Alkali complexes of non-steroidal anti-inflammatory drugs inhibit lung and oral cancers in vitro

Abstract

Non-steroidal anti-inflammatory drugs (NSAIDs) are widely used to treat pain, fever, and inflammation. In this paper, we report four new alkali metal complexes (1–4) containing 1,10-phenanthroline (phen), and two NASID drugs, that is, ibuprofen (ibu) and flurbiprofen (fibu). Complexes 1–4 were characterized by a variety of analytical techniques including infrared and UV-vis spectroscopy, ¹H NMR, and single-crystal and powder X-ray diffraction analysis. In complexes 1–4, phen binds via an N,N′-chelate pocket, while the ibu and fibu ligands coordinate in a bidentate fashion in all four complexes. The complexes were evaluated for their anticancer activity against lung and oral cancer cell lines. The complexes displayed excellent activities against oral and lung cancer cells with the least toxicity toward normal cells when compared to the standard drug 5-fluorouracil.

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Introduction

Metal complexes of drugs are an attractive area of research, where the goal is not only to enhance the bioactivity and bioavailability of the drugs but also to search for new targets.^1–9 Several drug-based metal complexes have been prepared and, in some cases, exhibited remarkable performances.¹⁰ For instance, a Ru²⁺ complex of chloroquine, an antiviral drug, is reported to exhibit activity two to five times higher than chloroquine.¹¹ Moreover, the interaction of drugs with metal ions may lead to a better understanding of metal ion antagonism.¹² Cancer is a collection of diseases characterized by unconstrained multiplication of any of the various types of cells in the body.¹³ Cisplatin, an expensive drug and associated with severe side effects, is still the drug of choice for different types of cancers.^14–16 One of the major side effects of cisplatin is binding with proteins at the positions occupied by zinc ions, which ultimately results in the loss of Zn²⁺ ions.^17,18 Therefore, the search for new but cheap metal-based drugs with less side effects is a challenging task for bioinorganic chemists.

Nonsteroidal anti-inflammatory drugs (NSAIDs) are an important class of drugs that are used to reduce pain, fever, and inflammation.¹⁹ Most NSAIDs possess carboxylic groups that can bind to metal ions through oxygen atoms. In this respect, several metal complexes of NSAIDs have been prepared and their biological properties have been explored.^20–23 Among NSAIDs, ibuprofen (ibu) and flurbiprofen (fibu) (Fig. 1) possess analgesic and antipyretic properties,²⁴ and are used in inflammatory and painful diseases of rheumatic and non-rheumatic origin.²⁵ It is reported that a regular dose of ibu may be effective against breast²⁶ and lung cancer.²⁷ Both ibu and fibu have also been studied for their antimicrobial and antibacterial properties.^28–31 Moreover, metal–organic frameworks derived from ibu/fibu with organic linkers have been evaluated for controlled drug release.^32–42

Fig. 1 Structural representation of ibuprofen (ibu), flurbiprofen (fibu) and 1,10-phenanthroline (phen).

We started a research program and aimed to synthesize and structurally characterize new alkali/alkaline earth metal complexes of biologically active compounds and to evaluate their biological activity for potentially useful pharmaceutical applications. We recently reported alkali/alkaline earth metal complexes of diclofenac, an NSAID, which exhibited excellent cytotoxic activity against lung and oral cancer cell lines.⁴³ In the present work, we selected ibu and fibu as O-donor ligands and 1,10-phenanthroline (phen) as an N-donor ligand for complexation reactions with selected alkali metals (lithium, potassium, and caesium). Phen is not only a powerful predictable ligand^44–46 but is also known for its enzyme inhibition properties.^47,48 The alkali complexes of ibu/fibu with phen as a co-ligand are unknown (CSD 2019).⁵² There is only one example in the literature, where the luminescence properties of an inner transition metal (Re²⁺) complex containing ibu⁻ and phen were reported.⁴⁹ The bioactivities of coordination complexes of cheaper metals (i.e., alkali/alkaline earth metals) did not receive much attention compared to transition metals. Alkali metals play a vital role in many cellular pathways, apoptosis, homeostasis, adjusting electrolyte balance, and nerve impulses.^50,51

In the current new study, we have investigated the reactions of phen and ibu/fibu with M₂CO₃ (where M = Li, K, and Cs) and described the structures of a mononuclear complex [Li(phen)(ibu)(H₂O)] (1), a dinuclear complex [Li₂(phen)₂(fibu)(H₂O)₃] (2), and 1D polymer chains of [K(phen)(ibu)(ibuH)]_n (3) and [Cs(phen)(fibu)]_n (4). The structures of 1–4 were analysed by single-crystal X-ray diffraction and detailed descriptions of their crystal structures are given. The bulk compositions were confirmed by ¹H NMR, UV-vis and IR spectroscopy, and powder X-ray diffraction analysis. The anticancer activities against lung and oral cancer cell lines were studied to check their therapeutic potential.

