Thomas Weyhermüller

Aerobic Oxidation of Primary Alcohols by a New Mononuclear CuII-Radical Catalyst

Primary alcohols such as ethanol or benzyl alcohol are selectively and catalytically oxidized by the mononuclear copper(II) radical complex 1-a functional model of the metalloenzyme galactose oxidase-with oxygen from air at 20°C to give the corresponding aldehydes and H2 O2 in about 60 % yield.

Neutral Bis(α-iminopyridine)metal Complexes of the First-Row Transition Ions (Cr, Mn, Fe, Co, Ni, Zn) and Their Monocationic Analogues: Mixed Valency Involving a Redox Noninnocent Ligand System

A series of bis(alpha-iminopyridine)metal complexes featuring the first-row transition ions (Cr, Mn, Fe, Co, Ni, and Zn) is presented. It is shown that these ligands are redox noninnocent and their paramagnetic pi radical monoanionic forms can exist in coordination complexes. Based on spectroscopic and structural characterizations, the neutral complexes are best described as possessing a divalent metal center and two monoanionic pi radicals of the alpha-iminopyridine. The neutral M(L*)2 compounds undergo ligand-centered, one-electron oxidations generating a second series, [(L(x))2M(THF)][B(ArF)4] [where L(x) represents either the neutral alpha-iminopyridine (L)0 and/or its reduced pi radical anion (L*)-]. The cationic series comprise mostly mixed-valent complexes, wherein the two ligands have formally different redox states, (L)0 and (L*)-, and the two ligands may be electronically linked by the bridging metal atom. Experimentally, the cationic Fe and Co complexes exhibit Robin-Day Class III behavior (fully delocalized), whereas the cationic Zn, Cr, and Mn complexes belong to Class I (localized) as shown by X-ray crystallography and UV-vis spectroscopy. The delocalization versus localization of the ligand radical is determined only by the nature of the metal linker. The cationic nickel complex is exceptional in this series in that it does not exhibit any ligand mixed valency. Instead, its electronic structure is consistent with two neutral ligands (L)0 and a monovalent metal center or [(L)2Ni(THF)][B(ArF)4]. Finally, an unusual spin equilibrium for Fe(II), between high spin and intermediate spin (S(Fe) = 2 <--> S(Fe) = 1), is described for the complex [(L*)(L)Fe(THF)][B(ArF)4], which consequently is characterized by the overall spin equilibrium (S(tot) = 3/2 <--> S(tot) = 1/2). The two different spin states for Fe(II) have been characterized using variable temperature X-ray crystallography, EPR spectroscopy, zero-field and applied-field Mössbauer spectroscopy, and magnetic susceptibility measurements. Complementary DFT studies of all the complexes have been performed, and the calculations support the proposed electronic structures.

X-ray Emission Spectroscopy to Study Ligand Valence Orbitals in Mn Coordination Complexes

We discuss a spectroscopic method to determine the character of chemical bonding and for the identification of metal ligands in coordination and bioinorganic chemistry. It is based on the analysis of satellite lines in X-ray emission spectra that arise from transitions between valence orbitals and the metal ion 1s level (valence-to-core XES). The spectra, in connection with calculations based on density functional theory (DFT), provide information that is complementary to other spectroscopic techniques, in particular X-ray absorption (XANES and EXAFS). The spectral shape is sensitive to protonation of ligands and allows ligands, which differ only slightly in atomic number (e.g., C, N, O...), to be distinguished. A theoretical discussion of the main spectral features is presented in terms of molecular orbitals for a series of Mn model systems: [Mn(H(2)O)(6)](2+), [Mn(H(2)O)(5)OH](+), and [Mn(H(2)O)(5)NH(3)](2+). An application of the method, with comparison between theory and experiment, is presented for the solvated Mn(2+) ion in water and three Mn coordination complexes, namely [LMn(acac)N(3)]BPh(4), [LMn(B(2)O(3)Ph(2))(ClO(4))], and [LMn(acac)N]BPh(4), where L represents 1,4,7-trimethyl-1,4,7-triazacyclononane, acac stands for the 2,4-pentanedionate anion, and B(2)O(3)Ph(2) represents the 1,3-diphenyl-1,3-dibora-2-oxapropane-1,3-diolato dianion.

