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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Inorganica Chim Acta. Author manuscript; available in PMC 2010 July 7.
Published in final edited form as:
Inorganica Chim Acta. 2000 September 11; 307(1-2): 149–153.
PMCID: PMC2898268
NIHMSID: NIHMS214553

Schiff base chemistry of the {ReO}3+ core: structural characterization of the unusual ‘3 + 2’ complex [ReO(η3-OC6H4–CH=NC6H4-2-S)(η2-OC6H4An external file that holds a picture, illustration, etc.
Object name is nihms214553ig1.jpg)]

Abstract

The reaction of [ReOCl3(PPh3)2] with salicylaldehyde-2-mercaptoanil (1) in methanol yields [ReO(OCH3)(PPh3)(η3-OC6H4CH=NC6H4S)] (3). Reaction of 3 with 2-(2-hydroxyphenyl)benzothiazole (2) produces [ReO(η3-OC6H4CH=NC6H4S)( η2-OC6H4An external file that holds a picture, illustration, etc.
Object name is nihms214553ig2.jpg)] (4) in good yield. Compound 4 may also be prepared directly from the reaction of [ReOCl3(PPh3)2] with excess 1, after prolonged reaction times in the presence of atmospheric oxygen. The structure of 3 exhibits a distorted octahedral Re(V) center coordinated to a terminal oxo-group, a methoxy ligand, a triphenylphosphine ligand, and the N, O and S donors of the tridentate Schiff base ligand. The octahedral geometry of 4 is defined by the terminal oxo-group, the N, O and S donors of the tridentate Schiff base ligand, and the N and O donors of the bidentate 2-(2-hydroxophenyl)benzothiazole ligand. Oxidation and cyclization of the Schiff base salicylaldehyde-2-mercaptoanil to produce 2-(2-hydroxyphenyl)benzothiazole is precedented and in this instance may be driven by the formation of the robust ‘3+2’ complex 4.

Keywords: Crystal structures, Oxorhenium complexes, Schiff base complexes

1. Introduction

The development of rhenium coordination chemistry has gained significant impetus from the potential applications of the radioisotopes 186Re and 188Re to radiotherapy [15]. One approach to the design of novel reagents has focused on polydentate ligands with thiolate and nitrogen donors with the robust {ReO}3+ core [6]. The ‘3+1’ concept of ligand addition provides a strategy for the consistent synthesis of {ReO}3+ core complexes, allowing a systematic investigation of the size, shape, molecular weight, and charge of the rhenium-thiolate complexes [710]. The concept is based on ligation of a dinegative tridentate unit with the (S–X–Y) donor set (X = O, S, NR; Y = S, O) in combination with a mononegative thiol, so as to preserve the oxometal core and the formal +5 oxidation state of the metal. More recently, the ‘3+1’ concept has been extended to Schiff base ligands with (O–N–S) donor groups, a strategy which provides a general and facile method for high yield synthesis of robust complexes with the {ReO}3+ core [11].

As part of these investigations on Schiff base materials of the {ReO}3+ unit, we have studied the stepwise addition of salicylaldehyde-2-mercaptoanil, HOC6H4-CH=NC6H4SH (1), to [ReOCl3(PPh3)2] to yield [ReO(OCH3)(PPh3)(η3-OC6H4CH=NC6H4S)] (3) and [ReO(η3 - OC6H4CH=NC6H4S)( η2 - OC6H4An external file that holds a picture, illustration, etc.
Object name is nihms214553ig3.jpg)] (4). Compound 4 is an example of a ‘3+2’ complex of the {ReO}2+ core [12,13] in which one ligand is present in the tridentate open chain imine form, while the second has been oxidized to the bidentate 2-(2-hydroxophenyl) benzothiazole form.

2. Experimental

2.1. General considerations

NMR spectra were recorded on a Bruker DPX 300 (1H 300.10 MHz) spectrometer in CDCl3 (δ 7.27 ppm). IR spectra were recorded as KBr pellets with a Perkin–Elmer series 1600 FTIR. Elemental analysis for carbon, hydrogen and nitrogen were carried out by Oneida Research Services, Whitesboro, NY. Ammonium perrhenate, salicylaldehyde and 2-aminobenzenethiol were purchased from Aldrich and used without further purification. Reagent grade solvents, distilled and dried by standard methods, were used in all cases. [ReOCl3(PPh3)2] was prepared according to the literature [14]. Schiff base salicylaldehyde-2-mercaptoanil (1) was prepared previously in this laboratory [13] and by Corbin and Work [15]. 2-(2-Hydroxyphenyl)benzothiazole (2) was synthesized according to a reported procedure [16].

