Exceptional Structural Compliance of the B₁₂F₁₂^2– Superweak Anion

The single-crystal X-ray structures, thermogravimetric analyses, and/or FTIR spectra of a series of salts of the B₁₂F₁₂^2– anion and homoleptic Ag­(L)_<i>n</i>⁺ cations are reported (L = CH₂Cl₂, <i>n</i> = 2; L = PhCH₃, <i>n</i> = 3; L = CH₃CN; <i>n</i> = 2–4; L = CO, <i>n</i> = 1, 2). The superweak-anion nature of B₁₂F₁₂^2– (Y^2–) was demonstrated by the rapid reaction of microcrystalline Ag₂(Y) with 1 atm of CO to form a nonclassical silver­(I) carbonyl compound with an FTIR ν­(CO) band at 2198 cm^–1 (and with the proposed formula [Ag­(CO)_<i>n</i>]₂[Y]). In contrast, microcrystalline Ag₂(B₁₂Cl₁₂) did not exhibit ν­(CO) bands and therefore did not form Ag­(CO)⁺ species, even after 32 h under 24 atm of CO. When Ag₂(Y) was treated with carbon monoxide pressures higher than 1 atm, a new ν­(CO) band at 2190 cm^–1 appeared, which is characteristic of a Ag­(CO)₂⁺ dicarbonyl cation. Both Ag₂(CH₃CN)₈(Y) and Ag₂(CH₃CN)₅(Y) rapidly lost coordinated CH₃CN at 25 °C to form Ag₂(CH₃CN)₄(Y), which formed solvent-free Ag₂(Y) only after heating above 100 °C. Similarly, Ag₂(PhCH₃)₆(Y) rapidly lost coordinated PhCH₃ at 25 °C to form Ag₂(PhCH₃)₂(Y), which formed Ag₂(Y) after heating above 150 °C, and Ag₂(CH₂Cl₂)₄(Y) rapidly lost three of the four coordinated CH₂Cl₂ ligands between 25 and 100 °C and formed Ag₂(Y) when it was heated above 200 °C. Solvent-free Ag₂(Y) was stable until it was heated above 380 °C. The rapid evaporative loss of coordinated ligands at 25 °C from nonporous crystalline solids requires equally rapid structural reorganization of the lattice and is one of three manifestations of the structural compliance of the Y^2– anion reported in this work. The second, more quantitative, manifestation is that Ag⁺ bond-valence sums for Ag₂(CH₃CN)_<i>n</i>(Y) are virtually constant, 1.20 ± 0.03, for <i>n</i> = 8, 5, 4, because the Y^2– anion precisely compensated for the lost CH₃CN ligands by readily forming the necessary number of weak Ag–F­(B) bonds. The third, and most exceptional, manifestation is that the idealized structural reorganization accompanying the conceptual transformations Ag₂(CH₃CN)₈(Y) → Ag₂(CH₃CN)₅(Y) → Ag₂(CH₃CN)₄(Y) involve close-packed layers of Y^2– anions that sandwich the Ag­(CH₃CN)₄⁺ complexes splitting into staggered flat ribbons of interconnected (Y^2–)₃ triangles that surround the Ag₂(CH₃CN)₅²⁺ complexes on four sides, conceptually re-forming close-packed layers of anions that sandwich the Ag­(CH₃CN)₂⁺ complexes. The interconnected (Y^2–)₃ triangle lattice of anions in Ag₂(CH₃CN)₅(Y) may be the first example of this structure type

Formation of a Cationic Vinylimido Group upon C–H Activation of Nitriles by Trialkylamines in the Presence of TaCl₅

We report a new CH₃CN activation mode where an imido group is directly formed by deprotonation of the nitrile coordinated to the highly Lewis acidic Ta^V center. The unexpected deprotonation of TaCl₅(CH₃CN) by NEt₃ resulted in isolation of the triethylammonium vinylimido complex [HNEt₃]­[Ta­(NC­(CH₂)­NEt₃)­Cl₅]. The reaction is proposed to proceed through rearrangement of the initial nucleophilic carbanion to the electrophilic azaallene/carbocation intermediate. The use of more sterically hindered (<i>i</i>-Pr)­CN and weakly nucleophilic N­(<i>i</i>-Pr)₂Et resulted in the isolation of a vinylimido group formed upon dimerization of deprotonated nitriles, suggesting deprotonation as the first step of the transformation

