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Published in final edited form as: J Organomet Chem. 2021 Jan 27;936:121718. doi: 10.1016/j.jorganchem.2021.121718

Microwave-Assisted Synthesis of Organometallic Rhenium (I) Pentylcarbonato Complexes: New Synthon for Carboxylato, Sulfonato and Chlorido Complexes

Angela J Winstead †, Khayra Alabrash †, Brent V Powell †, Sabreea J Parnell †, Tiara V Hinton †, Tijesunimi Odebode †, Jiangnan Peng †, Jeanette A Krause ‡, Peter Y Zavalij §, Santosh K Mandal †
PMCID: PMC8095682  NIHMSID: NIHMS1668467  PMID: 33953436

Abstract

Tricarbonylrhenium(I)(α-diimine) complexes are of importance because of their strong cytotoxic and fluorescence properties. Syntheses of such complexes were achieved through a two-step process. First, the pentylcarbonato complexes, fac-(CO)3(α-diimine)ReOC(O)OC5H11 were synthesized through a microwave-assisted reaction of Re2(CO)10, α-diimine, 1-pentanol and CO2 in a few hours. Second, the pentylcarbonato complexes are treated with carboxylic, sulfonic and halo acids to obtain the corresponding carboxylato, sulfonato and halido complexes. This is the first example of conversion of Re2(CO)10 into a rhenium carbonyl complex through microwave-assisted reaction.

Keywords: Rhenium decacarbonyl, α-diimines, microwave-assisted, pentyl carbonate, carboxylate, sulfonate, chloride

Graphical abstract

graphic file with name nihms-1668467-f0008.jpg

1. Introduction

The carboxylato and sulfonato derivatives of rhenium(I) tricarbonyl complexes containing α-diimine ligands (N~N) exhibit exceptionally strong cytotoxic properties. The low IC50 (concentration of the compound required for 50% inhibition) values of some of these complexes have been compiled in a recent review article [1]. Almost invariably all rhenium(I) tricarbonyl complexes are obtained from the starting material, rhenium pentacarbonyl chloride, Re(CO)5Cl, possibly due to its ready availability from commercial sources. Customarily, carboxylato rhenium(I) tricarbonyl complexes, Fac-(CO)3(N~N)ReOC(O)R, are obtained from the reaction of the corresponding chlorido complexes, A, with silver triflate as shown in Scheme 1 [2].

Scheme 1.

Scheme 1.

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Conventional synthesis of carboxylato complexes

This method produces undesirable heavy metal salt (AgCl). We took a slightly different but simple synthetic route to carboxylato and sulfonato complexes. The tricarbonyl rhenium(I) pentylcarbonato complexes, Fac-(CO)3(N~N)ReOC(O)OC5H11, a new class of synthon, can be obtained from the treatment of the parent Re2(CO)10 and N~N in the presence of CO2 in 1-pentanol [3]. Subsequent treatment of the tricarbonyl rhenium(I) pentylcarbonato complexes with the corresponding carboxylic and sulfonic acids yield the carboxylato and sulfonato complexes quantitatively as shown in Scheme 2 [4]. The reactions of Re2(CO)10, N~N and CO2 in 1-pentanol afforded the pentylcarbonato complexes over a period of 24–36 h [3]. We observed that the reactions were very slow in low boiling alcohols. Previously we reported that it took more than two weeks to prepare a rhenium-diphosphine propylcarbonato complex from the reaction of Re2(CO)10 and a chelated diphosphine in 1-propanol [5].

Scheme 2.

Scheme 2.

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Synthesis of pentylcarbonato, carboxylato, and sulfonato complexes

Microwave-assisted synthesis of organic compounds has been a routine technique in industry and academia [6]. It is also being used in the synthesis of inorganic and organometallic compounds [7], nanoparticles [8], etc. It shortens the reaction time abruptly. Many organometallic rhenium complexes have been synthesized using microwave-assisted synthesis [9]. To the best of our knowledge, synthesis of any organometallic rhenium complex from parent Re2(CO)10 using microwave-assisted synthesis has not been reported to date. Herein, we report the microwave-assisted synthesis of three tricarbonyl rhenium(I) pentylcarbonato complexes (1–3) directly from the reactions of a mixture of Re2(CO)10, α-diimines, CO2 and 1-pentanol (Scheme 3).

Scheme 3.

Scheme 3.

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Microwave assisted synthesis of pentylcarbonato complexes, 1–3

Additionally, we report the synthesis of 2-pyridinesulfonato (4), chlorido (5) and acetylsalicylato (aka, aspirinato) (6) derivatives from the new synthons, 1–3 (Scheme 4).

