Showing papers by "José Elguero published in 2016"

Cooperativity in Tetrel Bonds

Abstract: A theoretical study of the cooperativity in linear chains of (H3SiCN)n and (H3SiNC)n complexes connected by tetrel bonds has been carried out by means of MP2 and CCSD(T) computational methods. In all cases, a favorable cooperativity is observed, especially in some of the largest linear chains of (H3SiNC)n, where the effect is so large that the SiH3 group is almost equidistant to the two surrounding CN groups and it becomes planar. In addition, the combination of tetrel bonds with other weak interactions (halogen, chalcogen, pnicogen, triel, beryllium, lithium, and hydrogen bond) has been explored using ternary complexes, (H3SiCN)2:XY and (H3SiNC)2:XY. In all cases, positive cooperativity is obtained, especially in the (H3SiNC)2:ClF and (H3SiNC)2:SHF ternary complexes, where, respectively, halogen and chalcogen shared complexes are formed.

Host–Guest Chemistry: Oxoanion Recognition Based on Combined Charge‐Assisted C−H or Halogen‐Bonding Interactions and Anion⋅⋅⋅Anion Interactions Mediated by Hydrogen Bonds

Abstract: Several bis-triazolium-based receptors have been synthesized and their anion-recognition capabilities have been studied. The central chiral 1,1'-bi-2-naphthol (BINOL) core features either two aryl or ferrocenyl end-capped side arms with central halogen- or hydrogen-bonding triazolium receptors. NMR spectroscopic data indicate the simultaneous occurrence of several charge-assisted aliphatic and heteroaromatic C-H noncovalent interactions and combinations of C-H hydrogen and halogen bonding. The receptors are able to selectively interact with HP2 O7 (3-) , H2 PO4 (-) , and SO4 (2-) anions, and the value of the association constant follows the sequence: HP2 O7 (3-) >SO4 (2-) >H2 PO4 (-) . The ferrocenyl end-capped 7(2+) ⋅2 BF4 (-) receptor allows recognition and differentiation of H2 PO4 (-) and HP2 O7 (3-) anions by using different channels: H2 PO4 (-) is selectively detected through absorption and emission methods and HP2 O7 (3-) by using electrochemical techniques. Significant structural results are the observation of an anion⋅⋅⋅anion interaction in the solid state (2:2 complex, 6(2+) ⋅[H2 P2 O7 ](2-) ), and a short C-I⋅⋅⋅O contact is observed in the structure of the complex [8(2+) ][SO4 ]0.5 [BF4 ].

Cation–cation and anion–anion complexes stabilized by halogen bonds

Abstract: Stable minima showing halogen bonds between charged molecules with the same sign have been explored by means of theoretical calculations. The dissociation transition states and their corresponding barriers have also been characterized. In all cases, the results indicate that the complexes are thermodynamically unstable but kinetically stable with respect to the isolated monomers in gas phase. A corrected binding energy profile by removing the charge-charge repulsion of the monomers shows a profile similar to the one observed for the dissociation of analogous neutral systems. The nature of the interaction in the minima and TSs has been analyzed using the symmetry adapted perturbation theory (SAPT) method. The results indicate the presence of local favorable electrostatic interactions in the minima that vanish in the TSs. Natural bond orbital (NBO) and "atoms-in-molecules" (AIM) theories were used to analyze the complexes, obtaining good correlations between Laplacian and electron density values with both bond distances and charge-transfer energy contributions E(2). The largest E(2) orbital interaction energies for cation-cation and anion-anion complexes are 561.2 and 197.9 kJ mol-1, respectively.

Modulating intramolecular P···N pnictogen interactions.

Abstract: A computational study of the intramolecular pnictogen bond in 8-phosphinonaphthalen-1-amine derivatives (1-NX2, 8-PX2 with X = H, F, Cl, Br, CH3, CN and NC), proton sponge analogues, has been carried out to determine their structural and geometric parameters, interaction energies and electronic properties such as the electron density of the intramolecular interaction. Our results show that substitution of H atoms in the PH2 group by electron withdrawing groups on the Lewis acid moiety strengthens the P⋯N pnictogen bond, evidenced by the increasing electron density values at the bond critical point and by shorter distances. However, substitutions on the Lewis base moiety (NX2) show weaker P⋯N interactions than when the substitution is done on the Lewis acid counterpart (PX2). Nevertheless, in all cases, pnictogen bonds are enhanced upon substitution with respect to the parent 1-NH2, 8-PH2 system. Second-order orbital interaction energies, electron density maps, electron delocalization functions and charge transfer corroborate the evolution of the P⋯N strength upon substitution.

