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We report the synthesis of a semiquinoid-bridged single-chain magnet, as generated through a thermally induced metal-ligand electron transfer. Reaction of FeCl 3 with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH 2 ) in the presence of (NMe 4 )Cl gave the compound (NMe 4 ) 2 [LFeCl 2 ]. Together, variable-temperature X-ray diffraction, Mössbauer spectra, Raman spectra, and dc magnetic susceptibility reveal a transition from a chain containing (L 2- )Fe II units to one with (L 3-• )Fe III upon decreasing temperature, with a transition temperature of T 1/2 = 213 K. The dc magnetic susceptibility measurements show strong metal-radical coupling within the chain, with a coupling constant of J = -81 cm -1 , and ac susceptibility data reveal slow magnetic relaxation, with a relaxation barrier of Δ τ = 55(1) cm -1 . To our knowledge, this compound provides the first example of a semiquinoid-bridged single-chain magnet.

A fundamental challenge in the design of bioresponsive (or bioactivated) Gd III -based magnetic resonance (MR) imaging probes is the considerable background signal present in the "preactivated" state that arises from outer-sphere relaxation processes. When sufficient concentrations of a bioresponsive agent are present (i.e., a detectable signal in the image), the inner- and outer-sphere contributions to r 1 may be misinterpreted to conclude that the agent has been activated, when it has not. Of the several parameters that determine the observed MR signal of an agent, only the electron relaxation time ( T 1e ) impacts both the inner- and outer-sphere relaxation. Therefore, strategies to minimize this background signal must be developed to create a near zero-background (or truly "off" state) of the agent. Here, we demonstrate that intramolecular magnetic exchange coupling when Gd III is coupled to a paramagnetic transition metal provides a means to overcome the contribution of second- and outer-sphere contributions to the observed relaxivity. We have prepared a series of complexes with the general formula LMLn(μ-O 2 CCH 3 )(O 2 CCH 3 ) 2 (M = Co, Cu, Zn). Solid-state magnetic susceptibility measurements reveal significant magnetic coupling between Gd III and the transition metal ion. Nuclear magnetic relaxation dispersion (NMRD) analysis confirms that the observed differences in relaxivity are associated with the modulation of T 1e at Gd III . These results clearly demonstrate that magnetic exchange coupling between Gd III and a transition metal ion can provide a significant decrease in T 1e (and therefore the relaxivity of Gd III ). This design strategy is being exploited to prepare new generations of preclinical bioresponsive MR imaging probes with near zero-background.

Success in the projects aimed at providing an advanced understanding of the brain is directly predicated on making critical advances in nanotechnology. This Perspective addresses the unique interface of neuroscience and nanomaterials by considering the foundational problem of sensing neuron membrane voltage and offers a potential solution that may be facilitated by a prototypical nanomaterial. Despite substantial improvements, the visualization of instantaneous voltage changes within individual neurons, whether in cell culture or in vivo, at both the single-cell and network level at high speed remains complex and problematic. The unique properties of semiconductor quantum dots (QDs) have made them powerful fluorophores for bioimaging. What is not widely appreciated, however, is that QD photoluminescence is exquisitely sensitive to proximal electric fields. This property should be suitable for sensing voltage changes that occur in the active neuronal membrane. Here, we examine the potential role of QDs in addressing the important challenge of real-time optical voltage imaging.

The role of peroxometal species as reactive intermediates in myriad biological processes has motivated the synthesis and study of analogous molecular model complexes. Peroxomanganese(iv) porphyrin complexes are of particular interest, owing to their potential ability to form from reversible O 2 binding, yet have been exceedingly difficult to isolate and characterize in molecular form. Alternatively, immobilization of metalloporphyrin sites within a metal-organic framework (MOF) can enable the study of interactions between low-coordinate metal centers and gaseous substrates, without interference from bimolecular reactions and axial ligation by solvent molecules. Here, we employ this approach to isolate the first rigorously four-coordinate manganese(ii) porphyrin complex and examine its reactivity with O 2 using infrared spectroscopy, single-crystal X-ray diffraction, EPR spectroscopy, and O 2 adsorption analysis. X-ray diffraction experiments reveal for the first time a peroxomanganese(iv) porphyrin species, which exhibits a side-on, η 2 binding mode. Infrared and EPR spectroscopic data confirm the formulation of a peroxomanganese(iv) electronic structure, and show that O 2 binding is reversible at ambient temperature, in contrast to what has been observed in molecular form. Finally, O 2 gas adsorption measurements are employed to quantify the enthalpy of O 2 binding as h ads = -49.6(8) kJ mol -1 . This enthalpy is considerably higher than in the corresponding Fe- and Co-based MOFs, and is found to increase with increasing reductive capacity of the M II/III redox couple.

An Fe II complex that features a pH-dependent spin state population, by virtue of a variable ligand protonation state, is described. This behavior leads to a highly pH-dependent 19 F NMR chemical shift with a sensitivity of 13.9(5) ppm per pH unit at 37 °C, thereby demonstrating the potential utility of the complex as a 19 F chemical shift-based pH sensor.

We report a Co 2 -based magnetic resonance (MR) probe that enables the ratiometric quantitation and imaging of pH through chemical exchange saturation transfer (CEST). This approach is illustrated in a series of air- and water-stable Co II 2 complexes featuring CEST-active tetra(carboxamide) and/or hydroxyl-substituted bisphosphonate ligands. For the complex bearing both ligands, variable-pH CEST and NMR analyses reveal highly shifted carboxamide and hydroxyl peaks with intensities that increase and decrease with increasing pH, respectively. The ratios of CEST peak intensities at 104 and 64 ppm are correlated with solution pH in the physiological range 6.5-7.6 to construct a linear calibration curve of log(CEST 104 ppm /CEST 64 ppm ) versus pH, which exhibits a remarkably high pH sensitivity of 0.99(7) pH unit -1 at 37 °C. In contrast, the analogous Co II 2 complex with a CEST-inactive bisphosphonate ligand exhibits no such pH response, confirming that the pH sensitivity stems from the integration of amide and hydroxyl CEST effects that show base- and acid-catalyzed proton exchange, respectively. Importantly, the pH calibration curve is independent of the probe concentration and is identical in aqueous buffer and fetal bovine serum. Furthermore, phantom images reveal analogous linear pH behavior. The Co II 2 probe is stable toward millimolar concentrations of H 2 PO 4 - /HPO 4 2- , CO 3 2- , SO 4 2- , CH 3 COO - , and Ca 2+ ions, and more than 50% of melanoma cells remain viable in the presence of millimolar concentrations of the complex. The stability of the probe in physiological environments suggests that it may be suitable for in vivo studies. Together, these results highlight the ability of dinuclear transition metal PARACEST probes to provide a concentration-independent measure of pH, and they provide a potential design strategy toward the development of MR probes for ratiometric pH imaging.