Scheme 4.

Scheme 4.

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Synthesis of sulfonato, chlorido and acetylsalicylato complexes, 4–6

2. Experimental Section

Synthesis of 1–3. Re2(CO)10 (0.050 g, 0.076 mmol), 4-methyl-1,10-phenanthroline (0.0382 g, 0.16 mmol) or 4,7-dimethyl-1,10-phenanthroline (0.033 g, 0.16 mmol) or 3,4,7,8-tetramethyl-1,10-phenanthroline (0.038 g, 0.16 mmol), 4.5 mL of 1-pentanol, and a magnetic stirring bar were placed into a CEM microwave vial. The vial was loaded with CO2, capped, and the contents were stirred for 30 minutes. It was then microwaved into the CEM microwave oven for 3 hours and 30 minutes at 160°C. After the reaction, the vial was cooled in the freezer for several hours. The contents of the vial were transferred into a 100mL round-bottomed flask containing 50mL of hexanes. It was then stirred in the presence of CO2 gas for several hours and cooled in the freezer. Filtration afforded a yellow solid which was washed with hexanes to remove the residual 1-pentanol. The filtrate was kept aside for further examination. The yields range from 68–80%. Characterization of 1: FT-IR (cm−1, CH2Cl2) ʋ(C≡O) 2023(vs), 1918(s) and 1894(s), and ʋ(C=O) 1660(m). 1H NMR (400 MHz, CD2Cl2) δ 9.31 (dd, J = 5.1, 1.5 Hz, 1H), 9.15 (d, J = 5.2 Hz, 1H), 8.48 (dd, J = 8.3, 1.5 Hz, 1H), 8.06 (d, J = 9.0 Hz, 1H), 7.93 (d, J = 9.1 Hz, 1H), 7.75 (dd, J = 8.2, 5.1 Hz, 1H), 7.60 (dt, J = 5.3, 0.9 Hz, 1H), 3.44 (t, J = 6.9 Hz, 2H), 2.79 (d, J = 0.9 Hz, 3H), 1.23 – 1.11 (m, 2H), 1.09 – 0.89 (m, 4H), 0.65 (t, J = 7.2 Hz, 3H). 13C NMR (101 MHz, DMSO-d6) δ196.43, 191.90, 157.90, 154.58, 153.77, 150.51, 146.51, 146.01, 140.08, 130.58, 127.87, 127.41, 126.87, 125.08, 124.65, 65.31, 28.75, 27.91, 22.20, 19.24, 14.28. Anal. Calcd for C22H21N2O6Re.0.1 CH2Cl2 (%): C, 43.93; H,3.53; N, 4.63. Found: C, 43.67; H, 3.49; N, 4.72. Characterization of 2: FT-IR (cm−1, CH2Cl2) ʋ(C≡O) 2022(vs), 1916(s) and 1892(s), and ʋ(C=O) 1660 (m). 1H NMR (400 MHz, CD2Cl2) δ 9.27 (d, J = 5.3 Hz, 2H), 8.20 (s, 2H), 7.70 (dd, J = 4.5, 0.8 Hz, 2H), 3.56 (t, J = 4.3 Hz, 2H), 2.91 (s, 3H), 1.31–1.26 (m, 2H), 1.17–1.04 (m, 4H), 0.78 (t, J = 7.3 Hz, 3H). 13C NMR (101 MHz, CD2Cl2) δ 198.22, 194.24, 158.69, 153.14, 149.05, 146.91, 130.17, 126.34, 123.68, 65.86, 28.91, 28.12, 22.43, 19.08, 13.79. Anal. Calcd for C23H23N2O6Re (%): C, 45.31; H,3.80; N, 4.60. Found: C, 45.15; H, 3.68; N, 4.45. Characterization of 3: FT-IR (cm−1, CH2Cl2) ʋ(C≡O) 2021(vs), 1915(s) and 1890(s), and ʋ(C=O) 1660. 1H NMR (400 MHz, CD2Cl2) δ 9.19 (s, 2H), 8.21 (s, 2H), 3.64 (t, J = 7.1 Hz, 2H), 2.82 (s, 6H), 2.66 (s, 6H), 1.37–1.33 (m, 2H), 1.25 – 1.11 (m, 4H), 0.84 (t, J = 6.9 Hz, 3H). 13C NMR (101 MHz, CD2Cl2) δ 198.31, 194.48, 158.56, 154.02, 146.43, 145.61, 134.30, 129.06, 123.38, 65.83, 28.90, 28.09, 22.40, 17.60, 14.91, 13.75. Anal. Calcd for C25H27N2O6Re (%): C, 47.09; H,4.27; N, 4.39. Found: C, 46.84; H, 4.16; N, 4.37.

