Hemiacetal formation from a Schiff base in the presence of dysprosium(III)

: The formation of hemiacetals from aldehydes and alcohols is quite well known, as these are usually developed as intermediates in the preparation of acetals from aldehydes or ketones. Nevertheless, as far as we know, the examples of transformation of imines into hemiacetals are very scarce, and this reaction seems to be promoted by coordination to a metal ion. In this work, we describe the partial hydrolysis of a Schiff base, and its subsequent evolution to an hemiacetal donor in the presence of dysprosium(III) in an alcoholic medium. Full characterization of the final product, including single X-ray studies, is reported.


Introduction
The reversible acid-catalyzed reaction of aldehydes with alcohols to form acetals occurs via the hemiacetal intermediate. However, isolation of this intermediate is relatively far difficult compared to that of the corresponding stable acetal, being obtained in very small amounts if it is not stabilized by structural effects [1]. One way of stabilizing hemiacetals may be promoted via coordination of metal ions [1][2][3][4]. Accordingly, the synthesis of hemiacetal bound metal complexes is a great challenge for coordination chemistry. As far as we know, there are not many complexes containing hemiacetals as ligands reported up to now. These scarce examples usually contain d-metals but, to the best of our knowledge, no hemiacetal complexes have been described with lanthanoid metal ions. Besides, most of the reported complexes were obtained by conversion of aldehydes, but only one case was reported where the hemiacetal donor comes from an imine ligand [5].
Accordingly the conversion and stabilization of hemiacetals in the presence of metal ions is a research field scarcely explored. We describe herein an unusual example of an hemiacetal ligand generated in situ from an imine donor in the presence of dysprosium(III).

Materials and general methods
All chemical reagents and solvents were purchased from commercial sources and used as received without further purification. Elemental analyses of C, H and N were performed on a FISONS EA 1108 analyzer. Infrared spectra were recorded in the ATR mode on a Varian 670 FT/IR spectrophotometer in the range 4000-500 cm -1 . 1 H NMR spectrum of H2L was recorded on a Bruker DPX-250 spectrometer, using DMSO-d6 as solvent.
Single X-ray data for 1 were collected at 100 K on a Bruker D8 VENTURE PHOTON III-14 diffractometer, employing graphite monochromated Mo-kα (λ = 0.71073 Å) radiation. Multi scan absorption corrections were applied using SADABS [6]. The structure was solved by standard direct methods, employing SHELXT [7], and then refined by full matrix least-squares techniques on F 2 , using SHELXL from the program package SHELX 2014 [7]. Powder diffractograms for 1 were recorded with a Philips diffractometer with a control unity type "PW1710", a vertical goniometer type "PW1820/00" and a generator type "Enraf Nonius FR590", operating at 40 kV and 30 mA, using monochromated Cu-Kα (λ = 1.5418 Å) radiation. A scan was performed in the range 2 < 2θ < 30° with t = 3 s and Δ2θ = 0.02°. LeBail refinement was obtained with the aid of HighScore Plus Version 3.0d.

Syntheses
2.2.1. Synthesis of H2L. The imine ligand H2L (Scheme 1) was obtained by a variation of a previously described method [8], where the ethanol solvent of the reaction is changed by a mixture of chloroform and ethanol, and the ligand was satisfactorily characterized by elemental analysis, IR and 1 H NMR spectroscopy. Yield: 97%. Elemental anal. calcd. for C19H15O2N3 (317)

Scheme 1. Reaction scheme for isolation of complex 1
This ionic compound shows that in the [Dy(HL')2)] + cation, the dysprosium ion is surrounded by two (HL') -ligands, which derive from the Schiff base L 2-(Scheme 1), and that show a hemiacetal functional group. As far as we know, there are not many examples of conversion of imines into hemiacetals, but this reaction, summarized in Scheme 2, seems to be catalyzed by coordination to the metal ion [5]. This can readily be inferred from the fact that the Schiff base H2L can be obtained in alcohols with very high yield, and without any evidence of decomposition. Accordingly, the initial step in the formation of the hemiacetal donor should be the partial hydrolysis of the Schiff base, due to the presence of small amounts of water in the reaction medium, followed by a nucleophilic attack of the methanol solvent on the recently formed aldehyde function, all the steps being promoted and stabilized by the dysprosium(III) ion. Scheme 2. Proposed mechanism for the formation of hemiacetal from imine functional group Complex 1 is a red solid, which was unequivocally characterized by microanalysis, IR spectroscopy, single X-ray diffraction studies, and powder X-ray diffraction analyses.
The infrared spectrum of 1 shows two intense bands at 1583 and 1539 cm -1 , assigned to ν(C=N) vibrations of the imine and pyridine moieties, respectively [9], which agree with the presence of at least one imine group. These bands undergo negative shifts of ca. 45 cm -1 respect to the free ligand, indicating that the imine and pyridine nitrogen atoms are coordinated to the metal ion. Besides, the spectrum shows one quite sharp band at 3327 cm -1 , in accordance with the presence of the non-deprotonated alcohol function of the hemiacetal group.

