Biomedical subjects
Publications and source records attributed to Douglas R Powell.
A terpyridine ligand is covalently linked to a hexamolybdate cluster through the Mo-N imido bond, paving the way for the preparation of hybrids containing covalently bonded transition metal complexes and polyoxometalate clusters.
The silver(I) coordination chemistry of 2,6-diarylpyrazines is reported. Discrete coordination complexes and two-dimensional coordination networks were characterized. The substitution pattern on the pendant aryl groups controlled the type of coordination chemistry involved. Thus, o-methyl-substituted aryl groups held the aryl groups orthogonal to the central pyrazine ring, opening the "hindered" nitrogen atoms to complexation, and polymeric networks were characterized. In the absence of the o-methyl groups, discrete coordination complexes were characterized. Thus, a dimeric 2:1 ligand-silver(I) complex was isolated and characterized on reaction of 2,6-bis(3',5'-dimethylphenyl)pyrazine with silver(I) trifluoroacetate in acetonitrile solvent, while a 2:2 complex was isolated from dichloromethane solvent. Two trifluoroacetate ligands bridge two silver cations in both complexes. Reaction of the same pyrazine ligand with silver(I) tetrafluoroborate yielded a discrete 2:1 complex. A 2:1 complex was isolated on reaction of 2,6-diphenylpyrazine with silver(I) nitrate. These complexes were interlinked by weakly coordinating nitrate anions to form interwoven one-dimensional ribbons. Two-dimensional networks were obtained on reaction of silver(I) trifluoroacetate with either 2,6-bis(2',6'-dimethylphenyl)pyrazine or 2-(2',6'-dimethylphenyl)-6-(3',5'-dimethylphenyl)pyrazine. The networks comprised pyrazine-silver(I) strands cross-linked with complex bridged silver(I) trifluoroacetates.
An efficient synthesis of remarkably stable 6-isocyanoazulene (CN6Az), the first nonbenzenoid organic isocyanide, is described. Its superb pi-accepting potential as a ligand was demonstrated through cyclic voltammetry studies on the binary complexes [Cr(CN6Az)6]0/+. The paramagnetic shift patterns for [Cr(CN6Az)6]+, the only azulenic pi-system studied by paramagnetic NMR, suggest that Cr(dpi) --> CN6Az(ppi*) electron delocalization involves both rings of the azulenyl substituents in [Cr(CN6Az)6]+. This conclusion is supported by density functional theory calculations on the complex [Cr(CN6Az)6][BF4].
The synthesis and structure of a rigid, cavity containing tetra-cobalt(III) [2 x 2] grid complex using an unusual bis(bipyridine)dimethoxynaphthyridine ligand is described.
Interaction of aminoferrocene with a 65/35 mol % mixture of phenyl formate/phenol followed by subsequent dehydration of the resulting ferrocenylformamide with POCl3 produced a high yield of air- and thermally stable, peach-colored isocyanoferrocene (CNFc). Combining 6 equiv of CNFc with bis(naphthalene)chromium(0) afforded orange-red Cr(CNFc)6, the first homoleptic complex of CNFc. Successive one-electron oxidations of the latter with Ag+ quantitatively produced saddle-brown [Cr(CNFc)6]+ and forest-green [Cr(CNFc)6]2+. The compounds [Cr(CNFc)6]0,1+,2+ are air-, light-, and thermally robust and are remarkable due to the incorporation of seven potentially electroactive transition metal ions within a relatively compact ML6 motif. Their properties demonstrate that the pi-accepting potential of the ferrocenyl substituent, customarily compared to an alkyl group, may have been underestimated. Indeed, the donor/acceptor ratio of the CNFc ligand appears to be similar to those of aryl isocyanides but quite different from those of alkyl isocyanides. Spectroscopic (including multinuclear paramagnetic NMR), magnetic, structural, and electrochemical characteristics of the above substances are discussed. In addition, a DFT analysis of the Frontier molecular orbitals of CNFc is presented.
