Synthesis, structural characterization, and antimicrobial evaluation of new mononuclear mixed ligand complexes based on furfural-type imine ligand, and 2,2′-bipyridine

The present investigation goal was to investigate the chemistry of four new mononuclear mixed ligand Fe(III), Co(II), Cu(II), and Cd(II) complexes constructed from furfural-type imine ligand (L), and the co ligand 2,2′-bipyridine in addition to assessing their antimicrobial activity against some bacterial, and fungi strains. The structure of the complexes was interpreted by different spectroscopic techniques such as MS, IR, 1H NMR, UV–Vis, elemental analysis, TG-DTG, conductivity, and magnetic susceptibility measurements. The correlation of all results revealed that ligand (L) acts as a neutral ONNO tetradentate whereas the co ligand acts as a neutral NN bidentate. The coordination of the ligands with the metal ions in a molar ratio of 1:1:1 leads to formation of an octahedral geometry around the metal ions. The octahedral geometry has been validated and optimized by DFT analysis. Conductivity data showed the electrolytic nature of all complexes. The thermal stability of all complexes was deduced in addition to evaluating some thermodynamic, and kinetic parameters using Coats–Redfern method. Furthermore, all complexes in comparison to their parent ligands were tested for their biological potency against some pathogenic bacterial, and fungi strains using the paper disk diffusion method. [CdL(bpy)](NO3)2 complex revealed the highest antimicrobial activity.

Due to their ability to coordinate with different metal ions, forming stable complexes with attractive structural variance, Schiff bases as organic compounds represent a great class of characteristic ligands in coordination chemistry. Their corresponding complexes are characterized by remarkable structure, and significant geometry has adapted them to be useful for various applications in medicine, and pharmaceutical fields in addition to their potential applications in catalysis, material science, and others. Moreover, the physical, and chemical properties of Schiff bases metal frameworks in addition to their geometrical structures are affected by the nature of the ligand structure, and the metal ion as building blocks [1][2][3] . Hence, it is fundamental to select suitable ligands, and metal ions to discover new metal complexes that display beneficial properties with a diverse structure. In continuance of our previously achieved mixed ligand complexes [4][5][6][7][8][9][10][11][12] , the current research aims to synthesize new mixed-ligand complexes based on furfural-type imine ligand (L) as primary ligand, and 2,2′-bipyridine (2,2′-bpy) as a co-ligand in the presence of iron(III), cobalt(II), copper(II), and cadmium(II) ions. Selection of 2,2′-bpy to act as a co-ligand because it is an electron-conjugated heterocyclic aromatic ligand has a higher coordinating ability due to its N-donor chelation nature forming stable complexes 13 . Several studies have focused on the use of 2,2′-bpy as a co-ligand in mixed ligand complex synthesis revealing that 2,2′-bpy is a strong chelating ligand capable of strongly binding, in a bidentate manner to several types of metal ions forming very stable complexes with five-membered chelate rings through the two pyridine ring nitrogen atoms 14 . Furthermore, the mixed-ligand technique is pleasurable as a result of its design which allows different functional groups with variable binding sites, and tightly binds to the metal ion forming stable mixed-ligand complexes. These mixed-ligand complexes  www.nature.com/scientificreports/ IR spectra studies. The IR spectra of the four synthesized mixed ligand complexes, in addition to the imine ligand (L), and 2,2′-bpy are listed in Table 2. Two intense absorption peaks at 1602-1598, and 1309-1286 cm -1 observed in the IR spectrum of the mixed ligand complexes (Fig. 3) could be ascribed to the imine group ν(C=N), and furanyl ring moiety ν(C-O-C), respectively. The observed negative, and positive shifts by 12-16, and 2-25 cm -1 for ν(C=N), and ν(C-O-C) of the complexes as compared to 1614, and 1284 cm -1 , respectively, of the free imine ligand (L) clearly point to the participation of azomethine nitrogen 20 , and furanyl oxygen atoms in coordination with the metal ions 21 . Furthermore, the characteristic peak of C=N associated with the free 2,2′bpy co-ligand located at 1553 cm -1 showed that all four synthesized mixed ligand complexes in a range of 1587-1567 cm -1 agree with the pyridyl nitrogen atoms chelation around the metal ions 22 . In addition, all synthesized mixed ligand complexes showed new peaks in the range of 596-472 cm −1 refer to M-N, and M-O vibrations that confirmed the assumption of the coordination sites about the metal ions 23 . Only, the two Fe(III), and Co(II) mixed ligand complexes show broad bands at about 3411 and 3440 cm -1 , respectively, in their IR spectra referred to as the ν(OH 2 ) stretching vibration mode for water molecules 24 . This assumption is compatible with the elemental analysis results and confirmed by thermal analysis as given later. Also, the Cd(II) complex spectrum showed a characteristic frequency for the ionic nitrate as an intense peak appeared at 1384 cm −125 confirmed by the conductance measurements. These findings, and the changes in profile of the stretching frequencies for the four synthesized mixed ligand complexes as compared with those observed for the ligand, L, and 2,2′-bipyridine co-ligand confirm the chelation sites of the ligands with the metal ions, and the formation of metal-ligand bonds through these sites, which are the possible binding sites for the coordination.
