RESUMEN

When handling metallic centers of higher coordination numbers, one is commonly deluded with the presumption that any assembled metal complex geometry (including a crystallographic one) is good enough as a starting structure for computational chemistry calculations; all oblivious to the fact that such a structure is nothing short of just one out of several, sometimes dozens, or even thousands of other stereoisomers. Moreover, coordination chirality, so frequently present in complexes of higher coordination numbers, is another often overlooked property, rarely recognized as such. The Complex Build algorithm advanced in this article has been designed with the purpose of generating starting structures for molecular modeling calculations with full stereochemical control, including stereoisomer complete identification and coordination chirality recognition. Besides being in the chosen correct stereochemistry, the ligands are positioned by the Complex Build algorithm in a very unobstructed and unclogged manner, so that their degrees of freedom do not hinder or even choke one another, something that would otherwise tend to lead to negative force constants after further geometry optimizations by more advanced computational model chemistries. The Complex Build algorithm has been conceived for any metallic center, but at present is targeting primarily lanthanoids whose coordination numbers range mostly from 5 to 12 and often lead to a combinatorial explosion of stereoisomers.

RESUMEN

By combining NMR data (nuclear Overhauser effect and pseudocontact shifts) with luminescence measurements, we uncover how the structure of an anionic europium complex adapts to different solvent polarities as a result of the different relative proximities of the ion pairs. In nonpolar solvents, the detected contact ion pairs, CIPs, indicate that the ions remain in proximity, with the molecular cation, and then perturbing and distorting the coordination polyhedron of the anion complex to a low symmetry configuration, which promotes luminescence. Differently, solvent separated ion pairs occur in polar solvents, indicating that the molecular ions have been disconnected. Thus, in polar solvents, the europium complex anion becomes free from the close influence of the molecular cation, allowing the coordination polyhedron to increase its symmetry, which in turn reduces the luminescence of the anionic europium complex. This reduction of coordination polyhedron symmetry by the close proximity of the molecular cation in nonpolar solvents was confirmed by additional photophysical measurements combined with quantum chemical RM1 calculations, suggesting that, in nonpolar solvents, the symmetry point group of the coordination polyhedron is C1, whereas in polar solvents it is either D2d or S4. The nonpolar solvents used were benzene, chloroform and dichloromethane; and the polar ones were acetone and acetonitrile. The synthesized ionic liquids were tetrakis [C5mim][La(BTFA)4] and [C5mim][Eu(BTFA)4], where BTFA stands for 4,4,4-trifluoro-1-phenyl-1,3-butanedione, lanthanoids (La3+ and Eu3+) and C5mim stands for 1-methyl-3-isopentylimidazolium. They were synthesized by a microwave methodology that is both fast and green (the synthetic reaction takes about 15 min) and also leads to more easily purifiable crystals.

RESUMEN

We address the use of Euler's theorem and topological algorithms to design 18 polyhedral hydrocarbons of general formula CnHn that exist up to 28 vertexes containing four- and six-membered rings only; compounds we call "nuggets". Subsequently, we evaluated their energies to verify the likelihood of their chemical existence. Among these compounds, 13 are novel systems, of which 3 exhibit chirality. Further, the ability of all nuggets to perform fusion reactions either through their square faces, or through their hexagonal faces was evaluated. Indeed, they are potentially able to form bottom-up derived molecular hyperstructures with great potential for several applications. By considering these fusion abilities, the growth of the nuggets into 1D, 2D, and 3D-scaffolds was studied. The results indicate that nugget24a (C24H24) is predicted to be capable of carrying out fusion reactions. From nugget24a, we then designed 1D, 2D, and 3D-scaffolds that are predicted to be formed by favorable fusion reactions. Finally, a 3D-scaffold generated from nugget24a exhibited potential to be employed as a voxel with a chemical structure remarkably similar to that of MOF ZIF-8. And, such a voxel, could in principle be employed to generate any 3D sculpture with nugget24a as its level of finest granularity.

