Complexation of selenomethionine and its derivatives with some dimeric rhodium(II) tetracarboxylates: 1H and 13C nuclear magnetic resonance spectroscopy

Journal of Molecular Structure 1198 (2019) 126908

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Complexation of selenomethionine and its derivatives with some dimeric rhodium(II) tetracarboxylates: 1H and 13C nuclear magnetic resonance spectroscopy  ski* Rafał Głaszczka, Jarosław Ja zwin Institute of Organic Chemistry, Polish Academy of Sciences 01-224 Warszawa, Kasprzaka 44/52, Poland

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 April 2019 Received in revised form 22 July 2019 Accepted 6 August 2019 Available online 7 August 2019

Complexation of three dimeric rhodium(II) tetracarboxylates: tetraacetylate, tetrakistrifluoroacetylate and optically pure rhodium(II) Mosher's acid derivative, with selenomethionine, the methyl ester of selenomethionine and its N-phthaloyl and N-formyl derivatives, have been investigated by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy. Studies were performed using the NMR titration procedure, in D2O or CDCl3 solution depending on the solubility of substrates and ligands. In the presence of rhodium substrate, ligands formed subsequently 1:1 and 1:2 complexes, consisting of one dirhodium moiety and one or two ligand molecules, respectively. The parameter complexation shift Dd, defined as a difference between the chemical shift of a nucleus in a complex and corresponding chemical shift in the free ligand, varied from
0.3 to þ0.6 ppm (1H) and from
3.3 to þ5.4 ppm (13C). Complexation shift pattern indicated the complexation via the selenium atom. For chiral substrate and racemic ligands, diastereomeric dispersions Dn(1H) reached the value up to 140 Hz. © 2019 Elsevier B.V. All rights reserved.

Keywords: Rhodium(II) tetracarboxylate Selenomethionine Nuclear magnetic resonance Complexes Chiral recognition

Rhodium(II) dimeric compounds having a characteristic paddle wheel structure consist of two rhodium atoms and four carboxylic RCO2, thioacid (RCS2 or COS), amide (RCONH), or imide (RNCHNR) moieties [1]. One class of these species, rhodium tetracarboxylates, Rh2(RCO2)4 (Fig. 1), exhibits many useful properties. These compounds are stable in the air, and there is no need to use a protective atmosphere when working with them. Most of the organic ligands bind to axial sites of dirhodium substrates and form adducts in the solution (in situ). Complexed ligands can be easily exchanged or removed. Adducts are usually stable although some substrateligand combinations lead to further reactions and decomposition of the dirhodium core. Rhodium tetracarboxylates and their adducts are coloured and diamagnetic, so can be investigated in solution by the UV/Vis absorption spectroscopy and nuclear magnetic resonance (NMR) techniques. Two dirhodium compounds, tetraacetate Rh2(AcO)4 and tetrkistrifluoroacetate Rh2TFA4 (Fig. 1, R ¼ CH3, CF3) have been used in circular dichroism spectroscopy as auxiliary compounds, acting as a source of a chromophore (Vis range), reflecting

* Corresponding author. Institute of Organic Chemistry Polish Academy of Sciences ul. Kasprzaka 44/52 01-224 Warszawa, Poland.  ski). E-mail address: [email protected] (J. Ja zwin https://doi.org/10.1016/j.molstruc.2019.126908 0022-2860/© 2019 Elsevier B.V. All rights reserved.

configuration (R or S) of complexed chiral ligand [2e7]. Chiral, optically pure, rhodium(II) tetracarboxylates, deriving from amino acids or Mosher's acid (Rh2Mosh4), have been used as chiral recognition reagents to determine the optical purity of organic ligands by NMR [8e16], and, in some cases, to find the configuration (R or S) of ligands [17,18]. Some of the rhodium(II) tetracarboxylates have been used as catalysts in organic chemistry, including asymmetric synthesis [19e27]. The discovery about the anti-cancer effect of rhodium tetracarboxylates caused interest in research on the interaction of these substrates and related compounds with molecules of biological importance [28e35]. Investigations included nucleic bases, DNA and DNA fragments [36e43]. Polymer complexes of rhodium tetracarboxylates with bi- and multifunctional ligands, such as diamines, aromatic diazines and triazine, appeared to be another interesting research objects. Depending on ligands, polymer complexes consisted of two- or three-dimensional networks in the solid phase. These materials attract industry interest due to gas-occlusion properties and catalytic activities in olefins hydrogenation [44e51]. Previously, we have investigated the formation of complexes of Rh2(AcO)4, Rh2TFA4 and chiral, optically pure Rh2Mosh4 with aromatic heterocycles [52,53] and amines [54e57], in the liquid phase using 1H, 13C and 15N NMR spectroscopy. Measurements were

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 ski / Journal of Molecular Structure 1198 (2019) 126908 R. Głaszczka, J. Jazwin

Investigations had two purposes: examining which atom was involved in the formation of complex, and the effect of complexation on NMR chemical shifts.

