Diaroyl Tellurides: Synthesis, Structure and NBO Analysis of (2-MeOC6H4CO)2Te – Comparison with Its Sulfur and Selenium Isologues. The First Observation of [MgBr][R(C=Te)O] Salts

A series of aromatic diacyl tellurides were prepared in moderate to good yields by the reactions of sodium or potassium arenecarbotelluroates with acyl chlorides in acetonitrile. X-ray structure analyses and theoretical calculations of 2-methoxybenzoic anhydride and bis(2-methoxybenzoyl) sulfide, selenide and telluride were carried out. The two 2-MeOC6H4CO moieties of bis(2-methoxybenzoyl) telluride are nearly planar and the two methoxy oxygen atoms intramolecularly coordinate to the central tellurium atom from both side of C(11)-Te(11)-C(22) plane. In contrast, the oxygen and sulfur isologues (2-MeOC6H4CO)2E (E = O, S), show that one of the two methoxy oxygen atoms contacts with the oxygen atom of the carbonyl group connected to the same benzene ring. The structure of di(2-methoxybenzoyl) selenide which was obtained by MO calculation resembles that of tellurium isologues rather than the corresponding oxygen and sulfur isologues. The reactions of di(aroyl) tellurides with Grignard reagents lead to the formation of tellurocarboxylato magnesium complexes [MgBr][R(C=Te)O].


Introduction
In contrast to diacyl sulfides and selenides, the isolation of diacyl tellurides is very difficult due to their instability toward oxygen and thermal conditions. The first isolation of diacyl telluride was reported in 1978 by Bergman and Engman, who isolated phthaloyl telluride [1]. In 1985, du Mont and his coworkers successfully isolated two aliphatic diacyl tellurides [(RCO) 2 Te: R = Me, t-Bu] by using (Me 3 Si) 2 Te as a tellurium source [2] and reported a molecular structural analysis of di(adamanthoyl) tellurides [3]. In 1986 we also reported the isolation of two aromatic diacyl tellurides from the reaction of sodium carbotelluroate with acyl chlorides [4]. In this paper, we report an improved synthesis and some reactions of diaroyl tellurides, and NBO (natural bond orbital) analysis of (2-MeOC 6 H 4 ) 2 E (E= O, S, Se, Te).

Synthesis
The reactions of sodium telluride with two molar equivalents of acyl chlorides in ether, benzene and tetrahydrofuran etc. gave di(acyl) tellurides 4 in yields below 50% [5], due to the difficulty of separating by-products such as diacyl ditelluride, acid anhydride and black tellurium. We then found that the use of acetonitrile as a solvent led to good yields of 4. For example, two molar equivalents of 4-methylbenzoyl chloride in acetonitrile were added dropwise at 0 ºC to a suspension of freshly prepared sodium telluride in the same solvent, and the mixture was stirred for 2 h. The precipitates and excess of sodium telluride were filtered out and the solvent was removed under reduced pressure to give bis(4-methylbenzoyl) telluride (4e) in 54% yield.

Scheme 1. Synthesis of diacyl tellurides 4a-k.
Under similar conditions, the reactions of other sodium or potassium arenecarbotelluroates with the corresponding aroyl chlorides led to the expected diacyl tellurides 4d, 4f−4k in isolated yields of 39−84% (Scheme 1). In contrast, the preparation of aliphatic compounds 4a−4c resulted in low yields of 21−34%, most likely due to their instability.
We were unable to obtain single-crystals of the selenium isologue 7 even after several attempts. Therefore, the structure of 7 was optimized by using the structure parameters of bis(2methoxybenzoyl) diselenide [10]. As shown in Figure 2, the structure of 7 resembles that of tellurium isologue 4f rather than the corresponding sulfur isologue 6, although the 2-MeOC 6  The C-E-C and O=C---C=O interplanar angles in the -CO-E-CO-moieties of (2-MeOC 6 H 4 CO) 2 E (E = O, S, Se, Te) are shown in Table 3. Both the C-E-C angles and O=C---C=O interplanar angles are narrow, in the order E = O > S > Se > Te. Narrowing of the C-Te-C angle reduces the intramolecular repulsion of the carbonyl oxygen atoms.