Metal- and Ligand-Centered Reactivity of <i>meta</i>-Carboranyl-Backbone Pincer Complexes of Rhodium

We report the synthesis of the chelating phosphinite-arm carboranyl POBOP-H (POBOP = 1,7-OP­(<i>i</i>-Pr)₂-<i>m</i>-carboranyl) ligand precursor, preparation of its rhodium complexes, and their reactivity in oxidative addition/reductive elimination reactions. The oxidative addition of iodobenzene to the low-valent (POBOP)­Rh­(PPh₃) resulted in the selective formation of the 16-electron complex (POBOP)­Rh­(Ph)­(I), featuring a highly strained exohedral rhodium–boron bond. The complex (POBOP)­Rh­(Ph)­(I) is the first example of a B-carboranyl aryl metal complex, which is a proposed intermediate in metal-promoted B–C coupling reactions. The complex (POBOP)­Rh­(Ph)­(I) was selectively and directly converted, in the presence of acetonitrile, to (POB­(BPh)­OP)­Rh­(H)­(I)­(CH₃CN) (POB­(BPh)­OP = 1,7-OP­(<i>i</i>-Pr)₂-2-Ph-<i>m</i>-carboranyl) through unprecedented cascade reductive elimination of the phenyl-<i>B</i>-carboranyl and the oxidative addition of a vicinal B–H bond of the boron cluster to the metal center, exhibiting both metal- and cluster-centered reactivity

Opening of Carborane Cages by Metal Cluster Complexes: The Reaction of a Thiolate-Substituted Carborane with Triosmium Carbonyl Cluster Complexes

The reaction of Os₃(CO)₁₀(NCMe)₂ with closo-<i>o</i>-(1-SCH₃)­C₂B₁₀H₁₁ has yielded the complex Os₃(CO)₉[μ₃-η³-C₂B₁₀H₉(SCH₃)]­(μ-H)₂, <b>1</b>, by the loss of the two NCMe ligands and one CO ligand from the Os₃ cluster and the coordination of the sulfur atom and the activation of two B–H bonds with transfer of the hydrogen atoms to the cluster. Reaction of <b>1</b> with a second equivalent of Os₃(CO)₁₀(NCMe)₂ yielded the complex Os₃(CO)₉(μ-H)­[(μ₃-η³-1,4,5-μ₃-η³-6,10,11-C₂B₁₀H₈S­(CH₃)]­Os₃(CO)₉(μ-H)₂, <b>2</b>, that contains two triosmium triangles attached to the same carborane cage. The carborane cage was opened by cleavage of two B–C bonds and one B–B bond. The B–H group that was pulled out of the cage became a triply bridging group on one of the Os₃ triangles but remains bonded to the cage by two B–B bonds. When heated to 150 °C, <b>2</b> was transformed into the complex Os₃(CO)₉(μ-H)­[(μ₃-η³-μ₃-η³-C₂B₁₀H₇S­(CH₃)]­Os₃(CO)₉(μ-H), <b>3</b>, by the loss of two hydrogen atoms and a rearrangement that led to further opening of the carborane cage. Reaction of <b>1</b> with a second equivalent of closo-<i>o</i>-(1-SCH₃)­C₂B₁₀H₁₁ has yielded the complex Os₃(CO)₆)­(μ₃-η³-C₂B₁₀H₉-<i>R</i>-SCH₃) (μ₃-η³-C₂B₁₀H₁₀-<i>S</i>-SCH₃)­(μ-H)₃, <b>4a</b>, containing two carborane cages coordinated to one Os₃ cluster. Compound <b>4a</b> was isomerized to the compound Os₃(CO)₆(μ₃-η³-C₂B₁₀H₉-<i>R</i>-SCH₃)­(μ₃-η³-C₂B₁₀H₁₀-<i>R</i>-SCH₃)­(μ-H)₃, <b>4b</b>, by an inversion of stereochemistry at one of the sulfur atoms by heating to 174 °C