RESUMEN

The concept of random coordination ratios, RCRs, is advanced for lanthanide complexes. RCRs describe the relative probabilities of occurrence of subsets of stereoisomers of same-symmetry point groups in the limiting situation when energetic effects are equivalent. We then introduce a method to uniquely identify the stereoisomer of the coordination polyhedron of a given crystallographic structure and introduce a notation that fully characterizes its stereochemistry in an unambiguous manner, from which absolute configuration naturally follows. De facto, the coordination chirality in lanthanide complexes is a frequently overlooked property, even though these compounds often exhibit, when luminescent, high dissymmetry factors. With our methodology, we even managed to recognize a known dilanthanide complex as a meso compound, with both metal ions functioning as stereogenic centers. To achieve these results, we enumerate all possible stereoisomers of lanthanide complexes with coordination numbers from 4 to 9 for all combinations of monodentate, symmetric and asymmetric bidentate ligands, and for several shapes of coordination polyhedra. We confirmed the number of stereoisomers for each case by means of Pólya's theorem. We further classified all stereoisomers according to their symmetry point groups and generated their Cartesian coordinates. This collection of all coordination polyhedra stereoisomer geometries, which is made available in the Supporting Information , can also be used to easily build starting-point geometries for theoretical calculations of metal complexes.

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We advance the concept that a single efficient antenna ligand substituted in or added to an otherwise weakly luminescent europium complex is enough to significantly boost its luminescence. Our results, on the basis of photophysical measurements on 5 novel europium complexes and 15 known ones, point in the direction that ligand dissimilarity and ligand diversity are all concepts that clearly play a fundamental role in the luminescence of europium complexes. We show that it is important that a symmetry breaker ligand exists in the complex to enhance ligand dissimilarity and ligand diversity, all mainly affecting the nonradiative decay rate by reducing it. Because the presence of at least one antenna ligand is also obviously necessary, the optimal and the most cost-effective situation can be achieved by adding a single coordination symmetry breaker that is also an efficient antenna, such as 1-(2-thenoyl)-3,3,3-trifluoroacetone or 4,4,4-trifluoro-1-phenyl-1,3-butanedione. In such cases the quantum efficiency, η, is decidedly boosted, as can be verified by going from complex [EuCl2(TPPO)4]Cl·3H2O with η = 0% to the novel complex [EuCl2(BTFA)(TPPO)3], where TPPO stands for triphenylphosphine oxide, with η = 62%.

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The RM1 quantum chemical model for the calculation of complexes of Tm(III), Yb(III) and Lu(III) is advanced. Subsequently, we tested the models by fully optimizing the geometries of 126 complexes. We then compared the optimized structures with known crystallographic ones from the Cambridge Structural Database. Results indicate that, for thulium complexes, the accuracy in terms of the distances between the lanthanide ion and its directly coordinated atoms is about 2%. Corresponding results for ytterbium and lutetium are both 3%, levels of accuracy useful for the design of lanthanide complexes, targeting their countless applications.

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RESUMEN

The spontaneous emission coefficient, Arad, a global molecular property, is one of the most important quantities related to the luminescence of complexes of lanthanide ions. In this work, by suitable algebraic transformations of the matrices involved, we introduce a partition that allows us to compute, for the first time, the individual effects of each ligand on Arad, a property of the molecule as a whole. Such a chemical partition thus opens possibilities for the comprehension of the role of each of the ligands and their interactions on the luminescence of europium coordination compounds. As an example, we applied the chemical partition to the case of repeating non-ionic ligand ternary complexes of europium(III) with DBM, TTA, and BTFA, showing that it allowed us to correctly order, in an a priori manner, the non-obvious pair combinations of non-ionic ligands that led to mixed-ligand compounds with larger values of Arad.

RESUMEN

ß-diketonates are customary bidentate ligands in highly luminescent ternary europium complexes, such as Eu(ß-diketonate)3(L)2, where L stands for a nonionic ligand. Usually, the syntheses of these complexes start by adding, to an europium salt such as EuCl3(H2O)6, three equivalents of ß-diketonate ligands to form the complexes Eu(ß-diketonate)3(H2O)2. The nonionic ligands are subsequently added to form the target complexes Eu(ß-diketonate)3(L)2. However, the Eu(ß-diketonate)3(H2O)2 intermediates are frequently both difficult and slow to purify by recrystallization, a step which usually takes a long time, varying from days to several weeks, depending on the chosen ß-diketonate. In this article, we advance a novel synthetic technique which does not use Eu(ß-diketonate)3(H2O)2 as an intermediate. Instead, we start by adding 4 equivalents of a monodentate nonionic ligand L straight to EuCl3(H2O)6 to form a new intermediate: EuCl3(L)4(H2O)n, with n being either 3 or 4. The advantage is that these intermediates can now be easily, quickly, and efficiently purified. The ß-diketonates are then carefully added to this intermediate to form the target complexes Eu(ß-diketonate)3(L)2. For the cases studied, the 20-day average elapsed time reduced to 10 days for the faster synthesis, together with an improvement in the overall yield from 42% to 69%.

