Homocoenzyme B12 and Bishomocoenzyme B12: Covalent Structural Mimics for Homolyzed,Enzyme‐Bound Coenzyme B12

Efficient electrochemical syntheses of “homocoenzyme B₁₂” ( 2 , Co_β‐(5′‐deoxy‐5′‐adenosyl‐methyl)‐cob(III )alamin) and “bishomocoenzyme B₁₂” ( 3 , Co_β‐[2‐(5′‐deoxy‐5′‐adenosyl)‐ethyl]‐cob(III )alamin) are reported here. These syntheses have provided crystalline samples of 2 and 3 in 94 and 77 % yield, respectively. In addition, in‐depth investigations of the structures of 2 and 3 in solution were carried out and a high‐resolution crystal structure of 2 was obtained. The two homologues of coenzyme B₁₂ ( 2 and 3 ) are suggested to function as covalent structural mimics of the hypothetical enzyme‐bound “activated” (that is, “stretched” or even homolytically cleaved) states of the B₁₂ cofactor. From crude molecular models, the crucial distances from the corrin‐bound cobalt center to the C5′ atom of the (homo)adenosine moieties in 2 and 3 were estimated to be about 3.0 and 4.4 Å, respectively. These values are roughly the same as those found in the two “activated” forms of coenzyme B₁₂ in the crystal structure of glutamate mutase. Indeed, in the crystal structure of 2 , the cobalt center was observed to be at a distance of 2.99 Å from the C5′ atom of the homoadenosine moiety and the latter was found to be present in the unusual syn conformation. In solution, the organometallic moieties of 2 and 3 were shown to be rather flexible and to be considerably more dynamic than the equivalent group in coenzyme B₁₂. The homoadenosine moiety of 2 was indicated to occur in both the syn and the anti conformations.     

Electrochemical Synthesis and Structure Analysis of Neocoenzyme B12 – An Epimer of Coenzyme B12 with a Remarkably Flexible Organometallic Group

In neocoenzyme B₁₂ (=(5′-deoxy-5′-adenosyl)-13-epicob(III)alamin; 5 ), an epimer of coenzyme B₁₂ ( 1 ), the organometallic group and a propanamide side chain of the vitamin-B₁₂ ligand compete for the same region in space. Interesting consequences for structure and organometallic reactivity of this isomer of 1 are to be expected. Neocoenzyme B₁₂ ( 5 ; 89% yield) and methyl-13-epicobalamin ( 6 ; 88% yield) were prepared from neovitamin B₁₂ ( 4 ) by electrochemical means (Fig. 3). The solution structure of the organometallic neovitamin-B₁₂ derivative 5 was analyzed by homonuclear and heteronuclear NMR spectroscopy. Comparison of the structures of 1 and 5 informed on the structural consequences of the epimerization at C(13) and revealed a remarkable flexibility of the organometallic group in 5 . In 5 , both sterically interacting functionalities (organometallic group and propanamide side chain at C(13)) adapt their conformations dynamically to avoid significant mutual clashes. As one consequence of this structural adaptation, the major conformations of 5 feature counterclockwise and clockwise reorientations of the organometallic ligand with respect to its crystallographically determined position in coenzyme B₁₂ ( 1 ). One of the dominant conformers of 5 exhibits an orientation of the organometallic functionality similar to that found in the crystal structure of the coenzyme-B₁₂-dependent methylmalonyl CoA mutase. The present NMR study also revealed the significant population of syn-conformers of the organometallic adenosine group, another remarkable feature of the solution structure of 5 .     

Strukturelle Untersuchungen an Cs2[B12H12]

Structural Investigations on Cs₂[B₁₂H₁₂] The crystal structure of Cs₂[B₁₂H₁₂] has been determined from X‐ray single‐crystal data collected at room temperature. Dicesium dodecahydro‐closo‐dodecaborate crystallizes as colourless, face‐rich crystals (cubic, Fm 3; a = 1128.12(7) pm; Z = 4). Its synthesis is based on the reaction of Na[BH₄] with BF₃(O(C₂H₅)₂) via the decomposition of Na[B₃H₈] in boiling diglyme, followed by subsequent separations, precipitations (with aqueous CsOH solution) and recrystallizations. The crystal structure is best described as anti‐CaF₂‐type arrangement with the Cs⁺ cations in all tetrahedral interstices of the cubic closest‐packed host lattice of the icosahedral [B₁₂H₁₂]^2–‐cluster dianions. The intramolecular bond lengths are in the range usually found in closo‐hydroborates: 178 pm for the B–B and 112 pm for the B–H distance. Twelve hydrogen atoms belonging to four [B₁₂H₁₂]^2– icosahedra provide an almost perfect cuboctahedral coordination sphere to the Cs⁺ cations, and their distance of 313 pm (12 ×) attests for the salt‐like character of Cs₂[B₁₂H₁₂] according to {(Cs⁺)₂([B₁₂H₁₂]^2–)}. The ¹¹B{¹H}‐NMR data in aqueous (D₂O) solution are δ = –12,70 ppm (¹J_B–H = 125 Hz), and δ = –15,7 ppm (linewidth: δν_1/2 = 295 Hz) for the solid state ¹¹B‐MAS‐NMR.     