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Dimerization of the alkynylcarbene complex Cp(CO)(2)Re=C(Tol)C(triple bond)CCH(3) (8) occurs at 100 degrees C to give a 1.2:1 mixture of enediyne complexes [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)CC(CH(3))=C(CH(3))C(triple bond)CTol] (10-Eand 10-Z), showing no intrinsic bias toward trans-enediyne complexes. The cyclopropyl-substituted alkynylcarbene complex Cp(CO)(2)Re=C(Tol)C(triple bond)CC(3)H(5) (11) dimerizes at 120 degrees C to give a 5:1 ratio of enediyne complexes [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)C(C(3)H(5))C=C(C(3)H(5))C(triple bond)CTol] (12-E and 12-Z); no ring expansion product was observed. This suggests that if intermediate A formed by a [1,1.5] Re shift and having carbene character at the remote alkynyl carbon is involved, then interaction of the neighboring Re with the carbene center greatly diminishes the carbene character as compared with that of free cyclopropyl carbenes. The tethered bis-(alkynylcarbene) complex Cp(CO)(2)Re=C(Tol)C(triple bond)CCH(2)CH(2)CH(2)C(triple bond)CC(Tol)= Re(CO)(2)Cp (13) dimerizes rapidly at 12 degrees C to give the cyclic cis-enediyne complex [Cp(CO)(2)Re](2)[eta(2),eta(2)-TolC(triple bond)CC(CH(2)CH(2)CH(2))=CC(triple bond)CTol] (15). Attempted synthesis of the 1,8-disubstituted naphthalene derivative 1,8-[Cp(CO)(2)Re=C(Tol)C(triple bond)C](2)C(10)H(6) (16), in which the alkynylcarbene units are constrained to a parallel geometry, leads to dimerization to [Cp(CO)(2)Re](2)(eta(2),eta(2)-1,2-(tolylethynyl)acenaphthylene] (17). The very rapid dimerizations of both 13 and 16 provide compelling evidence against mechanisms involving cyclopropene intermediates. A mechanism is proposed which involves rate-determining addition of the carbene center of A to the remote alkynyl carbon of a second alkynylcarbene complex to generate vinyl carbene intermediate C, and rearrangement of C to the enediyne complex by a [1,1.5] Re shift.
Ruthenium benzylidene metathesis catalysts react with 2,3-dicarbomethoxymethylene-cyclopropane, eliminating styrene and dimethyl fumarate, and producing the first terminal ruthenium carbide complexes. The products are diamagnetic, air-stable, and moderately soluble in hydrocarbon solvents. An X-ray study of Ru(C:)Cl2(P(C6H11)3) (1,3-dimesityl-4,5-dihydroimidazol-2-ylidene) shows a Ru-C distance of 1.650(2) A, consistent with the presence of a very short Ru-C triple bond.
The preparation of calix[n]naphthalenes from derivatives of 2,7-dihydroxynaphthalene is described. 1,8-Dialkyl substitution is used to direct the regiochemistry of the acid-catalyzed condensation reactions. Acyclic peri substituents lead to a 3,5-linked calix[3]naphthalene, whereas cyclic peri substituents give predominantly a calix[5]naphthalene with the corresponding 3,6-linkage. The 3,6-linked calix[4]naphthalene is prepared in pure form by a dimerization strategy.
D-eritadenine (DEA) is a potent inhibitor (IC(50) = 7 nm) of S-adenosyl-l-homocysteine hydrolase (AdoHcyase). Unlike cyclic sugar Ado analogue inhibitors, including mechanism-based inhibitors, DEA is an acyclic sugar Ado analogue, and the C2' and C3' have opposite chirality to those of the cyclic sugar Ado inhibitors. Crystal structures of DEA alone and in complex with AdoHcyase have been determined to elucidate the DEA binding scheme to AdoHcyase. The DEA-complexed structure has been analyzed by comparing it with two structures of AdoHcyase complexed with cyclic sugar Ado analogues. The DEA-complexed structure has a closed conformation, and the DEA is located near the bound NAD(+). However, a UV absorption measurement shows that DEA is not oxidized by the bound NAD(+), indicating that the open-closed conformational change of AdoHcyase is due to the substrate/inhibitor binding, not the oxidation state of the bound NAD. The adenine ring of DEA is recognized by four essential hydrogen bonds as observed in the cyclic sugar Ado complexes. The hydrogen bond network around the acyclic sugar moiety indicates that DEA is more tightly connected to the protein than the cyclic sugar Ado analogues. The C3'-H of DEA is pointed toward C4 of the bound NAD(+) (C3'...C4 = 3.7 A), suggesting some interaction between DEA and NAD(+). By placing DEA into the active site of the open structure, the major forces to stabilize the closed conformation of AdoHcyase are identified as the hydrogen bonds between the backbone of His-352 and the adenine ring, and the C3'-H...C4 interaction. DEA has been believed to be an inactivator of AdoHcyase, but this study indicates that DEA is a reversible inhibitor. On the basis of the complexed structure, selective inhibitors of AdoHcyase have been designed.