Synthesis of the complexes

Ligands ibu and fibu and metal carbonates were obtained commercially and were used without further purification. The reactions of phen, ibu, and fibu with M₂CO₃ (where M = Li, K, and Cs) in MeOH/H₂O produced a mononuclear Li complex (1), a dinuclear Li complex (2), and 1D chains of both K (3) and Cs (4) complexes. All complexes were characterized by single-crystal and powder X-ray diffraction, and UV-vis, IR and ¹H NMR spectroscopy. X-ray quality single crystals were grown directly from the mother liquor at room temperature.

Analytical data and spectral characterization

Complexes 1–4 were obtained in good yields (64–73%) in an analytically pure form. The complexes were soluble in MeOH and DMSO. The IR spectra of complexes 1–4 are presented in Fig. S1 of the ESI.† The broad peaks in the range 2920–3619 cm⁻¹ are assigned to the O–H stretching frequency, which are broadened due to hydrogen-bonding. The peaks in the range of 1618–1722 cm⁻¹ reflected the presence of the carbonyl group, while the peaks in the range of 1549–1621 cm⁻¹ are attributed to the aromatic carbon–carbon stretching. The heterocyclic C–N (phen) bond stretching appeared in the range of 1505–1514 cm⁻¹. All these assignments with their values are in good agreement with literature data.^52,53 The UV-vis data collection for complexes 1–4 in non-coordinating solvents (such as CH₂Cl₂) was hindered by insolubility. Therefore, the spectral data for the complexes were recorded in MeOH. The UV-vis data for complexes 1–4 were measured in methanol and are presented in Fig. S2 of the ESI.† The UV-vis spectra are dominated by intense π–π* ligand-centred absorption bands which are attributed to ligand-based transitions by comparison with the free ligands (Fig. S2, ESI†). The increase in the absorption coefficient (ε) of complexes 1–4 (ε = 36 233–65 604 M⁻¹ cm⁻¹) compared to the free ligand supports the suggestion that these ligands remained coordinated in the solution, with metal coordination enhancing the extinction coefficient. The ¹H NMR spectra of complexes 1–4 were recorded in MeOD/DMSO-d₆ (Fig. S3–S6, ESI†). The signals of the free ligands are present in the ¹H NMR spectra of the complexes when compared to the free ligands. The signals for the phen protons are unchanged, while there is a small shift in the ibuH/ibu⁻/fibu⁻ protons, indicating some binding of the anion to the metal ion in solution.

PXRD on complex 1–4

The experimental powder diffraction patterns of complexes 1–4 (Fig. S7–S10, ESI†) are in good agreement with those calculated based on the single-crystal data. Small differences between the intensity profiles may be accounted for by the texture (preferred orientation) of the samples.

Description of the crystal structures

Crystallographic details for 1–4 are listed Table 1.

Table 1 Crystal data for complexes 1–4

Complexes	1	2	3	4
Chemical formula	C25H27LiN2O3	C54H46F2Li2N4O7	C38H43KN2O4	C27H20CsFN2O2
M r (g mol^−1)	410.42	914.83	630.84	556.36
Crystal system, space group	Triclinic, P1̄	Monoclinic, I12/c	Triclinic, P1̄	Monoclinic, P21/n
a (Å)	7.1787 (5)	19.4525 (11)	10.3951 (10)	18.108 (3)
b (Å)	10.2822 (6)	6.6516 (3)	10.6836 (12)	7.1063 (12)
c (Å)	16.1772 (10)	35.989 (2)	17.2357 (19)	19.003 (3)
α (°)	96.057 (3)	90	72.414 (5)	90
β (°)	101.245 (3)	98.923 (3)	77.519 (5)	109.543 (5)
γ (°)	106.314 (3)	90	88.313 (5)	90
V (Å^3)	1107.59 (12)	4600.3 (4)	1780.2 (3)	2304.5 (6)
Z	2	4	2	4
μ (mm^−1)	0.08	0.09	0.19	1.64
T min, Tmax	0.630, 0.745	0.706, 0.745	0.599, 0.745	0.453, 0.745
No. of measured, independent and observed [I > 2σ(I)] reflections	18 297, 4508, 2645	30 472, 4655, 3566	35 049, 6246, 4257	25 136, 4721, 3667
R int	0.059	0.040	0.064	0.059
(sin θ/λ) max (Å^−1)	0.626	0.625	0.595	0.627
R[F^2 > 2σ(F^2)], wR(F^2), S	0.062, 0.184, 1.04	0.050, 0.151, 1.04	0.093, 0.293, 1.10	0.041, 0.108, 1.00
No. of reflections	4508	4655	6246	4721
No. of parameters	420	377	441	328
No. of restraints	436	63	42	14
Δρmax, Δρmin (e Å^−3)	0.24, −0.19	0.24, −0.25	1.05, −0.37	0.63, −1.05
CCDC	2015737	2015738	2015739	2015740

