Mono- and poly-nuclear copper(II) carboxylates with flourous ligands: Synthesis, structure and improved properties

Inorganica Chimica Acta 498 (2019) 119177

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Research paper

Mono- and poly-nuclear copper(II) carboxylates with flourous ligands: Synthesis, structure and improved properties

T

⁎

Muhammad Iqbala, , Saqib Alib, Muhammad N. Tahirc, Arif Nawaza, Paul A. Andersond, Wilayat Khane a

Department of Chemistry, Bacha Khan University, Charsadda 24420, KPK, Pakistan Department of Chemistry, Quaid-i-Azam University, Islamabad 45320, Pakistan c Department of Physics, University of Sargodha, Sargodha, Pakistan d School of Chemistry, The University of Birmingham, Birmingham, UK e Department of Physics, Bacha Khan University, Charsadda 24420, KPK, Pakistan b

A R T I C LE I N FO

A B S T R A C T

Keywords: Flourous copper(II) complexes Structural study DFT Biological importance

Polynuclear copper(II) complexes [Cu(para-fluorophenyl acetate)2]n (1) and [Cu(para-nitrophenyl acetate)2]n (2) were converted in-situ to mono-nuclear complexes [Cu(para-fluorophenyl acetate)2(1,10-Phenanthroline) (H2O)] (3) and [Cu(para-nitrophenyl acetate)2(2,2́ -Bipyridine)(H2O)].3H2O (4). The complexes were isolated in quantitative yield, purified and characterized using FT-IR, absorption, electrochemical, electron paramagnetic resonance and powder XRD techniques yielding results in accordance with structural data. Structures of 1 and 2 consist of directly interlinked paddlewheel units with square pyramidal geometry while those of 3 and 4 are mononuclear with octahedral and square pyramidal geometry around Cu. Fluorinated complexes 1 and 3 were found to consist of extensive intermolecular interactions while 2 and 4 contained no such interactions. Their optical band gap was around 1.4 eV which indicates semiconducting property of the complexes. Moreover, the flourous complex 3 was found to possess significant activity against Bacillus subtilis and Escherichia coli and good activity against Micrococcus luteus. A similar trend was observed in their DNA-binding potency studied through absorption as well as electrochemical solution studies yielding Kb values 1.494 × 104, 1.342 × 104 and 1.411 × 104 M−1 and 7.547 × 104, 2.457 × 104 and 3.667 × 104 M−1 for 1–3, respectively. The electronic structures of the complexes elucidated using density functional theory ab-initio calculations confirmed the Intermediate states which play important role in the enhancement of the optoelectronic properties. The structure and properties of the flourous complexes were found different than their non-flourous analogues which was attributable to fluorine.

1. Introduction The properties of metal complexes are a function of metal as well as the attached ligands. The ligand centered properties depend mostly on the attached substitutes. Since there is lot of diversity in the functional groups, these affect the properties to variable extent. Replacing fluorine for other substituents usually brings striking variation in properties of the resultant complexes. The special chemistry of fluorine originated from its small size, low polarizability, nuclear spin = 1/2 and being the strongest grabber of electron. Once, substituted in a molecule, it exhibits its strong inductive effect via sigma bond. It also establishes strong intermolecular interactions with the sideling molecules in the lattice. The latter type of bonding arises from the ability of the fluorine to act as π-donor. Moreover, as a substituent on aromatic ring, its effect ⁎

is different at para-position (mostly de-activating) than at ortho- and meta-positions (usually activating) [1]. It has been inferred from the coupling constant values of NMR spectroscopy of the aromatic substituted compounds that fluorine acts as a π-donor at ortho-position while σ-acceptor at meta- or para-positions [2]. A comprehensive study on the recent progress in fluorine chemistry of the bio-active complexes has been carried out [3]. F is the favorite heteroatom after nitrogen whose substitution enhances the desired biological activity of the molecule such as its binding to the target molecule and membrane permeability [4]. Importance of fluorinecontaining compounds in bio-medicinal chemistry can be judged form the fact that several F-containing compounds are extensively used as chemotherapeutic agents [5] and many more are in various stages of clinical trials [6].

Corresponding author. E-mail addresses: [email protected], [email protected] (M. Iqbal), [email protected] (S. Ali).

https://doi.org/10.1016/j.ica.2019.119177 Received 1 July 2019; Received in revised form 2 September 2019; Accepted 26 September 2019 Available online 27 September 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved.

