Josep M. Oliva

Modulation of the CC distance in disubstituted l,2-R2-o-carboranes. Crystal structure of closo l,2-(SPh)2-l,2-C2B10H10

Abstract 1,2-Cc-substituted o-carboranes, where Cc represents the cluster carbon atoms, display a range of experimental/computed Cc⋯Cc distances from 1.629/1.624 A for the unsubstituted parent compound 1,2-C2B10H12 (1) to 1.798/1.818 A for l,2-(SPh)2-l,2-C2B10H10 (5) 1 or 1.858(5)/1.826(5) for l,2-μ-SCH2(CH2OCH2)2CH2S-1,2-C2B10H10 (7). Different Cc⋯Cc distances can be achieved by modifying the substituents on the carbon of the cluster compound. The crystal structure of a new disubstituted o-carborane 1,2-(SPh)2-l,2-C2B10H10 (5) was elucidated by single crystal X-ray diffraction. Purely alkyl substituents alter the Cc⋯Cc distance of the parent compound very little, which implies that steric effects, although relevant, are not the major cause of the lengthening. In contrast, substituents with lone pairs alter the Cc⋯Cc distance substantially. Our calculations suggest that the cause of the elongation is the transfer of electron density from the available lone pairs on the substituents to the Ψ* low-lying virtual orbitals mainly distributed around Cc, producing a decrease in the Cc⋯Cc bond order and, thereby, an increase in the Cc⋯Cc distance. A Bader analysis shows that the electron density at the bond-critical point, which is found at the mid-point of the Cc⋯Cc distance, decreases considerably with the presence of lone pairs of the sulfur atoms bound to the Cc atoms of the carborane cage.

Design of carborane molecular architectures with electronic structure computations: From endohedral and polyradical systems to multidimensional networks

The 12 cage-anchoring points of the very stable icosahedral ortho-, meta-, and para-carborane allow the design of multidimensional architectures provided new self-assembling routes are devised. We provide bases for constructing carborane molecular architectures through high-level quantum chemical computations. We consider ejection mechanisms for the inner atom/ion in endohedral carborane complexes, singlet-triplet energy gaps in carborane biradicals, as well as geometry reorganization in carborane neutral and dianionic triplet states. These features, explored in monomers, are starting points for the design of molecular architectures based on electronic structure properties of carborane assemblies.

Complexation of stable carbenes with alkali metals

Stable diaminocarbenes, including imidazol-2-ylidenes, undergo complexation with lithium, sodium and potassium species; the crystal structure of a complex of 1,3-diisopropyl-3,4,5,6-tetrahydropyrimid-2-ylidene 1 with KN(SiMe3)2 is reported.

What Electronic Structures and Geometries of Carborane Mono- and ortho-, meta-, and para-Diradicals are Preferred?

Structures, relative stabilities, singlet-triplet gaps, and the ground-state character of mono- and diradicals derived from the three icosahedral carborane cage isomers have been computed by unrestricted broken-symmetry DFT and by CASPT2 methods. Whereas the bond dissociation energies (BDE) leading to the carborane monoradicals are close to the benzene BDE, the most stable carborane radicals are derived from dissociations of hydrogens farthest away from the carbon atoms. All the monomeric carborane diradicals are determined to have singlet ground states. However, the energetic accessibility of triplet states in some of the species offers the potential of building diradical multidimensional carborane network architectures with interesting magnetic properties.

Organometallic gold complexes of carborane. Theoretical comparative analysis of ortho, meta, and para derivatives and luminescence studies

The synthesis and X-ray analysis of complexes [(micro-1,12-C2B10H10){Au(PPh3)}2] and [(micro-1,2-C2B10H10){Au(PMe3)}2] have provided the experimental data needed to analyse two points. The first point is the use of these data to carry out a computational study with the aim of comparing the electronic structures and relative stabilities of the organometallic isomers [(micro-1,n-C2B10H10){Au(PR3)}2] (n=2, 7, 12; R=Ph, Me) with those of the parent carborane clusters ortho-, meta- and para-carborane and the influence of the monophosphine substituents. The second point is focused in the influence of the steric demand of the monophosphine in the presence or not of aurophilic interactions in the ortho derivatives [(micro-1,2-C2B10H10){Au(PR3)}2] (R=Me, Ph). The photoluminescent behaviour of both the carboranes and the organometallic complexes is presented.

ANALYSIS OF THE CHANGES ON THE POTENTIAL ENERGY SURFACE OF MENSHUTKIN REACTIONS INDUCED BY EXTERNAL PERTURBATIONS

Abstract A quantitative analysis of the changes induced by solvation and static uniform electric fields on the potential energy surface of the SN2 Menshutkin-type reaction between ammonia and methyl chloride has been performed with the help of different indexes. These indexes have been defined to account for the structural and electronic degree of advance of the transition state with respect to the reactant complex and ion pair product through the use of geometrical parameters, dipole moments and electron density distributions. Indexes reveal that external perturbations yield transition states which are both electronically and structurally advanced as compared to the transition state in the gas phase. The overall study is complemented with HOMO-LUMO orbital considerations, quantification of the global charge density redistributions by means of quantum molecular self-similarity measures and an analysis of the topological features of electron density distributions.

