Ligand Polarization Research Articles

N-Heterocyclic Carbene-Phosphinidene and Carbene-Phosphinidenide Transition Metal Complexes.

Half-sandwich complexes of the N-heterocyclic carbene-phosphinidene adduct [(IPr)PH] (1, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were prepared by its reaction with dimeric complexes of the type [LMCl2]2, which afforded the three-legged piano-stool complexes [LMCl2{HP(IPr)}] (9a/9b: M = Ru/Os, L = η6-p-cymene; 10a/10b: M = Rh/Ir, L = η5-C5Me5). Their conversion into the corresponding carbene-phosphinidenide complexes [LMCl{P(IPr)}] (11a/11b: M = Ru/Os; 12a/12b: M = Rh/Ir) with a two-legged piano stool geometry was studied by NMR spectroscopy in the presence of the strong base 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). Alternatively, the complexes 11 and 12 were isolated in high yields from the reactions of the carbene-phosphinidene adduct [(IPr)PTMS] (2) with [LMCl2]2, whereby formation of the metal-phosphorus bonds was accompanied by elimination of trimethylsilyl chloride (Me3SiCl). Theoretical calculations reveal a strong polarization of the phosphorus ligands upon metal complexation, which can be ascribed to the ability of the imidazole moiety to effectively stabilize a positive charge. Dehydrohalogenation of complexes 9/10 to 11/12 affords a significant increase of the metal-phosphorus bond order, with the carbene-phosphinidenide ligand acting as a strong 2σ,2π-electron donor.

Nickel-Catalyzed Enantioselective Reductive Cross-Coupling of Styrenyl Aziridines.

A Ni-catalyzed reductive cross-coupling of styrenyl aziridines with aryl iodides is reported. This reaction proceeds by a stereoconvergent mechanism and is thus amenable to asymmetric catalysis using a chiral bioxazoline ligand for Ni. The process allows facile access to highly enantioenriched 2-arylphenethylamines from racemic aziridines. Multivariate analysis revealed that ligand polarizability, among other features, influences the observed enantioselectivity, shedding light on the success of this emerging ligand class for enantioselective Ni catalysis.

Remarkable Anion-Dependent Spin-State Switching in Diiron(III) μ-Hydroxo Bisporphyrins: What Role do Counterions Play?

Addition of 2,4,6-trinitrophenol (HTNP) to an ethene-bridged diiron(III) μ-oxo bisporphyrin (1) in CH2 Cl2 initially leads to the formation of diiron(III) μ-hydroxo bisporphyrin (2⋅TNP) with a phenolate counterion that, after further addition of HTNP or dissolution in a nonpolar solvent, converts to a diiron(III) complex with axial phenoxide coordination (3⋅(TNP)2 ). The progress of the reaction from μ-oxo to μ-hydroxo to axially ligated complex has been monitored in solution by using 1 H NMR spectroscopy because their signals appear in three different and distinct spectral regions. The X-ray structure of 2⋅TNP revealed that the nearly planar TNP counterion fits perfectly within the bisporphyrin cavity to form a strong hydrogen bond with the μ-hydroxo group, which thus stabilizes the two equivalent iron centers. In contrast, such counterions as I5 , I3 , BF4 , SbF6 , and PF6 are found to be tightly associated with one of the porphyrin rings and, therefore, stabilize two different spin states of iron in one molecule. A spectroscopic investigation of 2⋅TNP has revealed the presence of two equivalent iron centers with a high-spin state (S=5/2) in the solid state that converts to intermediate spin (S=3/2) in solution. An extensive computational study by using a range of DFT methods was performed on 2⋅TNP and 2+ , and clearly supports the experimentally observed spin flip triggered by hydrogen-bonding interactions. The counterion is shown to perturb the spin-state ordering through, for example, hydrogen-bonding interactions, switched positions between counterion and axial ligand, ion-pair interactions, and charge polarization. The present investigation thus provides a clear rationalization of the unusual counterion-specific spin states observed in the μ-hydroxo bisporphyrins that have so far remained the most outstanding issue.

Distinctive Coordination of CO vs N2 to Rhodium Cations: An Infrared and Computational Study.