1. Experimental section

Fig. 1. Dimeric rhodium(II) tetracarboxylates. Asterisks indicate axial sites of the substrate, able to bind ligand molecules. Acyl groups RCO2 are called equatorial substituents. Depending on the number of ligand molecules, rhodium tetracarboxylates form 1:1 and 1:2 adducts.

carried out employing titration technique, in D2O or CDCl3 as solvents of choice. Usually, the 1:1 and 1:2 complexes were formed in solution subsequently, depending on the reagents molar ratio. The complexation via nitrogen atom resulted in the change of 15N chemical shits by a few ppm for amines and from
40 to
70 ppm for aromatic heterocycles (dcomp. e dlig. < 0 ppm). The largest change of the corresponding 1H and 13C chemical shifts were observed for the nuclei in the vicinity of complexing sites. These trend has been supported by quantum-chemical calculations of NMR parameters [58,59]. In the case of oxazole, thiazole, and ester derivatives of aziridine, dirhodium substrates were bound by a nitrogen atom, although oxygen ligands also can form complexes [9,12,13,60,61]. Ligands having a rigid structure and two or three equivalent nitrogen complexation sites (diazines, triazine, 3,30 - and 4,40 -bipyridines) tended to produce polymer materials insoluble in common organic solvents [62e64]. Cysteine and methionine as well as their Nformyl and N-phthaloyl derivatives bound dirhodium substrates by sulphur atoms. However, NMR experiments provided inconclusive results in the case of N,N-dimethyl derivatives of these ligands, probably due to the presence of a mixture of species in solution [65e67]. In the present work, we report on the formation of complexes of other compounds of biological importance, selenomethionine and its derivatives (Fig. 2). Selenium is an essential trace element in living organisms, as well as corresponding selenium-containing amino acids and proteins [68]. Only in 2019, about 20 works on the role and importance of selenomethionine in biology appeared. One of these work concerned the interaction of selenoproteins with Cu and Fe cations [69]. Our work follows this direction of research.

Seleno-L-methionine ((S)-amino-4-(methylseleno) butyric acid) as well as its racemic analogue, commercially available, were purchased. Derivatives of selenomethionine were obtained from amino acid according to literature procedures. The methyl ester was obtained by the reaction of the amino acid with 2,2dimethoxypropane in the presence of hydrochloric acid [70]. Ester was converted either to N-formyl derivative by the reaction with HCO2NH2, and to N-phthaloyl derivative by the reaction with phthalic anhydride [71,72]. Rhodium tetracarboxylates were obtained from commercially available rhodium(II) tetraacetate Rh2(AcO)4 by the conversion to rhodium carbonate and then by the reaction with a corresponding carboxylic acid, trifluoroacetic acid or (R)-Mosher's acid, Ph(CF3)(OCH3)CCO2H [73,74]. All compounds were identified by nuclear magnetic resonance spectroscopy. All NMR experiments were performed on Varian VNMRS 600 and Bruker DRX 500 spectrometers, using 5 mm broadband probe equipped with the z-gradient coil. The measurements were carried out in D2O or CDCl3 solution applying NMR titration technique. Portions of ligand solution were added to the solution of ca. 5e10 mg of dirhodium substrate in NMR tube, to obtain substrate to ligand molar ratios of 1: 0.5, 1:1, 1:1.5, 1:2, and 1: 2.5. The signals of 13 15 C, N and 77Se nuclei, due to diluted samples, were registered by inverse 2D techniques (1H,13C HSQC and 1H,xxX HMBC, where xx X ¼ 13C, 15N and 77Se). Complexes were characterized by the parameter complexation shift (Dd, in ppm) being the difference between the chemical shift of a signal in a complex and corresponding chemical shift in a free ligand. Selected Dd parameters were collected in Table 1. Diastereomeric dispersion Dn (customarily given in Hz, [8]) was estimated as a difference between signals of two diastereomers, using the spectra acquired on a 600 MHz spectrometer. The 1H and 13 C NMR chemical shifts in the spectra acquired in D2O were given with reference to the 1H and 13C signals of the SiCH3 group of sodium 3-(trimethylsilyl)propanoate-d4 (0 ppm). The 1H and 13C NMR spectra acquired in CDCl3 were referenced using TMS signals (0 ppm for d(1H) and d(13C) signals). In the case of 15N and 77Se NMR, the “spectral reference” method was used. Initially, the spectra of reference compounds (nitromethane, CH3NO2, d(15N) ¼ 0 ppm, or 1,2-diphenyldiselane, PhSeSePh, d(77Se) ¼ 461 ppm with respect to CH3SeCH3 signal [75]) were acquired, and the frequencies of 0 ppm were read off. Then, these values were transferred onto the spectra of samples in question acquired at the same experimental conditions. All NMR data, as well as examples of the spectra, were collected as Supplementary Materials.

