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A new family of low-coordinate Co complexes supported by three redox-noninnocent tridentate [OCO] pincer-type bis(phenolate) N-heterocyclic carbene (NHC) ligands are described. Combined experimental and computational data suggest that the charge-neutral four-coordinate complexes are best formulated as Co(II) centers bound to closed-shell [OCO] 2- dianions, of the general formula [(OCO)Co II L] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms of the [(OCO)Co II L] complexes reveal three oxidations accessible at potentials below 1.2 V vs Fc + /Fc, corresponding to generation of formally Co(V) species, but the true physical/spectroscopic oxidation states are much lower. Chemical oxidations afford the mono- and dications of the imidazoline NHC-derived complex, which were examined by computational and magnetic and spectroscopic methods, including single-crystal X-ray diffraction. The metal and ligand oxidation states of the monocationic complex are ambiguous; data are consistent with formulation as either [( S OCO)Co III (THF) 2 ] + containing a closed-shell [ S OCO] 2- diphenolate ligand bound to a S = 1 Co(III) center, or [( S OCO • )Co II (THF) 2 ] + with a low-spin Co(II) ion ferromagnetically coupled to monoanionic [ S OCO • ] - containing a single unpaired electron distributed across the [OCO] framework. The dication is best described as [( S OCO 0 )Co II (THF) 3 ] 2+ , with a single unpaired electron localized on the d 7 Co(II) center and a doubly oxidized, charge-neutral, closed-shell S OCO 0 ligand. The combined data provide for the first time unequivocal and structural evidence for [OCO] ligand redox activity. Notably, varying the degree of unsaturation in the NHC backbone shifts the ligand-based oxidation potentials by up to 400 mV. The possible chemical origins of this unexpected shift, along with the potential utility of the [OCO] pincer ligands for base-metal-mediated organometallic coupling catalysis, are discussed.

The synthesis and characterization of soluble azaiptycenes is reported. Optical and physical properties were studied and compared with those of the structurally consanguine azaacenes. Electrochemical experiments and quantum-chemical calculations revealed the electronic structure of the iptycene derivatives. Their crystallization behavior was examined. A highly fluorescent amorphous diazatetracene derivative was integrated into a simple organic light-emitting diode, showing enhanced performance compared with that of previously reported, structurally similar tetracenes.

Terminal oxo complexes of the late transition metals Pt, Pd, and Au have been reported by us in Science and Journal of the American Chemical Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 analytical methods in one case), we have continued to study these structures. Initial work on these systems was motivated by structural data from X-ray crystallography and neutron diffraction and (17)O and (31)P NMR signatures which all indicated differences from all previously published compounds. With significant new data, we now revisit these studies. New X-ray crystal structures of previously reported complexes K(14)[P(2)W(19)O(69)(OH(2))] and "K(10)Na(3)[Pd(IV)(O)(OH)WO(OH(2))(PW(9)O(34))(2)]" and a closer examination of these structures are provided. Also presented are the (17)O NMR spectrum of an (17)O-enriched sample of [PW(11)O(39)](7-) and a careful combined (31)P NMR-titration study of the previously reported "K(7)H(2)[Au(O)(OH(2))P(2)W(20)O(70)(OH(2))(2)]." These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogues with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data have been re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.

[P(4)W(35)O(124){Re(CO)(3)}(2)](16-) (1), a Wells-Dawson [α(2)-P(2)W(17)O(61)](10-) polyoxometalate (POM)-supported [Re(CO)(3)](+) complex containing covalent W(VI)-O-Re(I) bonds has been synthesized and characterized by several methods, including X-ray crystallography. This complex shows a high visible absorptivity (ε(470 nm) = 4000 M(-1) cm(-1) in water) due to the formation of a Re(I)-to-POM charge transfer (MPCT) band. The complex was investigated by computational modeling and transient absorption measurements in the visible and mid-IR regions. Optical excitation of the MPCT transition results in instantaneous (<50 fs) electron transfer from the Re(I) center to the POM ligand., (© 2011 American Chemical Society)

Dirhodium tetrakis-(R)-(1-(4-bromophenyl)-2,2-diphenylcyclopropanecarboxylate) (Rh(2)(R-BTPCP)(4)) was found to be an effective chiral catalyst for enantioselective reactions of aryl- and styryldiazoacetates. Highly enantioselective cyclopropanations, tandem cyclopropanation/Cope rearrangements and a combined C-H functionalization/Cope rearrangement were achieved using Rh(2)(R-BTPCP)(4) as catalyst. The advantages of Rh(2)(R-BTPCP)(4) include its ease of synthesis, its tolerance to the size of the ester group in the styryldiazoacetates, and its compatibility with dichloromethane as solvent. Computational studies suggest that the catalyst adopts a D(2)-symmetric arrangement, but when the carbenoid binds to the catalyst, two of the p-bromophenyl groups on the ligands rotate outward to make room for the carbenoid and the approach of the substrate to the carbenoid.

A combination of polyanion size and charge allows the Keggin-type polyoxometalate (POM), [CuPW(11)O(39)](5-), a catalyst for some air-based organic oxidations, to fit snuggly in the pores of MOF-199 (HKUST-1), a metal-organic framework (MOF) with the POM countercations residing in alternative pores. This close matching of POM diameter and MOF pore size in this POM-MOF material, [Cu(3)(C(9)H(3)O(6))(2)](4)[{(CH(3))(4)N}(4)CuPW(11)O(39)H] (1), results in a substantial synergistic stabilization of both the MOF and the POM. In addition, this heretofore undocumented POM-MOF interaction results in a dramatic increase in the catalytic turnover rate of the POM for air-based oxidations. While 1 catalyzes the rapid chemo- and shape-selective oxidation of thiols to disulfides and, more significantly, the rapid and sustained removal of toxic H(2)S via H(2)S + 1/2 O(2) → 1/8 S(8) + H(2)O (4000 turnovers in <20 h), the POM or the MOF alone is catalytically slow or inactive. Three arguments are consistent with the catalytic reactions taking place inside the pores. POM activation by encapsulation in the MOF likely involves electrostatic interactions between the two components resulting in a higher reduction potential of the POM.

