Mössbauer Spectroscopic and Computational Investigation of An Iron Cyclopentadienone Complex.

(Cyclopentadienone)iron carbonyl complexes have recently received particular attention for their use as catalysts in hydrogenation or transfer hydrogenation reactions including the N-alkylation of amines with alcohols. This is due to their easy synthesis from simple and cheap materials, air and water stabilities, and the crucial metal-ligand cooperation giving rise to unique catalytic properties. Here, we report a Mössbauer spectroscopic and computational investigation of such a complex and its corresponding activated species for dehydrogenation and hydrogenation reactions. This study affords a deeper understanding of the species formed by the reaction with Me3NO and their distribution upon the added amount of an oxidant.

Effect of Distortions on the Geometric and Electronic Structures of One-Electron Oxidized Vanadium(IV), Copper(II), and Cobalt(II)/(III) Salen Complexes.

The ligands N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,2-ethanediamine and N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,3-propanediamine were chelated to V(IV)═O (1, 2), Cu(II) (3, 4), Co(II) (5), and Co(III) (6). The X-ray crystal structures of 1-6 were solved. The vanadium center in 1-2 resides in square pyramidal geometry, with an axially bound oxo ligand, whereas the metal ion displays a tetrahedrally distorted square planar geometry in 3-5. The extent of distortion is correlated to the length of the diamine spacer: The longer the linker, the larger the tetrahedral distortions. Complex 6 is octahedral with a bidentate acetate molecule that completes the coordination sphere. All the complexes were characterized by UV-vis and EPR spectroscopies, as well as DFT calculations and electrochemistry. Complexes 1-6 exhibit a reversible one-electron oxidation wave in the range -0.11-0.26 V vs Fc+/Fc. The cations 1+ and 2+ were structurally characterized, showing an octahedral V(V) ion with one oxo and one water molecule coordinated in axial positions. Their vis-NIR spectra are dominated by a band at 727 and 815 nm, respectively, which is assigned to a phenolate-to-vanadium(V) charge transfer (CT) transition. The crystal structures of 3+ and 4+ are congruent with Cu(II)-radical species, wherein the metal center remains four-coordinated. Both feature a Class II (Robin-Day classification scale) IVCT transition at around 1200 nm (ε > 1 mM cm-1), indicative of partial localization of the radical. The structure of 5+ displays a square pyramidal cobalt ion, where the fifth (axial) coordination is occupied by a water molecule. It displays a NIR feature at 1244 nm and is described as intermediate between high spin Co(III) and Co(II) radical. In the presence of acetate the dimer [(5)2(µ-OAc)]+ forms, which was structurally characterized and shows a blue shift and lowering in intensity of the NIR absorption band in comparison to 5+. Complex 6+ is a genuine Co(III) radical complex, wherein the phenoxyl moiety is localized on one side of the molecule.

Electrochemical Water Oxidation and Stereoselective Oxygen Atom Transfer Mediated by a Copper Complex.

Water oxidation by copper-based complexes to form dioxygen has attracted attention in recent years, with the aim of developing efficient and cheap catalysts for chemical energy storage. In addition, high-valent metal-oxo species produced by the oxidation of metal complexes in the presence of water can be used to achieve substrate oxygenation with the use of H2 O as an oxygen source. To date, this strategy has not been reported for copper complexes. Herein, a copper(II) complex, [(RPY2)Cu(OTf)2 ] (RPY2=N-substituted bis[2-pyridyl(ethylamine)] ligands; R=indane; OTf=triflate), is used. This complex, which contains an oxidizable substrate moiety (indane), is used as a tool to monitor an intramolecular oxygen atom transfer reaction. Electrochemical properties were investigated and, upon electrolysis at 1.30 V versus a normal hydrogen electrode (NHE), both dioxygen production and oxygenation of the indane moiety were observed. The ligand was oxidized in a highly diastereoselective manner, which indicated that the observed reactivity was mediated by metal-centered reactive species. The pH dependence of the reactivity was monitored and correlated with speciation deduced from different techniques, ranging from potentiometric titrations to spectroscopic studies and DFT calculations. Water oxidation for dioxygen production occurs at neutral pH and is probably mediated by the oxidation of a mononuclear copper(II) precursor. It is achieved with a rather low overpotential (280 mV at pH 7), although with limited efficiency. On the other hand, oxygenation is maximum at pH 8-8.5 and is probably mediated by the electrochemical oxidation of an antiferromagnetically coupled dinuclear bis(µ-hydroxo) copper(II) precursor. This constitutes the first example of copper-centered oxidative water activation for a selective oxygenation reaction.

