On the basis of density functional theoretical approach, we have assessed the ground state geometries and absorption spectra of recently synthesized monometallic ruthenium (II) complex of composition [(bpy)2Ru(H3Imbzim)](ClO4)2·2H2O where bpy = 2,2′-bypyridine and H3Imbzim = 4,5-bis(benzimidazol-2-yl)imidazole. The all different kinds of charge transfers such as ligand-ligand, and metal-ligand have been quantified, compared, and contrasted with the experimental results. In addition, the effect of solvent on excitation energies has been evaluated. In spite of some digital discrepancies in calculated and observed geometries, as well as in absorption spectra, the density functional theory (DFT) seems to explain the main features of this complex.
In recent times, the importance of inorganic complexes has been reported extensively keeping in mind the wide range of applicability these complexes possess in different domains of life. Thanks to the synthetic chemists for synthesizing, characterizing, and demonstrating the wide range of applicability of these complexes. Among the entire applicability domain, the recognition and sensing of anions is one of the recently emerged and challenging areas in the field of research. This is due to the important role played by anions in the field of biological, industrial, agricultural, and environmental processes [
The present paper reports the comparison between gas phase optimized geometry with X-ray geometry and determination of position of hydrogen atoms involved in hydrogen bonding, which is not possible by single crystal XRD due to its low electron density. Furthermore the assignment of gas phase electronic spectra using TD-DFT calculations and the effect of solvents on excitation energies has been carried out.
All the theoretical calculations were performed with the Gaussian-03 program package [
It is important to compare and contrast the essential structural parameters obtained as a result of geometry optimization with respect to experimentally reported X-ray structure [
Numbering scheme of the complex molecule.
The experimental data for this complex is available, results for related molecule in terms of structural parameters (bond lengths and bond angles) are included in Tables
(a) Comparison of selected bond lengths and (b) comparison of selected bond angles.
Serial number | Name of bond lengths | Bond lengths | |
---|---|---|---|
Computed | Experimental | ||
1 | Ru-N6 | 2.060 | 2.033 |
2 | Ru-N4 | 2.072 | 2.039 |
3 | Ru-N3 | 2.067 | 2.039 |
4 | Ru-N5 | 2.074 | 2.074 |
5 | Ru-N8 | 2.112 | 2.077 |
6 | Ru-N1 | 2.093 | 2.093 |
Serial number | Name of bond angles | Bond angles (in degree) | |
---|---|---|---|
Computed | Experimental | ||
1 | N6-Ru-N4 | 89.84 | 89.20 |
2 | N6-Ru-N3 | 97.06 | 96.47 |
3 | N6-Ru-N5 | 78.44 | 78.88 |
4 | N4-Ru-N3 | 78.44 | 79.21 |
5 | N4-Ru-N5 | 98.45 | 100.60 |
6 | N3-Ru-N5 | 174.61 | 175.35 |
7 | N6-Ru-N8 | 173.26 | 172.98 |
8 | N4-Ru-N8 | 95.45 | 95.51 |
9 | N3-Ru-N8 | 88.08 | 89.54 |
10 | N5-Ru-N8 | 96.63 | 95.10 |
11 | N6-Ru-N1 | 97.23 | 97.48 |
12 | N4-Ru-N1 | 171.38 | 170.99 |
13 | N3-Ru-N1 | 95.75 | 93.97 |
14 | N5-Ru-N1 | 87.80 | 86.67 |
15 | N8-Ru-N1 | 77.85 | 78.42 |
However, computationally calculated longest Ru–N distance involves the central imidazole nitrogen with the value equal to 2.112 Å. The next longest Ru–N distance involves the benzimidazole nitrogen atom with the value equal to 2.110 Å. The bipyridine ligand provides the shortest Ru–N distance with average value of 2.060 Å. The deviation of metal complex from the idealized octahedral geometry is reflected in their bond angles. The
In monometallic RuII complex it has been experimentally found that the NH proton of metal coordinated benzimidazole side is involved in an intramolecular hydrogen bonding interaction with nitrogen atom of the free benzimidazole moiety with N–H
The experimentally determined electronic spectra of the Ru (II) complex exhibit a number of absorption bands in the UV-visible region. The two most intense bands are observed at around 242 and 290 nm due to
The interpretation and assignment of the theoretically calculated spectra of this complex have been done on the basis of shapes of Kohn-Sham orbitals. All types of charge transfers have been found in this complex, these include ligand centered transfer of bridged ligands towards the central metal atom (LMCT) or between the ligands (LL), and sometimes it has been found that the charge transfer occurs from the central metal atom to the ligands (MLCT). In addition to these charge transfers, the complex also displays
Electronic transitions computed at the level of time-dependent density functional theory for [(bpy)2Ru(H3Imbzim)]+2 complex.