The addition of an [X](+) electrophile to the five-coordinate oxorhenium(V) anion [Re(V)(O)(ap(Ph))(2)](-) {[ap(Ph)](2-) = 2,4-di-tert-butyl-6-(phenylamido)phenolate} gives new products containing Re-X bonds. The Re-X bond-forming reaction is analogous to oxo transfer to [Re(V)(O)(ap(Ph))(2)](-) in that both are 2e(-) redox processes, but the electronic structures of the products are different. Whereas oxo addition to [Re(V)(O)(ap(Ph))(2)](-) yields a closed-shell [Re(VII)(O)(2)(ap(Ph))(2)](-) product of 2e(-) metal oxidation, [Cl](+) addition gives a diradical Re(VI)(O)(ap(Ph))(isq(Ph))Cl product ([isq(Ph)](•-) = 2,4-di-tert-butyl-6-(phenylimino)semiquinonate) with 1e(-) in a Re d orbital and 1e(-) on a redox-active ligand. The differences in electronic structure are ascribed to differences in the π basicity of [O](2-) and Cl(-) ligands. The observation of ligand radicals in Re(VI)(O)(ap(Ph))(isq(Ph))X provides experimental support for the capacity of redox-active ligands to deliver electrons in other bond-forming reactions at [Re(V)(O)(ap(Ph))(2)](-), including radical additions of O(2) or TEMPO(•) to make Re-O bonds. Attempts to prepare the electron-transfer series monomers between Re(VI)(O)(ap(Ph))(isq(Ph))X and [Re(V)(O)(ap(Ph))(2)](-) yielded a symmetric bis(μ-oxo)dirhenium complex. Formation of this dimer suggested that Re(VI)(O)(ap(Ph))(isq(Ph))Cl may be a source of an oxyl metal fragment. The ability of Re(VI)(O)(ap(Ph))(isq(Ph))Cl to undergo radical coupling at oxo was revealed in its reaction with Ph(3)C(•), which affords Ph(3)COH and deoxygenated metal products. This reactivity is surprising because Re(VI)(O)(ap(Ph))(isq(Ph))Cl is not a strong outer-sphere oxidant or oxo-transfer reagent. We postulate that the unique ability of Re(VI)(O)(ap(Ph))(isq(Ph))Cl to effect oxo transfer to Ph(3)C(•) arises from symmetry-allowed mixing of a populated Re≡O π bond with a ligand-centered [isq(Ph)](•-) ligand radical, which gives oxyl radical character to the oxo ligand. This allows the closed-shell oxo ligand to undergo a net 2e(-) oxo-transfer reaction to Ph(3)C(•) via kinetically facile redox-active ligand-mediated radical steps. Harnessing intraligand charge transfer for radical reactions at closed-shell oxo ligands is a new strategy to exploit redox-active ligands for small-molecule activation and functionalization. The implications for the design of new oxidants that utilize low-barrier radical steps for selective multielectron transformations are discussed.

Two inorganic-organic hybrid clusters with one or two covalently linked pyrene fluorescent probes, [(n-C(4)H(9))(4)N](2)[V(6)O(13){(OCH(2))(3)C(NH(CO)CH(2)CH(2)CH(2)C(16)H(9))}{(OCH(2))(3)C-(NH(2))}] ((TBA(+))(2)1) and [(n-C(4)H(9))(4)N](2)[V(6)O(13){(OCH(2))(3)C(NH(CO)CH(2)CH(2)CH(2)C(16)H(9))}(2)] ((TBA(+))(2)2), respectively, are synthesized from Lindqvist type polyoxometalates (POMs). The incorporation of pyrene into POMs results in amphiphilic hybrid molecules and simultaneously offers a great opportunity to study the interaction between hybrid clusters and their counterions. 2D-NOESY NMR and fluorescence techniques have been used to study the role of counterions such as tetrabutyl ammonium (TBA) in the vesicle formation of the hybrid clusters. The TBA(+) ions not only screen the electrostatic repulsions between the POM head groups but also are involved in the hydrophobic region of the vesicular structure where they interrupt the formation of pyrene excimers that greatly perturbs the luminescence signal from the vesicle solution. By replacing the TBA(+) counterions with protons, the new vesicles demonstrate interesting pH-dependent fluorescence properties.

A diiron(II) complex containing two μ-1,3-(κN:κO)-amidate linkages has been synthesized using the 2,2',2''-tris(isobutyrylamido)triphenylamine (H(3)L(iPr)) ligand. The resulting diiron complex, 1, reacts with dioxygen (or iodosylbenzene) to effect intramolecular C-H bond activation at the methine position of the ligand isopropyl group. The ligand-activated product, 2, has been isolated and characterized by a variety of methods including X-ray crystallography. Electrospray ionization mass spectroscopy of 2 prepared from(18)O(2) was used to confirm that the oxygen atom incorporated into the ligand framework is derived from molecular oxygen.

Dirhodium tetrakis((S)-N-(dodecylbenzenesulfonyl)prolinate) (Rh2(S-DOSP)4) is an effective catalyst for highly enantioselective cyclopropenation reactions between terminal alkynes and arylvinyldiazoacetates. The resulting vinylcyclopropenes can undergo rhodium-catalyzed regioselective rearrangement to cyclopentadienes. Computational studies indicate that the high enantioselectivity of the process is governed by the specific orientation of the alkyne during its approach to the carbenoid through a relatively late transition state. The specific orientation occurs due to the presence of a hydrogen bonding interaction between the alkyne hydrogen and a carboxylate ligand on the dirhodium catalyst.

Square planar cobalt(III) complexes with redox-active amidophenolate ligands are strong nucleophiles that react with alkyl halides, including CH(2)Cl(2), under gentle conditions to generate stable square pyramidal alkylcobalt(III) complexes. The net electrophilic addition reactions formally require 2e(-) oxidation of the metal fragment, but there is no change in metal oxidation state because the reaction proceeds with 1e(-) oxidation of each amidophenolate ligand. Although the four-coordinate complexes are very strong nucleophiles, they are mild outer-sphere reductants. Accordingly, addition of alkyl- or phenylzinc halides to the five-coordinate organometallic complexes regenerates the square planar starting materials and extrudes C-C coupling products. The net 2e(-) reductive elimination reaction also occurs without a oxidation state change at the cobalt(III) center. Together these reactions comprise a complete, well-defined cycle for cobalt Negishi-like cross-coupling of alkyl halides with organozinc reagents.

