Nitride-Bridged Triiron Complex and Its Relevance to Dinitrogen Activation

Using a simple metathesis approach, the triiron­(II) tribromide complex Fe₃Br₃L (<b>1</b>) reacts with tetrabutylammonium azide to afford the monoazide dibromide analogue Fe₃(Br)₂(N₃)­L (<b>2</b>) in high yield. The inclusion of azide was confirmed by IR spectroscopy with a ν­(N₃) = 2082 cm^–1 as well as combustion analysis and X-ray crystallography. Heating <b>2</b> in the solid state results in the complete loss of the azide vibration in the IR spectra and the isolation of the olive-green mononitride complex Fe₃(Br)₂(N)­L (<b>3</b>). Solution magnetic susceptibility measurements support that the trimetallic core within <b>2</b> is oxidized upon generation of <b>3</b> (5.07 vs 3.09 μ_B). Absorption maxima in the UV–visible–near-IR (NIR) spectra of <b>2</b> and <b>3</b> support the azide-to-nitride conversion, and a broad NIR absorption centered at 1117 nm is similar to that previously reported for the intervalence charge-transfer band for a mixed-valent nitridodiiron cluster. The cyclic voltammograms recorded for <b>3</b> are comparable to those of <b>1</b> with no reductive waves observed between ∼0 and −2.5 V (vs Fc/Fc⁺), whereas a reversible one-electron redox process is observed for Fe₃(NH₂)₃L (<b>4</b>). These results suggest that intercluster cooperativity is unlikely to predominate the dinitrogen reduction mechanism when <b>1</b> is treated with KC₈ under N₂

Synthesis of Trinuclear Tin(II), Germanium(II), and Aluminum(III) Cyclophane Complexes

The synthesis and characterization of trinuclear Ge^II and Sn^II chlorides and a trialuminum complex supported by a trinucleating tris­(β-diketiminate) cyclophane ligand (<b>L</b>^<b>3</b>–) are reported. The in situ deprotonation of H₃<b>L</b> with benzylpotassium and subsequent reaction with GeCl₂·dioxane or SnCl₂ afforded (GeCl)₃<b>L</b> (<b>1</b>) and (SnCl)₃<b>L</b> (<b>2</b>) in 42 and 60% yields, respectively. Each Ge^II and Sn^II atom is three-coordinate and exhibit pseudotrigonal pyramidal geometry as anticipated for three-coordinate divalent group 14 cations. UV/visible spectra collected on THF solutions of <b>1</b> and <b>2</b> display a bathochromic shift in the absorption from <b>1</b> to <b>2</b> (from 361 to 375 nm). Addition of AlMe₃ to toluene solutions of H₃<b>L</b> resulted in the formation of (AlMe₂)₂AlMe₃H<b>L</b> (<b>3</b>), which possesses two NCCCN chelated AlMe₂ moieties. The third β-diketimine arm remains protonated and adopts an atypical <i>trans</i> conformation with an AlMe₃ coordinated to the solvent exposed imine N atom

Synthesis of Trinuclear Tin(II), Germanium(II), and Aluminum(III) Cyclophane Complexes

The synthesis and characterization of trinuclear Ge^II and Sn^II chlorides and a trialuminum complex supported by a trinucleating tris­(β-diketiminate) cyclophane ligand (<b>L</b>^<b>3</b>–) are reported. The in situ deprotonation of H₃<b>L</b> with benzylpotassium and subsequent reaction with GeCl₂·dioxane or SnCl₂ afforded (GeCl)₃<b>L</b> (<b>1</b>) and (SnCl)₃<b>L</b> (<b>2</b>) in 42 and 60% yields, respectively. Each Ge^II and Sn^II atom is three-coordinate and exhibit pseudotrigonal pyramidal geometry as anticipated for three-coordinate divalent group 14 cations. UV/visible spectra collected on THF solutions of <b>1</b> and <b>2</b> display a bathochromic shift in the absorption from <b>1</b> to <b>2</b> (from 361 to 375 nm). Addition of AlMe₃ to toluene solutions of H₃<b>L</b> resulted in the formation of (AlMe₂)₂AlMe₃H<b>L</b> (<b>3</b>), which possesses two NCCCN chelated AlMe₂ moieties. The third β-diketimine arm remains protonated and adopts an atypical <i>trans</i> conformation with an AlMe₃ coordinated to the solvent exposed imine N atom

Observation of Radical Rebound in a Mononuclear Nonheme Iron Model Complex

A nonheme iron­(III) terminal methoxide complex, [Fe^III(N3PyO^2Ph)­(OCH₃)]­ClO₄, was synthesized. Reaction of this complex with the triphenylmethyl radical (Ph₃C^•) leads to formation of Ph₃COCH₃ and the one-electron-reduced iron­(II) center, as seen by UV–vis, EPR, ¹H NMR, and Mössbauer spectroscopy. These results indicate that homolytic Fe–O bond cleavage occurs together with C–O bond formation, providing a direct observation of the “radical rebound” process proposed for both biological and synthetic nonheme iron centers