Inorganica Chimica Acta 498 (2019) 119177

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Effect of fluorine and other substituted ligands on structure and spectroscopic properties of the resulting complexes has been studied [7–10]. There is a pronounced effect on the non-covalent interactions of the molecules of the complexes on replacing F for other substituents. The anion-π interactions arising from F are strongest of all halogens and other substituents [11]. This type of interaction adds cumulatively when a complex has aromatic π-π stacking interactions arising from the phenyl rings and planar ligands such as 1,10-phenanthroline and 2,2′bipridine. Effect of substituent is so important that these secondary interactions are absent altogether on changing the substituent [12]. Similar two pairs of copper(II) complexes having flouro- and nitrosubstituted phenyl rings along with other hetero-aromatic planar ligands have been synthesized. Interestingly, the non-flourous pair of the complexes has no intermolecular non-covalent interactions and the flourous pair has ample amount of these stacking interactions. Moreover, the difference has been reflected in DNA-binding as well as antifungal properties as well. The electronic structures of the complexes were also studied theoretically using DFT calculations to understand the enhancement in the photocatalytic and optoelectronic properties.

atom were clearly indicated in the respective spectra of all the four complexes. 2.2. Electron paramagnetic resonance X-Band ESR spectra of powdered samples of 3 and 4 (Fig. 1B, C) showed typical copper(II) spectra with g⊥ values = 2.13246 and 2.07085, respectively. These spectra clearly indicate that copper is in 2+ oxidation state in these complexes. These spectra are similar to the X-band spectra observed for other mono-nuclear copper(II) complexes with two carboxylate ligands and 1,10-phenanthroline and 2,2́ -bipyridine [16,17]. The spectrum of 2 (Fig. 1A) has an observable curve between G values 3000 and 4000. The signal of copper(II) is usually observed in this region. Such diffused signal was observed for the other structural analogue of 2 as well [18]. 2.3. Powder XRD studies Being NMR-silent, the single crystalline samples of copper(II) complexes were subjected to powder XRD technique and the resulting spectra of complexes 1, 3 and 4 are shown in Fig. 2A–C. These spectra indicate several well defined peaks between 2 theta values 5 and 30 which show the homogenous crystalline nature of these complexes. The theoretical spectra were obtained from the respective single crystal XRD data which were superimposed on the experimental powder XRD spectra. These spectra show the bulk purity of the synthesized complexes.

2. Results and discussion Four complexes have been synthesized and purified in quantitative yield and characterized structurally. 2.1. FT-IR spectroscopy results The most pronounced peaks in the spectra of all the four complexes were near 1600 and 1400 cm−1 which were attributed to the stretching vibrations of the carboxylate moieties attached to copper. The difference in the asymmetric and symmetric stretching vibrations were 195, 179, 136 and 252 cm−1, respectively for 1–4. These values indicate bridging bidentate coordination mode of carboxylate ligand in 1 and 2, chelating bidentate binding mode in 3 and mono-dentate coordination mode in 4. These assignments were in accordance to the structural data of the complexes 1–4. There was a peak of low intensity in spectra of each complex between 400 and 420 cm−1 attributable to the Cu–O bond stretch. Nitro-group in 2 and 4 was indicated by two intense bands observed in the region 1341–1435 cm−1. The appearance of C]N stretching band of the complexes 3 and 4 in a frequency range 1586–1600 cm−1 instead of its normally observed characteristic region (1610–1625 cm−1) [13,14] indicated the involvement of the nitrogen atom of 1,10-pnhenanthroline and 2,2́ -bipyridine in bonding with copper(II) ion [15]. In support to this, Cu–N bonding was indicated by bands at 468 and 481 cm−1 for 2 and 4 which indicated the attachment of heteroaromatic ligands through N. All the rest of the functionalities such as methylene-H, aromatic CeH and the absence of carboxylate-H

2.4. Electronic spectra, optical band gap and stability studies Electronic spectra of complexes 1–4 indicated a broad absorption band at λmax = 725, 744, 691 and 658 nm, respectively as shown in Fig. 3A, indicating penta-coordinated square pyramidal geometry around copper in 1 and 2 [18] and hexa-coordinated octahedral geometry in 3 and 4 [19] as observed in similar regions for copper(II) complexes of same geometry and neuclearity. The absorption behavior of 1 and 2 indicate the stable nature of the paddlewheel subunits where the maximum number of ligands attached with Cu can be five and the inner site is always unoccupied. Absorption spectra of 3 and 4 are typical of the usual octahedral coordination around copper(II). The broad absorption bands of the complexes indicate that these can harvest a wide range of wavelengths in UV–Visible region of the electromagnetic spectrum. This property enables them to be used as photosensitizers in semiconductors when combined with nano-films of crystalline oxides [20]. The optical band gaps of the complexes have been calculated from the extrapolation of the plots (shown in Fig. 3B) between (Ahc)2 and hc/λ where A is the absorbance and the rest of the 2

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better than some previously reported Co, Ni, Cu and Zn-complexes [20]. The stability of the geometry of complexes in the solvent system has been checked by taking spectra at various intervals as shown in Fig. S1. These indicate that the complexes retain their geometry in solution.