First-row transition metal-halide complexes of tris(2-dimethylaminophenyl)amine, L(Me), have been synthesized and characterized. X-ray crystallographic studies on [Co(L(Me))Br]BPh(4), [Ni(L(Me))Cl]BPh(4), [Fe(L(Me))Cl]BPh(4), and [Cu(L(Me))Cl]BF(4) have been performed, and in all cases the ligand produces five-coordinate complexes with distorted trigonal bipyramidal coordination geometries. Where possible, comparisons have been made to the structures of related neutral tripodal ligands. Spectroscopic and magnetic studies of these complexes are also described. The Cu(I)-carbonyl complexes [Cu(L(Me))(CO)]PF(6) and [Cu(Me(6)tren)(CO)]PF(6) (Me(6)tren = tris(N,N-dimethylaminoethyl)amine) have also been prepared. Infrared spectroscopic investigations of these carbonyl complexes confirm that L(Me) is a less electron donating ligand than Me(6)tren and indicate that L(Me) can impart a different coordination number in the solid-state.

A new family of sandwich-type polytungstophosphates containing two different types of metals in the central belt, M'(2)M(2)(PW(9)O(34))(2)(12-) (M' = Na or Li, M = Mn(2+), Co(2+), Ni(2+), and Zn(2+)), have been synthesized and characterized by infrared spectroscopy, (31)P solution NMR spectroscopy, and elemental analysis. Compounds Na(2)Co(2)(PW(9)O(34))(2)(12-) (Na2Co2), Na(2)Ni(2)(PW(9)O(34))(2)(12-) (Na2Ni2), Li(2)Ni(2)(PW(9)O(34))(2)(12-) (Li2Ni2), Na(2)Mn(2)(PW(9)O(34))(2)(12-) (Na2Mn2), and Li(2)Zn(2)(PW(9)O(34))(2)(12-) (Li2Zn2) were characterized by X-ray crystallography. All these compounds have similar structures, in which two transition-metal ions and two alkali metal ions (sodium or lithium) are sandwiched between two [B-alpha-PW(9)O(34)](9-) units; the transition and alkali metals reside in the internal and external (solvent exposed) positions of the central belt, respectively. By adding LiCl to aqueous solutions of compounds Na2M2, lithium-sodium exchanges in the external belt positions are observed by (31)P solution NMR spectroscopy and X-ray crystallography. Magnetic measurements indicate ferromagnetic exchange interactions between the two Ni(2+) ions in Na2Ni2 at 10-300 K and the two Co(2+) ions in Na2Co2 at 6-30 K. In contrast, Na2Mn2 exhibits an antiferromagnetic interaction between the Mn(2+) ions at 2-50 K.

We achieved the stereoselective syntheses of two different structural patterns corresponding to the enantiomers of the marine natural products abudinol B and muzitone, by developing two-directional tandem biomimetic cyclizations of polyepoxides of squalene analogues in which one alkene was functionalized as an enolsilane. In the course of this work, we demonstrated that the structure of muzitone was misassigned.

Several key properties of the water oxidation catalyst Rb(8)K(2)[{Ru(IV)(4)O(4)(OH)(2)(H(2)O)(4)}(gamma-SiW(10)O(36))(2)] and its mechanism of water oxidation are given. The one-electron oxidized analogue [{Ru(V)Ru(IV)(3)O(6)(OH(2))(4)}(gamma-SiW(10)O(36))(2)](11-) has been prepared and thoroughly characterized. The voltammetric rest potentials, X-ray structures, elemental analysis, magnetism, and requirement of an oxidant (O(2)) indicate these two complexes contain [Ru(IV)(4)O(6)] and [Ru(V)Ru(IV)(3)O(6)] cores, respectively. Voltammetry and potentiometric titrations establish the potentials of several couples of the catalyst in aqueous solution, and a speciation diagram (versus electrochemical potential) is calculated. The potentials depend on the nature and concentration of counterions. The catalyst exhibits four reversible couples spanning only ca. 0.5 V in the H(2)O/O(2) potential region, keys to efficient water oxidation at low overpotential and consistent with DFT calculations showing very small energy differences between all adjacent frontier orbitals. The voltammetric potentials of the catalyst are evenly spaced (a Coulomb staircase), more consistent with bulk-like properties than molecular ones. Catalysis of water oxidation by [Ru(bpy)(3)](3+) has been examined in detail. There is a hyperbolic dependence of O(2) yield on catalyst concentration in accord with competing water and ligand (bpy) oxidations. O(2) yields, turnover numbers, and extensive kinetics data reveal several features and lead to a mechanism involving rapid oxidation of the catalyst in four one-electron steps followed by rate-limiting H(2)O oxidation/O(2) evolution. Six spectroscopic, scattering, and chemical experiments indicate that the catalyst is stable in solution and under catalytic turnover conditions. However, it decomposes slowly in acidic aqueous solutions (pH < 1.5).

Enlarged acenothiadiazoles, which are easily prepared, display attractive optical and electrochemical properties. The annulation of thiadiazole to anthracene gives a stable material with optical properties similar to those of substituted pentacenes.

The first structurally characterized iridium-substituted polyoxometalate, K(14)[(IrCl(4))KP(2)W(20)O(72)].23H(2)O [1; orthorhombic Pnnm, with a = 18.6546(7) A, b = 19.5192(6) A, c = 14.8670(5) A, V = 5413.4(3) A(3), and Z = 2, final R = 0.0730], is reported. Elemental analysis, X-ray crystallography, and NMR all indicate one Ir atom in each molecule, while IR and electronic absorption spectroscopy, thermal gravimetric analysis, and electrochemistry all indicate its purity in both solid and solution states. Complex 1 is a molecular model of iridium supported on redox-active metal oxides, and aqueous solutions of 1 catalyze oxidation of water to O(2).

1,5-D-mannoseptanosyl di- and trisaccharide ring-size isomers of the corresponding mannopyranosyl oligosaccharides have been prepared. Remarkably, these compounds show no inhibition of the alpha-mannosidase-catalyzed hydrolysis of p-nitrophenyl-alpha-D-mannopyranoside.