Sensory, motor and cognitive operations involve the coordinated action of large neuronal populations across multiple brain regions in both superficial and deep structures. Existing extracellular probes record neural activity with excellent spatial and temporal (sub-millisecond) resolution, but from only a few dozen neurons per shank. Optical Ca 2+ imaging offers more coverage but lacks the temporal resolution needed to distinguish individual spikes reliably and does not measure local field potentials. Until now, no technology compatible with use in unrestrained animals has combined high spatiotemporal resolution with large volume coverage. Here we design, fabricate and test a new silicon probe known as Neuropixels to meet this need. Each probe has 384 recording channels that can programmably address 960 complementary metal-oxide-semiconductor (CMOS) processing-compatible low-impedance TiN sites that tile a single 10-mm long, 70 × 20-μm cross-section shank. The 6 × 9-mm probe base is fabricated with the shank on a single chip. Voltage signals are filtered, amplified, multiplexed and digitized on the base, allowing the direct transmission of noise-free digital data from the probe. The combination of dense recording sites and high channel count yielded well-isolated spiking activity from hundreds of neurons per probe implanted in mice and rats. Using two probes, more than 700 well-isolated single neurons were recorded simultaneously from five brain structures in an awake mouse. The fully integrated functionality and small size of Neuropixels probes allowed large populations of neurons from several brain structures to be recorded in freely moving animals. This combination of high-performance electrode technology and scalable chip fabrication methods opens a path towards recording of brain-wide neural activity during behaviour.

We demonstrate the ability of a molecular Fe 2 complex to enable magnetic resonance (MR)-based ratiometric quantitation of redox status, namely through redox-dependent paramagnetic chemical exchange saturation transfer (PARACEST). Metalation of a tetra(carboxamide) ligand with Fe II and/or Fe III in the presence of etidronate ion affords analogous FeII2, Fe II Fe III , and FeIII2 complexes. Both FeII2 and Fe II Fe III complexes give highly-shifted, sharp, and non-overlapping NMR spectra, with multiple resonances for each complex corresponding to exchangeable carboxamide protons. These protons can be selectively irradiated to give CEST peaks at 74 and 83 ppm vs. H 2 O for the Fe II Fe III complex and at 29, 40 and 68 ppm for the FeII2 complex. The CEST spectra obtained from a series of samples containing mixtures of FeII2 and Fe II Fe III are correlated with independently-determined open-circuit potentials to construct a Nernstian calibration curve of potential vs. CEST peak intensity ratio. In addition, averaged intensities of phantom images collected on a 9.4 T MRI scanner show analogous Nernstian behavior. Finally, both the FeII2 and Fe II Fe III forms of the complex are stable to millimolar concentrations of H 2 PO 4 - /HPO 4 2- , CO 3 2- , SO 4 2- , CH 3 COO - , and Ca 2+ ions, and the FeIII2 form is air-stable in aqueous buffer and shows >80% viability in melanoma cells at millimolar concentration. The stability suggests the possible application of this or related complexes for in vivo studies. To our knowledge, this concentration-independent method based on a single Fe 2 probe provides the first example of MR-based ratiometric quantitation of redox environment.

Here, we describe new detachable floating glass micropipette electrode devices that provide targeted action potential recordings in active moving organs without requiring constant mechanical constraint or pharmacological inhibition of tissue motion. The technology is based on the concept of a glass micropipette electrode that is held firmly during cell targeting and intracellular insertion, after which a 100-µg glass microelectrode, a "microdevice," is gently released to remain within the moving organ. The microdevices provide long-term recordings of action potentials, even during millimeter-scale movement of tissue in which the device is embedded. We demonstrate two different glass micropipette electrode holding and detachment designs appropriate for the heart (sharp glass microdevices for cardiac myocytes in rats, guinea pigs, and humans) and the brain (patch glass microdevices for neurons in rats). We explain how microdevices enable measurements of multiple cells within a moving organ that are typically difficult with other technologies. Using sharp microdevices, action potential duration was monitored continuously for 15 min in unconstrained perfused hearts during global ischemia-reperfusion, providing beat-to-beat measurements of changes in action potential duration. Action potentials from neurons in the hippocampus of anesthetized rats were measured with patch microdevices, which provided stable base potentials during long-term recordings. Our results demonstrate that detachable microdevices are an elegant and robust tool to record electrical activity with high temporal resolution and cellular level localization without disturbing the physiological working conditions of the organ. NEW & NOTEWORTHY Cellular action potential measurements within tissue using glass micropipette electrodes usually require tissue immobilization, potentially influencing the physiological relevance of the measurement. Here, we addressed this limitation with novel 100-µg detachable glass microelectrodes that can be precisely positioned to provide long-term measurements of action potential duration during unconstrained tissue movement., (Copyright © 2017 the American Physiological Society.)

Realizing atomic-level spatial control over qubits, the fundamental units of both quantum information processing systems and quantum sensors, constitutes a crucial cross-field challenge. Toward this end, embedding electronic-spin-based qubits within the framework of a crystalline porous material is a promising approach to create precise arrays of qubits. Realizing porous hosts for qubits would also impact the emerging field of quantum sensing, whereby porosity would enable analytes to infuse into a sensor matrix. However, building viable qubits into a porous material is an appreciable challenge because of the extreme sensitivity of qubits to local magnetic noise. To insulate these frameworks from ambient magnetic signals, we borrowed from atomic physics the idea to exploit clock transitions at avoided level crossings. Here, sensitivity to magnetic noise is inherently limited by the flat slope of the so-called clock transition. More specifically, we created an array of clocklike qubits within a metal-organic framework by combining coordination chemistry considerations with the fundamental concept of atomic clock transitions. Electron paramagnetic resonance studies verify a clocklike transition for the hosted cobalt(II) spins in the framework [(TCPP)Co 0.07 Zn 0.93 ] 3 [Zr 6 O 4 (OH) 4 (H 2 O) 6 ] 2 , the first demonstration in any porous material. The clocklike qubits display lifetimes of up to 14 μs despite abundant local nuclear spins, illuminating a new path toward proof-of-concept quantum sensors and processors with high inherent structural precision.

High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido [{PhB(Im R ) 3 }FeN], R = t Bu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, [{PhB(Im R ) 3 }Fe(NR')] + , R = Mes, R' = 1-adamantyl (3), t Bu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, [(TIMEN Ar )FeN] + (TIMEN Ar = tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine; Ar = Xyl (xylyl), Mes), were investigated by 57 Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3d z 2 (a 1 ) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices.

The variable-temperature CO binding at a four-coordinate cobaltous porphyrin complex within a metal-organic framework is investigated using a combined array of infrared, X-ray diffraction, EPR, and CO adsorption analysis. Single-crystal X-ray diffraction experiments provide the first crystallographically characterized example of a noniron first-row transition metal porphyrin carbonyl adduct. Variable-temperature electron paramagnetic resonance spectroscopy supports the structural observation and reveals conversion of the dicarbonyl complex to a monocarbonyl species as temperature is increased. Finally, CO adsorption analysis data enable quantification of the Co-CO interaction, providing a binding enthalpy of h ads = -29(2) kJ/mol. This value is nearly twice that observed for O 2 binding in the same compound and is attributed to a stronger σ interaction for Co and CO vs O 2 . These results demonstrate the ability of MOFs to enable a thorough investigation of CO binding in metalloporphyrins.

