Article pubs.acs.org/Organometallics
Synthesis and Structure of an Intramolecularly Coordinated Diaryltelluronic Acid and Its Dimethyl Ester Jens Beckmann,*,†,‡ Jens Bolsinger,† Andrew Duthie,§ and Pamela Finke‡ †
Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34/36, 14195 Berlin, Germany Institut für Anorganische und Physikalische Chemie, Universität Bremen, Leobener Straße, 28359 Bremen, Germany § School of Life and Environmental Sciences, Deakin University, Pigdons Road, Waurn Ponds 3217, Australia ‡
S Supporting Information *
ABSTRACT: The oxidation of the telluroxane cluster (8Me2NC10H6Te)6O8(OH)2 (4) or the diaryl ditelluride (8-Me2NC10H6Te)2 (7) using H2O2 provided the diarylditelluronic acid [8-Me2NC10H6Te(O)(OH)2]2(O) (6), which is the second member of this compound class and the first one to contain an intramolecularly coordinated substituent. Attempts at recrystallizing 6 from Methanol provided the partial ester [8Me2NC10H6Te(O)(OH)(OMe)]2(O) (8). In addition structural motifs of known diaryltelluronic acids were compared using DFT calculations.
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INTRODUCTION Sulfonic acids, RS(O)2(OH) (R = alkyl, aryl), are a wellestablished compound class that find extensive utility in organic synthesis.1 In contrast, little is known about the related selenonic acids, RSe(O)2(OH) (R = alkyl, aryl), which is arguably due to difficulties in their preparation and handling. Most known selenonic acids are characterized by a very high acidity, a strong oxidizing power, and a low thermal stability.2 Even less information is available on telluronic acids. Only very recently we reported that the reaction of the dinuclear aryltellurinic acid [2,6-Mes2C6H3Te(O)(OH)]2 (1) with sodium hydride afforded the tetranuclear sodium aryltellurinate [Na(2,6-Mes2C6H3Te(O)2)]4 (2), which was oxidized by molecular oxygen in the presence of a crown ether to give the dinuclear telluronic acid [2,6-Mes2C6H3Te(O)(OH)3]2 (3) (Scheme 1).3 The kinetic stabilization of 1−3 was assured by applying bulky m-terphenyl substituents shielding the inorganic core structure. It is noted that all attempts at directly converting 1 to 3 using strong oxidizing reagents, such as H2O2, KMnO4, and NaIO4, failed (Scheme 1).3 Very recently, we also described the intramolecularly coordinating telluroxane cluster (8Me2NC10H6Te)6O8(OH)2 (4) and the telluroxane polymer [(8-Me2NC10H6Te)2O3]n (5), which can both be regarded as (partial) anhydrides of the same elusive aryltellurinic acid.4 Compounds 4 and 5 dissolve in water to give the same solution of oligomeric telluroxane species having an average degree of aggregation of 2.5. © 2011 American Chemical Society
RESULTS AND DISCUSSION We have now found that an aqueous solution of (8Me2NC10H6Te)6O8(OH)2 (4) can be easily oxidized using H2O2 to give the diarylditelluronic acid [8-Me2NC10H6Te(O)(OH)2]2(O) (6) in 76% yield (Scheme 2). Alternatively, 6 can be prepared by the oxidation of an aqueous suspension of the parent diaryl ditelluride (8Me2NC10H6Te)2 (7) with H2O2 in 34% yield (Scheme 2). Compound 6 was isolated by crystallization from the aqueous solution and obtained as colorless prisms when recrystallized from water. It is worth mentioning that the photooxidation of the same diaryl ditelluride (8-Me2NC10H6Te)2 (7) with molecular oxygen gave rise to a complex product mixture from which the tetranuclear telluroxane cluster (8-MeNC10H5TeO)4 was isolated in 17% yield.5 The molecular structure of 6·4H2O is shown in Figure 1 and contains two crystallographically independent, albeit similar, aryltellurium(VI) sites, which are linked by an oxygen bridge (Te−O−Te = 127.0(2)°). The spatial arrangement of the tellurium atoms is distorted octahedral and is defined by a CNO4 donor set. The 125Te MAS NMR spectrum of 6 shows one signal at δiso 864.7 ppm. Each Te atom of 6 is involved in one (formal) Te−O double bond (average 1.835(3) Å) and three Te−O single bonds (average 1.936(4) Å), which compare reasonably well with the bond lengths in (2,4,6-i-Pr3C6H2)2TeO2 (average 1.802(3) Å)6 and cubic Te(OH)6 (1.913(6) Å).7 The intramolecular Te···N Received: September 26, 2011 Published: December 14, 2011 289