Synthese,Kristallstruktur und thermischer Abbau von Mg(H2O)6[B12H12] · 6 H2O

Synthesis, Crystal Structure, and Thermal Decomposition of Mg(H₂O)₆[B₁₂H₁₂] · 6 H₂O By reaction of an aqueous solution of the free acid (H₃O)₂[B₁₂H₁₂] with MgCO₃ and subsequent isothermic evaporation of the resulting solution to dryness, colourless, bead‐shaped single crystals of the dodecahydrate of magnesium dodecahydro closo‐dodecaborate Mg(H₂O)₆[B₁₂H₁₂] · 6 H₂O (cubic, F4₁32; a = 1643.21(9) pm, Z = 8) emerge. The crystal structure is best described as a NaTl‐type arrangement in which the centers of gravity of the quasi‐icosahedral [B₁₂H₁₂]^2— anions (d(B—B) = 178—180 pm, d(B—H) = 109 pm) occupy the positions of Tl^— while the Mg²⁺ cations occupy the Na⁺ positions. A direct coordinative influence of the [B₁₂H₁₂]^2— units at the Mg²⁺ cations is however not noticeable. The latter are octahedrally coordinated by six water molecules forming isolated hexaaqua complex cations [Mg(H₂O)₆]²⁺ (d(Mg—O) = 206 pm, 6×). In addition, six “zeolitic” water molecules are located in the crystal structure for the formation of a strong O—H^δ+···^δ—O‐hydrogen bridge‐bonding system. The evidence of weak B—H^δ—···^δ+H—O‐hydrogen bonds between water molecules and anionic [B₁₂H₁₂]^2— clusters is also considered. Investigations on the dodecahydrate Mg[B₁₂H₁₂] · 12 H₂O (≡ Mg(H₂O)₆[B₁₂H₁₂] · 6 H₂O) by DTA/TG measurements showed that its dehydration takes place in two steps within a temperature range of 71 and 76 °C as well as at 202 °C, respectively. Thermal treatment eventually leads to the anhydrous magnesium dodecahydro closo‐dodecaborate Mg[B₁₂H₁₂].     

Synthesis,Characterization, and Crystal Structures of [N(CH3)4]2[B12H12] and [N(CH3)4]2[B12H12] · CH3CN

Bis(tetramethylammonium) dodecahydrododecaborate, [(CH₃)₄N]₂[B₁₂H₁₂], and bis(tetramethylammonium) dodecahydrododecaborate acetonitrile, [(CH₃)₄N]₂[B₁₂H₁₂] · CH₃CN, were synthesized and characterized via Infrared, ¹H and ¹¹B NMR spectroscopy. [(CH₃)₄N]₂[B₁₂H₁₂] crystallizes isopunctual to the alkali metal dodecaborates. The crystal structure of [(CH₃)₄N]₂[B₁₂H₁₂] · CH₃CN was determined from single crystal data and refined in the orthorhombic crystal system (Pcmn, no. 62, a = 898.68(8), b = 1312.85(9) c = 1994.5(1) pm, R(|F| , 4σ) = 5.9%, wR(F²) = 18.3%). Here, the geometry of the dodecaborate anion is that of an almost ideal icosahedron, less distorted than most other dodecaborates known. By low‐temperature Guinier‐Simon diffractometry phase transitions were detected for [(CH₃)₄N]₂[B₁₂H₁₂] and [(CH₃)₄N]₂[B₁₂H₁₂] · CH₃CN at –70 and –15 °C, respectively.     

