Metal-ligand Binding Energies Research Articles

Metal–ligand binding energies in copper (II) and nickel (II) complexes with tetradentate N2O2 Schiff base ligands

This work constitutes a new contribution for understanding the relationship between the metal–ligand bonding and, indirectly, the inherent reactivity of metallic complexes with tetradentate N2O2 Schiff base ligands, being reported the energetic characterization of two transition metal complexes – (N,N'́-bis(salicylaldehydo)tetramethylenediiminate)nickel(II) and (N,N'-bis(salicylaldehydo)propylenediiminate)copper(II). The standard molar enthalpies of formation of these complexes were determined by solution-reaction calorimetry measurements. Their standard molar enthalpies of sublimation, at T = 298.15 K, were obtained by an effusion method. From these studies, the gas-phase enthalpies of formation of Ni(II) and Cu(II) complexes, at T = 298.15 K, were derived. Differences between the metal–ligand and mean hydrogen-ligand bond dissociation enthalpies were derived and discussed in structural terms, in comparison with identical parameters for complexes of the same metals with analogous tetradentate Schiff bases. High-level quantum chemical calculations have also been conducted, complementing the results obtained experimentally.

Beryllium (II) Chloride Complexes with Phosphoryl Ligands: A DFT Study

Beryllium complexes of the types [BeCl2L2] (L = (Me2N)3P(O) (1), (Me2N)2P(O)F (2), Me2NP(O)F2 (3) and P(O)F3 (4)) have been theoretically studied by means of DFT geometry optimization and NMR chemical shift calculations (B3LYP/6-31G(d)). A good correlation was found between calculated and experimental data for complex 2. On going from complex 1 to 4, the Be-L bond underwent considerable lengthening, while that of Be-Cl was shortened (Be-O: 1.646 in 1 vs. 1.740 A° in 4; Be-Cl: 2.043 in 1 vs. 1.953 A° in 4). In the same way, the Be-O-P bond angle was found to decrease from 135° for 1 to 124° for 4. The trends are in good agreement with the calculated metal-ligand binding energies of complexes 1-4. Interestingly, the structural changes are accompanied by increased 9Be chemical shifts towards higher frequencies as the Me2N groups in the ligand are substituted by fluorine atoms. The results were compared to corresponding complexes with tin (IV) chloride, [SnCl4L2]. The theoretical data showed that the use of the 6-31G* basis set could efficiently predict the 9Be NMR chemical shifts in the complexes [BeCl2L2].

Macrocyclic Ligands with an Unprecedented Size-Selectivity Pattern for the Lanthanide Ions.

Lanthanides (Ln3+) are critical materials used for many important applications, often in the form of coordination compounds. Tuning the thermodynamic stability of these compounds is a general concern, which is not readily achieved due to the similar coordination chemistry of lanthanides. Herein, we report two 18-membered macrocyclic ligands called macrodipa and macrotripa that show for the first time a dual selectivity toward both the light, large Ln3+ ions and the heavy, small Ln3+ ions, as determined by potentiometric titrations. The lanthanide complexes of these ligands were investigated by NMR spectroscopy and X-ray crystallography, which revealed the occurrence of a significant conformational toggle between a 10-coordinate Conformation A and an 8-coordinate Conformation B that accommodates Ln3+ ions of different sizes. The origin of this selectivity pattern was further supported by density functional theory (DFT) calculations, which show the complementary effects of ligand strain energy and metal-ligand binding energy that contribute to this conformational switch. This work demonstrates how novel ligand design strategies can be applied to tune the selectivity pattern for the Ln3+ ions.