We report the magnetism and conductivity for a redox pair of iron-quinoid metal-organic frameworks (MOFs). The oxidized compound, (Me 2 NH 2 ) 2 [Fe 2 L 3 ]·2H 2 O·6DMF (LH 2 = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) was previously shown to magnetically order below 80 K in its solvated form, with the ordering temperature decreasing to 26 K upon desolvation. Here, we demonstrate this compound to exhibit electrical conductivity values up to σ = 1.4(7) × 10 -2 S/cm (E a = 0.26(1) cm -1 ) and 1.0(3) × 10 -3 S/cm (E a = 0.19(1) cm -1 ) in its solvated and desolvated forms, respectively. Upon soaking in a DMF solution of Cp 2 Co, the compound undergoes a single-crystal-to-single-crystal one-electron reduction to give (Cp 2 Co) 1.43 (Me 2 NH 2 ) 1.57 [Fe 2 L 3 ]·4.9DMF. Structural and spectroscopic analysis confirms this reduction to be ligand-based, and as such the trianionic framework is formulated as [Fe III 2 (L 3-• ) 3 ] 3- . Magnetic measurements for this reduced compound reveal the presence of dominant intralayer metal-organic radical coupling to give a magnetically ordered phase below T c = 105 K, one of the highest reported ordering temperatures for a MOF. This high ordering temperature is significantly increased relative to the oxidized compound, and stems from the overall increase in coupling strength afforded by an additional organic radical. In line with the high critical temperature, the new MOF exhibits magnetic hysteresis up to 100 K, as revealed by variable-field measurements. Finally, this compound is electrically conductive, with values up to σ = 5.1(3) × 10 -4 S/cm with E a = 0.34(1) eV. Taken together, these results demonstrate the unique ability of metal-quinoid MOFs to simultaneously exhibit both high magnetic ordering temperatures and high electrical conductivity.

The potential utility of paramagnetic transition metal complexes as chemical shift 19 F magnetic resonance (MR) thermometers is demonstrated. Further, spin-crossover Fe II complexes are shown to provide much higher temperature sensitivity than do the high-spin analogues, owing to the variation of spin state with temperature in the former complexes. This approach is illustrated through a series of Fe II complexes supported by symmetrically and asymmetrically substituted 1,4,7-triazacyclononane ligand scaffolds bearing 3-fluoro-2-picolyl derivatives as pendent groups (L x ). Variable-temperature magnetic susceptibility measurements, in conjunction with UV-vis and NMR data, show thermally-induced spin-crossover for [Fe(L 1 )] 2+ in H 2 O, with T 1/2 = 52(1) °C. Conversely, [Fe(L 2 )] 2+ remains high-spin in the temperature range 4-61 °C. Variable-temperature 19 F NMR spectra reveal the chemical shifts of the complexes to exhibit a linear temperature dependence, with the two peaks of the spin-crossover complex providing temperature sensitivities of +0.52(1) and +0.45(1) ppm per °C in H 2 O. These values represent more than two-fold higher sensitivity than that afforded by the high-spin analogue, and ca. 40-fold higher sensitivity than diamagnetic perfluorocarbon-based thermometers. Finally, these complexes exhibit excellent stability in a physiological environment, as evidenced by 19 F NMR spectra collected in fetal bovine serum.

Unlabelled: While commonplace in clinical settings, DNA-based assays for identification or enumeration of drinking water pathogens and other biological contaminants remain widely unadopted by the monitoring community. In this study, shotgun metagenomics was used to identify taste-and-odor producers and toxin-producing cyanobacteria over a 2-year period in a drinking water reservoir. The sequencing data implicated several cyanobacteria, including Anabaena spp., Microcystis spp., and an unresolved member of the order Oscillatoriales as the likely principal producers of geosmin, microcystin, and 2-methylisoborneol (MIB), respectively. To further demonstrate this, quantitative PCR (qPCR) assays targeting geosmin-producing Anabaena and microcystin-producing Microcystis were utilized, and these data were fitted using generalized linear models and compared with routine monitoring data, including microscopic cell counts, sonde-based physicochemical analyses, and assays of all inorganic and organic nitrogen and phosphorus forms and fractions. The qPCR assays explained the greatest variation in observed geosmin (adjusted R(2) = 0.71) and microcystin (adjusted R(2) = 0.84) concentrations over the study period, highlighting their potential for routine monitoring applications. The origin of the monoterpene cyclase required for MIB biosynthesis was putatively linked to a periphytic cyanobacterial mat attached to the concrete drinking water inflow structure. We conclude that shotgun metagenomics can be used to identify microbial agents involved in water quality deterioration and to guide PCR assay selection or design for routine monitoring purposes. Finally, we offer estimates of microbial diversity and metagenomic coverage of our data sets for reference to others wishing to apply shotgun metagenomics to other lacustrine systems., Importance: Cyanobacterial toxins and microbial taste-and-odor compounds are a growing concern for drinking water utilities reliant upon surface water resources. Specific identification of the microorganism(s) responsible for water quality degradation is often complicated by the presence of co-occurring taxa capable of producing these undesirable metabolites. Here we present a framework for how shotgun metagenomics can be used to definitively identify problematic microorganisms and how these data can guide the development of rapid genetic assays for routine monitoring purposes., (Copyright © 2016, American Society for Microbiology. All Rights Reserved.)

The ability of magnetic exchange coupling to enable observation of paramagnetic chemical exchange saturation transfer (PARACEST) in transition metal ions with long electronic relaxation times (τs) is demonstrated. Metalation of the dinucleating, tetra(carboxamide) ligand HL with Cu(2+) in the presence of pyrophosphate (P2O7)(4-) affords the complex [LCu(II)2(P2O7)](-). Solution-phase variable-temperature magnetic susceptibility data reveal weak ferromagnetic superexchange coupling between the two S = 1/2 Cu(II) centers, with a coupling constant of J = +2.69(5) cm(-1), to give an S = 1 ground state. This coupling results in a sharpened NMR line width relative to a GaCu analogue, indicative of a shortening of τs. Presaturation of the amide protons in the Cu2 complex at 37 °C leads to a 14% intensity decrease in the bulk water (1)H NMR signal through the CEST effect. Conversely, no CEST effect is observed in the GaCu complex. These results provide the first example of a Cu-based PARACEST magnetic resonance contrast agent and demonstrate the potential to expand the metal ion toolbox for PARACEST agents through introduction of magnetic exchange coupling.

We describe and employ a high-throughput screening method to accelerate the synthesis and identification of pure-phase, nanocrystalline metal-organic frameworks (MOFs). We demonstrate the efficacy of this method through its application to a series of porphyrinic zirconium MOFs, resulting in the isolation of MOF-525, MOF-545, and PCN-223 on the nanoscale.

We demonstrate that incorporation of a redox-active benzoquinoid ligand into a one-dimensional chain compound can give rise to a material that exhibits simultaneous solid-state redox switching of optical, magnetic, and electronic properties. Metalation of the ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol ((N,O)LH4) with Mn(III) affords the chain compound Mn((N,O)L)(DMSO). Structural and spectroscopic analysis of this compound show the presence of Mn(II) centers bridged by (N,O)L(2-) ligands, resulting partially from a spontaneous ligand-to-metal electron transfer. Upon soaking in a solution of the reductant Cp2Co, Mn((N,O)L)(DMSO) undergoes a ligand-centered solid-state reduction to [Mn((N,O)L)](-), as revealed by a suite of techniques, including Raman and X-ray absorption spectroscopy. The ligand-based reduction engenders a dramatic modulation of the physical properties of the chain compound. An electrochromic response, evidenced by a color change from dark green to dark purple is accompanied by a nearly 40-fold increase in magnetic coupling strength, from J = -0.38(1) to -15.6(2) cm(-1), and a 10,000-fold increase in electronic conductivity, from σ = 2.33(1) × 10(-12) S/cm (Ea = 0.64(1) eV) to 8.61(1) × 10(-8) S/cm (Ea = 0.39(1) eV). Importantly, the chemical reduction is reversible: treatment of the reduced compound with [Cp2Fe](+) regenerates the oxidized chain. Taken together, these results highlight the ability of benzoquinoid ligands to facilitate solid-state ligand-based redox reactions in nonporous coordination solids, giving rise to reversible switching of optical properties, magnetic exchange interactions, and electronic conductivity.