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[Li(ibu)(phen)(H₂O)] (1). The molecular structure of 1 is shown in Fig. 2 and selected interatomic distances and angles are listed in Table 2. The complex crystallizes in the triclinic space group P1̄, with one phen ligand, a deprotonated ibu⁻ anionic ligand, a Li⁺ cation, and a coordinated water molecule in the asymmetric unit. The Li⁺ ion adopts a four-coordinated slightly disordered tetrahedral geometry (Fig. S11, ESI†) comprising a [N₂O₂] donor set. The phen ligand adopts an N,N′-chelate mode through the N1 and N2 atoms, making a five-membered ring, with Li–N distances of 2.077 (5) and 2.147 (5) Å. The Li–N bond lengths lie within the normal observed range of 2.077 (5)–2.29 Å for related lithium complexes with phen ligands.⁴⁷ In complex 1, the ibu⁻ anion is disordered and was modeled over two sites (Fig. S12, ESI†). The bond angles around Li⁺ are in the range 79.6 (2)–118.1 (2)°. The Li–O_water distance is 1.893 (5) Å, consistent with hydrated Li⁺ complexes reported by us⁵⁴ and others.⁴⁴ Lithium complexes of phen with carboxylate as co-ligands are rare. A CSD (2019) search resulted in only two hits,^55,56 where the Li⁺ coordination geometry is similar to that of 1. Complex 1 exhibits hydrogen-bonding interactions which involve the carboxylate moiety and the water molecule. The ibu⁻ carboxylate O atoms (O1A and O3A) form hydrogen bonds with water molecules, which are coordinated to neighboring molecules (O2–H2O⋯O1Ai, and O2–H3O⋯O3Aii), resulting in a supramolecular hydrogen-bonded 1D infinite chain parallel to the crystallographic a-axis (Fig. 3). The structure is additionally stabilized by intermolecular π⋯π stacking between phen ligands, with centroid–centroid (cg⋯cg) distances of 3.544 Å (Fig. S13, ESI†).

Fig. 2 The molecular structure of [Li(ibu)(phen)(H₂O)] (1). Displacement ellipsoids are drawn at the 50% probability level. The disordered part of ibu⁻ is truncated for clarity.

Table 2 Selected geometric parameters (Å, °) for 1

Li1–O2	1.893 (5)	Li1–N1	2.148 (5)
Li1–O1B	1.72 (2)	Li1–N2	2.077 (5)
Li1–O1A	1.934 (7)
O1B–Li1–O2	121.0 (10)	O1B–Li1–N1	100.5 (16)
O2–Li1–O1A	113.5 (3)	O2–Li1–N1	113.9 (2)
O1B–Li1–N2	114.0 (8)	O1A–Li1–N1	118.0 (3)
O2–Li1–N2	118.1 (2)	N2–Li1–N1	79.60 (16)
O1A–Li1–N2	109.8 (2)

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Fig. 3 The hydrogen-bonded polymeric arrangement of 1. The disordered part of ibu⁻ is truncated for clarity.

[Li₂(phen)₂(H₂O)₃][fibu]₂ (2). The molecular structure of 2 is shown in Fig. 4 and selected interatomic distances and angles are listed in Table 3. This complex is a binuclear centrosymmetric dimer with one half of the molecule comprising the asymmetric unit of the structure about an inversion centre. The Li⁺ cations are charge-balanced by two deprotonated fibu⁻ anions to satisfy the overall charge balance on the complex. Each Li⁺ ion adopts a four coordinated distorted spherical square pyramid geometry (Fig. S14 and Table S2, ESI†) comprising a [N₂O₅] donor set. The bond angles around Li⁺ are in the range 76.9 (1)–114.70°. The phen ligand adopts an N,N′-chelate mode through the N1 and N2 atoms, making a five-membered ring, with Li–N distances of 2.174 (4) and 2.176 (3) Å. The Li–N bond lengths lie within the normal observed range of 2.04–2.29 Å for related lithium complexes with phen ligands.⁴⁴ The Li–O_water distances are 2.027 (5) and 2.152 (3) Å, slightly larger than the hydrated Li⁺ complexes reported by others.⁴⁴ The two crystallographically identical Li⁺ are connected to each other via three bridging water ligands leading to a Li⋯Li separation of 2.661 (4) Å. The un-coordinated carboxylate oxygen (O2) of fibu⁻ forms hydrogen bonds with two water molecules, each of which are coordinated to neighboring molecules (d_O–H⋯O = 2.732 and 2.822 (2) Å, and θ_O–H⋯O = 173.4 and 175.7°), resulting in a supramolecular hydrogen-bonded chain parallel to the crystallographic b axis (Fig. 5). Lithium complexes containing both phen and fibu mixed ligands are unknown (CSD 2019).