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For better insight into the optoelectronic properties, the partial density of states (PDoS) in both regions (valence and conduction bands), especially in the conduction band region of the complexes has been described as shown in Fig. 4. Considering different chemical compositions of 1–4, figures describe different intermediate bands around the Fermi level contributed by different electronic states of the respective complexes. Where different transitions can take place and exhibit different absorbing peaks in UV spectra (Fig. 3). The Cu-d-orbitals have different intensities and divergence positions around the Fermi level corresponding to their elemental composition in complexes 2, 3 and 4, respectively. But in all cases the contribution of Cu-d-orbitals is predominant to the intermediate bands in addition to the minor contribution of O-p-orbitals (in case of 2 and 4) and of O-p/Np orbitals (in case of 3). Other orbitals from different elements of the complexes have minor contributions shown in the inset of the Fig. 4AD. The O-p-orbitals and Cu-d-orbitals in case of 2 and 4 and O-p-orbitals, Cu-d-orbitals and N-p-orbitals show interaction in the region around Fermi level and from −2 to −4 eV. The position of np/ns orbitals of non-metals, transition metals and halogens is relatively constant and they follow different trends in intensities (see insets of Fig. 4). To compare the experimental spectra with the theoretically simulated PDoS, our findings have nicely explained the peaks. The main peaks in the UV spectra come from the transitions of electrons from the orbitals lying in the main band gap between valence band and conduction band, while the small peaks originated from the transition of electrons from the orbitals lying in the intermediate band gap.

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Oxidation and reduction behavior of the synthesized complexes was checked by cyclic voltammetry which yielded cyclic voltammograms shown in Fig. 5. Since the electrochemistry is metal centered, and every complex has Cu2+ center, therefore, all the four complexes have given similar voltamograms. The oxidation signals of the complexes were in potential range 0–0.3 V while the reduction signals were in potential ranging from −0.1 to − 0.3 V. the signals of polymeric complexes 1 and 2 were assigned to CuIICuII/CuIICuI process while those of the mononuclear complexes 3 and 4 were ascribed to CuII/CuI process as observed in structurally similar copper(II) carboxylate complexes [18,19]. The peak separation values (ΔE = Epa – Epc) and peak current ratios (Ipc/Ipa) of complexes 1–4 are 250, 350, 250 and 450 mV, and 0.758, 0.843, 1.3 and 3, respectively. These parameters as well as the shifting of peak potential with scan rate indicate that the redox processes can be safely termed as irreversible processes [21]. Owing to the far lying substituents on the aromatic ring, their relative effect was not obvious on the redox properties of central copper ion however, the observed electrochemical behavior was typical of the copper(II) complexes and helped to indicate stable Cu(II) center in solutions of the complexes.

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Fig. 2. experimental and simulated powder XRD spectra of the complexes 1(A), 3(B) and 4(C) showing coincidence of the peaks on 2 theta values.

Molecular structures of the complexes have been shown in Figs. 6–9 while their refinement parameters and selected bond lengths and angles have been listed in Tables 1–3. 1 and 2 adopt polynuclear motifs where the secondary paddlewheel (pw) units are interlinked with one another directly through their Cu and O atoms. The inter-pw Cu – O distance is 2.21 and 2.195 Å for 1 and 2. There is a debate whether this is a genuine covalent bond or a secondary interaction. It was pointed out that a

parameters have usual meanings. The extrapolation to the x-axis of the plot showed the band gap values between 1.3 and 1.5 eV. These values clearly indicate that these complexes can find potential applications in photovoltaic materials. Moreover, the band gap values were found 3

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Fig. 3. A, Absorption peaks of the complexes showing broad bands. B, optical band gap of the complexes.