The synthesis and X-ray characterization of two new dialkynylated diazatetracenes and the corresponding N, N-dihydrodiazatetracenes are reported. The dialkynylated heteroacenes are packed in a brick-wall motif that enforces significant overlap of their pi-faces. Cyclic voltammetry indicates that the dehydrogenated forms are easily reduced to their radical anions in solution. The planarity of these species validates the discussion of their aromaticity. Nucleus Independent Chemical Shift (NICS) computations demonstrate that both of these 20 pi and 24 pi electron systems are aromatic. Both experimental and computational results suggest that the aromaticity of the dihydroheteroacenes is reduced.

New five- and six-coordinate complexes containing the [Mn(III)(Br4cat)2](-) core (Br4cat(2-) = tetrabromo-1,2-catecholate) have been prepared. Homoleptic [Mn(III)(Br4cat)3](3-) reacts rapidly with O2 to produce tetrabromo-1,2-benzoquinone (Br4bq). The [Mn(III)(Br4cat)2](-) fragment is a robust catalytic platform for the aerobic conversion of catechols to quinones. The oxidase activity apparently derives from the coupling of metal- and ligand-centered redox events.

The complex [Ru(tpy)(CO)(2)TFA]+[PF(6)]- (where tpy = 2,2':6',2' '-terpyridine and TFA = CF(3)CO(2)-) (1) has been synthesized and fully characterized spectroscopically. The X-ray structure of the complex has been determined. The photopysical properties of the ruthenium complex and the free ligand tpy have been investigated at room temperature and at 77 K in acetonitrile solution and in the solid state. Their electronic spectra are highly influenced by intermolecular stacking interactions, both in solution and in the solid state. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations have been performed to characterize the electronic structure and the excited states of [Ru(tpy)(CO)(2)TFA]+[PF(6)]- and tpy. TDDFT calculations on three different conformations of free ligand have been performed as well. Absorption and emission spectra of tpy have been studied at different temperatures and concentrations in order to have a better understanding of this ruthenium derivative's properties. The absorption spectrum of 1 is characterized by metal-perturbed ligand-centered (LC) bands in the UV region. No metal-to-ligand charge transfer (MLCT) bands are observed in the visible for the complex. Only at high concentrations (10(-4) M) does a very weak band appear at 470 nm. At 77 K and low concentrations, solutions of 1 exhibit a major 3LC emission band centered at 468 nm (21.4 x 10(-3) cm(-1)). When the concentration of the complex is increased, an unstructured narrow emission at 603 nm (16.6 x 10(-3) cm(-1)), with a lifetime of 10 micros, dominates the emission spectrum in glassy acetonitrile. This emission originates from a pi-pi stacked dimeric (or oligomeric) species. TDDFT calculations performed on a tail-to-tail dimer structure, similar to that seen in the solid state, ascribe the transition to a triplet excited state, where intermolecular metal (d) --> ligand (pi*, polypyridine) charge transfer occurs. A good estimate of the transition energy is also obtained (623 nm, 1.94 eV).

In contradiction to current bonding paradigms, two terminal Au-oxo molecular complexes have been synthesized by reaction of AuCl3 with metal oxide-cluster ligands that model redox-active metal oxide surfaces. Use of K10[alpha2-P2W17O61].20H2O and K2WO4 (forming the [A-PW9O34]9- ligand in situ) produces K15H2[Au(O)(OH2)P2W18O68].25H2O (1); use of K10[P2W20O70(OH2)2].22H2O (3) produces K7H2[Au(O)(OH2)P2W20O70(OH2)2].27H2O (2). Complex 1 crystallizes in orthorhombic Fddd, with a=28.594(4) A, b=31.866(4) A, c=38.241(5) A, V=34844(7) A3, Z=16 (final R=0.0540), and complex 2 crystallizes in hexagonal P6(3)/mmc, with a=16.1730(9) A, b=16.1730(9) A, c=19.7659(15) A, V=4477.4(5) A3, Z=2 (final R=0.0634). The polyanion unit in 1 is disorder-free. Very short (approximately 1.76 A) Au-oxo distances are established by both X-ray and 30 K neutron diffraction studies, and the latter confirms oxo and trans aqua (H2O) ligands on Au. Seven findings clarify that Au and not W is present in the Au-oxo position in 1 and 2. Five lines of evidence are consistent with the presence of d8 Au(III) centers that are stabilized by the flanking polytungstate ligands in both 1 and 2: redox titrations, electrochemical measurements, 17 K optical spectra, Au L2 edge X-ray absorption spectroscopy, and Au-oxo bond distances. Variable-temperature magnetic susceptibility data for crystalline 1 and 2 establish that both solids are diamagnetic, and 31P and 17O NMR spectroscopy confirm that both remain diamagnetic in solution. Both complexes have been further characterized by FT-IR, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and other techniques.

We report the synthesis and characterization of five novel phenothiazine-containing cruciforms (5-9). The targets were prepared by a NaH-promoted Horner reaction of tetraethyl(2,5-diiodo-1,4-phenylene)bis(methylene)diphosphonate with 10-hexyl-10H-phenothiazine-3-carbaldehyde. The formed intermediary 3,3'-(1E,1'E)-2,2'-(2,5-diiodo-1,4-phenylene)bis(ethene-2,1-diyl)bis(10-hexyl-10H-phenothiazine) was reacted with several different aromatic alkynes (1-tert-butyl-4-ethynylbenzene, N,N-dibutyl-4-ethynylaniline, 1-ethynyl-3-(trifluoromethyl)benzene, and 1-ethynyl-3,5-bis(trifluoromethyl)benzene) to give the corresponding cruciform fluororphores (XF). The XFs were fully characterized by NMR and IR spectroscopy and then exposed to trifluoroacetic acid as well as to several metal triflates. The XFs show dramatic shifts in emission and to a lesser extent in absorption when exposed to magnesium triflate or zinc triflate. In the case of magnesium triflate, a blue shift in emission was observed; in contrast, addition of zinc triflate results in either quenching or a red-shifted emission. Due to the electronic situation, these XFs display spatially separated frontier molecular orbitals, allowing the HOMO or the LUMO of the XFs to be addressed independently by addition of zinc or magnesium ions. Phenothiazine XFs may have potential in array-type sensory applications for metal cations.