On the Oxidation of the Three‐Dimensional Aromatics [B12X12]2− (X=F,Cl, Br,I)

The perhalogenated closo‐dodecaborate dianions [B₁₂X₁₂]^2? (X=H, F, Cl, Br, I) are three‐dimensional counterparts to the two‐dimensional aromatics C₆X₆ (X=H, F, Cl, Br, I). Whereas oxidation of the parent compounds [B₁₂H₁₂]^2? and benzene does not lead to isolable radicals, the perhalogenated analogues can be oxidized by chemical or electrochemical methods to give stable radicals. The chemical oxidation of the closo‐dodecaborate dianions [B₁₂X₁₂]^2? with the strong oxidizer AsF₅ in liquid sulfur dioxide (lSO₂) yielded the corresponding radical anions [B₁₂X₁₂] ^{? ?} (X=F, Cl, Br). The presence of radical ions was proven by EPR and UV/Vis spectroscopy and supported by quantum chemical calculations. Use of an excess amount of the oxidizing agent allowed the synthesis of the neutral perhalogenated hypercloso‐boranes B₁₂X₁₂ (X=Cl, Br). These compounds were characterized by single‐crystal X‐ray diffraction of dark blue B₁₂Cl₁₂ and [Na(SO₂)₆][B₁₂Br₁₂] ? B₁₂Br₁₂. Sublimation of the crude reaction products that contained B₁₂X₁₂ (X=Cl, Br) resulted in pure dark blue B₁₂Cl₁₂ or decomposition to red B₉Br₉, respectively. The energetics of the oxidation processes in the gas phase were calculated by DFT methods at the PBE0/def2‐TZVPP level of theory. They revealed the trend of increasing ionization potentials of the [B₁₂X₁₂]^2? dianions by going from fluorine to bromine as halogen substituent. The oxidation of all [B₁₂X₁₂]^2? dianions was also studied in the gas phase by mass spectrometry in an ion trap. The electrochemical oxidation of the closo‐dodecaborate dianions [B₁₂X₁₂]^2? (X=F, Cl, Br, I) by cyclic and Osteryoung square‐wave voltammetry in liquid sulfur dioxide or acetonitrile showed very good agreement with quantum chemical calculations in the gas phase. For [B₁₂X₁₂]^2? (X=F, Cl, Br) the first and second oxidation processes are detected. Whereas the first process is quasi‐reversible (with oxidation potentials in the range between +1.68 and +2.29 V (lSO₂, versus ferrocene/ferrocenium (Fc^0/+))), the second process is irreversible (with oxidation potentials ranging from +2.63 to +2.71 V (lSO₂, versus Fc^0/+)). [B₁₂I₁₂]^2? showed a complex oxidation behavior in cyclic voltammetry experiments, presumably owing to decomposition of the cluster anion under release of iodide, which also explains the failure to isolate the respective radical by chemical oxidation.     

Synthese und Kristallstruktur von Cadmium‐Dodekahydro‐closo‐Dodekaborat‐Hexahydrat,Cd(H2O)6[B12H12]

Synthesis and Crystal Structure of Cadmium Dodecahydro closo‐Dodecaborate Hexahydrate, Cd(H₂O)₆[B₁₂H₁₂] Through neutralization of the aqueous free acid (H₃O)₂[B₁₂H₁₂] with cadmium carbonate (CdCO₃) and after isothermic evaporation of the resulting solution, colourless lath‐shaped single crystals of Cd(H₂O)₆[B₁₂H₁₂] are obtained. Cadmium dodecahydro closo‐dodecaborate hexahydrate crystallizes at room temperature in the monoclinic system (space group: C2/m) with the lattice constants a = 1413.42(9), b = 1439.57(9), c = 749.21(5) pm and β = 97.232(4)° (Z = 4). The crystal structure of Cd(H₂O)₆[B₁₂H₁₂] can be regarded as a monoclinic distortion variant of the CsCl‐type structure. Two crystallographically different [Cd(H₂O)₆]²⁺ octahedra (d(Cd–O) = 227–230 pm) are present which only differ in their relative orientation. The intramolecular bond lengths for the quasi‐icosahedral [B₁₂H₁₂]^2? cluster anions range in the intervals usually found for dodecahydro closo‐dodecaborates (d(B–B) = 177–179 pm, d(B–H) = 103–116 pm). The hydrogen atoms of the [B₁₂H₁₂]^2? clusters have no direct coordinative influence on the Cd²⁺ cations. Due to the fact that no “zeolitic” crystal water molecules are present, a stabilization of the lattice takes place mainly via the B–H^δ?···^+δH–O hydrogen bonds.     

