David M. Pham

Crystallization and Interconversions of Vapor-Sensitive, Luminescent Polymorphs of [(C6H11NC)2AuI](AsF6) and [(C6H11NC)2AuI](PF6)

The remarkable, vapor-induced transformation of the yellow polymorphs of [(C(6)H(11)NC)(2)Au(I)](AsF(6)) and [(C(6)H(11)NC)(2)Au(I)](PF(6)) into the colorless forms are reported along with related studies of the crystallization of these polymorphs. Although the interconversion of these polymorphs is produced by vapor exposure, molecules of the vapor are not incorporated into the crystals. Thus, our observations may have broad implications regarding the formation and persistence of other crystal polymorphs where issues of stability and reproducibility of formation exist. Crystallographic studies show that the colorless polymorphs, which display blue luminescence, are isostructural and consist of linear chains of gold(I) cations that self-associate through aurophilic interactions. Significantly, the yellow polymorph of [(C(6)H(11)NC)(2)Au(I)](AsF(6)) is not isostructural with the yellow polymorph of [(C(6)H(11)NC)(2)Au(I)](PF(6)). Both yellow polymorphs exhibit green emission and have the gold cations arranged into somewhat bent chains with significantly closer Au···Au separations than are seen in the colorless counterparts. Luminescence differences in these polymorphs clearly enhance the ability to detect and monitor their phase stability.

Blue or Green Glowing Crystals of the Cation [Au{C(NHMe)2}2]+. Structural Effects of Anions, Hydrogen Bonding, and Solvate Molecules on the Luminescence of a Two-Coordinate Gold(I) Carbene Complex

Depending upon the crystallization conditions, [Au{C(NHMe) 2} 2](AsF 6) forms colorless crystals that display a blue or green luminescence. The difference involves the type of solvate molecule that is incorporated into the crystal and the structure of the chains of cations that are formed upon crystallization. The crystallographically determined structures of blue-glowing [Au{C(NHMe) 2} 2](AsF 6).0.5(benzene), blue-glowing [Au{C(NHMe) 2} 2](AsF 6).0.5(acetone), green-glowing [Au{C(NHMe) 2} 2](AsF 6).0.5(chlorobenzene), and blue-glowing, solvate-free [Au{C(NHMe) 2} 2](EF 6), E = P, As, Sb are reported. All pack with the cations forming extended columns, which may be linear or bent, but all show significant aurophilic interactions. The blue-glowing crystals have ordered stacks of cations with some variation in structural arrangement whereas the green-glowing crystals have disorder in their stacking pattern. Although there is extensive hydrogen bonding between the cations and anions in all structures, in the solvated crystals, the solvate molecules occupy channels but make no hydrogen-bonded contacts. The emission spectra of these new salts taken at 298 and 77 K are reported.

Modifications of boronic ester pro-chelators triggered by hydrogen peroxide tune reactivity to inhibit metal-promoted oxidative stress

Several new analogs of salicylaldehyde isonicotinoyl hydrazone (SIH) and salicylaldehyde benzoyl hydrazone (SBH) that contain an aryl boronic ester (BSIH, BSBH) or acid (BASIH) in place of an aryl hydroxide have been synthesized and characterized as masked metal ion chelators. These pro-chelators show negligible interaction with iron(III), although the boronic acid versions exhibit some interaction with copper(II), zinc(II) and nickel(II). Hydrogen peroxide oxidizes the aryl boronate to phenol, thus converting the pro-chelators to tridentate ligands with high affinity metal binding properties. An X-ray crystal structure of a bis-ligated iron(III) complex, [Fe(SBH(m-OMe)(3))(2)]NO(3), confirms the meridonal binding mode of these ligands. Modifications of the aroyl ring of the chelators tune their iron affinity, whereas modifications on the boron-containing ring of the pro-chelators attenuate their reaction rates with hydrogen peroxide. Thus, the methoxy derivative pro-chelator (p-OMe)BASIH reacts with hydrogen peroxide nearly 5 times faster than the chloro derivative (m-Cl)BASIH. Both the rate of pro-chelator to chelator conversion as well as the metal binding affinity of the chelator influence the overall ability of these molecules to inhibit hydroxyl radical formation catalyzed by iron or copper in the presence of hydrogen peroxide and ascorbic acid. This pro-chelator strategy has the potential to improve the efficacy of medicinal chelators for inhibiting metal-promoted oxidative stress.

Interactions of metalloporphyrins as donors with the electron acceptors C60, tetracyanoquinomethane (TCNQ) and trinitrofluorenylidenemalonitrile

Crystals of C60·PtII(OEP)·2(C6H6), TCNQ·CuII(OEP), TCNQ·H2(OEP), TCNQ·2CuII(OEP), TCNQ·2ZnII(OEP) and TNFM·CoII(OEP) [OEP is the dianion of octaethylporphyrin, TCNQ is 7,7,8,8-tetracyanoquinodimethane, TNFM is (2,4,7-trinitrofluorenylidene)malonitrile] have been obtained by diffusion of a solution of the porphyrin as donor into a solution of the respective acceptor molecule. The structure of C60·PtII(OEP)·2(C6H6) consists of an ordered C60 cage nestled against the platinum porphyrin which makes close face-to-face contact with another PtII(OEP) molecule. In contrast, there are no close face-to-face contacts between porphyrins in the crystal structures of TCNQ·CuII(OEP), TCNQ·H2(OEP), and TNFM·CoII(OEP). These compounds consist of classical donor–acceptor stacks of interleaved porphyrin and TCNQ or TNFM molecules with separations of ca. 3.3 A between adjacent molecules. However with TCNQ·2CuII(OEP) and TCNQ·2ZnII(OEP) the structures involve TCNQ (A) and MII(OEP) (D) molecules that crystallize in stacks with a DDA(DDA)nDDA arrangement. Within these stacks there are pairwise contacts between MII(OEP) molecules and these pairs are compared to those found in C60·PtII(OEP)·2(C6H6) and related fullerene-containing crystals.

Variation in crystallization conditions allows the isolation of trimeric as well as dimeric and monomeric forms of [(alkyl isocyanide)4RhI]+

Trimeric green [(i-PrNC)12Rh(I)3]Cl3.4.5H2O, monomeric [(C6H11NC)4Rh(I)](BPh4) and [(i-PrNC)4Rh(I)](BPh4) (both yellow), and red, dimeric [(C6H11NC)8Rh(I)2]Cl2.0.5C6H6.2H2O have been crystallized.