Naina Deibel

Paramagnetic Palladacycles with PdIII Centers Are Highly Active Catalysts for Asymmetric Aza-Claisen Rearrangements

A combination of spectroscopic and electrochemical methods--XANES, EXAFS, X-ray, (1)H NMR, EPR, Mössbauer, and cyclic voltammetry--demonstrate that the most efficient Pd catalysts for the asymmetric rearrangement of allylic trifluoroacetimidates unexpectedly possess in the activated oxidized form a Pd(III) center bound to a ferrocene core which remains unchanged (Fe(II)) during the oxidative activation. These are the first recognized Pd(III) complexes acting as enantioselective catalysts.

Redox-Induced Spin-State Switching and Mixed Valency in Quinonoid-Bridged Dicobalt Complexes

The complexes [{(tmpa)Co(II) }2 (μ-L(1) )(2-) ](2+) (1(2+) ) and [{(tmpa)Co(II) }2 (μ-L(2) )(2-) ](2+) (2(2+) ), with tmpa=tris(2-pyridylmethyl)amine, H2 L(1) =2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, and H2 L(2) =2,5-di-[2-(trifluoromethyl)-anilino]-1,4-benzoquinone, were synthesized and characterized. Structural analysis of 2(2+) revealed a distorted octahedral coordination around the cobalt centers, and cobalt-ligand bond lengths that match with high-spin Co(II) centers. Superconducting quantum interference device (SQUID) magnetometric studies on 1(2+) and 2(2+) are consistent with the presence of two weakly exchange-coupled high-spin cobalt(II) ions, for which the nature of the coupling appears to depend on the substituents on the bridging ligand, being antiferromagnetic for 1(2+) and ferromagnetic for 2(2+) . Both complexes exhibit several one-electron redox steps, and these were investigated with cyclic voltammetry and UV/Vis/near-IR spectroelectrochemistry. For 1(2+) , it was possible to chemically isolate the pure forms of both the one-electron oxidized mixed-valent 1(3+) and the two-electron oxidized isovalent 1(4+) forms, and characterize them structurally as well as magnetically. This series thus provided an opportunity to investigate the effect of reversible electron transfers on the total spin-state of the molecule. In contrast to 2(2+) , for 1(4+) the metal-ligand distances and the distances within the quinonoid ligand point to the existence of two low-spin Co(III) centers, thus showing the innocence of the quintessential non-innocent ligands L. Magnetic data corroborate these observations by showing the decrease of the magnetic moment by roughly half (neglecting spin exchange effects) on oxidizing the molecules with one electron, and the disappearance of a paramagnetic response upon two-electron oxidation, which confirms the change in spin state associated with the electron-transfer steps.

Experimental and Theoretical Investigations of the Existence of Cu(II), Cu(III), and Cu(IV) in Copper Corrolato Complexes.

The most common oxidation states of copper in stable complexes are +I and +II. Cu(III) complexes are often considered as intermediates in biological and homogeneous catalysis. More recently, Cu(IV) species have been postulated as possible intermediates in oxidation catalysis. Despite the importance of these higher oxidation states of copper, spectroscopic data for these oxidation states remain scarce, with such information on Cu(IV) complexes being non-existent. We herein present the synthesis and characterization of three copper corrolato complexes. A combination of electrochemistry, UV/Vis/NIR/EPR spectroelectrochemistry, XANES measurements, and DFT calculations points to existence of three distinct redox states in these molecules for which the oxidation states +II, +III, and +IV can be invoked for the copper centers. The present results thus represent the first spectroscopic and theoretical investigation of a Cu(IV) species, and describe a redox series where Cu(II), Cu(III), and Cu(IV) are discussed within the same molecular platform.

Electrochemistry, Chemical Reactivity, and Time-Resolved Infrared Spectroscopy of Donor-Acceptor Systems [(Q(x))Pt(pap(y))) (Q = Substituted o-Quinone or o-Iminoquinone; pap = Phenylazopyridine)

