Quantum Chemical Study of Mixed-Ligand Monometallic Ruthenium(II) Complex of Composition [(bpy)2Ru(H3Imbzim)](ClO4)2⋅2H2O

On the basis of density functional theoretical approach, we have assessed the ground state geometries and absorption spectra of recently synthesizedmonometallic ruthenium (II) complex of composition [(bpy) 2 Ru(H 3 Imbzim)](ClO 4 ) 2 ⋅2H 2 Owhere bpy = 2,2bypyridine and H 3 Imbzim = 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.


Introduction
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 [1][2][3][4][5][6][7][8].This importance can be visualized from the facts that majority of enzymes bind anions as either substrate or cofactor and many act as ubiquitous nucleophiles, bases, redox agents, and phase transfer catalysts [9,10].Diseases like cystic fibrosis and Alzheimer's are induced by the malfunction of natural anion regulation processes [11,12].Even, from environmental point of view, the eutrophication of water is an important issue which is caused by phosphate and nitrate ions and used in agriculture fertilizers [13][14][15][16].The advantage of the transition metal complex of composition [(bpy) 2 Ru(H 3 Imbzim)] 2+ where bpy = 2,2  -bypyridine and H 3 Imbzim = 4,5-bis(benzimidazol-2-yl)imidazole to act as a sensor is that it contains three potent NH bonds that can be donated for the hydrogen bonding to the anions.Given this importance, it is not surprising that extensive experimental studies have been dedicated to design of simple artificial anion receptors and sensors [17][18][19][20].However, there is not very clear understanding about the guiding principles about the selection of right kind of moiety as a chromophore and ligand as an anion sensor.In such situation, quantum chemical based electronic structure calculations can be expected to provide us valuable information about its structural features which in turn could be further used for better understanding of structural features and hence for enhanced appreciative of the factors responsible for such properties.Recently, Debasish et al. [21] have synthesized and characterized monometallic ruthenium (II) complex of composition [(bpy) 2 Ru(H 3 Imbzim)](ClO 4 ) 2 ⋅2H 2 O using standard analytical and spectroscopic techniques.On the basis of their study, they have proposed molecular structure from single crystal X-ray measurements.Furthermore, they have experimentally explored in detail the electronic absorption and emission behavior in different solvents.In this work, our aim is to complement experimental work with quantum chemical calculations in order to have better understanding of it in the one hand while developing it with some confidence for better predicting power of some experimentally unexplored properties on the other hand.The advancement in DFT as computational methodology has reached a point where predicted properties can be expected to be of reasonable chemical accuracy.
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.

Methods of Calculations
All the theoretical calculations were performed with the Gaussian-03 program package [22].Full geometry optimization in gas and solvent phase were carried out using B3LYP functional and 6-31G * * [23] as basis set except for ruthenium for which we used LANL2DZ [24] pseudopotential.The hessian matrix has been calculated to make sure that the given structure is at its minima.Thereafter, the absorption spectra have been calculated as vertical electronic excitations from the ground state using TD-DFT approach with PBE1 as functional of the choice and 6-31G * * basis set for all the elements present in the complex except for ruthenium for which LANL2DZ [24] has been employed as implemented in Gaussian-03 [22].The choice of PBE1 hybrid functional [25][26][27] has been made as its efficiency on a wide range of compounds has been shown in the many earlier reports in literature [28].The effect of solvent on absorption spectra was taken care by using polarizable continuum model (PCM) of Tomasi and coworkers [29].Default parameters were used for the solvents dimethyl sulfoxide (DMSO) and acetonitrile (CH 3 CN).