Comparative Study of Charge-Assisted Hydrogen- and Halogen-Bonding Capabilities in Solution of Two-Armed Imidazolium Receptors toward Oxoanions

Abstract: Two-armed imidazolium-based anion receptors have been prepared. The central 2,7-disubstituted naphthalene ring features two photoactive anthracene end-capped side arms with central 2-bromoimidazolium or hydrogen-bonding imidazolium receptors. Combined emission and 1H and 31P NMR studies carried out in the presence of a wide variety of anions reveal that only HP2O73–, H2PO4–, SO42–, and F– anions promoted noticeable changes. The halogen receptor 62+·2PF6– acts as a selective fluorescent molecular sensor for H2PO4– anions, since only this anion promotes the appearance of the anthracene excimer emission band, whereas it remains unchanged in the presence of the other tested anions. In addition this halogen receptor behaves as a chemodosimeter toward HP2O73– anion, through its transformation into the corresponding bis-imidazolone after debromination by the action of the basic anion. The association constant values of the halogen-bonding complexes in a competitive solvent CD3CN/MeOD (8/2) mixture with H2PO4– an...

Beryllium-Based Anion Sponges: Close Relatives of Proton Sponges.

Abstract: Through the use of high-level ab initio and density functional calculations it is shown that 1,8-diBeX-naphthalene (X=H, F, Cl, CN, CF3 , C(CF3 )3 ) derivatives behave as anion sponges, very much as 1,8-bis(dimethylamino)naphthalene derivatives behave as proton sponges. The electron-deficient nature of the BeX substituents, which favors strong charge transfer from the anion towards the former, results in anion affinities that are among the largest ones reported for single neutral molecules.

Exergonic and Spontaneous Production of Radicals through Beryllium Bonds.

Abstract: High-level ab initio calculations show that the formation of radicals, by the homolytic bond fission of Y-R (Y=F, OH, NH2 ; R=CH3 , NH2 , OH, F, SiH3 , PH2 , SH, Cl, NO) bonds is dramatically favored by the association of the molecule with BeX2 (X=H and Cl) derivatives. This finding is a consequence of two concomitant effects, the significant activation of the Y-R bond after the formation of the beryllium bond, and the huge stabilization of the F(.) (OH(.) , NH2 (.) ) radical upon BeX2 attachment. In those cases where R is an electronegative group, the formation of the radicals is not only exergonic, but spontaneous.

Boron as an Electron-Pair Donor for B⋅⋅⋅Cl Halogen Bonds

Abstract: MP2/aug'-cc-pVTZ calculations were performed to investigate boron as an electron-pair donor in halogen-bonded complexes (CO)2 (HB):ClX and (N2 )2 (HB):ClX, for X=F, Cl, OH, NC, CN, CCH, CH3 , and H. Equilibrium halogen-bonded complexes with boron as the electron-pair donor are found on all of the potential surfaces, except for (CO)2 (HB):ClCH3 and (N2 )2 (HB):ClF. The majority of these complexes are stabilized by traditional halogen bonds, except for (CO)2 (HB):ClF, (CO)2 (HB):ClCl, (N2 )2 (HB):ClCl, and (N2 )2 (HB):ClOH, which are stabilized by chlorine-shared halogen bonds. These complexes have increased binding energies and shorter B-Cl distances. Charge transfer stabilizes all complexes and occurs from the B lone pair to the σ* Cl-A orbital of ClX, in which A is the atom of X directly bonded to Cl. A second reduced charge-transfer interaction occurs in (CO)2 (HB):ClX complexes from the Cl lone pair to the π* C≡O orbitals. Equation-of-motion coupled cluster singles and doubles (EOM-CCSD) spin-spin coupling constants, 1x J(B-Cl), across the halogen bonds are also indicative of the changing nature of this bond. 1x J(B-Cl) values for both series of complexes are positive at long distances, increase as the distance decreases, and then decrease as the halogen bonds change from traditional to chlorine-shared bonds, and begin to approach the values for the covalent bonds in the corresponding ions [(CO)2 (HB)-Cl]+ and [(N2 )2 (HB)-Cl]+ . Changes in 11 B chemical shieldings upon complexation correlate with changes in the charges on B.

