RESEARCH ARTICLE


Hydridothiazole Rhodium Complexes as a Result of C-H Bond Activation in Iminothiazoles Chelating Ligands



Jehan Al-hamidi1, Abdulhamid Alsaygh2, Ibrahim Al-Najjar2, *
1 Chemistry Department, College of Science, Princess Nora Bent Abdullrahman University, Riyadh, Saudi Arabia
2 Petrochemicals Research Institute, King Abdulaziz City for Science and Technology, P. O. Box 6086, Riyadh – 11442,Saudi Arabia


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© Al-hamidi et al; Licensee Bentham Open.

open-access license: This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.

* Address correspondence to this author at the Petrochemicals ResearchInstitute, King Abdulaziz City for Science and Technology, P. O. Box 6086, Riyadh – 11442, Saudi Arabia; Tel: +966 504269199; E-mail; alnajjar@kacst.edu.sa


Abstract

A series of 20 Schiff base ligands derived from 2-aminothiazole and its derivatives and aryl aldehydes with either [RhCl(PPh3)3] or [Rh(µ-Cl)(COD)]2 in the presence of 4 equivalents of PPh3 lead to an Rh(III) cyclometallated complex and the imine ligand (C-H) bond has been added to the metal (C-M-H). The complexes were investigated by using I.R., 1H, 13C and 31P NMR Spectroscopic techniques. The signal of the (C-H) ligand was observed as trans to the nitrogen atom in the complex which is a donor ligand.

Graphical Abstract:

Total synthesis of hydridothiazole rhodium complexes possessing rhodium hydride signal at δ (-14.60 to-15.04) ppm, trans to N-donor ligand and iminoyl carbon (7C=N) signal in Rh (III) observed at δ (220.1-237.6)ppm, lower field and suggestive of carbine like properties.

Keywords:: Hydrido complexes, ligand substitution, IR,1H,13C,31P NMR, oxidative-addition, phosphine complexes, rhodium, Schiff-bases.



1. INTRODUCTION

The oxidative addition of C-H bond activated by transition metal has been reported in many recent studies in organometallic chemistry [1-4]. Benzylic imines (Ph-CH = NR) are the most studied ligands in the cyclometallation of transition metals [5, 6]. It has been reported that intramolecular C-H activation examples of rhodium with N-donar ligands and C-X (X: halogen) activation with imines [2-4, 7-9]. The first report of ortho-metallation of imines at rhodium via C-H activation has appeared only recently [2-4, 10]. Interestingly, cyclometallation reaction of imine [11] C-H bonds lead us to study some of the important chemistry related to the imines, derivatives from 2-aminothiazole and its derivatives and aryl aldehydes. In most recent application for ruthenium, rhodium and iridium complexes have been used as therapeutic agents and a number of kinetically inert ruthenium(II), iridium(III) and rhodium(III) complexes have been reported as inhibitors of protein kinases [12-16]. Chung-Hang Leung and Dik-Lung Ma group have also actively pursued the development of kinetically inert metal complexes as inhibitors of various bimolecular targets, including DNA, enzymes and protein-protein interactions [13]. Here, we report the synthesis and characterization of many new rhodium (III) complexes of (X - (substituted benzylidene amino) thiazoles.

2 . MATERIAL AND METHODOLOGY

CHEMICALS: All techniques and all operations were performed under nitrogen using Schlenk techniques. Substituted aryl aldehydes, RhCl3. xH2O, Ph3P, cyclo-1, 5-octadien, and 2- aminothiazoles derivatives were purchased from Winlab, Aldrich and Strem chemicals, and were used as received.

2.1. SCHIFF BASES SYNTHESES:

The Schiff bases were synthesized by adding equivalent amounts of aryl aldehydes and 2-aminothiazole derivatives in 80 ml methanol. The resulting mixture was boiled under reflux and stirred for 9 h at 80°C in an oil bath, and then the solvent was concentrated by using rotary evaporation to give brown viscous liquid. Then n-hexane was added to precipitate the crude product, which was then recrystallized in dichloromethane and with n-hexane to give white precipitate, dried, yield 50-70% scheme (1), (the full characterization of the resulting Schiff base was submitted for publication in Arab Gulf Journal of Scientific Research (AGJSR), 2014.

