The Explosive “Inert” Anion CB11(CF3)12-

Three-Body Excitations in Fock-Space Coupled-Cluster: Fourth Order Perturbation Correction to Electron Affinity and Its Relation to Bondonic Formalism

In this paper, we present a formulation of highly correlated Fock-space multi-reference coupled-cluster (FSMRCC) methods, including approximate triples on top of the FSMRCC with singles and doubles, which correct the electron affinities by at least at third and up to the fourth order in perturbation. We discuss various partial fourth-order schemes, which are reliable and yet computationally more efficient than the full fourth-order triples scheme. The third-order scheme is called MRCCSD+T^*(3). We present two approximate fourth-order schemes, MRCCSD+T^*−a(4) and MRCCSD+T^*(4). The results that are presented allow one to choose an appropriate fourth-order scheme, which is less expensive and right for the problem. All these schemes are based on the effective Hamiltonian scheme, and provide a direct calculation of the vertical electron affinities. We apply these schemes to a prototype Li₂ molecule, using four different basis sets, as well as BeO and CH⁺. We have calculated the vertical electron affinities of Li₂ at the geometry of the neutral Li₂ molecule. We also present the vertical ionization potentials of the Li₂ anion at the geometry of the anion ground state. We have also shown how to calculate adiabatic electron affinity, though in that case we lose the advantages of direct calculation. BeO has been examined in two basis sets. For CH⁺, four different basis sets have been used. We have presented the partial fourth-order schemes to the EA in all the basis sets. The results are analyzed to illustrate the importance of triples, as well as highlight computationally efficient partial fourth-order schemes. The choice of the basis set on the electron affinity calculation is also emphasized. Comparisons with available experimental and theoretical results are presented. The general fourth-order schemes, which are conceptually equivalent with the Fock-space multi-reference coupled-cluster singles, doubles, and triplets (MRCCSD+T) methods, based on bondonic formalism, are also presented here in a composed way, for quantum electronic affinity.

A Super-Oxidized Radical Cationic Icosahedral Boron Cluster

While the icosahedral closo-[B₁₂H₁₂]^2- cluster does not display reversible electrochemical behavior, perfunctionalization of this species via substitution of all 12 B-H vertices with alkoxy or benzyloxy (OR) substituents engenders reversible redox chemistry, providing access to clusters in the dianionic, monoanionic, and neutral forms. Here, we evaluated the electrochemical behavior of the electron-rich B₁₂(O-3-methylbutyl)₁₂ (1) cluster and discovered that a new reversible redox event that gives rise to a fourth electronic state is accessible through one-electron oxidation of the neutral species. Chemical oxidation of 1 with [N(2,4-Br₂C₆H₃)₃]^•+ afforded the isolable [1]^•+ cluster, which is the first example of an open-shell cationic B₁₂ cluster in which the unpaired electron is proposed to be delocalized throughout the boron cluster core. The oxidation of 1 is also chemically reversible, where treatment of [1]^•+ with ferrocene resulted in its reduction back to 1. The identity of [1]^•+ is supported by EPR, UV-vis, multinuclear NMR (¹H, ¹¹B), and X-ray photoelectron spectroscopic characterization.

Superhalogen and Superacid

A superhalogen F@C₂₀ (CN)₂₀ and a corresponding Brønsted superacid were designed and investigated on DFT and DLPNO-CCSD(T) levels of theory. Calculated compounds have outstanding electron affinity and deprotonation energy, respectively. We consider superacid H[F@C₂₀ (CN)₂₀ ] to be able to protonate molecular nitrogen. The stability of these structures is discussed, while some of the previous predictions concerning neutral Brønsted superacids of record strength are doubted. © 2019 Wiley Periodicals, Inc.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

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.

Reactive p-block cations stabilized by weakly coordinating anions

The chemistry of the p-block elements is a huge playground for fundamental and applied work. With their bonding from electron deficient to hypercoordinate and formally hypervalent, the p-block elements represent an area to find terra incognita. Often, the formation of cations that contain p-block elements as central ingredient is desired, for example to make a compound more Lewis acidic for an application or simply to prove an idea. This review has collected the reactive p-block cations (rPBC) with a comprehensive focus on those that have been published since the year 2000, but including the milestones and key citations of earlier work. We include an overview on the weakly coordinating anions (WCAs) used to stabilize the rPBC and give an overview to WCA selection, ionization strategies for rPBC-formation and finally list the rPBC ordered in their respective group from 13 to 18. However, typical, often more organic ion classes that constitute for example ionic liquids (imidazolium, ammonium, etc.) were omitted, as were those that do not fulfill the - naturally subjective -"reactive"-criterion of the rPBC. As a rule, we only included rPBC with crystal structure and only rarely refer to important cations published without crystal structure. This collection is intended for those who are simply interested what has been done or what is possible, as well as those who seek advice on preparative issues, up to people having a certain application in mind, where the knowledge on the existence of a rPBC that might play a role as an intermediate or active center may be useful.

