Computed Properties of the CB9H10 and CB11H12 Free Radicals

Divalent closo-monocarborane solvates for solid-state ionic conductors

Li-ion batteries have held the dominant position in battery research for the last 30+ years. However, due to inadequate resources and the cost of necessary elements (e.g., lithium ore) in addition to safety issues concerning the components and construction, it has become more important to look at alternative technologies. Multivalent metal batteries with solid-state electrolytes are a potential option for future battery applications. The synthesis and characterisation of divalent hydrated closo-monocarborane salts - Mg[CB₁₁H₁₂]₂·xH₂O, Ca[CB₁₁H₁₂]₂·xH₂O, and Zn[CB₁₁H₁₂]₂·xH₂O - have shown potential as solid-state electrolytes. The coordination of a solvent (e.g. H₂O) to the cation in these complexes shows a significant improvement in ionic conductivity, i.e. for Zn[CB₁₁H₁₂]₂·xH₂O dried at 100 °C (10^-3 S cm^-1 at 170 °C) and dried at 150 °C (10^-5 S cm^-1 at 170 °C). Solvent choice also proved important with the ionic conductivity of Mg[CB₁₁H₁₂]₂·3en (en = ethylenediamine) being higher than that of Mg[CB₁₁H₁₂]₂·3.1H₂O (2.6 × 10^-5 S cm^-1 and 1.7 × 10^-8 S cm^-1 at 100 °C, respectively), however, the oxidative stability was lower (<1 V (Mg²⁺/Mg) and 1.9 V (Mg²⁺/Mg), respectively). Thermal characterisation of the divalent closo-monocarborane salts showed melting and desolvation, prior to high temperature decomposition.

Anodic Oxidation of 18 Halogenated and/or Methylated Derivatives of CB11H12. Inorg Chem 2016;56:269-276. [PMID: 27936641 DOI: 10.1021/acs.inorgchem.6b02126]

Anodic oxidation of [CB₁₁H₁₂]^- and 18 of its halogenated and/or methylated derivatives was examined. Reversible oxidation was found for four of the anions in liquid SO₂ and for four more in 1,1,1,3,3,3-hexafluoroisopropyl alcohol. The oxidation occurred at ∼1 V (for [CB₁₁Me₁₂]^-) up to more than 4 V (for [1-H-(2-6)-F₅-(7-12)-(CF₃)₆-CB₁₁]^-) relative to ferrocene/ferricinium. The anodic peak potentials are reproduced by a set of additive position-sensitive substituent increments.

STRUCTURE AND STABILITY OF $CLOSO-B_{n}H_{n-1}N_{2}^{-} (n=5-12)$

[Formula: see text], framework-isoelectronic to [Formula: see text], have been investigated at the B3LYP/6-311+G** level of theory. The most stable positional isomers of individual [Formula: see text] are similar to those of [Formula: see text] and [Formula: see text] in framework configuration. The energy analysis reveals that the cage [Formula: see text] with the icosahedron framework, [Formula: see text] with the octahedron framework and [Formula: see text] with the bicapped square antiprism framework are, in order, the most stable in the [Formula: see text] system. Furthermore, three-dimensional aromaticity of all [Formula: see text] is characterized. The most stable cages, [Formula: see text], [Formula: see text] and [Formula: see text], also have relatively large aromaticity. All the predicted N–N stretching frequencies (2127–2303 cm−1) of the most stable positional isomers in the [Formula: see text] system are lower than the computational N–N stretching frequency (2445 cm−1) of free N2 at the same level, which results from the σ donation of N2 to B frameworks. The stretching frequencies of N–N in the [Formula: see text] cages increase with the decrease of N–N distances or the increase of B–N distance. In addition, B n−1 H n−1 fragment has larger affinity for group CH than that for group BNN, namely, it favors to form the [Formula: see text] rather than the [Formula: see text].