Fig. 4. A, B, Calculated partial density of states of Cu-d-orbitals, Cu/O/N/C np/ns orbitals and H 1s orbitals (complexes 2 (B) and 4 (D)) and C, calculated partial density of states of Cu-d-orbitals, Cu/O/N/C/F np/ns orbitals and H 1s orbitals (complexes 1 (A) and 3 (C)).

distance of Cu – O/N/F up to 2.4 Å might be considered a definite covalent bond; 2.4–2.8 Å indicates a weaker secondary Cu∙∙∙L bond primarily electrostatic in nature; while > 2.8 Å is mere van der Waals bond [22]. As per this criterion, the structure of 1 and 2 are definitely polymeric. This pw-pw distance was found typical of other structural analogue without a substituent on phenyl ring [18]. This shows that the electronic effect of the substituents on aromatic ring have little effect on electronic properties of central copper ion as observed in electrochemical behavior. However, the effect of substituent is observable in

the form of the small contraction in Cu⋯Cu distance within the pw units of substituted (NO2 and F in 1 and 2) and non-substituted [15] phenyl rings of the phenyl acetate ligands. The former pair has Cu⋯Cu distance = 2.574 and 2.576 Å compared to 2.584 Å observed for the later [18]. This contraction is in accordance to the slightly higher iconicity of the substituted than the non-substituted phenyl acetate ligands. Cu∙∙∙Cu distance within pw is further increased if donating ability of ligand is increased as observed in dinuclear pw complexes with soft pyridine ligands [23]. The decrease in carboxylate donor strength of 1 4

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Fig. 9. Structure of complex 4 with atom numbering scheme.

copper(II) is 5-coordinated (CuO5) square pyramidal with elongated Cu – O bond on the apical position. The longer Cu – O bond on the apical position is weaker which is broken easily when a stronger Lewis base is added to solution of the polymeric complex where the added ligand competes successfully for this bond and the structure is converted to binuclear pw complex [24]. The solubility of 1 and 2 in common solvents contrasted to MOF type copper(II) complexes [25] also indicated that the weaker bond is broken and the structure is discrete binuclear in solution form rather than intact polymeric. The electrochemical solution and absorption spectroscopic studies yielded results which were typical of binuclear pw–complexes [18,24]. Complexes 3 and 4 are the mono-nuclear structural analogues of the 1 and 2 with reference to the carboxylate functionality. These were resulted owing to the relative preference of the intermediate hard Cu2+ ion for the N-donor ligands over the O-donor ones. Complete replacement of the substituted carboxylate ligands by simple heterocyclic rings was probably rendered difficult by the overriding solvation effect. Both complexes contain two carboxylate ligands and a 1,10-phenanthroline (3) and 2,2́ -bipyridine (4) moieties. Complex 3 has Jahn-Teller distorted octahedral geometry with elongated Cu–O and Cu–N bonds = 2.429 and 2.124 Å compared to the normal Cu–O and Cu–N bonds = 2.147 and 2.006 Å. The trans O-Cu–N angle was found to be 156.14°. The striking angle deformation from 180° may be attributed to the chelating carboxylate and phenanthroline moieties. One of the carboxylate ligands is monodentate as compared to the para-methyl analogue which has bidentate carboxylate ligands [19] and, probably owing to this reason, the latter complex has more distortion from the ideal octahedral geometry as far as angles and distances are concerned. Complex 4 has both carboxylate ligands as mono-dentate and a water molecule as fifth coordination member. Thus the geometry is distorted square pyramidal. The octahedral analogue of this complex has both carboxylate ligands acting in bidentate fashion [26] without water molecule. The packing interactions are quite different from one another owing to variation in the structure and substituents. F atom of 1 is actively involved in packing interactions and there are ample interlayer contacts as compared to 2 which has no interlayer contacts as shown in Figs. 10–13 similar picture is presented by 3 and 4 where the molecules in 3 are held together by extensive OeH⋯C and CeH⋯C interactions while no such inert-molecular contacts are found in 4 as shown in Figs. 9–12. Moreover, owing to the small distance of 3.692 Å between phenanthroline ligands, there are π–π interactions in 3 while no such interactions are found in 4 because of the bipyridine rings are far apart from each other. The 1D polymeric chain structures of 1 and 2 are shown in Figs. S2 and S3.

Fig. 6. Structure of a paddlewheel unit of complex 1 with atom numbering scheme.

Fig. 7. Structure of a paddlewheel unit of complex 2 with atom numbering scheme.

