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Results of polyhedral expansion of 12-vertex carboranes to 13-vertex metallacarboranes. For para-carborane {M} = {Ru(p-cymene)}, {C} = {CPh} relating to ref. 4; for ortho-carborane, e.g. {M} = {Ru(p-cymene)} or {CoCp}, {C} = e.g. {CH}.2,5 Computed energies of [nido-C2B10H12]²⁻ species given relative to 7,9 in kcal mol⁻¹. See inset for formal numbering
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Dianionic nido-[C2B10]2- species are key intermediates in the polyhedral expansion from 12- to 13-vertex carboranes and metallacarboranes, and the isomer adopted by these nido intermediates dictates the isomeric form of the 13-vertex product that is produced. Upon reduction and metallation of para-carborane up to five MC2B10 metallacarboranes can b...
Citations
... On that basis, the reaction of boron hydrides may involve many competing pathways [5]. Consequently, relatively little progress has been made so far in the understanding of the reaction mechanisms of boron hydrides and carboranes of various molecular shapes [6][7][8]. Apart from the reaction of the icosahedral carboranes [1,2], a few conversions of ten-vertex closo carboranes have already emerged. ...
Modern computational protocols based on the density functional theory (DFT) infer that polyhedral closo ten-vertex carboranes are key starting stationary states in obtaining ten-vertex cationic carboranes. The rearrangement of the bicapped square polyhedra into decaborane-like shapes with open hexagons in boat conformations is caused by attacks of N-heterocyclic carbenes (NHCs) on the closo motifs. Single-point computations on the stationary points found during computational examinations of the reaction pathways have clearly shown that taking the “experimental” NHCs into account requires the use of dispersion correction. Further examination has revealed that for the purposes of the description of reaction pathways in their entirety, i.e., together with all transition states and intermediates, a simplified model of NHCs is sufficient. Many of such transition states resemble in their shapes those that dictate Z-rearrangement among various isomers of closo ten-vertex carboranes. Computational results are in very good agreement with the experimental findings obtained earlier.
... On that basis, the reaction of boron hydrides may involve many competing pathways [3]. For that reason, relatively little progress has so far been made in the understanding of the reaction mechanisms of boron hydrides and carboranes of various molecular shapes [4][5][6]. To our knowledge, the reaction pathways associated with ten-vertex closo carboranes, for instance closo-1,2-C 2 B 8 H 10 (see Scheme 1), have not yet been explored. ...
... All of the stationary points in the reactions of closo-1,2-, 1,6-, and 1,10-C2B8H10 with OH (−) were optimized and frequencies were calculated at the SMD(water) [15,16]/B3LYP/6-311+G(2d,p) level, a model chemistry well-established for this class of materials [4][5][6]. The entries in Table 1 for B3LYP/6-311+G(2d,p) are single-point energies at SMD(water)/B3LYP/6-311+G(2d,p)-optimized geometries. ...
... All of the stationary points in the reactions of closo-1,2-, 1,6-, and 1,10-C 2 B 8 H 10 with OH (−) were optimized and frequencies were calculated at the SMD(water) [15,16]/B3LYP/6-311+G(2d,p) level, a model chemistry well-established for this class of materials [4][5][6]. The entries in Table 1 for B3LYP/6-311+G(2d,p) are single-point energies at SMD(water)/B3LYP/6-311+G(2d,p)-optimized geometries. ...
On the basis of the direct transformations of closo-1,2-C2B8H10 with OH(−) and NH3 to arachno-1,6,9-OC2B8H13(−) and arachno-1,6,9-NC2B8H13, respectively, which were experimentally observed, the DFT computational protocol was used to examine the corresponding reaction pathways. This work is thus a computational attempt to describe the formations of 11-vertex arachno clusters that are formally derived from the hypothetical closo-B13H13(2−). Moreover, such a protocol successfully described the formation of arachno-4,5-C2B6H11(−) as the very final product of the first reaction. Analogous experimental transformations of closo-1,6-C2B8H10 and closo-1,10-C2B8H10, although attempted, were not successful. However, their transformations were explored through computations.
