Novel Hydrido-Rhodium (III) Complexes with Some Schiff Bases Derived from Substituted Pyridines and Aryl Amines

A Series of rhodium (III) cyclometallated complexes of the type (RhHCl(NC5H2C=N Ar (PPh3)2} (Ar=Substituted aryl), have been synthesized and characterized. Schiff bases derived from a substituted benzaldehyde and 2-amino pyridine substituents were allowed to react with [RhCl(PPh3)3] or [Rh(μ-Cl)(COD)]2 in the presence of 4 equivalents of PPh3 (or Ph2BzP) to give Rh(III) Cyclometallated complexes, in which the imine C-H bond was added oxidatively to the rhodium metal to give (H-M-C). The complexes were characterized using IR and NMR spectroscopy confirmed by elemental micro-analysis. The absorption of the hydride ligand was inferred as trans to N-donar ligand. *Corresponding author: Ibrahim Al-Najjar, Petrochemicals Research Institute, King Abdulaziz city for Science and Technology, P.O.Box 6086, Riyadh 11442, Kingdom of Saudi Arabia, Tel: 965-226-36626; E-mail: alnajjar@kacst.edu.sa Received August 17, 2014; Accepted September 27, 2014; Published October 01, 2014 Citation: Alsaygh A, Al-Humaidi J, Al-Najjar I (2014) Novel Hydrido-Rhodium (III) Complexes with Some Schiff Bases Derived from Substituted Pyridines and Aryl Amines. Mod Chem appl 2: 139. doi:10.4172/2329-6798.1000139 Copyright: © 2014 Alsaygh A, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


Introduction
Although the Cyclometallation of aromatic and to a lesser extent aliphatic C-H groups is widely recognized [1,2], these are relatively little known concerning with the cyclometallation of aldehydes [3] and imine functions [4][5][6]. We have shown that Schiff bases of 2-substituted benzylideneaminothiazoles [5], and 2-(benzylideneamino) pyridines [6], can be form cyclometallated complexes at the imine carbon by using Rh (I) complex. A number of studies have exploited ligands such as quindine-8-carbaldehyde [3,7] and 2-(benzylideneamino) pyridines [8]. Complexation of the metal with aromatic nitrogen gives a favorable geometry for the insertion of the metal into the neighboring C-H or C-C bond [4,7,9,10]. 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 [11][12][13][14][15]. Chung-Hang Leung and Dik-Lung Ma group [14] has also actively pursued the development of kinetically inert metal complexes as inhibitors of various bimolecular targets, including DNA, enzymes and protein-protein interactions [13]. The synthesis and characterization of a variety of new rhodium (III) complexes of {N-benzylideneamino} pyridines, in which the imine C-H bond has undergone oxidative addition to the metal, are reported here.

Instruments
Open capillaries were used to determine melting points and were uncorrected using Gallenkamp Melting Points Apparatus. Elemental microanalysis of the separated solid chelates for C, H, N, were performed at Perkin Elmer 2400 CHN. The analyses were repeated twice to check the accuracy of the results obtained. Infrared spectra were recorded on a Nexus 470-670-760 spectrometer and FT-IR Spectrometer, Spectrum 8400s. The 1 H, 13 CNMR and 31 P NMR spectra were recorded using 400 MHz Joel Spectrometer.

Synthesis of ligands
All experiments were carried out under an atmosphere of nitrogen by Schlenk techniques. The Schiff bases were prepared by mixing equivalent amount of substituted benzaldehydes and 2-amino pyridine derivatives in methanol solution. This mixture was boiled under reflux with stirring for 8h, at 80 o C in an oil bath, and then the mixture was concentrated by rotary evaporation to give yellow precipitate. Which was filtered off, dried, yields are 70%-80% (Scheme 1, Table 1). The results of UV, IR, 1 H and 13 C, Spectroscopy and elemental analyses for Schiff 's bases were published elsewhere [16].
Rhodium compounds of {RhCl(COD)} 2 and {RhCl(PPh 3 ) 3 } were prepared by literature procedures [17,18]. In this work rhodium cyclometallated complexes, were prepared by the reaction of the Schiff After cooling, addition of n-hexane led to precipitation of the product as a yellow powder which was filtered off (the product recrystallized twice from CH 2 Cl 2 /hexane, yield 40%-50% (Table 2 and Scheme 2).

Results and Discussion
The physical, analytical data and UV, IR, 1 H, 13 C-NMR Spectroscopy for Schiff bases were published elsewhere [16]. The corresponding Rh-complexes of different Schiff base ligand are investigated also by analytical, physical and different spectroscopy methods (Tables 3-5).

Characterization of Rh-Complexes
Infrared Spectra: Infrared spectra of the complexes were recorded to confirm their structure. The vibration frequencies and their tentative assignments for imines ligand (Scheme 1) and their Rh-complexes were assigned by comparison with the vibrational frequencies of the free ligand and their related complexes. The main futures in the infrared of the complexes is the shift of the stretching frequencies of the azomethine (-C=N-) group of the transition metal complexes to lower frequencies   in the range, 1600-1576 cm -1 , compared with free imine ligand, v(1690-1620 cm -1 ) due to the coordination of the azomethine moiety, v(C=N) to the metal [19]. Further evidence of the bonding is given by the observation of new bands in the spectra of the metal complexes of medium or week intensity at the region 467-435 cm -1 due to v(M-N) stretching vibration supporting the involvement of the nitrogen atom of the azomethine group via coordination [20,21] (Figure 1), complex (22). Further evidence come from the spectra of 1 H, 13 C and 31 P NMR (Tables 4 and 5).