Electronic Structure and Reactivity of One-Electron-Oxidized Copper(II) Bis(phenolate)-Dipyrrin Complexes.

The sterically hindered bis(phenol)-dipyrrin ligands HLH3 and PhLH3 were reacted with 1 equiv of copper(II) under ambient conditions to produce the copper radical complexes [Cu(HL)] and [Cu(PhL)]. Their X-ray crystal structures show relatively short C-O bond distances (mean bond distances of 1.287 and 1.291 Å), reminiscent of mixed pyrrolyl-phenoxyl radical species. Complexes [Cu(HL)] and [Cu(PhL)] exhibit rich electronic spectra, with an intense near-IR (NIR) band (ε > 6 mM-1 cm-1) at 1346 and 1321 nm, respectively, assigned to a ligand-to-ligand charger-transfer transition. Both show a reversible oxidation wave ( E1/21,ox = 0.05 and 0.04 V), as well as a reversible reduction wave ( E1/21,red = -0.40 and -0.56 V versus ferrocenium/ferrocene, respectively). The cations ([Cu(HL)]+ and [Cu(PhL)]+) and anions ([Cu(HL)]- and [Cu(PhL)]-) were generated. They all display an axial ( S = 1/2) signal with a copper hyperfine structure in their electron paramagnetic resonance spectra, consistent with ligand-centered redox processes in both reduction and oxidation. Complex [Cu(HL)](SbF6) was cocrystallized with [Cu(HL)]. Oxidation is accompanied by a slight contraction of both the C-O bonds (mean bond distance of 1.280 Å) and the C-C bonds connecting the peripheral rings to the dipyrrin. The cations show vis-NIR bands of up to 1090 nm due to their quinoidal nature. The anions do not show a significant band above 700 nm, in agreement with their bis(phenolate)-dipyrrin character. The radical complexes efficiently catalyze the aerobic oxidation of benzyl alcohol, 1-phenylethanol, and unactivated 2-phenylethanol in basic conditions.

A Structurally Characterized CuIII Complex Supported by a Bis(anilido) Ligand and Its Oxidative Catalytic Activity.

Three copper(II) complexes of the (R,R)-N,N'-bis(3,5-di-tert-butyl-2-aminobenzylidene)-1,2-diaminocyclohexane ligand, namely [Cu(N L)], [Cu(N LH)]+ and [Cu(N LH2 )]2+ , were prepared and structurally characterized. In [Cu(N LH2 )]2+ the copper ion lies in an octahedral geometry with the aniline groups coordinated in equatorial positions. In [Cu(N L)] the anilines are deprotonated (anilido moieties) and coordinated to an almost square-planar metal ion. Complex [Cu(N L)] displays two oxidation waves at E1/2ox, 1 =-0.14 V and E1/2ox, 2 =0.36 V vs. Fc+ /Fc in CH2 Cl2 . Complex [Cu(N LH2 )]2+ displays an irreversible oxidation wave at high potential (1.21 V), but shows a readily accessible and reversible metal-centered reduction at E1/2red =-0.67 V (CuII /CuI redox couple). Oxidation of [Cu(N L)] by AgSbF6 produces [Cu(N L)](SbF6 ), which was isolated as single crystals. X-ray structure analysis discloses a contraction of the coordination sphere by 0.05 Šupon oxidation, supporting a metal-centered process. Complex [Cu(N L)](SbF6 ) displays an intense NIR band at 1260 nm corresponding to an anilido-to-copper(III) charge transfer transition. This compound slowly evolves in CH2 Cl2 solution towards [Cu(N LH)](SbF6 ), which is a copper(II) complex comprised of both anilido and aniline groups coordinated to the metal center. The copper(III) complex [Cu(N L)](SbF6 ) is an efficient catalyst for benzyl alcohol oxidation, with 236 TON in 24 h at 298 K, without additives other than oxygen and a base.

Combined Spectroscopic and Electrochemical Detection of a Ni(I) ⋠⋠⋠H-N Bonding Interaction with Relevance to Electrocatalytic H2 Production.

