Lewis-base stabilized diiodine adducts with N-heterocyclic chalcogenamides †

Oxidation reactions of stable chalcogenamides with iodine are intriguing due to their broad application in various organic syntheses. In the present study we report on the utilization of N-heterocyclic carbene and cyclic-alkyl-amino carbenes L: (L: = :C[N(2,6-iPr2-C6H3)CH]2, L : = :C(CH2)(CMe2)(C6H10)N-2,6-iPr2C6H3, L : = :C(CH2)(CMe2)2N-2,6-iPr2-C6H3) for the syntheses of chalcogenamides L vE (E = S, Se, Te) 1–4 and zwitterionic adducts LvE–I–I 5–8. The synthesis of compounds 1–4 involved the addition reaction of ligand L: and elemental chalcogen. Treatment of 1–4 with iodine resulted in the formation of adducts 5–8. Compounds 5–8 are well characterized with various spectroscopic methods and singlecrystal X-ray structural analysis.


Introduction
The reactions of elemental halogens (I 2 , Br 2 ) and interhalogens (IBr, ICl) with organic molecules (L) containing group 16 donor atoms of composition LvE (L = organic framework, E = S, Se, Te) have gained particular interest in recent years.][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] The adduct formation of a donor (group 16 atom) with diiodine (LvE-I-I) stabilizes the lone pair of electrons of the donor by mixing its orbital with the σ* anti-bonding orbital of I 2 . 18As a result a coordination bond forms and the double bond of the chalcogenone group as well as the iodine-iodine single bond are lengthened.A direct relationship was found to exist between these three bond lengths: if the E-I bond length is short, then the C-E bond length and I-I bond length will be long and the donor functionality is strong.Furthermore the ν I-I frequencies are linearly correlated with the I-I bond lengths.It is also worth mentioning that a zwitterionic structure is formed with the positive charge at the organic framework and the negative charge at the I 2 unit with the result that a lengthening of the CvE double bond is observed.Therefore, chalcogenamides are particularly good donors.Generally, the greater the stabilizing ability of the N-alkyl or -aryl groups, the stronger the resulting diiodine complex will be. 19e motivation for using L 1-3 vE (L 1 : = :C[N(2,6-iPr 2 -C 6 H 3 )-CH] 2 , L 2 : = :C(CH 2 )(CMe 2 )(C 6 H 10 )N-2,6-iPr 2 -C 6 H 3 , L 3 : = :C(CH 2 )-(CMe 2 ) 2 N-2,6-iPr 2 -C 6 H 3 ) compounds as precursors for the reaction with iodine was the possibility of forming an oxidized diiodide E(I) 2 instead of the non-oxidized E-I-I adduct.

Results and discussion
The N-heterocyclic carbene ligands L 1-3 : are treated with elemental chalcogens (S, Se, Te) to form carbon-chalcogen double bonds (Scheme 1).The rate of reaction of carbenes with chalcogens increases in the order Se < Te < S in accordance with the increase of the chalcogen solubility.All the prepared chalcogenamides can be extracted with hot hexane and recrystallized from THF. Chalcogenamides 1-4 are further treated with diiodine in a molar ratio of 1 : 1 in toluene at room temperature to give the diiodine adducts 5-8 (Scheme 2).The present research is devoted to the comparison of the structural data between diiodine adducts of thio-(1, 2), seleno-(3) and telluro-(4) amides.
The 13 C NMR measurements showed distinct differences in the chemical shifts between free carbenes L 1-3 : and chalcogenamides L 1-3 vE (E = S, Se, Te).The 'carbene' carbon resonance is shifted more than 50 ppm upfield on converting the ligand L to a respective chalcogenamide.For instance, in the 13 C NMR spectra, the difference in 'carbene' carbon resonance between L 1 : (220.6 ppm) and 1 (167 ppm) is 53.6 ppm, and between L 2 : (309.4 ppm) and 2 (213.6 ppm) is 95.8 ppm.Comparable chemical shifts are observed for the 'carbene' carbon atoms of compounds 3 (219.8ppm) and 4 (215.4ppm) while the corresponding resonance in L 3 : is 304.2 ppm.
The 1 H NMR chemical shift of the CH(CH 3 ) 2 proton in compound 5 is shifted upfield to 2.65 ppm when compared with that of 1 (2.75 ppm).In the case of compound 6, the chemical shift of the characteristic N-C(Me 2 )CH 2protons is shifted to low field (2.67 ppm) when compared with that of 2 (2.27 ppm).
The 'carbene' carbon atom in compound 7 shows the resonance at 231.7 ppm which is more than 10 ppm downfield compared with that of 3 (219.8ppm).However, the 77 Se NMR resonance for selenoamide 3 (492.43ppm) and the corresponding diiodine adduct 7 (491.26ppm) are nearly the same.Compound 8 has a very weak solubility in most of the organic solvents, and therefore the 13 C NMR spectrum as well as the 125 Te NMR spectrum were silent.
All the chalcogenamides are stable in open air.All the diiodine adducts of chalcogenamides are stable under a nitrogen atmosphere.
In order to establish unambiguously the structural features of compounds 5-8, single crystal X-ray structural analyses were carried out.Suitable single crystals of 5-8 were obtained from saturated toluene solutions.

