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The magic roundabout: The B19− cluster behaves like a molecular Wankel engine (see picture) in which the two concentric boron rings rotate in opposite directions. During the rotation the cluster remains planar owing to a marginal rotational energy barrier.

Abstract:The lack of reference aromatic systems in the realm of inorganic aromatic compounds makes the evaluation of aromaticity in all-metal and semimetal clusters a difficult task. To date, calculation of nucleus-independent chemical shifts (NICS) has been the most widely used method to discuss aromaticity in these systems. In the first part of this work, we briefly review our previous studies, showing some pitfalls of the NICS indicator of aromaticity in organic molecules. Then, we refer to our study on the performance of some aromaticity indices in a series of 15 aromaticity tests, which can be used to analyze the advantages and drawbacks of aromaticity descriptors. It is shown that indices based on the study of electron delocalization are the most accurate among those analyzed in the series of proposed tests, while NICS(1) zz and NICS(0) πzz present the best behavior among NICS indices. In the second part, we discuss the use of NICS and electronic multicenter indices (MCI) in inorganic clusters. In particular, we evaluate the aromaticity of two series of all-metal and semimetal clusters with predictable aromaticity trends by means of NICS and MCI. Results show that the expected trends are generally better reproduced by MCI than NICS. It is concluded that NICS(0) π and NICS(0) πzz are the kind of NICS that perform the best among the different NICS indices analyzed for the studied series of inorganic compounds.

[3] whose combination of nucleophilic and electrophilic sites are well-suited to combine with the carbon and oxygen atoms of CO2, respectively ( Figure 1A). Another common mechanism of main-group-based CO2 activation is the (initial) 2+2 cycloaddition of one C=O bond of CO2 with another E-E multiple bond, e.g. P=N (i.e. the Aza-Wittig reaction), Si=O, Si=N, Ge=O, Sn=O, and B=N bonds ( Figure 1B). [4] To our knowledge, no CO2 fixation or activation has been observed by solely utilizing a nonpolar multiple bond, despite the fact that a range of highly reactive compounds with E-E multiple bonds are known.[5] However, in 2011 Kato and Baceiredo reported [6] the reaction of CO2 with a disilyne bisphosphine adduct, a compound thought to possess some multiple bonding character between its two silicon atoms despite the clearly non-planar geometry around the silicon atoms. Herein we present fixation and splitting reactions of CO2 through its interaction with distinctly non-polar multiple bonds of two significantly different diboron species:[5c,d,e] a doubly basestabilized diborene [5,7] with tricoordinate boron atoms and a B=B double bond, and a linear diboryne species bearing strongly π-acidic cyclic (alkyl)(amino)carbene (CAACs) donors, [8] effectively a diboracumulene [9] species with a B-B bond order between two and three. Interestingly, in the reaction of CO2, we were also able to isolate the thermally unstable 2+2 (C=O + B=B) cycloaddition product, which slowly undergoes cleavage of one C=O bond. The apparently facile reaction of CO2 with B-B multiply bound species is attributed to the high reactivity of the latter, which is able to overcome the lack of polarity in the bond and effect the initial cycloaddition step. Dibromodiborenes (L(Br)B=B(Br)L), very few of which exist in the literature, [10,11] were chosen as candidates for CO2 binding due to their sterically unhindered B=B bonds and thus presumed high reactivity. Upon treatment with one atmosphere of CO2 at room temperature, after 7 min the 11 B NMR spectroscopic signal of diborene 1 [10] (dB 20) was found to have completely disappeared, replaced by two broad signals (dB ca. 0, -10). Removal of the solvent from this mixture and extraction of the residue into hexane provided a solution from which orange crystals (2) were grown. The solid-state structure of 2 (Figure 2, middle) confirms the combination of the diborene 1 with CO2 to form a dibora-b-lactone structure in which the two boron atoms form a slightly puckered four-membered B-B-C-O ring with one carbon and one oxygen atom of the CO2 unit. The remaining oxygen atom is part of a carbonyl group with a short C-O distance of 1.20(1) Å but a relatively wide O-C-B angle (136.2(8)º). Interestingly, the endocyclic B-B-C angle is strongly acute (73.7(8)º). The NHC and Br groups are each oriented in a trans fashion with respect to the ring.

As compared to classical organic aromatic compounds, the evaluation of aromaticity in all-metal and semimetal clusters is much more complex. For a series of these clusters, it is frequently found that different methods used to discuss aromaticity lead to divergent conclusions. For this reason, there is a need to evaluate the reliability of the different descriptors of aromaticity to provide correct trends in all-metal and semimetal aromatic clusters. This work represents the first attempt to assess the performance of aromaticity descriptors in all-metal clusters. To this end, we introduce the series of all-metal and semimetal clusters [X n Y4−n ] q± (X, Y = Al, Ga, Si, and Ge; n = 0−4) and [X n Y5−n ]4−n (X = P and Y = S and Se; n = 0−5) with predictable aromaticity trends. Aromaticity, in these series, is quantified by means of nucleus-independent chemical shifts (NICS) and electronic multicenter indices (MCI). Results show that the expected trends are generally better reproduced by MCI than by NICS. It is found that NICS(0)π is the kind of NICS that performs better among the different NICS indices analyzed.

In this work, we used the induced magnetic field (B ind) to study the degree of aromaticity of the planar (HF)3 ring. The induced magnetic field analysis shows that the degree of electron delocalization in the hydrogen-bonded cyclic trimer of HF is very low. This result is in agreement with those obtained using GIMIC and is opposite to the Rehaman’s suggestion. Our results demonstrate a clear limitation of the NICS index when a strong anisotropy is exhibited and suggest that the NICS values should be used carefully to discuss aromaticity in systems without an important p z -orbital overlap that produces the π clouds. In view of the fact that the NICS index is extensively used by computationally and theoretically oriented experimental chemists, this is an important warning.

A new bis(cyclopentadienyl) dialane is prepared, which shows controlled, selective dialumination reactions with a conventional alkyne, an electron-rich alkyne, and an azide. The reactions provide structurally diverse products, featuring a range of aluminium coordination numbers, cyclopentadienyl binding modes, and cyclic motifs. The variable nature of the bonding in the Cp*Al units allows a range of binding modes depending on the electronic requirements of the Al atom and provides new possibilities to the chemistry of dialanes, as demonstrated by the isolation of a double internal Lewis adduct with "ring-slipped" Cp* rings in this work.

Polar and apolar boron-based triple bonds promote the single and double C–H activation of acetone following similar coordination-deprotonation mechanisms.