Diethyl 2,5-Dihydroxy-3,6-diiodoterephthalate

The title compound has been characterised for the first time by the full range of spectroscopic methods, and its X-ray structure shows hydrogen bonded stacks with iodine atoms aligned.


Introduction
The simple aromatic compound diethyl 2,5-dihydroxy-3,6-diiodoterephthalate 1 (Scheme 1) was first reported in the literature in 1899 [1]. Rather remarkably, there have been no further references to it since then, and the only experimental data for it is a melting point. In the course of studies on new linkers for metal organic framework (MOF) materials, we prepared the compound, intending to hydrolyse it to give the so-far unknown dihydroxydiiodoterephthalic acid. In fact, we were unable to hydrolyse 1 to the corresponding diacid, and this prompted further investigation of its structure including analysis by NMR and X-ray diffraction. In this paper, we present full characterisation of 1 for the first time, including UV, IR 1 H and 13 C NMR spectra and its X-ray structure (see Supplementary Materials).

Introduction
The simple aromatic compound diethyl 2,5-dihydroxy-3,6-diiodoterephthalate 1 (Scheme 1) was first reported in the literature in 1899 [1]. Rather remarkably, there have been no further references to it since then, and the only experimental data for it is a melting point. In the course of studies on new linkers for metal organic framework (MOF) materials, we prepared the compound, intending to hydrolyse it to give the so-far unknown dihydroxydiiodoterephthalic acid. In fact, we were unable to hydrolyse 1 to the corresponding diacid, and this prompted further investigation of its structure including analysis by NMR and X-ray diffraction. In this paper, we present full characterisation of 1 for the first time, including UV, IR 1 H and 13 C NMR spectra and its X-ray structure (see Supplementary Materials). Scheme 1. Synthesis and structure of 1.

Results
Compound 1 was readily prepared by reaction of the dibromo analogue 2 [2] with an excess of potassium iodide in boiling ethanol for 6 h. Its UV spectrum (see Supporting Material) showed four strong absorptions in the range 200-354 nm, while its IR spectrum showed a strong C=O absorption at 1686 cm -1 . The NMR spectra were in agreement with expectation and featured a 1 H signal at 9.55 ppm for OH as well as a remarkably shielded 13 C signal for C-I at 88.6 ppm. This can be compared with a value of 110 ppm for C-Br in compound 2 and results from the well-known heavy atom shielding effect of iodine.
Crystals of compound 1 as prepared were directly suitable for X-ray diffraction, and the resulting molecular structure is shown in Figure 1. This showed the expected high

Results
Compound 1 was readily prepared by reaction of the dibromo analogue 2 [2] with an excess of potassium iodide in boiling ethanol for 6 h. Its UV spectrum (see Supporting Material) showed four strong absorptions in the range 200-354 nm, while its IR spectrum showed a strong C=O absorption at 1686 cm −1 . The NMR spectra were in agreement with expectation and featured a 1 H signal at 9.55 ppm for OH as well as a remarkably shielded 13 C signal for C-I at 88.6 ppm. This can be compared with a value of 110 ppm for C-Br in compound 2 and results from the well-known heavy atom shielding effect of iodine.
Crystals of compound 1 as prepared were directly suitable for X-ray diffraction, and the resulting molecular structure is shown in Figure 1. This showed the expected high degree of steric congestion around the central six-membered ring, resulting in the ester groups being almost orthogonal to the plane of the benzene ring. degree of steric congestion around the central six-membered ring, resulting in the ester groups being almost orthogonal to the plane of the benzene ring. In the crystal, adjacent molecules had iodine atoms in a similar orientation but with alternating positions of the hydroxyl and ester groups, allowing efficient hydrogen bonding ( Figure 2, Table 1).   (7) 0.98 (2) 1.79 (2) 2.746 (3) 163 (4) Overall, the crystal structure consisted of stacks of molecules with the iodine atoms aligned and joined by hydrogen bonds alternately at the top and bottom (Figure 3). This is a type of structure that has been described before for halogenated dihydroxyterephthalate esters. In fact, the solid-state structure of such compounds has been of considerable interest ever since the early observation by Hantzsch of different coloured forms of dimethyl dichlorodihydroxyterephthalate [3]. A summary of the different structures determined for such compounds together with the CSD reference codes and literature references is shown in Figure 4. In the crystal, adjacent molecules had iodine atoms in a similar orientation but with alternating positions of the hydroxyl and ester groups, allowing efficient hydrogen bonding ( Figure 2, Table 1).