Carbonyl and nitrogen complexes with Rh+ are produced in a molecular beam using laser ablation and a pulsed-nozzle source. Mass-selected ions of the form Rh(CO)n+ and Rh(N2)n+ are investigated via infrared laser photodissociation spectroscopy. The fragmentation patterns and infrared spectra provide information on the coordination and geometries of these complexes. The shifts in vibrational frequencies relative to the uncoordinated ligands give insight into the nature of the bonding interactions involved. Experimental band positions and intensities are compared to those predicted by density functional theory (DFT). Rh+ coordinates only four nitrogen molecules, whereas it can accommodate five carbonyl ligands. The fifth CO ligand resides in an axial site with bonding intermediate between coordination and solvation. The carbonyl stretch in Rh(CO)4+ (2160 cm-1) is blue-shifted with respect to the molecular CO vibration (2143 cm-1). Conversely, the N-N stretch in Rh(N2)4+ (2297 cm-1) is red-shifted with respect to the free N2 vibration (2330 cm-1). The opposite directions of these frequency shifts is explained by a combination of σ donation and electrostatic ligand polarization.

Impact of metal ion's charge on the interatomic Coulombic decay widths in microsolvated clusters.

Interatomic Coulombic decay (ICD) is an efficient electronic decay mechanism of electronically excited atoms and molecules embedded in an environment. For the series of isoelectronic Na(+), Mg(2+), and Al(3+) ions in aqueous solution, ultrashort ICD lifetimes of 3.1 fs, 1.5 fs, and 0.9 fs, respectively, were observed experimentally. The magnitude of the ICD lifetimes and their variation within the series were qualitatively explained by shortening metal-oxygen equilibrium distances and the increasing polarization of the water molecules as the metal's charge grows. We carried out an extensive ab initio investigation of the variation of the ICD widths with the metal-oxygen distances and the number of water neighbors in Na(+)-(H2O)m (m = 1-4) and Mg(2+)-(H2O)n (n = 1-6) clusters including and excluding polarization effects in the decaying state. We demonstrated that the effect of the induced polarization of the water ligand and the equilibrium cation-oxygen distance are equally important in determining the ordering and ratios of the ICD lifetimes in the series. Moreover, we showed that the induced polarization of the water molecules leads to a slower than linear growth of ICD width with the number of equivalent water neighbors; the non-linearity is stronger for Mg(2+). The ab initio ICD widths in microsolvated Na(+)-(H2O)4 and Mg(2+)-(H2O)6 clusters are found to be in good agreement with the experimental values.

Interaction of amines with electrodes modified by polymeric complexes of Ni with salen-type ligands

Chemically modified electrodes based on complexes of transition metals with N,N′-ethylenebis(salicylimine) (salen) type ligands are reported to act as redox mediators due to axial coordination of exogeneous ligands to metal atom in oxidized form of complex. Thus such electrodes can be used as amperometric and voltammetric detectors of various compounds. However, effects of ligand structure and ligand-analyte specific interactions on the electrochemical response of said complexes have not been studied yet for organic analytes. In this article we demonstrate that some aromatic amines can interact with salen-type ligands by several routes, depending on the ligand and amine structure. It leads to the ligand polarization, and thus to the unique shape of voltammetric curves of chemically modified electrodes, based on polymeric salen complexes, immersed in amines solutions. Such feature allows using the electrodes as sensors for molecular recognition. The same electrodes can act as electron transfer catalysts of the amines oxidation, and may be applicable to the quantitative analysis of amines with the detection limit about 10μmoll−1.

Optical Activity in the Near-IR Region: The λ=980 nm Multiplet of Chiral Yb3+ Complexes.

We used a very simplified electrostatic model based on charge and polarizability of atoms and groups on an organic ligand around a lanthanide ion to predict the near-infrared electronic circular dichroism (NIR ECD) spectra of Yb3+ (a monoelectronic ion). We tuned our method by using two widely different complexes. The first was the heterobimetallic species CsYb(hfbc)4 [hfbc=(-)-3-heptafluorobutyrylcamphorate], in which the ligand is a diketonate and, as such, is endowed with a chromophore with strong UV absorption (π-π* ). Its oxygen atoms define a square antiprism, which provides a symmetric coordination polyhedron. The second system was Yb DOTMA [DOTMA=(1R,4R,7R,10R)-α,α',α'',α'''-tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid], a chiral Yb analogue of Gd DOTA (DOTA=1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), in which the ligand lacks relevant electronic transitions and provides a dissymmetric cage. The relative weights of dynamic (ligand polarization) and static contributions to Yb NIR ECD were evaluated, and the spectra appear to have been well predicted by theory through the introduction of a heuristic weight factor. To validate the approach and to confirm the value of the weight factor, we applied it to two other compounds, namely, Na3 Yb(BINOLate)3 and Yb(BINOLAM)3 [BINOLate=2,2'-dihydroxy-1,1'-binaphthyl; BINOLAM=3,3'-bis(diethylaminomethyl)-1-1'-bi-2-naphthol].