DFT and TD-DFT Study of [Tris(dithiolato)M]3- Complexes[M= Cr, Mn and Fe]: Electronic Structures, Properties and Analyses

Using Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) methods, transition metal complexes of benzene-1, 2-dithiolate (L2-) ligand from Cr to Fe have been studied theoretically. The ground state geometries, binding energies, UV-Visible spectra (UV-Vis), frontier molecular orbitals (FMOs) analysis, charge analysis and natural bond orbital (NBO) have been calculated. The structural parameters are in good accord with the experimental data. The metal-ligand binding energies are one (1) order of magnitude higher than the physisorption energy of a benzene-1, 2-dthiolate molecule on a metallic surface. In accordance with experiment the calculated electronic spectra of these tris complexes show bands at 565, 559 and 546 nm for Cr3+, Mn3+, and Fe3+ respectively which are mainly qualified to ligand-to metal charge transfer (LMCT) transitions. The electronic properties analysis demonstrate that the highest occupied molecular orbital (HOMO) is mostly centered on metal coordinated sulfur atoms whereas the lowest unoccupied molecular orbital (LUMO) is mainly located on the metal surface. By calculating natural bond orbital (NBO), the intramolecular interactions and electron delocalization was obtained. The results of NBO analysis illustrated the significant charge transfer from sulfur to central metal ions, as well as to the benzene of the complex. The calculated charges on metal ions are also reported at various charge schemes. The calculations show encouraging agreement with the available experimental data.
 Dhaka Univ. J. Sci. 67(1): 63-68, 2019 (January)

Investigation of Structural and Electronic Properties of [Tris(Benzene-1,2-Dithiolato)M]<sup>3-</sup> (M = V, Cr, Mn, Fe and Co) Complexes: A Spectroscopic and Density Functional Theoretical Study

In this study, the first raw transition metals from V to Co complexes with benzene-1,2-dithiolate (L2-) ligand have been studied theoretically to elucidate the geometry, electronic structure and spectroscopic properties of the complexes. Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) methods have been used. The ground state geometries, binding energies, spectral properties (UV-vis), frontier molecular orbitals (FMOs) analysis, charge analysis and natural bond orbital (NBO) have been investigated. The geometrical parameters are in good agreement with the available experimental data. The metal-ligand binding energies are 1 order of magnitude larger than the physisorption energy of a benzene-1, 2-dthiolate molecule on a metallic surface. The electronic structures of the first raw transition metal series from V to Co have been elucidated by UV-vis spectroscopic using DFT calculations. In accordance with experiment the calculated electronic spectra of these tris complexes show bands at 522, 565, 559, 546 and 863 nm for V3+, Cr3+, Mn3+, Fe3+ and Co3+ respectively which are mainly attributed to ligand to metal charge transfer (LMCT) transitions. The electronic properties analysis shows that the highest occupied molecular orbital (HOMO) is mainly centered on metal coordinated sulfur atoms whereas the lowest unoccupied molecular orbital (LUMO) is mainly located on the metal surface. From calculation of intramolecular interactions and electron delocalization by natural bond orbital (NBO) analysis, the stability of the complexes was estimated. The NBO results showed significant charge transfer from sulfur to central metal ions in the complexes, as well as to the benzene. The calculated charges on metal ions are also reported at various charge schemes. The calculations show encouraging agreement with the available experimental data.

Superatomic nature of alkaline earth metal-water complexes: the cases of Be(H2O) and Mg(H2O).

Beryllium- and magnesium-water complexes are shown to accommodate peripheral electrons around their Be2+(H2O)4 and Mg2+(H2O)6 cores in hydrogenic type orbitals. The lowest energy state of these tetra- and hexa-coordinated complexes possess one (cationic species) and two electrons (neutral species) in a pseudo spherical s-orbital, and populate p-, d-, f-, and g-type orbitals in their low-lying excited electronic states. High level quantum chemical calculations are performed to study the electronic structure of these complexes belonging to the category of solvated electrons precursors (SEPs). The observed Aufbau principle is in harmony with the previously introduced series for metal-ammonia complexes. In the current study we are able to expand the previously proposed shell model of SEPs beyond the 2d level. The observed shell model for Mg(H2O)6+ is found to be 1s, 1p, 1d, 2s, 2p, 1f, 2d, 3s, and 1g. The stability of the Be(H2O)0/+m and Mg(H2O)0/+n systems with m = 1-4 and n = 1-6 is also examined and the higher stability of metal-ammonia SEPs over metal-water SEPs is explained in terms of the metal-ligand binding energies, hydrogen bonding, and the activation energy barrier leading to H2 release.