Solution and computed structure of O-lithium N,N-diisopropyl-P,P-diphenylphosphinic amide. Unprecedented Li-O-Li-O self-assembly of an aryllithium.

The structural characterization of an ortho-lithiated diphenylphosphinic amide is described for the first time. Multinuclear magnetic resonance ((1)H, (7)Li, (13)C, (31)P) studies as a function of temperature and concentration employing 1D and 2D methods showed that the anion exists as a mixture of one monomer and two diastereomeric dimers. In the dimers the chiral monomer units are assembled in a like and unlike manner through oxygen-lithium bonds, leading to fluxional ladder structures. This self-assembling mode leads to the formation of Li(2)O(2) four-membered rings, a structural motif unprecedented in aryllithium compounds. DFT computations of representative model compounds of ortho-lithiated phosphinic amide monomer and Li(2)C(2) and Li(2)O(2) dimers with different degrees of solvation by THF molecules showed that Li(2)O(2) dimers are thermodynamically favored with respect to the alternative Li(2)C(2) structures by 4.3 kcal mol(-1) in solvent-free species and by 2.3 kcal mol(-1) when each lithium atom is coordinated to one THF molecule. Topological analysis of the electron density distribution revealed that the Li(2)O(2) four-membered ring is characterized by four carbon-lithium bond paths and one oxygen-oxygen bond path. The latter divides the Li-O-Li-O ring into two Li-O-Li three-sided rings, giving rise to two ring critical points. On the contrary, the bond path network in the Li(2)C(2) core includes a catastrophe point, suggesting that this molecular system can be envisaged as an intermediate in the formation of Li(2)O(2) dimers. The computed (13)C chemical shifts of the C-Li carbons support the existence of monomeric and dimeric species containing only one C-Li bond and are consistent with the existence of tricoordinated lithium atoms in all species in solution.

Study of the electronic states of the allyl radical using spin-coupled valence bond theory

The spin-coupled valence bond method is used to study the doublet valence excited states and the n=3 Rydberg excited states of the allyl radical below the first ionization potential 8.13 eV. The calculations included three π electrons in the active space. Our results are close to those resulting from the most extensive MO-CI calculations reported to date. The spin-coupled VB approach has an advantage of providing a compact description of the various states.

What Is the Limit of Atom Encapsulation for Icosahedral Carboranes

The stability of endohedral carboranes X@{1,n-C2B10H12} (X = Li(+), Be(2+); n = 2, 7, 12) and X@{CB11H12(-)} (X = Li(+), Be(2+)) is studied using electronic structure calculations with the B3LYP/6-311+G(d,p) model. Our calculations suggest that all endohedral compounds are local energy minima; for the exohedral complexes X···cage, the global energy minimum always corresponds to the X atom above a triangular face of the icosahedron. In the latter the X atom is furthest apart from the carbon atoms of the cage. As opposite to exohedral {Be(2+)···cage} complexes, no global energy minima were found for exohedral complexes {Li(+)···cage} whereby a carbon atom is present in the triangular face of the icosahedron below the Li(+) cation.

A computational study of the lowest singlet and triplet states of neutral and dianionic 1,2-substituted icosahedral and octahedral o-carboranes

This work introduces a calibrated B3LYP/6‐31G(d) study on the electronic structure of singlet and triplet neutral species of 1,2‐substituted icosahedral 1,2‐R2‐1,2‐C2B10H10 and octahedral 1,2‐R2‐1,2‐C2B4H4 molecules with R = {H, OH, SH, NH2, PH2, CH3, SiH3} and their respective dianions formed by proton removal on each R group. A variety of small adiabatic singlet‐triplet gaps ΔEST are obtained from these systems ranging from 2.93 eV (R = NH2) ≤ ΔEST ≤ 3.98 eV (R = SiH3) for the icosahedral neutrals and 1.56 eV (R = NH2) ≤ ΔEST ≤ 4.13 eV (R = SiH3) for the octahedral neutrals, these gaps being globally smaller for the dianionic systems, ranging from 0.94 eV (R− = CH2−) ≤ ΔEST ≤ 2.01 (R− = e−) for the icosahedral dianions 1,2‐(R−)2‐C2B10H10 and 0.91 (R− = CH2−) ≤ ΔEST ≤ 2.41 (R = SiH2−) for the octahedral dianions 1,2‐(R−)2C2B4H4 (R− = e−, O−, S−, NH−, PH−, CH2−, PH2−). The different gaps lie within or in the neighborhood of the visible region of the electromagetic spectrum. The optimized geometries for the molecular cage remain similar in substituted icosahedral and octahedral singlet states, while a rich variety of different structures can be found for the optimized triplet states.