... 38 Taking this into account, what occurs with the nido-[B n H n ] 4− series, to which nido-[C 2 B 9 H 11 ] 2− belongs? 39 These have one electron pair more of electrons than closo-[B n H n ] 2− , and therefore, if closo-[B n H n ] 2− are aromatic, it is expected that nido-[B n H n ] 4− should be nonaromatic or antiaromatic, if the parallelism between closo-boranes and flat Huckel's rule abiders holds. This would be the case for typical hydrocarbon aromatic compounds. ...
o-C2B10H12 isomerizes to m-C2B10H12 upon heating at 400 ºC. Deboronation in o-C2B10H12 is a relatively easy process, whereas it is more difficult in m-C2B10H12. These two experimental facts indicate that m-C2B10H12 is thermodynamically more stable than o-C2B10H12. On the other hand, it is widely accepted that closo boranes and carboranes are aromatic compounds. In this work, we relate difficulty in the deboronation of the carboranes with stability and aromaticity. We do this by combining lab work and by means of DFT calculations. Computationally, our results show that the higher thermodynamic stability of m-C2B10H12 is not related to aromaticity differences but to the location of the C atoms in the carborane structure. It is also demonstrated that the aromaticity observed in closo boranes and carboranes is also present in their nido counterparts and, consequently, we conclude that aromaticity in boron clusters survives radical structural changes. Further, sandwich metallocenes (e.g. ferrocene) and sandwich metallabis(dicarbollides) (e.g. [Co(C2B9H11)2]-) have traditionally been considered similar. Here it is shown that they are not. Metallabis(dicarbollides) display global aromaticity whereas metallocenes present local aromaticity in the ligands. Remarkable and unique is the double probe given by 1H- and 11B-NMR tracing the reciprocally antipodal endocyclic open face Hec and B1. These magnetic studies have permitted to correlate both nuclei and relate them to a diatropic current in the plane at the middle of the nido [C2B9H12]-. This observation is the first and unique data that proves experimentally the existence of diatropic currents, thence aromaticity, in clusters and is comparable to the existence of diatropic currents in planar aromatic compounds. Additionally, heteroboranes with two carbon atoms have been compared to heterocycles with two nitrogen or boron atoms, C2B10H12 carboranes against planar N2C4H4 diazines or [B2C4H4]2- diboratabenzenes, proving the higher persistence of the aromaticity of the tri-dimensional compounds in heteroatom substituted species. This research accounts very well for the “Paradigm for the Electron Requirements of Clusters” in which a closo-cluster that is aromatic upon addition of 2e- becomes also an aromatic nido species and explains the nice schemes by R.W. Rudolph and R. E. Williams
... Again, the larger nido clusters are rare. The nido-[C 2 B 10 H 12 ] 2À system [15] has been detected as the adduct C 2 B 10 H 12 .HNP(NMe 2 ) 3 . [16] The same molecular shape was identified when the so-called ocarborane, closo-1,2-C 2 B 10 H 12 , was reacted with N-heterocyclic carbenes. ...
The concept of icosahedral barrier has been expanded from the chemistry of carbaboranes to the area of thiaboranes. Both representatives of this barrier, i. e., closo‐1,2‐C2B10H12 and closo‐1‐SB11H11, are similar in their electron distribution, which is dominated by positive charge in the midpoint of the C−C vector and on the sulfur atom with experimentally determined dipole moments of 4.50 D and 3.64 D, respectively. This is a driving force for their reactivity as exemplified by their reactions with different carbon functionalities. Icosahedral closo‐1‐SB11H11 reacts both with an electron sextet containing carbon (in the form of N‐heterocyclic carbenes), reported earlier, and with methyl iodide with an electron octet on the carbon. The latter reaction provides hexamethylated thiaborane on the basis of methylation so far unknown in this area of heteroborane chemistry. The computations of the heat of formation (ΔHf²⁹⁸) make it possible to estimate the height of the barrier as well as to propose closo‐thiaboranes beyond the barrier. Eleven and twelve vertex thiaboranes with nido electron count are known experimentally for breaking the barrier. These computations also suggest that the larger nido‐thiaboranes are promising candidates for the corresponding experimental availability, i. e., the ΔHf²⁹⁸ of a 13‐vertex nido‐thiaborane cluster has been computed to be more negative than that of the well‐known nido‐SB10H11⁻ cluster (−6.7 and −5.6 kcal mol⁻¹ per vertex, respectively).