Vinyltrifluoroborate Complexes of Silver Supported by N‐Heterocyclic Carbenes

Silver complexes involving vinyltrifluoroborate, (IPr)Ag(CH2=CHBF3) (4), [(IPr)2Ag][CH2=CHBF3] (5), (SIPr)Ag(CH2=CHBF3) (6) and (IPr*)Ag(CH2=CHBF3) (7) have been synthesized by using (NHC)AgNO3 [NHC = IPr (1), SIPr (2) and IPr* (3)] and K(CH2=CHBF3) in a metathesis process. NMR spectroscopic details of 1–7 and X‐ray crystal structures of compounds 5–7 as well as that of a precursor (IPr)AgNO3 are reported. Compounds 6 and 7 are more stable in solution compared to 4 (which forms 5) indicating a supporting ligand effect in the stability of monomeric (NHC)Ag(CH2=CHBF3). Calculated metal–ligand binding energies show that [CH2=CHBF3]− is a significantly better ligand for AgI than CH2=CH2 or isoelectronic CH2=CHCF3. IPr, SIPr and IPr* carbenes coordinate to Ag(CH2=CHBF3) moiety forming silver adducts 4, 6 and 7, with binding energies of about –72.1, –72.1 and –83.8 kcal/mol, respectively. The C=C bond length of [CH2=CHBF3]− (calculated value of 1.341 Å) increases upon coordination to the silver ion. Experimental data show a similar trend for structurally characterized 6 and 7. Electrostatic attraction dominates the bonding between the alkene ligand [C2H3BF3]− and the metal fragments [Ag(IPr)]+ and [Ag(SIPr)]+ in 4 and 6. The strongest orbital interaction involves σ‐donation from the occupied π‐orbital of the [C2H3BF3]− ligand to the vacant orbital of the metal fragment, while AgI→[C2H3BF3]− π‐backbonding is relatively weak but not insignificant.

Synthesis, characterization, computational studies and biological activities of Co(II), Ni(II) and Cu(II) complexes of 2-Amino-1,3,4-thiadiazole derivatives

Co(II), Ni(II), and Cu(II) complexes of 2-Amino-5-ethyl-1,3,4-thiadiazole (AET) and 2-Amino-5-(ethylthio)-1,3,4-thiadiazole (AEST) have been synthesized and characterized based on elemental analysis, magnetic susceptibility, infrared (4000–400 cm−1), mass spectrometry (ESI and MALDI), UV–Vis (200–1100 nm) and thermal analysis (TGA/DTA). Molar conductance measurements proved that [M(L)2(H2O)2]Cl2·H2O are electrolytic complexes where M represents Co, Ni, and Cu divalent metal ions. The geometrical isomerism of [M(L)2(H2O)2]2+ ions were investigated by DFT-B3LYP calculations incorporated in Gaussian09 package; it favored the all trans isomers due to having the lowest energy points on the potential energy surface. The outcome of DFT-B3LYP quantum mechanical calculations using 6-31G(d) basis set favor six-coordinate sites via a bidentate ligand through exo amino and adjacent endo thiadiazole nitrogen (N3) donors. These results were consistent with magnetic measurements combined with infrared and UV–Vis spectral interpretations. The predicted metal–ligand binding energies from B3LYP/6-31G(d) calculations follow the trend Cu2+>Ni2+>Co2+, in agreement with the Irving–Williams series. Both AET and AEST ligands and the synthesized complexes were screened for their antibacterial activity and the outcome was high antimicrobial activity of the complexes compared to the free ligands against one or more microbial species and in some cases (copper complexes) higher activity than standard drugs.