Evaporation of a small amount of the filtrate yielded a yellow residue that might contain pentyl hydrogen carbonate. Because ethyl hydrogen carbonate is a solid, it is likely that pentyl hydrogen carbonate is solid. The IR spectrum of ethyl hydrogen carbonate shows a ʋ(C=O) at 1730 cm−1 [10]. The IR spectrum of the yellow solid residue in dichloromethane did not show any ʋ(C=O) in 1700–1800 cm−1 region. Likewise, the IR spectra of 1–3 lack ʋ(C=O)’s in the same region. Therefore, the microwave heating of 1-pentanol with carbon dioxide might not produce any pentyl hydrogen carbonate.

Synthesis of 4–6. A mixture of 0.1000 g (1.68 × 10−4 mol) of 1, 0.0267 g (1.68 × 10−4 mol) of pyridine-2-sulfonic acid or 0.1000 g (1.64 × 10−4 mol) of 2, 0.0600 g (136.5 μL, 1.64 × 10−3 mol) of concentrated hydrochloric acid (37%, density = 1.185 g/mL) or 0.1000 g (1.57 × 10−4 mol) of 3, 0.0283 g (1.57 × 10−4 mol) of acetylsalicylic acid and 10 mL of dichloromethane were stirred for about six (6) hours in a 50 mL round bottom flask. The reactions were monitored with infrared spectroscopy. We did not detect any pentyl hydrogen carbonate in the dichloromethane solutions during the reactions of 1–3 with the corresponding acids. The IR spectrum of pentyl hydrogen carbonate is expected to exhibit a ʋ(C=O) in 1700–1800 cm−1 region [10]. When the reaction was complete, the solution was concentrated to about 2 mL on a rotary evaporator. It was then carefully mixed with hexane so that no precipitation occurred. The mixture was cooled to −5° C for a day or two. The pale-yellow crystals were obtained through filtration. The yields of 4–6 range from 90 to 100%. Characterization of 4: FT-IR (cm−1, CH2Cl2) ʋ(C≡O) 2030(vs), 1926(s) and 1905(s). 1H NMR (400 MHz, CD2Cl2) δ 9.36 (dd, J = 8, 4 Hz, 1H), 9.21 (d, J = 4 Hz, 1H), 8.63 (d, J = 8 Hz, 1H), 8.34 (m, 1H), 8.20 (d, J = 4 Hz, 1H), 8.07 (d, J = 4Hz, 1H), 7.89 (dd, J = 8, 4 Hz, 1H), 7.73 (d, J = 4 Hz, 1H), 7.68 (m, 2H), 7.26 (m, 1H), 2.93 (s, 3H). 13C NMR (101 MHz, CD2Cl2) δ 159.81, 154.48, 153.81, 150.18, 149.52, 147.95, 147.28, 139.42, 137.52, 130.97, 130.90, 127.68, 127.25, 126.27, 125.45, 124.73, 121.99, 19.71. Anal. Calcd for C21H14N3O6ReS. 0.1 C6H14 (%): C, 41.10; H, 2.46; N, 6.65. Found: C, 41.17; H, 2.55; N, 6.46. Characterization of 5: FT-IR (cm−1, CH2Cl2) ʋ(C≡O) 2022(vs), 1917(s) and 1895(s). 1H NMR (400 MHz, CD2Cl2) δ 9.21 (d, J = 4 Hz, 1H), 8.22 (s, 1H), 7.70 (d, J = 4 Hz, 1H), 2.93 (s, 3H). 13C NMR (101 MHz, CD2Cl2) δ 197.91, 190.12, 152.45, 148.78, 146.82, 130.37, 126.59, 123.74, 19.06. Anal. Calcd for C17ClH12N2O3Re (%): C, 39.73; H, 2.35; N, 5.45. Found: C, 39.34; H, 2.22; N, 5.38. Characterization of 6: FT-IR (cm−1, CH2Cl2) ʋ(C≡O) 2019(vs), 1913(s) and 1889(s), ʋ(C=O) 1753 (br, m), and ʋ(C=O) 1630 (br, m). 1H NMR (400 MHz, CD2Cl2) δ 9.24 (s, 2H), 8.21 (s, 2H), 7.23 – 7.11 (m, 1H), 7.00 – 6.73 (m, 3H), 2.82 (s, 6H), 2.66 (s, 6H), 2.11 (s, 3H). 13C NMR (101 MHz, CD2Cl2) δ 169.62, 169.43, 154.66, 149.65, 146.58, 146.06, 134.51, 131.23, 130.37, 130.12, 129.36, 125.09, 123.67, 122.72, 21.28, 18.00, 15.30. Anal. Calcd for C28H23N2O7Re.0.1CH2Cl2 CH2Cl2 (%): C, 48.61; H, 3.36; N, 4.03. Found: C, 48.32; H, 3.18; N, 3.93.