The structure of β-diketones related to curcumin determined by X-ray crystallography, NMR (solution and solid state) and theoretical calculations

Abstract: Structural data are reported on sixteen ketoenols of β-diketones: solution NMR, solid-state NMR (CPMAS and MAS) and X-ray crystallography (four compounds, where three are new). The emphasis is on the tautomerism between both ketoenols, in solution and in the solid state. GIAO/B3LYP/6-311++G(d,p) and Quantum ESPRESSO (QE) calculations were used and compared. For average values, the GIAO/DMSO-PCM is enough, but splittings can only be approached by using QE. A case of rotational disorder has been analyzed. Some anomalies related to C–F bonds and to the C–CF3 group have been detected.

Unusual acid–base properties of the P4 molecule in hydrogen-, halogen-, and pnicogen-bonded complexes

Abstract: Ab initio MP2/aug′-cc-pVTZ calculations have been carried out to investigate hydrogen bonding, halogen bonding, and pnicogen bonding involving tetrahedral P4 and the FH, ClH, and FCl molecules. P4 has three unique interaction sites: at a vertex (designated the P1 atom); at an edge (the P2–P3 bond); and at the P2–P3–P4 face. The uniqueness of molecular P4 is its ability to act as an electron donor and an electron acceptor at the same site, except for the P2–P3 bond, which is only an electron donor. FCl and FH form five different complexes with P4, but ClH forms only three. The type of complex formed and its binding energy depend on both the interaction site of molecular P4 and the interacting molecule. For all complexes with FH, ClH, and FCl, the binding energies at a given site with the P4 molecule acting as the base are greater than the binding energies when P4 is the acid. Thus, P4 is a better electron donor than an electron acceptor. Charge-transfer interactions and EOM-CCSD spin–spin coupling constants across hydrogen, halogen, and pnicogen bonds are reported for all of the P4 complexes. Relative to 1J(Pi–Pj) in molecular P4, 1J(P1–P2) coupling constants decrease in absolute value and 1J(P2–P3) coupling constants increase in pnicogen-bonded complexes and the complex with FCl that has a P⋯F halogen bond. Absolute values of 1J(P1–P2) increase and those of 1J(P2–P3) decrease in hydrogen-bonded complexes and complexes with P⋯Cl halogen bonds. 1J(P1–P2) and 1J(P2–P3) exhibit a single linear correlation with the corresponding Pi–Pj distances.

Properties of cationic pnicogen-bonded complexes F4-nHnP+:N-base with H–P···N linear and n = 1–4

Abstract: Ab initio MP2/aug'-cc-pVTZ calculations have been carried out to investigate the pnicogen-bonded complexes F4-nHnP+:N-base, for n = 1–4, each with a linear or nearly linear Hax–P···N alignment. The sp3-hybridised nitrogen bases include NH3, NClH2, NFH2, NCl2H, NCl3, NFCl2, NF2H, NF2Cl, and NF3, and the sp bases are NCNH2, NCCH3, NP, NCOH, NCCl, NCH, NCF, NCCN, and N2. Binding energies increase as the P–N distance decreases, with an exponential curve showing this relationship when complexes with sp3 and sp hybridised bases are treated separately. However, the correlations are not as good as they are for the complexes F4-nHnP+:N-base for n = 0–3 with F–P···N linear. Different patterns are observed for the change in the binding energies of complexes with a particular base as the number of F atoms in the acid changes. Thus, the particular acid–base pair is a factor in determining the binding energies of these complexes.Three different charge-transfer interactions stabilise these complexes, namely Nlp→...

B4H4 and B4(CH3)4 as Unique Electron Donors in Hydrogen-Bonded and Halogen-Bonded Complexes.

