Macrocylic tetraamido-N ligands that stabilize high valent complexes of chromium, manganese, iron, cobalt, nickel, and copper

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. High valent middle and later transition metal centers tend to oxidatively degrade their ligands. A series of ligand structural features that prevent discovered decomposition routes is presented. The result of the iterative design, synthesis, and testing process described are the macrocyclic tetraamides H4MAC* and H4DEMAMPA-DCB. H4MAC* and H4DEMAMPA-DCB are the parent acids of the macrocyclic tetraamido-N ligands [...] and [...], which are shown to stabilize high valent middle and later transition metal complexes unavailable in other systems. The crystal structures of H4MAC* and a copper complex of one of its synthetic precursors reveal intramolecular and intermolecular hydrogen-bonding patterns which are relevant to recent developments in the ordering effects of hydrogen-bonding on solution and solid state structures. The synthetic value of these ordering effects is discussed. Two chromium(V) oxo complexes, [...] and [...], have been synthesized and characterized by X-ray crystallography and IR and EPR spectroscopies. Because exchange of the oxo ligand with water is slow, the easily synthesized, stable, crystalline [...]O-labeled diperoxide [...], was prepared and used to conveniently synthesize [...]O-labeled oxo complexes in high yields. The bonding of the two unique oxidation resistant macrocyclic tetraamides to chromium is compared. The structural and EPR properties are consistent with a chromium centered radical in each case and suggest that a chromium(V) oxidation state assignment is equally appropriate whether the ancillary ligand is the innocent [...] or the potentially noninnocent [...]. Both oxo complexes contain nonplanar amide groups. The distortions in [...] are more marked, and it is a unique species in containing four distinctly nonplanar amides. The synthesis and characterization of the first water-stable Mn(V) monooxo complex is described. [...] has been isolated as a green crystalline material and characterization includes [...] NMR, IR, resonance Raman, and an X-ray crystal structure determination. The first definitive assignment of a [...] IR stretch [...] is made. The first mononuclear five-coordinate Fe(IV) complex to be isolated as a crystalline compound and structurally characterized, [...], is described. The species has a square pyramidal structure. Cyclic voltammetry of [...] shows a reversible [...] couple at [...] vs. [...] with [...] (0.1 M) as supporting electrolyte. The Mossbauer spectrum of [...] obtained at 150 K has [...] and an isomer shift, [...], lending film support to the iron(IV) oxidation state assignment. Data on [...] are provided for comparison. The synthesis and characterization of the first macrocyclic square planar Co(III) complex is described. [...] has been isolated as a red crystalline material and characterization includes a [...] spectrum of the lithium salt and an X-ray crystal structure determination of the [...] salt. The cobalt complex contains one significantly nonplanar amido-N ligand where the source of the nonplanarity appears to be a simple mismatch between the geometrical features of the macrocycle and the structural requirements of the square planar metal center, a unique case of amide nonplanarity in inorganic chemistry. The first crystallographically characterized neutral square planar complex of cobalt in an oxidation state higher than [...], [...], is reported. Structural data for this new class of compounds indicate that the macrocycle in [...] is noninnocent; however, EPR data in toluene at 5.9 K [...] show that the metal center is the primary residence site of the unpaired electron. [...] is a stable, yet potent, oxidant which is soluble in benzene and slightly soluble in pentane. The [...] couple is reversible and found at 0.550 V vs. [...] in [...]. [...] slowly oxidizes water, yielding [...], which may also be prepared by the reaction of [...] with [...]. Both the redox and the acid/base chemistries of [...] are reversible. Electrochemical and EPR data are also presented for other derivatives. The first square planar nickel(III) complex with an innocent ligand to be structurally characterized, [...], is described: All four Ni(III)N bond distances are significantly shorter [...] than any known Ni(III)-N bond distance. The average Ni-N distance in [...] is [...] and the shortest of the four is 1.825(4). [...] EPR and UV-vis evidence are presented which indicate that the four-coordinate structure found for the solid state form is also present in solutions of non-coordinating solvents, and in noncoordinating glasses at 4 K. In [...], five-coordinate species can be produced at [...] with cyanide ion [...] = [...], and at 77 K with pyridine, 2,6-lutidine, and [...], but not with [...], [...], THF (neat), [...] (neat), MeCN (neat), [...] (neat), or acetone (neat) which do not coordinate. The EPR spectra of the four coordinate complex in glasses of [...]/toluene or [...] are of significance to nickel bioinorganic chemistry since [...], the opposite to the usually accepted EPR signature for square planar nickel(III). The Ni(III/II) couple of [...] is reversible and occurs at -0.58 V vs. [...] supporting electrolyte, 0.13 V vs. NHE showing that an abiological square planar ligand complement consisting of four deprotonated amides can produce a marked stabilization of four-coordinate nickel(III). Data on [...] are provided for comparison. The synthesis of copper(III) complexes with reversible cathodic and anodic waves is described. The low values for the Cu(III/II) couples (-0.815 V vs. [...]) and the high potentials for the [...] couples (ca. +0.830 V vs. [...]) are presented. The Dunitz amide nonplanarity analysis is applied to the crystal structures presented to illustrate the unique bonding that occurs in some of the macrocyclic compounds