STRUCTURE AND STABILITY OF HYDROGEN POOR CLOSO-${\rm B}_{n}{\rm H}_{n-x}^{-/0/+}$ (n = 5 - 12)

Structures and stabilities of hydrogen poor closo-[Formula: see text], closo- B n H n-2, and closo-[Formula: see text] (n = 5 - 12) have been investigated at the B3LYP/6-311+G** density functional level of theory. It is found that [Formula: see text], B 5 H 3, [Formula: see text], and [Formula: see text] have open instead of the expected closo structures, and the other isomers have the same structural pattern as closo-[Formula: see text]. Energetic analysis identifies closo-[Formula: see text] (n = 7, 9), closo- B n H n-2 (n = 7, 11), and closo-[Formula: see text] (n = 6, 8, 10) as the most stable clusters. The CO affinities of the most stable closo- B n H n-1 CO -, closo- B n H n-2( CO )2, and closo-[Formula: see text] (n = 5 - 12) have been computed.

Synthesis of the Isolable Biradicals •(CH3B)11C−C⋮C−C(BCH3)11• and trans-•(CH3B)11C−CHCH−C(BCH3)11•

A synthesis of ethene and ethyne derivatives carrying the anionic -C(BCH3)11- substituent on one or both carbon atoms is described. Two-electron oxidation of the dianions yielded the stable and isolable electroneutral title biradicals.

Polyethyl substituted weakly coordinating carborane anions: a sequential dehydrogenative borylation–hydrogenation route

Polyethylated carborane monoanions based on [closo-CB11H12]- with up to five B-ethyl groups can be prepared by a sequential Rh-catalysed dehydrogenative borylation then hydrogenation.

Metal Cation−Methyl Interactions in CB11Me12- Salts of Me3Ge+, Me3Sn+, and Me3Pb+

Oxidation of Me(6)M(2) (M = Ge, Sn) and Me(4)Pb with the CB(11)Me(12)(*) radical in alkane solvents produced the insoluble salts Me(3)M(+)CB(11)Me(12)(-), characterized by CP-MAS NMR and EXAFS. The cations interact with methyl groups of CB(11)Me(12)(-) with coordination strength increasing from Pb to Ge. Density functional theory (DFT) calculations for the isolated ion pairs, Me(3)M(+)CB(11)Me(12)(-) (M = Ge, Sn), revealed three isomers with the cation above methyl 2, 7, or 12, and not above a BB edge or a BBB triangle. The interaction has a considerable covalent component, with the cation attempting to perform a backside S(E)2 substitution on the methyl carbon. In a fourth less favorable isomer the cation is near methyl 1, inclined toward methyl 2, and interacts with hydrogens. DFT atomic charge distributions and plots of the electrostatic potential on the surface of spheres centered at the CB(11)H(12)(-) and CB(11)Me(12)(-) icosahedra display the effects of uneven charge distribution within the anion and contradict the common belief that the negative charge of the cage anion is concentrated primarily on the cage boron atoms 7-12; in CB(11)Me(12)(-), roughly half is on the cage carbon and the rest on methyls 7-12.

Silver−Phosphine Complexes of the Highly Methylated Carborane Monoanion [closo-1-H-CB11Me11]-

