Structural Prediction for a Chiral Copper(II) Complex in Solution by Circular Dichroism Spectra



1 Introduction

It is much more difficult to determine molecular structures in solution than in the solid state. In the case of chiral transition metal complexes, circular dichroism (CD) spectra are observed in the visible region, and the spectra contain abundant information about central metal ions [1]. Using the information from CD spectra for structural prediction will be very beneficial. Therefore, our purpose in this study was to predict the structures of transition metal complexes on the basis of their CD spectra. In this study we demonstrate the structural prediction for a chiral copper(II) complex with an N-substituted ethylenediamine derivative R-mben [R-mben = N-(methylbenzyl)ethylenediamine] (Figure 1) [2].

Figure 1. Chemical structure (left) and ball-and-stick model (right) of R-mben.

When the chirality is introduced on the ethylene backbone of ethylenediamine derivatives, the conformation of the five-membered chelate ring is known to be fixed in either the d or the l form in metal complexes (Figure 2) [3]. In this way, chirality can be introduced in metal complexes, and the CD spectrum is expected to be observed in their d-d transition region. The relationship between the CD spectra and the structures has been extensively studied; however, it is not easy to predict the structure from the CD spectrum alone. Nowadays, we can predict the CD spectra on the basis of the structure by a molecular orbital method, such as TD-DFT. Therefore, if a CD spectral prediction is made for all possible structures, it will be possible to choose the most probable structure as the one whose CD spectrum matches the observed data.

Figure 2. Five-membered ring conformations of the d form (left) and the l form (right).

2 Computational methods

Gaussian curve analyses of the circular dichroism (CD) spectra were made using the AbSimCD software developed by Yuka Matsukawa and Hiroshi Sakiyama. All the ab initio and DFT computations were performed using Gaussian 03 software (Gaussian, Inc.) [4 - 12]. Structural optimization was performed with the MP2/6-31G(d,p) and B3LYP/LANL2DZ methods, and the CD spectral prediction was performed with the TD-DFT(B3LYP) method using the LANL2DZ basis set. In all the computations, the solvents around complex cations were not taken into account.

3 Results and discussion

3. 1 CD spectral analysis and coordination geometry determination

A chiral copper(II) complex [Cu(R-mben)2](ClO4)2, was prepared earlier, and its CD spectra were measured in acetonitrile and in pyridine [2]. In this study, the CD spectra were analyzed to obtain the spectral components assuming that the CD spectrum was composed of several spectral components described by Gaussian functions. To simplify the discussion, the spectrum in acetonitrile will be discussed first. The CD spectrum in acetonitrile is shown in Figure 3 together with the spectral components and the sum of all the components. In the visible region, three apparent spectral components were obtained: the first component was negative and appeared at 678 nm, the second component was also negative and appeared at 548 nm, and the third component was positive and appeared at 455 nm. Hereafter we call this pattern (-, -, +). These visible-region components are assigned to "d-d transition" bands, and they are strongly related to the "coordination geometry" around the copper(II) ion. It should be noted here that the coordination geometry is a shape around a central metal ion with directly bound donor atoms. First, to find out the coordination geometry, we compared the observed components with the computationally obtained components. In the computation, we considered three coordination geometries for a simplified complex [Cu(en)2] where "en" represents an ethylenediamine: a tetra-coordinated square-planar geometry, a penta-coordinated square-pyramidal geometry with an acetonitrile molecule, and a hexa-coordinated octahedral geometry with two acetonitrile molecules. Each structure was optimized using the MP2/6-31G(d,p) method, and the resulting energy diagram is described in Figure 4 together with the optimized structures and several shapes of the molecular orbitals. In the case of the D2 symmetry, the highest d-orbital is dxy, and it is singly occupied. It should be noted here that molecular orbitals in which the d character is dominant are called d-orbitals. In the case of the square-planar complex, the other four filled d-orbitals are almost in the same energy level, while in the cases of square-pyramidal and octahedral complexes, the dz2 orbital becomes higher than the others due to the coordination of acetonitrile. In this diagram the levels of dxz and dyz are higher than that of dx2-y2. The main reason in the square-pyramidal complex is that the position of the copper(II) ion is above the N4 plane. Other reasons are the p-orbital effect of acetonitrile and the tetrahedral distortion of the N4 plane.

Figure 3. CD spectrum of [Cu(R-mben)2]2+ in acetonitrile: Observed data (···), spectral components (- -), and sum of the spectral components ().

