Controlled hydrolysis of aryltellurium trichlorides using 2-pyrrolidinones: isolation and structural characterization of monomeric aryltellurium(IV) monohydroxides

Abstract

The 2-pyrrolidinone (Pyrr)-assisted hydrolysis of aryltellurium trichlorides, provided their first-step hydrolysis products as 1 : 1 molecular adducts, ArTeCl₂OH.Pyrr (Ar = 1-C₁₀H₇, Npl, 1; 2,4,6-Me₃C₆H₂, Mes, 2). The hydroxo ligand occupies an equatorial position in the ψ-trigonal bipyramidal geometry around Te(IV) and is involved in a strong H-bonding interaction with the amido O of the base. As NplTeBr₃ did not react with 2-pyrrolidinone under similar conditions, the bromo analogue of 1, NplTeBr₂OH.Pyrr (3), was prepared by a metathetical reaction with NaBr. Both 1 and 2 undergo electrophilic substitution with acetone and pinacolone to afford Ar(RCOCH₂)TeCl₂ (R = Me, t-Bu), but do not react with iodoacetic acid, suggesting that the hydroxo ligand acts as a nucleophile rather than a base. The Te–OH bond in 1 is also cleaved by CH₃COCl to give the parent trihalide NplTeCl₃, while reaction with BrCH₂COBr results in NplTeBr₃ (4) as the ultimate tellurium containing product. The reaction of NplTeCl₃ with N-Methyl protected 2-pyrrolidinone (PyrrMe), under similar conditions, gave an ionic complex [(PyrrMe)₂H] [NplTeCl₄] (5), but did not react with N-acyl protected 2-pyrrolidinone, PyrrCOCH₂Br. However, the latter added oxidatively to elemental tellurium to give (PyrrCOCH₂)₂TeBr₂ (6). In the crystal lattice of 5, discreet five-coordinate square pyramidal CTeX₄ units are present, while in the case of 4 the same units are realized via intermolecular Te⋯Br interactions in a one-dimensional supramolecular architecture. N-(2-pyrrolidinone)amidomethyl ligands in 6 adopt a rare, if not the first, 1,6-(C,O) mode of chelation in preference to 1,4-(C,O) mode often observed in similar Te(IV) compounds.

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Introduction

Hydrolysis of organotellurium(IV) di- and trihalides has been studied over the last century and the extent of hydrolysis is described to dependent upon the reaction conditions and electronic and steric influence of the halo and organic ligands.¹ The chlorides are hydrolyzed completely in alkaline medium to afford telluroxides, R₂TeO and tellurinic acids, RTe(O)OH, or the corresponding anhydrides (RTeO)₂O when treated with dilute acids. Hydrolysis of aryltellurium trichlorides in neutral medium, however, affords the acid chlorides ArTe(O)Cl and seems to proceed in a stepwise manner. Monohydroxotellurium(IV) compounds, such as Ar₂TeClOH or ArTeCl₂OH formed in the first step of hydrolysis of Ar₂TeCl₂ or ArTeCl₃ respectively, have not been isolated, indicating the intervening stages are fast. The isolated products of hydrolysis remained structurally ill-defined for a long time due to their low volatility and poor solubility in common organic solvents.

The structure and reactivity of telluroxides and tellurinic acids has recently been the subject matter of some research groups due to their importance as potential precursors to telluroxanes, the oligomeric species that bear Te–O–M backbone linkages.^2–6 This has given impetus to the development of telluroxane chemistry in analogy to the chemistry of siloxanes whose importance as molecular precursors to zeolite materials is well known.⁷ Precise crystallographic data that remained elusive for a long time is now known for some diaryltelluroxanes.⁸ The well defined dimeric tellurinic acid [2,6-Mes₂C₆H₃Te(O)(OH)]₂, stabilized kinetically with a bulky m-terphenyl ligand that limits the condensation and aggregation process,⁹ and monomeric [2-(C₆H₅N:N)C₆H₄Te(O)(OH)], stabilized by an intramolecular Te–N interaction,¹⁰ have recently been isolated and characterized structurally. Some novel anionic and neutral homometallic organotellurium oxide clusters bearing rings and cages (involving Te–O bonds exclusively) in their structures have also been characterized by X-ray crystallography and may be viewed as the condensation products of organotellurium trihydroxides.^11,12 During the course of our study, Beckmann et al. communicated¹³ the crystal structure of kinetically stabilized trans-2,4,6-t-Bu₃C₆H₂TeCl₂(OH). This monohydroxide was obtained in a small amount by the solid-state hydrolysis of a secondary modification of the parent trichloride. Attempts to prepare the monohydroxide by controlled hydrolysis of the trichloride in solution failed. The hydrolysis of intramolecularly coordinated 8-Me₂NC₁₀H₆TeCl₃ with water afforded the dinuclear telluroxane (8-Me₂NC₁₀H₆TeCl₂)₂O.¹⁴

In view of the hydrolytic lability of the metal and metalloid amides [M]–NR₂, often obtained by aminolysis of [M]–Cl bonds,¹⁵ reactions between ArTeCl₃ and 2-pyrrolidinone (γ-lactam) have been examined with the intention of initially preparing N-(aryldichlorotelluro)-pyrrolidinone, which on subsequent hydrolysis would then provide mononuclear aryltellurium(IV) hydroxides. The paramount strong hydrogen bonding tendency of γ-lactams,¹⁶ which even stabilizes pyrrolidinonium ion in the solid-state¹⁷ was expected to prevent intermolecular condensation of Te–OH bonds. This paper details (i) the preparation and structural characterization of stable aryltellurium(IV) monohydroxides and subsequent reactions involving cleavage of the Te–OH bond and (ii) the structure elucidation of products from (a) the reaction between NplTeCl₃ and N-methyl-2-pyrrolidinone and (b) oxidative insertion of elemental tellurium into C–Br bond of α-bromo-N-(2-pyrrolidinone)acetamide. Also included is the X-ray crystal structure of NplTeBr₃, which has surprisingly escaped elucidation so far despite those of its iodo analogue (NplTeI₃)¹⁸ and other aryltellurium tribromides, viz.PhTeBr₃,¹⁹ MesTeBr₃²⁰ and 2-PhC₆H₄TeBr₃²¹ being known for a long time.