Reduced N-alkyl substituted bis(imino)pyridine cobalt complexes: molecular and electronic structures for compounds varying by three oxidation states.

The stepwise 1-3 electron reduction of the N-alkyl substituted bis(imino)pyridine cobalt dichloride complexes, ((R)APDI)CoCl(2), was studied where (R)APDI = 2,6-(RN=CMe)(2)C(5)H(3)N, R = C(6)H(11) (Cy), CHMe(2) ((i)Pr). One electron reduction with either zinc metal or NaBEt(3)H furnished the bis(imino)pyridine cobalt monochloride compounds, ((R)APDI)CoCl. X-ray diffraction on the ((iPr)APDI)CoCl derivative established a distortion from square planar geometry where the chloride ligand is lifted out of the idealized cobalt-chelate plane. Superconducting Quantum Interference Device (SQUID) magnetometry on both compounds established spin crossover behavior with an S = 1 state being predominant at room temperature. Computational studies, in combination with experimental results, establish that the triplet spin isomer arises from a high spin Co(II) center (S(Co) = 3/2) antiferromagnetically coupled to a bis(imino)pyridine chelate radical anion, [PDI](-) (S(PDI) = 1/2). At lower temperatures, the Co(II) ion undergoes a spin transition to the low spin form (S(Co) = 1/2) and antiferromagnetic coupling gives rise to the observed diamagnetic ground state. Replacing the chloride ligand with a methyl group, namely ((R)APDI)CoCH(3), also yielded distorted compounds, albeit less pronounced, that are diamagnetic at room temperature. Two electron reduction of the ((R)APDI)CoCl(2) derivatives with excess 0.5% sodium amalgam or 2 equiv of NaBEt(3)H furnished the bis(chelate)cobalt complexes, ((R)APDI)(2)Co, while three electron reduction with 3 equiv of sodium naphthalenide yielded the cobalt dinitrogen anions, [Na(solv)(3)][((R)APDI)CoN(2)] (solv = THF, Et(2)O). Both bis(chelate) compounds were crystallographically characterized and determined to have S = 3/2 ground states by SQUID magnetometry and electron paramagnetic resonance (EPR) spectroscopy. Computational studies, in combination with metrical parameters determined from X-ray diffraction, establish a high spin (S(Co) = 3/2) cobalt(II) center with two bis(imino)pyridine chelate radical anions. Antiferromagnetic coupling between the two chelate centered radicals is mediated by a doubly occupied t(2g) cobalt orbital and gives rise to the observed overall quartet ground state.

Biomimetic metal-radical reactivity: aerial oxidation of alcohols, amines, aminophenols and catechols catalyzed by transition metal complexes.

Abstract The contributions of the authors to the research program ‘Radicals in Enzymatic Catalysis’ over the last ca. 5 years are summarized. Significant efforts were directed towards the design and testing of phenol-containing ligands for synthesizing radical-containing transition metal complexes as potential candidates for catalysis of organic substrates like alcohols, amines, aminophenols and catechols. Functional models for different copper oxidases, such as galactose oxidase, amine oxidases, phenoxazinone synthase and catechol oxidase, are reported. The copper complexes synthesized can mimic the function of the metalloenzymes galactose oxidase and amine oxidases by catalyzing the aerial oxidation of alcohols and amines. Even methanol could be oxidized, albeit with a low conversion, by a biradical-copper(II) compound. The presence of a primary kinetic isotope effect, similar to that for galactose oxidase, provides compelling evidence that H-atom abstraction from the α-C-atom of the substrates is the rate-limiting step. Although catechol oxidase and phenoxazinone synthase contain copper, manganese(IV) complexes containing radicals have been found to be useful to study synthetic systems and to understand the naturally occurring processes. An ‘on-off’ mechanism of the radicals without redox participation from the metal centers seems to be operative in the catalysis involving such metal-radical complexes.