2.2. Preparation of ReO(OCH3)(PPh3)[η3-O–C6H4 –CH=N–C6H4 -2 -S] (3)

To a solution of Schiff base 1 (23 mg, 0.1 mmol) in CHCl3 (20 cm3) was added [ReOCl3(PPh3)2] (83 mg, 1 mmol) and subsequently Et3N (mg, mmol). The resulting solution was refluxed for 10 min and MeOH (10 cm3) was then added, and the red solution was refluxed for further 10 min. The reaction mixture was evaporated to dryness under reduced pressure. The brick-red residue was washed with ice-cold MeOH (2 × 5 cm3) (yield: 47 mg, 65%). IR (KBr, cm−1): 968, ν(Re=O).1H NMR (CDCl3, ppm): 3.49 (s, 3H, OCH3), 7.3 (s, 15H, PPh3), 7.4–7.9 (m, 8H, ArH). Anal. Calc. for C32H27NO3PSRe: C, 53.19; H, 3.74; N, 1.94. Found: C, 53.36; H, 3.68; N, 2.01%. Red crystals suitable for X-ray crystallography were obtained by recrystallization from CHCl3–MeOH.

2.3. Preparation of the ‘3+2’ complex [ReO(η3-O–C6H4 –CH=N–C6H4 -2 -S)-( η2-O–C6H4 –C=N–An external file that holds a picture, illustration, etc.
Object name is nihms214553ig4.jpg)] (4)

2.3.1. Method A

From the intermediate [ReO(OCH3)(PPh3)(η3-O–C6H4–CH=N–C6H4-2-S)] (3). To a solution of 3 (15 mg, 0.02 mmol) in CHCl3 was added 2-(2-hydroxyphenyl)benzothiazole (5 mg, 0.022 mmol) and Et3N (1 drop) was refluxed for 15 min, whereupon the color changed from red to dark brown. The solution was evaporated to dryness, rinsed with EtOH and Et2O. This was dissolved in a small portion of CHCl3 and purified over a silica gel column, using CHCl3 and then 80/20 CHCl3/acetone as the eluent. X-ray quality crystals were obtained from recrystallization from CH3CN (yield: 8 mg, 61%). IR (KBr, cm−1): 963, ν(Re=O). 1H NMR (CDCl3, ppm): 7.45–7.55 (m, 4H, ArH), 7.65–7.75 (m, 4H, ArH). Anal. Calc. for C26H17N2O3S2Re·CH3CN: C, 48.28; H, 2.87; N, 6.03. Found: C, 48.49; H, 2.78; N, 6.17%.

2.3.2. Method B

One-pot synthesis from [ReOCl3(PPh3)2]. A mixture of [ReOCl3(PPh3)2] (83 mg, 0.1 mmol), Schiff base 1 (35 mg, 0.15 mmol), NaOAc (83 mg, 1 mmol) in MeOH (20 cm3) and CHCl3 (5 cm3) was refluxed overnight. Re OCl3(PPh3)2 dissolved slowly and the yellowish green solution turned to red and finally dark brown. The solvent was evaporated and the residue crystallized from CH3CN (yield: 74%).

2.4. X-ray crystallography

Crystallographic data for 3 and 4 were collected with a Siemens P4 diffractometer equipped with the SMART CCD system [17] and using Mo Kα radiation (λ = 0.71073 Å ) (Table 1). The data were collected at 89 K. The data were corrected for Lorentz and polarization effects, and absorption corrections were made using sadabs [18]. The structure solutions and refinements were carried out using the shelxl96 [19] software package. The structures were solved using direct methods and all of the non-hydrogen atoms were located from the initial solution. After locating all of the non-hydrogen atoms in each structure, the models were refined against F2, initially using isotropic and later anisotropic thermal displacement parameters until the final value of Δ/σmax was less than 0.001. At this point the hydrogen atoms were located from the electron density difference map and a final cycle of refinements was performed, until the final value of Δ/σmax was again less than 0.001.