Results and discussion

Reaction of ArTeCl₃ with an excess of commercial 2-pyrrolidinone at room temperature, after work-up and crystallization from dichloromethane gave the adducts, ArTeCl₂OH.Pyrr (Ar = 1 − C₁₀H₇, Npl, 1; 2,4,6-Me₃C₆H₂, Mes, 2; Pyrr = 2-pyrrolidinone) in nearly quantitative yield. The use of dried 2-pyrrolidinone resulted in a sluggish reaction with low yield. Formation of the hydroxide, ArTeCl₂OH, in the first instance, appears to involve initial formation of the amide (Eqn 1a) which is hydrolyzed by the adventitious incursion of moisture present in the commercial lactam/solvent. This is substantiated by the fact that N-methylpyrrolidinone, (PyrrMe), which cannot cause aminolysis, forms an anionic complex [(PyrrMe)₂H][NplTeCl₄] under the same conditions. However, base assisted direct hydrolysis of the trichlorides ArTeCl₃ by the moisture to give the monohydroxides, cannot be ruled out (Eqn 1b). This finds support from the fact that NplTeCl₃ when stirred with water in THF affords a colorless solid, which darkens at ∼192 °C but does not melt. It is insoluble in common organic solvents and analyses for (NplTeO_1.5)_x. In an attempt to prepare the dihydroxide, NplTeCl(OH)₂, 1 was treated separately with one mole of NaOH or 2-pyrrolidinone in water/THF. In each case the same insoluble, product was obtained. It shows that the condensation/aggregation process in the dihydroxide could not be prevented even by the H-bond formation as in case of 1.

Both 1 and 2 are colorless crystalline solids soluble in dichloromethane and chloroform. They melt without decomposition, but when stirred in refluxing toluene more than 70% of adduct 1 decomposed to elemental tellurium and some insoluble, high melting solid which was not characterized as it was impossible to separate it from the Te powder completely. The chloro ligands in 1 undergo halogen exchange with NaBr to give the bromo analogue, NplTeBr₂OH.Pyrr (3), which is otherwise not accessible by the reaction of NplTeBr₃ with 2-pyrrolidinone. Reactions of 1 with acetone and pinacolone yield Npl(RCOCH₂)TeCl₂ (R = Me, t-Bu), which have been obtained earlier²² in the reaction of NplTeCl₃ with the respective ketones. Formation of Npl(RCOCH₂)TeCl₂ from 1 would require electrophilic substitution of the ketones involving cleavage of the Te–OH bond (unless Te–Cl bond is cleaved and the liberated HCl then converts Te–OH into Te–Cl bond, which appears unlikely due to the presence of pyrrolidinone moiety). The Te–OH bond in 1 is also cleaved by acetyl chloride to afford NplTeCl₃, while refluxing of 1 with α-iodoacetic acid in chloroform for 5 h left the reactants unchanged. It shows that the monohydroxide behaves as a nucleophile rather than a base. The reaction of 1 with bromoacetyl bromide is interesting as it not only cleaves the Te–OH bond but also exchanges halogens to afford NplTeBr₃ instead of the mixed halogen complex NplTeCl₂Br.

The N-acyl protected 2-pyrrolidinone, PyrrCOCH₂Br (prepared from 2-pyrrolidinone and BrCOCH₂Br), unlike PyrrMe, did not react with NplTeCl₃ but adds oxidatively to elemental tellurium to give colourless crystalline (PyrrCOCH₂)₂TeBr₂ (6).

Spectroscopic studies

Compounds 1 and 2 show νCO at ∼1635 cm⁻¹, which is lower than that in 2-pyrrolidinone (1703 cm⁻¹) due to its involvement in the formation of H-bonding simultaneously with the N-atom of the second molecule of pyrrolidinone and oxygen atom of the Te–OH group (vide infra). The ¹H NMR spectrum of 1 consists of signals due to aryl protons between δ 7.63 and 8.60, methylene protons of the pyrrolidinone group at δ 2.16–3.42 and a broad singlet for the N–H proton. The o-Me protons as well as the two ring protons in 2 appear both as two singlets which can be explained in terms of restricted rotation along the Te–C(Mes) bond. This is also substantiated by the appearance of two signals for the o-Me carbons in the ¹³C NMR spectrum. The carbonyl carbon appears in the low field (δ ∼187) in 1 as well as 2 and the rest of the three carbon atoms of the pyrrolidinone group are observed as three singlets in the range δ 19–43 ppm. In compound 6, protons of methylene group bound to Te(IV) are deshielded, though marginally, compared to the parent bromide, PyrrCOCH₂Br (δ 4.51). The ¹²⁵Te NMR spectra of 1–6 show only a single resonance in CDCl₃ suggesting the presence of only one Te containing species in solution. Interestingly the Te atom in 1 (δ 1326) is shielded compared to 2 (δ 1409) which can be attributed to greater steric requirement of mesityl group. While the electronegativity of halo ligands in NplTeCl₃ and NplTeBr₃ justify the observed δ¹²⁵Te values (δ 1372 and 1348 respectively), the moderate shielding of Te atom in 1 compared to its bromo analogue is surprising. The δ¹²⁵Te value for the anionic complex 5, as expected, is appreciably at higher field compared to that for NplTeCl₃.²³