Fig. 4 Molecular structure of [Li₂(phen)₂(H₂O)₃][fibu]₂ (2). Displacement ellipsoids are drawn at the 50% probability level.

Table 3 Selected geometric parameters (Å, °) for 2

Symmetry code: (i) −x, y, −z + 1/2
O5–Li1i	2.113 (3)	O5–H5B	0.8186
O5–Li1	2.151 (3)	O7–Li1i	2.027 (3)
O5–H5A	0.8281	O7–Li1	2.027 (3)
O7–Li1–O5i	83.59 (11)	O5–Li1–N2	162.66 (15)
O7–Li1–O5	82.63 (10)	N1–Li1–N2	76.88 (10)
O5i–Li1–O5	85.83 (11)	O7–Li1–Li1i	48.98 (8)
O7–Li1–N1	119.66 (14)	O5i–Li1–Li1i	52.04 (10)
O5i–Li1–N1	156.62 (15)	O5–Li1–Li1i	50.76 (10)
O5–Li1–N1	94.36 (13)	N1–Li1–Li1i	141.1 (2)
O7–Li1–N2	114.70 (14)	N2–Li1–Li1i	141.4 (2)
O5i–Li1–N2	96.32 (13)

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Fig. 5 The hydrogen-bonded polymeric arrangement of 2.

[K(phen)(ibu)(ibuH)]_n (3). Complex 3 crystallizes in the triclinic space group P1̄; the asymmetric unit is shown in Fig. 6, and selected interatomic distances and angles in Table 4. The K⁺ adopt a distorted [N₂O₄] geometry with one phen ligand acting as an N₂ donor (K–N_phen = 2.826 (4) and 2.830 (6) Å) and protonated and deprotonated ibu ligands adopting a 1,3-O,O′-bridging mode (K–O = 2.681 (3)–2.875 (4) Å). The oxygen atoms O3 and O3′ (and O1/O1′) are related via an inversion center and appeared to be very close to each other (O3⋯O3i = 2.464 (5) Å and O1⋯O1i = 2.473 (5) Å). An H-atom of half occupancy is associated with each O1⋯H⁺⋯O1 and O3⋯H⁺⋯O3 unit. The K–N bond lengths lie within the normal observed range of 2. 2.748 (4)–2.872 (2) Å for related potassium complexes with phen ligands.⁴⁴ The K⁺ cation is charge-balanced by an ibu⁻ anion, which satisfies the overall charge on complex 3. The six-coordinate coordination geometry around K⁺ is highly distorted and can be described as between a trigonal prism and a pentagonal pyramid (Fig. S15 and Table S3, ESI†). The phen ligand is virtually planar, and has similar bond distances and angles comparable to those found in the structure of free phen.⁵⁷ The K⁺ ion lies almost planar to the phen mean plane (δ_K = 0.061 Å); the phen ligand tilts with respect to the K1–N1–N2 plane by a dihedral angle of 2.05°. The bridging mode of the ibu⁻ (ibuH) ligands results in a 1-D infinite chain structure of 3 parallel to the crystallographic a-axis (Fig. 7). The K⁺ ions are connected to inversion related K⁺ ions via bridging of O_carboxylates (O1, O2, O3, and O4) atoms resulting in a zig-zag arrangement, so that an eight-membered ring K(μ-O)₄K is formed, in which the distance between the two K⁺ ions is 6.418 (2) Å. The structure is additionally stabilized by π⋯π interactions with cg⋯cg distances of 3.541 and 3.468 Å (depicted in Fig. S16, ESI†). Potassium complexes of phen are known; however, those with ibu as a co-ligand are not known (CSD 2019).

Fig. 6 The asymmetric unit in [K(phen)(ibu)(ibuH)]_n (3): displacement ellipsoids are drawn at the 50% probability level. The disordered part of ibu⁻ is truncated for clarity.

