Synthesis,structure and magnetic properties of unsymmetrical dodecanuclear Mn–Ln clusters

Reaction of manganese acetate and lanthanide nitrates in the presence of excess of PhCOOH affords highly asymmetric dodecanuclear mixed-metal [Mn₁₀Ln₂(OH)(O)₈(PhCOOH)(PhCOO)₁₉] (Ln = Pr^III (1), Nd^III (2)) clusters. The similar reaction, but with only 2 equiv. of PhCOOH resulted in the compounds with higher nuclearity [Mn₁₁Eu₄(O)₈(OH)₈(PhCOO)₁₈(NO₃)₂(H₂O)₆]NO₃ · 4CH₃CN (3). Variable-temperature solid-state magnetic susceptibility of 1 and 2 in the temperature range 1.8–300 K were carried out, and for both complexes antiferromagnetic exchange interactions between the metal centers were observed, giving an estimated S = 17/2 ground state. AC magnetic susceptibility data have revealed out-of-phase signals, which suggest that these complexes exhibit a slow relaxation of magnetization as observed in single-molecule magnets.     

Direct observation of the preference of hole transfer over electron transfer for radical ion pair recombination in donor-bridge-acceptor molecules

Understanding how the electronic structures of electron donor-bridge-acceptor (D-B-A) molecules influence the lifetimes of radical ion pairs (RPs) photogenerated within them (D+*-B-A-*) is critical to designing and developing molecular systems for solar energy conversion. A general question that often arises is whether the HOMOs or LUMOs of D, B, and A within D+*-B-A-* are primarily involved in charge recombination. We have developed a new series of D-B-A molecules consisting of a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) electron donor linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor via a series of Phn oligomers, where n = 1-4, to give DMJ-An-Phn-NI. The photoexcited charge transfer state of DMJ-An acts as a high-potential photoreductant to rapidly and nearly quantitatively transfer an electron across the Phn bridge to produce a spin-coherent singlet RP 1(DMJ+*-An-Phn-NI-*). Subsequent radical pair intersystem crossing yields 3(DMJ+*-An-Phn-NI-*). Charge recombination within the triplet RP then gives the neutral triplet state. Time-resolved EPR spectroscopy shows directly that charge recombination of the RP initially produces a spin-polarized triplet state, DMJ-An-Phn-3*NI, that can only be produced by hole transfer involving the HOMOs of D, B, and A within the D-B-A system. After the initial formation of DMJ-An-Phn-3*NI, triplet-triplet energy transfer occurs to produce DMJ-3*An-Phn-NI with rate constants that show a distance dependence consistent with those determined for charge separation and recombination.     

Anion-perturbed magnetic slow relaxation in planar {Dy4} clusters

Two planar tetranuclear dysprosium(III) complexes, [Dy(4)(mu(3)-OH)(2)(hmmpH)(2)(hmmp)(2)(Cl)(4)].3MeCN.MeOH (1) and [Dy(4)(mu(3)-OH)(2)(hmmpH)(2)(hmmp)(2)(N(3))(4)].4MeOH (2) {hmmpH(2) = 2-[(2-hydroxyethylimino)methyl]-6-methoxyphenol}, which exhibit an anion-dependent magnetic slow relaxation behavior, have been synthesized by in situ condensation of o-vanillin and 2-aminoethanol. The higher energy barrier observed in 2 could be the result of a more favorable crystal field and/or orientations of single-ion easy axes of magnetization of the Dy(III) ions.     

Competition between Hydrophilic and Argyrophilic Interactions in Surface Enhanced Raman Spectroscopy

The competition for binding and charge‐transfer (CT) from the nitrogen containing heterocycle pyrimidine to either silver or to water in surface enhanced Raman spectroscopy (SERS) is discussed. The correlation between the shifting observed for vibrational normal modes and CT is analyzed both experimentally using Raman spectroscopy and theoretically using electronic structure theory. Discrete features in the Raman spectrum correspond to the binding of either water or silver to each of pyrimidine's nitrogen atoms with comparable frequency shifts. Natural bond orbital (NBO) calculations in each chemical environment reveal that the magnitude of charge transfer from pyrimidine to adjacent silver atoms is only about twice that for water alone. These results suggest that the choice of solvent plays a role in determining the vibrational frequencies of nitrogen containing molecules in SERS experiments.     