dx.doi.org/10.1021/om2008953 | Organometallics 2012, 31, 289−293
Organometallics
Article
Scheme 1. Synthesis of the Sodium Aryltellurinate [Na(2,6-Mes2C6H3Te(O)2)]4 (2) and the Diarylditelluronic Acid [2,6Mes2C6H3Te(O)(OH)3]2 (3)
The molecular structure of 6 displays two asymmetric intramolecular hydrogen bonds of the type Te−O−H···OTe. The donor−acceptor distance (average O···O = 2.683(6) Å) is consistent with medium-strength hydrogen bonding.10 Most oxygen atoms of 6 are also involved in intermolecular hydrogen bonding with adjacent water molecules in the crystal lattice. Once isolated from the aqueous layer, the solubility of 6 in water is rather poor. Compound 6 is most soluble in very polar organic solvents such as DMSO but virtually insoluble in weakly polar or nonpolar solvents. The 125Te NMR spectrum (d6-DMSO) of 6 exhibits a signal at δ 867.0 ppm, which resembles the 125Te MAS NMR chemical shift. The 1H NMR spectrum of 6 in d6-DMSO reveals one set of signals for the 8(dimethylamino)naphthyl substituents with the two methyl groups being chemically and magnetically inequivalent, indicating that the solid-state structure is retained in this solvent. However, the 1H NMR spectrum of 6 in D2O shows only one signal for the two methyl groups, implying magnetic equivalence or an exchange process that is fast on the NMR time scale in water. Perhaps in water the telluroxane linkage is disrupted, giving rise to the reversible formation of the aryltelluronic acid 8-Me2NC10H6Te(O)(OH)3 (6a). The ESI MS spectrum (MeCN, positive mode) of 6 shows six prominent mass clusters, which were assigned to the dinuclear cations [(RTe)2O4(OH)]+ (m/z 676.99), [(RTe)2O3(OH)3]+ (m/z 695.00), and [(RTe)2O2(OH)5]+ (m/z 713.01), the trinuclear cation [(RTe)3O4(OH)6]+ (m/z 1062.00), and the tetranuclear cations [(RTe)4O6(OH)7]+ (m/z 1409.02) and [(RTe)4O5(OH)9]+ (m/z 1427.00), respectively (R = 8Me2NC10H6). Apparently these cations are formed by autoionization and condensation processes, which might be interpreted in terms of a rather high kinetic lability of the Te−O bonds. No evidence was found that 6 possesses any oxidizing power toward alcohols. Attempts at recrystallizing 6 from methanol led to the selective esterification of two hydroxyl groups and formation of the partial methyl ester [8Me2NC10H6Te(O)(OH)(OMe)]2(O) (8), which was obtained as colorless needles in 68% yield (Scheme 2). The molecular structure of 8 is shown in Figure 2 and closely resembles that of 6, with the essential bond parameters being practically identical. It is noted that the esterification occurred selectively at those hydroxyl groups of 6 that are not involved in intramolecular hydrogen bonding and that are situated in positions trans to the oxygen bridge. Interestingly, the minute structural change on going from 6 to 8 significantly increased the solubility in common organic solvents, such as CHCl3. The 125Te NMR spectrum (CDCl3) of 8 shows one signal at δ 885.5 ppm, which
Scheme 2. Synthesis and Partial Esterification of the Diarylditelluronic Acid [8-Me2NC10H6Te(O)(OH)2]2(O) (6)
Figure 1. Molecular structure of 6 showing 30% probability ellipsoids and the crystallographic numbering scheme. Selected bond lengths (Å) and angles (deg): Te1−O1 1.969(3), Te1−O2 1.921(4), Te1−O3 1.836(3), Te1−O4 1.903(4), Te1···N1 2.376(4), Te1−C10 2.106(4), Te2−O1 1.983(3), Te2−O5 1.928(3), Te2−O6 1.834(3), Te2−O7 1.912(4), Te2···N2 2.366(4), Te2−C20 2.104(4), O3···O7 2.642(5), O4···O6 2.724(6); Te1−O1−Te2 127.0(2).