1‐Oxa‐nido‐dodecaborate [OB11H12]− from the Controlled Oxidation of the closo‐Borates [B11H11]2− and [B12H12]2−

The closo‐dodecaborate [B₁₂H₁₂]^2? is degraded at room temperature by oxygen in an acidic aqueous solution in the course of several weeks to give B(OH)₃. The degradation is induced by Ag²⁺ ions, generated from Ag⁺ by the action of H₂S₂O₈. Oxa‐nido‐dodecaborate(1?) is an intermediate anion, that can be separated from the reaction mixture as [NBzlEt₃][OB₁₁H₁₂] after five days in a yield of 18 %. The action of FeCl₃ on the closo‐undecaborate [B₁₁H₁₁]^2? in an aqueous solution gives either [B₂₂H₂₂]^2? (by fusion) or nido‐B₁₁H₁₃(OH)^? (by protonation and hydration), depending on the concentration of FeCl₃. In acetonitrile, however, [B₁₁H₁₁]^2? is transformed into [OB₁₁H₁₂]^? by Fe³⁺ and oxygen. The radical anions [B₁₂H₁₂] ^{˙ ?} and [B₁₁H₁₁] ^{˙ ?} are assumed to be the primary products of the oxidation with the one‐electron oxidants Ag²⁺ and Fe³⁺, respectively. These radical anions are subsequently transformed into [OB₁₁H₁₂]^? by oxygen. The crystal structure analysis shows that the structure of [OB₁₁H₁₂]^? is derived from the hypothetical closo‐oxaborane OB₁₂H₁₂ by removal of the B3 vertex, leaving a non‐planar pentagonal aperture with a three‐coordinate O vertex, as predicted by NMR spectra and theory.     

The electronic structure ofα-B12, B12P2 and B12AS2

The electronic band structures of the rhombohedral-based boron compounds -B₁₂, B₁₂P₂ and B₁₂ As₂ have been investigated along all symmetry directions. The calculations show that the band gap, in all cases, is of the order of 2 eV, which correlates with the known color of -rhombohedral boron. The materials should be intrinsic semi-conductors, as has recently been shown experimentally. The states around the band gap in -B₁₂ are dominated by the boron 2p atomic states. The bonding in the icosahedra, as illuminated by cluster calculations, is shown to be rather similar to that in the isolated B₁₂ icosahedron. Of the intericosahedron interactions, those between B(2) and B(2) atoms are the strongest and have a bond index just above unity. In B₁₂P₂ the orbitals of the P₂ moiety make a significant contribution to the valence band edge states and the conduction band edge states also incorporate considerable (55%) phosphorus 3d orbital character. In B₁₂As₂ the arsenic 4d orbitals do not have as much effect in that crystal as do the 3d orbitals in B₁₂P₂.     

Die Kristallstrukturen der Dicaesium-Dodekahalogeno-closo-Dodekaborate Cs2[B12X12] (X = Cl,Br, I) und ihrer Hydrate

The Crystal Structures of the Dicesium Dodecahalogeno-closo-Dodecaborates Cs₂[B₁₂X₁₂] (X = Cl, Br, I) and their Hydrates The perhalogenated derivatives Cs₂[B₁₂X₁₂] (X = Cl - I) have been synthesized by reaction of Cs₂[B₁₂H₁₂] with the respective elemental halogens (Cl₂, Br₂ and I₂). Upon recrystallization from aqueous solution colourless, face-rich single crystals of the dihydrates (Cs₂[B₁₂X₁₂] · 2 H₂O) are obtained first which can be dehydrated topotactically via the monohydrates (Cs₂[B₁₂X₁₂] · H₂O) leaving to the solvent-free compounds (Cs₂[B₁₂X₁₂]) behind without loss of their crystallinity. The ionic cesium salts were characterized by single crystal X-ray diffraction. All three halogenoborates are isostructural and they crystallize at room temperature in the trigonal space group (Cs₂[B₁₂Cl₁₂]: a = 959.67(3) pm, c = 4564.2(2) pm; Cs₂[B₁₂Br₁₂]: a = 997.92(3) pm, c = 4766.4(3) pm; Cs₂[B₁₂I₁₂]: a = 1047.05(4) pm, c = 5018.3(3) pm; Z = 6). The crystal structures consist of a cubic closest packed host lattice formed by two crystallographically inequivalent quasi-icosahedral [B₁₂X₁₂]^2- anions (Cs₂[B₁₂Cl₁₂]: d(B-B) = 178 - 179 pm, d(B-Cl) = 179 - 180 pm; Cs₂[B₁₂Br₁₂]: d(B-B) = 176 - 180 pm, d(B-Br) = 195 - 197 pm; Cs₂[B₁₂I₁₂]: d(B-B) = 177 - 182 pm, d(B-I) = 214 - 217 pm). By ordered occupation of half of the tetrahedral and formally all octahedral interstices in every intermediate layer with Cs⁺ cations, a structure emerges where (Cs1)⁺ is trigonally non-planar coordinated by three (CN = 9) and (Cs2)⁺ tetrahedrally coordinated by four (CN = 12) [B₁₂X₁₂]^2- anions. Thereby triangular faces of halogen atoms of the icosahedral clusters are coordinatively effective in both cases. In their mono- and dihydrates the incomplete coordination sphere of (Cs1)⁺ is completed by one and two water molecules, respectively. The thermal decomposition of the dicesium dodecahalogeno-closo-dodecaborate hydrates and their dehydration products was investigated using DTA/TG methods in a temperature range between room temperature and 1200 °C. Additionally the compounds were also characterized by ¹¹B-NMR spectroscopy in aqueous solution.     