Packing
The unit cells and intermolecular short contacts of compounds 4f, 5 and 6 are shown in

Ab initio calculations
To elucidate the nature of the nonbonding attractions, ab initio geometry optimizations were performed at the B3LYP/6-311G(2d,p) level for compounds 5 and sulfur 6 and at the RHF/LANL2DZ level+p for compounds 4f and 7 with the Gaussian 03 program [12,13] using their X-ray geometries [10,14]. The Beck-style three-parameter density functional theory [15] with the Lee/Yang/Parr [16] gradient-corrected correlation function was used in our calculation. Effective core potentials (ECP) with an appropriate valence basis set [LANL2DZ+polarization functions (d) and diffusion functions (sp)] were used for Te, Se, S and O, and the 04-31G* basis set was used for C and H [17]. At all levels and with all of the basis sets used, two conformational energy minima were seen for (2- (7), Te (4f)], which are conformers with RCO groups [syn-(C s symmetry) and anti-conformer (C 2 symmetry)] ( Figure 5). The structure parameters of the optimized geometries of these compounds are consistent with the absence of intermolecular interactions in crystal structures with these XRD values (detail data: ESI- Table 1).
To understand the orbital interactions of the -C(O)EC(O)-(E = O, S, Se, Te) moiety, the NBO analysis of (2-MeOC 6 H 4 CO) 2 E [E = O (5), S (6), Se (7), Te (4f)] was performed using the X-ray geometries of 5 and 6 ( Table 4). For the -C(O)EC(O)-moiety, orbital interactions are observed for the lone-pair electrons, which indicates delocalization of the lone-pair electrons on the carbonyl oxygen to the π* of a chalcogen atom (E) -carbon bond and on the central atom (E) to the π* orbital of the carbonyl groups. In addition, for 4f, there are nonbonding intramolecular interactions n O2 →σ* Te-C2 and n O4 →σ* Te-C1 , which indicates a large stabilization compared to other derivatives without an orthomethoxy substituent.  No. E a Stabilization energy associated with delocalization. b Calculation level: RHF3-21G for compounds 5 and 6. c Calculation level: RHF/LANL2dz(p.d) for compounds 7 and 4f.
Interestingly, when a tetrahydrofuran solution of phenylmagnesium bromide was added, the orangecolored solution of 4f immediately changed to dark green. The electron and 13 C-NMR spectra of this dark green solution showed a characteristic absorption maximum at λ max 732 nm and a new signal at δ 232 ppm, respectively. In addition, treatment of the dark green solution with excess of iodomethane led to Te-methyl 2-methoxybenzenecarbotelluroate (9). Several attempts to isolate the dark green compound with an absorption maximum at λ max 732 nm were unsuccessful. As for the cause of this change, du Mont and his coworkers have reported that the n-π* transitions of the C=Te group of Otrimethylsilyl ethanecarbotelluroate appear at 732 nm [2a]. We have also found that O-triorganylsilyl arenecarbotelluroates [20a] and aromatic carbotelluroic OH-acid (ArCTeOH) in polar solvents [20b] show a similar dark green to deep blue coloration and that their n-π* transitions of the C=Te group and tellurocarbonyl carbon chemical shifts appear in the ranges of 600−750 nm and δ 230−250 ppm, respectively [20]. Presumably, the absorption maximum at 732 nm observed in the reaction of bis(2-methoxybenzoyl) telluride with Grignard reagent may be attributable to the n-π* transitions of the C=Te group, indicating the formation of a complex 13, in which the oxygen atom of the carbotelluroato moiety is connected to the central magnesium ion. (Scheme 3) At present stage, it is unclear whether or not the ditelluride 12 (R = 2-MeOC 6 H 4 ) forms via 13, since no color change to dark green or deep sky blue was observed when t-BuMgBr was added.