Why Does the Active Form of Galactose Oxidase Possess a Diamagnetic Ground State

The relative orientation of the two magnetic orbitals, the CuII d x 2-y 2 orbital and the half-occupied π orbital of the tyrosyl radical, is the key to answering the question in the title. The arrangement shown (CuII -O-C bond angle of about 130° and a dihedral angle of about 90° between the x,y plane of the CuII polyhedron and the tyrosyl ring plane) leads to an overlap of the orbitals, which results in a singlet ground state.

Phenoxyl radical complexes of chromium(III), manganese(III), cobalt(III), and nickel(II)

Abstract The tetradentate monoanionic macrocycles 1,4-dimethyl-7-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane, H(L1), and 1,4-di-iso-propyl-7-(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane, H(L2), form in the presence of a bidentate coligand 1,3-diphenyl-1,3-propanedionate, Ph2acac, 1,3-dimethyl-1,3-propanedionate, acac, or 1,3-di-tert-butyl-1,3-dionate, Bu2acac−, very stable octahedral complexes with di- or trivalent metal ions: [MIII(L1)(Ph2acac)](ClO4) M=CoIII (1), CrIII (3), MnIII (7), GaIII (11); [MII(L1)(Ph2acac)] M=NiII (9); [MIII(L2)(Bu2acac)](ClO4) M=CoIII (2), CrIII (4), MnIII (8), GaIII (12), [MII(L1)(Bu2acac)] M=NiII (10); [CrIII(L2)(acac)](ClO4) (5), [CrIII(L2)(C2O4)] (6). All of these monophenolatometal complexes can be electrochemically, reversibly one-electron oxidized yielding stable phenoxyl radical complexes in solution. Electronic, resonance Raman and EPR spectra prove that the phenoxyl ligand is coordinated to the corresponding metal ion. The electronic ground states of complexes containing a paramagnetic transition metal ion with dn electron configuration and a bound phenoxyl radical (S=1/2) are attained via an intramolecular anti- or ferromagnetic exchange coupling mechanism the nature of which depends on the actual dn configuration of the central metal ion: a half-filled t2g subshell allows antiferromagnetic coupling, for example [3] 2+, [4] 2+, [5] 2+ and [6] + have an St=1 ground state. In contrast, unpaired electrons in an eg subshell enforce a ferromagnetic coupling: [9] 2+, [10] 2+ have an St=3/2 ground state. The crystal structures of 1, 5, and 9 are reported.

Phenoxyl-copper(II) complexes: models for the active site of galactose oxidase

Abstract The reaction of the macrocycles 1,4,7-tris (3,5-di-tert-butyl-2-hydroxy-benzyl)-1,4,7-triazacyclononane, L1H3, or 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxy-benzyl)-1,4,7-triazacyclononane, L2H3, with Cu(ClO4)2·6H2O in methanol (in the presence of Et3N) affords the green complexes [CuII(L1H)] (1), [CuII(L2H)]·CH3OH (2) and (in the presence of HClO4) [CuII(L1H2)](ClO4) (3) and [CuII(L2H2)] (ClO4) (4). The CuII ions in these complexes are five-coordinate (square-base pyramidal), and each contains a dangling, uncoordinated pendent arm (phenol). Complexes 1 and 2 contain two equatorially coordinated phenolato ligands, whereas in 3 and 4 one of these is protonated, affording a coordinated phenol. Electrochemically, these complexes can be oxidized by one electron, generating the phenoxyl-copper(II) species [CuII(L1H)]+ ·, [Cu(L2H)]+ ·, [CuII(L1H2)]2+ ·, and [CuII(L2H2)]2+ ·, all of which are EPR-silent. These species are excellent models for the active form of the enzyme galactose oxidase (GO). Their spectroscopic features (UV-VIS, resonance Raman) are very similar to those reported for GO and unambiguously show that the complexes are phenoxyl-copper(II) rather than phenolato-copper(III) species.