Scheme 1.

Describes Schematic preparation and structural of iminothiazoles (1-20) chelating ligands.


Eq. (1).

Describes schematic diagram for the preparation of Schiff bases (No. 1-15),: X, 1 = -H, 2 = 2-OH, 3 = 4-NO2, 4 = 4-Br, 5 = 3-OH, 6= 2-NO2, 7 = 3-Me (compounds from: 1-7, Y = H) 8, Y = 4-CH3 (X = H), 9, Y = 4-Me (X = 2-OH), 10, Y = 4-Me (X = 4-NO2), 11, Y = 4-Me, (X = 4-Br), 12, Y = 5-Me, (X = H), 13, Y = 5-Me, (X = 2-OH), 14, Y = 5-Me, (X = 4-NO2), 15, Y = 5-Me, (X = 4-Br).


Eq. (2).

Compound (No.16), X = 2-OH.


Eq. (3).

Compound (No.17):, X = H,(18), X = 2-OH,(19), X = 4-NO2, (20), X = 4-Br.


2.2. SYNTHESES of the CYCLOMETALLATED SCHIFF BASE COMPLEXES.

The rhodium (III) complexes were synthesized by reaction of the Schiff base with either {RhCl(Ph3P)3} or [Rh (µ-Cl)(COD)]2. [17, 18]. Here, we report two examples:

  1. A solution of {Rh Cl(PPh3)3} (300 mg, 0.325 mmol) mixed with an equivalent amount of thiazole imines in 20 cm of dry THF was refluxed for1 h under nitrogen atmosphere, then allowed to cool. Addition of n-hexane precipitated the product, the resulting product gave yellow solid, which was separated and (recrystallized from CH2Cl2/hexane).
  2. A solution of {Rh(µCl)(COD)}2 (200 mg, 0.28 mmol), thiazole Schiff base (0.56 mmol) and PPh3 (293 mg, 1.12 mmol) in ca. 20 cm3 of dry THF was refluxed for 1 h, and by addition of n-hexane, the product was precipitated, which was separated by filtration and (recrystallized from CH2Cl2/hexane).

2.3. SPECTROSCOPY:

I.R. spectra were measured using Nexus spectrophotometer FT IR. The N.M.R. spectra were recorded at R.T. on a JEOL 400 MHz. The 1H, 13C (1H) and 31P [1H] -n. m. r. frequencies observated at 400, 100 and 161.08 MHz respectively (at JEOL). Positive values for 31P- [1H] representing deshielding. The cyclometallated complexes) were dissolved in CDCl3.

3. RESULTS AND DISCUSSION:

Introducing metal into C-H bond have been observed in many compounds like quinoline and Schiff base substrates [19]. A significant amount of work has been done on the heterocyclic aromatic species, 8-substituted quinoline [20-24] and 2- (benzylidenamino) pyridines [25-29]. Coordination of the metal with nitrogen atom in aryl amines results in a favorable geometry for introducing of the metal into neighboring C-H or C-C bond [21, 25, 26, 30]. Rhodium complexes (Table 1)

Table 1.

Describes the rhodium complexes obtained from Schiff bases derived from 2-amino thiazoles and benzaldehydes (complexes No. (21-29), 2-aminobenzothiazole and benzaldehyde (complex 30) and 2-amino-5-t-butyl-1, 3, 4-thiadiazoleand benzaldehydes (complex 31).