The synthesis of the silver(I) salt of the highly methylated carborane anion [closo-1-H-CB(11)Me(11)](-) is described, Ag[closo-1-H-CB(11)Me(11)] 1, which in the solid state shows close intermolecular Ag...H(3)C contacts. Addition of various monodentate phosphines to 1 results in the formation of the complexes (R(3)P)Ag[closo-1-H-CB(11)Me(11)] [R = Ph, 2; cyclohexyl (C(6)H(11)), 3; (3,5-Me(2)-C(6)H(3)), 4]. All these complexes show close intermolecular Ag.H(3)C contacts in the solid state that are considerably shorter than the sum of the van der Waals radius of methyl (2.00 A) and the ionic radius of silver(I) (1.29 A). For 2 and 3 there are other close intermolecular Ag...H(3)C contacts in the solid state, arising from proximate carborane anions in the crystal lattice. Addition of methyl groups to the periphery of the phosphine ligand (complex 4) switches off the majority of these interactions, leaving essentially a single cage interacting with the cationic silver-phosphine fragment through three CH(3) groups. In solution (CD(2)Cl(2)) Ag...H(3)C contacts remain, as evidenced by both the downfield chemical shift change and the significant line-broadening observed for the cage methyl signals. These studies also show that the metal fragment is fluxional over the surface of the cage. The Ag...H(3)C interactions in solution may be switched off by addition of a stronger Lewis base than [closo-1-H-CB(11)Me(11)](-). Thus, addition of [NBu(4)][closo-1-H-CB(11)H(5)Br(6)] to 2 affords (Ph(3)P)Ag[closo-1-H-CB(11)H(5)Br(6)], while adding Et(2)O or PPh(3) affords the well-separated ion-pairs [(Ph(3)P)(L)Ag][closo-1-H-CB(11)Me(11)] (L = OEt(2) 5, PPh(3) 6,) both of which have been crystallographically characterized. DFT calculations on 2 (at the B3LYP/DZVP level) show small energy differences between the possible coordination isomers of this compound, with the favored geometry being one in which the [(Ph(3)P)Ag](+) fragment interacts with three of the [BCH(3)] vertices on the lower surface of the cage, similar to the experimentally observed structure of 4.

Ion-pairing ability, chemical stability, and selectivity behavior of halogenated dodecacarborane cation exchangers in neutral carrier-based ion-selective electrodes

Recently, it has been discovered that carba-closo-dodecaborates can be used as cation exchangers in neutral carrier-based ion-selective chemical sensors. Because of their inherent chemical stability and versatile functionalization chemistries, they offer many advantages that may potentially be exploited for ion analyses that require nontraditional sample conditions, including strongly acidic media. In this work, trimethylammonium salts of undecachlorinated (UCC), undecabrominated (UBC), hexabrominated (HBC), and undecaiodinated (UIC) carborane anions were prepared and evaluated for their potential use in solvent polymeric membrane-based sensors. Computational methods including Natural population analysis and electrostatic mapping were used to predict the ion-exchanging ability of each lipophilic anion. In addition, the sandwich membrane technique was used to evaluate the ion-pairing ability of each carborane anion in situ (i.e., within bis(2-ethylhexyl) sebacate (DOS)- and 2-nitrophenyl octyl ether (o-NPOE)-plasticized ISE membranes). The results of the computational and potentiometric studies found that binding affinity of the anions followed the generalized trend HBC > UCC > UBC > UIC. PVC-DOS bulk optode thin films containing the chromoionophore ETH 5315 and a respective anion were used to determine the chemical stability/lipophilicity of the carboranes and tetrakis[3,5-bis(trifluoromethyl)phenyl] borate (TFPB) in acidic media (0.2 M HOAc) under flowing conditions. The studies found that in terms of stability/lipophilicity UIC > UBC > TFPB approximately UCC >> HBC. Electrodes containing a Pb(2+)-selective ionophore, tert-butylcalix[4]arene-tetrakis(N,N-dimethylthioacetamide)(lead IV), were used to evaluate the functionality of each cation exchanger. An evaluation of response characteristics such as slope and selectivity found that UIC and UBC were quite comparable to the behavior of TFPB. Interestingly, both UIC and UBC showed a marked selectivity improvement over cadmium, with log K(pot)(pb),(Cd) values of -7.19 and -7.29, respectively, with TFPB giving a value of -5.89. Demonstrating excellent stability and suitable electrostatic properties, the carboranes, UIC in particular, are a very promising alternative to the tetraphenylborates and should find widespread application in the field of chemical sensors.

Proposed fluorination mechanism of CB(5)H(6)(-) and CB(9)H(10)(-) with HF. Evidence of kinetic control in the formation of 2-CB(5)H(5)F(-) and 6-CB(9)H(9)F(-)

Two pathways have been considered in the fluorination of CB(5)H(6)(-) and CB(9)H(10)(-) by HF. In the ionic HF fluorination pathway, the monocarborane anion cage is first protonated in a BBB face followed by H(2) elimination and fluoride anion addition. In the covalent HF fluorination pathway, HF is first coordinated through hydrogen to the BBB face. Next, the fluorine can add to either an axial or equatorial boron atom which opens the cage to a nido structure with an endo fluoride substituent. Endo to exo rearrangement occurs with a small activation barrier followed by H(2) elimination. In both pathways, fluorination at the equatorial boron position is predicted to have smaller activation barriers even though substitution at the axial position leads to the more stable products.