The [Ni(P(R) 2 N(R') 2 )2 ](2+) family of complexes are exceptionally active catalysts for proton reduction to H2 . In this manuscript, we explore the first protonation step of the proposed catalytic cycle by using a catalytically inactive Ni(I) complex possessing a sterically demanding variation of the ligand. Due to the paramagnetic nature of the Ni(I) oxidation state, the protonated Ni(I) intermediate has been characterized through a combination of cyclic voltammetry, electron nuclear double resonance (ENDOR) spectroscopy, and hyperfine sublevel correlation (HYSCORE) spectroscopy. Both the electrochemical and spectroscopic studies indicate that the Ni(I) complex is protonated at a pendant amine that is endo to Ni, which suggests the presence of an intramolecular Ni(I) ⋅⋅⋅HN bonding interaction. Using density functional theory, the hydrogen bond was found to involve three doubly-occupied, localized molecular orbitals: the 3dxz , 3d z 2, and 3dyz orbitals of nickel. These studies provide the first direct experimental evidence for this critical catalytic intermediate, and implications for catalytic H2 production are discussed.

Iron- and Organo-Catalyzed Borrowing Hydrogen for the Stereoselective Construction of Tetrahydropyrans.

Stereocontrolled oxa-Michael additions are challenging, given the high reversibility of the process, which ultimately leads to racemization of the newly formed stereocenters. When iron-catalyzed borrowing hydrogen from allylic alcohols was combined with a stereocontrolled organocatalytic oxa-Michael addition, a wide array of chiral tetrahydropyrans were efficiently prepared. The reaction could be performed in a diastereoselective manner from pre-existing stereocenters or enantioselectively from achiral substrates. The key to success was the reactivity of the iron complex, which was selective for allylic alcohol dehydrogenation and irreversibly led the reaction to the final product.

Stable anilinyl radicals coordinated to nickel: X-ray crystal structure and characterization.

Two anilinosalen and a mixed phenol-anilinosalen ligands involving sterically hindered anilines moieties were synthesized. Their nickel(II) complexes 1, 2, and 3 were prepared and characterized. They could be readily one-electron oxidized (E(1/2)=-0.30, -0.26 and 0.10 V vs. Fc(+)/Fc, respectively) into anilinyl radicals species [1](+), [2](+), and [3](+), respectively. The radical complexes are extremely stable and were isolated as single crystals. X-ray crystallographic structures reveal that the changes in bond length resulting from oxidation do not exceed 0.02 Å within the ligand framework in the symmetrical [1](+) and [2](+). No quinoid bond pattern was present. In contrast, larger structural rearrangements were evidenced for the unsymmetrical [3](+), with shortening of one C(ortho)-C(meta) bond. Radical species [1](+) and [2](+) exhibit a strong absorption band at around 6000 cm(-1) (class III mixed valence compounds). This band is significantly less intense than [3](+), consistent with a rather localized anilinyl radical character, and thus a classification of this species as class II mixed-valence compound. Magnetic and electronic properties, as well as structural parameters, have been computed by DFT methods.

Structural and spectroscopic investigation of an anilinosalen cobalt complex with relevance to hydrogen production.

A Co(II) anilinosalen catalyst containing proton relays in the first coordination sphere has been synthesized that catalyzes the electrochemical production of hydrogen from acid in dichloromethane and acetonitrile solutions. The complex has been spectroscopically and theoretically characterized in different protonation and redox states. We show that both coordinated anilido groups of the neutral Co(II) complex can be protonated into aniline form. Protonation induces an anodic shift of more than 1 V of the reduction wave, which concomitantly becomes irreversible. Hydrogen evolution that originates from the aniline protons located in the first coordination sphere is observed upon bulk electrolysis at -1.5 V of the protonated complex in absence of external acid. Structures for intermediates in the catalytic reaction have been identified based on this data.

Ligand-centered redox activity in cobalt(II) and nickel(II) bis(phenolate)-dipyrrin complexes.

One for all: a trianionic ligand containing the biologically relevant moieties phenolate and porphyrin was designed and synthesized. One-electron oxidation of the nickel and cobalt complexes of these ligands affords an unprecedented and highly stable hybrid porphyrinyl-phenoxyl radical bound to the metal center. Two-electron oxidation of these complexes leads to the M(2+) -(close-shell two-electron oxidized ligand) species.

Radical localization in a series of symmetric Ni(II) complexes with oxidized salen ligands.

