Ana K. Valdo

Experimental and theoretical second harmonic generation and photoluminescence from the pseudo-centrosymmetric dihydrochloride salt dihydrate of trans-1,2-bis(4-pyridyl)ethene

Here we have prepared a dihydrochloride salt dihydrate of the well-known trans-1,2-bis(4-pyridyl)ethene (BPE) featuring both photoluminescent and nonlinear optical properties. In its triclinic lattice (space group P1), BPE cations are stacked face-to-tail through π⋯π interactions between the spacer double-bonded carbons and the protonated pyridyl ring, with a slippage of 3.45 A always towards the same direction, which is common in other NLO crystals. The existence of inversion symmetry was suggested in its crystal structure, which was ruled out by the SHG emission centred at 487 nm upon excitation at 974 nm. While the fully optimized single molecule of the divalent BPE cation in the gas phase had almost null μ, βtot, βCT and βvec values calculated at the CAM-B3LYP/NLO-V//B3LYP/6-31G* level of theory, these values differed from zero in the crystal conformation. More interestingly, a ca. 4-fold increase in βtot, α and μ was observed for the π⋯π stacked four-molecules as found in its crystal structure. Lastly, this BPE material presents high photoluminescence emission centred at 425 nm under excitation at 366 nm, being therefore a multifunctional optical crystal form.

Competition between coordination bonds and hydrogen bonding interactions in solvatomorphs of copper(II), cadmium(II) and cobalt(II) complexes with 2,2′-bipyridyl and acetate

Abstract The delicate balance among conformation, coordination bonds and hydrogen bonding has been probed in solvatomorphs of known metal-organic molecules synthesised from copper(II), cadmium(II) and cobalt(II) with acetate (OAc) and 2,2′-bipyridine (bipy). The Cu(OAc)2(bipy) complex, isolated as a pentahydrate, has the acetate ligands oriented to opposite sides of the coordination square plane. DFT calculations show the energy difference between this structure and a syn form amount to approximately 16 kJ/mol. The presence of lattice water enables the formation of O–H···O hydrogen bonds with the acetate ligands. Different coordination numbers and energies are found as a function of the number of water molecules co-crystallising in the Cd(OAc)2(bipy)(OH2)·3H2O and [Co(OAc)(bipy)2](OAc)·3H2O complexes.

Why lamivudine assembles into double-stranded helices in crystals: salt heterosynthon versus base-pairing homosynthon

Here we were interested in obtaining a better understanding of the competition between the salt heterosynthon and the base-pairing homosynthon formed by the anti-HIV drug lamivudine in the presence of strong acids. Even though the preparation of the multicomponent crystal forms of this drug with weak or moderate acids is well investigated, the crystallization of lamivudine with strong acids is still little explored. Besides filling this crystallization screening gap, the driving forces of the so-called lamivudine duplexes could be drawn from the complete structural landscape and the theoretical thermodynamic parameters of the salt heterosynthon, calculated at the M06-2X/6-31+G** level of theory. Five new crystal structures of lamivudine were obtained, wherein two of them were assembled as base-paired helically-stacked strands in the presence of trifluoroacetic and trichloroacetic acids (lamivudine duplexes V and VI, respectively). Besides, three salts were prepared by crystallization of lamivudine with sulfuric and perchloric acids. Finally, the theoretical approach showed that there is no energy trend regarding the formation of lamivudine duplexes with aliphatic organic acids or lamivudine salts with aromatic acids, which is usually observed in practice.

Synthesis, crystal structure and magnetic properties of [Co(bpcam)2]ClO4·dmso·H2O, [Co(bpcam)2]2[Co(NCS)4]·dmso·H2O and [Ni(bpcam)2]·H2O [Hbpcam = bis(2-pyrimidylcarbonyl)amide]

The preparation, spectroscopic characterization, structural study and magnetic investigation of three new complexes of formula [Co(bpcam)2]ClO4·dmso·H2O (1), [Co(bpcam)2]2[Co(NCS)4]·dmso·H2O (2) and [Ni(bpcam)2]·H2O (3) [Hbpcam = bis(2-pyrimidylcarbonyl)amide] are reported. Each bpcam group in 1–3 acts as a tridentate ligand being coordinated to the cobalt(III) (1 and 2)/nickel(II) (3) ions through three nitrogen atoms in a mer-arrangement. Six-coordinate cobalt(III) and nickel(II) occur in 1 and 3 respectively, whereas six-coordinate cobalt(III) and four-coordinate cobalt(II) coexist in 2. Cyclic voltammetry of 1 in acetonitrile shows the occurrence of one quasi reversible CoIII ↔ CoII process. Direct current (dc) variable-temperature magnetic susceptibility measurements on polycrystalline samples of 2 and 3 were carried out in the temperature range 1.9–295 K. The magnetic behaviour of 2 obeys to the zero-field splitting effects (DCo) of the 4A2 ground term of the pseudotetrahedral [Co(NCS)4]2− complex anion. A Curie law for a magnetically isolated nickel(II) ion in the high temperatures domain occurs for 3, the small decrease of χMT below 10 K being due zero-field splitting effects. The analysis of the magnetic susceptibility data of 2 and 3 through the spin Hamiltonian H = DM[Sz2 − S(S + 1)/3] + gMβHS [M = Co(2) and Ni(3)] led to the following set of values: gM = 2.37 (2) and 2.12 (3) and |DM| = 34.7 (2) and 1.72 cm−1 (3).

Synthon trends according to acid strength and geometry in salts of N-heterocyclic bases

The hierarchy and robustness of homosynthons and heterosynthons formed by N-heterocyclic bases were assessed experimentally in salts of aminopyrazine (ampyz) and trans-1,2-bis(4-pyridyl)ethane (BPE) with common strong, moderate and weak acids, and theoretically at the M06-2X/6-31+G** level of theory. A trend for a base-pairing primary homosynthon to assemble in ampyz salts as the acid strength increases can be drawn. This homosynthon is present in chloride and bromide salts of ampyz, which is compatible with protonation at N4 and the formation of an accessory four-point heterosynthon engaging two (ampyz)+ cations and two halides. This robust synthon is also present in chloride, bromide and dihydrogen phosphate salts of BPE. Among all our BPE multicomponent crystal forms, it is not found only in the uncommon phosphoric acid cocrystal of the dihydrogen phosphate salt. When ampyz was crystallized with weaker acids such as trifluoroacetic, trichloroacetic and phosphoric acids, the primary homosynthon disappears gradually as the strength acid decreases. In the last two cases, this homosynthon is not found, but the trifluoroacetate salt of ampyz is found to have both the base-pairing primary homosynthon and the two-point heterosynthon with carboxylate. Both monoprotonated forms at N1 and N4 are found together in this structure. Another trend of N1 protonation in the presence of counterions from inorganic tetrahedral oxoacids such as isopropyl sulfuric and phosphoric acids is also outlined, regardless of their acid strength.