Axel Knödler

Copper complexes with non-innocent ligands: probing CuII/catecholato-CuI/o-semiquinonato redox isomer equilibria with EPR spectroscopy

Abstract The temperature-dependent intramolecular metal/ligand electron transfer equilibrium (L)Cu II (Q 2− )⇌(L)Cu I (Q − ) has been quantitatively analyzed by EPR as a function of the non-innocent o -quinonoid ligand Q and of the co-ligand L. Suitable co-ligands were 1-methyl-(2-methylthiomethyl)-1 H -benzimidazole and 1-diphenylphosphino-2-dimethylaminoethane (dde) which can tolerate both copper oxidation states. EPR hyperfine data were extracted for a variety of alkyl- and methoxy-substituted o -semiquinones and their copper(I) complexes in solution. A close match of Q and Cu redox orbitals is essential for observing the redox isomer equilibrium which has been similarly reported for aminoxidase enzymes. Efforts to isolate the mixed-ligand compounds from solution failed due to coordination disproportionation. The new [Cu(dde) 2 ](ClO 4 ) was structurally characterized with comparable CuN and CuP distances of 2.24–2.27 A and a PCuP angle of 142.41(5) o , the ‘2+2’ coordination pattern could be reproduced by DFT calculations.

Rhenium(I) coordinated lumazine and pterin derivatives: structure and spectroelectrochemistry of reversibly reducible (6-ATML)Re(CO)3Cl (6-ATML=6-acetyl-1,3,7-trimethyllumazine)

Abstract A number of complexes between substituted lumazines (2,4-dioxo-(1H,3H)pteridines) or pterins (2-amino-4-oxo-(3H)pteridines) and Re(CO)3Cl have been synthesized and characterized electrochemically and spectroscopically. The structure of the (6-ATML)Re(CO)3Cl derivative (6-ATML=6-acetyl-1,3,7-trimethyllumazine) could be determined in [fac-(6-ATML)Re(CO)3Cl]·3C6H6. The rhenium(I) center coordinates in a rather symmetrical fashion through the O4–N5 α-carbonylimino chelate site of 6-ATML; the potentially available acetyl function is not involved in the metal coordination. The acetyl acceptor substituent facilitates the reversible one-electron reduction of this compound to a persistent anion radical complex [(6-ATML)Re(CO)3Cl] −, which is accompanied by variable low-frequency shifts of the six carbonyl stretching bands and by the appearance of a broad EPR signal.

Tripropylarsane Complexes of Palladium(II) and Platinum(II) — Syntheses, Spectroscopy, and Structures

Several palladium(II) and platinum(II) complexes of tripropylarsanes (AsR3; R = Pr, iPr) with the formulae, [MCl2(AsR3)2], [M2Cl2(μ-Cl)2(AsR3)2], [Pd2Me2(μ-Cl)2(AsR3)2], [Pd2X2(μ-Pz)2(AsR3)2] (X = Cl or Me, Pz = pyrazolate), [Pd2Cl2(μ-Y)2(AsR3)2] (Y = OAc or SPh), [MCl(S2CNEt2)(AsR3)] and [PdCp(Cl)(AsiPr3)] (M = Pd or Pt) have been prepared. All the complexes have been characterised by elemental analyses, IR and 1H NMR spectroscopy. The stereochemistry of the complexes has been deduced from the spectroscopic data. The structures of [Pd2Me2(μ-X)2(AsiPr3)2] (X = Cl or Pz) have been established by single crystal X-ray diffraction analyses. Both of the complexes have sym-trans configuration. Strong trans influence of the methyl group is reflected on the Pd—X bond distances. Tripropylarsan-Komplexe von Palladium(II) und Platin(II) — Synthesen, Spektroskopie und Strukturen Eine Reihe von Komplexen des Palladium(II) und Platin(II) mit den Tripropylarsanen AsR3 (R = Pr, iPr) in den Zusammensetzungen [MCl2(AsR3)2], [M2Cl2(μ-Cl)2(AsR3)2], [Pd2Me2(μ-Cl)2(AsR3)2], [Pd2X2(μ-Pz)2(AsR3)2] (X = Cl oder Me, Pz = Pyrazolat), [Pd2Cl2(μ-Y)2(AsR3)2] (Y = OAc oder SPh), [MCl(S2CNEt2)(AsR3)] und [PdCp(Cl)(AsiPr3)] (M = Pd oder Pt) wurde hergestellt und elementaranalytisch sowie IR- und 1H-NMR-spektroskopisch charakterisiert. Die Stereochemie der Komplexe konnte den spektroskopischen Daten entnommen werden. Die Strukturen von [Pd2Me2(μ-X)2(AsiPr3)2] (X = Cl oder Pz) wurden kristallstrukturanalytisch bestimmt. Beide Komplexe weisen eine sym-trans-Konfiguration auf. Der starke trans-Einflus der Methylgruppen spiegelt sich in den Pd-X-Bindungslangen wider.

