Decarbonylation Products of Binuclear Methylphosphinidene Complexes of Cyclopentadienyliron Carbonyls: Triplet and Quintet Structures Are Favored Energetically over Singlet Structures with Iron-Iron Multiple Bonding.

The structures, energetics, and energetically preferred spin states of methylphosphinidene-bridged binuclear cyclopentadienyliron carbonyl complexes MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) related to the experimentally known (µ-RP)Fe2(µ-CO)(CO)2Cp2 (R = cyclohexyl, phenyl, mesityl, and 2,4,6-tBu3C6H2) complexes have been investigated by density functional theory. Singlet structures having a pyramidal pseudotetrahedral phosphorus environment with 18-electron iron configurations are energetically preferred in the tricarbonyl and tetracarbonyl systems MePFe2(CO)nCp2 (n = 4 and 3) with the lowest energy structures of the tricarbonyl very closely resembling the experimentally determined structures. For the more unsaturated dicarbonyl and monocarbonyl systems MePFe2(CO)nCp2 (n = 2 and 1), higher spin state triplet and quintet structures are energetically preferred over singlet structures. These more highly unsaturated structures can be derived from the lowest energy singlet MePFe2(CO)nCp2 (n = 4, 3) by the removal of carbonyl groups. The iron atoms giving up carbonyl groups in their 16- and 14-electron configurations bear the spin density of the unpaired electrons in the higher spin states. The lowest energy singlet structure of the monocarbonyl MePFe2(CO)Cp2, although a relatively high energy isomer, is unusual among the collection of MePFe2(CO)nCp2 (n = 4, 3, 2, and 1) structures by having both the formal Fe=Fe double bond and the four-electron donor MeP unit with the planar phosphorus coordination required to allow each of its iron atoms to attain the favored 18-electron configuration.

Polyhedral Ferraboranes with Iron Carbonyl Vertices: Carbonyl Migration Processes in the Iron Tetracarbonyl Derivatives.

The structures and energetics of the neutral Bn-1Hn-1Fe(CO)x (x = 4, 3) and the dianions [Bn-1Hn-1Fe(CO)3]2- (n = 6-14) have been investigated by density functional theory. The low-energy structures of the tricarbonyl dianions [Bn-1Hn-1Fe(CO)3]2- are all found to have closo deltahedral structures in accordance with their 2n+2 skeletal electrons. The low-energy structures of the neutral tricarbonyls Bn-1Hn-1Fe(CO)3 (n = 6-14) with only 2n skeletal electrons are based on capped (n-1)-vertex closo deltahedra (n = 6, 7, 8) or isocloso deltahedra with a degree 6 vertex for the iron atom. The closo 8- and 9-vertex deltahedra are also found in low-energy Bn-1Hn-1Fe(CO)3 structures relating to the nondegeneracy of their frontier molecular orbitals. Carbonyl migration occurs in most of the low-energy structures of the tetracarbonyls Bn-1Hn-1Fe(CO)4. Thus, migration of a carbonyl group from an iron atom to a boron atom gives closo Bn-2Hn-2(BCO)(µ-H)Fe(CO)3 structures with a BCO vertex and a hydrogen atom bridging a B-B deltahedral edge. In other low-energy Bn-1Hn-1Fe(CO)4 structures, a carbonyl group is inserted into the central n-vertex FeBn-1 deltahedron to give a Bn-1Hn-1(CO)Fe(CO)3 structure with a central (n+1)-vertex FeCBn-1 deltahedron that can be an isocloso deltahedron or a µ3-BH face-capped n-vertex FeCBn-2 closo deltahedron. Other low-energy Bn-1Hn-1Fe(CO)4 structures include Bn-1Hn-1Fe(CO)2(µ-CO)2 structures with two of the carbonyl groups bridging FeB2 faces (n = 6, 7, 10) or Fe-B edges (n = 12) or structures in which a closo Bn-1Hn-1 ligand (n = 6, 7, 10, 12) is bonded to an Fe(CO)4 unit with exclusively terminal carbonyl groups through B-H-Fe bridges.

Polyhedral Dicobaltadithiaboranes and Dicobaltdiselenaboranes as Examples of Bimetallic Nido Structures without Bridging Hydrogens.