Abstract: Ab initio MP2/aug′-cc-pVTZ calculations have been carried out on B4H4 and B4(CH3)4 to investigate the base properties of these molecules with Td symmetry. Each face of the tetrahedral structure of B4H4 and B4(CH3)4 is stabilized by a two-electron, three-center B–B–B bond. The face uses these two electrons to act uniquely as an electron-pair donor for the formation of stable hydrogen-bonded and halogen-bonded complexes with C3v symmetry. The hydrogen-bonded complexes are B4H4:HY and B4(CH3)4:HY, with HY = HNC, HF, HCl, HCN, and HCCH; the halogen-bonded complexes are B4H4:ClY and B4(CH3)4:ClY, with ClY = ClF, ClCl, ClNC, ClCN, ClCCH, and ClH. The absolute values of the binding energies of the hydrogen-bonded complexes B4(CH3)4:HY and of the halogen-bonded complexes B4(CH3)4:ClY are significantly greater than the binding energies of the corresponding complexes with B4H4. The binding energies of each series correlate with the distance from the hydrogen-bonded H atom or halogen-bonded Cl atom to the centroid o...

A computational study of azaphospholes: anions and neutral tautomers

Abstract: One hundred and eleven azaphospholes (31 anions and 80 neutral compounds) have been studied theoretically at the B3LYP/6-311++G(d,p) level. Among the analyzed properties, there are the geometries, mainly the non-planarity of the PH tautomers, the isomerism of the anions, the NH/PH tautomerism of neutral compounds, the chemical shifts and spin–spin coupling constants. The aromaticity has been assessed through Schleyer’s NICS values, preferably NICS(1). Finally, we have compared our calculations with previous ones as well as with the available experimental data for the calculated compounds and for related ones.

A computational study on [(PH2X)2]·+ homodimers involving intermolecular two-center three-electron bonds

Abstract: A computational study at CCSD(T) theoretical level has been carried out on radical cation [(PH2X)2]·+ homodimers. Four stable minima configurations have been found for seven substituted phosphine derivatives, X = H, CH3, CCH, NC, OH, F and Cl. The most stable minimum presents an intermolecular two-center three-electron P···P bond except for X = CCH. The other three minima correspond to an alternative P···P pnicogen bonded complex, to a P···X contact and the last one to the complex resulting from a proton transfer, PH3X+:PHX·. The complexes obtained have been compared with those of the corresponding neutral ones, (PH2X)2, and the analogous protonated ones, PH3X+:PH2X, recently described in the literature. The spin and charge densities of the complexes have been examined. The electronic characteristics of the complexes have been analyzed with the NBO and AIM methods. The results obtained for the spin density, charge and NBO are coherent for all the complexes.

Competition between intramolecular hydrogen and pnictogen bonds in protonated systems

Abstract: A theoretical study of the competition between hydrogen (HB) and pnictogen bonds (ZB) in three different families of compounds, (Z)-1,2-disubstituted ethenes (Eth), 1,2-disubstituted benzenes (Phe) and 1,8-disubstituted naphthalenes (Naph), with a charged group, ZH3 + and a neutral one, Z′H2 (Z, Z′ = N, P, As) as interacting moieties, has been carried out. In those structures with a NH3 + motif, intramolecular hydrogen bond structures are minima while pnictogen interactions are transition states. The opposite is true for PH3 + and AsH3 + moieties. An analysis of isodesmic energies (E iso), interaction energies (E b) and deformation energies (E def) shows that in Eth derivatives, the most stable compound corresponds to P+–N ZB while in the Phe and Naph ones, the N+–N HB interaction presents the largest negative isodesmic energy. Also, Eth and Phe derivatives show negative E iso values for all the compounds under study; however, in some cases of Naph derivatives positive isodesmic energies have been found. The Atoms in Molecules (AIM) analysis of the electron density, natural bond orbital (NBO) second-order orbital energies and electron density shift maps (EDS) have been used to better understand these intramolecular interactions.