A New Homogeneous Catalyst for the Dehydrogenation of Dimethylamine Borane Starting with Ruthenium(III) Acetylacetonate

The catalytic activity of ruthenium(III) acetylacetonate was investigated for the first time in the dehydrogenation of dimethylamine borane. During catalytic reaction, a new ruthenium(II) species is formed in situ from the reduction of ruthenium(III) and characterized using UV-Visible, Fourier transform infrared (FTIR), ¹H NMR, and mass spectroscopy. The most likely structure suggested for the ruthenium(II) species is mer-[Ru(N₂Me₄)₃(acac)H]. Mercury poisoning experiment indicates that the catalytic dehydrogenation of dimethylamine-borane is homogeneous catalysis. The kinetics of the catalytic dehydrogenation of dimethylamine borane starting with Ru(acac)₃ were studied depending on the catalyst concentration, substrate concentration and temperature. The hydrogen generation was found to be first-order with respect to catalyst concentration and zero-order regarding the substrate concentration. Evaluation of the kinetic data provides the activation parameters for the dehydrogenation reaction: the activation energy E_a = 85 ± 2 kJ·mol⁻¹, the enthalpy of activation ∆H^# = 82 ± 2 kJ·mol⁻¹ and the entropy of activation; ∆S^# = −85 ± 5 J·mol⁻¹·K⁻¹. The ruthenium(II) catalyst formed from the reduction of ruthenium(III) acetylacetonate provides 1700 turnovers over 100 hours in hydrogen generation from the dehydrogenation of dimethylamine borane before deactivation at 60 °C.

Chemistry of the three-dimensionally aromatic CB11 cage

After a brief introduction to the electronic structure of the three-dimensionally aromatic icosahedral closo-monocarbadodecaborate anion CB11H12-, some recent results for its permethylated version, CB11Me12- and three highly reactive electroneutral analogs are presented and discussed. These are the radical CB11Me12·, the boronium ylide CB11Me11 with a naked boron vertex, and the isomeric carbonium ylide with a naked carbon vertex. These ylides are probably better viewed as unusual types of singlet borylene and carbene, respectively.

Condensed Two- and Three-Dimensional Aromatic Systems: A Theoretical Study on the Relative Stabilities of Isomers of CB19H16+, B20H15Cl, and B20H14Cl2 and Comparison to B12H10Cl22-, C6H4Cl2, C10H7Cl, and C10H6Cl2

DFT studies (B3LYP/6-31G) on mono- and dichloro derivatives of benzene, naphthalene, B12H12(2-), four-atom-sharing condensed systems B20H16, and monocarborane isomers of B20H16 are used to compare the variation of relative stability and aromaticity between condensed aromatics. The trends in the variation of the relative energies and aromaticity in these two- and three-dimensional systems are similar. Aromaticity, estimated by NICS values, does not change considerably with condensation or substitution. The minor variation in the relative energies of the isomers of chloro derivatives is explained by the topological charge stabilization rule of Gimarc. The compatibility of the cap and ring orbitals decides the relative stability of CB19H16+.

Carborane acids. New “strong yet gentle” acids for organic and inorganic chemistry

Icosahedral carborane anions such as CHB11Cl11- are amongst the least coordinating, most chemically inert anions known. They are also amongst the least basic, so their conjugate acids, H(carborane), are superacids (i.e. stronger than 100% H2SO4). Acidity scale measurements indicate that H(CHB11Cl11) is the strongest pure Brønsted acid presently known, surpassing triflic and fluorosulfuric acid. Nevertheless, it is also an extremely gentle acid--because its conjugate base engages in so little chemistry. Carborane acids separate protic acidity from anion nucleophilicity and destructive oxidative capacity in the conjugate base, to a degree not previously achieved. As a result, many long-sought, highly acidic, reactive cations such as protonated benzene (C6H7+), protonated C60(HC60+), tertiary carbocations (R3C+), vinyl cations (R2C=C(+)-R), silylium ions (R3Si+) and discrete hydronium ions (H3O+, H5O2+ etc.) can be readily isolated as carborane salts and characterized at room temperature by X-ray crystallography.

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.