1 H NMR studies. In the 1 H NMR spectrum of the diamagnetic Cd(II) complex (Fig. 4), two singlet signals were observed at δ 2.72, and 8.99 ppm in comparison to δ 2.56, and 8.47 ppm in the ligand L spectrum. The first signal is due to the methyl protons, whereas the second one is due to the azomethein proton (-CH=N-) 26,27 . The observed higher chemical shift value of the signal due to the azomethein proton affirms the bonding of the ligand L, to the Cd(II) ion through the imine nitrogen atom. Additionally, the ligand L, and Cd(II) complex reveal some multiplet signals due to phenyl, furanyl, and pyridyl rings protons that were well established in their predictable regions as given in Table 2 with their coupling constant J (Hz) 28,29 .
Conductance behavior study. The molar conductance measurements for the synthesized mixed ligand complexes were performed at ambient temperature on a freshly prepared solution of each metal complex in DMF as a solvent at a concentration of 1 × 10 -3 M. The obtained conductance data establish the ionic nature of the synthesized mixed ligand complexes as a result to the presence of chloride or nitrate anions outside the coordination sphere. The obtained measurement values (Table 1) for the mixed ligand Co(II), Cu(II), and Cd(II) complexes, were in the range of 143-176 Ω −1 mol −1 cm 2 whereas for the Fe(III) complex, the value was 280 Ω −1 mol −1 cm 2 indicating that all complexes are electrolytic types. These values show a 1:3 electrolytic nature for the Fe(III) complex, and an electrolytic nature of the type 1:2 for other complexes 30 . Additionally, the presence of Cl − ion outside the coordination sphere is confirmed by a white precipitate formation upon addition of AgNO 3 solution 31 . These results are also supported by comparing them with similar work elsewhere and confirm the assumption that the nonparticipation of the counter anions in the coordination sphere is about the metal ions.
Magnetic susceptibility measurements. The measurements of the magnetic susceptibility for the metal complexes were done at room temperature to estimate the effective magnetic moment value (µ eff ) that help in the geometrical structural investigation via the determination of the number of unpaired electrons. Magnetic susceptibility measurements for all complexes, Table 3 display their paramagnetic properties except for the Cd(II) complex. For the Fe(III) complex, the estimated µ eff was 5.97 B.M which is near the theoretical value of 5.92 B.M indicating a high-spin complex with five unpaired electrons, and octahedral geometry 32 . The Co(II) complex has a µ eff value of 4.31 B.M. This value arises from the existence of three unpaired electrons but it is noticeably higher than those described for the spin only value (3.87 B.M), and can be attributed to d 7 -system with an orbital angular momentum contribution 33 . The estimated µ eff value for the Cu(II) complex was 1.65 B.M which is in compliance with the Cu(II) ion containing one unpaired electron, and near the theoretical value (1.73 B.M) corresponding to one unpaired electron 34 . The magnetic susceptibility measurement for the Cd(II) complex showed its diamagnetic character, which illustrates that it has no unpaired electron 35 .  Table 3 at a range of 248-300 nm assigned to the transition π → π* due to the presence of π-electrons in the conjugated system of phenyl, furanyl, and pyridyl rings as well as the C=N chromophore connected with imine, and pyridyl moieties. Also, the observed peaks in the range of 307-343 nm can be assigned to the n → π* transition as a result of the presence of the nonbonding electrons of the imine nitrogen, pyridyl nitrogen, and furanyl oxygen atoms functionalities. The electronic absorption spectrum of the Fe(III) mixed ligand complex exhibited two weak absorption peaks at 494, and 534 nm attributed to the two transitions 6 A 1g → 4 T 1g , and 6 A 1g → 4 T 2g , respectively. In