The donor–acceptor complex [(O,NQ2–)Pt(pap0)] (1; pap = phenylazopyridine, O,NQ0 = 4,6-di-tert-butyl-N-phenyl-o-iminobenzoquinone), which displays strong π-bonding interactions and shows strong absorption in the near-IR region, has been investigated with respect to its redox-induced reactivity and electrochemical and excited-state properties. The one-electron-oxidized product [(O,NQ•–)Pt(pap0)](BF4) ([1]BF4) was chemically isolated. Single-crystal X-ray diffraction studies establish the iminosemiquinone form of O,NQ in [1]+. Simulation of the cyclic voltammograms of 1 recorded in the presence of PPh3 elucidates the mechanism and delivers relevant thermodynamic and kinetic parameters for the redox-induced reaction with PPh3. The thermodynamically stable product of this reaction, complex [(O,NQ•–) Pt(PPh3)2](PF6) ([2]PF6), was isolated and characterized by X-ray crystallography, electrochemistry, and electron paramagnetic resonance spectroscopy. Picosecond time-resolved infrared spectroscopic studies on complex 1b (one of the positional isomers of 1) and its analogue [(O,OQ2–)Pt(pap0)] (3; O,OQ = 3,5-di-tert-butyl-o-benzoquinone) provided insight into the excited-state dynamics and revealed that the nature of the lowest excited state in the amidophenolate complex 1b is primarily diimine-ligand-based, while it is predominantly an interligand charge-transfer state in the case of 3. Density functional theory calculations on [1]n+ provided further insight into the nature of the frontier orbitals of various redox forms and vibrational mode assignments. We discuss the mechanistic details of the newly established redox-induced reactivity of 1 with electron donors and propose a mechanism for this process.

Silver corrole complexes: unusual oxidation states and near-IR-absorbing dyes.

Macrocycles such as porphyrins and corroles have important functions in chemistry and biology, including light absorption for photosynthesis. Generation of near-IR (NIR)-absorbing dyes based on metal complexes of these macrocycles for mimicking natural photosynthesis still remains a challenging task. Herein, the syntheses of four new Ag(III) corrolato complexes with differently substituted corrolato ligands are presented. A combination of structural, electrochemical, UV/Vis/NIR-EPR spectroelectrochemical, and DFT studies was used to decipher the geometric and electronic properties of these complexes in their various redox states. This combined approach established the neutral compounds as stable Ag(III) complexes, and the one-electron reduced species of all the compounds as unusual, stable Ag(II) complexes. The one-electron oxidized forms of two of the complexes display absorptions in the NIR region, and thus they are rare examples of mononuclear complexes of corroles that absorb in the NIR region. The appearance of this NIR band, which has mixed intraligand charge transfer/intraligand character, is strongly dependent on the substituents of the corrole rings. Hence, the present work revolves round the design principles for the generation of corrole-based NIR-absorbing dyes and shows the potential of corroles for stabilizing unusual metal oxidation states. These findings thus further contribute to the generation of functional metal complexes based on such macrocyclic ligands.

Luminescent Dirhenium(I)-Double-Heterostranded Helicate and Mesocate

The semirigid ligands 1,4-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)benzene (H2-pBC) and 1,3-bis(2-(2-hydroxyphenyl)benzimidazol-1-ylmethyl)-2,4,6-trimethylbenzene (H2-mBC), containing two hydroxyphenylbenzimidazolyl units as bis-chelating (or bis(bidentate)) N∩OH donor, were synthesized and were used to assemble neutral, luminescent heteroleptic, unsaturated double-hetero-stranded, rhenium(I)-based helicate (1) and mesocate (2) with the flexible bis(monodentate) nitrogen donor (1,4-bis(benzimidazol-1-ylmethyl)benzene/1,3-bis(benzimidazol-1-ylmethyl)benzene), and Re2(CO)10. The photophysical properties of the complexes were studied. Both complexes 1 and 2 exhibit dual emissions in both solution and solid state. In solution, these complexes show both fluorescence and phosphorescence. Complex 1 undergoes a predominantly ligand-centered oxidation, resulting in the generation of phenoxyl radicals.

Three‐Way Cooperativity in d8 Metal Complexes with Ligands Displaying Chemical and Redox Non‐Innocence

Reversible proton- and electron-transfer steps are crucial for various chemical transformations. The electron-reservoir behavior of redox non-innocent ligands and the proton-reservoir behavior of chemically non-innocent ligands can be cooperatively utilized for substrate bond activation. Although site-decoupled proton- and electron-transfer steps are often found in enzymatic systems, generating model metal complexes with these properties remains challenging. To tackle this issue, we present herein complexes [(cod-H)M(μ-L(2-)) M(cod-H)] (M = Pt(II), [1] or Pd(II), [2], cod = 1,5-cyclooctadiene, H2L = 2,5-di-[2,6-(diisopropyl)anilino]-1,4-benzoquinone), in which cod acts as a proton reservoir, and L(2-) as an electron reservoir. Protonation of [2] leads to an unusual tetranuclear complex. However, [1] can be stepwise reversibly protonated with up to two protons on the cod-H ligands, and the protonated forms can be stepwise reversibly reduced with up to two electrons on the L(2-) ligand. The doubly protonated form of [1] is also shown to react with OMe(-) leading to an activation of the cod ligands. The site-decoupled proton and electron reservoir sources work in tandem in a three-way cooperative process that results in the transfer of two electrons and two protons to a substrate leading to its double reduction and protonation. These results will possibly provide new insights into developing catalysts for multiple proton- and electron-transfer reactions by using metal complexes of non-innocent ligands.