Scott Lovell

Crystal violet’s shoulder

The symmetry of the crystal violet cation has been the subject of ongoing discussion since G. N. Lewis and co-workers called attention to the unexpected shoulder in its optical absorption spectrum. Strikingly absent from this debate is a description of the X-ray crystal structure of the common chloride salt, that compound to which the vast majority of the spectroscopic analyses pertain, and whose large crystals account for the familiar name. This absence is especially curious since a preliminary structure determination was made as early as 1943. Here, we present single crystal structures of two forms of crystal violet chloride, a monoclinic monohydrate and a trigonal nonahydrate. In principle, the pair is well suited to address whether desymmetrization is responsible for the shoulder since the cation is asymmetric in the monoclinic crystal, while it is axially symmetric in the trigonal crystal. Absorption spectra of the two crystalline forms were identical and dominated by peaks assigned to aggregates. Raman spectra also could not distinguish the hydrates from one another. Mixed crystals of benzene-1,2-dicarboxylic acid (phthalic acid) containing mM concentrations of crystal violet were prepared in order to study the cation fixed in a dissymmetric medium, yet free from aggregate absorption. Through this work, and a reevaluation of the literature, we show that the two prevailing theories for the shoulder, that there are two isomers in equilibrium, and that desymmetrization gives rise to two excited states, are not mutually exclusive.

DIE OXIDATION VON C-H- UND O-H-BINDUNGEN DURCH KUPFER(III)-KOMPLEXE

Bei Oxidationen durch Kupferenzyme werden immer haufiger Kupfer(III)-Spezies als Intermediate vorgeschlagen. Ein gutes Modell fur derartige Reaktionen ist die Oxidation von Dihydroanthracen zu Anthracen durch einen isolierbaren Kupfer(III)-imin/oxim-Komplex [Gl. (1)].

Intrasectoral Zoning of Proteins and Nucleotides in Simple Crystalline Hosts

Oriented gases of biopolymers in simple, single crystal hosts might be used to measure anisotropic molecular properties of analytes that could not otherwise be crystallized. Here we show two types of crystals as examples of the single crystal matrix isolation of biopolymers: green fluorescent protein in α-lactose monohydrate as a model system for studying the kinetic stabilization of biopharmaceuticals, and adenosine phosphates in potassium dihydrogen phosphate, a first step in the matrix isolation of oligonucleotides. In each case, the hosts undergo compositional zoning – both intersectoral and intrasectoral – during growth from solution. Intrasectoral zoning is evident by the selective luminescence of adjacent vicinal slopes of growth active hillocks. Nucleotides furthermore distinguish between symmetry related growth sectors enantioselectively.

Osmium(IV) complexes TpOs(X)Cl2 and their Os(III) counterparts: oxidizing compounds with an unusual resistance to ligand substitution

TpOsIV(X)Cl2 complexes (Tp = hydrotris(1-pyrazolyl)borate) are formed upon treatment of TpOs(NPPh3)Cl2 with the protic acids HX: triflic acid (HOTf), HCl, HBr, CF3COOH, and CH3COOH. The reaction with acetic acid is slow but is catalyzed by HOTf. The triflate ligand in TpOs(OTf)Cl2 (1) is remarkably inert and does not undergo simple substitution reactions. For instance, no reaction is observed between 1 and anhydrous Cl− salts, but conversion to TpOsCl3 occurs upon addition of small amounts of H2O or HCl. Substitution appears to be catalyzed by protic reagents. Treatment of 1 with PPh3 or pyridine (py) yields the substituted osmium(III) complexes TpOs(L)Cl2 (L = PPh3, py). A more general route to Os(III) complexes involves reduction of 1 by decamethylferrocene (Cp*2Fe) to give [Cp*2Fe][TpOs(OTf)Cl2], which undergoes substitution at 65 °C forming the Os(III) complexes TpOs(L)Cl2 (L = MeCN, C6H5CN, PPh3, py, imidazole, and NH3) in 70–90% yields. Oxidation of the neutral Os(III) complexes with [NO]BF4 in CH2Cl2 affords Os(IV) salts of the formula [TpOs(L)Cl2]BF4 in near quantitative yields. This indirect synthetic approach yields osmium(IV) complexes that are not accessible by direct substitution. The Os(IV) complexes are strong oxidants, with E1/2 values from +0.00 to +0.70 V in MeCN vs. Cp2Fe+/0. The inertness of the triflate ligand in 1, and the acetonitrile ligand in [TpOs(NCMe)Cl2]BF4, appear to be in part a consequence of the electrophilic character of the Os(IV) center.