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Aurides Chemistry – New Paper in Organometallics


Compound 2 represents the first structural example of a 12 e− auride complex, with a pseudohalide/hydride nature in bonding. According to our NBO calculations, this electron deficient gold center is stabilized by weak intramolecular interactions between Au p orbitals and σC−C and σC−H bonds of adjacent aromatic rings together with a Ga−Au−Ga 3 centers−2 electrons bond (I like the term ‘banana bond‘, don’t you?).

Fig. 1 Crystal structure for Compound 2. Au in the center is effectively an auride.

I was invited to participate in this wonderful venture by my good friend and colleague Dr. José Oscar Carlos Jiménez-Halla, from the University of Guanajuato, Mexico, with whom we’re now working with Prof. Rong Shang at the Hiroshima University. Prof. Shang has synthesized this portentous Auride complex and over the last year, Leonardo “Leo” Lugo has worked with Oscar and I in calculating their electronic structure and bonding properties.

Gold catalysis is an active area of research but low valent Au compounds are electron deficient and therefore highly reactive and elusive; that’s why researchers prefer to synthesize these compounds in situ, to harness their catalytic properties before they’re lost. Power’s digalladeltacyclane was used as a ligand framework to bind to a Au(I) center, which became reduced after the addition and breaking of the Ga−Ga bond while the opposite face of the metallic center became blocked by the bulky aromatic groups on the main ligand. NBO calculations at the M05-2X/[LANL2TZ(f),6-311G(d,p)] and QTAIM BCP analysis show the main features of Au bonding in 2, noteworthy features are the 3c−2e bond (banana) and the σC−C and σC−H donations (See figure 2).

Fig.2 Natural Hybrid Composition for the Ga−Au−Ga ‘banana‘ bond (left). Bond Critical Points (BCPs) for Au in 2 (right).

One of the most interesting features of this compound is the fact that Au(PPh3)Cl reacts differently to the digallane ligand than it does to analogous B−B, Si−Si, or Sn−Sn bonds. The Au−Cl bond does not undergo metathesis as with B−B, nor does it undergo an oxidative addition, so to further understand the chemistry of−and leading to−compound 2, the reaction mechanism energy profile was calculated in a rather painstakingly effort (Kudos, Leo, and a big shoutout to my friend Dr. Jacinto Sandoval for his one on one assistance). Figure 3 shows the energy profile for the reaction mechanism for the formation of 2 from Power’s digallane reagent and Au(PPh3)Cl.

Fig. 3 Free Energy profile for the formation of 2. All values, kcal/mol

You can read more details about this research in Organometallics DOI:10.1021/acs.organomet.0c00557. Thanks again to Profs. Rong Shang and Óscar Jiménez-Halla for bringing me on board of this project and to Leo for his relentless work getting those NBO calculations done; this is certainly the beginning of a golden opportunity for us to collaborate on a remarkable field of chemistry, it has certainly made me go bananas over Aurides chemistry. OK I’ll see myself out.

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

Imagen1

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’

Imagen2.jpg

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.

Comparing the Relative Stability of Non-Equivalent Molecules


How do you compare the stability of two or more compounds which differ in some central atom(s)?

If you simply calculate the energy of both compounds you get a misleading answer since the number of electrons is different from one to the next -in fact, the answer is not so much misleading as it is erroneous. Take compounds 1 and 2 shown in figure 1, for example. Compound 1 was recently synthesized characterized through X-Ray crystallography by my friend Dr. Monica Moya’s group; compound 2 doesn’t exist and we want to know why – or at least know if it is relatively unstable respect to 1.

Figure 1. Compound 1 exists but compound 2 is apparently less stable. Is it?

Figure 1. Compound 1 exists but compound 2 is apparently less stable. Is it?

Although stoichiometry is the same, varying only by the substitution of Ga by Al the number of electrons is quite different. We then made the following assumption: Since the atomic radii of Ga and Al are quite similar (according to the CCDC their respective covalent radii are 122[4] and 121[3] pm), relative stability must rely on the bonding properties rendering 2 harder to obtain, at least through the method used for 1. The total energy for compound 1 was calculated at the M06-2X/6-31G(d,p) level of theory; then both Al atoms were changed by Ga and the total energy was calculated again at the same level. Separately, the energy of isolated Ga and Al atoms were calculated. Compensating the number of electrons was now a simple algebraic problem:

ΔE = E(MnBxOy) – nEM + nEM’ – E(M’nBxOy)

 The absolute energy difference E1 – E2 is staggering due to the excess of 36 electrons in 2. But after this compensation procedure we now have a more reliable result of ΔE value of ca. 81 kcal/mol in favor of compound 1. In strict sense, we performed geometry optimizations at various stages: first on compound 1 to remove the distorsions due to the crystal field and then on the substituted compound 2 to make sure Ga atoms would find a right fit in the molecule but since their covalent radii are similar, no significant changes in the overall geometry were observed confirming the previous assumption.

We now have the value of the energy difference between 1 and 2 and other similar cases, the next step is to find the distal causes of the relative stability which may rely on the bonding properties of the Ga-O bond respect to the Al-O bonds.

What do you think? Is there another method you can share for tackling this problem? Please share your thoughts on the comments section.

Thanks for reading.

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