The asymmetric redox-active ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol ((N,O)LH4) is prepared and metalated to afford the hexanuclear complex [Mn6((N,O)L)6](6-). Structural analysis and magnetic measurements reveal this complex to feature Mn(II) ions bridged by (N,O)L(3-)˙ radicals, which are antiferromagnetically coupled to give an S = 12 ground state.

The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) with Fe(II) affords the solid (Me2NH2)2[Fe2L3]·2H2O·6DMF. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of Fe(III) centers with mixed-valence ligands of the form (L3)(8-) that result from a spontaneous electron transfer from Fe(II) to L(2-). Upon removal of DMF and H2O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me2NH2)2[Fe2L3], and a fit to N2 adsorption data of this activated compound gives a BET surface area of 885(105) m(2)/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally characterized extended solid that contains tetraoxolene radical ligands.

The ability of tetraazalene radical bridging ligands to mediate exceptionally strong magnetic exchange coupling across a range of transition metal complexes is demonstrated. The redox-active bridging ligand N , N ', N '', N '''-tetra(2-methylphenyl)-2,5-diamino-1,4-diiminobenzoquinone ( NMePh LH 2 ) was metalated to give the series of dinuclear complexes [(TPyA) 2 M 2 ( NMePh L 2- )] 2+ (TPyA = tris(2-pyridylmethyl)amine, M = Mn II , Fe II , Co II ). Variable-temperature dc magnetic susceptibility data for these complexes reveal the presence of weak superexchange interactions between metal centers, and fits to the data provide coupling constants of J = -1.64(1) and -2.16(2) cm -1 for M = Mn II and Fe II , respectively. One-electron reduction of the complexes affords the reduced analogues [(TPyA) 2 M 2 ( NMePh L 3- ˙)] + . Following a slightly different synthetic procedure, the related complex [(TPyA) 2 CrIII2( NMePh L 3- ˙)] 3+ was obtained. X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy indicate the presence of radical NMePh L 3- ˙ bridging ligands in these complexes. Variable-temperature dc magnetic susceptibility data of the radical-bridged species reveal the presence of strong magnetic interactions between metal centers and ligand radicals, with simulations to data providing exchange constants of J = -626(7), -157(7), -307(9), and -396(16) cm -1 for M = Cr III , Mn II , Fe II , and Co II , respectively. Moreover, the strength of magnetic exchange in the radical-bridged complexes increases linearly with decreasing M-L bond distance in the oxidized analogues. Finally, ac magnetic susceptibility measurements reveal that [(TPyA) 2 Fe 2 ( NMePh L 3- ˙)] + behaves as a single-molecule magnet with a relaxation barrier of U eff = 52(1) cm -1 . These results highlight the ability of redox-active tetraazalene bridging ligands to enable dramatic enhancement of magnetic exchange coupling upon redox chemistry and provide a rare opportunity to examine metal-radical coupling trends across a transmetallic series of complexes.

The ability of a benzoquinonoid bridging ligand to mediate double-exchange coupling in a mixed-valence Fe2 complex is demonstrated. Metalation of the bridging ligand 2,5-di(2,6-dimethylanilino)-3,6-dibromo-1,4-benzoquinone (LH2) with Fe(II) in the presence of the capping ligand tris((6-methyl-2-pyridyl)methyl)amine (Me3TPyA) affords the dinuclear complex [(Me3TPyA)2Fe(II)2(L)](2+). The dc magnetic measurements, in conjunction with X-ray diffraction and Mössbauer spectroscopy, reveal the presence of weak ferromagnetic superexchange coupling between Fe(II) centers through the diamagnetic bridging ligand to give an S = 4 ground state. The ac magnetic susceptibility measurements, collected in a small dc field, show this complex to behave as a single-molecule magnet with a relaxation barrier of U(eff) = 14(1) cm(-1). The slow magnetic relaxation in the Fe(II)2 complex can be switched off through one-electron oxidation to the mixed-valence congener [(Me3TPyA)2Fe2(L)](3+), where X-ray diffraction and Mössbauer spectroscopy indicate a metal-centered oxidation. The dc magnetic measurements show an S = 9/2 ground state for the mixed-valence complex, stemming from strong ferromagnetic exchange coupling that is best described considering electron hopping through a double-exchange coupling mechanism, with a double-exchange parameter of B = 69(4) cm(-1). In accordance with double-exchange, an intense feature is observed in the near-infrared region and is assigned as an intervalence charge-transfer band. The rate of intervalence electron hopping is comparable to that of the Mössbauer time scale, such that variable-temperature Mössbauer spectra reveal a thermally activated transition from a valence-trapped to detrapped state and provide an activation energy for electron hopping of 63(8) cm(-1). These results demonstrate the ability of quinonoid ligands to mediate electron hopping between high-spin metal centers, by providing the first example of an Fe complex that exhibits double-exchange through an organic bridging ligand and the largest metal-metal separation yet observed in any metal complex with double-exchange coupling.

A series of four isostructural Fe(II)2 complexes, [(TPyA)2Fe2((X)L)](2+) (TPyA = tris(2-pyridylmethyl)amine; (X)L(2-) = doubly deprotonated form of 3,6-disubstituted-2,5-dianilino-1,4-benzoquinone; X = H, Br, Cl, and F), were synthesized to enable a systematic study of electronic effects on spin crossover behavior. Comparison of X-ray diffraction data for these complexes reveals the sole presence of high-spin Fe(II) at 225 K and mixtures of high-spin and low-spin Fe(II) at 100 K, which is indicative of incomplete spin crossover. In addition, crystal packing diagrams show that these complexes are well-isolated from one another in the solid state, owing primarily to the presence of bulky tetra(aryl)borate counteranions, such that spin crossover is likely not significantly affected by intermolecular interactions. Variable-temperature dc magnetic susceptibility data confirm the structural observations and reveal that 54(1), 56(1), 62(1), and 84(1)% of Fe(II) centers remain high-spin even below 65 K. Moreover, fits to magnetic data provide crossover temperatures of T1/2 = 160(1), 124(1), 121(1), and 110(1) K for X = H, Br, Cl, and F, respectively, along with enthalpies of ΔH = 11.4(3), 8.5(3), 8.3(3), and 7.5(2) kJ/mol, respectively. These parameters decrease with increasing electronegativity of X and thus increasing electron-withdrawing character of (X)L(2-), suggesting that the observed trends originate primarily from inductive effects of X. Moreover, when plotted as a function of the Pauling electronegativity of X, both T1/2 and ΔH undergo a linear decrease. Further analyses of the low-temperature magnetic data and variable-temperature Mössbauer spectroscopy suggest that the incomplete spin crossover behavior in [(TPyA)2Fe2((X)L)](2+) is best described as a transition from purely [FeHS-FeHS] (HS = high-spin) complexes at high temperature to a mixture of [FeHS-FeHS] and [FeHS-FeLS] (LS = low-spin) complexes at low temperature, with the number of [FeHS-FeHS] species increasing with decreasing electron-withdrawing character of (X)L(2-).