Tautomerism in novel oxocorrologens

A novel corrole-type macrocycle, oxocorrologen (2), substituted with hemiquinone groups, has been synthesized. It was found to undergo multiple tautomerism of its exchangeable protons between electronegative atom sites at the macrocyclic core (nitrogen atoms) and periphery (phenol oxygen atoms). Alkylation at one macrocyclic nitrogen atom with a 4-nitrobenzyl group gave 3, which can exist in only two tautomeric forms depending on the solvent. Tautomerism has been studied by means of (1)H NMR spectroscopy in a variety of solvents and solvent mixtures. Tautomer structure assignments have been supported by DFT calculations of the relative energies of the tautomers. X-ray crystallography of the N-nitrobenzyl derivative has revealed that intramolecular hydrogen bonding may be responsible for stabilizing the observed tautomers. The solvent dependence of the tautomerism of 2 and 3 confers solvatochromism. Electrochemical measurements on 2 and 3 in their respective quinone forms have revealed irreversible processes, but indicate that they are both electron-deficient with a small HOMO-LUMO gap and first reduction potentials close to those of fullerene electron acceptors.     

High-performance liquid chromatography-diode array detection/electrospray ionization mass spectrometry for the simultaneous analysis of cis-, trans- and dihydro-2-glucosyloxycinnamic acid derivatives from Dendrobium medicinal plants

Sensitive, selective and reliable high-performance liquid chromatography (HPLC)-diode array detection (DAD)/electrospray ionization multi-stage mass spectrometry (ESI-MSn) methods have been developed for the characterization of nine 2-glucosyloxycinnamic acid derivatives and quantitative analysis of three of the major 2-glucosyloxycinnamic acids, cis-melilotoside, trans-melilotoside and dihydromelilotoside, present in Dendrobium medicinal plants. The identities of the latter three major 2-glucosyloxycinnamic acids were confirmed by comparing their retention times, UV and mass spectra with those of the reference standards. The characteristic ESI-MSn fragmentation patterns of the remaining six 2-glucosyloxycinnamic acid derivatives, which are similar to the three major compounds, have allowed the putative elucidation of their structures. The concentrations of the cis-, trans- and dihydromelilotosides were simultaneously determined by HPLC/ESI-MS2 using the multiple reaction monitoring (MRM) mode in extracts of Dendrobium species. The method was validated with respect to the overall intra- and inter-day variation (RSD less than 8%) and the limits of quantification for the cis-, trans- and dihydromelilotosides were 0.09, 0.09 and 0.01 microg/mL, respectively.     

Alpha-glucosidase inhibitors from Millettia conraui

A new geranylated isoflavone, 7-O-geranyl-6-methoxypseudobaptigenin (1) was isolated from the stem barks of Millettia conraui, along with known compounds 5-methoxydurmillone (2), conrauinone A (3), beta-amyrine (4), sitosterol (5), 3-O-beta-D-glucopyranosyl sitosterol (6) and n-docosanol (7). Compounds 1 and 4 showed a significant alpha-glucosidase inhibitory activity. The structures of the compounds were determined by analysis of their spectroscopic data.     

Selective reductions in the sphere of organophosphorus compounds

Recent results on the selective reduction of cyclic vinylphosphine oxides and vinylphosphinates, as well as their refunctionalization by the use of borane, are summarized. The selective reduction of the phosphorus moiety of unsaturated phosphonates, phosphinates, and phosphine oxides is also discussed. © 2001 John Wiley & Sons, Inc. Heteroatom Chem 12:161–167, 2001