Approaches to the Molecular Recognition of Lanthanides Using Azulenyl-, Phenyl-, Beta-Naphtyl- and Vinyl-Malonate Derivatives

Molecular recognition remains a very active field in the supramolecular chemistry research [1, 2] as well as the elaboration of novel improved ligands. The main focus of our group was on the azulene compounds characterized by an electron rich five-atoms cyclic moiety and a electron-poor seven-atoms cyclic moiety. This "push-pull" double structure where the azulenyl is the electron donating group and the substituent is the electron withdrawing group offers some interesting properties to the molecule. Due to this particular structure the azulenic derivatives could be involved in both oxidation and reduction processes while the nature of the substituent and its’ orientation greatly influences the electrochemical properties (number of waves and the oxydo-reduction potential). The azulenic derivatives are also polyvalent starting materials and the formation of electrode-deposed polymers with electro-activity has been widely reported. In this regard certain new derivatives containing azulenyl and a vinyl-ester group have been synthesized and then physically and chemically characterized [3]. Their ability to interact with lanthanide trivalent cations have been studied by means of electrochemical and UV-Vis procedures. Those interactions are based on the high polarizability of azulenic ligands which can form strong dipole-dipole coordination bonds with the metallic cations. This polarisation is favored by the stabilisation of the polar structure as tropylium ion (Figure 1). Starting from the positive results obtained towards some lanthanide cations (Sm3+, Eu3+, Yb3+, Tb3+) in solution, with this new class of azulenyl-malonates and –malonimides [4, 5], we also considered for this purpose their more simple phenyl- and β-naphtyl-vynilmalonate derivatives. The compounds were characterized by electrochemical methods (cyclic voltammetry and differential pulse voltammetry) and the polymeric film formation was also tested by potential scanning or controlled potential electrochemistry. The titration of the ligand solution with several lanthanides offered a view on their complexing properties. UV-Vis spectroscopy and 1D-NMR further indicated the formation of architectures with various stoichiometries and the existence of multiple binding possibilities, supporting a pseudo-cooperative binding. X-ray diffraction couldn’t offer more relevant information as the crystals obtained were of poor quality. This is also confirmed by the observation that the structure in the assumed complexing region greatly influences the ligand behaviour, the complexing ability increasing in the order: Nf-CH = C(COOEt)2 > Ph-CH = CH-CH = C(COOEt)2 > Ph-CH = C (COOEt)2> Nf, Ph-CH = C (CN) COOEt. Acknowledgments: POSDRU/137390 project for the financial support in realising this work. References (1) J.-C. Bunzli, C. Piguet, Chem. Rev., 2002, 102, 1897–1928. (2) C. Coluccini, D. Pasini, P. Righetti, D. A. Vander Griend, Tetrahedron, 2009, 65, 10436–10440. (3) A. C. Razus, S. Nica, L. Cristian, M. Raicopol, L. Birzan, A. E. Dragu, Dyes Pigm., 2011, 91, 55-61. (4) C.-A. Amarandei, G. O. Buica, G. A. Inel, L. Birzan, E. M. Ungureanu, Acta Chimica Slovenica, 2014, 61, 894-899. (5) E.-M. Ungureanu, G.-O. Buica, A. C. Razus, L. Birzan, R. Weisz, M.-R. Bujduveanu, Rev. Chim.(Buc), 2012, 63(1), 27-33. Figure 1

