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

Equilibrium Thermodynamics To Form a Rhodium Formyl Complex from Reactions of CO and H₂: Metal σ Donor Activation of CO

A rhodium­(II) dibenzo­tetra­methyl­aza­[14]­annulene dimer ([(tmtaa)­Rh]₂) (<b>1</b>) reacts with CO and H₂ in toluene and pyridine to form equilibrium distributions with hydride and formyl complexes ((tmtaa)­Rh–H (<b>2</b>); (tmtaa)­Rh–C­(O)H (<b>3</b>)). The rhodium formyl complex ((tmtaa)­Rh–C­(O)­H) was isolated under a CO/H₂ atmosphere, and the molecular structure was determined by X-ray diffraction. Equilibrium constants were evaluated for reactions of (tmtaa)­Rh–H with CO to produce formyl complexes in toluene (<i>K</i>_{2(298 K)(tol)} = 10.8 (1.0) × 10³) and pyridine (<i>K</i>_{2(298 K)(py)} = 2.2 (0.2) × 10³). Reactions of <b>1</b> and <b>2</b> in toluene and pyridine are discussed in the context of alternative radical and ionic pathways. The five-coordinate 18-electron Rh­(I) complex ([(py)­(tmtaa)­Rh^I]⁻) is proposed to function as a nucleophile toward CO to give a two-electron activated bent Rh–CO unit. Results from DFT calculations on the (tmtaa)­Rh system correlate well with experimental observations. Reactions of <b>1</b> with CO and H₂ suggest metal catalyst design features to reduce the activation barriers for homogeneous CO hydrogenation

Evaluation of the Rh^(II)–Rh^(II) Bond Dissociation Enthalpy for [(TMTAA)Rh]₂ by ¹H NMR T₂ Measurements: Application in Determining the Rh–C(O)– BDE in [(TMTAA)Rh]₂CO

Toluene solutions of the rhodium­(II) dimer of dibenzotetramethylaza[14]­annulene ([(TMTAA)­Rh]₂; (<b>1</b>)) manifest an increase in the line widths for the singlet methine and methyl ¹H NMR resonances with increasing temperature that result from the rate of dissociation of the diamagnetic Rh^II–Rh^II bonded dimer (<b>1</b>) dissociating into paramagnetic Rh^II monomers (TMTAA) Rh (<b>2</b>). Temperature dependence of the rates of Rh^II–Rh^II dissociation give the activation parameters for bond homolysis Δ<i>H</i>^⧧_app = 24(1) kcal mol^–1 and Δ<i>S</i>^⧧_app = 10 (1) cal K^–1 mol^–1 and an estimate for the Rh^II–Rh^II bond dissociation enthalpy (BDE) of 22 kcal mol^–1. Thermodynamic values for reaction of <b>1</b> with CO to form (TMTAA)­Rh–C­(O)–Rh­(TMTAA) (<b>3</b>) Δ<i>H</i>₁° = −14 (1) kcal mol^–1, Δ<i>S</i>₁°= −30(3) cal K^–1 mol^–1) were used in deriving a (TMTAA)­Rh–C­(O)– BDE of 53 kcal mol^–1

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-N^<i>t</i>Bu)₄(N^<i>t</i>Bu)₄ (<b>1</b>) and its one-electron-oxidized analogue, the 3:1 Mn^IV/Mn^V cluster [Mn­(μ₃-N^<i>t</i>Bu)₄(N^<i>t</i>Bu)₄]⁺[PF₆]⁻ (<b>1</b>⁺[PF₆]), are described. The <i>S</i> = 0 spin quantum number of <b>1</b> is explained by a diamagnetic electronic structure where all metal-based <i>d</i> electrons are paired in Mn–Mn bonding orbitals. Temperature- and power-dependent studies of the <i>S</i> = ¹/₂ electron paramagnetic resonance signal of <b>1</b>⁺ are consistent with an electronic structure described as a delocalized one-electron radical

Heterobimetallic Complexes of Rhodium Dibenzotetramethylaza[14]annulene [(tmtaa)Rh-M]: Formation, Structures, and Bond Dissociation Energetics

A rhodium­(II) dibenzotetramethylaza[14]­annulene dimer ([(tmtaa)­Rh]₂) undergoes metathesis reactions with [CpCr­(CO)₃]₂, [CpMo­(CO)₃]₂, [CpFe­(CO)₂]₂, [Co­(CO)₄]₂, and [Mn­(CO)₅]₂ to form (tmtaa)­Rh-M complexes (M = CrCp­(CO)₃, MoCp­(CO)₃, FeCp­(CO)₂, Co­(CO)₄, or Mn­(CO)₅). Molecular structures were determined for (tmtaa)­Rh-FeCp­(CO)₂, (tmtaa)­Rh-Co­(μ-CO)­(CO)₃, and (tmtaa)­Rh-Mn­(CO)₅ by X-ray diffraction. Equilibrium constants measured for the metathesis reactions permit the estimation of several (tmtaa)­Rh-M bond dissociation enthalpies (RhCr = 19 kcal mol^–1, RhMo = 25 kcal mol^–1, and RhFe = 27 kcal mol^–1). Reactivities of the bimetallic complexes with synthesis gas to form (tmtaa)­Rh-C­(O)H and M-H are surveyed