Table 1
Summary of crystallographic data for [ReO(OCH3)(PPh3)(η3-OC6H4CH=NC6H4S)] (3) and [ReO(η3-OC6H4CH=NC6H4S)(η2-OC6H4C=NC6H4S)] (4)

3. Discussion

The ligand salicylaldehyde-2-mercaptoanil (1) is synthesized in the condensation reaction of 2-hydroxybenzaldehyde and 2-aminothiophenol (Scheme 1). Prolonged reaction times and exposure to oxygen results in the formation of 2-(2-hydroxyphenyl)benzothia-zole (2) as a minor product. Compound 2 may be prepared directly as a monophasic material from the reaction of 2-cyanophenol and 2-aminophenylthiol (Scheme 2) [16].

The reaction of ligand 1 with [ReOCl3(PPh3)2] in methanol yields [ReO(OCH3)(PPh3)(η3-OC6H4-CH=NC6H4S)] (3) in good yield. Subsequent reaction of 3 with 2-(2-hydroxyphenyl)benzothiazole 2 yields the ‘3+2’ complex [ReO(η3-OC6H4-CH=NC6H4S)(η2-OC6H4An external file that holds a picture, illustration, etc.
Object name is nihms214553ig5.jpg)] (4) (Scheme 3). Compound 4 could also be prepared directly from the reaction of [ReOCl3(PPh3)2] and 1 in the presence of sodium acetate and prolonged reaction times. This suggests that oxidation and ring closure of the open chain imine into the benzothiazole form is facilitated by introducing adventitious oxygen and extending the duration of the reaction.

The structure of 3, shown in Fig. 1, consists of discrete distorted octahedral molecules. The coordination geometry at the rhenium(V) site is defined by the terminal oxo-group, the oxygen donor of a methoxo ligand, the phosphorus of the PPh3 group, and the N, O and S donors of the tridentate Schiff base ligand. It is noteworthy that the oxo-group of 3 does not exert a significant trans influence, as indicated by the Re–O distance of 1.904 Å to the trans oxygen of the methoxy group (Table 2). The relatively long bond distances associated with the equatorial plane, defined by the N, O and S donors of the tridentate ligand and the P donor of the PPh3 may reflect steric congestion along the molecular girdle.

Fig. 1
A view of the structure of 3, showing the atom labeling scheme and 50% thermal ellipsoids.
Table 2
Selected bond lengths (Å) and angles (°) for [ReO(OCH3)(PPh3)(η3-OC6H4CH=NC6H4S)] (3)

As shown in Fig. 2, the structure of 4 consists of molecular oxorhenium(V) monomers. The distorted octahedral geometry of the Re(V) center is defined by the terminal oxo-group, the N, O and S donors of the tridentate Schiff base ligand, and the N and O donors of the bidentate 2-(2-hydroxophenyl)benzothiazole ligand. Once again, it is noted that the Re–O3 distance along the axial vector O1–Re–O3 is significantly shorter than the Re–O2 distance in the equatorial plane, defined by O2, N1, S1 and N2 (1.989(3) and 2.076(3) Å, respectively). The N1–C7 distance of 1.330(6) Å and the valence angles at N1 of 118.6(4), 121.4(3) and 119.3(3)° confirm that the N1 site is sp2 hybridized and the ligand in the Schiff base form. Similarly, the N2–C20 distance of 1.330(6) Å and the valence angles at N2 of 111.0(4), 124.1(3) and 124.7(3)° are consistent with an N2–C20 double bond and sp2 hybridization at the N2 site, confirming that the bidentate ligand is in the oxidized 2-(2-hydroxo-phenyl)-benzothiazole form (Table 3).

