Head-to-head cross-linked adduct between the antitumor unit bis(mu-N,N'-di-p-tolylformamidinato)dirhodium(II,II) and the DNA fragment d(GpG)

Reactions of the compound cis-[Rh2(DTolF)2(CH3CN)6](BF4)2, a formamidinate derivative of the class of antitumor compounds [Rh2(O2CR)4] (R=Me, Et, Pr), with 9-ethylguanine (9-EtGuaH) or the dinucleotide d(GpG) proceed by substitution of the acetonitrile groups, with the guanine bases spanning the Rh--Rh bond, in a bridging fashion, through sites N7/O6. In the case of 9-EtGuaH, both head-to-head (HH) and head-to-tail (HT) isomers are formed, whereas with the tethered bases in d(GpG), only one right-handed conformer HH1R [Rh2(DTolF)2{d(GpG)}] is present in solution. For both cis-[Rh2(DTolF)2(9-EtGuaH)2](BF4)2 and [Rh2(DTolF)2{d(GpG)}], the absence of N7 protonation at low pH and the substantial decrease of the pKa values for N1-H deprotonation, support N7/O6 binding of the bases to the dirhodium core. The N7/O6 binding of the bases is further corroborated by the downfield shift by Deltadelta approximately 4.0 ppm of the 13C NMR resonances for the C6 nuclei as compared to the corresponding resonances of the free ligands. The HH arrangement of the guanine bases in [Rh2(DTolF)2{d(GpG)}] is indicated by the intense H8/H8 ROE cross-peaks in the 2D ROESY NMR spectrum. Complete characterization of the [Rh2(DTolF)2{d(GpG)}] conformer by 2D NMR spectroscopy supports anti-orientation and N (C3'-endo) conformation for both deoxyribose residues. The N-pucker for the 5'-G base is universal in such cross-links, but it is very unusual for platinum and unprecedented for dirhodium HH cross-linked adducts to have both deoxyribose residues in the N-type conformation. The bulk, the nonlabile character, and the electron-donating ability of the formamidinate bridging groups spanning the dirhodium core affect the nature of the preferred dirhodium DNA adducts. Molecular modeling studies performed on [Rh2(DTolF)2{d(GpG)}] corroborate the structural features obtained by NMR spectroscopy.     

Hydrothermal synthesis and structure of a three-dimensional cobalt(II) triazolate magnet

The reaction of CoCl2.6H2O, 1,2,4-triazole, and H(2)O at 200 degrees C for 96 h yields purple rods of [Co2(trz)3Cl] (1; trz = 1,2,4-triazolate) in 90% yield. The structure of 1 consists of {Co(trz)3}n(n-) chains of Co(II) sites linked through tetrahedral sites of {CoN(3)Cl} geometry into a three-dimensional framework. The competing antiferromagnetic interactions of the octahedral and tetrahedral Co(II) centers result in weak ferromagnetic behavior below Tc = 9 K.     

IRMPD spectroscopy of metal-ion/tryptophan complexes

Infrared multiple-photon dissociation (IRMPD) spectroscopy is employed to obtain detailed binding information on singly charged silver and alkali metal-ion/tryptophan complexes in the gas phase. For these complexes the presence of the salt bridge (i.e. zwitterionic) tautomer can be virtually excluded, except for perhaps a few percent in the case of Li+. Two low-energy structures having the charge solvation form are shown to be likely, where the metal cation is either in a tridentate N/O/Ring conformation or in a bidentate O/Ring conformation. These two structures can be conveniently discriminated and their relative quantities can be approximated by IR spectroscopy, based principally on diagnostic modes near approximately 1150 (N/O/Ring) and 1400 (O/Ring) cm(-1). Interestingly, the smaller cation complexes (i.e. Ag+ and Li+) display exclusively the N/O/Ring conformation, whereas the O/Ring conformer becomes progressively more abundant with increasing alkali metal size, eventually representing the majority of the ion population for Rb+ and Cs+. These spectroscopic findings are in excellent agreement with thermochemical density functional theory (DFT) calculations, giving support to the utility of high-level quantum-chemical calculations for such systems. Moreover, in contrast to other mass spectrometry-based techniques, IRMPD spectroscopy allows clear differentiation and semi-quantitative approximation of these metal-ligand binding motifs, thereby underlining its importance in thermochemical model benchmarking.     

Syntheses and reactivity studies of solvated dirhenium acetonitrile complexes

Fully and partially solvated triply-bonded [Re2]4+ complexes have been synthesized and their X-ray structures are described. A fully solvated dirhenium salt with BArf [tetrakis(3,5-bis(trifluoromethyl)phenyl)borate] as the counter anion [Re2(CH3CN)10][BArf]4 () has been characterized. The solubility of the complex in CH2Cl2 and THF in addition to CH3CN offers the possibility of improved reactivity. The structure of [Re2(micro-O)(CH3CN)10][BF4]4 () that possesses a linear [Re(III)-O-Re(III)]4+ unit is reported. Protonation reactions of cis-Re2Cl2(dppm)2(O2CCH3)2 and trans-Re2Cl4(dppm)2 with HBF4.Et2O in acetonitrile afforded cis and trans [Re2(dppm)2(CH3CN)6][BF4]4 ( and ), respectively. Prolonging the reaction time, however, does not lead to fully solvated complex [Re2(CH3CN)10][BF4]4. The neutral nitrogen donor ligands pynp (2-(2-pyridyl)-1,8-naphthyridine) and tznp (2-(2-thiazolyl)-1,8-naphthyridine) react readily with [Re2(CH3CN)10][BF4]4 to provide trans-[Re2(pynp)2(CH3CN)4][BF4]4 and trans-[Re2(tznp)2(CH3CN)4][BF4]4. The X-ray structures trans-[Re2(pynp)2(CH3CN)4][BF4]4 () and trans-[Re2(tznp)2(CH3CN)4][BF4]3[PF6] () have been determined.     

