The first cobalt molecule to function as a single-molecule magnet, [Co 4 (hmp) 4 (MeOH) 4 Cl 4 ], where hmpis the anion of hydroxymethylpyridine, is reported.The core of the molecule consists of four Co(II) cations and four hmpoxygen atoms ions at the corners of a cube. Variable-field and variable-temperature magnetization data have been analyzed to establish that the molecule has a S=6 ground state with considerable negative magnetoanisotropy. Single-ion zero-field interactions (DS z 2 ) at each cobalt ion are the origin of the negative magnetoanisotropy. A single-crystal of the compound was studied by means of a micro-SQUID magnetometer in the range of 0.040-1.0K.Hysteresis was found in the magnetization versus magnetic field response of this single crystal. It is concluded that this is the first cobalt molecule to function as a singlemolecule magnet.

A Raman spectroscopic evaluation of numerous crystalline solvates with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI or LiN(SO2CF3)2) has been conducted over a wide temperature range. Four new crystalline solvate structures-(PHEN)3:LiTFSI, (2,9-DMPHEN)2:LiTFSI, (G3)1:LiTFSI and (2,6-DMPy)1/2:LiTFSI with phenanthroline, 2,9-dimethyl[1,10]phenanthroline, triglyme, and 2,6-dimethylpyridine, respectively-have been determined to aid in this study. The spectroscopic data have been correlated with varying modes of TFSI(-)···Li(+) cation coordination within the solvate structures to create an electrolyte characterization tool to facilitate the Raman band deconvolution assignments for the determination of ionic association interactions within electrolytes containing LiTFSI. It is found, however, that significant difficulties may be encountered when identifying the distributions of specific forms of TFSI(-) anion coordination present in liquid electrolyte mixtures due to the wide range of TFSI(-)···Li(+) cation interactions possible and the overlap of the corresponding spectroscopic data signatures.

Copper(I)-dioxygen interactions are of great interest due to their role in biological O2-processing as well as their importance in industrial oxidation processes. We describe here the study of systems which lead to new insights concerning the factors which govern Cu(II)-mu-eta2:eta2 (side-on) peroxo versus Cu(III)-bis-mu-oxo species formation. Drastic differences in O2-reactivity of Cu(I) complexes which differ only by a single -CH3 versus -H substituent on the central amine of the tridentate ligands employed are observed. [Cu(MeAN)]B(C6F5)4 (1) (MeAN = N,N,N',N',N'-pentamethyl-dipropylenetriamine) reacts with O2 at -80 degrees C to form almost exclusively the side-on peroxo complex [{CuII(MeAN)}2(O2)]2+ (3) in CH2Cl2, tetrahydrofuran, acetone, and diethyl ether solvents, as characterized by UV-vis and resonance Raman spectroscopies. In sharp contrast, [Cu(AN)]B(C6F5)4 (2) (AN = 3, 3'-iminobis(N,N-dimethyl-propylamine) can support either Cu2O2 structures in a strongly solvent-dependent manner. Extreme behavior is observed in CH2Cl2 solvent, where 1 reacts with O2 giving 3, while 2 forms exclusively the bis-mu-oxo species [{CuIII(AN)}2(O)2]2+ (4Oxo). Stopped-flow kinetics measurements also reveal significant variations in the oxygenation reactions of 1 versus 2, including the observations that 4Oxo forms much faster than does 3; the former decomposes quickly, while the latter is quite stable at 193 K. The solvent-dependence of the bis-mu-oxo versus side-on peroxo preference observed for 2 is opposite to that reported for other known copper(I) complexes; the factors which may be responsible for the unusual behavior of 1/O2 versus 2/O2 (possibly N-H hydrogen bonding in the AN chemistry) are suggested. The factors which affect bis-mu-oxo versus side-on peroxo formation continue to be of interest.