Fig. 2. Selenomethionine and its derivatives investigated in the present work.

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Table 1 1 H and13C NMR complexation shifts Dd (ppm) for complexes of some rhodium tetracarboxylates with 1, 1.HCl, 2.HCl, 3 and 4a. Complex

SeCH3

SeCH2

SeCH2CH2

CH

1, Rh2(AcO)4 298 K, D2O 1, Rh2(AcO)4 333 K, D2O 1.HCl, Rh2(AcO)4 298 K, D2O 2.HCl, Rh2(AcO)4 298 K, D2O 3, Rh2(AcO)4b 298 K, CDCl3 3, Rh2(AcO)4b 228 K, CDCl3 3, Rh2TFA4b 228 K, CDCl3 3, Rh2Mosh4 228 K, CDCl3 4, Rh2(AcO)4b 228 K, CDCl3 4, Rh2TFA4 228 K, CDCl3 4, Rh2Mosh4 298 K, CDCl3

0.36

0.42

0.19, 0.25

0.08

0.65 [1.8]

0.72 [2.0]

0.55, 0.60 [-0.8]

0.47 [0.1]

[3.5] (C]O)

0.34 [3.0]

0.39 [3.5]

0.19, 0.17 [-0.6]


0.08 [0.9]

[1.0] (C]O)

0.23

0.27

0.09, 0.07


0.06


0.15 (CH3)

0.38

0.40

0.36, 0.32


0.01

0.01 (CH3)

0.41 [3.5]

0.48, 0.36 [3.5]

0.42, 0.32, [-0.4]

0.12 [0.9]

0.01 [0.3] (CH3) [0.1] (C]O)

0.02 [0.1]

0.01 [0.7] (CH3) [-0.2] (C]O)

c

CO2H (CO2CH3)

0.46 [4.1]

0.3e0.4 [1.5]

0.45 [4.8]

0.55, 0.52 [5.4]

0.48, 0.38 [-1.0]


0.09 [0.0]

0.01 [-0.1] (CH3) [0.1] (C]O)

0.37 [4.0]

0.52 [3.8]

0.27, 0.06 [-0.1]


0.10 [0.0]


0.10 [0.1] (CH3) [-0.4] (C]O)

0.41 [3.4]

0.47 [4.6]

0.15, 0.00 [-3.3]

0.02 [-1.5]


0.10 [-0.4] (CH3) [-0.1] (C]O)

0.38

0.58

0.35, 0.37

0.23


0.19 (CH3)

a Complexation shifts were given as a difference (ppm) between the chemical shift of a signal in a complex and corresponding chemical shift in a free ligand (ppm);13C NMR data, if available, were given in square brackets. All samples contained 1:0.5 substrate to ligand molar ratios unless not marked otherwise. Dd parameters of N-protective groups, NPht and NHCHO, were enclosed as Supplementary Materials. b The 1:1 substrate to ligand molar ratio. c Signals overlapped.

