Stereoactive lone pair of electrons on bismuth(III): tris(3-hydroxy-2-methyl-4 H -pyran-4-onato)bismuth(III)

Another rare example of stereoactive lone pair of electrons on six-coordinated bismuth(III) – previously unknown tris(3-hydroxy-2-methyl-4 H -pyran-4-onato)bismuth(III)


Introduction
Currently 3-hydroxy-2-methyl-4H-pyran-4-one (maltol), an non-toxic (LD 50 1400 mg/kg) 1 chelating ligand of natural [2][3][4][5] origin, attracts an ever-increasing interest.][13] Inorganic compounds of bismuth (trioxide, nitrate) or bismuth acetate are usually used for the treatment of various gastrointestinal diseases as well as the preparation of antiseptic and woundhealing products. 14,15 ut these drugs possess various side-effects (bismuth stomatitis, dermatitis, nephropathy) related to their toxicity and slow and incomplete excretion from organism. 15As a result, bismuth complexes prepared on the basis of natural non-toxic maltol, showing antibacterial 1 and antioxidant [16][17][18] properties may be of special medicinal interest.Thus, the detailed information on structure of maltol-based complexes of bismuth is gaining in importance.This information may also hold a fundamental stereochemical aspect: the stereoactive lone pair on bismuth(III).To the best of our knowledge, only a few compounds of six-coordinated bismuth(III) having a stereochemically active lone pair of electrons (stereoactive lone pair) have been reported, [19][20][21] while the overwhelming majority of bismuth compounds have no such lone pair. 21,22

Results and Discussion
Tris(3-hydroxy-2-methyl-4H-pyran-4-onato)bismuth(III) was prepared by the reaction of maltol with bismuth nitrate pentahydrate.It is known that the ion-coordinative chelatation of maltol is favored by its ionization at the О-Н bond to form one-charge maltolate-anion С 6 Н 5 О 3 ¯ (L). 23herefore, the reaction was carried out in the presence of alkali (2N NaOH), which leads to maltol ionization (рК а in water 8.6).At the same time the reaction equilibrium was shifted towards the formation of target chelate complex due to the neutralization of acid released during the reaction (Scheme 1). .

