Figure 1. Chemical structures of bomp- (left) and [Zn2(bomp)(OCOMe)2]+ complex cation (right).
Figure 2. Structures of the [Zn2(bomp)(OCOMe)2]+ complex cation; crystallographically observed structure (a) and structures computed using the B3LYP/LANL2DZ method (b), the PM5 method (c), and the MMFF94s-(5,1) method (d).
Computed structures for the complex cation were obtained using several computational methods, such as ab initio methods, DFT methods, semi-empirical methods, and empirical MM methods. The ab initio and DFT computations were made using HF and B3LYP methods with STO-3G, 3-21G, and LANL2DZ basis sets, and the results are summarized in Tables 1, - 3. When STO-3G was used, the octahedral coordination geometry around each zinc atom was broken, and the results were not satisfactory. Other basis sets gave successful results, and the B3LYP/LANL2DZ method gave the best result among them. The structure obtained by B3LYP/LANL2DZ is shown in Figure 2b. Deviations from the crystal structure were, at most, -8~7% in the bond lengths of non-hydrogen atoms and -5~9% in the bond angles of non-hydrogen atoms (see Table 1). The Zn...Zn separation and the twisted angle were almost reproduced (see Table 2). The computed axial elongation (4.9%) was slightly smaller than the crystal structure value (see Table 3), but the elongation tendency was reproduced. The MP2 computations were also performed using the result of B3LYP/LANL2DZ method. The result by MP2/LANL2DZ was not good so much; however, the result by MP2/3-21G could improve the initial structure obtained by the B3LYP/LANL2DZ method (see Tables 1, - 3). This indicates the importance of the configuration interaction.
Table 1. Deviations from the crystal structure for the computed structures.
|Method||Deviations in bond distances/%||Deviations in bond angles/%|
|Around zinc||Non-hydrogen atoms except for zinc||Around zinc||Non-hydrogen atoms except for zinc|
Table 2. Comparison of the crystal structure and the computed structures.
|Method||Zn...Zn separation/A||Dihedral angle between the aromatic ring plane and the Zn-O-Zn plane/°|
Table 3. Equatorial and axial Zn-O(ether) distances and axial elongation. a
|Method||Equatorial Zn-O distance/A||Axial Zn-O distance/A||Axial elongation/%|
Semi-empirical computational optimization was made on the basis of the AM1, PM3, PM5, and PM6 methods. When the structure was optimized using the AM1 or the PM3 method, the octahedral coordination geometry around each zinc atom was broken, and the results were not satisfactory. On the other hand however, the PM5 and the PM6 methods gave successful results; the result obtained by the PM5 method was slightly better, and the structure is shown in Figure 2c. This might have happened because the AM1 and the PM3 method may not be suitable for the metal-ether bonds, whereas the PM5 method may be very suitable. For the PM5 method, deviations from the crystal structure were, at most, -9~4% in the bond lengths of non-hydrogen atoms and -10~15% in the bond angles of non-hydrogen atoms. The Zn...Zn separation and the twisted angle were close to the crystal structure values. The computed axial elongation was 1.3~1.4%, smaller than the crystal structure value, but a tendency towards elongation was observed. The quality of the PM5 computation seems to be slightly worse than that using the B3LYP/LANL2DZ method, but the PM5 method was determined to be applicable for zinc complexes of this kind.
In MM computations, parameters around zinc do not exist in general . Thus, in this study, several models were made to reproduce the crystal structure. The first ones were to assume standard zinc-donor bond lengths for all six types of coordination bonds. One was based on the MM2 force field (MM2-(6,0) model) and the other on the MMFF94s force field (MMFF94s-(6,0) model). Parameters additionally defined for both models are summarized in the appendices. The two numbers in parentheses (n, m) indicate that the additional parameters are used for m of the six donor atoms but not used for n of the donor atoms (0 <= m <= 6, n = 6 - m). In both resultant structures, the Zn...Zn separation and the twisted angle were good; however, no axial elongation was observed. In a second trial using MMFF94s parameters, no additional parameters were introduced around zinc (MMFF94s-(0,6) model); however, an axial elongation of the order of 4.3~4.5% was unexpectedly observed. In this computation, only van der Waals interactions and electrostatic interactions were considered around zinc. Therefore the result may suggest that the zinc-ether bonds are ionic. Thus, in the next model using MMFF94s parameters, the additional pseudo-potentials were introduced for four non-ether donor atoms (MMFF94s-(4,2) model). The obtained structure was really good judging from the deviations of the bond lengths and angles, the Zn...Zn separation, and the twist angle. The axial elongation was 4.3~4.4%. Finally, in a last model using MMFF94s parameters, the additional pseudo-potentials were introduced for four non-ether donor atoms and equatorial ether oxygen atoms (MMFF94s-(5,1) model). This result was the best, and the axial elongation was 8.8~8.9%, which was quite similar to the crystal structure value. In conclusion, it has been found that the coordination geometry around zinc can be simulated by the molecular mechanics method if we assume ionic zinc-ether bonds.
Additional stretching parameters for MM2 methods
Additional bending parameters for MM2 methods
Additional out-of-plane bending parameters for MM2 methods
Additional torsion parameters for MM2 methods
Additional pseudo-potential parameters for MMFF94s method