2. Result and discussion A solvent for the study of complexation should not complex rhodium tetracarboxylates and compete with ligands. Thus, solvents commonly used in NMR spectroscopy such as acetonitrile-d3, DMSO‑d6 or acetone-d6 should be avoided. Two chlorinated solvents were suitable for our purposes, dichloromethane-d2 and chloroform-d. The first was difficult to handle (volatility) and more expensive. Therefore, chloroform appeared to be a good solvent of choice and has been used in many previous investigations [2e16,52e61]. However, due to the insolubility of some ligands in chloroform, several NMR studies of rhodium complexes were performed in water-d2 or methanol-d4. The use of water or methanol as the solvent was a kind of compromise because solvent molecules formed complexes with Rh2(AcO)4 and Rh2TFA4. As a consequence, dirhodium substrates in solution, in the absence of other ligands, were anticipated to form 1:2 adducts containing two solvent molecules. A 1:1 complex with a ligand L, in turn, was expected to contain both solvent and ligand moieties. In the case of our investigations, we supposed that the complexation via the selenium atom would be more effective than via oxygen atoms, analogously to methionine and cysteine derivatives where complexation took place via S atoms [65,66]. Due to the low solubility of some substrates, incomplete solubility of complexes and the precipitation of species, the molar ratio of substrate to a ligand in solution often did not correspond to the amount of substance added to the sample. It is a case of Rh2(AcO)4, which itself is poorly soluble in CDCl3. However, the presence of a ligand in solution increases the solubility of substrate: the concentration of Rh2(AcO)4 is governed by complex formation [55]. The combination of Rh2(AcO)4 and 3 can serve as an example. Signal integration in the spectrum of the 1:0.5 sample revealed the 1:1.4 M ratio of substrate to the ligand, instead of the anticipated 1:0.5 proportion. Such inaccuracy was not observed in the case of 1:2 sample where the proportion of added reagents agreed with reagents ratio arising from signal integration. These divergences usually did not occur if substrates and ligands are soluble in the

solvent used. Measurements at reduced temperatures allowed to observe signals of all species in solutions. However, such experiments suffered from broad signals caused by dynamic processes in samples, such as ligand exchange and variation of ligand conformation, medium-fast on the NMR timescale (complexes of cysteine and methionine derivatives behaved similarly [65e67]). Broad and overlapped signals in 1H NMR spectra often made interpretation very difficult. In such a case, 1H,13C HSQC 2D spectra appeared to be very helpful, but 2D experiments often provided incomplete data. The HSQC sequence comprises two INEPT transfer, from 1H to 13C and then from 13C to 1H [76]. Exchange of ligands and broad signals make transfer ineffective. Besides, broad signals are characterized by low intensity and an unfavourable signal-to-noise ratio. In contrast, the spectra acquired at increased temperatures showed narrow signals, but due to fast ligand exchange in such conditions, information about individual species in solution was lost. Moreover, some samples decomposed rapidly at increased temperatures and excluded long 2D experiments (cf Fig. 3). 2.1. Complexation of 1, 1∙ ∙HCl and 2∙ ∙HCl in D2O Due to the low solubility of 1, 1∙ ∙HCl and 2∙ ∙HCl in chloroform, their complexes have been studied in D2O. Two dirhodium tetracarboxylates, Rh2(AcO)4 and Rh2TFA4, were used as substrates; the third one, Rh2Mosh4, was hydrophobic and insoluble in water. After adding the ligand to the Rh2(AcO)4 solution, the green mixture adopted purple or pale purple colour, depending on the substrate to ligand ratio. The 1H NMR signals were broad, usually without subtle structures. The spectra acquired at the increased temperature showed narrow signals, but the samples decomposed rapidly, within 2 h (Fig. 3). One set of signals in spectra and lack of the signals of individual species pointed to the rapid ligand exchange. However, broad signals suggest the exchange rate just above the coalescent point. Complexation shifts Dd did not decrease during the titration (up to the 1:2 samples), due to continuous consuming of ligands by the