accordance with that, the high spin octahedral arrangement of the Fe(III) cation characterized by the 6 A 1g ground term and a series of weak transitions 37 . The spectrum of the Co(II) mixed ligand complex exhibited two absorption peaks at 571, and 628 nm connected with 4 T 1g (F) → 4 A 2g (F), and 4 T 1g (F) → 4 T 1g (P) transitions, respectively, suggesting an octahedral shape around the Co(II) ion 38 . Moreover, the ligand field parameters; (B), (D q ), (β), (β %), and (LFSE) have also been directly evaluated for this complex from the spectra data, and are summarized in Table 3. The obtained results reveal that, the B-value (Racah interelectronic repulsion) is smaller than that reported for the free transition metal ion (1120 cm −1 ) suggesting a remarkable orbital overlap, and delocalization of metal d-electrons onto the ligands. Further, β-value (nephelauxetic ratio) is less than unity assuming a partial covalent metal ligand bonding 39 . The recorded spectrum for the Cu(II) mixed ligand complex displayed a low Table 2. IR spectral (cm −1 ), and 1 H-NMR (δ, ppm) data of the imine ligand (L), the co-ligand (bpy), and its mixed ligand complexes.
Compound under study IR spectra (cm −1 )  www.nature.com/scientificreports/ intensity broad peaks at 707 nm assigned to 2 B 1g → 2 A 1g , and 2 B 1g → 2 E g transitions suggest a distorted octahedral shape 40 . The Cd(II) complex spectrum does not furnish any d-d transition due to its completely filled d 10 orbital.

H-NMR (δ, ppm) ν(OH 2 ) ν(C = N) azomethine / pyridyl ν(C-O-C) (furan) δ(py) (bending) υ(M-O) υ(M-N) Additional bands
Optical properties for the mixed ligand complexes. Band gap energy (E g , eV) is the energy magnitude required for excitation of an electron between the valence, and conduction bands. Also, it describes the difference in energy between the top, and the bottom of these two bands. Estimation of the band gap energy of the materials is important to predict the potential use and performance of such materials in technologies, especially in optoelectronic applications. Additionally, UV-VIS absorption spectra are used to determine band gap energy 41,42 . Actually, to determine the E g value of the synthesized mixed ligand complexes, sequence steps are required. The absorption coefficient (α, cm −1 ) is evaluated at first using the data of absorbance (A) and thickness (t) as given in Eq. (1) 43 .
The dependence of (α) values on the photon energy (hv) of the investigated complexes is presented graphically in Fig. 5. The fundamental absorption edge (E e , eV) of the investigated mixed ligand complexes was obtained by extrapolating the linear region in (α)-(hv) plot to the x-axis of the curve (α = zero) 44 . The values of E e of the investigated complexes were found to be 3.94, 3.46, 4.59, and 3.34 eV for iron(III), cobalt(II), cupper(II), and cadmium(II) mixed ligand complexes, respectively, and listed in Table 4. Continuously, the calculated values of (α) are used to construct Tauc's plot to obtain the band gap energy (E g ) of the investigated complexes.
where (h), (ʋ), and (B) are Planck's constant, incident light frequency, and proportionality constant, whereas (n) is the optical frequency that expresses the nature of the transition process to be direct or indirect as: n = 1/2, and 2 for direct, and indirect allowed transitions, and n = 3/2, and 3 for direct, and indirect forbidden transitions, respectively. Tauc's module is drawn as shown in Fig. 6 in which the incident photon energy (hν) along the x-axis is plotted against (αhν) n along the Y-axis for the different possible values of n = 1/2, and 2. In continuation, by extending  www.nature.com/scientificreports/ the linear portion of the graph onto the photon energy axis, the direct (E d g ), and indirect (E i g ) energy band gaps were determined at the cut-off point 46 . The obtained values of (E d g ), and (E i g ) energy band gap are summarized in Table 4.