Infrarotspektroskopische Untersuchung des Borcarbids und seiner isotypen Derivate B12O2, B12P2 und B12As2

Infrared Spectra of Boron Carbide and its Isotypic Derivatives B₁₂O₂, B₁₂P₂, and B₁₂As₂ From very finely powdered samples of B₄C [ = B₁₁C(CBC)], B₁₂O₂, B₁₂P₂, and B₁₂As₂ infrared spectra with very well shaped bands could be obtained. The spectrum of B₁₂O₂ with the smallest number of bands was assigned with the aid of a normal coordinate analysis. This gave a good basis for the interpretation of the spectra of normal and ¹⁰B-enriched B₄C and of B₁₂P₂. Both are exhibiting some bands in addition to those expected for the ideal structures, which are explained by the assumption of disordering within the C? B? C resp. P? P groups. At boron carbide this disorder seems to depend on the ratio B:C within its range of homogeneity.     

Untersuchungen zur Kristallstruktur von Lithium‐dodecahydro‐closo‐dodecaborat aus wäßriger Lösung: Li2(H2O)7[B12H12]

Investigations on the Crystal Structure of Lithium Dodecahydro‐closo‐dodecaborate from Aqueous Solution: Li₂(H₂O)₇[B₁₂H₁₂] By neutralization of an aqueous solution of the acid (H₃O)₂[B₁₂H₁₂] with lithium hydroxide (LiOH) and subsequent isothermic evaporation of the resulting solution to dryness, it was possible to obtain the heptahydrate of lithium dodecahydro‐closo‐dodecaborate Li₂[B₁₂H₁₂] · 7 H₂O (≡ Li₂(H₂O)₇[B₁₂H₁₂]). Its structure has been determined from X‐ray single crystal data at room temperature. The compound crystallizes as colourless, lath‐shaped, deliquescent crystals in the orthorhombic space group Cmcm with the lattice constants a = 1215.18(7), b = 934.31(5), c = 1444.03(9) pm and four formula units in the unit cell. The crystal structure of Li₂(H₂O)₇[B₁₂H₁₂] can not be described as a simple AB₂‐structure type. Instead it forms a layer‐like structure analogous to the well‐known barium compound Ba(H₂O)₆[B₁₂H₁₂]. Characteristic feature is the formation of isolated cation pairs [Li₂(H₂O)₇]²⁺ in which the water molecules form two [Li(H₂O)₄]⁺ tetrahedra with eclipsed conformation, linked to a dimer via a common corner. The bridging oxygen atom (∢(Li‐ O ‐Li) = 112°) thereby formally substitutes Ba²⁺ in Ba(H₂O)₆[B₁₂H₁₂] according to (H₂ O )Li₂(H₂O)₆[B₁₂H₁₂]. A direct coordinative influence of the [B₁₂H₁₂]^2— cluster anions to the Li⁺ cations is not noticeable, however. The positions of the hydrogen atoms of both the water molecules and the [B₁₂H₁₂]^2— units have all been localized. In addition, the formation of B‐H^δ—···^δ+H‐O‐hydrogen bonds between the water molecules and the hydrogen atoms from the anionic [B₁₂H₁₂]^2— clusters is considered and their range and strength is discussed. The dehydratation of the heptahydrate has been investigated by DTA‐TG measurements and shown to take place in two steps at 56 and 151 °C, respectively. Thermal treatment leads to the anhydrous lithium dodecahydro‐closo‐dodecaborate Li₂[B₁₂H₁₂], eventually.     

A Density Functional Investigation of Fluorinated B12N12 Clusters

Density functional calculations have been carried out on a series of fluorinated B₁₂N₁₂ molecules. The fluorine atoms are more prone to absorb on the boron atoms than the nitrogen atoms in B₁₂N₁₂. The 1,3 addition is an energetically favorable adsorption site in one‐fluorine‐molecule adsorption. We found that the average bond energy of fluorine molecule is decreased with n increasing, but significantly larger than that of B₁₂N₁₂F. The energy gap of B₁₂N₁₂ is controllable by introducing fluorine molecules. Moreover, calculation of the Gibbs free energy of the B₁₂N₁₂+12F₂→B₁₂N₁₂F₂₄ reaction showed that this reaction is exothermic at low temperatures.     