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solubility of Rh2(AcO)4 in chloroform is poor, but its complexes with organic ligands are usually soluble. For this reason, studies on complex formation have been carried out in CDCl3 solutions. Solutions of 3 and 4 in the presence of Rh2(AcO)4 adopted a pale purple or purple colour depending on the substrate to ligand molar ratios. The same ligands with Rh2TFA4 produced pale orange or orange solutions. In a case of Rh2Mosh4, the solutions turned from blue-green (1:0.5 mixture) via purple to red (1:2 mixture). The spectra measured at 298 K (room temperature) consisted of narrow signals, in contrast to the spectra acquired at 228 K, revealing broad signals only. However, reduced temperatures of measurements gave better inside into complexation process, enabling to observe the signals of individual species. Complexation shift Dd(1H) of 3 and 4 in the presence of Rh2(AcO)4 or Rh2TFA4 did not exceed 0.5 ppm, whereas Dd(13C) varied from þ4.6 to e 3.3 ppm. Maximum positive Dd(1H) and Dd(13C) values were noted for CH3 and CH2 groups, in the vicinity of the selenium atom; Dd(13C) for the remaining CH2 group adopted the values less than zero. Thus, complexes of 3 and 4 exhibited Dd(13C) patterns similar to those observed for methionine derivatives [65], i.e. positive values for the H3CSeCH2 and negative for Fig. 3. The 1H NMR spectra of 1 in the absence (a) and the presence (b) of Rh2(AcO)4 (D2O, 1:0.5 substrate to ligand molar ratio). The presence of substrate caused the change of signal positions. At 298 K, signals are broad (b), whereas increasing the temperature narrows the signals (c). However, at 333 K, the sample decomposed rapidly (d).

substrate. Such behaviour indicated indirectly the formation of two complexes, 1:1 and 1:2. Two parameters, Dd(1H) and Dd(13C), adopted the values from
0.08e0.72 ppm and from
0.8e3.5 ppm, respectively. The maximum positive Dd(1H) and Dd(13C) values were noted for SeCH3 and SeCH2 groups (Fig. 4), whereas Dd(13C) of the SeCH2CH2 carbon atom adopted a negative value of
0.8 ppm. Similar Dd(13C) patterns have been observed for Rh2(AcO)4 complexes of methionine and methionine hydrochloride [65], where the sulphur atom acted as the binding site. The above data pointed to the selenium atom as the centre of complexation. However, large Dd(13C), of 3.5 ppm, observed for carbonyl group in the complex of 1, was intriguing. Possibly, this value indicated the interactions of Rh2(AcO)4 with the C]O group. Unfortunately, we could not acquire complete 13C data for complex of 2.HCl, but Dd(1H) pattern also indicated the complexation via the selenium atom. All ligands, 1, 1∙ ∙HCl and 2∙ ∙HCl, with Rh2TFA4, produced insoluble brown or orange amorphous material. Only traces of signals, of non-complexed ligands, were detected in solutions. Presumably, this material had a polymeric structure where complexation took place via both selenium and nitrogen or oxygen atoms.

SeCH2CH2 carbon atoms. Such features pointed to the complexation via the selenium atom. The Dd parameters remained practically unchanged regardless of the substrate to ligand molar ratio, indicating the subsequent formation of 1:1 and 1:2 complexes. The signal of non-complexed ligand appeared in 1H NMR spectra of 1:1.5 substrate and ligand mixture. An intriguing phenomenon has been observed for 1:0.5 mixture of Rh2(AcO)4 and 3 (Fig. 5). Corresponding 1H NMR spectrum slightly, but noticeably differed from the spectra of remaining samples, having higher ligand to substrate ratio. The 1H NMR spectrum of 1:0.5 mixture of Rh2(AcO)4 and 3 revealed two broad CH3 signals of the substrate. Because Rh2(AcO)4 is poorly soluble in chloroform and its 1H peak is practically undetectable in the absence of ligands, these signals must come from a complex different than 1:1 one. We assigned this broad signal to the hypothetical 2:1 complex [3-(Rh2(AcO)4)2], having one dirhodium molecule attached to the selenium atom and the second one either to the nitrogen or C]O oxygen atom. This 2:1 adduct was expected to appear at the beginning of the titration, and then to transform to 1:1 and to 1:2 complexes as the ligand was added. A similar phenomenon has been observed in the case of Rh2TFA4 and 3, where the chemical shifts of some signals in the spectrum of the 1:0.5 mixture deviated from those observed for other samples. Effects were subtle but observable in the stack plot (Fig. 6). 2.3. Complexation of 3 and 4 with Rh2Mosh4 in CDCl3

2.2. Complexation of 3 and 4 with Rh2(AcO)4 and Rh2TFA4 in CDCl3 Ligands 3 and 4, as well as two substrates, Rh2TFA4 and Rh2Mosh4, were soluble in CDCl3. As it was mentioned above, the

The third substrate, optically pure Rh2Mosh4 has been previously used as a chiral discrimination agent, enabling to estimate optical purity of complexed ligand [8e16]. The combination of

Fig. 4. Complexation shifts Dd(1H) and [Dd(13C)] (ppm) of 1 (a) and 1·HCl (b) in the presence of Rh2(AcO)4, 1:0.5 substrate and ligand mixture in D2O, at 333 K and 298 K, respectively.

