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

Authors: Meng Guan Tay; Thareni Lokanathan; Kok Tong Ong; Ruwaida Asyikin Abu Talip; Ying Ying Chia
Journal: International Journal of Inorganic Chemistry
 (2016)

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## Abstract

The present paper reports the use of31P NMR spectroscopy to predict the isomer structures of [bis{4-methoxy-phenyl-[3-(4-methoxy-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-p-anisole)-1,4-azabutadiene (compound1), 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. 31P NMR 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.


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## Body

## 1. 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 [1–3]. For inorganic chemist, the use of 31P 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 31P 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 (PPh3) as the ligands to react with ruthenium trichloride under reflux condition. The products formed were checked by using 31P NMR spectroscopy and the results found in the spectra are worth to be discussed in the present communication.


## 2. Methodology

The ruthenium complexes were characterized using UV/Vis, FTIR, and31P NMR spectroscopy. The IR spectra were recorded using a Thermo Scientific Nicolet iS10 in KBr disc. 1H NMR spectrum for compound1 and 31P 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.


### 2.1. Preparation of (4-Methoxy-phenyl)-[3-(4-methoxy-phenyl)-allylidene]-amine or (di-p-Anisole)-1,4-azabutadiene (1)

4-Methoxycinnamaldehyde (1.62 g, 10.00 mmol) was dissolved in 10 mL of ethanol and followed by 4-methoxyaniline (1.23 g, 10.00 mmol) which was then added to the solution. The reaction mixture was stirred for 4 hours and resulted in green-yellow solid. The solid was filtered, washed with 5 mL of ethanol, and driedin vacuo. The solid was purified by dissolving in DCM and layered with hexanevia slow diffusion: yield: 2.368 g (88.7%); IR (KBr, cm−1)ν: 3036 (C-H stretching), 1627 (C=N- stretching), 1601 (C=C stretching, aliphatic), 1575 and 1468 (C=C stretching, aromatic), and 1110 (OCH3 stretching); 1H NMR (500 MHz, CDCl3,) δ: 8.25 (d, 1H, J
=
8 Hz, -CH=N-), 7.47 (d, 2H, J
=
8 Hz, H
a
r
o
m), 7.18 (d, 2H, J
=
8 Hz, H
a
r
o
m), 7.05 (t, 1H, J
=
16 Hz, H-Cα), 6.99 (m, 1H, H-Cβ), 6.90 (d, 4H, J
=
7 Hz, H
a
r
o
m), 3.83 (s, 3H, OCH3), and 3.81 (s, 3H, OCH3); UV-Vis (DCM, λ
m
a
x/nm): 273, 373; Anal. Calc. for C17H17O2N (%): C, 76.38; H, 6.41; N, 5.24; found (%): C, 76.75; H, 6.31; N, 5.05.


### 2.2. Preparation of [Bis{4-methoxy-phenyl-[3-(4-methoxy-phenyl)-allylidene]-amino}]-bis-[triphenylphosphate]ruthenium(II) or Bis{(di-p-anisole)-1,4-azabutadiene}-bis[triphenylphosphine]ruthenium(II) Complexes

RuCl3·xH2O (2.070 g, 1.0 mmol) and PPh3 (0.525 g, 2.0 mmol) were added to a round bottom flask containing 10 mL ethanol and the mixture was then refluxed for 5 h. Compound1(0.316 g, 2.0 mmol) was then added to the round bottom flask and refluxed for another 5 h. Pale maroon solids were formed, filtered, and washed with hexane. The precipitate was dried invacuo: IR (KBr, cm−1) ν: 3034 (C-H stretching), 1661 (C=N), 1576 (merge IR band of C=C stretching from aliphatic and aromatic), 1469 (C=C stretching of aromatic ring), and 654 (Ru-C), 577 (Ru-N); 31P NMR (202.5 MHz, CDCl3) δ: 49.7 (d, 1P, J
P
-
P
=
38 Hz), 47.4 (d, 1P, J
P
-
P
=
38 Hz), 41.7 (d, 1P, J
P
-
P
=
21 Hz), 39.7 (d, 1P, J
P
-
P
=
21 Hz), 35.1 (s, Ph3P=O), and 29.9 (s, 1P); UV-Vis (DCM) (λ
m
a
x
/
n
m): 321 and 382.


## 2.1. Preparation of (4-Methoxy-phenyl)-[3-(4-methoxy-phenyl)-allylidene]-amine or (di-p-Anisole)-1,4-azabutadiene (1)

4-Methoxycinnamaldehyde (1.62 g, 10.00 mmol) was dissolved in 10 mL of ethanol and followed by 4-methoxyaniline (1.23 g, 10.00 mmol) which was then added to the solution. The reaction mixture was stirred for 4 hours and resulted in green-yellow solid. The solid was filtered, washed with 5 mL of ethanol, and driedin vacuo. The solid was purified by dissolving in DCM and layered with hexanevia slow diffusion: yield: 2.368 g (88.7%); IR (KBr, cm−1)ν: 3036 (C-H stretching), 1627 (C=N- stretching), 1601 (C=C stretching, aliphatic), 1575 and 1468 (C=C stretching, aromatic), and 1110 (OCH3 stretching); 1H NMR (500 MHz, CDCl3,) δ: 8.25 (d, 1H, J
=
8 Hz, -CH=N-), 7.47 (d, 2H, J
=
8 Hz, H
a
r
o
m), 7.18 (d, 2H, J
=
8 Hz, H
a
r
o
m), 7.05 (t, 1H, J
=
16 Hz, H-Cα), 6.99 (m, 1H, H-Cβ), 6.90 (d, 4H, J
=
7 Hz, H
a
r
o
m), 3.83 (s, 3H, OCH3), and 3.81 (s, 3H, OCH3); UV-Vis (DCM, λ
m
a
x/nm): 273, 373; Anal. Calc. for C17H17O2N (%): C, 76.38; H, 6.41; N, 5.24; found (%): C, 76.75; H, 6.31; N, 5.05.


