Above: Along the crystal a axis of [Eu(thf)3]2(µ-n1:n1-N2Ph2)2, the structure used in the computational analysis reported in the Molecules paper below. See text for more info. Image hastily made with Vesta.
Published somewhat recently in Molecules. And Greek letters are being difficult in WordPress at the moment – those in the know please forgive the use of "n" and "w" where inappropriate).
Reminded of a phrase often spoken by the late Marshall Nye – actor, director (one of the first shows I ever drummed for was his directing of "Grease"), thespian, all-around good guy and ninth grade math teacher.
"Fools rush in where angels dare to tread."
For the record, my remembering this had to do with the modeling of europium as a theoretical chemist, not the deep dive that has been taken in terms of my inbox after having an article submitted to an Open Access journal. Maybe.
The Math
To the technical details – with a particular authorship deadline in mind for its publication, the theoretical section turned into part one of what is hoped to be an extended set of density functional theory comparisons for their assessment w.r.t. a selection of alkali and alkali earth metal coordination complexes with specific families of coordinating ligands. The extended set of the structures not reported in this first paper spans the Ca/Sr/Ba series, with Eu being a particularly similar metal in terms of ionic radius and coordination behavior (no surprise). The question to be addressed was how well/differently several recent density functionals reproduced structural parameters of the coordination complexes, this as a wide stepping stone to some rational design of ligands to alter various properties (vapor pressure, binding strengths (most easily through steric modification), intermolecular interactions for potential extended coordination networks, etc.) in these smaller model systems based on theoretical work.
As these structural details are all from crystal structures of these complexes, and because the units cells are large enough to not want to treat them by periodic methods for the time being at the employed levels of theory, there is also the obvious issue of if disagreements with the calculations are attributable to the crystal environment changing the geometries or the theory levels actually over/underestimating something (the clear concern for any coordination interactions to the europium, to the extent those differences matter to the behavior of the systems).
A few aside takeaways from the work itself:
- The calculated and crystal geometries are quite close all around, indicating that these are well-defined complexes that then pack in the solid-state (my interpretation).
- I tried to use the same as-large-as-possible/methodologically-appropriate basis set for single-point calculations, which then required going to the Basis Set Exchange for Def2-SVP and Def2-TZVP. Convergence of the wavefunction in Gaussian 09 was an awful fight down to default "tight" values. Part of the solution was the reuse of the checkpoint file from the previous run (guess=read – good practice for reducing the time to convergence generally), part was reducing the convergence threshold to 10^-7 (consider Q-Chem's default for single-point calcs!).
- The LRC-wPBEh density functional was generally excellent for these systems. The one oddity was the way that it split select N=N stretching modes compared to CAM-B3LYP and wB97XD (as discussed in the paper).
With time constraints in mind, this work only covered the smaller of the two complexes, but some tests on the larger system play out equally well (although it is even more sensitive to crystal packing given how some of the ligands binds and are prone to local disorder, so the more rigid structure RMSD for structure 1 needs to be tempered with analyses at the bond and angle levels for 2).
All that said, Molecules is Open Access – I don't need to summarize what you can read for free. Speaking of…
The Aftermath
This was my first foray into the open access model, where you get an invite, a reduced rate for publishing, and hopefully a good set of reviewers committed to (a) the open access philosophy and (b) scientific rigor.
I have, since publication of this article, received 5x more invitations to submit my research to journals than I had prior. It is an obvious leap in invite count. Might be fine if the emails did not look like the following:
Dear Dr. Allis Damian,
Hope you are doing well!
Can you kindly let us know if you can submit any type of article towards our Journal? As we are short of one article to close the issue.
Waiting to receive your reply as early as possible.
Sincerely,
Annals of Chemical Science Research
ISSN: 2688-8394 | Impact Factor: 1.599
I realize it's been a long year since publishing something last, but are other folks in this same boat? Are the bots that busier? Are folks sitting on these journals waiting to scrape the pages for @ symbols?
Damian G. Allis, Ana Torvisco, Cody C. Webb, Jr., Miriam M. Gillett-Kunnath, and Karin Ruhlandt-Senge
Abstract: The preparation and characterization of two novel europium–azobenzene complexes that demonstrate the effectiveness of this ligand for stabilizing reactive, redox-active metals are reported. With the family of rare earth metals receiving attention due to their potential as catalysts, critical components in electronic devices, and, more recently, in biomedical applications, a detailed understanding of factors contributing to their coordination chemistry is of great importance for customizing their stability and reactivity. This study introduces azobenzene as an effective nonprotic ligand system that provides novel insights into rare earth metal coordination preferences, including factors contributing to the coordinative saturation of the large, divalent europium centers. The two compounds demonstrate the impact of the solvent donors (tetrahydrofuran (THF) and dimethoxyethane (DME)) on the overall coordination chemistry of the target compounds. Apart from the side-on coordination of the doubly-reduced azobenzene and the anticipated N-N bond elongation due to decreased bond order, the two compounds demonstrate the propensity of the europium centers towards limited metal-pi interactions. The target compounds are available by direct metallation in a straightforward manner with good yields and purity. The compounds demonstrate the utility of the azobenzene ligands, which may function as singly- or doubly-reduced entities in conjunction with redox-active metals. An initial exploration into the computational modeling of these and similar complexes for subsequent property prediction and optimization is performed through a methodological survey of structure reproduction using density functional theory.