Table 4 Selected geometric parameters (Å, °) for 3

Symmetry codes: (i) −x + 1, −y + 2, −z + 1; (ii) −x, −y + 2, −z + 1.
N1–K1	2.826 (4)	K1–O1	2.695 (3)
N2–K1	2.830 (4)	K1–O4i	2.852 (3)
K1–O3	2.681 (3)	K1–O2ii	2.876 (3)
O3–K1–O1	105.77 (12)	N1–K1–O4i	77.99 (12)
O3–K1–N1	116.75 (13)	N2–K1–O4i	134.99 (12)
O1–K1–N1	127.59 (12)	O3–K1–O2ii	91.23 (11)
O3–K1–N2	133.89 (13)	O1–K1–O2ii	68.15 (10)
O1–K1–N2	111.12 (13)	N1–K1–O2ii	135.40 (12)
N1–K1–N2	57.33 (12)	N2–K1–O2ii	78.15 (12)
O3–K1–O4i	69.05 (10)	O4i–K1–O2ii	146.56 (13)
O1–K1–O4i	90.81 (11)

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Fig. 7 The oxygen bridged polymer chain in 3. An H-atom of half occupancy is associated with each O1⋯H⁺⋯O1 and O3⋯H⁺⋯O3 unit. The disordered part of ibu is truncated for clarity.

[Cs(phen)(fibu)]_n (4). The complex crystallizes in the monoclinic space group P2₁/n with one phen, a fibu⁻, and a Cs⁺ cation in the asymmetric unit (Fig. 8). Selected interatomic distances and angles are listed in Table 5. The fundamental repeat units in 4 are binuclear [Cs(phen)(fibu)] motifs which are connected to each other via μ₂-O_carboxylate (O1A and O2A) bridging. The Cs⁺ ion lies almost planar to the phen mean plane. The Cs⁺ ion is six-coordinate and adopts a distorted [N₂O₄] trigonal prism (Fig. S17 and Table S4, ESI†) with one phen ligand acting as an N2 donor [Cs–N_phen (N1, N2) = 3.184 (5) and 3.220 (3) Å] and two fibu⁻ ligands acting as μ₂-O donors [μ₂-O, Cs1–O1A = 3.24 (3) Å, and Cs1–O2A = 3.14 (3) Å]. The Cs⁺ ions are connected to each other via the bridging of O_carboxylate atoms, resulting in a zig-zag arrangement (Fig. 9) The distance between the two Cs⁺ ions is 4.639 (7) Å. The complex is additionally stabilized by π⋯π interactions between phen rings with cg⋯cg distances in the range 3.693–3.721 Å (Fig. S18, ESI†). Cs⁺ coordination chemistry with phen and/or fibu ligands is unknown and complex 4 is the first example containing phen and/or fibu⁻ ligands.

Fig. 8 Asymmetric unit in [Cs(phen)(fibu)]_n (4). Displacement ellipsoids are drawn at the 50% probability level.

Table 5 Selected geometric parameters (Å, °) for 4

Symmetry codes: (i) x, y − 1, z; (ii) −x + 1/2, y − 1/2, −z + 1/2; (iii) −x + 1, −y, −z + 1; (iv) −x + 1/2, y + 1/2, −z + 1/2; (v) x, y + 1, z.
Cs1–O2Bi	2.821 (16)	Cs1–N1	3.183 (5)
Cs1–O1A	2.892 (11)	Cs1–N2	3.221 (4)
Cs1–O1B	2.974 (19)	Cs1–O1Aii	3.24 (2)
Cs1–O2Ai	3.000 (17)	Cs1–O2Bii	3.31 (5)
Cs1–O1Bii	3.05 (3)	Cs1–C13ii	3.468 (4)
Cs1–O2Aii	3.14 (3)	Cs1–C6iii	3.907 (5)
O1A–Cs1–O2Ai	119.9 (7)	O1Bii–Cs1–N2	135.1 (6)
O1A–Cs1–O1Bii	119.0 (4)	O2Aii–Cs1–N2	125.7 (5)
O2Ai–Cs1–O1Bii	81.0 (7)	N1–Cs1–N2	50.62 (11)
O1A–Cs1–O2Aii	81.9 (5)	O1A–Cs1–O1Aii	111.9 (5)
O2Ai–Cs1–O2Aii	116.3 (7)	O2Ai–Cs1–O1Aii	78.6 (6)
O2Bi–Cs1–N1	113.8 (10)	O2Aii–Cs1–O1Aii	39.8 (3)
O1A–Cs1–N1	117.6 (5)	N1–Cs1–O1Aii	94.2 (4)
O1B–Cs1–N1	127.8 (6)	N2–Cs1–O1Aii	144.6 (4)
O2Ai–Cs1–N1	120.4 (6)	O1A–Cs1–O2Bii	80.4 (5)
O1Bii–Cs1–N1	84.9 (6)	O2Ai–Cs1–O2Bii	111.0 (6)
O2Aii–Cs1–N1	85.7 (5)	O2Aii–Cs1–O2Bii	7.1 (12)
O2Bi–Cs1–N2	106.4 (7)	N1–Cs1–O2Bii	92.5 (8)
O1A–Cs1–N2	91.2 (5)	N2–Cs1–O2Bii	132.5 (8)
O1B–Cs1–N2	101.1 (6)	O1Aii–Cs1–O2Bii	36.9 (4)
O2Ai–Cs1–N2	113.6 (4)

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Fig. 9 The oxygen bridged polymer chain in 4. The disordered part of fibu⁻ is truncated for clarity.