Based on the structures obtained by the MP2/6-31G(d,p) method, the spectra were predicted by the TD-DFT method using B3LYP/LANL2DZ; the results are summarized in Table 1. The computed result is consistent with our knowledge that the square-planar bis(ethylenediamine)copper(II) complex is reddish and that the square-pyramidal and octahedral complexes are blue or purple. Comparing the observed spectral components with the computed results, we found that the observed data were very close to that for the penta-coordinated square-pyramidal geometry. The coordination of acetonitrile molecules to a central copper(II) ion is possible and the square-pyramidal geometry is reasonable, since the coordination to the sixth site is blocked by bulky N-substituents. Therefore, we concluded that the main species in the acetonitrile solution had a square-pyramidal geometry.

Figure 4. Energy diagram for the [Cu(en)2]2+ complex cation obtained using MP2/6-31G(d,p); octahedral trans-[Cu(en)2(MeCN)2]2+, square-pyramidal [Cu(en)2(MeCN)]2+, and square-planar [Cu(en)2]2+ complex cations. Common names of d-orbitals are used for molecular orbitals.

Table 1. Predicted CD components for bis(ethylenediamin)copper(II) complexes.
Complexl /nm (De)
1st band2nd band3rd band4th band
Observed for [Cu(R-mben)2]2+ in acetonitrile681 (-0.042)541 (-0.024)481 (+0.018)
d-[Cu(en)2]2+449 (+0.005)414 (+0.32)411 (-0.35)406 -0.45)
d-[Cu(en)2(CH3CN)]2+682 (+0.22)520 (+0.064)495 (-0.29)464 (+0.19)
d-[Cu(en)2(CH3CN)2]2+894 (+0.02)607 (+0.15)587 (+1.8)536 (+0.4)

3. 2 Conformation analysis and CD spectral prediction for [Cu(R-mben)2(CH3CN)]2+

Next, possible conformers for the square-pyramidal [Cu(R-mben)2(CH3CN)]2+ complex cation will be considered assuming a pseudo-C2 symmetry along the Cu-N(acetonitrile) bond. For the chelating ring, there are two conformers, the d form and the l form (Figure 2). In each form, there are four patterns concerning the position of the chiral N-substituent: an equatorial position far from the acetonitrile (eq-far), an equatorial position close to the acetonitrile (eq-close), an axial position far from acetonitrile (ax-far), and an axial position close to the acetonitrile (ax-close) (Figure 5); however, two of the patterns, eq-close and ax-close, were excluded because they were less favorable due to the steric repulsions between the N-substituent and the coordinating acetonitrile. Therefore we considered four patterns d-eq-far, d-ax-far, l-eq-far, and l-ax-far, and for each pattern we considered three conformers along the N-C(asymmetric) bond. At this stage, twelve conformers were considered in total: conformers d-eqA, d-eqB, d-eqC, d-axA, d-axB, d-axC, l-eqA, l-eqB, l-eqC, l-axA, l-axB, and l-axC, where d and l represent the conformation of chelating rings, eq and ax represent the position of N-substituents, and A, B, and C are used to identify conformers along the N-C(asymmetric) bond.

Figure 5. Eight diastereomers for [Cu(R-mben)2(MeCN)]2+.

For the twelve conformers described above, structural optimization was performed by the B3LYP/LANL2DZ method; the optimized structures are shown in Figure 6. Then, the CD spectrum was predicted for each optimized conformer by the TD-DFT method. The computed energies and CD spectral components are summarized in Table 2.

Figure 6. Perspective views of the twelve conformers for [Cu(R-mben)2(CH3CN)]2+. All the structures were optimized by the B3LYP/LANL2DZ method.

Table 2. CD spectral prediction for the twelve conformers of [Cu(R-mben)2(CH3CN)]2+.
ComplexEnergy/HartreeL /nm (De)
1st band2nd band3rd band4th band
Observed681 (-0.042)541 (-0.024)481 (+0.018)
d-eqA-1328.74299640 (-0.68)573 (-0.92)550 (+1.28)505 (+1.65)
d-eqB-1328.72723738 (+0.22)626 (-0.1)511 (+1.2)501 (+1.58)
d-eqC-1328.73554733 (+0.016)516 (-0.38)490 (+0.42)475 (+0.32)
l-axA-1328.74455635 (-0.18)530 (-1.9)482 (+2.5)475 (-2.5)
l-axB-1328.72878647 (+0.24)654 (-0.36)497 (-1.48)493 (+2.2)
l-axC-1328.73113712 (-0.65)688 (+0.15)495 (-0.83)485 (+2.17)

