Dysprosium(III)-Mediated Carboxylate Formation from a Schiff Base ^†

Abstract

This work describes an uncommon new case of imine hydrolysis, leading to the formation of a carboxylate group. The conditions of the media that favor this conversion, which seem to be mediated by the presence of Dy^III, are analyzed. The process brings about the formation of a dysprosium complex of formula {[(DyL₂)₂Na₂(H₂O)₈][Et₃NH]₂[DyL₂]₂}·14H₂O (1). Single X-ray characterization of the final product is reported.

1. Introduction

Hydrolysis of Schiff bases usually produces the amine and aldehyde starting materials [1]. In Schiff bases containing two imine groups, partial hydrolysis is quite common, and it can generate imine−amine [2,3] or imine−aldehyde derivatives [4]. However, although it is unusual, sometimes it is observed that such hydrolysis does not stop in the mentioned products, and they continue reacting to form new compounds. In light of this, we recently described the synthesis and stabilization of a hemiacetal from a Schiff base [5]. As a continuation of this work, we describe the formation of an imine−carboxylate ligand from a diimine donor. The obtention of carboxylic acids from aldehydes is well known, and some synthetic methods that do not use any oxidants in addition to the air itself are reported for this organo-catalyzed [6,7] or metal-catalyzed [8] reaction. Additionally, the Cannizzaro reaction also leads to the isolation of carboxylates by aldehyde disproportionation, usually in a strong basic medium [9].

Nevertheless, in spite of these well-known processes, as far as we know, no partial hydrolysis of Schiff base metal complexes has been previously reported to yield coordination compounds containing imine−carboxylate ligands. Accordingly, the herein reported in situ transformation of a diimine donor into an imine−carboxylate ligand, in the presence of Dy^III, is an uncommon reaction.

2. Materials and Methods

2.1. Materials and General Methods

All chemical reagents and solvents were purchased from commercial sources and used as received without further purification. The Schiff base H₂L^SB (Scheme 1) was obtained as previously reported [5] and satisfactorily characterized by elemental analysis, IR and ¹H NMR spectroscopy.

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. Multiscan absorption corrections were applied using SADABS [10]. The structure was solved by standard direct methods, employing SHELXT [11], and then refined by the full matrix least-squares techniques on F², using SHELXL [12] from the program package SHELX.

2.2. Syntheses

{[(DyL₂)₂Na₂(H₂O)₈][Et₃NH]₂[DyL₂]₂}·14H₂O (1): to a solution of H₂L^SB (0.154 g, 0.487 mmol) in ethanol (20 mL), NaOH (0.038 g, 0.969 mmol) and triethylamine (0.098 g, 0.969 mmol) were added. The newly obtained solution was mixed with an ethanol (10 mL) solution of dysprosium triflate (0.295 g, 0.487 mmol). The mixture was stirred for 3 h. During this time, an orange solid formed. The suspension was centrifuged, and the orange solid collected and recrystallized in ethanol. The formed single crystals of 1, suitable for X-ray diffraction studies, were collected by filtration and air dried. Yield: 0.216 g (55% based on Dy). Crystal data (at 100(2) K): triclinic, P-1, C₁₁₆H₁₄₀Dy₄N₁₈Na₂O₄₆, MW = 3218.43, with a = 10.264(2) Å, b = 13.553(4) Å, c = 23.507(6) Å, α = 95.859(8) ^ο, β = 91.785(8) ^ο, γ = 106.110(7) ^ο, V = 3119.0(14) ų, Z = 1, R₁ = 0.0966 and wR₂ = 0.2061 (I > 2σI).

3. Results and Discussion

3.1. Synthesis and Spectroscopic Characterization

The reaction of the dysprosium(III) triflate with a solution of H₂L^SB in ethanol and basified with triethylamine and sodium hydroxide leads to the isolation of the complex {[(DyL₂)₂Na₂(H₂O)₈][Et₃NH]₂[DyL₂]₂}·14H₂O (1) (Scheme 1).

This compound contains the newly generated in situ ligand [L]²⁻, which derives from the Schiff base [L^SB]²⁻ (Scheme 1) by partial hydrolysis and where a carboxylate functional group is present. The hydrolysis of imines giving rise to aldehydes is well known, but, seemingly, there are not previous examples of the transformation of coordinated imines into carboxylates. This reaction, summarized in Scheme 2, seems to be catalyzed by the coordination to the Dy^III ion as no evidence of such hydrolysis is observed when a basic ethanolic solution of H₂L^SB is left to stand for one week. Accordingly, the initial step in the formation of the carboxylate group should be the partial hydrolysis of the Schiff base, due to the presence of small amounts of water in the reaction medium (Scheme 1), followed by a nucleophilic attack of the hydroxide ion on the recently generated aldehyde function [9,13]. This leads to the formation of the acidic function, which, in a basic medium, is deprotonated, and all the steps seem to be promoted by Dy^III.

Complex 1 was an orange solid, apparently stable in air in a solid state and in the solution, which was unequivocally characterized by single X-ray diffraction studies.

