INTRODUCTION:
Bipyridine molecules have been extensively used as multidentate ligands in coordination, and supramolecular chemistry due to their strong binding affinities to metal ion[1]. Moreover, the metal complexes of these ligands and their derivatives play important roles in photochemical and photophysical processes, synthetic chemistry, biological and medicinal applications[2-3]. For example, the metal complexes of bipyridines are widely used for investigating fundamental photophysical processes and have been considered to be potentially valuable systems for solar energy conversion and nanotechnology[4-8]. In addition, some transition metal bipyridine complexes can serve as the active sites of many catalysts and are essential for the activities of several enzymes. 22BIPY (Figure-1a) is one of the most important representatives of the bipyridine ligands with the presence of two adjacent coordinating sites[9]. The stereochemistry of 22BIPY has been the subject of many experimental and theoretical investigations. 44BIPY (Figure-1b) has two nitrogen atoms at the opposite ends and, therefore, is considered as a bifunctional, non-chelating molecule used as a bridging ligand in coordination chemistry. Several previous experimental and theoretical investigations have been carried out on the rotational conformations of 44BIPY.
Copper plays an important role in organisms as one of the so-called “essential” metals. The ubiquitous Cu-organonitrogen complexes and their extensive applications have inspired much interest in the bonding, structures, and properties of Cu and nitrogen containing molecules. This research paper presents a systematic quantum chemical study of the metal-ligand binding interaction in Cu-bipyridine complexes.
The extensive applications of these metal-polypyridine complexes have inspired considerable interests in their reactions, bonding and structures. A large number of experimental and theoretical investigations about the complexes of transition metal cations with bipyridines and their derivatives have been reported previously[10-20] except copper.Therefore, it would be interesting to determine the bonding interactionbetween bipyridines and other transition metal atoms such as copper in this research paper.
Figure-1: Structures of 2,2’-bipyridine (a) and 4,4’-bipyridine (b)
Quantum chemical calculations
With a rapid growth in CPU speed, quantum chemical calculations using available program packages have become increasingly more sophisticated and valuable for predicting, modelling, and understanding experimental measurements. Quantum chemical calculations can provide quite reliable results about molecular geometries, energies, electronic states, vibrational frequencies, and reaction dynamics. In our work, theoretical calculations are employed to predict the molecular structures, electronic states, bond strengths, and vibrational frequencies of the neutral and ionic metal complexes as well as the AIEs of the neutral metal complexes.
In this work, the second order MP method (MP2) is used in the theoretical calculations, as it yields good predictions for the Cu complexes. In addition to the MP2 method, the MP3 and MP4 methods include higher order energy corrections. However, the MP3 method yields little improvement, and both MP3 and MP4 methods are much more expensive. Therefore, we mainly used the MP2 method for the theoretical calculations in this work. In our work, all the calculations are carried out with6-311+G (d,p) basis sets. The geometry optimization by the MP2 method is usually started from an optimized structure obtained from faster density functional theory (DFT) calculations. DFT methods are different from the wave function-based ab initio methods where the energy of an electronic system is expressed in terms of its electron density. DFT methods have a lower computational cost and a wider range of applications. Although there have been some concerns about the accuracy and reliability of DFT, these calculations have become very popular in modelling various types of molecular systems[14-16]and have yielded reasonable agreement with the experimental results.
Computational strategy and procedure
In this work, both MP2 and DFT calculations were performed with the GAUSSIAN2003 program package[21]. In the DFT calculations, Becke’s three parameter hybrid exchange functional (B3)was combined with the gradient-corrected correlation functional of Perdew (P86)or Lee, Yang and Parr (LYP). People’s triple zeta basis set with polarization and diffuse functions, 6-311+G(d,p), was used in most calculations. Electron density maps were generated and vibrational modes were identified with GaussView3.09[22]. AIEs were calculated to be the energy difference between the ionic and neutral complexes, where vibrational zero-point energy corrections were included. Metal-ligand bond dissociation energies were calculated to be the energy difference of the metal atoms and ligands from their complexes.