A DFT Investigation of Tris-(4´-(Amino)(1,1´-biphenyl)-3,4-diol) Fe(III) Complex

Phenolic compounds, known as the pyrocatechol act as a metal chelating agent. Molecular details of cross-linking of pyrocatechol by transition metal ions are largely unknown. In the present study, the molecular properties of the tris-(4´-(amino)(1,1´-biphenyl)-3,4-diol)- Fe(III) complex have been investigated using density functional theory (DFT) at 6-311G(d,p). Calculated molecular properties of the optimized structure, the binding energies and spectroscopic properties are compared with the available experimental results. For the tris-complex investigated, the binding of Fe (III) with the catechol derivative is not as strong as the binding of other metal ions with catechol. The IR stretching bands show that the strong IR intensities is due to large charge polarization. Calculated electronic band gap is 2.45 eV which is in the range of transition metal ion-tris-(4´-(amino)(1,1´-biphenyl)-3,4-diol) complexes. The metal-ligand binding energy is 513.54 kcal mol-1, which could be used in understanding the speciation of Fe(III)-catechol complex. Structural parameters obtained from the DFT calculations are in good agreement with the crystallographic data.
 Dhaka Univ. J. Sci. 66(1): 67-71, 2018 (January)

Structural, Spectral (IR and UV/Visible) and Thermodynamic Properties of Some 3d Transition Metal(II) Chloride Complexes of Glyoxime and Its Derivatives: A DFT and TD-DFT Study

Metal complexes bearing vic-dioxime ligands have been extensively used as analytical and biochemical reagents, and are well-known antimicrobial agents. Herein is reported a DFT study on the molecular structures, thermodynamic properties, chemical reactivity and spectral properties of some 3d metal(II) chloride complexes of glyoxime. The functionals B3LYP and CAM-B3LYP have each been used in conjunction with LANL2DZ for the metal(II) ions (Fe2+, Co2+, Ni2+ and Cu2+) and the Poplestyle basis set 6-31G+(d,p) for the rest of the elements, to perform theoretical calculations. The metal complexation abilities of the glyoxime ligands studied in this work have been evaluated on the basis of metal-ligand binding energies. These ligands were found to have high affinities towards Ni(II) and Fe(II) ions, and all complexation reactions were found to be thermodynamically feasible. Ligand-to-metal electron donations in the complexes studied have been revealed by natural population analysis. The fully optimized geometries of the complexes have adopted square planar structures around the central metal ions. On the basis of orbital composition analysis, the UV-Vis electronic absorption bands of these molecules have been attributed mainly to MLCT, LMCT and d-d electronic transitions involving metal-based orbitals.

Metal-ion effects on the polarization of metal-bound water and infrared vibrational modes of the coordinated metal center of Mycobacterium tuberculosis pyrazinamidase via quantum mechanical calculations.

Mycobacterium tuberculosis pyrazinamidase (PZAse) is a key enzyme to activate the pro-drug pyrazinamide (PZA). PZAse is a metalloenzyme that coordinates in vitro different divalent metal cofactors in the metal coordination site (MCS). Several metals including Co2+, Mn2+, and Zn2+ are able to reactivate the metal-depleted PZAse in vitro. We use quantum mechanical calculations to investigate the Zn2+, Fe2+, and Mn2+ metal cofactor effects on the local MCS structure, metal–ligand or metal–residue binding energy, and charge distribution. Results suggest that the major metal-dependent changes occur in the metal–ligand binding energy and charge distribution. Zn2+ shows the highest binding energy to the ligands (residues). In addition, Zn2+ and Mn2+ within the PZAse MCS highly polarize the O–H bond of coordinated water molecules in comparison with Fe2+. This suggests that the coordination of Zn2+ or Mn2+ to the PZAse protein facilitates the deprotonation of coordinated water to generate a nucleophile for catalysis as in carboxypeptidase A. Because metal ion binding is relevant to enzymatic reaction, identification of the metal binding event is important. The infrared vibrational mode shift of the C=Nε (His) bond from the M. tuberculosis MCS is the best IR probe to metal complexation.

