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Marion Kerscher

Publications and source records attributed to Marion Kerscher.

6 recordsLinked to original sources
PubMed2005

Coordination chemistry of a new rigid, hexadentate bispidine-based bis(amine)tetrakis(pyridine) ligand.

The hexadentate bispidine-based ligand 2,4-bis(2-pyridyl)-3,7-bis(2-methylenepyridine)-3,7-diazabicyclo[3.3.1]nonane-9-on-1,5-bis(carbonic acid methyl ester), L(6m), with four pyridine and two tertiary amine donors, based on a very rigid diazaadamantane-derived backbone, is coordinated to a range of metal ions. On the basis of experimental and computed structural data, the ligand is predicted to form very stable complexes. Force field calculations indicate that short metal-donor distances lead to a buildup of strain in the ligand; that is, the coordination of large metal ions is preferred. This is confirmed by experimentally determined stability constants, which indicate that, in general, stabilities comparable to those with macrocyclic ligands are obtained with the relative order Cu(2+) > Zn(2+) >> Ni(2+) < Co(2+), which is not the typical Irving-Williams behavior. The preference for large M-N distances also emerges from relatively high redox potentials (the higher oxidation states, that is, the smaller metal ions, are destabilized) and from relatively weak ligand fields (dd-transition, high-spin electronic ground states). The potentiometric titrations confirm the efficient encapsulation of the metal ions since only 1:1 complexes are observed, and, over a large pH range, ML is generally the only species present in solution.

Journal Article↗
PubMed2003

Structural properties of cyclopentanone-bridged bis-macrocyclic ligand dicopper(II) complexes in the solid and in solution: a successful test of the MM-EPR method.

The copper(II)-assisted condensation of 2,3,2-tet (3,7-diazanonane-1,9-diamine) with formaldehyde and cyclopentanone yields the mono- and bis-macrocyclic Mannich condensation products L(1) and L(2), as well as the Schiff-base product L(3), all with cyclam-type tetraaza macrocycles, coordinated to copper(II). The combination of molecular mechanics and EPR spectroscopy (MM-EPR) reveals that all three isomers of [Cu(2)(L(2))(OH(2))(n)](2+) (n = 0-4), with the expected trans-III (R,R,S,S) configuration of the 14-membered tetraaza macrocycles, are of similar stability, and that the isomer whose structure is solved by X-ray crystallography has a different structure in solution.

Journal Article↗
PubMed2002

Structural variation in transition-metal bispidine compounds.

The experimentally determined molecular structures of 40 transition metal complexes with the tetradentate bispyridine-substituted bispidone ligand, 2,4-bis(2-pyridine)-3,7-diazabicyclo[3.3.1]nonane-9-one [M(bisp)XYZ]n+; M = CrIII, MnII, FeII, CoII, CuII, CuI, ZnII; X, Y, Z = mono- or bidentate co-ligands; penta-, hexa- or heptacoordinate complexes) are characterized in detail, supported by force-field and DFT calculations. While the bispidine ligand is very rigid (N3...N7 distance = 2.933 +/- 0.025 A), it tolerates a large range of metal-donor bond lengths (2.07 A < sigma(M-N)/4 < 2.35 A). Of particular interest is the ratio of the bond lengths between the metal center and the two tertiary amine donors (0.84 A < M-N3/M-N7 < 1.05 A) and the fact that, in terms of this ratio there seem to be two clusters with M-N3 < M-N7 and M-N3 > or = M-N7. Calculations indicate that the two structural types are close to degenerate, and the structural form therefore depends on the metal ion, the number and type of co-ligands, as well as structural variations of the bispidine ligand backbone. Tuning of the structures is of importance since the structurally differing complexes have very different stabilities and reactivities.

Journal Article↗
PubMed2002

Copper-bispidine coordination chemistry: syntheses, structures, solution properties, and oxygenation reactivity.

Copper(I) and copper(II) complexes of two mononucleating and four dinucleating tetradentate ligands with a bispidine backbone (2,4-substituted (2-pyridyl or 4-methyl-2-pyridyl) 3,7-diazabicyclo[3.3.1]nonanone) have been prepared and analyzed structurally, spectroscopically, and electrochemically. The structures of the copper chromophores are square pyramidal, except for two copper(I) compounds which are four-coordinate with one noncoordinated pyridine. The other copper(I) structures have the two pyridine donors, the co-ligand (NCCH(3)), and one of the tertiary amines (N3) in-plane with the copper center and the other amine (N7) coordinated axially (Cu-N3 > Cu-N7, approximately 2.25 A vs 2.20 A). The copper(II) compounds with pyridine donors have a similar structure, but the axial amine has a weaker bond to the copper(II) center (Cu-N3 < Cu-N7, approximately 2.03 A vs 2.30 A). The structures with methylated pyridine donors are also square pyramidal with the co-ligands (Cl(-) or NCCH(3)) in-plane. With NCCH(3) the same structural type as for the other copper(II) complexes is observed, and with the bulkier Cl(-) the co-ligand is trans to N7, leading to a square pyramidal structure with the pyridine donors rotated out of the basal plane and only a small difference between axial and in-plane amines (2.15, 2.12 A). These structural differences, enforced by the rigid bispidine backbone, lead to large variations in spectroscopic and electrochemical properties and reactivities. Oxygenation of the copper(I) complexes with pyridine-substituted bispidine ligands leads to relatively stable mu-peroxo-dicopper(II) complexes; with a preorganization of the dicopper chromophores, by linking the two donor sets, these peroxo compounds are stable at room temperature for up to 1 h. The stabilization of the peroxo complexes is to a large extent attributed to the square pyramidal coordination geometry with the substrate bound in the basal plane, a structural motif enforced by the rigid bispidine backbone. The stabilities and structural properties are also seen to correlate with the spectroscopic (UV-vis and Raman) and electrochemical properties.

Chemical Phenomena↗