Direct Visualization of Energy-transferred Excited State in Eu Complex

Highly luminescent lanthanide (Ln) complexes have attracted much attention because Ln3+ions show long-lived ff-emissions with narrow band shape. Their unique photo-optical properties are promising for the design of light-emitting materials and sensing devices. Although the ff-emissions are essentially weak because of Laporte forbidden, chelate ligands is effective to strengthen the intramolecular energy transfer from photo-excited organic ligands to Ln3+ions. The direct evidence of energy transfer from ligands to Ln3+and details of excited state, however, are still veiled. Here, we report direct visualization of energy-transferred excited state in Eu complex with a hexadentate ligand (L) consisting of two bipyridine moieties bridged by an ethylendiamine unit, [Eu3+(L)(NO3)2](PF6) (Eu(L))[1] by Maximum Entropy Method (MEM) charge density[2] and electrostatic potential analysis[3] based on SR X-ray diffraction. First, we confirmed that the electron numbers of Eu and ligand L in the excited state are the same as those in the ground state, which is a direct evidence of energy transfer instead of charge transfer. Next, we observed charge re-distribution in the Eu ion and the ligand L. The electrostatic potential distributions calculated using MEM charge density give an experimental evidence for the existence of polarization of ligand L both in the ground and photo-excited states. The orientation polarization in the ground state changed during pumping at 315 nm, and the charge re-distribution are qualitatively consistent with a theoretical prediction. This characteristic luminescence behavior based on the energy relaxation process have not been detected by fundamental crystal structural analysis. We have succeeded in visualization of subtle but important change due to energy transfer in the mononuclear Europium complex with hexadentate ligand at the first time.

Structural Determinants of Tau Aggregation Inhibitor Potency

Small-molecule Tau aggregation inhibitors are under investigation as potential therapeutic agents against Alzheimer disease. Many such inhibitors have been identified in vitro, but their potency-driving features, and their molecular targets in the Tau aggregation pathway, have resisted identification. Previously we proposed ligand polarizability, a measure of electron delocalization, as a candidate descriptor of inhibitor potency. Here we tested this hypothesis by correlating the ground state polarizabilities of cyanine, phenothiazine, and arylmethine derivatives calculated using ab initio quantum methods with inhibitory potency values determined in the presence of octadecyl sulfate inducer under reducing conditions. A series of rhodanine analogs was analyzed as well using potency values disclosed in the literature. Results showed that polarizability and inhibitory potency directly correlated within all four series. To identify putative binding targets, representative members of the four chemotypes were added to aggregation reactions, where they were found to stabilize soluble, but SDS-resistant Tau species at the expense of filamentous aggregates. Using SDS resistance as a secondary assay, and a library of Tau deletion and missense mutants as targets, interaction with cyanine was localized to the microtubule binding repeat region. Moreover, the SDS-resistant phenotype was completely dependent on the presence of octadecyl sulfate inducer, but not intact PHF6/PH6* hexapeptide motifs, indicating that cyanine interacted with a species in the aggregation pathway prior to nucleus formation. Together the data suggest that flat, highly polarizable ligands inhibit Tau aggregation by interacting with folded species in the aggregation pathway and driving their assembly into soluble but highly stable Tau oligomers.

Effect of Ligand Polarization on Asymmetric Structural Formation for Strongly Luminescent Lanthanide Complexes

AbstractThe effect of ligand polarization on the formation of strongly luminescent lanthanide complexes with asymmetric structures is described for the first time. The lanthanide complexes are composed of EuIII ions, three hexafluoroacetylacetonate (hfa) ligands, and two monodentate phosphine oxide ligands, namely, triphenylphosphine oxide (TPPO), p‐tolyldiphenylphosphine oxide (TPPO‐Me), tri‐p‐tolylphosphine oxide (TPPO‐3Me) or o‐phenoxyphenyldiphenylphosphine oxide (TPPO‐OPh). The luminescence properties of the EuIII complexes are characterized by their emission quantum yields, emission lifetimes, and their radiative (kr) and nonradiative (knr) rate constants. The EuIII complex with TPPO‐OPh ligands offers a markedly high emission quantum yield (72 % in [D6]acetone, 85 % in the solid state) owing to enhancement of the electric dipole transition and suppression of vibrational relaxation, which are directly related to kr and knr. The coordination geometries of the EuIII complexes are categorized by shape‐measure calculations. The EuIII complexes exhibit characteristic square‐antiprismatic or trigonal‐dodecahedral structures, depending on the ligand polarization. Strongly luminescent Eu(hfa)3(TPPO‐OPh)2 has an asymmetric dodecahedron structure. The formation of distorted dodecahedral structures with low vibrational frequencies for the enhancement of luminescence is elucidated in terms of the ligand polarization of the monodentate phosphine oxide ligands in the EuIII complexes.