3.2. Single X-ray Diffraction Studies

Single crystals of {[(DyL₂)₂Na₂(H₂O)₈][Et₃NH]₂[DyL₂]₂}·14H₂O (1) were obtained as detailed above. An ellipsoid diagram for 1 is shown in Figure 1, and the main distances and angles are recorded in

Table 1. Main bond distances (Å) and angles (º) for 1.

Dy1-O11	2.216(15)	Dy2-O21	2.285(13)
Dy1-O12	2.354(14)	Dy2-O22	2.388(12)
Dy1-O14	2.297(12)	Dy2-O24	2.263(13)
Dy1-O15	2.349(13)	Dy2-O25	2.317(13)
Dy1-N11	2.502(14)	Dy2-N23	2.514(16)
Dy1-N12	2.494(15)	Dy2-N24	2.502(14)
Dy1-N13	2.475(15)	Dy2-N21	2.501(16)
Dy1-N14	2.496(14)	Dy2-N22	2.508(15)
Na1-O1OW	2.34(2)	Na1-O13	2.347(18)
Na1-O11W	2.392(19)	Na1-O13W	2.614(18)
Na1-O12W	2.383(17)	Na1-O13W #1	2.436(17)
N12-Dy1-N11	63.5(5)	N24-Dy2-N23	63.1(5)
O14-Dy1-O15	163.7(5)	O24-Dy2-O25	165.2(5)

^#1: −x, −y + 2, −z.

.

The crystal structure of 1 shows that it is composed of two Et₃N⁺ cations, two crystallographically identical mononuclear [DyL₂]⁻ anions, which are called 1a, and a neutral [(DyL)₂Na₂(H₂O)₈] moiety, which is called 1b. In addition, water as a solvate is present in the unit cell.

In the 1a anion (Figure 1), the dysprosium ion is surrounded by two imine−carboxylate ligands that act as dianionic tetradentate N₂O₂ donors, linking the metal ion through both nitrogen atoms of the imine and pyridine functions, the deprotonated phenolic oxygen atom and the deprotonated oxygen atom of the carboxylic group. Accordingly, the dysprosium ion is octacoordinated in an N₄O₄ environment. Calculations of the degree of distortion of the DyN₄O₄ core, with respect to an ideal eight-vertex polyhedron with the SHAPE software [14,15,16], gives rise to shape measurements closer to a snub diphenoid but distorts towards a triangular dodecahedron. The [DyL₂]⁻ moieties develop an intricate hydrogen bond scheme with the [Et₃NH]⁺ cations and the water solvates, which involves the phenolate and carboxylate oxygen donors of the anions. Thus, a zig-zag double chain of [DyL₂]⁻ anions is formed, and both chains are joined by hydrogen bonds between them. In addition, the double chain holds the [Et₃NH]⁺ cations through hydrogen bonds via water molecules (Scheme 2). In this assembly, the shortest intrachain and interchain Dy2···Dy2 distances are 10.264(2) Å and 7.583(2) Å, respectively.

The [(DyL)₂Na₂(H₂O)₈] (1b) moiety can be understood as two crystallographically related [DyL₂]⁻ anions (named 1b₁, Figure 1), assembled through a [Na₂(OH₂)₈]²⁺ moiety. In the [DyL₂]⁻ block, both imine−carboxylate ligands act as in 1a, providing an N₄O₄ environment to the Dy^III ion.

Once more, SHAPE calculations for the octacoordinated Dy^III indicate that the geometry around the metal (Figure 1) is a snub diphenoid distorted towards a triangular dodecahedron.

The [Na₂(OH₂)₈]²⁺ moiety can be understood as two symmetry-related [Na(OH₂)₄]⁺ cations, where the sodium atoms are surrounded by four water molecules. Two of these cations are bridged through the oxygen atom of one of the water ligands (O13W), thus giving rise to an O₅ environment for the sodium ion. This generates the [Na₂(OH₂)₈]²⁺ cation, where each one of the Na⁺ centres additionally binds to the uncoordinated carboxylate oxygen atom of one of the [L]²⁻ ligands belonging to the [DyL₂]⁻ anion. Accordingly, this joins the two [DyL₂]⁻ blocks through the [Na₂(OH₂)₈]²⁺ glue and gives rise to an O₆ environment for the sodium centres. This leads to Na···Na distances of 3.52(2) Å and Dy1···Dy1 distances of 16.452(3) Å within the 1b assembly.

Both 1a and 1b blocks are also connected through an intricate hydrogen bond scheme (Figure 2), which involves the coordinated water, the water solvate and the phenolate oxygen atoms, which results in the shortest intermolecular Dy1···Dy2 distances of 7.531(2) Å.

4. Conclusions

This work reports the formation and stabilization of an imine−carboxylate donor from a diimine ligand in a basic medium and in the presence of dysprosium(III). Accordingly, this work contributes to increasing the number of functional groups that can be produced from the hydrolysis of imine metal complexes, given that, as far as we know, there are no reports of previous examples of carboxylate donors obtained from imine ligands.