Molecular and crystal structures

The crystallographically determined molecular structure is unambiguous in the case of 1, while in 2, the hydroxo and one of the chloro (Cl2) ligands are two-fold disordered with occupancies 0.703 : 0.297. The asymmetric unit in each case comprises of a 1 : 1 molecular adduct of the tellurium(IV) monohydroxide with 2-pyrrolidinone, held together by polarization-assisted strong (Te)O–H⋯O(amido) hydrogen bonding interaction (Fig. 1–2). The geometry around Te(IV) in the aryltellurium(IV) monohydroxide moiety, ArTeCl₂OH, of 1 and 2 resembles a distorted trigonal bipyramid with chloro ligands at the apical sites. The equatorial Te–OH bond distances of 1.884(1) Å in 1 and 1.814(4) Å in 2 are comparable to 1.993(2) Å, the value reported in case of the supermesityl analogue, trans-2,4,6-t-Bu₃C₆H₂TeCl₂OH (wherein the TeCl₂OH group of atoms is two-fold disordered¹³). These bonds in 1 and 2 are among the shortest Te–O distances observed¹⁰ and are also shorter than the sum of covalent radii for Te and O (Σr_cov(Te,O) = 2.15 Å). The greater steric demand of the mesityl ligand bound to Te(IV) in 2, is obvious from the wider equatorial C–Te–O angle of 108.80(14)° compared to 94.42(5)° in 1. Strong repulsion among the equatorial ligands and the lone pair in 2 is also evident from the exocyclic angles of 125.9(2)° and 113.7(2)° at the ipso C1 atom of the mesityl ligand that deviate from the putative value of 120°. As a consequence, local C₂ symmetry of the mesityl group about the Te–C axis is lost, accounting for separate signals for the orthomethyl protons in the ¹H NMR spectrum (vide supra).

Fig. 1 Molecular structure of 1 showing 50% probability displacement ellipsoids and the atom numbering scheme. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Te–C1 2.117(13), Te–Cl1 2.546(4), Te–Cl2 2.462(4), Te–O1 1.884(12), O1–H1O 0.85(3), O1–Te–C1 94.42(5), Cl1–Te–Cl2 176.68(1), Cl1–Te–C1 89.23(4), Cl1–Te–O1 89.51(4), Cl2–Te–C1 89.90 (4),Cl2–Te–O1 87.36(4).

Fig. 2 Molecular structure of 2 showing 20% probability displacement ellipsoids and the atom numbering scheme. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Te–C1 2.101(3), Te–Cl1 2.490(1), Te–Cl2A 2.520(1), Te–O1AA 1.814(4), O1AA–H1AB 0.82, O1AA–Te–C1 108.80(14), Cl1–Te–Cl2A 175.91(4), Cl1–Te–C1 89.05(7), Cl1–Te–O1AA 94.23(12), Cl2A–Te–C1 87.32 (7), Cl2A–Te–O1AA 85.16(12).

In the lattices of compounds 1 and 2 the molecular adducts form dimers. The 2-pyrrolidinone molecules with cis amido configuration have propensity to form cyclic dimer via reciprocatory resonance-assisted N–H⋯O(=C) hydrogen bonding in solution¹⁶ as well as in the metastable solid-state phase.²⁴ Thus, dimeric structures of 1 and 2 in the crystalline state (Fig. S1 and S2, ESI†) may alternatively be described as the cyclic dimer of the lactam holding two molecules of ArTeCl₂OH, one each to its amido O atoms by means of O⋯H–O hydrogen bonding interactions. The strong H-bonding interactions appear to prevent intermolecular condensation of Te–OH bonds. Each amido O atom in the centrosymmetric dimeric units of 1 and 2 holds the two protons (OH and NH) simultaneously at an angle of 120.6°, which is reminiscent of angular separation between the two lone pairs on a sp²oxygen atom. Although the eight-member ring-centered inversion point in the solid state dimeric unit of the parent lactam²⁴ is retained in the lattices of 1 and 2, additional strong O–H⋯O hydrogen bonding interactions present in the latter bring a noticeable change in its geometry. The residual pyramidality of N atoms observed in the structure of cyclic dimer of the lactam vanishes and all its atoms (excluding C3A and C3Aⁱ; symmetry code: i = 1.5 − x, 0.5 − y, 1 − z for 1, i = −x, −y, 1 − z for 2) become approximately coplanar (r.m.s. deviation: 0.0168 Å for 1, 0.0295 Å for 2). The dimeric lactam in adducts 1 and 2, thus adopts a nearly flat-seat chair conformation. An interesting one-dimensional supramolecular motif extending along b axis in the crystal lattice of 1 can be identified (Fig. S3†), wherein the centrosymmetric dimeric units are further associated via reciprocatory Te⋯Cl1 secondary bonding interactions of 3.550(1) Å (symmetry code: a = 1.5 − x, 1.5 − y, 1 − z). The equatorial hydroxo ligands are trans with respect to the Te₂Cl₂ parallelogram in the dimeric units of NplTeCl₂OH (Fig. S4†) as against cis disposition, observed in the analogous dimer of 2,4,6-t-Bu₃C₆H₂TeCl₂OH.¹³ The role of each centrosymmetric dimeric unit of NplTeCl₂OH, providing OH groups as H-bond donors in the supramolecular assembly of 1, may be compared with that of a dicarboxylic acid molecule in the lattices of the adducts of the 2-pyrrolidinone with succinic or fumaric acids.²⁵