Static and Dynamic Properties of Fluorinated 4‐Aryl‐1,5‐Benzodiazepinones

Abstract: Six 4-aryl-6,7,8,9-tetrafluoro-1,5-benzodiazepine-2-ones have been synthesized and fully characterized by 1H, 13C, 15N and 19F NMR in solution and in the solid state. In addition, the structures of three of them have been determined by X-ray crystallography. The imine/enamine tautomerism has been experimentally studied by NMR in solution (in the solid state, only the imino tautomer is present). Three barriers corresponding to the inversion of the seven-membered ring have been determined (N1-unsubstituted derivatives). Both for the tautomerism and for the conformational aspects, theoretical calculations at the B3LYP/6-311++G(d,p) level including GIAO calculations of the NMR chemical shifts have been carried out with satisfying results. Different energy profiles have been considered and several transition states located.

A theoretical and experimental NMR study of BODIPY 493/503: difluoro{2-[1-(3,5-dimethyl-2H-pyrrol-2-ylidene-N)ethyl]-3,5-dimethyl-1H-pyrrolato-N}boron.

Abstract: This work was carried out with financial support from the Ministerio de Economia y Competitividad (Project Nos. CTQ2014-56833-R and CTQ2015-63997-C2-2-P) and Comunidad Autonoma de Madrid (Project FOTOCARBON, ref. S2013/MIT-2841). Computer, storage and other resources from the CTI (CSIC) are gratefully acknowledged.

Weak interactions within nitryl halide heterodimers

Abstract: A theoretical study of nitryl halide heterodimers has been carried out using SCS-RI-MP2 and CCSD(T) at the complete basis set (CBS) calculations. For this purpose, 66 heterodimers have been characterized as minima and arranged in six groups depending on the interactions involved and their geometrical arrangements. The CCSD(T)/CBS interaction energies vary between −0.6 and −11.1 kJ mol−1. The heavier the halogen atoms, the larger the interaction energies. Natural bond orbital (NBO) and “atoms-in-molecules” (AIM) theories were then used to analyze the complexes, confirming the presence of halogen, chalcogen, and π-hole interaction bonds. The largest charge-transfer energy contributions were found for halogen bonded complexes (up to 29.1 kJ mol−1). Furthermore, the physical nature of the interactions was studied using symmetry-adapted perturbation theory (SAPT) calculations, and it was concluded that dispersion was the major source of attraction, although electrostatics is important in halogen bonded complexes.

Saddle-Shaped Cyclic Indole Tetramers: 3D Electroactive Molecules

Abstract: We present a joint theoretical and experimental study of a series of cyclic indole tetramers aimed at understanding the fundamental electronic properties of this 3D platform and evaluating its potential in the construction of new semiconductors. To this end, we combined absorption and Raman spectroscopy, cyclic voltammetry, and spectroelectrochemistry with DFT calculations. Our results suggest that this platform can be easily and reversibly oxidized. Additionally, it has a HOMO that matches very well with the workfunction of gold, therefore charge injection from a gold electrode is expected to occur without significant barriers. Interestingly, the cyclic tetraindoles allow for good electron delocalization in spite of their saddle-shaped structures. The steric constraints introduced by N-substitution significantly inhibits ring inversion of the central cyclooctatetraene unit, whereas it only barely affects the optical and electrochemical properties (a slightly higher oxidation potential and a blueshifted absorption upon alkylation are observed).

Tautomerism: A Historical Perspective

Abstract: When I started my PhDThesis, some 60 years ago, I was involved with pyrazoles tautomerism (annular of pyrazoles and indazoles and functional of pyrazolinones, then called pyrazolones). The knowledge about tautomerism was empirical, qualitative, and unsystematic. Owing to the efforts of Alan Roy Katritzky and those of my thesis supervisor, Robert Jacquier, the situation improved considerably resulting in the publication in 1976 of the book “TheTautomerismofHeterocycles” [1].1) One can say that, in 1976, there was good understanding of the thermodynamic aspects of the tautomerism of heteroaromatic compounds as well as that of compounds like β-diketones and related structures. Besides, thanks to the works of Hammett, Taft, Exner, and many others, the influence of substituents and solvents on the equilibrium constants can be quantified through extrathermodynamic relationships. We are now in the process of changing the paradigm and moving to a complete picture of the kinetic and thermodynamic aspects of tautomerism in the three phases, gas, solution, solid, and including ground and excited states.This is due to the increasing power of physicochemical methods as well as to the enormous success of theoretical chemistry. It is not finished yet andmany inputs andmany years of research will be necessary to declare that the task is completed.The subject has been reviewed in 2000 [2] and in 2014 [3] and the present book, to be published in 2016 [4], will further actualize it. An interesting effort is being made for buildingup tools for the automatic generation of all tautomeric forms of a given organic compound, for instance, Ambit-Tautomer [5]. To display a panorama of tautomerism today, a practical approach is to divide the knowledge into three sections: high, medium, and low. This warrants a separation of the thermodynamic aspects from the kinetic aspects, since often the first are well known but not so much the latter. Unless stated differently, this introduction will deal with prototropic tautomerism, which is by far the most common. It corresponds to the proton