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We advance the concept that the charge factors of the simple overlap model and the polarizabilities of Judd-Ofelt theory for the luminescence of europium complexes can be effectively and uniquely modeled by perturbation theory on the semiempirical electronic wave function of the complex. With only three adjustable constants, we introduce expressions that relate: (i) the charge factors to electronic densities, and (ii) the polarizabilities to superdelocalizabilities that we derived specifically for this purpose. The three constants are then adjusted iteratively until the calculated intensity parameters, corresponding to the (5)D0→(7)F2 and (5)D0→(7)F4 transitions, converge to the experimentally determined ones. This adjustment yields a single unique set of only three constants per complex and semiempirical model used. From these constants, we then define a binary outcome acceptance attribute for the adjustment, and show that when the adjustment is acceptable, the predicted geometry is, in average, closer to the experimental one. An important consequence is that the terms of the intensity parameters related to dynamic coupling and electric dipole mechanisms will be unique. Hence, the important energy transfer rates will also be unique, leading to a single predicted intensity parameter for the (5)D0→(7)F6 transition.

RESUMEN

The RM1 model for the lanthanides is parameterized for complexes of the trications of lanthanum, cerium, and praseodymium. The semiempirical quantum chemical model core stands for the [Xe]4fn electronic configuration, with n =0,1,2 for La(III), Ce(III), and Pr(III), respectively. In addition, the valence shell is described by three electrons in a set of 5d, 6s, and 6p orbitals. Results indicate that the present model is more accurate than the previous sparkle models, although these are still very good methods provided the ligands only possess oxygen or nitrogen atoms directly coordinated to the lanthanide ion. For all other different types of coordination, the present RM1 model for the lanthanides is much superior and must definitely be used. Overall, the accuracy of the model is of the order of 0.07Å for La(III) and Pr(III), and 0.08Å for Ce(III) for lanthanide-ligand atom distances which lie mostly around the 2.3Å to 2.6Å interval, implying an error around 3% only.

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RESUMEN

Complexes of dysprosium, holmium, and erbium find many applications as single-molecule magnets, as contrast agents for magnetic resonance imaging, as anti-cancer agents, in optical telecommunications, etc. Therefore, the development of tools that can be proven helpful to complex design is presently an active area of research. In this article, we advance a major improvement to the semiempirical description of lanthanide complexes: the Recife Model 1, RM1, model for the lanthanides, parameterized for the trications of Dy, Ho, and Er. By representing such lanthanide in the RM1 calculation as a three-electron atom with a set of 5 d, 6 s, and 6 p semiempirical orbitals, the accuracy of the previous sparkle models, mainly concentrated on lanthanide-oxygen and lanthanide-nitrogen distances, is extended to other types of bonds in the trication complexes' coordination polyhedra, such as lanthanide-carbon, lanthanide-chlorine, etc. This is even more important as, for example, lanthanide-carbon atom distances in the coordination polyhedra of the complexes comprise about 30% of all distances for all complexes of Dy, Ho, and Er considered. Our results indicate that the average unsigned mean error for the lanthanide-carbon distances dropped from an average of 0.30 Å, for the sparkle models, to 0.04 Å for the RM1 model for the lanthanides; for a total of 509 such distances for the set of all Dy, Ho, and Er complexes considered. A similar behavior took place for the other distances as well, such as lanthanide-chlorine, lanthanide-bromine, lanthanide, phosphorus and lanthanide-sulfur. Thus, the RM1 model for the lanthanides, being advanced in this article, broadens the range of application of semiempirical models to lanthanide complexes by including comprehensively many other types of bonds not adequately described by the previous models.