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

Magnetism and EPR Studies of Binuclear Ruthenium Hydride Binuclear Species Bearing Redox-Active Ligands

The binuclear complex {[N₃]­Ru­(H)}₂­(μ-η¹:η¹-N₂) ([N₃] = 2,6-(ArylNCMe)₂­C₅H₃N and Aryl = mesityl or xylyl) contains two formally Ru­(I), d⁷ centers linked by a bridging dinitrogen ligand, although the odd electrons are substantially delocalized onto the redox non-innocent pincer ligands. The complex exhibits paramagnetic behavior in solution, but is diamagnetic in the solid state. This difference is attributed to intermolecular “π-stacking” observed in the solid state, which essentially couples unpaired electrons on each half of the complex to form delocalized 22-center-2-electron covalent bonds. Introduction of a bulky <i>t-</i>butyl group on the ligand pyridine ring prevents this intermolecular association and allows further investigation of the magnetic behavior and electronic structure of the binuclear species. The interaction of the unpaired electrons in the two halves of the complex has been probed with magnetic susceptibility and perpendicular and parallel mode EPR measurements, revealing a weakly antiferromagnetically coupled system with a thermally accessible triplet excited state. In addition, the mixed valent, <i>S</i> = ¹/₂, {[N₃]­Ru­(H)}­(μ-η¹:η¹-N₂)­{[N₃]­Ru} system has also been observed via perpendicular mode EPR and was used to quantify the growth of the thermally accessible triplet state of the dihydride complex using parallel mode EPR

Mechanistic Elucidation of the Stepwise Formation of a Tetranuclear Manganese Pinned Butterfly Cluster via N–N Bond Cleavage, Hydrogen Atom Transfer, and Cluster Rearrangement

A mechanistic pathway for the formation of the structurally characterized manganese-amide-hydrazide pinned butterfly complex, Mn₄(μ₃‑PhN-NPh-κ³<i>N</i>,<i>N</i>′)₂­(μ‑PhN-NPh-κ²-<i>N</i>,<i>N</i>′)­(μ‑NHPh)₂L₄ (L = THF, py), is proposed and supported by the use of labeling studies, kinetic measurements, kinetic competition experiments, kinetic isotope effects, and hydrogen atom transfer reagent substitution, and via the isolation and characterization of intermediates using X-ray diffraction and electron paramagnetic resonance spectroscopy. The data support a formation mechanism whereby bis­[bis­(trimethylsilyl)­amido]­manganese­(II) (Mn­(NR₂)₂, where R = SiMe₃) reacts with <i>N</i>,<i>N</i>′-diphenyl­hydrazine (PhNHNHPh) via initial proton transfer, followed by reductive N–N bond cleavage to form a long-lived Mn^IV imido multinuclear complex. Coordinating solvents activate this cluster for abstraction of hydrogen atoms from an additional equivalent of PhNHNHPh resulting in a Mn­(II)­phenylamido dimer, Mn₂­(μ‑NHPh)₂­(NR₂)₂L₂. This dimeric complex further assembles in fast steps with two additional equivalents of PhNHNHPh replacing the terminal silylamido ligands with η¹-hydrazine ligands to give a dimeric Mn₂­(μ‑NHPh)₂­(PhN-NHPh)₂L₄ intermediate, and finally, the addition of two additional equivalents of Mn­(NR₂)₂ and PhNHNHPh gives the pinned butterfly cluster

Frustrated Solvation Structures Can Enhance Electron Transfer Rates

Polar surfaces can interact strongly with nearby water molecules, leading to the formation of highly ordered interfacial hydration structures. This ordering can lead to frustration in the hydrogen bond network, and, in the presence of solutes, frustrated hydration structures. We study frustration in the hydration of cations when confined between sheets of the water oxidation catalyst manganese dioxide. Frustrated hydration structures are shown to have profound effects on ion-surface electron transfer through the enhancement of energy gap fluctuations beyond those expected from Marcus theory. These fluctuations are accompanied by a concomitant increase in the electron transfer rate in Marcus’s normal regime. We demonstrate the generality of this phenomenonenhancement of energy gap fluctuations due to frustrationby introducing a charge frustrated XY model, likening the hydration structure of confined cations to topological defects. Our findings shed light on recent experiments suggesting that water oxidation rates depend on the cation charge and Mn-oxidation state in these layered transition metal oxide materials