Anticancer activity

The cytotoxic potential of complexes 1–4 was evaluated against cancer cells [oral (CAL-27); lung (NCI-H460)], and a normal mouse fibroblast cell line (NIH-3T3). The synthesized alkali complexes showed potent anticancer activities against cancer cells and low toxicity toward normal cells, comparatively, Table 6. All complexes 1–4 showed potent anticancer potential against oral cancer with IC₅₀ values in the range of 1.93 ± 0.18–6.83 ± 0.19 μM, as compared to 5-Fluorouracil. Complexes 3 and 4 were found to be the most active for oral cancer having IC₅₀ values of 1.93 ± 0.18 and 2.74 ± 0.28 μM, respectively. Complexes 1 and 2 almost equally inhibited both cancer cells (oral and lung) with an IC₅₀ of 5.92 ± 1.97 and 6.83 ± 0.19 μM against CAL-27 (oral cancer), and 3.75 ± 0.097 and 6.60 ± 0.46 μM against NCI-H460 (lung cancer), respectively. However, both complexes 3 and 4 also inhibited lung cancer cells but at higher concentrations i.e. IC₅₀ values of 10.00 ± 4.93 and 46.60 ± 5.74 μM, respectively. All four complexes were found to be less cytotoxic against normal cells (NIH-3T3), which signifies their anticancer potential. Our results clearly suggested that complexes 1–4 have enhanced cytotoxic activity compared to free phen, ibu and fibu ligands or their equimolar mixtures of phen/ibu and phen/fibu (Table 6).

Table 6 Percent inhibitory concentration of the compounds on different cancer and normal cell lines

Complexes	Cytotoxic activities (μM)
	CAL-27	NCI-H460	NIH-3T3
	IC50 ± S.D.	IC50 ± S.D.	IC50 ± S.D.
phen (100 μM)	>100	>100	>100
fibu (100 μM)	>100	>100	>100
ibu (100 μM)	>100	>100	>100
phen (50 μM) + fibu (50 μM)	75.0 ± 1.90	68.00 ± 3	>100
phen (50 μM) + ibu (50 μM)	85.0 ± 2	84.00 ± 4	>100
1	6.00 ± 2	3.80 ± 0.10	143.00 ± 0.20
2	6.85 ± 0.20	6.60 ± 0.46	131.30 ± 6.50
3	2.75 ± 0.30	10.00 ± 5.0	108 ± 4
4	2.00 ± 0.20	46.60 ± 5.74	102 ± 3
5-Florouracil	2.40 ± 0.20	4.54 ± 1.67	>100

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Conclusions

We have successfully prepared four novel lithium, potassium and caesium complexes containing phen, and ibu/fibu co-ligands. In all four complexes, phen coordinated in an N,N-chelate mode, while the ibu-H/ibu⁻ and fibu⁻ ligands coordinated in a bidentate fashion. All complexes (1–4) showed significant anticancer activities against oral cancer (IC₅₀ = 1.93 ± 0.18–6.83 ± 0.19 μM), while complexes 1 and 2 have excellent anti-lung cancer activity (IC₅₀ = 3.75 ± 0.097 and 6.60 ± 0.46 μM), when compared to normal mouse fibroblast cells. The significant anticancer potential of these complexes clearly indicated that these complexes can be used as anti-cancer drug candidates for new drugs, but more detailed in vivo studies are required to understand their mechanism of action. Future reactions of the above mixed ligand systems with Group I and Group II metal ions are currently under investigation.

Experimental section

General considerations

Solvents, ligands and metal salts were obtained from commercial suppliers and used without further purification.