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RESUMEN

All versions of our previous Sparkle Model were very accurate in predicting lanthanide-lanthanide distances in complexes where the two lanthanide ions directly face each other, and mainly lanthanide-oxygen, and lanthanide-nitrogen distances, which are by far the most common ones in lanthanide complexes. In this article, we are advancing for the first time the RM1 model for lanthanides. Designed to be a much more general NDDO model, the RM1 model for lanthanides is capable of predicting geometries of lanthanide complexes for the cases when the central lanthanide trication is directly coordinated to any other atoms, not only oxygen or nitrogen. The RM1 model for lanthanides is defined by three important attributes: (a) the orbitals, the lanthanide ion has now three electrons and a NDDO basis set made of 5d, 6s, and 6p functions; (b) the parametrization, via cluster analysis and an adequate sampling; and (c), the statistical validation of the parameters to make sure the errors behave as random around a mean. All three aspects are described in detail in the article. Results indicate that the RM1 model does extend the accuracy of the previous Sparkle Models to types of coordinating bonds other than Ln-O and Ln-N; the most common ones for Eu, Gd, and Tb, being Ln-C, Ln-S, Ln-Cl, and Ln-Br. Overall, these other coordinating bonds are now predicted within 0.06 Å of their correct values. Therefore, the RM1 model here presented is capable of predicting geometries of lanthanide complexes, materials, metal-organic frameworks, etc., with useful accuracy.

RESUMEN

The recently published Parametric Method number 7, PM7, is the first semiempirical method to be successfully tested by modeling crystal structures and heats of formation of solids. PM7 is thus also capable of producing results of useful accuracy for materials science, and constitutes a great improvement over its predecessor, PM6. In this article, we present Sparkle Model parameters to be used with PM7 that allow the prediction of geometries of metal complexes and materials which contain lanthanide trications. Accordingly, we considered the geometries of 224 high-quality crystallographic structures of complexes for the parameterization set and 395 more for the validation of the parameterization for the whole lanthanide series, from La(III) to Lu(III). The average unsigned error for Sparkle/PM7 for the distances between the metal ion and its coordinating atoms is 0.063Å for all lanthanides, ranging from a minimum of 0.052Å for Tb(III) to 0.088Å for Ce(III), comparable to the equivalent errors in the distances predicted by PM7 for other metals. These distance deviations follow a gamma distribution within a 95% level of confidence, signifying that they appear to be random around a mean, confirming that Sparkle/PM7 is a well-tempered method. We conclude by carrying out a Sparkle/PM7 full geometry optimization of two spatial groups of the same thulium-containing metal organic framework, with unit cells accommodating 376 atoms, of which 16 are Tm(III) cations; the optimized geometries were in good agreement with the crystallographic ones. These results emphasize the capability of the use of the Sparkle Model for the prediction of geometries of compounds containing lanthanide trications within the PM7 semiempirical model, as well as the usefulness of such semiempirical calculations for materials modeling. Sparkle/PM7 is available in the software package MOPAC2012, at no cost for academics and can be obtained from http://openmopac.net.

RESUMEN

Lanthanide luminescence has many important applications in anion sensing, protein recognition, nanosized phosphorescent devices, optoelectronic devices, immunoassays, etc. Luminescent europium complexes, in particular, act as light conversion molecular devices by absorbing ultraviolet (UV) light and by emitting light in the red visible spectral region. The quantum yield of luminescence is defined as the ratio of the number of photons emitted over the number of UV photons absorbed. The higher the quantum yield of luminescence, the higher the sensitivity of the application. Here we advance a conjecture that allows the design of europium complexes with higher values of quantum yields by simply increasing the diversity of good ligands coordinated to the lanthanide ion. Indeed, for the studied cases, the percent boost obtained on the quantum yield proved to be strong: of up to 81%, accompanied by faster radiative rate constants, since the emission becomes less forbidden.