An antiaromatic-aromatic pair of peralkynylated heterocycles readily interconvert via redox reactions. [reaction: see text]

A series of dendrimeric conjugates based on a PAMAM (polyamidoamine) backbone with macrocyclic Gd-DO3A-P(ABn) complexes (monophosphinated analogue of DOTA) was prepared. The chelates were covalently attached to the G1-, G2-, and G4-PAMAM dendrimers through a thiourea linker in high loads (>90%). The prepared conjugates G1-(Gd-DO3A-P(BnN{CS}))(8), G2-(Gd-DO3A-P(BnN{CS}))(16), and G4-(Gd-DO3A-P(BnN{CS}))(59) showed relaxivities of 10.1, 14.1, and 18.6 s(-)(1) mM(-)(1) at 20 MHz and 37 degrees C and pH = 7.5, respectively. A variable-pH study (range 2-12) revealed up to 30% increase in the relaxivity at low pH for the G2-(Gd-DO3A-P(BnN{CS}))(16) conjugate. As confirmed by (1)H NMR titration of the unmodified G2 dendrimer, this is due to protonation of core tertiary amines leading to a more open and rigid structure. The variable-temperature (17)O NMR and (1)H NMRD relaxometric studies confirmed that the relaxivity is not controlled by water exchange but by rotational dynamics. A multiparametrical data evaluation using the Lipari-Szabo approach revealed that the water residence lifetime, (298)tau(M), for the conjugates studied was ca. 45-70 ns, which is longer than the value found for the monomeric model compound Gd-DO3A-P(ABn) (16 ns) but short enough so as not to limit the relaxivity. The global rotational correlation time, (298)tau(Rg), varied from 1.5 to 3.1 ns and seemed to indicate a sufficiently slow molecular tumbling to achieve the high relaxivities measured; however, the rigidity factor S(2) (approximately 0.26), describing the internal flexibility, was far from optimum. The overall relaxivity was significantly increased (e.g. by a factor of 1.8 for the G1-(Gd-DO3A-P(BnN{CS}))(8) conjugate) when a positively charged polyaminoacid like poly(Arg) or poly(Lys) was added to the conjugate solutions. The electrostatic interactions partially "freeze" the internal mobility of the conjugate and also slow down global motion. This assumption was confirmed by an evaluation of (1)H relaxometric data obtained for the G2-(Gd-DO3A-P(BnN{CS}))(16)-poly(Lys)(59) adduct. Importantly, it was proved that the adduct formation did not hamper the water exchange process.

An acetylene dye rotaxane with alpha-CD has been synthesized using the Heck-Cassar-Sonogashira-Hagihara-type reaction in aqueous solution. Free dye with tetracarboxylic acids is found to be highly sensitive to various metal ions, showing high Stern-Volmer constants, KSV. As CD encapsulation protects and stabilizes the threaded chromophore against an outside quencher, metal-insensitive biological tags are an obvious application for this class of molecules.

Side-chain-functionalized lactide analogues have been synthesized from commercially available amino acids and polymerized using stannous octoate as a catalyst. The synthetic strategy presented allows for the incorporation of any protected amino acid for the preparation of functionalized diastereomerically pure lactide monomers. The resulting functionalized cyclic monomers can be homopolymerized and copolymerized with lactides and then quantitatively deprotected forming new functional poly(lactide)-based materials. This strategy allows for the introduction of functional groups along a poly(lactide) (PLA) backbone that after deprotection can be viewed as chemical handles for further functionalization of PLA, yielding improved biomaterials for a variety of applications.

Fully oxidized alpha-AlIIIW12O40(5-) (1ox), and one-electron-reduced alpha-AlIIIW12O40(6-) (1red), are well-behaved (stable and free of ion pairing) over a wide range of pH and ionic-strength values at room temperature in water. Having established this, 27Al NMR spectroscopy is used to measure rates of electron exchange between 1ox (27Al NMR: 72.2 ppm relative to Al(H2O)63+; nu(1/2) = 0.77 Hz) and 1red (74.1 ppm; nu(1/2) = 0.76 Hz). Bimolecular rate constants, k, are obtained from line broadening in 27Al NMR signals as ionic strength, mu, is increased by addition of NaCl at the slow-exchange limit of the NMR time scale. The dependence of k on is plotted using the extended Debye-Hückel equation: log k = log k0 + 2alphaz1z2mu(1/2)/(1 + betarnu(1/2)), where z1 and z2 are the charges of 1ox and 1red, alpha and beta are constants, and r, the distance of closest contact, is fixed at 1.12 nm, the crystallographic diameter of a Keggin anion. Although not derived for highly charged ions, this equation gives a straight line (R2 = 0.996), whose slope gives a charge product, z1z2, of 29 +/- 2, statistically identical to the theoretical value of 30. Extrapolation to mu = 0 gives a rate constant k11 of (6.5 +/- 1.5) x 10(-3) M(-1) s(-1), more than 7 orders of magnitude smaller than the rate constant [(1.1 +/- 0.2) x 10(5) M(-1) s(-1)] determined by 31P NMR for self-exchange between P(V)W12O40(3-) and its one-electron-reduced form, P(V)W12O40(4-). Sutin's semiclassical model reveals that this dramatic difference arises from the large negative charges of 1ox and 1red. These results, including independent verification of k11, recommend 1red as a well-behaved electron donor for investigating outer-sphere electron transfer to molecules or nanostructures in water, while addressing a larger issue, the prediction of collision rates between uniformly charged nanospheres, for which 1ox and 1red provide a working model.

[structures: see text] Three diterpene-benzoate natural products, with novel carbon skeletons and an unusual proposed biosynthesis, were isolated from extracts of the Fijian red alga Callophycus serratus and identified by a combination of X-ray crystallographic, NMR, and mass spectral analyses. Bromophycolide A (1) displayed cytotoxicity against several human tumor cell lines via specific apoptotic cell death. This represents the first discovery of natural products incorporating a diterpene and benzoate skeleton into a macrolide system.

