Humberto C. Garcia

On the Azo/Hydrazo Equilibrium in Sudan I Azo Dye Derivatives

In this study, Raman, infrared, UV/vis, NMR, and single crystal X-ray diffraction spectroscopies are used to elucidate the tautomeric equilibrium of azo dyes derived from 1-phenyl-azo-2-naphthol (Sudan I). A new crystallographic structure is described for Sudan I, revealing the presence of intramolecular hydrogen bonds and supramolecular interactions, such as the unconventional C-H···O hydrogen bond type, π-stacking, and charge-dipole interactions. All of these weak intermolecular interactions play a role in the stability of the crystalline structure. Theoretical calculations are also reported for geometries, energy, and spectroscopic properties. The predicted spectra are in accordance with the experiments carried out in the solid state and in solution of dichloromethane, carbon tetrachloride, and chloroform, suggesting the hydrazo form as the preferable tautomer in gas and condensate phases for Sudan I and its derivatives.

Supramolecular structures of metal complexes containing barbiturate and 1,2-bis(4-pyridyl)-ethane

This work describes the synthesis, thermal, spectroscopic properties (Raman and infrared), and crystal structures of five new supramolecular compounds [Mn(bpa)(H2O)4]B2 · 4H2O (1), [Fe(bpa)(H2O)4]B2 · 4H2O (2), [Co(bpa)(H2O)4]B2 · 4H2O (3), [Zn(bpa)(H2O)4]B2 · 4H2O (4), and Co2mal2bpa · 2H2O (5), where B is the anion of barbituric acid, bpa is 1,2-bis(4-pyridyl)-ethane, and mal is malonate ion. Compounds 1–4 are isostructural, showing covalent linear 1-D [M(bpa)(H2O)4]2+ chains, which interact by hydrogen-bonding and π-stacking interactions with barbiturate and crystallization water molecules resulting in a 3-D arrangement, belonging to Pbcn space group. Compound 5 has been obtained from the opening of the barbituric acid ring, with the formation of malonate, coordinated simultaneously to three cobalts in a 1-D chain along the c-axis, whereas bpa ligand gives rise to another 1-D chain along the a- and b-axes, resulting in a 3-D coordination polymer containing cavities. The vibrational spectra of 1–4 are also very similar; Raman spectra display two intense bands related to bpa at 1616 and 1020 cm−1, assigned to the (ν CC/ν CN) and ring stretching modes, respectively. The barbiturate is also confirmed by a band at 684 cm−1; the interesting point to be emphasized is this vibrational mode is not observed for 5, corroborating the absence of this building block in the structure.

An intriguing hydrogen bond arrangement of polymeric 1D chains of 4,4′-bipyridine coordinated to Co2+, Ni2+, Cu2+ and Zn2+ ions having barbiturate as counterions in a 3D network

In this work, four new transition metal complexes of general formula {[M(bipy)(H2O)4](B)2}(H2O)6 (where MCo2+, Ni2+, Cu2+ and Zn2+, B is barbiturate anion and bipy is 4,4′-bipyridine) were obtained and investigated by means of single crystal X-ray diffraction and vibrational (Raman and infrared) spectroscopic techniques. The CoB2bipy·10H2O, NiB2bipy·10H2O and CuB2bipy·10H2O basic units give rise to polymers, where Ni(II) and Cu(II) compounds are isomorphous, however all compounds present the same molecular structure. The metal sites appear in a distorted octahedral geometry coordinated by two pyridine nitrogen atoms and four oxygen atoms from coordinated water molecules. Each structure showing one covalent linear [M(bipy)(H2O)4]2+ chains one-dimensional, which interact by hydrogen-bonding with the barbiturate anion and crystallization water resulting in a 3D arrangement. The analysis of the Ni2+ and Cu2+ complexes’ structures show flexible 2D hydrogen bonded networks being constructed by the barbiturate anions and the crystallization waters molecules, which may be deemed to be the host, while the robust 1D [M(bipy)(H2O)4]2+ chains may be deemed to be the guest, in a very intriguing and interesting structure. The vibrational spectra of the compounds are very similar, in agreement to the crystal data. In all infrared spectra a medium band at 1690 cm−1 has been observed, assigned to the CO stretch of the barbiturate anion. In the Raman spectra the most important bands referring to 4,4′-bipyridine are the ones at 1616, 1290 and 1020 cm−1, assigned to δCC/CN, δring and δCH modes, respectively. The vibrational results strongly suggest the Zn2+ compound presents the same molecular arrangement, based on the fact that the spectra of all complexes are similar.

An interesting pseudo-honeycomb supramolecular arrangement obtained from the interaction between 4-aminosalicylic acid, trans-1,2-bis(4-pyridyl)ethylene and transition metal ions†

In this work, three new compounds containing coordination polymers named [Co(bpe)(H2O)4]AS2·4H2O (1), [Ni(bpe)(H2O)4]AS2·4H2O (2) and [Zn(bpe)(H2O)4]AS2·4H2O (3) (where AS is aminosalicylate anion and bpe is trans-1,2-bis(4-pyridyl)ethylene) were synthesized and characterized by X-ray diffraction and vibrational spectroscopy (infrared and Raman). All complexes are isomorphous, crystallizing in a monoclinic system with space groupP21/c. The crystalline arrangement shows the presence of a cavity called pseudo-honeycomb imperfect, parallel to the bc-plane formed through hydrogen bonding between aminosalicylate anions and water molecules of crystallization. The vibrational spectra of the compounds are very similar, in agreement with the crystal data. The Raman spectra show bands for bpe at 1642 and 1614 cm−1, assigned to ν(CC) and ν(CC)/ν(CN), respectively. This study can be considered as the first work in the literature where the aminosalicylate anion is associated with coordination polymers just filling the cavities of pseudo-honeycomb structures.

