Metal-Free Reversible Double Cyclization of Cyanuric Diazide to an Asymmetric Bitetrazolate via Cleavage of the Six-Membered Aromatic Ring.

Crystallization of the reaction mixture of 2-amino-4,6-diazido-1,3,5-triazine and excess tert-butylamine results in the isolation of tert-butylammonium N,N-[1'H-(1,5'-bitetrazol)-5-yl]cyanamidate, suggesting a complex decyclization/cyclization rearrangement involving breakage of the six-membered aromatic ring and the formation of two new five-membered azole rings mediated by deprotonation of the precursor by the amine. The addition of tert-butylamine to 2-amino-4,6-diazido-1,3,5-triazine gives spectroscopic indication of thermodynamically unfavorable reactivity in low-dielectric solvents, and high-level quantum chemical computations also suggest its formation to be unfavorable. A computed interconversion pathway describes the likely reaction mechanism and supports the general thermodynamic unfavorability of the reaction and the requirement for a high-dielectric environment to template formation of the ionic product and its trapping by crystallization.

A Rearrangement Reaction to Yield a NH4 + Ion Driven by Polyoxometalate Formation.

Triethylamine is a volatile liquid and exists in the atmosphere in the gas phase. It is a hazardous air pollutant and identified as a toxic air contaminant. Thus, producing ammonia (a vital chemical for fertilizer production) from the vapor state of this toxic substance is a challenging task. Diffusion of the vapor of triethylamine, (C2H5)3N, into an acidified aqueous solution of sodium molybdate results in the formation of single crystals of compound [(C2H5)3NH]2[(C2H5)4N][NaMo8O26] (1). Notably, compound 1 includes a [(C2H5)4N]+ cation, even though the concerned reaction mixture was not treated with any tetraethylammonium salt. The formation of the [(C2H5)4N]+ cation from (C2H5)3N in an acidic aqueous medium is logically possible only when an ammonium cation (NH4 +) is formed in the overall reaction: 4(C2H5)3N + 4H+ = 3[(C2H5)4N]+ + [NH4]+. Although the resulting NH4 + cation (identified by Nessler's reagent test) is not included in the crystals of compound 1 as a cation, it can be made associated with a crown ether in the isolation of single crystals of compound [NH4⊂B15C5]3[PMo12O40]·B15C5 (2), (B15C5 = benzo-15-crown-5). Crystal structure analysis and 1H NMR studies of compound 2 have established the presence of an H-bonded NH4 + ion in compound 2, thereby established the rearrangement reaction.

Transition-metal-mediated reduction and reversible double-cyclization of cyanuric triazide to an asymmetric bitetrazolate involving cleavage of the six-membered aromatic ring.

Cyanuric triazide reacts with several transition metal precursors, extruding one equivalent of N2 and reducing the putative diazidotriazeneylnitrene species by two electrons, which rearranges to N-(1'H-[1,5'-bitetrazol]-5-yl)methanediiminate (biTzI2-) dianionic ligand, which ligates the metal and dimerizes, and is isolated from pyridine as [M(biTzI)]2Py6 (M = Mn, Fe, Zn, Cu, Ni). Reagent scope, product analysis, and quantum chemical calculations were combined to elucidate the mechanism of formation as a two-electron reduction preceding ligand rearrangement.

Activation of C-H, N-H, and O-H Bonds via Proton-Coupled Electron Transfer to a Mn(III) Complex of Redox-Noninnocent Octaazacyclotetradecadiene, a Catenated-Nitrogen Macrocyclic Ligand.

Reaction of 1,3-diazidopropane with an electron-rich Mn(II) precursor results in oxidation of the metal center to a Mn complex with concomitant assembly of the macrocyclic ligand into the 1,2,3,4,8,9,10,11-octaazacyclotetradeca-2,9-diene-1,4,8,11-tetraido (OIM) ligand. Although describable as a Werner Mn(V) complex, analysis by X-ray diffraction, magnetic measurements, X-ray photoelectron spectroscopy, cyclic voltammetry, and density functional theory calculations suggest an electronic structure consisting of a Mn(III) metal center with a noninnocent OIM diradical ligand. The resulting complex, (OIM)Mn(NH tBu), reacts via proton-coupled electron transfer (PCET) with phenols to form phenoxyl radicals, with dihydroanthracene to form anthracene, and with (2,4-di tert-butyltetrazolium-5-yl)amide to extrude a tetrazyl radical. PCET from the latter generates the isolable corresponding one-electron reduced compound with a neutral, zwitterionic axial 2,4-di tert-butyltetrazolium-5-yl)amido ligand. Electron paramagnetic resonance and density functional theoretical analyses suggest an electronic structure wherein the manganese atom remains Mn(III) and the OIM ligand has been reduced by one electron to a monoradical noninnocent ligand. The result indicates PCET processes whereby the proton is transferred to the axial ligand to extrude tBuNH2, the electron is transferred to the equatorial ligand, and the central metal remains relatively unperturbed.