Rearrangement Reactions of the Transient Lewis Acids (CF3)3B and (CF3)3BCF2: An Experimental and Theoretical Study

Short-lived (CF(3))(3)B and (CF(3))(3)BCF(2) are generated as intermediates by thermal dissociation of (CF(3))(3)BCO and F(-) abstraction from the weak coordinating anion [B(CF(3))(4)](-), respectively. Both Lewis acids cannot be detected because of their instability with respect to rearrangement reactions at the B-C-F moiety. A cascade of 1,2-fluorine shifts to boron followed by perfluoroalkyl group migrations and also difluorocarbene transfer reactions occur. In the gas phase, (CF(3))(3)B rearranges to a mixture of linear perfluoroalkyldifluoroboranes C(n)()F(2)(n)()(+1)BF(2) (n = 2-7), while the respective reactions of (CF(3))(3)BCF(2) result in a mixture of linear (n = 2-4) and branched monoperfluoroalkyldifluoroboranes, e.g., (C(2)F(5))(CF(3))FCBF(2). For comparison, the reactions of [CF(3)BF(3)](-) and [C(2)F(5)BF(3)](-) with AsF(5) are studied, and the products in the case of [CF(3)BF(3)](-) are BF(3) and C(2)F(5)BF(2) whereas in the case of [C(2)F(5)BF(3)](-), C(2)F(5)BF(2) is the sole product. In contrast to reports in the literature, it is found that CF(3)BF(2) is too unstable at room temperature to be detected. The decomposition of (CF(3))(3)BCO in anhydrous HF leads to a mixture of the new conjugate Brønsted-Lewis acids [H(2)F][(CF(3))(3)BF] and [H(2)F][C(2)F(5)BF(3)]. All reactions are modeled by density functional calculations. The energy barriers of the transition states are low in agreement with the experimental results that (CF(3))(3)B and (CF(3))(3)BCF(2) are short-lived intermediates. Since CF(2) complexes are key intermediates in the rearrangement reactions of (CF(3))(3)B and (CF(3))(3)BCF(2), CF(2) affinities of some perfluoroalkylfluoroboranes are presented. CF(2) affinities are compared to CO and F(-) affinities of selected boranes showing a trend in Lewis acidity, and its influence on the stability of the complexes is discussed. Fluoride ion affinities are calculated for a variety of different fluoroboranes, including perfluorocarboranes, and compared to those of the title compounds.

Synthesis and characterization of ammonioundecafluoro-closo-dodecaborates(1-). New superweak anions

The ammonioborane monoanion H(3)NB(12)H(11)(-) was per-B-fluorinated with elemental fluorine in liquid hydrogen fluoride to yield the first member of a new class of weakly coordinating anions, H(3)NB(12)F(11)(-) (isolated as [N(n-Bu)(4)](2)[H(2)NB(12)F(11)] in 41% yield). The pK(a) of the H(3)NB(12)F(11)(-) anion is 9.6. Several salts of the tri-N-alkylated anions Me(3)NB(12)F(11)(-) and Dd(3)NB(12)F(11)(-) (Dd = n-C(12)H(25)) were also prepared. The structure of [CPh(3)][Me(3)NB(12)F(11)] was determined by single-crystal X-ray diffraction: monoclinic, space group P2(1)/c, a = 18.053(3) A, b = 33.139(5) A, c = 9.600(2) A, beta = 91.459(4) degrees, V = 5742(2) A(3), Z = 8, T = 173(2) K, R(1) = 0.045. It revealed that the only direct interactions between the undecafluoroammonioborate monoanions and the trityl cations in the two independent ion pairs were long and weak BF...CPh(3) interactions of 2.992(6) and 2.942(6) A. Salts of the new anions were chemically, electrochemically, and thermally stable. The conductivity of Li(Me(3)NB(12)F(11)) in dimethoxyethane was comparable to that of LiPF(6) but less than half the value of Li(1-Me-CB(11)F(11)).

Density functional theory study of anionic and neutral per-substituted 12-vertex boron cage systems, B(12)X(12)(n-) (n = 2, 1, 0)

The 12(12) closomers form a rapidly expanding class of compounds where a 12-vertex cage is surrounded by 12 identical substituents. Density functional theory (B3LYP/6-31G(d)) is used to study a number of these closomers in different states of oxidation (dianion, radical anion, and neutral cages). The cage stability increases as the group electronegativity of the substituent increases. Also, the 12(12) closomer becomes easier to oxidize as the Hammett sigma(p) parameter becomes more negative (electron-donating). As the closomer is oxidized, the size of the cage increases and the B-B distances become more asymmetric. The Raman-active breathing mode in the 404-434 cm(-1) range moves to lower frequency as the cage is oxidized, which is caused by removing one or two electrons from a cage-bonding molecular orbital.