Fig. 8. Structure of complex 3 with atom numbering scheme.

and 2 is also exhibited in slight shortening of Cu – O bond distances (1.95 Å on the average for both) of the square base around Cu(II) compared to the unsubstituted analogue (1.96 Å average) [18]. Within the pw unit, Cu – O bond distances are shorter i.e., 1.95 Å on the average compared to the pw-pw Cu – O bond. Overall, geometry around 5

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Table 1 Structure refinement parameters of complexes 1–4. Complex

1

2

3

4

Empirical formula Formula weight (g mol−1) Temperature (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) α (°) β (°) γ (°) Volume (Å3) Z ρ (calculated) (Mg/m3) Absorption coeff. (mm−1) F(0 0 0) Crystal size (mm3) θ range (°) Index ranges

C32H24F4Cu2O8 739.59 296(2) 0.71073 Monoclinic C 2/c

C16H12CuN2O8 423.82 296(2) 0.71073 Monoclinic P 21/c

C28H22CuF2N2O5 568.01 296(2) 0.71073 Monoclinic P 21/n

C26H24CuN4O10 616.03 296(2) 0.71073 triclinic P-1

26.1472(19) 5.1647(3) 22.2109(14) 90.00 98.892(4) 90.00 2963.4(3) 4 1.658 1.513 1496 0.25 × 0.20 × 0.18 2.24 to 25.25 −30 ≤ h ≤ 30 −6 ≤ k ≤ 6 −26 ≤ l ≤ 26 10,610 2673 99.9 Full-matrix LS on F2 2673/24/242 0.981 R1 = 0.0957, wR2 = 0.0502 R1 = 0.1170, wR2 = 0.1039

5.1547(7) 16.031(3) 20.523(3) 90 92.427(4) 90 1694.5(4) 4 1.661 1.339 860 0.30 × 0.25 × 0.18 2.358 to 27.735 −5 ≤ h ≤ 6 −20 ≤ k ≤ 20 −26 ≤ l ≤ 26 10,974 3843 96.80 Full-matrix LS on F2 3843/0/244 0.927 R1 = 0.0544, wR2 = 0.0966 R1 = 0.1299, wR2 = 0.1240

13.0522(10) 12.0762(11) 16.4526(13) 90 101.533(5) 90 2540.9(4) 4 1.485 0.916 1164 0.30 × 0.24 × 0.23 1.824 to 26.000 −16 ≤ h ≤ 16 −14 ≤ k ≤ 14 −6 ≤ l ≤ 20 19,689 4985 99.90 Full-matrix LS on F2 4985/0/350 1.016 R1 = 0.0422, wR2 = 0.0859 R1 = 0.0745, wR2 = 0.0985

6.9891(2) 14.2334(4) 15.3188(5) 65.5750(10) 82.9650(20) 79.2140(10) 1361.26(7) 2 1.503 0.866 634 0.38 × 0.30 × 0.30 2.56 to 25.25 −8 ≤ h ≤ 8 −17 ≤ k ≤ 16 −18 ≤ l ≤ 17 20,979 4940 99.90 Full-matrix LS on F2 4940/3/282 1.047 R1 = 0.0294, wR2 = 0.0744 R1 = 0.0345, wR2 = 0.0798

Reflections collected Independent reflections Completeness to θ % Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2σ (I)] R indices (all data)

Table 2 Selected bond lengths and angles of complexes 1 and 2. Bond

Table 3 Selected bond lengths and angles of complexes 3 and 4.

Complex 1

Complex 2

3

Distances, Å Cu(1)-O(1) Cu(1)-O(2) Cu(1)-O(3) Cu(1)-O(4) Cu(1)-O(5) Cu(1)-O(2) Cu(1)-Cu(1) Cu(1)-O(6)

1.957(3) 2.015(3) 1.935(4) 1.942(4) — 2.211(3) 2.5743(10) —

O(4)-Cu(1)-O(1) O(2)-Cu(1)-O(3) O(1)-Cu(1)-O(2) O(1)-Cu(1)-O(3) O(4)-Cu(1)-O(2) O(3)-Cu(1)-O(4) O(2)-Cu(1)-O(3) O(4)-Cu(1)-O(2) O(1)-Cu(1)-O(2) O(2)-Cu(1)-O(2) O(5)-Cu(1)-O(6) O(5)-Cu(1)-O(1) O(6)-Cu(1)-O(1) O(5)-Cu(1)-O(2) O(6)-Cu(1)-O(2)

Angles, ° 90.19(15) 91.81(15) 169.68(12) 88.21(16) 87.92(14) 169.51(15) 96.05(15) 94.26(14) 109.62(12) 80.64(13) — — — — —

1.948(3) 2.011(3) — — 1.924(3) 2.195(3) 2.5760(10) 1.940(3) — — 169.84(11) — — — — — — 81.36(12) 169.74(12) 89.98(13) 88.67(13) 88.81(13) 90.72(13)