A One-Pot, High-Yield Synthesis of a Paramagnetic Nickel Square from Divergent Precursors by Anion Template Assembly

A paramagnetic Ni-containing molecular square has been synthesized in high yield from divergent precursors. An X-ray structure (shown in the picture) reveals the presence of an encapsulated tetrafluoroborate anion, which appears to be a requisite condition for the formation of the cyclic oligomer.     

CLIP: similarity searching of 3D databases using clique detection

This paper describes a program for 3D similarity searching, called CLIP (for Candidate Ligand Identification Program), that uses the Bron-Kerbosch clique detection algorithm to find those structures in a file that have large structures in common with a target structure. Structures are characterized by the geometric arrangement of pharmacophore points and the similarity between two structures calculated using modifications of the Simpson and Tanimoto association coefficients. This modification takes into account the fact that a distance tolerance is required to ensure that pairs of interatomic distances can be regarded as equivalent during the clique-construction stage of the matching algorithm. Experiments with HIV assay data demonstrate the effectiveness and the efficiency of this approach to virtual screening.     

Structural,magnetic, and optoelectronic properties of (diimine)(dithiolato)platinum(II) and -palladium(II) complexes and their charge-transfer adducts with nitrile acceptors

Two new diimine dithiolato complexes, (dbbpy)Pt(dmid), 1, and (dbbpy)Pd(dmid), 2, were prepared and characterized (dbbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; dmid = 2-oxo-1,3-dithiole-4,5-dithiolate). Both complexes interact with the nitrile acceptor TCNQ, and 1 also interacts with TCNQF(4) and TCNE (TCNQ = 7,7,8,8-tetracyanoquinodimethane; TCNQF(4) = 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane; TCNE = tetracyanoethylene) to form supramolecular 2:1 charge-transfer solids that stack in the manner -DDADDADDA- (D = electron donor; A = electron acceptor). All compounds have been fully characterized by magnetic, spectroscopic, electrochemical, and single-crystal X-ray crystallographic analyses. Magnetic susceptibility studies of the charge-transfer compounds revealed that the platinum-based complexes exhibit temperature-independent paramagnetism of approximately 10(-3) emu/mol. The donor complexes exhibit continuous absorption bands across the UV/visible and into the NIR region. Upon interaction with the nitrile acceptors, the extinction coefficients of the absorption bands increase and the energies of some d-d transitions in the NIR region change. The donor-acceptor compounds possess desired spectral features for solar cell dyes, but low conversion efficiencies resulted when a representative compound was tested in a TiO(2) solar cell. The results, however, serve to illustrate that the donor-acceptor interactions persist in solution and the adsorption of the dye molecules to the semiconductor surface occurs in the absence of typical anchoring groups. Evaluation of the spectral and electrochemical data for the title compounds and the results of the preliminary solar cell study serve as guides for future research in choosing promising candidates for efficient solar cell dyes.     

Metal-metal bonded diruthenium(II, III) assemblies with the polycyano anionic linkers N(CN)2-, C(CN)3-, and 1,4-dicyanamido-2,5-dimethylbenzene (DM-dicyd2-): syntheses, structures, and magnetic properties

The new metal-metal bonded diruthenium(II,III) compounds [Ru2(O2CCH3)4(mu-L)]infinity (L = N(CN)2-, 1; C(CN)3-, 2) and [[Ru2(O2CCH3)2(mhp)2]2(mu-DM-Dicyd)] (3) (mhp = 2-oxy-6-methylpyridinate, DM-Dicyd = 1,4-dicyanamido-2,5-dimethylbenzene dianion) have been synthesized and fully characterized. Compounds 1 and 2 were synthesized by the reaction of [Ru2(O2CCH3)4(NCCH3)2](BF4) with NaN(CN)2 and KC(CN)3, respectively. The "dimer-of-dimers", 3, was synthesized by a 2:1 reaction of [Ru2(O2CCH3)2(mhp)2(MeOH)](BF4) with [As(Ph)4]2[DM-Dicyd]. Compound 1 crystallizes in the monoclinic space group C2/m with a = 10.174(2) A, b = 13.016(3) A, c = 7.0750(14) A, beta = 101.83(3) degrees, and Z = 2. Compound 2 crystallizes in the orthorhombic space group Fdd2 with a = 29.679(6) A, b = 31.409(6) A, c = 7.3660(15) A, V = 6866(2) A3, and Z = 16. In compound 1, dicyanamide anions (N(CN)2-) bridge the [Ru2(O2CCH3)4]+ units in an end-to-end bridging mode, thereby forming an alternating one-dimensional chain. In compound 2, two cyano groups of tricyanomethanide anion (C(CN)3-) are coordinated to independent [Ru2(O2CCH3)4]+ units to give a chain similar to that found in 1. The Ru-Ru bond distances in 1 and 2 are 2.2788(14) and 2.2756(5) A, respectively, which are typical values for Ru2(O2CR)4Cl and [Ru2(O2CR)4]+ compounds. The Ru-N distances are 2.257(8) A in 1 and 2.259(4) and 2.283(4) A in 2. The temperature dependence of the magnetic susceptibilities of compounds 1-3 reveals a weak antiferromagnetic interaction between Ru2 units (S = 3/2) through each polycyano anionic linker: g = 2.16, zJ = -0.33 cm(-1), D = 63.3 cm(-1) for 1; g = 2.15, zJ = -0.22 cm(-1), D = 58.0 cm(-1) for 2; and g = 2.10, zJ = -0.90 cm(-1), D = 75.0 cm(-1) for 3.