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Fig. 5. (a) The 1H,13C HSQC spectrum of 3 in the presence of Rh2(AcO)4 (1:1 mixture, 228 K, CDCl3). (b) NMR titration of Rh2(AcO)4 with 3 (CDCl3, 228 K). (left) CH signals of 3, the signal of non-complexed ligand (*) appeared in the 1:2 sample. (right) The CH3 signal of the substrate. The spectra of 1:0.5 and 1:1 mixtures are different. Broad signal (#) was expected to arise from the hypothetical 2:1 complex [3-(Rh2(AcO)4)2].

Fig. 6. (a) The 1H,13C HSQC spectrum of 3 in the presence of Rh2TFA4 (1:1 mixture, 228 K, CDCl3). (b) NMR titration of Rh2TFA4 with 3 (CDCl3, 228 K); CH and OCH3 signals of the ligand were shown. The signal of non-complexed ligand (*) appeared in the 1:1.5 sample. The chemical shift of OCH3 in the spectra of 1:0.5 and 1:1 samples differed by 14 Hz. Inconsistency was explained by the presence of 2:1 complex [3-(Rh2TFA4)2].

Rh2Mosh4 and optically pure ligand yielded one 1:1 and one 1:2 adducts, whereas racemic ligand produced two 1:1 and three 1:2 complexes, namely (S,4R,S), (R,4R,R) and (R,4R,S) [56,57]. These 1:2 complexes contained, in total, four ligand molecules recognizable by NMR in the liquid phase. We expected that Rh2Mosh4 would show the same action for 3 and 4. The spectra of 3 and 4 in the presence of Rh2Mosh4 have been acquired at 228 K. As it was mentioned, the use of low temperatures

resulted in signal broadening, but gave better insight into complexation process and enabled to observe the signals of individual species in spectra (Fig. 7). Broad signals caused difficulties in spectra registration and analysis, especially in the case of 2D NMR inverse techniques. This phenomenon was particularly troublesome for the spectra of Rh2Mosh4 complexes, acquired at low temperatures. Certainly, signal broadening was caused by mediumfast ligand exchange in solution, but another factor was expected to

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Fig. 7. (a) (from the bottom) Part of the spectrum of Rh2Mosh4 and 3 mixture (1:0.5) acquired at room temperature; the same spectrum recorded at 228 K, and the spectrum of the 1:2.5 Rh2Mosh4 and 3 mixture, at 228 K. The signals marked with frames were shown in the stack plot. (b) Titration of Rh2Mosh4 with 3, at 228 K. Asterisks (*) denote the signals of free ligand, hash (#) refers to the signal of the free substrate.

be more important. It was hindered rotation of bulky groups within the molecule, such as a ligand and R groups of Mosher's acid residues. In such a case, the solution was expected to contain a mixture of rotamers being in equilibrium. The signals of methyl groups of substrate and ligand appeared to be the most diagnostic. At the beginning of titration, the 1H NMR spectrum of 1:0.5 mixture of Rh2Mosh4 and 3 showed three OCH3 signals of the substrate, a singlet coming from the non-complexed Rh2Mosh4 and two signals of two 1:1 diastereomers revealing Dn of 14 Hz (Fig. 7). The 1H signal of CO2CH3 of the ligand was also split (Dn of 10.5 Hz), as well as the signal of SeCH3 (Dn of 2 Hz). Interestingly, the spectra of complexes of racemic ligand and the ligand expected to be optically pure did not differ. Such a result indicated that the transformation of optically pure selenomethionine to 3 yielded racemic compound, despite the mild condition of reactions. In the course of titration, the OCH3 signal of free Rh2Mosh4 and signals of the 1:1 complex gradually vanished, instead of the signals

of the 1:2 complex appeared. The spectrum of 1:2 mixture revealed the signals of the 1:2 complex only, whereas the spectrum of 1:2.5 sample contained also the signals of non-complexed 3. The 1H NMR spectrum of the 1:2 complex showed OCH3 signals of substrate and ligand as broad singlets (half-width of ca. 7 Hz) without subtle structure. In contrast, CH and SeCH3 signals were broad (linewidth of ca. 35 Hz) and split, revealing Dn of 138 and 50 Hz, respectively. What draws attention is the large difference between the CH and SeCH3 signals in the spectra of 1:1 and 1:2 complexes. The spectra of 4 in the presence of Rh2Mosh4, acquired at 228 K, contained a few diagnostic signals only. As above, the 1H NMR spectrum of 1:0.5 mixture of Rh2Mosh4 and 4 revealed the signals of the complexed and free substrate. The signal of the noncomplexed substrate disappeared after adding the next portion of 4. The 1H signal of CH hydrogen revealed the subsequent formation of 1:1 and 1:2 complexes (Fig. 8a). The CH signal splitting (Dn of 72 Hz, at 228 K) occurred in the spectra of both complexes, of