Structure of salts of lithium chloride and lithium hexafluorophosphate as solvates with pyridine and vinylpyridine and structural comparisons: (C5H5N)LiPF6, [p-(CH2=CH)C5H4N]LiPF6, [(C5H5N)LiCl]n, and [p-(CH2=CH)C5H4N]2Li(µ-Cl)2Li[p-(CH2=CH)C5H4N]2.

Due to the flammability of liquid electrolytes used in lithium ion batteries, solid lithium ion conductors are of interest to reduce danger and increase safety. The two dominating general classes of electrolytes under exploration as alternatives are ceramic and polymer electrolytes. Our group has been exploring the preparation of molecular solvates of lithium salts as alternatives. Dissolution of LiCl or LiPF6 in pyridine (py) or vinylpyridine (VnPy) and slow vapor diffusion with diethyl ether gives solvates of the lithium salts coordinated by pyridine ligands. For LiPF6, the solvates formed in pyridine and vinylpyridine, namely tetrakis(pyridine-κN)lithium(I) hexafluorophosphate, [Li(C5H5N)4]PF6, and tetrakis(4-ethenylpyridine-κN)lithium(I) hexafluorophosphate, [Li(C7H7N)4]PF6, exhibit analogous structures involving tetracoordinated lithium ions with neighboring PF6- anions in the I-4 and Aea2 space groups, respectively. For LiCl solvates, two very different structures form. catena-Poly[[(pyridine-κN)lithium]-µ3-chlorido], [LiCl(C5H5N)]n, crystalizes in the P212121 space group and contains channels of edge-fused LiCl rhombs templated by rows of π-stacked pyridine ligands, while the structure of the LiCl-VnPy solvate, namely di-µ-chlorido-bis[bis(4-ethenylpyridine-κN)lithium], [Li2Cl2(C7H7N)4], is described in the P21/n space group as dinuclear (VnPy)2Li(µ-Cl)2Li(VnPy)2 units packed with neighbors via a dense array of π-π interactions.

Covalent Metal-Metal-Bonded Mn4 Tetrahedron Inscribed within a Four-Coordinate Manganese Cubane Cluster, As Evidenced by Unexpected Temperature-Independent Diamagnetism.

The electronic structures of the manganese(IV) cubane cluster Mn(µ3-NtBu)4(NtBu)4 (1) and its one-electron-oxidized analogue, the 3:1 MnIV/MnV cluster [Mn(µ3-NtBu)4(NtBu)4]+[PF6]- (1+[PF6]), are described. The S = 0 spin quantum number of 1 is explained by a diamagnetic electronic structure where all metal-based d electrons are paired in Mn-Mn bonding orbitals. Temperature- and power-dependent studies of the S = 1/2 electron paramagnetic resonance signal of 1+ are consistent with an electronic structure described as a delocalized one-electron radical.

Reactive Pendant Mn═O in a Synthetic Structural Model of a Proposed S4 State in the Photosynthetic Oxygen Evolving Complex.

The molecular mechanism of the Oxygen Evolving Center of photosystem II has been under debate for decades. One frequently cited proposal is the nucleophilic attack by water hydroxide on a pendant Mn═O moiety, though no chemical example of this reactivity at a manganese cubane cluster has been reported. We describe here the preparation, characterization, and a reactivity study of a synthetic manganese cubane cluster with a pendant manganese-oxo moiety. Reaction of this cluster with alkenes results in oxygen and hydrogen atom transfer reactions to form alcohol- and ketone-based oxygen-containing products. Nitrene transfer from core imides is negligible. The inorganic product is a cluster identical to the precursor, but with the pendant Mn═O moiety replaced by a hydrogen abstracted from the organic substrate, and is isolated in quantitative yield. 18O and 2H isotopic labeling studies confirm the transfer of atoms between the cluster and the organic substrate. The results suggest that the core cubane structure of this model compound remains intact, and that the pendant Mn═O moiety is preferentially reactive.