coordination in 6 (average 2.371(4) Å) is significantly shorter than those of the parent compounds (8Me2NC10H6Te)6O8(OH)2 (4, average 2.647(9) Å)4 and (8Me2NC10H6Te)2 (7, average 2.721(5) Å)8, pointing to strong attractive interactions. In fact, the intramolecular Te···N coordination in 6 is almost as short as that of the aryltellurenyl chloride 8-Me2NC10H6TeCl (2.350(3) Å),4 possessing the shortest value observed for intramolecular Te···N bonds in 8(dimethylamino)naphthyltellurium compounds.4,8,9 290
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[(RTe)2O2(OMe)3(OH)2]+ (m/z 755.05), and the trinuclear cations [(RTe) 3 O3(OMe) 2(OH)6]+ (m/z 1106.04) and [(RTe)3O2(OMe)3(OH)7]+ (m/z 1140.07), respectively (R = 8-Me2NC10H6). Thus, the degree of aggregation is slightly reduced in comparison to that in 6. DFT Calculations. The observation of different structure types for the aryltelluronic acids [2,6-Mes2C6H3Te(O)(OH)3]2 (3) and [8-Me2NC10H6Te(O)(OH)2]2(O) (6) prompted us to calculate the relative stabilities of the meta-, meso-, ortho-, and para-telluronic acids counterbalanced with water: 2RTe(O)2OH + 4H2O, 2RTe(O)(OH)3 + 2H2O, 2RTe(OH)5, [RTe(O)(OH)3]2 + 2H2O, and [RTe(O)(OH)2]2(O) + 3H2O (R = 8-Me2NC10H6, Ph), which are collected in Figure 3. The energies of the most stable forms within the series, namely [PhTe(O)(OH)3]2 and [8-Me2NC10H6Te(O)(OH)2]2(O) (6), were arbitrarily set to 0 kJ mol−1. The tellurium atoms of these compounds are characterized by a coordination number of 6. In comparison to the para-phenyltelluronic acid [PhTe(O)(OH)3]2, the ortho-phenyltelluronic acid PhTe(OH)5, having the same coordination number, is only marginally less stable (6 kJ mol−1), while all other structure types, which are characterized by lower coordination numbers, are significantly less stable (153, 203, and 345 kJ mol−1). In comparison to the para-aryltelluronic acid [8Me2NC10H6Te(O)(OH)2]2(O) (6), the ortho-aryltelluronic acid 8-Me2NC10H6Te(OH)5 and the para-aryltelluronic acid [8-Me2NC10H6Te(O)(OH)3]2, having coordination numbers of 7, are slightly less stable (both 17 kJ mol−1), whereas the meso-aryltelluronic acid 8-Me2NC10H6Te(O)(OH)3 (56 kJ mol−1) and the meta-aryltelluronic acid 8-Me2NC10H6Te (O)2(OH) (186 kJ mol−1) are considerably less stable. Unlike the tetracoordinate sulfonic acids and selenonic acids, the related telluronic acids favor hexacoordinate dinuclear
Figure 2. Molecular structure of 8 showing 30% probability ellipsoids and the crystallographic numbering scheme. Selected bond lengths (Å) and angles (deg): Te1−O1 1.981(5), Te1−O2 1.974(5), Te1−O3 1.830(6), Te1−O4 1.923(6), Te1···N1 2.406(8), Te1−C10 2.110(7), Te2−O2 1.976(5), Te2−O5 1.948(6), Te2−O6 1.837(7), Te2−O7 1.898(6), Te2···N2 2.384(7), Te2−C20 2.142(9), O3···O7 2.637(8), O4···O6 2.711(5); Te1−O1−Te2 125.3(3).