Complex No. X Y Complex No. X Y
21 H H 27 4-NO2 4-Me
22 2-OH H 28 H 5-Me
23 4-NO2 H 29 2-OH 5-Me
24 4-Br H 30 2-OH 1*
25 2-OH 4-Me 31 2-OH 2*
26 H 4-Me
were synthesized either by mixing and refluxing equimolar amount of the Schiff base with {RhC1(Ph3P)} in THF for 0.5h [31], or by boiling a solution of one equivalent of {Rh(µ-Cl)(COD)}2, with two equivalents of prepared Schiff base with four equivalents of phosphine in THF for 1h, as showed in Scheme (2). The 1H NMR spectrum of each of the new rhodium complexes in CDCl3 shows a hydride resonance between δ -11.49 and δ -13.27 ppm (Table 3). The signals of the starting Schiff bases, C-H observed at δ 8.20-9.44 ppm and in the resulting complex, these signals are absent, providing evidence for insertion of Rh complex into the C-H bond of the imines. Strong confirmation evidence comes from appearance of the resonance of the hydride signal in each complex at high field [24, 29] ca. δ -12.38 ppm. The hydride signals in the complexes are split by coupling to an equivalent of two 31P nuclei and the 103Rh nucleus. The spin-spin couplings are frequently ca. 12.0 Hz, the hydride multiplet observed as a pseudo quartet, but higher resolution frequency are usually appear as the expected doublet of triplets. The 31P, of rhodium complexes show a 31P signal at ca. δ 28.2-37.78 with 1J (103Rh-31P) 102.00-120.00 Hz as a doublet (Table 3) in keeping with previous report [30]. The majority of the rhodium imines hydride complexes are only moderately soluble in chloroform-d and dichlomethane-d2 solvents (complexes are soluble in DMSO, but decomposed during NMR processing).

Table 2.

CHN-Elemental Analyses for Complexes (No.21-31).


No. X M.P. (°C) M.F. Calculated (%) Found (%)
C H N C H N
21. H 162 RhC46H38N2SP2Cl 64.9 farazfaiza4.51 3.3 65.30 4.63 3.40
22. 2-OH 190 RhC46H38N2SOP2Cl 63.70 4.53 3.23 64.70 5.01 3.34
23. 4-NO2 185 RhC46H37N3SO2P2Cl 61.7 4.16 4.7 60.66 4.26 4.34
24 4-Br 192 RhC46H37N2SP2ClBr 59.40 4.00 3.00 60.40 4.30 3.32
25. H 202 RhC47H40N2SP2Cl 65.24 4.65 3.23 64.98 3.93 3.7
26. 2-OH 192 RhC47H40N2SOP2Cl 64.06 4.89 3.17 63.13 4.44 2.95
27. 4-NO2 214 RhC47H40N3SO2P2Cl 60.63 4.42 4.61 60.33 4.38 4.53
28. H 198 RhC47H41N2SP2Cl 65.24 4.65 3.23 64.98 3.93 3.7
29. 2-OH 200 RhC47H42N2SOP2Cl 64.06 4.89 3.17 64.2 4.13 3.52
30. 2-OH 70 RhC50H40N2SOP2Cl 66.63 4.47 3.11 65.98 4.35 3.93
31. 2-OH 160 RhC49H46N3SOP2Cl 64.47 5.09 4.62 65.01 4.89 4.35
Table 3.

1H, and 31P -N.M.R. chemical shifts and 2J (31P-1H), 1J (103Rh-1H) and 1J (1Rh-31P) coupling constants of complexes (No.21-31). (In CDCl3).


Comp. No. X Y δ1H Hydride(p.p.m.) δ31P{1H}(p.p.m.) 2J(31P-1H)(Hz) 1J(103Rh-1H)(Hz) 1J(103Rh-31P)(Hz)
21. H H -12.09 28.7 12.0 12.4 102
22. 2-OH H -12.30 31.4 12.0 12.4 107
23. 4-NO2 H -12.05 29.0 12.0 12.7 107
24. 4-Br H -12.16 28.2 12.0 12.5 103
25. 3-OH 4-Me -12.36 32.35 12.0 12.50 111
26. H 4-Me -13.27 29.51 12.0 12.45 108
27. 4-NO2 4-Me -13.27 29.60 12.0 12.54 111
28. H 5-Me -12.15 32.30 11.0 12.50 111
29. 2-OH 5-Me -12.36 32.33 11.0 12.45 111
30 2-OH - -13.08 29.20 11.0 13.3 120
31 2-OH - -11.49-12.36 37.8729.54 11.011.0 12.5012.80 111118
Scheme 2.

Describes schematic diagram for preparation of “hydridothiazole” rhodium complexes.


