Spectroscopic, Physical, Thermal, and Magnetic Studies of N-[Tris(Hydroxyl Methyl)Methyl]Glycine (Tricine, L) Complexes and Their Applications Against Tumor Activity

New metal complexes derived from the interaction of tricine with some metal salts (Cu2+, Co2+, Zn2+, Cd2+, and Ni2+) were synthesized and characterized by spectral (IR, UV-vis., EPR, mass, 1H-NMR), magnetic, conductance, and thermal (TGA measurements) analyses. The results suggest that L coordinates in a mono-, biand/or tridentate manners via the COO, NH, and OH groups. Also, the results suggest that the carboxylate group is bonded to the metal ions in two forms depending on the type of solvent and the pH of the reaction mixture. Spectral and magnetic studies suggest an octahedral geometry around the investigated metal ions. Moreover, L coordinates in a tridentate manner. Material studio program has been used for calculating HUMO, LUMO, and DFT parameters on the atoms to confirm the geometry of complexes. The cytotoxic activities of complexes against human tumor cells have been screened. The Cu2+ complex showed the highest activities using colorimetric assay.


Introduction
The structure of N-[tris(hydroxyl methyl)methyl]glycine (L) shows several coordination sites (COO, OH, and NH) and thence it acts as an excellent chelating agent [1].Literature survey reveals that L has the ability to bind metal ions in mono-, bi-and/or tridentate manners [2,3,4,5,6,7,8,9,10,11,12].In continuation of our earlier work [13] and others [14,15], we extend this work to throw the light on the importance of tricine in different fields in particular biological studies.We previously reported that the participation of the coordination sites depends on the pH [13] during complex formation but we reported herein that the coordination sites depend also on the type of the solvent used.Moreover, the aim of this work is to synthesize and characterize new series of its complexes involving Zn 2+ , Cu 2+ , Cd 2+ , Ni 2+ , and Co 2+ salts, which are not reported earlier in literature, involving structural explication, thermal, physical behavior, spectral and molecular modeling of complexes.Finally, one of our goals is to study the cytotoxic activity of the metal complexes.

Instrumentation and materials
All the chemicals and solvents and instrumentation were carried out as reported earlier [13].

Synthesis of solid complexes
Two categories of solid complexes were synthesized and characterized.The first type of solid complexes was prepared in absolute EtOH while the second type was isolated from redistilled H 2 O.The complexes separated in presence of H 2 O are accompanied by losing a proton from the carboxylic group as in case of Co 2+ and Ni 2+ complexes at pH = 8 using NaOH and NaOAc as buffering agents, respectively.On the other hand, the Co 2+ and Ni 2+ complexes separated in presence of absolute EtOH as a solvent and the ligand participates without losing a proton from the carboxylic group at pH above 8.

Synthesis of complexes in EtOH
A hot EtOH solution of the metal chlorides CuCl

Results and discussion
3.1.IR spectra, electronic spectra, magnetic moments, and conductance studies All the complexes derived from L and reported earlier in literature [1,2,3,4,5,6,7,8,9,10,11,12,13,14] show that the ligand coordinates to the metal ions in a mononuclear, binuclear, and tridentate manner.In our case, the results of spectral and magnetic measurements suggest also that L coordinates in a mono, bi-and/or tridentate manner and the isolated complexes have an octahedral structure • 2 H 2 O occurred on using H 2 O as a solvent while with the rest of complexes the ligand reacted without losing a proton.This phenomenon is explained on the basis that the water molecules form weak hydrogen bonding with the active centers and hence it acts as a strong acid while the presence of ethanol makes tricine acts as a weak acid.This behavior causes a strong stabilization of the Zwiterrion.This behavior agrees with the results reported by Bates et al. [18].
The electronic spectra of all complexes were carried out in Nujol mull as shown in Figures S10, S11, S12, and S13.The results of electronic spectra as well as the values of magnetic moments indicate that the complexes have octahedral geometry around the metal ions [19,20,21].The values of conductance (0-8 ohm −1 cm 2 mol −1 ) confirm that all the complexes are nonelectrolyte in nature [22] except the Cu 2+ complex with the general formula, [Cu(tric) 2 ]Cl 2 • 3 H 2 O, which is electrolyte in nature (1:2).