The Crystal Structure of Solvent‐Free Silver Dodecachloro‐closo‐dodecaborate Ag2[B12Cl12] from Aqueous Solution

Colourless octahedral single crystals of solvent‐free Ag₂[B₁₂Cl₁₂] (cubic, Pa3¯; a = 1238.32(7) pm, Z = 4) are obtained by the metathesis reaction of Cs₂[B₁₂Cl₁₂] with an aqueous solution of silver nitrate (AgNO₃) and recrystallization of the crude product from water. The crystal structure is best described as a distorted anti‐CaF₂‐type arrangement in which the quasi‐icosahedral [B₁₂Cl₁₂]^2— anions (d(B—B) = d(B—Cl) = 177—180 pm) are arranged in a cubic closest‐packed fashion. The tetrahedral interstices are filled with Ag⁺ cations which are strongly displaced from their ideal positions. Thereby each silver atom gets coordinated by six chlorine atoms from the edges of three [B₁₂Cl₁₂]^2— anions providing a distorted octahedral coordination sphere to the Ag⁺ cations (d(Ag—Cl) = 283—285 pm, CN = 6).     

Glycerol as a Substrate and Inactivator of Coenzyme B12-Dependent Diol Dehydratase

Diol dehydratase, dependent on coenzyme B₁₂ (B₁₂-dDDH), displays a peculiar feature of being inactivated by its native substrate glycerol (GOL). Surprisingly, the isofunctional enzyme, B₁₂-independent glycerol dehydratase (B₁₂-iGDH), does not undergo suicide inactivation by GOL. Herein we present a series of QM/MM and MD calculations aimed at understanding the mechanisms of substrate-induced suicide inactivation in B₁₂-dDDH and that of resistance of B₁₂-iGDH to inactivation. We show that the first step in the enzymatic transformation of GOL, hydrogen abstraction, can occur from both ends of the substrate (either C1 or C3 of GOL). Whereas C1 abstraction in both enzymes leads to product formation, C3 abstraction in B₁₂-dDDH results in the formation of a low energy radical intermediate, which is effectively trapped within a deep well on the potential energy surface. The long lifetime of this radical intermediate likely enables its side reactions, leading to inactivation. In B₁₂-iGDH, by comparison, C3 abstraction is an endothermic step; consequently, the resultant radical intermediate is not of low energy, and the reverse process of reforming the reactant is possible.     

Das Lanthan‐Dodekahydro‐closo‐Dodekaborat‐Hydrat [La(H2O)9]2[B12H12]3·15 H2O und sein Oxonium‐Chlorid‐Derivat [La(H2O)9](H3O)Cl2[B12H12]·H2O

The Lanthanum Dodecahydro‐closo‐Dodecaborate Hydrate [La(H₂O)₉]₂[B₁₂H₁₂]₃·15 H₂O and its Oxonium‐Chloride Derivative [La(H₂O)₉](H₃O)Cl₂[B₁₂H₁₂]·H₂O By neutralization of an aqueous solution of the free acid (H₃O)₂[B₁₂H₁₂] with basic La₂O₃ and after isothermic evaporation colourless, face‐rich single crystals of a water‐rich lanthanum(III) dodecahydro‐closo‐dodecaborate hydrate [La(H₂O)₉]₂[B₁₂H₁₂]₃·15 H₂O are isolated. The compound crystallizes in the trigonal system with the centrosymmetric space group (a = 1189.95(2), c = 7313.27(9) pm, c/a = 6.146; Z = 6; measuring temperature: 100 K). The crystal structure of [La(H₂O)₉]₂[B₁₂H₁₂]₃·15 H₂O can be characterized by two of each other independent, one into another posed motives of lattice components. The [B₁₂H₁₂]²⁻ anions (d(B–B) = 177–179 pm; d(B–H) = 105–116 pm) are arranged according to the samarium structure, while the La³⁺ cations are arranged according to the copper structure. The lanthanum cations are coordinated in first sphere by nine oxygen atoms from water molecules in form of a threecapped trigonal prism (d(La–O) = 251–262 pm). A coordinative influence of the [B₁₂H₁₂]²⁻ anions on La³⁺ has not been determined. Since “zeolitic” water of hydratation is also present, obviously the classical H–O^δ–···^+δH–O‐hydrogen bonds play a significant role in the stabilization of the crystal structure. During the conversion of an aqueous solution of (H₃O)₂[B₁₂H₁₂] with lanthanum trichloride an anion‐mixed salt with the composition [La(H₂O)₉](H₃O)Cl₂[B₁₂H₁₂]·H₂O is obtained. The compound crystallizes in the hexagonal system with the non‐centrosymmetric space group (a = 808.84(3), c = 2064.51(8) pm, c/a = 2.552; Z = 2; measuring temperature: 293 K). The crystal structure can be characterized as a layer‐like structure, in which [B₁₂H₁₂]²⁻ anions and H₃O⁺ cations alternate with layers of [La(H₂O)₉]³⁺ cations (d(La–O) = 252–260 pm) and Cl⁻ anions along [001]. The [B₁₂H₁₂]²⁻ (d(B–B) = 176–179 pm; d(B–H) = 104–113 pm) and Cl⁻ anions exhibit no coordinative influence on La³⁺. Hydrogen bonds are formed between the H₃O⁺ cations and [B₁₂H₁₂]²⁻ anions, also between the water molecules of [La(H₂O)₉]³⁺ and Cl⁻ anions, which contribute to the stabilization of the crystal structure.     