## 2.2. Preparation of [Bis{4-methoxy-phenyl-[3-(4-methoxy-phenyl)-allylidene]-amino}]-bis-[triphenylphosphate]ruthenium(II) or Bis{(di-p-anisole)-1,4-azabutadiene}-bis[triphenylphosphine]ruthenium(II) Complexes

RuCl3·xH2O (2.070 g, 1.0 mmol) and PPh3 (0.525 g, 2.0 mmol) were added to a round bottom flask containing 10 mL ethanol and the mixture was then refluxed for 5 h. Compound1(0.316 g, 2.0 mmol) was then added to the round bottom flask and refluxed for another 5 h. Pale maroon solids were formed, filtered, and washed with hexane. The precipitate was dried invacuo: IR (KBr, cm−1) ν: 3034 (C-H stretching), 1661 (C=N), 1576 (merge IR band of C=C stretching from aliphatic and aromatic), 1469 (C=C stretching of aromatic ring), and 654 (Ru-C), 577 (Ru-N); 31P NMR (202.5 MHz, CDCl3) δ: 49.7 (d, 1P, J
P
-
P
=
38 Hz), 47.4 (d, 1P, J
P
-
P
=
38 Hz), 41.7 (d, 1P, J
P
-
P
=
21 Hz), 39.7 (d, 1P, J
P
-
P
=
21 Hz), 35.1 (s, Ph3P=O), and 29.9 (s, 1P); UV-Vis (DCM) (λ
m
a
x
/
n
m): 321 and 382.


## 3. Results and Discussion

The appearance of two pairs of doublets and one singlet in the31P 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.Figure 1
31P NMR spectrum for ruthenium(II) complexes.The singlet at 29.88 ppm reveals that the two PPh3 are magnetically equivalent in ruthenium(II) complex. In this case, the two PPh3 are either located at axial position, which istransto each other (Figure 2(a)) [7], or located at equatorial plane, which is onlytrans 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 atrans-isomer, whereas the two isomers in Figures 2(b) and 2(c) arecis-isomer. Unfortunately, we cannot identify which one is the correct structure represented by the singlet at 29.88 ppm at this stage.Figure 2
Postulated structure of (a)trans- and ((b) and (c))cis-[bis{(di-p-anisole)-1,4-azabutadiene}]-bis[triphenylphosphine]ruthenium(II).Meanwhile, a pair of doublets at 41.84 and 39.74 ppm withJ
P
-
P coupling constant of 21 Hz is assigned to acis-isomer of ruthenium(II) complex as shown in Figure 3(a). Lastly, another pair of doublets at 49.80 and 47.36 ppm with J
P
-
P coupling constant of 38 Hz is assigned to atrans-ruthenium(II) complex (Figure 3(b)). The difference in J
P
-
P coupling between ruthenium(II) complexes in Figures 3(a) and 3(b) is due to the positions of PPh3 ligands. The smaller coupling constant, namely, 21 Hz, is assigned to thecis-isomer because both PPh3 ligands are in the equatorial plane. The presence of doublets for the PPh3 ligands in the complex is shown in Figure 3(a) because both PPh3 ligands aretrans to different atoms, that is, nitrogen and carbon atoms. For ruthenium(II) complex as shown in Figure 3(b), the two PPh3 ligands are located at axial position andtrans to each other. Unlike thetrans complex in Figure 2(a), the magnetic field of these two PPh3 in Figure 3(b) is different because the two ligands of (di-p-anisole)-1,4-azabutadiene aretrans 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].Figure 3
Postulated structure of (a)cis- and (b)trans-[bis{(di-p-anisole)-1,4-azabutadiene}-bis[triphenylphosphine]ruthenium(II)].On the other hand, the binding of compound1 to ruthenium(II) metal centre can be confirmed using FTIR and UV-Vis spectroscopy. Comparing the IR spectra between compound1 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 compound1 to 1661 cm−1 in ruthenium complex [9, 10], whereas for C=C stretching, the IR band appears at 1601 cm−1 in compound1 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].Figure 4
IR spectra of compound1 (a) and ruthenium(II) complexes (b).
(a)
(b)The complexation of compound1to ruthenium(II) metal centre can be further supported by the UV-Vis data as shown in Figure 5. For compound1, two absorption bands were observed at 273 and 372 nm which are assigned to π
→
π
∗ transition of the benzene ring and n
→
π
∗ 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 compound1 was successfully bound to ruthenium(II) metal centrevia 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 π electrons from Ru to the antibonding orbitals of C=C-C=N moiety in compound1. This, in turn, has weakened the bond in C=C-C=N [13].Figure 5
UV-Vis spectra of compound1 (a) and ruthenium(II) complex (b).
(a)
(b)


## 4. Conclusion

The evidence from31P 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 compound1 has bound to ruthenium(II) metal centre.



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*Source: 7095624-2016-11-10.xml*