Fullerene and corannulene derivatives acting as insulators of Cl-and BeH2

Abstract: The capacity of corannulene and its benzo-derivatives CxH10 (x = 20–60) as prototypes of non-planar π-aromatic systems which mimic the end of carbon nanotubes to act as insulators between BeH2 and Cl− chemical entities has been explored by means of M06-2x/cc-pVDZ and M06-2x/6-311+G(d) calculations. For the sake of completeness, the set investigated includes also fullerene, C60. All these aromatic derivatives lead to stable binary complexes either with BeH2 or halogen (Cl−) anions. For BeH2, however, only the complexes in which the interaction involves the convex face of the aromatic system are stable. No significant changes are observed when the binding energies (BEs) of the BeH2 complexes are compared to those involving planar aromatic compounds, but the ones involving Cl− with the concave face of the aromatic moiety can be very large, because its curvature favors many contacts of the anion with the carbon atoms of the π-aromatic system. The formation of these binary complexes changes to a large extent the electrostatic potential on the free face of the aromatic system leading to a mutual reinforcement of both interactions, the beryllium bond and the interaction with Cl−, when the ternary complexes are formed. As a result, the BEs for the triads are larger than the sum of the BEs of the corresponding binary complexes and the distances between the aromatic subunit and BeH2 or Cl− become shorter in the triads than in the binary complexes. A MBIE analysis also indicates that the enhanced stability of ternary complexes arises mainly from the reinforcement of the beryllium bonds as well as from the three-body terms. An exploration of all the minima for BeH2:C60H10:Cl− shows that BeH2 binds preferentially to the peripheral aromatic rings than those in the more curved region.

The (1) H NMR spectrum of pyrazole in a nematic phase.

Abstract: This work was supported by the Spanish Ministry of Economy and Competitiveness (grant CTQ2012-13129-C02-02) and the Comunidad Autonoma de Madrid (Project Fotocarbon, ref. S2013/MIT-2841). F. R. acknowledges a CSIC JAE-Doc contract cofunded by EU (ESF, European Social Fund).

The structures of two scorpionates: thallium tetrakis(3-phenyl-1H-pyrazol-1-yl)borate and potassium tetrakis(3-cyclopropyl-1H-pyrazol-1-yl)borate

Abstract: The introduction of poly(1H-pyrazol­yl)borate anions, better known as scor­pion­ates, as negatively charged ligands for a great diversity of metal cations has had a tremendous influence in coordination chemistry. The structures of two salts of tetra­kis­pyrazolylborate, namely [tetra­kis­(3-phenyl-1H-pyrazol-1-yl)borato]thallium(I), [Tl(C36H28BN8)], and catena-poly[potassium-[μ2-tetra­kis­(3-cyclo­propyl-1H-pyrazol-1-yl)borato]], [K(C24H28BN8)]n, have been determined at 296 K in the monoclinic P21/c and C2/c space groups, respectively. In their crystal structures, the thallium salt presents discrete mol­ecular motifs, while the potassium salt shows infinite polymeric chains. The 13C and 15N CPMAS (cross polarization magic angle spinning) NMR spectra of these compounds were recorded and the chemical shifts compared with theoretically calculated ones at the GIAO/B3LYP/6-311++G(d,p) level. Both techniques are complementary and mutually consistent.