Conclusions
The use of acetonitrile as a solvent was found to be effective for the synthesis of aromatic diacyl tellurides from the reaction of acyl chlorides with sodium and potassium tellurides. X-ray crystal structure analyses and theoretical calculations for 2-methoxybenzoic anhydride (5) and bis(2methoxybenzoyl) sulfide (6), selenide 7 and telluride 4f revealed that the two 2-MeOC 6 H 4 CO moieties of 4f are nearly planar, where the two carbonyl oxygen atoms are in the same direction, and the two methoxy oxygen atoms intramolecularly coordinate to the central tellurium atom from both sides of the C(11)-Te(11)-C(22) plane. For the corresponding acid anhydride 5 and diacyl sulfide 6, one of the two methoxy oxygen atoms is in contact with the central oxygen or sulfur atom and the other methoxy oxygen atom is in contact the oxygen atom of a carbonyl group connected to the same benzene ring. The structure of bis(2-methoxybenzoyl) selenide (7) obtained by theoretical calculations resembles that of tellurium isologue 4f, rather than the corresponding oxygen and sulfur isologues 5 or 6. Thus, the two carbonyl oxygen atoms are in the same direction and the two methoxy oxygen atoms intramolecularly contact the central selenium atom from both sides of the C(11)-Se(11)-C(22) plane. Natural bond orbital (NBO) analysis of the telluride 4f and selenide 7 revealed that two types of orbital interactions, n O2 →σ* E-C2 /n O4 →σ* E-C1 and n O2 →σ* E-C1 /n O4 →σ* E-C2 , are important and the former particularly play a predominant role. The reactions of diaroyl tellurides with Grignard reagents led to the formation of the corresponding carbotelluroato magnesium complexes in which the oxygen atoms of carbotelluroato ligands are connected to the magnesium ion.

General
The melting points were measured by a Yanagimoto micromelting point apparatus and uncorrected. The IR spectra were measured on a PERKIN ELMER FT-IR 1640 and a JASCO grating IR spectrophotometer IR-G. The 1 H-NMR spectra were recorded on JEOL JNM-GX 270 (270 MHz) or JNM-α400 (399.7 MHz) instruments, respectively, with tetramethylsilane as an internal standard. The 13 C-NMR spectra were obtained from a JEOL JMN-GX 270 (67.9 MHz). The 125 Te NMR spectra for aromatic diacyl tellurides 4d−4k were obtained from a JEOL JNM-GX-270 (85.9 MHz) with Ph 2 Te as an external standard: their chemical shifts were determined relative to Me 2 Te with δ (Ph 2 Te) = 420 ppm relative to Me 2 Te. The 125 Te-NMR spectra for aliphatic diacyl tellurides 4a-4c were obtained on a JEOL JNM-α400 (126.0 MHz) instrument with dimethyl telluride as an external standard. The electron spectra were obtained with Hitachi 124 and 310 spectrometers. The mass spectra were recorded on Shimadzu GCMS QP1000 (A) (EI/CI, model) mass spectrometers. The high resolution mass spectra (HRMS) were recorded on Shimadzu GCMS 9020DF high resolution mass spectrometer. 1 and 2 [4b,20d] were prepared according to the literature. Diethyl ether and hexane were refluxed and distilled from sodium metal using benzophenone as indicator before use. Dichloromethane and acetonitrile were distilled over phosphorus pentoxide. All manipulations were carried out under argon.

X-ray Measurements [20-24]
The measurements were carried out on a Rigaku AFC7R four-circle diffract meter with graphitemonochromated Mo-K radiation (= 0.71069 Å). All of the structures were solved and refined using the teXsan crystallographic software package on an IRIS Indigo computer. X-ray quality crystals of 4f, 5 and 6 were obtained by recrystallization from ether/petroleum ether. The crystal of 4f was cut from the grown needles and coated with an epoxy resin and mounted on a glass fiber. The cell dimensions were determined from a least-squares refinement of the setting diffract-meter angles for 25 automatically centered reflections. The crystals of 5 and 6 were obtained by recrystallization from a mixed solvent of ether/hexane. Lorentz and polarization corrections were applied to the data, and empirical absorption corrections (ψ-scans [21]) were also applied. The structures were solved by direct methods using SIR97 [22] and refined by using SHELXL97 [23]. Scattering factors for neutral atoms were from Cromer and Waber [24] and anomalous dispersion [25]

Syntheses of di(acyl) tellurides 4a-k
The preparation of bis(4-methylbenzoyl) telluride (4e) is described in detail as a typical procedure. Except were indicated, potassium carbotelluroates 2 were used for the preparation of compounds 4. The IR spectra of the latter were consistent with those of the corresponding authentic samples, respectively, which were prepared by the reaction of the corresponding piperidinium or potassium salts with acyl chlorides.