Analysis of vacuum ultraviolet electronic spectra of Ce3+ and Pr3+ ions in Ca9Lu(PO4)7: crystal-field calculations and simulation of optical spectra

The 4f–5d excitation and emission spectra of Ce3+ and Pr3+ ions in Ca9Lu(PO4)7 as recently reported (2012 J. Phys.: Condens. Matter 24 385502) were further analyzed and simulated by employing the effective Hamiltonian model for the 4fN and 4fN−15d electronic configurations of impurity lanthanide ions and the exchange charge model of crystal-field theory. The multi-site effect on the 4f–5d transition spectra was explicitly discussed from the points of view of the local structure and site occupation ratios of lanthanide ions in Ca9Lu(PO4)7. An excellent agreement between the predicted and measured spectra confirms the validity of the performed calculations. Based on these energy level and intensity calculation results, the radiative lifetimes of the 5d–4f emissions of Ce3+ and Pr3+ ions have been modeled to show nearly independent temperature trends. Comparison with the measured lifetimes suggests the nonradiative relaxation process in this host is probably related to the intrinsic defect states. In addition to the studies of the 4f–5d transitions, a general theoretical scheme to calculate the lowest 4f–6s transition energy of the Ce3+ ion was proposed for the first time on the basis of the ligand polarization model. The predicted 6s energy position of the Ce3+ ion in Ca9Lu(PO4)7 is solid evidence corroborating our previous spectroscopic assignment.

Speciation of La(III) Chloride Complexes in Water and Acetonitrile: A Density Functional Study

Car-Parrinello molecular dynamics (CMPD) simulations and static computations are reported at the BLYP level of density functional theory (DFT) for mixed [LaCl(x)(H(2)O)(y)(MeCN)(z)](3-x) complexes in aqueous and nonaqueous solution (acetonitrile). Both methodologies predict coordination numbers (i.e., x + y + z) that are successively lower than nine as the Cl content increases from x = 0 to 3. While the static DFT method with implicit solvation through a polarizable continuum model overestimates the binding strength of chloride and erroneously predicts [LaCl(2)(H(2)O)(5)](+) as global free-energy minimum, constrained CPMD simulations with explicit solvent and thermodynamic integration reproduce the weak binding of chloride in water reasonably well. Special attention is called to the dipole moments of coordinated water molecules as function of coligands and solvent, evaluated through maximally localized Wannier function centers along the CPMD trajectories. Cooperative polarization of these water ligands by the metal cation and the surrounding solvent is remarkably sensitive to fluctuations of the La-O distances and, to a lesser extent, on the La-water tilt angles. The mean dipole moment of water ligands is rather insensitive to the other coligands, oscillating around 3.2 D, 3.5 D, and 3.3 D in MeCN, water, and [dmim]Cl solution, respectively, the latter being an archetypical ionic liquid.

Exploration of the binding of DNA binding ligands to Staphylococcal DNA through QM/MM docking and molecular dynamics simulation

DNA binding ligands (DBL) were reported to bind the minor groove of bacterial DNA. In the present study, DBL were analyzed and screened for their Staphylococcus inhibitory activity by inhibiting the Staphylococcal DNA replication. The orientation and the ligand-receptor interactions of DBL within the DNA-binding pocket were investigated applying a multi-step docking protocol using Glide and QM/MM docking. The polarization of ligands with QM/MM for DNA-ligand docking with Staphylococcal DNA minor groove was performed in order to understand their possible interactions. Molecular dynamics simulation techniques were employed to obtain the dynamic behavior of the DBL with Staphylococcal DNA. Computational docking and simulation represented a promising alternative to bridge the gap, and so that DNA and gyrase interactions were blocked by DBL. The results revealed the importance of the DBL for strong interactions with the DNA minor groove region and blocking the bacterial replication.

Temperature Sensitive Luminescence of Ce3+ in Two Different Sites of Na3LuSi2O7

Ce3+ doped sodium lutetium pyrosilicates phosphors Na3Lu1-xCexSi2O7 were prepared through a high-temperature solid-state reaction technique. The studies on VUV-vis luminescent properties and fluorescent decays indicate that Ce3+ ions occupy two distinct Lu3+ sites in the host lattice. The excitation and emission bands of Ce3+ in these two lattice sites are assigned in terms of crystal structure and an improved ligand polarization model. The emission color and chromaticity coordinates of phosphors can be tuned by altering temperature due to the temperature-dependent characteristics of two luminescence centers, which suggest the potential application of the phosphors in temperature sensors.