A copper(I) dimer [Cu(L)(CH3CN)]2(BF4)2 with the new chelating and bridging tridentate ligand 1-(2-pyridylmethyl)-2-(2-pyridyl)benzimidazole (L)

Abstract Reaction of 1,2-phenylenediamine with 2-pyridinecarboxaldehyde gave the new compound 1-(2-pyridylmethyl)-2-(2-pyridyl)benzimidazole (L) which reacted with [Cu(CH3CN)4](BF4) to form the title complex. Although the imine/2-(2-pyridyl) (‘α-diimine’) coordination setting in L appears well suited for forming a five-membered chelate ring, the crystal structure analysis of the bis(methanol) solvate revealed that the system rather opts for the formation of a partially saturated eight-membered chelate ring involving both pyridyl groups with a twist angle of 61.8° between the benzimidazole and 2-(2-pyridyl) moieties. In addition, L acts as a tridentate ligand, effecting dimerization of two chelate rings through the imine nitrogen centers of the imidazole groups. This dimerization gives rise to a ten-membered dimetalla ring. Tetracoordination at copper(I) is complemented by acetonitrile. The preference for this structure is attributed to the formation of unstrained metal centers with N–Cu–N angles between 105° and 118° and Cu–N distances of 2.03–2.05 A.

Dimethylplatinum(II) and tetramethylplatinum(IV) complexes of 1-methyl-(2-alkylthiomethyl)-1H-benzimidazoles: experimental and DFT-calculated structures and NMR spectra

Abstract The compounds (L)PtMen where n=2 or 4 and L=mmb (1-methyl-(2-methylthiomethyl)-1H-benzimidazole) or L=mtb (1-methyl-(2-tert-butylthiomethyl)-1H-benzimidazole) were characterized by X-ray crystallography (except for (mtb)PtMe2) and by 1H- and 195Pt-NMR spectroscopy. The tetramethylplatinum(IV) complexes exhibit a variable degree of dynamic 1H-NMR behavior due to the mobility at the thioether sulfur atom in the non-planar five-membered chelate ring, as supported by structural analysis. Density-functional theory (DFT) calculations were used to reproduce the structural features and the 1H-NMR chemical shifts. In comparison with other late transition metal complexes of these NS chelate ligands the Me4Pt and especially the Me2Pt compounds exhibit a relatively stronger preference of the metal for the sulfur donor.

Experimental and DFT calculated structure of the diruthenium(2.5) complex [(Me3TACN)Ru(μ‐Cl)3Ru(Me3TACN)](PF6)2

The diruthenium(2.5) complex [(Me 3 TACN)Ru(μ-Cl) 3 Ru(Me 3 TACN)]-(PF 6 ) 2 , Me 3 TACN = 1,4,7-trimethyl-1,4,7-triazacyclononane, has been crystallized for structural characterization. The results are reproduced by density functional theory (DFT) calculations and confirm the sensitivity of the central Ru(μ-Cl) 3 Ru core to contacts between the Cl bridging atoms and the co-ligands. The singly occupied MO is characterized as a σ* MO involving the metal d z 2 orbitals and a small halide contribution by DFT calculations and EPR.

Coordination ambivalence of the electroactive bis-chelate ligand bis(1-methyl-2-imidazolyl)glyoxal (BIG) in mononuclear and dinuclear complexes with Re(CO)3Cl

Bis(1-methyl-2-imidazolyl)glyoxal (BIG) is reversibly reduced to an EPR, IR and UV-VIS spectroelectrochemically detectable α-diketyl radical anion. As a ligand BIG can coordinate through the carbonyl oxygen centres O and O′ and through the imine nitrogen atoms N and N′ of the imidazolyl rings. For fac-(BIG)Re(CO)3Cl metal coordination occurs via both imine nitrogen centres, leading to a non-planar seven-membered chelate ring with a free α-dicarbonyl moiety as evident from X-ray structural analysis. For dinuclear (μ-BIG)[fac-Re(CO)3Cl]2 three essentially different structural coordination alternatives A–C are possible in a bis-chelate situation: Separate N,N′- and O,O′-coordination with one seven-membered and one five-membered chelate ring (A), N,O′- and N′,O-coordination with two edge-sharing six-membered chelate rings (B), or N,O- and N′,O′-coordination with two five-membered chelate rings and free rotation around the C(O)–C′(O′) single bond. Spectroscopic, electrochemical and spectroelectrochemical results obtained for the mono- and di-nuclear complexes suggest the π-conjugated structure alternative B for (μ-BIG)[fac-Re(CO)3Cl]2.