The 13C 1H NMR spectrum, in particular the signal from the metal- carbon bonded atom, is consistent with the presence of the cyclometallated ring [24, 31, 32] the signal from the metal-bonded carbon, C (7) (iminoyl carbon-13C=N) appears as a doublet of triplets owing to coupling to two equivalent 31P nuclei and the 103Rh nucleus, whereas the corresponding signal from the uncomplexed Schiff base was found at δ 159.25-164.97 ppm [24]. For C (7), δ- has been observed at low-field position in which a chelating atom is incorporated in a five member-ring [33], and this was expected for a cyclometallated sp2 carbon [11, 34] (similar to carbene-carbon. The remaining 1H and 13C NMR data are as expected. Table 4, demonstrate the 13C-NMR chemical shifts for C (7) (iminoyl carbon-13C=N), and coupling constants 1J (103Rh-31P) Hz and 2J (103Rh-31P) Hz. Complex No. X, C (7) (ppm) 1J (103Rh-31P) (Hz) 2J (103Rh-31P) (Hz), respectively. No. 21, X = H; 220.1; 33; 8. No 22; X = 2-OH; 222.8; 32; 8. No.23; X = 4-NO2; 224.1; 33. 8 .No. 24; X = 4-Br; 222.0 ; 32.0; 9 .The positions of the Rh-H signals in both IR (v Rh-H) 2073.48 cm-1 for compound (24) and 1H NMR (δ -11.49 to -13.27 ppm) spectra, are as expected for a Rh-H bond located trans to the nitrogen-donor ligand. In addition, the 2J (31P-1H) value is consistent with a hydride located cis to two magnetically equivalent of PPh3 groups [35-37], which in turn are mutually trans, as confirmed from the 13P (1H) NMR spectrum. Interestingly, the hydride 1H NMR spectrum of compound (31) presented in two type of the hydride spectrum which appear at δ -11.49 ppm and -12.36 ppm. Also, two 31P-spectrum appears at δ 37.87 ppm and δ 29.54 ppm with 2J (31P-1H), 11.0 and 11.0 Hz and 1J (103Rh-1H) 12.50 Hz and 12.80 Hz with 1J (103Rh-31P), of 111 Hz and 118 Hz respectively. This result may be due to the present of t-butyl group at C-5 of the thiazole ring, which lead to different geometric structure. It was also observed that the signal for C (7) (iminoyl carbon- 13C=N) is at low magnetic field, at δ 224.1 ppm, this may be due to the substitution of 4-NO2 (at para-position in aryl ring), as electron withdrawing group, which lead to decrease the electronic density on C-7 led δ-moved to low magnetic field, compared with other groups on aryl ring of the same complexes.

Table 4.

13C-NMR chemical shifts for C (7) (iminoyl carbon-13C=N), and coupling constants 1J (103Rh-31P) Hz and 2J (103Rh-31P) Hz.


Complex No. X C(7) (ppm) 1J (103Rh-31P) (Hz) 2J (103Rh-31P)(Hz)
21. H 220.1 33 8
22. 2-OH 222.8 32 8
23. 4-NO2 224.1 33 8
24. 4-Br 222.0 32 9

4. CONCLUSION

The new cyclometallated complexes have been characterized by elemental analysis, IR, 1H, 13C-NMR and 31P (only the more soluble complexes were recorded in CDCl3) spectroscopy. Interestingly, the hydride ligand signals in both IR (2040 cm-1) and 1H-NMR, δ ((-14.60) - (-15.04)) ppm. The result obtained from the spectra was expected for Rh -hydride atom trans position to the N-donor ligand. However, the 31P-NMR for some cyclometallated complexes shows signal at δ (30.20-34.67) ppm. The 2J (31P-1H) value consistent for H cis is to two magnetically equivalent PPh3-groups, which indicate mutually trans, as observed from the 31P (1H) NMR spectrum. This result supported from 1H and 13C-NMR spectra. Interestingly, the 13C-NMR of the iminoyl carbon (7C=N) signal in Rh (III) observed at δ (220.1-237.6) ppm. This low-field position for cyclometallated complexes is suggestive of carbene -like properties.

CONFLICT OF INTEREST

The authors confirm that this article content has no conflict of interest.

ACKNOWLEDGEMENTS

The authors would like to thank the Research Center, College of Science, Princess Nora University and King Abdulaziz City for Science and Technology for the financial support to this Research Project (AT-17-171).

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