(BB)-Carboryne Complex of Ruthenium: Synthesis by Double B–H Activation at a Single Metal Center

The first example of a transition metal (BB)-carboryne complex containing two boron atoms of the icosahedral cage connected to a single exohedral metal center (POBBOP)­Ru­(CO)₂ (POBBOP = 1,7-OP­(<i>i</i>-Pr)₂-2,6-dehydro-<i>m</i>-carborane) was synthesized by double B–H activation within the strained <i>m</i>-carboranyl pincer framework. Theoretical calculations revealed that the unique three-membered (BB)>Ru metalacycle is formed by two bent B–Ru σ-bonds with the concomitant increase of the bond order between the two metalated boron atoms. The reactivity of the highly strained electron-rich (BB)-carboryne fragment with small molecules was probed by reactions with electrophiles. The carboryne–carboranyl transformations reported herein represent a new mode of cooperative metal–ligand reactivity of boron-based complexes

(BB)-Carboryne Complex of Ruthenium: Synthesis by Double B–H Activation at a Single Metal Center

The first example of a transition metal (BB)-carboryne complex containing two boron atoms of the icosahedral cage connected to a single exohedral metal center (POBBOP)­Ru­(CO)₂ (POBBOP = 1,7-OP­(<i>i</i>-Pr)₂-2,6-dehydro-<i>m</i>-carborane) was synthesized by double B–H activation within the strained <i>m</i>-carboranyl pincer framework. Theoretical calculations revealed that the unique three-membered (BB)>Ru metalacycle is formed by two bent B–Ru σ-bonds with the concomitant increase of the bond order between the two metalated boron atoms. The reactivity of the highly strained electron-rich (BB)-carboryne fragment with small molecules was probed by reactions with electrophiles. The carboryne–carboranyl transformations reported herein represent a new mode of cooperative metal–ligand reactivity of boron-based complexes

Structures of M₂(SO₂)₆B₁₂F₁₂ (M = Ag or K) and Ag₂(H₂O)₄B₁₂F₁₂: Comparison of the Coordination of SO₂ versus H₂O and of B₁₂F₁₂^2– versus Other Weakly Coordinating Anions to Metal Ions in the Solid State

The structures of three solvated monovalent cation salts of the superweak anion B₁₂F₁₂^2– (Y^2–), K₂(SO₂)₆Y, Ag₂­(SO₂)₆Y, and Ag₂­(H₂O)₄Y, are reported and discussed with respect to previously reported structures of Ag⁺ and K⁺ with other weakly coordinating anions. The structures of K₂(SO₂)₆Y and Ag₂­(SO₂)₆Y are isomorphous and are based on expanded cubic close-packed arrays of Y^2– anions with M­(OSO)₆⁺ complexes centered in the trigonal holes of one expanded close-packed layer of B₁₂ centroids (⊙). The K⁺ and Ag⁺ ions have virtually identical bicapped trigonal prism MO₆F₂ coordination spheres, with M–O distances of 2.735(1)–3.032(2) Å for the potassium salt and 2.526(5)–2.790(5) Å for the silver salt. Each M­(OSO)₆⁺ complex is connected to three other cationic complexes through their six μ-SO₂-κ¹<i>O</i>,κ²<i>O</i>′ ligands. The structure of Ag₂­(H₂O)₄Y is unique [different from that of K₂­(H₂O)₄Y]. Planes of close-packed arrays of anions are offset from neighboring planes along only one of the linear ⊙···⊙···⊙ directions of the close-packed arrays, with [Ag­(μ-H₂O)₂­Ag­(μ-H₂O)₂)]_∞ infinite chains between the planes of anions. There are two nearly identical AgO₄F₂ coordination spheres, with Ag–O distances of 2.371(5)–2.524(5) Å and Ag–F distances of 2.734(4)–2.751(4) Å. This is only the second structurally characterized compound with four H₂O molecules coordinated to a Ag⁺ ion in the solid state. Comparisons with crystalline H₂O and SO₂ solvates of other Ag⁺ and K⁺ salts of weakly coordinating anions show that (i) N­[(SO₂)₂­(1,2-C₆H₄)]⁻, BF₄^–, SbF₆^–, and Al­(OC­(CF₃)₃)₄^– coordinate much more strongly to Ag⁺ than does Y^2–, (ii) SnF₆^2– coordinates somewhat more strongly to K⁺ than does Y^2–, and (iii) B₁₂Cl₁₂^2– coordinates to K⁺ about the same as, if not slightly weaker than, Y^2–