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RESUMEN

In this article, we advance the foundations of a strategy to develop a molecular mechanics method based not on classical mechanics and force fields but entirely on quantum mechanics and localized electron-pair orbitals, which we call quantum molecular mechanics (QMM). Accordingly, we introduce a new manner of calculating Hartree-Fock ab initio wavefunctions of closed shell systems based on variationally preoptimized nonorthogonal electron pair orbitals constructed by linear combinations of basis functions centered on the atoms. QMM is noniterative and requires only one extremely fast inversion of a single sparse matrix to arrive to the one-particle density matrix, to the electron density, and consequently, to the ab initio electrostatic potential around the molecular system, or cluster of molecules. Although QMM neglects the smaller polarization effects due to intermolecular interactions, it fully takes into consideration polarization effects due to the much stronger intramolecular geometry distortions. For the case of methane, we show that QMM was able to reproduce satisfactorily the energetics and polarization effects of all distortions of the molecule along the nine normal modes of vibration, well beyond the harmonic region. We present the first practical applications of the QMM method by examining, in detail, the cases of clusters of helium atoms, hydrogen molecules, methane molecules, as well as one molecule of HeH(+) surrounded by several methane molecules. We finally advance and discuss the potentialities of an exact formula to compute the QMM total energy, in which only two center integrals are involved, provided that the fully optimized electron-pair orbitals are known.

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PM6 is the first semiempirical method to be released already parametrized for the elements of the periodic table, from hydrogen to bismuth (Z = 83), with the exception of the lanthanides from cerium (Z = 58) to ytterbium (Z = 70). In order to fill this gap, we present in this article a generalization of our Sparkle Model for the quantum chemical semiempirical calculation of lanthanide complexes to PM6. Accordingly, we present Sparkle/PM6 parameters for all lanthanide trications from La(III) to Lu(III). The validation procedure again considered only high-quality crystallographic structures and included 633 complexes. Sparkle/PM6 unsigned mean errors UME(Ln-L)s, corresponding to all the interatomic distances between the lanthanide ion and the atoms directly coordinated to it, range from 0.066 to 0.086 Šfor Gd(III) and Ce(III), respectively. These minimum and maximum UME(Ln-L)s across the lanthanide series are comparable to the Sparkle/AM1 values of 0.054 and 0.085 Šfor Ho(III) and Pr(III), respectively, as well as to the values for Sparkle/PM3 of 0.064 and 0.093 Šfor Gd(III) and Pr(III), respectively. Moreover, for all 15 lanthanide ions, these interatomic distance deviations follow a γ distribution within a 95% level of confidence, indicating that these errors appear to be random around a mean, freeing the model of systematic errors, at least within the validation set. Sparkle/PM6 presented here, therefore, broadens the range of applicability of PM6 to the coordination compounds of the rare earth metals.

RESUMEN

The recently defined Sparkle model for the quantum chemical prediction of geometries of lanthanum(III) and lutetium(III) complexes within AM1 (J. Phys. Chem. A 2006, 110, 5897) has been extended to PM3. As training sets, we used the same two groups, one for each lanthanide, of 15 high-crystallographic-quality (R factor < 0.05 A) complexes as was previously chosen to parametrize Sparkle/AM1. Likewise, in the validation procedure, we used the same Sparkle/AM1 validation sets of 60 additional La(III) and 15 additional Lu(III) complexes. The Sparkle/PM3 unsigned mean errors for all interatomic distances between the metal ions and the ligand atoms of the first sphere of coordination proved to be random around the means of 0.068 A for lanthanum(III) and 0.076 A for lutetium(III), thus being comparable to the respective Sparkle/AM1 values of 0.078 and 0.075 A. Furthermore, effective-core-potential ab initio calculations on smaller subsets of such complexes led to similar accuracies. Sparkle/PM3 and Sparkle/AM1 are therefore made available to the researcher who must decide which of the models to use based on considerations of the impact of either PM3 or AM1 on the description of the ligands and the consequence of such a choice on the properties of interest.