Concise Syntheses of bis-Strychnos Alkaloids (-)-Sungucine, (-)-Isosungucine, and (-)-Strychnogucine†B from (-)-Strychnine.

The first chemical syntheses of complex, bis-Strychnos alkaloids (-)-sungucine (1), (-)-isosungucine (2), and (-)-strychnogucine B (3) from (-)-strychnine (4) is reported. Key steps included (1) the Polonovski-Potier activation of strychnine N-oxide; (2) a biomimetic Mannich coupling to forge the signature C23-C5' bond that joins two monoterpene indole monomers; and (3) a sequential HBr/NaBH3 CN-mediated reduction to fashion the ethylidene moieties in 1-3. DFT calculations were employed to rationalize the regiochemical course of reactions involving strychnine congeners.

Electronic Structure of Manganese Complexes of the Redox-Non-innocent Tetrazene Ligand and Evidence for the Metal-Azide/Imido Cycloaddition Intermediate.

The first synthetic manganese tetrazene complexes are described as a redox pair comprising anionic [Mn(N4 Ad2 )2 ](-) (1) and neutral Mn(N4 Ad2 )2 (2) complexes (N4 Ad2 =[Ad-N-N=N-N-Ad](2-) ). Compound 1 is obtained in two forms as lithium salts, one as a cationic Li2 Mn cluster, and one as a Mn-Li 1D ionic polymer. Compound 1 is electronically described as a Mn(III) center with two [N4 Ad2 ](2-) ligands. The one-electron oxidized 2 is crystalized in two morphologies, one as pure 2 and one as an acetonitrile adduct. Despite similar composition, the behavior of 2 differs in the two morphologies. Compound 2-MeCN is relatively air and temperature stable. Crystalline 2, on the other hand, exhibits a compositional, dynamic disorder wherein the tetrazene metallacycle ring-opens into a metal imide/azide complex detectable by X-ray crystallography and FTIR spectroscopy. Electronic structure of 2 was examined by EPR and XPS spectroscopies and DFT calculations, which indicate 2 is best described as a Mn(III) ion with an anion radical delocalized across the two ligands through spin-polarization effects.

Biomimetic Total Syntheses of (-)-Leucoridines†A and C through the Dimerization of (-)-Dihydrovalparicine.

Concise biomimetic syntheses of the Strychnos-Strychnos-type bis-indole alkaloids (-)-leucoridine A (1) and C (2) were accomplished through the biomimetic dimerization of (-)-dihydrovalparicine (3). En route to 3, the known alkaloids (+)-geissoschizoline (8) and (-)-dehydrogeissoschizoline (10) were also prepared. DFT calculations were employed to elucidate the mechanism, which favors a stepwise aza-Michael/spirocyclization sequence over the alternate hetero-Diels-Alder cycloaddition reaction.

Acceleration of an aromatic Claisen rearrangement via a designed spiroligozyme catalyst that mimics the ketosteroid isomerase catalytic dyad.

A series of hydrogen-bonding catalysts have been designed for the aromatic Claisen rearrangement of a 1,1-dimethylallyl coumarin. These catalysts were designed as mimics of the two-point hydrogen-bonding interaction present in ketosteroid isomerase that has been proposed to stabilize a developing negative charge on the ether oxygen in the migration of the double bond.1 Two hydrogen bond donating groups, a phenol alcohol and a carboxylic acid, were grafted onto a conformationally restrained spirocyclic scaffold, and together they enhance the rate of the Claisen rearrangement by a factor of 58 over the background reaction. Theoretical calculations correctly predict the most active catalyst and suggest that both preorganization and favorable interactions with the transition state of the reaction are responsible for the observed rate enhancement.

Formation of the tetranuclear, tetrakis-terminal-imido Mn4(IV)(N(t)Bu)8 cubane cluster by four-electron reductive elimination of (t)BuN=N(t)Bu. The role of the s-block ion in stabilization of high-oxidation state intermediates.

Mn4(IV)(µ3-N(t)Bu)4(N(t)Bu)4 is obtained from a previously reported asymmetric Mn(IV/V)-Li-(NR)(N) cluster by the removal of Li from the starting cluster by ion metathesis, which triggers reductive elimination of azo-tert-butane to give a tetranuclear heterocubane cluster.

Architectural spiroligomers designed for binuclear metal complex templating.