Activation of C–H Bonds of Alkyl- and Arylnitriles by the TaCl₅–PPh₃ Lewis Pair

A new pathway of activation of C–H bonds of alkyl- and arylnitriles by a cooperative action of TaCl₅ and PPh₃ under mild conditions is reported. Coordination of nitriles to the highly Lewis acidic Ta­(V) center resulted in an activation of their aliphatic and aromatic C–H bonds, allowing nucleophilic attack and deprotonation by the relatively weak base PPh₃. The propensity of Ta­(V) to form multiple bonds to nitrogen-containing ligands is an important driving force of the reaction as it led to a sequence of bond rearrangements and the emergence of, in the case of benzonitrile, a zwitterionic enediimido complex of Ta­(V) through CC double bond formation between two activated nitrile fragments. These transformations highlight the special role of the high-valent transition metal halide in substrate activation and distinguish the reactivity of the TaCl₅–PPh₃ system from both non-metal- and late transition metal-based frustrated Lewis pairs

Comparison of the Coordination of B₁₂F₁₂^2–, B₁₂Cl₁₂^2–, and B₁₂H₁₂^2– to Na⁺ in the Solid State: Crystal Structures and Thermal Behavior of Na₂(B₁₂F₁₂), Na₂(H₂O)₄(B₁₂F₁₂), Na₂(B₁₂Cl₁₂), and Na₂(H₂O)₆(B₁₂Cl₁₂)

The synthesis of high-purity Na₂B₁₂F₁₂ and the crystal structures of Na₂(B₁₂F₁₂) (5 K neutron powder diffraction (NPD)), Na₂(H₂O)₄(B₁₂F₁₂) (120 K single-crystal X-ray diffraction (SC-XRD)), Na₂(B₁₂Cl₁₂) (5 and 295 K NPD), and Na₂(H₂O)₆­(B₁₂Cl₁₂) (100 K SC-XRD) are reported. The compound Na₂(H₂O)₄(B₁₂F₁₂) contains {[(Na­(μ-H₂O)₂Na­(μ-H₂O)₂)]²⁺}_∞ infinite chains; the compound Na₂(H₂O)₆­(B₁₂Cl₁₂) contains discrete [(H₂O)₂Na­(μ-H₂O)₂Na­(H₂O)₂]²⁺ cations with OH···O hydrogen bonds linking the terminal H₂O ligands. The structures of the two hydrates and the previously published structure of Na₂(H₂O)₄(B₁₂H₁₂) are analyzed with respect to the relative coordinating ability of B₁₂F₁₂^2–, B₁₂H₁₂^2–, and B₁₂Cl₁₂^2– toward Na⁺ ions in the solid state (i.e., the relative ability of these anions to satisfy the valence of Na⁺). All three hydrated structures have distorted octahedral NaX₂(H₂O)₄ coordination spheres (X = F, H, Cl). The sums of the four Na–O bond valence contributions are 71, 75, and 89% of the total bond valences for the X = F, H, and Cl hydrated compounds, respectively, demonstrating that the relative coordinating ability by this criterion is B₁₂Cl₁₂^2– ≪ B₁₂H₁₂^2– < B₁₂F₁₂^2–. Differential scanning calorimetry experiments demonstrate that Na₂(B₁₂F₁₂) undergoes a reversible, presumably order–disorder, phase transition at ca. 560 K (287 °C), between the 529 and 730 K transition temperatures previously reported for Na₂(B₁₂H₁₂) and Na₂(B₁₂Cl₁₂), respectively. Thermogravimetric analysis demonstrates that Na₂(H₂O)₄­(B₁₂F₁₂) and Na₂(H₂O)₆­(B₁₂Cl₁₂) undergo partial dehydration at 25 °C to Na₂(H₂O)₂­(B₁₂F₁₂) and Na₂(H₂O)₂­(B₁₂Cl₁₂) in ca. 30 min and 2 h, respectively, and essentially complete dehydration to Na₂(B₁₂F₁₂) and Na₂(B₁₂Cl₁₂) within minutes at 150 and 75 °C, respectively (the remaining trace amounts of H₂O, if any, were not quantified). The changes in structure upon dehydration and the different vapor pressures of H₂O needed to fully hydrate the respective Na₂(B₁₂X₁₂) compounds provide additional evidence that B₁₂Cl₁₂^2– is more weakly coordinating than B₁₂F₁₂^2– to Na⁺ in the solid state. Taken together, the results suggest that the anhydrous, halogenated <i>closo</i>-borane compounds Na₂(B₁₂F₁₂) and Na₂(B₁₂Cl₁₂), in appropriately modified forms, may be viable component materials for fast-ion-conducting solid electrolytes in future energy-storage devices