... Through these investigations, a control of enhancement of diverse molecular properties by substituent changes at specific sites can be performed. The unusual stability [20,27] and reactivity [21] of cobalt bis(dicarbollide) derivatives are proven here by noteworthy aromaticity. ...
In this work, a novel study on aromaticity for a series of cobalt bis(dicarbollide) derivatives: \([{\hbox {3-Co-}}({\hbox {1,2-C}}_{2}{\hbox {B}}_9{\hbox {H}}_9)_{2}{\hbox {RX}]^-}\, ({\hbox {R, X}}={\hbox {H}},{\hbox {NH}}_2,{\hbox {NO}}_2,{\hbox {CH}}_3, {\hbox {COH}},{\hbox {OCH}}_3,{\hbox {C}}_6{\hbox {H}}_5,{\hbox {Cl}}\)) is presented. For this goal the nucleus-independent chemical shift (NICS) calculations were used. Calculated negative NICS is found to be more than two times larger than the relatively high values obtained for other sandwich compounds of transition metals previously studied (Gribanova et al. in Chem Eur J 16(7):2272, 2010).
Icosahedral closo-dodecaboranes have the ability to accept two electrons, opening into a dianionic nido-cluster. This transformation can be utilized to store electrons, drive bond activation, or alter coordination to metal cations. In this feature article, we present cases for each of these applications, wherein the redox activity of carborane facilitates the generation of unique products. We highlight the effects of exohedral substituents on reactivity and the stability of the products through conjugation between the cluster and exohedral substituents. Futher, the utilization of the redox properties and geometry of carborane clusters in the ligand design is detailed, both in the stabilization of low-valent complexes and in the tuning of ligand geometry.
On the basis of the experimentally observable transformation of closo-1,6-C2B8H10 to arachno-4,6-C2B7H12⁻ with NBu4F, the DFT computational protocol has been used to examine the corresponding reaction pathway. Therefore, this work is a joint synthetic/computational attempt to describe the formation of such a 9-vertex arachno cluster. Analogous experimental transformations of closo-1,2-C2B8H10 and closo-1,10-C2B8H10 have been attempted and the corresponding arachno clusters have been afforded only computationally as the results of energetically demanding processes. For comparison, the identical reaction of icosahedral carboranes provided the fluoroborate anions BF4⁻ and BHxF4−x⁻ (x=1–3) similarly as in the course of the reactions under scrutiny, but in the case of the 1,2- and 1,10-isomers as the only detectable products. The negative charge of the arachno-4,6-C2B7H12⁻ product with respect to the neutral arachno-4,6-C2B7H13 is caused by a CH group instead of two CH2, detected for the latter.
We report metal-free bond activation by the carboranyl diphosphine 1-PtBu2-2-PiPr2-C2B10H10. This main group element system contains basic binding sites and possesses the ability to cycle through two-electron redox states. The reported reactions with selected main group hydrides and alcohols occur via the formal oxidation of the phosphine groups and concomitant reduction of the boron cage. These transformations, which are driven by the cooperation between the electron-donating exohedral substituents and the electron-accepting cluster, differ from those of "regular" phosphines and are reminiscent of oxidative addition to transition metal centers, thus representing a new approach to metal-free bond activation.