Supramolecular compounds of azo dyes derived from 1-phenylazo-2-naphthol and their nickel and copper complexes

In this study, experimental techniques including Raman, infrared and X-ray crystal diffraction, as well as quantum chemistry calculations, are used to investigate two new azo dyes supramolecular complexes: 1-phenylazo-2-naphthol (Sudan I) with nickel(II) ion and 1-(2,4-xylylazo)-2-naphthol (Sudan II) with copper(II) ion. The crystallographic structures have been solved for bis-1-(pheny-l-azo)-2-naphtholatenickel(II) and bis-1-(2,4-xylylazo)-2-naphtholatecopper(II) complexes, revealing the presence of supramolecular interactions, such as the centroid–centroid π-stacking interactions and CH…π hydrogen interactions. These weak intermolecular interactions appear to play an important role on the crystal structure stability for both compounds. Density functional theory calculations at B3LYP/6-311++G(d,p) level were performed in order to help understanding such molecular interactions and to assign the vibrational spectra. The experimental and theoretical data have allowed the analysis of the packing forces, revealing charge accumulation at key molecular regions.

Supramolecular structures of Mn2+, Co2+ and Zn2+ complexes containing 1,3-bis(4-pyridyl)propane and aminosalicylate anion

In this study, the synthesis, spectroscopic properties and crystal structures of three new supramolecular compounds named [Mn2(bpp)4(H2O)4](AS)4·H2O (1), [Co2(bpp)4(H2O)4](AS)4·H2O (2) and [Zn(bpp)(AS)2] (3), have been described, where bpp is 1,3-bis(4-pyridil)propane and AS− is aminosalicylate anion. By analysing the similarities between the X-ray powder diffraction results, it has been observed that compounds 1 and 2 are isomorphous, exhibiting an orthorhombic system with space group Pccn; for compound 3, another orthorhombic system was observed, with space group Aba2, which displays coordination between the Zn2+ metal ion and the aminosalicylate anion; this can be considered the first case in the literature involving the direct coordination to the metal ion. The vibrational spectra of compounds 1 and 2 are very similar. In the Raman spectra, the main bands are observed at ca. 1625 and 1020 cm− 1, referring to the − O–C = O and CC/CN stretching modes of AS− and bpp ligands, respectively.

Squaraines: crystal structures and spectroscopic analysis of hydrated and anhydrous forms of squaric acid-isoniazid species.

The crystal structures, synthesis procedure, thermal behavior, and spectroscopic properties of a new squaraine SqINH·H2O and its anhydrous arrangement are described. This squaraine is obtained through an acid-base reaction using squaric acid (H2Sq) and isoniazid (INH) as precursors. Both squaraines crystallize in the monoclinic system, but in different space groups: the hydrated and anhydrous arrangement crystallizes in the P2₁ and P2₁/c space group, respectively. The crystallographic data strongly suggest that the structures present an expressive increase in their electronic delocalization all over the molecular structure of both compounds, when compared with the reagents. The bond distances for both structures present an average value intermediate between a single and double character (1.463(3) Å for SqINH·H2O and 1.4959(3) Å for SqINH). The vibrational and electronic data also corroborate with this proposal, since the band shifts indicate that the conjugation over the system is increased, as indicated by the blue shift observed for the carbonyl stretching bands for both compounds. The presence of the water molecule is responsible for a decrease in fluorescence emission, as determined by the emission spectra recorded for both compounds.

Two New Supramolecular Compounds that Involve Trans-1,2-bis(4-pyridyl)ethylene (bpe), Barbituric Acid and Fe(II) ions

In this study, the synthesis, spectroscopic properties (infrared and Raman) and crystal structures of two new compounds named (H2bpe)B2 . 2H2O (1) and (Fe(bpe)(H2O)4)B2·4H2O (2) have been reported, where bpe is trans-1,2- bis(4-pyridil)-ethylene, H2bpe is protonated bpe and B - is the barbiturate anion. For both complexes, we observed the presence of supramolecular interactions, such as electrostatic, hydrogen bonding and � -stacking interactions. Compound 1 crystallised in a triclinic system with the space group P-1, which was generated by cationic and anionic units formed by protonated bpe, the barbiturate anion and water molecules. These molecules interacted through weak intermolecular inter- actions and were responsible for the crystalline stability of the synthesised compounds. For compound 2, an orthorhombic crystal system was observed with a Pbcn space group. In this crystal, the metallic site adopted a distorted octahedral ge- ometry that was coordinated by two pyridine nitrogen atoms and four oxygen atoms from coordinated water molecules. The three-dimensional arrangement of the structure consisted of one covalent one-dimensional linear cationic (Fe(bpe)(H2O)4) 2+ chain, which interacts by forming hydrogen-bonding, � -stacking and electrostatic interactions with bar- biturate anions and water molecules that were trapped in the crystal. The vibrational spectra of the two compounds were very similar despite the different crystalline arrangements that were adopted by the two structures. In the infrared spectra, two intense bands at 1684 and 1630 cm -1 were observed and assigned to the CO stretching ( CO) of the barbiturate anion and  C=C of the bpe ligand, respectively. In the Raman spectra of the compounds, the most important bands were attrib- uted to the bpe ligand at 1629 and 1006 cm -1 , which were assigned to the  C=C and  ring, respectively. The presence of the barbiturate anion was confirmed by the presence of a band of low intensity located at 680 cm -1 and was assigned as the