shifted only marginally from that of 6 (867.0 ppm). Like 6, the 1 H NMR spectrum (CDCl3) of 8 exhibits one set of signals for the 8-(dimethylamino)naphthyl substituents, with the two methyl groups being chemically and magnetically inequivalent. The ESI MS spectrum (MeCN, positive mode) of 8 exhibits eight prominent mass clusters, which were assigned to the mononuclear cations [(RTe)O(OMe)(OH)]+ (m/z 364.02), [(RTe)O(OMe)(ONa)] + (m/z 386.00), and [(RTe)(OMe) 2 (OH) 2 ] + (m/z 396.05), the dinuclear cations [(RTe)2O3(OMe)2(OH)]+ (m/z 723.03), [(RTe)2O3(OMe)2(ONa)]+ (m/z 745.01), and
Figure 3. Relative stability of meta-, meso-, ortho-, and para-aryltelluronic acids. 291
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positions for all structures using a riding model. Crystal and refinement data are collected in Table 1. Figures were created using
structures, whereby the structural type may be influenced by the choice of the organic substituent.
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Table 1. Crystal Data and Structure Refinement of 6·4H2O and 8·MeOH
EXPERIMENTAL SECTION
General Considerations. The starting materials (8Me 2 NC 10 H 6 Te) 6 O 8 (OH) 2 (4) 4 and diaryl ditelluride (8Me 2 NC 10H6 Te)2 (7) 8 were prepared according to literature procedures. The 1H and 125Te NMR spectra were recorded using JEOL GX 270 and Varian 300 Unity Plus spectrometers and are referenced to SiMe4 (1H, 13C) and Me2Te (125Te). The 125Te CP MAS NMR spectra were obtained using a JEOL Eclipse Plus 400 MHz NMR spectrometer equipped with a 6 mm rotor operating at spinning frequencies between 8 and 10 kHz. A 30 s recycle delay was used, and typically 5000−10 000 transitions were accumulated to obtain adequate signal-to-noise ratios. The isotropic chemical shifts δiso were determined by comparison of two acquisitions measured at sufficiently different spinning frequencies and were referenced using solid Te(OH)6 as the secondary reference (δiso 692.2/685.5 ppm). Infrared (IR) spectra were recorded using a Nexus FT-IR spectrometer with a Smart DuraSamplIR. Microanalyses were obtained from a Vario EL elemental analyzer. Synthesis of [8-Me2NC10H6Te(O)(OH)2]2(O) (6). Method A. To a solution of (8-Me2NC10H6Te)6O8(OH)2 (4; 175 mg, 0.09 mmol) in water (20 mL) was added H2O2 (35%, 2 mL), and the mixture was stirred for 1 h. Colorless prisms of 6·4H2O were obtained upon standing overnight (yield 160 mg, 0.20 mmol, 76%; mp 225− 227 °C). Method B. To a suspension of (8-Me2NC10H6Te)2 (7; 200 mg, 0.34 mmol) in water (10 mL) was added H2O2 (35%, 2 mL), and the mixture was stirred for 1 day. A brownish solid precipitated that was collected by filtration, washed with water (20 mL), and dried under vacuum. Recrystallization of the crude product from water yielded colorless prisms of 6·4H2O (yield 90 mg, 0.11 mmol, 34%; mp 225− 227 °C). 