The porphyrinic metal-organic framework (MOF) PCN-224 is metalated with Fe(II) to yield a 4-coordinate ferrous heme-containing compound. The heme center binds O2 at -78 °C to give a 5-coordinate heme-O2 complex. For the first time, this elusive species is structurally characterized, revealing an Fe(III) center coordinated to superoxide via an end-on, η(1) linkage. Mössbauer spectroscopy supports the structural observations and indicates the presence of a low-spin electronic configuration for Fe(III). Finally, variable-temperature O2 adsorption data enable quantification of the Fe-O2 interaction, providing a binding enthalpy of -34(4) kJ/mol. This value is nearly half of that observed for comparable 6-coordinate, imidazole-bound heme-O2 complexes, a difference that further illustrates the importance of axial ligands in biological heme-mediated O2 transport and storage. These results demonstrate the ability of a MOF, by virtue of its rigid solid-state structure, to enable isolation and thorough characterization of a species that can only be observed transiently in molecular form.

Monitoring representative fractions of neurons from multiple brain circuits in behaving animals is necessary for understanding neuronal computation. Here, we describe a system that allows high-channel-count recordings from a small volume of neuronal tissue using a lightweight signal multiplexing headstage that permits free behavior of small rodents. The system integrates multishank, high-density recording silicon probes, ultraflexible interconnects, and a miniaturized microdrive. These improvements allowed for simultaneous recordings of local field potentials and unit activity from hundreds of sites without confining free movements of the animal. The advantages of large-scale recordings are illustrated by determining the electroanatomic boundaries of layers and regions in the hippocampus and neocortex and constructing a circuit diagram of functional connections among neurons in real anatomic space. These methods will allow the investigation of circuit operations and behavior-dependent interregional interactions for testing hypotheses of neural networks and brain function.

One-electron reduction of the complex [(TPyA)2Fe(II)2((NPh)L(2-))](2+) (TPyA = tris(2-pyridylmethyl)amine, (NPh)LH2 = azophenine = N,N',N",N'''-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone) affords the complex [(TPyA)2Fe(II)2((NPh)L(3-•))](+). X-ray diffraction and Mössbauer spectroscopy confirm that the reduction occurs on (NPh)L(2-) to give an S = 1/2 radical bridging ligand. Dc magnetic susceptibility measurements demonstrate the presence of extremely strong direct antiferromagnetic exchange between S = 2 Fe(II) centers and (NPh)L(3-•) in the reduced complex, giving an S = 7/2 ground state with an estimated coupling constant magnitude of |J| ≥ 900 cm(-1). Mössbauer spectroscopy and ac magnetic susceptibility reveal that this complex behaves as a single-molecule magnet with a spin relaxation barrier of U(eff) = 50(1) cm(-1). To our knowledge, this complex exhibits by far the strongest magnetic exchange coupling ever to be observed in a single-molecule magnet.

The subcellular locations of synapses on pyramidal neurons strongly influences dendritic integration and synaptic plasticity. Despite this, there is little quantitative data on spatial distributions of specific types of synaptic input. Here we use array tomography (AT), a high-resolution optical microscopy method, to examine thalamocortical (TC) input onto layer 5 pyramidal neurons. We first verified the ability of AT to identify synapses using parallel electron microscopic analysis of TC synapses in layer 4. We then use large-scale array tomography (LSAT) to measure TC synapse distribution on L5 pyramidal neurons in a 1.00 × 0.83 × 0.21 mm(3) volume of mouse somatosensory cortex. We found that TC synapses primarily target basal dendrites in layer 5, but also make a considerable input to proximal apical dendrites in L4, consistent with previous work. Our analysis further suggests that TC inputs are biased toward certain branches and, within branches, synapses show significant clustering with an excess of TC synapse nearest neighbors within 5-15 μm compared to a random distribution. Thus, we show that AT is a sensitive and quantitative method to map specific types of synaptic input on the dendrites of entire neurons. We anticipate that this technique will be of wide utility for mapping functionally-relevant anatomical connectivity in neural circuits.

Three complexes of Mn(III) with "scorpionate" type ligands have been investigated by a variety of physical techniques. The complexes are [Tp(2)Mn]SbF(6) (1), [Tp(2)*Mn]SbF(6) (2), and [{PhB(MeIm)(3)}(2)Mn](CF(3)SO(3)) (3a), where Tp(-) = hydrotris(pyrazolyl)borate anion, Tp*(-) = hydrotris(3,5-dimethylpyrazolyl)borate anion, and PhB(MeIm)(3)(-) = phenyltris(3-methylimidazol-2-yl)borate anion. The crystal structure of 3a is reported; the structures of 1 and 2 have been previously reported, but were reconfirmed in this work. The synthesis and characterization of [{PhB(MeIm)(3)}(2)Mn]Cl (3b) are also described. These complexes are of interest in that, in contrast to many hexacoordinate (pseudo-octahedral) complexes of Mn(III), they exhibit a low-spin (triplet) ground state, rather than the high-spin (quintet) ground state. Solid-state electronic absorption spectroscopy, SQUID magnetometry, and high-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy were applied. HFEPR, in particular, was useful in characterizing the S = 1 spin Hamiltonian parameters for complex 1, D = +19.97(1), E = 0.42(2) cm(-1), and for 2, D = +15.89(2), E = 0.04(1) cm(-1). In addition, frequency domain Fourier-transform THz-EPR spectroscopy, using coherent synchrotron radiation, was applied to 1 only and gave results in good agreement with HFEPR. Variable-temperature dc magnetic susceptibility measurements of 1 and 2 were also in good agreement with the HFEPR results. This magnitude of zero-field splitting (zfs) is over 4 times larger than that in comparable hexacoordinate Mn(III) systems with S = 2 ground states. Complexes 3a and 3b (i.e., regardless of counteranion) have a yet much larger magnitude zfs, which may be the result of unquenched orbital angular momentum so that the spin Hamiltonian model is not appropriate. The triplet ground state is rationalized in each complex by ligand-field theory (LFT) and by quantum chemistry theory, both density functional theory and unrestricted Hartree-Fock methods. This analysis also shows that spin-crossover behavior is not thermally accessible for these complexes as solids. The donor properties of the three different scorpionate ligands were further characterized using the LFT model that suggests that the tris(carbene)borate is a strong σ-donor with little or no π-bonding.

A new cyanido-bridged Re(IV)-Mn(III) heterometallic 1D system, [Mn(III)(5-Me-saltmen)](2)[Re(IV)Cl(4)(CN)(2)]·3CH(3)CN (1), was designed and structurally characterized. Interchain interactions stabilize a canted antiferromagnetic ordered state below 6.2 K that does not prevent slow relaxation of the magnetization reminiscent of the single-chain magnet properties of the individual chains.