The theoretical calculations of metal complexes begin from geometry optimization of the free ligand. For ligands with several possible conformations, a systematic local minimum search needs to be performed starting from a number of initial guesses. The global minimum energy structure can be located by comparing the energies of all structures. During this search for the minimum energy structure, weak interactions, such as intramolecular hydrogen bonding, should be considered. Multidimensional FC factors were computed from the theoretical equilibrium geometries, harmonic frequencies, and normal modes of the neutral and ionic complexes.
RESULTS AND DISCUSSION:
Cu-22BIPY Complex
To calculate the structures of Cu-(22BIPY), we first considered stable rotational conformers of the free ligand. Figure -2 illustrates two conformers of 22BIPY in cis- and trans- configurations, and Table -1 lists their symmetries, geometries and relative energies from the B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) calculations. The trans- conformer has a planar structure in the C2h point group and is about 2000 cm-1 more stable than the cis-conformer. The cis-conformer is calculated to be in C2 symmetry with a rotation angle of 40.50 by the B3LYP method and a rotation angle of 47.0 0 by the MP2 method.
Table-1: Point groups, relative energies (Ee, cm-1),geometric structures(R, Ĺ;°)of 2,2’-bipyridine isomers from B3LYP/6-311+G(d,p)and MP2/6-311+G(d,p) calculations:
|
Methods |
B3LYP/6-311+G(d,p) |
MP2/6-311+G(d,p) |
||
|
Point group |
C2h |
C2 |
C2h |
C2 |
|
Relative energies |
0 |
2253 |
0 |
1725 |
|
Geometric structures |
||||
|
R (C2 –C2’) |
1.491 |
1.494 |
1.488 |
1.486 |
|
R (C2 – N) |
1.342 |
1.340 |
1.350 |
1.348 |
|
R (N – C6) |
1.333 |
1.333 |
1.342 |
1.343 |
|
R (C6 – C5) |
1.394 |
1.394 |
1.399 |
1.400 |
|
R (C5 – C4) |
1.393 |
1.391 |
1.398 |
1.397 |
|
R (C4 – C3) |
1.389 |
1.391 |
1.395 |
1.396 |
|
R (C3 – C2) |
1.402 |
1.401 |
1.405 |
1.404 |
|
|
117.0 |
117.1 |
116.6 |
116.1 |
|
|
118.3 |
118.1 |
117.6 |
117.1 |
|
|
123.6 |
123.8 |
123.8 |
124.0 |
|
|
118.0 |
118.0 |
118.2 |
118.4 |
|
|
119.0 |
118.8 |
118.8 |
118.4 |
|
|
118.8 |
119.1 |
118.8 |
119.0 |
|
|
122.2 |
122.2 |
122.8 |
123.1 |
|
ϕa |
180.0 |
40.5 |
180.0 |
47.0 |
|
R (C2 –C2’) |
1.491 |
1.494 |
1.488 |
1.486 |
aфis the rotation angle between the two pyridine rings.
Considering both the trans- and cis-conformations of 22BIPY, Cu may bind with one nitrogen in the trans- conformer to form a monodentate structure [Figure -3 (a)] or with both nitrogen atoms in the cis conformer to form a bidentate structure [Figure -3 (b)]. Table -2 lists the low-lying electronic states, point groups, relative energies, metal-ligand BDEs, and equilibrium geometries of the optimized structures of Cu-22BIPY from the B3LYP/6-311+G(d,p) and MP2/6-311+G(d,p) calculations. A twisted monodentate trans- structure and a planar bidentate cis- structure are predicted by the B3LYP method[23-24]. The trans- structure has no symmetry and is about 3000 cm-1 higher in electronic energy than the cis structure. In this trans- configuration, the two pyridine rings are twisted by about 220 compared to the free ligand, and the Cu atom bends off the pyridine plane to which it is attached by ~ 310. The AIE of this trans- isomer is calculated to be 42625 cm-1 and it is close to the measured ionization energy of the monodentate Cu-pyridine complex (43703 cm-1).
The bidentate cis isomer has C2v symmetry with the ground electronic states of 2B1 for the neutral species and 1A1 for the ion[25].
