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

A series of 20 Schiff base ligands derived from 2-aminothiazole and its derivatives and aryl aldehydes with either [RhCl(PPh3)3] or [Rh( -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., 1 H, 13 C and 31 P 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 (-14.60 to-15.04) ppm, trans to N-donor ligand and iminoyl carbon ( 7 C=N) signal in Rh (III) observed at (220.1-237.6) ppm, lower field and suggestive of carbine like properties.


INTRODUCTION
The oxidative addition of C-H bond activated by transition metal has been reported in many recent studies in organometallic chemistry [1][2][3][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 Ndonar ligands and C-X (X: halogen) activation with imines [2][3][4][7][8][9].The first report of ortho-metallation of imines at rhodium via C-H activation has appeared only recently [2][3][4]10].Interestingly, cyclometallation reaction of imine [11] C-H bonds leads 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][13][14][15][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.

MATERIAL AND METHODOLOGY
CHEMICALS: All techniques and operations were performed under nitrogen using Schlenk techniques.Substituted aryl aldehydes, RhCl 3 .xH 2 O, Ph 3 P, cyclo-1, 5-octadien, and 2-aminothiazoles derivatives were purchased from Winlab, Aldrich and Strem chemicals, and were used as received.

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 o 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.

Syntheses of the Cyclometallated Schiff Base Complexes
The rhodium (III) complexes were synthesized by reaction of the Schiff base with either {RhCl(Ph 3 P) 3 } or [Rh ( -Cl)(COD)] 2 .[17,18].Here, we report two examples: a.A solution of {Rh Cl(PPh 3 ) 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 CH 2 Cl 2 /hexane).

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][21][22][23][24] and 2-(benzylidenamino) pyridines [25][26][27][28][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) were synthesized either by mixing and refluxing equimolar amount of the Schiff base with {RhC1(Ph 3 P)} in THF for 0.5h [31], or by boiling a solution of one equivalent of {Rh( -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 1 H NMR spectrum of each of the new rhodium complexes in CDCl 3 shows a hydride resonance between -11.49 and -13.27 ppm (Table 3).The signals of the starting Schiff bases, C-H observed at 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.-12.38 ppm.The hydride signals in the complexes are split by coupling to an equivalent of two 31 P nuclei and the 103 Rh 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 31 P, of rhodium complexes show a 31 P signal at ca. 28.2-37.78with 1J ( 103 Rh-31 P) 102.00-120.00Hz 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).
The 13 C [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 103 Rh nucleus, whereas the corresponding signal from the uncomplexed Schiff base was found at 159.25-164.97ppm [24].For C (7), -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 sp 2 carbon [11,34] (similar to carbene-carbon.The remaining 1 H and 13 C NMR data are as expected.Hz respectively.This result may be due to the presence of tbutyl group at C-5 of the thiazole ring, which leads to different geometric structure.It was also observed that the signal for C (7) (iminoyl carbon-13 C=N) is at low magnetic field, at 224.1 ppm, this may be due to the substitution of 4-NO 2 (at para-position in aryl ring), as electron withdrawing group, which leads to decrease in the electronic density on C-7 led -moved to low magnetic field, compared with other groups on aryl ring of the same complexes.

CONCLUSION
The new cyclometallated complexes have been characterized by elemental analysis, IR, 1 H, 13 C-NMR and 31 P (only the more soluble complexes were recorded in CDCl 3 ) spec-troscopy.Interestingly, the hydride ligand signals in both IR (2040 cm -1 ) and 1 H-NMR, ((-14.60)-(-15.04))ppm.The result obtained from the spectra was expected for Rhhydride atom trans position to the N-donor ligand.However, the 31 P-NMR for some cyclometallated complexes shows signal at (30.20-34.67)ppm.The 2J ( 31 P-1 H) value consistent for H cis is to two magnetically equivalent PPh 3 -groups, which indicate mutually trans, as observed from the 31 P ( 1 H) NMR spectrum.This result supported from 1 H and 13 C-NMR spectra.Interestingly, the 13 C-NMR of the iminoyl carbon (7C=N) signal in Rh (III) observed at (220.1-237.6)ppm.This low-field position for cyclometallated complexes is suggestive of carbene -like properties.