The geometries and energetics of the n-vertex polyhedral dicobaltadithiaboranes and dicobaltadiselenaboranes Cp2Co2E2Bn-4Hn-4 (E = S, Se; n = 8 to 12) have been investigated via the density functional theory. Most of the lowest-energy structures in these systems are generated from the (n + 1)-vertex most spherical closo deltahedra by removal of a single vertex, leading to a tetragonal, pentagonal, or hexagonal face depending on the degree of the vertex removed. In all of these low-energy structures, the chalcogen atoms are located at the vertices of the non-triangular face. Alternatively, the central polyhedron in most of the 12-vertex structures can be derived from a Co2E2B8 icosahedron with adjacent chalcogen (E) vertices by breaking the E-E edge and 1 or more E-B edges to create a hexagonal face. Examples of the arachno polyhedra with two tetragonal and/or pentagonal faces derived from the removal of two vertices from isocloso deltahedra were found among the set of lowest-energy Cp2Co2E2Bn-4Hn-4 (E = S, Se; n = 8 and 12) structures.

Aromaticity in P8 allotropes and (CH)8 analogues: significance of their 40 valence electrons?

The currently unknown phosphorus allotrope P8 is of interest since its 40 total valence electrons is a "magic number" corresponding to a filled 1S21P61D101S21F142P6 shell such as found in the relatively stable main group element clusters Al13- and Ge94-. However, P8 still remains as an elusive structure not realized experimentally. The lowest energy P8 structure by a margin of ∼9 kcal mol-1 is shown by density functional theory to be a cuneane analogue with no PP double bonds and two each of P5, P4, and P3 rings. Higher energy P8 structures are polycyclic systems having at most a single PP double bond. These P8 systems are not "carbon copies" of the corresponding (CH)8 hydrocarbons with exactly one hydrogen atom bonded to each carbon atom. Thus the lowest energy (CH)8 structure is cyclooctatetraene with four CC bonds followed by benzocyclobutene with three CC bonds. The cuneane (CH)8 structure is a relatively high energy isomer lying ∼36 kcal mol-1 above cyclooctatetraene. The cubane P8 and (CH)8 structures are even higher energy structures, lying ∼37 and ∼74 kcal mol-1 in energy above the corresponding global minima. Our results demonstrate differences in medium sized aggregates of elemental phosphorus and isolobal hydrocarbon species.

Triplet Spin-State Capped Deltahedral Structures Rather than Singlet Spin-State Oblatocloso Structures as Energetically Favored Dimanganaborane Structures.

Density functional studies show that the singlet spin-state flattened oblatocloso deltahedral structures found experimentally in the dimetallaboranes Cp*2Re2Bn-2Hn-2 (Cp* = Me5C5; n = 8-12) of the third row group 7 element rhenium are not favored for analogous dimetallaboranes Cp2Mn2Bn-2Hn-2 (n = 8-14) of its first row congener manganese. Instead, the energetically preferred structures for the dimanganaboranes are higher spin-state triplet and quintet spin-state structures. This appears to be related to the lower ligand field splittings in complexes of the first row transition-metal manganese relative to analogous complexes of the third row transition-metal rhenium. The lowest-energy Cp2Mn2Bn-2Hn-2 (n = 8-13) structures typically have a central MnBn-2 closo deltahedron with one face capped by the second CpMn unit. However, for the 14-vertex Cp2Mn2B12H12 system the lowest-energy structures consist of B12 icosahedra with faces capped by both CpMn units. The thermochemistry of cluster buildup reactions of the type Cp2Mn2Bn-2Hn-2 + BH → Cp2Mn2Bn-1Hn-1 suggests that the 11- and 13-vertex structures are likely to be favored products in reactions of cyclopentadienylmanganese derivatives with borane sources. The paramagnetism of the predicted triplet and quintet spin states for the lowest-energy dimanganaboranes Cp2Mn2Bn-2Hn-2 (n = 8-14) suggests possible applications in novel magnetic materials.

Enhancement of Ion Pairing of Sr(II) and Ba(II) Salts by a Tritopic Ion-Pair Receptor in Solution.