Scheme 1
According to the elemental analysis data the complex of maltol with bismuth(III) has a stoichiometric ratio metal : ligand of 1 : 3.
In the UV spectrum of the tris(3-hydroxy-2-methyl-4H-pyran-4-onato)bismuth(III) (Fig. 1  (b)) in addition to the band at 270 nm (caused partial complex dissociation in the protonodonor solvent resulting to free maltol) appears the band at 312 nm provided π→π* transition of maltol at anion bonded with Bi 3+ .That confirmed the position coincidence of observed absorption ARKAT USA, Inc. maxima at 270 and 312 nm with those of initial maltol (Figure 1a) and maltol in alkaline buffer (Fig. 1 (с)) (where it is present mainly as anion), 23,24 respectively.Decrease in the excitation energy is caused by the shift of π-electrons across the conjugated chain (polarization) under maltol ionization with formation of the chelate.The bands of charge-transfer (metal-ligand) and d→d* transitions, apparently, have low intensity or these transitions are spin forbidden.Since, they are not registered even at increase of concentration and light way.Besides the position of an observable absorption band at 312 nm does not depend on an electronic configuration of maltol complexes, 24 therefore it can not be referred to d→d* transitions or to the charge-transfer band.The fact that maltol is bound with metal ion through the oxygen atom of carbonyl group is confirmed by a significant decrease (by 50 cm -1 ) of valence vibration frequency ν(С=О) in IR spectrum of the complex as compared to the spectrum of free lignand (Table 1). 25Disappearance of the absorption band ν(О-Н) at 3240 cm -1 , assigned to maltol hydroxyl, is indicative of the hydroxyl group deprotonation and its coordination with respect to bismuth.The appearance of the bands at 445 cm -1 and 251 cm -1 proves the availability of the metal-oxygen bond. 26,27,28edistribution of electron density in the chelate results in shift of absorption bands, which correspond to conjugated double bonds of γ-pyrone, toward low-frequency (Table 1).  1 Н NMR spectra of maltol and its bismuth complex (Table 2) provide support for the involvement of OH-group proton in the reaction: the spectrum of its metal derivative show no wide resonant signal at 7.31 ppm, caused by hydroxyl proton.In addition, the resonance signal of Н 5 , Н 6 and СН 3 -group protons are shifted with respect their position in maltol that also point to the redistribution of electron density in new metal chelate molecule.So, the signals of Н 5 and Н 6 protons are shifted towards high field, while signal of the СН 3 -group protons -towards low field.Signals of Н 5 and Н 6 protons of both maltol and its complex are observed as doublets.
To assign the carbon signals the 13 С NMR spectra (without decoupling from protons) and twodimensional (2D) HMBC 1 H - 13 C spectra were recorded (Table 2).As is seen from the table the values of coupling constants 1 J CH of С 6 atom are higher than those of С 5 atom that can be explained by neighborhood of the С 6 atom with oxygen atom.Comparative analysis of 13 С NMR spectra of maltol and its bismuth complex (Table 2) also proves the formation of chelate compound.So, resonance signals of carbon atoms located near to the coordination center (С 2 , С 3 , С 4 , С 5 ) are shifted towards lower field.The complex formation seems to be accompanied by the partial deshielding of the ligand carbon atoms that leads to the signals shift observed.The resonance signal of the С 6 atom is shifted (like the signal of the Н 6 atom in 1 Н NMR spectrum) towards higher field.This effect can be explained by the decrease of conjugation in the γ-pyrone ring.
Crystal structure of the complex is formed by one crystallographic independent molecule C 18 H 15 BiO 9 (Fig. 2), taking general position in the unit cell.The conformation of the molecule looks like a butterfly: the heterocycles with О1, О2, О6 and О7 atoms -«wings», are planar.The dihedral angle formed by BiО1С1С2О2 and BiО6С15С16О7 planes is 157.7°.The perpendicular BiО3С8С9О4 plane is the «body of the butterfly».The dihedral angles between this plane and «wings» are 68.9 and 90.7°, for BiО1С1С2О2 and BiО6С15С16О7 planes, respectively.
Table 2.The values of 1 H and 13 С NMR (δ, ppm) chemical shifts and coupling constants (J, Hz) of maltol and tris(3-hydroxy-2-methyl-4H-pyran-4-onato)bismuth(III) The metal atom is hexa-coordinated in a distorted pentagonal pyramid.The О3 atom is located on the vertex of the pyramid, while О1, О2, О4, О7 and О6 atoms are placed at base of the pyramid.These atoms form distorted pentagon with bismuth atom in the center.][21] Important bond distances and bond angles for complex appear in Table 4 and 5 3) Å, respectively (sum of van der Waals radii of the corresponding atoms is 3.9 Å). 35 In addition, shorten interaction Bi…Bi having the distance of 3.721(1) Å was found (sum of van der Waals radii is 4.8 Å). 35

Table 4. Bond distances (d) for tris(3-hydroxy
This explains the fact that the bismuth atom, being involved in the additional non-valence interactions, deviates from the base of the pyramid (0.44 Å) in the direction opposite to apical atom О3 towards the metal atom of the neighbor molecule.Due to these contacts dimer associates are formed (Figs. 3, 4), while Bi atom becomes nine-coordinated.In this paper, we report the synthesis and characterisation of previously unknown tris(3hydroxy-2-methyl-4H-pyran-4-onato)bismuth(III), a rare example of the complexes of hexacoordinated bismuth(III) with stereoactive lone pair.In crystal packing, molecules of the complex are dimers due to the Bi…Bi and Bi…O contacts.

Experimental Section
General Procedures.Maltol was extracted from needles of Abies sibirica Ledeb.and purified according to the published procedure. 2The other reagents employed were reagent-grade.IR spectra were recorded on a Specord 75 IR (4000 -800 cm -1 ) and on Specord М-82 (800 -200 cm -1 ) instruments.UV spectra were recorded on Specord UV/VIS spectrophotometer. 1 Н NMR and 13 C NMR spectra were recorded on the Bruker DPX-400 and AV-400 at 400.13 MHz and 101.61MHz, respectively, using HMDS as an internal standard. 13С NMR spectra were recorded under complete decoupling from protons as well as without decoupling.Spectral assignment in the 13 С NMR spectrum was carried out based on the 2D correlation diagrams HMBC.The X-ray diffractions studies were carried out on Kuma Diffraction KM-4 diffractometer at room temperature (ω/2θ-scan mode, Mo Kα radiation, graphite monochromator).The crystal structure was solved by direct methods followed by Fourier syntheses with SHELXS-97 36 and refined using anisotropic full-matrix least-squares approximation for all nonhydrogen atoms using SHELXL-97 36 .Hydrogen atoms were placed geometrically.An absorption correction was applied 37 , T min = 0.294, T max = 0.736.Crystal data collection and details of the structure determination are summarized in Table 3.