Results and Discussion
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 [21].The optimized geometry and the corresponding figure with numbering scheme for the title complex have been depicted in Figure 1.
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 1(a) and 1(b) for comparison, and different conclusions may be drawn by analyzing the data.It is clear from crystal structure that the monometallic complex consists of hexa-coordinated Ru II center in which the Ru II (bpy) 2 unit is coordinated to one of the two imidazole (central) nitrogen N8 and one benzimidazole nitrogen N1 of 4,5-bis(benzimidazol-2yl)imidazole.In this complex, three different types of metalnitrogen distances are observed.The longest Ru-N distance that involves the benzimidazole nitrogen atom is 2.093 Å.
The next longest Ru-N distance, pertaining to the central imidazole nitrogen atom, is 2.077 Å.The bipyridine ligand provides shortest Ru-N distances with average value of 2.033 Å.
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 cis-angle varies from 78.42 ∘ to 79.21 ∘ , while the trans-angles vary between 170.99 ∘ and 175.35 ∘ .However, it is also reflected from the computed bond angles that this complex deviates from the regular octahedral geometry.It could be due to electronic repulsions as well as due to steric hindrances amongst the ligands.Both experimental and theoretical methods are in agreement with their observations and predictions.To be more precise, the largest deviations in bond lengths and bond angles are 0.033 Å and 2.15 ∘ , respectively.These results reflect how much theoretical methods of calculation are in agreement with the experimental methods.The comparison of different bond angles for the title complex is summarized in Table 1(b).
In monometallic Ru II 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⋅ ⋅ ⋅ N distance of 2.096 Å.However, theoretically calculated N-H⋅ ⋅ ⋅ N distance is 1.864 Å and N-H distance is 1.0313 Å.In addition, the complex also possesses a pair of externally directed NH protons of H 3 Imbzim − which adopt cis-arrangement and could be used for the formation of adduct with anion via hydrogen-bonding interactions.The N-H and N-H⋅ ⋅ ⋅ N distances of both the protons have not been explored experimentally because of the low electron density at hydrogen atom.However, theoretically calculated N-H and N-H⋅ ⋅ ⋅ N distances of both the protons are 1.030 Å and 1.8475 Å, respectively.Since the predicted parameters for this complex are in good agreement with experimental data, thereby providing strong support for the use of these theoretical methods for structural and spectroscopic studies.
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 - * transition of bpy, while the next higher wavelength absorption occurring between 340 and 370 nm is due to ligand centered transitions of bridging ligand.In addition to these transitions, this complex exhibits two fairly intense absorptions at 475 and 425 nm and the next higher energy bands are due to Ru (d)-to-bpy( * ) charge-transfer (MLCT) transitions.
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 d-d transitions.
Even though, theoretically, it is possible to explain nature of each and every transition, but here we will try to compare the transitions of prime interest.This can be explained on the basis of the fact that the - * transitions of the bridged ligand are lower in energy than those of bpy.In addition to these bands, theoretically, an additional intense band is observed at 412 nm and come up because of the transitions from 163 → 168 and 163 → 169.Although, the nature of transitions is comparable with the experimentally observed results, the wavelengths of absorption corresponding to these transitions in gas phase are slightly different from the actual absorptions.These transitions are responsible for the explanations of many properties including colour, sensing capabilities, binding capacity, and many more.These different types of charge transfers can be visualized using MO diagrams, provided in Figures 2(a) to 2(q) with the molecular orbital number given in the parenthesis and the nature of transitions in Table 2, respectively.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, CH 3 CN 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 3, it is found that there is small increase in the values of excitation energies in case of CH 3 CN and DMSO in comparison to the excitation energies of gas phase geometry.The maximum shift in excitation energies obtained in case of CH 3 CN and DMSO is 0.1201 and 0.1180 eV, respectively, from the gas phase excitation energies values, amongst the first ten excitations.On the other hand the maximum shift obtained in the excitation energy values between gas and solvent phase optimized geometries for the same solvent (CH 3 CN) is 0.0315 eV.The difference between excitation energies of gas phase and solvent phase optimized geometry is very small and is almost in the range of gas phase optimized excitation energy values.However, small alterations in the values of excitation energies could be attributed to the interaction of fields produced by ligands and metal atom with the field of solvent molecules which affect the energy gap between the various molecular orbitals and lead to the shift in the values of excitation energies.Since the strength of these interactions varies from solvent to solvent, this leads to the shift in excitation energies accordingly.One of the main components for such interactions is the strength of hydrogen bonding, which is different for different solvents.These shifts are almost equal in magnitude because of similar dipole moments for both of these solvents.However, slight variation may be due to different hydrogen bonding capacities and different arrangement of atoms in the space, which are responsible for the production of field interactions.These solvent interactions will lead to the increase in the electron density at the metal center resulting in decrease of the band energies which leads to shift in the values of excitation energies.
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 CH 3 CN to orange-brown in DMSO.

Conclusions
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 [1][2][3][4][5][6][7][8].In this study, optimization was carried at DFT level, while electronic absorption spectra were computed as vertical electronic excitations from the ground state using TD-DFT approach.We have chosen PBE1 hybrid functional for the calculation of ground state and excited state properties and the effect of solvent on excitation energies was taken carefully by using polarizable continuum model [26] (PCM).
The comparison of optimized geometry with X-ray geometry, effect of CH 3 CH and DMSO on the excitation energies, comparison of absorption spectra, and the assignment of nature of different charge transfers have been done in the present work.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

Figure 1 :
Figure 1: Numbering scheme of the complex molecule.
Figure 2 The intense bands due to - * transitions of ligands are observed at 235 and 286 nm.

Table 1 :
(a) Comparison of selected bond lengths and (b) comparison of selected bond angles.

Table 3 :
Effect of solventson excitation energies.Excitation energy values obtained from solvent phase optimized geometry.DFT and TD-DFT methods, future research studies should consider this method of calculating ground and excited state properties. *