Paper Dalton Transactions
Compound 7 crystallizes in the triclinic space group P1 ˉand compound 8 in the monoclinic space group P2 1 /n (Table 1).The molecular structure of compounds 7 and 8 are shown in Fig. 3 and 4, respectively.Notable differences in the corresponding bond lengths and angles in compounds From the above discussed structural details the bonding situation in compounds 5-8 can be explained as follows.The bond formation between chalcogenamide and iodine occurs through donation of electron density from the chalcogen atom to the iodine unit.This results in a partial positive charge at the chalcogen atom and some negative charge in the iodine

Dalton Transactions Paper
part (Scheme 2).The involvement of the nitrogen lone pair in a CvN double bond causes a positive charge on the nitrogen with partial compensation of the electron density of the chalcogen atom resulting in a final weak zwitterionic structure with the positive nitrogen atom and the negative I-I part.As a result of this electronic delocalization, the C-N and C-E bond lengths show a partial double bond character.The C-N bond lengths in compounds 5-8 are very close to the CvN double bond distance which indicates that the positive charge density is mainly located on the nitrogen atom.This is further evidenced by the almost planar geometry around the nitrogen atom in compounds 5-8.In 5, the C-N bond distance is 1.358(3) Å which is longer than the corresponding bond lengths in 6 (1.324(2) Å), 7 (1.315(3)Å) and 8 (1.305(3) Å).This is due to the two nitrogen atoms which are present in compound 5, and both of them participate in the charge delocalization whereas in the latter compounds only one nitrogen atom is present.

Experimental section
All manipulations were performed in a dry and oxygen free atmosphere (N 2 ) using standard Schlenk-line techniques and inside an MBraun MB 150-GI glove box maintained at or below 1 ppm of O 2 and H 2 O.All solvents were dried by an MBraun solvent purification system prior to use.N-Heterocyclic carbene L 1 : and cyclic-alkyl-amino carbene L 2,3 : were synthesized using reported procedures. 20,21Sulfur was sublimed before using, and grey selenium, tellurium and iodine were purchased and used as received.The 1 H, 13 C, 77 Se, and 127 Te NMR spectra were recorded on a Bruker Avance DRX instrument (300 or 500 MHz).The chemical shifts δ are given in ppm with tetramethylsilane as an external standard.Elemental analyses were performed at the Analytisches Labor des Instituts für Anorganische Chemie der Universität Göttingen.

Synthesis of L 1 vS (1)
Compound 1 was synthesized from the reaction of N-heterocyclic carbene L 1 : (

Synthesis of L 3 vTe (4)
Compound 4 was synthesized by the reaction of L 3 : (1.00 g, 3.5 mmol) in THF with tellurium (0.58 g, 4.55 mmol) at room temperature under a nitrogen atmosphere.The mixture was stirred for 2 days until the color of the solution turned dark.The solution was evaporated to dryness.The residue was extracted with boiling hexane (2 × 40 mL), filtered and dried to yield orange pure 4 (yield 0.90 g, 62% Synthesis of L 1 vS-I-I (5) For the synthesis of compound 5 a mixture of crystalline iodine (0.097 g, 0.38 mmol) and L1 vS (1) (0.16 g, 0.38 mmol) was dissolved in toluene (20 mL) at room temperature.The color of the solution turned dark immediately.The solution was stirred for 3 days and then filtered and the filtrate was evaporated to dryness.The residue was dissolved in toluene (10 mL) and stored at −32 °C for 3 days in a freezer to give brown crystals of 5 suitable for single crystal X-ray structural analysis (yield 0.25 g, 99%).Mp 180 °C (dec.

Crystal structure determination
Suitable single crystals for X-ray structural analysis of 5, 6, 7, and 8 were mounted at low temperature in inert oil under an argon atmosphere by applying the X-Temp2 device. 22The data were collected on a Bruker D8 three circle diffractometer equipped with a SMART APEX II CCD detector and an INCOA-TEC Ag microfocus source with INCOATEC Quazar mirror optics. 23The data were integrated with SAINT 24 and a semiempirical (for 5, 7) and an analytical (for 6, 8) absorption correction with SADABS 25 were applied.The structure was solved by direct methods (SHELXS-97) and refined against all data by full-matrix least-squares methods on F 2 (SHELXL-97). 26All non-hydrogen-atoms were refined with anisotropic displacement parameters.The hydrogen atoms were refined isotropically on calculated positions using a riding model with their U iso values constrained to 1.5U eq of their pivot atoms for terminal sp 3 carbon atoms and 1.2 times for all other carbon atoms.The disordered parts were refined using distance restraints and restraints for the anisotropic displacement parameters.

Conclusions
The synthesis and reactivity of compounds with heavier group 16 elements is a rapidly growing field in chemistry because of their extensive use in various applications.In the present study we show that an N-heterocyclic carbene L 1 : as well as cyclic-alkyl-amino carbenes L 2-3 : react with chalcogens to form air stable chalcogenamides 1-4.All the chalcogenamides regardless of the organic framework or chalcogen are able to form diiodine adducts with the general formula LvE-I-I.These experiments show the easy access to this class of compounds.Compounds 5-8 are well-characterized with various spectroscopic methods and single-crystal X-ray analyses.These compounds might prove to be promising candidates for the synthesis of iodine containing compounds, which are sensitive to elemental iodine.The formation of an oxidized E(I) 2 compound which we were aiming at was not observed due to the low oxidation potential of iodine.