Physical measurements

Melting points were determined using a Stanford Research Systems MPA120 EZ-Melt automated melting point apparatus. UV-vis spectra were measured on an Agilent 8453 spectrophotometer using 10⁻⁵–10⁻⁶ M solutions in MeOH in the range 200–500 nm. IR spectra were obtained using a Bruker Alpha FT-IR spectrometer equipped with a Platinum single reflection diamond ATR module. The ¹H NMR spectra were recorded on a Bruker 600 MHz in DMSO-d₆. The degree of distortion of the coordination polyhedrons in complexes 1–4 with respect to an ideal polyhedron was calculated by continuous shape measure (CshM) theory utilizing SHAPE software.⁵⁸

X-Ray crystallography

Crystal data are summarized in Table 1. Single crystals of 1–4 were mounted on a MiTeGen loop with grease and examined on a Bruker D8 Venture APEX diffractometer equipped with a Photon 100 CCD area detector at 296 (2) K using graphite-monochromated Mo K_α radiation (λ = 0.71073 Å). Data were collected using APEX-II software,⁵⁹ integrated using SAINT⁶⁰ and corrected for absorption using a multi-scan approach (SADABS).⁶¹ The final cell constants were determined from full least squares refinement of all observed reflections. The structure was solved using intrinsic phasing (SHELXT).⁶² All non-H atoms were located in subsequent difference maps and refined anisotropically. H-atoms on carbons were added at calculated positions and refined with a riding model. O–H atoms were located in the difference map and refined isotropically with U(iso) riding on the O with DFIX constraints applied. The structures of 1–4 have been deposited with the CCDC (CCDC deposition numbers 2015737–2015740).† A powder X-ray diffraction (PXRD) scan was performed using a Bruker D8 Discover instrument operated at 40 kV with 2θ ranging from 5 to 50°.

Cell cytotoxicity assay

The cell cytotoxic potential of the synthesized complexes was evaluated by using 3-(4′,5′-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay as described previously against NCI-H460 (lung cancer), CAL-27 (oral cancer), and NIH-3T3 (mouse normal fibroblast) cell lines.⁶³ The cell lines were obtained from our institute biobank facility (PCMD, ICCBS, University of Karachi). The cells were passaged and maintained in Dulbecco's modified Eagle's medium (Gibco, USA) as described previously. The respective cells (10⁴ per well) were seeded in a 96 well microtiter plate and incubated overnight to form a uniform monolayer. The next day, the wells were washed and the monolayer was treated with different concentrations of alkali metal metallic complexes followed by incubation for a further 48 h. Following incubation, 10 μL of MTT dye (0.5 mg mL⁻¹) was added in each well and incubated for 4 h. After incubation the dye was removed carefully and the formed insoluble formazan crystals were dissolved in DMSO. The absorbance of the plate was recorded at 570 nm, and the percent inhibition of cells was calculated. The inhibitory concentration i.e. IC₅₀ was calculated by using Ez-Fit Enzyme Kinetics software (Perrella Scientific Inc., Amherst, NH, USA).

Preparation of [Li(phen)(H₂O)(Ibu)] (1)

Li₂CO₃ (0.0369 g, 0.5 mmol) and ibu (0.206 g, 1.0 mmol) were stirred together in an MeOH/H₂O (4 : 1) mixture. The reaction mixture was refluxed for ½ hour. Phen (0.198 g, 1 mmol) in 10 mL of MeOH solution was added dropwise to the above solution. The reaction mixture was further refluxed for ½ hour. The reaction mixture was then filtered and left undisturbed at room temperature. In two days small colourless transparent crystals appeared. The crystals were quite suitable for X-ray diffraction. Yield 0.289 g, 70%. Melting point 127–130 °C, selected IR data (cm⁻¹): ν_OH = 3683–3110, ν_C–H = 2957, ν_C=O = 1656, ν_C=C = 1590, ν_C=N = 1511. UV-vis (1.59 × 10⁻⁵ M, MeOH; λ_max, nm; ε, M⁻¹ cm⁻¹): 225 (5.1 × 10⁴), 263 (3.1 × 10⁴). ¹H NMR (600 MHz, MeOD) δ 9.11 (d, J = 3.8 Hz, 1H), 8.48 (d, J = 8.0 Hz, 1H), 7.79 (d, J = 8.0 Hz, 1H), 7.79 (d, J = 8.0 Hz, 1H), 7.28 (d, J = 7.9 Hz, 1H), 7.04 (d, J = 7.9 Hz, 1H), 3.57 (q, J = 7.1 Hz, 1H), 2.43 (d, J = 7.1 Hz, 1H), 1.83 (t, J = 13.5 Hz, 1H), 1.83 (d, J = 13.5 Hz, 1H), 1.41 (d, J = 7.1 Hz, 1H), 0.90 (d, J = 6.6 Hz, 1H).