The crystal structure of the ionic complex 5 is built of discrete tetrachloro(1-naphthyl)tellurate anions and protonated dimeric N-methyl-2-pyrrolidinone cations (Fig. 3). The five-coordinate tellurium atom in the anion [NplTeCl₄]⁻ bears slightly distorted square pyramidal geometry with mutually transoid Cl1/Cl3, Cl2/Cl4 atoms in the basal positions and ipso C1 atom of the aryl ligand in the apical position. The central Te atom lies in the basal plane (∑∠Cl–Te–Cl = 360.0°) with negligible deviation (0.0008 Å) from the mean plane of the TeCl₄ unit and a narrow Te–Cl bond length range of 2.5090(4) to 2.5449(4) Å. The Te–C_aryl bond length of 2.173(1) Å in 5 is slightly longer in comparison to the values reported for the analogous tetrachloroaryltellurate anions [PhTeCl₄⁻, 2.126(2) Ų⁶, 2.135(2) Ų⁷; 4-PhOC₆H₄TeCl₄⁻, 2.127(5) Ų⁸ and in the zwitterion 4-H₃N⁺-3,5-(i-Pr)₂C₆H₂TeCl₄⁻, 2.127(7) Ų⁹]. It is also greater than that observed in the tetraiodo(2-naphthyl)tellurate anion (2.141(8) Å)³⁰ or (1-naphthyl)tellurium trichloride (2.115(5) Å)²³ and approaches the value for Te–C_alkyl bond length, 2.178(4) Å, reported in the zwitterionic species, nacnacTeCl₄.³¹ The Te⋯Cl and Cl⋯Cl secondary bonding interactions that are reported²⁶ to give rise to oligomeric as well as polymeric species, [(ArTeCl₄)⁻]_n (n = 2, 3 or ∞), are absent in the crystal lattice of 5. Instead, weak intermolecular (naphthyl)C7–H7⋯Cl2 interactions self-assemble anions into chains (Fig. S5†). While centrosymmetric pairs of anions drawn from adjacent chains with face-to-face eclipsed basal planes can be identified, the internuclear distances between Te atoms [4.2335(6) Å] and eclipsed Cl atoms [either 4.1845(6) Å or 4.3293(6) Å] are too long to be of major significance. The two N-methyl-2-pyrrolidinone molecules, each of which is two-fold disordered, are held together via a strong (carbonyl)O⋯H⁺⋯O(carbonyl) hydrogen bond in the cation. Each N-protected molecule of the γ-lactam in the cation involves a weak (ring methylene)C2B–H2BA⋯Cl1 interaction with an anion in the neighboring one-dimensional arrays. Examples of similar protonated dimeric cations where a proton bridges (the amido O atoms unsymmetrically or ethereal O atoms symmetrically) a pair of solvent molecules by a strong hydrogen bond are available in the literature.^32–34

Fig. 3 Molecular structure of 5 showing 50% probability displacement ellipsoids and the atom numbering scheme. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Te–C1 2.1729(14), Te–Cl1 2.5090(4), Te–Cl2 2.5298(4), Te–Cl3 2.5449(4), Te–Cl4 2.5175(4), Cl1–Te–Cl3 177.18(1), Cl2–Te–C4 177.31(1), Cl1–Te–Cl2 88.93(1), Cl2–Te–Cl3 88.70(1), Cl3–Te–Cl4 93.75(1), Cl4–Te–Cl1 88.65(1).

The asymmetric unit in the crystal lattice of NplTeBr₃ (4) contains two crystallographically independent molecules, each conforming to the usual ψ-trigonal bipyramidal geometry. An elliposoid diagram representing the asymmetric unit of 4 is shown in Fig. 4. While Br atom at the equatorial site is closest of the three attached to Te atom in each molecule, [d(Te1,Br12), 2.517(1)Å and d(Te2,Br23), 2.524(1)Å], the axial Te–Br bonds are unequal and substantially longer compared to Σr_cov(Te,Br) value of 2.51 Å.³⁵ Such an almost linear Te(IV)Br₂ fragment among organotellurium(IV) di- and trihalides is routinely described as a three-centre four-electron bond (3c–4e), similar to that introduced by Rundle and Pimentel for trihalide anions³⁶ and explains the hypervalency of tellurium atom without violating the octet rule or invoking ionic bonding. As two of the four electrons occupy a non-bonding molecular orbital, only one bonding pair is available for the two Te–Br bonds in 4 as well, thereby accounting for their bond orders typically less than 1.0 for a single electron pair bond (2c–2e) between the same elements. The significant disparity in the Te–Br_axial bond distances in both the molecules of 4 may be due to the involvement of one of the axial Br atoms in strong intermolecular Te⋯Br secondary bonding interaction (SBI). In the crystal lattice, units of each of the independent molecules form separate polymeric zig-zag chains via μ₂-bromo bridging (Fig. S6†) which are interlinked by means of Br⋯Br SBIs (Fig. S7†). The intermolecular secondary bond lengths [d(Te1⋯Br11#1) = 2.953(2)Å, d(Te2⋯Br21#2) = 2.937(2)Å] are only slightly longer (∼5%) than the longer of the two axial Te–Br bonds [d(Te1⋯Br11) = 2.800(1)Å, d(Te2⋯Br21) = 2.811(2)Å] and are shorter than Σr_vdr(Te,Br) value of 4.0 Å.³⁷ Each tellurium atom in the crystalline state of 4 thus adopts five-coordinate square pyramidal geometry with the aryl ligand at the apical site. The nearly coplanar TeBr₄ {deviation from the mean plane is limited to 0.11 Å (Br12 atom) and 0.08 Å (Br23 atom) in molecules 1 and 2 respectively} occupies the basal plane. Formation of similar square pyramidal, TeX₄Y units in the solid state has been observed invariably for the anions [TeX₄Y]⁻ (Y = organic ligand, X = Cl, Br or Y = X = Cl^29,38) and among the oligomeric or polymeric supramolecular motifs in the solid state of similar organotellurium tribromides¹⁹ as well as organotellurium trichlorides³⁹ including the chloro analogue of 4.²³ Among the solid state structures of aryltellurium trichlorides and -tribromides, square pyramidal CTeX₄ units are formed when a chloro or bromo ligand of an adjacent molecule approaches the central Te atom in a direction trans to the equatorial Cl(Br)–Te bond in preference to that of the C–Te bond resulting in two quasi linear X–Te–X alignments. Formation of such a planar covalent TeX₄ moiety among 12-Te-5 hypervalent species may be said to represent a five-centre eight-electron bonding (5c–8e) system where two atomic orbitals from the Te atom and one each from four halogen atoms constitute three doubly degenerate molecular orbitals. Of the eight electrons that occupy bonding and non-bonding molecular orbitals, only four account for the four Te–Br bonds, hence weaker than a normal 2c–2e bond. Such halo-bridged square pyramidal CTeX₄ units, however, are not realized in the lattices of the iodo analogue of 4,¹⁸mesityltellurium tribromide,²⁰2-biphenylyltellurium tribromide²¹ and also among those of the functionalized aryltellurium tribromides possessing intramolecular Te⋯A SBI with the central Te atom.⁴⁰ A slight variation in the steric and electronic features of the ligands bound to the Te(IV) atom could lead to subtle modifications in the supramolecular motifs and so it is unsurprising that a diverse combination of crystal packings have been observed among organotellurium trihalides.