Fig. 2
A view of the structure of 4, showing the atom labeling scheme and 50% thermal ellipsoids.
Table 3
Selected bond lengths (Å) and angles (°) for [ReO(η3-OC6H4CH=NC6H4S)(η2-OC6H4C=NC6H4S)] (4)

4. Conclusions

Tridentate Schiff bases with mixed O, N, S donor groups provide effective chelates for the preparation of complexes with the {ReO}3+ core. Most significantly, the oxorhenium-Schiff base structural unit may be exploited to prepare both ‘3+1’ and ‘3+2’ complexes depending on the identity of the coligand. Thus, [ReO(η3-OC6H4CH=N–C6N4S)(η2-OC6H4An external file that holds a picture, illustration, etc.
Object name is nihms214553ig6.jpg)] (4) may be prepared from [ReO(OCH3)(PPh3)(η3-OC6H4CH=NC6H4S)] (3) and 2-(2-hydroxyphenyl)-benzothiazole or directly from [ReOCl3(PPh3)2] and the Schiff base salicylaldehyde-2-mercaptoanil. These results suggest that the Schiff base chemistry of the {ReO}3+ core may be used to develop a diverse chemistry of materials whose size, shape, molecular weight, and charge may be systematically varied.

5. Supplementary material

All atomic and thermal parameters and all interatomic angles are available from the authors upon request. Crystallographic data (excluding structure factor) for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Center as publication Nos. CCDC 140064 and 139496. Copies of the data can be obtained free of charge on application to The Director, CCDC, 12 Union Road, Cambridge, CB2 1EQ, UK (fax: +44-1223-336033; e-mail: ku.ca.mac.cdcc@tisoped or www: http://www.ccdc.cam.ac.uk).

Acknowledgements

This work was supported by a grant from the Department of Energy, Office of Health and Environmental Research (D2-FG02-99ER62791).

References

1. Volkert W, Goeckeler WF, Ehrhardtetring GJ. Nucl. Med. 1991;32:174. [PubMed]
2. DiZio JP, Fiaschi R, Davison A, Jones AG, Katzenellenbogen JA. Bioconj. Chem. 1991;2:353. [PubMed]
3. Deutsch EA, Libson K, Vanderheyden J-L. In: Technetium and Rhenium in Chemistry and Nuclear Medicine. Nicolini M, Bandoli G, Mazzi U, editors. Vol. 3. New York: Raven Press; 1990. pp. 13–22.
4. Volkert WA, Hoffman TJ. Chem. Rev. 1999;99:2269. [PubMed]
5. Schubiger PA, Alberto R, Smith A. Bioconj. Chem. 1996;7:165. [PubMed]
6. Breitz H, Ratliff B, Schroff R, Vanderheyden J-L, Fritzberg AR, Appelbaum J, Fisher DR, Abrams P, Weiden P. J. Nucl. Med. 1990;31:725.
7. Pietzsch HJ, Spies H, Hoffman S, Stach J. Inorg. Chim. Acta. 1989;161:15.
8. Spies H, Fietz T, Pietzsch HJ, Johannsen B, Leibnitz P, Reck G, Scheller D, Klostermann K. J. Chem. Soc., Dalton Trans. 1995:227. and references. therein.
9. Papadopoulous M, Pirmettis I, Tsoukalas C, Nock B, Maina T, Raptopoulou CP, Pietzsch H-J, Friebe M, Spies H, Johannsen B, Chiotellis E. Inorg. Chim. Acta. 1999;295:1.
10. Maresca KP, Bonavia GH, Babich JW, Zubieta J. Inorg. Chim. Acta. 1999;284:252. and references. therein.
11. Femia FJ, Chen X, Babich JW. J. Zubieta, Inorg. Chim. Acta. 2000;300–302:517. [PMC free article] [PubMed]
12. Nock B, Maina T, Tisato F, Papadopoulos M, Raptopoulou CP, Terzis A, Chiotellis E. Inorg. Chem. 1999;38:4197.
13. Femia FJ, Babich JW, Zubieta J. Inorg. Chim. Acta. 2000;300–302:462. [PMC free article] [PubMed]
14. Chatt J, Rowe GA. J. Chem. Soc. 1962:4019.
15. Corbin JL, Work DE. Can. J. Chem. 1974;52:1054.
16. Shuter E, Hoveyda HR, Karunaratne V, Rettig SJ, Orwig C. Inorg. Chem. 1996;35:368. [PubMed]
17. Siemens, SMART Software Reference Manual. Madison, WI: Siemens Analytical X-ray Instruments; 1994.
18. Sheldrick GM. SADABS: Program for Empirical Absorption Corrections. Germany: University of Göttingen; 1996.
19. Sheldrick GM. SHELXL96, Program for the Refinement of Crystal Structures. Germany: University of Göttingen; 1996.