Transamination of divalent transition metal starting materials (M(2)(N(SiMe(3))(2))(4), M = Mn, Co) with hexadentate ligand platforms (R)LH(6) ((R)LH(6) = MeC(CH(2)NPh-o-NR)(3) where R = H, Ph, Mes (Mes = Mesityl)) or (H,Cy)LH(6) = 1,3,5-C(6)H(9)(NHPh-o-NH(2))(3) with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type ((R)L)Mn(3)(py)(3) and ((R)L)Co(3)(PMe(2)R')(3) (R' = Me, Ph). While the sterically less encumbered ligand varieties, (H)L or (Ph)L, give rise to local square-pyramidal geometries at each of the bound metal atoms, with four anilides forming an equatorial plane and an exogenous pyridine or phosphine in the apical site, the mesityl-substituted ligand ((Mes)L) engenders local tetrahedral coordination. Both the neutral Mn(3) and Co(3) clusters feature S = (1)/(2) ground states, as determined by direct current (dc) magnetometry, (1)H NMR spectroscopy, and low-temperature electron paramagnetic resonance (EPR) spectroscopy. Within the Mn(3) clusters, the long internuclear Mn-Mn separations suggest minimal direct metal-metal orbital overlap. Accordingly, fits to variable-temperature magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange interactions through the bridging anilide ligands with exchange couplings ranging from J = -16.8 to -42 cm(-1). Conversely, the short Co-Co interatomic distances suggest a significant degree of direct metal-metal orbital overlap, akin to the related Fe(3) clusters. With the Co(3) series, the S = (1)/(2) ground state can be attributed to population of a single molecular orbital manifold that arises from mixing of the metal- and o-phenylenediamide (OPDA) ligand-based frontier orbitals. Chemical oxidation of the neutral Co(3) clusters affords diamagnetic cationic clusters of the type [((R)L)Co(3)(PMe(2)R)(3)](+). Density functional theory (DFT) calculations on the neutral (S = (1)/(2)) and cationic (S = 0) Co(3) clusters reveal that oxidation occurs at an orbital with contributions from both the Co3 core and OPDA subunits. The predicted bond elongations within the ligand OPDA units are corroborated by the ligand bond perturbations observed by X-ray crystallography.

The model compounds (NBu(4))(2)[ReCl(4)(CN)(2)] (1), (DMF)(4)ZnReCl(4)(CN)(2) (2), and [(PY5Me(2))(2)Mn(2)ReCl(4)(CN)(2)](PF(6))(2) (3) have been synthesized to probe the origin of the magnetic anisotropy barrier in the one-dimensional coordination solid (DMF)(4)MnReCl(4)(CN)(2) (4). High-field electron paramagnetic resonance spectroscopy reveals the presence of an easy-plane anisotropy (D > 0) with a significant transverse component, E, in compounds 1-3. These findings indicate that the onset of one-dimensional spin correlations within the chain compound 4 leads to a suppression of quantum tunneling of the magnetization within the easy plane, resulting in magnetic bistability and slow relaxation behavior. Within this picture, it is the transverse E term associated with the Re(IV) centers that determines the easy axis and the anisotropy energy scale associated with the relaxation barrier. The results demonstrate for the first time that slow magnetic relaxation can be achieved through optimization of the transverse anisotropy associated with magnetic ions that possess easy-plane anisotropy, thus providing a new direction in the design of single-molecule and single-chain magnets., (© 2012 American Chemical Society)

Two-photon probe excitation data are commonly presented as absorption cross section or molecular brightness (the detected fluorescence rate per molecule). We report two-photon molecular brightness spectra for a diverse set of organic and genetically encoded probes with an automated spectroscopic system based on fluorescence correlation spectroscopy. The two-photon action cross section can be extracted from molecular brightness measurements at low excitation intensities, while peak molecular brightness (the maximum molecular brightness with increasing excitation intensity) is measured at higher intensities at which probe photophysical effects become significant. The spectral shape of these two parameters was similar across all dye families tested. Peak molecular brightness spectra, which can be obtained rapidly and with reduced experimental complexity, can thus serve as a first-order approximation to cross-section spectra in determining optimal wavelengths for two-photon excitation, while providing additional information pertaining to probe photostability. The data shown should assist in probe choice and experimental design for multiphoton microscopy studies. Further, we show that, by the addition of a passive pulse splitter, nonlinear bleaching can be reduced--resulting in an enhancement of the fluorescence signal in fluorescence correlation spectroscopy by a factor of two. This increase in fluorescence signal, together with the observed resemblance of action cross section and peak brightness spectra, suggests higher-order photobleaching pathways for two-photon excitation., (Copyright © 2012 Biophysical Society. Published by Elsevier Inc. All rights reserved.)

Reaction of NO(2)(-) with the octahedral cluster ((H)L)(2)Fe(6) in the presence of a proton source affords the hexanitrosyl cluster ((H)L)(2)Fe(6)(NO)(6). This species forms via a proton-induced reduction of six nitrite molecules per cluster, utilizing each site available on the polynuclear core. Formation of the hexanitrosyl cluster is accompanied by a near 2-fold expansion of the ((H)L)(2)Fe(6) core volume, where intracore Fe-Fe interactions are overcome by strong π-bonding between Fe centers and NO ligands. A core volume of this magnitude is rare in octahedral metal clusters not supported by interstitial atoms. Moreover, the structural flexibility afforded by the ((H)L)(2)Fe(6) platform highlights the potential for other reaction chemistry involving species with metal-ligand multiple bonds. Carrying out the reaction of the cluster [((H)L)(2)Fe(6)(NCMe)(6)](4+) with nitrite in the absence of a proton source serves to forestall the nitrite reduction and enables clean isolation of the intermediate hexanitro cluster [((H)L)(2)Fe(6)(NO(2))(6)](2-).

A series of potent and selective β1-adrenoreceptor ligands were identified (IC50 range, 0.04-0.25 nM; β1/β2 selectivity range, 65-450-fold), labeled with the PET radioisotope fluorine-18 and evaluated in normal Sprague-Dawley rats. Tissue distribution studies demonstrated uptake of each radiotracers from the blood pool into the myocardium (0.48-0.62% ID/g), lung (0.63-0.97% ID/g), and liver (1.03-1.14% ID/g). Dynamic μPET imaging confirmed the in vivo dissection studies.

Oxidation of the nominally all-ferrous hexanuclear cluster ((H)L)(2)Fe(6) with six equivalents of ferrocenium in the presence of bromide ions results in a six-electron oxidation of the Fe(6) core to afford the nominally all-ferric cluster ((H)L)(2)Fe(6)Br(6). The hexabromide cluster is also structurally characterized in a 4+ core oxidation state. A structural comparison of these two clusters provides an insight into the Fe(6) core electronic structure., (This journal is © The Royal Society of Chemistry 2011)