Latent Porosity in Alkali-Metal M₂B₁₂F₁₂ Salts: Structures and Rapid Room-Temperature Hydration/Dehydration Cycles

Structures of the alkali-metal hydrates Li₂(H₂O)₄Z, LiK­(H₂O)₄Z, Na₂(H₂O)₃Z, and Rb₂(H₂O)₂Z, unit cell parameters for Rb₂Z and Rb₂(H₂O)₂Z, and the density functional theory (DFT)-optimized structures of K₂Z, K₂(H₂O)₂Z, Rb₂Z, Rb₂(H₂O)₂Z, Cs₂Z, and Cs₂(H₂O)­Z are reported (Z^2– = B₁₂F₁₂^2–) and compared with previously reported X-ray structures of Na₂(H₂O)_0,4Z, K₂(H₂O)_0,2,4Z, and Cs₂(H₂O)­Z. Unusually rapid room-temperature hydration/dehydration cycles of several M₂Z/M₂(H₂O)_<i>n</i>Z salt hydrate pairs, which were studied by isothermal gravimetry, are also reported. Finely ground samples of K₂Z, Rb₂Z, and Cs₂Z, which are not microporous, exhibited latent porosity by undergoing hydration at 24–25 °C in the presence of 18 Torr of H₂O­(g) to K₂(H₂O)₂Z, Rb₂(H₂O)₂Z, and Cs₂(H₂O)­Z in 18, 40, and 16 min, respectively. These hydrates were dehydrated at 24–25 °C in dry N₂ to the original anhydrous M₂Z compounds in 61, 25, and 76 min, respectively (the exact times varied from sample to sample depending on the particle size). The hydrate Na₂(H₂O)₂Z also exhibited latent porosity by undergoing multiple 90 min cycles of hydration to Na₂(H₂O)₃Z and dehydration back to Na₂(H₂O)₂Z at 23 °C. For the K₂Z, Rb₂Z, and Cs₂Z transformations, the maximum rate of hydration (rh_max) decreased, and the absolute value of the maximum rate of dehydration (rd_max) increased, as <i>T</i> increased. For K₂Z ↔ K₂(H₂O)₂Z hydration/dehydration cycles with the same sample, the ratio rh_max/rd_max decreased 26 times over 8.6 °C, from 3.7 at 23.4 °C to 0.14 at 32.0 °C. For Rb₂Z ↔ Rb₂(H₂O)₂Z cycles, rh_max/rd_max decreased from 0.88 at 23 °C to 0.23 at 27 °C. For Cs₂Z ↔ Cs₂(H₂O)­Z cycles, rh_max/rd_max decreased 20 times over 8 °C, from 6.7 at 24 °C to 0.34 at 32 °C. In addition, the reversible substitution of D₂O for H₂O in fully hydrated Rb₂(H₂O)₂Z in the presence of N₂/16 Torr of D₂O­(g) was complete in only 60 min at 23 °C