3. Results and Discussion

Previously we reported the synthesis of analogous pentylcarbonato complexes [3]. It was suggested that the ligand substitution reactions followed by homolytic cleavage and subsequent reactions with 1-pentanol and carbon dioxide yielded the pentylcarbonato complexes. The microwave reactions probably follow similar pathways. An average yield of 75% was obtained for 1–3. The IR spectrum of each (1–3) exhibits the characteristic ʋ(C=O) around 1660 cm−1 [3]. The 1H NMR spectrum of each confirms the presence of the pentyl group. Likewise, the 13C NMR spectrum of each shows a resonance due to C=O around δ165. A single crystal of 3 was successfully grown through crystallization in a mixture of dichloromethane and hexane in the presence of carbon dioxide. 3 crystallizes in monoclinic space group P21/n (Fig. 1). The molecule crystallizes as three independent molecules in the lattice (Fig. S7). The molecules differ in torsion angles about the pentyl group. Two of the independent molecules show disorder in the pentyl groups. No solvent is present in the lattice. The crystallographic data (Table S1) and related other information are available in Supplementary materials. The axial Re-CO bond length of 1.894(10) Å is slightly shorter than the equatorial Re-CO bond lengths of 1.917(9) and 1.915(8) Å. A similar trend was previously observed with related other carboxylato complexes [2,3,5,11]. Treatments of 1, 2 and 3 with 2-pyridinesulfonic acid, hydrochloric acid and acetylsalicylic acid in dichloromethane afforded the corresponding sulfonato (4), chlorido (5) and acetylsalicylato (6) complexes respectively. 4–6 were characterized through IR and NMR spectroscopic techniques. The IR spectrum of each exhibits three strong ʋ(C≡O)’s consistent with facial geometry of each complex [3]. Also, 6 shows a broad medium intensity ʋ(C=O) at 1753 cm−1 for the methyl ester group and a second a broad medium intensity ʋ(C=O) at 1630 cm−1 for the -C(=O)O-Re-moiety. The 1H NMR spectrum of 4–6 each confirms the methyl groups present in the aromatic rings and an additional methyl of the acetylsalicylate group. The 13C NMR spectrum of each exhibits the expected number of aromatic and aliphatic carbons. Two down-field resonances due to three terminal carbonyls are observed for 5; however, weak resonances are observed for 4 and 6. For 6, the peaks at δ 169.62 and 169.43 are due to the two C=O groups. Single crystals of 4 – 6 were grown through crystallization in a mixture of dichloromethane and hexane. 4 crystallizes in monoclinic C2/c space group (Fig. 2), 5 crystallizes in triclinic space group P-1 (Fig. S8) and 6 crystallizes in monoclinic space group P21 (Fig. 3 for molecule 1 and Fig. S9 for molecule 2). The crystallographic data and related other information are available in Supplementary material. As expected, the average Re-N lengths of 2.162 Å in 3 and 2.176 Å in 6 are longer the corresponding Re-O lengths of 1.116 Å in 3 and 2.148 Å in 6. A similar trend was also observed in related rhenium (I) carboxylato complexes [12]. In 5, the length of 1.090(4) Å for the C1-O1 bond trans to the Cl atom is shorter than those of 1.149(4) and 1.153(4) Å for the CO bonds cis to the Cl atom. This was also observed in other rhenium chlorido complexes [13]. The Re1-Cl1 bond length of 2.4807(8) Å is comparable to those of 2.478(1) Å [14], 2.491(1) Å [14], 2.459(1) Å [14], and 2.497(2) Å [15] observed for the Re-Cl lengths of similar rhenium(I) chlorido complexes. Re1-O11 bond length of 2.170 (4) Å in 4 is very similar to the Re-O bond distances in related sulfonato complexes [4,16]. However, Re1-O11-S1 bond angle of 127.3(2)° in 4 is smaller than the corresponding bond angles of 130.47(11)° and 130.66(12)° observed in other sulfonato complexes [4,16].