Comparison of the fundamental absorption edge (E e ) values with the direct (E d g ), and indirect (E i g ) energy band gap values, the direct allowed transitions were suggested for all synthesized mixed ligand complexes.
A literature survey shows that, an accurate determination of the energy band gap can be achieved using the absorption spectra fitting (ASF) method in which the parameter (Aλ −1 ) 1/n along the y-axis is plotted against the parameter (λ −1 ) along the x-axis for the different possible values of n = 1/2, and 2 followed by a linear portion extrapolation of the graph onto the x-axis, Fig. 7. After that, λ g values were determined from the cut-off point. So, the energy band gap values (E g ASF ) of the studied complexes, Table 4 can be deduced using Eq. (3)   www.nature.com/scientificreports/ Additionally, plotting of ln(α) versus hv (Fig. 8) gives a straight line. The inverse value of the slope for the straight line gives the value of Urbach energy (E U , eV), Table 4. The Urbach energy value gives information about the defects that may have originated during the preparation process 48 .
As clear from Table 4, the complexes can serve as semiconductor materials with band gap energy in the range of semiconductor materials 49 .  Table 4 as the extinction coefficient (k), refractive index (n), and steepness coefficient using the fundamentally Eqs. (4-6) 50,51 . www.nature.com/scientificreports/ The variation of the extinction coefficient (k) with the wavelength (λ) for all complexes was represented graphically in Fig. 9. The value of (k) is increased with the increase of (λ) describing the increase in scattering rate for all complexes.  www.nature.com/scientificreports/ Thermal analyses. The thermal analysis study is the main study to detect the thermal stability of the synthesized metal complexes, and to investigate their structural building where, the water molecules nature; coordinated/hydrated, and the metal content is achieved through their mass decomposition 52 . The decomposition steps, temperature range, mass loss (%), mass loss assignments, leaving species, and the final residue are listed in Table 5, and TG-DTG thermograms of the prepared complexes are depicted in Fig. 10a. The thermal decomposition of the [FeL(bpy)]Cl 3 .H 2 O complex displayed two decomposition pathways. The first thermal breakdown occurs between 45, and 388 °C corresponds to a loss of one lattice water molecule in addition to 1½ Cl 2 gas with estimated mass loss of 20.27% (calcd. 20.23%). The second decomposition step occurs in the 388-710 °C temperature range, coincident with the decomposition of the organic species with a mass loss of 70.81% (calcd. 70.69%) leaving Fe metal as a final residue within a weight loss of 8.92% (calc. 9.08%), and an overall mass loss equivalent to 91.08% (calcd. 90.92%).
For the [CoL(bpy)]Cl 2 .3H 2 O complex, its decomposition passed through five breakdown steps. The first, and second decomposition steps with a temperature range of 46-84, and 85-155 °C with an estimated mass loss of 2.88% (calcd. 2.91%), and 5.82% (calcd. 5.83%) resulted in the loss of one, and two lattice water molecules, respectively. The third decomposition step at (155-357 °C) due to the release of 1 2 N 2 , and Cl 2 gases in addition to the furan moiety (C 4 H 4 O) with mass loss of 24.63% (calcd. 24.74%). The fourth decomposition step involved a 2.23% mass loss due to the liberation of half a mole of N 2 gas (calcd. 2.26%). The five-step starts at 379 °C and ends at 642 °C due to the loss of 2HC≡CH, C 7 H 6 , and C 10 H 8 N 2 with a mass loss of 48.32% (calcd. 48.26%) leaving CoO + 2C as a residue within a weight loss of 16.12% (calc. 16.00%) and an overall mass loss of 83.88% (calcd. 84.00%).
The thermal degradation of the [CuL(bpy)]Cl 2 complex showed two stages of decomposition with a temperature range of 62-448, and 448-612 °C, respectively. The first one involved a 49.46% mass loss, which corresponds to losses of ½Cl 2 , ½N 2 , C 4 H 3 CN, and C 10 H 8 N 2 (calcd. 49.70%). The second step specified the removal of HC≡CH, C 4 H 4 O, and C 6 H 5 -Cl fragments with a mass loss of 36.28% (calcd. 36.33%) leaving CuO residue with a weight loss of 14.26% (calcd. 13.97%) and an overall mass loss of 85.74% (calcd. 86.03%).