Communication: Quantum polarized fluctuating charge model: A practical method to include ligand polarizability in biomolecular simulations

We present a simple and practical method to include ligand electronic polarization in molecular dynamics (MD) simulation of biomolecular systems. The method involves periodically spawning quantum mechanical (QM) electrostatic potential (ESP) calculations on an extra set of computer processors using molecular coordinate snapshots from a running parallel MD simulation. The QM ESPs are evaluated for the small-molecule ligand in the presence of the electric field induced by the protein, solvent, and ion charges within the MD snapshot. Partial charges on ligand atom centers are fit through the multi-conformer restrained electrostatic potential (RESP) fit method on several successive ESPs. The RESP method was selected since it produces charges consistent with the AMBER/GAFF force-field used in the simulations. The updated charges are introduced back into the running simulation when the next snapshot is saved. The result is a simulation whose ligand partial charges continuously respond in real-time to the short-term mean electrostatic field of the evolving environment without incurring additional wall-clock time. We show that (1) by incorporating the cost of polarization back into the potential energy of the MD simulation, the algorithm conserves energy when run in the microcanonical ensemble and (2) the mean solvation free energies for 15 neutral amino acid side chains calculated with the quantum polarized fluctuating charge method and thermodynamic integration agree better with experiment relative to the Amber fixed charge force-field.

Proton Transfer Reactions for Methanol and Water Containing Manganese Ion Complexes

Under considerations in the current study are reactions of the type [Mn(LOH)(2)](2+) → [Mn(LO)](+) + LOH(2)(+), where the ligand LOH represents water or/and methanol. Preferential proton transfer reactions and loss of any ligand fragments are discussed in the light of ligand polarizability, dipole moment, dissociation energy, proton affinity, differences in ligand-ion ionization energy, and ion radii. The results indicate the proton affinity and dissociation energy of the O-H bond are more important for the overall proton transfer reaction than differences in the first ionization energy of the ligand and the second ionization energy of the metal ion.

DFT calculations of crystal-field parameters for the lanthanide ions in theLaCl3 crystal

We present herein a calculation of crystal-field (CF) parameters for the lanthanide ions in theLaCl3 crystal, using the density functional theory-based orbital-free embedding formalism inconjunction with the effective Hamiltonian method. The calculated values for thesecond- and fourth-rank CF parameters agree fairly well with the experimental data. It isfound that the effect of ligand polarization plays a crucial role in determining the sign andmagnitude of the second-rank CF parameters, while the effect of Pauli repulsion between 4felectrons and the ligands is important for the fourth-rank CF parameters. The usefulnessof the present approach is illustrated by a study of the relative preference ofEu3 + occupation onthe two distinct Pb2 + sites in the KPb2Cl5 crystal. The computational approach and results discussed in this work are useful for abetter understanding and simulation of the CF effect in the lanthanide-containing systems.

Role of polarization in ligand docking and binding affinity prediction for inhibitors of dengue virus

Molecular docking methodology is useful in predicting comparative binding affinity of library of different ligands whose co-crystal structure in complex form is already known. However, scope of this methodology is not reliable for cross docking of different ligands due to incorrect prediction of binding pose if co-crystal structure is unknown. In the present work, we have studied the ligand polarization due to protein environment during the docking of seven ligands in envelope protein of dengue virus. We have used six kinase inhibitors which are active for dengue virus as well and a detergent molecule whose crystal structure is already known. We observed major change in docking scores due to polarization of ligands. The charges of the ligands were calculated by ab initio methods for the accuracy of our results. We observed increased hydrogen bonding due to polarization in protein environment. These results are more significant for inhibitors containing electronegative elements like chlorine and fluorine.

A Theoretical Study on Interaction of Cyclopentadienyl Ligand with Alkali and Alkaline Earth Metals

Ab initio and density functional theory calculations are performed on half-sandwich (M-Cp) and sandwich (Cp-M-Cp) complexes of alkali and alkaline earth metals (M = Li, Na, K, Mg, and Ca) with cyclopentadienyl ligand (Cp). A comparison of dissociation energies demonstrates the ease of dissociation of the complex as ions in solvent phase and preference for dissociation as radicals in gas phase. Atoms in molecules analysis is used to characterize this cation-π interaction based on electron density values obtained at the cage critical point. The contribution of various components to the complex energy is estimated using reduced variational space analysis confirming maximum contribution from the coulomb exchange followed by contributions from polarization and charge transfer components of cyclopentadienyl ligand.