Fig. 4 Molecular structure of 4 showing 50% probability displacement ellipsoids and the atom numbering scheme. Hydrogen atoms are omitted for clarity. Symmetry codes used to generate equivalent atoms: i = −x, 0.5 + y, 0.5 − z; ii = 1 − x, −0.5 + y, 1.5 − z. Selected bond distances (Å) and angles (°): Te1–C1A 2.130(10), Te1–Br11 2.953(2), Te1⋯Br11ⁱ 2.799(1), Te1–Br12 2.517(1), Te1–Br13 2.554(1), Te2–C1B 2.118(11), Te2–Br21 2.811(2), Te2⋯Br21ⁱⁱ 2.937(1), Te2–Br22 2.553(1), Te2–Br23 2.524(1); C1A–Te1–Br11 90.9(3), C1A–Te1⋯Br11ⁱ 90.4(3), C1A–Te1–Br12 96.1(3), C1A–Te1–Br13 92.5(3), Br11–Te1⋯Br11ⁱ 107.39(4), Br11–Te1–Br12 84.37(5), Br11–Te1–Br13 173.11(5), Br12–Te1⋯Br11ⁱ 84.37(5), Br12–Te1–Br13 89.12(5), Br13–Te1⋯Br11ⁱ 173.10(5), Te1⋯Br11ⁱ–Te1ⁱ 119.13(5), C1B–Te2–Br21 90.7(3), C1B–Te2⋯Br21ⁱⁱ 90.7(3), C1B–Te2–Br22 94.0(3), C1B–Te2–Br23 95.1(3), Br21–Te2⋯Br21ⁱⁱ 106.98(3), Br21–Te2–Br22 172.94(5), Br21–Te2–Br23 84.87(5), Br22–Te2⋯Br21ⁱⁱ 79.01(5), Br22–Te2–Br23 88.82(5), Br23–Te2⋯Br21ⁱⁱ 166.87(5), Te2⋯Br21ⁱⁱ–Te2ⁱⁱ 120.23(5).

The molecular structure of bis(N-(2-pyrrolidinone)amidomethyl)tellurium dibromide (6), depicted in Fig. 5, presents an interesting case of intramolecular Te⋯O secondary bonding interactions. The stereochemically active lone pair at the central Te(IV) atom gives the usual ψ-trigonal bipyramidal geometry around it with bromo ligands at the apical positions. In the C-bonded organic ligands that have extended O–C–N–C–O conjugation, all the skeletal atoms are almost coplanar (r.m.s. deviations: 0.0480 Å, 0.0270 Å). The organic ligands are transoid with respect to the equatorial C–Te–C plane and subtend nearly equal angles (51.8° and 53.3°) with it resulting in an approximate C₂ symmetry. The ring carbonyl O atom of each amidomethyl ligand is trans to a Te–C bond [∠O⋯Te–C = 153.97(8)°, 156.06(8)°] that makes (C=O)n → σ*(Te–C) orbital interaction possible. The 1,6-(C,O) mode of chelation of the organic ligands in 6 is substantiated by the internuclear distances between the central Te and O atoms (d(Te–O2A) 2.903(2), d(Te–O2B) 2.829(2)Å) which are significantly shorter than Σr_vdr(Te,O) (3.58 Å).³⁵ Interestingly, formation of six-membered chelate rings is preferred to the four-membered alternatives, even though it results in the significant angular distortion at C_sp3 atoms bound to tellurium [∠Te–C_sp3–C = 114.8(2)°, 117.8(2)°]. While bis(amidomethyl)- and bis(acylmethyl)tellurium(IV) dihalides invariably exhibit 1,4-Te⋯O intramolecular attractive secondary bonding interaction,^22,41 molecular structure of 6 represents one of the few (if not the only) examples of a 1,6-Te⋯O intramolecular interaction.

Fig. 5 Molecular structure of 6 showing 50% probability displacement ellipsoids and the atom numbering scheme. Hydrogen atoms are omitted for clarity. Selected bond distances (Å) and angles (°): Te–C1A 2.159(2), Te–C1B 2.145(3), Te–Br1 2.6488(3), Te–Br2 2.6784(3), Te⋯O2A 2.904(2), Te⋯O2B 2.829(2); C1A–Te–C1B 93.12(11), Br1–Te–Br2 175.98(1), ClA–Te⋯O2A 64.35(8), C1B–Te⋯O2B 65.96(9), C1A–Te⋯O2B 156.06(8), C1B–Te⋯O2A 153.97(8).