6

4

Bond

Distances, Å

Cu(1)-O(1) Cu(1)-O(2) Cu(1)-O(3) Cu(1)-O(5) Cu(1)-N(1) Cu(1)-N(2) Cu(1)-O(9)

2.147(3) 2.429(3) 1.937(2) 2.085(3) 2.123(3) 2.006(2) —

1.9383(14) — — 1.9443(16) 2.0000(17) 2.0094(16) 2.3149(15)

O(1)-Cu(1)-O(2) O(1)-Cu(1)-O(3) O(1)-Cu(1)-N(2) O(3)-Cu(1)-N(2) O(1)-Cu(1)-N(1) O(3)-Cu(1)-N(1) N(2)-Cu(1)-N(1) O(2)-Cu(1)-O(3) O(5)-Cu(1)-O(3) O(5)-Cu(1)-N(2) O(5)-Cu(1)-N(1) O(5)-Cu(1)-O(1) O(5)-Cu(1)-O(2) O(1)-Cu(1)-O(9) O(5)-Cu(1)-O(9) O(9)-Cu(1)-N(1) O(9)-Cu(1)-N(2) O(2)-Cu(1)-N(1)

Angles, ° 93.40(9) 91.71(10) 87.64(10) 174.60(10) 101.38(10) 95.16(10) 79.74(10) 93.40(9) 94.11(10) 88.82(10) 103.61(10) 153.69(11) 97.93(10) — — — — 156.14(10)

— — 170.81(6) — 94.24(7) — 80.77(7) — — 90.23(7) 165.53(6) 93.06(6) — 89.78(6) 90.54(6) 101.95(6) 98.77(6) —

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Fig. 10. Packing diagram of the complex 1 where each molecule represents a 1D-layer as shown in Fig. S1. The layers are held together by C⋯HeC interactions.

Fig. 13. packing diagram of complex 4 showing no intermolecular interactions.

2.8. DNA-binding study In order to explore the complexes for their biological relevance, these were screened for their ability to interact with DNA. The decrease in the active concentration of the complexes on interaction with added amount of DNA was indicated by the absorption spectroscopy as well as cyclic voltammetry. Complex 1 exhibited clear red shift of 29 nm with DNA addition as shown in Fig. 14 A. This is indicative of predominantly an intercalative mode of the interaction of the complex with DNA. The intercalation would result from the aromatic rings being able to get inserted between the DNA base pairs. Other complexes also exhibited red shift which is indicative of the same intercalative mode of interaction with DNA but of smaller extent (Fig. 15A and B) while 2 exhibited small blue shift (Fig. 14B). The latter is attributed to the concomitant electrostatic with groove binding mode of interaction along with intercalative mode. This mode of binding might be due to the suitably oriented nitro group giving rise to ionic interactions with polar groups of DNA [27]. The quantitative binding abilities of the complexes with DNA were determined from the slope to intercept ratio of the respective plots between reciprocal molar concentration of DNA and the relative absorbance values of the respective complexes as shown in Figs. S4–S7. These values have been listed in Table 4 which are similar to those calculated spectrophotometrically for some already reported copper complexes with similar structures [18,19,26]. Since the complexes are electroactive owing to the redox-active metal center, the decrease in the concentrations of the complexes in solution can be followed in cyclic voltammetry as well where the net current decreases as a result of addition of DNA. The resulting voltammograms before and after DNA addition have been shown in Figs. S8–S11. Since there is a change in current as a function of concentration of DNA, a plot of logarithmic values of the relative current vs reciprocal molar concentration of DNA (shown in Figs. S12–S15) gives a constant value of intercept. This value is called binding constant which are listed in Table 4 which are higher than some other copper complexes determined through the same technique [18,19,26]. Since there is reduction in concentration of the free electro-active complex moving to the electrode surface, the value of diffusion co-efficient Do should suffer diminution as a result of addition of DNA. This has been proved to be so where Do values have been calculated for all the complexes before and after DNA addition using Randles–Sevcik equation [28]. These values have been listed in Table 4 which show a considerable loss in Do values on addition of DNA indicating that the complexes have potent DNA binding abilities.

Fig. 11. Packing diagram of the complex 2 where each molecule represents a 1D-layer as shown in Fig. S2. The layers are held together by C⋯HeC interactions.

Fig. 12. packing diagram of complex 3 showing intermolecular interactions.

7

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Fig. 14. Absorption peaks of complexes 1(A) and 2(B) before (uppermost peak) and after DNA addition (lower peaks) whereby the significant reduction in absorbance as well as shift in λmax on addition of DNA indicate complex-DNA interaction.

