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81.
Helton JS Matan K Shores MP Nytko EA Bartlett BM Yoshida Y Takano Y Suslov A Qiu Y Chung JH Nocera DG Lee YS 《Physical review letters》2007,98(10):107204
We have performed thermodynamic and neutron scattering measurements on the S=1/2 kagomé lattice antiferromagnet ZnCu3(OH)6Cl2. The susceptibility indicates a Curie-Weiss temperature of theta CW approximately = -300 K; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-T follows a power law temperature dependence. These results suggest that an unusual spin liquid state with essentially gapless excitations is realized in this kagomé lattice system. 相似文献
82.
A series of manganese Hangman salen ligand platforms functionalized by tert-butyl groups in the 3 and 3' positions using the Suzuki cross-coupling methodology are presented. The Hangman platforms support multielectron chemistry mediated by proton-coupled electron transfer (PCET), as demonstrated by their ability to promote the catalytic disproportionation of hydrogen peroxide to oxygen and water via a high-valent metal oxo. The addition of the steric groups to the salen macrocycle leads to enhanced catalase activity by circumventing side reactions that sequester the catalyst off pathway. The stereochemistry imposed by the cyclohexanediamine backbone of the salen platform is revealed by the epoxidation of 1,2-dihydronapthalene by a variety of oxidants. Improved enantiomeric excess and catalase activity as compared to sterically unmodified counterparts establishes the efficacy of the tert-butyl groups in promoting PCET catalysis on the Hangman platform. 相似文献
83.
84.
Rosenthal J Chng LL Fried SD Nocera DG 《Chemical communications (Cambridge, England)》2007,(25):2642-2644
Incorporation of a disparate set of aryl groups appended to the meso-positions of Hangman porphyrin xanthene architectures dramatically impacts the ability of such systems to catalyze the disproportionation of H2O(2)via the catalase reaction. 相似文献
85.
Rui Sun Dr. Yangzhong Qin Prof. Dr. Daniel G. Nocera 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2020,132(24):9614-9620
Self-sustained NiI/III cycles are established as a potentially general paradigm in photoredox Ni-catalyzed carbon–heteroatom cross-coupling reactions through a strategy that allows us to recapitulate photoredox-like reactivity in the absence of light across a wide range of substrates in the amination, etherification, and esterification of aryl bromides, the latter of which has remained, hitherto, elusive under thermal Ni catalysis. Moreover, the accessibility of esterification in the absence of light is especially notable because previous mechanistic studies on this transformation under photoredox conditions have unanimously invoked energy-transfer-mediated pathways. 相似文献
86.
Holder PG Pizano AA Anderson BL Stubbe J Nocera DG 《Journal of the American Chemical Society》2012,134(2):1172-1180
Incorporation of 2,3,6-trifluorotyrosine (F(3)Y) and a rhenium bipyridine ([Re]) photooxidant into a peptide corresponding to the C-terminus of the β protein (βC19) of Escherichia coli ribonucleotide reductase (RNR) allows for the temporal monitoring of radical transport into the α2 subunit of RNR. Injection of the photogenerated F(3)Y radical from the [Re]-F(3)Y-βC19 peptide into the surface accessible Y731 of the α2 subunit is only possible when the second Y730 is present. With the Y-Y established, radical transport occurs with a rate constant of 3 × 10(5) s(-1). Point mutations that disrupt the Y-Y dyad shut down radical transport. The ability to obviate radical transport by disrupting the hydrogen bonding network of the amino acids composing the colinear proton-coupled electron transfer pathway in α2 suggests a finely tuned evolutionary adaptation of RNR to control the transport of radicals in this enzyme. 相似文献
87.