Synthesis, characterization and DNA binding studies of new ruthenium(II)bisterpyridine complexes

Two new ruthenium(II) complexes, [Ru(itpy)2](PF6)2, 1 and [Ru(bitpy)2](PF6)2 2, were synthesized and characterized by ESI-Mass, UV-Visible, 1H NMR, fluorescence spectroscopy and cyclic voltammetry. Complex 1 has been characterized crystallographically. Interaction of these complexes with CT-DNA has been studied using absorption and CD spectra. Absorption spectral titration and CD spectral measurements show that complex 1 binds with DNA through intercalation. Complex 2 on the other hand shows dual mode of binding to DNA, groove binding as well as intercalation. Photo nuclease activity of these complexes has been studied using agarose gel electrophoresis and both the complexes have been shown to exhibit photonuclease activity. However, complex 1 has been found to show higher DNA cleaving efficiency compared to complex 2.

Electronic Structure of the [Tris(dithiolene)chromium]z(z=0,1―, 2―, 3―) Electron Transfer Series and Their Manganese(IV) Analogues. An X-ray Absorption Spectroscopic and Density Functional Theoretical Study

From the reaction mixture of 3,6-dichlorobenzene-1,2-dithiol, H(2)(Cl(2)-bdt), [CrCl(3)(thf)(3)], and NEt(3) in tetrahydrofuran (thf) in the presence of air, dark green crystals of [N(n-Bu)(4)](2)[Cr(Cl(2)-bdt)(3)] (S = 1) (1) were isolated upon addition of [N(n-Bu)(4)]Br. Oxidation of the AsPh(4)(+) salt of 1 with [Fc]PF(6) yielded microcrystals of [AsPh(4)][Cr(Cl(2)-bdt)(3)] (S = (1)/(2)) (2) whereas the reduction of 1 with sodium amalgam produced light green crystals of [N-(n-Bu)(4)](3)[Cr(Cl(2)-bdt)(3)].thf (S = (3)/(2)) (3). The corresponding maleonitriledithiolato complexes [PPh(4)](2)[Cr(mnt)(3)] (S = 1) (4) and [PPh(4)](3)[Cr(mnt)(3)] (S = (3)/(2)) (5) have been synthesized. Isoelectronic manganese complexes of 3 and 5, namely, [NEt(4)](2)[Mn(Cl(2)-bdt)(3)] (S = (3)/(2)) (6) and [PPh(4)](2)[Mn(mnt)(3)] (S = (3)/(2)) (7), have also been prepared. Complexes 1, 6, and 7 have been characterized by single crystal X-ray crystallography. Complexes 1-7 have been electrochemically studied and their UV-vis and electron paramagnetic resonance spectra (EPR) have been recorded; magnetic properties have been elucidated by temperature-dependent susceptibility measurements. It is shown by chromium K-edge and sulfur K-edge X-ray absorption spectroscopy (XAS) that the oxidation state of the central Cr ion in each compound is the same (+III, d(3)) and that all one-electron redox processes are ligand-based, involving one, two, or three ligand pi radical monoanions. Complexes 6 and 7 possess a Mn(IV) ion with three dianionic ligands. The results have been corroborated by broken symmetry (BS) density functional theoretical (DFT) calculations by using the B3LYP functional. Time-dependent DFT calculations have been performed to calculate the metal and sulfur K-pre-edges. It is suggested that the neutral complexes [Cr(dithiolene)(3)](0) S = 0 possess octahedral rather than trigonal prismatic CrS(6) polyhedra. Three ligand pi radicals (S(rad) = (1)/(2)) couple antiferromagnetically to the central Cr(III) ion (d(3)) yielding the observed diamagnetic ground state. It is established that the four members of the [Cr(dithiolene)(3)](z) (z = 0, 1-, 2-, 3-) electron transfer series are related by ligand-based one-electron transfer processes; for each of the four members it is shown that they contain a central Cr(III) (d(3)) ion, and the CrS(6) polyhedron is a (distorted) octahedron.