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Fig. 8. Parts of 1H NMR spectra of 4 in the presence of Rh2Mosh4 (titration experiments). (a) The CH signal in the spectra acquired at 228 K. The signal of the 1:1 complex in the 1: 0.5 sample (the first trace from the bottom) is split due to the presence of two diastereomers. (b) Selected 1H NMR signals (CHO, NH and CH groups) of 4 in the presence of Rh2Mosh4 (titration experiment, 298 K, CDCl3).

racemic ligand, and the ligand expected to be optically pure. Therefore, similarly to 3, the transformation of selenomethionine to derivative 4 yielded the racemic product. The 1H NMR spectra of 4 in the presence of Rh2Mosh4, acquired at the room temperature (298 K), contained one set of signals only, due to the fast exchange of species on the NMR timescale. Some signals exhibited interesting Dd(1H) trend. Depending on the substrate to ligand ratio in mixtures, 1:0.5, 1:1, 1:1, and 1:2, Dd(1H) parameter adopted the values of 0.23,
0.01,
0.09, and
0.11 ppm for CH, 0.32,
0.07,
0.18, and
0.19 ppm for NH, and 0.07,
0.17,
0.24,
0.25 ppm for CHO signals, respectively. The change of sign of Dd(1H) parameter at the beginning of titration, 1:0.5 vs. 1:1 mixtures, was noteworthy. This phenomenon suggested that Dd values depend not only on the formation of a rhodium-selenium heteroatom bond but also on other factors (cf. complexes of Rh2(AcO)4 and Rh2TFA4 with 3).

Rh2TFA4 produced insoluble material with unknown stoichiometry. N-phthaloyl 3 and N-formyl 4 derivatives of selenomethionine, insoluble in D2O, formed complexes with Rh2(AcO)4, Rh2TFA4 and Rh2Mosh4 in CDCl3 solution. Typically, the 1:1 and 1:2 complexes were formed subsequently, depending on reagent molar ratios. Some phenomena observed in the spectra of mixtures containing a ligand and an excess of substrate suggested the presence of 2:1 complex which was composed of two substrate and one ligand molecules. Supposedly, in these complexes, substrate molecules were bonded via selenium and oxygen atoms of the ligand. The change of NMR chemical shifts due to complexation (Dd parameter) varied from
0.3 to þ0.6 ppm for 1H and from
3.3 to þ5.4 ppm for 13 C NMR signals. Complexation shift pattern, i.e. maximum positive Dd values of 1H and 13C nuclei in the CH3SeCH2 fragment indicated the complexation via the selenium atom. Acknowledgements

2.4.

15

N and

77

Se NMR measurements

Broad signals in spectra, diluted samples and unfavourable natural abundance of nuclei of interest provided difficulties in NMR measurements, and the measurement by direct techniques of 15N and 77Se nuclei was unsuccessful. The measurements by 2D inverse techniques often resulted in the fragmentary and ambiguous data, not suitable for thorough analysis. Practically, unambiguous 15N and 77Se NMR chemical shifts were collected for free ligands only. The 15N NMR signals appeared within the range from
343.6 to e 341.3 ppm (1, 1·HCl, 2·HCl) and at
219.9 and
260.2 ppm for 3 and 4, respectively. The 77Se NMR spectra revealed the signals within the range from 65.4 to 87.3 ppm. Selenium chemical shifts depended on the temperature of measurements. For instance, the signal of the selenium atom of 3 appeared at 75.5 ppm (298 K) and 65.4 ppm (228 K). 3. Conclusions Selenomethionine 1, its hydrochloride 1∙ ∙HCl as well as hydrochloride of the methyl ester of selenomethionine 2∙ ∙HCl, in D2O solution, formed complexes with rhodium tetraacetate Rh2(AcO)4. Above ligands in the presence of rhodium tetrakistrifluoroacetate

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