D-H…A
Molbank 2022, 2022, x FOR PEER REVIEW 2 of 5 degree of steric congestion around the central six-membered ring, resulting in the ester groups being almost orthogonal to the plane of the benzene ring. In the crystal, adjacent molecules had iodine atoms in a similar orientation but with alternating positions of the hydroxyl and ester groups, allowing efficient hydrogen bonding ( Figure 2, Table 1).  (7) 0.98 (2) 1.79 (2) 2.746 (3) 163 (4) Overall, the crystal structure consisted of stacks of molecules with the iodine atoms aligned and joined by hydrogen bonds alternately at the top and bottom (Figure 3). This is a type of structure that has been described before for halogenated dihydroxyterephthalate esters. In fact, the solid-state structure of such compounds has been of considerable interest ever since the early observation by Hantzsch of different coloured forms of dimethyl dichlorodihydroxyterephthalate [3]. A summary of the different structures determined for such compounds together with the CSD reference codes and literature references is shown in Figure 4.  Overall, the crystal structure consisted of stacks of molecules with the iodine atoms aligned and joined by hydrogen bonds alternately at the top and bottom (Figure 3). This is a type of structure that has been described before for halogenated dihydroxyterephthalate esters. In fact, the solid-state structure of such compounds has been of considerable interest ever since the early observation by Hantzsch of different coloured forms of dimethyl dichlorodihydroxyterephthalate [3]. A summary of the different structures determined for such compounds together with the CSD reference codes and literature references is shown in Figure 4.
In diethyl dihydroxyterephthalate 3, the molecules are planar and form chains featuring both intra-and intermolecular hydrogen bonding [4]. This is also the case for the dihydro analogue 4, for which three separate, but essentially identical, structures have been reported [5][6][7]. Introduction of a bulky substituent such as aryloxy at the remaining ring positions removes the intermolecular hydrogen bonding, and compounds 5-8 retain only the two intramolecular hydrogen bonds [8].     . Reported X-ray structures of dihydroxyterephthalates and substituted derivatives [4][5][6][7][8][9][10][11][12]. When we come to ring halogenated derivatives, the situation is considerably more interesting and provides an explanation on a molecular level for Hantzsch's early observation. Thus, the yellow form of dimethyl dichlorodihydroxyterephthalate 9 has the molecules approximately planar and forming chains featuring both intra-and intermolecular hydrogen bonding, as for 3 and 4 [9]. There is also a further "pale yellow" form that has ester groups twisted out of plane by around 40 • , but still retains the same chain structure. However, Hatzsch's white form of 9 has the ester groups orthogonal to the ring with no intramolecular hydrogen bonding, and actually has the same stack structure as we have found for 1 [9]. A similar structure has also been reported for dimethyl dibromodihydroxyterephthalate 10 [11]. When we come to diethyl dibromodihydroterephthalate 2, the situation is slightly different again with two reports of the stack structure lacking intramolecular hydrogen bonding [8,12], but also a different structure that has a chain of molecules in which one ester group is in plane and involved in both intra-and intermolecular hydrogen bonding, but the second ester group is orthogonal and not involved in hydrogen bonding [12].
In summary, the X-ray crystal structure of 1, the first for a dihydroxydiiodoterephthalate, has the ester groups orthogonal to the benzene ring and adopts the hydrogen bonded stack structure also previously described for the related dichloro and dibromo esters 2, 9 and 10.

Experimental Section
Melting points were recorded on a Reichert hot-stage microscope (Reichert, Vienna, Austria) and are uncorrected. IR spectra were recorded on a Perkin-Elmer 1420 instrument (Perkin-Elmer, Waltham, MA, USA). NMR spectra were obtained for 1 H at 300 MHz and for 13 C at 75 MHz using a Bruker AV300 instrument (Bruker, Billerica, MA, USA). Spectra were run at 25 • C on solutions in CD 3 SOCD 3 with internal Me 4 Si as the reference. Chemical shifts are reported in ppm to high frequency of the reference, and coupling constants J are in Hz.

Supplementary Materials:
The following are available online. Figure S1: UV-Vis spectrum of 1; Figure S2: IR spectrum of 1; Figure S3: 1 H NMR spectrum of 1; Figure S4: 13 C NMR spectrum of 1. Cif and check-cif files for 1.