Single-molecule magnets: a new class of tetranuclear manganese magnets

The preparation, X-ray structure, and detailed physical characterization are presented for a new type of single-molecule magnet [Mn4(O2CMe)2(pdmH)6](ClO4)2 (1). Complex 1.2MeCN.Et2O crystallizes in the triclinic space group P1, with cell dimensions at 130 K of a = 11.914(3) A, b = 15.347(4) A, c = 9.660(3) A, alpha = 104.58(1) degree, beta = 93.42(1) degree, gamma = 106.06(1) degree, and Z = 1. The cation lies on an inversion center and consists of a planar Mn4 rhombus that is mixed-valent, MnIII2MnII2. The pdmH- ligands (pdmH2 is pyridine-2,6-dimethanol) function as either bidentate or tridentate ligands. The bridging between Mn atoms is established by either a deprotonated oxygen atom of a pdmH- ligand or an acetate ligand. The solvated complex readily loses all acetonitrile and ether solvate molecules to give complex 1, which with time becomes hydrated to give 1.2.5H2O. Direct current and alternating current magnetic susceptibility data are given for 1 and 1.2.5H2O and indicate that the desolvated complex has a S = 8 ground state, whereas the hydrated 1.2.5H2O has a S = 9 ground state. Ferromagnetic interactions between MnIII-MnII and MnIII-MnIII pairs result in parallel spin alignments of the S = 5/2 MnII and S = 2 MnIII ions. High-frequency EPR spectra were run for complex 1.2.5H2O at frequencies of 218, 328, and 436 GHz in the 4.5-30 K range. A magnetic-field-oriented polycrystallite sample was employed. Fine structure is clearly seen in this parallel-field EPR spectrum. The transition fields were least-squares-fit to give g = 1.99, D = -0.451 K, and B4 degrees = 2.94 x 10(-5) K for the S = 9 ground state of 1.2.5H2O. A molecule with a large-spin ground state with D < 0 can function as a single-molecule magnet, as detected by techniques such as ac magnetic susceptibility. Out-of-phase ac signals (chi' M) were seen for complexes 1 and 1.2.5H2O to show that these complexes are single-molecule magnets. A sample of 1 was studied by ac susceptibility in the 0.4-6.4 K range with the ac field oscillating at frequencies in the 1.1-1000 Hz range. A single peak in chi' M vs temperature plots was seen for each frequency; the temperature of the chi' M peak varies from 2.03 K at 995 Hz to 1.16 K at 1.1 Hz. Magnetization relaxation rates were evaluated in this way. An Arrhenius plot gave an activation energy of 17.3 K, which, as expected, is less than the 22.4 K value calculated for the thermodynamic barrier for magnetization direction reversal for an S = 8 complex with D = -0.35 K. The 1.2.5H2O complex with an S = 9 ground state has its chi' M peaks at higher temperatures.     

Osmium(VI) complexes as a new class of potential anti-cancer agents

A nitridoosmium(VI) complex [Os(VI)(N)(sap)(OH(2))Cl] (H(2)sap = N-salicylidene-2-aminophenol) displays prominent in vitro and in vivo anti-cancer properties, induces S- and G2/M-phase arrest and forms a stable adduct with dianionic 5'-guanosine monophosphate.     

Solid-state dinuclear-to-trinuclear conversion in an oxalato-bridged chromium(III)-cobalt(II) complex as a new route toward single-molecule magnets

A novel bis(oxalato)chromium(III) salt of a ferromagnetically coupled, oxalato-bridged dinuclear chromium(III)-cobalt(II) complex of formula [CrL(ox)(2)CoL'(H(2)O)(2)][CrL(ox)(2)]·4H(2)O (1) has been self-assembled in solution using different aromatic α,α'-diimines as blocking ligands, such as 2,2'-bipyridine (L = bpy) and 2,9-dimethyl-1,10-phenanthroline (L' = Me(2)phen). Thermal dehydration of 1 leads to an intriguing solid-state reaction between the S = 3/2 Cr(III) anions and the S = 3 Cr(III)Co(II) cations to give a ferromagnetically coupled, oxalato-bridged trinuclear chromium(III)-cobalt(II) complex of formula {[CrL(ox)(2)](2)CoL'} (2). Complex 2 possesses a moderately anisotropic S = 9/2 Cr(III)(2)Co(II) ground state, and it exhibits slow magnetic relaxation behavior at very low temperatures (T(B) < 2.0 K).     

Diruthenium(II,III) tetramidates as a new class of oxygenation catalysts

Two new diruthenium(II,III) tetramidate compounds, Ru(2)(NHOCC(CH(3))(2))(4)Cl (1) and Ru(2)(NHOCCH(2)CH(3))(4)Cl (2) have been prepared and structurally characterized by X-ray crystallography. The activity of promoting sulfide oxygenation using simple oxidants such as hydrogen peroxide (H(2)O(2)) and tert-butyl hydroperoxide (TBHP) was studied. A UV-kinetics study indicated that the initial rates of 1 and 2 are comparable to the previously studied diruthenium tetracarboxylates in promoting TBHP oxygenation of methyl phenyl sulfide (MPS). Using excess oxidant and CH(3)CN as the solvent, organic sulfides MPS and diphenyl sulfide (PPS) were oxidized using 1 mol% of the catalytic species. Compound 1 is more effective than 2 in converting sulfides to sulfoxide under the same conditions. Fast conversion was achieved when the reactions were carried out in the solvent-free conditions, and the major oxidation product was the sulfoxide. The electronic structure of the title compounds was studied with DFT calculations to gain an understanding of the activation of peroxy reagents.     