A terminal Pd-oxo unit is reported. The unit is encapsulated in a cavity defined by two [A-alpha-PW9O34]9- units fused together by one [WO(OH2)]4+ center and forms from Pd(II) in buffered media in the presence of O2. Both X-ray diffraction and EXAFS data are consistent with a Pd-oxo bond distance of ca. 1.65 A. 17O NMR studies confirm that the solid-state structure is maintained in solution.

The biogenesis of trans,syn,trans-fused polycyclic ether substructures of neurotoxic natural products of the brevetoxin-ciguatoxin family may involve regio- and stereoselective oxacyclizations of polyepoxides. We report that a biomimetic Lewis acid-promoted cyclization of 1 to 2 proceeds with endo-regioselectivity and anti-stereospecificity at each position of nucleophilic addition (C3, C7, C11, and C15 in 1) when the tandem cyclization cascade is terminated by a carbonyl oxygen nucleophile. In contrast to expectations from earlier reports with simpler epoxide substrates, alkyl substituents are not required at internal epoxide positions corresponding to C7 and C11 in polyepoxide 1. Furthermore, we report that trimethylsilyl serves as a removable regioselectivity-directing substituent at positions corresponding to C3 of 1. These results greatly expand the potential applications of this biomimetic oxacyclization cascade to the synthesis of fused polycyclic ether natural products.

The presence of an alkenyl substituent on the terminal epoxide of a polyepoxide substrate enhances the yield of all-endo-regioselective tandem oxacyclization to trans-syn-trans-fused polycyclic ethers. For a substrate in which the epoxide and alkene functional groups are separated by two methylene substituents, a novel bromonium ion-induced endo-regioselective cyclization to bromooxepane is also described. [reaction: see text]

The reaction of Fe(III) with Na(+) and K(+) salts of the trivacant [alpha-SiW(9)O(34)](10)(-) ligand have been investigated at pH 6 and pH 1. A new dimer, [(alpha-SiFe(3)W(9)(OH)(3)O(34))(2)(OH)(3)](11-) (1), is synthesized by reacting Na(7)H(3)[alpha-SiW(9)O(34)] or K(10)[alpha-SiW(9)O(34)] with exactly 3 equiv of Fe(III) in a 0.5 M sodium acetate solution (pH 6). The structure of 1, determined by single-crystal X-ray diffraction (a = 22.454(2) A, b = 12.387(2) A, c = 37.421(2), beta = 100.107(8) degrees , monoclinic, C2/c, Z = 4, R(1) = 5.11% based on 12739 independent reflections), consists of two [alpha-SiFe(3)W(9)(OH)(3)O(34)](4-) units linked by three Fe-mu-OH-Fe bonds. Reaction of K(10)[alpha-SiW(9)O(34)] with 3 equiv of Fe(III) in water (pH 1) yields [(alpha-Si(FeOH(2))(2)FeW(9)(OH)(3)O(34))(2)](8)(-2). The structure of 2 was also determined by single-crystal X-ray diffraction (a = 36.903(2) A, b = 13.9868(9) A, c = 21.7839(13) A, beta = 122.709(1) degrees , monoclinic, C2/c, Z = 4, R(1) = 4.57% based on 11787 independent reflections). It consists of two [alpha-Si(FeOH(2))(2)FeW(9)(OH)(3)O(34)](4-) Keggin units linked by a single edge. The terminal ligand on Fe1 in each trisubstituted Keggin unit becomes a mu(2) oxo ligand bridging to a [WO(6)](2-) moiety. The UV-vis spectra of both complexes show the characteristic oxygen-to-metal-charge-transfer bands of polyoxometalates as well as an Fe(III)-centered band at 436 nm (epsilon = 146 M(-1) cm(-1)) and 456 nm (epsilon = 104 M(-1) cm(-1)) for complexes 1 and 2, respectively. Differential scanning calorimetry data show that complex 1 decomposes between 575 and 600 degrees C whereas no decomposition is observed for complex 2 up to temperatures of 600 degrees C.

A new type of coordination network polymer involving the redox-active polyanion, PV(2)Mo(10)O(40)(5)(-), and bridging -Cu(II)(OH(2))(4)- units, [[(Cu(II)(OH(2))(4))(3)(OH)]PV(2)Mo(10)O(40)](n)() (1), has been characterized by X-ray crystallography and several other methods. It is the first efficient heterogeneous (insoluble) catalyst for selective and rapid sulfoxidation using only the ambient environment (air at room temperature). Catalytic activity is enhanced by soluble nitrate in nontoxic perfluoropolyether (PFPE) media.

[reaction: see text] The stereoselectivity of Lewis acid-induced endo-regioselective oxacyclizations of 1,4-diepoxides is dependent upon the nature of the terminating nucleophile. For instance, the tert-butyl carbonate-substituted diepoxide of 3,6-dimethylhepta-2,5-dien-1-ol provides a cis-fused bicyclic product, whereas the N,N-dimethylcarbamate derivative affords the trans-fused diastereomer. Stereospecific and regioselective conversion of the tertiary carbamate-terminated 1,4,7-triepoxide (I) to tricyclic all-trans-fused polypyran (II) is also demonstrated.

Reaction of the trivacant lacunary complex, alpha-Na(12)[As(2)W(15)O(56)], with an aqueous solution of Fe(NO(3))(3).9H(2)O yields the sandwich-type polyoxometalate, alphabetabetaalpha-Na(12)(Fe(III)OH(2))(2)Fe(III)(2)(As(2)W(15)O(56))(2) (Na1). The structure of this complex, determined by single-crystal X-ray crystallography (a = 13.434(1) A, b = 13.763(1) A, c = 22.999(2) A, alpha = 90.246(2) degrees, beta = 102.887(2) degrees, gamma = 116.972(1) degrees, triclinic, Ponemacr;, R1 = 5.5%, based on 25342 independent reflections), consists of an Fe(III)(4) unit sandwiched between two trivacant alpha-As(2)W(15)O(56)(12)(-) moieties. UV-vis, infrared, cyclic voltammetry, and elemental analysis data are all consistent with the structure determined from X-ray analysis. Magnetization studies confirm that the four Fe(III) centers are antiferromagnetically coupled. A cyclic voltammogram of Na1 reveals that a three-wave W(VI) system replaces the two-wave W(VI) system found in the precursor alpha-As(2)W(15)O(56)(12)(-) complex. The observed modifications in the CV patterns of Na1 and alpha-As(2)W(15)O(56)(12)(-) are most likely due to subsequent changes in the acid-base properties of two reduced POMs that occur as a result of Fe(III) incorporation. Na1 is shown to be more efficient than the monosubstituted complex alpha(2)-As(2)(Fe(III)OH(2))W(17)O(61)(7)(-) in the electrocatalytic reduction of dioxygen. This is attributed to cooperativity effects among the adjacent Fe(III) centers in Na1.