Paramagnetic Cluster Ions [B(6)Hal(n)()Hal'(6)(-)(n)()](*)(-) (Hal, Hal' = Cl, Br, I). EPR Evidence for Radical Stabilization through Electronic Effects of the Halogen Substituents

Monoanionic hexaborate cluster radicals [B(6)Hal(n)()Hal'(6)(-)(n)()](*)(-) with mixed halogen substitution were prepared from oxidizable dianionic precursors and were characterized by vibrational and UV-vis spectroscopy. EPR studies of these and the structurally established homoleptic species [B(6)Hal(6)](*)(-) (Hal = Cl, Br, I) reveal strongly increasing g anisotropy and relaxation rate on replacing Cl by Br and especially I substituents; the very stable B(6)I(6)(*)(-) ion (g(1) = 2.04, g(2) = 1.66, g(3) = 1.15) thus exhibits an EPR spectrum only at 4 K. The extent of these effects is attributed to the Jahn-Teller situation in [B(6)X(6)](*)(-) with only partial occupancy of a degenerate MO. Both the absence of B-H bonds and the evidently strong participation of the halogen substituents in the singly occupied MO contribute to the extraordinary stability of these cluster radicals.

Stability and Three-Dimensional Aromaticity of closo-Monocarbaborane Anions, CB(n)()(-)(1)H(n)(-), and closo-Dicarboranes, C(2)B(n)()(-)(2)H(n)()

Comprehensive ab initio calculations RMP2(fc)/6-31G on the closo-monocarbaboranes, CB(n)()(-)(1)H(n)()(-) (n = 5-12), and the closo-dicarboranes, C(2)B(n)()(-)(2)H(n)() (n = 5-12), show that the relative energies of all the positional isomers agree with the qualitative connectivity considerations of Williams and with the topological charge stabilization rule of Gimarc. The reaction energies (DeltaH) of the most stable positional isomers, 1-CB(4)H(5)(-), CB(5)H(6)(-), 2-CB(6)H(7)(-), 1-CB(7)H(8)(-), 5-CB(8)H(9)(-), 1-CB(9)H(10)(-), 2-CB(10)H(11)(-), CB(11)H(12)(-), as well as 1,5-C(2)B(3)H(5), 1,6-C(2)B(4)H(6), 2,4-C(2)B(5)H(7), 1,7-C(2)B(6)H(8), 4,5-C(2)B(7)H(9), 1,10-C(2)B(8)H(10), 2,3-C(2)B(9)H(11), and 1,12-C(2)B(10)H(12) (computed using the equations, CBH(2)(-) + (n - 1)BH(increment) --> CB(n)()H(n)()(+1)(-) (n = 4-11) and C(2)H(2) + nBH(increment) --> C(2)B(n)()H(n)()(+2) (n = 3-10)), show that the stabilities of closo-CB(n)()(-)(1)H(n)()(-) and of closo-C(2)B(n)()(-)(2)H(n)() generally increase with increasing cluster size from 5 to 12 vertexes. This is a characteristic of three-dimensional aromaticity. There are variations in stabilities of individual closo-CB(n)()(-)(1)H(n)()(-) and closo-C(2)B(n)()(-)(2)H(n)() species, but these show quite similar trends. Moreover, there is rough additivity for each carbon replacement. The rather large nucleus independent chemical shifts (NICS) and the magnetic susceptibilities (chi), which correspond well with one another, also show all closo-CB(n)()(-)(1)H(n)()(-) and closo-C(2)B(n)()(-)(2)H(n)() species to exhibit "three-dimensional aromaticity". However, the aromaticity ordering based on these magnetic properties does not always agree with the relative stabilities of positional isomers of the same cluster, when other effects such as connectivity and charge considerations are important.