The thermal degradation of the [CdL(bpy)](NO 3 ) 2 complex proceeds via in three stages. The first one from 63 to 225 °C is associated with an evolution of HCN gas with a mass loss of 4.05% (calcd. 4.02%). The next breakdown stage within a temperature range 225-413 °C correlates with the elimination of two moles of NO 2 and one mole of O 2 gases with bipyridine, and C 3 H 4 with a mass loss of 47.70% (calcd. 47.74%). The third decomposition phase within a range of temperature 413-524 °C corresponds to the loss of the furan ring and C 6 H 5 -CN species with a mass loss of 24.64% (calcd. 25.52%) leaving CdO + 2C residue with a weight loss of 23.61% (calcd. 22.72%) and an overall mass loss of 76.39% (calcd. 77.28%).
Kinetic and thermodynamic study. All the synthesized mixed ligand complexes were subjected to kinetic analyses to study the kinetic, and thermodynamic parameters for their different thermal events over the studied temperature range using the formula of the Coats-Redfern (CR) Eq. (7) 53 .
In this equation W, and W f are the sample weights before degradation, and at temperature T, respectively, A is Arrhenius pre-exponential factor, R denotes the universal gas constant equal to 8.314 J mol −1 K −1 , θ is the heating rate, and E* is the activation energy. Additionally, a graphical representation of this equation as given in Fig. 10b gives a straight line. The intercept of this line gives an A value whereas its slope gives an E* value 54 .
Besides this, the activation enthalpy (ΔH*), activation entropy (ΔS*), and activation free energy change (ΔG*) were determined by the Eyring equation 55 , and tabulated in Table 6. www.nature.com/scientificreports/ The obtained data, Table 6 reveal that there is an increase in the activation energy, and the free energy change of activation values from step to step may be specified to the higher the stability of complexes, and the rigidity of the fragment resulting through the decomposition process in comparison to the original state. Furthermore, the positive ΔH, and negative ΔS values demonstrate the endothermic degradation process, and the non-spontaneous process at all 56 . . This distortion is due to the fact that the nearest coordination environment of the metal cation consists of two nitrogen atoms belong to the 2,2′-bpy co-ligand, other two nitrogen atoms belonging to the imine group, and two oxygen atoms belonging to the furan moiety. The total electronic energy, highest occupied molecular orbital energy (E HOMO ), the lowest unoccupied molecular orbital energy (E LUMO ), and dipole moment (D) were Table 6. Thermodynamic parameters of the thermal decomposition of the mixed ligand complexes.  www.nature.com/scientificreports/ estimated for the complexes, in addition to the imine ligand (L), and the co-ligand (2,2′-bpy), and tabulated in Table 7. Besides, depending upon E HOMO , and E LUMO values, additional parameters such as HOMO-LUMO gap energy (ΔE), chemical potential (µ), global softness (S), absolute hardness (η), absolute electronegativity (χ), absolute softness (σ), global electrophilicity (ω), electron affinity (EA), ionization potential (IP), and the additional electronic charge (ΔN max ) values can be estimated via the functionally equations given below, and also tabulated in Table 7 57 .