Square-planar nickel(II) complexes of salen ligands, N,N'-bis(3-tert-butyl-(5R)-salicylidene)-1,2-cyclohexanediamine), in which R=tert-butyl (1), OMe (2), and NMe(2) (3), were prepared and the electronic structure of the one-electron-oxidized species [1-3](+·) was investigated in solution. Cyclic voltammograms of [1-3] showed two quasi-reversible redox waves that were assigned to the oxidation of the phenolate moieties to phenoxyl radicals. From the difference between the first and second redox potentials, the trend of electronic delocalization 1(+·) >2(+·) >3(+·) was obtained. The cations [1-3](+·) exhibited isotropic g tensors of 2.045, 2.023, and 2.005, respectively, reflecting a lower metal character of the singly occupied molecular orbital (SOMO) for systems that involve strongly electron-donating substituents. Pulsed-EPR spectroscopy showed a single population of equivalent imino nitrogen atoms for 1(+·), whereas two distinct populations were observed for 2(+·). The resonance Raman spectra of 2(+·) and 3(+·) displayed the ν(8a) band of the phenoxyl radicals at 1612 cm(-1), as well as the ν(8a) bands of the phenolates. In contrast, the Raman spectrum of 1(+·) exhibited the ν(8a) band at 1602 cm(-1), without any evidence of the phenolate peak. Previous work showed an intense near-infrared (NIR) electronic transition for 1(+·) (Δν(1/2) =660 cm(-1), ε=21,700 M(-1) cm(-1)), indicating that the electron hole is fully delocalized over the ligand. The broader and moderately intense NIR transition of 2(+·) (Δν(1/2) =1250 cm(-1) , ε=12,800 M(-1) cm(-1)) suggests a certain degree of ligand-radical localization, whereas the very broad NIR transition of 3(+·) (Δν(1/2) =8630 cm(-1), ε=2550 M(-1) cm(-1)) indicates significant localization of the ligand radical on a single ring. Therefore, 1(+·) is a Class III mixed-valence complex, 2(+·) is Class II/III borderline complex, and 3(+·) is a Class II complex according to the Robin-Day classification method. By employing the Coulomb-attenuated method (CAM-B3LYP) we were able to predict the electron-hole localization and NIR transitions in the series, and show that the energy match between the redox-active ligand and the metal d orbitals is crucial for delocalization of the radical SOMO.

Ligand contributions to the electronic structures of the oxidized cobalt(II) salen complexes.

Square planar cobalt(II) complexes of salen ligands N,N'-bis(3-tert-butyl-5R-salicylidene)-1,2-cyclohexanediamine), where R = OMe (1) and tert-butyl (2), were prepared. 1 and 2 were electrochemically reversibly oxidized into cations [1-H(2)O](+) and [2-H(2)O](+) in CH(2)Cl(2). The chemically generated [1-H(2)O](SbF(6))·0.68 H(2)O·0.82CH(2)Cl(2) and [2-H(2)O](SbF(6))·0.3H(2)O·0.85CH(2)Cl(2) were characterized by X-ray diffraction and NIR spectroscopy. Both complexes are paramagnetic species containing a square pyramidal cobalt ion coordinated at the apical position by an exogenous water molecule. They exhibit remarkable NIR bands at 1220 (7370 M(-1) cm(-1)) and 1060 nm (5560 M(-1) cm(-1)), respectively, assigned to a CT transition. DFT calculations and magnetic measurements confirm the paramagnetic (S = 1) ground spin state of the cations. They show that more than 70% of the total spin density in [1-H(2)O](+) and [2-H(2)O](+) is localized on the metal, the remaining spin density being distributed over the aromatic rings (30% phenoxyl character). In the presence of N-methylimidazole 1 and 2 are irreversibly oxidized by air into the genuine octahedral cobalt(III) bis(phenolate) complexes [1-im(2)](+) and [2-im(2)](+), the former being structurally characterized. Neither [1-im(2)](+) nor [2-im(2)](+) exhibits a NIR feature in its electronic spectrum. 1 and 2 were electrochemically two-electron oxidized into [1](2+) and [2](2+). The cations were identified as Co(III)-phenoxyl species by their characteristic absorption band at ca. 400 nm in the UV-vis spectrum. Coordination of the phenoxyl radical to the cobalt(III) metal ion is evidenced by the EPR signal centered at g = 2.00.

Characterization of a Dinuclear Copper(II) Complex and Its Fleeting Mixed-Valent Copper(II)/Copper(III) Counterpart.