Fig. 15. Absorption peaks of complexes 3(A) and 4(B) before (uppermost peak) and after DNA addition (lower peaks) whereby the significant reduction in absorbance as well as shift in λmax on addition of DNA indicate complex-DNA interaction.

It is evident that the fluorous complexes in both polymeric and monomeric pairs of complexes give rise to higher DNA-binding constants and thus exhibit better DNA binding activity. This might be due to the small size and higher electrostatic influence of the fluorine responsible for the electrostatic groove binding interactions of the fluorous complexes with DNA-helix. Such increased activity of the floro-substituted copper complexes relative to the unsubstituted, methyl- and methoxy-substituted copper complexes has been observed already [24]. The mode of DNA-binding behavior was further supported from the viscosity measurement of the complex-bound-DNA solution. The increase observed in the relative viscosity of DNA solution with incremental amounts of the complexes, indicated classical intercalation mode of interaction of the complexes with DNA helix as shown in Fig. 16. This results from the lengthening of the DNA-helix as a result of insertion of the planar moieties of the complexes into the DNA-base pairs.

Table 4 Do and Kb values of complexes 1–4. Complex

Do before DNA addition × 10−8

Do after DNA addition × 10−8

Cyclic voltammetry Kb × 104

Absorption Kb × 104

1 2 3 4

23.170 121.101 5.778 1.807

0.891 36.192 0.022 1.337

7.547 2.457 3.667 1.820

1.494 1.342 1.411 1.351

1.2 3 4

Relative Viscosity

1.0

2 1

0.8 0.6

2.9. Anti-bacterial study

0.4

The complexes were screened for their anti-bacterial activity against gram positive as well as gram negative bacterial strains. It was noticed that only complex 3, the mononuclear fluorous complex showed excellent anti-bacterial activity as given in Table 5 This may be due to the small size of 3 as well as the F-substituent which gives rise to optimum penetrability to the molecule through the bacterial cell wall.

0.2 0.0

0.6

0.9

1.2

1.5

1.8

2.1

2.4

2.7

[Complex] / [DNA] Fig. 16. Plots of the [complex]/[DNA] vs. relative viscosity of complexes 1–4. The increase in viscosity indicated intercalative mode of complexes.

3. Conclusions Four new (two polynuclear and two mononuclear) complexes of 8

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Table 5 Antibacterial data of complex 3: Average zone of inhibition (mm) and Minimum Inhibitory Concentration (mg/mL). Bacterial strain Parameter Average zone of inhibition (mm) Minimum Inhibitory Concentration (mg/mL)

Complex 3 Cefixime Complex 3 Cefixime

Micrococcus luteus

Bacillus subtilis

Escherichia coli

Staphylococcus aureus

18 33 0.25 0.05

23 29 0.25 NA

26 30 0.25 0.03

18 35 0.5 0.02

Maximum Concentration: 1 mg/mL in DMSO. Reference drug, Cefixime: 1 mg/mL.

chloroform and methanol (1:1) yielding crystals of 1. Complex 2: The same procedure was followed for 2 except replacing 4–nitrophenyl acetic acid (6 mmol, 0.90 g) for 4–florophenyl acetic acid (6 mmol, 0.925 g). Complex 3: 3 was prepared by adding 1,10-phenanthroline (0.540 g, 3 mmol) to the reaction mixture after 3 h stirring of the appearance of precipitates of 1. the resulting precipitates were washed with distilled water and air-dried. Complex 4: 4 was prepared by adding 2,2́ -bipyridine (0.468 g, 3 mmol) to the reaction mixture after 3hstirring of the appearance of precipitates of complex 2. The resulting precipitates were washed with distilled water and air-dried. The dried product was purified and crystallized from a mixture of chloroform and methanol (1:1).