Experimental

General

The reactions were performed under atmospheric conditions. Solvents were dried and freshly distilled prior to use. 2-Pyrrolidinone (Merck, India) and N-Methyl-2-pyrrolidinone (Alfa-Aesar, India) were used as received. Dry 2-pyrrolidinone was obtained by storing it over CaH₂ and distilling under reduced pressure. 1-Napthyltellurium trichloride and mesityltellurium trichloride were prepared by the chlorination of their corresponding ditellurides with SO₂Cl₂. Melting points were recorded in capillary tubes and are uncorrected. IR spectra were recorded as KBr pellets using a Perkin-Elmer RX1 spectrometer. ¹H NMR spectra were recorded in CDCl₃ on a Varian DRX 300 spectrometer at 300.13 MHz using Me₄Si as internal standard. ¹³C{¹H} (100.54 MHz) and ¹²⁵Te{¹H}(126.19MHz) NMR spectra were recorded in CDCl₃ on a JEOL Eclipse Plus 400 NMR spectrometer, using Me₄Si and Me₂Te as internal standards. C, H, N analysis was performed on Carlo Erba model 1108 elemental analyzer. The Electrospray mass spectra were recorded on a MICROMASS QUATTRO II triple quadrupole mass spectrometer.

Reactions of ArTeCl₃ with 2-pyrrolidinone

Compound 1. A mixture of 1-naphthyltellurium trichloride (0.36 g, 1.00 mmol) and 2-pyrrolidinone (0.43 g, 5.0 mmol) was stirred at room temperature under dry atmosphere. The yellow color of the trichlorides gradually disappeared within 15 min. The reaction mixture was stirred for ∼3 h and the resulting paste washed with a 2 : 1 mixture of hexane and ethanol (3 × 3 mL). The off-white residue thus obtained was dissolved in dichloromethane (25 mL) and filtered through a short silica column. Concentration of the extract and addition of pet-ether (40°–60°) afforded a pale yellow solid which was recrystallized from chloroform to give 1 as needle shaped crystals (0.36 g, 85%); mp 120 °C. (Found: C, 39.4; H, 3.6; N, 3.1. Calc. for C₁₄H₁₅Cl₂NO₂Te: C, 39.3; H, 3.5; N, 3.2%); ν_max/cm⁻¹ 1629.0 (CO). ¹H NMR: δ 2.16 (2H, s, CH₂), 2.35 (2H, s, CH₂), 3.42 (2H, s, CH₂), 6.03 (1H, s, NH), 7.63–8.6 (m, 7H, aryl). ¹³C{¹H} NMR: δ 19.47, 28.99, 40.78 (CH₂), 124.73, 125.39, 125.55, 125.73, 128.15, 130.32, 130.65, 132.94, 145.23 (Aryl C), 194.24 (CO). ¹²⁵Te{¹H} NMR: δ 1326; m/z (ESI) 559 [(NplTe)₂O₂(OH)]⁺, 577 [(NplTe)₂O₂Cl]⁺, 829 [(NplTe)₃O₄]⁺, 865 [(NplTe)₃O₃(OH)Cl]⁺, 883 [(NplTe)₃O₂(OH)₃Cl]⁺.

When dried 2-pyrrolidinone was used in place of the commercial sample, under similar conditions, the reaction was sluggish with low yield (70%).

Compound 2. A mixture of mesityltellurium trichloride (0.35 g, 1.00 mmol) and 2-pyrrolidinone (0.43 g, 5.0 mmol) was stirred slowly, under dry atmosphere, at room temperature for ∼3 h. The resulting suspension was washed with a 2 : 1 mixture of hexane and absolute ethanol (3 × 3 mL) and dissolved in chloroform (10 mL). Addition of hexane (1 mL) and evaporation at room temperature afforded needle shaped colorless crystals of 2 (0.33 g, 80%); mp 85 °C. (Found: C, 36.76; H, 4.42; N, 3.14. Calc. for C₁₃H₁₉Cl₂NO₂Te: C, 37.19; H, 4.56; N, 3.34%); ν_max/cm⁻¹ 1647.0 (CO). ¹H NMR: δ 2.14 (2H, s, CH₂), 2.67 (2H, s, CH₂), 3.42 (2H, s, CH₂), 6.38 (1H, s, NH), 2.81 (3H, s, o-Me), 2.66 (3H, s, o-Me), 2.31 (3H, s, p-Me), 6.94(1 H, s, Aryl H), 7.01 (1H, s, Aryl H). ¹³C{¹H} NMR: δ 20.55 (p-Me), 21.04 (o-Me), 21.47 (o-Me), 19.80, 30.16, 42.62 (CH₂), 130.27, 133.09, 137.41, 141.39, 142.27, 142.98 (Aryl C), 180.18 (CO). ¹²⁵Te{¹H} NMR: δ 1403; m/z (ESI) 543 [(MesTe)₂O₂(OH)]⁺, 561 [(MesTe)₂O₂Cl]⁺, 859 [(MesTe)₃O₂(OH)Cl₃]⁺.

Reaction of NplTeCl₃ with water. 0.12 g of NplTeCl₃ (0.33 mmol) was dissolved in THF (3 mL) and water was added dropwise till no more white precipitate appeared to be formed. It was further stirred for 2 h. The solid was filtered, washed with water and ether and dried over anhydrous CaCl₂ in a vacuum dessicator. (NplTeO_1.5)_x, Yield 0.08 g (Found: C, 42.56; H, 2.55. Calc. for C₁₀H₇O_1.5Te: C, 43.09; H, 2.53%);

Reaction of 1 with NaBr. Compound 1 (0.42 g, 1.00 mmol) was dissolved in dichloromethane (20 mL) and dry sodium bromide (0.45 g, 3.00 mmol) added. The mixture was stirred for 12 h at room temperature and filtered. The filtrate was concentrated to about 5 mL and petroleum ether (40–60°, 10 mL) was added. The precipitated solid was filtered and recrystallized from chloroform to get light yellow crystals of NplTeBr₂OH.Pyrr (3). (0.34 g, 70%); mp 108–9 °C. (Found: C, 32.1; H, 2.7; N, 2.6. Calc. for C₁₄H₁₅Br₂NO₂Te: C, 32.5; H, 2.9; N, 2.75); ¹H NMR: δ 3.41 (2H, t, CH₂), 2.34 (2H, t, CH₂), 2.18 (2H, m, CH₂), 5.75 (1H, br, NH), 7.52–8.67 (m, 7H, aryl), ¹²⁵Te{¹H} NMR: δ 1354.