Mass spectra
The mass spectrum of [Cu(tric) 2 ]Cl 2 • 3 H 2 O at 120 °C (Figure S14) shows a molecular ion peak [m/z] at 546.66 and matches with the theoretical value (546.83).This proposes that the suggested structure of this complex is correct.The fragmentation pattern of the Cu 2+ complex is shown in Scheme 1. Also, the results of elemental analyses and thermal analyses are taken as additional evidences for the proposed structure.The spectrum shows that the dissociation of Cu complex started with losing Cl   S1.

UV-vis. spectra
The electronic spectra of the Ni 2+ and Co 2+ complexes with the general formulae, [Ni(tric as an examples of the two types of the isolated solid complexes, show bands as shown in the experimental section.These bands suggest an octahedral geometry around the two metal ions (Ni 2+ and Co 2+ ).Racah parameters (B and β) were calculated as reported earlier [13,19].
The results of this complex are g // = 2.21, g ⊥ = 2.1, G = 2.578, and A = 97.5.The observed g // for the Cu complex is less than 2.3, suggesting important covalent character of the metal-ligand bond [23].The direct g // > g ⊥ > ge (2.0023) viewed for this complex suggests that d x2−y2 is the ground state of the Co 2+ ion [24].

Chemical reactivity
The assignment of energies of HOMO (π-donor) and LUMO (π-acceptor) are important parameters in quantum compound counts.The HOMO is the orbital that generally goes about as electron giver and the LUMO is the orbital that fundamentally goes about as electron acceptor.These molecular orbitals are also called frontier molecular orbitals (FMOs).The all negative values of E HOMO , E LUMO , and their neighboring orbitals show that the prepared molecules are steady.The energy gap (E HOMO -E LUMO ) is an important stability index which serves to portray the chemical reactivity and kinetic stability of the molecule [30].The gap (E HOMO -E LUMO ) is connected to build up a hypothetical pattern for illustrating the structure in many molecular systems.The small gap in molecule means that the molecule is more polarized and the molecule is known as soft molecule.The responsive of soft molecules is more than that of the hard ones as they easily offer electrons to an acceptor.The small energy gap in tricine shows that charge transfer easily happens in it.The ability of the molecule to give electron is weaker if the HOMO energy value is low.On the opposite, the ability of the molecule is good if the HOMO energy value is high [31].All the data are shown in Tables 1 and 2.

Figure 1 :
Figure 1: TGA, DTG curves of [Co(tric) 2 Cl 2 ] • 2.5H 2 O. 3.3.Thermal studies The thermal analyses (TGA and DTG) curves were performed under a temperature range from 20 °C up to 1,000 °C.The mass losses were estimated and computed up on the results of the TGA of the calculated mass loss using the results of the microanalyses.The four steps of the decomposition of [Co(tric) 2 Cl 2 ] • 2.5H 2 O complex is shown in Figure 1.The temperature of the first step from 25 °C to 135 °C corresponds to the loss of two H 2 O molecules and CH 2 (Found: 8.632%, Calcd.: 9.55%).The temperature of the second and third steps from 135 °C to 800 °C is referred to the loss of the fragments (C 9 H 22 N 2 O 6 + 2 HCl) (Found: 62.199%, Calcd.: 62.036%).Finally, the residue appraises in the temperature range 800 °C-1,000 °C corresponds to CoNO 4 C, in which the calculated loss 28.412% which is matching the found loss 29.1%.The thermal analyses curves of the other complexes are shown in Figures S15, S16, S17, and S18.All the thermal decomposition steps are tabulated in TableS1.