Fig. 1.

Fig. 1.

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Molecular structure of one of three independent molecules in the lattice of 3 (50% probability ellipsoids). Hydrogen atoms are omitted for clarity. Select bond distances (Å) and angles (deg): Re(1)-C(1), 1.917(9); Re(1)-C(2), 1.915(8); Re(1)-C(3), 1.894(10); Re(1)-N(1), 2.166(5); Re(1)-N(2), 2.158(6); Re(1)-O(4), 2.116(5); C(3)-Re(1)-O(4), 172.2(3); C(2)-Re(1)-N(2), 173.4(3); C(1)-Re(1)-N(1), 169.6(3); C(20)-O(4)-Re(1), 125.6(4); C(1)-Re(1)-O(4), 96.8(3); C(2)-Re(1)-O(4), 95.1(3); N(1)-Re(1)-O(4), 76.3(2); N(2)-Re(1)-O(4), 79.92(19). See the Supplemental materials for details of the two independent molecules in the lattice that exhibit disorder in the pentyl group.

Fig. 2.

Fig. 2.

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Molecular structure of 4. Select bond distances (Å) and angles (deg): Re1-C1, 1.921(7); Re1-C2, 1.912(7); Re1-C3, 1.890(7); Re1-N22, 2.172(5); Re1-N34, 2.169(5); Re1-O11, 2.170(4); C3-Re1-O11, 174.0(2); C2-Re1-N22, 171.5(3); C1-Re1-N34, 175.4(3); S1-O11-Re1, 127.3(2); C1-Re1-O11, 97.8(2); C2-Re1-O11, 96.3(2); N22-Re1-O11, 78.69(17); N34-Re1-O11, 81.90(16).

Fig. 3.

Fig. 3.

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Molecular structure of one of the two molecules in the lattice of 6. Select bond distances (Å) and angles (deg): Re1-C1A, 1.902(3); Re1-C3A, 1.939(4); Re1-C5A, 1.919(4); Re1-N21A, 2.170(3); Re1-N36A, 2.181(3); Re1-O7A, 2.148(2); C1A-Re1-O7A, 175.73(11); C3A-Re1-N21A, 171.69(13); C5A-Re1-N36A, 172.87(12); Re-O7A-C8A, 120.41(19); C3A-Re1-O7A, 94.26(11); C5A-Re1-O7A, 93.39(12); O7A-Re1-N21A, 80.55(9); O7A-Re1-N36A, 83.33(9). See the Supplemental materials for details of the second molecule in the lattice that exhibit disorder in the acetoxy group.

4. Conclusion and perspectives

The pentylcarbonato complexes serve as very important and convenient synthon. They are readily available through microwave-assisted synthesis. They can be manipulated in ordinary laboratory conditions. They are stable at ambient temperature for years if they are stored in stoppered vials. A wide variety of carboxylato, sulfonato, and related other complexes can be obtained with an almost quantitative yield in a very short period of time. Research is in progress to cut short the microwave reaction times using high boiling alcohols such as 1-hexanol and 1-heptanol.

Supplementary Material

Supplementary materials

Highlights.

  • Microwave heating of a mixture of rhenium decacarbonyl, α-diimines, 1-pentanol and carbon dioxide yields tricarbonylrhenium(I)(α-diimine) pentylcarbonato complexes.

  • The pentylcarbonato complexes cleanly convert to the corresponding carboxylato, sulfonato and chlorido complexes in the presence of carboxylic acid, sulfonic acid and hydrochloric acid.

  • The tricarbonylrhenium (3,4,7,8-tetramethylphen) pentylcarbonato complex crystallizes as three independent molecules in the lattice.

  • The tricarbonylrhenium (3,4,7,8-tetramethylphen) aspirinato complex crystallizes as two independent molecules in the lattice.

Acknowledgements

This research was partially supported by the NIH Grant No. 5R25GM0058904 to AJW.

Footnotes

Supplementary material

IR and NMR spectra of 1–6. Molecular structures of the 3 independent molecules in the lattice of 3. X-ray structure of 5. X-ray structure of 6 – molecule 2. Crystallographic data for 3 - 6 (PDF)

CCDC 1949461, 1995496, 1952462, and 2008663 (for 3 - 6, respectively) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Declaration of competing interest

The authors declare no competing financial interest.

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