unveil the impact of such complexes on the cell cycle and other cellular effects in A2780 human ovarian endometroid adenocarcinoma cells.