Zu den Kristallstrukturen der Übergangsmetall(II)‐Dodekahydro‐closo‐Dodekaborat‐Hydrate Cu(H2O)5,5[B12H12]·2,5 H2O und Zn(H2O)6[B12H12]·6 H2O

On the Crystal Structures of the Transition‐Metal(II) Dodecahydro‐closo‐Dodecaborate Hydrates Cu(H₂O)_5.5[B₁₂H₁₂]·2.5 H₂O and Zn(H₂O)₆[B₁₂H₁₂]·6 H₂O By neutralization of an aqueous solution of the free acid (H₃O)₂[B₁₂H₁₂] with basic copper(II) carbonate or zinc carbonate, blue lath‐shaped single crystals of the octahydrate Cu[B₁₂H₁₂]·8 H₂O (≡ Cu(H₂O)_5.5[B₁₂H₁₂]·2.5 H₂O) and colourless face‐rich single crystals of the dodecahydrate Zn[B₁₂H₁₂]·12 H₂O (≡ Zn(H₂O)₆[B₁₂H₁₂]·6 H₂O) could be isolated after isothermic evaporation. Copper(II) dodecahydro‐closo‐dodecaborate octahydrate crystallizes at room temperature in the monoclinic system with the non‐centrosymmetric space group Pm (Cu(H₂O)_5.5[B₁₂H₁₂]·2.5 H₂O: a = 768.23(5), b = 1434.48(9), c = 777.31(5) pm, β = 90.894(6)°; Z = 2), whereas zinc dodecahydro‐closo‐dodecaborate dodecahydrate crystallizes cubic in the likewise non‐centrosymmetric space group F23 (Zn(H₂O)₆[B₁₂H₁₂]·6 H₂O: a = 1637.43(9) pm; Z = 8). The crystal structure of Cu(H₂O)_5.5[B₁₂H₁₂]·2.5 H₂O can be described as a monoclinic distortion variant of the CsCl‐type arrangement. As characteristic feature the formation of isolated [Cu₂(H₂O)₁₁]⁴⁺ units as a condensate of two corner‐linked Jahn‐Teller distorted [Cu(H₂O)₆]²⁺ octahedra via an oxygen atom of crystal water can be considered. Since “zeolitic” water of hydratation is also present, obviously both classical H–O^δ?···^+δH–O and non‐classical B–H^δ?···^+δH–O hydrogen bonds play a significant role for the stabilization of the structure. A direct coordinative influence of the quasi‐icosahedral [B₁₂H₁₂]^2? anions on the Cu²⁺ cations has not been determined. The zinc compound Zn(H₂O)₆[B₁₂H₁₂]·6 H₂O crystallizes in a NaTl‐type related structure. Two crystallographically different [Zn(H₂O)₆]²⁺ octahedra are present, which only differ in their relative orientation within the packing of the [B₁₂H₁₂]^2? anions. The stabilization of the crystal structure takes place mainly via H–O^δ?···^+δH–O hydrogen bonds, since again the hydrogen atoms of the [B₁₂H₁₂]^2? anions have no direct coordinative influence on the Zn²⁺ cations.     