RESUMEN

The Sparkle/PM3 model is extended to neodymium(III), promethium(III), and samarium(III) complexes. The unsigned mean error, for all Sparkle/PM3 interatomic distances between the trivalent lanthanide ion and the ligand atoms of the first sphere of coordination, is 0.074 Å for Nd(III); 0.057 Å for Pm(III); and 0.075 Å for Sm(III). These figures are similar to the Sparkle/AM1 ones of 0.076 Å, 0.059 Å, and 0.075 Å, respectively, indicating they are all comparable models. Moreover, their accuracy is similar to what can be obtained by present-day ab initio effective potential calculations on such lanthanide complexes. Hence, the choice of which model to utilize will depend on the assessment of the effect of either AM1 or PM3 on the quantum chemical description of the organic ligands. Finally, we present a preliminary attempt to verify the geometry prediction consistency of Sparkle/PM3. Since lanthanide complexes are usually flexible, we randomly generated 200 different input geometries for the samarium complex QIPQOV which were then fully optimized by Sparkle/PM3. A trend appeared in that, on average, the lower the total energy of the local minima found, the lower the unsigned mean errors, and the higher the accuracy of the model. These preliminary results do indicate that attempting to find, with Sparkle/PM3, a global minimum for the geometry of a given complex, with the understanding that it will tend to be closer to the experimental geometry, appears to be warranted. Therefore, the sparkle model is seemingly a trustworthy semiempirical quantum chemical model for the prediction of lanthanide complexes geometries.

RESUMEN

Twenty years ago, the landmark AM1 was introduced, and has since had an increasingly wide following among chemists due to its consistently good results and time-tested reliability--being presently available in countless computational quantum chemistry programs. However, semiempirical molecular orbital models still are of limited accuracy and need to be improved if the full potential of new linear scaling techniques, such as MOZYME and LocalSCF, is to be realized. Accordingly, in this article we present RM1 (Recife Model 1): a reparameterization of AM1. As before, the properties used in the parameterization procedure were: heats of formation, dipole moments, ionization potentials and geometric variables (bond lengths and angles). Considering that the vast majority of molecules of importance to life can be assembled by using only six elements: C, H, N, O, P, and S, and that by adding the halogens we can now build most molecules of importance to pharmaceutical research, our training set consisted of 1736 molecules, representative of organic and biochemistry, containing C, H, N, O, P, S, F, Cl, Br, and I atoms. Unlike AM1, and similar to PM3, all RM1 parameters have been optimized. For enthalpies of formation, dipole moments, ionization potentials, and interatomic distances, the average errors in RM1, for the 1736 molecules, are less than those for AM1, PM3, and PM5. Indeed, the average errors in kcal x mol(-1) of the enthalpies of formation for AM1, PM3, and PM5 are 11.15, 7.98, and 6.03, whereas for RM1 this value is 5.77. The errors, in Debye, of the dipole moments for AM1, PM3, PM5, and RM1 are, respectively, 0.37, 0.38, 0.50, and 0.34. Likewise, the respective errors for the ionization potentials, in eV, are 0.60, 0.55, 0.48, and 0.45, and the respective errors, in angstroms, for the interatomic distances are 0.036, 0.029, 0.037, and 0.027. The RM1 average error in bond angles of 6.82 degrees is only slightly higher than the AM1 figure of 5.88 degrees, and both are much smaller than the PM3 and PM5 figures of 6.98 degrees and 9.83 degrees, respectively. Moreover, a known error in PM3 nitrogen charges is corrected in RM1. Therefore, RM1 represents an improvement over AM1 and its similar successor PM3, and is probably very competitive with PM5, which is a somewhat different model, and not fully disclosed. RM1 possesses the same analytical construct and the same number of parameters for each atom as AM1, and, therefore, can be easily implemented in any software that already has AM1, not requiring any change in any line of code, with the sole exception of the values of the parameters themselves.

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RESUMEN

The sparkle/AM1 model for the quantum chemical prediction of coordination polyhedron crystallographic geometries from isolated lanthanide complex ion calculations, defined recently for Eu(III), Gd(III), and Tb(III) (Inorg. Chem. 2005, 44, 3299) is now extended to La(III) and Lu(III). Thus, for each of the metal ions we chose a training set of 15 complexes that possess various representative ligands of high crystallographic quality (R factor < 0.05 Angstroms) and oxygen and/or nitrogen as coordinating atoms. In the validation procedure we used a set of 60 more La(III) coordination compound structures, as well as 15 more Lu(III) coordination compound structures, all of high crystallographic quality. For both the 75 La(III) compounds and the 30 Lu(III) compounds, the Sparkle/AM1 unsigned mean error, for all interatomic distances between the metal ions and the ligand atoms of the first sphere of coordination, is 0.08 Angstroms, thus comparable to the accuracy normally achievable by present day ab initio/ECP calculations, while being hundreds of times faster.