Structural Prediction of Bis { ( dip-anisole )-1 , 4-azabutadiene }-bis [ triphenylphosphine ] ruthenium ( II ) Using 31 P NMR Spectroscopy

The present paper reports the use of P NMR spectroscopy to predict the isomer structures of [bis{4-methoxy-phenyl-[3-(4methoxy-phenyl)-allylidene]-amino}]-bis[triphenylphosphine]ruthenium(II), also known as bis{(di-p-anisole)-1,4-azabutadiene}bis[triphenylphosphine]ruthenium(II), complexes. The complexation reaction was carried out under refluxing condition of (di-panisole)-1,4-azabutadiene (compound 1), triphenylphosphine (PPh3), and ruthenium chloride in the ratio of 2 : 2 : 1 for five hours. In addition, ruthenium(II) complexes were also characterized using FTIR and UV-Vis spectroscopy to support the formation of ruthenium(II) complexes. PNMR spectroscopic study on ruthenium(II) complexes suggested that there are three isomers present after the complexation reaction and all the ruthenium complexes demonstrate octahedral geometry.


Introduction
Nuclear magnetic resonance (NMR) spectroscopy is an essential instrument in chemistry as it can determine the structure of a molecule, the presence of impurities in a sample and the rate of formation as well as degradation of a compound. Even in 1970s, NMR has already been used to determine the cancer formation which offered a simple, fast, and low cost method to identify cancer formation [1][2][3]. For inorganic chemist, the use of 31 P NMR to identify the structure of a complex containing phosphine ligands is very common [4,5]. One of the well-known examples is the use of 31 P NMR spectroscopy to determine the Wilkinson hydrogenation mechanism by identifying the coupling patterns among phosphine ligands and also the coupling constants between phosphine ligands as well as rhodium(I) metal centre [6].
In our long term research interest in ruthenium(II) complexes synthesis, we used (di-p-anisole)-1,4-azabutadiene (1) and triphenylphosphine (PPh 3 ) as the ligands to react with ruthenium trichloride under reflux condition. The products formed were checked by using 31 P NMR spectroscopy and the results found in the spectra are worth to be discussed in the present communication.

Methodology
The ruthenium complexes were characterized using UV/Vis, FTIR, and 31 P NMR spectroscopy. The IR spectra were recorded using a Thermo Scientific Nicolet iS10 in KBr disc. 1 H NMR spectrum for compound 1 and 31 P NMR spectrum for ruthenium(II) complexes were recorded using JEOL JNM-ECA 500 spectrometer with TMS as an internal standard. The absorption spectra were recorded with Jasco V-630 spectrophotometer.

Results and Discussion
The appearance of two pairs of doublets and one singlet in the 31 P NMR spectrum for ruthenium complexes (Figure 1) indicates that there are three isomers present in the complexation reaction with the ratio of 1 : 1 : 1.
The singlet at 29.88 ppm reveals that the two PPh 3 are magnetically equivalent in ruthenium(II) complex. In this case, the two PPh 3 are either located at axial position, which is trans to each other (Figure 2(a)) [7], or located at equatorial plane, which is only trans to either C atom from C=C or N atom from N=C (Figure 2(b)). Apparently, the one shown in Figure 2(a) is a trans-isomer, whereas the two isomers in Figures 2(b) and 2(c) are cis-isomer. Unfortunately, we cannot identify which one is the correct structure represented by the singlet at 29.88 ppm at this stage.
Meanwhile, a pair of doublets at 41.84 and 39.74 ppm with P-P coupling constant of 21 Hz is assigned to a cisisomer of ruthenium(II) complex as shown in Figure 3(a). Lastly, another pair of doublets at 49.80 and 47.36 ppm with P-P coupling constant of 38 Hz is assigned to a transruthenium(II) complex (Figure 3(b)). The difference in P-P coupling between ruthenium(II) complexes in Figures 3(a) and 3(b) is due to the positions of PPh 3 ligands. The smaller coupling constant, namely, 21 Hz, is assigned to the cis-isomer because both PPh 3 ligands are in the equatorial plane. The presence of doublets for the PPh 3 ligands in the complex is shown in Figure 3(a) because both PPh 3 ligands are trans to different atoms, that is, nitrogen and carbon atoms. For ruthenium(II) complex as shown in Figure 3(b), the two PPh 3 ligands are located at axial position and trans to each other. Unlike the trans complex in Figure 2(a), the magnetic field of these two PPh 3 in Figure 3(b) is different because the two ligands of (di-p-anisole)-1,4-azabutadiene are trans to each other at the equatorial plane (Figure 3(b)). Lastly, the single peak observed at 35.14 ppm is attributed to the presence of the triphenylphosphine oxide [8].
On the other hand, the binding of compound 1 to ruthenium(II) metal centre can be confirmed using FTIR and UV-Vis spectroscopy. Comparing the IR spectra between compound 1 and ruthenium complexes (Figure 4), the vibrations of C=N and C=C stretching bands have been shifted after binding to ruthenium(II) metal centre. For C=N stretching band, it shifted from 1627 cm −1 in compound 1 to 1661 cm −1 in ruthenium complex [9,10], whereas for C=C stretching, the IR band appears at 1601 cm −1 in compound 1 but it is not clearly shown in the complex because the IR bands of C=C bands for aliphatic and aromatic were merging into one board IR band centred at 1576 cm −1 . Nevertheless, two additional IR peaks are present in the finger print region at 577 and 654 cm −1 indicating the formation of respective Ru-N and Ru-C bonds [11].
The complexation of compound 1 to ruthenium(II) metal centre can be further supported by the UV-Vis data as shown in Figure 5. For compound 1, two absorption bands were observed at 273 and 372 nm which are assigned to → * transition of the benzene ring and → * transition of the imine group [12], respectively. After the complexation,  both absorption bands shifted to 321 and 382 nm, respectively. Significant shifts of these two absorption bands have proven compound 1 was successfully bound to ruthenium(II) metal centre via the nitrogen atom from C=N group and carbon atom from C=C aliphatic group in C=C-C=N moiety. The bathochromic shift of these two absorption bands was due to the backbonding of electrons from Ru to the antibonding orbitals of C=C-C=N moiety in compound 1. This, in turn, has weakened the bond in C=C-C=N [13].

Conclusion
The evidence from 31 P NMR spectrum has shown the presence of three isomers of bis{(di-p-anisole)-1,4-azabutadiene}bis[triphenylphosphine]ruthenium(II) complex in the ratio of 1 : 1 : 1. In addition, the data from IR and UV-Vis revealed that compound 1 has bound to ruthenium(II) metal centre.