Azobenzene as an Effective Ligand in Europium Chemistry—A Synthetic and Theoretical Study
Abstract
:1. Introduction
2. Results and Discussion
2.1. Synthesis
2.2. X-Ray Crystal Structures
3. Discussion
3.1. Synthetic and Spectral Aspects
3.2. Structural Aspects
3.3. Computational Modeling
- For 1, across these ranges of theory, increasing the basis set size (and computational cost) from Def2-SVP to Def2-TVZ does not improve agreement with the crystal structure geometry except when using the LC-ωHPBE density functional with no dispersion correction.
- The LC-ωHPBE density functional with the GD3(BJ) dispersion correction produces geometries with RMSDs nearly half those of the remainder of the survey. We note that this does not mean that the LC-ωHPBE density functional is the means to producing the best gas-phase geometry until it can be determined that the geometry of the complex is only minimally impacted by crystal packing interactions. In the case of 1, where inter-complex interactions are predominantly between THF C-H bonds and azobenzene phenyl rings, such a condition might be completely reasonable.
- Except for the Eu-N1 distance (one of the two longer Eu-N distances), the inclusion of dispersion corrections uniformly improves agreement with the experiment for all of the considered theory levels.
- NPA comparisons show a uniform sensitivity to the natural charges on the Eu atoms with a choice of SVP or TZV but only a fractional difference between N atoms in either case (and similar for the O atoms, which uniformly have predicted natural charges in the −0.55 to −0.59 e− range).
- Considering the Eu-C(π) distances among the calculations reveals that using the smaller Def2-SVP basis set leads to markedly better agreement with the crystal geometry than Def2-TZV, and that dispersion corrections provide only fractional additional improvements in agreement. In terms of the generation of molecular structures for computational assessment, the crystal geometry of 1 can be better reproduced with theory levels differing only in the basis set that reduces the compute time per SCF cycle by 50% or more.
- With some small variation across all methods, the calculated Eu-C(π) distances are all in good agreement with the experiment and within the range of accepted values for these interactions. The removal of these complexes from the crystal environment and optimization as gas-phase species preserved these distances, indicating that their formation is not directly attributable to crystal packing but instead some energetic preference for including these coordinative interactions as part of the overall environment around each metal.
- Geometry comparisons (Figure 4) reveal that the shifting of the THF C4H8 chains during energy minimization produces the largest differences between gas-phase theory and crystal geometry. This is by no means unexpected, as the complete encapsulation of the Eu metals by the most electronegative atoms in these complexes leaves only the ligand fragments capable of the weakest of electrostatic interactions to undergo the structurally large but energetically small changes as part of crystal packing.
- There is clear disagreement in the prediction of the vibrational mode energy for the N=N stretch in the isolated cis-azobenzene using Def2-SVP vs. Def2-TZV, where Def2-SVP calculations can differ from the accepted 1511 cm−1 value by 11% to 20%. In the complex, however, Def2-SVP and Def2-TZV predicted mode energies for the symmetric and asymmetric N=N stretching pair differ by only a few cm−1 in all cases. Again, the time savings from using Def2-SVP across both optimization and normal mode analysis are considerable over Def2-TZV for the same density functionals and dispersion corrections.
- The one notable difference between LC-ωHPBE and the other three density functionals is to be found in the prediction that the N=N stretching modes are not localized to a single dominant stretching mode. With LC-ωHPBE, the lower-energy pair of stretches is coupled to phenyl ring expansion modes, while the higher-energy pair is coupled to phenyl lateral wagging modes. This splitting is 10 cm−1 with Def2-SVP and 40 cm−1 with Def2-TZV.
4. Materials and Methods
4.1. General Methods and Synthesis
4.2. Single-Crystal X-Ray Diffraction
4.3. Computational Methods
5. Conclusions
Supplementary Materials
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
References
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Metal Cation (2+) | Ionic Radii (Å) | Electronegativity (Pauling) | E° (V) (M2+ (aq) + 2e− = M(s)) |
---|---|---|---|
Ca | 1.12 | 1.00 | −2.87 |
Yb | 1.14 | 1.10 | −2.22 |
Sr | 1.25 | 0.95 | −2.89 |
Sm | 1.26 | 1.17 | −2.30 |
Eu | 1.24 | 1.20 | −1.99 |
Ba | 1.42 | 0.89 | −2.90 |
Compound | N-N (Å) Ligand | Eu-N (Å) Ligand | Eu-O (Å) Donor | N-Eu-N (°) Ligand | Dihedral (ɸ), Ph-NN-Ph (°) | |
---|---|---|---|---|---|---|
[Eu(thf)3]2(N2Ph2)2 | 1.471(3) | 2.469(3) 2.675(2) 2.456(2) 2.675(2) | ― | 2.592(2)– 2.659(2) | 33.00(7) 32.91(7) | 83.52(1) |
[Eu(dme)2]2(N2Ph2)2 | 1.472(8), 1.479(7) | Eu1 2.707(6) 2.494(6) 2.644(6) 2.499(6) | Eu2 2.455(6) 3.237(5) 2.445(6) 2.731(5) | 2.620(5)– 2.733(5) | 32.5(2) 33.2(2) 32.6(2) | 75.5(7) 78.8(7) |
Compound | Eu···C(π) (Å) | Eu···C(π) (Å) | Eu···H-C (Å) | ||
---|---|---|---|---|---|
[Eu(thf)3]2(N2Ph2)2 | Eu1 3.19 (C1) 3.28 (C7) | Eu1 3.19 (C1) 3.28 (C7) | 2.98–3.29 | ― | |
[Eu(dme)2]2(N2Ph2)2 | Eu1 3.09 (C1) | Eu2 3.13 (C19) | ― | Eu1 ― | Eu2 3.21 (H34B, DME) 3.25 (H6, Ph2N2) |
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Allis, D.G.; Torvisco, A.; Webb, C.C., Jr.; Gillett-Kunnath, M.M.; Ruhlandt-Senge, K. Azobenzene as an Effective Ligand in Europium Chemistry—A Synthetic and Theoretical Study. Molecules 2024, 29, 5187. https://doi.org/10.3390/molecules29215187
Allis DG, Torvisco A, Webb CC Jr., Gillett-Kunnath MM, Ruhlandt-Senge K. Azobenzene as an Effective Ligand in Europium Chemistry—A Synthetic and Theoretical Study. Molecules. 2024; 29(21):5187. https://doi.org/10.3390/molecules29215187
Chicago/Turabian StyleAllis, Damian G., Ana Torvisco, Cody C. Webb, Jr., Miriam M. Gillett-Kunnath, and Karin Ruhlandt-Senge. 2024. "Azobenzene as an Effective Ligand in Europium Chemistry—A Synthetic and Theoretical Study" Molecules 29, no. 21: 5187. https://doi.org/10.3390/molecules29215187
APA StyleAllis, D. G., Torvisco, A., Webb, C. C., Jr., Gillett-Kunnath, M. M., & Ruhlandt-Senge, K. (2024). Azobenzene as an Effective Ligand in Europium Chemistry—A Synthetic and Theoretical Study. Molecules, 29(21), 5187. https://doi.org/10.3390/molecules29215187