Better understanding of the species with the shortest Re2(6+) bonds and related Re2)(7+) species with tetraguanidinate paddlewheel structures.

A series of compounds has been made containing quadruply bonded Re2(hpp)4X2 species (hpp = the anion of 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2a]pyrimidine), where X is CF3SO3 (1), CF3CO2 (2), and F (3). The distances of 2.1562(7), 2.1711(5), and 2.1959(4) A for 1-3 show significant effects of the sigma and pi electron donating ability of the axial ligands on the metal-metal distance. With the weakly coordinating triflate ligand the Re-Re distance is the shortest for any quadruple bonded species known. In addition to examining the effects of axial ligands on the Re2(hpp)42+ core, our study of the Re2(hpp)43+ core is being extended beyond the preliminary results previously reported in only one compound [Re2(hpp)4Cl2]PF6 (Dalton Trans. 2003, 1218). We now report the structural characterization by both X-ray and neutron diffraction of the compound [Re2(hpp)4F](TFPB)2, 4 (TFPB = the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate), and a detailed study by EPR spectroscopy of [Re2(hpp)4Cl2]PF6 at 9.5, 34.5, and 95 GHz frequencies, using dilute fluid solutions, frozen glass, and neat powder, show that the unpaired electron in the [Re2(hpp)4Cl2]+ ion is in an MO of predominant metal character with little mixing from the guanidinate ligands.

Proof of large positive zero-field splitting in a Ru2 5+ paddlewheel.

We present the synthesis, as well as the structural and magnetic characterization, of [Ru2(D(3,5-Cl2Ph)F)4Cl(0.5H2O)].C6H14 (D(3,5-Cl2Ph)F = N,N'-di(3,5-dichlorophenyl)formamidinate), a Ru2(5+) compound having a 4B(2u) ground state derived from a sigma2pi4delta2pi2delta electron configuration. The persistence of this configuration from 27 to 300 K is shown by the invariance of the Ru-Ru distance. Orientation-dependent magnetic susceptibility (chiT) and magnetization (M(H)) data are in accord with a spin quartet ground state with large magnetocrystalline anisotropy associated with a large axial zero-field splitting (D) parameter. Theoretical fits to chiT and M(H) plots yielded D/kB = +114 K, implying an S = +/-1/2 Kramers doublet ground state at low temperature. Single-crystal and powder EPR data are consistent with this result, as the only observed transition is between the M(s) = +/1/2 Zeeman levels. The g values are g(perpendicular) = 2.182, g(parallel) = 1.970, and D = 79.8 cm(-1). The totality of the results demands D >> 0.

Crystalline and water soluble Cr(4+) and Cr(5+) model compounds for chromium toxicity studies.

We discuss two complexes of Cr(4+) for their possible utility as models for Cr toxicity studies. They are Cr(dien)(O2)2(.)H2O (dien = diethylenetriamine) and Cr(NH3)3(O2)2, which have been recently characterized by x-ray diffraction and magnetic techniques. We present their optical and infrared absorption spectra as quick aids in their identification procedure. We also summarize the general properties of some well-characterized Cr(5+) compounds. All of these compounds are water soluble with the Cr(5+) samples being fairly stable in basic solutions. They can all be prepared as pure crystals with a shelf life of over 2 years when stored in a refrigerator.

Observation of symmetry lowering and electron localization in the doublet-states of a spin-frustrated equilateral triangular lattice: Cu3(O2C16H23) x 1.2C6H12.

Cu3(O2C16H23)6.1.2C6H12, containing a Cu36+ core in an equilateral triangle geometry, has been found to be a versatile model system for investigating the spin-frustration phenomenon in a triangular lattice. It affords well-resolved EPR spectra from both of the two possible (Stotal = 1/2 and 3/2) spin states of the Cu36+ core. From 295 to 100 K, the spectra consist of a triplet, but with the central line overlapped by an additional, sharp peak, which replaces the triplet at 30 K and below. The triplet was thus assigned to the excited state with Stotal = 3/2, located at 324 +/- 5 K ( approximately 225 cm-1), with the zero-field parameters D = -535 G, E = 0, g parallel = 2.209 and g perpendicular = 2.057. The singlet was attributed to the Stotal = 1/2 state, with gxx = 2.005, gyy = 2.050, gzz = 2.282, and, surprisingly, a hyperfine splitting arising from a single Cu2+ nucleus, with Azz = 157 G. The detailed magnetic measurements on a three-electron, equilateral triangular system, and the observation of symmetry lowering in the doublet ground state, should be of broad theoretical and experimental interest in molecular magnetism.

Applications of high-field (W-band) EPR to M-M bonded units (M = Cr, Mo): the first confirmed oxidation of a Cr2 4+ paddlewheel complex to a stable isostructural Cr2 5+ product.

The EPR spectra of [Cr(2)[(PhN)(2)CN(CH(2))(4)](4)]PF(6) and [Mo(2)(TiPB)(4)]PF(6) (TiPB = anion of 2,4,6-triisopropylbenzoic acid) at W-band are shown to have g values significantly lower than 2.00 and exhibit parallel and perpendicular components (not resolved at X-band). Therefore the unpaired electrons of the M(2)(5+) units must reside on metal-based (not ligand-based) orbitals. Thus, the chromium compound must be considered as the first confirmed oxidation product of a Cr(2)(4+) paddlewheel complex comparable to the Mo(2)(5+) compounds.

Probing triplex formation by EPR spectroscopy using a newly synthesized spin label for oligonucleotides.

Spin labels have been extensively used to study the dynamics of oligonucleotides. Spin labels that are more rigidly attached to a base in an oligonucleotide experience much larger changes in their range of motion than those that are loosely tethered. Thus, their electron paramagnetic resonance spectra show larger changes in response to differences in the mobility of the oligonucleotides to which they are attached. An example of this is 5-(2,2,5,5-tetramethyl-3-ethynylpyrrolidine-1-oxyl)-uridine (1). How ever, the synthesis of this modified DNA base is quite involved and, here, we report the synthesis of a new spin-labeled DNA base, 5-(2,2,6,6-tetramethyl-4-ethynylpiperidyl-3-ene-1-oxyl)-uridine (2). This spin label is readily prepared in half the number of steps required for 1, and yet behaves in a spectroscopically analogous manner to 1 in oligonucleotides. Finally, it is shown here that both spin labels 1 and 2 can be used to detect the formation of both double-stranded and triplex DNA.