Preparation of [Li(phen)₂(H₂O)₃(fibu)₂] (2). Li₂CO₃ (0.0369 g, 0.5 mmol) and fibu (0.244 g, 1.0 mmol) were refluxed for half an hour in 8 mL of MeOH. Phen (0.198 g, 1 mmol) in 5 mL of MeOH solution was added dropwise to the above reaction mixture. The reaction mixture was further heated and stirred for 30 minutes. The reaction mixtures were then cooled down, filtered and left undisturbed for crystallization at room temperature. In one week colorless transparent crystals were collected. Yield 0.312 g, 68%. Selected IR data (cm⁻¹): ν_OH = 3587–3133, ν_C–H = 2957, ν_C=O = 1722, ν_C=C = 1621, ν_C=N = 1514. UV-vis (1.19 × 10⁻⁵ M, MeOH; λ_max, nm; ε, M⁻¹ cm⁻¹): 229 (3.8 × 10⁴), 260 (4.5 × 10⁴). ¹H NMR (600 MHz, MeOD) δ 9.10 (d, J = 4.0 Hz, 1H), 8.47 (d, J = 8.0 Hz, 1H), 7.78 (d, J = 12.3 Hz, 1H), 7.78 (d, J = 3.7 Hz, 1H), 7.52 (d, J = 7.6 Hz, 1H), 7.42 (t, J = 7.6 Hz, 1H), 7.36 (d, J = 8.0 Hz, 1H), 7.25 (d, J = 8.0 Hz, 1H), 7.22 (d, J = 12.4 Hz, 1H), 3.64 (q, J = 7.1 Hz, 1H), 1.47 (d, J = 7.1 Hz, 1H).

Preparation of [K(phen)(ibu)(ibuH)]_n (3). K₂CO₃ (0.069 g, 0.5 mmol) and ibu (0.206 g, 1.0 mmol) were dissolved in 5 mL of an MeOH/H₂O (4 : 1) mixture. The reaction mixture was refluxed for one hour. Phen (0.198 g, 1 mmol) in 10 mL of MeOH solution was added dropwise to the above solution. The reaction mixture was further refluxed for an additional 30 minutes. The reaction mixture was then cooled down and the solution was filtered and left undisturbed at room temperature. Colorless transparent crystals appeared in a week. Yield 0.327 g, 71%. Selected IR data (cm⁻¹): ν_OH = 3646–3130, ν_C–H = 2951, ν_C=O = 1701, ν_C=C = 1618, ν_C=N = 1505. UV-vis (9.80 × 10⁻⁵ M, MeOH; λ_max, nm; ε, M⁻¹ cm⁻¹): 224 (6.5 × 10⁴), 262 (3.6 × 10⁴). ¹H NMR (600 MHz, MeOD) δ 9.11 (d, J = 4.1 Hz, 1H), 8.48 (d, J = 8.0 Hz, 1H), 7.80 (d, J = 4.3 Hz, 1H), 7.78 (d, J = 4.3 Hz, 1H), 7.26 (d, J = 7.8 Hz, 3H), 7.07 (d, J = 7.8 Hz, 3H), 3.62 (q, J = 7.1 Hz, 2H), 2.45 (d, J = 7.1 Hz, 3H), 1.85 (d, J = 13.4 Hz, 1H), 1.85 (t, J = 13.5 Hz, 2H), 1.43 (d, J = 7.1 Hz, 5H), 0.90 (d, J = 6.6 Hz, 10H).

Preparation of [Cs(phen)(fibu)]_n (4). Cs₂CO₃ (0.165 g, 0.5 mmol) and fibu (0.244 g, 1.0 mmol) were refluxed together in 12 mL of H₂O for 30 min. To this solution, phen (0.198 g, 1 mmol) in 5 mL of MeOH was added dropwise and the reaction mixture was further refluxed for an additional 30 min. The reaction mixture was then cooled down, filtered and left undisturbed at room temperature. Colorless crystals appeared after several days. Yield 0.302 g, 64%. Selected IR data (cm⁻¹): ν_C–H = 2964, ν_C=O = 1626, ν_C=C = 1580, ν_C=N = 1508. UV-vis (1.48 × 10⁻⁵ M, MeOH; λ_max, nm; ε, M⁻¹ cm⁻¹): 229 (5.5 × 10⁴), 260 (4 × 10⁴). ¹H NMR (600 MHz, DMSO) δ 9.09 (d, J = 4.1 Hz, 1H), 8.49 (d, J = 8.0 Hz, 1H), 7.77 (d, J = 8.0 Hz, 1H), 7.76 (d, J = 8.0 Hz, 1H), 7.44 (t, J = 7.5 Hz, 1H), 7.30 (t, J = 8.3 Hz, 1H), 7.13 (d, J = 18.7 Hz, 1H), 3.22 (q, J = 7.1 Hz, 1H), 1.23 (d, J = 7.1 Hz, 1H).

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

The authors would like to thank the University of Nizwa and The Oman Research Council (TRC) for their generous support.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 2015737 (1), 2015738 (2), 2015739 (3) and 2015740 (4). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0nj04585c

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