copper(II) with para-floro- and nitro-substituted-2-phenyl acetates have been synthesized, characterized and their properties explored. The polynuclear complexes have been successful converted to the corresponding mononuclear analogues on addition of N-donor bidentate ligand to the reaction mixture containing the polynuclear complex. Both polynuclear complexes contained paddlewheel secondary building units (with square pyramidal copper) linearly interlinked via Cu and Oatoms without intervening moieties. However, the corresponding mononuclear analogues contained octahedral copper(II) centers with two carboxylates and an N-donor ligand. The powder XRD spectra indicated the uniformity of the crystalline samples. While the +2 oxidation state of the metal was confirmed from the ESR spectra of the complexes. Each complex gave an optical band gape near 1.4 eV which is a good indication that complexes can find applications in the field catalysis as well. The complexes exhibited irreversible metal centered electro-activity assignable to CuII/CuI processes. The complexes have given rise to excellent DNA-binding activity checked by absorption spectroscopy and cyclic voltammetry with DNA-binding constant values 1.494 × 104, 1.342 × 104 and 1.411 × 104 M−1 and 7.547 × 104, 2.457 × 104 and 3.667 × 104 M−1 for 1–3, respectively. Moreover, the fluorous complex 3 was found to possess significant activity against Bacillus subtilis and Escherichia coli and good activity against Micrococcus luteus. The fluorine substituted analogues have marked difference in their structures, supra-molecular synthons, DNA-binding ability and biological activity. The ab-initio calculations of DFT to determine the electronic structure revealed intermediate states around the Fermi level, which endorses that these complexes can be efficiently used in photocatalytic materials.

Appendix A. Supplementary data CCDC 934916, 1880883, 951572 and 937195 correspond to the crystallographic data of complexes 1–4, respectively, reported in this manuscript. For free acquisition of the data: Fax: +44-1223-336-033; E-Mail: [email protected], http://www.ccdc.cam.ac.uk. Complex 1: Blue crystals; m.p. 218–220 °C; yield (60%). FT-IR (cm−1): 1592 ν(OCO)asym, 1395 ν(OCO)sym, Δν = 197, 2948 νCH2, 3020 ν(Ar–H), 1505 νAr(C]C), 1211 ν(Ar–F), 420 ν(Cu–O). Elemental analysis: Calculated (%): C, 51.89; and H, 3.24. Found (%):C, 52.09; and H, 3.19. Complex 2: Light blue crystals; m.p. 190–191 °C; yield (65%). FT–IR (cm−1): 1615 ν(OCO)asym, 1436 ν(OCO)sym, Δν = 179, 2920 νCH2, 3052 ν(Ar–H), 1580, 1437 νAr(C]C), 1431, 1341 ν(NO2), 414 ν(Cu–O). Elemental analysis: Calculated (%): C, 45.30; H, 2.83; and N, 6.60. Found (%): C, 45.28; H, 2.79; and N, 6.55. Complex 3: Blue crystals; m.p. 178–180 °C; yield (65%). FT–IR (cm−1): 1580 ν(OCO)asym, 1444 ν(OCO)sym, Δν = 136, 2962 νCH2, 3084 ν(Ar–H), 1595, 1473 νAr(C]C), 1206 ν(Ar–F), 414 ν(Cu–O), 468 ν(Cu–N). Elemental analysis: Calculated (%): C, 59.15; H, 3.87; and N, 4.93. Found (%): C, 59.08; H, 3.90; and N, 4.89. Complex 4: Blue crystals; m.p. 169–170 °C; yield (65%). FT–IR (cm−1): 1570 ν(OCO)asym, 1439 ν(OCO)sym, Δν = 131, 2950 νCH2, 3032 ν(Ar–H), 1595, 1447 νAr(C]C), 1435, 1340 ν(NO2), 420 ν(Cu–O), 481 ν(Cu–N). Elemental analysis: Calculated (%): C, 50.65; H, 3.89; and N, 9.09. Found (%): C, 50.68; H, 3.82; and N, 8.99. Supplementary data to this article can be found online at https:// doi.org/10.1016/j.ica.2019.119177.

4. Experimental 4.1. Chemicals and methods Analytical grade materials, used in this work were purchased from suppliers. Melting point was determined by an electrothermal apparatus and FT-IR spectra recorded on a Nicolet-6700 spectrophotometer fitted with ATR. Powder XRD data was acquired using a PANalytical, X́ Pert PRO diffractometer having Cu-Kα radiation (λ = 1.540598 Å) at 298 K. X-band electron spin resonance (ESR) spectra were obtained using a Bruker ESP-300 spectrometer having X-band frequency (−9.5 GHz). See supplementary materials section for experimental protocols of single crystal XRD study, electrochemistry, absorption spectroscopy, computational details and antibacterial studies.

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4.2. Synthesis of the complex Complex 1: Sodium bicarbonate (0.504 g, 6 mmol) was reacted with an equimolar quantity of 4–florophenyl acetic acid (6 mmol, 0.925 g) at 60 °C in distilled water. After complete neutralization of the acid with base, the aqueous solution of copper sulphate (0.240 g, 3 mmol) was added drop wise. The reaction mixture was stirred for 4 h at 60 °C and the resulting precipitates were filtered, washed thoroughly with distilled water and air dried. The solid was recrystallized from a mixture of 9

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