Extraction complexes of Pu(IV) with carbamoylmethylphosphine oxide ligands: A relativistic density functional study

Abstract The extraction complexes of Pu(IV) with n-octyl(phenyl)-N,N-diisobutyl-methylcarbamoyl phosphine oxide (CMPO) and diphenyl-N,N-diisobutyl carbamoyl phosphine oxide (Ph2CMPO) have been studied by using density functional theory (DFT) combined with relativistic small-core pseudopotentials. For most complexes, the CMPO and Ph2CMPO molecules are coordinated as bidentate chelating ligands through the carbonyl oxygen and phosphoric oxygen atoms. The metal-ligand bonding is mainly ionic for all of these complexes. The neutral PuL(NO3)4 and PuL2(NO3)4 complexes are predicted to be the most thermodynamically stable molecules according to the metal-ligand complexation reactions. In addition, hydration energies may also play a significant role in the extractability of CMPO and Ph2CMPO for the plutonium cations. In most cases, the complexes with CMPO possess qualitatively similar geometries and electron structures to those with Ph2CMPO, and they also have comparable metal-ligand binding energies. Thus, replacement of alkyl groups by phenyl groups at the phosphorus atom of CMPO seems to have no obvious influence on the extraction of Pu(IV).

Complexation Behavior of Eu(III) and Am(III) with CMPO and Ph2CMPO Ligands: Insights from Density Functional Theory

A series of extraction complexes of Eu(III) and Am(III) with CMPO (n-octyl(phenyl)-N,N-diisobutyl-methylcarbamoyl phosphine oxide) and its derivative Ph2CMPO (diphenyl-N,N-diisobutyl carbamoyl phosphine oxide) have been studied using density functional theory (DFT). It has been found that for the neutral complexes of 2:1 and 3:1 (ligand/metal) stoichiometry, CMPO and Ph2CMPO predominantly coordinate with metal cations through the phosphoric oxygen atoms. Eu(III) and Am(III) prefer to form the neutral 2:1 and 3:1 type complexes in nitrate-rich acid solutions, and in the extraction process the reactions of [M(NO3)(H2O)7](2+) + 2NO3(-) + nL → ML(n)(NO3)3 + 7H2O (M = Eu, Am; n = 2, 3) are the dominant complexation reactions. In addition, CMPO and Ph2CMPO show similar extractability properties. Taking into account the solvation effects, the metal-ligand binding energies are obviously decreased, i.e., the presence of solvent may have an significant effect on the extraction behavior of Eu(III) and Am(III) with CMPOs. Moreover, these CMPOs reagents have comparable extractability for Eu(III) and Am(III), confirming that these extractants have little lanthanide/actinide selectivity in acidic media.

Density Functional Theory Studies of UO22+ and NpO2+ Complexes with Carbamoylmethylphosphine Oxide Ligands

The UO(2)(2+) and NpO(2)(+) extraction complexes with n-octyl(phenyl)-N,N-diisobutylmethylcarbamoyl phosphine oxide (CMPO) and diphenyl-N,N-diisobutylcarbamoyl phosphine oxide (Ph(2)CMPO) have been investigated by density functional theory (DFT) in conjunction with relativistic small-core pseudopotentials. For these extraction complexes, especially the complexes of 2:1 (ligand/metal) stoichiometry, UO(2)(2+) and NpO(2)(+) predominantly coordinate with the phosphoric oxygen atoms. The CMPO and Ph(2)CMPO ligands have higher selectivity for UO(2)(2+) over NpO(2)(+), and for all of the extraction complexes, the metal-ligand interactions are mainly ionic. In most cases, the complexes with CMPO and Ph(2)CMPO ligands have comparable metal-ligand binding energies, that is, the substitution of a phenyl ring for the n-octyl group at the phosphoryl group of CMPO has no obvious influence on the extraction of UO(2)(2+) and NpO(2)(+). Moreover, hydration energies might play an important role in the extractability of CMPO and Ph(2)CMPO for these actinyl ions.