Depending upon the basic concept of the molecular orbital theory, and the data listed in Table 7, there are some important notes that can be summarized as follows: The estimated E total was found to be − 916 (imine ligand, L), − 489.44 (co-ligand, 2,2′-bpy), and − 1597 to − 1525 (metal complexes). The more negative values of metal complexes suggest their better stability than their parent ligands. The estimated values of the energy gap of the ligand L and co-ligand, 2,2′-bpy (ΔE = 2.803, and 11.08 eV, respectively) are greater than all synthesized complexes, so the complexes are more reactive than their parent ligands. Additionally, the [CoL(bpy)]Cl 2 ·3H 2 O complex (ΔE = 0.327 eV) is the most reactive complex, while [FeL(bpy)]Cl 3 .H 2 O (ΔE = 1.687 eV) is the least reactive complex. The higher energy band gap (ΔE, 1.687 eV) of the iron (III) complex in comparison to the other complexes indicates its higher molecular chemical stability 58 . Also, among all complexes, the iron(III) complex has the highest value of the absolute hardness (η, 0.844 eV), showing it to be the chemically the hardest complex 59 . The observed trend in η values of complexes is iron(III) complex > copper(II) complex > cobalt(II) complex. Additionally, cobalt(II) complex has the highest absolute softness value (σ, 0.844 eV −1 ), with order of cobalt(II) complex > copper(II) complex > iron(III) complex. So, cobalt(II) complex is the soft one 60 . This trend is similar to the order of ΔE where, the hard molecules possess large energy gap values whereas, the soft molecules possess small energy gap values 61 . The higher value of the ionization potential (IP, 2.721 eV) of the iron(III) complex confirms its higher stability in comparison to other complexes 62 . Figure 12 shows that the electron densities of HOMO, and LUMO are localized on the metal, and coordinated atoms of ligands.
Antimicrobial evaluation. All synthesized mixed ligand complexes in addition to the imine ligand (L), and co-ligand 2,2′-bpy were tested for in vitro antimicrobial activity against different species of bacterial, and fungal strains using the method of paper disk diffusion and measuring the relevant inhibition zone value (IZV) as mentioned in the experimental part. The results obtained in addition to the estimated activity index (AI) were tabulated in Table 8, and depicted in Fig. 13. It is clear from Table 8, the observed results varied in terms of IZV, and AI, and generally the imine ligand (L), co-ligand (2,2′-bpy), and all mixed ligand complexes exhibited a variable degree of antimicrobial activity against the selected bacterial, and fungal pathogens. According to these results, some remarkable points were summarized for bacterial and fungal assays. Table 7. The calculated quantum chemical parameters for of the imine ligand (L), the co-ligand (bpy), and its mixed ligand complexes.  Figure 12. The frontier molecular orbitals of the imine ligand (L), the co-ligand (2,2′-bpy), and its mixed ligand complexes in the gas phase. subtilis. This is an indication to sensitivity of these mixed ligand complexes, and fairly can be applied in treatment of certain diseases caused by certain organism than others.
• The imine ligand (L), and co-ligand (2,2′-bpy) showed remarkable antifungal action against A. flavus, and C. albicans but to different degrees. The imine ligand (L) showed better activity with an IZV of 25 mm against C. albicans. This value is higher than the co-ligand (2,2′-Bipy) ( Finally, as per the literature review, the diversity in the effectiveness of the tested compounds towards the bacteria, and fungi species can be related to the easy penetration, and more interference of the sample into the cell wall. This is based on different factors such as cell membrane, cell permeability, and disruption of the cytoplasmic membrane as a result of protein synthesis inhibition. Also, metal ion nature, donor site nature, and metal complex formation have great effect as described through Tweedy's chelation theory. The easy penetration, more interference of the complex into the cell through H-bond formation are the concepts of this theory cells [63][64][65] .

Conclusion
In the present paper, four new mononuclear mixed ligand Fe(III), Co(II), Cu(II), and Cd(II) complexes have been successfully prepared using the furfural-type imine ligand (L), and the co-ligand 2,2′-bipyridine (2,2′-bpy), and structurally characterized using different techniques. Correlation of all obtained data confirmed their suggested composition, and structure. The aim to synthesis such complexes is the combination of different bioactive www.nature.com/scientificreports/ molecules in addition to incorporation some metal ions as Fe(III), Co(II), Cu(II), and Cd(II) forming significant complexes as antimicrobial agents characterize with enhanced antimicrobial activity. All the synthesized mixed ligand complexes in comparison to their parent ligands; the imine ligand (L), and co-ligand (2,2′-bpy) were subject to study their inhibitor activity toward some bacterial, and fungi strains. The novelty of this research article is the screening results of the biological evaluation of these complexes indicating the sensitivity of some of these mixed ligand complexes toward some pathogenic under study in addition to the higher action than the standard antibiotic. This outcome provides a type of complexes of medicinal value consider as an attractive target for antimicrobial drug development.

Data availability
The data used to support the findings of this study are included in the article.