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The preparation and characterization of three new donor-bridge-acceptor biradical complexes are described. Using variable-temperature magnetic susceptibility, EPR hyperfine coupling constants, and the results of X-ray crystal structures, we evaluate both exchange and electronic couplings as a function of bridge length for two quintessential molecular bridges: oligo(para-phenylene), β = 0.39 Å(-1) and oligo(2,5-thiophene), β = 0.22 Å(-1). This report represents the first direct comparison of exchange/electronic couplings and distance attenuation parameters (β) for these bridges. The work provides a direct measurement of superexchange contributions to β, with no contribution from incoherent hopping. The different β values determined for oligo(para-phenylene) and oligo(2,5-thiophene) are due primarily to the D-B energy gap, Δ, rather than bridge-bridge electronic couplings, H(BB). This is supported by the fact that the H(BB) values extracted from the experimental data for oligo(para-phenylene) (H(BB) = 11,400 cm(-1)) and oligo(2,5-thiophene) (12,300 cm(-1)) differ by <10%. The results presented here offer unique insight into the intrinsic molecular factors that govern H(DA) and β, which are important for understanding the electronic origin of electron transfer and electron transport mediated by molecular bridges.

A series of four anti-disposed dinuclear platinum(II) complexes featuring metal-metal-to-ligand charge-transfer (MMLCT) excited states, bridged by either 2-hydroxy-6-methylpyridine or 2-hydroxy-6-phenylpyridine and cyclometalated with 7,8-benzoquinoline or 2-phenylpyridine, are presented. The 2-hydroxypyridine bridging ligands control intramolecular d-d metal-metal σ interactions, affecting the frontier orbitals' electronic structure, resulting in marked changes to the ground- and excited-state properties of these complexes. Three of these molecules possess reversible one-electron oxidations in cyclic voltammetry experiments as a result of strong intramolecular metallophilic interactions. In this series of molecules, X-ray crystallography revealed Pt-Pt distances ranging between 2.815 and 2.878 Å; the former represents the shortest reported metal-metal distance for platinum(II) dimers possessing low-energy MMLCT transitions. All four molecules reported here display visible absorption bands beyond 500 nm and feature MMLCT-based red photoluminescence (PL) above 700 nm at room temperature with high PL quantum yields (up to 4%) and long excited-state lifetimes (up to 341 ns). The latter were recorded using both transient PL and transient absorption experiments that self-consistently yielded quantitatively identical excited-state lifetimes. The energy-gap law was successfully applied to this series of chromophores, documenting this behavior for the first time in molecules possessing MMLCT excited states. The combined data illustrate that entirely new classes of MMLCT chromophores can be envisioned using bridging pyridyl hydroxides in cooperation with various C^N cyclometalates to achieve photophysical properties suitable for excited-state electron- and energy-transfer chemistry.

Partially and fully aromatic molecular spacers bearing two symmetrically bound terpyridyl chelators have been prepared. These spacer-chelators were constructed to dispose the two terpyridyl ligands and their complexes with square planar metals cofacially with a separation of about 7 A between the two metals. Dipalladium(II) complexes of these spacer-chelators were prepared and characterized. These palladium complexes readily form large molecular rectangles with a linear linker such as 4,4'-dipyridyl. The dichlorodipalladium complex of the partially reduced spacer-chelator is capable of incarcerating planar aromatic and coordination compounds as guests. A crystal structure showing the incorporation of 9-methylanthracene has been determined. A 9-methylanthracene lies completely within the approximately 7 A space provided by the cleft formed by the two cofacially disposed chloro-palladium-terpyridyl units. The crystal structure shows additional pi-stacking interactions between a second 9-methylanthracene and neighboring receptors.

Elemental sulfur (S8) was found to react with [(TMPA)CuI(CH3CN)]+ to form the trans-mu-1,2 end-on disulfide complex [(TMPA)Cu-S-S-Cu(TMPA)]2+. The X-ray structure of this centrosymmetric disulfide complex shows a Cu(1)-S(1) bond length of 2.280(2) A and a S(1)-S(1A) bond length of 2.044(4) A.