Treatment of 2-phenylhex-5-enal with benzylamine followed by sequential reaction with ethylthioacetyl chloride and sodium periodate oxidation afforded a E/Z mixture of alpha-sulfinylamides. As anticipated from a 4pi-conrotatory mechanism, cyclization of each olefin afforded fused isoquinoline lactams as single diastereomers epimeric at the ethylthio position without any cross contamination. Some preliminary studies were directed toward the synthesis of mesembrine using a 3,4-dimethoxy aryl group. In this case, the Z-enamide prefers to undergo electrophilic aromatic substitution to give a substituted azepinone as the preferred product in 87% yield. In contrast, the E-enamide isomer provided the desired hydroindolone. The convergency and stereochemical control associated with the tandem Pummerer /Mannich cyclization make it particularly suited for the assembly of jamtine, a tetrahydroisoquinoline alkaloid reputed for its therapeutic properties. The key step in the synthesis involves a domino thionium/N-acyliminium ion cyclization to provide the tricyclic ring skeleton 27a as the major diastereomer. Deprotonation of 27a with NaH gave 28a, which contains the fully assembled skeleton of jamtine. Completion of the synthesis entailed installation of the double bond and reduction of the lactam. Oxidation of a synthetic sample of jamtine with MCPBA afforded the corresponding N-oxide, which does not match the spectral data reported in the literature for this alkaloid. Our synthetic efforts raise the possibility of a revision of the earlier assignment.

beta-[SiW(12)O(40)](4)(-) (C(3)(v) symmetry) is sufficiently higher in energy than its alpha-isomer analogue that effectively complete conversion to alpha-[SiW(12)O(40)](4)(-) (T(d)) is observed. By contrast, beta- and alpha-[AlW(12)O(40)](5)(-) (beta- and alpha-1; C(3)(v) and T(d), respectively) are sufficiently close in energy that both isomers are readily seen in (27)Al NMR spectra of equilibrated (alpha-beta) mixtures. Recently published DFT calculations ascribe the stability of beta-1 to an electronic effect of the large, electron-donating [AlO(4)](5)(-) (T(d)) moiety encapsulated within the polarizable, fixed-diameter beta-W(12)O(36) (C(3)(v)) shell. Hence, no unique structural distortion of beta-1 is needed or invoked to explain its unprecedented stability. The results of these DFT calculations are confirmed by detailed comparison of the X-ray crystal structure of beta-1 (beta-Cs(4.5)K(0.5)[Al(III)W(12)O(40)].7.5H(2)O; orthorhombic, space group Pmc2(1), a = 16.0441(10) A, b = 13.2270(8) A, c = 20.5919(13) A, Z = 4 (T = 100(2) K)) with previously reported structures of alpha-1, alpha- and beta-[SiW(12)O(40)](4)(-), and beta(1)-[SiMoW(11)O(40)](4)(-).

Reaction of alpha-P(2)W(15)O(56)(12-) and Fe(III) in a saturated NaCl solution produces a trisubstituted Wells-Dawson structure with three low-valent metals, alpha-(Fe(III)Cl)(2)(Fe(III)OH(2))P(2)W(15)O(59)(11-) (1). Dissolution of this species into 1 M NaBr (Br(-) is non-coordinating) gives the triaquated species alpha-(Fe(III)OH(2))(3)P(2)W(15)O(59)(9-) (2). Ionic strength values of 1 M or greater are necessary to avoid decomposition of 1 or 2 to the conventional sandwich-type complex, alpha beta beta alpha-(Fe(III)OH(2))(2)Fe(III)(2)(P(2)W(15)O(56))(2)(12-) (3). If the pH is greater than 5, a new triferric sandwich, alpha alpha beta alpha-(NaOH(2))(Fe(III)OH(2))Fe(III)(2)(P(2)W(15)O(56))(2)(14-) (4), forms rather than 3. Like the previously reported Wells-Dawson-derived sandwich-type structures with three metals in the central unit ([TM(II)Fe(III)(2)(P(2)W(15)O(56))(P(2)TM(II)(2)W(13)O(52))],(16-) TM = Cu, Co), this complex has a central alpha-junction and a central beta-junction. Thermal studies suggest that 4 is more stable than 3 over a wide range of temperatures and pH values. The intrinsic Jahn-Teller distortion of d-electron-containing metal ions incorporated into the external sites of the central multi-metal unit impacts the stoichiometry of their incorporation (with a consequent change in the inter-POM-unit connectivity, where POM = polyoxometalate). Reaction of non-distorting Ni(II) with the diferric lacunary sandwich-type POM alpha alpha alpha alpha-(NaOH(2))(2)Fe(III)(2)(P(2)W(15)O(56))(2)(16-) (5) produces alpha beta beta alpha-(Ni(II)OH(2))(2)Fe(III)(2)(P(2)W(15)O(56))(2)(14-) (6), a Wells-Dawson sandwich-type structure with two Ni(II) and two Fe(III) in the central unit. All structures are characterized by (31)P NMR, IR, UV-vis, magnetic susceptibility, and X-ray crystallography. Complexes 4 and 6 are highly selective and effective catalysts for the H(2)O(2)-based epoxidation of alkenes.