Tritopic ion-pair receptors can bind bivalent salts in solution; yet, these salts have a tendency to form ion-pairs even in the absence of receptors. The extent to which such receptors can enhance ion pairing has however remained elusive. Here, we study ion pairing of M2+ (Ba2+ , Sr2+ ) and X- (I- , ClO4- ) in acetonitrile with and without a dichlorooxacalix[2]arene[2]triazine-related receptor containing a pentaethylene-glycol moiety. We find marked ion association already in receptor-free solutions. When present, most of the MX+ ion-pairs are bound to the receptor and the overall degree of ion association is enhanced due to coordinative, hydrogen-bonding, and anion-π interactions. The receptor shows higher selectivity for iodides but also stabilizes perchlorates, despite the latter are often considered as weakly coordinating anions. Our results show that ion-pair binding is strongly correlated to ion pairing in these solutions, thereby highlighting the importance of taking ion association in organic solvents into account.

Correction: Pseudo electron-deficient organometallics: limited reactivity towards electron-donating ligands.

Correction for 'Pseudo electron-deficient organometallics: limited reactivity towards electron-donating ligands' by Anaïs Pitto-Barry et al., Dalton Trans., 2017, 46, 15676-15683.

Nonsphericity in diferratetracarbaboranes having 2n + 2 Wadean skeletal electrons: deviations from closo deltahedral geometries and high-energy kinetically stable isomers.

The diferratetracarbaboranes Cp2Fe2C4Bn-6Hn-2 (n = 10 to 14; Cp = η5-C5H5) as well as the experimentally known C-tetramethyl derivatives Cp2Fe2C4Me4B8H8 have been studied by density functional theory methods. For the Cp2Fe2C4Me4B8H8 system, the three structurally characterized isomers produced under relatively mild conditions having an "open" tetragonal or pentagonal face correspond to the lowest energy structures not based on the 14-vertex closo deltahedron, namely the bicapped hexagonal antiprism. These structures provide examples of kinetically favored but thermodynamically disfavored high-energy metallacarborane structures. Thus the lowest energy such structure lies ∼22 kcal mol-1 above the global minimum, namely a C2vcloso structure with no C-C deltahedral edges. This latter global minimum 14-vertex closo structure is found experimentally to be the ultimate pyrolysis product in the Cp2Fe2C4Me4B8H8 system at 300 °C. The lowest energy structures for the smaller 11 to 13 vertex Cp2Fe2C4Bn-6Hn-2 systems are the corresponding most spherical closo deltahedra as expected by the Wade-Mingos rules for these 2n + 2 skeletal electron systems. However, for the 11- and 12-vertex systems, less spherical deltahedral structures providing additional degree 6 vertices for the iron atoms and degree 4 vertices for the carbon atoms become energetically competitive. For the 10-vertex Cp2Fe2C4B4H8 system a relatively non-spherical deltahedral structure with four degree 4 vertices for the carbon atoms and two degree 6 vertices for the iron atoms is energetically preferred by a substantial margin. Thus such a structure lies ∼23 kcal mol-1 in energy below the isomeric 10-vertex closo bicapped tetragonal antiprism structure expected from the Wade-Mingos rules for this 2n + 2 skeletal electron system.

The tetracapped truncated tetrahedron in 16-vertex tetrametallaborane structures: spherical aromaticity with an isocloso rather than a closo skeletal electron count.