Judging from the computed energies, we conclude that conformer l-axA was the most stable one, and conformer d-eqA was the second best among the twelve conformers. If we compare the CD spectral components, only conformers l-axA and d-eqA reproduced the observed spectral pattern (-, -, +). The structures of the two conformers are different; however, the orientations of the chiral N-substituents are very similar. Therefore, we may conclude that the orientation of the N-substituents is the predominant factor to determine the CD spectral pattern in this case. In the case of square-planar complexes with ethylenediamine derivatives, substituents on the ethylene backbone are generally in equatorial positions to avoid steric repulsions. On the other hand, in the case of N-substituents in square-pyramidal complexes, steric repulsion between the apical ligand and N-substituents plays an important role in determining the whole structure. In the case of conformer d-eqA, the square-pyramidal geometry seems to have deformed into a trigonal bipyramidal geometry to avoid the steric repulsions. In the case of the conformer l-axA, the geometry is square-pyramidal, but the N-substituents are in the axial positions. These positions seem to be favorable for avoiding steric repulsions. Judging from the reproducibility of the CD spectrum and the Cu-N bond distances, we conclude that conformer l-axA is much more probable.

3. 3 Structural prediction for [Cu(R-mben)2(C6H5N)]2+

In the above sections, we discussed a structure in acetonitrile; in this section, we will discuss a structure in pyridine. The CD spectrum of the copper(II) complex in pyridine is shown in Figure 7 together with the spectral components and the sum of all the components. In the visible region, four spectral components were obtained: the first positive component at 746 nm, the second negative component at 615 nm, the third positive component at 519 nm, and the fourth negative component at 436 nm, thus the CD spectral pattern is (+, -, +, -). As in the case of acetonitrile, CD spectra were predicted for twelve conformers for [Cu(R-mben)2(C6H5N)]2+, and the results are summarized in Table 3.

Figure 7. CD spectrum of [Cu(R-mben)2]2+ in pyridine: Observed data (···), spectral components (- -), and sum of the spectral components ().

Figure 8. Perspective views of the twelve conformers for [Cu(R-mben)2(C5H5N)]2+. All structures were optimized by the B3LYP/LANL2DZ method.

Table 3. CD spectral prediction for the six conformers of [Cu(R-mben)2(C5H5N)]2+.
ComplexEnergy/HartreeL /nm (De)
1st band2nd band3rd band4th band
Observed748 (+0.014)614 (-0.046)520 (+0.020)438 (-0.0047)
d-eqA-1444.25088808 (-0.47)678 (+1.2)653(+1.2)543 (-1.4)
d-eqB-1444.23325768 (+0.031)558 (+0.4)539 (-0.33)509 (+0.075)
d-eqC-1444.24270726 (+0.51)577 (+0.62)531 (-1.03)500 (+0.28)
l-axA-1444.25106829 (+0.18)530 (-4.2)523 (+2.6)487 (-0.9)
569 (+1.5)
l-axB-1444.23355778 (+0.12)665 (+0.24)635 (-0.3)518 (-0.38)
665 (+0.24)
l-axC-1444.23786819 (+0.325)701 (-0.38)553 (+1.98)493 (+0.55)
660 (-0.52)

The observed CD pattern (+, -, +, -) was reproduced by four conformers: d-axA, l-eqA, l-eqC, and l-axA; l-axA was the most stable of the twelve conformers. As in the case of acetonitrile, the most stable conformer is l-axA, and the predicted CD spectrum for this conformer is similar to the observed data. In the case of the present complex [Cu(R-mben)2]2+, the octahedral geometry seems to be impossible due to the bulky N-substituent, and the square-pyramidal geometry seems to be preferred. In the square-pyramidal structure, the N-substituents seem to prefer axial positions in order to avoid steric repulsions.

4 Conclusion

The structures were predicted for the chiral copper(II) complex [Cu(R-mben)2]2+ in acetonitrile and in pyridine on the basis of their CD spectra. In both cases the most stable conformer was found to be "l-axA" by DFT computation; the computed CD spectra for the most stable conformers were similar to the observed data. The present method will be useful for the structural prediction of chiral transition metal complexes.


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