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.

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.

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.

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.

Affinity and Effect of Anticancer Drugs on the Redox Reactivity of Hemoglobin.

Hemoglobin's redox reactivity is affected by anticancer drugs of the antitubulin class. Direct binding of these drugs to hemoglobin, with biomedically relevant affinities, is demonstrated. While this interaction is mostly allosteric, in the case of docetaxel, a direct redox reaction is also observed-correlating well with structural differences between the four compounds. A role for Tyr145 in this reactivity is proposed, in line with previous observations of the importance of this amino acid in the reactivity of Hb toward agents of oxidative stress. A susceptibility of vinorelbin (and to a lower extent of paclitaxel) toward peroxide and peroxidase is shown.

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.

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.

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.

The Reaction of Oxy Hemoglobin with Nitrite: Mechanism, Antioxidant-Modulated Effect, and Implications for Blood Substitute Evaluation.

The autocatalytic reaction between nitrite and the oxy form of globins involves free radicals. For myoglobin (Mb), an initial binding of nitrite to the iron-coordinated oxygen molecule was proposed; the resulting ferrous-peroxynitrate species was not detected, but its decay product, the high-valent ferryl form, was demonstrated in stopped-flow experiments. Reported here are the stopped flow spectra recorded upon mixing oxy Hb (native, as well as chemically-derivatized in the form of several candidates of blood substitutes) with a supraphysiological concentration of nitrite. The data may be fitted to a simple kinetic model involving a transient met-aqua form, in contrast to the ferryl detected in the case of Mb in a similar reaction sequence. These data are in line with a previous observation of a transient accumulation of ferryl Hb under auto-catalytic conditions at much lower concentrations of nitrite (Grubina, R. et al. J. Biol. Chem. 2007, 282, 12916). The simple model for fitting the stopped-flow data leaves a small part of the absorbance changes unaccounted for, unless a fourth species is invoked displaying features similar to the oxy and tentatively assigned as ferrous-peroxynitrate. Density functional theory (DFT) calculations support this latter assignment. The reaction allows for differentiating between the reactivities of various chemically modified hemoglobins, including candidates for blood substitutes. Polymerization of hemoglobin slows the nitrite-induced oxidation, in sharp contrast to oxidative-stress type reactions which are generally accelerated, not inhibited. Sheep hemoglobin is found to be distinctly more resistant to reaction with nitrite compared to bovine Hb, at large nitrite concentrations (stopped-flow experiments directly observing the oxy + nitrite reaction) as well as under auto-catalytic conditions. Copolymerization of Hb with bovine serum albumin (BSA) using glutaraldehyde leads to a distinct increase of the lag time compared to native Hb as well as to any other form of derivatization examined in the present study. The Hb-BSA copolymer also displays a slower initial reaction with nitrite under stopped-flow conditions, compared to native Hb.

The exocyclic amino group of adenine in PtII and PdII complexes: a critical comparison of the X-ray crystallographic structural data and gas phase calculations.

A detailed computational (DFT level of theory) study regarding the nature of the exocyclic amino group, N6H2, of the model nucleobase 9-methyladenine (9MeA) and its protonated (9MeAH+) and deprotonated forms (9MeA-H), free and metal-complexed, has been conducted. The metals are PtII and PdII, bonded to nitrogen-containing co-ligands (NH3, dien, bpy), with N1, N6, and N7 being the metal-binding sites, individually or in different combinations. The results obtained from gas phase calculations are critically compared with X-ray crystallography data, whenever possible. In the majority of cases, there is good qualitative agreement between calculated and experimentally determined C6-N6 bond lengths, but calculated values always show a trend to larger values, by 0.02-0.08 Å. Both methods indicate, with few exceptions, a high degree of double-bond character of C6-N6, consistent with an essentially sp2-hybridized N6 atom. The shortest values for C6-N6 distances in X-ray crystal structures are around 1.30 Å. Exceptions refer to cases in which DFT calculations suggest the existence of a hydrogen bond with N6H2 acting as a H bond acceptor, hence a situation with N6 having undergone a substantial hybridization shift toward sp3. Nevertheless, even in these cases the C6-N6 bond (1.392 Å) is still halfway between a typical C-N single bond (1.48 Å) and a typical C=N double bond (1.28 Å). This scenario is, however, not borne out by X-ray crystallographic results, and is attributed to the absence of counter anions and solvent molecules in the calculated structures.

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.

Biicosahedral metallaboranes: aromaticity in metal derivatives of three-dimensional analogues of naphthalene.

Consideration of the well-known very stable icosahedral B12H12(2-) as a three-dimensional analogue of benzene was extended by the recent synthesis of the biicosahedral B21H18(-) as a three-dimensional analogue of naphthalene. The preferred structures of metallaboranes derived from B21H18(-) have now been examined by density functional theory. The isoelectronic species CpNiB20H17 and CpCoCB19H17 have the 46 skeletal electrons expected by the Wade-Mingos and Jemmis rules for a structure consisting of two face-sharing fused icosahedra. The CpM units in these structures energetically prefer to be located at a meta vertex of the biicosahedron. The analogous ferraboranes CpFeB20H17 with only 44 skeletal electrons also have related biicosahedral structures. The presence of an agostic hydrogen atom bridging an Fe-B edge compensates for the two-electron deficiency in CpFeB20H17 relative to CpNiB20H17. The nucleus-independent chemical shift (NICS) values of these systems indicate them to be strongly aromatic.

Nonspherical Deltahedra in Low-Energy Dicarbalane Structures Testing the Wade-Mingos Rules: The Regular Icosahedron Is Not Favored for the 12-Vertex Dicarbalane.

Theoretical studies on the dicarbalanes C2Al(n-2)Men (n = 7-14; Me = methyl) predict both carbon atoms to be located at degree 4 vertices of a central C2Al(n-2) deltahedron in the lowest energy structures. As a consequence, deltahedra having two degree 4 vertices, two degree 6 vertices, and eight degree 5 vertices rather than the regular icosahedron having exclusively degree 5 vertices are found for the 12-vertex dicarbalane C2Al10Me12. However, the lowest energy C2Al(n-2)Men (n = 7-11) structures are based on the same most spherical (closo) deltahedra as the corresponding deltahedral boranes. The lowest energy structures for the 13- and 14-vertex systems C2Al(n-2)Men (n = 13 and 14) are also deltahedra having exactly two degree 4 vertices for the carbon atoms. The six-vertex C2Al4Me6 system is exceptional since bicapped tetrahedral and capped square pyramidal structures with degree 3 vertices for the carbon atoms are energetically preferred over the octahedral structure suggested by the Wade-Mingos rules.

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.

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.

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.