Density functional theory studies on the experimentally known Cp*3Rh3B12H12Rh(B4H9RhCp*) as well as the model compounds Cp4Rh4B12H12 and Cp3Rh3B12H12Rh(η3-C3H5) indicate low energy structures with central Rh4B12 tetracapped tetratruncated tetrahedra (TTT) for these 32 Wadean skeletal electron systems. This skeletal electron count corresponds to 2k2 (k = 4) skeletal electrons suggesting a spherical aromatic system with filled 1s + 1p + 1d + 1f molecular orbitals as well as an isocloso 2n (= 32 for n = 16) skeletal electron count. Similar TTT structures are found for the valence isoelectronic 32 skeletal electron systems [Cp4M''4B12H12]4+ (M'' = Ni, Pd, Pt) and [Cp4M'4B12H12]4- (M' = Fe, Ru, Os). The preferred structures of the 34 skeletal electron systems [Cp4M4B12H12]2- (M = Co, Rh, Ir), [Cp4M''4B12H12]2+ (M'' = Ni, Pd, Pt) are not the most spherical TTT despite their 2n + 2 skeletal electron count (= 34 for n = 16) for a closo structure by the Wade-Mingos rules. Instead they are prolate (elongated) polyhedra with two degree 6 and two degree 5 metal vertices with a central M4 macrobutterfly having one long MM distance of ∼5.0 Šbetween the wingtips. The preferred structures for the still electron richer 36 skeletal electron systems Cp4M''4B12H12 (M'' = Pd, Pt) are derived from triple square antiprisms with two open M''2B2 square faces. A distorted version of this polyhedron is the deltahedral structure with four degree 5 metal vertices and four degree 6 boron vertices found in the valence isoelectronic 36 skeletal electron first row transition metal derivatives Cp4Ni4B12H12 and [Cp4Co4B12H12]4-. However, this polyhedron is not found in the 36 skeletal electron [Cp4M4B12H12]4- (M = Rh, Ir), that instead have symmetrical central M4B12 TTTs. For some 16-vertex [Cp4M4B12H12]z systems deviating from the favored 32 skeletal electron count, low-energy structures are found in which hydrogen atoms migrate to bridge B-B edges or bend over to bridge M-B edges. In addition, the hypoelectronic hexacations [Cp4M4B12H12]6+ (M = Co, Rh, Ir; Ni, Pd, Pt) are found to have low-energy structures in which three of the four Cp rings are hydrogenated to give tetrahapto cyclopentadiene η4-C5H6 rings.

Cationic gold clusters with eight valence electrons: possible spherical aromatic systems with Sigma holes.

The energetically preferred structures of the gold clusters Au9+, Au113+, and Au124+ with eight skeletal electrons have been studied by density functional theory for comparison with the 8-electron Au102+ cluster shown previously to have a highly favored Td tetracapped octahedral structure. The low-energy structures for the Au9+ and Au113+ clusters are found to be similar relatively spherical polyhedra. Such systems can be considered to exhibit spherical aromaticity in accord with their filled 1S21P6 shells, their diatropic NICS(0) values ranging from -21.4 to -44.3 ppm, and their shielding cone surfaces. However, the preferred spherical polyhedra for Au9+ and Au113+ are not the same as the closo deltahedra found in the BnHn2- borane dianions. Instead they have smaller internal cavities formed by capping faces of smaller deltahedra or by formation of internal Au-Au bonds. The lowest energy Au124+ structures are not similar nearly spherical polyhedral structures. Instead they are derived from planar gold subclusters by adding more gold atoms to form tetrahedral Au4 bubbles. The planar origin of the low-energy Au124+ structures relates to the energetic preference for neutral Au<14 clusters for planar structures or nearly planar structures containing small polyhedral bubbles. The presence of σ-holes has been identified on the surfaces of the complete series of the Aun(n-8)+ (n = 9 to 12) clusters. The strength of their electrostatic interactions is predicted to increase upon increasing cluster size.

Versatile coordination behaviour of the chloro-tetrazine-picolylamine ligand: mixed-valence binuclear Cu(i)/Cu(ii) complexes.

The 3-Cl-6-amino-(2'-picolyl)-1,2,4,5-tetrazine ligand HL1 has been synthesized and structurally characterized. Its versatile coordination behavior has been evidenced through reactions with Cu(Hfac)2 and Cu(triflate)2 precursors, which provided mixed-valence bimetallic Cu1.5Cu1.5 complexes [Cu2(µ-Cl)(L1)2] 2 and [Cu2(µ-triflate)(L1)2] 5. Changing the Cu(ii) precursor and the solvent leads to the formation of mononuclear octahedral Cu(ii) complexes [CuCl2(HL1)2]·2CH3CN 3 and [Cu(Hfac)2(HL1)] 4, in which only the amino-pyridine unit is involved in the coordination of the metal center. In contrast, in complexes 2 and 5, the ligands are deprotonated and bridge the metal centers as pyridine-amido-tetrazine fragments, while a bridging chloride or triflate ligand completes the coordination sphere of the metal ions. The Cu-Cu distances of 2.4313(4) Å in 2 and 2.5198(10) Å in 5 lie among the shorter values within mixed-valence bimetallic Cu complexes. Mixed-valence character is strongly supported by DFT calculations, showing the equal repartition of the unpaired electron between the two metal centers.

Magnesium(II) d-Gluconate Complexes Relevant to Radioactive Waste Disposals: Metal-Ion-Induced Ligand Deprotonation or Ligand-Promoted Metal-Ion Hydrolysis?