Poly(guanidyl)silanes as a new class of chelating, N-based ligand

The series of poly(guanidyl)silanes RnSi[hpp](4-n) [hpp = 1,3,4,6,7,8-hexahydro-2Hpyrimido[1,2-a]pyrimidinate; R = Me, n= 1, 2; R = Ph, n= 1] have been synthesised and their coordination behaviour at copper(I) halides investigated.     

Physiologically stable vanadium(IV) porphyrins as a new class of anti-HIV agents

The water soluble oxovanadium(IV) tetraarylporphyrin has demonstrated excellent solution stability against glutathione reduction and high potency (5 microM, 97% inhibition) in inhibiting HIV-1 replication in Hut/CCR5 cells.     

Coumarinyl(thienyl)thiazoles as new fluorescent molecular photoswitches

A number of new photochromic 3-(4-phenylthiazol-5-yl)-and 3-(4-hetarylthiazol-5-yl)- coumarins has been synthesized. These compounds possess properties of molecular photo- switches providing a reversible change of the fluorescence intensity in the visible region of the spectrum upon alternating irradiation of their solutions with the visible and UV light. Irradiation with the UV light (λ < 400 nm) leads to their electrocyclization and loss of fluorescence, whereas irradiation of the cyclic form with the visible light (λ < 400 nm) returns the system to the state with the original absorption and fluorescence spectra. Switching of fluorescence is also observed in polymer matrices.     

A novel class of tetrairon(III) single-molecule magnets with graphene-binding groups

Tripods of general formula R’–O–CH₂C(CH₂OH)₃ are excellent site-specific ligands for the preparation of functionalized Fe₄ single-molecule magnets. Herein, we describe the synthesis and characterization of two novel complexes designed to bind graphene surfaces, in which the R group consists of an alkyl spacer –(CH₂)_n– (n = 6 and 10) and a terminal pyrenyl moiety. The site-specific ligand substitution on [Fe₄(OMe)₆(dpm)₆] (Hdpm = dipivaloylmethane) with the new tripods has been studied with ²H NMR on isotopically-enriched samples, revealing that, once formed, these clusters are stable in solution over long timescales. It was not possible to isolate the new compounds as crystalline solids, nevertheless they were chemically characterized by elemental analysis and ¹H NMR. The presence of the pyrenyl ending groups prompted us to investigate the effect of metal complexation on fluorescence, and a full pyrene-to-iron cluster excitation energy transfer was observed. The analysis of the magnetic behaviour revealed an S = 5 ground spin state with a negative zero-field splitting parameter D = ?0.42 cm^?1.     

Design of molecular magnets

The conventional magnetic materials used in present-day technology, such as Fe, Fe₂O₃, Cr₂O₃, SmCo₅, Nd₂Fe₁₄B, etc. are all atom-based, whose synthesis requires high-temperature routes. Employing ambient-temperature synthetic organic chemistry, it has become possible to engineer a bulk molecular material with long-range magnetic order, primarily due to the weak nature of intermolecular interactions in it. Typical synthetic approach to design molecule-based magnets consists of choosing molecular precursors, each bearing an unpaired spin, and assembling them in such a way that there is no compensation of spins at the scale of the crystal lattice. Magnetism being a co-operative effect, the spin-spin interaction must extend to all the three dimensions, either through space or through bonds. Specific occurrence of ‘spin delocalisation’ and ‘spin polarisation’ in molecular lattices is helpful in bringing about ferromagnetic interaction by facilitating necessary intermolecular exchange interactions. Since the first successful synthesis of molecular magnets in 1986, a large variety of them have been synthesized, which can be classified on the basis of the chemical nature of the magnetic units involved: organic systems, metal-based systems, hetero-bimetallic assemblies, or mixed organic-inorganic systems. The design of molecular magnets has also opened the doors for the unique possibility of designing polyfunctional molecular materials, such as magnets exhibiting second-order optical nonlinearity, liquid crystalline magnets, or chiral magnets. Solubility of molecular magnets, their low density and biocompatibility are attractive features. Being weakly colored, unlike the opaque classic magnets, possibilities of photomagnetic switching can be envisaged. Persistent efforts continue to design the ever-elusive polymer magnets for applications in industry. While providing a brief overview of the field of molecular magnetism, we highlight some recent developments, with emphasis on a few studies from the author's own lab.     