Using a trinucleating hexaamide ligand platform, the all-ferrous hexanuclear cluster ((H)L)(2)Fe(6) (1) is obtained from reaction of 3 equiv of Fe(2)(Mes)(4) (Mes = 2,4,6-Me(3)C(6)H(2)) with 2 equiv of the ligand ((H)L)H(6). Compound 1 was characterized by X-ray diffraction analysis, (57)Fe Mössbauer, SQUID magnetometry, mass spectrometry, and combustion analysis, providing evidence for an S=6 ground state and delocalized electronic structure. The cyclic voltammogram of [((H)L)(2)Fe(6)](n+) in acetonitrile reveals a rich redox chemistry, featuring five fully reversible redox events that span six oxidation states ([((H)L)(2)Fe(6)](n+), where n=-1→4) within a 1.3 V potential range. Accordingly, each of these species is readily accessed chemically to provide the electron-transfer series [((H)L)(2)Fe(6)(NCMe)(m)][PF(6)](n) (m=0, n=-1 (2); m=2, n=1 (3); m=4, n=2 (4); m=6, n=3 (5); m=6, n=4 (6)). Compounds 2-6 were isolated and characterized by X-ray diffraction, (57)Fe Mössbauer and multinuclear NMR spectroscopy, and combustion analysis. Two-electron oxidation of the tetracationic cluster in 6 by 2 equiv of [NO](+) generates the thermally unstable hexacationic cluster [((H)L)(2)Fe(6)(NCMe)(m)](6+), which is characterized by NMR and (57)Fe Mössbauer spectroscopy. Importantly, several stepwise systematic metrical changes accompany oxidation state changes to the [Fe(6)] core, namely trans ligation of solvent molecules and variation in Mössbauer spectra, spin ground state, and intracluster Fe-Fe separation. The observed metrical changes are rationalized by considering a qualitative, delocalized molecular orbital description, which provides a set of frontier orbitals populated by Fe 3d electrons., (© 2011 American Chemical Society)

The magnetic, electronic, and Mössbauer spectral properties of [Fe(2)L(μ-OAc)(2)]ClO(4), 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H(2)L, indicate that 1 is a class III mixed valence iron(II∕III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe(2)](V) cationic configuration with a S(t) = 9∕2 ground state. Fits of the dc magnetic susceptibility between 2 and 300 K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of -32(2) cm(-1) for an electron transfer parameter, B, of 950 cm(-1), a zero-field uniaxial D(9∕2) parameter of -0.9(1) cm(-1), and g = 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, U(eff), of 9.8 cm(-1). At 5.25 K the Mössbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 × 10(-8) s. A fit of the temperature dependence of the average effective hyperfine field yields |D(9∕2)| = 0.9 cm(-1). An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm(-1).

The four-coordinate iron(II) phosphoraniminato complex PhB(MesIm)(3)Fe-N═PPh(3) undergoes an S = 0 to S = 2 spin transition with T(C) = 81 K, as determined by variable-temperature magnetic measurements and Mössbauer spectroscopy. Variable-temperature single-crystal X-ray diffraction revealed that the S = 0 to S = 2 transition is associated with an increase in the Fe-C and Fe-N bond distances and a decrease in the N-P bond distance. These structural changes have been interpreted in terms of electronic structure theory.

Reaction of the high-magnetic anisotropy building unit [ReCl(4)(CN)(2)](2-) with [Cu(MeCN)(6)](2+) and hydrotris(pyrazol-1-yl)borate (Tp(-)) affords the zigzag chain compound (Bu(4)N)[TpCuReCl(4)(CN)(2)]. Dc magnetic susceptibility measurements reveal the presence of ferromagnetic exchange coupling between Re(IV) and Cu(II) centers along each chain and a fit to the data gives an exchange constant of J/k(B) = +41 K (+29 cm(-1)), representing the strongest ferromagnetic coupling yet observed through cyanide. Below 11.4 K and at applied fields of less than 3600 Oe, the compound undergoes a phase transition to an antiferromagnetic ground state, stemming from weak π-π interchain interactions of strength J(⊥)/k(B) = -1.7 K (-1.2 cm(-1)). This metamagnetic behavior is fully elucidated using both experimental and theoretical methods. In addition, theoretical modeling provides a detailed determination of the local anisotropy tensors corresponding to the [ReCl(4)(CN)(2)](2-) units and demonstrates that the zigzag arrangement of the Re(IV) centers significantly reduces the effective anisotropy of the chain. These results demonstrate the utility of the Re(IV)-CN-Cu(II) linkage and the importance of anisotropic spin orientation in designing strongly coupled systems, which will aid in both the realization of single-chain magnets with higher relaxation barriers and in the construction of high-dimensional cyano-bridged materials exhibiting higher ordering temperatures.

We present a family of trigonal pyramidal iron(II) complexes supported by tris(pyrrolyl-α-methyl)amine ligands of the general formula [M(solv)(n)][(tpa(R))Fe] (M = Na, R = tert-butyl (1), phenyl (4); M = K, R = mesityl (2), 2,4,6-triisopropylphenyl (3), 2,6-difluorophenyl (5)) and their characterization by X-ray crystallography, Mössbauer spectroscopy, and high-field EPR spectroscopy. Expanding on the discovery of slow magnetic relaxation in the recently reported mesityl derivative 2, this homologous series of high-spin iron(II) complexes enables an initial probe of how the ligand field influences the static and dynamic magnetic behavior. Magnetization experiments reveal large, uniaxial zero-field splitting parameters of D = -48, -44, -30, -26, and -6.2 cm(-1) for 1-5, respectively, demonstrating that the strength of axial magnetic anisotropy scales with increasing ligand field strength at the iron(II) center. In the case of 2,6-difluorophenyl substituted 5, high-field EPR experiments provide an independent determination of the zero-field splitting parameter (D = -4.397(9) cm(-1)) that is in reasonable agreement with that obtained from fits to magnetization data. Ac magnetic susceptibility measurements indicate field-dependent, thermally activated spin reversal barriers in complexes 1, 2, and 4 of U(eff) = 65, 42, and 25 cm(-1), respectively, with the barrier of 1 constituting the highest relaxation barrier yet observed for a mononuclear transition metal complex. In addition, in the case of 1, the large range of temperatures in which slow relaxation is observed has enabled us to fit the entire Arrhenius curve simultaneously to three distinct relaxation processes. Finally, zero-field Mössbauer spectra collected for 1 and 4 also reveal the presence of slow magnetic relaxation, with two independent relaxation barriers in 4 corresponding to the barrier obtained from ac susceptibility data and to the 3D energy gap between the M(S) = ±2 and ±1 levels, respectively.

Treatment of the cyanometalate building unit [Re(CN)(7)](3-) with [(PY5Me(2))M(MeCN)](2+) (M = Co, Ni, Cu) affords a series of pentanuclear clusters of formulas [(PY5Me(2))(4)M(4)Re(CN)(7)](5+) (M = Co, Ni, Cu) and [(PY5Me(2))(4)Cu(4)Re(CN)(7)](4+). Single crystal X-ray diffraction analyses of the clusters reveal a star-like structure in which four [(PY5Me(2))M](2+) moieties are linked to a central [Re(CN)(7)](3-) unit via bridging cyanide ligands. An intramolecular Co(II) → Re(IV) charge-transfer accompanies the formation of the Co(II)(4)Re(IV) cluster, giving a Co(II)(3)Co(III)Re(III) species. Spectroelectrochemical methods and irradiation experiments are used to characterize the metal-metal charge-transfer bands of this compound. A rhenium-based thermally induced one-electron reduction is observed for the Cu(II)(4)Re(IV) cluster to give a Cu(II)(4)Re(III) complex; however, this reduction may be forestalled at low temperature. Finally, magnetic measurements reveal intracluster ferromagnetic exchange coupling, strong uniaxial magnetic anisotropy, and slow magnetic relaxation in the Ni(II)(4)Re(IV) and Cu(II)(4)Re(IV) clusters.