The complexation equilibria between Mg2+ and d-gluconate (Gluc-) ions are of particular importance in modeling the chemical speciation in low- and intermediate-level radioactive waste repositories. NMR measurements and potentiometric titrations conducted at 25 °C and 4 M ionic strength revealed the formation of the MgGluc+, MgGlucOH0, MgGluc(OH)2-, and Mg3Gluc2(OH)40 complexes. The trinuclear species provides indirect evidence for the existence of multinuclear magnesium(II) hydroxido complexes, whose formation was proposed earlier but has not been confirmed yet. Additionally, speciation calculations demonstrated that MgCl2 can markedly decrease the solubility of thorium(IV) at low ligand concentrations. Regarding the structure of MgGluc+, both IR spectra and density functional theory (DFT) calculations indicate the monodentate coordination of Gluc-. By the potentiometric data, the acidity of the water molecules is higher in the MgGluc+ and MgGlucOH0 species than in the Mg(H2O)62+ aqua ion. On the basis of DFT calculations, this ligand-promoted hydrolysis is caused by strong hydrogen bonds forming between Gluc- and Mg(H2O)62+. Conversely, metal-ion-induced ligand deprotonation takes place in the case of calcium(II) complexes, giving rise to salient variations on the NMR spectra in a strongly alkaline medium.

Spherical Closo Deltahedra with Surface Metal-Metal Multiple Bonding versus Oblate Deltahedra with Internal Metal-Metal Bonding in Dichromadicarbaborane Structures: The Nature of Stone's Icosahedral Dichromadicarbaborane.

The dichromadicarbaboranes Cp2Cr2C2B n-4H n-2 ( n = 8-12) related to the icosahedral structure reported in 1983 by Stone and co-workers and formulated by them as Cp2Cr2C2B8H10 have been investigated using density functional theory. In most cases, the lowest-energy structures are flattened oblatocloso structures with degree 6 and 7 chromium vertices similar to the lowest-energy and experimental structures of the isoelectronic dirhenaboranes Cp2Re2B n-2H n-2. However, most isomeric spherical closo deltahedral structures with surface Cr≣Cr quadruple bonds as well as isocloso structures with surface metal-metal Cr≡Cr triple bonds lie at accessible energies, typically lower than those in the corresponding dirhenaborane systems. However, for the 11-vertex Cp2Cr2C2B7H9 system, the most spherical closo/ isocloso deltahedral structure with a degree 6 metal vertex and degree 4 carbon vertices as well as a surface M≡M triple bond lies energetically below the lowest-energy oblatocloso structure. Calculations of the Cr-Cr distances in an icosahedral Cp2Cr2C2B8H10 structure and in a dihydrogenated icosahedral Cp2Cr2(µ-H)2C2B8H10 structure suggest the latter structure for "Cp2Cr2C2B8H10" reported by Stone and co-workers.

First-in-class allosteric inhibitors of bacterial IMPDHs.

Inosine-5'-monophosphate dehydrogenase (IMPDH) is an essential enzyme in many bacterial pathogens and is considered as a potential drug target for the development of new antibacterial agents. Our recent work has revealed the crucial role of one of the two structural domains (i.e. Bateman domain) in the regulation of the quaternary structure and enzymatic activity of bacterial IMPDHs. Thus, we have screened chemical libraries to search for compounds targeting the Bateman domain and identified first in-class allosteric inhibitors of a bacterial IMPDH. These inhibitors were shown to counteract the activation by the natural positive effector, MgATP, and to block the enzyme in its apo conformation (low affinity for IMP). Our structural studies demonstrate the versatility of the Bateman domain to accommodate totally unrelated chemical scaffolds and pave the way for the development of allosteric inhibitors, an avenue little explored until now.

Neutral Rhenadicarbaboranes with Re(CO)2(NO) Vertices: A Theoretical Study of Building Blocks for Rhenacarborane-Based Drug Delivery Agents.