Reactions of lithium, sodium, and potassium salts of 2,4,6-trimethyl-s-triazine (1) with 2-halomethyl-4,6-dimethyl-s-triazine (2) (X = Cl, Br) in glyme have been studied and found to give 1,2-bis(4,6-dimethyl-s-triazin-2-yl)ethane (3), 1,2-bis(4,6-dimethyl-s-triazin-2-yl)ethene (5), 1,2,3-tris(4,6-dimethyl-s-triazin-2-yl)cyclopropane (6), 1,2,3-tris(4,6-dimethyl-s-triazin-2-yl)propane (7), and 1,2,3,4-tetrakis(4,6-dimethyl-s-triazin-2-yl)butane (8). It is proposed that product 3 is formed primarily via an S(N)2 reaction, whereas the remaining products are formed primarily via carbenoid reactions that are enumerated.

Boron trifluoride-etherate promotes the endo-selective oxacyclization of polyepoxides derived from various acyclic terpenoid polyalkenes, including geraniol, farnesol, and geranylgeraniol, providing an efficient and stereoselective synthesis of substituted oxepanes and fused polyoxepanes. The mechanism of the oxacyclization reaction probably involves intramolecular nucleophilic addition of epoxide oxygen to open another epoxide that is activated as an electrophile by the Lewis acid. These oxacyclizations proceed stereospecifically with inversion of configuration upon opening of each epoxide to provide trans-fused polycyclic products. The oxacyclization cascade is terminated by a tethered nucleophile, which may be the carbonyl oxygen of a ketone, ester, or carbonate, or a trisubstituted alkene. The best oxacyclization yields are generally observed with tert-butyl carbonate as the terminating nucleophile, although in some cases the oxacyclization products include formation of tert-butyl ethers as a minor product. The oxacyclization transformations described herein may mimic ring-forming steps in the biosynthesis of trans-syn-trans-fused polycyclic ether marine natural products.

Trends in the stability of alpha- and beta-Keggin heteropolytungstates of the second-row main-group heteroatoms Al(III), Si(IV), and P(V) are elaborated by data that establish the roles of kinetic and thermodynamic control in the formation and isomerization of Keggin tungstoaluminates. Slow, room-temperature co-condensation of Al(III) and W(VI) (2:11 molar ratio) in water gives a pH 7 solution containing beta(1) and beta(2) isomers of [Al(AlOH(2))W(11)O(39)](6)(-) (beta(1)- and beta(2)-1). Partial equilibration of this kinetic product mixture by gentle heating (2 h at 100 degrees C) or, alternatively, co-condensation of Al(III) and W(VI) for 2.5 h at 100 degrees C both give mixtures of beta(2)-, beta(3)-, and alpha-1. Full equilibration, by prolonged heating (25 days at 100 degrees C), gives an isomerically pure solution of alpha-1, thus demonstrating that isomerization occurs in the direction beta(1) --> beta(2) --> beta(3) --> alpha. Furthermore, kinetically controlled conversions of 1 to H(5)[AlW(12)O(40)] (2)-achieved by heating pH 0-0.2 solutions of 1 for 5 days at 100 degrees C-occur with retention of isomeric integrity, such that alpha-1 is converted to alpha-2 (92%; 8% beta), while mixtures of beta(2)- and beta(3)-1 are converted to beta-2 (87%; 13% alpha). These data, when combined with previously reported observations (equilibria between alpha- and beta-2, kinetically controlled hydrolyses of alpha-2 to alpha-[AlW(11)O(39)](9)(-) (alpha-3) and of beta-2 to beta(2)-3, and equilibria between beta(3)- and alpha-3), provide a comprehensive picture regarding the roles of kinetic and thermodynamic control. Finally, a general method for preparation of the isomerically pure derivatives alpha-K(9)(-)(n)()[AlM(n)()(+)W(11)O(39)] (4), M(n)()(+) = Al(III), [V(IV)O](2+), [V(V)O](3+), Mn(II), Mn(III), Mn(IV), Co(II), and Co(III), is provided. The presence of Mn(IV) is confirmed by cyclic voltammetry, pK(a) values of the aquo ligands on 4 are determined by pH titration, and the isomeric structure of these derivatives is established by (27)Al, (51)V, and (183)W NMR and IR spectroscopies and X-ray crystallography.

Reaction of the diferric sandwich-type polyoxometalate (NaOH(2))(2)Fe(III)(2)(P(2)W(15)O(56))(2)(16-)(1) with excess aqueous Cu(II) or Co(II) yields a new type of d-electron-metal substituted polyoxometalate, [TM(II)Fe(III)(2)(P(2)W(15)O(56)) (P(2)TM(II)(2)W(13)O(52))](16-), TM = Cu (2), Co (3), respectively. The structure of the sodium salt of 2 (Na2), determined by single-crystal X-ray diffraction analysis (a = 13.4413(9) A, b = 21.2590(15) A, c = 25.5207(18) A, alpha = 80.475(2) degrees, beta = 85.555(2) degrees, gamma = 89.563(2) degrees, triclinic, P(-)1, R1 = 5.42%, based on 43097 independent reflections), consists of a defect Fe(2)Cu central unit sandwiched between two different trivacant Wells-Dawson-type units, P(2)W(15) and P(2)Cu(2)W(13), where the latter unit has two octahedral Cu(II) ions substituted for two adjacent belt W(VI) atoms. The CuO(5)OH(2) octahedron in the central unit shows pronounced Jahn-Teller distortion. A low-resolution X-ray structure of Na3 is included in the Supporting Information. UV-visible, infrared, (31)P NMR, cyclic voltammetric, and elemental analysis data are all consistent with the structure determined from the X-ray analysis. Cyclic voltammograms of 2 and 3 exhibit multiple electron-transfer processes under ambient conditions, and copper or cobalt incorporation into the framework of 1 results in a substantial pertubation of the electrochemical properties of the polyoxotungstate framework. The tetra-n-butylammonium salts of 2 and 3 (readily prepared by metathesis) are stable and effective catalysts for the oxidation of some alkenes with high yields based on H(2)O(2).

[reaction: see text] This communication describes the first examples of tandem endo-regioselective and stereospecific oxacyclizations of 1, 5-diepoxides to oxepane products and a similar tandem oxacyclization of 1,5,9-triepoxides to fused bisoxepane cyclic carbonates. A mechanism for these biomimetic oxacyclizations is proposed in which the epoxides act as both electrophilic and nucleophilic reaction partners.