Electronic states | Excitation energy (eV) | Wavelength (nm) | Oscillator strength | Types of transition | Nature of transition Assignment | |
---|---|---|---|---|---|---|
01 | 2.5529 | 485.67 | 0.2010 |
|
−0.41477 | MLCT |
|
0.47766 | MLCT | ||||
|
||||||
02 | 2.8490 | 435.19 | 0.1570 |
|
−0.28104 | MLCT |
|
0.34023 | MLCT | ||||
|
−0.29161 | Mixed | ||||
|
0.30828 | LL | ||||
|
||||||
03 | 3.0068 | 412.35 | 0.0807 |
|
0.54778 | MLCT |
|
0.28194 | MLCT | ||||
|
||||||
04 | 3.2830 | 377.66 | 0.1804 |
|
−0.30132 | MLCT |
|
0.51815 | MLCT | ||||
|
||||||
05 | 3.4396 | 360.46 | 0.2244 |
|
0.35197 | MLCT |
|
−0.26725 | MLCT | ||||
|
0.42918 | MLCT | ||||
|
0.25544 | LL | ||||
|
||||||
06 | 3.5832 | 346.01 | 0.1440 |
|
−0.26963 | MLCT |
|
−0.28653 | MLCT | ||||
|
0.31145 | MLCT | ||||
|
0.36692 | MLCT | ||||
|
||||||
07 | 4.3353 | 286 | 0.0648 |
|
0.43704 | LL |
|
0.12355 | LL | ||||
|
0.21078 | LL | ||||
|
0.32326 | LL | ||||
|
0.13110 | LL | ||||
|
−0.13259 | LL | ||||
|
−0.10353 | LL | ||||
|
−0.12937 | LL | ||||
|
||||||
08 | 5.2682 | 235 | 0.0226 |
|
−0.16029 | LL |
|
−0.11813 | LL | ||||
|
0.39092 | LL | ||||
|
−0.18365 | LL | ||||
|
0.20017 | LL | ||||
|
0.25480 | LL | ||||
|
0.12910 | LL | ||||
|
0.10284 | LL | ||||
|
−0.12647 | LL | ||||
|
−0.12578 | LL |
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One of the important aspects of this work is to study the effect of solvents on the excitation energies. We adopt two-step approach to study the effect of solvent on excitation energies. In the first step we calculate excitation energies in two different solvents, namely, CH3CN and DMSO, on gas phase optimized structure. We further go on calculating excitation energies on solvent phase optimized geometry in the second step. On the basis of calculated results reported in Table
Effect of solventson excitation energies.
Electronic state | Excitation energy in gas phase (eV) | Excitation energy in DMSO (eV) | Excitation energy in CH3CN (eV) | *Excitation energy in CH3CN (eV) |
---|---|---|---|---|
1 | 2.5529 | 2.6503 | 2.6486 | 2.6485 |
2 | 2.6545 | 2.6702 | 2.6682 | 2.6711 |
3 | 2.7310 | 2.7901 | 2.7887 | 2.7849 |
4 | 2.8490 | 2.8370 | 2.8356 | 2.8239 |
5 | 3.0068 | 2.9916 | 2.9943 | 2.9758 |
6 | 3.0859 | 3.1035 | 3.1070 | 3.0976 |
7 | 3.1545 | 3.1715 | 3.1719 | 3.1631 |
8 | 3.2052 | 3.2905 | 3.2937 | 3.2835 |
9 | 3.2830 | 3.4015 | 3.4031 | 3.3822 |
10 | 3.4396 | 3.5296 | 3.5283 | 3.4968 |
From the above discussion it may also be concluded that there are two main factors responsible for the change in energy gap of various molecular orbitals which are polarity and hydrogen bonding ability of the solvent. Due to difference in energy gap, the complex absorbs at different wavelengths and displays different colour in different solvents. This conclusion is reflected from the experimental observations that the complex changes its colour from yellow-orange in CH3CN to orange-brown in DMSO.
The recognition and sensing of anions has emerged recently as a key research area within the generalized area of supramolecular chemistry for the important role played by anions in biological, industrial, and environmental processes [
Finally, it is observed that DFT and TD-DFT calculations performed on this complex are adequate in the reproduction of excitation and absorption energies and thus can be used in the design of anion sensors. Based on good reliability of DFT and TD-DFT methods, future research studies should consider this method of calculating ground and excited state properties.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors would like to thank Central University of Gujarat, Gandhinagar, for providing basic computational facility.