1
H, 13 C and 31 P NMR Spectra: The 1 H, 13 C and 31 P NMR spectra of the rhodium complexes have been studied in CDCl 3 . The 1 H NMR spectrum of each of the new rhodium complexes in CDCl 3 , shows a hydride resonance between δ11.19-11.78 ppm ( Table 4). The imines C-H signals for the starting free imines appear at δ 9.01-9.44 ppm and after complexation these signals are absent, providing evidence for insertion of Rh metal 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 [22,23], ca. (average) δ -11.29 ppm. The hydride signals in the complexes are split by compiling to two equivalent 31 P nuclei of the rhodium complex. As both of these spin-spin couplings are ca. 11.00-14.52Hz, frequently. 1 J ( 10 3 Rh-1 H). Hz, and 2J ( 31 P-1 H), ca. 11.00 -12.45 Hz ( Table 4). The hydride multiple often appears as a pseudo quartet, but at higher resolution studies usually reveal the expected doublet of triplets (Figure 2 and Figure  3), complexes (23 and 24) .The phosphine (PPh 3 ) rhodium complexes    (17)(18)(19)(20).  (Table 4), with 1 J( 103 Rh-31 P) 98.7-118.0 Hz as a doublet in keeping with previous report [3,10,16], depending on the type of the substituent group on pyridine ring ( Table 4). The majority of the rhodium imine hydride complexes are only moderately soluble in most organic solvents. The signal of 13 C=N of the imino group is observed at ca. δ 225.06-237.60ppm (Table 5). The 13 C [ 1 H] NMR spectrum, in particular the signal from the metal-bonded carbon atom, is consistent with the presence of the cyclometallated ring [22,23]. The signal from the metal-bonded carbon, C(7) (iminoyl carbon), appear as a doublet or triplets owing to coupling of two equivalent 31 P nuclei and the 103 Rh nucleus, whereas the corresponding signal from the uncomplexed imines is found at ca. δ146.24-164.97 ppm [22]. This low-field position for C (7) has been observed in other cases in what a chelating atom is incorporated in a five member-ring [24], and is not unusual for a cyclometallated sp 2 carbon [25], similar to carbenecarbon. The remaining 1 H and 13 C data are as expected. Steric effects are extremely important to structures, spectroscopic properties, and   chemical behavior of phosphorus ligands and their complexes [26]. In this study two types of phosphorus ligands (PPh 3 and PBzPh 2 ) were used with different steric and electronic effects. The cone-angle data of Tolman [27] allows some comparisons of relative ligand steric effects to be made and demonstrates phosphine ligands such as PBzPh 2 (ca. 153 o ) and PPh 3 (ca. 145 o ). Increasing the size of the substituents on phosphorus will tend to reduce the s character in the phosphorus long pair, thus decreasing 1 J(M-P) [21]. Data from  [27,28]. the 1 J( 31 P-1H) value is consistent with a hydride located cis to two magnetically equivalent PPh 3 groups [29], which in turn are mutually trans, as inferred from 31 P [ 1 H]NMR spectrum (Table 4).
This result may be due to complex instability. The similarity of present of Cl-atom at C5 results of two or three 31 P absorption spectrum. By substitution of Br-atom at C-4 of aryl ring ( Figure 5) a significant change in signal of 31 P was recorded in Figures 4,5 and Table  4. It was also observed that the signal for C-7 (iminoyl carbon 13  at low magnetic field, at δ225.16-237.60ppm with 1 J ( 103 Rh-13 C), 32-33 Hz and 2 J ( 31 P-13 C), 8-9Hz (Table 5).
The rhodium complexes are only moderately soluble in organic solvents, and so we have not obtained many 13 C spectra, however, some 13 C (7) data for few complexes are shown in Table 5. The signal for C-7 is all at 225. .60 ppm, whereas the uncomplexed imines C-7 signal is found at δ159.39-164.97 ppm. This low field position is suggestive of carbine-like properties; however, the δ 13 C=N for complex (24) is observed at low magnetic field at δ 237.67ppm (Table 5 and Figure 6).  The chromatographic results show no indication of forming hex-5 'enylketimine. These results indicated that the bond between rhodium and hydrogen is not active enough, very stable and can't go for further reactions.

Conclusion
The new cyclometallated rhodium complexes have been characterized by elemental analysis, UV, IR, 1 H, 31 P (occasionally) and 13 C-NMR-spectroscopy. Interestingly the hydride ligand signal in IR (v 2034.9 cm -1 and 1 H-NMR (δ -11.29 ppm), complex (22). The result obtained from the spectra was expected for Rh-H group trans position to the N-donor ligand.
However, the 31 P-NMR for some cyclometallated complexes shows signal at δ 31.86ppm, complex (22). Furthermore, the 2 J ( 31 P-1H) value account for H cis to two magnetically equivalent PPh 3 -groups, which in turn are mutually trans, as inferred from 31 P(1H) NMR spectrum. This result is supported from 1 H and 13 C NMR spectra.
Interestingly, the 13 C-NMR of the iminoyl carbon ( 13 C=N) signal in Rh(III) (δ 225.16-237.60 ppm). This low-field position for cyclometallated complexes is suggestive of carbene-like properties. The result from the study indicated that the bond between rhodium and hydrogen is not active enough, very stable and can't go for further reactions.