Observation of Radical Rebound in a Mononuclear Nonheme Iron Model Complex

A nonheme iron­(III) terminal methoxide complex, [Fe^III(N3PyO^2Ph)­(OCH₃)]­ClO₄, was synthesized. Reaction of this complex with the triphenylmethyl radical (Ph₃C^•) leads to formation of Ph₃COCH₃ and the one-electron-reduced iron­(II) center, as seen by UV–vis, EPR, ¹H NMR, and Mössbauer spectroscopy. These results indicate that homolytic Fe–O bond cleavage occurs together with C–O bond formation, providing a direct observation of the “radical rebound” process proposed for both biological and synthetic nonheme iron centers

Observation of Radical Rebound in a Mononuclear Nonheme Iron Model Complex

A nonheme iron­(III) terminal methoxide complex, [Fe^III(N3PyO^2Ph)­(OCH₃)]­ClO₄, was synthesized. Reaction of this complex with the triphenylmethyl radical (Ph₃C^•) leads to formation of Ph₃COCH₃ and the one-electron-reduced iron­(II) center, as seen by UV–vis, EPR, ¹H NMR, and Mössbauer spectroscopy. These results indicate that homolytic Fe–O bond cleavage occurs together with C–O bond formation, providing a direct observation of the “radical rebound” process proposed for both biological and synthetic nonheme iron centers

A Family of Tri- and Dimetallic Pyridine Dicarboxamide Cryptates: Unusual <i>O</i>,<i>N</i>,<i>O</i>‑Coordination and Facile Access to Secondary Coordination Sphere Hydrogen Bonding Interactions

A series of tri- and dimetallic metal complexes of pyridine dicarboxamide cryptates are reported in which changes to the base and metal source result in diverse structure types. Addition of strong bases, such as KH or KN­(SiMe₃)₂, followed by divalent metal halides allows direct access to trinuclear complexes in which each metal center is coordinated by a dianionic <i>N</i>,<i>N</i>,<i>N</i>-chelate of each arm. These complexes bind a guest K⁺ cation within the central cavity in a trigonal planar coordination environment. Minor changes to the solvent and equivalents of base used in the syntheses of the triiron­(II) and tricobalt­(II) complexes affords two trinuclear clusters with atypical <i>O</i>,<i>N</i>,<i>O-</i>coordination by each pyridine dicarboxamide arm; the amide carbonyl O atoms are oriented toward the interior of the cavity to coordinate to each metal center. Finally, varying the base enables the selective synthesis of dinuclear nickel­(II) and copper­(II) complexes in which one pyridine dicarboxamide arm remains protonated. These amide protons are at one end of a hydrogen bonding network that extends throughout the internal cavity and terminates at a metal bound hydroxide, carbonate, or bicarbonate donor. In the dinickel complex, the bicarbonate cannot be liberated as CO₂ either thermally or upon sparging with N₂, which differs from previously reported monometallic complexes. The carbonate or bicarbonate ligands likely arise from sequestration of atmospheric CO₂ based on the observed reaction of the di­(hydroxonickel) analog

A Family of Tri- and Dimetallic Pyridine Dicarboxamide Cryptates: Unusual <i>O</i>,<i>N</i>,<i>O</i>‑Coordination and Facile Access to Secondary Coordination Sphere Hydrogen Bonding Interactions

A series of tri- and dimetallic metal complexes of pyridine dicarboxamide cryptates are reported in which changes to the base and metal source result in diverse structure types. Addition of strong bases, such as KH or KN­(SiMe₃)₂, followed by divalent metal halides allows direct access to trinuclear complexes in which each metal center is coordinated by a dianionic <i>N</i>,<i>N</i>,<i>N</i>-chelate of each arm. These complexes bind a guest K⁺ cation within the central cavity in a trigonal planar coordination environment. Minor changes to the solvent and equivalents of base used in the syntheses of the triiron­(II) and tricobalt­(II) complexes affords two trinuclear clusters with atypical <i>O</i>,<i>N</i>,<i>O-</i>coordination by each pyridine dicarboxamide arm; the amide carbonyl O atoms are oriented toward the interior of the cavity to coordinate to each metal center. Finally, varying the base enables the selective synthesis of dinuclear nickel­(II) and copper­(II) complexes in which one pyridine dicarboxamide arm remains protonated. These amide protons are at one end of a hydrogen bonding network that extends throughout the internal cavity and terminates at a metal bound hydroxide, carbonate, or bicarbonate donor. In the dinickel complex, the bicarbonate cannot be liberated as CO₂ either thermally or upon sparging with N₂, which differs from previously reported monometallic complexes. The carbonate or bicarbonate ligands likely arise from sequestration of atmospheric CO₂ based on the observed reaction of the di­(hydroxonickel) analog