For the copper(II) complexes of the bispa ligands B1a and B1b an angular overlap model
(AOM) based ligand field analysis of the electronic and ESR spectra has been carried out. This study was performed for a detailed comparison of the solid state structures and the structures in solution as well as to understand differences in the degree of preorganization of the two isomeric ligands for the JAHN-TELLER labile CuII ion.
Term energies of transition metal complexes are governed by the metal center, the donor atoms and the coordination geometry.[385] The AOM uses a simplified molecular orbital (MO) approach to calculate the spectroscopic properties of such complexes.[386-388] In this model metal-ligand (M-L) bonds are described by covalent σ, π and δ interactions using two-atom overlap integrals of the metal d orbitals and the donor orbitals. Each M-L interaction has to be parameterized individually with the respective eσ, eπ and eδ values. These parameters are positive for donor atoms and negative for acceptors, and their values decrease in the order |eσ| > |eπ| > |eδ|. The interaction between a free electron pair at the ligand and a metal orbital with suitable symmetry has a destabilizing effect resulting in an upward shift in energy of the metal atomic orbital and eσ is thus always positive. The π parameter, on the other hand, can adopt both signs, depending on the character of the bond with respect to π-forward and π-backward donation. In the case of an anisotropic π bond, eπ is divided into two parts, i.e. eπx and eπy. Due to the minor overlap integral of δ bonds, the values for eδ are relatively small and are hence often neglected. Moreover, the ligand systems investigated here do not possess orbitals suitable for the formation of δ bonds. The AOM parameters depend on the metal-ligand distance r in a way that their values approximately decrease with the sixth power of r.[386-389] In tetragonally distorted copper(II) complexes the energy of the dz2 orbital is decreased by configurational
CuII N7 Npy4 N3 Npy3 Npy2 Npy1 CuII N7 Opa N3 Npa Npy2 Npy1 CuII N7 Npa N3 Opa Npy2 Npy1
interaction with the metal 4s orbital. This effect, known as ds mixing, is considered in the AOM by assigning each ligand an additional bonding parameter eds with eds = 1/4 eσ.[330,390] The AOM calculations reported here were performed with the computer program CAMMAG.[391] The coordinates of the CuII center and the donor atoms were taken from the X-ray structures of CuII-B1a and CuII-B1b. In both cases, the z axis was defined along the pseudo JAHN-TELLER elongated axis (see also Figure 49). Details of the AOM
analysis together with examples of input-files are given in Appendix F. AOM eσ, eπ and eds parameters are not transferable in principle but in a series of studies, it has been shown that the error made by assuming transferability generally is acceptable.[315,379,392] However, with published parameters for CuII-amine, CuII-pyridine and CuII-carboxylate bonds, adjusted to the observed CuII-donor distances with 1/r6,[385,390,393] the computed and experimentally determined electronic transitions are only in fair agreement. Thus, modifications of the ligand field parameters were introduced and led to acceptable predictions of the dd transitions and EPR g tensor parameters as shown in Table 10. One
reason for the discrepancy between the published and moderately adjusted AOM parameters is that those for the carboxylates are merely the result of an ad-hoc parameterization for one particular set of complexes,[393] while the parameterization for amines and pyridines were deduced from a larger set of spectroscopic studies.[385] Also, transferability is an assumption with limited applicability, and electron transfer from anionic ligands may be problematic in this context.[394] In addition and most importantly, small structural changes upon dissolution of the complexes, in particular along the JAHN-TELLER vibration, may lead to significant changes of the ligand field. However, while the adjustments to the AOM parameters seemed to be necessary for esthetic reasons, the qualitative interpretation that there are only minor structural changes upon dissolution of the complexes does not change with the two sets of parameters used.
The structural data suggest that CuII-B1a is elongated along the N7-Cu-Opa axis, while the elongation observed for CuII-B1b is along Npy1-Cu-Npy2. Based on both sets of AOM parameters, the solid state structure is confirmed to also be the structure in solution and this has some implication for the relative stabilities (see below). The splitting of the three dπ orbitals is not resolved for either isomer but, from the relative line width of the dxy,xz,yz→dx2-y2 transitions, it appears that the splitting is slightly larger for CuII-B1a, and this agrees with the geometry observed in the solid state structures, which for CuII-B1a suggests interaction of the carboxylate π donor with the CuII d
xz,yz orbitals. The in-plane carboxylate in CuII-B1b leads to a relative destabilization of the d
stabilization of the dx2-y2 orbital with respect to CuII-B1a with an axial carboxylate, emerging in a generally lower ligand field but a higher energy dz2→dx2-y2 transition.
The conservation of the solid state structures in solution, i.e. the different orientations of the pseudo JAHN-TELLER axes, is also well supported by the g values resulting from simulation of the frozen solution X-band ESR spectra. As already discussed in Chapter 4.1.4, tertiary amines and pyridine donors exhibit very similar ligand field parameters,[140,379] while those for carboxylate are significantly different and therefore lead to a larger in-plane asymmetry in CuII-B1b.
In metal complexes the donor atoms N7 and N3 of the bispidine scaffold are part of rather flexible six- and very rigid five-membered chelate rings, respectively.[375] This structural difference in general leads to long M-N7 and shorter M-N3 bonds and therefore to a rigid bispidine ligand shape that is highly complementary for copper(II). For the N2py3-type isomers similar to B1a and B1b this results in distinct stability differences between the two
corresponding isomeric CuII complexes and an interesting CuII selectivity for one of the two isomers.[303] However, “JAHN-TELLER isomerism” was observed for the CuII complexes of a number of bispidine derivatives,[329,378] and the ligand field properties of the two isomers CuII-B1a and CuII-B1b as well as the stronger CuII-Opa interaction in CuII-B1b suggest that this isomer might lead to a slightly more stable CuII complex.
Table 10. Comparison of experimental and calculated (AOM) spectroscopic data of the
copper(II) complexes based on B1a and B1b.
UV-vis-NIR (dd transitions) ESR
λ [cm-1] g
x, gy, gz
[CuII(B1a)](TFA) exp. 14880, 7790 2.059, 2.059, 2.243
AOMpub(a) 17870, 17330, 15640, 9110 2.029, 2.038, 2.136 AOMadj(b) 16131, 15668, 13421, 8114 2.028, 2.046, 2.154
[CuII(B1b)](TFA) exp. 15360, 11060 2.016, 2.068, 2.236
AOMpub(a) 18530, 18260, 17440, 11200 2.014, 2.055, 2.120 AOMadj(b) 17306, 16978, 15713, 10681 2.015, 2.060, 2.128
(a) For AOM
pub published AOM parameters were used.[385,390,393] (b) AOMadj gives data based on
moderately adjusted parameters offering improved matching to the spectroscopic data (see Appendix F).