Portable X-ray Fluorescence and Infrared Fluorescence Imaging Studies of Cadmium Yellow Alteration in Paintings by Edward Munch and Henri matisse in Oslo, Copenhagen, and San Francisco

The identification of altered cadmium yellow paints in early modernist works is critical to their stabilization and to the long-term preservation of the paintings in which they occur. The identification of incipient photoalteration of these pigments, before there is visual evidence of their chemical degradation, is of particular concern. The alteration of these pigments causes chalking, flaking, fading, and darkening of the yellow paints, leading to irreversible changes in the physical and chemical structure of the paint layer and dramatically altering the appearance of the work. Standoff methods for the identification of this phenomenon are desired to rapidly and efficiently survey the condition of the pigment across an entire work and also to minimize invasive and destructiveanalyses wherever possible. Such methods are a particular need for collections with large holdings of Impressionist and early modernist works from the 1880s to the 1920s, for which these cadmium yellow alterations are particularly problematic and a rapid surveying method for the collection is needed. To address this challenge, four standoff methods were attempted (both aloneand in concert): ultraviolet-induced visible fluorescence, ultraviolet-induced infrared fluorescence, multispectral imaging, and X-ray fluorescence. Questions addressed included the following: Is the imaging method being tested comprehensive? Is it efficient at surveying an entire painting? Does it reveal the state of preservation of the pigment? Does it reliably discriminate among intact versus altered cadmium yellow pigments? To answer these questions, the methods were tested on Henri Matisse’s Le Bonheur de vivre (1905–1906, the Barnes Foundation, Philadelphia) and oil sketches for this work in the San Francisco Museum of Modern Art and the Statens Museum for Kunst, Copenhagen. They were also tested on Edvard Munch’s The Scream (ca. 1910?, Munch Museum, Oslo). It was found that ultraviolet-induced visible fluorescence has the best ability to discriminate between altered and unaltered cadmium yellow paints (even before alteration is visible to the unaided eye), whereas multispectral imaging allows for the most efficient and comprehensive localization of the cadmium pigments in a work

Conservation Science Education Online (CSEO) – A heritage science resource

Conservation Science Education Online (CSEO) is a new online resource that shares strategies for teaching science in art conservation and related cultural heritage fields. An overview will be given of how undergraduate chemistry curricula in the United States have used examples from cultural heritage. The field of art conservation will then be described with an emphasis on the science curricula taught in art conservation programs around the world. Challenges include relating theoretical learning to real-world applications and teaching scientific terminology and concepts to students who may have limited science backgrounds; as well, there is a lack of textbooks and resources with appropriate case studies. The newly launched CSEO online resource offers freely available, effective teaching methods in the form of modules developed by international educators in the field. The inaugural CSEO Conference 2022 served as an introduction to the online resource for a global audience and was the first dedicated conference to bring together heritage science educators to discuss challenges and teaching strategies with the goal of building such a resource. The conference facilitated discussions among participants about teaching strategies, with the intention that these topics would become modules for the online resource, available to all science educators