Reaction of 1 with ketones. To 1 (0.42 g, 1.00 mmol) was added an excess of dry acetone (10 mL) and stirred vigorously. White solid separated slowly which was filtered, washed with diethylether and recrystallized from chloroform to give colorless crystals which were characterized as Npl(MeCOCH₂)TeCl₂. (0.26 g, 68%); mp 157 °C (lit.^22a 156 °C), authentic ¹H NMR.

Npl(t-BuCOCH₂)TeCl₂ was obtained and characterized similarly (0.29 g, 69%); mp 168 °C (erroneously reported as 200 °C earlier^22b), authentic ¹H NMR.

Reaction of 1 with acetyl chloride and bromoacetyl bromide. Compound 1 (0.42 g, 1.00 mmol) was dissolved in dichloromethane (20 mL) and acetyl chloride (0.21 mL, 3.00 mmol) in dichloromethane (10 mL) added dropwise. The color of the mixture changes to bright yellow and was stirred for 4 h at room temperature. The filtrate was concentrated to about 5 mL and petroleum ether (40–60°, 10 mL) added. The precipitated solid was filtered and recrystallized with chloroform, giving yellow crystals of NplTeCl₃ (0.21 g, 58%); mp 178 °C. (lit.⁴² 175–180 °C).

Under similar conditions, 1 (0.42 g, 1.00 mmol) and bromoacetyl bromide (0.17 mL, 4.00 mmol) afforded orange red crystals of NplTeBr₃ (4) (0.29 g, 57%); mp 156–7 °C (lit.⁴² 159–160 °C) (Found: C, 24.3; H, 1.5. Calc. for C₂₀H₁₄Br₆Te₂: C, 24.3; H, 1.4%), ¹³C{¹H} NMR: δ 124.79, 127.51, 127.75, 128.03, 129.66, 131.57, 133.26, 134.62 (Aryl C). ¹²⁵Te{¹H} NMR: δ 1348.

Reaction of NplTeCl₃ with N-protected-2-pyrrolidinones. A mixture of napthyltellurium trichloride (0.36 g, 1.00 mmol) and N-Methyl-2-pyrrolidinone (0.50 g, 5.0 mmol) was stirred slowly at room temperature for ∼12 h. The resulting suspension was washed with a 2 : 1 mixture of hexane and absolute ethanol (3 × 3 mL) and dissolved in chloroform (10 mL). Addition of hexane (1 mL) and evaporation at room temperature afforded colorless chunks of 5 (0.32 g, 54%); mp 118 °C. (Found: C, 40.1; H, 4.2; N, 4.6. Calc. for C₂₀H₂₅Cl₄N₂O₂Te: C, 40.4; H, 4.2; N, 4.7%); ¹H NMR: δ 2.91 (3H, s, CH₃), 3.69 (2H, t, CH₂), 2.99 (2H, t, CH₂), 2.01 (2H, t, CH₂), 7.61–8.6 (m, 7H, aryl). ¹³C{¹H} NMR: δ 17.30, 30.69, 31.14 (CH₂), 51.69 (CH₃), 125.53, 125.80, 126.01, 128.96, 129.47, 131.95, 132.51, 134.62, 136.92, 137.02, (Aryl C), 178.17 (CO). ¹²⁵Te{¹H} NMR: δ 1285.

When N-acyl-2-pyrrolidinone (PyrrCOCH₂Br) was reacted with NplTeCl₃, under similar conditions, the reactants were recovered unchanged.

Reaction of N-(bromoacetyl)-2-pyrrolidinone with elemental Te. Freshly ground tellurium powder (0.64 g, 5.0 mmol) and PyrrCOCH₂Br (2.06 g, 10.0 mmol) were stirred together at ∼70 °C for ∼36 h. The reaction mixture was then washed with ether (3 × 10 mL) and the resulting grey solid extracted with dichloromethane (50 mL). The extract was passed through a short silica column and the eluent concentrated to about 10 mL. Addition of petroleum ether and cooling afforded 6 as a colorless crystalline solid (3.8 g, 70%); mp 140 °C. (Found: C, 26.9; H, 2.9; N, 5.1. Calc. for C₁₂H₁₆Br₂N₂O₄Te: C, 26.7; H, 3.0; N, 5.2%); ¹H NMR: δ 4.50 (2H, s, CH₂Te), 3.92 (2H, t, CH₂), 2.67 (2H, t, CH₂), 2.11 (2H, q, CH₂). ¹³C{¹H} NMR: δ 17.11, 33.05, 45.49 (CH₂), 45.90 (CH₂Te) , 165.17 (CO–N), 177.63 (CO). ¹²⁵Te{¹H} NMR: δ 780.

Conclusion

First step hydrolysis products, ArTeCl₂OH are isolated in good yield as hydrogen bonded adducts with 2-pyrrolidinone. However, in the absence of 2-pyrrolidinone, aqueous hydrolysis of ArTeCl₃ affords (NplTeO_1.5)_x as an insoluble and infusible solid. The Te–OH bond in 1 is cleaved by methyl ketones and acid halides, but not iodoacetic acid. Reaction of NplTeCl₃ with N-Methyl-2-pyrrolidinone yields an anionic complex [(PyrrMe)₂H][NplTeCl₄]. The N-acyl-2-pyrrolidinone, PyrrCOCH₂Br, does not react with NplTeCl₃ but adds oxidatively to elemental Te to give crystalline (PyrrCOCH₂)₂TeBr₂ which provides the rare, if not only, example of 1,6-Te⋯O intramolecular interaction.