The first structurally, spectroscopically, and electronically characterized metal-spiroligomer complexes are reported. The binuclear [M2L2](4+) ions (M = Mn, Zn) are macrocyclic "squares" and are characterized by X-ray diffraction, (1)H and (13)C NMR, electronic absorption, emission, and mass spectroscopies. The manganese complex contains two spin-independent Mn(II) ions and is additionally characterized using EPR and CD spectroscopies and CV.

Synthesis, structure, and magnetic studies of manganese-oxygen clusters of reduced coordination number, featuring an unchelated, 5-coordinate octanuclear manganese cluster with water-derived oxo ligands.

The synthesis of reduced coordination (less than 6), unchelated manganese oxygen cluster systems is described. Addition of phenols to Mn(NR(2))(2) (R = SiMe(3)) results in protolytic amide ligand replacement, and represents the primary entry into the described chemistry. Addition of PhOH to Mn(NR(2))(2) results in the formation of the heteroleptic dimer Mn(2)(µ-OPh)(2)(NR(2))(2)(THF)(2) (1). Usage of the sterically larger 2,6-diphenylphenol (Ph(2)C(6)H(3)OH) as the ligand source results in the formation of a 3-coordinate heteroleptic dimer without THF coordination, Mn(2)(µ-OC(6)H(3)Ph(2))(2)(NR(2))(2) (2). Attempts to generate 2 in the presence of THF or Et(2)O resulted in isolation of monomeric Mn(OC(6)H(3)Ph(2))(2)L(2) (3, L = THF, Et(2)O). Use of the sterically intermediate 2,4,6-trimethylphenol (MesOH) resulted in formation of the linear trinuclear cluster Mn(3)(µ-OMes)(4)(NR(2))(2)(THF)(2) (4). Reaction of Mn(NR(2))(2) with PhOH in the presence of water, or reaction of 1 with water, results in the formation of a 5-coordinate, unchelated Mn-O cluster, Mn(8)(µ(5)-O)(2)(µ-OPh)(12)(THF)(6) (5). Preparation, structures, steric properties, and magnetic properties are presented. Notably, complex 5 exhibits a temperature-dependent phase transition between a 4-spin paramagnetic system at low temperature, and an 8-spin paramagnetic system at room temperature.

Electronic structure and solution behavior of a tris(N,N'-diphenylhydrazido)manganese(IV) propeller complex.

The electronic structure and magnetic properties of the manganese(IV) trihydrazide propeller complex, Li(2)Mn(κ(2)-PhN-NPh)(3)L(2) (1, L = tetrahydrofuran, diethyl ether), are explored. EPR and solid-state magnetometry studies are indicative of a high spin Mn(IV) with a S = 3/2 spin state. Solution-phase magnetic measurements result in a measured µ(eff) less than that expected for a S = 3/2, indicating a solution-phase equilibrium with a lower-spin species. Concentration-dependent magnetic susceptibility measurements identify clustering of 1 to an antiferromagnetically coupled multinuclear complex as the most likely explanation for the solution behavior. Comparative infrared spectroscopy in solution and solid phase are described which support speciation in solution.

Synthesis of a high-valent, four-coordinate manganese cubane cluster with a pendant Mn atom: photosystem II-inspired manganese-nitrogen clusters.

High-valent, four-coordinate manganese imido- and nitrido-bridged heterodicubane clusters have been prepared and characterized by single-crystal X-ray diffraction and spectroscopic techniques. The title compound, a corner-nitride-fused dicubane with the chemical formula [Mn(5)Li(3)(µ(6)-N)(N)(µ(3)-N(t)Bu)(6)(µ-N(t)Bu)(3)(N(t)Bu)] (1), has been prepared as an adduct with a nearly isostructural tetramanganese cluster with one Mn atom replaced by Li. An important feature of the reported chemistry is the formation of nitride from tert-butylamide, indicative of N-C bond cleavage facilitated by manganese.

An isolable, metastable, geometrically unique manganese(IV) trihydrazide complex poised for reactivity.

A metastable Mn(IV) tris(N,N')diphenylhydrazide complex, Li(2)Mn(κ(2)-N(2)Ph(2))(3) (1) has been prepared, and characterized by X-ray diffraction analysis and spectroscopic techniques. Mn precursors are reacted with LiNH(t)Bu to form an intermediate manganese-imido mixture, which reacts with N,N'-diphenylhydrazine, oxidizing the metal, and forming 1. Despite ease of formation, 1 is poised for reactivity, as loss of ether ligand triggers reductive elimination of azobenzene.