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 [Mn₅Li₃(μ₆-N)­(N)­(μ₃-N^<i>t</i>Bu)₆(μ-N^<i>t</i>Bu)₃(N^<i>t</i>Bu)] (<b>1</b>), 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 <i>tert</i>-butylamide, indicative of N–C bond cleavage facilitated by manganese

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 [Mn₅Li₃(μ₆-N)­(N)­(μ₃-N^<i>t</i>Bu)₆(μ-N^<i>t</i>Bu)₃(N^<i>t</i>Bu)] (<b>1</b>), 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 <i>tert</i>-butylamide, indicative of N–C bond cleavage facilitated by manganese

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

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, ¹H and ¹³C 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(NR2)2 (R = SiMe3) results in protolytic amide ligand replacement, and represents the primary entry into the described chemistry. Addition of PhOH to Mn­(NR2)2 results in the formation of the heteroleptic dimer Mn2(μ-OPh)2(NR2)2(THF)2 (1). Usage of the sterically larger 2,6-diphenylphenol (Ph2C6H3OH) as the ligand source results in the formation of a 3-coordinate heteroleptic dimer without THF coordination, Mn2(μ-OC6H3Ph2)2(NR2)2 (2). Attempts to generate 2 in the presence of THF or Et2O resulted in isolation of monomeric Mn­(OC6H3Ph2)2L2 (3, L = THF, Et2O). Use of the sterically intermediate 2,4,6-trimethylphenol (MesOH) resulted in formation of the linear trinuclear cluster Mn3(μ-OMes)4(NR2)2(THF)2 (4). Reaction of Mn­(NR2)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, Mn8(μ5-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

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, ¹H and ¹³C 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

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

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(NR₂)₂ (R = SiMe₃) results in protolytic amide ligand replacement, and represents the primary entry into the described chemistry. Addition of PhOH to Mn­(NR₂)₂ results in the formation of the heteroleptic dimer Mn₂(μ-OPh)₂(NR₂)₂(THF)₂ (<b>1</b>). Usage of the sterically larger 2,6-diphenylphenol (Ph₂C₆H₃OH) as the ligand source results in the formation of a 3-coordinate heteroleptic dimer without THF coordination, Mn₂(μ-OC₆H₃Ph₂)₂(NR₂)₂ (<b>2</b>). Attempts to generate <b>2</b> in the presence of THF or Et₂O resulted in isolation of monomeric Mn­(OC₆H₃Ph₂)₂L₂ (<b>3</b>, L = THF, Et₂O). Use of the sterically intermediate 2,4,6-trimethylphenol (MesOH) resulted in formation of the linear trinuclear cluster Mn₃(μ-OMes)₄(NR₂)₂(THF)₂ (<b>4</b>). Reaction of Mn­(NR₂)₂ with PhOH in the presence of water, or reaction of <b>1</b> with water, results in the formation of a 5-coordinate, unchelated Mn–O cluster, Mn₈(μ₅-O)₂(μ-OPh)₁₂(THF)₆ (<b>5</b>). Preparation, structures, steric properties, and magnetic properties are presented. Notably, complex <b>5</b> exhibits a temperature-dependent phase transition between a 4-spin paramagnetic system at low temperature, and an 8-spin paramagnetic system at room temperature

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. 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

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. 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