The discovery that individual molecules can function as magnets provided a new, "bottom-up" approach to nanoscale magnetic materials, [1][2][3] and such molecules have since been called single-molecule magnets (SMMs). [4] Each molecule is a single-domain magnetic particle that, below its blocking temperature, exhibits the classical macroscale property of a magnet, namely magnetization hysteresis. In addition, SMMs straddle the classical/quantum interface in also displaying quantum tunneling of magnetization (QTM) [5,6] and quantum phase interference, [7] which are the properties of the microscale. SMMs have various potential applications, including very high-density information storage with each bit stored as the magnetization orientation of an individual molecule, and as quantum bits for quantum computing [8] by taking advantage of the quantum superposition of states provided by the QTM. For a number of reasons, including facilitating development of techniques for addressing individual SMMs, we have sought to synthesize SMMs of very-large dimensions (by molecular standards). In effect, can the molecular (or bottomup) approach reach the size regime of the classical (or topdown) approach to nanoscale magnetic materials? Indeed, herein we report a giant Mn 84 SMM. It has a 4 nm diameter torus structure, exhibits both magnetization hysteresis and QTM, and crystallizes as supramolecular nanotubes.The (2) with (N n Bu 4 )(MnO 4 ) in MeOH that contained a little acetic acid, followed by filtration and layering of the filtrate with chloroform. After several weeks, the well-formed reddish-brown crystals were isolated in a 20 % overall yield, based on the total available Mn equivalents. The compound crystallizes in hexagonal space group P6 with the asymmetric unit containing 1/6 of the molecule and approximately 25 water and 2 chloroform solvent mole-

This report describes the synthesis and characterization of boronate ester-cross-linked hydrogels capable of self-healing behavior at neutral and acidic pH. This atypically wide pH range over which healing behavior is observed was achieved through the use of an intramolecular coordinating boronic acid monomer, 2-acrylamidophenylboronic acid (2APBA), where the internal coordination helped to stabilize cross-links formed at acidic and neutral pH. Two different hydrogels were formed from a 2APBA copolymer cross-linked with either poly­(vinyl alcohol) or a catechol-functionalized copolymer. The self-healing ability of these hydrogels was characterized through physical testing and rheological studies. Furthermore, the catechol cross-linked hydrogel was shown to be oxygen sensitive, demonstrating reduced self-healing and stress relaxation after partial oxidation. The synthesis of these hydrogels demonstrates a new strategy to produce boronic acid materials capable of self-healing at physiological pH.

The laboratory synthesis of the oxygen-evolving complex (OEC) of photosystem II has been the objective of synthetic chemists since the early 1970s. However, the absence of structural information on the OEC has hampered these efforts. Crystallographic reports on photosystem II that have been appearing at ever-improving resolution over the past ten years have finally provided invaluable structural information on the OEC and show that it comprises a [Mn 3 CaO 4 ] distorted cubane, to which is attached a fourth, external Mn atom, and the whole unit attached to polypeptides primarily by aspartate and glutamate carboxylate groups. Such a heterometallic Mn/Ca cubane with an additional metal attached to it has been unknown in the literature. This paper reports the laboratory synthesis of such an asymmetric cubane-containing compound with a bound external metal atom, [Mn IV 3 Ca 2 O 4 ðO 2 CBu t Þ 8 ðBu t CO 2 HÞ 4 (1)] . All peripheral ligands are carboxylate or carboxylic acid groups. Variable-temperature magnetic susceptibility data have established 1 to possess an S ¼ 9∕2 ground state. EPR spectroscopy confirms this, and the Davies electron nuclear double resonance data reveal similar hyperfine couplings to those of other Mn IV species, including the OEC S 2 state. Comparison of the X-ray absorption data with those for the OEC reveal 1 to possess structural parameters that make it a close structural model of the asymmetric-cubane OEC unit. This geometric and electronic structural correspondence opens up a new front in the multidisciplinary study of the properties and function of this important biological unit.crystal structure | magnetic properties | spectroscopic properties P hotosystem II (PSII) is a multicomponent assembly of proteins and cofactors that is located in the thylakoid membrane of plants, algae, and cyanobacteria. It carries out the sunlightdriven oxidation of water to O 2 , the generated protons driving ATP synthase and the electrons providing the reducing equivalents that ultimately lead to carbon dioxide fixation. The oxidation of water is an energetically demanding process, but the development approximately 2.7 billion years ago of a catalytic system capable of carrying it out efficiently made available as a raw material the vast quantities of water on the planet. The site of water oxidation to O 2 is the oxygen-evolving complex (OEC), and the determination of its structure has been the focus of much study by various biochemical and spectroscopic techniques. X-ray absorption spectroscopy (XAS), including extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge spectroscopy (XANES), have been invaluable tools in assessing possible metal topologies and distances between atoms that compose the OEC. These studies, along with those by EPR spectroscopy, have allowed for the description of Mn oxidation states at each of the S states (S n , where n ¼ 0-4) of the Kok cycle. Many groups have employed such spectroscopic data to guide their efforts toward the preparation of inorganic mod...

The reaction of MnCl2.4H2O (3 equiv), pyridine-2,6-dimethanol (pdmH2) (10 equiv), and NaN3 (10 equiv) in MeOH/MeCN (1:2 v/v) with NMe4OH (1 equiv) gave [Mn25O18(OH)2(N3)12(pdm)6(pdmH)6](Cl)2.12MeCN (1.12MeCN) in approximately 30% yield. The cation of complex 1 comprises five Mnx layers of three types in an ABCBA arrangement. Fitting of variable-temperature and -field magnetization data establishes that 1 has an S = 51/2 ground state, the largest value for a molecular species. The complex also displays hysteresis loops below 0.6 K in magnetization vs applied field sweeps, establishing it as the largest spin single-molecule magnet to date.