RESUMO

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.

RESUMO

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.

RESUMO

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.

RESUMO

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.

RESUMO

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%.

RESUMO

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.

Assuntos

RESUMO

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.

RESUMO

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%.

Assuntos

RESUMO

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.

RESUMO

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.

Assuntos