Oxygenic polyoxometalates: a new class of molecular propellers

The oxygen evolving catalyst [Ru(4)(μ-OH)(2)(μ-O)(4)(H(2)O)(4)(γ-SiW(10)O(36))(2)](10-) effects H(2)O(2) dismutation at rates (k = 36.8 ± 1.4 M(-1) s(-1)), one/two order of magnitude higher compared to related tetra-substituted Cu, Fe, Mn, Ni and even Co polyoxometalates, thus providing localised oxygen gas bursts to power nano-propulsion of composite materials.     

{(Mo)Mo5O21(H2O)3(SO4)}12(VO)30(H2O)20]36-: a molecular quantum spin icosidodecahedron

Self-assembly of aqueous solutions of molybdate and vanadate under reducing, mildly acidic conditions results in a polyoxomolybdate-based {Mo72V30} cluster compound Na8K16(VO)(H2O)5[K10 subset{(Mo)Mo5O21(H2O)3(SO4)}12(VO)30(H2O)20].150H2O, 1, a quantum spin-based Keplerate structure.     

Hydrothermal Synthesis and Structure of[{Mo_8V_4O_(36)(VO_4)(VO)_2}n]~(7n-)Bi-capped α-Keggin Fragments Linked to a Chain

The title compound, (H2en)3H3O{Mo8V4O36(VO4) (VO)2} ?4H2O, was hydrothermally synthesized and structurally characterized by means of IR, ESR spectrum and single crystal X-ray diffraction. It crystallized in a monoclinic system with space group P21/c, a=1. 980 4(4) nm, 6=2. 063 4(4) nm, c=l. 192 0(2) nm, =94. 76(3)?and deep black colour. The compound contains V-centered bi-capped a-Keggin fragments {Mo8V7O42} that are linked together by edge-shared units V O5 via V桹梀 bonds, forming a chain.     

LambdaO4 upside down: a new molecular structure for supported VO4 catalysts

Vanadium oxide (1 wt %) supported on gamma-Al(2)O(3) was used to investigate the interface between the catalytically active species and the support oxide. Raman, UV-vis-NIR DRS, ESR, XANES, and EXAFS were used to characterize the sample in great detail. All techniques showed that an isolated VO(4) species was present at the catalyst surface, which implies that no V-O-V moiety is present. Surprisingly, a Raman band was present at 900 cm(-1), which is commonly assigned to a V-O-V vibration. This observation contradicts the current literature assignment. To further elucidate on potential other Raman assignments, the exact molecular structure of the VO(4) entity (1 V=O bond of 1.58 A and 3 V-O bonds of 1.72 A) together with its position relative to the support O anions and Al cation of the Al(2)O(3) support has been investigated with EXAFS. In combination with a structural model of the alumina surface, the arrangement of the support atoms in the proximity of the VO(4) entity could be clarified, leading to a new molecular structure of the interface between VO(4) and Al(2)O(3). It was found that VO(4) is anchored to the support oxide surface, with only one V-O support bond instead of three, which is commonly accepted in the literature. The structural model suggested in this paper leaves three possible assignments for the 900 cm(-1) band: a V-O-Al vibration, a V-O-H vibration, and a V-(O-O) vibration. The pros and cons of these different options will be discussed.     

Bis(benzimidazole)pyridine derivative as a new class of G-quadruplex inducing and stabilizing ligand

Two new bis(benzimidazole)aryl derivatives have been prepared and one of them has been shown to induce and stabilize formation of a G-quadruplex.     

Syntheses and characterization of a new class of vanadia precursors of oxime-modified oxovanadium(V) isopropoxide,crystal and molecular structure of [VO{ONC10H16}3]

Modification of [VO(OPrⁱ)₃] with oximes in different molar ratios, yielded new class of vanadia precursors, [VO{OPrⁱ}_3?n{L}_n] {where, n = 1–3 and LH = C₉H₁₆C=NOH (1–3) and (CH₃)₂C=NOH (4–6)}.All the products are yellow in colour. (1) and (2) are liquid/viscous liquid, while others are solids. Molecular weight measurements of all these derivatives and the ESI-mass spectral studies of (1), (2), (3) and (5) indicate their monomeric nature. ¹H and ¹³C{¹H} NMR spectra suggest that the oximato moieties are monodentate in solution which was further confirmed by the ⁵¹V NMR signals, appeared in the region expected for tetra-coordinated oxo-vanadium atoms. On ageing, a disproportionation reaction occurs in (1) and some crystals appeared. Single crystal X-ray diffraction analyses of the crystals obtained from (1) as well as from (3) were found to be the same and indicate the presence of side-on {dihapto η ²-(N, O)} binding modes of the oximato ligands, leading to the formation of seven coordination environment around the vanadium atom. Thermogravimetric curve of (1) exhibits multi-step decomposition with the formation of V₂O₅ as the final product at ~850 °C. Sol–gel transformation of (3) yielded (a) VO₂ sintered at 300 °C and (b) V₂O₅ at 600 °C. Similarly, sol–gel transformations of (1) and (2) yielded V₂O₅ (c) and (d) at 600 °C, respectively. Formation of monoclinic phase in (a) and orthorhombic phase in (b), (c) and (d) were confirmed by powder XRD patterns.