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A new paper on the Weak Link Approach
Chemically actuating a molecule is a very cool thing to do and the Weak Link Approach (WLA) allows us to do precisely that through the reversible coordination of one or various organometallic centers to a longer ligand that opens or closes a macrocyclic cavity. All this leads to an allosteric effect so important in biological instances available in inorganic molecules. Once again, the Mirkin group at Nortwestern University in Evanston, Illinois, has given me the opportunity to contribute with the calculations to the energetic properties of these actuators as well as their electronic properties for their use as molecular scavengers or selective capsules for various purposes such as drug delivery agents.
As in the previous WLA work (full paper), the NBODel procedure was used at the B97D/LANL2DZ level of theory, only this time the macrocycle consisted of two organometallic centers and for the first time the asymmetric opening of the cavity was achieved, as observed by NMR. With the given fragments, all possibilities shown in scheme 1 were obtained. The calculated bond energies for the Pt – S bonds are around 60 – 70 kcal/mol whereas for the Pt – Cl bonds the values are closer to 90 kcal/mol. This allows for a selective opening of the cavity which can then be closed by removing the chlorine atoms with the help of silver salts.
For the case of complex mixture 4a, 4b, and 4c, the thermochemistry calculations show they are all basically isoenergetic with differences in the thousandths of kcal/mol. The possibilities for the groups in the weakly bonded ligands are enormous; currently, there is work being done about substituting those phenyl rings for calix[4]arenes in order to have a macrucyclic capsule made by macrocylic capusules.
Thanks to Andrea D’Aquino for taking me into her project, for all the stimulating discussions and her great ideas for expanding WLA into new avenues; I’m sure she’ll succeed in surprising us with more possibilities for these allosteric macrocycles.
The full paper is published in Inorganic Chemistry from the ACS (DOI: 10.1021/acs.inorgchem.7b02745). Thanks for reading and -if you made it this far into the post- happy new year!
Collaborations in Inorganic Chemistry
I began my path in computational chemistry while I still was an undergraduate student, working on my thesis under professor Cea at unam, synthesizing main group complexes with sulfur containing ligands. Quite a mouthful, I know. Therefore my first calculations dealt with obtaining Bond indexed for bidentate ligands bonded to tin, antimony and even arsenic; yes! I worked with arsenic once! Happily, I keep a tight bond (pun intended) with inorganic chemists and the recent two papers published with the group of Prof. Mónica Moya are proof of that.
In the first paper, cyclic metallaborates were formed with Ga and Al but when a cycle of a given size formed with one it didn’t with the other (fig 1), so I calculated the relative energies of both analogues while compensating for the change in the number of electrons with the following equation:
Fig 1
Under the same conditions 6-membered rings were formed with Ga but not with Al and 8-membered rings were obtained for Al but not for Ga. Differences in their covalent radii alone couldn’t account for this fact.
ΔE = E(MnBxOy) – nEM + nEM’ – E(M’nBxOy) Eq 1
A seamless substitution would imply ΔE = 0 when changing from M to M’
Hipothetical compounds optimized at the B3LYP/6-31G(d,p) level of theory
The calculated ΔE were: ΔE(3/3′) = -81.38 kcal/mol; ΔE(4/4′) = 40.61 kcal/mol; ΔE(5/5′) = 70.98 kcal/mol
In all, the increased stability and higher covalent character of the Ga-O-Ga unit compared to that of the Al analogue favors the formation of different sized rings.
Additionally, a free energy change analysis was performed to assess the relative stability between compounds. Changes in free energy can be obtained easily from the thermochemistry section in the FREQ calculation from Gaussian.
This paper is published in Inorganic Chemistry under the following citation: Erandi Bernabé-Pablo, Vojtech Jancik, Diego Martínez-Otero, Joaquín Barroso-Flores, and Mónica Moya-Cabrera* “Molecular Group 13 Metallaborates Derived from M−O−M Cleavage Promoted by BH3” Inorg. Chem. 2017, 56, 7890−7899
The second paper deals with heavier atoms and the bonds the formed around Yttrium complexes with triazoles, for which we calculated a more detailed distribution of the electronic density and concluded that the coordination of Cp to Y involves a high component of ionic character.
This paper is published in Ana Cristina García-Álvarez, Erandi Bernabé-Pablo, Joaquín Barroso-Flores, Vojtech Jancik, Diego Martínez-Otero, T. Jesús Morales-Juárez, Mónica Moya-Cabrera* “Multinuclear rare-earth metal complexes supported by chalcogen-based 1,2,3-triazole” Polyhedron 135 (2017) 10-16
We keep working on other projects and I hope we keep on doing so for the foreseeable future because those main group metals have been in my blood all this century. Thanks and a big shoutout to Dr. Monica Moya for keeping me in her highly productive and competitive team of researchers; here is to many more years of joint work.
Inorganic Chemistry paper
A new paper is coming out! These are always good news for someone whose productivity is evaluated by the number of his/her publications, and in my case the pleasure is double. It turns out that despite the fact a scientist is continuously working, there isn’t always the possibility of having results put “out there”. Back in Mexico we have the “National Researchers System” in which the National Council for Science and Technology encourages us to keep on working by providing economical stimuli and evaluating our productivity by, yes, the number of papers published each year. For three years I worked for a private research center in which fundamental science was tackled as far as it was economically possible and cost effective. Practically all of the work carried out there was confidential since the company it belonged to is leader in its market, not only in Mexico but in Latin America and southern USA! At this facility some papers were published from time to time (most of them came from research in molecular modeling) but not without struggle against the administrators. We can thank for most of the struggle (and the papers!) to Dr. Armando Gama-Goicochea, a great physicist as well as a great friend of mine.
Anyway, the bottom line here is that I’m excited about having a new paper coming out again, even if I’m nowhere close to being first author. This three year paper fastening seems to be over and let us hope it’s only the first of various now that I’m holding a postdoc position here in Romania.
In this paper we tackled the bonding properties of some Aluminum complexes with three chalcogeno triazoles. The electrostatic potential mapped onto a density surface of one of those compounds is currently shown in the header of this blog, btw. We concluded that the bonding in such compounds is mainly covalent as opposed to the more conventional electrostatic notion prevailing for such hard atoms. In order to get this information we resorted to Natural Bonding Analysis calculations with the RHF method and somewhat large basis sets in order to get a full description of the electronic density.
I very much like these systems in which several bonding possibilities occur. The fact that nature is chosing one out of many has always a reason which can be assessed by our models and may serve us to learn how to modify it’s behaviour.
Coordination Diversity of Aluminum Centers Molded by Triazole Based Chalcogen Ligands
Jocelyn Alcántara-García, Vojtech Jancik, Joaquín Barroso, Sandra Hidalgo-Bonilla, Raymundo Cea-Olivares, Rubén A. Toscano and Mónica Moya-Cabrera