The synthesis of a dinuclear copper(II) complex, supported by a 1,3-diamino-2-propanol-based tetraamide ligand, is reported. Structural properties in the solid state and in solution, by means of XRD analysis and NMR spectroscopy, respectively, provide evidence of a highly flexible complex that can display several conformations, leading to the image of the wings of a butterfly. The complex was fully characterized and the redox properties were investigated. Room-temperature spectro-electrochemistry was used to monitor the formation of a metastable mono-oxidized product that displayed an absorption band centered at λ=463 nm. EPR investigation of the low-temperature, chemically generated, mono-oxidized product reveals the presence of an intermediate described as a mixed-valent CuII CuIII species, which is a model of the possible highly oxidizing intermediate in particulate methane monooxygenase.

Nickel(ii) radical complexes of thiosemicarbazone ligands appended by salicylidene, aminophenol and aminothiophenol moieties.

The nickel(ii) complexes of three unsymmetrical thiosemicarbazone-based ligands featuring a sterically hindered salicylidene (1), aminophenol (2) or thiophenol (3) moiety were synthesized and structurally characterized. The metal ion lies in an almost square planar geometry in all the complexes. The cyclic voltammetry (CV) curve of 1 shows an irreversible oxidation wave at E = 0.49 V, which is assigned to the phenoxyl/phenolate redox couple. The CV curves of 2 and 3 display a reversible one-electron oxidation wave (E1/2 = 0.26 and 0.22 V vs. Fc(+)/Fc, respectively) and an one-electron reduction wave (E1/2 = -1.55 and -1.46 V, respectively). The cations 2(+) and 3(+) as well as the anions 2(-) and 3(-) were generated. The EPR spectra of the cations in THF show a rhombic signal at g1 = 2.034, g2 = 2.010 and g3 = 1.992 (2(+)) and g1 = 2.069, g2 = 2.018, g3 = 1.986 (3(+)) that is consistent with a main radical character of the complexes. The difference in anisotropy is assigned to the different nature of the radical, iminosemiquinonate vs. iminothiosemiquinonate. The anions display an isotropic EPR signal at giso = 2.003 (2(+)) and 2.006 (3(+)), which is indicative of a main α-diimine radical character of the compounds. Both the anions and cations exhibit charge transfer transitions of low to moderate intensity in their visible spectrum. Quantum chemical calculations (B3LYP) reproduce both the g-values and Vis-NIR spectra of the complexes. The radical anions readily react with dioxygen to give the radical cations. 2(+) catalyzes the aerobic oxidation of benzyl alcohol into benzaldehyde.

Characterization of one-electron oxidized copper(II)-salophen-type complexes; effects of electronic and geometrical structures on reactivities.

One-electron oxidized salophen-type complexes, [Cu(salophen)](+) (H2salophen = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diaminobenzene), and its methoxy derivatives, [Cu(MeO-salophen)](+) and [Cu(salophen-(MeO)2)](+) (H2MeO-salophen = N,N'-bis(3-tert-butyl-5-methoxysalicylidene)-1,2-diaminobenzene, H2salophen-(MeO)2 = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-diamino-4,5-dimethoxybenzene), have been synthesized and structurally characterized, and their reactivities have been investigated. The solid state structures of the one-electron oxidized forms of these complexes suggested that [Cu(salophen)](+) and [Cu(MeO-salophen)](+) can be assigned to relatively localized Cu(ii)-phenoxyl radical complexes, while [Cu(salophen-(MeO)2)](+) is the diiminobenzene radical complex. On the other hand, [Cu(salophen)](+) in solution showed a different electronic structure from that of the solid sample, the radical electron being delocalized over the whole π-conjugated ligand. The reaction of these oxidized complexes with benzyl alcohol has been investigated in the presence of a large excess of substrate, which revealed the difference in the kinetic behavior between the complexes. The mechanisms of the oxidation have been discussed on the basis of the electronic and geometrical structures and the reaction kinetics.

A singlet ground state for a cobalt(II)-anilinosalen radical complex.

The cobalt(II) anilinosalen complex [Co(II)(L)] was prepared and subsequently oxidized by one electron. The resulting cation comprises a square planar low spin Co(II) ion anti-ferromagnetically exchange coupled to an anilinyl radical.

Unsymmetrical one-electron oxidized Ni(II)-bis(salicylidene) complexes: a protonation-induced shift of the oxidation site.

The spin density in the nickel(II) radical salen complex (2*)(+) could be localized on a specific ring by controlling the acidity of the medium.