Halogen oxidation and halogen photoelimination chemistry of a platinum-rhodium heterobimetallic core
The heterobimetallic complexes, PtRh(tfepma)(2)(CN(t)Bu)X(3) (X = Cl, Br), are assembled by the treatment of Pt(cod)X(2) (cod =1,5-cyclooctadiene) with {Rh(cod)X}(2), in the presence of tert-butylisonitrile (CN(t)Bu) and tfepma (tfepma = bis(trifluoroethoxyl)phosphinomethylamine). The neutral complexes contain Pt-Rh single bonds with metal-metal separations of 2.6360(3) and 2.6503(7) ? between the square planar Pt and octahedral Rh centers for the Cl and Br complexes, respectively. Oxidation of the XPt(I)Rh(II)X(2) cores with suitable halide sources (PhICl(2) or Br(2)) furnishes PtRh(tfepma)(2)(CN(t)Bu)X(5), which preserves a Pt-Rh bond. For the chloride system, the initial oxidation product orients the platinum-bound chlorides in a meridional geometry, which slowly transforms to a facial arrangement in pentane solution as verified by X-ray crystal analysis. Irradiation of the mer- or fac-Cl(3)Pt(III)Rh(II)Cl(2) isomers with visible light in the presence of olefin promotes the photoelimination of halogen and regeneration of the reduced ClPt(I)Rh(II)Cl(2) core. In addition to exhibiting photochemistry similar to that of the chloride system, the oxidized bromide cores undergo thermal reduction chemistry in the presence of olefin with zeroth-order olefin dependence. Owing to an extremely high photoreaction quantum yield for the fac-ClPt(I)Rh(II)Cl(2) isomer, details of the X(2) photoelimination have been captured by transient absorption spectroscopy. We now report the first direct observation of the photointermediate that precedes halogen reductive elimination. The intermediate is generated promptly upon excitation (<8 ns), and halogen is eliminated from it with a rate constant of 3.6 × 10(4) s(-1). As M-X photoactivation and elimination is the critical step in HX splitting, these results establish a new guidepost for the design of HX splitting cycles for solar energy storage. 相似文献
88.
Teets TS Cook TR McCarthy BD Nocera DG 《Journal of the American Chemical Society》2011,133(21):8114-8117
The two-electron mixed-valence dirhodium complex Rh(2)(0,II)(tfepma)(2)(CN(t)Bu)(2)Cl(2) (tfepma = CH(3)N[P(OCH(2)CF(3))(2)](2)) reacts with HCl to furnish two isomeric dirhodium hydrido-chloride complexes, Rh(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)Cl(3)H. In the presence of HCl, the hydride complex effects the reduction of 0.5 equiv of O(2) to 1 equiv of H(2)O, generating Rh(2)(II,II)(tfepma)(2)(CN(t)Bu)(2)Cl(4), which can be prepared independently by chlorine oxidation of the Rh(2)(0,II) precursor. The starting Rh(2)(0,II) complex is regenerated photochemically to close an oxygen-to-water reduction photocycle. 相似文献
89.
T.M. McQueen T.H. Han P.W. Stephens D.G. Nocera 《Journal of solid state chemistry》2011,184(12):3319-3323
A new transition metal hydroxide chloride containing kagomé layers of magnetic ions, CdCu3(OH)6Cl2, has been synthesized and structurally characterized. The actual low symmetry P21/n structure can be derived from the ideal trigonal one with a change in cation distribution and coherent distortions of the anion framework. The result is a fundamentally different Cu2+ kagomé framework than found in the related Herbertsmithite and Kapellasite minerals. Magnetization measurements show no transition to long range magnetic order above T=2 K, despite strong antiferromagnetic interactions with a Weiss temperature of θw=−150 K. Furthermore, we show that the structure of CdCu3(OH)6Cl2 and related hydroxide chlorides can be rationalized on the basis of [(OH)3Cl]4− pseudopolyatomic anions that pack and rotate, in much the same way as do traditional polyatomic anions. This opens the door to rational design of new and useful hydroxide chloride materials. 相似文献
90.
S Rayaprol Krushna Mavani CM Thaker DS Rana Keka Chakravorty SK Paranjape M Ramanadham Nilesh A Kulkarni DG Kuberkar 《Pramana》2002,58(5-6):877-880
A new series of mixed oxide superconductors with the stoichiometric composition La2−x
Dy
x
Ca
y
Ba2Cu4+y
O
z
(x=0.0 − 0.5, y=2x) has been studied for structural and superconductiong properties. Our earlier studies on La2−x
(Y/Er)
x
Ca
y
Ba2Cu4+y
O
z
series, show a strong dependence of T
c on hole concentration (p
sh). In the present work, the results of the analysis of the neutron diffraction measurements at room temprerature on x=0.3 and 0.5 samples are reported. It is interesting to know that Ca substitutes for both La and Ba site with concomitant
displacement of La onto Ba site. Superconductivity studies show that maximum T
c is obtained for x=0.5, y=1.0 sample (T
c ∼ 75 K), for La1.5Dy0.5Ca1Ba2Cu5O
z
(La-2125). 相似文献