Crystallography

Single crystals suitable for X-ray crystallography were grown from CHCl₃ solutions. Intensity data were collected on Oxford Diffraction Xcalibur, Ruby and Gemini CCD diffractometers with graphite monochromated Mo-Kα (0.71073 Å). Data were reduced and corrected for absorption using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm in the CrysAlisPro, Oxford Diffraction Ltd., version 1.171.33.55 program. The structures were solved by direct methods and difference Fourier synthesis using SHELXL-97.⁴³ Full-matrix least-squares refinements on F², using all data, were carried out with anisotropic displacement parameters applied to non-hydrogen atoms. Hydrogen atoms attached to carbon were included in geometrically calculated positions using a riding model and were refined isotropically. Crystallographic data collection and refinement parameters are given in Table 1. ORTEP views of molecular structures of 1, 2, 4, 5 and 6 are shown in Fig. 1–5.

Table 1 Crystallographic data and structure refinement details for 1, 2, 4, 5, and 6

	1	2	4	5	6
Formula	C14H15Cl2NO2Te	C13H19Cl2NO2Te	C20H14Br6Te2	C20H25Cl4N2O2Te	C12H16Br2N2O4Te
Formula weight	427.77	419.79	988.97	594.82	539.69
Temperature (K)	110(2)	295(2)	123(2)	123(2)	274(2)
Wavelength, λ (Å)	0.71073	0.71073	0.71073	0.70173	0.7073
Crystal system	Monoclinic	Monoclinic	Monoclinic	Triclinic	Monoclinic
Space group	C2/c	P21/c	P21/c	P–1	C2/c
a (Å)	17.9609(5)	13.6291(4)	20.3174(8)	10.0476(4)	28.4086(10)
b (Å)	7.74529(16)	6.8780(3)	7.3678(3)	10.8133(3)	9.5781(2)
c (Å)	23.9193(7)	19.3019(7)	17.7571(8)	11.8837(5)	13.1481(4)
α	90	90	90	91.270(3)	90
β	112.645(3)	109.336(4)	115.656(5)	112.651(4)	112.596(4)
γ	90	90	90	95.227(3)	90
V (Å^3)	3070.95(13)	1707.33(11)	2396.07(17)	1184.28(8)	3303.11(17)
Z	8	4	4	2	8
ρ calcd (Mg/m^3)	1.850	1.633	2.742	1.668	2.170
Abs coeff. (mm^−1)	2.285	2.053	12.447	1.726	6.655
F(000)	1664	824	1793	590	2048
Crystal size (mm^3)	0.48 × 0.45 × 0.34	0.48 × 0.41 × 0.16	0.87 × 0.26 × 0.16	0.49 × 0.38 × 0.25	0.49 × 0.414 × 0.37
θ range (°)	5.06 to 32.79	5.11 to 32.68	5.02 to 32.72	5.15 to 32.79	5.04 to 32.73
Index ranges	−27 ≤ h ≤ 26	−20 ≤ h≤ 20	−29 ≤ h ≤ 24	−14 ≤ h ≤ 15	−43 ≤= h ≤ 40
	−11 ≤= k ≤ 11	−9 ≤ k ≤ 10	−10 ≤ k ≤ 11	−16 ≤ k ≤ 16	−14 ≤= k ≤ 14
	−36 ≤= l ≤ 36	−28 ≤ l ≤ 28	−21 ≤ l ≤ 26	−17 ≤ l ≤ 17	−18 ≤ l ≤ 19
Reflns collected	25706	13068	20066	16082	27863
Indep reflns	5313 [R(int) = 0.0169]	5677 [R(int) = 0.0337]	8027[R(int) = 0.0678]	7808[R(int) = 0.0218]	5736 [R(int) = 0.0516]
Completeness to θmax (%)	98.9	98.8	99.1	99.1	98.7
Abs. correction	Semi-empirical from equivalents		Analytical	Semi-empirical from equivalents
Max., min. transmissions	1.00000, 0.59137	1.00000, 0.53504	0.173 , 0.026	1.00000 , 0.47400	1.00000 , 0.32999
Restraints/params	5313/0/189	5677/12/188	8027/0/254	7808/32/395	5736/0/191
GoF(F^2)	1.239	0 .910	1.033	1.005	0.972
Final R indices [I>2σ(I)]	R1 = 0.0189, wR2 = 0.0442	R1 = 0.0438, wR2 = 0.1062	R1 = 0.0686, wR2 = 0.1748	R1 = 0.0249, wR2 = 0.0540	R1 = 0.0325, wR2 = 0.0653
R indices (all data)	R1 = 0.0202, wR2 = 0.0446	R1 = 0.1106, wR2 = 0.1234	R1 = 0.0963, wR2 = 0.1942	R1 = 0.0364, wR2 = 0.0564	R1 = 0.0535, wR2 = 0.0681
Refinement method	Full-matrix least-squares on F^2
Larg. diff. peak/hole(eÅ^−3)	0.555/−0.872	0.660/−0.663	4.068/−0.387	0.621/−0.649	1.590/−1.490

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Acknowledgements

Financial assistance by the Department of Science and Technology, Government of India, New Delhi, is gratefully acknowledged. SM is thankful to the DST, New Delhi for Fast Track-Young Scientist Fellowship.

References

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Footnote

† Electronic supplementary information (ESI) available: The supramolecular architectures identified in the crystal lattice of 1, 2, 4 and 5 (Fig. S1–S7) using Diamond⁴⁴ and H-bonding parameters in tabular form (Table S1). CCDC reference numbers for 1, 2, 4, 5 and 6796562, 796563, 825838–825840. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ra00409c

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