Nicholas A. Piro

Catalytic N2 Reduction to Silylamines and Thermodynamics of N2 Binding at Square Planar Fe

The geometric constraints imposed by a tetradentate P4N2 ligand play an essential role in stabilizing square planar Fe complexes with changes in metal oxidation state. The square pyramidal Fe0(N2)(P4N2) complex catalyzes the conversion of N2 to N(SiR3)3 (R = Me, Et) at room temperature, representing the highest turnover number of any Fe-based N2 silylation catalyst to date (up to 65 equiv N(SiMe3)3 per Fe center). Elevated N2 pressures (>1 atm) have a dramatic effect on catalysis, increasing N2 solubility and the thermodynamic N2 binding affinity at Fe0(N2)(P4N2). A combination of high-pressure electrochemistry and variable-temperature UV-vis spectroscopy were used to obtain thermodynamic measurements of N2 binding. In addition, X-ray crystallography, 57Fe Mössbauer spectroscopy, and EPR spectroscopy were used to fully characterize these new compounds. Analysis of Fe0, FeI, and FeII complexes reveals that the free energy of N2 binding across three oxidation states spans more than 37 kcal mol-1.

Synthesis, Characterization, and Catalytic Activity of a Series of Aluminium–Amidate Complexes*

The aluminium complexes {[κ2-N,O-(t-BuNCOPh)]AlMe2}2 (2), [κ2-N,O-(t-BuNCOPh)]2AlMe (3), and [κ2-N,O-(t-BuNCOPh)]3Al (4) were prepared through the protonolysis reaction between trimethylaluminium and one, two, or three equivalents, respectively, of N-tert-butylbenzamide. Complex 2 was also prepared via a salt metathesis reaction between K(t-BuNCOPh) and dimethylaluminium chloride. Complexes 2–4 were characterized using 1H and 13C NMR spectroscopy. Single-crystal X-ray diffraction analysis of the complexes corroborated ligandu2009:u2009metal stoichiometries and revealed that all the amidate ligands coordinate to the aluminium ion in a κ2 fashion. The Al–amidate complexes 2–4 were viable catalyst precursors for the Meerwein–Ponndorf–Verley–Oppenauer reduction–oxidation manifold, successfully interconverting several classes of carbonyl and alcohol substrates.

Ammonia Oxidation by Abstraction of Three Hydrogen Atoms from a Mo–NH3 Complex

We report ammonia oxidation by homolytic cleavage of all three H atoms from a [Mo-NH3]+ complex using the 2,4,6-tri-tert-butylphenoxyl radical to yield a Mo-alkylimido ([Mo═NR]+) complex (R = 2,4,6-tri-tert-butylcyclohexa-2,5-dien-1-one). Chemical reduction of [Mo═NR]+ generates a terminal Mo≡N nitride complex upon N-C bond cleavage, and a [Mo═NH]+ complex is formed by protonation of the nitride. Computational analysis describes the energetic profile for the stepwise removal of three H atoms from [Mo-NH3]+ and formation of [Mo═NR]+.

Reprint of: Structural, electronic and acid/base properties of [Ru(bpy)(bpy(OH)2)2]2+ (bpy = 2,2′-bipyridine, bpy(OH)2 = 4,4′-dihydroxy-2,2′-bipyridine)

Abstract The development of metal complexes with pH dependent ligands could lead to useful design principles in altering catalysis. We have synthesized the complex [Ru(bpy)(bpy(OH)2)2]2+ (bpyxa0=xa02,2′-bipyridine, bpy(OH)2xa0=xa04,4′-dihydroxy-2,2′-bipyridine) to better understand how the hydroxyl groups influence the electronic and structural properties of the complex as a function of protonation state. Both experimental and computational methods were utilized to study the complex in the protonated and deprotonated state. The most notable difference observed by X-ray diffraction studies, as well as by computational structural analysis, is the shortening of the modified ligand’s C–O bond length upon deprotonation due to increasing double bond character by resonance. Cyclic voltammetry studies of the complex revealed a 0.96xa0V decrease in RuIII/II potential upon deprotonation. Only one ligand redox wave is observed when deprotonated, assigned to the unmodified bpy ligand. The absorption spectrum of protonated [Ru(bpy)(bpy(OH)2)2]2+ is similar to that of [Ru(bpy)3]2+ with typical metal to ligand charge transfer bands at approximately 460xa0nm. Upon deprotonation, the absorption spectrum shifts dramatically, exhibiting a 4504xa0cm−1 red shift from λmaxxa0=xa0468xa0nm to λmaxxa0=xa0593xa0nm in acetonitrile. Computational studies indicate that the bpy(O−)2 ligand’s orbitals heavily mix with the Ru d-orbitals, leading to mixed metal–ligand to ligand charge transfer transitions. Luminescence studies reveal absolute quenching of the excited state upon deprotonation in accordance with the energy gap law. These studies alongside results from the previously studied [Ru(bpy)2(bpy(OH)2)]2+ and [Ru(bpy(OH)2)3]2+ complexes, provide useful insight into the impact of increasing electron donation to the metal center.