The physical chemistry of [M(H2O)4(NO3)2] (M = Mn2+, Co2+, Ni2+, Cu2+, Zn2+) complexes: computational studies of their structure, energetics and the topological properties of the electron density

The complexes of M2+ (M = Mn, Co, Ni, Cu, Zn), trans-[M(H2O)4(NO3)2] in their high-spin ground electronic states have been investigated theoretically for the first time using several correlated DFT levels as well as with the MP2 method in conjunction with two different basis sets, 6-311++G(d, p) and LANL2TZ+/6-311++G(d, p), to examine their equilibrium structures and stabilities. Among the correlated methods, the X3LYP level together with the 6-311++G(d, p) basis set gives the best estimate of geometries in these complexes. The metal–ligand binding energies obtained follow the trend Cu2+ > Ni2+ > Zn2+ > Co2+ > Mn2+ across the series examined, in precise agreement with the Irving-Williams series. The DFT methods largely overestimate the binding energies compared to the MP2 level and the trend follows the order PW91PW91 < PBEPBE < X3LYP < B3LYP < MP2. The use of an ECP basis on the central metal cation and a 6-311++G(d, p) basis set on the main group elements increases the binding energies of the complexes compared to that found using the full-core basis set 6-311++G(d, p) and the energy difference between them can be as large as 20 kcal mol−1. There are significant differences between the structures calculated in the gas phase and those calculated with the PCM model to simulate the effect of solvent. Solvation shortens the M–OH2 bonds and lengthens the M–ONO2 bonds such that the difference between the computed and the crystallographically observed bond lengths tends to decrease; it increases complex stability; and that it leads to the disappearance of two intramolecular H bonds between OH2 and NO3– ligands that are present in the gas-phase structures. While there are differences between the natural populated atomic charges and Bader’s approach of the quantum theory of atoms in molecules (QTAIM), all show charge transfer from the ligands to the metal ion. However, the MP2-level-computed charges were found to be unreliable compared with the DFT-derived charges. The metal–ligand bonding and the intramolecular H bonding in the complex are explored with QTAIM and the insight gained into the electronic structure of these complexes is discussed.

Conformations and vibrational spectroscopy of metal-ion/polylalanine complexes

The thermochemistry and structures of complexes of dialanine and trialanine with a series of singly and doubly charged metal ions have been examined by spectroscopic and computational approaches. Complexes with Li +, K +, Cs +, Ca 2+, Sr 2+ and Ba 2+ were formed by electrospray ionization, and studied by infrared multiple photon dissociation spectroscopy (IRMPD) in a Fourier-transform ion cyclotron resonance (FTICR) mass spectrometer using irradiation in the 1000–1900 cm −1 infrared (IR) region from the FELIX free electron laser. Spectra were correlated with computed thermochemistry and simulated IR spectra from density functional (DFT) calculations of likely candidate conformations to make conformational assignments and characterize the variation of characteristic normal mode frequencies as a function of metal-ion identity. All trialanine complexes were found to have the charge-solvated (CS) form of the ligand with all three carbonyl oxygens chelating the metal. Dialanine was more varied: the Ba complex had largely the salt-bridge (SB) form, the Sr and Ca complexes showed a mixture of CS and SB, the Cs complex was CS with (OO) chelation, and the K, Na and Li complexes were CS in an undetermined mixture of (OO) and (NOO) chelation. Binding of the metal ion to the carboxyl carbonyl and to the amide carbonyl(s) (Amide I band) gave a red shift of the C O stretches, and a blue shift to the Amide II band. The frequency shifts were greater for more strongly interacting metal ions (smaller size, larger charge), and were found to be linearly proportional to the metal–ligand binding energy.

Tin tetrachloride adducts with phosphoryl ligands: A DFT study

Tin tetrachloride adducts of the type SnCl 4·2L (L = (Me 2N) 3P(O) ( 1), (Me 2N) 2P(O)F ( 2), Me 2NP(O)F 2 ( 3) and P(O)F 3 ( 4)) have been theoretically studied by means of DFT geometry optimisation (B3LYP/LANL2DZ) and 119Sn chemical shift calculations (B3LYP/SV). A good correlation was found between calculated and experimental data. On going from complex 1 to 4, the Sn L bond underwent considerable lengthening, while that of Sn Cl was shortened (Sn O: 2.11 in cis- 1 vs. 2.37 Å in cis- 4; Sn Cl: 2.43 in cis- 1 vs. 2.37 Å in cis- 4). In the same way, the Sn O P bond angle was found to decrease from 147° for cis- 1 to 136° for cis- 4. The trends are in good agreement with the calculated metal–ligand binding energies of complexes 1– 4. Interestingly, the structural changes are accompanied by increased 119Sn chemical shifts towards higher frequencies as the Me 2N groups in the ligand are substituted by fluorine atoms. The theoretical results showed that the use of the all-electron SV basis set for tin, together with the 6-31G∗ basis set for the other atoms could efficiently predict the 119Sn NMR chemical shifts in the complexes SnCl 4·2L.