The rhenadicarbaborane carbonyl nitrosyls (C2Bn-3Hn-1)Re(CO)2(NO), (n = 8 to 12), of interest in drug delivery agents based on the experimentally known C2B9H11Re(CO)2(NO) and related species, have been investigated by density functional theory. The lowest energy structures of these rhenadicarbaboranes are all found to have central ReC2Bn-3 most spherical closo deltahedra in accord with their 2n + 2 Wadean skeletal electrons. Carbon atoms are found to be located preferentially at degree 4 vertices in such structures. Furthermore, rhenium atoms are preferentially located at a highest degree vertex, typically a vertex of degree 5. Only for the 9-vertex C2B6H8Re(CO)2(NO) system are alternative isocloso deltahedral isomers found within ~8 kcal/mol of the lowest energy closo isomer. Such 9-vertex isocloso structures provide a degree 6 vertex for the rhenium atom flanked by degree 4 vertices for each carbon atom.

Reversible complexation of ammonia by breaking a manganese-manganese bond in a manganese carbonyl ethylenedithiolate complex: a theoretical study of an unusual type of Lewis acid.

The reaction of Mn(CO)5Br with sodium ethylenedithiolate was reported in 1968 to give a dark red binuclear H2C2S2Mn2(CO)6 complex possessing the unusual property of complexing reversibly with ammonia to give a yellow H2C2S2Mn2(CO)6·NH3 adduct. In order to provide some insight into the nature of this adduct, density functional studies were performed on the H2C2S2Mn2(CO)n (n = 4 to 8) systems as well as their relevant ammonia and trimethylphosphine adducts. These theoretical studies support the structure of H2C2S2Mn2(CO)6 originally suggested 50 years ago involving the binding of the ethylenedithiolate C[double bond, length as m-dash]C double bond as well as the sulfur atoms to the Mn2 unit with a bonding Mn-Mn distance of ∼2.8 Å. Complexation of H2C2S2Mn2(CO)6 with NH3 or Me3P preserves the complexed C[double bond, length as m-dash]C double bond of the ethylenedithiolate ligand but lengthens the MnMn distance to a non-bonding ∼3.6 Å. Thus H2C2S2Mn2(CO)6 represents a novel type of Lewis acid where reversible complexation with Lewis bases involves the rupture of a metal-metal bond. Carbonyl dissociation energies in the H2C2S2Mn2(CO)n series account for the formation of the hexacarbonyl H2C2S2Mn2(CO)6 as the stable product from the Mn(CO)5Br/ethylenedithiolate reaction.

The acidity and self-catalyzed lactonization of l-gulonic acid: Thermodynamic, kinetic and computational study.

Lactonization and proton dissociation of sugar acids take place simultaneously in acidic aqueous solutions. The protonation-deprotonation processes are always fast, whilst the formation and hydrolysis of γ- and δ-lactones are usually slower. Thus, both thermodynamic and kinetic information are required for the complete understanding of these reactions. The protonation constant (Kp) of l-gulonate (Gul-) was determined from potentiometric and polarimetric measurements, while the individual lactonization constants (KL,γ and KL,δ) for l-gulonic acid (HGul) were obtained via13C NMR experiments. The applicability of this method was proven by measuring these well-known constants for d-gluconic acid (HGluc) and by comparing them to literature data. l-gulonic acid γ-lactone (γ-HGul) has remarkable stability in contrast with δ-HGul as well as γ- and δ-HGluc. The polarimetric measurement implies that the main factor responsible for the enhanced stability of γ-HGul is that its hydrolysis is much slower than that of δ-HGul. This higher stability of the γ-HGul ring over its δ-isomer was also confirmed by quantum chemical calculations. A new confirmed feature of the reaction is that in parallel to H3O+, HGul also catalyzes the formation and reverse hydrolytic processes of γ-HGul, similarly to other general acid catalysts.

New Class of Hybrid Materials for Detection, Capture, and "On-Demand" Release of Carbon Monoxide.