Synthesis And Characterization Of Neutral Ligand α-Diimine Complexes Of Aluminum With Tunable Redox Energetics

The synthesis and full characterization of a series of neutral ligand α-diimine complexes of aluminum are reported. The compounds [Al(LAr)2Cl2)][AlCl4] [LAr = N, N-bis(4-R-C6H4)-2,3-dimethyl-1,4-diazabutadiene] are structurally analogous, as determined by multinuclear NMR spectroscopy and solid-state X-ray diffraction, across a range of electron-donating [R = Me (2), tBu (3), OMe (4), and NMe2 (5)] and electron-withdrawing [R = Cl (6), CF3 (7), and NO2 (8)] substituents in the aryl side arm of the ligand. UV-vis absorption spectroscopy and electrochemistry were used to access the optical and electrochemical properties, respectively, of the complexes. Both sets of properties are shown to be dependent on the R substituent. Density functional theory calculations performed on the [Al(LPh)2Cl2)][AlCl4] complex (1) indicate primarily ligand-based frontier orbitals and were used to help support our discussion of both the spectral and electrochemical data. We also report the reaction of the LPh ligand with both AlBr3 and AlI3 and demonstrate a different reactivity profile for the heavier halide relative to the lighter members of the group.

Synthesis of lemonose derivatives: methyl 4-amino-3-O,4-N-carbonyl-2,4,6-trideoxy-3-C-methyl-α-l-lyxo-pyranoside and its phenyl thioglycoside

Lemonose is a component of the antibiotic lemonomycin and other antibiotics and natural products. Three routes to the synthesis of the title compound, a protected, desmethyl derivative of lemonose, from l-rhamnose or its glycal, were investigated based on electrophilic cyclization, epoxidation-ring opening, and deoxygenation of an intermediate that was used in the synthesis of the amino sugar callipeltose. The deoxygenation route was successful and it provided the title compound, which was then converted to a phenyl thioglycoside.

Allyl 3,4,6-tri-O-acetyl-2-de­oxy-2-phthalimido-β-d-gluco­pyran­oside

The protected glycoside of 2-amino-2-dexadoxyxadglucose (glucosaxadmine), namely allyl 3,4,6-tri-O-acetyl-2-dexadoxy-2-phthalimido-β-d-glucoxadpyranxadoside, C23H25NO10, was synthesized from the glycosyl bromide. Crystallographic analysis confirmed the β-anomeric configuration and showed an approximately orthogonal orientation of the phthalimido group with respect to the pyranxadose ring. The absolute configuration of the molxadecule was known from the synthetic route and assigned accordingly.

Spectroscopic, structural and computational analysis of [Re(CO)3(dippM)Br](n+) (dippM = 1,1'-bis(diiso-propylphosphino)metallocene, M = Fe, n = 0 or 1; M = Co, n = 1).

While the redox active backbone of bis(phosphino)ferrocene ligands is often cited as an important feature of these ligands in catalytic studies, the structural parameters of oxidized bis(phosphino)ferrocene ligands have not been thoroughly studied. The reaction of [Re(CO)3(dippf)Br] (dippf = 1,1-bis(diiso-propylphosphino)ferrocene) and [NO][BF4] in methylene chloride yields the oxidized compound, [Re(CO)3(dippf)Br][BF4]. The oxidized species, [Re(CO)3(dippf)Br][BF4], and the neutral species, [Re(CO)3(dippf)Br], are compared using X-ray crystallography, cyclic voltammetry, visible spectroscopy, IR spectroscopy and zero-field (57)Fe Mössbauer spectroscopy. In addition, the magnetic moment of the paramagnetic [Re(CO)3(dippf)Br][BF4] was measured in the solid state using SQUID magnetometry and in solution by the Evans method. The electron transfer reaction of [Re(CO)3(dippf)Br][BF4] with acetylferrocene was also examined. For additional comparison, the cationic compound, [Re(CO)3(dippc)Br][PF6] (dippc = 1,1-bis(diiso-propylphosphino)cobaltocenium), was prepared and characterized by cyclic voltammetry, X-ray crystallography, and NMR, IR and visible spectroscopies. Finally, DFT was employed to examine the oxidized dippf ligand and the oxidized rhenium complex, [Re(CO)3(dippf)Br](+).

Crystal structures of fac-tri­carbonyl­chlorido­(6,6′-dihy­droxy-2,2′-bi­pyridine)­rhenium(I) tetra­hydro­furan monosolvate and fac-bromido­tricarbon­yl(6,6′-dihy­droxy-2,2′-bi­pyridine)­manganese(I) tetra­hydro­furan monosolvate

The structures of two facially coordinated Group VII metal complexes, fac-[ReCl(6,6′-dihydroxy-2,2′-bipyridine)(CO)3]·C4H8O and fac-[MnBr(6,6′-dihydroxy-2,2′-bipyridine)(CO)3]·C4H8O, are reported. These complexes are relevant to catalysis for CO2 reduction.