Copper(i) cyanide exposed to various liquid or vapor-phase amines (L) at ambient temperature produces a variety of visible photoluminescence colors via reversible formation of amine adducts. The adducts show phase matches to authentic (CuCN)L(n), n = 0.75-2.0, produced by heating CuCN with liquid amine.

An S = 3/2, high-anisotropy building unit, trans-[ReCl(4)(CN)(2)](2-), representing the first paramagnetic complex with a mixture of just cyanide and halide ligands, has been synthesized through the reaction of (Bu(4)N)CN with ReCl(4)(THF)(2). This species is characterized in detail and employed in directing the formation of a series of one-dimensional coordination solids of formula (DMF)(4)MReCl(4)(CN)(2) (M = Mn (2), Fe (3), Co (4), Ni (5)). Variable-temperature dc magnetic susceptibility measurements demonstrate the presence of intrachain antiferromagnetic (2) and ferromagnetic (3-5) exchange coupling within these solids. In addition, probing the ac magnetic susceptibility as a function of both temperature and frequency reveals that all of the chain compounds exhibit slow relaxation of the magnetization. The relaxation time is shown to be thermally activated, with energy barriers to relaxation of Delta(tau) = 31, 56, 17, and 20 cm(-1) for 2-5, respectively. Notably, the field-dependent magnetization of the iron congener exhibits a significant hysteresis effect at low temperature, with a coercive field of H(C) = 1.0 T, thus demonstrating magnetlike behavior in this one-dimensional system. Finally, the magnetization dynamics of all solids occur within the finite-size regime, where the magnetic domain growth is limited due to physical defects along the chains within the crystals.

Slow magnetic relaxation is observed for [(tpa(Mes))Fe](-), a trigonal pyramidal complex of high-spin iron(II), providing the first example of a mononuclear transition metal complex that behaves as a single-molecule magnet. Dc magnetic susceptibility and magnetization measurements reveal a strong uniaxial magnetic anisotropy (D = -39.6 cm(-1)) acting on the S = 2 ground state of the molecule. Ac magnetic susceptibility measurements indicate the absence of slow relaxation under zero applied dc field as a result of quantum tunneling of the magnetization. Application of a 1500 Oe dc field initiates slow magnetic relaxation, which follows a thermally activated tunneling mechanism at high temperature to give an effective spin-reversal barrier of U(eff) = 42 cm(-1) and follows a temperature-independent tunneling mechanism at low temperature. In addition, the magnetic relaxation time shows a pronounced dc-field dependence, with a maximum occurring at approximately 1500 Oe.

A nanosized heterometallic cluster containing 60 La(III) and 76 Ni(II) ions, which are arranged into a four-shell, nest-like framework structure, was obtained by the hydrolytic reaction of the mixed La(NO(3))(3)-Ni(NO(3))(2) system using iminodiacetate as an ancillary ligand to control the hydrolysis.

Recent progress in the assembly of actinide-containing coordination clusters has generated systems in which the first glimpses of magnetic exchange coupling can be recognized. Such systems are of interest owing to the prospects for involving 5f electrons in stronger magnetic exchange than has been observed for electrons in the more contracted 4f orbitals of the lanthanide elements. Here, we survey the actinide-containing molecules thought to exhibit magnetic exchange interactions, including multiuranium, uranium-lanthanide, uranium-transition metal, and uranium-radical species. Interpretation of the magnetic susceptibility data for compounds of this type is complicated by the combination of spin-orbit coupling and ligand-field effects arising for actinide ions. Nevertheless, for systems where analogues featuring diamagnetic replacement components for the non-actinide spin centers can be synthesized, a data subtraction approach can be utilized to probe the presence of exchange coupling. In addition, methods have been developed for employing the resulting data to estimate lower and upper bounds for the exchange constant. Emphasis is placed on evaluation of the linear clusters (cyclam)M[(mu-Cl)U(Me(2)Pz)(4)](2) (M = Co, Ni, Cu, Zn; cyclam = 1,4,8,11-tetraazacyclotetradecane; Me(2)Pz(-) = 3,5-dimethylpyrazolate), for which strong ferromagnetic exchange with 15 cm(-1) < or = J < or = 48 cm(-1) is observed for the Co(II)-containing species. Owing to the modular synthetic approach employed, this system in particular offers numerous opportunities for adjusting the strength of the magnetic exchange coupling and the total number of unpaired electrons. To this end, the prospects of such modularity are discussed through the lens of several new related clusters. Ultimately, it is hoped that this research will be of utility in the development of electronic structure models that successfully describe the magnetic behavior of actinide compounds and will perhaps even lead to new actinide-based single-molecule magnets.

The full promise of human genomics will be realized only when the genomes of thousands of individuals can be sequenced for comparative analysis. A reference sequence enables the use of short read length. We report an amplification-free method for determining the nucleotide sequence of more than 280,000 individual DNA molecules simultaneously. A DNA polymerase adds labeled nucleotides to surface-immobilized primer-template duplexes in stepwise fashion, and the asynchronous growth of individual DNA molecules was monitored by fluorescence imaging. Read lengths of >25 bases and equivalent phred software program quality scores approaching 30 were achieved. We used this method to sequence the M13 virus to an average depth of >150x and with 100% coverage; thus, we resequenced the M13 genome with high-sensitivity mutation detection. This demonstrates a strategy for high-throughput low-cost resequencing.

The integrin receptor alphavbeta3 is overexpressed on the endothelial cells of growing tumors and on some tumor cells themselves. A radiolabeled alphavbeta3 antagonists belonging to the quinolin-4-one class of peptidomimetics (TA138) was previously shown to exhibit high affinity for integrin alphavbeta3 and high selectivity versus other integrin receptors. 111In-TA138 exhibited high tumor uptake in the c-neu Oncomouse mammary adenocarcinoma model and produced excellent scintigraphic images. This study describes the synthesis of eight divalent versions of TA138 and their evaluation as potential tumor radiotherapeutic agents. The two main variables in this study were the length of the spacer bridging the biotargeting moieties and the total negative charge of the molecules imparted by the cysteic acid pharmacokinetic modifiers. Receptor affinity was evaluated in a panel of integrin receptor affinity assays, and biodistribution studies using the 111In-labeled derivatives were carried out in the c-neu Oncomouse model. All divalent agents maintained the high receptor affinity and selectivity of TA138, and six of the eight 111In derivatives exhibited blood clearance that was faster than 111In-TA138 at 24 h postinjection (PI). All divalent agents exhibited tumor uptake and retention at 24 h PI that was higher than 111In-TA138. Tumor/organ ratios were improved for most of the divalent agents at 24 h PI in critical nontarget organs marrow, kidney, and liver, with the agents having intermediate-length spacers (29-43 A) showing the largest improvement. As an example, 111In-15 showed tumor uptake of 14.3% ID/g at 24 h PI and tumor/organ ratios as follows: marrow, 3.24; kidney, 7.29; liver, 8.51. A comparison of therapeutic indices for 90Y-TA138 and 177Lu-15 indicate an improved therapeutic index for the divalent agent. The implications for radiotherapeutic applications and the mechanism of this multivalent effect are discussed.

Access to the new complex [TpCr(CN)3]- enables isolation of two metastable linkage isomers of a face-centered cubic cluster, [Tp8(H2O)6Cu6Cr8(CN)24]4+, both exhibiting an S=15 ground state.