Single X ray diffraction studies
Single crystals of [Dy(HL')2)][Dy(L)(Cl2)] (1) were obtained as detailed above. An ellipsoid diagram for 1 is shown in Figure 1 and main distances and angles are recorded in Table 1.  The crystal structure of 1 shows that it is a ionic compound, composed of [Dy(L ' )2)] + cations (1a) and [Dy(L)(Cl2)] -anions (1b). In the 1a cation (Figure 1), the dysprosium ion is surrounded by two imine-hemiacetal ligands that act as monoanionic N2O2 donors, linking the metal ion through both nitrogen atoms of the imine and pyridine functions, the deprotonated phenolic oxygen atom, and the protonated alcoholic oxygen atom of the hemiacetal group. Accordingly, the dysprosium ion is octacoordinated in a N4O4 environment. Calculations of the degree of distortion of the DyN4O4 core respect to an ideal eight vertex polyhedron with the SHAPE software [10], gives rise to shape measurements closer to triangular dodecahedron, but distorted towards snub diphenoid, as shown in Figure 2. This cation has two chiral centres, the carbon atoms of the hemiacetal group, but both S,S and R,R isomers of 1a are present in the unit cell in 1:1 ratio, thus giving rise to a racemic mixture. In the 1b anion (Figure 1), the bisdeprotonated Schiff base L 2-wraps the dysprosium(III) centre in its N3O2 pocket. The coordination sphere is completed by two chloride anions, and, accordingly, the dysprosium ion achieves coordination number 7, with slightly distorted pentagonal bipyramid geometry. In this pyramid, the pentadentate donor forms a nearly perfect plane (maximum deviation from any atom from the mean calculated N3O2 plane of 0.112 Å, with the Dy atom in the plane), and the angle Cl-Dy-Cl is ca. 177º.
Besides, this structure is further stabilized by two short hydrogen bonds (O···O distances of ca 2.5 Å) between the phenolic oxygen atoms of the 1b anion and the protonated alcoholic functions of the 1a cation, what generates a pseudodinuclear complex, with a Dy III ···Dy III distance of ca. 5.5 Å (Figure 3).

Powder X ray diffraction studies
Powder X-ray diffractograms for two microscrystalline samples of 1, obtained from two different syntheses, were recorded ( Figure 4). This study was done in order to demonstrate that 1 is the only product in the solid sample, and not a byproduct generated in the recrystallisation process, and that the experiment is reproducible, and not just the result of serendipity. When these diffractograms were compared with the calculated one using the single X-ray data (Figure 4), it could be concluded that both microcrystalline samples are exactly the same compound, thus demonstrating that the experiment is reproducible, and that they are the same product as the one crystallographically solved.

Figure 4.
Comparative powder X-ray diffractograms for 3: green and red: experimental diffractograms for two microcrystalline samples obtained from two different syntheses; grey: calculated diffractogram using the data obtained from single X-ray diffraction studies.
Accordingly, complex 1 is a pure product, obtained as the main product of a reproducible reaction. Thus, it should be noted that, as far as we know, this is the first dysprosium complex with a hemiacetal ligand generated in situ, and crystallographically characterised.

Conclusions
This work reports the formation and stabilization of a hemiacetal donor from an imine ligand in the presence of dysprosium(III). Accordingly, this research contributes to increase the scarce number of hemiacetals obtained from imine functions, and the number of metal complexes with hemiacetals as ligands. In addition, complex 1, obtained as the pure main product of a reproducible reaction, is the first lanthanoid hemiacetal complex to be crystallographically characterized.