Carbon monoxide (CO) is both a substance hazardous to health and a side product of a number of industrial processes, such as methanol steam reforming and large-scale oxidation reactions. The separation of CO from nitrogen (N2) in industrial processes is considered to be difficult because of the similarities of their electronic structures, sizes, and physicochemical properties (e.g., boiling points). Carbon monoxide is also a major poison in fuel cells because of its adsorption onto the active sites of the catalysts. It is therefore of the utmost economic importance to discover new materials that enable effective CO capture and release under mild conditions. However, methods to specifically absorb and easily release CO in the presence of contaminants, such as water, nitrogen, carbon dioxide, and oxygen, at ambient temperature are not available. Here, we report the simple and versatile fabrication of a new class of hybrid materials that allows capture and release of carbon monoxide under mild conditions. We found that carborane-containing metal complexes encapsulated in networks made of poly(dimethylsiloxane) react with CO, even when immersed in water, leading to dramatic color and infrared signature changes. Furthermore, we found that the CO can be easily released from the materials by simply dipping the networks into an organic solvent for less than 1 min, at ambient temperature and pressure, which not only offers a straightforward recycling method, but also a new method for the "on-demand" release of carbon monoxide. We illustrated the utilization of the on-demand release of CO from the networks by carrying out a carbonylation reaction on an electron-deficient metal complex that led to the formation of the CO-adduct, with concomitant recycling of the gel. We anticipate that our sponge-like materials and scalable methodology will open up new avenues for the storage, transport, and controlled release of CO, the silent killer and a major industrial poison.

Deviations from the Most Spherical Deltahedra in Rhenatricarbaboranes Having 2n + 2 Wadean Skeletal Electrons.

Density functional theory studies on the rhenatricarbaboranes C3Bn-4Hn-1Re(CO)3 (n = 7-12) show that the lowest energy polyhedra for n-vertex metallaboranes having 2n + 2 skeletal electrons and sufficiently dissimilar vertex atoms can deviate from the most spherical closo deltahedra predicted by application of the Wade-Mingos rules. Furthermore, the lowest energy structures of these rhenatricarbaboranes are found to avoid C-C edges and have carbon atoms located at degree 4 rather than degree 5 vertices. The lowest energy structures for the 7-vertex C3B3H6Re(CO)3 system all have a central C3B3Re closo deltahedron, namely the pentagonal bipyramid with the rhenium atom at a degree 5 axial vertex and all three carbon atoms at degree 4 equatorial vertices. However, the lowest energy structure for the 8-vertex C3B4H7Re(CO)3 is not the most spherical closo 8-vertex deltahedron, namely the bisdisphenoid, but instead a central C3B4Re hexagonal bipyramid with mutually nonadjacent degree 4 vertices for the carbon atoms. Similarly, the lowest energy 10-vertex C3B6H9Re(CO)3 structures are derived from isocloso deltahedra having three degree 4 vertices for all three carbon atoms rather than from the most spherical 10-vertex closo deltahedron, namely the bicapped square antiprism with only two degree 4 vertices. However, for the 9-vertex C3B5H8Re(CO)3 system, the most spherical closo deltahedron, namely the tricapped trigonal prism, has three mutually nonadjacent degree 4 vertices, which is ideal for the three carbon atoms and thus is the preferred deltahedron. The preferred deltahedron for the 11-vertex C3B7H10Re(CO)3 remains the most spherical closo deltahedron despite having only two degree 4 vertices for the carbon atoms. Furthermore, the six lowest energy 12-vertex C3B8H11Re(CO)3 structures are all based on the regular icosahedron generally favored in polyhedral borane chemistry despite its complete lack of degree 4 vertices for the carbon atoms.

Paramagnetism in Metallacarboranes: The Polyhedral Chromadicarbaborane Systems.

The chromadicarbaboranes CpCrC2Bn-3Hn-1 (8 ≤ n ≤ 12) are of interest in providing stable paramagnetic deltahedral metallaboranes among which the 12-vertex CpCrC2B9H11 has been synthesized by Hawthorne and co-workers. Density functional theory shows that the lowest-energy such structures are quartet spin-state Cr(III) structures in which the central CrC2Bn-3 units exhibit most spherical closo deltahedral geometries similar to those found in the borane dianions BnHn2-. Higher-energy doublet CpCrC2Bn-3Hn-1 (8 ≤ n ≤ 11) structures are found exhibiting central CrC2Bn-3 isocloso deltahedral geometries, thereby providing a degree 6 vertex for the chromium atom. The lowest-energy CpCrC2Bn-3Hn-1 (8 ≤ n ≤ 11) structures all have both carbon atoms at degree 4 vertices. However, the lowest-energy CpCrC2B9H11 structures all have central CrC2B9 icosahedra and thus lack degree 4 vertices for the carbon atoms. For all of the CpCrC2Bn-3Hn-1 (8 ≤ n ≤ 12) systems the lowest-energy isomers are those with the maximum number of Cr-C edges in contrast to the related CpCoC2Bn-3Hn-1 systems.