Calculating interaction energies in transition metal complexes with local electron correlation methods

The results of density fitting and local approximations applied to the calculation of transition metal-ligand binding energies using second order Møller-Plesset perturbation theory are reported. This procedure accurately reproduces counterpoise corrected binding energies from the canonical method for a range of test complexes. While counterpoise corrections for basis set superposition error are generally small, this procedure can be time consuming, and in some cases gives rise to unphysical dissociation of complexes. In circumventing this correction, a local treatment of electron correlation offers major efficiency savings with little loss of accuracy. The use of density fitting for the underlying Hartree-Fock calculations is also tested for sample Ru complexes, leading to further efficiency gains but essentially no loss in accuracy.

Theoretical Study of the Complexes of Horminone with Mg2+and Ca2+Ions and Their Relation with the Bacteriostatic Activity

The coordination of the horminone molecule with hydrated magnesium and calcium divalent ions was studied by means of the density functional theory. All-electron calculations were performed with the B3LYP/6-31G method. The first layer of the water molecules surrounding the metallic cations was included. It was found that the octahedral [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) complex is more stable than [Mg(H(2)O)(6)](2+). That is, horminone is able to displace two water units from the hexahydrated complex. This behavior does not occur for Ca(2+). Consistently, [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) has a greater metal-ligand binding energy than [horminone(O(a)-O(d))-Ca-(H(2)O)(4)](2+). The preference of horminone by Mg(2+) is enlightened by these results. Moreover, its electronic structure, as shown by huge changes in the atomic populations, is strongly perturbed by Mg(2+). Indeed, horminone, bonded to [Mg(H(2)O)(4)](2+), is able to cross the bacterial membrane cell. Once inside, [horminone(O(a)-O(d))-Mg-(H(2)O)(4)](2+) binds to rRNA phosphate groups yielding [horminone(O(a)-O(d))-Mg-(H(2)O)(PO(4)H(2))(PO(4)H(3))(2)](+). These results give insights into how horminone may inhibit the initial steps of protein synthesis. The stability of the studied systems is accounted for in terms of the calculated structural and electronic properties: Mg-O and Ca-O bond lengths, charge transfers, and binding energies.

Density Functional Theory Studies of Actinide(III) Motexafins (An-Motex2+, An = Ac, Cm, Lr). Structure, Stability, and Comparison with Lanthanide(III) Motexafins

Newly developed relativistic energy-consistent 5f-in-core actinide pseudopotentials and corresponding (7s6p5d1f)/[5s4p3d1f] basis sets in the segmented contraction scheme, combined with density functional theory methods, have been used to study the molecular structure and chemical properties of selected actinide(III) motexafins (An-Motex2+, An = Ac, Cm, Lr). Structure and stability are discussed, and a comparison to the lanthanide(III) motexafins (Ln-Motex2+, Ln = La, Gd, Lu) is made. The actinide element is found to reside above the mean N5 motexafin plane, and the larger the cation, the greater the observed out-of-plane displacement. It is concluded that the actinium(III), curium(III), and lawrencium(III) cations are tightly bound to the macrocyclic skeleton, yielding stable structures. However, the calculated metal-ligand gas-phase binding energy for An-Motex2+ is about 1-2 eV lower than that of Ln-Motex2+, implying a lower stability of An-Motex2+ compared to Ln-Motex2+. Results including solvent effects imply that Ac-Motex2+ is the most stable complex in aqueous solution and should be the best candidate for experimentalists to get stable actinide(III) motexafin complexes.