1 H NMR (D2O): δ 8.22 (d, 2H; Ar), 8.20 (d, 2H; Ar), 8.04 (d, 2H; Ar), 7.94 (d, 2H; Ar), 7.81 (t, 2H; Ar), 7.77 (t, 2H; Ar), 3.17 ppm (s, 12H; NMe). 1H NMR (d6-DMSO): δ 8.08 (d, 2H; Ar), 7.96 (d, 2H; Ar), 7.80 (d, 2H; Ar), 7.76 (d, 2H; Ar), 7.67 (t, 2H; Ar), 7.51 (t, 2H; Ar), 3.09 (s, 6H; NMe), 2.65 ppm (s, 6H; NMe). 125Te-NMR (d6DMSO): δ 867.0 ppm. 125Te CP MAS NMR: δiso 864.7 ppm. ESI MS (MeCN, positive mode): m/z 1426.00 [C48H57O14N4Te4]+, 1409.02 [C 4 8 H 5 5 O 1 3 N 4 Te 4 ] + , 1062.00 [C 3 6 H 4 2 O 1 0 N 3 Te 3 ] + , 713.01 [C 2 4 H 2 9 O 7 N 2 T e 2 ] + , 695.00 [C 2 4 H 2 7 O 6 N 2 T e 2 ] + , 676.99 [C24H25O5N2Te2]+. IR: ν̃(OH) 3156 cm−1 (broad). Anal. Calcd for 6·4H2O (C24H36N2O11Te2, 783.75): C, 36.78; H, 4.63; N, 3.57. Found: C, 37.01; H, 4.32; N, 3.59. Synthesis of [8-Me2NC10H6Te(O)(OH)(OMe)]2(O) (8). Recrystallization of 6·4H2O (78 mg, 0.10 mmol) from methanol (20 mL) gave a slightly pink solution, from which off-color needles of 8·MeOH crystallized. Drying under high vacuum afforded microcrystalline 8 (yield 50 mg, 0.07 mmol, 68%; mp 212−214 °C). 1 H NMR (CDCl3): δ 8.24, 7.81, 7.69−7.55, 7.42−7.34 (m, 12H; Ar), 3.83 (s, 6H; OMe), 3.16 (s, 6H; NMe), 2.81 ppm (s, 6H; NMe). 125 Te NMR (CDCl3): δ 885.8 ppm. ESI MS (MeCN, positive mode): m/z 1140.07 [C39H52O12N3Te3]+, 1106.04 [C38H48O11N3Te3]+, 755.05 [C27H35O7N2Te2]+, 745.01 [C26H30O6N2NaTe2]+, 723.03 [C26H31O6N2Te2]+, 396.05 [C14H20O4NTe]+, 386.00 [C13H15O3NNaTe]+, 364.02 [C13H16O3NTe]+. IR: ν̃(OH) 3101 cm−1 (broad). Anal. Calcd for 8 (C26H32N2O7Te2, 739.74): C, 42.21; H, 4.36; N, 3.79. Found: C, 42.69; H, 4.04; N, 3.62. Crystallography. Intensity data were collected on a STOE IPDS 2T diffractometer (6) at 150 K and a Bruker SMART 1000 CCD diffractometer (8) at 173 K with graphite-monochromated Mo Kα (0.7107 Å) radiation. The structures were solved by direct methods and difference Fourier synthesis using SHELXS-97 implemented in the program WinGX 2002.11 Full-matrix least-squares refinements on F2, using all data, were carried out with anisotropic displacement parameters applied to all non-hydrogen atoms. Hydrogen atoms attached to carbon atoms were included in geometrically calculated
formula formula wt cryst syst cryst size, mm space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z ρcalcd, Mg m−3 μ(Mo Kα), mm−1 F(000) θ range, deg index ranges
no. of rflns collected completeness to θmax, % no. of indep rflns no. of obsd rflns (I > 2σ(I)) no. of refined params GOF (F2) R1 (F) (I > 2σ(I)) wR2 (F2) (all data) (Δ/σ)max largest diff peak/hole, e Å−3 CCDC no.
6·4H2O
8·MeOH
C24H36N2O11Te2 783.75 triclinic 0.32 × 0.25 × 0.16 P1̅ 10.81(1) 10.94(1) 12.70(1) 108.88(8) 90.79(8) 95.28(9) 1413(3) 2 1.823 2.125 756 1.70−29.23 −13 ≤ h ≤ 14 −14 ≤ k ≤ 14 −17 ≤ l ≤ 17 15 933 98.2 7526 5611 352 0.932 0.0334 0.0945