The Cofactor of Tetrachloroethene Reductive Dehalogenase of Dehalospirillum multivorans Is Norpseudo‐B12, a New Type of a Natural Corrinoid

The corrinoid cofactor of the tetrachloroethene reductive dehalogenase of Dehalospirillum multivorans was isolated in its Coβ‐cyano form. This cofactor represents the main corrinoid found in D. multivorans cells. Analysis of the isolated cyano‐corrinoid by a combination of HPLC and UV/VIS‐absorbance spectroscopy revealed it to be nonidentical to a variety of known natural B₁₂ derivatives. From high‐resolution mass‐spectrometric analysis, the molecular formula of the corrinoid isolated from D. multivorans could be deduced as C₅₈H₈₁CoN₁₇O₁₄P. The sample of the novel corrinoid from D. multivorans was further analyzed by UV/VIS, CD, and one‐ and two‐dimensional ¹H‐, ¹³C‐, and ¹⁵N‐NMR spectroscopy, which indicated its structure to be closely related to that of pseudovitamin B₁₂ (Coβ‐cyano‐7″‐adeninylcobamide). By the same means, the corrinoid could be shown to differ from pseudovitamin B₁₂ only by the lack of the methyl group attached to carbon 176, and, therefore, it was named norpseudovitamin B₁₂ (or, more precisely, 176‐norpseudovitamin B₁₂). Norpseudovitamin B₁₂ represents the first example of a ‘complete’ B₁₂‐cofactor that lacks one of the methyl groups of the cobamide moiety, indicating that the B₁₂‐biosynthetic pathway in D. multivorans differs from that of other organisms. X‐Ray crystal‐structures were determined for norpseudovitamin B₁₂ from D. multivorans and the analogues pseudovitamin B₁₂ and factor A (Coβ‐cyano‐7″‐[2‐methyl]adeninylcobamide). These first accurate crystal structures of complete corrinoids with an adeninyl pseudonucleotide confirmed the expected coordination properties around Co and corroborated the close conformational similarity of the nucleotide moieties of norpseudovitamin B₁₂ and its two homologues.     

Darstellung und spektroskopische Charakterisierung der Monofluorohydro-closo-Borate [B6H5F]2− und [B12H11F]2−

Preparation and Spectroscopic Characterization of the Monofluorohydro-closo-borates [B₆H₅F]^2? and [B₁₂H₁₁F]^2? By treatment of [B₆H₆]^2? with 1-(chloromethyl)-4-fluoro-1,4-diazabicyclo[2.2.2]octane-bis(tetrafluoroborate)in acetonitrile monofluorohydro-closo-hexaborate [B₆H₅F]^2? ( 1 ) is formed in good yields. [B₁₂H₁₂]^2? reacts with unhydrous HF yielding the monofluorododecaborate [B₁₂H₁₁F]^2? ( 2 ). These compounds are separated by ion exchange chromatography on diethylaminoethyl(DEAE) cellulose from by-products. The ¹¹B nmr spectra exhibit the characteristic patterns (1 : 4 : 1) of a monosubstituted B₆ octahedron and (1 : 5 : 5 : 1) of a monosubstituted B₁₂ icosahedron with strong downfield shifts of the ipso-B nuclei at +9.3 ppm ( 1 ) and at +9.0 ppm ( 2 ). The ¹⁹F nmr spectra reveal quartets at ?212 ppm ( 1 ) and ?209 ppm ( 2 ) proving a B? F bonding. In the i.r. spectra, for ( 1 ) in the Raman spectrum too, cage vibrations depending on the F substituent at 1195 ( 1 ) and at 1182/1154 cm^?1 ( 2 ) are observed. The Raman spectra show the B₆F stretching mode at 535 cm^?1 and the B₁₂F stretching vibration at 445 cm^?1.     

Synthesis and Structure of Vitamin B12-Derivatives with a Modified Ribose-Unit

The acylation at the 5′-OH group of the ribose-unit of vitamin B₁₂ (cyanocobalamin) or of aquocobalamin with two conventional reagents gave mono-acylated B₁₂-derivatives with good to very high selectivity. The site of the modification was deduced from spectral data of the products and was further supported by the crystal structure data of three such modified B₁₂-derivatives. These three B₁₂-derivatives were found to crystallize in the space group P2₁2₁2₁, irrespective of the nature of the appendage. Acylation at 5′-OH has been used to protect (or block) this group in the context of functionalization of 2′-OH or elsewhere in the B₁₂-molecule. Attachment of the bifunctional succinyl-unit has allowed the preparation of further modified derivatives of vitamin B₁₂ and binding of B₁₂-derivatives to biological carriers and other macromolecules. In aqueous solution, 5′-acylcobalamins turned out to be rather susceptible to hydrolytic loss of the acyl-functionality.