Synthesis of 12-Hydroxy and 12-Dioxane Derivatives of the closo-1-Carbadodecaborate(1-) Ion. Variations on the Plešek's Cobalt Bis(dicarbollide) Pattern

Synthesis of two new B(12) substituted derivatives of the closo-[1-CB11H12]- anion (1) is reported. The zwitterionic derivative containing dioxane ring {12-[O(CH2CH2)2O]+-1-CB11H11-}0 (2) was obtained in moderate yield using reaction of anion 1 with dioxane as solvent, induced by dimethyl sulfate. The anion [12-(HO)-1-CB11H11]- (3) was obtained in high yield upon treatment of the parent anion with 80% H2SO4 at high temperature. Both compounds containing reactive functionalities were designed to serve as useful synthons for a variety of synthetic purposes. The compounds were characterized by 11B and 1H high-field NMR, IR spectroscopy, mass spectrometry, melting points and TLC. Molecular structure of compound 2 was determined by single crystal X-ray diffraction analysis.

Dodecamethyl-closo-dodecaborate(2-)

Bis(tetraethylammonium) dodecamethyl-closo-dodecaborate(2-), [NEt(4)](2)[closo-B(12)Me(12)], [NEt(4)](2)2, was prepared employing modified Friedel-Crafts reaction conditions from [NEt(4)](2)[closo-B(12)H(12)], [NEt(4)](2)1, trimethylaluminum, and methyl iodide. The [NEt(4)](2)2 salt provides sufficient solubility in water to allow the synthesis of the important alkali metal salts A(2)2 (A = Li, Na, K, Rb, Cs) using cation-exchange procedures. The solid state structure of colorless [AsPh(4)](2)2 reveals a nearly perfect icosahedral B(12) cluster with B-B bonds ranging from 1.785(3) to 1.807(3) A and B-C bonds of 1.597(3)-1.625(3) A. In contrast, the crystal structure of dark-red [Py(2)CH(2)]2 (obtained from [NEt(4)](2)2 and [Py(2)CH(2)]Br(2)) contains a distorted icosahedral dianion [B-B = 1.740(13)-1.811(14) A, B-C = 1.591(13)-1.704(13) A]. In the [Py(2)CH(2)]2 salt, the dianion 2(2-) and its dipositive dipyridiniomethane counterion form a red charge-transfer complex. One-electron oxidation of 2(2)(-) by ceric(IV) ammonium nitrate affords the blue, air-stable radical [hypercloso-B(12)Me(12)](*-), dodecamethyl-hypercloso-dodecaborate(1-), 2(*-), isolated as the PPN salt. X-ray crystallography reveals that the geometries of the B(12) clusters observed in hypercloso-[PPN]2 and closo-[AsPh(4)](2)2 are identical and essentially undistorted icosahedra. The anion in the [PPN]2 structure contains B-B bonds ranging from 1.784(8) to 1.806(7) A and a range of B-C bonds from 1.596(7) to 1.616(7) A.

Novel Systems for Selective Cation Detection: Application of a Lipophilic Carborane Anion in Polymeric Ion-Selective Electrodes

Selectivity and sensitivity of PVC membrane electrode are dramatically influenced not only by the specific receptor, but also, mainly for neutral ionophores, by additional lipophilic co-receptor. For cation detection, tetraphenylborate derivatives have been commonly used. Here, we describe novel systems based on conjunction of the classic cation receptors and a highly lipophilic cobalticarborane anionic species. The superiority of this novel co-receptor is demonstrated by application of dibenzo-18-crown-6 and dicyclohexano-18-crown-6 as ionophores in conjunction with the classic lipophilic additive and the novel additive cobalticarborane. The presented data show much higher efficiency which makes this additive co-receptor ideal for cation detection and development of novel sensors.

Preparation of [closo-CB11H12]- by Dichlorocarbene Insertion Into [nido-B11H14]-

In strongly basic media, the [nido-B11H14]- anion reacts with the haloforms. Dehydrogenation to [closo-B11H11]2- is the only reaction observed with iodoform. With chloroform and bromoform, the cage is expanded by dihalocarbene insertion. The dominant products are the [closo-CB11H12]- and the [2-Br-closo-CB11H12]- anion, respectively. The chief side product is the [closo-B11H11]2- anion, which results from dehydrogenation of the starting material. It was identified by 11B NMR spectroscopy and isolated after acidic aqueous workup in the form of the [nido-7-OH-B11H13]- anion. Since the starting [nido-B11H14]- anion is available from NaBH4 and BF3·Et2O in 50% yield, its conversion to [closo-CB11H12]- with chloroform and base in a 40% yield represents a useful laboratory route to the numerous known but previously very expensive derivatives of [closo-CB11H12]-, highly prized for their very low nucleophilicity.